Network Working Group F. Templin

Network Working Group F. Templin
Request for Comments: 4214 Nokia
Category: Experimental T. Gleeson
Cisco Systems K.K.
M. Talwar
D. Thaler
Microsoft Corporation
October 2005

Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)

Status of This Memo

This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2005).

IESG Note

The content of this RFC was at one time considered by the IETF, and
therefore it may resemble a current IETF work in progress or a
published IETF work. This RFC is not a candidate for any level of
Internet Standard. The IETF disclaims any knowledge of the fitness
of this RFC for any purpose, and in particular notes that the
decision to publish is not based on IETF review for such things as
security, congestion control or inappropriate interaction with
deployed protocols. The RFC Editor has chosen to publish this
document at its discretion. Readers of this RFC should exercise
caution in evaluating its value for implementation and deployment.
See RFC 3932 for more information.

Abstract

The Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) connects
IPv6 hosts/routers over IPv4 networks. ISATAP views the IPv4 network
as a link layer for IPv6 and views other nodes on the network as
potential IPv6 hosts/routers. ISATAP supports an automatic tunneling
abstraction similar to the Non-Broadcast Multiple Access (NBMA)
model.

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1. Introduction

This document specifies a simple mechanism called the Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP) that connects IPv6
hosts/routers over IPv4 networks. Dual-stack (IPv6/IPv4) nodes use
ISATAP to automatically tunnel IPv6 packets in IPv4, i.e., ISATAP
views the IPv4 network as a link layer for IPv6 and views other nodes
on the network as potential IPv6 hosts/routers.

ISATAP enables automatic tunneling whether global or private IPv4
addresses are used, and presents a Non-Broadcast Multiple Access
(NBMA) abstraction similar to [RFC2491][RFC2492][RFC2529][RFC3056].

The main objectives of this document are to: 1) describe the domain
of applicability, 2) specify addressing requirements, 3) specify
automatic tunneling using ISATAP, 4) specify the operation of IPv6
Neighbor Discovery over ISATAP interfaces, and 5) discuss Site
Administration, Security, and IANA 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 [BCP14].

This document also uses 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 in order to demonstrate protocol behavior. An
implementation is not required to have them in the exact form
described here, as long as its external behavior is consistent with
that described in this document.

3. Terminology

The terminology of [RFC2460][RFC2461] applies to this document. The
following additional terms are defined:

ISATAP node:
A node that implements the specifications in this document.

ISATAP interface:
An ISATAP node's Non-Broadcast Multi-Access (NBMA) IPv6 interface,
used for automatic tunneling of IPv6 packets in IPv4.

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ISATAP interface identifier:
An IPv6 interface identifier with an embedded IPv4 address
constructed as specified in Section 6.1.

ISATAP address:
An IPv6 unicast address that matches an on-link prefix on an
ISATAP interface of the node, and that includes an ISATAP
interface identifier.

locator:
An IPv4 address-to-interface mapping; i.e., a node's IPv4 address
and its associated interface.

locator set:
A set of locators associated with an ISATAP interface. Each
locator in the set belongs to the same site.

4. Domain of Applicability

The domain of applicability for this technical specification is
automatic tunneling of IPv6 packets in IPv4 for ISATAP nodes within
sites that observe the security considerations found in this
document, including host-to-router, router-to-host, and host-to-host
automatic tunneling in certain enterprise networks and 3GPP/3GPP2
wireless operator networks. (Other scenarios with a sufficient trust
basis ensured by the mechanisms specified in this document also fall
within this domain of applicability.)

Extensions to the above domain of applicability (e.g., by combining
the mechanisms in this document with those in other technical
specifications) are out of the scope of this document.

5. Node Requirements

ISATAP nodes observe the common functionality requirements for IPv6
nodes found in [NODEREQ] and the requirements for dual IP layer
operation found in ([MECH], Section 2). They also implement the
additional features specified in this document.

6. Addressing Requirements

6.1. ISATAP Interface Identifiers

ISATAP interface identifiers are constructed in Modified EUI-64
format ([RFC3513], Section 2.5.1 and Appendix A) by concatenating the
24-bit IANA OUI (00-00-5E), the 8-bit hexadecimal value 0xFE, and a
32-bit IPv4 address in network byte order as follows:

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|0 1|1 3|3 6|
|0 5|6 1|2 3|
+----------------+----------------+--------------------------------+
|000000ug00000000|0101111011111110|mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm|
+----------------+----------------+--------------------------------+

When the IPv4 address is known to be globally unique, the "u" bit
(universal/local) is set to 1; otherwise, the "u" bit is set to 0.
"g" is the individual/group bit, and "m" are the bits of the IPv4
address.

6.2. ISATAP Interface Address Configuration

Each ISATAP interface configures a set of locators consisting of IPv4
address-to-interface mappings from a single site; i.e., an ISATAP
interface's locator set MUST NOT span multiple sites.

When an IPv4 address is removed from an interface, the corresponding
locator SHOULD be removed from its associated locator set(s). When a
new IPv4 address is assigned to an interface, the corresponding
locator MAY be added to the appropriate locator set(s).

ISATAP interfaces form ISATAP interface identifiers from IPv4
addresses in their locator set and use them to create link-local
ISATAP addresses ([RFC2462], Section 5.3).

6.3. Multicast/Anycast

It is not possible to assume the general availability of wide-area
IPv4 multicast, so (unlike 6over4 [RFC2529]) ISATAP must assume that
its underlying IPv4 carrier network only has unicast capability.
Support for IPv6 multicast over ISATAP interfaces is not described in
this document.

Similarly, support for Reserved IPv6 Subnet Anycast Addresses is not
described in this document.

7. Automatic Tunneling

ISATAP interfaces use the basic tunneling mechanisms specified in
([MECH], Section 3). The following sub-sections describe additional
specifications.

7.1. Encapsulation

ISATAP addresses are mapped to a link-layer address by a static
computation; i.e., the last four octets are treated as an IPv4
address.

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7.2. Handling ICMPv4 Errors

ISATAP interfaces SHOULD process ARP failures and persistent ICMPv4
errors as link-specific information indicating that a path to a
neighbor may have failed ([RFC2461], Section 7.3.3).

7.3. Decapsulation

The specification in ([MECH], Section 3.6) is used. Additionally,
when an ISATAP node receives an IPv4 protocol 41 datagram that does
not belong to a configured tunnel interface, it determines whether
the packet's IPv4 destination address and arrival interface match a
locator configured in an ISATAP interface's locator set.

If an ISATAP interface that configures a matching locator is found,
the decapsulator MUST verify that the packet's IPv4 source address is
correct for the encapsulated IPv6 source address. The IPv4 source
address is correct if:

- the IPv6 source address is an ISATAP address that embeds the
IPv4 source address in its interface identifier, or

- the IPv4 source address is a member of the Potential Router
List (see Section 8.1).

Packets for which the IPv4 source address is incorrect for this
ISATAP interface are checked to determine whether they belong to
another tunnel interface.

7.4. Link-Local Addresses

ISATAP interfaces use link-local addresses constructed as specified
in Section 6 of this document.

7.5. Neighbor Discovery over Tunnels

ISATAP interfaces use the specifications for neighbor discovery found
in the following section of this document.

8. Neighbor Discovery for ISATAP Interfaces

ISATAP interfaces use the neighbor discovery mechanisms specified in
[RFC2461]. The following sub-sections describe specifications that
are also implemented.

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8.1. Conceptual Model of a Host

To the list of Conceptual Data Structures ([RFC2461], Section 5.1),
ISATAP interfaces add the following:

Potential Router List (PRL)
A set of entries about potential routers; used to support router
and prefix discovery. Each entry ("PRL(i)") has an associated
timer ("TIMER(i)"), and an IPv4 address ("V4ADDR(i)") that
represents a router's advertising ISATAP interface.

8.2. Router and Prefix Discovery - Router Specification

Advertising ISATAP interfaces send Solicited Router Advertisement
messages as specified in ([RFC2461], Section 6.2.6) except that the
messages are sent directly to the soliciting node; i.e., they might
not be received by other nodes on the link.

8.3. Router and Prefix Discovery - Host Specification

The Host Specification in ([RFC2461], Section 6.3) is used. The
following sub-sections describe specifications added by ISATAP
interfaces.

8.3.1. Host Variables

To the list of host variables ([RFC2461], Section 6.3.2), ISATAP
interfaces add the following:

PrlRefreshInterval
Time in seconds between successive refreshments of the PRL after
initialization. The designated value of all ones (0xffffffff)
represents infinity.
Default: 3600 seconds

MinRouterSolicitInterval
Minimum time in seconds between successive solicitations of the
same advertising ISATAP interface. The designated value of all
ones (0xffffffff) represents infinity.

8.3.2. Potential Router List Initialization

ISATAP nodes initialize an ISATAP interface's PRL with IPv4 addresses
discovered via manual configuration, a DNS Fully Qualified Domain
Name (FQDN) [STD13], a DHCPv4 option, a DHCPv4 vendor-specific
option, or an unspecified alternate method. FQDNs are established
via manual configuration or an unspecified alternate method. FQDNs
are resolved into IPv4 addresses through a static host file lookup,

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querying the DNS service, querying a site-specific name service, or
with an unspecified alternate method.

After initializing an ISATAP interface's PRL, the node sets a timer
for the interface to PrlRefreshInterval seconds and re-initializes
the interface's PRL as specified above when the timer expires. When
an FQDN is used, and when it is resolved via a service that includes
TTLs with the IPv4 addresses returned (e.g., DNS 'A' resource records
[STD13]), the timer SHOULD be set to the minimum of
PrlRefreshInterval and the minimum TTL returned. (Zero-valued TTLs
are interpreted to mean that the PRL is re-initialized before each
Router Solicitation event; see Section 8.3.4.)

8.3.3. Processing Received Router Advertisements

To the list of checks for validating Router Advertisement messages
([RFC2461], Section 6.1.1), ISATAP interfaces add the following:

- IP Source Address is a link-local ISATAP address that embeds
V4ADDR(i) for some PRL(i).

Valid Router Advertisements received on an ISATAP interface are
processed as specified in ([RFC2461], Section 6.3.4).

8.3.4. Sending Router Solicitations

To the list of events after which Router Solicitation messages may be
sent ([RFC2461], Section 6.3.7), ISATAP interfaces add the following:

- TIMER(i) for some PRL(i) expires.

Since unsolicited Router Advertisements may be incomplete and/or
absent, ISATAP nodes MAY schedule periodic Router Solicitation events
for certain PRL(i)s by setting the corresponding TIMER(i).

When periodic Router Solicitation events are scheduled, the node
SHOULD set TIMER(i) so that the next event will refresh remaining
lifetimes stored for PRL(i) before they expire, including the Router
Lifetime, Valid Lifetimes received in Prefix Information Options, and
Route Lifetimes received in Route Information Options [DEFLT].
TIMER(i) MUST be set to no less than MinRouterSolicitInterval seconds
where MinRouterSolicitInterval is configurable for the node, or for a
specific PRL(i), with a conservative default value (e.g., 2 minutes).

When TIMER(i) expires, the node sends Router Solicitation messages as
specified in ([RFC2461], Section 6.3.7) except that the messages are
sent directly to PRL(i); i.e., they might not be received by other
routers. While the node continues to require periodic Router

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Solicitation events for PRL(i), and while PRL(i) continues to act as
a router, the node resets TIMER(i) after each expiration event as
described above.

8.4. Neighbor Unreachability Detection

Hosts SHOULD perform Neighbor Unreachability Detection ([RFC2461],
Section 7.3). Routers MAY perform neighbor unreachability detection,
but this might not scale in all environments.

After address resolution, hosts SHOULD perform an initial
reachability confirmation by sending Neighbor Solicitation messages
and receiving a Neighbor Advertisement message. Routers MAY perform
this initial reachability confirmation, but this might not scale in
all environments.

9. Site Administration Considerations

Site administrators maintain a Potential Router List (PRL) of IPv4
addresses representing advertising ISATAP interfaces of routers.

The PRL is commonly maintained as an FQDN for the ISATAP service in
the site's name service (see Section 8.3.2). There are no mandatory
rules for the selection of the FQDN, but site administrators are
encouraged to use the convention "isatap.domainname" (e.g.,
isatap.example.com).

When the site's name service includes TTLs with the IPv4 addresses
returned, site administrators SHOULD configure the TTLs with
conservative values to minimize control traffic.

10. Security Considerations

Implementors should be aware that, in addition to possible attacks
against IPv6, security attacks against IPv4 must also be considered.
Use of IP security at both IPv4 and IPv6 levels should nevertheless
be avoided, for efficiency reasons. For example, if IPv6 is running
encrypted, encryption of IPv4 would be redundant unless traffic
analysis is felt to be a threat. If IPv6 is running authenticated,
then authentication of IPv4 will add little. Conversely, IPv4
security will not protect IPv6 traffic once it leaves the ISATAP
domain. Therefore, implementing IPv6 security is required even if
IPv4 security is available.

The threats associated with IPv6 Neighbor Discovery are described in
[RFC3756].

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There is a possible spoofing attack in which spurious ip-protocol-41
packets are injected into an ISATAP link from outside. Since an
ISATAP link spans an entire IPv4 site, restricting access to the link
can be achieved by restricting access to the site; i.e., by having
site border routers implement IPv4 ingress filtering and ip-
protocol-41 filtering.

Another possible spoofing attack involves spurious ip-protocol-41
packets injected from within an ISATAP link by a node pretending to
be a router. The Potential Router List (PRL) provides a list of IPv4
addresses representing advertising ISATAP interfaces of routers that
hosts use in filtering decisions. Site administrators should ensure
that the PRL is kept up to date, and that the resolution mechanism
(see Section 9) cannot be subverted.

The use of temporary addresses [RFC3041] and Cryptographically
Generated Addresses [CGA] on ISATAP interfaces is outside the scope
of this specification.

11. IANA Considerations

The IANA has specified the format for Modified EUI-64 address
construction ([RFC3513], Appendix A) in the IANA Ethernet Address
Block. The text in Appendix A of this document has been offered as
an example specification. The current version of the IANA registry
for Ether Types can be accessed at:

http://www.iana.org/assignments/ethernet-numbers

12. Acknowledgements

The ideas in this document are not original, and the authors
acknowledge the original architects. Portions of this work were
sponsored through SRI International internal projects 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, and Vlad Yasevich.

The following are acknowledged for their significant contributions:
Alain Durand, Hannu Flinck, Jason Goldschmidt, Nathan Lutchansky,
Karen Nielsen, Mohan Parthasarathy, Chirayu Patel, Art Shelest,
Markku Savela, Pekka Savola, Margaret Wasserman, and Brian Zill.

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The authors acknowledge the work of Quang Nguyen on "Virtual
Ethernet", 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.

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Appendix A. Modified EUI-64 Addresses in the IANA Ethernet Address
Block

Modified EUI-64 addresses ([RFC3513], Section 2.5.1 and Appendix A)
in the IANA Ethernet Address Block are formed by concatenating the
24-bit IANA OUI (00-00-5E) with a 40-bit extension identifier and
inverting the "u" bit; i.e., the "u" bit is set to one (1) to
indicate universal scope and set to zero (0) to indicate local scope.

Modified EUI-64 addresses have the following appearance in memory
(bits transmitted right-to-left within octets, octets transmitted
left-to-right):

0 23 63
| OUI | extension identifier |
000000ug00000000 01011110xxxxxxxx xxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxx

When the first two octets of the extension identifier encode the
hexadecimal value 0xFFFE, the remainder of the extension identifier
encodes a 24-bit vendor-supplied id as follows:

0 23 39 63
| OUI | 0xFFFE | vendor-supplied id |
000000ug00000000 0101111011111111 11111110xxxxxxxx xxxxxxxxxxxxxxxx

When the first octet of the extension identifier encodes the
hexadecimal value 0xFE, the remainder of the extension identifier
encodes a 32-bit IPv4 address as follows:

0 23 31 63
| OUI | 0xFE | IPv4 address |
000000ug00000000 0101111011111110 xxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxx

Normative References

[BCP14] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[STD13] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.

[RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.

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RFC 4214 ISATAP October 2005

[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.

[MECH] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.

Informative References

[RFC2491] Armitage, G., Schulter, P., Jork, M., and G. Harter, "IPv6
over Non-Broadcast Multiple Access (NBMA) networks", RFC
2491, January 1999.

[RFC2492] Armitage, G., Schulter, P., and M. Jork, "IPv6 over ATM
Networks", RFC 2492, January 1999.

[RFC2529] Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
Domains without Explicit Tunnels", RFC 2529, March 1999.

[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.

[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.

[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May
2004.

[CGA] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.

[DEFLT] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", Work in Progress, December 2003.

[NODEREQ] Loughney, J., Ed., "IPv6 Node Requirements", Work in
Progress, May 2004.

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Authors' Addresses

Fred L. Templin
Nokia
313 Fairchild Drive
Mountain View, CA 94110
US

EMail: [email protected]

Tim Gleeson
Cisco Systems K.K.
Shinjuku Mitsui Building
2-1-1 Nishishinjuku, Shinjuku-ku
Tokyo 163-0409
Japan

EMail: [email protected]

Mohit Talwar
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
US

Phone: +1 425 705 3131
EMail: [email protected]

Dave Thaler
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
US

Phone: +1 425 703 8835
EMail: [email protected]

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