Network Working Group A. Conta
Request for Comments: 3122 Transwitch Corporation
Category: Standards Track June 2001
Extensions to IPv6 Neighbor Discovery for Inverse Discovery
Specification
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This memo describes extensions to the IPv6 Neighbor Discovery that
allow a node to determine and advertise an IPv6 address corresponding
to a given link-layer address. These extensions are called Inverse
Neighbor Discovery. The Inverse Neighbor Discovery (IND) was
originally developed for Frame Relay networks, but may also apply to
other networks with similar behavior.
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RFC 3122 Extensions to IPv6 Neighbor Discovery June 2001
Table of Contents
1. Introduction.................................................... 3
2. Inverse Neighbor Discovery Messages............................. 3
2.1 Inverse Neighbor Discovery Solicitation Message............. 3
2.2 Inverse Neighbor Discovery Advertisement Message............ 5
3. Inverse Neighbor Discovery Options Format....................... 6
3.1 Target Address List......................................... 6
4. Inverse Neighbor Discovery Protocol............................. 9
4.1 Sender Node Processing...................................... 9
4.2 Receiver Node Processing.................................... 9
4.2.1 Processing Inverse Neighbor Discovery Solicitations..... 9
4.2.2 Processing Inverse Neighbor Discovery Advertisements... 10
4.3 Message Validation......................................... 10
4.3.1 Validation of Inverse Neighbor Discovery Solicitations. 10
4.3.2 Validation of Inverse Neighbor Discovery Advertisements 11
5. Security Considerations........................................ 12
6. IANA Considerations............................................ 13
7. Acknowledgments................................................ 13
8. References..................................................... 13
9. Authors' Addresses............................................. 14
Appendix A........................................................ 15
Full Copyright Statement.......................................... 20
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RFC 3122 Extensions to IPv6 Neighbor Discovery June 2001
1. Introduction
This document defines extensions to the IPv6 Neighbor Discovery
(ND)[IPv6-IND]. The extensions are called IPv6 Inverse Neighbor
Discovery (IND). The IPv6 Inverse Neighbor Discovery (IND) allows a
node that knows the link-layer address of a directly connected remote
node to learn the IPv6 addresses of that node. A node using IND
sends solicitations and receives advertisements for one or more IPv6
addresses corresponding to a known link-layer address.
The Inverse Neighbor Discovery (IND) was originally developed for
Frame Relay networks, but may also apply to other networks with
similar behavior.
The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED,
SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as defined
in [KEYWORDS].
There are a number of similarities and differences between the
mechanisms described here and those defined for Inverse ARP for IPv4
in [INV-ARP] or its replacement documents.
2. Inverse Neighbor Discovery Messages
The following messages are defined:
2.1. Inverse Neighbor Discovery Solicitation Message
A node sends an Inverse Neighbor Discovery Solicitation message to
request an IPv6 address corresponding to a link-layer address of the
target node while also providing its own link-layer address to the
target. Since the remote node IPv6 addresses are not known, Inverse
Neighbor Discovery (IND) Solicitations are sent as IPv6 all-node
multicasts [IPv6], [IPv6-FR], [ENCAPS]. However, at link layer
level, an IND Solicitation is sent directly to the target node,
identified by the known link-layer address.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
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Source Address
An IPv6 address assigned to the interface from which this message
is sent.
Destination Address
The IPv6 all-node multicast address. This address is specified in
its link-scope format, which is FF02::1.
Hop Limit 255
Authentication Header
If a Security Association for the IP Authentication Header exists
between the sender and the destination, then the sender SHOULD
include this header.
ICMP Fields:
Type 141
Code 0
Checksum The ICMP checksum. See [ICMPv6].
Reserved This field is unused. It MUST be initialized to
zero by the sender and MUST be ignored by the
receiver.
Required options:
The sender node MUST send the following options in the Solicitation
message:
Source Link-Layer Address
The link-layer address of the sender.
Target Link-Layer Address
The link-layer address of the target node.
Other valid options:
The sender node MAY choose to add the following options in the
Solicitation message:
Source Address List
The list of one or more IPv6 addresses of the interface identified
by the Source Link-Layer Address. This option is defined in
section 3.
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MTU
The MTU configured for this link [IPv6-ND].
Future versions of this protocol may add other option types.
Receivers MUST silently ignore any options they do not recognize and
continue processing the message.
2.2 Inverse Neighbor Discovery Advertisement Message
A node sends Inverse Neighbor Discovery Advertisements in response to
Inverse Neighbor Discovery Solicitations.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
Source Address
An address assigned to the interface from which the advertisement
is sent.
Destination Address
The Source Address of an invoking Inverse Discovery Neighbor
Solicitation.
Hop Limit 255
Authentication Header
If a Security Association for the IP Authentication Header exists
between the sender and the destination address, then the sender
SHOULD include this header.
ICMP Fields:
Type 142
Code 0
Checksum The ICMP checksum. See [ICMPv6].
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Reserved 32-bit unused field. It MUST be initialized to
zero by the sender and MUST be ignored by the
receiver.
Required options:
The sender node MUST send the following options in the Advertisement
message:
Source Link-Layer Address The link-layer address of the sender.
Target Link-Layer Address
The link-layer address of the target, that is, the sender of
the advertisement.
Target Address List
The list of one or more IPv6 addresses of the interface
identified by the Target Link-Layer Address in the Inverse
Neighbor Discovery Solicitation message that prompted this
advertisement. This option is defined in Section 3.
Other valid options:
The sender node MAY choose to add the following option in the
Advertisement message:
MTU
The MTU configured for this link [IPv6-ND].
Future versions of this protocol may add other option types.
Receivers MUST silently ignore any options they do not recognize and
continue processing the message.
3. Inverse Neighbor Discovery Options Formats
Inverse Neighbor Discovery messages include Neighbor Discovery
options [IPv6-ND] as well as an Inverse Neighbor Discovery specific
options: the Source Address List and the Target Address List.
3.1 Source/Target Address List
The Source Address List and the Target Address List option are TLV
options (type, length, variable size field) (see Section 4.6 of
[IPv6-ND] with the following fields:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - - - +
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ IPv6 Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
~
|
+-+-+-+-+...
Fields:
Type 9 for Source Address List
10 for Target Address List
Note: These Option Type values should be assigned from the IPv6
Neighbor Discovery family of values.
Length The length of the option (including the Type,
Length, and the Reserved fields) in units of 8
octets. The minimum value for Length is 3, for one
IPv6 address.
Reserved This field is unused. It MUST be initialized to
zero by the sender and MUST be ignored by the
receiver.
IPv6 Addresses One or more IPv6 addresses of the interface.
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Description:
The Source Address List contains a list of IPv6 addresses of the
interface identified by the Source Link-Layer Address.
The Target Address List contains a list of IPv6 addresses of the
interface identified by the Target Link-Layer Address.
The number of addresses "n" in the list is calculated based on the
length of the option:
n = (Length - 1)/2 (Length is the number of groups of 8 octets)
The Source Address List MUST fit in one IND Solicitation message.
Therefore in case all IPv6 addresses of an interface do not fit in
one messages, the option does not contain a complete list. For a
complete list of IPv6 addresses, a node should rely on the IND
Advertisement message.
The Target Address List SHOULD be the complete list of addresses of
the interface identified by the Target Link-Layer Address. If the
list of IPv6 addresses of an interface does not fit in one IND
Advertisement message, one or more IND Advertisement messages, with
the same fields as the first message, SHOULD follow. The Target
Address List option(s) of the second, and subsequent message(s)
SHOULD contain the rest of the IPv6 addresses of the interface
identified by the Target Link-Layer Address, which did not fit in the
first message.
Note 1: The scope of the Inverse Neighbor Discovery mechanism is
limited to IPv6 address discovery, that is, providing address mapping
information. Therefore, it does not make any provisions or rules
regarding how a node uses the addresses that were returned in an
Inverse Discovery message. Furthermore, it does not exclude any
particular type of IPv6 address from the Source or Target Address
List. For example, if an interface has manually configured, and
autoconfigured addresses, including temporary ones, unicast,
multicast, etc..., the list should not exclude any.
Note 2: An implementation MUST NOT send duplicates in the IPv6
address list.
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4. Inverse Neighbor Discovery Protocol
IND operates essentially the same as ND [IPv6-ND]: the solicitor of a
target IP address sends on an interface a solicitation message, the
target node responds with an advertisement message containing the
information requested. The information learned MAY be stored in the
Neighbor Discovery cache [IPv6-ND], as well as IPv6 address
structures which may be associated with the interface.
4.1 Sender Node Processing
A soliciting node formats an IND Solicitation message as defined in a
previous section, encapsulates the packet for the specific link-layer
and sends it directly to the target node. Although the destination
IP address is the all-node multicast address, the message is sent
only to the target node. The significant fields for the IND protocol
are the Source IP address, the Source link-layer address, the Target
link-layer address, and the MTU. The latter can be used in setting
the optimum value of the MTU for the link.
While awaiting a response, the sender SHOULD retransmit IND
Solicitation messages approximately every RetransTimer
(expiration)[IPv6-ND], even in the absence of additional traffic to
the neighbor. Retransmissions MUST be rate-limited to at most one
solicitation per neighbor every RetransTimer.
If no IND Advertisement is received after MAX_MULTICAST_SOLICIT
[IPv6-ND] solicitations, inverse address resolution has failed. If
the sending of the Solicitation was required by an upper-layer, the
sender module MUST notify the error to the upper-layer through an
appropriate mechanism (e.g., return value from a procedure call).
4.2 Receiver Node Processing
4.2.1 Processing Inverse Neighbor Solicitation Messages
For every IND Solicitation, the receiving node SHOULD format in
response a proper IND Advertisement using the link-layer source and
target address pair as well as the IPv6 source address from the IND
Solicitation message.
If a node updates the Neighbor Discovery Cache with information
learned from IND messages, the receiver node of the IND Solicitation
SHOULD put the sender's IPv6 address/link-layer address mapping -
i.e., the source IP address and the Source link-layer address from
the solicitation message - into its ND cache [IPv6-ND] as it would
for a ND solicitation.
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Because IPv6 nodes may have multiple IPv6 addresses per interface, a
node responding to an IND Solicitation SHOULD return in the Target
Address List option a list containing one or more IPv6 addresses
corresponding to the interface identified by the Target Link-Layer
Address field in the solicitation message. The list MUST not contain
duplicates.
4.2.2 Processing Inverse Neighbor Advertisement Messages
If a node updates The Neighbor Discovery Cache with information
learned from IND messages, the receiver node of the IND advertisement
SHOULD put the sender's IPv6 address/link-layer address mapping -
i.e., the IP addresses from Target addresses list and the Source
link-layer address from the IND advertisement message - into its ND
cache [IPv6-ND] as it would for a ND advertisement.
4.3 Message Validation
Inverse Neighbor Discovery messages are validated as follows:
4.3.1 Validation of Inverse Neighbor Discovery Solicitations
A node MUST silently discard any received Inverse Neighbor
Solicitation messages that do not satisfy all of the following
validity checks:
- The IP Hop Limit field has a value of 255, i.e., the packet
could not possibly have been forwarded by a router.
- If the message includes an IP Authentication Header, the
message authenticates correctly.
- ICMP Checksum is valid.
- ICMP Code is 0.
- ICMP length (derived from the IP length) is 24 or more
octets.
- The Target Link-Layer Address is a required option and MUST
be present.
- The Source Link-Layer Address is a required option and MUST
be present.
- All included options have a length that is greater than
zero.
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The content of the Reserved field, and of any unrecognized options,
MUST be ignored. Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.
The contents of any Neighbor Discovery [IPv6-ND] options that are not
specified to be used with Inverse Neighbor Discovery Solicitation
messages MUST be ignored and the packet processed as normal. The
only defined option that may appear besides the required options is
the MTU option.
An Inverse Neighbor Solicitation that passes the validity checks is
called a "valid solicitation".
4.3.2 Validation of Inverse Neighbor Discovery Advertisements
A node MUST silently discard any received Inverse Neighbor Discovery
Advertisement messages that do not satisfy all of the following
validity checks:
- The IP Hop Limit field has a value of 255, i.e., the packet
could not possibly have been forwarded by a router.
- If the message includes an IP Authentication Header, the
message authenticates correctly.
- ICMP Checksum is valid.
- ICMP Code is 0.
- ICMP length (derived from the IP length) is 48 or more
octets.
- Source Link-Layer Address option is present.
- Target Link-Layer Address option is present.
- The Target Address List option is present.
- The length of the Target Address List option is at least 3.
- All other included options have a length that is greater
than zero.
The contents of the Reserved fields, and of any unrecognized options,
MUST be ignored. Future, backward-compatible changes to the protocol
may specify the contents of the Reserved fields or add new options;
backward-incompatible changes may use different Code values.
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The contents of any defined options [IPv6-ND] that are not specified
to be used with Inverse Neighbor Advertisement messages MUST be
ignored and the packet processed as normal. The only defined option
that may appear besides the required options is the MTU option.
An Inverse Neighbor Advertisement that passes the validity checks is
called a "valid advertisement".
5. Security Considerations
When being employed on point to point virtual circuits, as it is the
case with Frame Relay networks, Inverse Neighbor Discovery messages
are less sensitive to impersonation attacks from on-link nodes, as it
would be the case with broadcast links.
Like Neighbor Discovery, the protocol reduces the exposure to threats
from off-link nodes in the absence of authentication by ignoring IND
packets received from off-link senders. The Hop Limit field of all
received packets is verified to contain 255, the maximum legal value.
Because routers decrement the Hop Limit on all packets they forward,
received packets containing a Hop Limit of 255 must have originated
from a neighbor.
Inverse Neighbor Discovery protocol packet exchanges can be
authenticated using the IP Authentication Header [IPSEC-Auth]. A
node SHOULD include an Authentication Header when sending Inverse
Neighbor Discovery packets if a security association for use with the
IP Authentication Header exists for the destination address. The
security associations may have been created through manual
configuration or through the operation of some key management
protocol.
Received Authentication Headers in Inverse Neighbor Discovery packets
MUST be verified for correctness and packets with incorrect
authentication MUST be ignored.
In case of use with Frame Relay, to avoid an IP Security
Authentication verification failure, the Frame Relay specific
preprocessing of a Neighbor Discovery Solicitation message that
contains a DLCI format Source link-layer address option, MUST be done
by the receiver node after it completed IP Security processing.
It SHOULD be possible for the system administrator to configure a
node to ignore any Inverse Neighbor Discovery messages that are not
authenticated using either the Authentication Header or Encapsulating
Security Payload. Such a switch SHOULD default to allowing
unauthenticated messages.
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Confidentiality issues are addressed by the IP Security Architecture
and the IP Encapsulating Security Payload documents [IPSEC], [IPSEC-
ESP].
6. IANA Considerations
IANA was requested to assign two new ICMPv6 type values, as described
in Section 2.1 and 2.2. They were assigned from the Informational
range of messages, as defined in Section 2.1 of RFC 2463. There were
no ICMPv6 code values defined for these types (other than 0); future
assignments are to be made under Standards Action as defined in RFC
2434.
IANA was also requested to assign two new ICMPv6 Neighbor Discovery
Option types as defined in Section 3.1. No outside reviewing was
necessary.
7. Acknowledgments
Thanks to Steve Deering, Thomas Narten and Erik Nordmark for
discussing the idea of Inverse Neighbor Discovery. Thanks to Thomas
Narten, and Erik Nordmark, and also to Dan Harrington, Milan Merhar,
Barbara Fox, Martin Mueller, and Peter Tam for a thorough reviewing.
Also it should be acknowledged that parts of the text in this
specification derived from the IPv6 Neighbor Discovery text [IPv6-
ND].
8. References
[IPv6] Deering, S. and R. Hinden, "Internet Protocol Version 6
Specification", RFC 2460, December 1998.
[IPv6-ND] Narten, T., Nordmark, E. and W. Simpson "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
[ICMPv6] Conta, A., and S. Deering "Internet Control Message
Protocol for the Internet Protocol Version 6", RFC
2463, December 1998.
[IPv6-FR] Conta, A., Malis, A. and M. Mueller, "Transmission of
IPv6 Packets over Frame Relay Networks", RFC 2590, May
1999. December 1997.
[IPSEC] Atkinson, R. and S. Kent, "Security Architecture for
the Internet Protocol", RFC 2401, November 1998.
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[IPSEC-Auth] Atkinson, R. and S. Kent, "IP Authentication Header",
RFC 2402, December 1998.
[IPSEC-ESP] Atkinson, R. and S. Kent, "IP Encapsulating Security
Protocol (ESP)", RFC 2406, November 1998.
[ASSIGN] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,
RFC 1700, March 1994.
[ENCAPS] Brown, C. and A. Malis, "Multiprotocol Interconnect
over Frame Relay", RFC 2427, November 1998.
[INV-ARP] Bradley, T., Brown, C. and A. Malis "Inverse Address
Resolution Protocol", RFC 2390, August 1998.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9. Authors' Addresses
Alex Conta
Transwitch Corporation
3 Enterprise Drive
Shelton, CT 06484
Phone: +1-203-929-8810
EMail: aconta@txc.com
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Appendix A
A. Inverse Neighbor Discovery with Frame Relay Networks
This appendix documents the details of using the Inverse Neighbor
Discovery on Frame Relay Networks, which were too specific to be part
of the more general content of the previous sections.
A.1 Introduction
The Inverse Neighbor Discovery (IND) specifically applies to Frame
Relay nodes. Frame Relay permanent virtual circuits (PVCs) and
switched virtual circuits (SVCs) are identified in a Frame Relay
network by a Data Link Connection Identifier (DLCI). Each DLCI
defines for a Frame Relay node a single virtual connection through
the wide area network (WAN). A DLCI has in general a local
significance.
By way of specific signaling messages, a Frame Relay network may
announce to a node a new virtual circuit with its corresponding DLCI.
The DLCI identifies to a node a virtual circuit, and can be used as
the equivalent of a remote node link-layer address, allowing a node
to identify at link layer level the node at the other end of the
virtual circuit. For instance in Figure 1., node A (local node)
identifies the virtual circuit to node B (remote node) by way of DLCI
= 30. However, the signaling message does not contain information
about the DLCI used by a remote node to identify the virtual circuit
to the local node, which could be used as the equivalent of the local
link-layer address. For instance in Figure 1., node B (remote node)
may identify the virtual circuit to node A by way of DLCI = 62.
Furthermore, the message being transmitted at link-layer level and
completely independent of the IPv6 protocol does not include any IPv6
addressing information. The Inverse Neighbor Discovery is a protocol
that allows a Frame Relay node to discover the equivalent of a local
link layer address, that is, the identifier by way of which remote
nodes identify the node, and more importantly discover the IPv6
addresses of the interface at the other end of the virtual circuit,
identified by the remote link-layer address.
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~~~~~~~~~~~ Remote
{ } Node
+-----+ DLCI { } DLCI+-----+
| A |-30------{--+----+----+--}---------62-| B |
+-----+ { } +-----+
Local { } Frame Relay
Node ~~~~~~~~~~~ Network Cloud
Figure 1.
The IPv6 Inverse Neighbor Discovery (IND) protocol allows a Frame
Relay node to discover dynamically the DLCI by which a remote node
identifies the virtual circuit. It also allows a node to learn the
IPv6 addresses of a node at the remote end of a virtual circuit.
A.2. Inverse Neighbor Discovery Messages
Frame Relay nodes generate Inverse Neighbor Discovery messages as
follows:
A.2.1. Inverse Neighbor Discovery Solicitation Message
The sender of an Inverse Neighbor Discovery Solicitation does not
know the remote node's IPv6 addresses, but knows the equivalent of a
remote node link-layer address. Inverse Neighbor Discovery (IND)
Solicitations are sent as IPv6 all-node multicasts [IPv6], [IPv6-FR],
[ENCAPS]. However, at link layer level, an IND Solicitation is sent
directly to the target node, identified by the known link-layer
address (DLCI).
The fields of the message, which are filled following considerations
specific to Frame Relay are:
Source Link-Layer Address
For the sender Frame Relay node, the Source Link-Layer Address is
the equivalent of the link-layer address by which the remote node
identifies the source of this message. The sender may have no
knowledge of this information. If the sender knows the
information, it SHOULD include it in the field, otherwise it
SHOULD live it zero (empty). This information, if present, can be
used for network debugging purposes. Regardless of the sender's
action on this field, prior to any Inverse Neighbor Discovery
processing, the receiver of this message replaces this field,
whether filled in or not by the sender, with information carried
by the Frame Relay header in the DLCI field. The field is encoded
in DLCI format as defined by [IPv6-FR].
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Target Link-Layer Address
For sender Frame Relay node, the Target Link-Layer Address field
is filled with the value known as the equivalent of the target
node link-layer address. This value is the DLCI of the VC to the
target node. It is encoded in DLCI format [IPv6-FR].
To illustrate the generating of a IND Solicitation message by a
Frame Relay node, let's consider as an example Node A (Figure 1.)
which sends an IND solicitation to Node B. The Solicitation
message fields will have the following values:
At Node A (sender of the IND solicitation message).
Source Link-Layer Address
DLCI=unknown (overwritten by the receiver).
Target Link-Layer Address
DLCI=30.
At Node B (receiver of the IND solicitation message).
Source Link-Layer Address
DLCI=62 (filled in by the receiver).
Target Link-Layer Address
DLCI=30.
Note: For Frame Relay, both the above addresses are in Q.922 format
(DLCI), which can have 10 (default), or 23 significant addressing
bits [IPv6-FR]. The option length (link-layer address) is expressed
in 8 octet units, therefore, the DLCI will have to be extracted from
the 8 bytes based on the EA field (bit 0) of the second, third, or
forth octet (EA = 1). The C/R, FECN, BECN, DE fields in the Q.922
address have no significance for IND and are set to 0 [IPv6-FR].
MTU
The value filled in the MTU option is the MTU for the virtual
circuit identified by the known DLCI [IPv6-FR].
A.2.2 Inverse Neighbor Discovery Advertisement Message
A Frame Relay node sends Inverse Neighbor Discovery Advertisements in
response to Inverse Neighbor Discovery Solicitations.
The fields of the message, which are filled following considerations
specific to Frame Relay are:
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Source Link-Layer Address
For Frame Relay, this field is copied from the Target link-layer
address field of the Inverse Neighbor Discovery Solicitation. It
is encoded in DLCI format [IPv6-FR].
Target Link-Layer Address
For Frame Relay, this field is copied from the Source link-layer
address field of the Inverse Neighbor Discovery Solicitation. It
is encoded in DLCI format [IPv6-FR].
For example if Node B (Figure 1.) responds to an IND solicitation
sent by Node A. with an IND advertisement, these fields will have the
following values:
At Node B (sender of the advertisement message):
Source Link-Layer Address
DLCI=30 (was Target in Solicitation Message).
Target Link-Layer Address
DLCI=62 (was Source in Solicitation Message).
At Node A (receiver of the advertisement message from B).
Source Link-Layer Address
DLCI=30 (was Target in Solicitation Message).
Target Link-Layer Address
DLCI=62 (was Source in Solicitation Message).
Target Address List
The list of one or more IPv6 addresses of the interface identified
by the Target Link-Layer Address in the Inverse Neighbor Discovery
Solicitation message that prompted this advertisement.
MTU The MTU configured for this link (virtual circuit) [IPv6-ND].
Note: In case of Frame Relay networks, the IND messages are sent
on a virtual circuit, which acts like a virtual-link. If the
virtual circuit breaks, all participants to the circuit receive
appropriate link layer signaling messages, which can be propagated
to the upper layers, including IPv6.
A.3. Inverse Neighbor Discovery Protocol
This section of the appendix documents only the specific aspects of
Inverse Neighbor Discovery with Frame Relay Networks.
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A.3.1 Sender Node Processing
A soliciting Frame Relay node formats an IND solicitation message as
defined in a previous section, encapsulates the packet for the Frame
Relay link-layer [IPv6-FR] and sends it to the target Frame Relay
node. Although the destination IP address is the IPv6 all-node
multicast address, the message is sent only to the target Frame Relay
node. The target node is the known remote node on the link
represented by the virtual circuit.
A.3.2 Receiver Node Processing
A.3.2.1 Processing Inverse Neighbor Solicitation Messages
A Frame Relay node, before further processing, is replacing in the
Source link-layer address the existent DLCI value with the DLCI value
from the Frame Relay header of the frame containing the message. The
DLCI value has to be formatted appropriately in the Source link-layer
address field [IPv6-FR]. This operation is required to allow a
correct interpretation of the fields in the further processing of the
IND solicitation message.
For a Frame Relay node, the MTU value from the solicitation message
MAY be used to set the receiver's MTU to a value that is more
optimal, in case that was not already done at the interface
configuration time.
A.3.2.2 Processing Inverse Neighbor Advertisement Messages
The receiver Frame Relay node of the IND Advertisement MAY put the
sender's IPv6 address/link-layer address mapping - i.e., the Target
IP addresses and the Source link-layer address from the IND
advertisement message - into its ND cache [IPv6-ND] as it would for
a ND Advertisement.
Further, the receiver Frame Relay node of the IND Advertisement MAY
store the Target link-layer address from the message as the DLCI
value at the remote end of the VC. This DLCI value is the equivalent
of the link-layer address by which the remote node identifies the
receiver.
If the receiver node of the IND Advertisement has a pool of IPv6
addresses, and if the implementation allows, it may take decisions to
pairing specific local IPv6 addresses to specific IPv6 addresses from
the target list in further communications on the VC. More
specifically, such a pairing may be based on IPv6 addresses being on
the same subnet, that is having the same prefix.
Conta Standards Track [Page 19]
RFC 3122 Extensions to IPv6 Neighbor Discovery June 2001
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Conta Standards Track [Page 20]
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