WDIFF

draft-ietf-ngtrans-isatap-12.txt --> draft-ietf-ngtrans-isatap-13.txt

Network Working Group F. Templin
Internet-Draft Nokia
Expires: July September 25, 2003 T. Gleeson
Cisco Systems K.K.
M. Talwar
D. Thaler
Microsoft Corporation
January 24,
March 27, 2003

Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
draft-ietf-ngtrans-isatap-12.txt
draft-ietf-ngtrans-isatap-13.txt

Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

This Internet-Draft will expire on July September 25, 2003.

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 infrastructure as a link layer
for IPv6 with no requirement for IPv4 multicast. 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. Applicability Statement . . . . . . . . . . . . . . . . . . . 3
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Basic IPv6 Operation . . . . . . . . . . . . . . . . . . . . . 4
6. Automatic Tunneling . . . . . . . . . . . . . . . . . . . . . 5
7. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . . 7 6
8. Deployment Considerations . . . . . . . . . . . . . . . . . . 10 9
9. Site Administration Considerations . . . . . . . . . . . . . . 9
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
10. 10
11. Security considerations . . . . . . . . . . . . . . . . . . . 11
11. 10
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 10
Normative References . . . . . . . . . . . . . . . . . . . . . 12 11
Informative References . . . . . . . . . . . . . . . . . . . . 13 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15 12
A. Major Changes . . . . . . . . . . . . . . . . . . . . . . . . 16 13
B. Rationale for Interface Identifier Construction . . . . . . . 17
C. ISATAP Interface MTU Considerations . . . . . . . . . . . . . 18 15
Intellectual Property and Copyright Statements . . . . . . . . 23 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 [1] [RFC2460] within IPv4 [2] [RFC0791] sites.
ISATAP allows dual-stack nodes that do not share a physical 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
over
using ISATAP links 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. Applicability Statement

ISATAP provides the following features:

o treats site's IPv4 infrastructure as a link layer for IPv6 using
automatic IPv6-in-IPv4 tunneling

o enables incremental deployment of IPv6 hosts within IPv4 sites
with no aggregation scaling issues at border gateways

o requires no special IPv4 services within the site (e.g.,
multicast)

o supports both stateless address autoconfiguration and stateful autoconfiguration as well as
manual configuration

o supports networks that use non-globally unique IPv4 addresses
(e.g., when private address allocations [10] [RFC1918] are used)

o compatible with other NGTRANS mechanisms (e.g., 6to4 [11]) [RFC3056])

3. 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 [3]. [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

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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.

4. Terminology

The terminology of RFC 2460 [1] [RFC2460] applies to this document. The following
additional terms are defined:

link, on-link, off-link:
same definitions as ([4], ([RFC2461], section 2.1).

underlying link:
a link layer that supports IPv4 (for ISATAP), and MAY also support
IPv6 natively.

ISATAP link: interface:
an interface configured over one or more underlying links used for tunneling. The IPv4 network
layer addresses of the underlying links are used as link-layer
addresses on the ISATAP link.

ISATAP interface:
a node's attachment to an ISATAP link. underling links.

advertising ISATAP interface:
same meaning as "advertising interface" in ([4], ([RFC2461], section
6.2.2).

ISATAP address:
an on-link address on an ISATAP interface and with an interface
identifier constructed as specified in Section 5.1

5. Basic IPv6 Operation

ISATAP links transmit interfaces automatically tunnel IPv6 packets via automatic tunnels 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. The mechanisms in [RFC2491] are used, with the following considerations
noted exceptions for IPv6 on
ISATAP links are noted: ISATAP:

5.1 Interface Identifiers and Unicast Addresses

ISATAP interface identifiers use "modified EUI-64" format ([5], ([ARCH],
section 2.5.1) and are formed by appending an IPv4 address on the
ISATAP assigned
to an underlying link to the 32-bit string '00-00-5E-FE'. Appendix B
includes non-normative rationale for this construction rule.

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With reference to ([5], sections 2.5.4, 2.5.6),

IPv6 global and local-use ([ARCH], sections 2.5.4, 2.5.6) ISATAP
addresses are constructed as follows:

5.2 ISATAP Link/Interface Interface Configuration

ISATAP links consist of interfaces are configured over one or more underlying links
that support IPv4 for tunneling within a site.

ISATAP interfaces are configured over ISATAP links; site; each IPv4 address
assigned to an underlying link is seen as a link-layer address for
ISATAP.

Neighbor discovery on ISATAP links (see: Section 7) provides the
functional equivalent of unicast virtual circuits (VCs) required for
other NBMA media types ([6], section 4.6). Neighbor state
information MAY be kept in the Conceptual Neighbor Cache ([4],
section 5.1).

5.3 Link Layer Address Options

With reference to ([6], ([RFC2491], 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.

5.4 Multicast and Anycast

ISATAP interfaces recognize a an IPv6 node's required addresses as specified
in ([5],
([ARCH], section 2.8). 2.8), including certain multicast/anycast addresses.

Mechanisms for multicast/anycast emulation on ISATAP links interfaces
(e.g., adaptations of MLD [12], [RFC2710], PIM-SM [13], [RFC2362], MARS [14],
[RFC2022], etc.) are subject for future companion document(s).

6. Automatic Tunneling

The common tunneling mechanisms specified in ([7], ([MECH], sections 2 and
3) are used, with the following noted considerations for ISATAP:

6.1 Dual IP Layer Operation Tunnel MTU and Fragmentation

ISATAP uses the same specification found in ([7], section 2). That

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is, ISATAP nodes provide complete IPv4 and IPv6 implementations and
are able to send and receive both IPv4 and IPv6 packets.

Address configuration and DNS considerations are the same as ([7],
sections 2.1 through 2.3).

6.2 Encapsulation/Decapsulation

The specifications in ([7], sections 3.1 and 3.6) are used.
Additionally, the IPv6 next-hop address for packets encapsulated on
an ISATAP link MUST be an ISATAP address; other packets are discarded
and an ICMPv6 destination unreachable indication with code 3 (Address
Unreachable) ([8], section 3.1) is returned to the source.

6.3 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 ([1], Section 5), but 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

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o Commonly-deployed VPN interfaces use an MTU of 1400 bytes

To maximize efficiency and minimize IPv4 fragmentation for the
predominant deployment case, the ISATAP interface MTU, or "LinkMTU"
(see: [4], [RFC2461], Section 6.3.2 ), 6.3.2), 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
additional fragmentation in the site's IPv4 network is deemed
acceptable. See
Appendix C for non-normative ISATAP interface MTU considerations.

When a dynamic IPv4 MTU discovery mechanism is not used, the ISATAP link
layer
interface encapsulates IPv6 packets with the Don't Fragment (DF) bit
not set in the encapsualting IPv4 header.

6.4

6.2 Handling IPv4 ICMP Errors

IPv4 ICMP errors and

ARP failures are and persistent ICMPv4 errors SHOULD be processed as link error
notifications.

6.5
link-specific information indicating that a path to a neighbor has
failed ([RFC2461], section 7.3.3).

6.3 Local-Use IPv6 Unicast Addresses

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The specification in ([7], ([MECH], section 3.7) is not used. Instead, local
use IPv6 unicast addresses are formed as specified in Section 5.1.

6.6 Ingress Filtering

The used; the
specification in ([7], section 3.9) is used. In particular,
ISATAP nodes that forward decapsulated packets MUST verify the tunnel
source address Section 5.1 is acceptable. used instead.

7. Neighbor Discovery

The specification in ([7], ([MECH], section 3.8) applies only to configured
tunnels. RFC 2461 [4] [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 links interfaces SHOULD implement Redirect, Neighbor Unreachability
Detection, and next-hop determination exactly as specified in [4].
[RFC2461]. Address resolution and the mechanisms for delivering
Router Solicitations and Advertisements for ISATAP links are not specified by [4];
[RFC2461]; instead, they are specified in the following sections of
this document.

7.1 Address Resolution and Neighbor Unreachability Detection

ISATAP addresses are resolved to link-layer addresses (IPv4) addresses by a

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static computation, i.e., the last four octets are treated as an IPv4
address.

Following static address resolution, hosts

Hosts SHOULD perform an initial reachability confirmation by sending
Neighbor Solicitation (NS) message(s) and receiving a Neighbor
Advertisement (NA) message using
the mechanisms as specified in ([4], ([RFC2461], section 7.2.). When the ISATAP
interface provides 7.2).
Unless otherwise specified in a multicast emulation mechanism (see: Section 5.4) future document, solicitations are
sent to the solicited-node multicast address
corresponding to the target address. Otherwise, the solicitation is
sent to the target's unicast address.

Hosts SHOULD additionally perform Neighbor Unreachability Detection
(NUD) as specified in ([4], ([RFC2461], section 7.3). Routers MAY perform the
above-specified
these reachability detection confirmation and NUD procedures, but this might
not scale in all environments.

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All ISATAP nodes MUST send solicited neighbor advertisements ([4],
([RFC2461], section 7.2.4).

7.2 Duplicate Address Detection

Duplicate Address Detection ([9], ([RFC2462], section 5.4) is not required
for ISATAP addresses, since duplicate address detection is assumed
already performed for the IPv4 addresses from which they derive.

7.3 Router and Prefix Discovery

The following sections describe mechanisms to support the router and
prefix discovery process ([4], ([RFC2461], section 6) on ISATAP links: 6):

7.3.1 Conceptual Data Structures

ISATAP nodes use the conceptual data structures Prefix List and
Default Router List exactly as in ([4], ([RFC2461], section 5.1). ISATAP links
add
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. SHOULD be no less than 3,600 seconds.

Default: 3,600 seconds

A PRL is associated with every ISATAP link. interface. Each entry in the
PRL ("PRL(i)") has an IPv4 address ("V4ADDR(i)") that represents an
advertising ISATAP interface and an associated timer ("TIMER(i)").
The process for initializing and refreshing the PRL is described
below:

When a node enables an ISATAP link, interface, it initializes the PRL with
IPv4 addresses. The addresses MAY be discovered via a DHCPv4 [15]
[RFC2131] option for ISATAP (option code TBD), ISATAP, manual configuration, or an unspecified

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alternate method (e.g., DHCPv4 vendor-specific option).

When no other mechanisms are available, a DNS fully-qualified domain
name (FQDN) [16] [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

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one or more of the methods described above) after PrlRefreshInterval.

7.3.2 Validation of Router Advertisements Messages

The specification in ([4], ([RFC2461], section 6.1.2) is used.

Additionally, received RA messages that contain Prefix Information
options and/or encode non-zero values in the Cur Hop Limit, Router
Lifetime, Reachable Time, or Retrans Timer fields (see: [4], [RFC2461],
section 4.2) MUST satisfy the following validity check for ISATAP:

o the network-layer (IPv6) source address is an ISATAP address and
embeds V4ADDR(i) for some PRL(i)

7.3.3 Router Specification

Routers with advertising ISATAP interfaces behave the same as
described in ([4], ([RFC2461], section 6.2). As permitted by ([4], ([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.

7.3.4 Host Specification

When no unsolicited RA messages containing prefix information options
and/or non-zero router lifetime values are received, hosts MAY send
Router Solicitation (RS) messages using

Hosts behave the specification same as described in Section
7.3.4.1. RA messages (whether solicited or unsolicited) are
processed using ([RFC2461], section 6.3) and
([RFC2462], section 5.5) with the specification in Section 7.3.4.2. following additional considerations
for ISATAP:

7.3.4.1 Sending Soliciting Router Advertisements

Hosts solicit Router Advertisements (RAs) by sending Router
Solicitations

All PRL(i)'s are assumed (RSs) to represent active advertising ISATAP interfaces within the site, i.e., in the PRL provides trust basis only;
not reachability detection. Hosts periodically solicit information
from one or more PRL(i) by sending Router Solicitation (RS) messages. PRL. The
manner of selecting a PRL(i) PRL(i)'s for solicitation and/or deprecating
a previously-selected PRL(i) is outside up to the scope of this
specification.
implementation. Hosts add the following variable to support the
solicitation process:

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MinRouterSolicitInterval
Minimum time in seconds between successive solicitations of the
same advertising ISATAP interface. SHOULD be no less than 900
seconds.

Default: 900 seconds

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Solicitation consists of sending

RS messages using the interface's use a link-local unicast addresses address from the ISATAP
interface as the IPv6 source address. When the ISATAP
interface provides Unless otherwise specified in a multicast emulation mechanism (see: Section
5.4),
future document, RS messages are sent to the All-Routers multicast address.
Otherwise, they are sent to use the link-local ISATAP address
constructed from V4ADDR(i) for some the PRL(i) selected for solicitation. The RS
messages are otherwise sent exactly being solicited as in ([4], section 6.3.7). the IPv6
destination address.

7.3.4.2 Processing Router Advertisements

Hosts process received RA messages exactly as in ([4], section 6.3.4)
and ([9], section 5.5.3). (But, see Appendix C for non-normative
considerations for RA messages containing MTU options.)

When the source address of the Advertisement Processing

When the source address of an RA message is an ISATAP address that
embeds V4ADDR(i) for some PRL(i) selected for solicitation, PRL(i), hosts
additionally reset TIMER(i). TIMER(i) to schedule
the next solicitation event (see: Section 7.3.4.1). 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 to: as follows:

TIMER(i) = MAX((0.5 * MIN_LIFETIME), MinRouterSolicitInterval)

7.3.4.3 Stateful Autoconfiguration

If stateful autoconfiguration is invoked ([RFC2462], sections 5.5.2,
5.5.3), the "ALL_DHCP_Relay_Agents_and_Servers" multicast address
([DHCPV6], section 5.1) is resolved to the link-local ISATAP address
constructed from V4ADDR(i) for some PRL(i).

8. Deployment Considerations

8.1 Host And Router Deployment Considerations

For hosts, if an underlying link supports both IPv4 (over which
ISATAP is implemented) and also supports IPv6 natively, then ISATAP
MAY be enabled if the

Hosts may enable ISATAP, e.g., when native IPv6 layer does not receive Router
Advertisements (i.e., does not have connection with an IPv6 router).
After a non-link-local address has been configured and a default
router acquired on the service is
unavailable. When native link, the host IPv6 service is acquired, hosts SHOULD
discontinue the ISATAP router solicitation process described in the Host Specification (Section 7.3.4) and
and/or allow existing ISATAP address configurations associated state to expire as specified in ([4], (see: [RFC2461], section 5.3) 5.3
and ([9], [RFC2462], section 5.5.4). Any ISATAP associated addresses added to the
DNS for this host should also be removed. In this way, ISATAP use will gradually diminish as IPv6
routers are widely deployed throughout the site.

Routers MAY configure both a native IPv6 and ISATAP interface interfaces over the
same physical link. Routing will operate as usual between these
two domains. Note that the The prefixes used on the ISATAP and native
IPv6 interfaces each interface will be distinct. The IPv4 address(es) configured on
a router's advertising ISATAP interface(s) SHOULD be added (either
automatically or manually) to
distinct, and normal IPv6 routing between the site's address records for
advertising ISATAP interfaces.

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8.2 interfaces may occur.

9. Site Administration Considerations

The following considerations are noted for sites that deploy ISATAP:

ISATAP links sites are administratively defined by a set of advertising

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interfaces and set of nodes which discover that solicit those interface
addresses. interfaces. Thus,
ISATAP links sites are defined by administrative (not physical) boundaries.

o Hosts and routers that use ISATAP can be deployed in an ad-hoc
fashion. In particular, hosts can be deployed with little/no
advanced knowledge of existing routers, and routers can be
deployed with no reconfiguration requirements for hosts.

o Site administrators maintain a list

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 7.3.1. ISATAP nodes
use this list to initialize and periodically refresh the PRL. Responsible site
administration can reduce the control traffic.
At a minimum, administrators SHOULD ensure that dynamically
advertised information for the site's PRL is well maintained.

9. traffic overhead.

10. IANA Considerations

A DHCPv4 option code for ISATAP (TBD) [17] may be requested in the
event that this document or a derivative thereof is moved to
standards track.

Modifications to the IANA "ethernet-numbers" registry (e.g., based on
text in Appendix B) may be requested in the event that this document
or a derivative thereof is moved to standards track.

10. are requested.

11. Security considerations

ISATAP site border routers and firewalls MUST implement IPv6 ingress
filtering and MUST NOT forward packets IPv4
ingress filtering, including ip-protocol-41 filtering. Packets with site-local
local-use source and/or destination addresses MUST NOT be forwarded
outside of the site [18].

In addition to possible attacks against IPv6, security attacks
against IPv4 must also be considered. In particular, border routers
and firewalls MUST implement IPv4 ingress filtering and
ip-protocol-41 filtering. site.

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 (e.g., [19], [20], etc.) SHOULD be used in routers with

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interfaces. At a minimum, ISATAP site border gateways MUST SHOULD log potential source
address spoofing cases.

IPv6 Neighbor Discovery trust models and threats [21] apply also to
ISATAP. However, ([21], section 4.4.) shows that most of these
threats are mitigated in corporate networks that implement site
security mechanisms, i.e., the applicability space for ISATAP.

ISATAP addresses do not support privacy extensions for stateless
address autoconfiguration [22]. [RFC3041]. However, since the ISATAP
interface identifier is derived from the node's IPv4 address, ISATAP
addresses do not have the same level of privacy concerns as IPv6
addresses that use an interface identifier derived from the MAC
address. (This is especially true when private address allocations [10]
[RFC1918] are used.)

11.

12. Acknowledgements

Some of the ideas presented in this draft were derived from work at
SRI with internal funds and contractual support. Government sponsors
who supported the work include Monica Farah-Stapleton and Russell
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
Marcotullio, Lou Rodriguez, and Dr. Ambatipudi Sastry supported the
work and helped foster early interest.

The following peer reviewers are acknowledged for taking the time to
review a pre-release of this document and provide input: Jim Bound,
Rich Draves, Cyndi Jung, Ambatipudi Sastry, Aaron Schrader, Ole

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Troan, Vlad Yasevich.

The authors acknowledge members of the NGTRANS community who have
made significant contributions to this effort, including Rich Draves,
Alain Durand, Nathan Lutchansky, Karen Nielsen, Art Shelest, Margaret
Wasserman, and Brian Zill.

The authors also wish to acknowledge the work of Quang Nguyen [23] [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

[1] Deering, S. and R.

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

[2] Addressing
Architecture", draft-ietf-ipngwg-addr-arch-v3-11 (work in
progress), October 2002.

[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.

[3]

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement

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[4]

[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.

[5] Hinden, R. and

[RFC2462] Thomson, S. Deering, "IP Version 6 Addressing
Architecture", draft-ietf-ipngwg-addr-arch-v3-11 (work in
progress), October 2002.

[6] Armitage, G., Schulter, P., Jork, M. and G. Harter, T. Narten, "IPv6 over
Non-Broadcast Multiple Access (NBMA) networks", Stateless Address
Autoconfiguration", 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
progress), November 2002.

[8] 2462, December 1998.

[RFC2463] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC 2463, December 1998.

[9] Thomson, S.

[RFC2491] Armitage, G., Schulter, P., Jork, M. and T. Narten, G. Harter, "IPv6 Stateless Address
Autoconfiguration",
over Non-Broadcast Multiple Access (NBMA) networks", RFC 2462, December 1998.
2491, January 1999.

Informative References

[10]

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[DHCPV6] Droms, R., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress),
November 2002.

[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.

[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and
E. Lear, "Address Allocation for Private Internets", BCP
5, RFC 1918, February 1996.

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

[12] Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
Discovery (MLD)

[RFC2022] Armitage, G., "Support for IPv6", Multicast over UNI 3.0/3.1
based ATM Networks", RFC 2710, October 1999.

[13] 2022, November 1996.

[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.

[RFC2362] Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering,
S., Handley, M. and V. Jacobson, "Protocol Independent
Multicast-Sparse Mode (PIM-SM): Protocol Specification",
RFC 2362, June 1998.

[14] Armitage, G., "Support

[RFC2710] Deering, S., Fenner, W. and B. Haberman, "Multicast
Listener Discovery (MLD) for Multicast over UNI 3.0/3.1 based ATM
Networks", RFC 2022, November 1996.

[15] Droms, R., "Dynamic Host Configuration Protocol", IPv6", RFC 2131,
March 1997.

[16] Mockapetris, P., "Domain names - implementation 2710, October
1999.

[RFC3041] Narten, T. and

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specification", STD 13, R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 1035, November 1987.

[17] Droms, R., "Procedures 3041,
January 2001.

[RFC3056] Carpenter, B. and IANA Guidelines for Definition K. Moore, "Connection of
New DHCP Options and Message Types", BCP 43, RFC 2939,
September 2000.

[18] Hinden, R., "IPv6 Globally Unique Site-Local Addresses",
draft-hinden-ipv6-global-site-local-00 (work in progress),
December 2002.

[19] Savola, P., "Security Considerations for 6to4",
draft-savola-ngtrans-6to4-security-01 (work in progress), March
2002.

[20] Bellovin, S., Leech, M. and T. Taylor, "ICMP Traceback
Messages", draft-ietf-itrace-03 (work in progress), January
2003.

[21] Nikander, P., "IPv6 Neighbor Discovery trust models and
threats", draft-ietf-send-psreq-01 (work in progress), January
2003.

[22] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", IPv6 Domains
via IPv4 Clouds", RFC 3041, January 3056, February 2001.

[23]

[VET] Nguyen, Q., "http://irl.cs.ucla.edu/vet/report.ps", spring
1998.

[24] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.

[25] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.

[26] Postel, J., "Internet Control Message Protocol", STD 5, RFC
792, September 1981.

[27] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
June 1995.

[28] Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923,
September 2000.

[29] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for
IP version 6", RFC 1981, August 1996.

[30] Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992.

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[31] Templin, F., "Neighbor Affiliation Protocol for
IPv6-over-(foo)-over-IPv4", draft-templin-v6v4-ndisc-01 (work
in progress), November 2002.

Authors' Addresses

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

Phone: +1 650 625 2331
EMail: ftemplin@iprg.nokia.com

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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

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Appendix A. Major Changes

changes from version 11 12 to version 12: 13:

o Added comments from co-authors

o Text cleanup; removed extraneous text

o Revised PRL initialization ISATAP interface/link terminology

o Updated Returned to using symbolic reference names

o Revised MTU section section; moved non-normative MTU text to seperate
document

changes from version 10 earlier versions to version 11: 12:

o Added multicast/anycast subsection

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o Revised PRL initialization

o Updated neighbor discovery, security consideration sections

o Updated MTU section

changes from version 09 to version 10:

o Rearranged/revised sections 5, 6, 7

o updated MTU section

changes from version 08 to version 09:

o Added stateful autoconfiguration mechanism

o Normative references to RFC 2491, RFC 2462

o Moved non-normative MTU text to appendix C

changes from version 07 to version 08:

o updated MTU section

changes from version 06 to version 07:

o clarified address resolution, Neighbor Unreachability Detection

o specified MTU/MRU requirements

changes from earlier versions to version 06:

o Addressed operational issues identified in 05 based on discussion
between co-authors

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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

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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:

TSE TSD Interpretation
--- ------------------
0x00-0xFD RESERVED for future IANA use
0xFE TSD contains 24-bit EUI-48 intf id
0xFF RESERVED by IEEE/RAC

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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. ISATAP Interface MTU Considerations

ISATAP encapsulators and decapsulators are IPv6 neighbors that may be
separated by multiple link layer (IPv4) forwarding hops. Thus, the
path MTU of the underlying IPv4 network may determine the uni-
directional IPv6 per-neighbor MTU from the encapsulator to the
decapsulator. (Note that this constitutes the MTU of only one hop in
what may be a multiple-hop IPv6 path.) When the encapsulator's ISATAP
interface configures a large LinkMTU value (see: Section 6.3),
special considerations apply as described in the following
non-normative sections:

C.1 Stateless (Static) MTU Assignment

Nodes that connect to the Internet should be able to reassemble and/
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/
discard maximum-length IPv4 packets are vulnerable to buffer overrun
attacks. This issue may be obviated for nodes that are accessed only
within a site (i.e., do not connect directly to the Internet) since
site border gateways, etc. can filter and discard fragments of large
packets before they reach constrained node(s).

When the ISATAP encapsulator does not implement a dynamic link layer
mechanism to determine per-neighbor MTUs, all IPv6 packets are
encapsulated with the DF bit not set in the IPv4 header.
Additionally, LinkMTU may be set to a value that is no more than the
smallest Effective MTU to Receive (EMTU_R) (see: RFC 1122 [24],
section 3.3.2) for all potential decapsulators in the site. The
value chosen for LinkMTU must be at least 1280 bytes (the minimum
IPv6 MTU) and such that the potential worst-case level of
fragmentation in the underlying IPv4 network is deemed "acceptable"
by the site's standards.

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For example, when all decapsulators in the site are known to have an
EMTU_R of 10KB and the site's IPv4 routers are optimized for IPv4
fragmentation, encapsulators may be able to use LinkMTU values as
large as 10KB (minus 20 bytes for IPv4 encapsulation). Conversely,
when IPv4 fragmentation causes performance degradation along some
paths, LinkMTU should be set to a smaller value.

Nodes that use a static MTU assignment SHOULD copy the value in an
MTU option received in any Router Advertisement message into LinkMTU
for the ISATAP interface as specified in ([4], section 6.3.4).

C.2 Stateful (Dynamic) MTU Determination

When the encapsulator implements a dynamic MTU determination
mechanism it keeps a link layer cache of per-neighbor MTU values
(e.g., as ancillary data in the IPv6 neighbor cache, in the IPv4 path
MTU discovery cache, etc.). IPv4 path MTU discovery [25] uses ICMPv4
"fragmentation needed" messages, but these generally do not provide
enough information for stateless translation to ICMPv6 "packet too
big" messages (see: RFC 792 [26] and RFC 1812 [27], section 4.3.2.3).
Additionally, ICMPv4 "fragmentation needed" messages can be spoofed,
filtered, or not sent at all by some forwarding nodes. Thus, IPv4
Path MTU discovery used alone may be inadequate and can result in
black holes that are difficult to diagnose [28].

Alternate methods for determining per-neighbor MTUs should be used
when RFC 1191 path MTU discovery is deemed inadequate. In these
methods, the encapsulator uses periodic and/or on-demand probing of
the IPv4 path to the decapsulator to initialize and update cache
entries. The following three probing methods (among others) are
possible:

1. Encapsulator-driven - the encapsulator periodically sends probe
packets with the DF bit set in the IPv4 header and waits for a
positive acknowledgement from the decapsulator that the probe was
received

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)

3. Hybrid - the encapsulator and decapsulator engage in a dialogue
and use "intelligent" probing to monitor the path MTU

These methods are discussed in detail in the following subsections:

C.2.1 Encapsulator-driven Method

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In this method, the encapsulator sets the DF bit in the IPv4 header
of probe packets. Probe packets may be sent either when the
encapsulator's link layer forwards a large data packet to the
decapsulator (i.e., on-demand) or when the path MTU for the
decapsulator has not been verified for some time (i.e., periodic).
IPv6 Neighbor Solicitation (NS) or ICMPv6 ECHO_REQUEST packets with
padding bytes added could be used for this purpose, since successful
delivery results in a positive acknowledgement that the probe
succeeded vis-a-vis a response from the decapsulator.

While probing, the encapsulator maintains a queue of packets that
have the decapsulator as the IPv6 next-hop address. 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 that are
larger than the last known successful probe are dropped and an ICMPv6
"packet too big" message returned to the sender [29]. The queue
should be large enough to buffer the (delay*bandwidth) product for
the round-trip time to the decapsulator. When smaller queues are
used, loss of packets that are too big for the yet-to-be-determined
path MTU may occur with no ICMPv6 "packet too big" message returned.
Such loss may occur only in rare instances, but may result in
unpredictable behavior in senders that base their adaptation solely
on ICMPv6 "packet too big" messages.

This method has the advantage that the decapsulator need not
implement any special mechanisms, since standard IPv6 request/
response mechanisms are used. Additionally, the encapsulator is
assured that any packets that are too large for the decapsulator to
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, queues may become large on
Long, Fat Networks (LFNs) (see: RFC 1323 [30]).

C.2.2 Decapsulator-driven Method

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 advantage that the data packets themselves are
used as probes and no queuing on the encapsulator is necessary.
(When large data packets for probing are not available, smaller data

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packets can be null-padded to the desired probe size by artificially
inflating the length field in the IPv4 header; leaving the IPv6
length unchanged.) An additional advantage is that fewer packets will
be lost since the decapsulator will quite often be able to reassemble
packets fragmented by the network. The primary disadvantage for this
method is that, using the current specifications, the encapsulator
has no way of knowing whether a particular decapsulator implements
the "fragmentation experienced" signaling 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 decapsulator 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 decapsulator 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 a
neighbor discovery message header would provide a means for the
decapsulator to inform the encapsulator that dynamic MTU discovery is
supported.

C.2.3 Hybrid Method

In this method, the encapsulator and decapsulator engage in a
"neighbor affiliation" protocol to negotiate link-layer parameters
such as MTU. (See: [31] 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 signaling protocol between the
endpoints is complicated and additional state is required in both encapsulated as: '01-00-5E-FF-FE-01-02-03').
But, the
encapsulator and decapsultor. The hybrid method seems best suited to
implementation in specification also provides a reliable transport-layer protocol rather than at
the network/link layer.

C.2.4 Additional Notes

o In all dynamic 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 special TYPE (0xFE) to
indicate an IPv4 address is embedded. Thus, when the actual path MTU. Enlightened senders will
interpret the loss as due to link/buffer restrictions and
immediately reduce their MTU estimate. first four

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o In all dynamic methods, when a Router Advertisement (RA) message
includes

octets of an MTU option hosts SHOULD NOT copy the option's value
into LinkMTU for the ISATAP interface. Instead, when the ISATAP IPv6 interface uses a per-neighbor path MTU cache, hosts SHOULD copy
the MTU option's value into the cache entry for the neighbor that
sent the RA message. This leaves an ambiguous interpretation for
processing received RA messages which could be eliminated if [4]
were modified to allow Neighbor Advertisement (NA) messages to
carry MTU options.

o In all methods, a "minimum MTU" must be supported by all nodes for
multicast (i.e., even when multicast is emulated on the NBMA IPv4
network.) The mechanisms described above speak only to identifier are: '00-00-5E-FE' (note: the unicast
case for MTU determination.

o To avoid denial-of-service attacks that would cause superfluous
probing based on counting down/up by small increments, plateau
tables (e.g., [25], section 7) should
'u/l' bit MUST be used when 0) the actual MTU
value is indeterminant.

o ICMPv4 "fragmentation needed" messages may result when a link
restriction interface identifier is encountered but may also come from denial of
service attacks. Implementations should treat ICMPv4
"fragmentation needed" messages as "tentative" negative
acknowledgments and apply heuristics to determine when said to suspect
an actual link restriction be in
"ISATAP format" and when to ignore the messages. IPv6
packets lost due actual link restrictions are perceived as lost
due to congestion by the original source, but robust
implementations minimize instances of such packet loss without
ICMPv6 "packet too big" messages returned to the sender. next four octets embed an IPv4 address
encoded in network byte order.

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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement

Funding for the RFC Editor function is currently provided by the
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----