draft-ietf-roll-dao-projection-00.txt   draft-ietf-roll-dao-projection-01.txt 
ROLL P. Thubert, Ed. ROLL P. Thubert, Ed.
Internet-Draft J. Pylakutty Internet-Draft J. Pylakutty
Intended status: Standards Track Cisco Intended status: Standards Track Cisco
Expires: June 10, 2017 December 07, 2016 Expires: September 11, 2017 March 10, 2017
Root initiated routing state in RPL Root initiated routing state in RPL
draft-ietf-roll-dao-projection-00 draft-ietf-roll-dao-projection-01
Abstract Abstract
This document proposes a protocol extension to RPL that enables to This document proposes a protocol extension to RPL that enables to
install a limited amount of centrally-computed routes in a RPL graph, install a limited amount of centrally-computed routes in a RPL graph,
enabling loose source routing down a non-storing mode DODAG, or enabling loose source routing down a non-storing mode DODAG, or
transversal routes inside the DODAG. As opposed to the classical transversal routes inside the DODAG. As opposed to the classical
route injection by DAO messages, this draft projects the routes from route injection in RPL that are injected by the end devices, this
the root of the DODAG. draft enables the root of the DODAG to projects the routes that are
needed on the nodes where they should be installed.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on April 27, 2017. This Internet-Draft will expire on September 11, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. New RPL Control Message Options . . . . . . . . . . . . . . . 4 3. New RPL Control Message Options . . . . . . . . . . . . . . . 3
3.1. Via Information . . . . . . . . . . . . . . . . . . . . . 4 3.1. Via Information Option . . . . . . . . . . . . . . . . . 4
4. Loose Source Routing in Non-storing Mode . . . . . . . . . . 5 4. Projected DAO . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Centralized Computation of Optimized Peer-to-Peer Routes . . 9 4.1. Non-storing Mode Projected DAO . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12 4.2. Storing-Mode Projected DAO . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 5. Applications . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12 5.1. Loose Source Routing in Non-storing Mode . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.2. Transversal Routes in storing and non-storing modes . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 13 6. RPL Instances . . . . . . . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 13 7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Normative References . . . . . . . . . . . . . . . . . . 14
10.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 15
A.1. Using storing mode P-DAO in non-storing mode MOP . . . . 16
A.2. Projecting a storing-mode transversal route . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction 1. Introduction
The Routing Protocol for Low Power and Lossy Networks [RFC6550] (LLN) The Routing Protocol for Low Power and Lossy Networks (LLN)(RPL)
(RPL) specification defines a generic Distance Vector protocol that [RFC6550] is a generic Distance Vector protocol that is well suited
is designed for very low energy consumption and adapted to a variety for application in a variety of low energy Internet of Things (IoT)
of LLNs. RPL forms Destination Oriented Directed Acyclic Graphs networks. RPL forms Destination Oriented Directed Acyclic Graphs
(DODAGs) which root often acts as the Border Router to connect the (DODAGs) in which the root often acts as the Border Router to connect
RPL domain to the Internet. The root is responsible to select the the RPL domain to the Internet. The root is responsible to select
RPL Instance that is used to forward a packet coming from the the RPL Instance that is used to forward a packet coming from the
Internet into the RPL domain and set the related RPL information in Internet into the RPL domain and set the related RPL information in
the packets. the packets.
In the non-storing mode (NSM) of operation (MOP), the root also The 6TiSCH architecture [I-D.ietf-6tisch-architecture] leverages RPL
computes routes down the DODAG towards the end device and leverages for its routing operation and considers the Deterministic Networking
source routing to get there, while the default route via the root is Architecture [I-D.ietf-detnet-architecture] as one possible model
used for routing upwards within the LLN and to the Internet at large. whereby the device resources and capabilities are exposed to an
NSM is the dominant MOP because because networks may get arbitrary external controller which installs routing states into the network
large and in Storing Mode, the amount of memory in nodes close to the based on some objective functions that reside in that external
root may unexpectedly require memory beyond a node's capabilities. entity.
But as a network gets deep, the size of the source routing header
that the root must add to all the downward packets may also become an
issue for far away target devices. In some use cases, a RPL network
forms long lines and a limited amount of well-targeted routing state
would allow to make the source routing operation loose as opposed to
strict, and save packet size. Limiting the packet size is directly
beneficial to the energy budget, but, mostly, it reduces the chances
of frame loss and/or packet fragmentation, which is highly
detrimental to the LLN operation. Because the capability to store a
routing state in every node is limited, the decision of which route
is installed where can only be optimized with a global knowledge of
the system, a knowledge that the root has in non-storing mode.
Additionally, RPL storing mode is optimized or Point-to-Multipoint
(P2MP), root to leaves and Multipoint-to-Point (MP2P) leaves to root
operations, whereby routes are always installed along the RPL DODAG.
Transversal Peer to Peer (P2P) routes in a RPL network will generally
suffer from some stretch since routing between 2 peers always happens
via a common parent. In NSM, all peer-to-peer routes travel all the
way to the root, which adds a source routing header and forwards the
packet down to the destination, resulting in the longest stretch and
overload of the radio bandwidth near the root. A controller, for
instance collocated with the RPL root, with enough topological
awareness of the connectivity between nodes, would be able to compute
more direct routes, avoiding the vicinity of the root whenever
possible.
The 6TiSCH architecture [I-D.ietf-6tisch-architecture] leverages the
Deterministic Networking Architecture [I-D.finn-detnet-architecture]
as one possible model whereby the device resources and capabilities
are exposed to an external controller which installs routing states
into the network based on some objective functions that reside in
that external entity.
Based on heuristics of usage, path length, and knowledge of device Based on heuristics of usage, path length, and knowledge of device
capacity and available resources such as battery levels and capacity and available resources such as battery levels and
reservable buffers, a Path Computation Element ([PCE]) with a global reservable buffers, a Path Computation Element ([PCE]) with a global
visibility on the system could install additional P2P routes that are visibility on the system could install additional P2P routes that are
more optimized for the current needs as expressed by the objective more optimized for the current needs as expressed by the objective
function. function.
This draft enables a RPL root, with optionally the assistance of a This draft enables a RPL root, with optionally the assistance of a
PCE, to install and maintain additional storing mode routes within PCE, to install and maintain additional storing and non-storing mode
the RPL domain, along a selected set of nodes and for a selected routes within the RPL domain, along a selected set of nodes and for a
duration, thus providing routes from suitable than those obtained selected duration, thus providing routes more suitable than those
from the distributed operation of RPL in either storing and non- obtained with the distributed operation of RPL. Those routes may be
storing modes. installed in either storing and non-storing modes RPL instances,
resulting in potentially hybrid situations where the mode of the
projected routes is different from that of the other routes in the
instance.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
The Terminology used in this document is consistent with and The Terminology used in this document is consistent with and
incorporates that described in `Terminology in Low power And Lossy incorporates that described in `Terminology in Low power And Lossy
Networks' [RFC7102] and [RFC6550]. Networks' [RFC7102] and [RFC6550].
3. New RPL Control Message Options 3. New RPL Control Message Options
Section 6.7 of [RFC6550] specifies Control Message Options (CMO) to Section 6.7 of [RFC6550] specifies Control Message Options (CMO) to
be placed in RPL messages such as the DAO message. The RPL Target be placed in RPL messages such as the Destination Advertisement
Option and the Transit Information Option (TIO) are such options; the Object (DAO) message. The RPL Target Option and the Transit
former indicates a node to be reached and the latter specifies a Information Option (TIO) are such options; the former indicates a
parent that can be used to reach that node. Options may be node to be reached and the latter specifies a parent that can be used
factorized; one or more contiguous TIOs apply to the one or more to reach that node. Options may be factorized; one or more
contiguous Target options that immediately precede the TIOs in the contiguous TIOs apply to the one or more contiguous Target options
RPL message. that immediately precede the TIOs in the RPL message.
This specification introduces a new Control Message Option, the Via This specification introduces a new Control Message Option, the Via
Information option (VIO). Like the TIO, the VIO MUST be preceded by Information option (VIO). Like the TIO, the VIO MUST be preceded by
one or more RPL Target options to which it applies. Unlike the TIO, one or more RPL Target options to which it applies. Unlike the TIO,
the VIO are not factorized: multiple contiguous Via options indicate the VIO are not factorized: multiple contiguous Via options indicate
an ordered sequence of hops to reach the target(s), presented in the an ordered sequence of routers to reach the target(s), presented in
same order as they would appear in a routing header. the order of the packet stream, source to destination, and in which a
routing state must be installed.
3.1. Via Information The Via Information option MUST contain at least one Via Address.
3.1. Via Information Option
The Via Information option MAY be present in DAO messages, and its The Via Information option MAY be present in DAO messages, and its
format is as follows: format is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x0A | Option Length | Path Sequence | Path Lifetime | | Type = 0x0A | Option Length | Path Sequence | Path Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
+ + + +
. . . .
. Next-Hop Address . . Via Address 1 .
. .
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .... .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
. .
. Via Address n .
. . . .
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Eliding the RPLInstanceID Figure 1: Via Information option format
Option Type: 0x0A (to be confirmed by IANA) Option Type: 0x0A (to be confirmed by IANA)
Option Length: Variable, depending on whether or not Parent Address Option Length: In bytes; variable, depending on the number of Via
is present. Addresses.
Path Sequence: 8-bit unsigned integer. When a RPL Target option is Path Sequence: 8-bit unsigned integer. When a RPL Target option is
issued by the root of the DODAG (i.e. in a DAO message), that issued by the root of the DODAG (i.e. in a DAO message), that
root sets the Path Sequence and increments the Path Sequence root sets the Path Sequence and increments the Path Sequence
each time it issues a RPL Target option with updated each time it issues a RPL Target option with updated
information. The indicated sequence deprecates any state for a information. The indicated sequence deprecates any state for a
given Target that was learned from a previous sequence and adds given Target that was learned from a previous sequence and adds
to any state that was learned for that sequence. to any state that was learned for that sequence.
Path Lifetime: 8-bit unsigned integer. The length of time in Path Lifetime: 8-bit unsigned integer. The length of time in
Lifetime Units (obtained from the Configuration option) that Lifetime Units (obtained from the Configuration option) that
the prefix is valid for route determination. The period starts the prefix is valid for route determination. The period starts
when a new Path Sequence is seen. A value of all one bits when a new Path Sequence is seen. A value of all one bits
(0xFF) represents infinity. A value of all zero bits (0x00) (0xFF) represents infinity. A value of all zero bits (0x00)
indicates a loss of reachability. A DAO message that contains indicates a loss of reachability. A DAO message that contains
a Via Information option with a Path Lifetime of 0x00 for a a Via Information option with a Path Lifetime of 0x00 for a
Target is referred as a No-Path (for that Target) in this Target is referred as a No-Path (for that Target) in this
document. document.
Next-Hop Address: 8 or 16 bytes. IPv6 Address of the next hop Via Address: 16 bytes. IPv6 Address of the next hop towards the
towards the destination(s) indicated in the target option that destination(s) indicated in the target option that immediately
immediately precede the VIO. The /64 prefix can be elided if precede the VIO. TBD: See how the /64 prefix can be elided if
it is the same as that of (all of) the target(s). In that it is the same as that of (all of) the target(s). In that
case, the Next-Hop Address is expressed as the 8-bytes suffix case, the Next-Hop Address could be expressed as the 8-bytes
only, otherwise it is expressed as 16 bytes. suffix only, otherwise it is expressed as 16 bytes, at least in
storing mode.
4. Loose Source Routing in Non-storing Mode 4. Projected DAO
A classical RPL implementation in a very constrained LLN uses the This draft adds a capability to RPL whereby the root projects a route
non-storing mode of operation whereby a RPL node indicates a parent- through an extended DAO message called a Projected-DAO (P-DAO) to an
child relationship to the root, using a Destination Advertisement arbitrary router down the DODAG, indicating a next hop or a sequence
Object (DAO) that is unicast from the node directly to the root, and of routers via which a certain destination indicated in the Target
the root builds a path to a destination down the DODAG by Information option may be reached.
concatenating this information.
A P-DAO message MUST contain at least a Target Information option and
at least one VIA Information option following it.
Like a classical DAO message, a P-DAO is processed only if it is
"new" per section 9.2.2. "Generation of DAO Messages" of the RPL
specification [RFC6550]; this is determined using the Path Sequence
information from the VIO as opposed to a TIO. Also, a Path Lifetime
of 0 in a VIO indicates that a route is to be removed.
There are two kinds of P-DAO, the storing mode and the non-storing
mode ones.
The non-storing mode P-DAO discussed in section Section 4.1 has a
single VIO with one or more Via Addresses in it, the list of Via
Addresses indicating the source-routed path to the target to be
installed in the router that receives the message, which replies
to the root directly with a DAO-ACK message.
The storing mode P-DAO discussed in section Section 4.2 has at
least two Via Information options with one Via Address each, for
the ingress and the egress of the path, and more if there are
intermediate routers. The Via Addresses indicate the routers in
which the routing state to the target have to be installed via the
next Via Address in the sequence of VIO. In normal operations,
the P-DAO is propagated along the chain of Via Routers from the
egress router of the path till the ingress one, which confirms the
installation to the root with a DAO-ACK message. Note that the
root may be the ingress and it may be the egress of the path, that
it can also be neither but it cannot be both.
The root is expected to use these mechanisms optimally and with
required parsimony to limit the state installed in the devices to fit
within their resources, but how the root figures the amount of
resources that is available in each device is out of scope for this
document.
In particular, the draft expects that the root has enough information
about the capability for each node to store a number of routes, which
can be discovered for instance using a Network Management System
(NMS) and/or the RPL routing extensions specified in Routing for Path
Calculation in LLNs [RFC6551].
A route that is installed by a P-DAO is not necessarily installed
along the DODAG, though how the root and the optional PCE obtain the
additional topological information to compute other routes is out of
scope for this document
4.1. Non-storing Mode Projected DAO
As illustrated in Figure 2, the non-storing mode P-DAO enables the
root to install a source-routed path towards a target in any
particular router; with this path information the router can add a
source routed header reflecting the path to any packet for which the
current destination either is the said target or can be reached via
the target, for instance a loose source routed packet for which the
next loose hop is the target, or a packet for which the router has a
routing state to the final destination via the target.
------+--------- ------+---------
| Internet | Internet
| |
+-----+ +-----+
| | Border Router | | Border Router
| | (RPL Root) | | (RPL Root)
+-----+ ^ | | +-----+ | P ^ |
| | DAO | ACK | | | DAO | ACK | Loose
o o o o | | | Strict o o o o router V | | Source
o o o o o o o o o | | | Source o o o o o o o o o | P-DAO . Route
o o o o o o o o o o | | | Route o o o o o o o o o o | Source . Path
o o o o o o o o o | | | o o o o o o o o o | Route . From
o o o o o o o o | v v o o o o o o o o | Path . Root
o o o o o o o o o target V . To
o o o o | Desti-
o o o o | nation
destination V
LLN LLN
Figure 2: RPL non-storing operation Figure 2: Projecting a non-storing route
Nodes are not expected to store downward routing state via their A router that receives a non-storing P-DAO installs a source routed
children, and the routing operates in strict source routing mode as path towards each of the consecutive targets via a source route path
detailed in An IPv6 Routing Header for Source Routes with RPL indicated in the following VIO.
[RFC6554]
This draft proposes an addition whereby the root projects a route When forwarding a packet to a destination for which the router
through an extended DAO to an arbitrary node down the DODAG, determines that routing happens via the target, the router inserts
indicating a child or a direct sequence of children via which a the source routing header in the packet to reach the target.
certain destination (target) may be reached. The root is expected to
use the mechanism optimally and with required parsimony to fit within In order to do so, the router encapsulates the packet with an IP in
the device resources, but how the root figures the amount of IP header and a non-storing mode source routing header (SRH)
resources that are available is out of scope. [RFC6554].
In the uncompressed form the source of the packet would be self, the
destination would be the first Via Address in the VIO, and the SRH
would contain the list of the remaining Via Addresses and then the
target.
In practice, the router will normally use the IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Paging Dispatch [RFC8025] to
compress the RPL artifacts as indicated in the 6LoWPAN Routing Header
[I-D.ietf-roll-routing-dispatch] specification. In that case, the
router indicates self as encapsulator in an IP-in-IP 6LoRH Header,
and places the list of Via Addresses in the order of the VIO and then
the target in the SRH 6LoRH Header.
4.2. Storing-Mode Projected DAO
As illustrated in Figure 3, the storing mode P-DAO enables the root
to install a routing state towards a target in the routers along a
segment between an ingress and an egress router; this enables the
routers to forward along that segment any packet for which the next
loose hop is the said target, for instance a loose source routed
packet for which the next loose hop is the target, or a packet for
which the router has a routing state to the final destination via the
target.
------+--------- ------+---------
| Internet | Internet
| |
+-----+ +-----+
| | Border Router | | Border Router
| | (RPL Root) | | (RPL Root)
+-----+ | ^ | +-----+ | ^ |
| | DAO | ACK | | | DAO | ACK |
o o o o | | | Loose o o o o | | | Loose
skipping to change at page 6, line 32 skipping to change at page 8, line 30
| | Border Router | | Border Router
| | (RPL Root) | | (RPL Root)
+-----+ | ^ | +-----+ | ^ |
| | DAO | ACK | | | DAO | ACK |
o o o o | | | Loose o o o o | | | Loose
o o o o o o o o o | ^ | Source o o o o o o o o o | ^ | Source
o o o o o o o o o o | | DAO | Route o o o o o o o o o o | | DAO | Route
o o o o o o o o o | ^ | o o o o o o o o o | ^ |
o o o o o o o o v | DAO v o o o o o o o o v | DAO v
o o o o o o o o
LLN LLN
Figure 2: Non-Storing with Projected routes Figure 3: Projecting a route
When a RPL domain operates in non-storing Mode of Operation (NS-MOP), Based on available topological, usage and capabilities node
only the root possesses routing information about the whole network. information, the root or an associated PCE computes which segment
A packet that is generated within the domain first reaches the root, should be optimized and which relevant state should be installed in
which can then apply a source routing information to reach the which nodes. The algorithm is out of scope but it is envisaged that
destination. Similarly, a packet coming from the outside of the the root could compute the ratio between the optimal path (existing
domain for a destination that is expected to be in a RPL domain path not traversing the root, and the current path), the application
reaches the root. service level agreement (SLA) for specific flows that could benefit
from shorter paths, the energy wasted in the network, local
congestion on various links that would benefit from having flows
routed along alternate paths.
In NS-MOP, the root, or some associated centralized computation In order to install the relevant routing state along the segment
engine, can thus determine the amount of packets that reach a between an ingress and an egress routers, the root sends a unicast
destination in the RPL domain, and thus the amount of energy and P-DAO message to the egress router of the routing segment that must
bandwidth that is wasted for transmission, between itself and the be installed. The P-DAO message contains the ordered list of hops
destination, as well as the risk of fragmentation, any potential along the segment as a direct sequence of Via Information options
delays because of a paths longer than necessary (shorter paths exist that are preceded by one or more RPL Target options to which they
that would not traverse the root). relate. Each Via Information option contains a Path Lifetime for
which the state is to be maintained.
Additionally, the DAG root knows the whole DAG topology, so when the The root sends the P-DAO directly to the egress node of the segment,
source of a packet is also in the RPL domain, the root can determine which In that P-DAO, the destination IP address matches the Via
the common parent that would have been used in storing mode, and thus Address in the last VIO. This is how the egress recognizes its role.
the list of nodes in the path between the common parent and the In a similar fashion, the ingress node recognizes its role as it
destination. For instance in the below diagram, if the source is 41 matches Via Address in the first VIO.
and the destination 52, the common parent is the node 22.
------+--------- The egress node of the segment is the only node in the path that does
| Internet not install a route in response to the P-DAO; it is expected to be
| already able to route to the target(s) on its own. It may either be
+-----+ the target, or may have some existing information to reach the
| | Border Router target(s), such as a connected route or an already installed
| | (RPL Root) projected route. If one of the targets cannot be located, the node
+-----+ MUST answer to the root with a negative DAO-ACK listing the target(s)
| \ \____ that could not be located (suggested status 10 to be confirmed by
/ \ \ IANA).
o 11 o 12 o 13
/ | / \
o 22 o 23 o 24 o 25
/ \ | \ \
o 31 o 32 o o o 35
/ / | \ | \
o 41 o 42 o o o 45 o 46
| | | | \ |
o 51 o 52 o 53 o o 55 o 56
LLN
Figure 3: Non-Storing with Projected routes If the egress node can reach all the targets, then it forwards the
P-DAO with unchanged content to its loose predecessor in the segment
as indicated in the list of Via Information options, and recursively
the message is propagated unchanged along the sequence of routers
indicated in the P-DAO, but in the reverse order, from egress to
ingress.
With this draft, the root can install routing states along a segment The address of the predecessor to be used as destination of the
that is either itself to the destination, or from one or more common propagated DAO message is found in the Via Information option the
parents for a particular source/destination pair towards that precedes the one that contain the address of the propagating node,
destination (in our example, this would be the segment made of nodes which is used as source of the packet.
22, 32, 42).
The draft expects that the root has enough information about the Upon receiving a propagated DAO, an intermediate router as well as
capability for each node to store a number of routes, which can be the ingress router install a route towards the DAO target(s) via its
discovered for instance using a Network Management System (NMS) and/ successor in the P-DAO; the router locates the VIO that contains its
or the RPL routing extensions specified in Routing for Path address, and uses as next hop the address found in the Via Address
Calculation in LLNs [RFC6551]. Based on that information, the root field in the following VIO. The router MAY install additional routes
computes which segment should be routed and which relevant state towards the addresses that are located in VIOs that are after the
should be installed in which nodes. The algorithm is out of scope next one, if any, but in case of a conflict or a lack of resource, a
but it is envisaged that the root could compute the ratio between the route to a target installed by the root has precedence.
optimal path (existing path not traversing the root, and the current
path), the application SLA for specific flows that could benefit from
shorter paths, the energy wasted in the network, local congestion on
various links that would benefit from having flows routed along other
paths.
This draft introduces a new mode of operation for loose source The process recurses till the P-DAO is propagated to ingress router
routing in the LLN, the Non-Storing with Projected routes MOP. With of the segment, which answers with a DAO-ACK to the root.
this new MOP, the root sends a unicast DAO message to the last node
of the routing segment that must be installed. The DAO message
contains the ordered list of hops along the segment as a list of Via
Information options that are preceded by one or more RPL Target
options to which they relate. Each Via Information option contains a
lifetime for which state is to be maintained.
The root sends the DAO directly to the last node in the segment, Also, the path indicated in a P-DAO may be loose, in which case the
which is expected to be able to route to the targets on its own. reachability to the next hop has to be asserted. Each router along
the path indicated in a P-DAO is expected to be able to reach its
successor, either with a connected route (direct neighbor), or by
routing, for instance following a route installed previously by a DAO
or a P-DAO message. If that route is not connected then a recursive
lookup may take place at packet forwarding time to find the next hop
to reach the target(s). If it does not and cannot reach the next
router in the P-DAO, the router MUST answer to the root with a
negative DAO-ACK indicating the successor that is unreachable
(suggested status 11 to be confirmed by IANA).
The last node in the segment may have another information to reach A Path Lifetime of 0 in a Via Information option is used to clean up
the target(s), such as a connected route or an already installed the state. The P-DAO is forwarded as described above, but the DAO is
projected route. If it does not have such a route then the node interpreted as a No-Path DAO and results in cleaning up existing
should lookup the address on the relevant interfaces. If one of the state as opposed to refreshing an existing one or installing a new
targets cannot be located, the node MUST answer to the root with a one.
negative DAO-ACK listing the target(s) that could not be located
(suggested status 10), and continue the process for those targets
that could be located if any.
For the targets that could be located, last node in the segment 5. Applications
generates a DAO to its loose predecessor in the segment as indicated
in the list of Via Information options.
The node strips the last Via Information option which corresponds to 5.1. Loose Source Routing in Non-storing Mode
self, and uses it as source address for the DAO to the predecessor.
The address of the predecessor to be used as destination for the DAO
message is found in the now last Via Information option. The
predecessor is expected to have a route to the address used as
source, either connected, installed previously as another DAO, or
from other means.
The predecessor is expected to have a route to the address used as A RPL implementation operating in a very constrained LLN typically
source and that is his successor. If it does not and cannot locate uses the non-storing mode of operation whereby a RPL node indicates a
the successor, the predecessor node MUST answer to the root with a parent-child relationship to the root, using a Destination
negative DAO-ACK indicating the successor that could not be located. Advertisement Object (DAO) that is unicast from the node directly to
The DAO-ACK contains the list of targets that could not be routed to the root, and the root typically builds a source routed path to a
(suggested status 11). destination down the DODAG by recursively concatenating this
information.
If the predecessor can route to the successor node, then it installs ------+---------
a route to the targets via the successor. If that route is not | Internet
connected then a recursive lookup will take place to reach the |
target(s). From there, the node strips the last Via Information +-----+
option and either answers to the root with a positive DAO-ACK that | | Border Router
contains the list of targets that could be routed to, or propagates | | (RPL Root)
the DAO to its own predecessor. +-----+ ^ | |
| | DAO | ACK |
o o o o | | | Strict
o o o o o o o o o | | | Source
o o o o o o o o o o | | | Route
o o o o o o o o o | | |
o o o o o o o o | v v
o o o o
LLN
A NULL lifetime in the Via Information option along the segment is Figure 4: RPL non-storing mode of operation
used to clean up the state.
In the example below, say that there is a lot of traffic to nodes 55 Based on the parent-children relationships expressed in the non-
and 56 and the root decides to reduce the size of routing headers to storing DAO messages,the root possesses topological information about
those destinations. The root can first send a DAO to node 45 the whole network, though this information is limited to the
indicating target 55 and a Via segment (35, 45), as well as another structure of the DODAG for which it is the destination. A packet
DAO to node 46 indicating target 56 and a Via segment (35, 46). This that is generated within the domain will always reach the root, which
will save one entry in the routing header on both sides. The root can then apply a source routing information to reach the destination
may then send a DAO to node 35 indicating targets 55 and 56 a Via if the destination is also in the DODAG. Similarly, a packet coming
segment (13, 24, 35) to fully optimize that path. from the outside of the domain for a destination that is expected to
be in a RPL domain reaches the root.
Alternatively, the root may send a DAO to node 45 indicating target It results that the root, or then some associated centralized
55 and a Via segment (13, 24, 35, 45) and then a DAO to node 46 computation engine such as a PCE, can determine the amount of packets
indicating target 56 and a Via segment (13, 24, 35, 46), indicating that reach a destination in the RPL domain, and thus the amount of
the same DAO Sequence. energy and bandwidth that is wasted for transmission, between itself
and the destination, as well as the risk of fragmentation, any
potential delays because of a paths longer than necessary (shorter
paths exist that would not traverse the root).
5. Centralized Computation of Optimized Peer-to-Peer Routes As a network gets deep, the size of the source routing header that
the root must add to all the downward packets becomes an issue for
nodes that are many hops away. In some use cases, a RPL network
forms long lines and a limited amount of well-targeted routing state
would allow to make the source routing operation loose as opposed to
strict, and save packet size. Limiting the packet size is directly
beneficial to the energy budget, but, mostly, it reduces the chances
of frame loss and/or packet fragmentation, which is highly
detrimental to the LLN operation. Because the capability to store a
routing state in every node is limited, the decision of which route
is installed where can only be optimized with a global knowledge of
the system, a knowledge that the root or an associated PCE may
possess by means that are outside of the scope of this specification.
With the initial specifications of RPL [RFC6550], the P2P path from a This specification enables to store source-routed or storing mode
source to a destination is often stretched, as illustrated in state in intermediate routers, which enables to limit the excursion
[RFC6550]: of the source route headers in deep networks. Once a P-DAO exchange
has taken place for a given target, if the root operates in non
storing mode, then it may elide the sequence of routers that is
installed in the network from its source route headers to destination
that are reachable via that target, and the source route headers
effectively become loose.
- in non-storing mode, all packets routed within the DODAG flow 5.2. Transversal Routes in storing and non-storing modes
all the way up to the root of the DODAG. If the destination is in
the same DODAG, the root must encapsulate the packet to place a RPL is optimized for Point-to-Multipoint (P2MP), root to leaves and
Multipoint-to-Point (MP2P) leaves to root operations, whereby routes
are always installed along the RPL DODAG. Transversal Peer to Peer
(P2P) routes in a RPL network will generally suffer from some stretch
since routing between 2 peers always happens via a common parent, as
illustrated in Figure 5:
o in non-storing mode, all packets routed within the DODAG flow all
the way up to the root of the DODAG. If the destination is in the
same DODAG, the root must encapsulate the packet to place a
Routing Header that has the strict source route information down Routing Header that has the strict source route information down
the DODAG to the destination. This will be the case even if the the DODAG to the destination. This will be the case even if the
destination is relatively close to the source and the root is destination is relatively close to the source and the root is
relatively far off. relatively far off.
- in storing mode, unless the destination is a child of the o In storing mode, unless the destination is a child of the source,
source, the packets will follow the default route up the DODAG as the packets will follow the default route up the DODAG as well.
well. If the destination is in the same DODAG, they will If the destination is in the same DODAG, they will eventually
eventually reach a common parent that has a DAO route to the reach a common parent that has a route to the destination; at
destination; at worse, the common parent may also be the root. worse, the common parent may also be the root. From that common
From that common parent, the packet will follow a path down the parent, the packet will follow a path down the DODAG that is
DODAG that is optimized for the Objective Function that was used optimized for the Objective Function that was used to build the
to build the DODAG. DODAG.
It results that it is often beneficial to enable additional P2P
routes, either if the RPL route present a stretch from shortest path,
or if the new route is engineered with a different objective.
------+--------- ------+---------
| Internet | Internet
| |
+-----+ +-----+
| | Border Router | | Border Router
| | (RPL Root) | | (RPL Root)
+-----+ +-----+
X X
^ v o o ^ v o o
^ o o v o o o o o ^ o o v o o o o o
^ o o o v o o o o o ^ o o o v o o o o o
^ o o v o o o o o ^ o o v o o o o o
S o o o D o o o S o o o D o o o
o o o o o o o o
LLN LLN
Figure 4: Routing Stretch Figure 5: Routing Stretch between S and D via common parent X
It results that it is often beneficial to enable transversal P2P
routes, either if the RPL route presents a stretch from shortest
path, or if the new route is engineered with a different objective.
For that reason, earlier work at the IETF introduced the Reactive For that reason, earlier work at the IETF introduced the Reactive
Discovery of Point-to-Point Routes in Low Power and Lossy Networks Discovery of Point-to-Point Routes in Low Power and Lossy Networks
[RFC6997], which specifies a distributed method for establishing [RFC6997], which specifies a distributed method for establishing
optimized P2P routes. This draft proposes an alternate based on a optimized P2P routes. This draft proposes an alternate based on a
centralized route computation. centralized route computation.
It must be noted that RPL has a concept of instance but does not have
a concept of an administrative distance, which exists in certain
proprietary implementations to sort out conflicts between multiple
sources. This draft conforms the instance model as follows:
- if the PCE needs to influence a particular instance to add
better routes in conformance with the routing objectives in that
instance, it may do so. When the PCE modifies an existing
instance then the added routes must not create a loop in that
instance. This is achieved by always preferring a route obtained
from the PCE over a route that is learned via RPL.
- If the PCE installs a more specific (Traffic Engineering) route
between a particular pair of nodes then it should use a Local
Instance from the ingress node of that path. Only packets
associated with that instance will be routed along that path.
In all cases, the path is indicated by VIA options, and the flow is
similar to the flow used to obtain loose source routing.
The root sends the DAO with the target option and the Via Option to
the lest router in the path; the last router removes the last Via
Option and passes the DAO to the previous hop.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
| Projected DAO message to C
o | o o
o o o | o o o o o
o o o | o o o o o o
o o V o o o o o o
S A B C D o o o
o o o o
LLN
Figure 5: Projected DAO from root
The process recurses till the destination which sends a DAO-ACK to
the root. In the example above, for target D, the list of via
options is S, A, B and C. The projected DAO is sent by the root to
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
^ Projected DAO-ACK from S
/ o o o
/ o o o o o o o
| o o o o o o o o o
| o o o o o o o o
S A B C D o o o
o o o o
LLN
Figure 6: Projected DAO-ACK to root
The process recurses till the destination which sends a DAO-ACK to
the root. In the example above, for target D, the list of via
options is S, A, B and C. The projected DAO is sent by the root to
------+--------- ------+---------
| Internet | Internet
| |
+-----+ +-----+
| | Border Router | | Border Router
| | (RPL Root) | | (RPL Root)
+-----+ +-----+
| |
o o o o o o o o
o o o o o o o o o o o o o o o o o o
o o o o o o o o o o o o o o o o o o o o
o o o o o o o o o o o o o o o o o o
S>>A>>>B>>C>>>D o o o S>>A>>>B>>C>>>D o o o
o o o o o o o o
LLN LLN
Figure 7: Projected Transversal Route Figure 6: Projected Transversal Route
6. Security Considerations This specification enables to store source-routed or storing mode
state in intermediate routers, which enables to limit the stretch of
a P2P route and maintain the characteristics within a given SLA. An
example of service using this mechanism oculd be a control loop that
would be installed in a network that uses classical RPL for
asynchronous data collection. In that case, the P2P path may be
installed in a different RPL Instance, with a different objective
function.
6. RPL Instances
It must be noted that RPL has a concept of instance but does not have
a concept of an administrative distance, which exists in certain
proprietary implementations to sort out conflicts between multiple
sources. This draft conforms the instance model as follows:
o if the PCE needs to influence a particular instance to add better
routes in conformance with the routing objectives in that
instance, it may do so. When the PCE modifies an existing
instance then the added routes must not create a loop in that
instance. This is achieved by always preferring a route obtained
from the PCE over a route that is learned via RPL.
o If the PCE installs a more specific (Traffic Engineering) route
between a particular pair of nodes then it should use a Local
Instance from the ingress node of that path. Only packets
associated with that instance will be routed along that path.
In all cases, the path is indicated by a new Via Information option,
and the flow is similar to the flow used to obtain loose source
routing.
7. Security Considerations
This draft uses messages that are already present in [RFC6550] with This draft uses messages that are already present in [RFC6550] with
optional secured versions. The same secured versions may be used optional secured versions. The same secured versions may be used
with this draft, and whatever security is deployed for a given with this draft, and whatever security is deployed for a given
network also applies to the flows in this draft. network also applies to the flows in this draft.
7. IANA Considerations 8. IANA Considerations
This document updates the IANA registry for the Mode of Operation This document updates the IANA registry for the Mode of Operation
(MOP) (MOP)
4: Non-Storing with Projected routes [this] 4: Non-Storing with Projected routes [this]
This document updates IANA registry for the RPL Control Message This document updates IANA registry for the RPL Control Message
Options Options
0x0A: Via descriptor [this] 0x0A: Via descriptor [this]
8. Acknowledgments 9. Acknowledgments
The authors wish to acknowledge JP Vasseur and Patrick Wetterwald for The authors wish to acknowledge JP Vasseur and Patrick Wetterwald for
their contributions to the ideas developed here. their contributions to the ideas developed here.
9. References 10. References
9.1. Normative References
10.1. Normative References
[I-D.ietf-roll-routing-dispatch]
Thubert, P., Bormann, C., Toutain, L., and R. Cragie,
"6LoWPAN Routing Header", draft-ietf-roll-routing-
dispatch-05 (work in progress), October 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., [RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550, Low-Power and Lossy Networks", RFC 6550,
skipping to change at page 13, line 30 skipping to change at page 15, line 17
in Low-Power and Lossy Networks", RFC 6551, in Low-Power and Lossy Networks", RFC 6551,
DOI 10.17487/RFC6551, March 2012, DOI 10.17487/RFC6551, March 2012,
<http://www.rfc-editor.org/info/rfc6551>. <http://www.rfc-editor.org/info/rfc6551>.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6 [RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554, for Low-Power and Lossy Networks (RPL)", RFC 6554,
DOI 10.17487/RFC6554, March 2012, DOI 10.17487/RFC6554, March 2012,
<http://www.rfc-editor.org/info/rfc6554>. <http://www.rfc-editor.org/info/rfc6554>.
[RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and [RFC8025] Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power
Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January Wireless Personal Area Network (6LoWPAN) Paging Dispatch",
2014, <http://www.rfc-editor.org/info/rfc7102>. RFC 8025, DOI 10.17487/RFC8025, November 2016,
<http://www.rfc-editor.org/info/rfc8025>.
9.2. Informative References
[I-D.finn-detnet-architecture] 10.2. Informative References
Finn, N. and P. Thubert, "Deterministic Networking
Architecture", draft-finn-detnet-architecture-08 (work in
progress), August 2016.
[I-D.ietf-6tisch-architecture] [I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-10 (work of IEEE 802.15.4", draft-ietf-6tisch-architecture-11 (work
in progress), June 2016. in progress), January 2017.
[I-D.ietf-detnet-architecture]
Finn, N. and P. Thubert, "Deterministic Networking
Architecture", draft-ietf-detnet-architecture-00 (work in
progress), September 2016.
[PCE] IETF, "Path Computation Element", [PCE] IETF, "Path Computation Element",
<https://datatracker.ietf.org/doc/charter-ietf-pce/>. <https://datatracker.ietf.org/doc/charter-ietf-pce/>.
[RFC6997] Goyal, M., Ed., Baccelli, E., Philipp, M., Brandt, A., and [RFC6997] Goyal, M., Ed., Baccelli, E., Philipp, M., Brandt, A., and
J. Martocci, "Reactive Discovery of Point-to-Point Routes J. Martocci, "Reactive Discovery of Point-to-Point Routes
in Low-Power and Lossy Networks", RFC 6997, in Low-Power and Lossy Networks", RFC 6997,
DOI 10.17487/RFC6997, August 2013, DOI 10.17487/RFC6997, August 2013,
<http://www.rfc-editor.org/info/rfc6997>. <http://www.rfc-editor.org/info/rfc6997>.
Authors' Addresses [RFC7102] Vasseur, JP., "Terms Used in Routing for Low-Power and
Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
2014, <http://www.rfc-editor.org/info/rfc7102>.
Appendix A. Examples
A.1. Using storing mode P-DAO in non-storing mode MOP
In non-storing mode, the DAG root maintains the knowledge of the
whole DODAG topology, so when both the source and the destination of
a packet are in the DODAG, the root can determine the common parent
that would have been used in storing mode, and thus the list of nodes
in the path between the common parent and the destination. For
instance in the diagram shown in Figure 7, if the source is node 41
and the destination is node 52, then the common parent is node 22.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
| \ \____
/ \ \
o 11 o 12 o 13
/ | / \
o 22 o 23 o 24 o 25
/ \ | \ \
o 31 o 32 o o o 35
/ / | \ | \
o 41 o 42 o o o 45 o 46
| | | | \ |
o 51 o 52 o 53 o o 55 o 56
LLN
Figure 7: Example DODAG forming a logical tree topology
With this draft, the root can install a storing mode routing states
along a segment that is either from itself to the destination, or
from one or more common parents for a particular source/destination
pair towards that destination (in this particular example, this would
be the segment made of nodes 22, 32, 42).
In the example below, say that there is a lot of traffic to nodes 55
and 56 and the root decides to reduce the size of routing headers to
those destinations. The root can first send a DAO to node 45
indicating target 55 and a Via segment (35, 45), as well as another
DAO to node 46 indicating target 56 and a Via segment (35, 46). This
will save one entry in the routing header on both sides. The root
may then send a DAO to node 35 indicating targets 55 and 56 a Via
segment (13, 24, 35) to fully optimize that path.
Alternatively, the root may send a DAO to node 45 indicating target
55 and a Via segment (13, 24, 35, 45) and then a DAO to node 46
indicating target 56 and a Via segment (13, 24, 35, 46), indicating
the same DAO Sequence.
A.2. Projecting a storing-mode transversal route
In this example, say that a PCE determines that a path must be
installed between node S and node D via routers A, B and C, in order
to serve the needs of a particular application.
The root sends a P-DAO with a target option indicating the
destination D and a sequence Via Information option, one for S, which
is the ingress router of the segment, one for A and then for B, which
are an intermediate routers, and one for C, which is the egress
router.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
| Projected DAO message to C
o | o o
o o o | o o o o o
o o o | o o o o o o
o o V o o o o o o
S A B C D o o o
o o o o
LLN
Figure 8: Projected DAO from root
Upon reception of the P-DAO, C validates that it can reach D, e.g.
using IPv6 Neighbor Discovery, and if so, propagates the P-DAO
unchanged to B.
B checks that it can reach C and of so, installs a route towards D
via C. Then it propagates the P-DAO to A.
The process recurses till the P-DAO reaches S, the ingress of the
segment, which installs a route to D via A and sends a DAO-ACK to the
root.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
^ Projected DAO-ACK from S
/ o o o
/ o o o o o o o
| o o o o o o o o o
| o o o o o o o o
S A B C D o o o
o o o o
LLN
Figure 9: Projected DAO-ACK to root
As a result, a transversal route is installed that does not need to
follow the DODAG structure.
------+---------
| Internet
|
+-----+
| | Border Router
| | (RPL Root)
+-----+
|
o o o o
o o o o o o o o o
o o o o o o o o o o
o o o o o o o o o
S>>A>>>B>>C>>>D o o o
o o o o
LLN
Figure 10: Projected Transversal Route
Authors' Addresses
Pascal Thubert (editor) Pascal Thubert (editor)
Cisco Systems Cisco Systems
Village d'Entreprises Green Side Village d'Entreprises Green Side
400, Avenue de Roumanille 400, Avenue de Roumanille
Batiment T3 Batiment T3
Biot - Sophia Antipolis 06410 Biot - Sophia Antipolis 06410
FRANCE FRANCE
Phone: +33 4 97 23 26 34 Phone: +33 4 97 23 26 34
Email: pthubert@cisco.com Email: pthubert@cisco.com
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