rfc2364.txt

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RFC 2364                     PPP Over AAL5                     July 1998


   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
   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 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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RFC 2364                     PPP Over AAL5                     July 1998


        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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RFC 2364                     PPP Over AAL5                     July 1998


   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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RFC 2364                     PPP Over AAL5                     July 1998


   [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

   EMail: [e-mail]

Authors' Addresses

   Questions about this memo can also be directed to:

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

   Phone:   [phone]
   EMail: [e-mail]


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

   Phone: [phone]
   EMail: [e-mail]







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RFC 2364                     PPP Over AAL5                     July 1998


   Arthur Lin
   Shasta Networks Inc.
   249 Humboldt Court
   Sunnyvale, CA 94089-1300

   Phone: [phone]
   EMail: [e-mail]


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

   Phone: [phone]
   EMail: [e-mail]


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

   Phone: [phone]
   EMail: [e-mail]


























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RFC 2364                     PPP Over AAL5                     July 1998


Full Copyright Statement

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