RFC 1434 (rfc1434) - Page 2 of 33

Data Link Switching: Switch-to-Switch Protocol

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RFC 1434             DLS: Switch-to-Switch Protocol           March 1993


1. Introduction

   Data Link Switching (DLS) is a forwarding mechanism for the IBM SNA
   and IBM NetBIOS protocols.  It does not provide full routing, but
   instead provides switching at the Data Link layer and encapsulation
   in TCP/IP for transport over the Internet.  This memo documents the
   Switch-to-Switch Protocol (SSP) that is used between IBM 6611 Network
   Processors.

   Today, the IBM 6611 supports SNA (PU 2 and PU 4) systems and NetBIOS
   systems attached to token-ring networks, as well as SNA (PU 2)
   systems attached to SDLC links.  For the later case, the SDLC
   attached systems are provided with a LAN appearance within the IBM
   6611.  For the LAN attached systems, the IBM 6611 appears as a
   source-routing bridge.  Remote systems that are accessed through the
   IBM 6611 appear as systems attached to an adjacent ring.  This ring
   is a virtual ring that is manifested within each IBM 6611.

2. Overview

   Data Link Switching was developed to provide support for SNA and
   NetBIOS in multi-protocol routers.  Since SNA and NetBIOS are
   basically connection oriented protocols, the Data Link Control
   procedure that they use on the LAN is IEEE 802.2 Logical Link Control
   (LLC) Type 2.  Data Link Switching also accommodates SNA protocols
   over WAN links via the SDLC protocol.

   IEEE 802.2 LLC Type 2 was designed with the assumption that the
   network transit delay would be small and predictable (i.e., a local
   LAN).  Therefore the LLC elements of procedure use a fixed timer for
   detecting lost frames.  When bridging is used over wide area lines
   (especially at lower speeds), the network delay is larger and it can
   vary greatly based upon congestion.  When the delay exceeds the
   time-out value LLC attempts to retransmit.  If the frame is not
   actually lost, only delayed, it is possible for the LLC Type 2
   procedures to become confused.  And as a result, the link is
   eventually taken down.

   Given the use of LLC Type 2 services, Data Link Switching addresses
   the following bridging problems:

         DLC Time-outs
         DLC Acknowledgments over the WAN
         Flow and Congestion Control
         Broadcast Control of Search Packets
         Source-Route Bridging Hop Count Limits

   NetBIOS also makes extensive use of datagram services that use LLC



Dixon & Kushi
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