Network Working Group F. Templin
Internet-Draft Nokia
Expires: March 10, 2004 T. Gleeson
Cisco Systems K.K.
M. Talwar
D. Thaler
Microsoft Corporation
September 10, 2003
Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
draft-ietf-ngtrans-isatap-15.txt
Status of this Memo
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This Internet-Draft will expire on March 10, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document specifies an Intra-Site Automatic Tunnel Addressing
Protocol (ISATAP) that connects IPv6 hosts and routers within IPv4
sites. ISATAP treats the site's IPv4 unicast infrastructure as a link
layer for IPv6. ISATAP enables intra-site automatic IPv6-in-IPv4
tunneling whether globally assigned or private IPv4 addresses are
used.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Basic IPv6 Operation . . . . . . . . . . . . . . . . . . . . . 4
5. Automatic Tunneling . . . . . . . . . . . . . . . . . . . . . 5
6. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . . 6
7. Address Autoconfiguration . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Security considerations . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
Normative References . . . . . . . . . . . . . . . . . . . . . 10
Informative References . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
A. Major Changes . . . . . . . . . . . . . . . . . . . . . . . . 12
B. Rationale for Interface Identifier Construction . . . . . . . 14
C. Deployment Considerations . . . . . . . . . . . . . . . . . . 15
D. Other Considerations . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . 17
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1. Introduction
This document presents a simple approach called the Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP) that enables
incremental deployment of IPv6 [RFC2460] within IPv4 [RFC0791] sites.
ISATAP allows dual-stack nodes that do not share a link with an IPv6
router to automatically tunnel packets to the IPv6 next-hop address
through IPv4, i.e., the site's IPv4 infrastructure is treated as a
link layer for IPv6.
Specific details for the operation of IPv6 and automatic tunneling
using ISATAP are given, including an interface identifier format that
embeds an IPv4 address. This format supports IPv6 address
configuration and simple link-layer address mapping. Also specified
is the operation of IPv6 Neighbor Discovery and deployment/security
considerations.
2. Requirements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
This document also makes use of internal conceptual variables to
describe protocol behavior and external variables that an
implementation must allow system administrators to change. The
specific variable names, how their values change, and how their
settings influence protocol behavior are provided to demonstrate
protocol behavior. An implementation is not required to have them in
the exact form described here, so long as its external behavior is
consistent with that described in this document.
3. Terminology
The terminology of [RFC2460] applies to this document. The following
additional terms are defined:
site:
same as defined in [RFC3582], which is intended to be equivalent
to "enterprise" as defined in [RFC1918].
link, on-link, off-link:
same as defined in ([RFC2461], section 2.1).
underlying link:
a link layer that supports IPv4 (for ISATAP), and MAY also support
IPv6 natively.
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ISATAP interface:
an interface configured over one or more underlying links.
advertising ISATAP interface:
same meaning as "advertising interface" in ([RFC2461], section
6.2.2).
ISATAP address:
an address with an on-link prefix assigned on an ISATAP interface
and with an interface identifier constructed as specified in
Section 4.1.
4. Basic IPv6 Operation
ISATAP interfaces automatically tunnel IPv6 packets using the site's
IPv4 infrastructure as a link layer for IPv6, i.e., IPv6 treats the
site's IPv4 infrastructure as a Non-Broadcast, Multiple Access (NBMA)
link layer, with properties similar to [RFC2491]. The following
ISATAP-specific considerations are noted for basic IPv6 operation:
4.1 Interface Identifiers and Unicast Addresses
ISATAP interface identifiers use "modified EUI-64" format ([RFC3513],
section 2.5.1) and are formed by appending an IPv4 address assigned
to an underlying link to the 32-bit string '00-00-5E-FE'. Appendix B
includes non-normative rationale for this construction rule.
IPv6 global and local-use ([RFC3513], sections 2.5.4, 2.5.6) ISATAP
addresses are constructed as follows:
| 64 bits | 32 bits | 32 bits |
+------------------------------+---------------+----------------+
| global/local unicast prefix | 0000:5EFE | IPv4 Address |
+------------------------------+---------------+----------------+
4.2 ISATAP Interface Configuration
ISATAP interfaces are configured over one or more underlying links
that support IPv4 for tunneling within a site; each IPv4 address
assigned to an underlying link is seen as a link-layer address for
ISATAP.
4.3 Multicast and Anycast
ISATAP interfaces recognize an IPv6 node's required addresses
([RFC3513], section 2.8), including certain multicast/anycast
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addresses.
Mechanisms for multicast/anycast emulation on ISATAP interfaces
(e.g., MARS [RFC2022], etc.) are out of scope.
5. Automatic Tunneling
The common tunneling mechanisms specified in ([MECH], sections 2 and
3) are used, with the following noted considerations for ISATAP:
5.1 Tunnel MTU and Fragmentation
ISATAP automatic tunnel interfaces may be configured over multiple
underlying links with diverse maximum transmission units (MTUs). The
minimum MTU for IPv6 interfaces is 1280 bytes ([RFC2460], Section 5),
but the following considerations apply for ISATAP interfaces:
o Nearly all IPv4 nodes connect to physical links with MTUs of 1500
bytes or larger (e.g., Ethernet)
o Sub-IPv4 layer encapsulations (e.g., VPN) may occur on some paths
o Commonly-deployed VPN interfaces use an MTU of 1400 bytes
To maximize efficiency and minimize IPv4 fragmentation for the
predominant deployment case, LinkMTU ([RFC2461], Section 6.3.2) for
the ISATAP interface SHOULD be set to no more than 1380 bytes (1400
minus 20 bytes for IPv4 encapsulation).
LinkMTU MAY be set to larger values when a dynamic link layer MTU
discovery mechanism is used or when a static MTU assignment is used
and the anticipated/measured level of fragmentation in the site's
IPv4 network is deemed acceptable.
When a dynamic link layer MTU discovery mechanism is not used, the
ISATAP interface MUST NOT encapsulate IPv6 packets with the Don't
Fragment (DF) bit set in the encapsulating IPv4 header.
5.2 Handling IPv4 ICMP Errors
ARP failures and persistent ICMPv4 errors SHOULD be processed as
link-specific information indicating that a path to a neighbor has
failed ([RFC2461], section 7.3.3).
5.3 Local-Use IPv6 Unicast Addresses
The specification in ([MECH], section 3.7) is not used; the
specification in Section 4.1 is used instead.
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5.4 Ingress Filtering
The specification in ([MECH], section 3.9) is used.
Additionally, packets received on an ISATAP interface with an ISATAP
network-layer (IPv6) source address that does not embed the
link-layer (IPv4) source address in the interface identifier are
silently discarded.
6. Neighbor Discovery
The specification in ([MECH], section 3.8) applies only to configured
tunnels. [RFC2461] provides the following guidelines for
non-broadcast multiple access (NBMA) link support:
"Redirect, Neighbor Unreachability Detection and next-hop
determination should be implemented as described in this document.
Address resolution and the mechanism for delivering Router
Solicitations and Advertisements on NBMA links is not specified in
this document."
ISATAP interfaces SHOULD implement Redirect, Neighbor Unreachability
Detection, and next-hop determination exactly as specified in
[RFC2461]. Address resolution and the mechanisms for delivering
Router Solicitations and Advertisements are not specified by
[RFC2461]; instead, they are specified in the following sections of
this document.
6.1 Address Resolution and Neighbor Unreachability Detection
ISATAP addresses are resolved to link-layer (IPv4) addresses by a
static computation, i.e., the last four octets are treated as an IPv4
address.
Hosts SHOULD perform an initial reachability confirmation by sending
Neighbor Solicitation (NS) message(s) and receiving a Neighbor
Advertisement (NA) message as specified in ([RFC2461], section 7.2).
Unless otherwise specified in a future document, solicitations are
sent to the target's unicast address.
Hosts SHOULD additionally perform Neighbor Unreachability Detection
(NUD) as specified in ([RFC2461], section 7.3). Routers MAY perform
these reachability confirmation and NUD procedures, but this might
not scale in all environments.
All ISATAP nodes MUST send solicited neighbor advertisements
([RFC2461], section 7.2.4).
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6.2 Duplicate Address Detection
Duplicate Address Detection ([RFC2462], section 5.4) is not required
for ISATAP addresses, since duplicate address detection is assumed to
have been already performed for the IPv4 addresses from which they
derive.
6.3 Router and Prefix Discovery
The following sections describe mechanisms to support the router and
prefix discovery process ([RFC2461], section 6):
6.3.1 Conceptual Data Structures
ISATAP nodes use the conceptual data structures Prefix List and
Default Router List exactly as in ([RFC2461], section 5.1). ISATAP
adds a new conceptual data structure "Potential Router List" (PRL)
and the following new configuration variable:
PrlRefreshInterval
Time in seconds between successive refreshments of the PRL after
initialization. It SHOULD be no less than 3,600 seconds. The
designated value of all 1's (0xffffffff) represents infinity.
Default: 3,600 seconds
A PRL is associated with every ISATAP interface and supports the
mechanisms specified in Section 6.3.4. Each entry in the PRL
("PRL(i)") has an associated timer ("TIMER(i)"), and an IPv4 address
("V4ADDR(i)") that represents a site border router's advertising
ISATAP interface.
When a node enables an ISATAP interface, it initializes the PRL with
IPv4 addresses. The addresses MAY be discovered via a DHCPv4
[RFC2131] option for ISATAP, manual configuration, or an unspecified
alternate method (e.g., DHCPv4 vendor-specific option, etc.).
When no other mechanisms are available, a DNS fully-qualified domain
name (FQDN) [RFC1035] established by an out-of-band method (e.g.,
DHCPv4, manual configuration, etc.) MAY be used. The FQDN is resolved
into IPv4 addresses for the PRL through a static host file, a
site-specific name service, querying a DNS server within the site, or
an unspecified alternate method. There are no mandatory rules for the
selection of a FQDN, but manual configuration MUST be supported. When
DNS is used, client resolvers use the IPv4 transport.
After initialization, nodes periodically refresh the PRL (i.e., using
one or more of the methods described above) after PrlRefreshInterval.
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6.3.2 Validation of Router Advertisements Messages
The specification in ([RFC2461], section 6.1.2) is used.
6.3.3 Router Specification
Routers with advertising ISATAP interfaces behave the same as
described in ([RFC2461], section 6.2). As permitted by ([RFC2461],
section 6.2.6), advertising ISATAP interfaces SHOULD send unicast RA
messages to a soliciting host's unicast address when the
solicitation's source address is not the unspecified address.
6.3.4 Host Specification
Hosts behave the same as described in ([RFC2461], section 6.3) with
the following additional considerations for ISATAP:
6.3.4.1 Soliciting Router Advertisements
Hosts solicit Router Advertisements (RAs) by sending Router
Solicitations (RSs) to advertising ISATAP interfaces in the PRL. The
manner of selecting PRL(i)'s for solicitation is up to the
implementation. Hosts add the following variable to support the
solicitation process:
MinRouterSolicitInterval
Minimum time in seconds between successive solicitations of the
same advertising ISATAP interface. It SHOULD be no less than 900
seconds.
Default: 900 seconds
RS messages use a link-local unicast address from the ISATAP
interface as the IPv6 source address.
6.3.4.2 Router Advertisement Processing
RAs received from a member of the PRL (i.e., RAs with an ISATAP IPv6
source address that embeds V4ADDR(i) for some PRL(i)) are processed
exactly as specified in ([RFC2461], section 6.3.4). Additionally,
hosts reset TIMER(i) to schedule the next solicitation event (see:
Section 6.3.4.1). Let "MIN_LIFETIME" be the minimum value in the
Router Lifetime or the lifetime(s) encoded in options included in the
RA message. Then, TIMER(i) is reset as follows:
TIMER(i) = MAX((0.5 * MIN_LIFETIME), MinRouterSolicitInterval)
RAs received from a router other than a member of the PRL are
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processed as specified in ([RFC2461], section 6.3.4) except that any
RA contents ([RFC2461], section 6.2.3) that would alter ISATAP link
parameters are silently ignored. In particular, non-zero values in
the Router Lifetime, M and O flags, Cur Hop Limit, Reachable Time,
and Retrans Timer as well as prefix options with the L and/or A bits
set are ignored. If the MTU option is present, the option's value
SHOULD be stored in a per-neighbor cache entry for the source of the
RA; it MUST NOT be copied into LinkMTU for the ISATAP link.
7. Address Autoconfiguration
Hosts invoke stateless address autoconfiguration under the conditions
specified in ([RFC2462], sections 5.5).
Hosts invoke stateful address autoconfiguration under the conditions
specified in ([RFC2462], section 5.5). When DHCPv6 [RFC3315] is used,
hosts send messages to the "All_DHCP_Relay_Agents_and_Servers"
multicast address ([RFC3315], sections 1.2 and 1.3). Sending
implementations map the "All_DHCP_Relay_Agents_and_Servers" multicast
address to a link-layer (IPv4) address by selecting V4ADDR(i) for
some PRL(i).
When the site supports the DHCPv6 service, the server/relay function
MUST be deployed equally on each router that is a member of the PRL.
8. IANA Considerations
The IANA is advised to specify construction rules for IEEE EUI-64
addresses formed from the Organizationally Unique Identifier (OUI)
"00-00-5E" in the IANA "ethernet-numbers" registry. The non-normative
text in Appendix B is offered as an example specification.
9. Security considerations
ISATAP site border routers MUST implement IPv6 and IPv4 ingress
filtering and in particular MUST discard any packets originating from
outside of the site that use an IP address from the site as the
source address. Additionally, site border routers MUST implement
ip-protocol-41 filtering by not allowing packets for that protocol in
and out of the site. Finally, site border routers MUST NOT forward
any packets with local-use source or destination addresses outside of
the site ([RFC3513], section 2.5.6).
Even with IPv4 and IPv6 ingress filtering, reflection attacks can
originate from compromised nodes within an ISATAP site that spoof
IPv6 source addresses. Security mechanisms for reflection attack
mitigation SHOULD be used in routers with advertising ISATAP
interfaces. At a minimum, site border routers SHOULD log potential
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source address spoofing cases.
Site administrators maintain a list of IPv4 addresses representing
advertising ISATAP interfaces and make them available via one or more
of the mechanisms described in Section 6.3.1. The list can include
IPv4 anycast address(es) but administrators are advised to consider
operational implications of anycast (e.g., see: [RFC1546]).
ISATAP addresses do not support privacy extensions for stateless
address autoconfiguration [RFC3041].
10. Acknowledgements
Portions of this work were derived from SRI International internal
funds and government contracts. Government sponsors include Monica
Farah-Stapleton and Russell Langan (U.S. Army CECOM ASEO), and Dr.
Allen Moshfegh (U.S. Office of Naval Research). SRI International
sponsors include Dr. Mike Frankel, J. Peter Marcotullio, Lou
Rodriguez, and Dr. Ambatipudi Sastry.
The following are acknowledged for providing peer review input: Jim
Bound, Rich Draves, Cyndi Jung, Ambatipudi Sastry, Aaron Schrader,
Ole Troan, Vlad Yasevich.
The following additional individuals are acknowledged for their
contributions: Rich Draves, Alain Durand, Nathan Lutchansky, Karen
Nielsen, Mohan Parthasarathy, Art Shelest, Pekka Savola, Margaret
Wasserman, Brian Zill.
The authors also acknowledge the work of Quang Nguyen [VET] under the
guidance of Dr. Lixia Zhang that proposed very similar ideas to those
that appear in this document. This work was first brought to the
authors' attention on September 20, 2002.
Normative References
[MECH] Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-00
(work in progress), February 2003.
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
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[RFC2461] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.
Informative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1546] Partridge, C., Mendez, T. and W. Milliken, "Host
Anycasting Service", RFC 1546, November 1993.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and
E. Lear, "Address Allocation for Private Internets", BCP
5, RFC 1918, February 1996.
[RFC2022] Armitage, G., "Support for Multicast over UNI 3.0/3.1
based ATM Networks", RFC 2022, November 1996.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[RFC2491] Armitage, G., Schulter, P., Jork, M. and G. Harter, "IPv6
over Non-Broadcast Multiple Access (NBMA) networks", RFC
2491, January 1999.
[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and
M. Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[RFC3582] Abley, J., Black, B. and V. Gill, "Goals for IPv6
Site-Multihoming Architectures", RFC 3582, August 2003.
[VET] Nguyen, Q., "http://irl.cs.ucla.edu/vet/report.ps", spring
1998.
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Authors' Addresses
Fred L. Templin
Nokia
313 Fairchild Drive
Mountain View, CA 94110
US
Phone: +1 650 625 2331
EMail: ftemplin@iprg.nokia.com
Tim Gleeson
Cisco Systems K.K.
Shinjuku Mitsu Building
2-1-1 Nishishinjuku, Shinjuku-ku
Tokyo 163-0409
Japan
EMail: tgleeson@cisco.com
Mohit Talwar
Microsoft Corporation
One Microsoft Way
Redmond, WA> 98052-6399
US
Phone: +1 425 705 3131
EMail: mohitt@microsoft.com
Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
US
Phone: +1 425 703 8835
EMail: dthaler@microsoft.com
Appendix A. Major Changes
changes from version 14 to version 15:
o several editorial changes
o revised Security; IANA considerations
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o revised Section 6.3.4.2
o added new section on ingress filtering
o revised stateful autoconfiguration and moved to new section
o removed overly-restrictive text at end of Section 6.3.4.1
changes from version 13 to version 14:
o removed applicability statement; applicability TBD by v6ops
o updated deployment/site admin sections; moved to appendices
o new text on "L" bit in prefix options in section 7.3.4.2
o removed extraneous text in Security Considerations
o fixed "layering bug" in section 7.3.4.3
o revised "ISATAP address" definition
o updated references for RFC 3315; 3513
changes from earlier versions to version 13:
o Revised ISATAP interface/link terminology
o Returned to using symbolic reference names
o Revised MTU section; moved non-normative MTU text to separate
document
o Added multicast/anycast subsection
o Revised PRL initialization
o Updated neighbor discovery, security consideration sections
o Rearranged/revised sections 5, 6, 7
o Added stateful autoconfiguration mechanism
o Normative references to RFC 2491, RFC 2462
o Moved non-normative MTU text to appendix C
o clarified address resolution, Neighbor Unreachability Detection
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o specified MTU/MRU requirements
o Addressed operational issues identified in 05 based on discussion
between co-authors
o Clarified ambiguous text per comments from Hannu Flinck; Jason
Goldschmidt
o Moved historical text in section 4.1 to Appendix B in response to
comments from Pekka Savola
o Identified operational issues for anticipated deployment scenarios
o Included reference to Quang Nguyen work
Appendix B. Rationale for Interface Identifier Construction
ISATAP specifies an EUI64-format address construction for the
Organizationally-Unique Identifier (OUI) owned by the Internet
Assigned Numbers Authority (IANA). This format (given below) is used
to construct both native EUI64 addresses for general use and modified
EUI-64 format interface identifiers for IPv6 unicast addresses:
|0 2|2 3|3 3|4 6|
|0 3|4 1|2 9|0 3|
+------------------------+--------+--------+------------------------+
| OUI ("00-00-5E"+u+g) | TYPE | TSE | TSD |
+------------------------+--------+--------+------------------------+
Where the fields are:
OUI IANA's OUI: 00-00-5E with 'u' and 'g' bits (3 octets)
TYPE Type field; specifies use of (TSE, TSD) (1 octet)
TSE Type-Specific Extension (1 octet)
TSD Type-Specific Data (3 octets)
And the following interpretations are specified based on TYPE:
TYPE (TSE, TSD) Interpretation
---- -------------------------
0x00-0xFD RESERVED for future IANA use
0xFE (TSE, TSD) together contain an embedded IPv4 address
0xFF TSD is interpreted based on TSE as follows:
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TSE TSD Interpretation
--- ------------------
0x00-0xFD RESERVED for future IANA use
0xFE TSD contains 24-bit EUI-48 intf id
0xFF RESERVED by IEEE/RAC
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
interpretations of TSE, TSD) are reserved for future IANA use.
The above specification is compatible with all aspects of EUI64,
including support for encapsulating legacy EUI-48 interface
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').
But, the specification also provides a special TYPE (0xFE) to
indicate an IPv4 address is embedded. Thus, when the first four
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
"ISATAP format" and the next four octets embed an IPv4 address
encoded in network byte order.
Appendix C. Deployment Considerations
Hosts can enable ISATAP, e.g., when native IPv6 service is
unavailable. When native IPv6 service is acquired, hosts can
discontinue the ISATAP router solicitation process (Section 6.3.4)
and/or allow associated state to expire (see: [RFC2461], section 5.3
and [RFC2462], section 5.5.4). In this case, any associated addresses
added to the DNS should also be removed.
Routers can configure both native IPv6 and ISATAP interfaces over the
same physical link. The prefixes used on each interface will be
distinct, and normal IPv6 routing between the interfaces can occur.
Routers can obtain IPv6 prefix delegations from a server via an
ISATAP interface and advertise the delegated prefix(es) on other IPv6
interface(s).
Responsible administration can reduce control traffic overhead
associated with router and prefix discovery.
Appendix D. Other Considerations
The Potential Router List (PRL) contains the IPv4 addresses of
advertising ISATAP interfaces on site border routers, and the
specification mandates that nodes only accept Router Advertisement
(RA) parameters that alter the ISATAP link (e.g., default router
list, on-link prefix list, LinkMTU, etc.) if they are sent by a
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member of the PRL. However, the specification allows any node on the
ISATAP link to send "other" parameters in RAs and also allows any
node on the ISATAP link to act as a (non-default) IPv6 router, e.g.,
if the node is configured as a router for its other IPv6 links.
These aspects of the specification allow useful functionality,
including the ability for ISATAP nodes other than PRL members to
serve as routers for "stub" IPv6 networks, the ability for ISATAP
nodes to send IPv6 packets with non-ISATAP source addresses (e.g.,
RFC 3401 privacy addresses), etc. But, allowing this functionality
prevents ISATAP nodes from perform effective ingress filtering for
IPv6 source addresses in packets they receive. Instead, the nodes
must trust that: 1) site border routers are performing ingress
filtering, and 2) malicious nodes are effectively denied access to
the link.
Additionally, the specification expects that that IPv4 addresses are
uniquely assigned within the ISATAP site.
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