rfc2491.txt

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Network Working Group                                       G. Armitage
Request for Comments: 2491                          Lucent Technologies
Category: Standards Track                                   P. Schulter
                                              Bright Tiger Technologies
                                                                M. Jork
                                                 Digital Equipment GmbH
                                                              G. Harter
                                                                 Compaq
                                                           January 1999


        IPv6 over Non-Broadcast Multiple Access (NBMA) networks

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 (1999).  All Rights Reserved.

Abstract

   This document describes a general architecture for IPv6 over NBMA
   networks. It forms the basis for subsidiary companion documents that
   describe details for various specific NBMA technologies (such as ATM
   or Frame Relay). The IPv6 over NBMA architecture allows conventional
   host-side operation of the IPv6 Neighbor Discovery protocol, while
   also supporting the establishment of 'shortcut' NBMA forwarding paths
   when dynamically signaled NBMA links are available. Operations over
   administratively configured Point to Point NBMA links are also
   described.

   Dynamic NBMA shortcuts are achieved through the use of IPv6 Neighbor
   Discovery protocol operation within Logical Links, and inter-router
   NHRP for the discovery of off-Link NBMA destinations. Both flow-
   triggered and explicitly source-triggered shortcuts are supported.

1. Introduction.

   Non Broadcast Multiple Access (NBMA) networks may be utilized in a
   variety of ways. At one extreme, they can be used to simply provide
   administratively configurable point to point service, sufficient to
   interconnect IPv6 routers (and even IPv6 hosts, in certain



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   situations). At the other extreme, NBMA networks that support dynamic
   establishment and teardown of Virtual Circuits (or functional
   equivalents) may be used to emulate the service provided to the IPv6
   layer by conventional broadcast media such as Ethernet.  Typically
   this emulation requires complex convergence protocols, particularly
   to support IPv6 multicast.

   This document describes a general architecture for IPv6 over NBMA
   networks. It forms the basis for companion documents that provide
   details specific to various NBMA technologies (for example, ATM [17]
   or Frame Relay). The IPv6 over NBMA architecture allows conventional
   host-side operation of the IPv6 Neighbor Discovery protocol, while
   also supporting the establishment of 'shortcut' NBMA forwarding paths
   (when dynamically signaled NBMA links are available).

   The majority of this document focuses on the use of dynamically
   managed point to point and point to multipoint calls between
   interfaces on an NBMA network. These will be generically referred to
   as "SVCs" in the rest of the document. The use of administratively
   configured point to point calls will also be discussed. Such calls
   will be generically referred to as "PVCs". Depending on context,
   either may be shortened to "VC".

   Certain NBMA networks may provide a form of connectionless service
   (e.g. SMDS). In these cases, a "call" or "VC" shall be considered to
   implicitly exist if the sender has an NBMA destination address to
   which it can transmit packets whenever it desires.

1.1 Neighbor Discovery.

   A key difference between this architecture and previous IP over NBMA
   protocols is its mechanism for supporting IPv6 Neighbor Discovery.

   The IPv4 world evolved an approach to address resolution that
   depended on the operation of an auxiliary protocol operating at the
   'link layer' - starting with Ethernet ARP (RFC 826 [14]). In the
   world of NBMA (Non Broadcast, Multiple Access) networks ARP has been
   applied to IPv4 over SMDS (RFC 1209 [13]) and IPv4 over ATM (RFC 1577
   [3]). More recently the ION working group has developed NHRP (Next
   Hop Resolution Protocol [8]), a general protocol for performing
   intra-subnet and inter-subnet address resolution applicable to a
   range of NBMA network technologies.

   IPv6 developers opted to migrate away from a link layer specific
   approach, chosing to combine a number of tasks into a protocol known
   as Neighbor Discovery [7], intended to be non-specific across a
   number of link layer technologies.  A key assumption made by Neighbor
   Discovery's actual protocol is that the link technology underlying a



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   given IP interface is capable of native multicasting.  This is not
   particularly true of most NBMA network services, and usually requires
   convergence protocols to emulate the desired service.  (The MARS
   protocol, RFC 2022 [5], is an example of such a convergence
   protocol.) This document augments and optimizes the MARS protocol for
   use in support of IPv6 Neighbor Discovery, generalizing the
   applicability of RFC 2022 beyond ATM networks.

1.2 NBMA Shortcuts.

   A shortcut is an NBMA level call (VC) directly connecting two IP
   endpoints that are logically separated by one or more routers at the
   IP level. IPv6 packets traversing this VC are said to 'shortcut' the
   routers that are in the logical IPv6 path between the VC's endpoints.

   NBMA shortcuts are a mechanism for minimizing the consumption of
   resources within an IP over NBMA cloud (e.g. router hops and NBMA
   VCs).

   It is important that NBMA shortcuts are supported whenever IP is
   deployed across NBMA networks capable of supporting dynamic
   establishment of calls (SVCs or functional equivalent).  For IPv6
   over NBMA, shortcut discovery and management is achieved through a
   mixture of Neighbor Discovery and NHRP.

1.3 Key components of the IPv6 over NBMA architecture.

1.3.1 NBMA networks providing PVC support.

   When the NBMA network is used in PVC mode, each PVC will connect
   exactly two nodes and the use of Neighbor Discovery and other IPv6
   features is limited.  IPv6/NBMA interfaces have only one neighbor on
   each Link. The MARS and NHRP protocols are NOT necessary, since
   multicast and broadcast operations collapse down to an NBMA level
   unicast operation. Dynamically discovered shortcuts are not
   supported.

   The actual details of encapsulations and link token generation SHALL
   be covered by companion documents covering specific NBMA technology.
   They SHALL conform to the following guidelines:

      Both unicast and multicast IPv6 packets SHALL be transmitted over
      PVC links using the encapsulation described in section 4.4.1.

      Interface tokens for PVC links SHALL be constructed as described
      in section 5. Interface tokens need only be unique between the two
      nodes on the PVC link.




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      This use of PVC links does not mandate, nor does it prohibit the
      use of extensions to the Neighbor Discovery protocol which may be
      developed for either general use of for use in PVC connections
      (for example, Inverse Neighbor Discovery).

   NBMA-specific companion documents MAY additionally specify the
   concatenation of IPv6 over PPP and PPP over NBMA mechanisms as an
   OPTIONAL approach to point to point IPv6.

   Except where noted above, the remainder of this document focuses on
   the SVC case.

1.3.2 NBMA networks providing SVC support.

   When the NBMA network is used in SVC mode, the key components are:

      - The IPv6 Neighbor model, where neighbors are discovered through
        the use of messages multicast to members of an IPv6 interface's
        local IPv6 Link.
      - The MARS model, allowing emulation of general multicast using
        multipoint calls provided by the underlying NBMA network.
      - The NHRP service for seeking out the NBMA identities of IP
        interfaces who are logically distant in an IP topological sense.
      - The modeling of IP traffic as 'flows', and optionally using the
        existence of a flow as the basis for attempting to set up a
        shortcut link level connection.

   In summary:

      The IPv6 "Link" is generalized to "Logical Link" (LL) in NBMA
      environments (analogous to the generalization of IPv4 IP Subnet to
      Logical IP Subnet in RFC 1209 and subsequently RFC 1577).

      IPv6/NBMA interfaces utilize RFC 2022 (MARS) for general intra-
      Logical Link multicasting. The MARS itself is used to optimally
      distribute discovery messages within the Logical Link.

      For destinations not currently considered to be Neighbors, a host
      sends the packets to one of its default routers.

      When appropriately configured, the egress router from a Logical
      Link is responsible for detecting the existence of an IP packet
      flow through it that might benefit from a shortcut connection.

         While continuing to conventionally forward the flow's packets,
         the router initiates an NHRP query for the flow's destination
         IP address.




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         The last router/NHS before the target of the NHRP query
         ascertains the target interface's preferred NBMA address.

         The originally querying router then issues a Redirect to the IP
         source, identifying the flow's destination as a transient
         Neighbor.

      Host-initiated triggering of shortcut discovery, regardless of the
      existence of a packet flow, is also supported through specific
      Neighbor Solicitations sent to a source host's default router.

   A number of key advantages are claimed for this approach. These are:

      The IPv6 stacks on hosts do not implement separate ND protocols
      for each link layer technology.

      When the destination of a flow is solicited as a transient
      neighbor, the returned NBMA address will be the one chosen by the
      destination when the flow was originally established through hop-
      by-hop processing. This supports the existing ND ability for IPv6
      destinations to perform their own dynamic interface load sharing.

1.4 Terminology.

   The bit-pattern or numeric value used to identify a particular NBMA
   interface at the NBMA level will be referred to as an "NBMA address".
   (An example would be an ATM End System Address, AESA, when applying
   this architecture to ATM networks, or an E.164 number when applying
   this architecture to SMDS networks.)

   The call that, once established, is used to transfer IP packets from
   one NBMA interface to another will be referred to as an SVC or PVC
   depending on whether the call is dynamically established through some
   signaling mechanism, or administratively established. The specific
   signaling mechanisms used to establish or tear down an SVC will be
   defined in the NBMA-specific companion specifications.  Certain NBMA
   networks may provide a form of connectionless service (e.g. SMDS). In
   these cases, a "call" or "SVC" shall be considered to implicitly
   exist if the sender has an NBMA destination address to which it can
   transmit packets whenever it desires.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [16].







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1.5 Document Structure.

   The remainder of this document is structured as follows: Section 2
   explains the generalization of IPv6 Link to "Logical Link" when used
   over NBMA networks, and introduces the notion of the Transient
   Neighbor.  Section 3 describes the modifications to the MARS protocol
   for efficient distribution of ND messages within a Logical Link, and
   the rules and mechanisms for discovering Transient Neighbors.
   Section 4 covers the basic rules governing IPv6/NBMA interface
   initialization, packet and control message encapsulations, and rules
   for SVC management. Section 5 describes the general rules for
   constructing Interface Tokens, the Link Layer Address Option, and
   Link Local addresses.  Section 6 concludes the normative sections of
   the document.  Appendix A provides some non-normative descriptive
   text regarding the operation of Ipv6 Neighbor Discovery.  Appendix B
   describes some sub-optimal solutions for emulating the multicasting
   of Neighbor Discovery messages around a Logical Link.  Appendix C
   discusses shortcut suppression and briefly reviews the future
   relationships between flow detection and mapping of flows onto SVCs
   of differing qualities of service.

2. Logical Links, and Transient Neighbors.


   IPv6 contains a concept of on-link and off-link. Neighbors are those
   nodes that are considered on-link and whose link-layer addresses may
   therefore be located using Neighbor Discovery.  Borrowing from the
   terminology definitions in the ND text:

   on-link   - an address that is assigned to a neighbor's interface on
               a shared link.  A host considers an address to be on-
               link if:
                 - it is covered by one of the link's prefixes, or
                 - a neighboring router specifies the address as the
                   target of a Redirect message, or
                 - a Neighbor Advertisement message is received for the
                   target address, or
                 - a Neighbor Discovery message is received from the