Network Working Group B. Thompson
Request for Comments: 3336 T. Koren
Category: Standards Track Cisco Systems
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 (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 20021. Introduction
PPP over AAL5  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 .
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)  to allow multiple PPP payloads to be multiplexed
into a set of ATM cells.
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 .
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  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 20024. PPP Operation with AAL2
PPP over AAL2 defines an encapsulation that uses the Service Specific
Segmentation and Reassembly Sublayer (SSSAR)  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