draft-ietf-ngtrans-isatap-10.txt   draft-ietf-ngtrans-isatap-11.txt 
NGTRANS Working Group F. Templin Network Working Group F. Templin
Internet-Draft Nokia Internet-Draft Nokia
Expires: July 4, 2002 T. Gleeson Expires: July 18, 2003 T. Gleeson
Cisco Systems K.K. Cisco Systems K.K.
M. Talwar M. Talwar
D. Thaler D. Thaler
Microsoft Corporation Microsoft Corporation
January 03, 2002 January 17, 2003
Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
draft-ietf-ngtrans-isatap-10.txt draft-ietf-ngtrans-isatap-11.txt
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract Abstract
This document specifies an Intra-Site Automatic Tunnel Addressing This document specifies an Intra-Site Automatic Tunnel Addressing
Protocol (ISATAP) that connects IPv6 hosts and routers within IPv4 Protocol (ISATAP) that connects IPv6 hosts and routers within IPv4
sites. ISATAP treats the site's IPv4 infrastructure as a link layer sites. ISATAP treats the site's IPv4 infrastructure as a link layer
for IPv6 with no requirement for IPv4 multicast. ISATAP enables for IPv6 with no requirement for IPv4 multicast. ISATAP enables
intra-site automatic IPv6-in-IPv4 tunneling whether globally assigned intra-site automatic IPv6-in-IPv4 tunneling whether globally assigned
or private IPv4 addresses are used. or private IPv4 addresses are used.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Applicability Statement . . . . . . . . . . . . . . . . . . 3 2. Applicability Statement . . . . . . . . . . . . . . . . . . . 3
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Non-Broadcast, Multiple Access (NBMA) Operation . . . . . . 4 5. Basic IPv6 Operation . . . . . . . . . . . . . . . . . . . . . 4
5.1 Multicast . . . . . . . . . . . . . . . . . . . . . . . . . 5 6. Automatic Tunneling . . . . . . . . . . . . . . . . . . . . . 5
5.2 Interface Identifiers and Address Construction . . . . . . . 5 7. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . . 7
5.3 ISATAP Link/Interface Configuration . . . . . . . . . . . . 5 8. Deployment Considerations . . . . . . . . . . . . . . . . . . 10
5.4 Link Layer Address Options . . . . . . . . . . . . . . . . . 6 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Automatic Tunneling . . . . . . . . . . . . . . . . . . . . 6 10. Security considerations . . . . . . . . . . . . . . . . . . . 11
6.1 Dual IP Layer Operation . . . . . . . . . . . . . . . . . . 6 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
6.2 Encapsulation . . . . . . . . . . . . . . . . . . . . . . . 6 Normative References . . . . . . . . . . . . . . . . . . . . . 12
6.3 Tunnel MTU and Fragmentation . . . . . . . . . . . . . . . . 7 Informative References . . . . . . . . . . . . . . . . . . . . 13
6.4 Handling IPv4 ICMP Errors . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
6.5 Local-Use IPv6 Unicast Addresses . . . . . . . . . . . . . . 8 A. Major Changes . . . . . . . . . . . . . . . . . . . . . . . . 15
6.6 Ingress Filtering . . . . . . . . . . . . . . . . . . . . . 8 B. Rationale for Interface Identifier Construction . . . . . . . 17
7. Neighbor Discovery for ISATAP Links . . . . . . . . . . . . 8 C. Dynamic MTU Discovery . . . . . . . . . . . . . . . . . . . . 18
7.1 Address Resolution . . . . . . . . . . . . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . . 22
7.2 Router and Prefix Discovery . . . . . . . . . . . . . . . . 9
7.2.1 Conceptual Data Structures . . . . . . . . . . . . . . . . . 9
7.2.2 Validity Checks for Router Advertisements . . . . . . . . . 10
7.2.3 Router Specification . . . . . . . . . . . . . . . . . . . . 11
7.2.4 Host Specification . . . . . . . . . . . . . . . . . . . . . 11
8. ISATAP Deployment Considerations . . . . . . . . . . . . . . 12
8.1 Host And Router Deployment Considerations . . . . . . . . . 12
8.2 Site Administration Considerations . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13
10. Security considerations . . . . . . . . . . . . . . . . . . 13
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
Normative References . . . . . . . . . . . . . . . . . . . . 14
Informative References . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
A. Major Changes . . . . . . . . . . . . . . . . . . . . . . . 17
B. Rationale for Interface Identifier Construction . . . . . . 18
C. Dynamic Per-neighbor MTU Discovery . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . 21
1. Introduction 1. Introduction
This document presents a simple approach that enables incremental This document presents a simple approach called the Intra-Site
deployment of IPv6 [1] within IPv4-based [2] sites. We refer to this Automatic Tunnel Addressing Protocol (ISATAP) that enables
approach as the Intra-Site Automatic Tunnel Addressing Protocol incremental deployment of IPv6 [1] within IPv4 [2] sites. ISATAP
(ISATAP). ISATAP allows dual-stack nodes that do not share a allows dual-stack nodes that do not share a physical link with an
physical link with an IPv6 router to automatically tunnel packets to IPv6 router to automatically tunnel packets to the IPv6 next-hop
the IPv6 next-hop address through IPv4, i.e., the site's IPv4 address through IPv4, i.e., the site's IPv4 infrastructure is treated
infrastructure is treated as a link layer for IPv6. as a link layer for IPv6.
This document specifies details for the operation of IPv6 over ISATAP Specific details for the operation of IPv6 and automatic tunneling
links (i.e., automatic IPv6-in-IPv4 tunneling), including an over ISATAP links are given, including an interface identifier format
interface identifier format that embeds an IPv4 address. This format that embeds an IPv4 address. This format supports IPv6 address
supports IPv6 protocol mechanisms for address configuration as well configuration and simple link-layer address mapping. Also specified
as simple link-layer address mapping. Also specified in this is the operation of IPv6 Neighbor Discovery and deployment/security
document is the operation of IPv6 Neighbor Discovery for ISATAP. The considerations.
document finally presents deployment and security considerations.
2. Applicability Statement 2. Applicability Statement
ISATAP provides the following features: ISATAP provides the following features:
o treats site's IPv4 infrastructure as a link layer for IPv6 using o treats site's IPv4 infrastructure as a link layer for IPv6 using
automatic IPv6-in-IPv4 tunneling (i.e., no configured tunnel automatic IPv6-in-IPv4 tunneling
state)
o enables incremental deployment of IPv6 hosts within IPv4 sites o enables incremental deployment of IPv6 hosts within IPv4 sites
with no aggregation scaling issues at border gateways with no aggregation scaling issues at border gateways
o requires no special IPv4 services within the site (e.g., o requires no special IPv4 services within the site (e.g.,
multicast) multicast)
o supports both stateless address autoconfiguration and manual o supports both stateless address autoconfiguration and manual
configuration configuration
o supports networks that use non-globally unique IPv4 addresses o supports networks that use non-globally unique IPv4 addresses
(e.g., when private address allocations [3] are used), but does (e.g., when private address allocations [18] are used)
not allow the virtual ISATAP link to span a Network Address
Translator [4]
o compatible with other NGTRANS mechanisms (e.g., 6to4 [19]) o compatible with other NGTRANS mechanisms (e.g., 6to4 [19])
3. Requirements 3. Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [5]. document, are to be interpreted as described in [3].
This document also makes use of internal conceptual variables to This document also makes use of internal conceptual variables to
describe protocol behavior and external variables that an describe protocol behavior and external variables that an
implementation must allow system administrators to change. The implementation must allow system administrators to change. The
specific variable names, how their values change, and how their specific variable names, how their values change, and how their
settings influence protocol behavior are provided to demonstrate settings influence protocol behavior are provided to demonstrate
protocol behavior. An implementation is not required to have them in protocol behavior. An implementation is not required to have them in
the exact form described here, so long as its external behavior is the exact form described here, so long as its external behavior is
consistent with that described in this document. consistent with that described in this document.
4. Terminology 4. Terminology
The terminology of RFC 2460 [1] applies to this document. The The terminology of RFC 2460 [1] applies to this document. The
following additional terms are defined: following additional terms are defined:
link, on-link, off-link: link, on-link, off-link:
same definitions as ([6], section 2.1). same definitions as ([4], section 2.1).
underlying link: underlying link:
a link layer that supports IPv4 (for ISATAP), and MAY also support a link layer that supports IPv4 (for ISATAP), and MAY also support
IPv6 natively. IPv6 natively.
ISATAP link: ISATAP link:
one or more underlying links used for tunneling. The IPv4 network one or more underlying links used for tunneling. The IPv4 network
layer addresses of the underlying links are used as link-layer layer addresses of the underlying links are used as link-layer
addresses on the ISATAP link. addresses on the ISATAP link.
ISATAP interface: ISATAP interface:
a node's attachment to an ISATAP link. a node's attachment to an ISATAP link.
advertising ISATAP interface:
same meaning as "advertising interface" in ([4], section 6.2.2).
ISATAP address: ISATAP address:
an on-link address on an ISATAP interface and with an interface an on-link address on an ISATAP interface and with an interface
identifier constructed as specified in Section 5.2 identifier constructed as specified in Section 5.2
ISATAP router: 5. Basic IPv6 Operation
an IPv6 node that has an ISATAP interface over which it forwards
packets not explicitly addressed to itself.
ISATAP host:
any node that has an ISATAP interface and is not an ISATAP router.
5. Non-Broadcast, Multiple Access (NBMA) Operation
ISATAP links transmit IPv6 packets via automatic tunnels using the ISATAP links transmit IPv6 packets via automatic tunnels using the
site's IPv4 infrastructure as a link layer for IPv6, i.e., IPv6 site's IPv4 infrastructure as a link layer for IPv6, i.e., IPv6
treats the site's IPv4 infrastructure as a Non-Broadcast, Multiple treats the site's IPv4 infrastructure as a Non-Broadcast, Multiple
Access (NBMA) link layer. RFC 2491 [7] provides a general Access (NBMA) link layer. The following considerations for IPv6 on
architecture for IPv6 over NBMA networks that forms the basis for ISATAP links are noted:
companion documents such as the present. The following subsections
present NBMA considerations for IPv6 on ISATAP links:
5.1 Multicast
ISATAP links most closely meet the description for connectionless
service found in the last paragraph of ([7], section 1), i.e., ISATAP
addresses provide the sender with an NBMA destination address to
which it can transmit packets whenever it desires. Thus, multicast
emulation mechanisms are not required to support host-side operation
of the IPv6 neighbor discovery protocol.
5.2 Interface Identifiers and Address Construction 5.1 Interface Identifiers and Unicast Addresses
([7], section 5.1) requires companion documents to specify the exact ISATAP interface identifiers use "modified EUI-64" format ([5],
mechanism for generating interface tokens (i.e., identifiers). section 2.5.1) and are formed by appending an IPv4 address on the
Interface identifiers for ISATAP are compatible with the EUI-64 ISATAP link to the 32-bit string '00-00-5E-FE'. Appendix B includes
identifier format ([8], section 2.5.1), and are constructed by non-normative rationale for this construction rule.
appending an IPv4 address on the ISATAP link to the 32-bit string
'00-00-5E-FE'. (Appendix B includes non-normative text explaining
the rationale for this construction rule.)
Global and Local-use ISATAP addresses are constructed as follows: With reference to ([5], sections 2.5.4, 2.5.6), global and local-use
ISATAP addresses are constructed as follows:
| 64 bits | 32 bits | 32 bits | | 64 bits | 32 bits | 32 bits |
+------------------------------+---------------+----------------+ +------------------------------+---------------+----------------+
| global or local-use unicast | 0000:5EFE | IPv4 Address | | global or local-use unicast | 0000:5EFE | IPv4 Address |
| prefix | | of ISATAP link | | prefix | | of ISATAP link |
+------------------------------+---------------+----------------+ +------------------------------+---------------+----------------+
Figure 1 5.2 ISATAP Link/Interface Configuration
For example, the global unicast address:
3FFE:1A05:510:1111:0:5EFE:8CAD:8108
has a prefix of '3FFE:1A05:510:1111::/64' and an ISATAP interface
identifier with embedded IPv4 address: '140.173.129.8'. The address
is alternately written as:
3FFE:1A05:510:1111:0:5EFE:140.173.129.8
Examples for local-use addresses are obvious from the above and with
reference to ([8], section 2.5.6).
5.3 ISATAP Link/Interface Configuration
ISATAP Link/Interface configuration is consistent with ([7], sections
5.1.1 and 5.1.2).
An ISATAP link consists of one or more underlying links that support An ISATAP link consists of one or more underlying links that support
IPv4 for tunneling within a site. ISATAP interfaces are configured IPv4 for tunneling within a site.
over ISATAP links; each IPv4 address assigned to an underlying link
is seen as a link-layer address for ISATAP.
5.4 Link Layer Address Options ISATAP interfaces are configured over ISATAP links; each IPv4 address
assigned to an underlying link is seen as a link-layer address for
ISATAP.
([7], section 5.2) requires companion documents to specify the At least one link-layer address per advertising ISATAP interface
contents of the [NTL], [STL], [NBMA Number] and [NBMA Subaddress] SHOULD be added to the Potential Routers List (see Section 7.3.1).
fields for link layer address options. For ISATAP links:
o the [NTL] and [STL] fields MUST be zero 5.3 Link Layer Address Options
o the [NBMA Number] encodes a 4-octet IPv4 address With reference to ([6], section 5.2), when the [NTL] and [STL] fields
in an ISATAP link layer address option encode 0, the [NBMA Number]
field encodes a 4-octet IPv4 address.
o the [NBMA Subaddress] field is omitted 5.4 Multicast and Anycast
([7], section 5.2) does NOT require companion documents to specify As for any IPv6 interface, an ISATAP interface is required to
the value for [Length], i.e., the total length of the option in 8 recognize certain IPv6 multicast and anycast addresses ([5], section
octets. Senders may therefore set [Length] to any value between 1 2.8). Mechanisms for sending multicast and anycast packets (e.g.,
and 255; when [Length] is greater than 1, receivers treat any bytes [20]) are left as future work.
that follow the [NBMA Number] as null-padding.
6. Automatic Tunneling 6. Automatic Tunneling
The common tunneling mechanisms specified in ([9], sections 2 and 3) The common tunneling mechanisms specified in ([7], sections 2 and 3)
are used, with the following noted specific considerations for are used, with the following noted considerations for ISATAP:
ISATAP:
6.1 Dual IP Layer Operation 6.1 Dual IP Layer Operation
ISATAP uses the same specification found in ([9], section 2). That ISATAP uses the same specification found in ([7], section 2). That
is, ISATAP nodes provide complete IPv4 and IPv6 implementations and is, ISATAP nodes provide complete IPv4 and IPv6 implementations and
are able to send and receive both IPv4 and IPv6 packets. ISATAP are able to send and receive both IPv4 and IPv6 packets.
nodes operate with both their IPv4 and IPv6 stacks enabled.
Address configuration considerations are the same as for ([9],
section 2.1). Additionally, ISATAP nodes require that IPv4 address
configuration take place on at least one underlying link prior to
IPv6 address configuration on an ISATAP link.
DNS considerations are the same as ([9], sections 2.2 and 2.3). Address configuration and DNS considerations are the same as ([7],
sections 2.1 through 2.3).
6.2 Encapsulation 6.2 Encapsulation
The specification in ([9], section 3.1) is used. Additionally, the The specification in ([7], section 3.1) is used. Additionally, the
IPv6 next-hop address for packets sent on an ISATAP link MUST be an IPv6 next-hop address for packets sent on an ISATAP link MUST be an
ISATAP address; other packets are discarded and an ICMPv6 destination ISATAP address; other packets are discarded and an ICMPv6 destination
unreachable indication with code 3 (Address Unreachable) [10] is unreachable indication with code 3 (Address Unreachable) [8] is
returned to the source. returned to the source.
6.3 Tunnel MTU and Fragmentation 6.3 Tunnel MTU and Fragmentation
The specification in ([9], section 3.2) is NOT used; the ISATAP automatic tunnel interfaces may be configured over multiple
specification in this section is used instead. underlying links with diverse maximum transmission units (MTUs). The
minimum MTU for IPv6 interfaces is 1280 bytes ([1], Section 5), but
ISATAP uses automatic tunnel interfaces that may be configured over the following considerations apply for ISATAP interfaces:
multiple underlying links with diverse maximum transmission units
(MTUs). The minimum MTU for IPv6 interfaces is 1280 bytes ([1],
Section 5), but the following considerations for the MTU of ISATAP
interfaces apply:
o Nearly all IPv4 nodes connect to physical links with MTUs of 1500 o Nearly all IPv4 nodes connect to physical links with MTUs of 1500
bytes or larger (e.g., Ethernet) bytes or larger (e.g., Ethernet)
o Sub-IPv4 layer encapsulations (e.g., VPN) may occur on some paths o Sub-IPv4 layer encapsulations (e.g., VPN) may occur on some paths
o Commonly-deployed VPNs use an MTU of 1400 bytes o Commonly-deployed VPN interfaces use an MTU of 1400 bytes
Unless a dynamic per-neighbor MTU discovery mechanism is implemented,
ISATAP interfaces MUST use an MTU (ISATAP_MTU) of no more than 1380
bytes (1400 minus 20 bytes for IPv4 encapsulation) to maximize
efficiency and minimize IPv4 fragmentation for the predominant
deployment case. ISATAP_MTU MAY be set to a larger value when the
encapsulator implements a dynamic per-neighbor MTU discovery
mechanism, but this value SHOULD NOT exceed the largest MTU of all
underlying links (minus 20 bytes for IPv4 encapsulation). Appendix C
provides non-normative considerations for dynamic per-neighbor MTU
discovery.
The network layer (IPv6) forwards packets of size ISATAP_MTU or
smaller to the ISATAP interface. All other packets are dropped, and
an ICMPv6 "packet too big" message with MTU = ISATAP_MTU is returned
to the source [11]. The ISATAP link layer encapsulates packets of
size 1380 bytes or smaller with the Don't Fragment (DF) bit NOT set
in the encapsualting IPv4 header.
Nodes that configure ISATAP interfaces MUST have IPv4 reassembly
buffers large enough to receive packets with the DF bit not set in
the encapsulating IPv4 header. RFC 1122 [12], section 3.3.2
specifies that the Effective MTU to Receive (EMTU_R) for IPv4 nodes:
"...MUST be greater than or equal to 576, SHOULD be either To maximize efficiency and minimize IPv4 fragmentation for the
configurable or indefinite, and SHOULD be greater than or equal to predominant deployment case, ISATAP interfaces that do not use a
the MTU of the connected network(s)". dynamic MTU discovery mechanism SHOULD set LinkMTU ([4], Section
6.3.2 ) to no more than 1380 bytes (1400 minus 20 bytes for IPv4
encapsulation). LinkMTU MAY be set to larger values on ISATAP
interfaces that use a dynamic MTU discovery mechanism. Appendix C
provides non-normative considerations for dynamic MTU discovery.
With reference to this specification, the EMTU_R for nodes that The ISATAP link layer encapsulates packets of size 1380 or smaller
configure ISATAP interfaces MUST be greater than or equal to 1500 with the Don't Fragment (DF) bit not set in the encapsualting IPv4
bytes (i.e., the predominant deployment case for connected IPv4 header.
networks) and SHOULD be either configurable or indefinite.
6.4 Handling IPv4 ICMP Errors 6.4 Handling IPv4 ICMP Errors
The specification in ([9], section 3.4) MAY be used. IPv4 ICMP IPv4 ICMP errors and ARP failures are processed as link error
errors and ARP failures are otherwise processed as link error
notifications. notifications.
6.5 Local-Use IPv6 Unicast Addresses 6.5 Local-Use IPv6 Unicast Addresses
The specification in ([9], section 3.7) is NOT used. Instead, local The specification in ([7], section 3.7) is not used. Instead, local
use IPv6 unicast addresses are formed exactly as specified in ([8], use IPv6 unicast addresses are formed as specified in Section 5.1.
section 2.5.6).
6.6 Ingress Filtering 6.6 Ingress Filtering
The specification in ([9], section 3.9) is used on ISATAP router The specification in ([7], section 3.9) is used. In particular,
interfaces. (ISATAP host interfaces silently discard any packets ISATAP nodes that forward decapsulated packets MUST be configured
received with a foreign IPv6 destination address, i.e., an address with a list of source IPv4 address prefixes that are acceptable.
not configured on the local IPv6 stack.)
Additionally, packets received on ISATAP host and router interfaces
MUST satisfy at least one (i.e., one or both) of the following
validity checks:
o the network-layer (IPv6) source address is an on-link ISATAP
address with an interface identifier that embeds the link-layer
(IPv4) source address
o the link-layer (IPv4) source address is in the Potential Routers
List (see Section 7.2.1)
Packets that do not satisfy the above conditions are silently
discarded.
7. Neighbor Discovery for ISATAP Links 7. Neighbor Discovery
RFC 2461 [6] provides the following guidelines for non-broadcast RFC 2461 [4] provides the following guidelines for non-broadcast
multiple access (NBMA) link support: multiple access (NBMA) link support:
"Redirect, Neighbor Unreachability Detection and next-hop "Redirect, Neighbor Unreachability Detection and next-hop
determination should be implemented as described in this document. determination should be implemented as described in this document.
Address resolution and the mechanism for delivering Router Address resolution and the mechanism for delivering Router
Solicitations and Advertisements on NBMA links is not specified in Solicitations and Advertisements on NBMA links is not specified in
this document." this document."
ISATAP links SHOULD implement Redirect, Neighbor Unreachability ISATAP links SHOULD implement Redirect, Neighbor Unreachability
Detection, and next-hop determination exactly as specified in [6]. Detection, and next-hop determination exactly as specified in [4].
Address resolution and the mechanisms for delivering Router Address resolution and the mechanisms for delivering Router
Solicitations and Advertisements for ISATAP links are not specified Solicitations and Advertisements for ISATAP links are not specified
by [6]; instead, they are specified in this document. (Note that by [4]; instead, they are specified in this document.
these mechanisms MAY potentially apply to other types of NBMA links
in the future.)
7.1 Address Resolution 7.1 Address Resolution and Neighbor Unreachability Detection
ISATAP addresses are resolved to link-layer addresses (IPv4) by a ISATAP addresses are resolved to link-layer addresses (IPv4) by a
static computation, i.e., the last four octets are treated as an IPv4 static computation, i.e., the last four octets are treated as an IPv4
address. address.
Following static address resolution, ISATAP hosts SHOULD perform an Following static address resolution, hosts SHOULD perform an initial
initial reachability confirmation by sending unicast Neighbor reachability confirmation by sending unicast Neighbor Solicitations
Solicitations (NSs) and receiving a Neighbor Advertisement using the (NSs) and receiving a Neighbor Advertisement using the mechanisms
mechanisms specified in ([6], sections 7.2.2-7.2.8). (Note that specified in ([4], sections 7.2.2-7.2.8).
implementations MAY omit the source/target link layer options in NS/
NA messages when unicast is used.)
ISATAP hosts SHOULD additionally perform Neighbor Unreachability Hosts SHOULD additionally perform Neighbor Unreachability Detection
Detection (NUD) as specified in ([6], section 7.3). ISATAP routers (NUD) as specified in ([4], section 7.3). Routers MAY perform the
MAY perform the above-specified reachability detection and NUD above-specified reachability detection and NUD procedures, but this
procedures, but this might not scale in all environments. might not scale in all environments.
All ISATAP nodes MUST send solicited neighbor advertisements ([6], All ISATAP nodes MUST send solicited neighbor advertisements ([4],
section 7.2.4). section 7.2.4).
7.2 Router and Prefix Discovery 7.2 Duplicate Address Detection
Since NBMA multicast emulation mechanisms are not used, ISATAP nodes Duplicate Address Detection ([9], section 5.4) is not required for
will not receive unsolicited multicast Router Advertisements. Thus, ISATAP addresses, since duplicate address detection is assumed
alternate mechanisms are required and specified below: already performed for the IPv4 addresses from which they derive.
7.2.1 Conceptual Data Structures 7.3 Router and Prefix Discovery
Since ISATAP nodes will typically not receive unsolicited multicast
Router Advertisements, unicast mechanisms are required as specified
below:
7.3.1 Conceptual Data Structures
ISATAP nodes use the conceptual data structures Prefix List and ISATAP nodes use the conceptual data structures Prefix List and
Default Router List exactly as in ([6], section 5.1). ISATAP links Default Router List exactly as in ([4], section 5.1). ISATAP links
add a new conceptual data structures "Potential Router List" and the add a new conceptual data structure "Potential Router List" and the
following new configuration variable: following new configuration variable:
ResolveInterval ResolveInterval
Time between name service resolutions. Default and suggested Time between name service resolutions. Default and suggested
minimum: 1hr minimum: 1hr
A Potential Router List (PRL) is associated with every ISATAP link. A Potential Router List (PRL) is associated with every ISATAP link.
The PRL provides a trust basis for router validation (see security Each entry in the PRL has an IPv4 address and an associated timer.
considerations). Each entry in the PRL has an IPv4 address and an The IPv4 address represents an advertising ISATAP interface, and is
associated timer. The IPv4 address represents a router's ISATAP used to construct the link-local ISATAP address for that interface.
interface (likely to be an "advertising interface"), and is used to The following sections specify the process for initializing the PRL:
construct the ISATAP link-local address for that interface. The
following sections specify the process for initializing the PRL:
When a node enables an ISATAP link, it first discovers IPv4 addresses When a node enables an ISATAP link, it discovers IPv4 addresses for
for the PRL. The addresses SHOULD be established by a DHCPv4 [13] the PRL. The addresses MAY be established by a DHCPv4 [10] option
option for ISATAP (option code TBD), by manual configuration, or by for ISATAP (option code TBD), manual configuration, or an unspecified
an unspecified alternate method (e.g., DHCPv4 vendor-specific alternate method (e.g., DHCPv4 vendor-specific option).
option).
When no other mechanisms are available, a DNS fully-qualified domain When no other mechanisms are available, a DNS fully-qualified domain
name (FQDN) [20] MAY be used. In this case, the FQDN is resolved name (FQDN) [21] established by an out-of-band method (e.g., DHCPv4,
into IPv4 addresses for the PRL through a static host file, a manual configuration, etc.) MAY be used. The FQDN is resolved into
site-specific name service, or by querying an IPv4-based DNS server. IPv4 addresses through a static host file, a site-specific name
Unspecified alternate methods for domain name resolution may also be service, querying a DNS server within the site, or an unspecified
used. The following notes apply: alternate method. The following notes apply:
1. Site administrators maintain a list of IPv4 addresses 1. Site administrators maintain a list of IPv4 addresses
representing ISATAP router interfaces and make them available via representing advertising ISATAP interfaces and make them
one or more of the mechanisms described above. available via one or more of the mechanisms described above.
2. There are no mandatory rules for the selection of a FQDN, but 2. There are no mandatory rules for the selection of a FQDN, but
administrators are encouraged to use the convention manual configuration MUST be supported.
"isatap.domainname" (e.g., isatap.example.com).
3. After initialization, nodes periodically re-initialize the PRL 3. After initialization, nodes periodically re-initialize the PRL
(after ResolveInterval). When DNS is used, client DNS resolvers (e.g., after ResolveInterval). When DNS is used, client
use the IPv4 transport to resolve the names and follow the cache resolvers use the IPv4 transport.
invalidation procedures in [20] when the DNS time-to-live
expires.
7.2.2 Validity Checks for Router Advertisements 7.3.2 Validation of Router Advertisements Messages
A node MUST silently discard any Router Advertisement messages it The specification in ([4], section 6.1.2) is used.
receives that do not satisfy both the validity checks in ([6],
section 6.1.2) and the following additional validity check for Additionally, received RA messages that contain Prefix Information
ISATAP: options and/or encode non-zero values in the Cur Hop Limit, Router
Lifetime, Reachable Time, or Retrans Timer fields (see: [4], section
4.2) MUST satisfy the following validity check for ISATAP:
o the network-layer (IPv6) source address is an ISATAP address and o the network-layer (IPv6) source address is an ISATAP address and
embeds an IPv4 address from the PRL embeds an IPv4 address from the PRL
7.2.3 Router Specification 7.3.3 Router Specification
Advertising ISATAP interfaces of routers behave the same as Routers with advertising ISATAP interfaces behave the same as
advertising interfaces described in ([6], section 6.2). However, described in ([4], section 6.2). Advertising ISATAP interfaces send
periodic unsolicited multicast Router Advertisements are not used, RA messages to a node's unicast address, as permitted by ([4],
thus the "interval timer" associated with advertising interfaces is section 6.2.6).
not used for that purpose.
When an ISATAP router receives a valid Router Solicitation on an 7.3.4 Host Specification
advertising ISATAP interface, it replies with a unicast Router
Advertisement to the address of the node which sent the Router
Solicitation. The source address of the Router Advertisement is a
link-local unicast address associated with the interface. This MAY
be the same as the destination address of the Router Solicitation.
ISATAP routers MAY engage in the solicitation process described under
Host Specification below, e.g., if Router Advertisement consistency
verification ([6], section 6.2.7) is desired.
7.2.4 Host Specification 7.3.4.1 Sending Router Solicitations
All entries in the PRL are assumed to represent active ISATAP routers All entries in the PRL are assumed to represent active advertising
within the site, i.e., the PRL provides trust basis only; not ISATAP interfaces within the site, i.e., the PRL provides trust basis
reachability detection. ISATAP nodes SHOULD use stateful only; not reachability detection. Hosts periodically solicit
configuration to assign IPv6 prefixes and default router information. information from one or more entries in the PRL ("PRL(i)") by sending
When stateful configuration is not available, hosts MAY periodically unicast Router Solicitation (RS) messages using PRL(i)'s IPv4 address
solicit information from one or more entries in the PRL ("PRL(i)") by ("V4ADDR_PRL(i)") and associated timer ("TIMER(i)"). The manner of
sending unicast Router Solicitation messages using the IPv4 address selecting a PRL(i) for solicitation and/or deprecating a
("V4ADDR_PRL(i)") and associated timer in the entry. Hosts add the previously-selected PRL(i) is outside the scope of this
following variable to support the solicitation process: specification. Hosts add the following variable to support the
solicitation process:
MinRouterSolicitInterval MinRouterSolicitInterval
Minimum time between sending Router Solicitations to any router. Minimum time between sending Router Solicitations. Default and
Default and suggested minimum: 15min. suggested minimum: 15min.
When a PRL(i) is selected, the host sets its associated timer to When a PRL(i) is selected, the host sets TIMER(i) to
MinRouterSolicitInterval and initiates solicitation following a short MinRouterSolicitInterval and initiates solicitation following a short
delay as in ([6], section 6.3.7). The manner of choosing particular delay. Solicitation consists of sending RS messages to the ISATAP
routers in the PRL for solicitation is outside the scope of this link-local address constructed from V4ADDR_PRL(i), i.e., they are
specification. The solicitation process repeats when the associated sent to 'FE80::0:5EFE:V4ADDR_PRL(i)' instead of
timer expires. 'All-Routers-multicast'. They are otherwise sent exactly as in ([4],
section 6.3.7).
Solicitation consists of sending Router Solicitations to the ISATAP 7.3.4.2 Processing Router Advertisements
link-local address constructed from the entry's IPv4 address, i.e.,
they are sent to 'FE80::0:5EFE:V4ADDR_PRL(i)' instead of 'All-Routers
multicast'. They are otherwise sent exactly as in ([6], section
6.3.7).
Hosts process received Router Advertisements exactly as in ([6], Hosts process received RA messages exactly as in ([4], section 6.3.4)
section 6.3.4). Hosts additionally reset the timer associated with and ([9], section 5.5.3) except that, when an RA message contains an
the V4ADDR_PRL(i) embedded in the network-layer source address in MTU option, hosts SHOULD NOT copy the option's value into the ISATAP
each solicited Router Advertisement received. The timer is reset to interface LinkMTU. Instead, when the ISATAP link layer implements a
either 0.5 * (the minimum value in the router lifetime or valid per-neighbor path MTU cache, hosts SHOULD copy the MTU option's value
lifetime of any on-link prefixes received in the advertisement) or into the cache entry for the router that sent the RA message (see:
MinRouterSolicitInterval; whichever is longer. Appendix C).
8. ISATAP Deployment Considerations When the network-layer source address in an RA message is an ISATAP
address that embeds V4ADDR_PRL(i) for some PRL(i) selected for
solicitation, hosts additionally reset TIMER(i). Let "MIN_LIFETIME"
be the minimum value in the router lifetime or valid lifetime of any
prefixes advertised in the RA message. Then, TIMER(i) is reset to:
MAX((0.5 * MIN_LIFETIME), MinRouterSolicitInterval)
8. Deployment Considerations
8.1 Host And Router Deployment Considerations 8.1 Host And Router Deployment Considerations
For hosts, if an underlying link supports both IPv4 (over which For hosts, if an underlying link supports both IPv4 (over which
ISATAP is implemented) and also supports IPv6 natively, then ISATAP ISATAP is implemented) and also supports IPv6 natively, then ISATAP
MAY be enabled if the native IPv6 layer does not receive Router MAY be enabled if the native IPv6 layer does not receive Router
Advertisements (i.e., does not have connection with an IPv6 router). Advertisements (i.e., does not have connection with an IPv6 router).
After a non-link-local address has been configured and a default After a non-link-local address has been configured and a default
router acquired on the native link, the host SHOULD discontinue the router acquired on the native link, the host SHOULD discontinue the
router solicitation process described in the host specification and router solicitation process described in the host specification and
allow existing ISATAP address configurations to expire as specified allow existing ISATAP address configurations to expire as specified
in ([6], section 5.3) and ([14], section 5.5.4). Any ISATAP in ([4], section 5.3) and ([9], section 5.5.4). Any ISATAP addresses
addresses added to the DNS for this host should also be removed. In added to the DNS for this host should also be removed. In this way,
this way, ISATAP use will gradually diminish as IPv6 routers are ISATAP use will gradually diminish as IPv6 routers are widely
widely deployed throughout the site. deployed throughout the site.
Routers MAY configure an interface to simultaneously support both Routers MAY configure both a native IPv6 and ISATAP interface over
native IPv6, and also ISATAP (over IPv4). Routing will operate as the same physical link. Routing will operate as usual between these
usual between these two domains. Note that the prefixes used on the two domains. Note that the prefixes used on the ISATAP and native
ISATAP and native IPv6 interfaces will be distinct. The IPv4 IPv6 interfaces will be distinct. The IPv4 address(es) configured on
address(es) configured on a router's ISATAP interface(s) SHOULD be a router's advertising ISATAP interface(s) SHOULD be added (either
added (either automatically or manually) to the site's address automatically or manually) to the site's address records for
records for ISATAP router interfaces. advertising ISATAP interfaces.
8.2 Site Administration Considerations 8.2 Site Administration Considerations
The following considerations are noted for sites that deploy ISATAP: The following considerations are noted for sites that deploy ISATAP:
o ISATAP links are administratively defined by a set of router o ISATAP links are administratively defined by a set of advertising
interfaces and set of nodes which discover those interface and ISATAP interfaces and set of nodes which discover those interface
server addresses Thus, ISATAP links are defined by administrative addresses. Thus, ISATAP links are defined by administrative (not
(not physical) boundaries. physical) boundaries.
o ISATAP hosts and routers can be deployed in an ad-hoc and
independent fashion. In particular, ISATAP hosts can be deployed
with little/no advanced knowledge of existing ISATAP routers, and
ISATAP routers can deployed with no reconfiguration requirements
for hosts.
o When stateful autoconfiguration is not available, ISATAP nodes MAY o Hosts and routers that use ISATAP can be deployed in an ad-hoc
periodically send unicast Router Solicitations to and receive fashion. In particular, hosts can be deployed with little/no
unicast Router Advertisements from to one or more members of the advanced knowledge of existing routers, and routers can deployed
potential router list. A well-deployed stateful autoconfiguration with no reconfiguration requirements for hosts.
service within the site can minimize and/or eliminate the need for
periodic solicitation.
o ISATAP nodes periodically refresh the entries on the PRL. o ISATAP nodes periodically refresh the entries on the PRL.
Responsible site administration can reduce the control traffic. Responsible site administration can reduce the control traffic.
At a minimum, administrators SHOULD ensure that dynamically At a minimum, administrators SHOULD ensure that dynamically
advertised information for the site's PRL is well maintained. advertised information for the site's PRL is well maintained.
9. IANA Considerations 9. IANA Considerations
A DHCPv4 option code for ISATAP (TBD) [21] is requested in the event A DHCPv4 option code for ISATAP (TBD) [22] may be requested in the
that the IESG recommends this document for standards track. event that this document (or a derivative thereof) is moved to
standards track.
10. Security considerations 10. Security considerations
Site administrators are advised that, in addition to possible attacks ISATAP site border routers and firewalls MUST implement IPv6 ingress
against IPv6, security attacks against IPv4 MUST also be considered. filtering and MUST NOT allow packets with site-local source and/or
destination addresses (i.e., addresses with prefix FEC0::/10) to
Responsible IPv4 site security management is strongly encouraged. In enter or leave the site.
particular, border gateways SHOULD implement filtering to detect
spoofed IPv4 source addresses at a minimum; ip-protocol-41 filtering
SHOULD also be implemented.
If IPv4 source address filtering is not correctly implemented, the In addition to possible attacks against IPv6, security attacks
ISATAP validity checks will not be effective in preventing IPv6 against IPv4 must also be considered. In particular, border routers
source address spoofing. and firewalls MUST implement IPv4 ingress filtering and
ip-protocol-41 filtering.
If filtering for ip-protocol-41 is not correctly implemented, IPv6 Even with IPv4 and IPv6 ingress filtering, reflection attacks can
source address spoofing is clearly possible, but this can be originate from nodes within an ISATAP site that spoof IPv6 source
eliminated if both IPv4 source address filtering, and the ISATAP addresses. Security mechanisms for reflection attack mitigation
validity checks are implemented. (e.g., [11], [12], etc.) SHOULD be used in routers with advertising
ISATAP interfaces. At a minimum, ISATAP site border gateways MUST
log potential source address spoofing cases.
(RFC 2461 [6]), section 6.1.2 implies that nodes trust Router (RFC 2461 [4], section 6.1.2) implies that nodes trust received
Advertisements they receive from on-link routers, as indicated by a Router Advertisement (RA) messages from on-link routers, as indicated
value of 255 in the IPv6 'hop-limit' field. Since this field is not by a value of 255 in the IPv6 'hop-limit' field. ISATAP links
decremented when ip-protocol-41 packets traverse multiple IPv4 hops require an additional validation check for received RA messages (see:
([9], section 3), ISATAP links require a different trust model. In Section 7.3.2).
particular, ONLY those Router Advertisements received from a member
of the Potential Routers List are trusted; all others are silently
discarded. This trust model is predicated on IPv4 source address
filtering, as described above.
The ISATAP address format does not support privacy extensions for ISATAP addresses do not support privacy extensions for stateless
stateless address autoconfiguration [22]. However, since the ISATAP address autoconfiguration [23]. However, since the ISATAP interface
interface identifier is derived from the node's IPv4 address, ISATAP identifier is derived from the node's IPv4 address, ISATAP addresses
addresses do not have the same level of privacy concerns as IPv6 do not have the same level of privacy concerns as IPv6 addresses that
addresses that use an interface identifier derived from the MAC use an interface identifier derived from the MAC address. (This is
address. (This issue is the same for NAT'd addresses.) especially true when private address allocations [18] are used.)
11. Acknowledgements 11. Acknowledgements
Some of the ideas presented in this draft were derived from work at Some of the ideas presented in this draft were derived from work at
SRI with internal funds and contractual support. Government sponsors SRI with internal funds and contractual support. Government sponsors
who supported the work include Monica Farah-Stapleton and Russell who supported the work include Monica Farah-Stapleton and Russell
Langan from U.S. Army CECOM ASEO, and Dr. Allen Moshfegh from U.S. Langan from U.S. Army CECOM ASEO, and Dr. Allen Moshfegh from U.S.
Office of Naval Research. Within SRI, Dr. Mike Frankel, J. Peter Office of Naval Research. Within SRI, Dr. Mike Frankel, J. Peter
Marcotullio, Lou Rodriguez, and Dr. Ambatipudi Sastry supported the Marcotullio, Lou Rodriguez, and Dr. Ambatipudi Sastry supported the
work and helped foster early interest. work and helped foster early interest.
skipping to change at page 14, line 29 skipping to change at page 12, line 25
The following peer reviewers are acknowledged for taking the time to The following peer reviewers are acknowledged for taking the time to
review a pre-release of this document and provide input: Jim Bound, review a pre-release of this document and provide input: Jim Bound,
Rich Draves, Cyndi Jung, Ambatipudi Sastry, Aaron Schrader, Ole Rich Draves, Cyndi Jung, Ambatipudi Sastry, Aaron Schrader, Ole
Troan, Vlad Yasevich. Troan, Vlad Yasevich.
The authors acknowledge members of the NGTRANS community who have The authors acknowledge members of the NGTRANS community who have
made significant contributions to this effort, including Rich Draves, made significant contributions to this effort, including Rich Draves,
Alain Durand, Nathan Lutchansky, Karen Nielsen, Art Shelest, Margaret Alain Durand, Nathan Lutchansky, Karen Nielsen, Art Shelest, Margaret
Wasserman, and Brian Zill. Wasserman, and Brian Zill.
The authors also wish to acknowledge the work of Quang Nguyen [23] The authors also wish to acknowledge the work of Quang Nguyen [24]
under the guidance of Dr. Lixia Zhang that proposed very similar under the guidance of Dr. Lixia Zhang that proposed very similar
ideas to those that appear in this document. This work was first ideas to those that appear in this document. This work was first
brought to the authors' attention on September 20, 2002. brought to the authors' attention on September 20, 2002.
Normative References Normative References
[1] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) [1] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998. Specification", RFC 2460, December 1998.
[2] Postel, J., "Internet Protocol", STD 5, RFC 791, September [2] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981. 1981.
[3] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E. [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Lear, "Address Allocation for Private Internets", BCP 5, RFC
1918, February 1996.
[4] Egevang, K. and P. Francis, "The IP Network Address Translator
(NAT)", RFC 1631, May 1994.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[6] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery [4] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998. for IP Version 6 (IPv6)", RFC 2461, December 1998.
[7] Armitage, G., Schulter, P., Jork, M. and G. Harter, "IPv6 over [5] Hinden, R. and S. Deering, "IP Version 6 Addressing
Non-Broadcast Multiple Access (NBMA) networks", RFC 2491,
January 1999.
[8] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", draft-ietf-ipngwg-addr-arch-v3-11 (work in Architecture", draft-ietf-ipngwg-addr-arch-v3-11 (work in
progress), October 2002. progress), October 2002.
[9] Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms for [6] Armitage, G., Schulter, P., Jork, M. and G. Harter, "IPv6 over
Non-Broadcast Multiple Access (NBMA) networks", RFC 2491,
January 1999.
[7] Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms for
IPv6 Hosts and Routers", draft-ietf-ngtrans-mech-v2-01 (work in IPv6 Hosts and Routers", draft-ietf-ngtrans-mech-v2-01 (work in
progress), November 2002. progress), November 2002.
[10] Conta, A. and S. Deering, "Internet Control Message Protocol [8] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6) (ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 2463, December 1998. Specification", RFC 2463, December 1998.
[11] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for [9] Thomson, S. and T. Narten, "IPv6 Stateless Address
IP version 6", RFC 1981, August 1996. Autoconfiguration", RFC 2462, December 1998.
[12] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.
[13] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, [10] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997. March 1997.
[14] Thomson, S. and T. Narten, "IPv6 Stateless Address [11] Savola, P., "Security Considerations for 6to4",
Autoconfiguration", RFC 2462, December 1998. draft-savola-ngtrans-6to4-security-01 (work in progress), March
2002.
[15] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, [12] Bellovin, S., Leech, M. and T. Taylor, "ICMP Traceback
Messages", draft-ietf-itrace-03 (work in progress), January
2003.
[13] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990. November 1990.
[16] Postel, J., "Internet Control Message Protocol", STD 5, RFC [14] Postel, J., "Internet Control Message Protocol", STD 5, RFC
792, September 1981. 792, September 1981.
[17] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, [15] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
June 1995. June 1995.
[18] Droms, R., "Dynamic Host Configuration Protocol for IPv6 [16] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for
(DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress), IP version 6", RFC 1981, August 1996.
November 2002.
[17] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.
Informative References Informative References
[18] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E.
Lear, "Address Allocation for Private Internets", BCP 5, RFC
1918, February 1996.
[19] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via [19] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via
IPv4 Clouds", RFC 3056, February 2001. IPv4 Clouds", RFC 3056, February 2001.
[20] Mockapetris, P., "Domain names - implementation and [20] Thaler, D., "Support for Multicast over 6to4 Networks",
draft-ietf-ngtrans-6to4-multicast-01 (work in progress), July
2002.
[21] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[21] Droms, R., "Procedures and IANA Guidelines for Definition of [22] Droms, R., "Procedures and IANA Guidelines for Definition of
New DHCP Options and Message Types", BCP 43, RFC 2939, New DHCP Options and Message Types", BCP 43, RFC 2939,
September 2000. September 2000.
[22] Narten, T. and R. Draves, "Privacy Extensions for Stateless [23] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001. Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[23] Nguyen, Q., "http://irl.cs.ucla.edu/vet/report.ps", spring [24] Nguyen, Q., "http://irl.cs.ucla.edu/vet/report.ps", spring
1998. 1998.
[24] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923, [25] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923,
September 2000. September 2000.
[26] Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992.
[27] Templin, F., "Neighbor Affiliation Protocol for
IPv6-over-(foo)-over-IPv4", draft-templin-v6v4-ndisc-01 (work
in progress), November 2002.
Authors' Addresses Authors' Addresses
Fred L. Templin Fred L. Templin
Nokia Nokia
313 Fairchild Drive 313 Fairchild Drive
Mountain View, CA 94110 Mountain View, CA 94110
US US
Phone: +1 650 625 2331 Phone: +1 650 625 2331
EMail: ftemplin@iprg.nokia.com EMail: ftemplin@iprg.nokia.com
skipping to change at page 17, line 15 skipping to change at page 15, line 24
Microsoft Corporation Microsoft Corporation
One Microsoft Way One Microsoft Way
Redmond, WA 98052-6399 Redmond, WA 98052-6399
US US
Phone: +1 425 703 8835 Phone: +1 425 703 8835
EMail: dthaler@microsoft.com EMail: dthaler@microsoft.com
Appendix A. Major Changes Appendix A. Major Changes
changes from version 10 to version 11:
o Added multicast/anycast subsection
o Revised PRL initialization
o Updated neighbor discovery, security consideration sections
o Updated MTU section
changes from version 09 to version 10: changes from version 09 to version 10:
o Rearranged/revised sections 5, 6, 7 o Rearranged/revised sections 5, 6, 7
o updated MTU section o updated MTU section
changes from version 08 to version 09: changes from version 08 to version 09:
o Added stateful autoconfiguration mechanism o Added stateful autoconfiguration mechanism
skipping to change at page 18, line 44 skipping to change at page 17, line 43
0x00-0xFD RESERVED for future IANA use 0x00-0xFD RESERVED for future IANA use
0xFE (TSE, TSD) together contain an embedded IPv4 address 0xFE (TSE, TSD) together contain an embedded IPv4 address
0xFF TSD is interpreted based on TSE as follows: 0xFF TSD is interpreted based on TSE as follows:
TSE TSD Interpretation TSE TSD Interpretation
--- ------------------ --- ------------------
0x00-0xFD RESERVED for future IANA use 0x00-0xFD RESERVED for future IANA use
0xFE TSD contains 24-bit EUI-48 intf id 0xFE TSD contains 24-bit EUI-48 intf id
0xFF RESERVED by IEEE/RAC 0xFF RESERVED by IEEE/RAC
Figure 2
Thus, if TYPE=0xFE, TSE is an extension of TSD. If TYPE=0xFF, TSE is Thus, if TYPE=0xFE, TSE is an extension of TSD. If TYPE=0xFF, TSE is
an extension of TYPE. Other values for TYPE (thus, other an extension of TYPE. Other values for TYPE (thus, other
interpretations of TSE, TSD) are reserved for future IANA use. interpretations of TSE, TSD) are reserved for future IANA use.
The above specification is compatible with all aspects of EUI64, The above specification is compatible with all aspects of EUI64,
including support for encapsulating legacy EUI-48 interface including support for encapsulating legacy EUI-48 interface
identifiers (e.g., an IANA EUI-48 format multicast address such as: identifiers (e.g., an IANA EUI-48 format multicast address such as:
'01-00-5E-01-02-03' is encapsulated as: '01-00-5E-FF-FE-01-02-03'). '01-00-5E-01-02-03' is encapsulated as: '01-00-5E-FF-FE-01-02-03').
But, the specification also provides a special TYPE (0xFE) to But, the specification also provides a special TYPE (0xFE) to
indicate an IPv4 address is embedded. Thus, when the first four indicate an IPv4 address is embedded. Thus, when the first four
octets of an IPv6 interface identifier are: '00-00-5E-FE' (note: the octets of an IPv6 interface identifier are: '00-00-5E-FE' (note: the
'u/l' bit MUST be 0) the interface identifier is said to be in 'u/l' bit MUST be 0) the interface identifier is said to be in
"ISATAP format" and the next four octets embed an IPv4 address "ISATAP format" and the next four octets embed an IPv4 address
encoded in network byte order. encoded in network byte order.
Appendix C. Dynamic Per-neighbor MTU Discovery Appendix C. Dynamic MTU Discovery
ISATAP encapsulators and decapsulators are IPv6 neighbors that may be ISATAP encapsulators and decapsulators are IPv6 neighbors that may be
separated by multiple link layer (IPv4) forwarding hops. When separated by multiple link layer (IPv4) forwarding hops. When an
ISATAP_MTU is larger than 1380 bytes, the encapsulator must implement encapsulator's interface configures a LinkMTU ([4], Section 6.3.2)
a dynamic link layer mechanism to discover per-neighbor MTUs. value larger than 1380 bytes, a dynamic link layer (IPv4) mechanism
is required to discover per-neighbor path MTUs. The following text
gives non-normative considerations for dynamic MTU discovery.
IPv4 path MTU discovery [15] relies on ICMPv4 "fragmentation needed" IPv4 path MTU discovery [13] uses ICMPv4 "fragmentation needed"
messages, but these do not provide enough information for stateless messages, but these generally do not provide enough information for
translation into ICMPv6 "packet too big" messages (see: RFC 792 [16] stateless translation to ICMPv6 "packet too big" messages (see: RFC
and RFC 1812 [17], section 4.3.2.3). Additionally, ICMPv4 792 [14] and RFC 1812 [15], section 4.3.2.3). Additionally, ICMPv4
"fragmentation needed" messages can be spoofed, filtered, or not sent "fragmentation needed" messages can be spoofed, filtered, or not sent
at all by some forwarding nodes. Thus, IPv4 Path MTU discovery used at all by some forwarding nodes. Thus, IPv4 Path MTU discovery used
alone is inadequate and can result in black holes that are difficult alone may be inadequate and can result in black holes that are
to diagnose [24]. difficult to diagnose [25].
The ISATAP encapsulator may implement an alternate per-neighbor MTU Alternate methods for determining per-neighbor MTUs should be used
discovery mechanism, e.g., periodic and/or on-demand probing of the when RFC 1191 path MTU discovery is deemed inadequate. In any
IPv4 path to the decapsulator. Probing consists of sending packets method, the encapsulator uses periodic and/or on-demand probing of
larger than 1380 bytes to the neighbor and receiving positive the IPv4 path to the decapsulator. The following three methods are
confirmation of receipt. Two methods are possible: possible:
In the first method, the encapsulator does NOT set the DF bit in the 1. Encapsulator-driven - the encapsulator periodically sends probe
IPv4 header of probe packets. In this case, the encapsulator must packets with the DF bit set in the IPv4 header and waits for a
have a priori knowledge of the decapsulator's reassembly buffer size positive acknowledgement from the decapsulator that the probe was
and should have a priori knowledge of the decapsulator's link MTU. received
This method has the advantage that probe packets will be delivered
even if the network performs fragmentation, thus ordinary data
packets may be used for probing resulting in greater efficiency.
Disadvantages for this method include:
o special mechanisms required on both encapsulator and decapsulator 2. Decapsulator-driven - the encapsulator sends all packets with the
DF bit NOT set in the IPv4 header unless and until the
decapsulator sends a "Fragmentation Experienced" indication(s)
o extra state required on both encapsulator and decapsulator 3. Hybrid - the encapsulator and decapsulator engage in a dialogue
and use "intelligent" probing to monitor the path MTU
o complex protocol signalling between encapsulator and decapsulator These methods are discussed in detail in the following subsections:
o possible extended periods of network fragmentation C.1 Encapsulator-driven Method
In the second (and preferred) method, the encapsulator sets the DF
bit in the IPv4 header of probe packets. Neighbor Solicitation (NS)
packets with padding bytes added should be used for this purpose,
since successful delivery results in a positive acknowledgement that
the probe succeeded, i.e., in the form of a Neighbor Advertisement
(NA) from the decapsulator. Setting the DF bit prevents the network
from fragmenting the packets and protects decapsulators from
receiving packets that might overrun the IPv4 reassembly buffer.
Additionally, special mechanisms and state are needed only on the
encapsulator, and no complex protocol signalling between the
encapsulator and decapsulator is required.
In either method, implementations may choose to couple the probing In this method, the encapsulator sets the DF bit in the IPv4 header
process with neighbor cache management procedures ([6], section 7), of probe packets. Probe packets may be sent either when the
e.g. to maintain timers, state variables and/or a queue of packets encapsulator's link layer forwards a large data packet to the
waiting for probes to complete. Packets retained on the queue are decapsulator (i.e., on-demand) or when the path MTU for the
forwarded when probes succeed, and provide state for sending ICMPv6 decapsulator has not been verified for some time (i.e., periodic).
"packet too big" messages to the source when probes fail. IPv6 Neighbor Solicitation (NS) or ICMPv6 ECHO_REQUEST packets with
Implementations may choose to store per-neighbor MTU information in padding bytes added could be used for this purpose, since successful
the IPv4 path MTU discovery cache, in the ISATAP link layer's private delivery results in a positive acknowledgement that the probe
data structures, etc. succeeded vis-a-vis a response from the decapsulator.
Additional notes: While the decapsulator is being probed, the encapsulator maintains a
queue of packets that have the decapsulator as the IPv6 next-hop
address. The queue should be large enough to buffer the
(delay*bandwidth) product for the round-trip time (RTT) to the
decapsulator. If the probe succeeds, packets in the queue that are
no larger than the probe size are sent to the decapsulator. If the
probe fails, packets larger than the last known successful probe are
dropped and an ICMPv6 "packet too big" message returned to the sender
[16].
1. Per-neighbor MTUs may vary dynamically due to fluctuations in the This method has the advantage that the decapsulator need not
IPv4 forwarding path and/or multipath routing (e.g., when QoS implement any special mechanisms, since standard IPv6 request/
routing is used in the IPv4 network). For such neighbors, response mechanisms are used. Additionally, the encapsulator is
encapsulators should detect a "losing battle" and reduce the assured that any packets that are too large for the decapsulator to
per-neighbor MTU size to no more than 1380 bytes. receive will be dropped by the network. Disadvantages for this
method include the fact that probe packets do not carry data and thus
consume network resources. Additionally, packet queues may become
large on Long, Fat Networks (LFNs) (see: RFC 1323 [26]).
2. When not probing, encapsulators may send packets to a neighbor C.2 Decapsulator-driven Method
with MTU greater than 1380 bytes either with the DF bit set or
not set. When the DF bit is set, undetected packet loss may
occur in the network if the neighbor's MTU decreases. When the
DF bit is NOT set, undetected packet loss is less likely but may
occur either in the network or at the neighbor's reassembly
buffer.
3. ICMPv4 "fragmentation needed" messages may result when a link In this method, the encapsulator sends all packets with the DF bit
NOT set in the IPv4 header with the expectation that the decapsulator
will send a "Fragmentation Experienced" indication if the IPv4
network fragments packets. In other words, the decapsulator simply
sends all packets that are no larger than LinkMTU unless and until it
receives "Fragmentation Experienced" messages from the decapsulator.
The decapsulator can use IPv6 Router Advertisement (RA) messages with
an MTU option as the means for both reporting fragmentation and
informing the encapsulator of a new MTU value to use.
This method has the distinct advantages that the data packets
themselves are used as probes and no queueing on the encapsulator is
necessary. Additionally, fewer packets will be lost since the
decapsulator will quite often be able to reassemble packets
fragmented by the network. The primary disadvantage is that, using
the current specifications, the encapsulator has no way of knowing
whether a particular decapsulator implements the "fragmentation
experienced" signalling capability. However, the "fragmentation
experienced" indication can be trivially implemented in an
application on the decapsulator that uses the Berkeley Packet Filter
(aka, libpcap) to listen for fragmented packets from encapsulators.
When fragmented packets arrive, the application sends IPv6 RA
messages with an MTU option to inform the encapsulator that
fragmentation has been experienced and a new value for the neighbor's
MTU should be used. The application additionally sends ICMPv6
"packet too big" messages to the original source when a fragmented
packet is not correctly reassembled. This function need not be built
into the decapsulator's operating system and can be added as an
after-market feature. Finally, simply adding an extra bit in the RA
message header ([4], section 4.2) would provide a means for the
decapsulator to inform the encapsulator that dynamic MTU discovery is
supported.
C.3 Hybrid Method
In this method, the encapsulator and decapsulator engage in a
"neighbor affiliation" protocol to negotiate link-layer parameters
such as MTU. (See: [27] for an example of such an approach.) This
approach has the advantage that bi-directional links are used and
both ends of the link have unambiguous knowledge that the other end
implements the protocol. However, the signalling protocol between
the endpoints is complicated and additional state is required in both
the encapsulator and decapsultor.
C.4 Summary
In summary, the decapsulator-based approach in Appendix C.2 has
distinct efficiency advantages over methods that engage the
encapsulator. Additionally, probing methods which use IPv4
encapsulation with the DF bit NOT set may use LinkMTU values for the
ISATAP link that exceed the underlying link MTU size. Experimental
verification is called for which may eventually result in a
recommendation for proposed standard.
C.5 Additional Notes
o In all methods, some packet loss due to link/buffer restrictions
may occur with no ICMPv6 "packet too big" message returned to the
sender. Unenlightened senders will interpret such loss as loss
due to congestion, which may result in longer convergence to the
actual path MTU. Enlightened senders will interpret the loss as
loss due to link/buffer restrictions and immediately reduce their
MTU estimate.
o To avoid denial-of-service attacks that would cause superfluous
probing based on counting down/up by small increments, plateau
tables (e.g., [13], section 7) should be used when the actual MTU
value is indeterminant.
o ICMPv4 "fragmentation needed" messages may result when a link
restriction is encountered but may also come from denial of restriction is encountered but may also come from denial of
service attacks. Implementations should treat ICMPv4 service attacks. Implementations should treat ICMPv4
"fragmentation needed" messages as "tentative" negative "fragmentation needed" messages as "tentative" negative
acknowledgments and apply heuristics to determine when to suspect acknowledgments and apply heuristics to determine when to suspect
an actual link restriction and when to ignore the messages. IPv6 an actual link restriction and when to ignore the messages. IPv6
packets lost due actual link restrictions are perceived as lost packets lost due actual link restrictions are perceived as lost
due to congestion by the original source, but robust due to congestion by the original source, but robust
implementations minimize instances of such packet loss without implementations minimize instances of such packet loss without
ICMPv6 "packet too big" messages returned to the sender. ICMPv6 "packet too big" messages returned to the sender.
o Nodes that connect to the Internet are expected to be able to
reassemble or discard IPv4 packets up to 64KB in length when the
DF bit is not set in the encapsulating IPv4 header. Nodes that
cannot reassemble or discard maximum-length IPv4 packets are
vulnerable to attacks such as the "ping-of-death".
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