RFC 2491 (rfc2491) - Page 2 of 44

IPv6 over Non-Broadcast Multiple Access (NBMA) networks

RFC 2491                IPv6 over NBMA networks             January 1999


   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



Armitage, et. al.           Standards Track