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Versions: 01 02 03 04 05 rfc2364                                        
PPP Extensions Working Group           George Gross, Lucent Technologies
INTERNET DRAFT                                     Manu Kaycee, Paradyne
Expires May 18, 1998                       Arthur Lin, Benchmark Capital
                                     Andrew Malis, Ascend Communications
                                           John Stephens, Cayman Systems
                                                       November 18, 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
its working groups.  Note that other groups may also distribute working
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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.

2. Specification of Requirements

In this document, several words are used to signify the requirements of
the specification.  These words are often capitalized.

     MUST - This word, or the adjective "required", means that the
     definition is an absolute requirement of the specification.

     MUST NOT - This phrase means that the definition is an absolute
     prohibition of the specification.

     SHOULD - This word, or the adjective "recommended", means that
     there may exist valid reasons in particular circumstances to ignore
     this item, but the full implications MUST be understood and
     carefully weighed before choosing a different course.

     MAY - This word, or the adjective "optional", means that this item
     is one of an allowed set of alternatives.  An implementation which
     does not include this option MUST be prepared to interoperate with
     another implementation which does include the option.

3. 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

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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.

4. Multi-Protocol Encapsulation

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

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 5.  This technique is referred to as "VC-
     multiplexed PPP".

     2.  MAY use LLC encapsulated PPP payloads on PVCs as described in
     section 6 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.

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     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 7.

5. 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.

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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.

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

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

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

6. 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:

          +-------------------------+ --------
          |  Destination SAP (0xFE) |     ^
          +-------------------------+     |
          |  Source 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

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The fields in the above diagram are:

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

     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.

The end points MAY be bi-laterally provisioned to send other LLC-
encapsulated protocols besides PPP across the same virtual connection.
However, they MUST NOT send packets belonging to any protocol that has
an active NCP within the PPP session.  Implementations SHOULD do packet
scheduling that minimizes the performance impact on the quality of
service commitments associated with both the LLC-encapsulated PPP and
non-PPP protocol flows.

7. Out-Of-Band Control Plane Signaling

When originating a switched virtual circuit AAL5 connection, the caller
MUST request in the SETUP message either VC-multiplexed PPP, LLC-
encapsulated PPP, or else both VC-multiplexed and LLC-encapsulated PPP.
Note that to guarantee inter-operability, it is suggested that VC-
multiplexed PPP always be requested.  When a caller is offering both
techniques, the two B-LLI 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 the use of VC-multiplexed PPP.

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,

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the first bytes of the AAL5 frame's payload field will contain an LLC
header, followed by a NLPID and the PPP payload.

8. Detection And Recovery From Unsolicited PPP Encapsulation Transitions

When the virtual connection loses state, the PPP encapsulation technique
may uni-laterally and unexpectedly change across such transitions.
Detection and recovery procedures are defined for the following state

     VC-multiplexed PPP changing to LLC encapsulated PPP

     LLC encapsulated PPP changing to VC-multiplexed PPP

When LLC-encapsulated PPP is being used, the inital 6 octets of the LCP
packets contain the sequence: fe-fe-03-cf-c0-21.  This sequence
constitutes the first 6 octets of the AAL5 frame.  In the case of VC-
multiplexed PPP, initial LCP packets contain the sequence c0-21.  This
sequence constitutes the first 2 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
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.

9. LCP Configuration Options

The Magic Number LCP configuration option is recommended, and the
Protocol Field Compression (PFC) option is not recommended.  An
implementation MUST NOT request any of the following options, and MUST
reject a request for such an option:

     Field Check Sequence (FCS) Alternatives,

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     Address-and-Control-Field-Compression (ACFC),

     Asynchronous-Control-Character-Map (ACCM)

The Maximum-Receive-Unit (MRU) option MUST NOT be negotiated to a larger
size than the maximum CPCS-SDU size specified in the associated
direction for the virtual connection's traffic contract.

When viewed end to end, a PPP session may be bridged over multiple
physical layer segments.  For a PPP session having AAL5 segments, for
each such AAL5 physical segment the LCP options MUST be negotiated by
the peer bridging convertors independently of the LCP framing options in
use by that session's other physical layer segments.

10. 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

When using LLC-encapsulated PPP over a virtual connection, an end point
can not assume that the PPP session authentication and related security
mechanisms also secure the other LLC encapsulated flows on that same
virtual connection.


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

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

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[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)

11. 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.  Special thanks to Phil Rakity of Flowpoint, Tim Kwok
of Microsoft, and David Allan of Nortel for their constructive review
and commentary.

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

Questions about this memo can also be directed to:

     George Gross

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

     Manu Kaycee
     Paradyne Corporation
     21 Bear Meadow Road
     Londonderry, NH 03053-2168
     Tel: +1.603.434.6088
     Email: mjk@nj.paradyne.com

     Arthur Lin
     Benchmark Capital
     2480 Sand Hill Road
     Suite 200
     Menlo Park, CA 94025
     Tel:   +1.650.854.8180
     Email: artlin@pacbell.net

     Andrew Malis
     Ascend Communications, Inc.
     1 Robbins Road
     Westford, MA 01886
     Tel:  +1.978.952.7414
     Email: malis@ascend.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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