Multiprotocol Encapsulation over ATM Adaptation Layer 5
RFC 1483

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Network Working Group                                      Juha Heinanen
Reguest for Comments: 1483                               Telecom Finland
                                                               July 1993

            Multiprotocol Encapsulation over ATM Adaptation Layer 5

Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Abstract

   This memo describes two encapsulations methods for carrying network
   interconnect traffic over ATM AAL5.  The first method allows
   multiplexing of multiple protocols over a single ATM virtual circuit
   whereas the second method assumes that each protocol is carried over
   a separate ATM virtual circuit.

1.  Introduction

   Asynchronous Transfer Mode (ATM) based networks are of increasing
   interest for both local and wide area applications.  This memo
   describes two different methods for carrying connectionless network
   interconnect traffic, routed and bridged Protocol Data Units (PDUs),
   over an ATM network.  The first method allows multiplexing of
   multiple protocols over a single ATM virtual circuit.  The protocol
   of a carried PDU is identified by prefixing the PDU by an IEEE 802.2
   Logical Link Control (LLC) header.  This method is in the following
   called "LLC Encapsulation" and a subset of it has been earlier
   defined for SMDS [1].  The second method does higher-layer protocol
   multiplexing implicitly by ATM Virtual Circuits (VCs).  It is in the
   following called "VC Based Multiplexing".

   ATM is a cell based transfer mode that requires variable length user
   information to be segmented and reassembled to/from short, fixed
   length cells.  This memo doesn't specify a new Segmentation And
   Reassembly (SAR) method for bridged and routed PDUs.  Instead, the
   PDUs are carried in the Payload field of Common Part Convergence
   Sublayer (CPCS) PDU of ATM Adaptation Layer type 5 (AAL5) [2].

   Note that this memo only describes how routed and bridged PDUs are
   carried directly over the CPCS of AAL5, i.e., when the Service
   Specific Convergence Sublayer (SSCS) of AAL5 is empty.  If Frame

Heinanen                                                        [Page 1]
RFC 1483                Multiprotocol over AAL5                July 1993

   Relay Service Specific Convergence Sublayer (FR-SSCS), as defined in
   I.36x.1 [3], is used over the CPCS of AAL5, then routed and bridged
   PDUs are carried using the NLPID multiplexing method described in RFC
   1294 [4].  Appendix A (which is for information only) shows the
   format of the FR-SSCS-PDU as well as how IP and CLNP PDUs are
   encapsulated over FR-SSCS according to RFC 1294.

2.  Selection of the Multiplexing Method

   It is envisioned that VC Based Multiplexing will be dominant in
   environments where dynamic creation of large numbers of ATM VCs is
   fast and economical.  These conditions are likely to first prevail in
   private ATM networks.  LLC Encapsulation, on the other hand, may be
   desirable when it is not practical for one reason or another to have
   a separate VC for each carried protocol.  This is the case, for
   example, if the ATM network only supports (semi) Permanent Virtual
   Circuits (PVCs) or if charging depends heavily on the number of
   simultaneous VCs.

   When two ATM stations wish to exchange connectionless network
   interconnect traffic, selection of the multiplexing method is done
   either by manual configuration (in case of PVCs) or by B-ISDN
   signalling procedures (in case of Switched VCs).  The details of B-
   ISDN signalling are still under study in CCITT [5].  It can, however,
   be assumed that B-ISDN signalling messages include a "Low layer
   compatibility" information element, which will allow negotiation of
   AAL5 and the carried (encapsulation) protocol.

3.  AAL5 Frame Format

   No matter which multiplexing method is selected, routed and bridged
   PDUs shall be encapsulated within the Payload field of AAL5 CPCS-PDU.
   The format of the AAL5 CPCS-PDU is given below:

Heinanen                                                        [Page 2]
RFC 1483                Multiprotocol over AAL5                July 1993

                AAL5 CPCS-PDU Format
               +-------------------------------+
               |             .                 |
               |             .                 |
               |        CPCS-PDU Payload       |
               |     up to 2^16 - 1 octets)    |
               |             .                 |
               |             .                 |
               +-------------------------------+
               |      PAD ( 0 - 47 octets)     |
               +-------------------------------+ -------