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Versions: 00 01 02 RFC 3336

Point-to-Point Protocol Extensions Working Group         Bruce Thompson
Internet Draft                                           Bruce Buffam
November 16, 2001                                        Tmima Koren
Expires June 2002                                     Cisco Systems

                          PPP over AAL2

Status of this memo

This document is an Internet Draft and is in full conformance with all
provisions of Section 10 of RFC 2026. Internet Drafts are working
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Copyright Notice

Copyright (C) The Internet Society (1999-2000). All Rights Reserved.


   The Point-to-Point Protocol (PPP) [1] 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

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 or 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
        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.

4. PPP Operation with AAL2

PPP over AAL2 defines an encapsulation that uses the Segmentation and
Reassembly Service Specific Convergence Sublayer (SSSAR) [5] for AAL
type 2. The SSSAR sublayer is used to segment PPP packets into frames
that can be transported using the AAL2 CPS. The SSSAR sublayer 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 6.2.1.

An implementation of PPP over AAL2 MAY use a single AAL2 Channel
Identifier (CID) for transport of all PPP packets. A PPP over AAL2
implementation may also use multiple AAL2 CIDs to carry a single PPP
session. Multiple CIDs could be used to implement a multiple class
real time transport service for PPP using the AAL2 layer for link
fragmentation and interleaving. A companion document [10] describes
class extensions for PPP over AAL2 using multiple AAL2 CIDs.

5. Comparison of PPP over AAL2 with existing encapsulations

This document proposes the substitution of AAL2 transport for PPP in
scenarios where small packets are being transported over an ATM
network. This is most critical in applications such as voice transport
using RTP [9] where RTP Header compression [5] is used. In
applications such as voice transport, both bandwidth efficiency and
low delay are very important.

This section provides justification for the PPP over AAL2 service for
ATM transport by comparing it to existing PPP encapsulation formats
used for transport over ATM. Two encapsulation formats will be
examined here: PPP over AAL5 [2], and PPP with PPPMUX [8]
over AAL5.

5.1 Comparison with PPP over AAL5

This proposal uses ATM AAL2 rather than AAL5 as the transport for PPP.
The header efficiency of the payload encapsulation with SSSAR
and the AAL2 CPS provides for less ATM encapsulation overhead per PPP
payload. The payload encapsulation consists of a 2 byte CRC. The
AAL2 CPS header consists of 3 bytes, and the Offset field is 1 byte.
This is a total encapsulation overhead of 6 bytes. This compares to 8
bytes of overhead for the AAL5 trailer used for PPP over AAL5.

The multiplexing function of the AAL2 CPS layer allows more bandwidth
efficient transport of CRTP frames by multiplexing multiple CRTP
frames into one or more ATM cells using the AAL2 CPS function. This
removes the pad overhead of AAL5 when used to transport short frames.

5.2 Comparison with PPPMUX over AAL5

A new method for doing multiplexing in the PPP layer has been adopted
in the PPP Extensions working group. The draft is called the PPP
Multiplexed Frame Option [8]. PPP Multiplexing provides similar
functionality to the CPS based multiplexing function of AAL2. Using
PPP multiplexing, a PPP stack would look like PPP/PPPMUX/AAL5.

Both PPP/PPPMUX/AAL5 and PPP/AAL2 use multiplexing to reduce the
overhead of cell padding when frames are sent over an ATM virtual
circuit. However, the bandwidth utilization of PPP/AAL2 will typically
be better than the bandwidth used by PPP/PPPMUX/AAL5. This is because
multiplexed frames in PPP/PPPMUX/AAL5 must always be encapsulated
within an AAL5 frame before being sent. This encapsulation causes an
additional 8 bytes of AAL5 trailer to be added to the PPPMUX
encapsulation. In addition to the 8 bytes of AAL5 trailer, PPPMUX will
incur an average of 24 additional bytes of AAL5 PAD. These 2 factors
will end up reducing the effective efficiency of PPPMUX when it is
used over AAL5.

With PPP/AAL2, the AAL2 CPS layer treats individual PPP frames as a
series of CPS payloads that can be multiplexed. As long as PPP frames
arrive at the CPS layer before the CPS TIMER_CU expires, all ATM cells
coming from the CPS layer will be filled. Under these conditions,
PPP/AAL2 will have no PAD associated with it. When the AAL2 CPS
function causes no PAD to be generated, PPP/AAL2 will be more bandwidth
efficient than PPP/PPPMUX/AAL5.

In PPP/PPPMUX/AAL5, the AAL5 SAR and the PPP MUX/DEMUX are performed
in two different layers. Thus, the PPPMUX/AAL5 receiver must
reassemble a full AAL5 frame from the ATM layer before the PPPMUX
layer can extract the PPP payloads. To derive maximum PPP Multiplexing
efficiency, many PPP payloads may be multiplexed together. This
increases the size of the multiplexed frame to many ATM cells. If one
of these ATM cells is lost, the whole PPPMUX packet will be discarded.
Also, there may be a significant delay incurred while the AAL5 layer
waits for many ATM cell arrival times until a full frame has been
assembled before the full frame is passed up to the PPP Multiplexing
layer where the inverse PPP demux then occurs. This same issue incurs
for the PPPMUX/AAL5 frames progressing down the stack.

With AAL2, both the segmentation and reassembly and multiplexing
functions are performed in the AAL2 CPS layer. Because of the
definition of the AAL2 CPS function, a multiplexed payload will be
extracted as soon as it is received. The CPS receiver does not wait
until the many payloads of an AAL2 multiplexed frame are received
before removing payloads from the multiplexed stream. The same benefit
also applies to AAL2 CPS sender implementations. Also, the loss of an
ATM cell causes the loss of the packets that are included in that cell

The AAL2 CPS function provides multiplexing in AAL2. This function
often needs to be implemented in hardware for performance reasons.
Because of this, a PPP/AAL2 implementation that takes advantage of an
AAL2 SAR implemented in hardware will have significant performance
benefits over a PPP/PPPMUX/AAL5 implementation where PPPMUX is
implemented in software. Also, the AAL2 specification has been
available significantly longer than the PPP Multiplexing specification
and because of this, optimized software and hardware implementations
of the AAL2 CPS function are further in development than PPP
Multiplexing implementations.

6. Detailed Protocol Operation Description

6.1 Background

6.1.1 AAL2 Multiplexing

ITU-T I.363.2 specifies ATM Adaptation Layer Type 2.  This AAL type
provides for bandwidth efficient transmission of low-rate, short and
variable length packets in delay sensitive applications. More than one
AAL type 2 user information stream can be supported on a single ATM
connection.  There is only one definition for the sub-layer because it
implements the interface to the ATM layer and is shared by more than
one SSCS layer.

6.1.2 AAL2 Service Specific Convergence Sub-layers

ITU-T I.366.1 and I.366.2 define Service Specific Convergence Sub-
layers (SSCS) that operate above the Common Part Sub-layer defined in
I.363.2.  This layer specifies packet formats and procedures to encode
the different information streams in bandwidth efficient transport. As
the name implies, this sub-layer implements those elements of service
specific transport. While there is only one definition of the Common
Part Layer there can be more than one SSCS function defined to run
over the CPS Layer. Different CIDs within an AAL2 virtual circuit MAY
run different SSCSs.

This proposal uses the SSSAR sublayer of I.366.1 for transport.

6.1.3 AAL2 CPS-PKT Format

The CPS-PKT format over AAL2 as defined in I.363.2:

|           +          +         +         +                          |
|    CID    +    LI    +   UUI   +   HEC   +        CPS-INFO          |
|           +          +         +         +                          |
|           +          +         +         +                          |
|    (8)    +    (6)   +   (5)   +   (5)   +       (45/64 * 8)        |
      Note: The size of the fields denote bit-width

The Channel ID (CID) identifies the sub-stream within the AAL2
connection. The Length indication (LI) indicates the length of the
CPS-INFO payload. The User-to-User Indication (UUI) carries
information between the SSCS/Application running above the CPS. The
SSSAR sublayer as defined in I.366.1 uses the following code points:

   UUI Code-point       Packet Content
   ++++++++++++++       ++++++++++++++

   0-26              Framed mode data, final packet.

   27                Framed mode data, more to come.

This proposal uses two UUI code-points as follows:

   UUI Code-point       Packet Content
   ++++++++++++++       ++++++++++++++

   27                   non-final packet.

   26                   final packet.

6.1.4 AAL2 CPS-PDU Format

The CPS-PDU format over AAL2 as defined in I.363.2:

                      +CPS+ CPS-INFO+
                      +PKT+         +
                      +HDR+         +
                      |             |

                      |             +-+-+-+~+~+-+-+
                                    +CPS+ CPS-INFO+
                      |             +PKT+         +
                                    +HDR+         +
                      |             +-+-+-+~+~+-+-+

                      |             |             +-+-+-+~+~+-+-+
                                                  +CPS+ CPS-INFO+
                      |             |             +PKT+         +
                                                  +HDR+         +
                      |             |             +-+-+-+~+~+-+-+

                      V             V             V             V
  Cell    +       +S+ +                                         |     +
  Header  +  OSF  + +P+             CPS-PDU Payload             | PAD +
          +  (6)  +N+ +                                         |     +
        Note: The size of the fields denote bitwidth

The CPS-PDU format is used to carry one or more CPS-PKT's multiplexed
on a single CPS-PDU. The offset field (OSF) carries the binary value of
the offset, measured in number of octets, between the end of the STF
and the first start of a CPS-Packet, or in the absence of a first
start, to the start of the pad field. The SN bit is used to number (mod
2) the CPS-PDUs.  The Parity(P) bit is set to 1 if the parity over the
8 bit STF is odd.

6.2 PPP over AAL2 Encapsulation

PPP encapsulation over AAL2 uses the AAL2 CPS with no change.

Some PPP encapsulated protocols such as RTP header compression require
that the link layer provide packet error detection. Because of this,
PPP over AAL2 defines a 16-bit CRC that is used along with the SSSAR sublayer
of I.366.1 to provide packet error detection. The encapsulation format is
described below.

6.2.1 PPP Payload Encapsulation over AAL2 with 16-bit CRC (CRC-16).

The payload encapsulation of PPP appends a two byte CRC to each
PPP frame before using the SSSAR layer to send the PPP packet as a
series of AAL2 frames. The CRC-16 field is computed using the
polynomial x^16 + x^12 + x^5 + 1.

The format of a PPP over AAL2 packet is shown in the diagram below. Note that
the diagram below shows the payload encapsulation when the packet is
not segmented (UUI=26). When the PPP packet is segmented, the PPP
Protocol ID, Information field, and CRC-16 fields will be split across
multiple SSSAR frames. In this case, the UUI field will be set to 27
for all frames except the last frame. In the last frame, the UUI field
will be set to 26.

Payload Encapsulation
|           +          +         +         +          +             +        |
|    CID    +    LI    +   UUI   +   HEC   + Protocol +             +        |
|           +          +         +         +    ID    + Information + CRC-16 |
|           +          +         +         +          +             +        |
|    (8)    +    (6)   +   (5)   +   (5)   +  (8/16)  +             +  (16)  |
      Note: The size of the fields denote bit-width

The CRC-16 field is filled with the value of a CRC calculation which
is performed over the contents of the PPP packet, including the PPP
Payload ID and the information field. The CRC field shall contain the
ones complement of the sum (modulo 2) of:
1) the remainder of x^k (x^15 + x^14 + ... + x + 1) divided (modulo 2)
by the generator polynomial, where k is the number of bits of the
information over which the CRC is calculated; and
2) the remainder of the division (modulo 2) by the generator
polynomial of the product of x^16 by the information over which the
CRC is calculated.

The CRC-16 generator polynomial is:
G(x) = x^16 + x^12 + x^5 + 1

The result of the CRC calculation is placed with the least significant
bit right justified in the CRC field. As a typical implementation at
the transmitter, the initial content of the register of the device
computing the remainder of the division is preset to all "1"s and is
then modified by division by the generator polynomial (as described
above) on the information over which the CRC is to be calculated; the
ones complement of the resulting remainder is put into the CRC field.

As a typical implementation at the receiver, the initial content of
the register of the device computing the remainder of the division is
preset to all "1"s. The final remainder, after multiplication by x^16
and then division (modulo 2) by the generator polynomial of the serial
incoming PPP packet, will be (in the absence of errors):
C(x) = x^15 + x^14 + x^12 + x^11 + x^10 + x^8 + x^6 + x^5 + x^4 + x^3
+ x + 1

6.2 Use of AAL2 CPS-PKT CIDs

An implementation of PPP over AAL2 MAY use a single AAL2 Channel
Identifier (CID) or multiple CIDs for transport of all PPP packets. In
order for the endpoints of a PPP session to work with AAL2, they MUST
both agree on the number, SSCS mapping, and values of AAL2 CIDs that
will be used for a PPP session. The values of AAL2 CIDs to be used for
a PPP session MAY be obtained from either static provisioning in the
case of a dedicated AAL2 connection (PVC) or from Q.2630.1 [7]
signaling in the case of an AAL2 switched virtual ciruit (SPVC or

Using this proposal it is possible to support the use of conventional
AAL2 in CIDs that are not used to support PPP over AAL2. This proposal
allows the co-existence of multiple types of SSCS function within
the same AAL2 VCC.

6.4 PPP over AAL2 Operation

PPP operation with AAL2 will perform basic PPP encapsulation with the
PPP protocol ID. A 16-bit CRC is calculated as described above and appended
to the payload. The SSSAR sublayer of AAL2 is used for transport.

Applications implementing PPP over AAL2 MUST meet all the requirements
of PPP [1].

7. Example implementation of PPP/AAL2

This section describes an example implementation of how PPP can be
encapsulated over AAL2. The example shows two application stacks generating
IP packets that are sent to the same interface running PPP/AAL2. One
Application stack is generating RTP packets and another application is
generating IP Datagrams. The PPP/AAL2 interface shown in this example
is running an RFC 2508 compliant version of RTP header compression.

Here are the paths an Application packet can take in this

    +---+---+---+---+--+                                        +
    |   Application A  |                                        |
    +---+---+---+---+--+                                        |
    |       RTP        |                                        |
    +---+---+---+---+--+       +---+---+---+---+---+       Application
    |       UDP        |       |   Application B   |            |
    +---+---+---+---+--+       +---+---+---+---+---+            |
    |        IP        |       |        IP         |            |
    +---+---+---+---+--+       +---+---+---+---+---+            +
            |                            |
                  +---+---+---+---+---+--+                      +
                  |  Compression Filter  |                      |
                  +---+---+---+---+---+--+                      |
                            |                                   |
                            |                                   |
                  +---------+-----------+                       |
                  |                     |                       |
     Compression  |                     | Non-Compression       |
      Interface   V                     |  Interface            |
    +---+---+---+---+---+---+           |                       |
    |            CRTP       |           |                       |
    +---+---+---+---+---+---+---+---+---+---+---+---+       Transport
    |                      PPP                      |           |
    +---+---+---+---+---+---+---+---+---+---+---+---+           |
                            |                                   |
    +---+---+---+---+---+---+---+ +--+---+---+---+---+--+--+-+  |
    |               Segmentation (SSSAR)                     |  |
    +---+---+---+---+---+---+---+ +--+---+---+---+---+--+--+-+  |
    +---+---+---+---+---+---+---+---+---+---+---+---+---+----+  |
    |                   AAL2 CPS                             |  |
    +---+---+---+---+---+---+---+---+---+---+---+---+---+----+  |
    |                   ATM Layer                            |  |
    +---+---+---+---+---+---+---+---+---+---+---+---+---+----+  +

In the picture above, application A is an RTP application generating
RTP packets. Application B is an IP application generating IP
datagrams. Application A gathers the RTP data and formats an RTP
packet. Lower level layers of application A add UDP and IP headers to
form a complete IP packet. Application B is generating datagrams to
the IP layer. These datagrams have neither a UDP header or an RTP

In the above picture, a protocol stack is configured to apply CRTP/PPP/AAL2
compression on an interface to a destination host. All packets that are sent
to this interface will be tested to see if they can be compressed using RTP
header compression. As packets appear at the interface, they will be tested
by a compression filter to determine if they are candidates for header
compression. If the compression filter determines that the packet is a
candidate for compression, the packet will be sent to the CRTP compressor.
If the packet is not a candidate for compression, it will be sent directly
to the PPP layer for encapsulation as an IP packet encapsulated in PPP.

The destination UDP port number and packet length are examples of criteria
that may be used by the compression filter to select the interface.

Packets from application A will be sent to the compression interface. The
compression interface applies RFC 2508 compliant header compression and then
hands the compressed packet to the PPP layer for encapsulation as one of the
compressed header types of CRTP. The PPP layer will add the appropriate CRTP
payload type for the compressed packet.

Packets from application B will be sent directly to the PPP layer for
encapsulation as an IP/PPP packet. The PPP layer will add the PPP payload
type for an IP packet encapsulated in PPP.

PPP packets are then segmented using I.366.1 segmentation with SSSAR.

The resulting AAL2 frame mode PDU is passed down as a CPS SDU to the CPS
Layer for multiplexing accompanied by the CPS-UUI and the CPS-CID. The CPS
Layer multiplexes the CPS-PKT onto a CPS-PDU. CPS-PDUs are passed to the ATM
layer as ATM SDUs to be carried end-to-end across the ATM network.

At the receiving end, the ATM SDU's arrive and are passed up to the AAL2
CPS. As the AAL2 CPS PDU is accumulated, complete CPS-PKT's are reassembled
by the SSSAR SSCS. Reassembled packets are checked for errors using the CRC

At this point, the PPP layer on the receiving side uses the PPP payload type
to deliver the packet to either the CRTP decompressor or the IP layer
depending on the value of the PPP payload type.

8. LCP Configuration Options

By default, PPP over AAL2 will use the 16 bit CRC encapsulation for
all packets.

The default Maximum-Receive-Unit (MRU) is 1500 bytes.

9. Security Considerations

Generally, ATM networks are virtual circuit based, and security is implicit
in the public data networking service provider's administration of Permanent
Virtual Circuits (PVCs) between the network boundaries.  The probability of a
security breach caused by mis-routed ATM cells is considered to be negligible.

When a public ATM network supports Switched Virtual Circuits, the protocol model
becomes analogous to traditional voice band modem dial up over the Public
Switched Telephone Network (PSTN).  The same PAP/CHAP authentication protocols
that are already widely in use for Internet dial up access are leveraged.  As a
consequence, PPP over AAL2 security is at parity with those practices already
established by the existing Internet infrastructure.

Those applications that require stronger security are encouraged to use
authentication headers, or encrypted payloads, and/or ATM-layer security

When PPP over AAL2 is used on a set of CIDs in a virtual connection, there may
be other non PPP encapsulated AAL2 CIDs running on the same virtual connection.
Because of this, an end point cannot assume that the PPP session authentication
and related security mechanisms also secure the non PPP encapsulated CIDs on
that same virtual connection.

10. Acknowledgements

The authors would like to thank Rajesh Kumar, Mike Mclaughlin, Pietro
Schicker, James Carlson and John O'Neil for their contributions to this

11. References

   [1]   Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
         51, RFC 1661, July 1994.

   [2]   Gross, G., Editor, "PPP over AAL5", STD
         51, RFC 2364, July 1998.

   [3]   S. Casner, V. Jacobson, "Compressing IP/UDP/RTP Headers for
         Low-Speed Serial Links", RFC2508, February 1999.

   [4]   International Telecommunications Union, "BISDN ATM Adaptation layer
         specification: Type 2 AAL(AAL2)", ITU-T Recommendation I.363.2,
         September 1997.

   [5]   International Telecommunications Union, "Segmentation and Reassembly
         Service Specific Convergence Sublayer for the AAL type 2", ITU-T
         Recommendation I.366.1, June 1998.

   [6]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [7]   ITU-T, "DRAFT NEW ITU-T RECOMMENDATION Q.2630.1", July 1999

   [8]    R. Pazhyannur, I. Ali, Craig Fox, "PPP Multiplexed Frame
          Option", draft-ietf-pppext-pppmux-00.txt, January 2000.

   [9]    H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, "RTP: A
          Transport Protocol for Real-Time Applications", RFC1889, January
   [10]   B Thompson, T Koren, B Buffam, "Class Extensions for PPP over AAL2",
          draft-brucet-pppext-ppp-over-aal2-class-00.txt, March 2001

12. Authors' Addresses

   Bruce Thompson
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134
   Phone: +1 408 527-0446
   Email: brucet@cisco.com

   Bruce Buffam
   Cisco Systems, Inc.
   365 March Road
   Kanata, Ontario,
   Canada, K2K-2C9
   Phone: +1 613 271-3412
   Email: bbuffam@cisco.com

   Tmima Koren
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134
   Phone: +1 408 527-6169
   Email: tmima@cisco.com

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