RFC 1937 (rfc1937) - Page 1 of 8

"Local/Remote" Forwarding Decision in Switched Data Link Subnetworks

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Network Working Group                                         Y. Rekhter
Request for Comments: 1937                                 Cisco Systems
Category: Informational                                       D. Kandlur
                                  T.J. Watson Research Center, IBM Corp.
                                                                May 1996


  "Local/Remote" Forwarding Decision in Switched Data Link Subnetworks

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   The IP architecture assumes that each Data Link subnetwork is labeled
   with a single IP subnet number. A pair of hosts with the same subnet
   number communicate directly  (with no routers); a pair of hosts with
   different subnet numbers always communicate through one or more
   routers. As indicated in RFC 1620, these assumptions may be too
   restrictive for large data networks, and specifically for networks
   based on switched virtual circuit (SVC) based technologies (e.g. ATM,
   Frame Relay, X.25), as these assumptions impose constraints on
   communication among hosts and routers through a network.  The
   restrictions may preclude full utilization of the capabilities
   provided by the underlying SVC-based Data Link subnetwork.  This
   document describes extensions to the IP architecture that relaxes
   these constraints, thus enabling the full utilization of the services
   provided by SVC-based Data Link subnetworks.

1.  Background

   The following briefly recaptures the concept of the IP Subnet.  The
   topology is assumed to be composed of hosts and routers
   interconnected via links (Data Link subnetworks).  An IP address of a
   host with an interface attached to a particular link is a tuple
   , where host number is
   unique within the subnet address prefix.  When a host needs to send
   an IP packet to a destination, the host needs to determine whether
   the destination address identifies an interface that is connected to
   one of the links the host is attached to, or not.  This referred to
   as the "local/remote" decision. The outcome of the "local/remote"
   decision is based on (a) the destination address, and (b) the address
   and the prefix length associated with the the local interfaces.  If
   the outcome is "local", then the host resolves the IP address to a
   Link Layer address (e.g. by using ARP), and then sends the packet



Rekhter & Kandlur            Informational
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