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Versions: (draft-anavi-tdmoip) 00 01 02 03 04 05 06 RFC 5087

PWE3                                                          Y(J) Stein
Internet-Draft                                               R. Shashoua
Expires: January 17, 2005                                      R. Insler
                                                                M. Anavi
                                                 RAD Data Communications
                                                           July 19, 2004

                              TDM over IP

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   or will be disclosed, and any of which I become aware will be
   disclosed, in accordance with RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 17, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   This document describes methods for structure-aware transport of TDM
   traffic over packet switched networks using pseudowires.

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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   TDMoIP Encapsulation . . . . . . . . . . . . . . . . . . . .   5
   3.   Encapsulation Details for Specific PSNs  . . . . . . . . . .   8
   3.1  UDP/IP . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   3.2  MPLS . . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   3.3  L2TPv3 . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   3.4  Ethernet . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   4.   TDMoIP Payload types . . . . . . . . . . . . . . . . . . . .  15
   4.1  AAL1 Format Payload  . . . . . . . . . . . . . . . . . . . .  16
   4.2  AAL2 Format Payload  . . . . . . . . . . . . . . . . . . . .  17
   4.3  HDLC Format Payload  . . . . . . . . . . . . . . . . . . . .  18
   5.   TDMoIP Defect Handling . . . . . . . . . . . . . . . . . . .  19
   6.   Implementation Issues  . . . . . . . . . . . . . . . . . . .  22
   6.1  Jitter and Packet Loss . . . . . . . . . . . . . . . . . . .  22
   6.2  Timing Recovery  . . . . . . . . . . . . . . . . . . . . . .  23
   6.3  Quality of Service . . . . . . . . . . . . . . . . . . . . .  24
   7.   Security Considerations  . . . . . . . . . . . . . . . . . .  24
   8.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  25
   9.   Trademarks . . . . . . . . . . . . . . . . . . . . . . . . .  25
   10.  References . . . . . . . . . . . . . . . . . . . . . . . . .  26
   10.1 Normative References . . . . . . . . . . . . . . . . . . . .  26
   10.2 Informative References . . . . . . . . . . . . . . . . . . .  27
   11.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .  28
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  29
   A.   Sequence Number Processing . . . . . . . . . . . . . . . . .  30
   B.   AAL1 Review  . . . . . . . . . . . . . . . . . . . . . . . .  32
   C.   AAL2 Review  . . . . . . . . . . . . . . . . . . . . . . . .  36
   D.   Performance Monitoring Mechanisms  . . . . . . . . . . . . .  38
   D.1  TDMoIP Connectivity Verification . . . . . . . . . . . . . .  38
   D.2  OAM Packet Format  . . . . . . . . . . . . . . . . . . . . .  39
        Full Copyright Statement . . . . . . . . . . . . . . . . . .  42

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1. Introduction

   Telephony traffic is conventionally carried over connection-oriented
   synchronous or plesiochronous links (loosely called TDM circuits
   herein).  With the proliferation of packet switched networks (PSNs),
   integration of TDM services into a unified PSN infrastructure has
   become desirable.  Such integration requires emulation of TDM
   circuits within the PSN, a function that can be carried out using
   pseudowires (PWs), as described in the PWE3 architecture [PWE-ARCH].
   This emulation must ensure QoS and voice quality similar to those of
   existing TDM networks as well as preserving signaling features, as
   described in the TDM PW requirements [TDM-REQ].

   The interworking function that connects between the TDM and PSN
   worlds will be called a TDMoIP gateway (GW), and it may be situated
   at the provider edge (PE) or at the customer edge (CE).  The TDM
   gateway that encapsulates TDM and injects packets into the PSN will
   be called the PSN-bound gateway, while the gateway that extracts TDM
   data from packets and generates traffic on a TDM network will be
   called the TDM-bound gateway.  Emulated TDM circuits are always
   point-to-point, bidirectional, and transport the same TDM rate in
   both directions.

   Although TDM circuits can be used to carry arbitrary bit-streams,
   there are standardized methods for carrying constant-length blocks of
   data called "structures".  Familiar structures are the T1 or E1
   frames [G.704] of length 193 and 256 bits, respectively.  T3 and E3
   frames [G.704,G.751] are much larger than those of T1 and E1, and
   even larger structures are used in the GSM Abis channel described in
   [TRAU].  TDM structures contain TDM data plus structure overhead; for
   example, the 193-bit T1 frame contains a single bit of structure
   overhead and 24 bytes of data, while the 32-byte E1 frame contains a
   byte of overhead and 31 data bytes.

   TDM circuits are often used to transport multiplexed 64 kbps
   channels.  A frame of a channelized T1 carries 24 byte-sized
   channels, while an E1 frame consists of 32 channels.  By
   concatenation of consecutive T1 or E1 frames we can build higher
   level structures called superframes or multiframes.

   TDM structures are universally delimited by placing an easily
   detectable periodic bit pattern, called the Frame Alignment Signal
   (FAS), in the structure overhead.  The structure overhead may
   additionally contain error monitoring and defect indications.  We
   will use the term "structured TDM" to refer to TDM with any level of
   structure imposed by an FAS.  Unstructured TDM signifies a bit stream
   upon which no structure has been imposed, implying that all bits are
   available for user data.

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   SAToP [SAToP] is a structure-agnostic protocol for transporting TDM
   over PWs.  SAToP treats the TDM input as an arbitrary bit-stream,
   completely disregarding any structure that may exist in the TDM bit-
   stream.  Hence SAToP is ideal for transport of truly unstructured
   TDM, and also suitable for transport of structured TDM when there is
   no need to protect structure integrity nor interpret or manipulate
   individual channels during transport.  In particular, SAToP is the
   technique of choice for PSNs with negligible packet loss, and for
   applications that do not require discrimination between channels nor
   intervention in TDM signaling.

   When it is required or desirable to explicitly safeguard TDM
   structure during transport over the PSN, structure-aware TDM
   transport must be employed.  Structure-aware transport exploits at
   least some level of the TDM structure to enhance robustness to packet
   loss or other PSN shortcomings.  Structure-aware TDM PWs might not
   transport structure overhead across the PSN; in particular, the FAS
   MAY be stripped by the PSN-bound GW and MUST be regenerated by the
   TDM-bound GW.  However, structure overhead MAY be transported over
   the PSN, since in addition to FAS it can contain maintenance

   There are three conceptually distinct methods of ensuring TDM
   structure integrity, namely structure-locking, structure-indication,
   and structure-reassembly.  Structure-locking requires each packet to
   commence at the start of a TDM structure, and to contain an entire
   structure or integral multiples thereof.  Structure-indication allows
   packets to contain arbitrary fragments of basic structures, but
   employs pointers to indicate where each structure commences.
   Structure-reassembly is only meaningful for channelized TDM; the PSN-
   bound GW extracts and buffers the individual channels, and the
   original structure is reassembled from the received constituents by
   the TDM-bound GW.

   All three methods of TDM structure preservation have their
   advantages.  Structure-locking is described in [CESoPSN], while the
   present document specifies both structure-indication (see Section
   4.1) and structure-reassembly (see Section 4.2) approaches.
   Structure-indication is used when channels may be allocated
   statically, and/or when it is required to interwork with existing
   circuit emulation systems (CES) based on AAL1.  Structure-reassembly
   is used when dynamic allocation of channels is desirable and/or when
   it is required to interwork with existing loop emulation systems
   (LES) based on AAL2.

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   Operation, administration, and maintenance (OAM) mechanisms are vital
   for proper TDM depolyments.  As aforementioned, structure-aware
   mechanisms may refrain from transporting structure overhead across
   the PSN, disrupting OAM functionality.  It is beneficial to
   distinguish between two OAM cases, the trail terminated and the trail
   extended scenarios.  A trail is defined to be the combination of data
   and associated OAM information transfer.  When the TDM trail is
   terminated, OAM information such as error monitoring and defect
   indications are not transported over the PSN, and the TDM networks
   function as separate OAM domains.  In the trail extended case we
   transfer the OAM information over the PSN (although not necessarily
   in its native format).  This will be discussed further in Section 5.

   Despite its name, the TDMoIP(R) protocol herein described tolerates
   several types of PSN, including UDP over IPv4 or IPv6, MPLS, L2TPv3
   over IP, or pure Ethernet.  Implementation specifics for particular
   PSNs are discussed in Section 3.  Although the protocol should be
   more generally called TDMoPW and its specific implementations TDMoIP,
   TDMoMPLS, etc.  We will use the nomenclature TDMoIP for reasons of
   consistency with earlier usage.

2. TDMoIP Encapsulation

   The overall format of TDMoIP packets is shown in the following

          |    PSN Headers      |
          | TDMoIP Control Word |
          |   Adapted Payload   |

   The PSN-specific headers are those of UDP/IP, L2TPv3/IP, MPLS or
   layer 2 Ethernet, and contain all information necessary for
   forwarding the packet from the PSN-bound GW to the TDM-bound one.
   The PSN is assumed to be reliable enough and of sufficient bandwidth
   to enable transport of the required TDM data.

   A TDMoIP gateway may simultaneously support multiple TDM PWs, and the
   TDMoIP gateway MUST maintain context information for each TDM PW.
   Distinct PWs are differentiated based on PW labels, which are carried
   in the PSN-specific layers.  Since TDM is inherently bidirectional,
   the association of two PWs in opposite directions is required.  In
   general the PW labels of these PWs will take different values.

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   In addition to the aforementioned headers, an OPTIONAL 12-byte RTP
   header may appear in order to provide a mechanism for explicit
   transfer of timing information in the packet.  If RTP is used, the
   fixed RTP header described in [RTP], MUST immediately precede the
   control word for UDP/IP, and MUST immediately follow it for all other
   cases.  The P (padding), X (header extension), CC (CSRC count), and M
   (marker) fields in the RTP header MUST be set to zero, and the PT
   values MUST be allocated from the range of dynamic values.  The RTP
   sequence number MUST be identical to the sequence number in the
   TDMoIP control word (see below).  The RTP timestamp MUST be generated
   in accordance with the rules established in [RTP]; the clock
   frequency should be an integer multiple of 8 kHz, and MUST be chosen
   to enable timing recovery that conforms with the appropriate
   standards (see Section 6.2).  When the TDMoIP gateways have
   sufficiently accurate local clocks or can derive sufficiently
   accurate timing without explicit timestamps, the RTP header SHOULD be

   The 32-bit control word MUST appear in every TDMoIP packet.  Its
   format is depicted in the following figure.

       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
      |FORMID |L|R| M |RES|  Length   |         Sequence Number       |

   FORMID Format identifier (4 bits) is an OPTIONAL field that MAY be
      used to specify the payload format.  When it is not used it MUST
      be set to zero by the PSN-bound GW and ignored by the TDM-bound
      GW.  It SHOULD NOT be used when PSN forwarding mechanisms identify
      PWs based on the initial four bits of their packets being zero.
      The following values are presently defined (none of which alias an
      IP packet):

         1100 AAL1 unstructured
         1101 AAL1 structured
         1110 AAL1 structured with CAS
         1001 AAL2
         1111 HDLC mode

      The payload format for each of these cases will be described in
      Section 4.

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   L Local Failure (1 bit) The L flag being set indicates that the PSN-
      bound GW has detected or has been informed of a TDM physical layer
      fault impacting the TDM data being forwarded.  In the trail
      extended OAM scenario the L flag MUST be set when the GW detects
      loss of signal, loss of frame synchronization, or AIS.  When the L
      flag is set the contents of the packet may not be meaningful, and
      the payload MAY be suppressed in order to conserve bandwidth.
      Once set, if the TDM fault is rectified the L flag MUST be
      cleared.  Use of the L flag is further explained in Section 5.

   R Remote Failure (1 bit) The R flag being set indicates that the PSN-
      bound GW has detected or has been informed, that TDM data is not
      being received from the remote TDM network, indicating failure of
      the reverse direction of the bidirectional connection.  In the
      trail extended OAM scenario the R flag MUST be set when the GW
      detects RDI, and the GW SHOULD generate TDM RDI upon receipt of an
      R flag indication.  Use of the R flag is further explained in
      Section 5.

   Defect Modifier (2 bits) Use of the M field is optional, and when
      used supplements the meaning of the L flag.

      When L is cleared (indicating valid TDM data) the M field is used
      as follows:

       0 0  indicates no local defect modification.
       0 1  reserved.
       1 0  reserved.
       1 1  reserved.

      When L is set (indicating invalid TDM data) the M field is used as

       0 0  indicates a TDM defect that should trigger conditioning
            or AIS generation by the TDM-bound gateway.
       0 1  indicates idle TDM data that should not trigger any alarm.
            If the payload has been suppressed then the preconfigured idle
            code should be generated at egress.
       1 0  indicates corrupted but potentially recoverable TDM data.
       1 1  reserved.

      Use of the M field is further explained in Section 5.

   RES (2 bits) These bits are reserved and MUST be set to zero.

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   Length (6 bits) is used to indicate the length of the TDMoIP packet
      (control word and payload), in case padding is employed to meet
      minimum transmission unit requirements of the PSN.  It MUST be
      used if the total packet length (including PSN, optional RTP,
      control word, and payload) is less than 64 bytes, and MUST be set
      to zero when not used.

   Sequence number (16 bits) The TDMoIP sequence number provides the
      common PW sequencing function described in [PWE-ARCH], and enables
      detection of lost and misordered packets.  The sequence number
      space is a 16-bit, unsigned circular space; the initial value of
      the sequence number SHOULD be random (unpredictable) for security
      purposes, and its value is incremented modulo 2^16 separately for
      PW.  Pseudocode for a sequence number processing algorithm that
      could be used by a TDM-bound GW is provided in Appendix A.

   In order to form the TDMoIP payload, the PSN-bound GW extracts bytes
   from the continuous TDM stream, filling each byte from its most
   significant bit.  The extracted bytes are then adapted using one of
   two adaptation algorithms (see Section 4), and the resulting adapted
   payload is placed into the packet.

3. Encapsulation Details for Specific PSNs

   TDMoIP PWs may exploit various PSNs, including UDP/IP (both IPv4 and
   IPv6), L2TPv3 over IP (with no intervening UDP), MPLS, and layer-2
   Ethernet.  In the following subsections We depict the packet format
   for these cases.

   For MPLS PSNs, the format is identical to that specified in [Y1413].

3.1 UDP/IP

   The UDP/IP header as described in [UDP] and [IP] is prefixed to the
   TDMoIP data.  The TDMoIP packet structure is as follows:

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        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
       | IPVER |  IHL  |    IP TOS     |          Total Length         |
       |         Identification        |Flags|      Fragment Offset    |
       |  Time to Live |    Protocol   |      IP Header Checksum       |
       |                     Source IP Address                         |
       |                  Destination IP Address                       |
       | VER |        PW label         |   Destination Port Number     |
       |           UDP Length          |         UDP Checksum          |
    opt|RTV|P|X|  CC   |M|     PT      |      RTP Sequence Number      |
    opt|                            Timestamp                          |
    opt|                         SSRC identifier                       |
       |FORMID |L|R| M |RES|  Length   |         Sequence Number       |
       |                                                               |
       |                        Adapted Payload                        |
       |                                                               |

   The first five rows are the IP header, the sixth and seventh rows are
   the UDP header.  Rows 8 through 10 are the optional RTP header.  Row
   11 is the TDMoIP control word.

   IPVER (4 bits) is the IP version number, e.g.  for IPv4 IPVER=4.

   IHL (4 bits) is the length in 32-bit words of the IP header, IHL=5.

   IP TOS (8 bits) is the IP type of service.

   Total Length (16 bits) is the length in bytes of header and data.

   Identification (16 bits) is the IP fragmentation identification

   Flags (3 bits) are the IP control flags and MUST be set to Flags=010
      to avoid fragmentation.

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   Fragment Offset (13 bits) indicates where in the datagram the
      fragment belongs and is not used for TDMoIP.

   Time to Live (8 bits) is the IP time to live field.  Datagrams with
      zero in this field are to be discarded.

   Protocol (8 bits) MUST be set to 11 hex = 17 dec  to signify UDP.

   IP Header Checksum (16 bits) is a checksum for the IP header.

   Source IP Address (32 bits) is the IP address of the source.

   Destination IP Address (32 bits) is the IP address of the

   VER (3 bits) is the TDMoIP version number.  The original version
      (VER=000) was experimental and should no longer be used.
      Presently VER=001 when RTP is not used, and VER=011 when RTP is

   PW label (13 bits)  This field is usually dedicated to the Source
      Port Number, but here identifies the unique data stream emanating
      from a given TDM circuit and sharing a common destination.  This
      nonstandard use of a UDP port number is similar to RTP/RTCP's use
      of port numbers to uniquely identify sessions, and to allocation
      of arbitrary UDP port numbers for VoIP sessions.  Placing the PW
      label in the UDP header rather than the application area
      facilitates implementation.  The value 0 is reserved; a
      preconfigured (default 1FFF hex = 8191 dec) label value is used
      for PW-layer OAM messages carried in a separate PW (see Appendix
      D); other PW labels in the range 1-8191 are available for use.

   Destination Port Number (16 bits) MUST be set to 0x085E (2142), the
      user port number that has been assigned to TDMoIP by IANA.

   UDP Length (16 bits) is the length in bytes of UDP header and data.

   UDP Checksum (16 bits) is the checksum of UDP/IP header and data.  If
      not computed it must be set to zero.

3.2 MPLS

   ITU-T recommendation Y.1413 [Y1413] describes structure-agnostic and
   structure-aware mechanisms for transporting TDM over MPLS networks.
   Although the terminology used here differs slightly from that of the
   ITU, implementations of TDMoIP for MPLS PSNs as described herein will
   interoperate with implementations designed to comply with Y.1413.

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   The MPLS header as described in [MPLS] is prefixed to the control
   word and TDM payload.  The packet structure is as follows:

        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
       |            Tunnel Label               | EXP |S| TTL           |
       |              PW label                 | EXP |S| TTL           |
       |FORMID |L|R| M |RES|  Length   |         Sequence Number       |
    opt|RTV|P|X|  CC   |M|     PT      |      RTP Sequence Number      |
    opt|                            Timestamp                          |
    opt|                         SSRC identifier                       |
       |                                                               |
       |                        Adapted Payload                        |
       |                                                               |

   The first two rows depicted above are the MPLS header; the third is
   the TDMoIP control word.  Fields not previously described will now be

   Tunnel Label (20 bits) is the MPLS label that identifies the MPLS LSP
      used to tunnel the TDM packets through the MPLS network.  The
      label can be assigned either by manual provisioning or via an MPLS
      control protocol.  While transiting the MPLS network there may be
      zero, one or several tunnel label rows.  For label stack usage see

   EXP (3 bits) experimental field, may be used to carry DiffServ
      classification for tunnel labels.

   S  (1 bit)  the stacking bit indicates MPLS stack bottom.  S=0 for
      all tunnel labels, and S=1 for the PW label.

   TTL (8 bits) MPLS Time to live.  Should be set to 2 for the PW label.

   PW Label (20 bits) The value 0 is reserved; a preconfigured (default
      FFFFF hex = 1048575 dec) label value is used for PW-layer OAM
      messages carried in a separate PW (see Appendix D); other PW
      labels are available for use.

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3.3 L2TPv3

   The L2TPv3 header defined in [L2TPv3] is prefixed to the TDMoIP data.
   The packet structure is as follows:

        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
       | IPVER |  IHL  |    IP TOS     |          Total Length         |
       |         Identification        |Flags|      Fragment Offset    |
       |  Time to Live |    Protocol   |      IP Header Checksum       |
       |                     Source IP Address                         |
       |                  Destination IP Address                       |
       |                     Session ID = PW label                     |
       |                      cookie 1 (optional)                      |
       |                      cookie 2 (optional)                      |
       |FORMID |L|R|  RES  |  Length   |         Sequence Number       |
    opt|RTV|P|X|  CC   |M|     PT      |      RTP Sequence Number      |
    opt|                            Timestamp                          |
    opt|                         SSRC identifier                       |
       |                                                               |
       |                        Adapted Payload                        |
       |                                                               |

   Rows 6 through 8 are the L2TPv3 header.  Fields not previously
   described will now be explained.

   Protocol the IP protocol field must be set to 73 hex = 115 dec, the
      user port number that has been assigned to L2TP by IANA.

   Session ID (32 bits) is the locally significant L2TP session
      identifier, and contains the PW label.  The value 0 is reserved; a
      preconfigured (default FFFFFFFF hex) label value is used for PW-
      layer OAM messages carried in a separate PW (see Appendix D);
      other PW labels are available for use.

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   Cookie (32 or 64 bits) is an optional field that contains a randomly
      selected value that can be used to validate association of the
      received frame with the expected PW.

3.4 Ethernet

   The TDMoIP packet described in the previous subsections will
   frequently be further encapsulated in an Ethernet frame by prefixing
   the Ethernet preamble, destination and source MAC addresses, optional
   VLAN header, and Ethertype, and suffixing the four-byte frame check
   sequence.  TDMoIP implementations MUST be able to receive both
   industry standard (DIX) Ethernet and IEEE 802.3 [IEEE802.3] frames
   and SHOULD transmit Ethernet frames.

   Ethernet encapsulation introduces restrictions on both minimum and
   maximum packet size.  Whenever the entire TDMoIP packet is less than
   64 bytes, zero padding is introduced and the true length indicated by
   using the Length field in the control word.  In order to avoid
   fragmentation the TDMoIP packet must be restricted to the maximum
   payload size.  For example, the length of the Ethernet payload for a
   UDP/IP encapsulation of AAL1 format payload with 30 PDUs per packet
   is 1460 bytes, which falls below the maximal permitted payload size
   of 1500 bytes.

   Ethernet frames may be used for TDMoIP transport without intervening
   IP or MPLS layers.  In this case an MPLS-style label will always be
   present, but if VLAN tags are sufficient to identify the PW the MPLS
   label MUST be set to one.  The Ethertype SHOULD be set to the value
   allocated for CESoETH, but MAY be set to the Ethertype of MPLS.  The
   packet structure is as follows:

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        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
                                       |  Destination MAC Address
                           Destination MAC Address (cont)              |
       |                     Source MAC Address
           Source MAC Address  (cont)  |   VLAN Ethertype (opt)        |
       |VLP|C|      VLAN ID (opt)      |         Ethertype             |
       |              PW label                 |        RES            |
       |FORMID |L|R| M |RES|  Length   |         Sequence Number       |
    opt|RTV|P|X|  CC   |M|     PT      |      RTP Sequence Number      |
    opt|                            Timestamp                          |
    opt|                         SSRC identifier                       |
       |                                                               |
       |                        Adapted Payload                        |
       |                                                               |
       |                     Frame Check Sequence                      |

   Rows 1 through 6 are the (DIX) Ethernet header; for 802.3 there may
   be additional fields, depending on the value of the length field, see
   [IEEE802.3].  Fields not previously described will now be explained.

   Destination MAC Address (48 bits) is the globally unique address of a
      single station that is to receive the packet.  The format is
      defined in [IEEE802.3].

   Source MAC Address (48 bits) is the globally unique address of the
      station that originated the packet.  The format is defined in

   VLAN Ethertype (16 bits) a 8100 hex in this position indicates that
      optional VLAN tagging according to [IEEE802.1Q] is employed, and
      that the next two bytes contain the VLP, C and VLAN ID fields.
      VLAN tags may be stacked, in which case the two-byte field
      following the VLAN ID is once again a VLAN Ethertype.

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   VLP (3 bits) is the VLAN priority, see [IEEE802.1Q].

   C  (1 bit) the "canonical format indicator" being set, indicates that
      route descriptors appear; see [IEEE802.1Q].

   VLAN ID (12 bits) the VLAN identifier uniquely identifies the VLAN to
      which the frame belongs.  If zero only the VLP information is
      meaningful.  Values 1 and FFF are reserved.  The other 4193 values
      are valid VLAN identifiers.

   Ethertype (16 bits) is the protocol identifier, as allocated by the

   Frame Check Sequence (32 bits) is a CRC error detection field,
      calculated per [IEEE802.3].

4. TDMoIP Payload types

   As discussed at the end of Section 2, TDMoIP transports real-time
   streams by first extracting bytes from the stream, and then adapting
   these bytes.  TDMoIP offers two different adaptation algorithms, one
   for constant rate real-time traffic, and one for variable rate real-
   time traffic.

   Since native TDM is always constant bit-rate, why is a variable rate
   adaptation needed? For unstructured TDM, or structured but
   unchannelized TDM, of structured channelized TDM with all channels
   active all the time, there is indeed no need.  In such cases TDMoIP
   uses structure-indication to emulate the native TDM circuit,
   utilizing an adaptation known as circuit emulation.  However,
   individual "local loops" are frequently "on-hook" and thus inactive,
   and bandwidth may be conserved by transporting only channels
   corresponding to active loops.  This results in variable rate real-
   time traffic, for which TDMoIP uses structure-reassembly to emulate
   the individual loops, utilizing an adaptation known as loop

   TDMoIP uses constant-rate AAL1 [AAL1,CES] for circuit emulation,
   while variable-rate AAL2 [AAL2] is employed for loop emulation.  The
   AAL1 mode MUST be used for structured transport of unchannelized data
   and SHOULD be used for circuits with relatively constant usage.  In
   addition, AAL1 MUST be used when the TDM-bound GW is required to
   maintain a high timing accuracy (e.g.  when its timing is further
   distributed) and SHOULD be used when high reliability is required.
   AAL2 SHOULD be used for channelized TDM when bandwidth needs to be
   conserved, and MAY be used whenever usage of voice-carrying channels
   is expected to be highly variable.

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   Additionally, a third mode is defined specifically for efficient
   transport of HDLC-based CCS signaling carried in TDM channels.

   The AAL family of protocols is a natural choice for TDM emulation.
   Although originally developed to adapt various types of application
   data to the rigid format of ATM, the mechanisms are general solutions
   to the problem of transporting constant or variable rate real-time
   streams over a packet network.

   Since the AAL mechanisms are extensively deployed within and on the
   edge of the public telephony system, they have been demonstrated to
   reliably transfer voice-grade channels, data and telephony signaling.
   These mechanisms are mature and well understood, and implementations
   are readily available.

   Finally, simplified service interworking with legacy networks is a
   major design goal of TDMoIP.  Re-use of AAL technologies simplifies
   interworking with existing AAL1- and AAL2-based networks.

4.1 AAL1 Format Payload

   For the prevalent cases of unchannelized TDM, or channelized TDM for
   which the channel allocation is static, the payload can be
   efficiently encoded using constant rate AAL1 adaptation.  The AAL1
   format is described in [AAL1] and its use for circuit emulation over
   ATM in [CES].  We briefly review highlights of AAL1 technology in
   Appendix B.  In this section we describe the use of AAL1 in the
   context of TDMoIP.

        |control word |    AAL1 PDU    |

   Single AAL1 PDU per TDMoIP packet

        +-------------+----------------+   +----------------+
        |control word |    AAL1 PDU    |---|    AAL1 PDU    |
        +-------------+----------------+   +----------------+

   Multiple AAL1 PDUs per TDMoIP packet

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   In AAL1 mode the TDMoIP payload consists of between one and thirty
   48-byte "AAL1 PDUs".  The number of PDUs must be pre-configured and
   typically chosen according to latency and bandwidth constraints.
   Using a single PDU reduces latency to a minimum, but incurs the
   highest overhead, while using, for example, eight PDUs reduces the
   overhead percentage while increasing the latency by a factor of
   eight.  All TDMoIP implementations MUST support between 1 and 8 PDUs
   per packet for E1 and T1 circuits, and between 5 and 15 PDUs per
   packet for E3 and T3 circuits.

   AAL1 differentiates between unstructured and structured data
   transfer, which correspond to structure-agnostic and structure-aware
   transport.  For structure-agnostic transport, AAL1 provides no
   inherent advantage as compared to SAToP; however, there may be
   scenarios for which its use is desirable.  For example, when it is
   necessary to interwork with an existing AAL1 ATM circuit emulation
   system, or when clock recovery based on AAL1-specific mechanisms is

   For structure-aware transport, [CES] defines two modes, structured
   and structured with CAS.  Structured AAL1 maintains TDM frame
   synchronization by embedding a pointer to the beginning of the next
   frame in the AAL1 PDU header.  Similarly, structured AAL1 with CAS
   maintains TDM frame and multiframe synchronization by embedding a
   pointer to the beginning of the next multiframe.  Furthermore,
   structured AAL1 with CAS contains a substructure including the CAS
   signaling bits.

4.2 AAL2 Format Payload

   Although AAL1 may be configured to transport fractional E1 or T1
   circuits, the allocation of channels to be transported must be static
   due to the fact that AAL1 transports constant rate bit-streams.  It
   is often the case that not all the channels in a TDM circuit are
   simultaneously active ("off-hook"), and by observation of the TDM
   signaling channel activity status may be determined.  Moreover, even
   during active calls about half the time is silence that can be
   identified using voice activity detection (VAD).  Using the variable
   rate AAL2 mode we may dynamically allocate channels to be
   transported, thus conserving bandwidth.

   The AAL2 format is described in [AAL2] and its use for loop emulation
   over ATM is explained in [SSCS,LES].  We briefly review highlights of
   AAL2 technology in Appendix C.  In this section we describe the use
   of AAL2 in the context of TDMoIP.

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        +-------------+----------------+   +----------------+
        |control word |    AAL2 PDU    |---|    AAL2 PDU    |
        +-------------+----------------+   +----------------+

   Concatenation of AAL2 PDUs in a TDMoIP packet

   In AAL2 mode the TDMoIP payload consists of one or more variable-
   length "AAL2 PDUs".  Each AAL2 PDU contains 3 bytes of overhead and
   between 1 and 64 bytes of payload.  A packet may be constructed by
   inserting PDUs corresponding to all active channels, by appending
   PDUs ready at a certain time, or by any other means.  Hence, more
   than one PDU belonging to a single channel may appear in a packet.

   [PWE-ARCH] denotes as Native Service Processing (NSP) functions all
   processing of the TDM data before its use as payload.  Since AAL2 is
   inherently variable rate, arbitrary NSP functions MAY be performed
   before the channel is placed in the AAL2 loop emulation payload.
   These include testing for on-hook/off-hook status, voice activity
   detection, speech compression, fax/modem/tone relay, etc.

   All mechanisms described in [AAL2,SSCS,LES] may be used for TDMoIP.
   In particular, CID encoding according to [AAL2], encoding formats
   defined in [SSCS], and transport of CAS and CCS signaling as
   described in [LES] may all be used.  The overlap functionality and
   AAL-CU timer and related functionalities may not be required, and the
   STF field is NOT used.  Computation of error detection codes, namely
   the HEC in the AAL2 PDU header and the CRC in the CAS packet, is
   Superfluous if an appropriate error detection mechanism is provided
   by the PSN.  In such cases these fields MUST be set to zero.

4.3 HDLC Format Payload

   The motivation for handling HDLC in TDMoIP is to efficiently
   transport common channel signaling (CCS) such as SS7 [SS7] or ISDN
   PRI signaling [ISDN-PRI], embedded in the TDM stream.  This mechanism
   is not intended for general HDLC payloads, and assumes that the HDLC
   messages are always shorter than the maximum packet size.

   The HDLC mode should only be used when the majority of the bandwidth
   of the input HDLC stream is expected to be occupied by idle flags.
   Otherwise the CCS channel should be treated as an ordinary channel.

   The HDLC format is intended to operate in port mode, transparently
   passing all HDLC data and control messages over a separate PW.

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   The PSN-bound GW monitors flags until a frame is detected.  The
   contents of the frame are collected and the FCS tested.  If the FCS
   is incorrect the frame is discarded, otherwise the frame is sent
   after initial or final flags and FCS have been discarded and zero
   removal has been performed.  When an TDMoIP- HDLC frame is received
   its FCS is recalculated, and the original HDLC frame reconstituted.

5. TDMoIP Defect Handling

   Native TDM networks signify network faults by carrying indications of
   forward defects (AIS) and reverse defects (RDI) in the TDM bit
   stream.  Structure-agnostic TDM transport transparently transports
   all such indications; however, structure-aware mechanisms for which
   the PSN-bound GW may remove TDM structure overhead carrying defect
   indications, will require explicit signaling of TDM defect

   We saw in Section 2 that defects can be indicated by setting flags in
   the control word.  This insertion of defect reporting into the packet
   rather than in a separate stream mimics the behavior of native TDM
   OAM mechanisms that carry such indications as bit patterns embedded
   in the TDM stream.  The flags are designed to address the urgent
   messaging, i.e.  messages whose contents must not be significantly
   delayed with respect to the TDM data that they potentially impact.
   Mechanisms for slow OAM messaging are discussed in Appendix D.

    +---+   +-----+   +------+   +-----+   +------+   +-----+   +---+
    |TDM|->-|     |->-|TDMoIP|->-|     |->-|TDMoIP|->-|     |->-|TDM|
    |   |   |TDM 1|   |      |   | PSN |   |      |   |TDM 2|   |   |
    |ES1|-<-|     |-<-|  GW1 |-<-|     |-<-|  GW2 |-<-|     |-<-|ES1|
    +---+   +-----+   +------+   +-----+   +------+   +-----+   +---+

   Typical TDMoIP network configuration

   The operation of TDMoIP defect handling is best understood by
   considering the downstream TDM flow from TDM end system 1 (ES1)
   through TDM network 1, through TDMoIP gateway 1 (GW1), through the
   PSN, through TDMoIP gateway 2 (GW2), through TDM network 2, towards
   TDM end system 2 (ES2), as depicted in the figure.  We wish not only
   to detect defects in TDM network 1, the PSN, and TDM network 2, but
   to localize such defects in order to raise alarms only in the
   appropriate network.

   In the trail terminated OAM scenario, only user data is exchanged
   between TDM network 1 and TDM network 2.  The GW functions as a TDM
   trail termination function, and defects detected in TDM network 1 are
   not relayed to network 2, or vice versa.

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   In the trail extended OAM scenarion, if there is a defect (e.g.  loss
   of signal or loss of frame synchronization) anywhere in TDM network 1
   before the ultimate link, the following TDM node will generate AIS
   downstream (towards TDMoIP GW1).  If a break occurs in the ultimate
   link, the GW itself will detect the loss of signal.  In either case,
   GW1 having directly detected lack of validity of the TDM signal, or
   having been informed of an earlier problem, raises the local ("L")
   defect flag in the control word of the packets it sends across the
   PSN.  In this way the trail is extended to TDM network 2 across the

   Unlike forward defect indications that are generated by all network
   elements, reverse defect indications are only generated by trail
   termination functions.  In the trail terminated scenario, GW1 serves
   as a trail termination function for TDM network 1, and thus when GW1
   directly detects lack of validity of the TDM signal, or is informed
   of an earlier problem, it MAY generate TDM RDI towards TDM ES1.  In
   the trail extended scenario GW1 is not a trail termination, and hence
   MUST NOT generate TDM RDI, but rather, as we have seen, sets the "L"
   defect flag.  As we shall see, this will cause the AIS indication to
   reach ES2, which is the trail termination, and which MAY generate TDM

   When the "L" flag is set there are four possibilities for treatment
   of payload content.  The default is for GW1 to fill the payload with
   the appropriate amount of AIS (usually all-ones) data.  If the AIS
   has been generated before the GW this can be accomplished by copying
   the received TDM data; if the penultimate TDM link fails and the GW
   needs to generate the AIS itself.  Alternatively, structure-aware
   transport of channelized TDM MAY fill the payload with "trunk
   conditioning"; this involves placing a preconfigured "out of service"
   code in each individual channel (the "out of service" code may differ
   between voice and data channels).  Trunk conditioning MUST be used
   when channels taken from several TDM PWs are combined by the TDM-
   bound GW into a single TDM circuit.  The third possibility is to
   suppress the payload altogether.  Finally, if GW1 believes that the
   TDM defect is minor or correctable (e.g.  loss of multiframe
   synchronization, or initial phases of detection of incorrect frame
   sync), it MAY place the TDM data it has received into the payload
   field, and specify in the defect modification field (“M”) that the
   TDM data is corrupted, but potentially recoverable.

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   When GW2 receives a local defect indication without “M”-field
   modification, it forwards (or generates if the payload has been
   suppressed) AIS or trunk conditioning towards ES2 (the choice between
   AIS and conditioning being preconfigured).  Thus AIS has been
   properly delivered to ES2 emulating the TDM scenario from the TDM end
   system's point of view.  In addition, GW2 receiving the “L”
   indication uniquely specifies that the defect was in TDM network 1
   and not in TDM network 2, thus suppressing alarms in the correctly
   functioning network.

   If the M field indicates that the TDM has been marked as potentially
   recoverable, then implementation specific algorithms (not herein
   specified) may optionally be utilized to minimize the impact of
   transient defects on the overall network performance.  If the "M"
   field indicates that the TDM is "idle", no alarms should be raised
   and GW2 treats the payload contents as regular TDM data.  If the
   payload has been suppressed, trunk conditioning and not AIS MUST be
   generated by GW2.

   The second case is when the defect is in TDM network 2.  Such defects
   cause AIS generation towards ES2, which may respond by sending TDM
   RDI in the reverse direction.  In the trail terminated scenario this
   RDI is restricted to network 2.  In the trail extended scenario, GW2
   upon observing this RDI inserted into valid TDM data, MUST indicate
   this by setting the "R" flag in packets sent back across the PSN
   towards GW1.  GW1, upon receiving this indication, generates RDI
   towards ES1, thus emulating a single conventional TDM network.

   The final possibility is that of a unidirectional defect in the PSN.
   In such a case TDMoIP GW1 sends packets toward GW2, but these are not
   received.  GW2 MUST inform the PSN's management system of this
   problem, and furthermore generate TDM AIS towards ES2.  ES2 may
   respond with TDM RDI, and as before, in the trail extended scenario,
   when GW2 detects RDI it MUST raise the "R" flag indication.  When GW1
   receives packets with the "R" flag set it has been informed of a
   reverse defect, and MUST generate TDM RDI towards ES1.

   In all cases, if any of the above defects persist for a preconfigured
   period (default value of 2.5 seconds) a service failure is declared.
   Since TDM PWs are inherently bidirectional, a persistent defect in
   either directional results in a bidirectional service failure.  In
   addition, if signaling is sent over a distinct PW as per Section 4.3,
   both PWs are considered to have failed when persistent defects are
   detected in either.

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   When failure is declared the PW MUST be withdrawn, and both TDMoIP
   GWs commence sending AIS (and not trunk conditioning) to their
   respective TDM networks.  The GWs then engage in connectivity testing
   using TDMoIP OAM as described in Appendix D until connectivity is

6. Implementation Issues

   General requirements for transport of TDM over pseudo-wires are
   detailed in [TDM-REQ].  In the following subsections we review
   additional aspects essential to successful TDMoIP implementation.

6.1 Jitter and Packet Loss

   In order to compensate for packet delay variation that exists in any
   PSN, a jitter buffer MUST be provided.  A jitter buffer is a block of
   memory into which the data from the PSN is written at its variable
   arrival rate, and data is read out and sent to the destination TDM
   equipment at a constant rate.  Use of a jitter buffer partially hides
   the fact that a PSN has been traversed rather than a conventional
   synchronous TDM network, except for the additional latency.
   Customary practice is to operate with the jitter buffer approximately
   half full, thus minimizing the probability of its overflow or
   underflow.  Hence the additional delay equals half the jitter buffer
   size.  The length of the jitter buffer SHOULD be configurable and MAY
   be dynamic (i.e.  grow and shrink in length according to the
   statistics of the PDV).

   In order to handle (infrequent) packet loss and misordering a packet
   sequence integrity mechanism MUST be provided.  This mechanism MUST
   track the serial numbers of packets in the jitter buffer and MUST
   take appropriate action when anomalies are detected.  When missing
   packet(s) are detected the mechanism MUST output filler packet(s) in
   order to retain TDM timing.  Packets with incorrect serial numbers or
   other detectable header errors MUST be discarded.  Packets arriving
   in incorrect order SHOULD be swapped.  Processing of filler packets
   SHOULD ensure that proper FAS is sent to the TDM network.  An example
   sequence number processing algorithm is provided in Appendix A.

   While the insertion of arbitrary filler packets may be sufficient to
   maintain the TDM timing, for voice traffic it may lead to gaps or
   artifacts that result in choppy, annoying or even unintelligible
   speech, see [TDM-PLC].  An implementation MAY blindly insert a
   preconfigured constant value in place of any lost speech samples, and
   this value SHOULD be chosen to minimize the perceptual effect.

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   Alternatively one MAY replay the previously received packet.  Since a
   TDMoIP packet is usually declared lost following the reception of the
   next packet, when computational resources are available,
   implementations SHOULD conceal the packet loss event by properly
   estimating the missing speech sample values.

6.2 Timing Recovery

   TDM networks are inherently synchronous; somewhere in the network
   there will always be at least one extremely accurate primary
   reference clock, with long-term accuracy of one part in 10E-11.  This
   node provides reference timing to secondary nodes with somewhat lower
   accuracy, and these in turn distribute timing information further.
   This hierarchy of time synchronization is essential for the proper
   functioning of the network as a whole; for details see [G823,G824].

   Packets in PSNs reach their destination with delay that has a random
   component, known as packet delay variation (PDV).  When emulating TDM
   on a PSN, extracting data from the jitter buffer at a constant rate
   overcomes much of the high frequency component of this randomness
   ("jitter").  The rate at which we extract data from the jitter buffer
   is determined by the destination clock, and were this to be precisely
   matched to the source clock proper timing would be maintained.
   Unfortunately the source clock information is not disseminated
   through a PSN, and the destination clock frequency will only
   nominally equal the source clock frequency, leading to low frequency
   ("wander") timing inaccuracies.

   In broadest terms there are three methods of overcoming this
   difficulty.  In the first method timing information is provided by
   some means independent of the PSN.  This timing may be provided to
   the TDM end system or to the GWs.  In a second method a common clock
   is assumed available to both gateways, and the relationship between
   the TDM source clock and this clock is encoded in the packet.  This
   encoding may be take the form of RTP timestamps or may utilizing the
   SRTS bits in the AAL1 overhead.  In the final method (adaptive clock
   recovery) the timing must be deduced solely based on the packet
   arrival times.  Example scenarios are detailed in [TDM-REQ] and in

   Adaptive clock recovery utilizes only observable characteristics of
   the packets arriving from the PSN, such as the precise time of
   arrival of the packet at the TDM-bound GW, or the fill-level of the
   jitter buffer as a function of time.  Due to the packet delay
   variation in the PSN, filtering processes that combat the statistical
   nature of the observable characteristics must be employed.  Frequency
   Locked Loops (FLL) and Phase Locked Loops (PLL) are well suited for
   this task.

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   Whatever timing recovery mechanism is employed, the output of the
   TDM-bound GW MUST conform to the jitter and wander specifications of
   TDM traffic interfaces, as defined in [G823,G824].  For some
   applications, more stringent jitter and wander tolerances MAY be

6.3 Quality of Service

   TDMoIP does not provide mechanisms to ensure timely delivery or
   provide other quality-of-service guarantees; hence it is required
   that the lower-layer services do so.  Layer 2 priority can be
   bestowed upon a TDMoIP stream by using the VLAN priority field, MPLS
   priority can be provided by using EXP bits, and layer 3 priority is
   controllable by using TOS.  Switches and routers which the TDMoIP
   stream must traverse should be configured to respect these

   If the PSN is Diffserv-enabled then an EF-PHB (expedited forwarding)
   class based PDB SHOULD be used, in order to provide a low latency and
   minimal jitter service.  It is suggested that the transport LSP be
   somewhat overprovisioned.

   If the MPLS network is Intserv enabled, then GS (Guaranteed Service)
   with the appropriate bandwidth reservation SHOULD be used in order to
   provide a bandwidth BW guarantee equal or greater than that of the
   aggregate TDM traffic.  The delay introduced by the MPLS network
   SHOULD be measured prior to traffic flow, to ensure its compliance
   with latency requirements.

7. Security Considerations

   TDMoIP does not enhance or detract from the security performance of
   the underlying PSN, rather it relies upon the PSN's mechanisms for
   encryption, integrity, and authentication whenever required.  The
   level of security provided may be less than that of a native TDM

   TDMoIP does not provide protection against malicious users utilizing
   snooping or packet injection during setup or operation.  However,
   random initialization of sequence numbers makes known-plaintext
   attacks on link encryption methods more difficult.

   PW labels SHOULD be selected in an unpredictable manner rather than
   sequentially or otherwise in order to deter session hijacking.  When
   using L2TPv3, randomly selected cookies MAY be used to validate
   circuit origin.  Sequence numbers SHOULD be randomly initialized in
   order to increase the difficulty of decrypting based on packet

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8. IANA Considerations

   When used with UDP/IP the destination port number MUST be set to
   0x085E (2142), the user port number which has been assigned by IANA
   to TDMoIP.

9. Trademarks

   TDMoIP is a registered trademark of RAD Data Communications.  RAD
   Data Communications grants the IETF a perpetual license to reproduce
   this trademark solely in connection with the reproduction,
   distribution or publication of this contribution and derivative works
   thereof, in accordance with RFC 3667.

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10. References

10.1 Normative References

   [AAL1] ITU-T Recommendation I.363.1 (08/96) B-ISDN ATM Adaptation
      Layer (AAL) specification: Type 1

   [AAL2] ITU-T Recommendation I.363.2 (11/00) B-ISDN ATM Adaptation
      Layer (AAL) specification: Type 2

   [CES] ATM forum specification atm-vtoa-0078 (CES 2.0) Circuit
      Emulation Service Interoperability Specification Ver.  2.0

   [CONNECT] RFC 2678 IPPM Metrics for Measuring Connectivity

   [DELAY] RFC 2679 A One-way Delay Metric for IPPM

   [G704] ITU-T Recommendation G.704 (10/98) Synchronous frame
      structures used at 1544, 6312, 2048, 8448 and 44736 kbit/s
      hierarchical levels

   [G751] ITU-T Recommendation G.751 (11/88) Digital multiplex
      equipments operating at the third order bit rate of 34368 kbit/s
      and the fourth order bit rate of 139264 kbit/s and using positive

   [G823] ITU-T Recommendation G.823 (03/00) The control of jitter and
      wander within digital networks which are based on the 2048 Kbit/s

   [G824] ITU-T Recommendation G.824 (03/00) The control of jitter and
      wander within digital networks which are based on the 1544 Kbit/s

   [IEEE802.1Q] IEEE 802.1Q, IEEE Standards for Local and Metropolitan
      Area Networks — Virtual Bridged Local Area Networks (2003)

   [IEEE802.3] IEEE 802.3, IEEE Standard Local and Metropolitan Area
      Networks - Carrier Sense Multiple Access with Collision Detection
      (CSMA/CD) Access Method and Physical Layer Specifications (2002)

   [IPPM] RFC 2330 Framework for IP Performance Metrics

   [IPv4] RFC 791 (STD0005) Internet Protocol (IP)

   [LES] ATM forum specification atm-vmoa-0145 (LES) Voice and
      Multimedia over ATM - Loop Emulation Service Using AAL2

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   [L2TPv3] draft-ietf-l2tpext-l2tp-base-10.txt (08/03) Layer Two
      Tunneling Protocol (L2TPv3), J.  Lau et al., work in progress

   [MPLS] RFC 3032 MPLS Label Stack encoding

   [RTP] RFC 3550 RTP: Transport Protocol for Real-Time Applications

   [SAToP] draft-ietf-pwe3-satop-00.txt (09/03) Structure-Agnostic TDM
      over Packet (SAToP), A.  Vainshtein and Y.  Stein, work in

   [SSCS] ITU-T Recommendation I.366.2 (11/00) AAL type 2 service
      specific convergence sublayer for narrow-band services

   [TRAU] GSM 08.60 (10/01) Digital cellular telecommunications system
      (Phase 2+); Inband control of remote transcoders and rate adaptors
      for Enhanced Full Rate (EFR) and full rate traffic channels

   [UDP] RFC 768 (STD0006) User Datagram Protocol (UDP)

   [VCCV] draft-ietf-pwe3-vccv-03.txt (06/04) Pseudo Wire Virtual
      Circuit Connectivity Verification, T.  Nadeau and R.  Aggarwal,
      work in progress

   [Y1413] ITU-T Recommendation Y.1413 (03/04) TDM-MPLS network
      interworking - User plane interworking

10.2 Informative References

   [CESoPSN] draft-ietf-cesopsn-00.txt (01/04), TDM Circuit Emulation
      Service over Packet Switched Network, A.  Vainshtein et al, work
      in progress

   [ICMP] RFC 792 Internet Control Message Protocol.  (09/81)

   [ISDN-PRI] ITU-T Recommendation Q.931 (05/98) ISDN user-network
      interface layer 3 specification for basic call control

   [PWE3-ARCH] draft-ietf pwe3-arch-07.txt (3/04), PWE3 Architecture,
      Stewart Bryant et al, work in progress

   [SS7] ITU-T Recommendation Q.700 (03/93) Introduction to CCITT
      Signalling System No.  7

   [TDM-PLC] draft-stein-pwe3-tdm-packetloss-01.txt (10/03), The Effect
      of Packet Loss on Voice Quality for TDM over Pseudowires, Y(J)
      Stein and I.  Druker, work in progress

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   [TDM-REQ] draft-ietf-pwe3-tdm-requirements-04.txt (1/04),
      Requirements for Edge-to-Edge Emulation of TDM Circuits over
      Packet Switching Networks, M.  Riegel et al., work in progress

11. Acknowledgments

   The authors would like to thank Hugo Silberman, Shimon HaLevy, Tuvia
   Segal, and Eitan Schwartz of RAD Data Communications for their
   valuable contributions to the technology described herein.

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Authors' Addresses

   Yaakov (Jonathan) Stein
   RAD Data Communications
   24 Raoul Wallenberg St., Bldg C
   Tel Aviv  69719

   Phone: +972 3 645-5389
   EMail: yaakov_s@rad.com

   Ronen Shashoua
   RAD Data Communications
   24 Raoul Wallenberg St., Bldg C
   Tel Aviv  69719

   Phone: +972 3 645-5447
   EMail: ronen_s@rad.com

   Ron Insler
   RAD Data Communications
   24 Raoul Wallenberg St., Bldg C
   Tel Aviv  69719

   Phone: +972 3 645-5445
   EMail: ron_i@rad.com

   Motty (Mordechai) Anavi
   RAD Data Communications
   900 Corporate Drive
   Mahwah, NJ  07430

   Phone: +1 201 529-1100 Ext. 213
   EMail: motty@radusa.com

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Appendix A. Sequence Number Processing

   The sequence number field in the control word enables detection of
   lost and misordered packets.  Here we give pseudocode for an example
   algorithm in order to clarify the issues involved.  These issues are
   implementation specific and no single explanation can capture all the

   In order to simplify the description modulo arithmetic is
   consistently used in lieu of ad-hoc treatment of the cyclicity.  All
   differences between indexes are explicitly converted to the range
   [–2^15 ...  +2^15 – 1] to ensure that simple checking of the
   difference’s sign correctly predicts the packet arrival order.

   Furthermore, we introduce the notion of a playout buffer in order to
   unambiguously define packet lateness.  When a packet arrives after
   having previously having been assumed lost, the TDM-bound GW may
   discard it, and continue to treat it as lost.  Alternatively if the
   filler data that had been inserted in its place has not yet been
   played out, the option remains to insert the true data into the
   playout buffer.  Of course, the filler data may be generated upon
   initial detection of a missing packet or upon playout.  This
   description is stated in terms of a packet-oriented playout buffer
   rather than a TDM byte oriented one; however this is not a true
   requirement for re-ordering implementations since the latter could be
   used along with pointers to packet commencement points.

   Having introduced the playout buffer we explicitly treat over-run and
   under-run of this buffer.  Over-run occurs when packets arrive so
   quickly that they can not be stored for playout.  This is usually an
   indication of gross timing inaccuracy or misconfiguration, and we can
   do little but discard such early packets.  Under-run is usually a
   sign of network starvation, resulting from congestion or network

   The external variables used by the pseudocode are:

      received:  sequence number of packet received.
      played:    sequence number of the packet being played out (Note 1).
      over-run:  is the playout buffer full? (Note 3).
      under-run: has the playout buffer been exhausted? (Note 3).

   The internal variables used by the pseudocode are:

      expected: sequence number we expect to receive next.
      D: difference between expected and received sequence numbers (Note 2).
      L: difference between sequence numbers of packet being played out
         and that just received (Notes 1 and 2).

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   In addition, the algorithm requires one parameter:

      R: maximum lateness of packet recoverable (Note 1).

     Note 1: this is only required for the optional re-ordering.
     Note 2: this number is always in the range -2^15 ... +2^15 - 1.
     Note 3: the playout buffer is emptied by the TDM playout process,
             which runs asynchronously to the packet arrival processing,
             and which is not herein specified.

   Sequence Number Processing Algorithm

   Upon receipt of a packet
     if received = expected
       { treat packet as in-order }
       if not over-run
         place packet contents into playout buffer
         discard packet contents
       set expected = (received + 1) mod 2^16
       calculate D = ( (expected-received) mod 2^16 ) - 2^15
       if D > 0 then
         { packets expected, expected+1, ... received-1 are lost }
         while not over-run
           place filler (all-ones or interpolation) into playout buffer
           if not over-run
             place packet contents into playout buffer
             discard packet contents
           set expected = (received + 1) mod 2^16
        else  { late packet arrived }
          declare "received" to be a late packet
          do NOT update "expected"
            discard packet
            if not under-run
              calculate L = ( (played-received) mod 2^16 ) - 2^15
              if 0 < L <= R
                replace packet previously marked as lost with actual data
                discard packet
   Note: by choosing R=0 we always discard the late packet

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Appendix B. AAL1 Review

   The first byte of the 48-byte AAL1 PDU always contains an error-
   protected three-bit sequence number.

         1 2 3 4 5 6 7 8
        |C| SN  | CRC |P| 47 bytes of payload

   C  (1 bit) convergence sublayer indication, its use here is limited
      to indication of the existence of a pointer (see below); C=0 means
      no pointer, C=1 means a pointer is present.

   SN (3 bits) The AAL1 sequence number increments from PDU to PDU.

   CRC (3 bits) is a 3 bit error cyclic redundancy code on C and SN.

   P  (1 bit) even byte parity.

   As can be readily inferred this byte can only take on eight different
   values, and incrementing the sequence number forms an eight PDU
   sequence number cycle, the importance of which will become clear

   The structure of the remaining 47 bytes in the AAL1 PDU depends on
   the PDU type, of which there are three, corresponding to the three
   types of AAL1 circuit emulation service defined in [CES].  These are
   known as namely unstructured circuit emulation, structured circuit
   emulation and structured circuit emulation with CAS.

   The simplest PDU is the unstructured one, which is used for
   transparent transfer of whole circuits (T1,E1,T3,E3).  Although AAL1
   provides no inherent advantage as compared to SAToP for unstructured
   transport, in certain cases AAL1 may be required or desirable.  For
   example, when it is necessary to interwork with an existing AAL1-
   based network, or when clock recovery based on AAL1-specific
   mechanisms is favored.

   For unstructured AAL1 the 47 bytes after the sequence number byte
   contain the full 376 bits from the TDM bit stream.  No frame
   synchronization is supplied or implied, and framing is the sole
   responsibility of the end-user equipment.  Hence the unstructured
   mode can be used to carry data, and for circuits with nonstandard
   frame synchronization.  For the T1 case the raw frame consists of 193
   bits, and hence 1 183/193 T1 frames fit into each AAL1 PDU.  The E1
   frame consists of 256 bits, and so 1 5/8 E1 frames fit into each PDU.

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   When the TDM circuit is channelized according to [G704], and in
   particular when it is desired to fractional E1 or T1, it is
   advantageous to use one of the structured AAL1 circuit emulation
   services.  Structured AAL1 views the data not merely as a bit stream,
   but as a bundle of channels.  Furthermore, when CAS signaling is used
   it can be formatted so that it can be readily detected and

   In the structured circuit emulation mode without CAS, N bytes from
   the N channels to be transported are first arranged in order of
   channel number.  Thus if channels 2, 3, 5, 7 and 11 are to be
   transported the corresponding five bytes are placed in the PDU
   immediately after the sequence number byte.  This placement is
   repeated until all 47 bytes in the PDU are taken;

       byte     1  2  3  4  5  6  7  8  9 10 --- 41 42 43 44 45 46 47
       channel  2  3  5  7 11  2  3  5  7 11 ---  2  3  5  7 11  2  3

   the next PDU commences where the present PDU left off

       byte     1  2  3  4  5  6  7  8  9 10 --- 41 42 43 44 45 46 47
       channel  5  7 11  2  3  5  7 11  2  3 ---  5  7 11  2  3  5  7

   and so forth.  The set of channels 2,3,5,7,11 is the basic structure
   and the point where one structure ends and the next commences is the
   structure boundary.

   The problem with this arrangement is the lack of explicit indication
   of the byte identities.  As can be seen in the above example, each
   AAL1 PDU starts with a different channel, so a single lost packet
   will result in misidentifying channels from that point onwards,
   without possibility of recovery.  The solution to this deficiency is
   the periodic introduction of a pointer to the next structure
   boundary.  This pointer need not be used too frequently, as the
   channel identifications are uniquely inferable unless packets are

   The particular method used in AAL1 is to insert a pointer once every
   sequence number cycle of eight PDUs.  The pointer is seven bits and
   protected by an even parity MSB, and so occupies a single byte.
   Since seven bits are sufficient to represent offsets larger than 47,
   we can limit the placement of the pointer byte to PDUs with even
   sequence number.  Unlike most AAL1 PDUs that contain 47 TDM bytes,
   PDUs that contain a pointer (P-format PDUs) have the following

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         0                 1
         1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
        |C| SN  | CRC |P|E|   pointer   | 46 bytes of payload


   C  (1 bit) convergence sublayer indication, C=1 for P-format PDUs.

   SN (3 bits) is an even AAL1 sequence number.

   CRC (3 bits) is a 3 bit error cyclic redundancy code on C and SN.

   P  (1 bit) even byte parity LSB for sequence number byte.

   E  (1 bit) even byte parity MSB for pointer byte.

   pointer (7 bits) pointer to next structure boundary.

   Since P-format PDUs have 46 bytes of payload and the next PDU has 47
   bytes, viewed as a single entity the pointer needs to indicate one of
   93 bytes.  If P=0 it is understood that the structure commences with
   the following byte (i.e.  the first byte in the payload belongs to
   the lowest numbered channel).  P=93 means that the last byte of the
   second PDU is the final byte of the structure, and the following PDU
   commences with a new structure.  The special value P=127 indicates
   that there is no structure boundary to be indicated (needed when
   extremely large structures are being transported).

   The P-format PDU is always placed at the first possible position in
   the sequence number cycle that a structure boundary occurs, and can
   only occur once per cycle.

   The only difference between the structured circuit emulation format
   and structured circuit emulation with CAS is the definition of the
   structure.  Whereas in structured circuit emulation the structure is
   composed of the N channels, in structured circuit emulation with CAS
   the structure encompasses the superframe consisting of multiple
   repetitions of the N channels and then the CAS signaling bits.  The
   CAS bits are tightly packed into bytes and the final byte is padded
   with zeros if required.

   For example, for E1 circuits the CAS signaling bits are updated once
   per superframe of 16 frames.  Hence the structure for N*64 derived
   from an E1 with CAS signaling consists of 16 repetitions of N bytes,
   followed by N sets of the four ABCD bits, and finally four zero bits
   if N is odd.  For example, the structure for channels 2,3 and 5 will

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   be as follows

       2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 2 3 5
       2 3 5 2 3 5 2 3 5 2 3 5 2 3 5 [ABCD2 ABCD3] [ABCD5 0000]

   Similarly for T1 ESF circuits the superframe is 24 frames, and the
   structure consists of 24 repetitions of N bytes, followed by the ABCD
   bits as before.  For the T1 case the signaling bits will in general
   appear twice, in their regular (bit-robbed) positions and at the end
   of the structure.

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Appendix C. AAL2 Review

   The basic AAL2 PDU is :

       |    Byte  1    |    Byte  2    |    Byte  3    |
        0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
       |      CID      |     LI    |   UUI   |   HEC   |   PAYLOAD

   CID (8 bits) channel identifier is an identifier that must be unique
      for the PW.  The values below 8 are reserved and so there are 248
      possible channels.  The mapping of CID values to channels is
      beyond the scope of the TDMoIP protocol and must be configured
      manually or via network management.

   LI (6 bits) length indicator is one less than the length of the
      payload in bytes.  Note that the payload is limited to 64 bytes.

   UUI (5 bits) user-to-user indication is the higher layer
      (application) identifier and counter.  For voice data the UUI will
      always be in the range 0-15, and SHOULD be incremented modulo 16
      each time a channel buffer is sent.  The receiver MAY monitor this
      sequence.  UUI is set to 24 for CAS signaling packets.

   HEC (5 bits) the header error control

   Payload - voice A block of length indicated by LI of voice samples
      are placed as- is into the AAL2 packet.

   Payload - CAS signaling For CAS signaling the payload is formatted as
      a type 3 packet (in the notation of [AAL2]) in order to ensure
      error protection.  The signaling is sent with the same CID as the
      corresponding voice channel.  Signaling is sent whenever the state
      of the ABCD bits changes, and is sent with triple redundancy, i.e.
      sent three times spaced 5 milliseconds apart.  In addition, the
      entire set of the signaling bits is sent periodically to ensure

          |RED|       timestamp           |
          |  RES  | ABCD  |    type   | CRC
              CRC (cont)  |

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   RED (2 bits) is the triple redundancy counter.  For the first packet
      it takes the value 00, for the second 01 and for the third 10.
      RED=11 means non-redundant information and is used for periodic
      refresh of the CAS information.

   Timestamp (14 bits) The timestamp is the same for all three redundant

   RES (4 bits) is reserved and MUST be set to zero.

   ABCD (4 bits) are the CAS signaling bits.

   type (6 bits) for CAS signaling this is 000011.

   CRC-10 (10 bits) is a 10 bit CRC error detection code.

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Appendix D. Performance Monitoring Mechanisms

   PWs require OAM mechanisms to monitor performance measures that
   impact the emulated service.  Performance measures, such as packet
   loss ratio and packet delay variation, may be used to set various
   parameters and thresholds; for TDMoIP PWs adaptive timing recovery
   and packet loss concealment algorithms may benefit from such
   information.  In addition, OAM mechanisms may be used to collect
   statistics relating to the underlying PSN [IPPM], and its suitability
   for carrying TDM services.

   TDMoIP GWs may benefit from knowledge of PSN performance metrics,
   such as round trip time (RTT), packet delay variation (PDV) and
   packet loss ratio (PLR).  These measurements are conventionally
   performed by a separate flow of packets designed for this purpose,
   e.g.  ICMP packets [ICMP] with multiple timestamps.  For AAL1 mode
   TDMoIP sends packets across the PSN at a constant rate, and hence no
   additional OAM flow is required for measurement of PDV or PLR.
   However, separate OAM flows are required for RTT measurement, for
   AAL2 mode PWs, for measurement of parameters at setup, for monitoring
   of inactive backup PWs, and for low-rate monitoring of PSNs after PWs
   have been withdrawn due to service failures.

   If the underlying PSN has appropriate maintenance mechanisms that
   provide connectivity verification, RTT, PDV, and PLR measurements
   that correlate well with those of the PW, then these mechanisms
   SHOULD be used.  If such mechanisms are not available, either of two
   similar OAM signaling mechanisms may be used.  One, internal to the
   PW and based on inband VCCV [VCCV], and another that runs in a
   separate PW.  The latter is particularly efficient when a large
   number of TDM PWs are placed in a single PSN tunnel.

D.1 TDMoIP Connectivity Verification

   In most conventional IP applications a server sends some finite
   amount of information over the network after explicit request from a
   client.  With TDMoIP PWs the PSN-bound GW could send a continuous
   stream of packets towards the destination without knowing whether the
   TDM-bound GW is ready to accept them.  For layer-2 networks this may
   lead to flooding of the PSN with stray packets.

   This problem may occur when a TDMoIP GW is first brought up, when the
   TDM-bound GW fails or is disconnected from the PSN, or the PW is
   broken.  After an aging time the destination gateway disappears from
   the routing tables, and intermediate switches may flood the network
   with the TDMoIP packets in an attempt to find a new path.

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   The solution to this problem is to significantly reduce the number of
   TDMoIP packets transmitted per second when PW failure is detected,
   and to return to full rate only when the PW is available.  The
   detection of failure and restoration is made possible by the periodic
   exchange of one-way connectivity-verification messages, as defined in

   Connectivity is tested by periodically sending OAM messages from the
   source GW to the destination GW, and having the destination reply to
   each message.  The connectivity verification mechanism SHOULD be used
   during setup and configuration.  Without OAM signaling one must
   ensure that the destination GW is ready to receive packets before
   starting to send them.  Since TDMoIP gateways operate full-duplex,
   both would need to be set up and properly configured simultaneously
   if flooding is to be avoided.  When using connectivity verification,
   a configured gateway may wait until it detects its peer before
   transmitting at full rate.  In addition, configuration errors may be
   readily discovered by using the service specific field of the OAM PW

   In addition to one way connectivity, OAM signaling mechanisms can be
   used to request and report on various PSN metrics, such as one way
   delay, round trip delay, packet delay variation, etc.  They may also
   be used for remote diagnostics, and for unsolicited reporting of
   potential problems (e.g.  dying gasp messages).

D.2 OAM Packet Format

   When using inband performance monitoring, additional packets are sent
   using the same PW label.  These packets are identified by having
   FORMID=0001, and must be separated from TDM data packets before
   sequence number processing.

   When using a separate OAM PW, all OAM messages MUST use the PW label
   preconfigured to indicate OAM (the default value is the highest label
   available).  All PSN layer parameters (for example, MPLS label, IP
   addresses, TOS, EXP bits, and VLAN ID) MUST remain those of the PW
   being investigated.

   The format of an OAM PW message packet is depicted in the following
   figure.  Note that PSN-specific layers are identical to those used to
   carry the TDMoIP data, with the exception that the PW label MUST be
   set to the preconfigured value instead of the usual PW identifier.

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        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
        |         PSN-specific layers  (with preconfigured PW label)    |
        |FORMID |L|R| M |RES| Length    |     OAM Sequence Number       |
        | OAM Msg Type  | OAM Msg Code  | Service specific information  |
        |       Forward PW label        |      Reverse PW label         |
        |                   Source Transmit Timestamp                   |
        |                 Destination Receive Timestamp                 |
        |                Destination Transmit Timestamp                 |

   FORMID, L, R, and M are identical to those of the PW being tested.

   Length is the length in bytes of the OAM message packet.

   OAM Sequence Number (16 bits) is used to uniquely identify the
      message.  Its value is unrelated to the sequence number of the
      TDMoIP data packets for the PW in question.  It is incremented in
      query messages, and replicated without change in replies.

   OAM Msg Type (8 bits) indicates the function of the message.  At
      present the following are defined:

             0 for one way connectivity query message
             8 for one way connectivity reply message.

   OAM Msg Code (8 bits) is used to carry information related to the
      message, and its interpretation depends on the message type.  For
      type 0 (connectivity query) messages the following codes are

             0 validate connection.
             1 do not validate connection

       for type 8 (connectivity reply) messages the available codes are:

             0 acknowledge valid query
             1 invalid query (configuration mismatch).

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   Service specific information (16 bits) is a field that can be used to
      exchange configuration information between gateways.  If it is not
      used this field MUST contain zero.  Its interpretation depends on
      the payload type.  At present the following is defined for AAL1

         0                   1
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
        | Number of TSs | Number of SFs |

   Number of TSs (8 bits) is the number of channels being transported,
      e.g.  24 for full T1.

   Number of SFs (8 bits) is the number of 48-byte AAL1 PDUs per packet,
      e.g.  8 when packing 8 PDUs per packet.

   Forward PW label (16 bits) is the PW label used for TDMoIP traffic
      from the source to destination gateway.

   Reverse PW label (16 bits) is the PW label used for TDMoIP traffic
      from the destination to source gateway.

   Source Transmit Timestamp (32 bits) represents the time the PSN-bound
      GW transmitted the query message.  This field and the following
      ones only appear if delay is being measured.  All time units are
      derived from a clock of preconfigured frequency, the default being
      100 microseconds.

   Destination Receive Timestamp (32 bits) represents the time the
      destination gateway received the query message.

   Destination Transmit Timestamp (32 bits) represents the time the
      destination gateway transmitted the reply message.

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Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an


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

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