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

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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
   between communicating entities attached to an SVC-based Data Link
   subnetwork enables the dynamic dedication and redistribution of
   certain communication resources (e.g. bandwidth) among the entities.
   This dedication and redistribution of resources could be accomplished
   by relying solely on the mechanism(s) provided by the Data Link



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