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Versions: (draft-ieft-l2tpext-tdm) 03 04 05 06 07 RFC 5611

                           TDM over L2TPv3                 April 2009


   Network Working Group                                  A. Vainshtein
   Internet Draft                                           ECI Telecom
   Document: draft-ietf-l2tpext-tdm-07.txt                   S. Galtzur
                                                              Rebellion

   Creation Date:                                        April 14, 2009
   Intended Status:                                   Proposed Standard
   Expires:                                                October 2009


      Layer Two Tunneling Protocol version 3 - Setup of Time-Division
                       Multiplexing (TDM) Pseudowires

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
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   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Abstract

   This document defines extensions to the Layer Two Tunneling Protocol
   version 3 (L2TPv3) for support of structure-agnostic and structure-
   aware (CESoPSN style) Time-Division Multiplexing (TDM) pseudowires.
   Support of structure-aware (TDMoIP style) pseudowires over L2TPv3 is
   left for further study.


Legal

   This documents and the information contained therein are provided on
   an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
   REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
   IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
   WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
   WARRANTY THAT THE USE OF THE INFORMATION THEREIN WILL NOT INFRINGE
   ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
   FOR A PARTICULAR PURPOSE.

Conventions used in this document

   In this document we refer to control plane as the packets that
   contain control information (via Attribute-Value pairs (AVP)) and the
   mechanism that handles these packets.
   In this document we refer to the data plane as the packets that
   contain transported user data.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119 [RFC2119].

Table of Contents

   1. Introduction...................................................3
   2. L2TPv3 Extension...............................................3


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      2.1 TDM PW Attribute-Value Pair (AVP)(ICRQ, OCRQ)..............4
      2.2 RTP Attribute-Value Pair AVP  (ICRQ, OCRQ, ICRP, OCRP).....6
      2.3 Changes in the Control Connection AVPs.....................7
      2.4 Changes in the Session Connection AVPs.....................7
   3. Creation of the TDM Pseudowire Session.........................7
   4. IANA Considerations............................................8
   5. Congestion Control.............................................9
   6. Security Considerations........................................9
   7. Acknowledgements...............................................9
   Normative references..............................................9
   Informative references...........................................10
   Authors' Addresses...............................................10


1. Introduction

   This document defines extensions to the Layer Two Tunneling Protocol
   Version 3(L2TPv3) for support of structure-agnostic [RFC4553] and
   structure-aware (CESoPSN style, see [RFC5086]) Time-Division
   Multiplexing (TDM) pseudowires. Structure-agnostic encapsulation of
   TDM bit-streams over L2TPv3 is described in [RFC4553], Figure 2b, and
   Circuit Emulation Service  over packet-Switched Networks (CESoPSN)
   structure-aware encapsulation - in [RFC5086], Figures 1c (TDM data
   packets) and 4a (CE application signaling packets). However, the
   order of the CESoPSN Control Word (CW) and RTP header (if it is used)
   MUST match between the TDM data and CE signaling packets.

   Setup of structure-aware TDM pseudowires using encapsulations
   described in [RFC5087] has been left for further study.

   Setup and maintenance of TDM PWs in MPLS networks using LDP is
   described in [RFC5287].


2. L2TPv3 Extension

   The L2TPv3 Control Connection is responsible for 3 main operations:
   1. Establishment and validation of a pseudowire (PW) session.
   2. Ending (tearing down) of a pseudowire session.
   3. Transferring of End Point status.

   Tearing down of session for a TDM pseudowire performed as described
   in [RFC3931].

   [RFC5086] and [RFC4553] describe how to transfer the Attachment
   Circuit (AC) status via the data plane. Therefore the Set-Link-Info
   (SLI) message described in [RFC3931] SHOULD NOT be used for conveying
   this status for the PWs in question.



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   [RFC3931] specifies that the Circuit Status Attribute-Value Pair
   (AVP) MUST be present in the ICRQ/ICRP messages. It also specifies
   that the N bit in this AVP should be set during the PW setup even if
   the specific AC does not provide any way to convey the "new AC"
   indication. Accordingly, the Circuit Status AVP for the PWs in
   question, when used in the ICRQ/ICRP messages, MUST always have both
   N and A bits set.

   The next sections describe the extensions to L2TPv3 for establishment
   and validation of TDM pseudowire sessions.

   There are two new AVPs for the Session Management messages. One AVP
   describes the TDM pseudowire attributes. The second AVP describes the
   RTP attributes for this TDM pseudowire.

2.1 TDM PW Attribute-Value Pair (AVP)(ICRQ, OCRQ)

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |M|H| rsvd  |      Length       |           Vendor Id (IETF)    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Attribute Type (AVP-TBA-1)   |         Reserved      |SP |CAS|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Bit Rate              |        Payload Bytes          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This AVP MAY be hidden (the H bit MAY be 0 or 1).  The M bit for this
   AVP SHOULD be set to 0.  The Length (before hiding) of this AVP is
   12.

   The Bit Rate field contains the value that represents the bit rate of
   the local AC in the units of 64 Kbit/s encoded as an unsigned 16-bit
   integer. Its usage for all types of TDM PWs employs the following
   semantics:
  1) Only the following values MUST be specified for structure-
     agnostic emulation (see [RFC4553]):
     a) Structure-agnostic E1 emulation  - 32
     b) Structure-agnostic T1 emulation:
         i) MUST be set to 24 for the basic mode
         ii) MUST be set to 25 for the "Octet-aligned T1"
            mode
     c) Structure-agnostic E3 emulation  - 535
     d) Structure-agnostic T3 emulation  - 699
  2) For CESoPSN PWs this parameter MUST be set to the number of
     DS0 channels in the corresponding attachment circuit.

   Note: For structure-agnostic T1 emulation, the values 24 and 25 do
   not reflect the exact bit rate, and are used for convenience only.


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   Note: The semantics of the Bit Rate field defined above are
   consistent with those of the Bit Rate Interface Attribute as defined
   in [RFC5287].

   The Payload Bytes field contains the value representing the number of
   the TDM Payload bytes in the PW packet and is used with the following
   semantics:

  1) For structure-agnostic emulation any value of the payload
     bytes can be specified.
  2) For CESoPSN PWs:
     a) The specified value MUST be an integer multiple of the
         number of DS0 channels in the corresponding attachment
         circuit.
     b) In addition to that, for trunk-specific NxDS0 with CAS,
         the number of the trunk frames per multiframe fragment
         (value resulting from the Payload Bytes divided by the
         number of DS0 channels) MUST be an integer divisor of
         the number of frames per corresponding trunk
         multiframe.

   The Reserved bits MUST be set to 0 on transmission and MUST be
   ignored on reception.

   The SP bits define support for the CESoPSN application signaling
   packets (see [RFC5086]) and MUST be used as following:
  1) Set to '01' for the CESoPSN PWs carrying TDM data packets and
     expecting CE application signaling packets in a separate PW
  2) Set to '10' for a PW carrying CE application signaling packets
     with the data packets in a separate PW
  3) Set to '11' for e CESoPSN PW carrying both TDM data and
     signaling packets
  4) Set to '00' for SAToP PWs and for CESoPSN PWs not using
     separate signaling packets.

   The CAS bits define the trunk type for trunk-specific CESoPSN
   services with CAS. These bits:
  1) For trunk-specific CESoPSN with CAS these bits MUST be set to:
     a) '01' in the case of an E1 trunk
     b) '10' in the case of a T1/ESF trunk
     c) '11' in the case of a T1/SF trunk.
  2) MUST be set to '00' for all the other TDM pseudowire types.



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2.2 RTP Attribute-Value Pair AVP  (ICRQ, OCRQ, ICRP, OCRP)

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |M|H| rsvd  |      Length       |           Vendor Id (IETF)    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Attribute Type (AVP-TBA-2)  |D|     PT      |C|  Reserved   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Reserved              |   Timestamp Clock  Frequency  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              SSRC                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Presence of this AVP indicates that the RTP header is used in the TDM
   pseudowire encapsulation. Use or non-use of the RTP header MUST match
   for the two directions of a TDM PW. This AVP MAY be hidden (the H bit
   MAY be 0 or 1).  The M bit for this AVP SHOULD be set to 0.  The
   Length (before hiding) of this AVP is 16.

   The D bit indicates the timestamping mode (absolute or differential)
   in the RTP header. These modes are described in, e.g., in [RFC4553],
   Section 4.3.2. If the D bit is set to 1 then the Differential
   timestamping mode is used, otherwise the Absolute timestamping mode
   is used.  Timestamping modes can be used independently for the two
   directions of a TDM PW.

   The C bit indicates the ordering of the RTP header and the control
   word as following:

    o If the C bit is set to 1 the RTP header appears after the
     control word in the data channel of the TDM pseudowire. This
     mode is described as SAToP/CESoPSN encapsulation over
     IPv4/IPv6 PSN with L2TPv3 demultiplexing in [RFC4553] and
     [RFC5086] respectively.
    o If the C bit is set to 0 the RTP header appears before the
     control word.  This mode described as the old mode of the
     SAToP/CESoPSN encapsulation over L2TPv3 in [RFC4553], Appendix
     A, and [RFC5086], Annex C, respectively.

   PT is the payload type expected in the RTP header.  A value of zero
   indicates that the receiver shall not check payload type to detect
   malformed packets.

   Timestamp Clock Frequency is the clock frequency used for the time
   stamping in units of 8 KHz.

   SSRC indicates the expected value of SSRC ID in the RTP header.  A
   zero in this field means that SSRC ID will not be used for detecting


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   misconnections. Since L2TP provides an alternative security mechanism
   using cookies, if the cookie length is larger than zero the SSRC
   SHOULD be zero.

2.3 Changes in the Control Connection AVPs

   Control Connections that support TDM PWs MUST add the appropriate PW
   Type value(s) to the list in the Pseudowire Capabilities List AVP.
   The valid values are listed in the next section.

2.4 Changes in the Session Connection AVPs

   PW Type AVP should be set to one of the following values:
   1. Structure-agnostic emulation [RFC4553] of:
      a. E1 circuits - TBA-SAToP-E1 by IANA
      b. T1 circuits - TBA-SAToP-T1 by IANA
      c. E3 circuits - TBA-SAToP-E3 by IANA
      d. T3 circuits - TBA-SAToP-T3 by IANA
   2. Structure-aware emulation [RFC5086] of:
      a. CESoPSN basic mode - TBA-CESoPSN-Basic by IANA
      b. Trunk-specific CESoPSN service with CAS - TBA-CESoPSN-
         CAS by IANA

   TDM pseudowires use their own control word.  Therefore the L2-
   Specific Sublayer AVP MUST either be omitted or set to zero.

   TDM pseudowires use their own sequencing.  Therefore the Data
   Sequencing AVP MUST either be omitted or set to zero.

   Note: The Control Word (CW) used in the SAToP and CESoPSN
   encapsulations over L2TPv3 effectively represents a dedicated L2-
   Specific Sub-layer.

3. Creation of the TDM Pseudowire Session

   When LCCE wants to open a Session for TDM PW it MUST include the TDM
   PW AVP (in any case) and the RTP AVP (if and only if the RTP header
   is used) in the ICRQ or OCRQ message.  The LCCE peer must validate
   the TDM PW AVP and make sure it can meet the requirements derived
   from the RTP AVP (if it exists).  If the peer agrees with the TDM AVP
   it will send an appropriate ICRP or OCRP message with the matching
   RTP AVP (if needed). The Initiator need to validate that it can
   supply the requirements derived from the received RTP AVP.


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   The two peers MUST agree on the values in the TDM PW AVP:

   1. Bit Rate values MUST be equal on both sides. If they are
      different, the connection will be rejected with return code
      RC-TBD-1 and error code EC-TBD-1.
   2. In the case of trunk-specific CESoPSN with CAS, the trunk
      type (as encoded in the CAS bits of the TDM AVP) MUST be the
      same for the two sides. Otherwise the connection will be
      rejected with return code RC-TBD-1 and error code EC-TBD-2.
   3. If one side does not support the payload bytes value proposed
      by the other one, the connection will be rejected with return
      code RC-TBD-1 and error code EC-TBD-3.
   4. If one side cannot send RTP header as requested by the other
      side, the connection will be rejected with return code RC-
      TBD-1 and error code EC-TBD-4.
   5. If one side can send RTP header but not with the requested
      timestamp clock frequency, the connection will be rejected
      with return code RC-TBD-1 and error code EC-TBD-5.

  If CE signaling for a CESoPSN basic PW is transported in a separate PW
  instance, then the two PW instances:

   1. MUST use the same PW type
   2. MUST use the same values in all the fields of the TDM AVP
      excluding the SP field which must be set to '01' for the TDM
      data PW and to '10' for the PW carrying CE application
      signaling
   3. MUST both use or not use RTP header (and accordingly,
      include or not include the RTP AVP).


4. IANA Considerations

   This draft requires assignment of the following values by IANA:

   New L2TPv3 Pseudowire Types:

       0x0011 (TBA-SAToP-E1)      - Structure-agnostic E1 circuit
       0x0012 (TBA-SAToP-T1)      - Structure-agnostic T1 (DS1) circuit
       0x0013 (TBA-SAToP-E3)      - Structure-agnostic E3 circuit
       0x0014 (TBA-SAToP-T3)      - Structure-agnostic T3 (DS3) circuit
       0x0015 (TBA-CESoPSN-Basic) - CESoPSN basic mode
       0x0017 (TBA-CESoPSN-CAS)   - CESoPSN TDM with CAS

   Note that the values listed are suggested to match with the values
   defined in [RFC4446] for the MPLS Pseudowire Types.

   New attribute value pair IDs:


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   1. AVP-TBD-1 - TDM Pseudowire AVP
   2. AVP-TBD-2 - RTP AVP

   New return codes for the CDN message:

   1. RC-TBD-1 - return code to indicate connection refused
      because of TDM PW parameters. The error code indicates the
      problem.

   TDM PW Specific error codes, to be used with the RC-TDB-1 return code
   For the CDN message:

   This is a new registry for IANA to maintain within the Result Code
   AVP (Attribute Type 1) Values. Additional values may be assigned by
   Expert Review [RFC5226].

   0. 0 - Reserved
   1. EC-TBD-1 - Bit Rate values disagree.
   2. EC-TBD-2 - Different trunk types in the case of trunk-
     specific CESoPSN with CAS
   3. EC-TBD-3 - Requested payload size too big or too small.
   4. EC-TBD-4 - RTP header cannot be generated.
   5. EC-TBD-5 - Requested timestamp clock frequency cannot be
     generated

5. Congestion Control

   The congestion considerations from [RFC4553] and [RFC5086] apply
   respectively to the structure-agnostic and CESoPSN modes of this
   specification.

6. Security Considerations

   This document specifies only the L2TPv3-based control plane for setup
   of TDM PWs. Within this scope, there are no additional security
   considerations on top of those discussed in [RFC3931].

   Common data plane security considerations for the TDM PWs have been
   discussed in some detail in both [RFC4553] and [RFC5086]. On top of
   these, the L2TPv3-based data plane provides additional security
   mechanisms based on usage of cookies.

7. Acknowledgements
   The authors want to thank Carlos Pignataro, Ignacio Goyret and Yaakov
   Stein for careful review and useful suggestions.

Normative references



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   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997

   [RFC3931]      J. Lau, M. Townsley, I. Goyret, Layer Two Tunneling
   Protocol - Version 3 (L2TPv3), March 2005

   [RFC5086]   A. Vainshtein et al, Structure-aware TDM Circuit
               Emulation Service over Packet Switched Network
               (CESoPSN), RFC 5086, December 2007

   [RFC4553]   A. Vainshtein, Y. Stein, Structure-Agnostic TDM over
               Packet (SAToP), RFC 4553, June 2006

Informative references

   [RFC5087]   Y. Stein et al, TDM over IP, RFC 5087, December 2007.

   [RFC4446]   L. Martini, M. Townsley, IANA Allocations for Pseudo
               Wire Edge to Edge Emulation (PWE3), RFC 4446,
               April 2006

   [RFC5287]   A. Vainshtein, Y. Stein, Control Protocol Extensions
               for Setup of TDM Pseudowires in MPLS Networks, RFC 5287,
               August 2008

   [RFC5226]   T. Narten, H. Alvestrand, Guidelines for Writing an IANA
               Considerations Section in RFCs, RFC 5226, May 2008


Authors' Addresses

   Sharon Galtzur
   Rebellion Inc.
   29 The Chilterns, Gloucester Green,
   Oxford, OX1 2DF, UK
   Email: sharon.galtzur@rebellion.co.uk

   Alexander Vainshtein,
   ECI Telecom,
   30 ha-Sivim St. PO Box 500,
   Petah-Tiqva 49517, Israel
   Email: Alexander.Vainshtein@ecitele.com



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