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Versions: 01 02 03 04 05 rfc2364                                        
PPP Extensions Working Group                       Manu Kaycee, Paradyne
INTERNET DRAFT                         George Gross, Lucent Technologies
Expires February 25, 1998                      Arthur Lin, Cisco Systems
                                     Andrew Malis, Ascend Communications
                                           John Stephens, Cayman Systems
                                                           July 25, 1997

                             PPP Over AAL5


Status Of This Memo

This document is an Internet-Draft.  Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
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Distribution of this memo is unlimited.


     The Point-to-Point Protocol (PPP) [1] provides a standard method
     for transporting multi-protocol datagrams over point-to-point

     This document describes the use of ATM Adaptation Layer 5 (AAL5)
     for framing PPP encapsulated packets.


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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

ATM AAL5 protocol is designed to provide virtual connections between end
stations attached to the same network.  These connections offer a packet
delivery service that includes error detection, but does not do error

Most existing implementations of PPP use ISO 3309 HDLC as a basis for
their framing [3].

When an ATM network is configured with point-to-point connections, PPP
can use AAL5 as a framing mechanism, ignoring its other features.

2. AAL5 Layer Service Interface

The PPP layer treats the underlying ATM AAL5 layer service as a bit-
synchronous point-to-point link.  In this context, the PPP link
corresponds to an ATM AAL5 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/AAL5 service interface boundary MUST meet the
following requirements:

     Interface Format - The PPP/AAL5 layer boundary presents an octet
     service interface to the AAL5 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.

     Control Signals - The AAL5 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.

3. Multi-Protocol Encapsulation

This specification uses the principles, terminology, and frame structure
described in "Multiprotocol Encapsulation over ATM Adaptation Layer 5"

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The purpose of this specification is not to document what is already
standardized in [4], but to specify how the mechanisms described in [4]
are to be used to map PPP onto an AAL5-based  ATM network.  Section 1
within [4] defines the two mechanisms for identifying the Protocol Data
Unit (PDU) payload field's protocol type: virtual circuit based
multiplexing, and Logical Link Control (LLC) encapsulation.  In the
former technique, the payload's protocol type is implicitly agreed to by
the end points for each virtual circuit using provisioning or control
plane procedures.  When using the LLC encapsulation technique, the
payload's protocol type is explicitly identified on a per PDU basis by
an in-band LLC header, followed by the payload data.

When transporting a PPP payload over AAL5, an implementation:

     1. MUST support virtual circuit multiplexed PPP payloads as
     described in section 4.  This technique is referred to as "VC-
     multiplexed PPP".

     2.  MAY use LLC encapsulated PPP payloads on PVCs as described in
     section 5 below by mutual configuration or negotiation of both end
     points.  This technique is referred to as "LLC encapsulated PPP".

     3. If an implementation is connecting though a Frame Relay/ATM
     FRF.8 [7] service inter-working unit to an RFC 1973 [6] end point,
     then it MUST support LLC encapsulated PPP payloads.

     4. For SVC set up, an implementation MUST negotiate using the
     Q.2931 [9] Annex C procedure, encoding the Broadband Lower Layer
     Interface (B-LLI) information element to signal either VC-
     multiplexed PPP or LLC encapsulated PPP.  The details of this
     control plane procedure are described in section 6.

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4. Virtual Circuit Multiplexed PPP Over AAL5

The AAL5 PDU format is shown in figure 1:

                     AAL5 CPCS-PDU Format
               |             .                 |
               |             .                 |
               |        CPCS-PDU Payload       |
               |     up to 2^16 - 1 octets)    |
               |             .                 |
               |             .                 |
               |      PAD ( 0 - 47 octets)     |
               +-------------------------------+ -------
               |       CPCS-UU (1 octet )      |
               |         CPI (1 octet )        |
               +-------------------------------+CPCS-PDU Trailer
               |        Length (2 octets)      |
               |         CRC (4 octets)        |
               +-------------------------------+ -------
                                Figure 1

The Common Part Convergence Sub-layer (CPCS)-PDU Payload field contains
user information up to 2^16 - 1 octets.

The PAD field pads the CPCS-PDU to fit exactly into the ATM cells such
that the last 48 octet cell payload created by the SAR sublayer will
have the CPCS-PDU Trailer right justified in the cell.

The CPCS-UU (User-to-User indication) field is used to transparently
transfer CPCS user to user information.  The field has no function under
the multi-protocol ATM encapsulation described in this memo and can be
set to any value.

The CPI (Common Part Indicator) field aligns the CPCS-PDU trailer to 64
bits.  Possible additional functions are for further study in ITU-T.
When only the 64 bit alignment function is used, this field shall be
coded as 0x00.

The Length field indicates the length, in octets, of the Payload field.
The maximum value for the Length field is 65535 octets.  A Length field
coded as 0x00 is used for the abort function.

The CRC field protects the entire CPCS-PDU except the CRC field itself.

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A VC-multiplexed PPP frame SHALL constitute the CPCS-PDU payload and is
defined as:

                  | Protocol | Information | Padding |
                  | 8/16 bits|             |         |
                                Figure 2

Each of these fields are specifically defined in [1].

5. LLC Encapsulated PPP Over AAL5

LLC encapsulated PPP over AAL5 is the alternative technique to VC-
multiplexed PPP over AAL5.  LLC encapsulated PPP minimizes the ATM/Frame
Relay inter-working translation complexity that occurs when a VCC is
connected to an RFC 1973 compliant end point.

The AAL5 CPCS-PDU payload  field is encoded as shown in figure 3:

          +-------------------------+ --------
          |  Source SAP (0xFE)      |     ^
          +-------------------------+     |
          |  Destination SAP (0xFE) | LLC header
          +-------------------------+     |
          |  Frame Type = UI (0x03) |     V
          +-------------------------+ --------
          |  NLPID = PPP (0xCF)     |
          +-------------------------+ --------
          |  Protocol Identifier    |     ^
          |     (8 or 16 bits)      |     |
          +-------------------------+ PPP payload
          |          .              |     |
          |          .              |     |
          |  PPP information field  |     |
          |          .              |     |
          |          .              |     V
          +-------------------------+ --------

                                Figure 3

The fields in the above diagram are:

     1. LLC header: 2 bytes encoded to specify a source SAP and
     destination SAP of non-OSI routed PDU (values 0xFE 0xFE), followed
     by an Un-numbered Information (UI) frame type (value 0x03).

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     2. Network Layer Protocol IDentifier (NLPID) representing PPP,
     (value 0xCF).

     3. the PPP protocol identifier field, which can be either 1 or 2
     octets long.

     4. followed by the PPP information field.

6. Out-Of-Band Control Plane Signaling

When originating a switched virtual circuit AAL5 connection, the caller
MUST request in the SETUP message either one or else both of the RFC1483
protocol encapsulation techniques for PPP payload transport.  When a
caller is offering both techniques, the two BLLI IEs are encoded within
a Broadband Repeat Indicator IE in the order of their preferance.  The
called implementation MUST be able to accept an incoming call that
offers VC-multiplexed PPP in the caller's request.  The called
implementation MAY reject a call set up request that only offers LLC
encapsulated PPP.  Implementations originating a call offering both
protocol encapsulation techniques MUST be able to negotiate to the fall
back position of VC-multiplexed PPP and still inter-operate.

When originating a virtual circuit multiplexed call that is to carry a
PPP payload, the ITU Q.2931 [9] B-LLI element user information layer 3
protocol field is encoded to select ISO/IEC TR 9577 [5] in octet 7.  The
extension octets specify an IPI value of PPP (0xCF).  By definition, the
first bytes of the AAL5 frame's payload field will always contain a PPP
header followed by a packet.

When originating an LLC encapsulated call that is to carry a PPP
payload, the ITU Q.2931 B-LLI element user information layer 2 protocol
field is encoded to select LAN Logical Link Control (ISO/IEC8802-2) in
octet 6.  See RFC 1755 [8] appendix A for an example.  By definition,
the first bytes of the AAL5 frame's payload field will contain an LLC
header, followed by a NLPID and the PPP payload.

7. PPP Link Control Protocol Phase Transitions

Initial LCP packets contain the sequence cf-c0-21.  In the case of VC-
multiplexed PPP, this sequence constitute the first three octets of an
AAL5 frame.   When a LCP Configure-Request packet is received and
recognized, the PPP link enters Link Establishment phase.

Configuration requests received over multi-point connections SHOULD
result in (a) misconfiguration indication(s).  This can be detected by
multiple responses to the LCP Configure-Request with the same
Identifier, coming from different framing addresses.  Some

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implementations might be physically unable to either log or report such

Once PPP has entered the Network-layer Protocol phase, and successfully
negotiated a particular NCP for a PPP Protocol, if a frame arrives using
an alternate but equivalent data encapsulation defined in [4], the PPP
Link MUST re-enter Link Establishment phase and send a new LCP
Configure-Request.  This prevents "black-holes" that occur when the peer
loses state.

An implementation which requires PPP link configuration, and other PPP
negotiated features (such as authentication), MAY enter Termination
phase when configuration fails.

8. Configuration Options

The following Configuration Options are recommended:

         Magic Number

         Protocol Field Compression

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 Telephone Switched Network (PTSN).  The same PAP/CHAP
authentication protocols that are already widely in use for Internet
dial up access are leveraged.  As a consequence, PPP over AAL5 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


[1]   Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD

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      51, RFC 1661, July 1994.

[2]   The ATM Forum, "Frame based User-to-Network Interface (FUNI)
      Specification v2", af-saa-0088.000, May 1997.

[3]   Simpson, W., Editor, "PPP in HDLC-like Framing", STD 51,
      RFC 1662, July 1994.

[4]   Hienanan, J., "Multiprotocol Interconnect over AAL5",
      RFC 1483, July 1993.

[5]   ISO/IEC DTR 9577.2, "Information technology -
      Telecommunications and Information exchange between systems -
      Protocol Identification in the network layer", 1995-08-16.

[6]   Simpson, W., "PPP in Frame Relay", RFC 1973, June 1996.

[7]   The Frame Relay Forum, "Frame Relay/ATM PVC Service Inter-working
      Implementation Agreement", FRF.8, April 1995.

[8]   M. Perez, F. Liaw, A. Mankin, E. Hoffman, D. Grossman, A. Malis,
      "ATM Signaling Support for IP over ATM", RFC 1755, February 1995.

[9]   International Telecommunication Union, "Broadband Integrated Service
      Digital Network (B-ISDN) Digital Subscriber Signaling System No.2
      (DSS2) User Network Interface Layer 3 Specification for Basic
      Call/Connection Control", ITU-T Recommendation Q.2931, (International
      Telecommunication Union: Geneva, 2/95)

10. Acknowledgments

This design is based on work performed in ADSL Forum's Packet Mode
Working Group.  It is inspired by  "PPP in Frame Relay", RFC 1973, by
William Simpson.

Chair's Address The working group can be contacted via the current
           Karl Fox
           Ascend Communications
           3518 Riverside Drive, Suite 101
           Columbus, Ohio 43221

           EMail: karl@ascend.com

Author's Address

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Internet Draft             PPP Over ATM AAL5              July 25th 1997

Questions about this memo can also be directed to:

     Manu Kaycee
     Paradyne Corporation
     100 Shultz Drive
     Red Bank, NJ 07701
     Tel:   +1.732.345.7664
     Email: mjk@nj.paradyne.com

     George Gross
     Lucent Technologies, Inc
     184 Liberty Corner Road
     Warren, NJ 07059
     Tel:   +1.908.580.4589
     Email: gmg@garage.lucent.com

     Arthur Lin
     Cisco Systems, Inc.
     170 West Tasman Drive
     San Jose, CA 95134
     Tel:   +1.408.526.8260
     Email: alin@cisco.com

     Andrew Malis
     Ascend Communications, Inc.
     5 Carlisle Road
     Westford, MA 01886
     Tel:  +1.508.952.7414
     Email: malis@casc.com

     John Stephens
     Cayman Systems, Inc.
     100 Maple Street
     Stoneham, MA 02180
     Tel:   +1.617.279.1101
     Email: john@cayman.com

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