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Network Working Group B. Thompson Request for Comments: 3336 T. Koren Category: Standards Track Cisco Systems

                                                             B. Buffam
                                                       Seaway Networks
                                                         December 2002
   PPP Over Asynchronous Transfer Mode Adaptation Layer 2 (AAL2)

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 (2002).  All Rights Reserved.


 The Point-to-Point Protocol (PPP) 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

Thompson, et. al. Standards Track [Page 1] RFC 3336 PPP Over AAL2 December 2002

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 on 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 layer.
    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.

Thompson, et. al. Standards Track [Page 2] RFC 3336 PPP Over AAL2 December 2002

4. PPP Operation with AAL2

 PPP over AAL2 defines an encapsulation that uses the Service Specific
 Segmentation and Reassembly Sublayer (SSSAR) [5] for AAL type 2.  The
 SSSAR sub-layer is used to segment PPP packets into frames that can
 be transported using the AAL2 CPS.  The SSSAR sub-layer 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
 An implementation of PPP over AAL2 MAY use one or more AAL2 Channel
 Identifiers (CIDs) for transport of PPP packets associated with each
 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

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 [3] 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 [4] rather than AAL5 as the transport for
 PPP.  SSSAR along with the AAL2 CPS generates 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 AAL2
 Start Field (STF) 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.

Thompson, et. al. Standards Track [Page 3] RFC 3336 PPP Over AAL2 December 2002

 The multiplexing function of the AAL2 CPS layer allows more bandwidth
 efficient transport of PPP frames by multiplexing multiple PPP 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

 PPP Multiplexing (PPPMUX) [8] is a new method for doing multiplexing
 in the PPP layer. PPPMUX provides functionality similar 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 also applies to PPPMUX/AAL5 frames
 progressing down the stack.

Thompson, et. al. Standards Track [Page 4] RFC 3336 PPP Over AAL2 December 2002

 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 only.
 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 Sublayer for AAL2, there can be
 multiple SSCS functions defined to run over this CPS layer.
 Different CIDs within an AAL2 virtual circuit may run different

Thompson, et. al. Standards Track [Page 5] RFC 3336 PPP Over AAL2 December 2002

6.1.3 AAL2 CPS Packet (CPS-PKT) Format

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


+ + + +
+ + + +
+ + + +
(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 sub-layer 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.

Thompson, et. al. Standards Track [Page 6] RFC 3336 PPP Over AAL2 December 2002

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


Cell    |           |                                         |     |
Header  |    STF    |             CPS-PDU Payload             | PAD |
        |           |                                         |     |


            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 CPS header contains enough information to
 identify the CPS packets within a CPS-PDU. In the event of cell loss,
 the STF field is used to find the first CPS-PKT in the current cell.

Thompson, et. al. Standards Track [Page 7] RFC 3336 PPP Over AAL2 December 2002

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 sub-layer 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 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 algorithms used for computing and verifying the CRC-16 field are
 identical to the algorithms specified for the Frame Check Sequence
 (FCS) field in Q.921 [13]. The algorithms from Q.921 are included in
 this section for ease of access.
 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
 Protocol 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

Thompson, et. al. Standards Track [Page 8] RFC 3336 PPP Over AAL2 December 2002

 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 (including the Protocol ID, the
 information and the CRC fields), will be 0001110100001111 (x^15
 through x^0, respectively) in the absence of transmission errors.

6.3 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.2630.2 [7] signaling in the case of an AAL2 switched virtual
 circuit (SPVC or SVC).
 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.

Thompson, et. al. Standards Track [Page 9] RFC 3336 PPP Over AAL2 December 2002

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 sub-layer of AAL2 is used for
 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.

Thompson, et. al. Standards Track [Page 10] RFC 3336 PPP Over AAL2 December 2002

 Here are the paths an Application packet can take in this
     +---+---+---+---+--+                                        +
     |   Application A  |                                        |
     +---+---+---+---+--+                                        |
     |       RTP        |                                        |
     +---+---+---+---+--+       +---+---+---+---+---+      Application
     |       UDP        |       |   Application B   |            |
     +---+---+---+---+--+       +---+---+---+---+---+            |
     |        IP        |       |        IP         |            |
     +---+---+---+---+--+       +---+---+---+---+---+            +
             |                            |
                   +---+---+---+---+---+--+                      +
                   |  Compression Filter  |                      |
                   +---+---+---+---+---+--+                      |
                             |                                   |
                             |                                   |
                   +---------+-----------+                       |
                   |                     |                       |
           RTP     |                     |   Non-RTP             |
         Packets   V                     |   Packets             |
     +---+---+---+---+---+---+           |                       |
     |            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 may not have UDP or RTP headers.

Thompson, et. al. Standards Track [Page 11] RFC 3336 PPP Over AAL2 December 2002

 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
 The destination UDP port number and packet length are examples of
 criteria that may be used by the compression filter to select the
 In this example, packets from application A will be passed to the
 CRTP compressor which 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 algorithm.
 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.

Thompson, et. al. Standards Track [Page 12] RFC 3336 PPP Over AAL2 December 2002

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

 This memo defines mechanisms for PPP encapsulation over ATM.  There
 is an element of trust in any encapsulation protocol: a receiver
 should be able to trust that the sender has correctly identified the
 protocol being encapsulated and that the sender has not been spoofed
 or compromised.  A receiver should also be able to trust that the
 transport network between sender and receiver has not been
 A PPP session that runs over an ATM Virtual Circuit must follow the
 PPP link operation state machine described in RFC 1661 [1].  This
 state machine includes the ability to enforce the use of an
 authentication phase using the PAP/CHAP authentication protocols
 before any network layer packets are exchanged.  Using PPP level
 authentication, a PPP receiver can authenticate a PPP sender.
 System security may also be compromised by the attacks of the ATM
 transport network itself.  The ATM Forum has published a security
 framework [11] and a security specification [12] that define
 procedures to guard against common threats to an ATM transport
 PPP level authentication does not guard against man in the middle
 attacks.  These attacks could occur if an attacker was able to
 compromise the security infrastructure of an ATM switching network.
 Applications that require protection against threats to an ATM
 switching network are encouraged to use authentication headers, or
 encrypted payloads, and/or the ATM-layer security services described
 in [12].
 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.

Thompson, et. al. Standards Track [Page 13] RFC 3336 PPP Over AAL2 December 2002

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 proposal.

11. References

 [1]  Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD
      51, RFC 1661, July 1994.
 [2]  Gross, G., Kaycee, M., Li, A., Malis, A. and J. Stephens, "PPP
      over AAL5", STD 51, RFC 2364, July 1998.
 [3]  Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP Headers for
      Low-Speed Serial Links", RFC 2508, 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, "ITU-T RECOMMENDATION Q.2630.2", December 2000.
 [8]  Pazhyannur, R, Ali, I. and C. Fox, "PPP Multiplexing", RFC 3153,
      August 2001.
 [9]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
      "RTP:  A Transport Protocol for Real-Time Applications", RFC
      1889, January 1996.
 [10] Thompson, B., Koren, T. and B. Buffam, "Class Extensions for PPP
      over Asynchronous Transfer Mode Adaptation Layer 2", RFC 3337,
      December 2002.
 [11] The ATM Forum, "ATM Security Framework Version 1.0", af-sec-
      0096.000, February 1998.
 [12] The ATM Forum, "ATM Security Specification v1.1", af-sec-
      0100.002, March 2001.

Thompson, et. al. Standards Track [Page 14] RFC 3336 PPP Over AAL2 December 2002

 [13] International Telecommunications Union, ISDN User-Network
      Interface-Data Link Layer Specification, ITU-T Recommendation
      Q.921, March 1993.

12. Authors' Addresses

 Bruce Thompson
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA 95134
 Phone: +1 408 527-0446
 Tmima Koren
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA 95134
 Phone: +1 408 527-6169
 Bruce Buffam
 Seaway Networks
 One Chrysalis Way,
 Suite 300,
 Ottawa, Canada
 Phone: +1 613 723-9161

Thompson, et. al. Standards Track [Page 15] RFC 3336 PPP Over AAL2 December 2002

13. Full Copyright Statement

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 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Thompson, et. al. Standards Track [Page 16]

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