"Local/Remote" Forwarding Decision in Switched Data Link Subnetworks (RFC1937)
Original Publication Date: 1996-May-01
Included in the Prior Art Database: 2019-Feb-12
Internet Society Requests For Comment (RFCs)
Y. Rekhter: AUTHOR [+1]
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. This memo provides information for the Internet community. This memo does not specify an Internet standard of any kind.
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.
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.
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 exam...