WDIFF

draft-ietf-ngtrans-isatap-10.txt --> draft-ietf-ngtrans-isatap-11.txt

NGTRANS

Network Working Group F. Templin
Internet-Draft Nokia
Expires: July 4, 2002 18, 2003 T. Gleeson
Cisco Systems K.K.
M. Talwar
D. Thaler
Microsoft Corporation
January 03, 2002 17, 2003

Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
draft-ietf-ngtrans-isatap-10.txt
draft-ietf-ngtrans-isatap-11.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
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The list of current Internet-Drafts can be accessed at http://
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The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

This Internet-Draft will expire on July 4, 2002. 18, 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. Non-Broadcast, Multiple Access (NBMA) Basic IPv6 Operation . . . . . . 4
5.1 Multicast . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.2 Interface Identifiers and Address Construction . . . . . . . 5
5.3 ISATAP Link/Interface Configuration . . . . . . . . . . . . 5
5.4 Link Layer Address Options . . . . . . . . . . . . . . . . . 6 4
6. Automatic Tunneling . . . . . . . . . . . . . . . . . . . . 6
6.1 Dual IP Layer Operation . . . . . . . . . . . . . . . . . . 6
6.2 Encapsulation . . . . . . . . . . . . . . . . . 5
7. Neighbor Discovery . . . . . . 6
6.3 Tunnel MTU and Fragmentation . . . . . . . . . . . . . . . . 7
6.4 Handling IPv4 ICMP Errors . . . . . . . . . . . . . . . . . 8
6.5 Local-Use IPv6 Unicast Addresses .
8. Deployment Considerations . . . . . . . . . . . . . 8
6.6 Ingress Filtering . . . . . 10
9. IANA Considerations . . . . . . . . . . . . . . . . 8
7. Neighbor Discovery for ISATAP Links . . . . . 11
10. Security considerations . . . . . . . 8
7.1 Address Resolution . . . . . . . . . . . . 11
11. Acknowledgements . . . . . . . . . 9
7.2 Router and Prefix Discovery . . . . . . . . . . . . . . 12
Normative References . . 9
7.2.1 Conceptual Data Structures . . . . . . . . . . . . . . . . . 9
7.2.2 Validity Checks for Router Advertisements . . 12
Informative References . . . . . . . 10
7.2.3 Router Specification . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . 11
7.2.4 Host Specification . . . . . . . . . . . . . . . 14
A. Major Changes . . . . . . 11
8. ISATAP Deployment Considerations . . . . . . . . . . . . . . 12
8.1 Host And Router Deployment Considerations . . . . 15
B. Rationale for Interface Identifier Construction . . . . . 12
8.2 Site Administration Considerations . . 17
C. Dynamic MTU Discovery . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . . . . 13
10. Security considerations . . . . . . . . . . . . . . . . . . 13
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
Normative References . . . . . . . . . . . . . . . . . . . . 14
Informative References . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
A. Major Changes . . . . . . . . . . . . . . . . . . . . . . . 17
B. Rationale for Interface Identifier Construction . . . . . . 18
C. Dynamic Per-neighbor MTU Discovery . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . 21 22

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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] within IPv4-based IPv4 [2] 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 a link layer for IPv6.

This document specifies

Specific details for the operation of IPv6 and automatic tunneling
over ISATAP links (i.e., automatic IPv6-in-IPv4 tunneling), are given, including an interface identifier format
that embeds an IPv4 address. This format supports IPv6 protocol mechanisms for address
configuration as well
as and simple link-layer address mapping. Also specified in this
document
is the operation of IPv6 Neighbor Discovery for ISATAP. The
document finally presents deployment and security 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 (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] [18] are used), but does
not allow the virtual ISATAP link to span a Network Address
Translator [4] used)

o compatible with other NGTRANS mechanisms (e.g., 6to4 [19])

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

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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] applies to this document. The
following additional terms are defined:

link, on-link, off-link:
same definitions as ([6], ([4], 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.

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

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

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.

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

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 as a Non-Broadcast, Multiple
Access (NBMA) link layer. RFC 2491 [7] provides a general
architecture for IPv6 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 noted:

5.1 Interface Identifiers and Address Construction

([7], section 5.1) requires companion documents to specify the exact
mechanism for generating Unicast Addresses

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

Global rule.

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With reference to ([5], sections 2.5.4, 2.5.6), global and Local-use 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

Examples for local-use addresses are obvious from the above and with
reference to ([8], section 2.5.6).

5.3

5.2 ISATAP Link/Interface Configuration

ISATAP Link/Interface configuration is consistent with ([7], sections

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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.

5.4 Link Layer Address Options

([7], section 5.2) requires companion documents to specify the
contents of the [NTL], [STL], [NBMA Number] and [NBMA Subaddress]
fields for link layer

At least one link-layer address options. For per advertising ISATAP links:

o interface
SHOULD be added to the Potential Routers List (see Section 7.3.1).

5.3 Link Layer Address Options

With reference to ([6], section 5.2), when the [NTL] and [STL] fields MUST be zero

o
in an ISATAP link layer address option encode 0, the [NBMA Number]
field encodes a 4-octet IPv4 address

o the [NBMA Subaddress] field address.

5.4 Multicast and Anycast

As for any IPv6 interface, an ISATAP interface is omitted

([7], section 5.2) does NOT require companion documents required to specify
the value
recognize certain IPv6 multicast and anycast addresses ([5], section
2.8). Mechanisms for [Length], i.e., the total length of the option in 8
octets. Senders may therefore set [Length] to any value between 1 sending multicast and 255; when [Length] is greater than 1, receivers treat any bytes
that follow the [NBMA Number] anycast packets (e.g.,
[20]) are left as null-padding. future work.

6. Automatic Tunneling

The common tunneling mechanisms specified in ([9], ([7], sections 2 and 3)
are used, with the following noted specific considerations for ISATAP:

6.1 Dual IP Layer Operation

ISATAP uses the same specification found in ([9], ([7], 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. and DNS considerations are the same as ([9], ([7],
sections 2.2 and 2.1 through 2.3).

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6.2 Encapsulation

The specification in ([9], ([7], 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 and an ICMPv6 destination
unreachable indication with code 3 (Address Unreachable) [10] [8] is
returned to the source.

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 for the MTU of ISATAP
interfaces apply: interfaces:

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

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

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

Unless

To maximize efficiency and minimize IPv4 fragmentation for the
predominant deployment case, ISATAP interfaces that do not use a
dynamic per-neighbor MTU discovery mechanism is implemented,
ISATAP interfaces MUST use an MTU (ISATAP_MTU) of SHOULD set LinkMTU ([4], Section
6.3.2 ) to no more than 1380 bytes (1400 minus 20 bytes for IPv4 encapsulation) to maximize
efficiency and minimize IPv4 fragmentation for the predominant
deployment case. ISATAP_MTU
encapsulation). LinkMTU MAY be set to a larger value when the
encapsulator implements values on ISATAP
interfaces that use a dynamic per-neighbor MTU discovery
mechanism, but this value SHOULD NOT exceed the largest MTU of all
underlying links (minus 20 bytes for IPv4 encapsulation). mechanism. Appendix C
provides non-normative considerations for dynamic per-neighbor MTU discovery.

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

Nodes that configure ISATAP interfaces MUST have IPv4 reassembly
buffers large enough to receive packets with the DF bit not set in the encapsulating encapsualting IPv4
header. RFC 1122 [12], section 3.3.2
specifies that the Effective MTU to Receive (EMTU_R) for IPv4 nodes:

"...MUST be greater than or equal to 576, SHOULD be either
configurable or indefinite, and 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 3.4) MAY be used.

IPv4 ICMP errors and ARP failures are otherwise processed as link error
notifications.

6.5 Local-Use IPv6 Unicast Addresses

The specification in ([9], ([7], section 3.7) is NOT not used. Instead, local
use IPv6 unicast addresses are formed exactly as specified in ([8],
section 2.5.6). Section 5.1.

6.6 Ingress Filtering

The specification in ([9], ([7], section 3.9) is used on used. In particular,
ISATAP router
interfaces. (ISATAP host interfaces silently discard any nodes that forward decapsulated packets
received with a foreign IPv6 destination address, i.e., an address
not MUST be configured on the local IPv6 stack.)

Additionally, packets received on

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MUST satisfy at least one (i.e., one or both) January 2003

with a list of the following
validity checks:

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

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

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

7. Neighbor Discovery for ISATAP Links

RFC 2461 [6] provides are acceptable.

7. Neighbor Discovery

RFC 2461 [4] 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]. [4].
Address resolution and the mechanisms for delivering Router
Solicitations and Advertisements for ISATAP links are not specified
by [6]; [4]; instead, they are specified in this document. (Note that
these mechanisms MAY potentially apply to other types of NBMA links
in the future.)

7.1 Address Resolution and Neighbor Unreachability Detection

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

Following static address resolution, ISATAP hosts SHOULD perform an initial
reachability confirmation by sending unicast Neighbor Solicitations
(NSs) and receiving a Neighbor Advertisement using the mechanisms
specified in ([6], ([4], sections 7.2.2-7.2.8). (Note that
implementations MAY omit the source/target link layer options in NS/
NA messages when unicast is used.)

ISATAP hosts

Hosts SHOULD additionally perform Neighbor Unreachability Detection
(NUD) as specified in ([6], ([4], section 7.3). ISATAP routers 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], ([4],
section 7.2.4).

7.2 Duplicate Address Detection

Duplicate Address Detection ([9], 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

Since NBMA multicast emulation mechanisms are not used, ISATAP nodes will typically not receive unsolicited multicast
Router Advertisements. Thus,
alternate Advertisements, unicast mechanisms are required and as specified

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below:

7.2.1

7.3.1 Conceptual Data Structures

ISATAP nodes use the conceptual data structures Prefix List and
Default Router List exactly as in ([6], ([4], section 5.1). ISATAP links
add a new conceptual data structures structure "Potential Router List" and 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) 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"), advertising ISATAP interface, and is
used to construct the ISATAP link-local ISATAP address for that interface.
The following sections specify the process for initializing the PRL:

When a node enables an ISATAP link, it first discovers IPv4 addresses for
the PRL. The addresses SHOULD MAY be established by a DHCPv4 [13] [10] option
for ISATAP (option code TBD), by manual configuration, or by an unspecified
alternate method (e.g., DHCPv4 vendor-specific option).

When no other mechanisms are available, a DNS fully-qualified domain
name (FQDN) [20] [21] established by an out-of-band method (e.g., DHCPv4,
manual configuration, etc.) MAY be used. In this case, the The FQDN is resolved into
IPv4 addresses for the PRL through a static host file, a site-specific name
service, or by querying an IPv4-based a DNS server.
Unspecified server within the site, or an unspecified
alternate methods for domain name resolution may also be
used. method. The following notes apply:

1. Site administrators maintain a list of IPv4 addresses
representing advertising ISATAP router interfaces and make them
available via one or more of the mechanisms described above.

2. There are no mandatory rules for the selection of a FQDN, but
administrators are encouraged to use the convention
"isatap.domainname" (e.g., isatap.example.com).
manual configuration MUST be supported.

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

7.2.2 Validity Checks for transport.

7.3.2 Validation of Router Advertisements

A node MUST silently discard any Router Advertisement messages it
receives that do not satisfy both the validity checks Messages

The specification in ([6], ([4], section 6.1.2) and the following additional validity check for
ISATAP:

o is used.

Additionally, received RA messages that contain Prefix Information

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

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

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7.2.3

7.3.3 Router Specification

Advertising

Routers with advertising ISATAP interfaces of routers behave the same as
advertising interfaces
described in ([6], ([4], 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
advertising Advertising ISATAP interface, it replies with a unicast Router
Advertisement interfaces send
RA messages to the address of the node which sent the Router
Solicitation. The source address of the Router Advertisement is a
link-local node's unicast address associated with the interface. This MAY
be the same address, 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], permitted by ([4],
section 6.2.7) is desired.

7.2.4 6.2.6).

7.3.4 Host Specification

7.3.4.1 Sending Router Solicitations

All entries in the PRL are assumed to represent active advertising
ISATAP routers interfaces within the site, i.e., the PRL provides trust basis
only; not reachability detection. ISATAP nodes SHOULD use stateful
configuration to assign IPv6 prefixes and default router information.
When stateful configuration is not available, hosts MAY Hosts periodically solicit
information from one or more entries in the PRL ("PRL(i)") by sending
unicast Router Solicitation (RS) messages using the PRL(i)'s IPv4 address
("V4ADDR_PRL(i)") and associated timer in ("TIMER(i)"). The manner of
selecting a PRL(i) for solicitation and/or deprecating a
previously-selected PRL(i) is outside the entry. scope of this
specification. Hosts add the following variable to support the
solicitation process:

MinRouterSolicitInterval
Minimum time between sending Router Solicitations to any router. Solicitations. Default and
suggested minimum: 15min.

When a PRL(i) is selected, the host sets its associated timer TIMER(i) 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.
delay. Solicitation consists of sending Router Solicitations RS messages to the ISATAP
link-local address constructed from the entry's IPv4 address, V4ADDR_PRL(i), i.e., they are
sent to 'FE80::0:5EFE:V4ADDR_PRL(i)' instead of 'All-Routers
multicast'.
'All-Routers-multicast'. They are otherwise sent exactly as in ([6], ([4],
section 6.3.7).

7.3.4.2 Processing Router Advertisements

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

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and ([9], section 5.5.3) except that, when an RA message contains an
MTU option, hosts SHOULD NOT copy the timer associated with option's value into the V4ADDR_PRL(i) embedded in ISATAP
interface LinkMTU. Instead, when the ISATAP link layer implements a

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per-neighbor path MTU cache, hosts SHOULD copy the MTU option's value
into the cache entry for the router that sent the RA message (see:
Appendix C).

When the network-layer source address in
each solicited Router Advertisement received. The timer an RA message is an ISATAP
address that embeds V4ADDR_PRL(i) for some PRL(i) selected for
solicitation, hosts additionally reset to
either 0.5 * (the TIMER(i). Let "MIN_LIFETIME"
be the minimum value in the router lifetime or valid lifetime of any on-link
prefixes received advertised in the advertisement) or
MinRouterSolicitInterval; whichever RA message. Then, TIMER(i) is longer. reset to:

MAX((0.5 * MIN_LIFETIME), MinRouterSolicitInterval)

8. ISATAP 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 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], ([4], section 5.3) and ([14], ([9], 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 a native IPv6, IPv6 and also ISATAP (over IPv4). interface over
the same physical link. 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 advertising ISATAP interface(s) SHOULD be added (either
automatically or manually) to the site's address records for
advertising ISATAP router interfaces.

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

o ISATAP hosts Hosts and routers that use ISATAP can be deployed in an ad-hoc and
independent

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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
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.
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. IANA Considerations

A DHCPv4 option code for ISATAP (TBD) [21] is [22] may be requested in the
event that the IESG recommends this document for (or a derivative thereof) is moved to
standards track.

10. Security considerations

Site administrators are advised that, in

ISATAP site border routers and firewalls MUST implement IPv6 ingress
filtering and MUST NOT allow packets with site-local source and/or
destination addresses (i.e., addresses with prefix FEC0::/10) to
enter or leave the site.

In addition to possible attacks against IPv6, security attacks
against IPv4 MUST must also be considered.

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

If and
ip-protocol-41 filtering.

Even with IPv4 source address filtering is not correctly implemented, the and IPv6 ingress filtering, reflection attacks can
originate from nodes within an ISATAP validity checks will not be effective in preventing site that spoof IPv6 source address spoofing.

If filtering
addresses. Security mechanisms for ip-protocol-41 is not correctly implemented, IPv6
source address spoofing is clearly possible, but this can reflection attack mitigation
(e.g., [11], [12], etc.) SHOULD be
eliminated if both IPv4 used in routers with advertising
ISATAP interfaces. At a minimum, ISATAP site border gateways MUST
log potential source address filtering, and the ISATAP
validity checks are implemented. spoofing cases.

(RFC 2461 [6]), [4], section 6.1.2 6.1.2) implies that nodes trust received
Router
Advertisements they receive Advertisement (RA) messages 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 an additional validation check for 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 RA messages (see:
Section 7.3.2).

ISATAP address format does addresses do not support privacy extensions for stateless
address autoconfiguration [22]. [23]. However, since the ISATAP

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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.)
especially true when private address allocations [18] are used.)

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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 [23] [24]
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]

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

[7]

[5] Hinden, R. and 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, "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,

[7] Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms for

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IPv6 Hosts and Routers", draft-ietf-ngtrans-mech-v2-01 (work in
progress), November 2002.

[10]

[8] 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,

[9] Thomson, 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, T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 1122, October 1989.

[13] 2462, December 1998.

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

[14] Thomson, S.

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

[12] Bellovin, S., Leech, M. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.

[15] Taylor, "ICMP Traceback
Messages", draft-ietf-itrace-03 (work in progress), January
2003.

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

[16]

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

[17]

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

[18] Droms,

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

[17] Braden, R., "Dynamic Host Configuration Protocol "Requirements for IPv6
(DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress),
November 2002. Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.

Informative References

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

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

[20] Thaler, D., "Support for Multicast over 6to4 Networks",
draft-ietf-ngtrans-6to4-multicast-01 (work in progress), July
2002.

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[20] 2003

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

[21]

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

[22]

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

[23]

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

[24]

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

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

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

Authors' Addresses

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

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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 10 to version 11:

o Added multicast/anycast subsection

o Revised PRL initialization

o Updated neighbor discovery, security consideration sections

o Updated MTU section

changes from version 09 to version 10:

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

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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

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o Included reference to Quang Nguyen work

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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

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').
But, the specification also provides a special TYPE (0xFE) to
indicate an IPv4 address is embedded. Thus, when the first four

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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 an
encapsulator's interface configures a LinkMTU ([4], Section 6.3.2)
value larger than 1380 bytes, the encapsulator must implement a dynamic link layer (IPv4) mechanism
is required to discover per-neighbor path MTUs. The following text
gives non-normative considerations for dynamic MTU discovery.

IPv4 path MTU discovery [15] relies on [13] uses ICMPv4 "fragmentation needed"
messages, but these generally do not provide enough information for
stateless translation into to ICMPv6 "packet too big" messages (see: RFC
792 [16] [14] and RFC 1812 [17], [15], 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 may be inadequate and can result in black holes that are
difficult to diagnose [24].

The ISATAP encapsulator may implement an alternate [25].

Alternate methods for determining per-neighbor MTUs should be used
when RFC 1191 path MTU discovery mechanism, e.g., is deemed inadequate. In any
method, the encapsulator uses 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 The following three methods are
possible:

In the first method,

1. Encapsulator-driven - the encapsulator does NOT set periodically sends probe
packets with the DF bit set 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 waits for a priori knowledge of the decapsulator's link MTU.
This method has
positive acknowledgement from the advantage decapsulator that the probe packets will be delivered
even if was
received

2. Decapsulator-driven - the network performs fragmentation, thus ordinary data encapsulator sends all packets may be used for probing resulting with the
DF bit NOT set in greater efficiency.
Disadvantages for this method include:

o special mechanisms required on both encapsulator the IPv4 header unless and until the
decapsulator

o extra state required on both sends a "Fragmentation Experienced" indication(s)

3. Hybrid - the encapsulator and decapsulator

o complex protocol signalling between encapsulator engage in a dialogue
and decapsulator

o possible extended periods of network fragmentation

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In use "intelligent" probing to monitor the second (and preferred) path MTU

These methods are discussed in detail in the following subsections:

C.1 Encapsulator-driven Method

In this method, the encapsulator sets the DF bit in the IPv4 header
of probe packets. Neighbor Solicitation (NS) Probe packets with padding bytes may be sent either when the
encapsulator's link layer forwards a large data packet to the

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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 should could be used for this purpose, since successful
delivery results in a positive acknowledgement that the probe succeeded, i.e., in the form of
succeeded vis-a-vis a Neighbor Advertisement
(NA) response from the decapsulator. Setting

While the DF bit prevents decapsulator is being probed, the network
from fragmenting encapsulator maintains a
queue of packets that have the decapsulator as the IPv6 next-hop
address. The queue should be large enough to buffer the
(delay*bandwidth) product for the round-trip time (RTT) to the
decapsulator. If the probe succeeds, packets and protects decapsulators from
receiving in the queue that are
no larger than the probe size are sent to the decapsulator. If the
probe fails, packets larger than the last known successful probe are
dropped and an ICMPv6 "packet too big" message returned to the sender
[16].

This method has the advantage that might overrun the IPv4 reassembly buffer.
Additionally, decapsulator need not
implement any special mechanisms, since standard IPv6 request/
response mechanisms and state are needed only on the
encapsulator, and no complex protocol signalling between used. Additionally, the encapsulator and decapsulator is required.
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, packet queues may become
large on Long, Fat Networks (LFNs) (see: RFC 1323 [26]).

C.2 Decapsulator-driven Method

In either this method, implementations may choose to couple the probing
process encapsulator sends all packets with neighbor cache management procedures ([6], section 7),
e.g. to maintain timers, state variables and/or the DF bit
NOT set in the IPv4 header with the expectation that the decapsulator
will send a queue of "Fragmentation Experienced" indication if the IPv4
network fragments packets. In other words, the decapsulator simply
sends all packets
waiting 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 probes both reporting fragmentation and
informing the encapsulator of a new MTU value to complete. Packets retained on use.

This method has the queue distinct advantages that the data packets
themselves are
forwarded when used as probes succeed, and provide state no queueing on the encapsulator is
necessary. Additionally, fewer packets will be lost since the
decapsulator will quite often be able to reassemble packets
fragmented by the network. The primary disadvantage is that, using
the current specifications, the encapsulator has no way of knowing
whether a particular decapsulator implements the "fragmentation
experienced" signalling capability. However, the "fragmentation
experienced" indication can be trivially implemented in an
application on the decapsulator that uses the Berkeley Packet Filter

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(aka, libpcap) to listen for sending fragmented packets from encapsulators.

When fragmented packets arrive, the application sends IPv6 RA
messages with an MTU option to inform the encapsulator that
fragmentation has been experienced and a new value for the neighbor's
MTU should be used. The application additionally sends ICMPv6
"packet too big" messages to the original source when probes fail.
Implementations may choose to store per-neighbor MTU information a fragmented
packet is not correctly reassembled. This function need not be built
into the decapsulator's operating system and can be added as an
after-market feature. Finally, simply adding an extra bit in the IPv4 path RA
message header ([4], section 4.2) would provide a means for the
decapsulator to inform the encapsulator that dynamic MTU discovery cache, is
supported.

C.3 Hybrid Method

In this method, the encapsulator and decapsulator engage in a
"neighbor affiliation" protocol to negotiate link-layer parameters
such as MTU. (See: [27] for an example of such an approach.) This
approach has the advantage that bi-directional links are used and
both ends of the ISATAP link layer's private
data structures, etc.

Additional notes:

1. Per-neighbor MTUs may vary dynamically due to fluctuations in have unambiguous knowledge that the
IPv4 forwarding path and/or multipath routing (e.g., when QoS
routing other end
implements the protocol. However, the signalling protocol between
the endpoints is used complicated and additional state is required in both
the IPv4 network). For such neighbors,
encapsulators should detect a "losing battle" encapsulator and reduce decapsultor.

C.4 Summary

In summary, 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 decapsulator-based approach in Appendix C.2 has
distinct efficiency advantages over methods that engage the
encapsulator. Additionally, probing methods which use IPv4
encapsulation with the DF bit NOT set or
not set. When the DF bit is set, undetected packet loss may
occur in use LinkMTU values for the network if
ISATAP link that exceed the neighbor's underlying link MTU decreases. When the
DF bit size. Experimental
verification is NOT set, undetected called for which may eventually result in a
recommendation for proposed standard.

C.5 Additional Notes

o In all methods, some packet loss is less likely but due to link/buffer restrictions
may occur either with no ICMPv6 "packet too big" message returned to the
sender. Unenlightened senders will interpret such loss as loss
due to congestion, which may result in longer convergence to the network or at
actual path MTU. Enlightened senders will interpret the neighbor's reassembly
buffer.

3. loss as
loss due to link/buffer restrictions and immediately reduce their
MTU estimate.

o To avoid denial-of-service attacks that would cause superfluous
probing based on counting down/up by small increments, plateau
tables (e.g., [13], section 7) should be used when the actual MTU

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value is indeterminant.

o 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" messages as "tentative" negative
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.

o Nodes that connect to the Internet are expected to be able to
reassemble or discard IPv4 packets up to 64KB in length when the
DF bit is not set in the encapsulating IPv4 header. Nodes that
cannot reassemble or discard maximum-length IPv4 packets are
vulnerable to attacks such as the "ping-of-death".

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This document and the information contained herein is provided on an
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Funding for the RFC Editor function is currently provided by the
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----