"Local/Remote" Forwarding Decision in Switched Data Link Subnetworks
RFC 1937

Document Type RFC - Informational (May 1996; No errata)
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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 RFC1620, 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
   <prefix length, address prefix, host number>, 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                      [Page 1]
RFC 1937        Forwarding in Switched Data Link Subnets        May 1996

   directly to that destination (using the Link layer services).  If the
   outcome is "remote", then the host uses one of its first-hop routers
   (thus relying on the services provided by IP routing).

   To summarize, two of the important attributes of the IP subnet model
   are:

      hosts with a common subnet address prefix are assumed to be
      attached to a common link (subnetwork), and thus communicate with
      each other directly, without any routers - "local";

      hosts with different subnet address prefixes are assumed to be
      attached to different links (subnetworks), and thus communicate
      with each other only through routers - "remote".

   A typical example of applying the IP subnet architecture to an SVC-
   based Data Link subnetwork is "Classical IP and ARP over ATM"
   (RFC1577).  RFC1577 provides support for ATM deployment that follows
   the traditional IP subnet model and introduces the notion of a
   Logical IP Subnetwork (LIS).  The consequence of this model is that a
   host is required to setup an ATM SVC to any host within its LIS; for
   destinations outside its LIS the host must forward packets through a
   router.  It is important to stress that this "local/remote" decision
   is based solely on the information carried by the destination address
   and the address and prefix lengths associated with the local
   interfaces.

2.  Motivations

   The diversity of TCP/IP applications results in a wide range of
   traffic characteristics.  Some applications last for a very short
   time and generate only a small number of packets between a pair of
   communicating hosts (e.g. ping, DNS). Other applications have a short
   lifetime, but generate a relatively large volume of packets (e.g.
   FTP). There are also applications that have a relatively long
   lifetime, but generate relatively few packets (e.g.  Telnet).
   Finally, we anticipate the emergence of applications that have a
   relatively long lifetime and generate a large volume of packets (e.g.
   video-conferencing).

   SVC-based Data Link subnetworks offer certain unique capabilities
   that are not present in other (non-SVC) subnetworks (e.g. Ethernet,
   Token Ring).  The ability to dynamically establish and tear-down SVCs
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