RFC 3293 (rfc3293) - Page 2 of 9

General Switch Management Protocol (GSMP) Packet Encapsulations for Asynchronous Transfer Mode (ATM), Ethernet and Transmission Control Protocol (TCP)

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RFC 3293               GSMP Packet Encapsulations              June 2002


2. ATM Encapsulation

   GSMP packets are variable length and for an ATM data link layer they
   are encapsulated directly in an AAL-5 CPCS-PDU [3][4] with an
   LLC/SNAP header as illustrated:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               LLC (0xAA-AA-03)                |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
   |                   SNAP (0x00-00-00-88-0C)                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                         GSMP Message                          ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Pad (0 - 47 bytes)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +             AAL-5 CPCS-PDU Trailer (8 bytes)                  +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   (The convention in the documentation of Internet Protocols [5] is to
   express numbers in decimal.  Numbers in hexadecimal format are
   specified by prefacing them with the characters "0x".  Numbers in
   binary format are specified by prefacing them with the characters
   "0b".  Data is pictured in "big-endian" order.  That is, fields are
   described left to right, with the most significant byte on the left
   and the least significant byte on the right.  Whenever a diagram
   shows a group of bytes, the order of transmission of those bytes is
   the normal order in which they are read in English.  Whenever a byte
   represents a numeric quantity the left most bit in the diagram is the
   high order or most significant bit.  That is, the bit labelled 0 is
   the most significant bit.  Similarly, whenever a multi-byte field
   represents a numeric quantity the left most bit of the whole field is
   the most significant bit.  When a multi-byte quantity is transmitted,
   the most significant byte is transmitted first.  This is the same
   coding convention as is used in the ATM layer [2] and AAL-5 [3][4].)

   The LLC/SNAP header contains the bytes: 0xAA 0xAA 0x03 0x00 0x00 0x00
   0x88 0x0C.  (0x880C is the assigned Ethertype for GSMP.)

   The maximum transmission unit (MTU) of the GSMP Message field is 1492
   bytes.





Doria, et. al.              Standards Track
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