draft-ietf-manet-dsr-03.txt   draft-ietf-manet-dsr-04.txt 
IETF MANET Working Group Josh Broch
INTERNET-DRAFT David B. Johnson IETF MANET Working Group David B. Johnson, Rice University
David A. Maltz INTERNET-DRAFT David A. Maltz, AON Networks
Carnegie Mellon University 17 November 2000 Yih-Chun Hu, Carnegie Mellon University
22 October 1999 Jorjeta G. Jetcheva, Carnegie Mellon University
The Dynamic Source Routing Protocol for Mobile Ad Hoc Networks The Dynamic Source Routing Protocol for Mobile Ad Hoc Networks
<draft-ietf-manet-dsr-03.txt> <draft-ietf-manet-dsr-04.txt>
Status of This Memo Status of This Memo
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This Internet-Draft is a submission to the IETF Mobile Ad Hoc
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Abstract Abstract
Dynamic Source Routing (DSR) is a routing protocol designed The Dynamic Source Routing protocol (DSR) is a simple and efficient
specifically for use in mobile ad hoc networks. The protocol allows routing protocol designed specifically for use in multi-hop wireless
nodes to dynamically discover a source route across multiple network ad hoc networks of mobile nodes. DSR allows the network to be
hops to any destination in the ad hoc network. When using source completely self-organizing and self-configuring, without the need
routing, each packet to be routed carries in its header the complete, for any existing network infrastructure or administration. The
ordered list of nodes through which the packet must pass. A key protocol is composed of the two mechanisms of "Route Discovery"
advantage of source routing is that intermediate hops do not need and "Route Maintenance", which work together to allow nodes to
to maintain routing information in order to route the packets they discover and maintain source routes to arbitrary destinations in the
receive, since the packets themselves already contain all of the ad hoc network. The use of source routing allows packet routing
necessary routing information. This, coupled with the dynamic, to be trivially loop-free, avoids the need for up-to-date routing
on-demand nature of DSR's Route Discovery, completely eliminates the information in the intermediate nodes through which packets are
need for periodic router advertisements and link status packets, forwarded, and allows nodes forwarding or overhearing packets to
significantly reducing the overhead of DSR, especially during periods cache the routing information in them for their own future use. All
when the network topology is stable and these packets serve only as aspects of the protocol operate entirely on-demand, allowing the
keep-alives. routing packet overhead of DSR to scale automatically to only that
needed to react to changes in the routes currently in use. This
document specifies the operation of the DSR protocol for routing
unicast IP packets in multi-hop wireless ad hoc networks.
Contents Contents
Status of This Memo i Status of This Memo i
Abstract i Abstract ii
1. Introduction 1 1. Introduction 1
2. Changes 1 2. Assumptions 3
3. Assumptions 1 3. DSR Protocol Overview 5
3.1. Basic DSR Route Discovery . . . . . . . . . . . . . . . . 5
3.2. Basic DSR Route Maintenance . . . . . . . . . . . . . . . 7
3.3. Additional Route Discovery Features . . . . . . . . . . . 8
3.3.1. Caching Overheard Routing Information . . . . . . 8
3.3.2. Replying to Route Requests using Cached Routes . 9
3.3.3. Preventing Route Reply Storms . . . . . . . . . . 10
3.3.4. Route Request Hop Limits . . . . . . . . . . . . 12
3.4. Additional Route Maintenance Features . . . . . . . . . . 12
3.4.1. Packet Salvaging . . . . . . . . . . . . . . . . 12
3.4.2. Automatic Route Shortening . . . . . . . . . . . 13
3.4.3. Increased Spreading of Route Error Messages . . . 14
4. Terminology 2 4. Conceptual Data Structures 15
4.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 2 4.1. Route Cache . . . . . . . . . . . . . . . . . . . . . . . 15
4.2. Specification Language . . . . . . . . . . . . . . . . . 4 4.2. Route Request Table . . . . . . . . . . . . . . . . . . . 17
4.3. Send Buffer . . . . . . . . . . . . . . . . . . . . . . . 18
4.4. Retransmission Buffer . . . . . . . . . . . . . . . . . . 19
5. Protocol Overview 5 5. Packet Formats 20
5.1. Route Discovery and Route Maintenance . . . . . . . . . . 5 5.1. Destination Options Header . . . . . . . . . . . . . . . 21
5.2. Packet Forwarding . . . . . . . . . . . . . . . . . . . . 6 5.1.1. DSR Route Request Option . . . . . . . . . . . . 22
5.3. Multicast Routing . . . . . . . . . . . . . . . . . . . . 7 5.2. Hop-by-Hop Options Header . . . . . . . . . . . . . . . . 24
5.2.1. DSR Route Reply Option . . . . . . . . . . . . . 25
5.2.2. DSR Route Error Option . . . . . . . . . . . . . 27
5.2.3. DSR Acknowledgment Option . . . . . . . . . . . . 29
5.3. DSR Routing Header . . . . . . . . . . . . . . . . . . . 30
6. Conceptual Data Structures 7 6. Detailed Operation 33
6.1. Route Cache . . . . . . . . . . . . . . . . . . . . . . . 7 6.1. General Packet Processing . . . . . . . . . . . . . . . . 33
6.2. Route Request Table . . . . . . . . . . . . . . . . . . . 9 6.1.1. Originating a Packet . . . . . . . . . . . . . . 33
6.3. Send Buffer . . . . . . . . . . . . . . . . . . . . . . . 9 6.1.2. Adding a DSR Routing Header to a Packet . . . . . 34
6.4. Retransmission Buffer . . . . . . . . . . . . . . . . . . 9 6.1.3. Receiving a Packet . . . . . . . . . . . . . . . 36
6.1.4. Processing a Routing Header in a Received Packet 38
6.2. Route Discovery Processing . . . . . . . . . . . . . . . 40
6.2.1. Originating a Route Request . . . . . . . . . . . 40
6.2.2. Processing a Received Route Request Option . . . 42
6.2.3. Generating Route Replies using the Route Cache . 43
6.2.4. Originating a Route Reply . . . . . . . . . . . . 45
6.2.5. Processing a Route Reply Option . . . . . . . . . 46
6.3. Route Maintenance Processing . . . . . . . . . . . . . . 47
6.3.1. Using Network-Layer Acknowledgments . . . . . . . 47
6.3.2. Using Link Layer Acknowledgments . . . . . . . . 48
6.3.3. Originating a Route Error . . . . . . . . . . . . 48
6.3.4. Processing a Route Error Option . . . . . . . . . 49
6.3.5. Salvaging a Packet . . . . . . . . . . . . . . . 49
7. Packet Formats 11 7. Constants 50
7.1. Destination Options Headers . . . . . . . . . . . . . . . 11
7.1.1. DSR Route Request Option . . . . . . . . . . . . 12
7.2. Hop-by-Hop Options Headers . . . . . . . . . . . . . . . 14
7.2.1. DSR Route Reply Option . . . . . . . . . . . . . 15
7.2.2. DSR Route Error Option . . . . . . . . . . . . . 17
7.2.3. DSR Acknowledgment Option . . . . . . . . . . . . 18
7.3. DSR Routing Header . . . . . . . . . . . . . . . . . . . 20
8. Detailed Operation 23 8. IANA Considerations 51
8.1. Originating a Data Packet . . . . . . . . . . . . . . . . 23
8.2. Originating a Packet with a DSR Routing Header . . . . . 23
8.3. Processing a Routing Header . . . . . . . . . . . . . . . 24
8.4. Route Discovery . . . . . . . . . . . . . . . . . . . . . 25
8.4.1. Originating a Route Request . . . . . . . . . . . 25
8.4.2. Processing a Route Request Option . . . . . . . . 26
8.4.3. Generating Route Replies using the Route Cache . 27
8.4.4. Originating a Route Reply . . . . . . . . . . . . 28
8.4.5. Processing a Route Reply Option . . . . . . . . . 29
8.5. Route Maintenance . . . . . . . . . . . . . . . . . . . . 30
8.5.1. Using Network-Layer Acknowledgments . . . . . . . 30
8.5.2. Using Link Layer Acknowledgments . . . . . . . . 32
8.5.3. Originating a Route Error . . . . . . . . . . . . 32
8.5.4. Processing a Route Error Option . . . . . . . . . 33
8.5.5. Salvaging a Packet . . . . . . . . . . . . . . . 33
9. Optimizations 35 9. Security Considerations 52
9.1. Leveraging the Route Cache . . . . . . . . . . . . . . . 35
9.1.1. Promiscuous Learning of Source Routes . . . . . . 35
9.2. Preventing Route Reply Storms . . . . . . . . . . . . . . 36
9.3. Piggybacking on Route Discoveries . . . . . . . . . . . . 37
9.4. Discovering Shorter Routes . . . . . . . . . . . . . . . 37
9.5. Rate Limiting the Route Discovery Process . . . . . . . . 38
9.6. Improved Handling of Route Errors . . . . . . . . . . . . 39
9.7. Increasing Scalability . . . . . . . . . . . . . . . . . 39
10. Path-State and Flow-State Mechanisms 40 Appendix A. Location of DSR in the ISO Network Reference Model 53
10.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 40
10.2. Path-State and Flow-State Identifiers . . . . . . . . . . 41
10.3. Path-State Creation, Use, and Maintenance . . . . . . . . 42
10.3.1. Creating Path-State for Routing . . . . . . . . . 42
10.3.2. Monitoring Characteristics of the Path . . . . . 43
10.3.3. Candidate Metrics . . . . . . . . . . . . . . . . 45
10.4. Flow-State Creation, Use, and Maintenance . . . . . . . . 46
10.4.1. Requesting Promises along Existing Paths . . . . 46
10.4.2. Requesting Promises as Part of Route Discovery . 48
10.4.3. Providing Notifications of Changing Path Metrics 49
10.5. Expiration of State from Intermediate Nodes . . . . . . . 50
10.6. Packet Formats . . . . . . . . . . . . . . . . . . . . . 51
10.6.1. Identifier Option . . . . . . . . . . . . . . . . 51
10.6.2. Path-Metrics Option . . . . . . . . . . . . . . . 52
10.6.3. Flow Request Option . . . . . . . . . . . . . . . 54
10.6.4. Encoding Path-Metrics . . . . . . . . . . . . . . 55
11. Constants 58 Appendix B. Implementation and Evaluation Status 54
12. IANA Considerations 59 Acknowledgements 55
13. Security Considerations 60 References 56
Location of DSR Functions in the ISO Model 61 Chair's Address 59
Implementation Status 62 Authors' Addresses 60
Acknowledgments 63 1. Introduction
References 64 The Dynamic Source Routing protocol (DSR) [12, 13] is a simple and
efficient routing protocol designed specifically for use in multi-hop
wireless ad hoc networks of mobile nodes. Using DSR, the network
is completely self-organizing and self-configuring, requiring no
existing network infrastructure or administration. Network nodes
cooperate to forward packets for each other to allow communication
over multiple "hops" between nodes not directly within wireless
transmission range of one another. As nodes in the network move
about or join or leave the network, and as wireless transmission
conditions such as sources of interference change, all routing is
automatically determined and maintained by the DSR routing protocol.
Since the number or sequence of intermediate hops needed to reach any
destination may change at any time, the resulting network topology
may be quite rich and rapidly changing.
Chair's Address 66 The DSR protocol allows nodes to dynamically discover a source
route across multiple network hops to any destination in the ad hoc
network. Each data packet sent then carries in its header the
complete, ordered list of nodes through which the packet will pass,
allowing packet routing to be trivially loop-free and avoiding the
need for up-to-date routing information in the intermediate nodes
through which the packet is forwarded. By including this source
route in the header of each data packet, other nodes forwarding or
overhearing any of these packets may also easily cache this routing
information for future use.
Authors' Addresses 67 In designing DSR, we sought to create a routing protocol that had
very low overhead yet was able to react quickly to changes in the
network. The DSR protocol provides highly reactive service to help
ensure successful delivery of data packets in spite of node movement
or other changes in network conditions.
1. Introduction The DSR protocol is composed of two mechanisms that work together to
allow the discovery and maintenance of source routes in the ad hoc
network:
This document describes Dynamic Source Routing (DSR) [8, 9], a - Route Discovery is the mechanism by which a node S wishing to
protocol developed by the Monarch Project [10, 19] at Carnegie Mellon send a packet to a destination node D obtains a source route
University for routing packets in a mobile ad hoc network [5]. to D. Route Discovery is used only when S attempts to send a
packet to D and does not already know a route to D.
Source routing is a routing technique in which the sender of a packet - Route Maintenance is the mechanism by which node S is able
determines the complete sequence of nodes through which to forward to detect, while using a source route to D, if the network
the packet; the sender explicitly lists this route in the packet's topology has changed such that it can no longer use its route
header, identifying each forwarding "hop" by the address of the next to D because a link along the route no longer works. When Route
node to which to transmit the packet on its way to the destination Maintenance indicates a source route is broken, S can attempt to
node. use any other route it happens to know to D, or can invoke Route
Discovery again to find a new route for subsequent packets to D.
DSR offers a number of potential advantages over other routing Route Maintenance for this route is used only when S is actually
protocols for mobile ad hoc networks. First, DSR uses no periodic sending packets to D.
routing messages of any kind (e.g., no router advertisements and no
link-level neighbor status messages), thereby significantly reducing
network bandwidth overhead, conserving battery power, reducing the
probability of packet collision, and avoiding the propagation of
potentially large routing updates throughout the ad hoc network. Our
Dynamic Source Routing protocol is able to adapt quickly to changes
such as node movement, yet requires no routing protocol overhead
during periods in which no such changes occur.
In addition, DSR has been designed to compute correct routes in In DSR, Route Discovery and Route Maintenance each operate entirely
the presence of asymmetric (uni-directional) links. In wireless "on demand". In particular, unlike other protocols, DSR requires no
networks, links may at times operate asymmetrically due to sources periodic packets of any kind at any level within the network. For
of interference, differing radio or antenna capabilities, or the example, DSR does not use any periodic routing advertisement, link
intentional use of asymmetric communication technology such as status sensing, or neighbor detection packets, and does not rely on
satellites. Due to the existence of asymmetric links, traditional these functions from any underlying protocols in the network. This
link-state or distance vector protocols may compute routes that do entirely on-demand behavior and lack of periodic activity allows
not work. DSR, however, will always find a correct route even in the the number of overhead packets caused by DSR to scale all the way
presence of asymmetric links. down to zero, when all nodes are approximately stationary with
respect to each other and all routes needed for current communication
have already been discovered. As nodes begin to move more or
as communication patterns change, the routing packet overhead of
DSR automatically scales to only that needed to track the routes
currently in use. Network topology changes not affecting routes
currently in use are ignored and do not cause reaction from the
protocol.
2. Changes In response to a single Route Discovery (as well as through routing
information from other packets overheard), a node may learn and cache
multiple routes to any destination. This allows the reaction to
routing changes to be much more rapid, since a node with multiple
routes to a destination can try another cached route if the one it
has been using should fail. This caching of multiple routes also
avoids the overhead of needing to perform a new Route Discovery each
time a route in use breaks.
Changes from version 02 to version 03 (10/1999) The operation of both Route Discovery and Route Maintenance in DSR
are designed to allow uni-directional links and asymmetric routes
to be easily supported. In particular, as noted in Section 2, in
wireless networks, it is possible that a link between two nodes may
not work equally well in both directions, due to differing antenna
or propagation patterns or sources of interference. DSR allows such
uni-directional links to be used when necessary, improving overall
performance and network connectivity in the system.
- Added description of path-state and flow-state maintenance This document specifies the operation of the DSR protocol for routing
(Section 10). These extensions remove the need for every unicast IP packets in multi-hop wireless ad hoc networks. Advanced,
data packet to carry a source route, thereby decreasing optional features, such as Quality of Service (QoS) support and
the byte-overhead of DSR. They also provide a framework for efficient multicast routing, are covered in other documents. The
supporting QoS inside DSR networks. specification of DSR in this document provides a compatible base
on which such features can be added, either independently or by
integration with the DSR operation specified here.
3. Assumptions The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [4].
2. Assumptions
We assume that all nodes wishing to communicate with other nodes We assume that all nodes wishing to communicate with other nodes
within the ad hoc network are willing to participate fully in the within the ad hoc network are willing to participate fully in the
protocols of the network. In particular, each node participating in protocols of the network. In particular, each node participating in
the network should also be willing to forward packets for other nodes the network SHOULD also be willing to forward packets for other nodes
in the network. in the network.
We refer to the minimum number of hops necessary for a packet to The diameter of an ad hoc network is the minimum number of hops
reach from any node located at one extreme edge of the network to necessary for a packet to reach from any node located at one extreme
another node located at the opposite extreme, as the diameter of the edge of the ad hoc network to another node located at the opposite
network. We assume that the diameter of an ad hoc network will be extreme. We assume that this diameter will often be small (e.g.,
small (e.g., perhaps 5 or 10 hops), but may often be greater than 1. perhaps 5 or 10 hops), but may often be greater than 1.
Packets may be lost or corrupted in transmission on the wireless Packets may be lost or corrupted in transmission on the wireless
network. A node receiving a corrupted packet can detect the error network. We assume that a node receiving a corrupted packet can
and discard the packet. detect the error and discard the packet.
We assume that nodes can enable promiscuous receive mode on their Nodes within the ad hoc network MAY move at any time without notice,
wireless network interface hardware, causing the hardware to and MAY even move continuously, but we assume that the speed with
deliver every received packet to the network driver software without which nodes move is moderate with respect to the packet transmission
filtering based on link-layer destination address. Although we do latency and wireless transmission range of the particular underlying
not require this facility, it is for example common in current LAN network hardware in use. In particular, DSR can support very
hardware for broadcast media including wireless, and some of our rapid rates of arbitrary node mobility, but we assume that nodes do
optimizations take advantage of its availability. Use of promiscuous not continuously move so rapidly as to make the flooding of every
mode does increase the software overhead on the CPU, but we believe individual data packet the only possible routing protocol.
that wireless network speeds are more the inherent limiting factor
to performance in current and future systems. We also believe A common feature of many network interfaces, including most current
that portions of the protocol are also suitable for implementation LAN hardware for broadcast media such as wireless, is the ability
to operate the network interface in "promiscuous" receive mode.
This mode causes the hardware to deliver every received packet to
the network driver software without filtering based on link-layer
destination address. Although we do not require this facility, some
of our optimizations can take advantage of its availability. Use
of promiscuous mode does increase the software overhead on the CPU,
but we believe that wireless network speeds are more the inherent
limiting factor to performance in current and future systems; we also
believe that portions of the protocol are suitable for implementation
directly within a programmable network interface unit to avoid this directly within a programmable network interface unit to avoid this
overhead on the CPU. overhead on the CPU [13]. Use of promiscuous mode may also increase
the power consumption of the network interface hardware, depending
on the design of the receiver hardware, and in such cases, DSR can
easily be used without the optimizations that depend on promiscuous
receive mode, or can be programmed to only periodically switch the
interface into promiscuous mode. Use of promiscuous receive mode is
entirely optional.
4. Terminology Wireless communication ability between any pair of nodes can at
times not work equally well in both directions, due for example to
differing antenna or propagation patterns or sources of interference
around the two nodes [1, 17]. That is, wireless communications
between each pair of nodes will in many cases be able to operate
bi-directionally, but at times the wireless link between two nodes
may be only uni-directional, allowing one node to successfully send
packets to the other while no communication is possible in the
reverse direction. Although many routing protocols operate correctly
only over bi-directional links, DSR can successfully discover and
forward packets over paths that contain uni-directional links.
Some MAC protocols, however, such as MACA [16], MACAW [2], or IEEE
802.11 [10], limit unicast data packet transmission to bi-directional
links, due to the required bi-directional exchange of RTS and CTS
packets in these protocols and due to the link-level acknowledgement
feature in IEEE 802.11; when used on top of MAC protocols such as
these, DSR can take advantage of additional optimizations, such as
the easy ability to reverse a source route to obtain a route back to
the origin of the original route.
4.1. General Terms The IP address used by a node using the DSR protocol MAY be assigned
by any mechanism (e.g., static assignment or use of DHCP for dynamic
assignment [8]), although the method of such assignment is outside
the scope of this specification.
link 3. DSR Protocol Overview
A communication facility or medium over which nodes can 3.1. Basic DSR Route Discovery
communicate at the link layer, such as an Ethernet (simple or
bridged). A link is the layer immediately below IP.
interface When some node S originates a new packet destined to some other
node D, it places in the header of the packet a source route giving
the sequence of hops that the packet is to follow on its way to
D. Normally, S will obtain a suitable source route by searching
its "Route Cache" of routes previously learned, but if no route is
found in its cache, it will initiate the Route Discovery protocol
to dynamically find a new route to D. In this case, we call S the
"initiator" and D the "target" of the Route Discovery.
A node's attachment to a link. For example, suppose a node A is attempting to discover a route to
node E. The Route Discovery initiated by node A in this example
would proceed as follows:
prefix ^ "A" ^ "A,B" ^ "A,B,C" ^ "A,B,C,D"
| id=2 | id=2 | id=2 | id=2
+-----+ +-----+ +-----+ +-----+ +-----+
| A |---->| B |---->| C |---->| D |---->| E |
+-----+ +-----+ +-----+ +-----+ +-----+
| | | |
v v v v
A bit string that consists of some number of initial bits of an To initiate the Route Discovery, node A transmits a "Route Request"
address. message as a single local broadcast packet, which is received by
(approximately) all nodes currently within wireless transmission
range of A, including node B in this example. Each Route Request
message identifies the initiator and target of the Route Discovery,
and also contains a unique request identification (2, in this
example), determined by the initiator of the Request. Each
Route Request also contains a record listing the address of each
intermediate node through which this particular copy of the Route
Request message has been forwarded. This route record is initialized
to an empty list by the initiator of the Route Discovery. In this
example, the route record initially lists only node A.
interface index When another node receives a Route Request (such as node B in this
example), if it is the target of the Route Discovery, it returns
a "Route Reply" message to the initiator of the Route Discovery,
giving a copy of the accumulated route record from the Route Request;
when the initiator receives this Route Reply, it caches this route
in its Route Cache for use in sending subsequent packets to this
destination. Otherwise, if this node receiving the Route Request
has recently seen another Route Request message from this initiator
bearing this same request identification and target address, or if it
finds that its own address is already listed in the route record in
the Route Request message, it discards the Request. Otherwise, this
node appends its own address to the route record in the Route Request
message and propagates it by transmitting it as a local broadcast
packet (with the same request identification). In this example,
node B broadcast the Route Request, which is received by node C;
nodes C and D each also broadcast the Request in turn, resulting in a
copy of the Request being received by node E.
An 7-bit quantity which uniquely identifies an interface among In returning the Route Reply to the initiator of the Route Discovery,
a given node's interfaces. Each node can assign interface such as node E replying back to A in this example, node E will
indices to its interfaces using any scheme it wishes. typically examine its own Route Cache for a route back to A, and if
found, will use it for the source route for delivery of the packet
containing the Route Reply. Otherwise, E SHOULD perform its own
Route Discovery for target node A, but to avoid possible infinite
recursion of Route Discoveries, it MUST piggyback this Route Reply on
its own Route Request message for A.
The index IF_INDEX_MA is reserved for use by Mobile IP [14] It is also possible to piggyback other small data packets, such as a
mobility agents (home or foreign agents) to indicate that they TCP SYN packet [26], on a Route Request using this same mechanism.
believe they can reach a destination via a connected internet Node E could also simply reverse the sequence of hops in the route
infrastructure. The index IF_INDEX_ROUTER is reserved for record that it is trying to send in the Route Reply, and use this
use by routers not acting as Mobile IP mobility agents to as the source route on the packet carrying the Route Reply itself.
indicate that they believe they can reach the destination via a For MAC protocols such as IEEE 802.11 that require a bi-directional
connected internet infrastructure. frame exchange as part of the MAC protocol [10], this route reversal
is preferred as it avoids the overhead of a possible second Route
Discovery, and it tests the discovered route to ensure it is
bi-directional before the Route Discovery initiator begins using the
route. However, this technique will prevent the discovery of routes
using uni-directional links. In wireless environments where the use
of uni-directional links is permitted, such routes may in some cases
be more efficient than those with only bi-directional links, or they
may be the only way to achieve connectivity to the target node.
The distinction between the index for mobility agents and When initiating a Route Discovery, the sending node saves a copy of
the index for routers, allows mobility agents to advertise the original packet in a local buffer called the "Send Buffer". The
their existence ``for free''. A node that processes a routing Send Buffer contains a copy of each packet that cannot be transmitted
header listing the interface index IF_INDEX_MA, can then send by this node because it does not yet have a source route to the
a unicast Agent Solicitation to the corresponding address in packet's destination. Each packet in the Send Buffer is stamped with
the routing header to obtain complete information about the the time that it was placed into the Buffer and is discarded after
mobility services being provided. residing in the Send Buffer for some timeout period; if necessary
for preventing the Send Buffer from overflowing, a FIFO or other
replacement strategy MAY also be used to evict packets before they
expire.
link-layer address While a packet remains in the Send Buffer, the node SHOULD
occasionally initiate a new Route Discovery for the packet's
destination address. However, the node MUST limit the rate at which
such new Route Discoveries for the same address are initiated, since
it is possible that the destination node is not currently reachable.
In particular, due to the limited wireless transmission range and the
movement of the nodes in the network, the network may at times become
partitioned, meaning that there is currently no sequence of nodes
through which a packet could be forwarded to reach the destination.
Depending on the movement pattern and the density of nodes in the
network, such network partitions may be rare or may be common.
A link-layer identifier for an interface, such as IEEE 802 If a new Route Discovery was initiated for each packet sent by a
addresses on Ethernet links. node in such a situation, a large number of unproductive Route
Request packets would be propagated throughout the subset of the
ad hoc network reachable from this node. In order to reduce the
overhead from such Route Discoveries, a node MUST use an exponential
back-off algorithm to limit the rate at which it initiates new Route
Discoveries for the same target. If the node attempts to send
additional data packets to this same node more frequently than this
limit, the subsequent packets SHOULD be buffered in the Send Buffer
until a Route Reply is received giving a route to this destination,
but the node MUST NOT initiate a new Route Discovery until the
minimum allowable interval between new Route Discoveries for this
target has been reached. This limitation on the maximum rate of
Route Discoveries for the same target is similar to the mechanism
required by Internet nodes to limit the rate at which ARP Requests
are sent for any single target IP address [3].
packet 3.2. Basic DSR Route Maintenance
An IP header plus payload. When originating or forwarding a packet using a source route, each
node transmitting the packet is responsible for confirming that the
packet has been received by the next hop along the source route; the
packet SHOULD be retransmitted (up to a maximum number of attempts)
until this confirmation of receipt is received. For example, in the
situation shown below, node A has originated a packet for node E
using a source route through intermediate nodes B, C, and D:
piggybacking +-----+ +-----+ +-----+ +-----+ +-----+
| A |---->| B |---->| C |-- | D | | E |
+-----+ +-----+ +-----+ +-----+ +-----+
Including two or more conceptually different types of data in In this case, node A is responsible for receipt of the packet at B,
the same packet so that all data elements move through the node B is responsible for receipt at C, node C is responsible for
network together. receipt at D, and node D is responsible for receipt finally at the
destination E.
home address This confirmation of receipt in many cases may be provided at no cost
to DSR, either as an existing standard part of the MAC protocol in
use (such as the link-level acknowledgement frame defined by IEEE
802.11 [10]), or by a "passive acknowledgement" [15] (in which, for
example, B confirms receipt at C by overhearing C transmit the packet
to forward it on to D). If neither of these confirmation mechanisms
are available, the node transmitting the packet can explicitly
request a DSR-specific software acknowledgement be returned by the
next hop; this software acknowledgement will normally be transmitted
directly to the sending node, but if the link between these two nodes
is uni-directional, this software acknowledgement may travel over a
different, multi-hop path.
An IP address that is assigned for an extended period of time If no receipt confirmation is received after the packet has been
to a mobile node. It remains unchanged regardless of where retransmitted the maximum number of attempts by some hop, this node
the node is attached to the Internet [14]. If a node has more SHOULD return a "Route Error" message to the original sender of the
than one home address, it SHOULD select and use a single home packet, identifying the link over which the packet could not be
address when participating in the ad hoc network. forwarded. For example, in the example shown above, if C is unable
to deliver the packet to the next hop D, then C returns a Route Error
to A, stating that the link from C to D is currently "broken". Node
A then removes this broken link from its cache; any retransmission
of the original packet can be performed by upper layer protocols
such as TCP, if necessary. For sending such a retransmission or
other packets to this same destination E, if A has in its Route Cache
another route to E (for example, from additional Route Replys from
its earlier Route Discovery, or from having overheard sufficient
routing information from other packets), it can send the packet
using the new route immediately. Otherwise, it SHOULD perform a new
Route Discovery for this target (subject to the exponential back-off
described in Section 3.1).
source route 3.3. Additional Route Discovery Features
A source route from a node S to some node D is an ordered list 3.3.1. Caching Overheard Routing Information
of home addresses and interface indexes that contains all the
information that would be needed to forward a packet through
the ad hoc network. For each node that will transmit the
packet, the source route provides the index of the interface
over which the packet should be transmitted, and the address of
the node which is intended to receive the packet.
DSR Routing Headers as described in Section 7.3 use a more A node forwarding or otherwise overhearing any packet MAY add the
compact encoding of the source route and do not explicitly list routing information from that packet to its own Route Cache. In
address S in the Routing Header`, since it is carried as the IP particular, the source route used in a data packet, the accumulated
Source Address of the packet. route record in a Route Request, or the route being returned in a
Route Reply MAY all be cached by any node. Routing information from
any of these packets received can be cached, whether the packet
was addressed to this node, sent to a broadcast (or multicast)
MAC address, or received while the node's network interface is in
promiscuous mode.
A source route is described as ``broken'' when the specific One limitation, however, on caching of such overheard routing
path it describes through the network is not actually viable. information is the possible presence of uni-directional links in the
ad hoc network (Section 2). For example, in the situation shown
below, node A is using a source route to communicate with node E:
Route Discovery +-----+ +-----+ +-----+ +-----+ +-----+
| A |---->| B |---->| C |---->| D |---->| E |
+-----+ +-----+ +-----+ +-----+ +-----+
^
|
+-----+ +-----+ +-----+ +-----+ +-----+
| V |---->| W |---->| X |---->| Y |---->| Z |
+-----+ +-----+ +-----+ +-----+ +-----+
As node C forwards a data packet along the route from A to E, it
can always add to its cache the presence of the "forward" direction
links that it learns from the headers of these packets, from itself
to D and from D to E. However, the "reverse" direction of the links
identified in the packet headers, from itself back to B and from B to
A, may not work for it since these links might be uni-directional.
If C knows that the links are in fact bi-directional, for example due
to the MAC protocol in use, it could cache them but otherwise SHOULD
not.
The method in DSR by which a node S dynamically obtains a Likewise, node V in the example above is using a different source
source route to some node D that will be used by S to route route to communicate with node Z. If node C overhears node X
packets through the network to D. Performing a Route Discovery transmitting a data packet to forward it to Y (from V), node C SHOULD
involves sending one or more Route Request packets. consider whether the links involved can be known to be bi-directional
or not before caching them. If the link from X to C (over which this
data packet was received) can be known to be bi-directional, then C
could cache the link from itself to X, the link from X to Y, and the
link from Y to Z. If all links can be assumed to be bi-directional,
C could also cache the links from X to W and from W to V. Similar
considerations apply to the routing information that might be learned
from forwarded or otherwise overheard Route Request or Route Reply
packets.
Route Maintenance 3.3.2. Replying to Route Requests using Cached Routes
The process in DSR of monitoring the status of a source route A node receiving a Route Request for which it is not the target,
while in use, so that any link-failures along the source route searches its own Route Cache for a route to the target of the
can be detected and the broken link removed from use. Request. If found, the node generally returns a Route Reply to
the initiator itself rather than forwarding the Route Request. In
the Route Reply, it sets the route record to list the sequence of
hops over which this copy of the Route Request was forwarded to it,
concatenated with its own idea of the route from itself to the target
from its Route Cache.
4.2. Specification Language However, before transmitting a Route Reply packet that was generated
using information from its Route Cache in this way, a node MUST
verify that the resulting route being returned in the Route Reply,
after this concatenation, contains no duplicate nodes listed in the
route record. For example, the figure below illustrates a case in
which a Route Request for target E has been received by node F, and
node F already has in its Route Cache a route from itself to E:
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", +-----+ +-----+ +-----+ +-----+
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this | A |---->| B |- >| D |---->| E |
document are to be interpreted as described in RFC 2119 [3]. +-----+ +-----+ \ / +-----+ +-----+
\ /
\ +-----+ /
>| C |-
+-----+
| ^
v |
Route Request +-----+
Route: A - B - C - F | F | Cache: C - D - E
+-----+
5. Protocol Overview The concatenation of the accumulated route from the Route Request and
the cached route from F's Route Cache would include a duplicate node
in passing from C to F and back to C.
5.1. Route Discovery and Route Maintenance Node F in this case could attempt to edit the route to eliminate
the duplication, resulting in a route from A to B to C to D and on
to E, but in this case, node F would not be on the route that it
returned in its own Route Reply. DSR Route Discovery prohibits
node F from returning such a Route Reply from its cache for two
reasons. First, this limitation increases the probability that the
resulting route is valid, since F in this case should have received
a Route Error if the route had previously stopped working. Second,
this limitation means that a Route Error traversing the route is very
likely to pass through any node that sent the Route Reply for the
route (including F), which helps to ensure that stale data is removed
from caches (such as at F) in a timely manner. Otherwise, the next
Route Discovery initiated by A might also be contaminated by a Route
Reply from F containing the same stale route. If the Route Request
does not meet these restrictions, the node (node F in this example)
discards the Route Request rather than replying to it or propagating
it.
A source routing protocol must solve two challenges, which DSR terms 3.3.3. Preventing Route Reply Storms
Route Discovery and Route Maintenance. Route Discovery is the
mechanism whereby a node S wishing to send a packet to a destination
D obtains a source route to D.
Route Maintenance is the mechanism whereby S is able to detect, while The ability for nodes to reply to a Route Request based on
using a source route to D, if the network topology has changed such information in their Route Caches, as described in Section 3.3.2,
that it can no longer use its route to D because a link along the could result in a possible Route Reply "storm" in some cases. In
route no longer works. When Route Maintenance indicates a source particular, if a node broadcasts a Route Request for a target node
route is broken, S can attempt to use any other route it happens to for which the node's neighbors have a route in their Route Caches,
know to D, or can invoke Route Discovery again to find a new route. each neighbor may attempt to send a Route Reply, thereby wasting
bandwidth and possibly increasing the number of network collisions in
the area.
To perform Route Discovery, the source node S link-layer broadcasts For example, the figure below shows a situation in which nodes B, C,
a Route Request packet. Here, node S is termed the initiator of the D, E, and F all receive A's Route Request for target G, and each have
Route Discovery, and the node to which S is attempting to discover a the indicated route cached for this target:
source route, say D, is termed the target of the Discovery.
Each node that hears the Route Request packet forwards a copy of the +-----+ +-----+
Request, if appropriate, by adding its own address to a source route | D |< >| C |
being recorded in the Request packet and then rebroadcasting the +-----+ \ / +-----+
Route Request. Cache: C - B - G \ / Cache: B - G
\ +-----+ /
-| A |-
+-----+\ +-----+ +-----+
| | \--->| B | | G |
/ \ +-----+ +-----+
/ \ Cache: G
v v
+-----+ +-----+
| E | | F |
+-----+ +-----+
Cache: F - B - G Cache: B - G
The forwarding of Route Requests is constructed so that copies of the Normally, they would all attempt to reply from their own Route
Request propagate hop-by-hop outward from the node initiating the Caches, and would all send their Replys at about the same time since
Route Discovery, until either the target of the Request is found or they all received the broadcast Route Request at about the same time.
until another node is found that can supply a route to the target. Such simultaneous replies from different nodes all receiving the
Route Request may create packet collisions among some or all of these
Replies and may cause local congestion in the wireless network. In
addition, it will often be the case that the different replies will
indicate routes of different lengths, as shown in this example.
The basic mechanism of forwarding Route Requests forwards the Request If a node can put its network interface into promiscuous receive
if the node (1) is not the target of the Request, (2) is not already mode, it SHOULD delay sending its own Route Reply for a short period,
listed in the recorded source route in this copy of the Request, and while listening to see if the initiating node begins using a shorter
(3) has not recently seen another Route Request packet belonging to route first. That is, this node SHOULD delay sending its own Route
this same Route Discovery. A node can determine if it has recently Reply for a random period d = H * (h - 1 + r), where h is the length
seen such a Route Request, since each Route Request packet contains in number of network hops for the route to be returned in this node's
a unique identifier for this Route Discovery, generated by the Route Reply, r is a random number between 0 and 1, and H is a small
initiator of the Discovery. Each node maintains an LRU cache of the constant delay (at least twice the maximum wireless link propagation
unique identifier from each recently received Route Request. By not delay) to be introduced per hop. This delay effectively randomizes
propagating any copies of a Request after the first, the overhead of the time at which each node sends its Route Reply, with all nodes
forwarding additional copies that reach this node along different sending Route Replys giving routes of length less than h sending
paths is avoided. their Replys before this node, and all nodes sending Route Replys
giving routes of length greater than h sending their Replys after
this node. Within the delay period, this node promiscuously receives
all packets, looking for data packets from the initiator of this
Route Discovery destined for the target of the Discovery. If such
a data packet received by this node during the delay period uses a
source route of length less than or equal to h, this node may infer
that the initiator of the Route Discovery has already received a
Route Reply giving an equally good or better route. In this case,
this node SHOULD cancel its delay timer and SHOULD NOT send its Route
Reply for this Route Discovery.
In addition, the Time-to-Live field in the IP header of the packet 3.3.4. Route Request Hop Limits
carrying the Route Request MAY be used to limit the scope over which
the Request will propagate, using the normal behavior of Time-to-Live
defined by IP [17, 2]. Additional optimizations on the handling and
forwarding of Route Requests are also used to further reduce the
Route Discovery overhead.
When the target of the Request (e.g., node D) receives the Route Each Route Request message contains a "hop limit" that may be used
Request, the recorded source route in the Request identifies the to limit the number of intermediate nodes allowed to forward that
sequence of hops over which this copy of the Request reached D. copy of the Route Request. This hop limit is implemented using the
Node D copies this recorded source route into a Route Reply packet Time-to-Live (TTL) field in the IP header of the packet carrying
and sends this Route Reply back to the initiator of the Route Request the Route Request. As the Request is forwarded, this limit is
(e.g., node S). decremented, and the Request packet is discarded if the limit reaches
zero before finding the target.
All source routes learned by a node are kept in a Route Cache, which This Route Request hop limit can be used to implement a variety of
is used to further reduce the cost of Route Discovery. When a node algorithms for controlling the spread of a Route Request during a
wishes to send a packet, it examines its own Route Cache and performs Route Discovery attempt. For example, a node MAY send its first
Route Discovery only if no suitable source route is found in its Route Request attempt for some target node using a hop limit of 1,
Cache. such that any node receiving the initial transmission of the Route
Request will not forward it to other nodes by rebroadcasting it.
This form of Route Request is called a "non-propagating" Route
Request. It provides an inexpensive method for determining if the
target is currently a neighbor of the initiator or if a neighbor
node has a route to the target cached (effectively using the
neighbors' Route Caches as an extension of the initiator's own Route
Cache). If no Route Reply is received after a short timeout, then a
"propagating" Route Request (i.e., with no hop limit) MAY be sent.
Further, when some intermediate node B receives a Route Request from Another possible use of the hop limit in a Route Request is to
S for some target node D, B not equal D, B searches its own Route implement an "expanding ring" search for the target [13]. For
Cache for a route to D. If B finds such a route, it might not have example, a node could send an initial non-propagating Route Request
to propagate the Route Request, but instead return a Route Reply to as described above; if no Route Reply is received for it, the node
node S based on the concatenation of the recorded source route from could initiate another Route Request with a hop limit of 2. For
S to B in the Route Request and the cached route from B to D. The each Route Request initiated, if no Route Reply is received for it,
details of replying from a Route Cache in this way are discussed in the node could double the hop limit used on the previous attempt,
Section 9.1. to progressively explore for the target node without allowing the
Route Request to propagate over the entire network. However, this
expanding ring search approach could have the effect of increasing
the average latency of Route Discovery, since multiple Discovery
attempts and timeouts may be needed before discovering a route to the
target node.
As a node overhears routes being used by others, either on data 3.4. Additional Route Maintenance Features
packets or on control packets used by Route Discovery or Route
Maintenance, the node MAY insert those routes into its Route Cache,
leveraging the Route Discovery operations of the other nodes in
the network. Such route information MAY be learned either by
promiscuously snooping on packets or when forwarding packets.
5.2. Packet Forwarding 3.4.1. Packet Salvaging
To represent a source route within a packet's header, DSR uses a After sending a Route Error message as part of Route Maintenance as
Routing Header similar to the Routing Header format specified for described in Section 3.2, a node may attempt to "salvage" the data
IPv6, adapted to the needs of DSR and to the use of DSR in IPv4 (or packet that caused the Route Error rather than discarding it. To
in IPv6 in the future). The DSR Routing Header uses a unique Routing attempt to salvage a packet, the node sending a Route Error searches
Type field value to distinguish it from the existing Type 0 Routing its own Route Cache for a route from itself to the destination of the
Header defined within IPv6 [6]. packet causing the Error. If such a route is found, the node may
salvage the packet after returning the Route Error by replacing the
original source route on the packet with the route from its Route
Cache. The node then forwards the packet to the next node indicated
along this source route. For example, in the situation shown in the
example of Section 3.2, if node C has another route cached to node E,
it can salvage the packet by applying this route to the packet rather
than discarding the packet.
To forward a packet, a receiving node N simply processes the Routing When salvaging a packet in this way, a count is maintained in the
Header as specified in Section 8.3 and transmits the packet to packet of the number of times that it has been salvaged, to prevent a
the next hop. If a forwarding error occurs along the link to the single packet from being salvaged endlessly. Otherwise, it could be
next hop in the route, this node N sends a Route Error back to the possible for the packet to enter a routing loop, as different nodes
originator S of this packet informing S that this link is "broken". repeatedly salvage the packet and replace the source route on the
If node N's Route Cache contains a different route to the destination packet with routes to each other.
of the original packet, then the packet is salvaged using the new
source route (Section 8.5.5). Otherwise, the packet is dropped.
Each node overhearing or forwarding a Route Error packet also 3.4.2. Automatic Route Shortening
removes from its Route Cache the link indicated to be broken, thereby
cleaning the stale cache data from the network.
5.3. Multicast Routing Source routes in use may be automatically shortened if one or more
intermediate hops in the route become no longer necessary. This
mechanism of automatically shortening routes in use is somewhat
similar to the use of passive acknowledgements. In particular, if a
node is able to overhear a packet carrying a source route (e.g., by
operating its network interface in promiscuous receive mode), then
this node examines the unused portion of that source route. If this
node is not the intended next hop for the packet but is named in
the later unused portion of the packet's source route, then it can
infer that the intermediate nodes before itself in the source route
are no longer needed in the route. For example, the figure below
illustrates an example in which node D has overheard a data packet
being transmitted from B to C, for later forwarding to D and to E:
At this time DSR does not support true multicasting. However, it +-----+ +-----+ +-----+ +-----+ +-----+
does support the controlled flooding of a data packet to all nodes in | A |---->| B |---->| C | | D | | E |
the network that are within some number of hops of the originator. +-----+ +-----+ +-----+ +-----+ +-----+
While this mechanism does not support pruning of the broadcast \ ^
tree to conserve network resources, it can be used to distribute \ /
information to nodes in the network. ---------------------
When an application on a DSR node sends a packet to a multicast In this case, this node (node D) returns a "gratuitous" Route Reply
address, DSR piggybacks the data from the packet inside a Route message to the original sender of the packet (node A). The Route
Request packet targeted at the multicast address. The normal Route Reply gives the shorter route as the concatenation of the portion of
Request distribution scheme described in Sections 5.1 and 8.4.2 the original source route up through the node that transmitted the
will result in this packet being efficiently distributed to all overheard packet (node B), plus the suffix of the original source
nodes in the network within the specified TTL of the originator. route beginning with the node returning the gratuitous Route Reply
The receiving nodes can then do destination address filtering on (node D). In this example, the route returned in the gratuitous Route
the packet, discarding it if they do not wish to receive multicast Reply message sent from D to A gives the new route as the sequence of
packets destined to this multicast address. hops from A to B to D to E.
6. Conceptual Data Structures 3.4.3. Increased Spreading of Route Error Messages
In order to participate in the Dynamic Source Routing Protocol, a When a source node receives a Route Error for a data packet that
node needs four conceptual data structures: a Route Cache, a Route it originated, this source node propagates this Route Error to its
Request Table, a Send Buffer, and a Retransmission Buffer. These neighbors by piggybacking it on its next Route Request. In this way,
data structures MAY be implemented in any manner consistent with the stale information in the caches of nodes around this source node will
external behavior described in this document. not generate Route Replys that contain the same invalid link for
which this source node received the Route Error.
6.1. Route Cache For example, in the situation shown in the example of Section 3.2,
node A learns from the Route Error message from C, that the link from
C to D is currently broken. It thus removes this link from its own
Route Cache and initiates a new Route Discovery (if it doesn't have
another route to E in its Route Cache). On the Route Request packet
initiating this Route Discovery, node A piggybacks a copy of this
Route Error message, ensuring that the Route Error message spreads
well to other nodes, and guaranteeing that any Route Reply that it
receives (including those from other node's Route Caches) in response
to this Route Request does not contain a route that assumes the
existence of this broken link.
4. Conceptual Data Structures
This document describes the DSR protocol in terms of a number of
conceptual data structures. This section describes each of these
data structures and provides an overview of its use in the protocol.
In an implementation of the protocol, these data structures MAY be
implemented in any manner consistent with the external behavior
described in this document.
4.1. Route Cache
All routing information needed by a node participating in an ad hoc All routing information needed by a node participating in an ad hoc
network using DSR is stored in a Route Cache. Each node in the network using DSR is stored in a Route Cache. Each node in the
network maintains its own Route Cache. The node adds information network maintains its own Route Cache. A node adds information
to the Cache as it learns of new links between nodes in the ad hoc to its Route Cache as it learns of new links between nodes in the
network, for example through packets carrying either a Route Reply or ad hoc network; for example, a node may learn of new links when it
a Routing Header. Likewise, the node removes information from the receives a packet carrying either a Route Reply or a DSR Routing
cache as it learns existing links in the ad hoc network have broken, header. Likewise, a node removes information from its Route Cache as
for example through packets carrying a Route Error or through the it learns that existing links in the ad hoc network have broken; for
link-layer retransmission mechanism reporting a failure in forwarding example, a node may learn of a broken link when it receives a packet
a packet to its next-hop destination. The Route Cache is indexed carrying a Route Error or through the link-layer retransmission
logically by destination node address, and supports the following mechanism reporting a failure in forwarding a packet to its next-hop
operations: destination.
void Insert(Route RT) It is possible to interface a DSR network with other networks,
external to this DSR network. Such external networks may, for
example, be the Internet, or may be other ad hoc networks routed
with a routing protocol other than DSR. Such external networks may
also be other DSR networks that are treated as external networks
in order to improve scalability. The complete handling of such
external networks is beyond the scope of this document. However,
this document specifies a minimal set of requirements and features
necessary to allow nodes only implementing this specification to
interoperate correctly with nodes implementing interfaces to such
external networks. This minimal set of requirements and features
involve the First Hop External (F) and Last Hop External (L) bits in
a DSR Routing Header and a DSR Route Reply option, and the addition
of an External flag bit tagging each node in the Route Cache, copied
from the First Hop External (F) and Last Hop External (L) bits in the
Routing header or Route Reply from which the link to this node was
learned.
Inserts information extracted from source route RT into the The Route Cache SHOULD support storing more than one route to each
Route Cache. destination. In searching the Route Cache for a route to some
destination node, the Route Cache is indexed by destination node
address.
Route Get(Node DEST) - Each implementation of DSR at any node MAY choose any appropriate
strategy and algorithm for searching its Route Cache and
selecting a "best" route to the destination from among those
found. For example, a node MAY choose to select the shortest
route to the destination (the shortest sequence of hops), or it
MAY use an alternate metric to select the route from the Cache.
Returns a source route from this node to DEST (if one is - However, if there are multiple cached routes to a destination,
known). the selection of routes when searching the Route Cache SHOULD
prefer routes that do not have the External flag set on any
node. This will prefer routes that lead directly to the target
node instead of routes that attempt to reach the target via any
external networks connected to the DSR ad hoc network.
void Delete(Node FROM, Interface INDEX, Node TO) - In addition, any route selected when searching the Route Cache
MUST NOT have the External bit set for any nodes other than
possibly the first node, the last node, or both; the External bit
MUST NOT be set for any intermediate hops in the route selected.
Removes from the route cache any routes which assume that a An implementation of a Route Cache MAY provide a fixed capacity for
packet transmitted by node FROM over its interface with the the cache, or the cache size MAY be variable.
given INDEX will be received by node TO.
Each implementation MAY choose the cache replacement and cache search - Each implementation of DSR at each node MAY choose any
strategies for its Route Cache that are most appropriate for its appropriate policy for managing the entries in its Route Cache,
particular network environment. For example, some environments may such as when limited cache capacity requires a choice of which
choose to return the shortest route to a node (the shortest sequence entries to retain in the cache. For example, a node MAY chose a
of hops), while others may select an alternate metric for the Get() "least recently used" (LRU) cache replacement policy, in which
operation. the entry last used longest ago is discarded from the cache if a
decision needs to be made to allow space in the cache for some
new entry being added.
The Route Cache SHOULD support storing more than one source route for - However, the Route Cache replacement policy SHOULD allow routes
each destination. to be categorized based upon "preference", where routes with a
higher preferences are less likely to be removed from the cache.
For example, a node could prefer routes for which it initiated
a Route Discovery over routes that it learned as the result of
promiscuous snooping on other packets. In particular, a node
SHOULD prefer routes that it is presently using over those that
it is not.
If there are multiple cached routes to a destination, the Route Get() Any suitable data structure organization, consistent with this
operation SHOULD prefer routes that do not traverse a hop with an specification, MAY be used to implement the Route Cache in any node.
interface index of IF_INDEX_MA or IF_INDEX_ROUTER. This will prefer For example, the following two types of organization are possible:
routes that lead directly to the target node over routes that attempt
to reach the target via any internet infrastructure connected to the
ad hoc network.
If a node S is using a source route to some destination D that - In DSR, the route returned in each Route Reply that is received
includes intermediate node N, S SHOULD shorten the route to by the initiator of a Route Discovery (or that is learned from
destination D when it learns of a shorter route to node N than the the header of overhead packets, as described in Section 6.1.3)
one that is listed as the prefix of its current route to D. represents a complete path (a sequence of links) leading to the
destination node. By caching each of these paths separately,
a "path cache" organization for the Route Cache can be formed.
A path cache is very simple to implement and easily guarantees
that all routes are loop-free, since each individual route from
a Route Reply or Route Request or used in a packet is loop-free.
To search for a route in a path cache data structure, the sending
node can simply search its Route Cache for any path (or prefix of
a path) that leads to the intended destination node.
A node S using a source route to destination D through intermediate This type of organization for the Route Cache in DSR has
node N, MAY shorten the source route if it learns of a shorter path been extensively studied through simulation [5, 11, 19] and
from node N to node D. through implementation of DSR in a mobile outdoor testbed under
significant workload [20, 21].
The Route Cache replacement policy SHOULD allow routes to be - Alternatively, a "link cache" organization could be used for the
categorized based upon "preference", where routes with a higher Route Cache, in which each individual link in the routes returned
preferences are less likely to be removed from the cache. For in Route Reply packets (or otherwise learned from the header of
example, a node could prefer routes for which it initiated a Route overhead packets) is added to a unified graph data structure of
Discovery over routes that it learned as the result of promiscuous this node's current view of the network topology. To search
snooping on other packets. In particular, a node SHOULD prefer for a route in link cache, the sending node must use a more
routes that it is presently using over those that it is not. complex graph search algorithm, such as the well-known Dijkstra's
shortest-path algorithm, to find the current best path through
the graph to the destination node. Such an algorithm is more
difficult to implement and may require significantly more CPU
time to execute.
6.2. Route Request Table However, a link cache organization is more powerful than a
path cache organization, in its ability to effectively utilize
all of the potential information that a node might learn about
the state of the network: links learned from different Route
Discoveries or from the header of any overheard packets can be
merged together to form new routes in the network, but this
is not possible in a path cache due to the separation of each
individual path in the cache.
The Route Request Table is a collection of records about Route This type of organization for the Route Cache in DSR, including
Request packets that were recently originated or forwarded by this the effect of a range of implementation choices, has been studied
node. The table is indexed by the home address of the target of the through detailed simulation [9].
route discovery. A record maintained on node S for node D contains
the following:
- The time that S last originated a Route Discovery for D. The choice of data structure organization to use for the Route Cache
in any DSR implementation is a local matter for each node and affects
only performance; any reasonable choice of organization for the Route
Cache does not affect either correctness or interoperability.
- The remaining amount of time that S must wait before the next 4.2. Route Request Table
attempt at a Route Discovery for D.
- The Time-to-live (TTL) field in the IP header of last Route The Route Request Table records information about Route Requests that
Request originated by S for D. were recently originated or forwarded by this node. The table is
indexed by IP address.
- A FIFO cache of the last ID_FIFO_SIZE Identification values from The Route Request Table on a node records the following information
Route Request packets targeted at node D that were forwarded by about nodes to which this node has initiated a Route Request:
this node.
Nodes SHOULD use an LRU policy to manage the entries of in their - The time that this node last originated a Route Discovery for
Route Request Table. that target node.
ID_FIFO_SIZE MUST NOT be set to an unlimited value, since, in the - The number of consecutive Route Requests initiated for this
worst case, when a node crashes and reboots the first ID_FIFO_SIZE target since receiving a valid Route Reply giving a route to that
Route Request packets it sends may appear to be duplicates to the target node.
other nodes in the network.
6.3. Send Buffer - The remaining amount of time before which this node MAY next
attempt at a Route Discovery for that target node.
- The Time-to-Live (TTL) field used in the IP header of last Route
Request initiated by this node for that target node.
In addition, the Route Request Table on a node also records the
following information about initiator nodes from which this node has
received a Route Request:
- A FIFO cache of size REQUEST_TABLE_IDS entries containing the
Identification value and target address from the most recent
Route Requests received by this node from that initiator node.
Nodes SHOULD use an LRU policy to manage the entries in their Route
Request Table.
The number of Identification values to retain in each Route Request
Table entry, REQUEST_TABLE_IDS, MUST NOT be unlimited, since,
in the worst case, when a node crashes and reboots, the first
REQUEST_TABLE_IDS Route Requests it initiates could appear to be
duplicates to the other nodes in the network.
4.3. Send Buffer
The Send Buffer of some node is a queue of packets that cannot be The Send Buffer of some node is a queue of packets that cannot be
transmitted by that node because it does not yet have a source transmitted by that node because it does not yet have a source
route to each respective packet's destination. Each packet in the route to each respective packet's destination. Each packet in the
Send Buffer is stamped with the time that it is placed into the Send Buffer is stamped with the time that it is placed into the
Buffer, and SHOULD be removed from the Send Buffer and discarded Buffer, and SHOULD be removed from the Send Buffer and discarded
SEND_BUFFER_TIMEOUT seconds after initially being placed in the SEND_BUFFER_TIMEOUT seconds after initially being placed in the
Buffer. If necessary, a FIFO strategy SHOULD be used to evict Buffer. If necessary, a FIFO strategy SHOULD be used to evict
packets before they timeout to prevent the buffer from overflowing. packets before they timeout to prevent the buffer from overflowing.
Subject to the rate limiting defined in Section 8.4, a Route Subject to the rate limiting defined in Section 6.2, a Route
Discovery SHOULD be initiated as often as possible for the Discovery SHOULD be initiated as often as possible for the
destination address of any packets residing in the Send Buffer. destination address of any packets residing in the Send Buffer.
6.4. Retransmission Buffer 4.4. Retransmission Buffer
The Retransmission Buffer of a node is a queue of packets sent by The Retransmission Buffer of a node is a queue of packets sent by
this node that are awaiting the receipt of an acknowledgment from the this node that are awaiting the receipt of an acknowledgment from the
next hop in the source route (Section 7.3). next hop in the source route (Section 5.3).
For each packet in the Retransmission Buffer, a node maintains (1) a For each packet in the Retransmission Buffer, a node maintains (1) a
count of the number of retransmissions and (2) the time of the last count of the number of retransmissions and (2) the time of the last
retransmission. retransmission.
Packets are removed from the buffer when an acknowledgment Packets are removed from the buffer when an acknowledgment
is received, or when the number of retransmissions exceeds is received, or when the number of retransmissions exceeds
DSR_MAXRXTSHIFT. In the later case, the removal of the packet from DSR_MAXRXTSHIFT. In the later case, the removal of the packet from
the Retransmission Buffer SHOULD result in a Route Error being the Retransmission Buffer SHOULD result in a Route Error being
returned to the initial source of the packet (Section 8.5). returned to the original source of the packet (Section 6.3).
7. Packet Formats 5. Packet Formats
Dynamic Source Routing makes use of four options carrying control Dynamic Source Routing makes use of four options carrying control
information that can be piggybacked in any existing IP packet. information that can be piggybacked in any existing IP packet. The
mechanism used to represent these options in a packet is based on
the design of the Hop-by-Hop and Destination Options mechanisms in
IPv6 [7]. The ability to generate and process such options must
be added to an IPv4 protocol stack. Specifically, the Protocol
field in the IP header is used to indicate that a Hop-by-Hop Options
extension header or Destination Options extension header follows the
IP header, and the Next Header field in the extension header is used
to indicate the type of protocol header (such as a transport layer
header) following the extension header.
The mechanism used for these options is based on the design of the In addition, DSR makes use of one additional header type, to carry
Hop-by-Hop and Destination Options mechanisms in IPv6 [6]. The the source route for a packet. This DSR Routing header is based on
ability to generate and process such options must be added to an IPv4 the design of the Routing header defined for IPv6 [7]. DSR defines
protocol stack. Specifically, the Protocol field in the IP header a new value for the Routing Type field to distinguish a DSR Routing
is used to indicate that a Hop-by-Hop Options or Destination Options header from other types of Routing headers.
extension header exists between the IP header and the remaining
portion of a packet's payload (such as a transport layer header).
The Next Header field in each extension header will then indicate the
type of header that follows it in a packet.
7.1. Destination Options Headers For IPv6, all extension headers are a multiple of 8 bytes in length.
However, for use in IPv4 packets, all extension headers only MUST be
a multiple of 4 bytes long. This requirement preserves the alignment
of any following extension headers and of any additional header
(e.g., a TCP header [26]) following the last extension header.
The Destination Options header is used to carry optional information 5.1. Destination Options Header
that need be examined only by a packet's destination node(s). The
Destination Options header is identified by a Next Header (or The Destination Options extension header is used to carry optional
Protocol) value of 60 in the immediately preceding header, and has information that needs to be examined only by a packet's destination
the following format: node(s). The Destination Options extension header is identified by
a Next Header (or Protocol) value of 60 in the immediately preceding
header [7], and has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | | | Next Header | Hdr Ext Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| | | |
. . . .
. Options . . Options .
. . . .
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Next Header
8-bit selector. Identifies the type of header immediately 8-bit selector. Identifies the type of header immediately
following the Destination Options header. Uses the same values following the Destination Options header. Uses the same values
as the IPv4 Protocol field [20]. as the IPv4 Protocol field [27].
Hdr Ext Len Hdr Ext Len
8-bit unsigned integer. Length of the Destination Options 8-bit unsigned integer. Length of the Destination Options
header in 4-octet units, not including the first 8 octets. header in 4-octet units, not including the first 8 octets.
Options Options
Variable-length field, of length such that the complete Variable-length field, of length such that the complete
Destination Options header is an integer multiple of 4 octets Destination Options header is an integer multiple of 4 octets
long. Contains one or more TLV-encoded options. long. Contains one or more TLV-encoded options.
The following destination option is used by the Dynamic Source The following destination option type is used by the DSR protocol:
Routing protocol:
- DSR Route Request option (Section 7.1.1)
This destination option MUST NOT appear multiple times within a - DSR Route Request option (Section 5.1.1)
single Destination Options header.
7.1.1. DSR Route Request Option 5.1.1. DSR Route Request Option
The DSR Route Request destination option is encoded in The DSR Route Request destination option is encoded in
type-length-value (TLV) format as follows: type-length-value (TLV) format as follows:
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Identification | | Option Type | Option Length | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Target Address | | Target Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| IN Index[1] |C| IN Index[2] |C| IN Index[3] |C| IN Index[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|OUT Index[1] |C|OUT Index[2] |C|OUT Index[3] |C|OUT Index[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] | | Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] | | Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[3] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| IN Index[5] |C| IN Index[6] |C| IN Index[7] |C| IN Index[8]|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|OUT Index[5] |C|OUT Index[6] |C| OUT Index[7] |C|OUT Index[8]|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[5] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
IP fields: IP fields:
Source Address Source Address
MUST be the home address of the node originating this packet. MUST be set to the address of the node originating this packet.
Intermediate nodes that repropagate the request do not change Intermediate nodes that repropagate the Route Request MUST not
this field. change this field.
Destination Address Destination Address
MUST be the limited broadcast address (255.255.255.255). MUST be set to the limited broadcast address (255.255.255.255).
Hop Limit (TTL) Hop Limit (TTL)
Can be varied from 1 to 255, for example to implement Can be varied from 1 to 255, for example to implement
expanding-ring searches. non-propagating Route Requests and Route Request expanding-ring
searches (Section 3.3.4).
Route Request fields: Route Request fields:
Option Type Option Type
???. A node that does not understand this option MUST discard ???. The top three bits of this Option Type value are equal to
the packet and the Option Data may change en-route (the top 011, meaning that a node that does not understand this option
three bits are 011). MUST discard the packet, and that the Option Data may change
en-route [7].
Option Length Option Length
8-bit unsigned integer. Length of the option, in octets, 8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields. excluding the Option Type and Option Length fields.
Identification Identification
A unique value generated by the initiator (original sender) A unique value generated by the initiator (original sender) of
of the Route Request. This value allows a recipient to the Route Request. Nodes initiating a Route Request generate
determine whether or not it has recently seen this a copy of a new Identification value for each Route Request, for example
this Request; if it has, the packet is simply discarded. When based on a sequence number counter of all Route Requests
propagating a Route Request, this field MUST be copied from the initiated by the node.
received copy of the Request being forwarded.
This value allows a receiving node to determine whether it
has recently seen a copy of this Route Request: if this
Identification value is found by this receiving node in its
Route Request Table (in the cache of Identification values
in the entry there for this initiating node), this receiving
node MUST discard the Route Request. When propagating a Route
Request, this field MUST be copied from the received copy of
the Route Request being forwarded.
Target Address Target Address
The home address of the node that is the target of the Route The address of the node that is the target of the Route
Request. Request.
Change Interface (C) bit[1..n]
A flag associated with each interface index that indicates
whether or not the corresponding node repropagated the Request
over a different physical interface type than over which it
received the Request.
IN Index[1..n]
IN Index[i] is the index of the interface over which the node
indicated by Address[i] received the Route Request option.
These are used to record a reverse route from the target of
the request to the originator, over which a Route Reply MAY be
sent.
OUT Index[1..n]
OUT Index[i] is the interface index that the node indicated by
Address[i-1] used when rebroadcasting the Route Request option.
Address[1..n] Address[1..n]
Address[i] is the home address of the ith hop recorded in the Address[i] is the address of the i-th hop recorded in the
Route Request option. Route Request option. The number of addresses present in this
field is indicated by the Option Length field in the option
(n = (Option Length - 6) / 4). Each node repropagating the
Route Request adds its own address to this list, increasing the
Option Length value by 4.
7.2. Hop-by-Hop Options Headers The DSR Route Request destination option MUST NOT appear more than
once within any single Destination Options extension header.
The Hop-by-Hop Options header is used to carry optional information 5.2. Hop-by-Hop Options Header
that must be examined by every node along a packet's delivery path.
The Hop-by-Hop Options header is identified by a Next Header (or The Hop-by-Hop Options extension header is used to carry optional
Protocol) value of ??? in the IP header, and has the following information that must be examined by every node along a packet's
delivery path. The Hop-by-Hop Options extension header is identified
by a Protocol value of 0 in the IP header [7], and has the following
format: format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | | | Next Header | Hdr Ext Len | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| | | |
. . . .
. Options . . Options .
. . . .
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Next Header Next Header
8-bit selector. Identifies the type of header immediately 8-bit selector. Identifies the type of header immediately
following the Hop-by-Hop Options header. Uses the same values following the Hop-by-Hop Options header. Uses the same values
as the IPv4 Protocol field [20]. as the IPv4 Protocol field [27].
Hdr Ext Len Hdr Ext Len
8-bit unsigned integer. Length of the Hop-by-Hop Options 8-bit unsigned integer. Length of the Hop-by-Hop Options
header in 4-octet units, not including the first 8 octets. header in 4-octet units, not including the first 8 octets.
Options Options
Variable-length field, of length such that the complete Variable-length field, of length such that the complete
Hop-by-Hop Options header is an integer multiple of 4 octets Hop-by-Hop Options header is an integer multiple of 4 octets
long. Contains one or more TLV-encoded options. long. Contains one or more TLV-encoded options.
The following hop-by-hop options are used by the Dynamic Source If present in an IP packet, the Hop-by-Hop Options extension header
Routing protocol: MUST appear in the packet immediately following the IP header.
- DSR Route Reply option (Section 7.2.1) The following hop-by-hop option types are used by the DSR protocol:
- DSR Route Error option (Section 7.2.2) - DSR Route Reply option (Section 5.2.1)
- DSR Acknowledgment option (Section 7.2.3) - DSR Route Error option (Section 5.2.2)
All of these destination options MAY appear one or more times within - DSR Acknowledgment option (Section 5.2.3)
a single Hop-by-Hop Options header.
7.2.1. DSR Route Reply Option 5.2.1. DSR Route Reply Option
The DSR Route Reply hop-by-hop option is encoded in type-length-value The DSR Route Reply hop-by-hop option is encoded in type-length-value
(TLV) format as follows: (TLV) format as follows:
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Reserved | | Option Type | Option Length |L| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Target Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|OUT Index[1] |C|OUT Index[2] |C|OUT Index[3] |C|OUT Index[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] | | Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] | | Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[3] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|OUT Index[5] |C|OUT Index[6] |C|OUT Index[7] |C|OUT Index[8] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[5] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type Option Type
???. A node that does not understand this option should ignore ???. The top three bits of this Option Type value are equal to
this option and continue processing the packet, and the Option 000, meaning that a node that does not understand this option
Data does not change en-route (the top three bits are 000). SHOULD ignore this option and continue processing the packet,
and that the Option Data does not change en-route [7].
Option Length Option Length
8-bit unsigned integer. Length of the option, in octets, 8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields. excluding the Option Type and Option Length fields.
Reserved Last Hop External (L)
Sent as 0; ignored on reception.
Target Address
The home address of the node to which the Route Reply must be
delivered.
Change Interface (C) bit[1..n]
If the C bit associated with a node N is set, it implies N will Set to indicate that the last node indicated by the Route Reply
be forwarding the packet out a different interface than the one is actually in a network external to the DSR network; the exact
over which it was received (i.e., the node sending the packet sequence of hops leading to it outside the DSR network are not
to N should not expect a passive acknowledgment). represented in the Route Reply. Nodes caching this hop in
their Route Cache MUST flag the cached hop with the External
flag. Such hops MUST NOT be returned in a cached Route Reply
generated from this Route Cache entry, and selection of routes
from the Route Cache to route a packet being sent SHOULD prefer
routes that contain no nodes flagged as External.
OUT Index[1..n] Reserved
OUT Index[i] is the interface index of the ith hop listed in Sent as 0; ignored on reception.
the Route Reply option. It denotes the interface that should
be used by Address[i-1] to reach Address[i] when using the
specified source route.
Address[1..n] Address[1..n]
Address[i] is the home address of the ith hop listed in the The source route being returned by the Route Reply, indicating
Route Reply option. a route from the node with address Address[1] to the node with
address Address[n]. The number of addresses present in this
field is indicated by the Option Length field in the option
(n = (Option Length - 1) / 4).
7.2.2. DSR Route Error Option A DSR Route Reply destination option MAY appear one or more times
within a single Hop-by-Hop Options extension header.
5.2.2. DSR Route Error Option
The DSR Route Error hop-by-hop option is encoded in type-length-value The DSR Route Error hop-by-hop option is encoded in type-length-value
(TLV) format as follows: (TLV) format as follows:
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Index | Error Type | | Option Type | Option Length | Error Type |Reservd|Salvage|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Source Address | | Error Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Destination Address | | Error Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Unreachable Node Address | | Unreachable Node Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type Option Type
???. A node that does not understand this option should ignore ???. The top three bits of this Option Type value are equal to
the option and continue processing the packet, and the Option 000, meaning that a node that does not understand this option
Data does not change en-route (the top three bits are 000). SHOULD ignore this option and continue processing the packet,
and that the Option Data does not change en-route [7].
Option Length Option Length
8-bit unsigned integer. Length of the option, in octets, 8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields. excluding the Option Type and Option Length fields.
Index For the current definition of the DSR Route Error option, this
field MUST be set to 13. Extensions to the DSR Route Error
The interface index of the network interface over which the option format may be included after the fixed portion of the
node designated by Error Source Address tried to forward a DSR Route Error option specified above. The presence of such
packet to the node designated by Unreachable Node Address. extensions will be indicated by the Option Length field. When
the Option Length is greater than 13 octets, the remaining
octets are interpreted as extensions. Currently, no extensions
have been defined.
Error Type Error Type
The type of error encountered. Values between 0 and 127 The type of error encountered. Currently, the following type
inclusive indicate a hard failure of connectivity between the value is defined:
Error Source Address and the Unreachable Node Address. Values
between 128 and 255 inclusive indicate a soft failure.
NODE_UNREACHABLE 1 NODE_UNREACHABLE 1
PATH_STATE_NOT_FOUND 129 Other values of the Error Type field are reserved for future
use.
Reservd
Reserved. Sent as 0; ignored on reception.
Salvage
A 4-bit unsigned integer. Copied from the Salvage field in the
DSR Routing header of the packet triggering the Route Error,
incremented by the node returning the Route Error.
Error Source Address Error Source Address
The home address of the node originating the Route Error (e.g., The address of the node originating the Route Error (e.g., the
the node that attempted to forward a packet and discovered the node that attempted to forward a packet and discovered the link
link failure). failure).
Error Destination Address Error Destination Address
The home address of the node to which the Route Error must be The address of the node to which the Route Error must
delivered (e.g, the node that generated the routing information be delivered (e.g., the node that generated the routing
claiming that the hop Error Source Address to Unreachable Node information claiming that the hop Error Source Address to
Address was a valid hop). Unreachable Node Address was a valid hop).
Unreachable Node Address Unreachable Node Address
The home address of the node that was found to be unreachable The address of the node that was found to be unreachable
(the next hop neighbor to which the node at ``Error Source (the next hop neighbor to which the node with address
Address'' was attempting to transmit the packet). Error Source Address was attempting to transmit the packet).
7.2.3. DSR Acknowledgment Option A DSR Route Error destination option MAY appear one or more times
within a single Hop-by-Hop Options extension header.
5.2.3. DSR Acknowledgment Option
The DSR Acknowledgment hop-by-hop option is encoded in The DSR Acknowledgment hop-by-hop option is encoded in
type-length-value (TLV) format as follows: type-length-value (TLV) format as follows:
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification | | Option Type | Option Length | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Source Address | | ACK Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ACK Destination Address | | ACK Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type Option Type
???. A node that does not understand this option should ignore ???. The top three bits of this Option Type value are equal to
the option and continue processing the packet, and the Option 000, meaning that a node that does not understand this option
Data does not change en-route (the top three bits are 000). SHOULD ignore this option and continue processing the packet,
and that the Option Data does not change en-route [7].
Option Length Option Length
8-bit unsigned integer. Length of the option, in octets, 8-bit unsigned integer. Length of the option, in octets,
excluding the Option Type and Option Length fields. excluding the Option Type and Option Length fields.
Identification Identification
A 32-bit value that when taken in conjunction with Data Source Copied from the Identification field of the DSR Routing header
Address, uniquely identifies the packet being acknowledged.
The Identification value is computed as ((ip_id << 16) | ip_off)
where ip_id is the value of the 16-bit Identification field in
the IP header of the packet being acknowledged, and ip_off is
the value of the 13-bit Fragment Offset field in the IP header
of the packet being acknowledged. of the packet being acknowledged.
When constructing the Identification, ip_id and ip_off MUST be
in host byte-order. The entire Identification value MUST then
be converted to network byte-order before being placed in the
Acknowledgment option.
ACK Source Address ACK Source Address
The home address of the node originating the Acknowledgment. The address of the node originating the DSR Acknowledgment.
ACK Destination Address ACK Destination Address
The home address of the node to which the Acknowledgment must The address of the node to which the DSR Acknowledgment is to
be delivered. be delivered.
Data Source Address A DSR Acknowledgement destination option MAY appear one or more times
within a single Hop-by-Hop Options extension header.
The IP Source Address of the packet being acknowledged.
7.3. DSR Routing Header 5.3. DSR Routing Header
As specified for IPv6 [6], a Routing header is used by a source to As specified for IPv6 [7], a Routing header is used by a source to
list one or more intermediate nodes to be ``visited'' on the way to list one or more intermediate nodes to be "visited" on the way to
a packet's destination. This function is similar to IPv4's Loose a packet's destination. This function is similar to IPv4's Loose
Source and Record Route option, but the Routing header does not Source and Record Route option, but the Routing header does not
record the route taken as the packet is forwarded. The specific record the route taken as the packet is forwarded. The specific
processing steps required to implement the Routing header must be processing steps required to implement the Routing header must be
added to an IPv4 protocol stack. The Routing header is identified by added to an IPv4 protocol stack. The Routing header is identified by
a Next Header value of 43 in the immediately preceding header, and a Next Header value of 43 in the immediately preceding header, and
has the following format: has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len | Routing Type | Segments Left | | Next Header | Hdr Ext Len | Routing Type | Segments Left |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
. . . .
. type-specific data . . type-specific data .
. . . .
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The type specific data for a Routing Header carrying a DSR Source The type-specific data for a Routing Header carrying a DSR Source
Route is: Route is:
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|S| Reserved | |F|L| Reserved |Salvage| Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|OUT Index[1] |C|OUT Index[2] |C|OUT Index[3] |C|OUT Index[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[1] | | Address[1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[2] | | Address[2] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[3] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[4] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|OUT Index[5] |C|OUT Index[6] |C|OUT Index[7] |C|OUT Index[8] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address[5] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Routing Header Fields: | Address[n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Routing header fields:
Next Header Next Header
8-bit selector. Identifies the type of header immediately 8-bit selector. Identifies the type of header immediately
following the Routing header. following the Routing header.
Hdr Ext Len Hdr Ext Len
8-bit unsigned integer. Length of the Routing header in 8-bit unsigned integer. Length of the Routing header in
4-octet units, not including the first 8 octets. 4-octet units, not including the first 8 octets.
skipping to change at page 21, line 26 skipping to change at page 31, line 26
Routing Type Routing Type
??? ???
Segments Left Segments Left
Number of route segments remaining, i.e., number of explicitly Number of route segments remaining, i.e., number of explicitly
listed intermediate nodes still to be visited before reaching listed intermediate nodes still to be visited before reaching
the final destination. the final destination.
Type Specific Fields: Type-specific fields:
Acknowledgment Request (R) First Hop External (F)
The Acknowledgment Request (R) bit is set to request an Set to indicate that the first node indicated by the Routing
explicit acknowledgment from the next hop. After processing header is actually in a network external to the DSR network;
the Routing Header, The IP Destination Address lists the the exact sequence of hops leading from it outside the DSR
address of the next hop. network are not represented in the Routing header. Nodes
caching this hop in their Route Cache MUST flag the cached
hop with the External flag. Such hops MUST NOT be returned
in a Route Reply generated from this Route Cache entry, and
selection of routes from the Route Cache to route a packet
being sent SHOULD prefer routes that contain no hops flagged as
External.
Salvaged Packet (S) Last Hop External (L)
The Salvaged Packet (S) bit indicates that this packet has been Set to indicate that the last hop indicated by the Routing
salvaged by an intermediate node, and thus that this Routing header is actually in a network external to the DSR network;
Header was generated by Address[1] and not the IP Source the exact sequence of hops leading to it outside the DSR
Address (Section 8.5.5). network are not represented in the Routing header. Nodes
caching this hop in their Route Cache MUST flag the cached
hop with the External flag. Such hops MUST NOT be returned
in a Route Reply generated from this Route Cache entry, and
selection of routes from the Route Cache to route a packet
being sent SHOULD prefer routes that contain no hops flagged as
External.
Reserved Reserved
Sent as 0; ignored on reception. Sent as 0; ignored on reception.
Change Interface (C) bit[1..n] Salvage
If the C bit associated with a node N is set, it implies N will A 4-bit unsigned integer. Count of number of times that
be forwarding the packet out a different interface than the one this packet has been salvaged as a part of DSR routing
over which it was received (i.e., the node sending the packet (Section 3.4.1).
to N should not expect a passive acknowledgment and MAY wish to
set the R bit).
OUT Index[1..n] Identification
Index[i] is the interface index that the node indicated Used to request that a DSR Acknowledgement option be returned
by Address[i-1] must use when transmitting the packet to to this transmitting node for this hop. The special value of 0
Address[i]. Index[1] indicates which interface the node indicates that no DSR Acknowledgement is requested. Otherwise,
indicated by the IP Source Address uses to transmit the packet. the Identification field is set to a unique nonzero number
by this node transmitting the packet and is copied into the
Identification field of the DSR Acknowledgement option when
returned by the node receiving the packet over this hop.
Address[1..n] Address[1..n]
Address[i] is the home address of the ith hop in the Routing The sequence of addresses of the source route. In routing
header. and forwarding the packet, the source route is processed as
described in Sections 6.1.2 and 6.1.4.
Note that Address[1] is the first intermediate hop along the route.
The address of the originating node is the IP Source Address. The
only exception to this rule is for packets that are salvaged, as
described in Section 8.5.5. A packet that has been salvaged has an
alternate route placed on it by an intermediate node in the network,
and in this case, the address of the originating node (the salvaging
node) is Address[1]. Salvaged packets are indicated by setting the S
bit in the DSR Routing header.
8. Detailed Operation
8.1. Originating a Data Packet
When node A originates a packet, the following steps MUST be taken
before transmitting the packet:
1. If the destination address is a multicast address, piggyback the
data packet on a Route Request targeting the multicast address.
The following fields MUST be initialized as specified:
IP.Source_Address = Home address of node A
IP.Destination_Address = 255.255.255.255
Request.Target_Address = Multicast destination address
DONE.
2. Otherwise, call Route_Cache.Get() to determine if there is a
cached source route to the destination.
3. If the cached route indicates that the destination is directly
reachable over one hop, no Routing Header should be added to the
packet. Initialize the following fields:
IP.Source_Address = Home address of node A
IP.Destination_Address = Home address of the Destination
DONE.
4. Otherwise, if the cached route indicates that multiple hops are
required to reach the destination, insert a Routing Header into
the packet as described in Section 8.2. DONE.
5. Otherwise, if no cached route to the destination is found, insert
the packet into the Send Buffer and initiate Route Discovery as
described in Section 8.4.
8.2. Originating a Packet with a DSR Routing Header
When a node originates a packet with a Routing Header, the address
of the first hop in the source route MUST be listed as the IP
Destination Address as well as Address[1] in the Routing Header.
The final destination of the packet is listed as the last hop
in the Routing Header (Address[n]). At each intermediate hop i,
Address[i] is copied into the IP Destination Address and the packet
is retransmitted.
For example, suppose node A originates a packet destined for node D
that should pass through intermediate hops B and C. The packet MUST
be initialized as follows:
IP.Source_Address = Home address of node A
IP.Destination_Address = Home address of node B
RT.Segments_Left = 2
RT.Out_Index[1] = Interface index used by A to reach B
RT.Out_Index[2] = Interface index used by B to reach C
RT.Out_Index[3] = Interface index used by C to reach D
RT.Address[1] = Home address of node B
RT.Address[2] = Home address of node C
RT.Address[3] = Home address of node D
8.3. Processing a Routing Header
Excluding the exceptions listed here, a DSR Routing Header is
processed using the same rules as outlined for Type 0 Routing Headers
in IPv6 [6]. The Routing Header is only processed by the node whose
address appears as the IP destination of the packet. The following
additional rules apply to processing the type specific data of a DSR
Source Route:
Let
SegLft = the value of Segments Left when the packet was received
NumAddrs = the total number of addresses in the Routing Header
1. The address of the next hop, Address[NumAddrs - SegLft + 1],
is copied into the IP.Destination_Address of the packet. The
existing IP.Destination_Address is NOT copied back into the
Address list of the Routing Header.
2. The interface used to transmit the packet to its next hop from
this node MUST be the interface denoted by Index[NumAddrs -
SegLft + 1].
3. If the Acknowledgment Request (R) bit is set, the node MUST
transmit a packet containing the DSR Acknowledgment option to
the previous hop, Address[NumAddrs - SegLft - 1], performing
Route Discovery if necessary. (Address[0] is taken as the
IP.Source_Address)
4. Perform Route Maintenance by verifying that the packet was
received by the next hop as described in Section 8.5.
8.4. Route Discovery
Route Discovery is the on-demand process by which nodes actively
obtain source routes to destinations to which they are actively
attempting to send packets. The destination node for which a
Route Discovery is initiated is known as the "target" of the Route
Discovery. A Route Discovery for a destination SHOULD NOT be
initiated unless the initiating node has a packet in the Send Buffer
requiring delivery to that destination. A Route Discovery for a
given target node MUST NOT be initiated unless permitted by the
rate-limiting information contained in the Route Request Table.
After each Route Discovery attempt, the interval between successive
Route Discoveries for this target must be doubled, up to a maximum of
MAX_REQUEST_PERIOD.
Route Discoveries for a multicast address SHOULD NOT be rate limited,
and SHOULD always be permitted.
8.4.1. Originating a Route Request
The basic Route Discovery algorithm for a unicast destination is as
follows:
1. Originate a Route Request packet with the IP header Time-to-Live
field initialized to 1. This type of Route Request is called a
non-propagating Route Request and allows the originator of the
Request to inexpensively query the route caches of each of its
neighbors for a route to the destination.
2. If a Route Reply is received in response to the non-propagating
Request, use the returned source route to transmit all packets
for the destination that are in the Send Buffer. DONE.
3. Otherwise, if no Route Reply is received within
RING0_REQUEST_TIMEOUT seconds, transmit a Route Request
with the IP header Time-to-Live field initialized to
MAX_ROUTE_LEN. This type of Route Request is called a propagating
Route Request. Update the information in the Route Request
Table, to double the amount of time before any subsequent Route
Discovery attempt to this target.
4. If no Route Reply is received within the time interval indicated
by the Route Request Table, GOTO step 1.
The Route Request option SHOULD be initialized as follows:
IP.Source_Address = This node's home address
IP.Destination_Address = 255.255.255.255
Request.Target = Home address of intended destination
Request.OUT_Index[1] = Index of interface used to transmit the Request
The behavior of a node processing a packet containing both a Routing
Header and a Route Request Destination option is unspecified.
Packets SHOULD NOT contain both a Routing Header and a Route Request
Destination option. [This is not exactly true: A Route Request
option appearing in the second Destination Options header that IPv6
allows after the Routing Header would probably do-what-you-mean,
though we have not triple-checked it yet. Namely, it would allow the
originator of a route discovery to unicast the request to some other
node, where it would be released and begin the flood fill. We call
this a Route Request Blossom since the unicast portion of the path
looks like a stem on the blossoming flood-fill of the request.]
Packets containing a Route Request Destination option SHOULD NOT be
retransmitted, SHOULD NOT request an explicit DSR Acknowledgment by
setting the R bit, SHOULD NOT expect a passive acknowledgment, and
SHOULD NOT be placed in the Retransmission Buffer. The repeated
transmission of packets containing a Route Request Destination option
is controlled solely by the logic described in this section.
8.4.2. Processing a Route Request Option
When a node A receives a packet containing a Route Request option,
the Route Request option is processed as follows:
1. If Request.Target_Address matches the home address of this node,
then the Route Request option contains a complete source route
describing the path from the initiator of the Route Request to
this node.
(a) Send a Route Reply as described in Section 8.4.4.
(b) Continue processing the packet in accordance with the Next
Header value contained in the Destination Option extension
header. DONE.
2. Otherwise, if the combination (IP.Source_Address,
Request.Identification) is found in the Route Request
Table, then discard the packet, since this is a copy of a
recently seen Route Request. DONE.
3. Otherwise, if Request.Target_Address is a multicast address then:
(a) If node A is a member of the multicast group indicated by
Request.Target_Address, then create a copy of the packet,
setting IP.Destination_Address = REQUEST.Target_Address, and
continue processing the copy of the packet in accordance with
the Next Header field of the Destination option.
(b) If IP.TTL is non-zero, decrement IP.TTL, and retransmit the
packet. DONE.
(c) Otherwise, discard the packet. DONE.
4. Otherwise, if the home address of node A is already listed in
the Route Request (IP.Source_Address or Request.Address[]), then
discard the packet. DONE.
5. Let
m = number of addresses currently in the Route Request option
n = m + 1
6. Otherwise, append the home address of node A to the Route Request
option (Request.Address[n]).
7. Set Request.IN_Index[n] = index of interface packet was received
on.
8. If a source route to Request.Target_Address is found in our Route
Cache and the rules of Section 8.4.3 permit it, return a Cached
Route Reply as described in Section 8.4.3. DONE.
9. Otherwise, for each interface on which the node is configured to
participate in a DSR ad hoc network:
(a) Make a copy of the packet containing the Route Request.
(b) Set Request.OUT_Index[n+1] = index of the interface.
(c) If the outgoing interface is different from the incoming
interface, then set the C bit on both Request.OUT_Index[n+1]
and Request.IN_Index[n]
(d) Link-layer re-broadcast the packet containing the Route
Request on the interface jittered by T milliseconds, where
T is a uniformly distributed, random number between 0 and
BROADCAST_JITTER. DONE.
8.4.3. Generating Route Replies using the Route Cache
A node SHOULD use its Route Cache to avoid propagating a Route
Request packet received from another node. In particular, suppose a
node receives a Route Request packet for which it is not the target
and which it does not discard based on the logic of Section 8.4.2.
If the node has a Route Cache entry for the target of the Request,
it SHOULD append this cached route to the accumulated route record
in the packet and return this route in a Route Reply packet to
the initiator without propagating (re-broadcasting) the Route
Request. Thus, for example, if node F in the example network shown
in Figure 8.4.3 needs to send a packet to node D, it will initiate
a Route Discovery and broadcast a Route Request packet. If this
broadcast is received by node A, node A can simply return a Route
Reply packet to F containing the complete route to D consisting of
the sequence of hops: A, B, C, and D.
Before transmitting a Route Reply packet that was generated using
information from its Route Cache, a node MUST verify that:
1. The resulting route contains no loops.
2. The node issuing the Route Reply is listed in the route that it
specifies in its Reply. This increases the probability that the
route is valid, since the node in question should have received
a Route Error if this route stopped working. Additionally, this
requirement means that a Route Error traversing the route will
pass through the node that issued the Reply based on stale cache
data, which is critical for ensuring stale data is removed from
caches in a timely manner. Without this requirement, the next
Route Discovery initiated by the original requester might also be
contaminated by a Route Reply from this node containing the same
stale route.
8.4.4. Originating a Route Reply
Let REQPacket denote a packet received by node A that
contains a Route Request option which lists node A as the
REQPacket.Request.Target_Address. Let REPPacket be a packet
transmitted by node A that contains a corresponding Route Reply. The
Route Reply option transmitted in response to a Route Request MUST be
initialized as follows:
B->C->D
+---+ +---+ +---+ +---+
| A |---->| B |---->| C |---->| D |
+---+ +---+ +---+ +---+
+---+
| F | +---+
+---+ | E |
+---+
Figure 1: An example network where A knows a
route to D via B and C.
1. If REQPacket.Request.Address[] does not contain any hops, then
node A is only a single hop from the originator of the Route
Request. Build a Route Reply packet as follows:
REPPacket.IP.Source_Address = REQPacket.Request.Target_Address
REPPacket.Reply.Target = REQPacket.IP.Source_Address
REPPacket.Reply.OUT_Index[1] = REQPacket.Request.OUT_index[1]
REPPacket.Reply.OUT_C_bit[1] = REQPacket.Request.OUT_C_bit[1]
REPPacket.Reply.Address[1] = The home address of node A
GOTO step 3.
2. Otherwise, build a Route Reply packet as follows:
REPPacket.IP.Source_Address = The home address of node A
REPPacket.Reply.Target = REQPacket.IP.Source_Address
REPPacket.Reply.OUT_Index[1..n]= REQPacket.Request.OUT_index[1..n]
REPPacket.Reply.OUT_C_bit[1..n]= REQPacket.Request.OUT_C_bit[1..n]
REPPacket.Reply.Address[1..n] = REQPacket.Request.Address[1..n]
3. Send the Route Reply jittered by T milliseconds, where T
is a uniformly distributed random number between 0 and
BROADCAST_JITTER. DONE.
If sending a Route Reply packet to the originator of the Route
Request requires performing a Route Discovery, the Route Reply
hop-by-hop option MUST be piggybacked on the packet that contains the
Route Request. This prevents a loop wherein the target of the new
Route Request (which was itself the originator of the original Route
Request) must do another Route Request in order to return its Route
Reply.
If sending the Route Reply to the originator of the Route Request
does not require performing Route Discovery, a node SHOULD send a
unicast Route Reply in response to every Route Request targeted at
it.
8.4.5. Processing a Route Reply Option
Upon receipt of a Route Reply, a node should extract the source route
(Target_Address, OUT_Index[1]:Address[1], .. OUT_Index[n]:Address[n]
) and insert this route into its Route Cache. All the packets in the
Send Buffer SHOULD be checked to see whether the information in the
Reply allows them to be sent immediately.
8.5. Route Maintenance
Route Maintenance requires that whenever a node transmits a data
packet, a Route Reply, or a Route Error, it must verify that the next
hop (indicated by the Destination IP Address) correctly receives the
packet.
If the sender cannot verify that the next hop received the packet, it
MUST decide that its link to the next hop is broken and MUST send a
Route Error to the node responsible for generating the Routing Header
that contains the broken link (Section 8.5.3).
The following ways may be used to verify that the next hop correctly 6. Detailed Operation
received a packet:
- The receipt of a passive acknowledgment (Section 8.5.1). 6.1. General Packet Processing
- The receipt of an explicitly requested acknowledgment 6.1.1. Originating a Packet
(Section 8.5.1).
- By the presence of positive feedback from the link layer When originating any packet, a node using DSR routing MUST perform
indicating that the packet was acknowledged by the next hop the following sequence of steps:
(Section 8.5.2).
- By the absence of explicit failure notification from the link - Search the node's Route Cache for a route to the address given in
layer that provides reliable hop-by-hop delivery such as MACAW or the IP Destination Address field in the packet's header.
802.11 (Section 8.5.2).
Nodes MUST NOT perform Route Maintenance for packets containing a - If no such route is found in the Route Cache, then perform
Route Request option or packets containing only an Acknowledgment Route Discovery for the Destination Address, as described in
option. Sending Acknowledgments for packets containing only Section 6.2.
an Acknowledgment option would create an infinite loop whereby
acknowledgments would be sent for acknowledgments. Acknowledgments
should be always sent for packets containing a Routing Header with
the R bit set (e.g., packets which contain only an Acknowledgment
and a Routing Header for which the last forwarding hop requires an
explicit acknowledgment of receipt by the final destination).
8.5.1. Using Network-Layer Acknowledgments - If the packet contains a Route Request option, then replace the
IP Destination Address field with the IP "limited broadcast"
address (255.255.255.255) [3].
For link layers that do not provide explicit failure notification, - Else, this node must have a route to the Destination Address
the following steps SHOULD be used by a node A to perform Route of the packet (since otherwise a Route Request would have
Maintenance. been added to the packet). If the length of this route is
greater than 1 hop, or if the node determines to request a DSR
network-layer acknowledgement from the first hop of the route,
then insert a DSR Routing header into the packet, as described
in Section 6.1.2. The source route in the packet is initialized
from the route to the Destination Address found in the Route
Cache.
When receiving a packet: - Transmit the packet to the address given in the IP
Destination Address, using Route Maintenance to retransmit the
packet if necessary, as described in Section 6.3.
- If the packet contains a Routing Header with the R bit set, send 6.1.2. Adding a DSR Routing Header to a Packet
an explicit acknowledgment as described in Section 8.3.
- If the packet does not contain a Routing Header, the node MUST The design of the DSR Routing header is based on the design of a
transmit a packet containing the DSR Acknowledgment option Routing header in IPv6 [7]. A node originating a packet adds a
to the previous hop as indicated by the IP.Source_Address. DSR Routing header to the packet, if necessary, in order to carry
Since the receiving node is the final destination, there the source route of hops from this originating node to the final
will be no opportunity for the originator to obtain a destination address of the packet. Specifically, the node adding the
passive acknowledgment, and the receiving node must infer the DSR Routing header constructs the Routing header and modifies the IP
originator's request for an explicit acknowledgment. packet according to the following sequence of steps:
When sending a packet: - A DSR Routing header, as described in Section 5.3, is created
and added to the packet after the IP header and any Hop-by-Hop
Options header that may already be in the packet, but before any
Destination Options header (e.g., containing a DSR Route Reply
option) that may be present.
1. Before sending a packet, insert a copy of the packet into the - The number of Address fields to include in the DSR Routing
Retransmission Buffer and update the information maintained about header (n) is the number of intermediate nodes in the source
this packet in the Retransmission Buffer. route for the packet (i.e., excluding address of the originating
node and the final destination address of the packet). The
Segments Left field in the DSR Routing header is initialized
equal to n.
2. If after processing the Routing Header, RH.Segments_Left is equal - The Source Address from the IP header is copied into Address[n]
to 0, then node A MUST set the Acknowledgment Request (R) bit in in the DSR Routing header.
the Routing Header before transmitting the packet over its final
hop.
3. If after processing the Routing Header and copying - The first hop of the source route for the packet is copied into
RH.Address[n] to IP.Destination_Address, node A determines that the Source Address field in the IP header.
RH.OUT_C_bit[n+1] is set, then node A MUST set the Acknowledgment
Request (R) bit in the Routing Header before transmitting the
packet (since the C bit was set during Route Discovery by the
node now listed as the IP.Destination_Address to indicate that
it will propagate the packet out a different interface, and that
node A will not receive a passive acknowledgment).
4. Set the retransmission timer for the packet in the Retransmission - The remaining hops of the source route for the packet are copied
Buffer. into sequential Address[i] fields in the DSR routing header,
for i = 1, 2, ..., n-1.
5. Transmit the packet. - The First Hop External (F) bit in the Routing header is copied
from the External bit flagging the first hop node in the source
route for the packet, as indicated in the Route Cache.
6. If a passive or explicit acknowledgment is received before the - The Last Hop External (L) bit in the Routing header is copied
retransmission timer expires, then remove the packet from the from the External bit flagging the last hop node in the source
Retransmission Buffer and disable the retransmission timer. route for the packet, as indicated in the Route Cache.
DONE.
7. Otherwise, when the Retransmission Timer expires, remove the - All other fields in the type-specific data in the DSR Routing
packet from the Retransmission Buffer. header are initialized to 0.
8. If DSR_MAXRXTSHIFT transmissions have been done, then attempt - The Routing Type field in the DSR Routing header is initialized
to salvage the packet (Section 8.5.5). Also, generate a Route to ???.
Error. DONE.
9. GOTO step 1. - The Hdr Ext Len field in the DSR Routing header is initialized
to 4.
8.5.2. Using Link Layer Acknowledgments - Next Header field in the DSR Routing header is set equal to the
current value in the Protocol field in the IP header (or the
Next Header field in the preceding extension header), and the
Protocol field (or preceding Next Header field) is set equal
to 43 to indicate a Routing header extension header [7].
If explicit failure notifications are provided by the link layer, 6.1.3. Receiving a Packet
then all packets are assumed to be correctly received by the next hop
and a Route Error is sent only when a explicit failure notification
is made from the link layer.
Nodes receiving a packet without a Routing Header do not need to send When a node receives any packet, it MUST process the packet according
an explicit Acknowledgment to the packet's originator, since the to the following sequence of steps:
link layer will notify the originator if the packet was not received
properly.
8.5.3. Originating a Route Error - If the Destination Address in the packet's IP header does not
match any of this receiving node's own IP address(s), then the
processing of this packet depends on whether the packet contains
a DSR Routing header:
If the next hop of a packet is found to be unreachable as described * If the packet contains a DSR Routing header, then discard the
in Section 8.5, a Route Error packet (Section 7.2.2) MUST be returned
to the node whose cache generated the information used to route the
packet. packet.
When a node A generates a Route Error for packet P, it MUST * Else, if the packet contains a Hop-by-Hop Options extension
initialize the fields in the Route Error as follows: header (if present, this MUST immediately follow the packet's
IP header), then process the options contained in the
Error.Source_Address = Home address of node A Hop-by-Hop Options extension header. Forward the packet
Error.Unreachable_Address = Home address of the unreachable node using normal IP forwarding proceedures and do not process the
packet further.
- If the packet contains a DSR Routing Header and the S bit is NOT
set, the packet has been forwarded without the need for salvaging
up to this point.
Error.Destination_Address = P.IP.Source_Address
- Otherwise, if the packet contains a DSR Routing Header and the S
bit IS set, the packet has been salvaged by an intermediate node,
and thus this Routing Header was placed there by the salvaging
node.
Error.Destination_Address = P.RoutingHeader.Address[1]
- Otherwise, if the packet does not contain a DSR Routing Header,
the packet must have been originated by this node A.
Error.Destination_Address = Home address of node A
Send the packet containing the Route Error to Error.Destination_Address,
performing Route Discovery if necessary.
As an optimization, Route Errors that are discovered by the
packet's originator (such that Error.Source_Address is equal to
Error.Destination_Address) SHOULD be processed internally. Such
processing should invoke all the steps that would be taken if a Route
Error option was created, transmitted, received, and processed,
but an actual packet containing a Route Error option SHOULD NOT be
transmitted.
8.5.4. Processing a Route Error Option
Upon receipt of a Route Error via any mechanism, a node
SHOULD remove any route from its Route Cache that uses the hop
(Error.Source_Address, Error.Index to Error.Unreachable_Address).
This includes all Route Errors overheard, and those processed
internally as described in Section 8.5.3.
When the node identified by Error.Destination_Address receives
the Route Error, it SHOULD verify that the source route
responsible for delivering the Route Error includes the same
hops as the working prefix of the original packet's source route
(Error.Destination_Address to Error.Source_Address). If any
hop listed in the working prefix is not included in the Route
Error's source route, then the originator SHOULD forward the Route
Error back along the working prefix (Error.Destination_Address to
Error.Source_Address) so that each node along the working prefix will
remove the invalid route from its Route Cache.
If the node processing a Route Error option discovers its home
address is Error.Destination_Address and the packet contains
additional Route Error option(s) later on the inside of the Hop
by Hop options header, we call the additional Route Errors nested
Route Errors. The node MUST deliver the first nested Route Error
to Nested_Error.Destination_Address, performing Route Discovery if
needed. It does this by removing the Route Error option listing
itself as the Error.Destination_Address, finding the first nested
Route Error option, and originating the remaining packet to
Nested_Error.Destination_Address. This mechanism allows for the
proper handling of Route Errors that are discovered while delivering
a Route Error.
8.5.5. Salvaging a Packet
When node A attempts to salvage a packet originated at node S and
destined for node D, it MUST perform the following steps:
1. Generate and send a Route Error to S as explained in
Section 8.5.3.
2. Call Route_Cache.Get() to determine if it has a cached source
route to the packet's ultimate destination D (which is the last
Address listed in the Routing Header).
3. If node A does not have a cached route for node D, it MUST
discard the packet. DONE.
4. Otherwise, let Salvage_Address[1] through Salvage_Address[m] be
the sequence of hops returned from the Route Cache. Initialize
the following fields in the packet's header:
RT.Segments_Left = m - 2;
RT.S = 1
RT.Address[1] = Home address of Node A
RT.Address[2] = Salvage.Address[1]
...
RT.Address[n] = Salvage.Address[m]
The IP Source Address of the packet MUST remain unchanged. When the
Routing Header in the outgoing packet is processed, RT.Address[2],
will be copied to the IP Destination Address field.
9. Optimizations
A number of optimizations can be added to the basic operation of
Route Discovery and Route Maintenance as described in Sections 8.4
and 8.5 that can reduce the number of overhead packets and improve
the average efficiency of the routes used on data packets. This
section discusses some of those optimizations.
9.1. Leveraging the Route Cache
The data in a node's Route Cache may be stored in any format, but
the active routes in its cache form a tree of routes, rooted at
this node, to other nodes in the ad hoc network. For example, the
illustration below shows an ad hoc network of six mobile nodes, in
which mobile node A has earlier completed a Route Discovery for
mobile node D and has cached a route to D through B and C:
B->C->D
+---+ +---+ +---+ +---+
| A |---->| B |---->| C |---->| D |
+---+ +---+ +---+ +---+
+---+ - Examine and process each of the extension headers (if any) in
| F | +---+ the packet in the order in which they occur in the packet. By
+---+ | E | dispatching on the Protocol field in the packet's IP header,
+---+ and subsequently dispatching on the Next Header field of each
encountered extension header, the appropriate protocol module is
executed by the receiving node for each extension header.
Since nodes B and C are on the route to D, node A also learns the - If a Hop-by-Hop Options extension header or Destination Options
route to both of these nodes from its Route Discovery for D. If A extension headers is encountered in processing the packet, the
later performs a Route Discovery and learns the route to E through B receiving node MUST process any options given in this header in
and C, it can represent this in its Route Cache with the addition of the order in which they occur in the Options field within the
the single new hop from C to E. If A then learns it can reach C in a option.
single hop (without needing to go through B), A SHOULD use this new
route to C to also shorten the routes to D and E in its Route Cache.
9.1.1. Promiscuous Learning of Source Routes Any DSR routing information carried in a packet SHOULD be examined
and reflected in the node's Route Cache, even if the options in
the packet are not otherwise processed as described above. In
particular, the following routing information SHOULD be handled in
this way:
A node can add entries to its Route Cache any time it learns a new - In a DSR Route Request option, the accumulated route record,
route. In particular, when a node forwards a data packet as an represented by the IP Source Address of the packet and by the
intermediate hop on the route in that packet, the forwarding node is sequence of Address[i] entries in the Route Request option SHOULD
able to observe the entire route in the packet. Thus, for example, be added to the node's Route Cache.
when any intermediate node B forwards packets from A to D, B SHOULD
add the source route information from that packet's Routing Header
to its own Route Cache. If a node forwards a Route Reply packet, it
SHOULD also add the source route information from the route record
being returned in the Route Reply, to its own Route Cache.
In addition, since all wireless network transmissions at the physical - In a DSR Route Reply option, the route record being returned,
layer are inherently broadcast, it may be possible for a node to represented by the sequence of Address[i] entries in the Route
configure its network interface into promiscuous receive mode, such Request option and by the Destination Address in the packet's IP
that the node is able to receive all packets without link layer header SHOULD be added to the node's Route Cache.
address filtering. In this case, the node MAY add to its Route Cache
the route information from any packet it can overhear.
9.2. Preventing Route Reply Storms - In a DSR Acknowledgement option, the single link from the
ACK Source Address to the ACK Destination Address SHOULD be added
to the node's Route Cache.
The ability for nodes to reply to a Route Request not targeted at - In a DSR Route Error option, the single link from the
them by using their Route Caches can result in a Route Reply storm. Error Source Address to the Unreachable Node Address MUST be
If a node broadcasts a Route Request for a node that its neighbors removed from the node's Route Cache.
have in their Route Caches, each neighbor may attempt to send a
Route Reply, thereby wasting bandwidth and increasing the rate
of collisions in the area. For example, in the network shown in
Section 9.1, if both node A and node B receive F's Route Request,
they will both attempt to reply from their Route Caches. Both will
send their Replies at about the same time since they receive the
broadcast at about the same time. Particularly when more than the
two mobile nodes in this example are involved, these simultaneous
replies from the mobile nodes receiving the broadcast may create
packet collisions among some or all of these replies and may cause
local congestion in the wireless network. In addition, it will
often be the case that the different replies will indicate routes
of different lengths. For example, A's Route Reply will indicate a
route to D that is one hop longer than that in B's reply.
For interfaces which can promiscuously listen to the channel, mobile - In a DSR Routing header, the indicated source route SHOULD be
nodes SHOULD use the following algorithm to reduce the number of added to the node's Route Cache, subject to the conditions
simultaneous replies by slightly delaying their Route Reply: identified in Section 3.3.1. The full sequence of hops in the
DSR Routing header is as follows:
1. Pick a delay period * The Source Address in the packet's IP header is the first hop
(the sender of the packet).
d = H * (h - 1 + r) * Let n equal Hdr Ext Len. This is the number of addresses in
the Routing header. Let i equal n minus Segments Left.
where h is the length in number of network hops for the route * The sequence of hops
to be returned in this node's Route Reply, r is a random number
between 0 and 1, and H is a small constant delay to be introduced
per hop.
2. Delay transmitting the Route Reply from this node for a period Address[1], Address[2], ..., Address[i]
of d.
3. Within the delay period, promiscuously receive all packets at follow immediately after the IP Source Address in the source
this node. If a packet is received by this node during the delay
period that is addressed to the target of this Route Discovery
(the target is the final destination address for the packet,
through any sequence of intermediate hops), and if the length of
the route on this packet is less than h, then cancel the delay
timer and do not transmit the Route Reply from this node; this
node may infer that the initiator of this Route Discovery has
already received a Route Reply, giving an equally good or better
route. route.
9.3. Piggybacking on Route Discoveries * The Destination Address in the packet's IP header follows
immediately next in the source route.
As described in Section 5.1, when one node needs to send a packet
to another, if the sender does not have a route cached to the
destination node, it must initiate a Route Discovery, buffering the
original packet until the Route Reply is returned. The delay for
Route Discovery and the total number of packets transmitted can be
reduced by allowing data to be piggybacked on Route Request packets.
Since some Route Requests may be propagated widely within the ad hoc
network, though, the amount of data piggybacked must be limited. We
currently use piggybacking when sending a Route Reply or a Route
Error packet, since both are naturally small in size. Small data
packets such as the initial SYN packet opening a TCP connection [18]
could easily be piggybacked.
One problem, however, arises when piggybacking on Route Request
packets. If a Route Request is received by a node that replies
to the request based on its Route Cache without propagating the
Request (Section 9.1), the piggybacked data will be lost if the node
simply discards the Route Request. In this case, before discarding
the packet, the node must construct a new packet containing the
piggybacked data from the Route Request packet. The source route
in this packet MUST be constructed to appear as if the new packet
had been sent by the initiator of the Route Discovery and had been
forwarded normally to this node. Hence, the first portion of the
route is taken from the accumulated route record in the Route Request
packet and the remainder of the route is taken from this node's Route
Cache. The sender address in the packet MUST also be set to the
initiator of the Route Discovery. Since the replying node will be
unable to correctly recompute an Authentication header for the split
off piggybacked data, data covered by an Authentication header SHOULD
NOT be piggybacked on Route Request packets.
9.4. Discovering Shorter Routes
Once a route between a packet source and a destination has been
discovered, the basic DSR protocol MAY continue to use that route
for all traffic from the source to the destination as long as
it continues to work, even if the nodes move such that a shorter
route becomes possible. In many cases, the basic Route Maintenance
procedure will discover the shorter route, since if a node moves
enough to create a shorter route, it will likely also move out of
transmission range of at least one hop on the existing route.
Furthermore, when a data packet is received as the result of
operating in promiscuous receive mode, the node checks if the Routing
Header packet contains its address in the unprocessed portion of the
source route (Address[NumAddrs - SegLft] to Address[NumAddrs]). If
so, the node knows that packet could bypass the unprocessed hops
preceding it in the source route. The node then sends what is called
a gratuitous Route Reply message to the packet's source, giving it
the shorter route without these hops.
The following algorithm describes how a node A should process packets
with an IP.Destination_Address not addressed to A or the IP broadcast
address or a multicast address that are received as a result of A
being in promiscuous receive mode:
1. If the packet is not a data packet containing a Routing Header,
drop the packet. DONE.
2. If the home address of this node does not appear in the portion
of the source route that has not yet been processed (indicated by
Segments Left), then drop the packet. DONE.
3. Otherwise, the node B that just transmitted the packet (indicated
by Address[NumAddrs - SegLft - 1]) can communicate directly with
this node A. Create a Route Reply. The Route Reply MUST list
the entire source route contained in the received packet with the
exception of the intermediate nodes between node B and node A.
4. Send this gratuitous Route Reply to the node listed as the
IP.Source_Address of the received packet. If Route Discovery
is required it MAY be initiated, or the gratuitous Route Reply
packet MAY be dropped.
9.5. Rate Limiting the Route Discovery Process
One common error condition that must be handled in an ad hoc network
is the case in which the network effectively becomes partitioned.
That is, two nodes that wish to communicate are not within
transmission range of each other, and there are not enough other
mobile nodes between them to form a sequence of hops through which
they can forward packets. If a new Route Discovery was initiated
for each packet sent by a node in this situation, a large number of
unproductive Route Request packets would be propagated throughout the
subset of the ad hoc network reachable from this node. In order to
reduce the overhead from such Route Discoveries, we use exponential
back-off to limit the rate at which new Route Discoveries may be
initiated from any node for the same target. If the node attempts to
send additional data packets to this same node more frequently than
this limit, the subsequent packets SHOULD be buffered in the Send
Buffer until a Route Reply is received, but it MUST NOT initiate a
new Route Discovery until the minimum allowable interval between new
Route Discoveries for this target has been reached. This limitation
on the maximum rate of Route Discoveries for the same target is
similar to the mechanism required by Internet nodes to limit the rate
at which ARP requests are sent to any single IP address [2].
9.6. Improved Handling of Route Errors
All nodes SHOULD process all of the Route Error messages they
receive, regardless of whether the node is the destination of
the Route Error, is forwarding the Route Error, or promiscuously
overhears the Route Error.
Since a Route Error packet names both ends of the hop that is no
longer valid, any of the nodes receiving the error packet may update
their Route Caches to reflect the fact that the two nodes indicated
in the packet can no longer directly communicate. A node receiving
a Route Error packet simply searches its Route Cache for any routes
using this hop. For each such route found, the route is effectively
truncated at this hop. All nodes on the route before this hop are
still reachable on this route, but subsequent nodes are not.
An experimental optimization to improve the handling of errors is
to support the caching of "negative" information in a node's Route
Cache. The goal of negative information is to record that a given
route was tried and found not to work, so that if the same route
is discovered again shortly after the failure, the Route Cache can
ignore or downgrade the metric of the failed route.
We have not currently included this caching of negative information
in our simulations, since it appears to be unnecessary if nodes also
promiscuously receive Route Error packets.
9.7. Increasing Scalability
We recently designed and began experimenting with ways to integrate
ad hoc networks with the Internet and with Mobile IP [14]. In
addition to this, we are also exploring ways to increase the
scalability of ad hoc networks by taking advantage of their
cooperative nature and the fact that some hierarchy can be imposed
on an ad hoc network, just be assigning addresses to the nodes in a
reasonable way. These ideas are described in a workshop paper [4].
10. Path-State and Flow-State Mechanisms
This section describes the current design of our framework for
supporting better-than-best-effort Quality of Service in DSR
networks. The framework dovetails into DSR's existing mechanisms,
and, like DSR itself, is completely on-demand in nature --- no
packets are sent unless there is user data to transfer. The
framework is based on the introduction of two kinds of soft-state,
called path-state and flow-state, at the intermediate nodes along the
path between senders and destinations.
Taken together, the path-state and flow-state extensions extend the
Quality of Service provided by DSR networks in the following ways:
- They eliminate the need for all data packets to carry a source
route, increasing the efficiency of the protocol in general.
- They provide accurate measurements of the state of the network to
higher layers protocols for use in adaptation.
- They enable senders to explicitly manage the consumption of
resources across the network.
- They enable the network to provide better-than-best-effort
service via admission control and per-flow resource management.
10.1. Overview
Path-state allows intermediate nodes to forward packets according to
a predetermined source route, even when most packets do not include
the full source route. Conceptually, the originator of each data
packet initially includes both a source route and a unique path
identifier in each packet it sends. As intermediate nodes forward
the packet, they cache the source route from the packet, indexed by
the path identifier. The source can then send subsequent packets
carrying only the path identifier, since intermediate nodes will be
able to forward the packet based on the source route for the path
that they have cached.
While path-state allows the elimination of the source route from each
packet, thereby reducing the overhead of the DSR protocol, it also
provides a way for sources to monitor the state of each path through
the network. When a source wishes to know the characteristics of
a path through the network, it piggybacks a path-metrics header
onto any data or control packet traversing the path. As the
packet propagates through the network, each intermediate node
updates the set of path-metrics carried by the packet to reflect
the local network conditions seen at the node. These metrics are
reflected back to the sender by the destination, along with the path
identifier, and enable the sender to track the value of these metrics
for each of the source routes it is currently using.
We are currently experimenting with metrics that are easy for nodes
to measure, that require constant size to represent regardless of
source route length, and that would enable the sender's network
layer to provide useful feedback to higher layers on the state of
the network. For example, by including ``available bandwidth''
or ``battery level'' as a metric, senders can load balance
the consumption of resources across the network. We have also
considered the possibility of replacing the TCP congestion control
algorithm with a ``leaky-bucket'' system controlled by the reflected
path-metrics --- our measured performance results show this could
dramatically improve TCP throughput in environments where many
packets are lost due to packet corruption. The feedback could also
be used as inputs to other researcher's systems for improving the
transport layer, such as Liu and Singh's ATCP [11], or for adaptation
at higher layers, as in Odyssey [13].
Flow-state allows a source to differentiate its traffic into
flows, and then request better-than-best-effort handling for these
flows. With the additional information provided by the flow-state,
the network can provide admission control and promise a specific
Quality of Service (QoS) to each flow. Since the ad hoc network may
frequently change topology, the flow-state mechanisms are directly
integrated into the routing protocol to minimize their reaction time
and provide notifications to a flow when the network must break its
promise for a specific level of QoS.
10.2. Path-State and Flow-State Identifiers
The metadata that intermediate nodes in the network must process
is divided into path-state and flow-state, where path-state is
the fundamental unit of routing information and flow-state is the
fundamental unit of Quality of Service information.
Path-state is associated with a particular set of hops through the
network from some source S to a destination D (i.e., a particular
source route in the network). It consists of the information needed
to route packets along the path, and information about the carrying
capacity of the path, such as the unused bandwidth along the path or
the minimum latency of the path.
Flow-state is specific to a particular class of packets flowing
between S and D that is routed over a given path. Flow-state is
used to record Quality of Service promises that have been made for a
particular flow, and allows packets from S to D that take the same
path through the network to be treated differently by intermediate
nodes. For example, all the TCP connections between S and D that
take the same path will share the same path-state, but may have
independent flow-state. At any point in time, S may use multiple
paths for its traffic to D and each of these paths may be comprised
of many flows. However, a single network layer flow may not be
multiplexed over different paths.
To represent paths and flows inside the network, we use a scheme
inspired by the Virtual Path Index and Virtual Circuit Index of
ATM networks [23, p. 451]. Paths are identified by the logical
concatenation of the source node address and a 16-bit path identifier
where the low 5 bits are 0. Flows are identified by a path
identifier where the low 5 bits are used to distinguish between the
different flows that use the same path.
This scheme has two main advantages. First, each source node can
independently generate globally unique path- and flow-identifiers.
Second, the hierarchical relation of flow-identifiers to
path-identifiers means that many flows from the same source node can
share the same path-state, which reduces the overhead of maintaining
the routing information. The drawback is that if a flow must be
rerouted, its flow identifier will change. However, when a flow is
rerouted the QoS metadata must be renegotiated anyway, so changing
flow identifiers will not create needless additional work in the
network.
10.3. Path-State Creation, Use, and Maintenance
The path-state portion of the protocol has two major goals. The
first goal is to ensure sufficient state exists at the nodes along a
path from a source S to a destination D so that packets from S to D
do not need to carry the complete source route. The second goal is
to allow S to discover the characteristics of a particular path to D
so that it can adapt its sending pattern to the capabilities of the
path, or even choose a different path entirely.
The next sections describe how the path-state is created, how the
characteristics of the path are discovered, and what metrics can be
used to characterize the path.
10.3.1. Creating Path-State for Routing
To create the path-state, we assume that Route Discovery proceeds as
normal in DSR. Once the source node S has obtained a source route to
the destination D, it begins sending data packets to D as normally
done in DSR, with each packet carrying a full source route header.
Internally, S assigns a path-identifier to that particular source
route and stores the path-identifier in its route cache along with
the source route. S then includes the path-identifier as part of the
A -----------------> B -----------------> C -----------------> D
+-------------+ +-------------+ +-------------+
|src: A | |src: A | |src: A |
|dst: D | |dst: D | |dst: D |
|path-id: 15 | |path-id: 15 | |path-id: 15 |
|rt: A,(B),C,D| |rt: A,B,(C),D| |rt: A,B,C,(D)|
+-------------+ +-------------+ +-------------+
| payload | | payload | | payload |
(a) Packet with path identifier and source route.
A -----------------> B -----------------> C -----------------> D
+-------------+ +-------------+ +-------------+
|src: A | |src: A | |src: A |
|dst: D | |dst: D | |dst: D |
|path-id: 15 | |path-id: 15 | |path-id: 15 |
+-------------+ +-------------+ +-------------+
| payload | | payload | | payload |
(b) Packet with path identifier only.
Figure 2: Path identifiers assigned to a source route by the
originating node A enable later packets to omit the source route.
source route header as shown in Figure 2(a). As each intermediate
node processes the source route to forward the packet, it also stores
the source route in its route cache, indexed by the source and
path-identifier.
After sending a packet containing both the source route and the
path-identifier into the network, S can begin sending subsequent
packets to D without a full source route --- carrying only the
path-identifier as shown in Figure 2(b). Each intermediate node
receiving such a packet queries its route cache to find the route
the packet is supposed to take, and determines its next hop. As
explained in Section 10.5, if the cached source route is not
available at some intermediate node, S will receive a Route Error and
can then correct the situation.
10.3.2. Monitoring Characteristics of the Path
In order to support network layer services such as balancing the
traffic load across the network, end-systems must have a method for
determining the characteristics of the paths through the network that
they could use. While many schemes have been proposed by which the
end-systems themselves can measure the characteristics of a path
(e.g., TCP congestion window and RTT calculations [1, 22, 24] and
SPAND [21]), we hypothesize that, particularly in the in the dynamic
environment of an ad hoc network, more useful, more accurate, and
more timely information can be developed by enlisting the aid of the
nodes along the path to measure the path characteristics.
We propose that each node can measure the activity around itself,
and thereby determine information such as: the mean latency it adds
to the packets it forwards and the latency variation (jitter); the
number of additional packets per second it believes it can process;
or the unused amount of wireless media capacity in the air around
the node. Experimentation will be required to discover exactly
which metrics will prove to be accurately measurable and useful,
though Section 10.3.3 provides several proposals. If the metrics
kept by each node on a path are combined, the result should be a
characterization of the path that the packet sender can use to
organize or adapt its offered load.
To implement this scheme, we first define a new type of extension
header for DSR than can be piggybacked onto a packet in the same way
as the existing DSR headers. This new header is called the path
metrics header (written as Measure) and conceptually consists of the
path-identifier of the path along which the metrics are measured,
the type of the Measure, and the metrics themselves encoded in a TLV
format (Section 10.6.2).
Whenever a sender S wishes to measure the characteristics of a path
it is using, it includes the Measure header in any packet it sends
along that path, setting the type of the header to record. As each
node along the path forwards the packet, it updates the variables
inside the Measure header with the metrics it has measured locally.
When the header reaches the final destination D, D sets the type
of the Measure header to return and piggybacks the header into any
packet headed back to S. Since the path metrics header includes
the path-identifier of the path along which it was measured, S can
include the data into its route cache for future use, and can treat
the receipt of the path metrics header as a positive acknowledgment
that the path-state between S and D for the given path-identifier
is correctly set up. This could lead S to cease including source
routes in the packets it sends along the path, as described in
Section 10.3.1.
If we find that it is valuable to immediately provide S with the path
metrics of every discovered route, we could alter Route Discovery
slightly to generate this information. Currently, if an intermediate
node has a cached route that it can use to answer a Route Request,
it generates a Route Reply itself. Instead, we could require it to
place its proposed route on the Route Request (turning it from a
flood-fill broadcast into a unicast packet) and send the packet to
the destination so it will measure the metrics of the complete path.
The destination will then return the metrics to the source along with
the Route Reply as described above.
We have been intending to experiment with this alteration to
Route Discovery for some time, since it offers two benefits,
even without path-state metrics. It should decrease the
number of broken routes returned by Route Discovery since
each cached route is tested before being returned, and
it should save us from jeopardizing one data packet for
every bad route in someone's cache. The cost is some extra
latency on Route Discovery.
10.3.3. Candidate Metrics
In order to limit the additional overhead that collecting and
distributing path-state metrics will place on the network, all the
metrics must have the property that the amount of space required to
express the metric does not increase as the number of hops on the
path increases. Experimentation will be required to determine which
metrics are most accurately measured and most useful, but our initial
set of candidates includes the following:
- Interface queue length --- Our previous work [12] has shown that
this is a good estimator of local congestion.
- Rate of interface queue draining --- When an interface is
backlogged, the rate at which packets leave the queue directly
measures the usable capacity of that interface.
- Quiet time fraction --- When an interface is not backlogged,
the usable capacity of the interface can be estimated by
promiscuously listening to the media and measuring the fraction
of time during which it is not in use (though this will
overestimate the capacity).
- Fraction Free Air Time --- The fraction of time our interface
would be able to send a packet. That is, the fraction of time
the interface does not sense carrier, is not deferring, and is
not backed off. Current experiments show this is an excellent
predictor of congestion and available capacity.
- Forwarding latency and variation --- This can be measured
as the time between when a packet is received and when it is
acknowledged by the next hop.
- Unidirectional links --- Paths containing unidirectional links
are usable, but undesirable as they increase the overhead of
Route Maintenance.
- Packet loss rate --- Signal quality information from the
interface itself, or the frequency of hop-by-hop retransmission,
can be used to estimate the loss rate of each link.
- Likelihood of path breakage --- Intermediate nodes may know ahead
of time that they intend to shutdown or move such that paths
through them will no longer work.
These metrics all have the property that they can be expressed in
a single value that each node can measure locally. As a packet
with a path metrics header passes through a node, the metrics in
the header can be updated to reflect the node's metrics using a
combination function like minimum, maximum, sum, or weighted average
that produces another single value to replace the one already in
the header. This updating will be done at the last possible moment
before the packet is forwarded, in order to assure the packet has the
most current metrics on it when it leaves.
10.4. Flow-State Creation, Use, and Maintenance
The flow-state portion of the protocol enables a sender to obtain
promises from all nodes along a path to a destination that a
certain set of resources are available along the path, and that
the intermediate nodes are committed to making these resources
available for the particular flow. This allows a sender to obtain
better-than-best-effort Quality of Service for a flow by obtaining
promises from the intermediate nodes to reserve the resources needed
to provide that QoS.
Unlike prior QoS work in wired networks, at this point we cannot
formally characterize or bound exactly what type of services the
flow-state protocol will be able to offer. The goal is to provide
CBR and TCP streams with the ability to specify and obtain a
minimum bandwidth and delay/jitter bound. If the environment is
particularly harsh, it is possible that only best-effort service will
be offerable. It is this intuition that leads us to the system of
promises and notifications. Experimentally, we hope to determine
how stable and effective this system will be in a multi-hop ad hoc
network environment.
10.4.1. Requesting Promises along Existing Paths
Similar to the use of the path metrics header, at any time a promise
can be requested or changed along any path an originator is currently
using. Once an originating node has created a path-identifier
for a route through the network, it can request a promise of
network resources along that route by first generating a new
flow-identifier to identify the promise. The originator then fills
out a flow-request header (written as Flow Request) and inserts it
into any packet sent along that path.
Figure 3 shows the conceptual layout of a Flow Request, which
contains the new path-identifier assigned by the originator, the
flow-identifier of the promise that this request supersedes (if any),
the requested lifetime of the promise, and the QoS parameters that
describe the requested promise itself. Section 10.6.3 provides the
detailed packet format. The use of the minimum and requested fields
for the QoS parameters differs depending on whether the Flow Request
is piggybacked on a Route Request or not, as described below.
When a Flow Request piggybacked on a unicast packet is received by a * The sequence of hops
node, the node performs the following steps:
- If the node is the destination of the packet, it converts the Address[i+1], Address[i+2], ..., Address[n]
Flow Request into a Measure with type return and uses the current
values in the desired fields of the Flow Request to populate the
fields of the Measure. It then piggybacks the Measure onto any
packet being returned to the originator.
- Else if the intermediate node has available enough resources to follow next in the source route. The address Address[n]
meet the minimum requested promise in the Flow Request, it: above is the final hop in the source route.
* Sets aside the maximum of its available resources and the In addition to the processing of received packets described above, a
desired resources. The set aside resources are held in a node SHOULD examine the packet to determine if the receipt of this
tentative promise pool until the promise is confirmed, or a packet indicates an opportunity for automatic route shortening, as
relatively short timeout expires. described in Section 3.4.2. If the received packet satisfies the
tests described there, then this node SHOULD perform the following
sequence of steps:
* Nodes can recycle resources from listed old flow-id - Return a gratuitous Route Reply to the IP Source Address of the
packet, as described in Section 3.4.2.
+--------------------------------------+ - Discard the received packet, since the packet has been received
| flow-id | old flow-id | before its normal traversal of the packet's source route would
+--------------------------------------+ have caused it to reach this receiving node. Another copy of
| lifetime | the packet will normally arrive at this node as indicated in
+--------------------------------------+ the packet's source route; discarding this initial copy of the
| capacity | min | desired | packet, which triggered the gratuitous Route Reply, will prevent
| latency | min | desired | the duplication of this packet that would otherwise occur.
|variation | min | desired |
| loss | min | desired |
+--------------------------------------+
Figure 3: Conceptual layout of the Flow Request header. 6.1.4. Processing a Routing Header in a Received Packet
* Updates the desired fields of the Flow Request to reflect A Routing header in a packet is not examined or processed until the
the resources set aside (there is questionable value in a packet reaches the node identified in the Destination Address field
down stream node allocating more resources to a flow than an in the packet's IP header. In that node, dispatching on the Protocol
upstream node can currently handle). field in the packet's IP header (or the Next Header field in the
preceding extension header) causes the Routing header module in that
node's IP implementation to be invoked. The node then examines the
Routing Type field in the Routing header to determine the specific
type of processing for that type of Routing header. The processing
for a Routing header here in general follows the procedures specified
for IPv6 Routing headers, and the processing specifically for a DSR
Routing header in general follows the general procedures specified
for a Type 0 Routing header in IPv6 [7].
* Forward the packet and piggybacked Flow Request to the next If, while processing a received packet, a node encounters a Routing
node on the path. header with an unrecognized Routing Type value, the required behavior
of the node depends on the value of the Segments Left field, as
follows:
- Else, the node does not have enough resources to meet the - If Segments Left is 0, the node MUST ignore the Routing header
minimum requested promise, so it sends the originator a Route and proceed to process the next header in the packet, whose type
Error piggybacked with a Measure reflecting the minimum of the is identified by the Next Header field in the Routing header.
current values of the desired fields in the Flow Request and the
available resources. The type field is set to refused. Such a
Measure enables the originator to learn three things: that its
requested cannot be satisfied along the given path; the identity
of the bottleneck node; and the available resources up to and
through the bottleneck node.
When the originating node receives a Measure header of type return - If Segments Left is non-zero, the node MUST discard the packet
for a flow on which it has an outstanding Flow Request, it accepts and send an ICMP Parameter Problem, Code 0, message [24] to
the promised level of service by changing the type of the Measure the packet's Source Address, pointing to the unrecognized
header to confirm and piggybacking the header on any packet going Routing Type.
along the flow. This informs the intermediate nodes to move the set
aside resources from the tentative promise pool to the allocated
pool, and enables upstream nodes to free any set aside resources in
excess of the capacity of a bottleneck downstream node.
The use of the old flow-id to recycle resources is important for two If, after processing a Routing header in a received packet, an
reasons. First, it enables an originator to attempt to increase or intermediate node determines that the packet is to be forwarded onto
decrease the amount of a current promise without losing the resources a link whose link MTU is less than the size of the packet, the node
it already has promised. Second, both packet loss and the expanding MUST discard the packet and send an ICMP Packet Too Big message to
ring search of Route Discovery may result in several Flow Requests the packet's Source Address [24].
being sent for the same flow. If subsequent Flow Requests for a
flow were not able to notify intermediate nodes that they can reuse
resources set aside while processing earlier Flow Requests, the
network could quickly reach a state where admissible flows are being
needlessly rejected.
10.4.2. Requesting Promises as Part of Route Discovery A DSR Routing header is identified by a Routing Type value of ???
in the Routing header. A DSR Routing header for IPv4 is processed
according to the following sequence of steps:
The scheme for requesting promises described in the previous section - If the value of the Segments Left field in the Routing header
has the advantage that it enables an originator to request or update equals 0, then proceed to process the next header in the packet,
a promise for a flow along any route currently in its route cache, whose type is identified by the Next Header field in the Routing
regardless of how it obtained the route. For the common case in header. Do not process the Routing header further.
which a node wishes to obtain a resource promise for a new flow to
a previously unknown destination, we can integrate the flow request
with the Route Discovery for the destination.
Integrating the flow request with Route Discovery enables us to avoid - Else, let n equal Hdr Ext Len. This is the number of addresses
the inefficiency of discovering routes that will not be usable by the in the Routing header.
flow due to insufficient resources. The integration of flow requests
with Route Discovery also allows us to avoid a common pitfall of
QoS schemes that layer a reservation signaling protocol on top of
a unicast routing algorithm --- schemes without tight integration
will refuse admissible flows whenever the unicast routing algorithm
directs the request packets into a congested area of the network,
unless the signaling protocol also provides a method to backtrack
the request and route around the congested area. Utilizing the same
mechanisms currently used in Route Discovery, we can avoid the need
for backtracking.
We call the combination of flow requests with Route Discovery - If the value of the Segments Left field is greater than n, then
QoS-guided Route Discovery, which originating nodes can invoke simply send an ICMP Parameter Problem, Code 0, message [24] to the IP
by piggybacking a Flow Request on the Route Request. Each node Source Address, pointing to the Segments Left field, and discard
receiving the Flow Request uses the same algorithm described in the packet. Do not process the Routing header further.
Section 10.4.1, with two exceptions:
- Nodes silently discard the Route Request if they can not meet - Else, decrement the value of the Segments Left field by 1. Let i
minimum requirements equal n minus Segments Left. This is the index of the next
address to be visited in the Address vector.
- Unless the Route Request indicates that replying from cache is - If Address[i] or the IP Destination Address is a multicast
forbidden, nodes with a cached route to destination unicast the address, then discard the packet. Do not process the Routing
Route Request along the cached route. header further.
A node requiring a route with a QoS promise uses the following - Else, swap the IP Destination Address and Address[i].
algorithm. First, it sends a Route Request that permits intermediate
nodes to reply from cache. If the network is uncongested, this
should frequently and quickly succeed in returning both a Route Reply
and a Measure describing the available QoS along the discovered
path. If after a timeout, the originating node has not received a
Route Reply, it begins another Route Discovery, this time forbidding
replies from cache, which will force an exploration of all feasible
paths to the destination.
This scheme does risk an implosion of unicast Requests at the target - Forward the packet to the IP address specified in the
of the Route Discovery (e.g., if target is a popular server to which Destination Address field of the IP header, following normal IP
many nodes have cached routes). At the cost of additional complexity forwarding procedures, including checking and decrementing the
and soft-state, it would be possible to add hold-downs at the nodes Time-to-Live (TTL) field in the packet's IP header [25, 3]. In
surrounding the target so that only the first few Requests are this forwarding of the packet, the next hop node (identified by
forwarded towards the target. the Destination Address) MUST be treated as a direct neighbor
node; the transmission to that next node MUST be done in a single
IP forwarding hop, without Route Discovery and without searching
the Route Cache.
10.4.3. Providing Notifications of Changing Path Metrics - In forwarding the packet, perform Route Maintenance for the next
hop of the packet, by verifying that the packet was received by
that next hop, as described in Section 6.3.
When a node detects that it must break a promise, it must notify the Multicast addresses must not appear in a DSR Routing header or in
node to which it made that promise. It is an open question how the the IP Destination Address field of a packet carrying a DSR Routing
now reduced resources should be distributed among the flows. We header.
currently pick the minimum set of promises to break that leave the
other promises unchanged.
The difficulty in providing notification of a changed path metric is 6.2. Route Discovery Processing
getting this information back to the source. When promise must be
broken at a node B, it sends a Measure to the originator indicating
what resources are now available. The use of Measure headers to
determine the currently available resources along a path is more
problematic, however, as for every Measure sent by the originator,
the destination must send a response containing the measured metrics.
If the traffic is TCP, the overhead of the responses are low, as Route Discovery is the mechanism by which a node S wishing to send a
they can be piggybacked on the ACK stream. For one-way CBR traffic packet to a destination node D obtains a source route to D. Route
though, introducing the overhead of a reverse stream to carry the Discovery is used only when S attempts to send a packet to D and
changing metrics could be severe. does not already know a route to D. The node initiating a Route
Discovery is known as the "initiator" of the Route Discovery, and the
destination node for which the Route Discovery is initiated is known
as the "target" of the Route Discovery.
If the overhead of the responses becomes a problem, it may be Route Discovery operates entirely on demand, with a node initiating
possible to implement a enhanced piggyback mechanism. The approach Route Discovery based on its own origination of new packets for
is based on the fact that although no work has been exerted to create some destination address to which it does not currently know a
hop-by-hop routing information at each node, chances are good that route. Route Discovery does not depend on any periodic or background
each node can determine a next-hop for packets headed to any known exchange of routing information or neighbor node detection at any
destination by simply examining its route cache. By piggybacking layer in the network protocol stack at any node.
the Measure header for one hop onto any packet that is headed to
that next-hop, we can cheaply create a reverse flow of information
that will eventually reach the originator of the Measure. Each
node who receives a Measure with a type of return simply piggybacks
the Measure for one-hop on packets that seem to be flowing the
right direction back to the source. To insure the timeliness of
the information, each Measure being returned to an originator could
include a deadline by which the information is supposed to reach the
originator. If it appears that hop-by-hop propagation will result
in missing the deadline, the Measure can be unicast as a first-class
packet to the originator.
10.5. Expiration of State from Intermediate Nodes The Route Discovery procedure utilizes two types of messages, a DSR
Route Request (Section 5.1.1) and a DSR Route Reply (Section 5.2.1),
to actively search the ad hoc network for a route to the desired
destination. These DSR messages MAY be carried in any type of IP
packet, through use of extension headers as described in Section 5:
a Route Request is carried in a Destination options extension header,
and a Route Reply is carried in a Hop-by-Hop options extension
header.
Since there is no guarantee that either the source or destination of A Route Discovery for a destination SHOULD NOT be initiated unless
a packet flow will be able to communicate with all of the nodes that the initiating node has a packet in the Send Buffer requiring
carried the flow when they wish to terminate the flow, there must delivery to that destination. A Route Discovery for a given target
be time-based expiration mechanism by which intermediate nodes can node MUST NOT be initiated unless permitted by the rate-limiting
purge the path-state and flow-state from their caches and reclaim the information contained in the Route Request Table. After each
resources set aside to maintain it. However, if intermediate nodes Route Discovery attempt, the interval between successive Route
were to purge the state of an active flow, the intermediate nodes Discoveries for this target must be doubled, up to a maximum of
would find themselves with packets to forward that do not contain MAX_REQUEST_PERIOD.
a source route, but only contain a flow-identifier that references
state they no longer hold. Since intermediate nodes do not
necessarily know the timing with which the sender originates packets,
an inactivity timer alone would have to be set very conservatively to
prevent purging the path-state of low bit-rate connections.
To solve the expiration problem, we take advantage of the relatively 6.2.1. Originating a Route Request
``soft'' nature of the path-state and flow-state. When the state
is created, the source node specifies a time after which it should
be discarded (This time will typically be on the order of a hundred
seconds). The source node can thereby estimate how often it must
refresh the state, for example, by sending packets that contain a
full source route on them. Should the state have somehow expired
at an intermediate node when a packet labeled with a flow or path
identifier arrives, the intermediate node can return a Route Error to
the source node specifying ``missing state information'' as the cause
of the Error and elicit the sender to refresh the missing state.
Since all path-state information is guaranteed to have expired from A node initiating a Route Discovery for some target creates and
the network after a bounded amount of time, nodes can safely and initializes a DSR Route Request option in some IP packet. This
unambiguously reuse path- and flow-identifiers after that period. MAY be a separate IP packet, used only to carry this Route Request
option, or the node MAY include the Route Request option in some
existing packet it needs to send to the target node (e.g., the IP
packet originated by this node, that caused the node to attempt Route
Discovery for the destination address of the packet).
10.6. Packet Formats The Route Request option MUST be included in a Destination Options
extension header in the packet. To initialize the Route Request
option, the node performs the following sequence of steps:
10.6.1. Identifier Option - The Option Type in the option MUST be set to the value ???.
Path and flow identifiers are carried as an option inside the - The Option Length field in the option MUST be set to the value 6.
Hop-by-Hop options header. This option MAY NOT appear more than once The total size of the Route Request option when initiated is
in a single Hop-by-Hop Options header. 8 octets; the Option Length field excludes the size of the
Option Type and Option Length fields themselves.
0 1 2 3 - The Identification field in the option MUST be set to a new
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 value, different from that used for other Route Requests recently
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ initiated by this node. For example, each node MAY maintain a
| Option Type | Option Length | Path-ID | Flow-ID | single counter value for generating a new Identification value
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ for each Route Request it initiates.
Option Type - The Target Address field in the option MUST be set to the IP
address that is the target of this Route Discovery.
???. A node that does not understand this option should ignore The Source Address in the IP header of this packet MUST be the node's
this option and continue processing the packet, and the Option own IP address. The Destination Address in the IP header of this
Data does not change en-route (the top three bits are 000). packet MUST be the IP "limited broadcast" address (255.255.255.255).
Option Length A node MUST maintain in its Route Request Table, information about
Route Requests that it initiates. When initiating a new Route
Request, the node MUST use the information recorded in the Route
Request Table entry for the target of that Route Request, and it MUST
update that information in the table entry for use in the next Route
Request initiated for this target. In particular:
8-bit unsigned integer. Length of the option, in octets, - The Route Request Table entry for a target node records the
excluding the Option Type and Option Length fields. Time-to-Live (TTL) field used in the IP header of the last Route
Request initiated by this node for that target node. This
value allows the node to implement a variety of algorithms
for controlling the spread of its Route Request on each Route
Discovery initiated for a target. As examples, two possible
algorithms for this use of the TTL field are described in
Section 3.3.4.
Path-ID - The Route Request Table entry for a target node records the
number of consecutive Route Requests initiated for this target
since receiving a valid Route Reply giving a route to that target
node, and the remaining amount of time before which this node MAY
next attempt at a Route Discovery for that target node.
The identifier assigned to this path by the node listed as the These values MUST be used to implement an exponential back-off
IP Source Address (Section 10.2). algorithm to limit the rate at which this node initiates new
Route Discoveries for the same target address. Until a valid
Route Reply is received for this target node address, the timeout
between consecutive Route Discovery initiations for this target
node SHOULD increase by doubling the timeout value on each new
initiation.
Flow-ID The behavior of a node processing a packet containing both a Routing
Header and a Route Request Destination option is unspecified.
Packets SHOULD NOT contain both a Routing Header and a Route Request
Destination option. [This is not exactly true: A Route Request
option appearing in the second Destination Options header that IPv6
allows after the Routing Header would probably do-what-you-mean,
though we have not triple-checked it yet. Namely, it would allow the
originator of a route discovery to unicast the request to some other
node, where it would be released and begin the flood fill. We call
this a Route Request Blossom since the unicast portion of the path
looks like a stem on the blossoming flood-fill of the request.]
The identifier assigned by the node listed as the IP Source Packets containing a Route Request Destination option SHOULD NOT be
Address to a particular flow along the path identified by the retransmitted, SHOULD NOT request an explicit DSR Acknowledgment by
Path-ID. If this portion is 0, the option names a path, but not setting the R bit, SHOULD NOT expect a passive acknowledgment, and
a particular flow. SHOULD NOT be placed in the Retransmission Buffer. The repeated
transmission of packets containing a Route Request Destination option
is controlled solely by the logic described in this section.
Discussion: This encoding of the path and flow identifiers will cost 6.2.2. Processing a Received Route Request Option
8 bytes of additional header overhead in a data packet with no other
extensions or options (4 bytes for the Hop-by-Hop options header, and
4 bytes for the identifier option). A more compact encoding would be
to define that, in a DSR network, an IP destination address with a
first octet of 127 actually encodes the path and flow identifiers as
follows:
0 1 2 3 When a node receives a packet containing a Route Request option, the
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 node MUST process the option according to the following sequence of
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ steps:
|0 1 1 1 1 1 1 1| reserved | Path-ID | Flow-ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The DSR module of the final destination would replace the IP - If the Target Address field in the Route Request matches this
destination address with its actual value before passing the packet node's own IP address, then the node SHOULD return a Route Reply
up the stack for further processing. to the initiator of this Route Request (the Source Address in the
IP header of the packet), as described in Section 6.2.4. The
source route for this reply is the sequence of hops
This encoding has the advantage that it requires no additional initiator, Address[1], Address[2], ..., Address[n], target
overhead in a data packet. The disadvantage is that if the packet
was somehow received by a DSR-unaware node without first being
processed by a DSR gateway node [4], the DSR-unaware node will either
drop the packet or will attempt to receive it locally (since the IP
destination address belongs to the loopback subnet).
10.6.2. Path-Metrics Option where initiator is the address of the initiator of this Route
Request, each Address[i] is an address from the Route Request,
and target is the target of the Route Request (the Target Address
field in the Route Request).
Path-metrics are carried as an option inside the Hop-by-Hop options The node MUST then continue processing the packet normally,
header. including any following options or extension headers in the
packet. The node MUST NOT retransmit the Route Request to
propagate it to other nodes. Do not process the Route Request
option further.
0 1 2 3 - Else, the node MUST examine the route recorded in the Route
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 Request option (the IP Source Address field and the sequence of
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Address[i] fields) to determine if this node's own IP address
| Option Type | Option Length | Path-ID | Flow-ID | already appears in this list of addresses. If so, the node MUST
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ discard the entire packet carrying the Route Request option.
| Type | Metrics...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type
???. A node that does not understand this option should ignore - Else, the node MUST search its Route Request Table for an entry
this option and continue processing the packet, and the Option for the initiator of this Route Request (the IP Source Address
Data does change en-route (the top three bits are 001). field). If such an entry is found in the table, the node MUST
search the cache of Identification values of recently received
Route Requests in that table entry, to determine if an entry
is present in the cache matching the Identification value
and target node address in this Route Request. If such an
(Identification, target address) entry is found in this cache in
this entry in the Route Request Table, then the node MUST discard
the entire packet carrying the Route Request option.
Option Length - Else, this node SHOULD repropagate this Route Request. If it
does so, the node MUST do so according to the following sequence
of steps:
8-bit unsigned integer. Length of the option, in octets, * Add an entry for this Route Request in its cache of
excluding the Option Type and Option Length fields. (Identification, target address) values of recently received
Route Requests.
Path-ID and Flow-ID * Create a copy of this entire packet and perform the following
steps on the copy of the packet.
The path identifier of the path that the metrics correspond * Append this node's own IP address to the list of Address[i]
to. If the Path-Metrics Option Type equals Measure, then the values in the Route Request, and increase the value of the
Path-ID and Flow-ID fields MUST equal those in any Identifier Option Length field in the Route Request by 4 (the size of an
Option carried in the Hop-by-Hop Options Header. IP address).
Type * This node SHOULD search its own Route Cache for a route
(from itself, as if it were the source of a packet) to the
target of this Route Request. If such a route is found in
its Route Cache, then this node SHOULD follow the procedure
outlined in Section 6.2.3 to return a "cached Route Reply"
to the initiator of this Route Request, if permitted by the
restrictions specified there.
One of * If the node does not return a cached Route Reply, then this
node SHOULD link-layer re-broadcast this copy of the packet,
with a short jitter delay before the broadcast is sent. The
jitter period SHOULD be chosen as a random period, uniformly
distributed between 0 and BROADCAST_JITTER.
Measure 6.2.3. Generating Route Replies using the Route Cache
Each node processing the option should update the metrics As described in Section 3.3.2, it is possible for a node processing a
to reflect the conditions at that node. received Route Request to avoid propagating the Route Request further
toward the target of the Request, if this node has in its Route Cache
a route from itself to this target. Such a Route Reply generated by
a node from its own cached route to the target of a Route Request is
called a "cached Route Reply", and this mechanism can greatly reduce
the overall overhead of Route Discovery on the network by reducing
the flood of Route Requests. The general processing of a received
Route Request is described in Section 6.2.2; this section specifies
the additional requirements that MUST be met before a cached Route
Reply may be generated and returned and specifies the procedure for
returning such a cached Route Reply.
Reply While processing a received Route Request, for a node to possibly
return a cached Route Reply, it MUST have in its Route Cache a route
from itself to the target of this Route Request. However, before
generating a cached Route Reply for this Route Request, the node MUST
verify that there are no duplicate addresses listed in the route
accumulated in the Route Request together with the route from this
node's Route Cache. Specifically, there MUST be no duplicates among
the following addresses:
The metrics in this option SHOULD NOT be modified by any - The IP Source Address of the packet containing the Route Request,
intermediate node. They represent the metrics measured
along the identified path.
Confirm - The Address[i] fields in the Route Request, and
The metrics in this option MUST NOT be modified by any - The nodes listed in the route obtained from this node's Route
intermediate node. They represent a confirmation by Cache, excluding the address of this node itself (this node
the sender that will transmit traffic conforming to the itself is the common point between the route accumulated in the
listed Quality of Service metrics along the identified Route Request and the route obtained from the Route Cache).
flow.
Metrics If any duplicates exist among these addresses, then the node MUST NOT
send a cached Route Reply. The node SHOULD continue to process the
Route Request as described in Section 6.2.2.
The individual path-metrics, encoded as described in If the Route Request and the route from the Route Cache meet the
Section 10.6.4. Unknown metrics SHOULD be ignored. If a restriction above, then the node SHOULD construct and return a cached
single value is provide for the metric, it MUST be interpreted Route Reply as follows:
as the metrics value. If two values are provided for the
metric, they MUST be interpreted as the range of values taken
by the metric (low value first). It is undefined for there to
be more than two values for the metric.
10.6.3. Flow Request Option - The source route for this reply is the sequence of hops
Flow-requests are carried as an option inside the Hop-by-Hop options initiator, Address[1], Address[2], ..., Address[n], c-route
header. They allow a sender to request that intermediate nodes
reserve sufficient resources for a flow to provide that flow with the
QoS characteristics described by the metrics.
0 1 2 3 where initiator is the address of the initiator of this Route
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 Request, each Address[i] is an address from the Route Request,
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ and c-route is the sequence of hops in the source route to this
| Option Type | Option Length | Lifetime | target node, obtained from the node's Route Cache. In appending
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ this cached route to the source route for the reply, the address
| old | old | new | new | of this node itself MUST be excluded, since it is already listed
| Path-ID | Flow-ID | Path-ID | Flow-ID | as Address[n].
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metrics ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type - Send a Route Reply to the initiator of the Route Request, using
the procedure defined in Section 6.2.4. The initiator of the
Route Request is indicated in the Source Address field in the
packet's IP header.
???. A node that does not understand this option should ignore 6.2.4. Originating a Route Reply
this option and continue processing the packet, and the Option
Data does change en-route (the top three bits are 001).
Option Length A node originates a Route Reply in order to reply to a received and
processed Route Request, according to the procedures described in
Sections 6.2.2 and 6.2.3. The Route Reply is returned in a DSR Route
Reply option (Section 5.2.1). The Route Reply option MAY be returned
to the initiator of the Route Request in a separate IP packet, used
only to carry this Route Reply option, or it MAY be included in any
other IP packet being sent to this address.
8-bit unsigned integer. Length of the option, in octets, The Route Reply option MUST be included in a Hop-by-Hop Options
excluding the Option Type and Option Length fields. extension header in the packet returned to the initiator. To
initialize the Route Reply option, the node performs the following
sequence of steps:
old Path-ID and old Flow-ID - The Option Type in the option MUST be set to the value ???.
The flow identifier provide in a previous request which this - The Option Length field in the option MUST be set to the value
request supersedes. (n * 4) + 1, where n is the number of addresses in the source
route being returned (excluding the Route Discovery initiator
node's address).
new Path-ID and new Flow-ID - The Last Hop External (L) bit in the option MUST be initialized
to 0.
The flow identifier that will be used with to identify the - The Reserved field in the option MUST be initialized to 0.
packets that should receive the QoS described by the included
metrics.
Metrics - The sequence of addresses of the source route are copied into
the Address[i] fields of the option. Address[1] MUST be set
to the first hop of the route after the initiator of the Route
Discovery, Address[n] MUST be set to the last hop of the source
route (the address of the target node), and each other Address[i]
MUST be set to the next address in sequence in the source route
being returned.
The metrics that characterize the desired QoS, encoded as The Destination Address field in the IP header of the packet carrying
described in Section 10.6.4. Unknown metrics SHOULD be the Route Reply option MUST be set to the address of the initiator
ignored. If a range of values are provided for a metric, they of the Route Discovery (i.e., for a Route Reply being returned in
MUST be interpreted as the minimum acceptable value and the response to some Route Request, the IP Source Address of the Route
desired value. Request).
10.6.4. Encoding Path-Metrics After creating and initializing the DSR Route Reply option and
the IP packet containing it, send the Route Reply, jittered by
T milliseconds, where T is a uniformly distributed random number
between 0 and BROADCAST_JITTER.
Each path-metric is encoded in a modified Type-Length-Value form as If sending a Route Reply to the originator of the Route Request
requires performing a Route Discovery, the Route Reply hop-by-hop
option MUST be piggybacked on the packet that contains the Route
Request. This piggybacking prevents a loop wherein the target of the
new Route Request (which was itself the originator of the original
Route Request) must do another Route Request in order to return its
Route Reply.
0 1 2 3 If sending the Route Reply to the originator of the Route Request
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 does not require performing Route Discovery, a node SHOULD send a
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ unicast Route Reply in response to every received Route Request
| Type |R| Length | Data... targeted at it.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 6.2.5. Processing a Route Reply Option
The type of metric Upon receiving a Route Reply, a node SHOULD extract the source route
from the Route Reply and add this routing information to its Route
Cache. The source route from the Route Reply is the sequence of hops
R bit initiator, Address[1], Address[2], ..., Address[n]
If 0, the data is a list of discrete values the metric can where initiator is the value of the Destination Address field in
or did take. If 1, the data represent a range of values the IP header of the packet carrying the Route Reply (the address
the metric can or did take. If a single metric value is of the initiator of the Route Discovery), and each Address[i] is a
supplied, the range is assumed to be 0 <= metric <= value. If node through which the source route passes, in turn, on the route to
two metric values are supplied, the range is assumed to be the target of the Route Discovery. Address[n] is the address of the
value1 <= metric <= value2. target.
Option Length If the Last Hop External (L) bit is set in the Route Reply, the node
MUST flag the hop Address[n] in its Route Cache as External.
8-bit unsigned integer. Length of the metric, in octets, Each packet in the Send Buffer SHOULD then be checked to see whether
excluding the Type and Length fields. the information in the Route Reply and now in the Route Cache allows
it to be sent immediately.
The currently defined metric types follow: 6.3. Route Maintenance Processing
Padding Route Maintenance is the mechanism by which node S is able to detect,
while using a source route to D, if the network topology has changed
such that it can no longer use its route to D because a link along
the route no longer works. When Route Maintenance indicates a source
route is broken, S can attempt to use any other route it happens to
know to D, or can invoke Route Discovery again to find a new route
for subsequent packets to D. Route Maintenance for this route is
used only when S is actually sending packets to D.
Type: 0 When forwarding a packet, a node MUST attempt to receive an
acknowledgement for the packet from the next hop. If no
acknowledgement is received, the node SHOULD return a Route Error to
the IP Source Address of the packet, as described in Section 6.3.3
The padding metric is special in that it contains no length field and 6.3.1. Using Network-Layer Acknowledgments
no data.
Available Capacity When a node retransmits a packet or has no other way to ensure
successful delivery of a packet to the next hop, it SHOULD request
a network-layer acknowledgement by placing a non-zero value in the
Identification field of the DSR Routing header. Such a value MUST
be unique over all packets delivered to the same next hop which are
either unacknowledged or recently acknowledged.
Type: 1 A node receiving a DSR Routing header with a non-zero value in the
Data encoded as Identification field MUST send an acknowledgement to the previous hop
by performing the following sequence of steps:
0 1 - Create a packet and set the IP Source Address to the address
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 of this node, the IP Destination Address to the address of the
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ previous hop, and the IP Protocol field to the protocol number
| Mantissa | Shift | reserved for Hop-by-Hop Options extension headers.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where the value is (Mantissa << Shift) bits per second. - Set the Hop-by-Hop Options extension header's Next Header field
to be the "No Next Header" value. Set the Header Extension
Length to the size of a DSR Acknowledgement Option.
Delay and Delay Variation - Set the DSR Acknowledgement option's Option Type field to
the Option Type reserved for DSR Acknowledgements, and the
Option Length field to 10.
Data encoded as - Copy the Identification field from the Routing Header into
the Identification field in the DSR Acknowledgement Option.
Set the ACK Source Address field in the option to be the IP
Source Address and the ACK Destination Address field to the IP
Destination Address.
0 1 - Send the packet as described in Section 6.1.1.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Delay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type: 2 - Delay 6.3.2. Using Link Layer Acknowledgments
The value is Delay milliseconds. If explicit failure notifications are provided by the link layer,
then all packets are assumed to be correctly received by the
next hop, and a Route Error is sent only when an explicit failure
notification is made from the link layer.
Type: 3 - Delay Variation Nodes receiving a packet without a Routing Header do not need to send
an explicit Acknowledgment to the packet's originator, since the
link layer will notify the originator if the packet was not received
properly.
The value is the standard deviation of Delay, in milliseconds. 6.3.3. Originating a Route Error
Link Bidirectionality When a node is unable to verify successful delivery of a packet to
the next hop after a maximum number of retransmission attempts,
a node SHOULD send a Route Error to the IP Source Address of the
packet. When sending a Route Error for a packet containing either a
DSR Route Error option or a DSR Acknowledgement option, a node SHOULD
add these options to it's Route Error, subject to some limit on
lifetime. Specifically, we define the "salvage count" of an option
to be the sum of one plus the salvage count recorded in the DSR
Routing header plus the sum of the salvage counts of any DSR Route
Errors preceding that option.
Type: 16 - Link Bidirectionality A node transmitting a Route Error MUST follow the following steps:
Data encoded as - Create a packet and set the IP Source Address to the address of
this node, the IP Destination Address to the address IP Source
Address of the packet experiencing the error.
0 1 - Insert a Hop-by-Hop Options Header into the packet.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # Uni-links | #Explicit ACK |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where # Uni-links is the number of uni-directional links on the path, - Add a Route Error Option, setting the Error Type to
and # Explicit ACK is the number of hops which require explicit NODE_UNREACHABLE, the Reserved bits to 0, the Salvage value to
acknowledgments. one plus the Salvage value from the DSR Routing header, and the
Unreachable Node Address to the address of the next hop. Set
the Error Source Address to the IP Source Address and the Error
Destination to the IP Destination Address.
Packet Loss Rate - The node MAY append each DSR Route Error and DSR Acknowledgement,
in order, from the packet experiencing the error, though it MUST
exclude options with salvage counts greater than 15.
Data encoded as - Send the packet as described in Section 6.1.1.
0 1 6.3.4. Processing a Route Error Option
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| # Packets Lost |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where the loss rate is (# Packets Lost / 2 ** 16). A node receiving a Route Error MUST process it as follows:
Type: 17 - Path Packet Loss Rate - Delete all routes from the Route Cache that have a link from the
Route Error Source Address to the Unreachable Node Address.
The value is the expected packet loss rate of the entire path - If the Hop-by-Hop option following the Route Error is a DSR
Acknowledgement or DSR Route Error option sent by this node
(that is, with Acknowledgement or Error Source Address equal to
this node's address), copy the Hop-by-Hop options following the
current Route Error into a new packet with IP Source Address
equal to this node's own IP address and IP Destination Address
equal to the Acknowledgement or Error Destination Address.
Transmit this packet as described in Section 6.1.1, with the
salvage count in the DSR Routing header set to the Salvage value
of the Route Error.
Type: 18 - Worst Loss Rate 6.3.5. Salvaging a Packet
The value is the expected packet loss rate of the single worst link When a node is unable to verify successful delivery of a packet
in the path. to the next hop after a maximum number of retransmission attempts
and has transmitted a Route Error to the sender, it MAY attempt to
salvage the packet by examining its route cache. If the node can
find a route to the packet's IP Destination Address in its own Route
Cache, then this node replaces the packet's Routing header with a new
Routing Header in the same way as described in Section 6.1.2, except
that Address[1] MUST be set to the address of this node and the
Salvage field MUST be set to 1 plus the value of the Salvage field in
the Routing Header that caused the error.
11. Constants 7. Constants
BROADCAST_JITTER 10 milliseconds BROADCAST_JITTER 10 milliseconds
MAX_ROUTE_LEN 15 nodes MAX_ROUTE_LEN 15 nodes
Interface Indexes
IF_INDEX_INVALID 0x7F
IF_INDEX_MA 0x7E
IF_INDEX_ROUTER 0x7D
Route Cache Route Cache
ROUTE_CACHE_TIMEOUT 300 seconds ROUTE_CACHE_TIMEOUT 300 seconds
Send Buffer Send Buffer
SEND_BUFFER_TIMEOUT 30 seconds SEND_BUFFER_TIMEOUT 30 seconds
Request Table Route Request Table
MAX_REQUEST_ENTRIES 32 nodes REQUEST_TABLE_SIZE 64 nodes
MAX_REQUEST_IDS 8 identifiers REQUEST_TABLE_IDS 16 identifiers
MAX_REQUEST_REXMT 16 retransmissions MAX_REQUEST_REXMT 16 retransmissions
MAX_REQUEST_PERIOD 10 seconds MAX_REQUEST_PERIOD 10 seconds
REQUEST_PERIOD 500 milliseconds REQUEST_PERIOD 500 milliseconds
RING0_REQUEST_TIMEOUT 30 milliseconds NONPROP_REQUEST_TIMEOUT 30 milliseconds
Retransmission Buffer Retransmission Buffer
DSR_RXMT_BUFFER_SIZE 50 packets DSR_RXMT_BUFFER_SIZE 50 packets
Retransmission Timer Retransmission Timer
DSR_MAXRXTSHIFT 2 DSR_MAXRXTSHIFT 2
12. IANA Considerations 8. IANA Considerations
This document proposes the use of the Destination Options header and This document proposes the use in IPv4 of the Destination Options
the Hop-by-Hop Options header, originally defined for IPv6, in IPv4. extension header, the Hop-by-Hop Options extension header, and
The Next Header values indicating these two extension headers thus Routing header, which were originally defined for IPv6 [7]. The
must be reserved within the IPv4 Protocol number space. Next Header values indicating these three extension header types (60,
0, and 43, respectively) must therefore be reserved within the IPv4
Protocol number space. In addition, the "No Next Header" type value
of 69, defined for IPv6, must also be defined for use in IPv4. Other
protocols in IPv4 wishing to use these IPv6-style extension headers
can also make use of these Protocol number assignments.
Furthermore, this document defines four new types of destination For use within a Destination Options extension header, this document
options, each of which must be assigned an Option Type value: defines one new type of destination option, which must be assigned an
Option Type value:
- The DSR Route Request option, described in Section 7.1.1 - DSR Route Request option, described in Section 5.1.1. The top
three bits of this Option Type value MUST be 011.
- The DSR Route Reply option, described in Section 7.2.1 For use within a Hop-by-Hop Options extension header, this document
defines three new types of hop-by-hop options, each of which must be
assigned an Option Type value:
- The DSR Route Error option, described in Section 7.2.2 - DSR Route Reply option, described in Section 5.2.1. The top
three bits of this Option Type value MUST be 000.
- The DSR Acknowledgment option, described in Section 7.2.3 - DSR Route Error option, described in Section 5.2.2. The top
three bits of this Option Type value MUST be 000.
DSR also requires a routing header Routing Type be allocated for the - DSR Acknowledgment option, described in Section 5.2.3. The top
DSR Source Route defined in Section 7.3. three bits of this Option Type value MUST be 000.
In IPv4, we require two new protocol numbers be issued to identify For use within a Routing header, this document defines one new type
the next header as either an IPv6-style destination option, or an of routing header, which must be assigned an Routing Type value:
IPv6-style routing header. Other protocols can make use of these
protocol numbers as nodes that support them will processes any
included destination options or routing headers according to the
normal IPv6 semantics.
13. Security Considerations - DSR Routing Header, defined in Section 5.3.
9. Security Considerations
This document does not specifically address security concerns. This This document does not specifically address security concerns. This
document does assume that all nodes participating in the DSR protocol document does assume that all nodes participating in the DSR protocol
do so in good faith and with out malicious intent to corrupt the do so in good faith and with out malicious intent to corrupt the
routing ability of the network. In mission-oriented environments routing ability of the network. In mission-oriented environments
where all the nodes participating in the DSR protocol share a where all the nodes participating in the DSR protocol share a
common goal that motivates their participation in the protocol, the common goal that motivates their participation in the protocol, the
communications between the nodes can be encrypted at the physical communications between the nodes can be encrypted at the physical
channel or link layer to prevent attack by outsiders. channel or link layer to prevent attack by outsiders.
Location of DSR Functions in the ISO Reference Model Appendix A. Location of DSR in the ISO Network Reference Model
When designing DSR, we had to determine at what level within the When designing DSR, we had to determine at what layer within
protocol hierarchy to implement source routing. We considered two the protocol hierarchy to implement ad hoc network routing. We
different options: routing at the link layer (ISO layer 2) and considered two different options: routing at the link layer (ISO
routing at the network layer (ISO layer 3). Originally, we opted to layer 2) and routing at the network layer (ISO layer 3). Originally,
route at the link layer for the following reasons: we opted to route at the link layer for several reasons:
- Pragmatically, running the DSR protocol at the link layer - Pragmatically, running the DSR protocol at the link layer
maximizes the number of mobile nodes that can participate in maximizes the number of mobile nodes that can participate in
ad hoc networks. For example, the protocol can route equally ad hoc networks. For example, the protocol can route equally
well between IPv4 [17], IPv6 [6], and IPX [7] nodes. well between IPv4 [25], IPv6 [7], and IPX [28] nodes.
- Historically, DSR grew from our contemplation of a multi-hop ARP - Historically [12, 13], DSR grew from our contemplation of
protocol [8, 9] and source routing bridges [15]. ARP [16] is a a multi-hop propagating version of the Internet's Address
layer 2 protocol. Resolution Protocol (ARP) [23], as well as from the routing
mechanism used in IEEE 802 source routing bridges [22]. These
are layer 2 protocols.
- Technically, we designed DSR to be simple enough that that it - Technically, we designed DSR to be simple enough that it could
could be implemented directly in network interface cards, well be implemented directly in the firmware inside wireless network
below the layer 3 software within a mobile node. We see great interface cards [12, 13], well below the layer 3 software within
potential for DSR running between clouds of mobile nodes around a mobile node. We see great potential in this for DSR running
fixed base stations. DSR would act to transparently fill in the inside a cloud of mobile nodes around a fixed base station,
coverage gaps between base stations. Mobile nodes that would where DSR would act to transparently extend the coverage range
otherwise be unable to communicate with the base station due to to these nodes. Mobile nodes that would otherwise be unable
factors such as distance, fading, or local interference sources to communicate with the base station due to factors such as
could then reach the base station through their peers. distance, fading, or local interference sources could then reach
the base station through their peers.
Ultimately, however, we decided to specify DSR as a layer 3 protocol Ultimately, however, we decided to specify and to implement [20]
since this is the only layer at which we could realistically support DSR as a layer 3 protocol, since this is the only layer at which we
nodes with multiple interfaces of different types. could realistically support nodes with multiple network interfaces of
different types forming an ad hoc network.
Implementation Status Appendix B. Implementation and Evaluation Status
We have implemented Dynamic Source Routing (DSR) under the The DSR protocol has been implemented under the FreeBSD 2.2.7
FreeBSD 2.2.7 operating system running on Intel x86 platforms. operating system running on Intel x86 platforms. FreeBSD is based
FreeBSD is based on a variety of free software, including 4.4 BSD on a variety of free software, including 4.4 BSD Lite from the
Lite from the University of California, Berkeley. University of California, Berkeley. For the environments in which
we used it, this implementation is functionally equivalent to the
protocol specified in this draft.
During the 7 months from August 1998 to February 1999, we designed During the 7 months from August 1998 to February 1999, we designed
and implemented a full-scale physical testbed to enable the and implemented a full-scale physical testbed to enable the
evaluation of ad hoc network performance in the field. The last evaluation of ad hoc network performance in the field, in a actively
week of February and the first week of March included demonstrations mobile ad hoc network under realistic communication workloads.
of this testbed to a number of our sponsors and partners, including The last week of February and the first week of March included
Lucent Technologies, Bell Atlantic, and DARPA. A complete description demonstrations of this testbed to a number of our sponsors and
of the testbed is available as a Technical Report [12]. partners, including Lucent Technologies, Bell Atlantic, and DARPA.
A complete description of the testbed is available as a Technical
The software is currently being ported to FreeBSD 3.3. Report [20].
Implementors notes: The software was ported to FreeBSD 3.3, and a preliminary version
of Quality of Service (QoS) support was added. A demonstration of
this modified version of DSR was presented in July 2000. Those QoS
features are not included in this draft, and will be added later in a
seprate draft on top of the base protocol specified here.
- Added field to Route Error The DSR protocol has been extensively studied using simulation; we
have implemented DSR in the ns-2 simulator [5, 19] and conducted
evaluations of different caching strategies documented in this
draft [9].
Acknowledgments Several independant groups have also used DSR as a platform for their
own research, or and as a basis of comparison between ad hoc network
routing protocols.
The protocol described in this draft has been designed within Acknowledgements
the CMU Monarch Project, a research project at Carnegie Mellon
University which is developing adaptive networking protocols and
protocol interfaces to allow truly seamless wireless and mobile node
networking [10, 19]. The current members of the CMU Monarch Project
include:
- Robert V. Barron The protocol described in this draft has been designed and developed
within the Monarch Project, a research project at Rice University and
Carnegie Mellon University which is developing adaptive networking
protocols and protocol interfaces to allow truly seamless wireless
and mobile node networking [14, 6].
- Josh Broch The authors would like to acknowledge the substantial contributions
of Josh Broch in helping to design, simulate, and implement the DSR
protocol. Josh is currently on leave of absence from Carnegie Mellon
University at AON Networks. We thank him for his contributions to
earlier versions of this draft.
- Yih-Chun Hu We would also like to acknowledge the assistance of Robert V. Barron
at Carnegie Mellon University. Bob ported our DSR implementation
from FreeBSD 2.2.7 into FreeBSD 3.3.
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276--287, St. Malo, France, October 1997.
[14] Charles Perkins, editor. IP Mobility Support. RFC 2002, [18] S.B. Lee, A. Gahng-Seop, X. Zhang, and A.T. Campbell. INSIGNIA:
October 1996. An IP-Based Quality of Service Framework for Mobile Ad Hoc
Networks. Journal of Parallel and Distributed Computing,
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[15] Radia Perlman. Interconnections: Bridges and Routers. [19] David A. Maltz, Josh Broch, Jorjeta Jetcheva, and David B.
Johnson. The effects of on-demand behavior in routing protocols
for multi-hop wireless ad hoc networks. IEEE Journal on
Selected Areas of Communications, 17(8):1439--1453, August 1999.
[20] David A. Maltz, Josh Broch, and David B. Johnson. Experiences
designing and building a multi-hop wireless ad hoc network
testbed. Technical Report CMU-CS-99-116, School of Computer
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[21] David A. Maltz, Josh Broch, and David B. Johnson. Quantitative
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Or converting network protocol addresses to 48.bit Ethernet Or converting network protocol addresses to 48.bit Ethernet
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[24] Gary R. Wright and W. Richard Stevens. TCP/IP IIlustrated, The [28] Paul Turner. NetWare communications processes. NetWare
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Chair's Address Chair's Address
The Working Group can be contacted via its current chairs: The MANET Working Group can be contacted via its current chairs:
M. Scott Corson M. Scott Corson Phone: +1 301 405-6630
Institute for Systems Research Institute for Systems Research Email: corson@isr.umd.edu
University of Maryland University of Maryland
College Park, MD 20742 College Park, MD 20742
USA USA
Phone: +1 301 405-6630 Joseph Macker Phone: +1 202 767-2001
Email: corson@isr.umd.edu Information Technology Division Email: macker@itd.nrl.navy.mil
Joseph Macker
Information Technology Division
Naval Research Laboratory Naval Research Laboratory
Washington, DC 20375 Washington, DC 20375
USA USA
Phone: +1 202 767-2001
Email: macker@itd.nrl.navy.mil
Authors' Addresses Authors' Addresses
Questions about this document can also be directed to the authors: Questions about this document can also be directed to the authors:
Josh Broch David B. Johnson Phone: +1 713 348-3063
Carnegie Mellon University Rice University Fax: +1 713 348-5930
Electrical and Computer Engineering Computer Science Department, MS 132 Email: dbj@cs.rice.edu
5000 Forbes Avenue 6100 Main Street
Pittsburgh, PA 15213-3890 Houston, TX 77005-1892
USA USA
Phone: +1 412 268-3056 David A. Maltz Phone: +1 650 688-3128
Fax: +1 412 268-7196 AON Networks Fax: +1 650 688-3119
Email: broch@cs.cmu.edu 3045 Park Blvd. Email: dmaltz@cs.cmu.com
Palo Alto, CA 94306
USA
David B. Johnson Yih-Chun Hu Phone: +1 412 268-3075
Carnegie Mellon University Carnegie Mellon University Fax: +1 412 268-5576
Computer Science Department Computer Science Department Email: yihchun@cs.cmu.edu
5000 Forbes Avenue 5000 Forbes Avenue
Pittsburgh, PA 15213-3891 Pittsburgh, PA 15213-3891
USA USA
Phone: +1 412 268-7399 Jorjeta G. Jetcheva Phone: +1 412 268-3053
Fax: +1 412 268-5576 Carnegie Mellon University Fax: +1 412 268-5576
Email: dbj@cs.cmu.edu Computer Science Department Email: jorjeta@cs.cmu.edu
David A. Maltz
Carnegie Mellon University
Computer Science Department
5000 Forbes Avenue 5000 Forbes Avenue
Pittsburgh, PA 15213-3891 Pittsburgh, PA 15213-3891
USA USA
Phone: +1 412 268-3621
Fax: +1 412 268-5576
Email: dmaltz@cs.cmu.edu
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