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RFC 2364


Pages: 12
Proposed Standard

ToP   noToC   RFC2364 - Page 1
Network Working Group                                           G. Gross
Request for Comments: 2364                           Lucent Technologies
Category: Standards Track                                      M. Kaycee
                                                                  A. Lin
                                                         Shasta Networks
                                                                A. Malis
                                                   Ascend Communications
                                                             J. Stephens
                                                          Cayman Systems
                                                               July 1998

                             PPP Over AAL5

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 Notice

   Copyright (C) The Internet Society (1998).  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 5 (AAL5) 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
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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 correction.

   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. Conventions

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [10].

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 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 or the underlying ATM
        layer traffic descriptor parameters.

        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"

        "Down" events to the LCP state machine [1] within the PPP layer.
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4. Multi-Protocol Encapsulation

   This specification uses the principles, terminology, and frame
   structure described in "Multiprotocol Encapsulation over ATM
   Adaptation Layer 5" [4].

   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 below by mutual configuration or
        negotiation of both end points.  This technique is referred to
        as "VC-multiplexed PPP".

        2. MUST support 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. 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.

   If an implementation is connecting through a Frame Relay/ATM FRF.8
   [7] service inter-working unit to an RFC 1973 [6] end point, then it
   MUST use LLC encapsulated PPP payloads.  Frame Relay/ATM FRF.8
   inter-working units are exempted from the requirement to support VC-
   multiplexed PPP.  This exemption allows the FR/ATM IWU to remain
   compliant with FRF.8 when the PPP over AAL5 end point is inter-
   operating with an RFC 1973 end point.
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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)        |    V
                  +-------------------------------+ -------
                                   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
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   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.

   The AAL5 CPCS-PDU payload  field is encoded as shown in figure 3.
   The pertinent fields in that 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.  See reference [1].

        4. followed by the PPP information field as per Figure 2.
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                  +-------------------------+ --------
                  |  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  |     |
                  |          .              |     |
                  |          .              |     |
                  +-------------------------+     |
                  |        padding          |     V
                  +-------------------------+ --------
                  |  PAD ( 0 - 47 octets)   |
                  +-------------------------+ --------
                  |  CPCS-UU (1 octet )     |     ^
                  +-------------------------+     |
                  |    CPI (1 octet )       |     |
                  +-------------------------+CPCS-PDU Trailer
                  |   Length (2 octets)     |     |
                  +-------------------------|     |
                  |    CRC (4 octets)       |     V
                  +-------------------------+ --------

                             Figure 3

   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.  When a caller is offering both techniques, the two
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   B-LLI IEs are encoded within a Broadband Repeat Indicator IE in the
   order of their preference.  The called implementation MUST be able to
   accept an incoming call that offers LLC-encapsulated PPP in the
   caller's request.  The called implementation MUST reject a call set
   up request that only offers an encapsulation that it does not
   support.  Implementations originating a call offering both protocol
   encapsulation techniques MUST be able to negotiate the use of LLC-
   encapsulated 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, the first bytes of the AAL5 frame's payload
   field will contain an LLC header, followed by a NLPID and the PPP

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 transitions:

        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.

   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 as
   defined in [4], then the PPP Link MUST:
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        For a SVC, immediately clear the call with the cause value 111,
        "protocol error, unspecified".

        For a PVC: tear down the active NCPs, SHOULD generate an error
        message, enter the Termination state, and silently drop all
        received packets.

   These policies prevent "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 state 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,

        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 peer to peer, a PPP link may be bridged over multiple
   physical layer sections.  For each such AAL5 section, the LCP framing
   options MUST be actively negotiated by the bridging convertors
   independently of the LCP framing options in use by other physical
   layer sections.

   Implementation Note:
        When an ATM AAL5 PVC is in the "Stopped" state, it is
        RECOMMENDED that the implementation wait for Configure-Requests.
        See the implementation option in reference [1] section 4.2, the
        "Stopped State" sub-section.

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.
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   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 services.

   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.

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.

12. References

   [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.

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

   [4]   Heinanen, 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.
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   [8]   Perez, M., Liaw, F., Mankin, A., Hoffman, E., Grossman, D., and
         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]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

Chair's Address

   The working group can be contacted via the current chair:

   Karl Fox
   Ascend Communications
   3518 Riverside Drive, Suite 101
   Columbus, Ohio 43221


Authors' Addresses

   Questions about this memo can also be directed to:

   George Gross
   Lucent Technologies, Inc
   184 Liberty Corner Road
   Warren, NJ 07059

   Phone:   +1.908.580.4589

   Manu Kaycee
   Paradyne Corporation
   21 Bear Meadow Road
   Londonderry, NH 03053-2168

   Phone: +1.603.434.6088
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   Arthur Lin
   Shasta Networks Inc.
   249 Humboldt Court
   Sunnyvale, CA 94089-1300

   Phone: +1.408.747.5051

   Andrew Malis
   Ascend Communications, Inc.
   1 Robbins Road
   Westford, MA 01886

   Phone: +1.978.952.7414

   John Stephens
   Cayman Systems, Inc.
   100 Maple Street
   Stoneham, MA 02180

   Phone: +1.617.279.1101
ToP   noToC   RFC2364 - Page 12
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