RFC 1538 (rfc1538) - Page 3 of 10

Advanced SNA/IP : A Simple SNA Transport Protocol

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RFC 1538                    Advanced SNA/IP                 October 1993


3.  SNA/IP Protocol Specification

3.1.  Glossary

   Data Link Switching (DLSw) - This is best described as a routing
   protocol used for the conversion of LLC-based SNA sessions to an IP
   form.  The initial version of the DLSw protocol is documented in the
   informational RFC 1434 [1].

   Downstream Node (DSN) - An SNA Physical Unit (PU) type 2.0 or 2.1
   device connected to the SNA network via a LAN (802.5, 802.3, etc.) as
   opposed to an SDLC, X.25, or channel connection.

   SNA Gateway - A device that provides a data link control (DLC)
   conversion function for SNA PU type 5 (host) devices and LAN-
   attached DSNs.

   Subnet SNA Gateway - A device connected to both a traditional SNA
   token-ring segment and an IP network that performs local termination
   of the LLC connections, a mapping function of source address to
   destination IP address, and a conversion (switching) function of LLC
   to IP.

3.2.  Conventions and Assumptions

   Frame formats are shown starting with the IP header.  Other headers
   will, of course, appear in the actual frames sent, but these headers,
   and the numbers of them, will vary across MAC types.

   It is assumed the reader is familiar with both the standard SNA
   protocol (to the extent it applies to SNA Gateway and DSN functions)
   and the base set of TCP/IP protocols.  Where practical, the reader is
   asked to refer to appropriate SNA and TCP/IP documentation.

3.3.  The Protocol

   Conceptually, there are three phases to the Advanced SNA/IP protocol:
   the Connection Establishment phase, the Data Transfer phase, and the
   Connection Termination phase.

3.3.1.  Connection Establishment

   Connection Establishment involves the exchange of logical XID packets
   between the connecting end nodes and culminates in the establishment
   of a TCP connection.  This process is similar to the IBM-specified
   Test, XID, SABME and UA exchange used to establish a Type II 802.2
   connection for SNA traffic [2].  In place of the 802.2 Type I
   messages, SNA/IP defines the following set of UDP datagrams:



Behl, Sterling & Teskey
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