ION/IPng Working Groups A. Conta (Lucent) INTERNET-DRAFT R. Duggal (Lucent) June 1999 Extensions to IPv6 Neighbor Discovery for Inverse Discovery Specification draft-ietf-ion-ipv6-ind-02.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This memo describes extensions to the IPv6 Neighbor Discovery that allow a node to solicit and be advertised an IPv6 address corresponding to a given link-layer address. These extensions are called Inverse Neighbor Discovery. They specifically apply to Frame Relay networks but they may also apply to other networks with similar behavior. Conta Expires in six months [Page 1] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 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.........................7 3.1 Target Address List...........................................7 4. Inverse Neighbor Discovery Protocol...............................8 4.1 Sender Node Processing........................................8 4.2 Receiver Node Processing......................................9 4.2.1 Processing Inverse Neighbor Discovery Solicitations.......9 4.2.2 Processing Inverse Neighbor Discovery Advertisements......9 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. Acknowledgments..................................................13 7. References.......................................................13 8. Authors' Addresses...............................................15 Appendix A..........................................................16 Conta Expires in six months [Page 2] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 1. Introduction This document defines extensions to the IPv6 Neighbor Discovery (ND). 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) specifically applies to Frame Relay networks. It may apply also to other networks that have a 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 is 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 ... +-+-+-+-+-+-+-+-+-+-+-+- Conta Expires in six months [Page 3] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 IP Fields: 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 link-layer address, then the sender SHOULD include this header. ICMP Fields: Type [TBD] 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: Source Link-Layer Address The link-layer address of the sender. Target Link-Layer Address The link-layer address of the target node. Conta Expires in six months [Page 4] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 Other valid options: 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Res|O| 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 [TBD] Conta Expires in six months [Page 5] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 Code 0 Checksum The ICMP checksum. See [ICMPv6]. Res(erved) This two bit reserved field MUST be initialized to zero by the sender and MUST be ignored by the receiver. O Override flag. When set, the O-bit indicates that the advertisement should override an existing ND cache entry and update the cached mapping of the link-layer address to IPv6 address [ND]. When it is not set the advertisement will not update a cached link-layer address to IPv6 address mapping though it will update an existing Neighbor Cache entry for which no IPv6 address is known. It SHOULD be set. Reserved 29-bit unused field. It MUST be initialized to zero by the sender and MUST be ignored by the receiver. Required options: Source Link-Layer Address This field is copied from the Target link-layer address field of the Inverse Neighbor Discovery Solicitation. Target Link-Layer Address This field is copied from the Source link-layer address field of the Inverse Neighbor Discovery Solicitation. 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. Other valid options: MTU The MTU configured for this link [IPv6-ND]. Future versions of this protocol may add other option types. Conta Expires in six months [Page 6] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 Receivers MUST silently ignore any options they do not recognize and continue processing the message. 3. Inverse Neighbor Discovery Options Formats Inverse Neighbor Discovery advertisment messages include Neighbor Discovery options as well as an Inverse Neighbor Discovery specific option: the Target Address List. 3.1 Target Address List The Target Address List option is a TLV (type, length, variable size field) option, with the following fields: 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 [TBD] Conta Expires in six months [Page 7] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 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 Frame Relay 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) If the list of IPv6 addresses does not fit in one IND advertisement message, one or more IND advertisement messages with the same fields as the first message, MAY follow. The Target Address List option(s) of the second, and subsequent message(s) 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. 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 Neighbor Conta Expires in six months [Page 8] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 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 Neighbor Advertisement is received after MAX_MULTICAST_SOLICIT [IPv6-ND] solicitations, inverse address resolution has failed. The sender MUST return ICMP destination unreachable indications with code 3 (Address Unreachable) for each packet queued awaiting inverse address resolution. Since the error occurs on the packet originating node, an implementation is not required to actually create and send an ICMP error packet to the source, as long as the upper-layer sender is notified through an appropriate mechanism (e.g. return value from a procedure call). Note, however, that an implementation may find it convenient in some cases to return errors to the sender by taking the offending packet, generating an ICMP error message, and then delivering it (locally) through the generic error handling routines. 4.2 Receiver Node Processing 4.2.1 Processing Inverse Neighbor Solicitation Messages For every IND solicitation, the receiving node may 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 is unable or unwilling to advertise, it ignores the solicitation. Further, the receiver node of the IND solicitation may 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. Because IPv6 nodes may have multiple IPv6 addresses per interface, a node responding to an IND solicitation MUST 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. 4.2.2 Processing Inverse Neighbor Advertisement Messages The receiver node of the IND advertisement puts 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 Conta Expires in six months [Page 9] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 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. 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; Conta Expires in six months [Page 10] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 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 24 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. Conta Expires in six months [Page 11] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 - 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. 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 Neighbor Discovery packets MUST be verified for correctness and packets with incorrect authentication MUST be ignored. It SHOULD be possible for the system administrator to configure a node to ignore any Inverse Neighbor Discovery messages that are not Conta Expires in six months [Page 12] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 authenticated using either the Authentication Header or Encapsulating Security Payload. Such a switch SHOULD default to allowing unauthenticated messages. Confidentiality issues are addressed by the IP Security Architecture and the IP Encapsulating Security Payload documents [IPSEC], [IPSEC- ESP]. 6. Acknowledgments Thanks to Steve Deering, Thomas Narten and Eric Nordmark who spent time discussing the idea of Inverse Neighbor Discovery, and reviewing this specification. Also thanks to Dan Harrington, Milan Merhar, Barbara Fox, and Martin Mueller for a thorough reviewing. 7. References [IPv6] S. Deering, R. Hinden, "Internet Protocol Version 6 Specification", RFC 2460, December 1998. [IPv6-ND] T. Narten, E. Nordmark, W.Simpson "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. [ICMPv6] A. Conta, S. Deering "Internet Control Message Protocol for the Internet Protocol Version 6", RFC 2463, December 1998. [IPv6-FR] A. Conta, A. Malis, M. Mueller, "Transmission of IPv6 Packets over Frame Relay networks" Work in Progress, December 1997. [IPSEC] Atkinson, R., S. Kent, "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [IPSEC-Auth] Atkinson, R., S. Kent, "IP Authentication Header", RFC 2402, December 1998. [IPSEC-ESP] Atkinson, R., S. Kent, "IP Encapsulating Security Protocol (ESP)", RFC 2406, November 1998. [ASSIGN] J. Reynolds, J. Postel, "Assigned Numbers", RFC 1700. Conta Expires in six months [Page 13] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 [ENCAPS] C. Brown, A. Malis, "Multiprotocol Interconnect over Frame Relay", RFC 2427, November 1998. [INV-ARP] T. Bradley, C. Brown, 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. Conta Expires in six months [Page 14] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 8. Authors' Addresses Alex Conta Lucent Technologies Inc. 300 Baker Ave, Suite 100 Concord, MA 01742 +1-978-287-2842 email: aconta@lucent.com Raj Duggal Lucent Technologies Inc. 300 Baker Ave, Suite 100 Concord, MA 01742 +1-978-287-2805 email: rduggal@lucent.com Conta Expires in six months [Page 15] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 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 = 50. 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 = 70. Furthermore, the message being transmitted at link-layer level and completely independent of the IPv6 protocol does not include any IPv6 addressing information. Therefore it seems to be useful to define 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. Conta Expires in six months [Page 16] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 ~~~~~~~~~~~ Remote { } Node +-----+ DLCI { } DLCI+-----+ | A |-50------{--+----+----+--}---------70-| 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 The Inverse Neighbor Discovery messages are generated by Frame Relay nodes 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, and may leave this field empty. Therefore 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]. 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 Conta Expires in six months [Page 17] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 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=50. At Node B (receiver of the IND solicitation message). Source Link-Layer Address DLCI=70 (filled in by the receiver). Target Link-Layer Address DLCI=50. Note: For Frame Relay, both the above addresses are in Q.922 format (DLCI), which can have 10 (default), 17, 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: The "Override" Bit in the message header. For Frame Relay, the Inverse Neighbor Discovery Advertisement Conta Expires in six months [Page 18] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 messages unlike the Neighbor Discovery Advertisement messages carrying DLCI format link-layer addresses, SHOULD have the Override bit "O", in the message header set to 1. 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=50 (was Target in Solicitation Message). Target Link-Layer Address DLCI=70 (was Source in Solicitation Message). At Node A (receiver of the advertisement message from B). Source Link-Layer Address DLCI=50 (was Target in Solicitation Message). Target Link-Layer Address DLCI=70 (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. Conta Expires in six months [Page 19] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 A.3. Inverse Neighbor Discovery Protocol This section of the appendix documents only the specific aspects of Inverse Neighbor Discovery with Frame Relay Networks. 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 formated 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 puts 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. Conta Expires in six months [Page 20] INTERNET-DRAFT IPv6 Inverse Neighbor Discovery June 25, 1999 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. Conta Expires in six months [Page 21] 1239