PPP Over Asynchronous Transfer Mode Adaptation Layer 2 (AAL2)
RFC 3336

Document Type RFC - Proposed Standard (December 2002; No errata)
Authors bruce Buffam  , Thima Koren  , Bruce Thompson 
Last updated 2013-03-02
Stream IETF
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IESG IESG state RFC 3336 (Proposed Standard)
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IESG note RFCs 3336 & 3337
Send notices to <karlfox@columbus.rr.com>
Network Working Group                                        B. Thompson
Request for Comments: 3336                                      T. Koren
Category: Standards Track                                  Cisco Systems
                                                               B. Buffam
                                                         Seaway Networks
                                                           December 2002

     PPP Over Asynchronous Transfer Mode Adaptation Layer 2 (AAL2)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   The Point-to-Point Protocol (PPP) provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.

   This document describes the use of ATM Adaptation Layer 2 (AAL2) for
   framing PPP encapsulated packets.


   This specification is intended for those implementations which desire
   to use the facilities which are defined for PPP, such as the Link
   Control Protocol, Network-layer Control Protocols, authentication,
   and compression.  These capabilities require a point-to-point
   relationship between the peers, and are not designed for the multi-
   point relationships which are available in ATM and other multi-access

Thompson, et. al.           Standards Track                     [Page 1]
RFC 3336                     PPP Over AAL2                 December 2002

1. Introduction

   PPP over AAL5 [2] describes the encapsulation format and operation of
   PPP when used with the ATM AAL5 adaptation layer.  While this
   encapsulation format is well suited to PPP transport of IP, it is
   bandwidth inefficient when used for transporting small payloads such
   as voice.  PPP over AAL5 is especially bandwidth inefficient when
   used with RTP header compression [3].

   PPP over AAL2 addresses the bandwidth efficiency issues of PPP over
   AAL5 for small packet transport by making use of the AAL2 Common Part
   Sublayer (CPS) [4] to allow multiple PPP payloads to be multiplexed
   into a set of ATM cells.

2. Conventions

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [6].

3. AAL2 Layer Service Interface

   The PPP layer treats the underlying ATM AAL2 layer service as a bit-
   synchronous point-to-point link.  In this context, the PPP link
   corresponds to an ATM AAL2 virtual connection.  The virtual
   connection MUST be full-duplex, point to point, and it MAY be either
   dedicated (i.e., permanent, set up by provisioning) or switched (set
   up on demand).  In addition, the PPP/AAL2 service interface boundary
   MUST meet the following requirements.

      Interface Format - The PPP/AAL2 layer boundary presents an octet
      service interface to the AAL2 layer.  There is no provision for
      sub-octets to be supplied or accepted.

      Transmission Rate - The PPP layer does not impose any
      restrictions regarding transmission rate on the underlying ATM
      layer traffic descriptor parameters.

      Control Signals - The AAL2 layer MUST provide control signals to
      the PPP layer which indicate when the virtual connection link has
      become connected or disconnected.  These provide the "Up" and
      "Down" events to the LCP state machine [1] within the PPP layer.

      In the case of PPP over AAL2, the state of the link can be derived
      from the type 3 fault management packets carried in-band within a
      given AAL2 CID flow.

Thompson, et. al.           Standards Track                     [Page 2]
RFC 3336                     PPP Over AAL2                 December 2002

4. PPP Operation with AAL2

   PPP over AAL2 defines an encapsulation that uses the Service Specific
   Segmentation and Reassembly Sublayer (SSSAR) [5] for AAL type 2.  The
   SSSAR sub-layer is used to segment PPP packets into frames that can
   be transported using the AAL2 CPS.  The SSSAR sub-layer uses
   different AAL2 UUI code-points to indicate whether a segment is the
   last segment of a packet or not.

   The encapsulation of PPP over AAL2 provides a 16-bit CRC for PPP
   payloads.  There are 2 UUI code points assigned from SSSAR to
   indicate intermediate fragments of a packet and the last fragment of
   a packet.  Code point 27 indicates intermediate frames of a
   fragmented packet and code point 26 indicates the last frame of a
   packet.  The encapsulation format is more fully described in section
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