WDIFF

draft-ietf-ngtrans-isatap-09.txt --> draft-ietf-ngtrans-isatap-10.txt

NGTRANS Working Group F. Templin
Internet-Draft Nokia
Expires: July 1, 2003 4, 2002 T. Gleeson
Cisco Systems K.K.
M. Talwar
D. Thaler
Microsoft Corporation
December 31,
January 03, 2002

Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
draft-ietf-ngtrans-isatap-09.txt
draft-ietf-ngtrans-isatap-10.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
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Internet-Drafts are draft documents valid for a maximum of six months
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The list of current Internet-Drafts can be accessed at http://
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This Internet-Draft will expire on July 1, 2003. 4, 2002.

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 . . . . . . . . . . . . . . . . . . . . . . . . 4 3
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Basic IPv6 Non-Broadcast, Multiple Access (NBMA) Operation on ISATAP Links . . . . . . 4
5.1 Multicast . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1
5.2 Interface Identifiers and Address Construction . . . . . . . 5
5.2
5.3 ISATAP Link/Interface Configuration . . . . . . . . . . . . 5
5.3 Dual Stack Operation and
5.4 Link Layer Address Configuration . Options . . . . . . 6
5.4 Tunneling Mechanisms . . . . . . . . . . . 6
6. Automatic Tunneling . . . . . . . . . 6
5.4.1 Encapsulation . . . . . . . . . . . 6
6.1 Dual IP Layer Operation . . . . . . . . . . . . 6
5.4.2 Tunnel MTU and Fragmentation . . . . . . 6
6.2 Encapsulation . . . . . . . . . . 6
5.4.3 Handling IPv4 ICMP Errors . . . . . . . . . . . . . 6
6.3 Tunnel MTU and Fragmentation . . . . 7
5.4.4 Decapsulation . . . . . . . . . . . . 7
6.4 Handling IPv4 ICMP Errors . . . . . . . . . . . 7
5.4.5 Link-Local Addresses . . . . . . 8
6.5 Local-Use IPv6 Unicast Addresses . . . . . . . . . . . . . . 7
5.4.6 8
6.6 Ingress Filtering . . . . . . . . . . . . . . . . . . . . . 7
6. 8
7. Neighbor Discovery and Address Autoconfiguration for ISATAP Links . . . . . . . . . . . . 8
6.1
7.1 Address Resolution . . . . . . . . . . . . . . . . . . . . . 8
6.2 Address Autoconfiguration and 9
7.2 Router and Prefix Discovery . . . . . . . . . . . . . . . . 9
6.2.1
7.2.1 Conceptual Data Structures . . . . . . . . . . . . . . . . . 9
6.2.2
7.2.2 Validity Checks for Router Advertisements . . . . . . . . . 10
6.2.3
7.2.3 Router Specification . . . . . . . . . . . . . . . . . . . . 10
6.2.4 11
7.2.4 Host Specification . . . . . . . . . . . . . . . . . . . . . 11
7.
8. ISATAP Deployment Considerations . . . . . . . . . . . . . . 12
7.1
8.1 Host And Router Deployment Considerations . . . . . . . . . 12
7.2
8.2 Site Administration Considerations . . . . . . . . . . . . . 12
8.
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13
9.
10. Security considerations . . . . . . . . . . . . . . . . . . 13
10.
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
Normative References . . . . . . . . . . . . . . . . . . . . 14
Informative References . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
A. Major Changes . . . . . . . . . . . . . . . . . . . . . . . 17
B. Rationale for Interface Identifier Construction . . . . . . 18
C. Dynamic Per-neighbor MTU Discovery . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . 21

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1. Introduction

This document presents a simple approach that enables incremental
deployment of IPv6 [1] within IPv4-based [2] sites in a manner that
is compatible with inter-domain tunneling mechanisms, e.g., RFC 3056
(6to4) [18]. sites. We refer to this
approach as the Intra-Site Automatic Tunnel Addressing Protocol
(ISATAP). 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 an a link layer for IPv6.

This document specifies details for the transmission operation of IPv6 packets over ISATAP
links (i.e., automatic IPv6-in-IPv4 tunneling), including an
interface identifier format that embeds an IPv4 address. This format
supports IPv6 protocol mechanisms for address configuration as well
as simple link-layer address mapping. Simple validity checks
for received packets are given. Also specified in this
document is the operation of IPv6 Neighbor Discovery for ISATAP. The
document finally presents deployment and security considerations for ISATAP. 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 (i.e., no configured tunnel
state)

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 manual
configuration

o supports networks that use non-globally unique IPv4 addresses
(e.g., when private address allocations [3] are used), but does
not allow the virtual ISATAP link to span a Network Address
Translator [4]

o compatible with other NGTRANS mechanisms (e.g., 6to4 [18])

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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 [5].

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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.

4. Terminology

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

link; on-link:

link, on-link, off-link:
same definitions as ([6], section 2.1).

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

ISATAP link:
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.

ISATAP address:
an on-link address on an ISATAP interface and with an interface
identifier constructed as specified in Section 5.1 5.2

ISATAP router:
an IPv6 node that has an ISATAP interface over which it forwards
packets not explicitly addressed to itself.

ISATAP host:
any node that has an ISATAP interface and is not an ISATAP router.

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5. Basic IPv6 Non-Broadcast, Multiple Access (NBMA) Operation on ISATAP Links

ISATAP links transmit 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 is treated as a Non-Broadcast, Multiple
Access (NBMA) link layer. The following subsections outline basic
operational details RFC 2491 [7] provides a general
architecture for IPv6 on ISATAP links:

5.1 Interface Identifiers and Address over NBMA networks that forms the basis for

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companion documents such as the present. The following subsections
present NBMA considerations for IPv6 on ISATAP links:

5.1 Multicast

ISATAP links most closely meet the description for connectionless
service found in the last paragraph of ([7], section 1), i.e., ISATAP
addresses provide the sender with an NBMA destination address to
which it can transmit packets whenever it desires. Thus, multicast
emulation mechanisms are not required to support host-side operation
of the IPv6 neighbor discovery protocol.

5.2 Interface Identifiers and Address Construction

(RFC2491 [7],

([7], section 5.1) requires companion documents to specify the exact
mechanism for generating interface tokens (i.e., identifiers).
Interface identifiers for ISATAP are compatible with the EUI-64
identifier format ([8], section 2.5.1), and are constructed by
appending an IPv4 address on the ISATAP link to the 32-bit string
'00-00-5E-FE'. (Appendix B includes non-normative text explaining
the rationale for this construction rule.)

Global and Local-use ISATAP addresses are constructed as follows:

Figure 1

For example, the global unicast address:

3FFE:1A05:510:1111:0:5EFE:8CAD:8108

has a prefix of '3FFE:1A05:510:1111::/64' and an ISATAP interface
identifier with embedded IPv4 address: '140.173.129.8'. The address
is alternately written as:

3FFE:1A05:510:1111:0:5EFE:140.173.129.8

(Similar examples

Examples for local-use addresses are made obvious by from the above and with
reference to the IPv6 addressing architecture
document.)

5.2 ([8], section 2.5.6).

5.3 ISATAP Link/Interface Configuration

ISATAP Link/Interface configuration is consistent with (RFC2491 [7], ([7], sections 5.1.1 and 5.1.2).

Using the terminology of Section 4, an ISATAP link consists of one or

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5.1.1 and 5.1.2).

An ISATAP link consists of one or more underlying links that support
IPv4 for tunneling within a site. ISATAP interfaces are configured
over ISATAP links; each IPv4 address assigned to an underlying link
is seen as a link-layer address for ISATAP.

5.3 Dual Stack Operation and

5.4 Link Layer Address Configuration

ISATAP uses the same specification found in ([9], Options

([7], section 2). That
is, ISATAP nodes implement "IPv6/IPv4" or "dual-stack" configurations 5.2) requires companion documents to specify the
contents of the [NTL], [STL], [NBMA Number] and operate with both stacks enabled. Address configuration [NBMA Subaddress]
fields for link layer address options. For ISATAP links:

o the [NTL] and DNS
considerations are [STL] fields MUST be zero

o the same as [NBMA Number] encodes a 4-octet IPv4 address

o the [NBMA Subaddress] field is omitted

([7], section 5.2) does NOT require companion documents to specify
the value for ([9], sections 2.1 [Length], i.e., the total length of the option in 8
octets. Senders may therefore set [Length] to any value between 1
and 2.2)

5.4 255; when [Length] is greater than 1, receivers treat any bytes
that follow the [NBMA Number] as null-padding.

6. Automatic Tunneling Mechanisms

The common tunneling mechanisms specified in ([9], sections 3.1
through 3.7) 2 and 3)
are used, with the following noted specific
considerations:

5.4.1 Encapsulation

The considerations for
ISATAP:

6.1 Dual IP Layer Operation

ISATAP uses the same specification found in ([9], section 2). That
is, ISATAP nodes provide complete IPv4 and IPv6 implementations and
are able to send and receive both IPv4 and IPv6 packets. ISATAP
nodes operate with both their IPv4 and IPv6 stacks enabled.

Address configuration considerations are the same as for ([9],
section 2.1). Additionally, ISATAP nodes require that IPv4 address
configuration take place on at least one underlying link prior to
IPv6 address configuration on an ISATAP link.

DNS considerations are the same as ([9], sections 2.2 and 2.3).

6.2 Encapsulation

The specification in ([9], section 3.1) is used. Additionally, the

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IPv6 next-hop address for packets sent on an ISATAP link MUST be an
ISATAP address; other packets are discarded (i.e., not encapsulated) and an ICMPv6 destination
unreachable indication with code 3 (Address Unreachable) [10] is
returned to the source.

5.4.2

6.3 Tunnel MTU and Fragmentation

The specification in ([9], section 3.2) is NOT used; the
specification in this section is used instead.

ISATAP uses automatic tunnel interfaces that 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 the following considerations apply when IPv4 is used
as a link layer for IPv6: the MTU of ISATAP
interfaces apply:

o nearly Nearly all IPv4 nodes accept unfragmented packets up connect to physical links with MTUs of 1500
bytes or larger (e.g., Ethernet)

o sub-IPv4 Sub-IPv4 layer encapsulations (e.g., VPN) may occur on some paths

o commonly-deployed Commonly-deployed VPNs use an MTU of 1400 bytes

Thus,

Unless a dynamic per-neighbor MTU discovery mechanism is implemented,
ISATAP interfaces SHOULD MUST use an MTU (ISATAP_MTU) of no more than 1380
bytes (1400 minus 20 bytes for IPv4 encapsulation) to maximize
efficiency and minimize IPv4 fragmentation. fragmentation for the predominant
deployment case. ISATAP_MTU MAY be set to a larger values value when the
encapsulator uses

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ISATAP_MTU discovery
mechanism, but this value SHOULD NOT exceed the maximum largest MTU of all
underlying links
minus (minus 20 bytes for link layer encapsulation. (Appendix IPv4 encapsulation). Appendix C
provides non-normative considerations for dynamic per-neighbor MTU discovery.)

As with ordinary IPv6 interfaces, the
discovery.

The network layer (IPv6) forwards packets of size ISATAP_MTU or
smaller to the ISATAP interface. All other packets are dropped, and
an ICMPv6 "packet too big" message with MTU = ISATAP_MTU is returned
to the source [11]. The ISATAP interfaces send all link layer encapsulates packets of
size 1380 bytes or smaller with the Don't Fragment (DF) bit NOT set
in the encapsualting IPv4 header.

5.4.3 Handling

Nodes that configure ISATAP interfaces MUST have IPv4 ICMP Errors

The specification reassembly
buffers large enough to receive packets with the DF bit not set in ([9],
the encapsulating IPv4 header. RFC 1122 [12], section 3.4) MAY be used. 3.3.2
specifies that the Effective MTU to Receive (EMTU_R) for IPv4 ICMP
errors nodes:

"...MUST be greater than or equal to 576, SHOULD be either
configurable or indefinite, and ARP failures are otherwise processed as link error
notifications.

5.4.4 Decapsulation

The specification in ([9], SHOULD be greater than or equal to

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the MTU of the connected network(s)".

With reference to this specification, the EMTU_R for nodes that
configure ISATAP interfaces MUST be greater than or equal to 1500
bytes (i.e., the predominant deployment case for connected IPv4
networks) and SHOULD be either configurable or indefinite.

6.4 Handling IPv4 ICMP Errors

The specification in ([9], section 3.6) is 3.4) MAY be used.

5.4.5 Link-Local IPv4 ICMP
errors and ARP failures are otherwise processed as link error
notifications.

6.5 Local-Use IPv6 Unicast Addresses

The specification in ([9], section 3.7) is NOT used. Instead,
link-local local
use IPv6 unicast addresses are formed by appending an interface identifier, exactly as defined specified in Section 5.1, to the prefix FE80::/64.

5.4.6 ([8],
section 2.5.6).

6.6 Ingress Filtering

The network layer (IPv6) destination address of a packet received specification in ([9], section 3.9) is used on
an ISATAP interface is either local (i.e., matches router
interfaces. (ISATAP host interfaces silently discard any packets
received with a foreign IPv6 destination address, i.e., an address
not configured on the local IPv6 stack) or foreign. The decapsulator
MUST be configured with a list of IPv4 address prefixes that are
acceptable, i.e., an ingress filter list (default deny all). For
packets with foreign network layer (IPv6) destination addresses, the
link layer (IPv4) source address MUST be explicitly allowed by
ingress filtering. Packets that do not satisfy this condition are
silently discarded. stack.)

Additionally, all packets (whether foreign or local) received on ISATAP host and router interfaces
MUST satisfy at least one (i.e., one or both) of the following
validity checks:

o the network-layer (IPv6) source address is an on-link ISATAP
address with an interface identifier that embeds the link-layer
(IPv4) source address

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o the link-layer (IPv4) source address is in the Potential Routers
List (see Section 6.2.1) 7.2.1)

Packets that do not satisfy the above conditions are silently
discarded.

7. Neighbor Discovery and Address Autoconfiguration

RFC 2491 [7] provides a general architecture for IPv6 over NBMA
networks, including multicast mechanisms to support host-side
operation of the IPv6 neighbor discovery protocol. ISATAP links most
closely meet the description for connectionless service found in the
last paragraph of ([7], section 1), i.e., ISATAP addresses provide
the sender with an NBMA destination address to which it can transmit
packets whenever it desires. Thus, the RFC 2491 multicast mechanisms
are not required for address resolution and not otherwise implemented
on ISATAP links due to traffic scaling considerations (i.e., ISATAP
links are unicast-only). Links

RFC 2461 [6] 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

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Solicitations and Advertisements on NBMA links is not specified in
this document."

ISATAP links SHOULD implement Redirect, Neighbor Unreachability
Detection, and next-hop determination exactly as specified in [6].
Address resolution and the mechanisms for delivering Router
Solicitations and Advertisements on for ISATAP links use the
specifications found in this document.

6.1 Address Resolution

ISATAP addresses are resolved to link-layer addresses (IPv4) not specified
by a
static computation, i.e., the last four octets are treated as an IPv4
address. ([7], section 5.2) requires companion documents to specify
the format for link layer address options, however, link layer
address options [6]; instead, they are not needed for address resolution in ISATAP.
Thus, no format is specified and the following specification from
([9], section 3.8) applies:

"This means in this document. (Note that a sender
these mechanisms MAY potentially apply to other types of Neighbor Discovery packets

* SHOULD NOT include Source Link Layer Address options or Target
Link Layer Address options on NBMA links
in the tunnel link.

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7.1 Address Resolution

ISATAP December 2002

* MUST silently ignore any received SLLA or TLLA options on addresses are resolved to link-layer addresses (IPv4) by a
static computation, i.e., the
tunnel link." last four octets are treated as an IPv4
address.

Following static address resolution, ISATAP hosts SHOULD implement
the perform an
initial reachability confirmation specifications by sending unicast Neighbor
Solicitations (NSs) and receiving a Neighbor Advertisement using the
mechanisms specified in [6], ([6], sections
7.2.2-7.2.8 7.2.2-7.2.8). (Note that apply
implementations MAY omit the source/target link layer options in NS/
NA messages when unicast Neighbor Solicitations (NS) are
used. is used.)

ISATAP hosts SHOULD additionally perform Neighbor Unreachability
Detection (NUD) as specified in (RFC 2461 [6], ([6], section 7.3). ISATAP routers
MAY perform the above-specified reachability detection and NUD
procedures, but this might not scale in all environments.

All ISATAP nodes MUST send solicited neighbor advertisements ([6],
section 7.2.4).

ISATAP links disable Duplicate Address Detection, as permitted by
([12], section 4).

6.2 Address Autoconfiguration and

7.2 Router and Prefix Discovery

Since NBMA multicast emulation mechanisms are not used on used, ISATAP
links, nodes
will not receive unsolicited multicast Router Advertisements. (RFC 2462 [12], section 5.5.2) requires that hosts
use stateful autoconfiguration (i.e., DHCPv6 [13]) in the absence of
Router Advertisements. When statelful autoconfiguration is not
available, nodes use Thus,
alternate mechanisms (described below) for
router are required and prefix discovery.

6.2.1 specified below:

7.2.1 Conceptual Data Structures

ISATAP nodes use the conceptual data structures Prefix List and
Default Router List exactly as in ([6], section 5.1). ISATAP links
add two a new conceptual data structures "Potential Router List" and
"Stateful Autoconfiguration Server List". the
following new configuration variable:

ResolveInterval
Time between name service resolutions. Default and suggested
minimum: 1hr

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A Potential Router List (PRL) and Stateful Autoconfiguration Server
List (SASL) is associated with every ISATAP link.
The PRL provides a trust basis for router validation (see security
considerations). Each entry in the PRL has an IPv4 address and an
associated timer. The IPv4 address represents a router's ISATAP
interface (likely to be an "advertising interface"), and is used to
construct the ISATAP link-local address for that interface. Similarly, each entry in the
SASL has an IPv4 address and associated timer. The IPv4 address
represents a DHCPv6 server attached to the ISATAP link, and is used
to construct
following sections specify the ISATAP link-local address process for that DHCPv6 server. initializing the PRL:

When a node enables an ISATAP link, it first discovers IPv4 addresses
for the PRL and SASL. PRL. The addresses MAY SHOULD be established by a DHCPv4

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[14] [13]
option for ISATAP (option code TBD), by manual configuration, or by
an unspecified alternative alternate method (e.g., DHCPv4 vendor-specific
option; DNS ([19]) fully-qualified domain names).
option).

When no other mechanisms are available, a DNS fully-qualified domain names are used,
name (FQDN) [20] MAY be used. In this case, the FQDN is resolved
into IPv4 addresses for the PRL and SASL are discovered through a static host file file, a
site-specific name service, or by querying an IPv4-based DNS server to resolve the domain names into
address records (e.g., DNS 'A' resource records) containing IPv4
addresses. server.
Unspecified alternative alternate methods for domain name resolution may also be
used. The following notes apply when DNS
fully-qualified domain names are used: apply:

1. Site administrators maintain domain names and a list of IPv4 addresses for
the PRL and SASL for the site's
representing ISATAP service, e.g., as address
records in the site's name service. Administrators may also
advertise router interfaces and make them available via
one or more of the domain names in a DHCPv4 option for ISATAP. mechanisms described above.

2. There are no mandatory rules for the selection of domain names, a FQDN, but
administrators are encouraged to use the convention
"(list_name).isatap.domainname"
"isatap.domainname" (e.g., prl.isatap.example.com). isatap.example.com).

3. After initialization, nodes periodically re-initialize the PRL
and SASL, e.g., once per hour.
(after ResolveInterval). When DNS is used, client DNS resolvers
use the IPv4 transport to resolve the names and follow the cache
invalidation procedures in [19] [20] when the DNS time-to-live
expires.

6.2.2

7.2.2 Validity Checks for Router Advertisements

A node MUST silently discard any Router Advertisement messages it
receives that do not satisfy both the validity checks in ([6],
section 6.1.2) and the following additional validity check for
ISATAP:

o the network-layer (IPv6) source address is an ISATAP address and
embeds an IPv4 address from the PRL

6.2.3

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7.2.3 Router Specification

Advertising ISATAP interfaces of routers behave the same as
advertising interfaces described in ([6], section 6.2). However,
periodic unsolicited multicast Router Advertisements are not used,
thus the "interval timer" associated with advertising interfaces is
not used for that purpose.

When an ISATAP router receives a valid Router Solicitation on an

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advertising ISATAP interface, it replies with a unicast Router
Advertisement to the address of the node which sent the Router
Solicitation. The source address of the Router Advertisement is a
link-local unicast address associated with the interface. This MAY
be the same as the destination address of the Router Solicitation.
ISATAP routers MAY engage in the solicitation process described under
Host Specification below, e.g., if Router Advertisement consistency
verification ([6], section 6.2.7) is desired.

6.2.4

7.2.4 Host Specification

All entries in the PRL are assumed to represent active ISATAP routers
within the site, i.e., the PRL provides trust basis only; not
reachability detection. When stateful autoconfiguration is available
(i.e., when the SASL is non-null and at least one DHCPv6 server is
reachable), hosts may send unicast messages directly to the DHCPv6
server as specified in ([13], section 1.1). Hosts ISATAP nodes SHOULD attempt
stateful autoconfiguration for each entry in the SASL (i.e., until an
attempt succeeds) before concluding that use stateful autoconfiguration
is unavailable.
configuration to assign IPv6 prefixes and default router information.
When stateful autoconfiguration configuration is unavailable, not available, hosts MAY periodically
solicit information from one or more entries in the PRL ("PRL(i)") by
sending unicast Router Solicitation messages using the IPv4 address
("V4ADDR_PRL(i)") and associated timer in the entry. Hosts add the
following variable to support the solicitation process:

MinRouterSolicitInterval
Minimum time between sending Router Solicitations to any router.
Default and suggested minimum 800,000 milliseconds (15min). minimum: 15min.

When a PRL(i) is selected, the host sets its associated timer to
MinRouterSolicitInterval and initiates solicitation following a short
delay as in ([6], section 6.3.7). The manner of choosing particular
routers in the PRL for solicitation is outside the scope of this
specification. The solicitation process repeats when the associated
timer expires.

Solicitation consists of sending Router Solicitations to the ISATAP
link-local address constructed from the entry's IPv4 address, i.e.,
they are sent to 'FE80::0:5EFE:V4ADDR_PRL(i)' instead of 'All-Routers
multicast'. They are otherwise sent exactly as in ([6], section
6.3.7).

Hosts process received Router Advertisements exactly as in ([6],

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section 6.3.4). Hosts additionally reset the timer associated with
the V4ADDR_PRL(i) embedded in the network-layer source address in
each solicited Router Advertisement received. The timer is reset to

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either 0.5 * (the minimum value in the router lifetime or valid
lifetime of any on-link prefixes received in the advertisement) or
MinRouterSolicitInterval; whichever is longer.

8. ISATAP Deployment Considerations

7.1

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 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 native link, the host SHOULD discontinue the
router solicitation process described in the host specification and
allow existing ISATAP address configurations to expire as specified
in ([6], section 5.3) and ([12], ([14], section 5.5.4). Any ISATAP
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 an interface to simultaneously support both
native IPv6, and also ISATAP (over IPv4). Routing will operate as
usual between these two domains. Note that the prefixes used on the
ISATAP and native IPv6 interfaces will be distinct. The IPv4
address(es) configured on a router's ISATAP interface(s) SHOULD be
added (either automatically or manually) to the site's address
records for ISATAP router interfaces.

7.2

8.2 Site Administration Considerations

The following considerations are noted for sites that deploy ISATAP:

o ISATAP links are administratively defined by a set of router
interfaces, a set of stateful autoconfiguration servers,
interfaces and set of nodes which discover those interface and
server addresses Thus, ISATAP links are defined by administrative
(not physical) boundaries.

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

o When stateful autoconfiguration is not available, ISATAP nodes MAY

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periodically send unicast Router Solicitations to and receive
unicast Router Advertisements from to one or more members of the

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potential router list. A well-deployed stateful autoconfiguration
service within the site can minimize and/or eliminate the need for
periodic solicitation.

o ISATAP nodes periodically refresh the entries on the PRL and SASL. PRL.
Responsible site administration can reduce the control traffic.
At a minimum, administrators SHOULD ensure that dynamically
advertised information for the site's PRL and SASL are is well maintained.

9. IANA Considerations

A DHCPv4 option code for ISATAP (TBD) [20] [21] is requested in the event
that the IESG recommends this document for standards track.

10. Security considerations

Site administrators are advised that, in addition to possible attacks
against IPv6, security attacks against IPv4 MUST also be considered.

Responsible IPv4 site security management is strongly encouraged. In
particular, border gateways SHOULD implement filtering to detect
spoofed IPv4 source addresses at a minimum; ip-protocol-41 filtering
SHOULD also be implemented.

If IPv4 source address filtering is not correctly implemented, the
ISATAP validity checks will not be effective in preventing IPv6
source address spoofing.

If filtering for ip-protocol-41 is not correctly implemented, IPv6
source address spoofing is clearly possible, but this can be
eliminated if both IPv4 source address filtering, and the ISATAP
validity checks are implemented.

(RFC 2461 [6]), section 6.1.2 implies that nodes trust Router
Advertisements they receive from on-link routers, as indicated by a
value of 255 in the IPv6 'hop-limit' field. Since this field is not
decremented when ip-protocol-41 packets traverse multiple IPv4 hops
([9], section 3), ISATAP links require a different trust model. In
particular, ONLY those Router Advertisements received from a member
of the Potential Routers List are trusted; all others are silently
discarded. This trust model is predicated on IPv4 source address
filtering, as described above.

The ISATAP address format does not support privacy extensions for
stateless address autoconfiguration [21]. [22]. However, since the ISATAP
interface identifier is derived from the node's IPv4 address, ISATAP

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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 issue is the same for NAT'd addresses.)

10.

11. 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
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 [22] [23]
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. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.

[2] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.

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

[4] Egevang, K. and P. Francis, "The IP Network Address Translator
(NAT)", RFC 1631, May 1994.

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

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[6] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery

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for IP Version 6 (IPv6)", RFC 2461, December 1998.

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

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

[9] Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms for
IPv6 Hosts and Routers", draft-ietf-ngtrans-mech-v2-01 (work in
progress), November 2002.

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

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

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

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

[14] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.

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

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

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

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

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

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

Informative References

[18]

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

[19]

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

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[20]

[21] Droms, R., "Procedures and IANA Guidelines for Definition of
New DHCP Options and Message Types", BCP 43, RFC 2939,
September 2000.

[21]

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

[22]

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

[23]

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

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

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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 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

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

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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:

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

Figure 2

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,

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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').

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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. Dynamic Per-neighbor MTU Discovery

ISATAP encapsulators and decapsulators are IPv6 neighbors that may be
separated by multiple link layer (IPv4) forwarding hops. When
ISATAP_MTU is larger than 1380 bytes, the encapsulator must implement
a dynamic link layer mechanism to discover per-neighbor MTUs.

IPv4 path MTU discovery [15] relies on ICMPv4 "fragmentation needed"
messages, but these do not provide enough information for stateless
translation into ICMPv6 "packet too big" messages (see: RFC 792 [16]
and RFC 1812 [17], 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 is inadequate and can result in black holes that are difficult
to diagnose [23].

The ISATAP encapsulator are difficult
to diagnose [24].

The ISATAP encapsulator may implement an alternate per-neighbor MTU
discovery mechanism, e.g., periodic and/or on-demand probing of the
IPv4 path to the decapsulator. Probing consists of sending packets
larger than 1380 bytes to the neighbor and receiving positive
confirmation of receipt. Two methods are possible:

In the first method, the encapsulator does NOT set the DF bit in the
IPv4 header of probe packets. In this case, the encapsulator must
have a priori knowledge of the decapsulator's reassembly buffer size
and should have a priori knowledge of the decapsulator's link MTU.
This method has the advantage that probe packets will be delivered
even if the network performs fragmentation, thus ordinary data
packets may implement an alternate per-neighbor MTU
discovery mechanism, e.g., periodic and/or on-demand be used for probing resulting in greater efficiency.
Disadvantages for this method include:

o special mechanisms required on both encapsulator and decapsulator

o extra state required on both encapsulator and decapsulator

o complex protocol signalling between encapsulator and decapsulator

o possible extended periods of network fragmentation

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In the
IPv4 path to second (and preferred) method, the decapsulator. Probing consists of sending packets
larger than 1380 bytes with encapsulator sets the DF
bit set in the IPv4 header. header of probe packets. Neighbor Solicitation (NS)
packets with padding bytes added should be used for this purpose,
since successful delivery results in a positive acknowledgement that
the probe succeeded, i.e., in the form of a Neighbor Advertisement
(NA) from the decapsulator. (NB: Setting the DF bit prevents the network
from fragmenting the packets and protects decapsulators from
receiving probe packets that
would might overrun the receive buffer IPv4 reassembly buffer.
Additionally, special mechanisms and state are needed only on an underlying link, thus the
encapsulator, and no
maximum receive unit (MRU) complex protocol signalling between the
encapsulator and decapsulator is required.)

Implementations required.

In either method, implementations may choose to couple the probing
process with neighbor cache management procedures ([6], section 7),
e.g. to maintain timers, state variables and/or a queue of packets
waiting for probes to complete. Packets retained on the queue are
forwarded when probes succeed, and provide state for sending ICMPv6
"packet too big" messages to the source when probes fail.
Implementations may choose to store per-neighbor MTU information in
the IPv4 path MTU discovery cache, in the ISATAP link layer's private
data structures, etc.

Additional notes:

1. Per-neighbor MTUs may vary dynamically due to fluctuations in the
IPv4 forwarding path and/or multipath routing (e.g., when QoS
routing is used in the IPv4 network). For such neighbors,
encapsulators should detect a "losing battle" and reduce the
per-neighbor MTU size to no more than 1380 bytes.

2. When not probing, encapsulators may send packets to a neighbor
with MTU greater than 1380 bytes either with the DF bit set or
not set. When the DF bit is set, undetected packet loss may
occur in the network if the neighbor's MTU decreases. When the
DF bit is NOT set, undetected packet loss is less likely but may
occur either in the network or at the neighbor's reassembly
buffer.

3. ICMPv4 "fragmentation needed" messages may result when a link
restriction is encountered but may also come from denial of
service attacks. Implementations should treat ICMPv4
"fragmentation needed"

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acknowledgments and apply heuristics to determine when to suspect
an actual link restriction 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.

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