RFC 1483 (rfc1483) - Page 4 of 16

Multiprotocol Encapsulation over ATM Adaptation Layer 5

RFC 1483                Multiprotocol over AAL5                July 1993


   necessary to identify the protocol of the routed or bridged PDU.  In
   LLC Encapsulation this information is encoded in an LLC header placed
   in front of the carried PDU.

   Although this memo only deals with protocols that operate over LLC
   Type 1 (unacknowledged connectionless mode) service, the same
   encapsulation principle applies also to protocols operating over LLC
   Type 2 (connection-mode) service.  In the latter case the format
   and/or contents of the LLC header would differ from what is shown
   below.

4.1.  LLC Encapsulation for Routed Protocols

   In LLC Encapsulation the protocol of the routed PDU is identified by
   prefixing the PDU by an IEEE 802.2 LLC header, which is possibly
   followed by an IEEE 802.1a SubNetwork Attachment Point (SNAP) header.
   In LLC Type 1 operation, the LLC header consists of three one octet
   fields:

               +------+------+------+
               | DSAP | SSAP | Ctrl |
               +------+------+------+

   In LLC Encapsulation for routed protocols, the Control field has
   always value 0x03 specifying Unnumbered Information Command PDU.

   The LLC header value 0xFE-FE-03 identifies that a routed ISO PDU (see
   [6] and Appendix B) follows.  The Control field value 0x03 specifies
   Unnumbered Information Command PDU.  For routed ISO PDUs the format
   of the AAL5 CPCS-PDU Payload field shall thus be as follows:

                 Payload Format for Routed ISO PDUs
               +-------------------------------+
               |       LLC  0xFE-FE-03         |
               +-------------------------------+
               |             .                 |
               |           ISO PDU             |
               |     (up to 2^16 - 4 octets)   |
               |             .                 |
               +-------------------------------+

   The routed ISO protocol is identified by a one octet NLPID field that
   is part of Protocol Data.  NLPID values are administered by ISO and
   CCITT.  They are defined in ISO/IEC TR 9577 [6] and some of the
   currently defined ones are listed in Appendix C.

   An NLPID value of 0x00 is defined in ISO/IEC TR 9577 as the Null
   Network Layer or Inactive Set.  Since it has no significance within



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