RFC 3336 (rfc3336) - Page 2 of 16

PPP Over Asynchronous Transfer Mode Adaptation Layer 2 (AAL2)

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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

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   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
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