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rfc:rfc4553

Network Working Group A. Vainshtein, Ed. Request for Comments: 4553 Axerra Networks Category: Standards Track YJ. Stein, Ed.

                                               RAD Data Communications
                                                             June 2006
        Structure-Agnostic Time Division Multiplexing (TDM)
                        over Packet (SAToP)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2006).

Abstract

 This document describes a pseudowire encapsulation for Time Division
 Multiplexing (TDM) bit-streams (T1, E1, T3, E3) that disregards any
 structure that may be imposed on these streams, in particular the
 structure imposed by the standard TDM framing.

Vainshtein & Stein Standards Track [Page 1] RFC 4553 Structure-Agnostic TDM over Packet June 2006

Table of Contents

 1. Introduction ....................................................3
 2. Terminology and Reference Models ................................3
    2.1. Terminology ................................................3
    2.2. Reference Models ...........................................4
 3. Emulated Services ...............................................4
 4. SAToP Encapsulation Layer .......................................5
    4.1. SAToP Packet Format ........................................5
    4.2. PSN and PW Demultiplexing Layer Headers ....................5
    4.3. SAToP Header ...............................................6
         4.3.1. Usage and Structure of the Control Word .............8
         4.3.2. Usage of RTP Header .................................9
 5. SAToP Payload Layer ............................................10
    5.1. General Payloads ..........................................10
    5.2. Octet-Aligned T1 ..........................................11
 6. SAToP Operation ................................................12
    6.1. Common Considerations .....................................12
    6.2. IWF Operation .............................................12
         6.2.1. PSN-Bound Direction ................................12
         6.2.2. CE-Bound Direction .................................13
    6.3. SAToP Defects .............................................14
    6.4. SAToP PW Performance Monitoring ...........................15
 7. Quality of Service (QoS) Issues ................................16
 8. Congestion Control .............................................16
 9. Security Considerations ........................................18
 10. Applicability Statement .......................................18
 11. IANA Considerations ...........................................20
 12. Acknowledgements ..............................................20
 13. Co-Authors ....................................................20
 14. Normative References ..........................................21
 15. Informative References ........................................22
 Appendix A: Old Mode of SAToP Encapsulation over L2TPv3 ...........24
 Appendix B: Parameters That MUST Be Agreed upon during the PW
             Setup .................................................24

Vainshtein & Stein Standards Track [Page 2] RFC 4553 Structure-Agnostic TDM over Packet June 2006

1. Introduction

 This document describes a method for encapsulating Time Division
 Multiplexing (TDM) bit-streams (T1, E1, T3, E3) as pseudowires over
 packet-switching networks (PSN).  It addresses only structure-
 agnostic transport, i.e., the protocol completely disregards any
 structure that may possibly be imposed on these signals, in
 particular the structure imposed by standard TDM framing [G.704].
 This emulation is referred to as "emulation of unstructured TDM
 circuits" in [RFC4197] and suits applications where the PEs have no
 need to interpret TDM data or to participate in the TDM signaling.
 The SAToP solution presented in this document conforms to the PWE3
 architecture described in [RFC3985] and satisfies both the relevant
 general requirements put forward in [RFC3916] and specific
 requirements for unstructured TDM signals presented in [RFC4197].
 As with all PWs, SAToP PWs may be manually configured or set up using
 the PWE3 control protocol [RFC4447].  Extensions to the PWE3 control
 protocol required for setup and maintenance of SAToP pseudowires and
 allocations of code points used for this purpose are described in
 separate documents ([TDM-CONTROL] and [RFC4446], respectively).

2. Terminology and Reference Models

 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 [RFC2119].

2.1. Terminology

 The following acronyms used in this document are defined in [RFC3985]
 and [RFC4197]:
 ATM          Asynchronous Transfer Mode
 CE           Customer Edge
 CES          Circuit Emulation Service
 NSP          Native Service Processing
 PE           Provider Edge
 PDH          Plesiochronous Digital Hierarchy
 PW           Pseudowire
 SDH          Synchronous Digital Hierarchy
 SONET        Synchronous Optical Network
 TDM          Time Division Multiplexing

Vainshtein & Stein Standards Track [Page 3] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 In addition, the following TDM-specific terms are needed:
    o  Loss of Signal (LOS) - a condition of the TDM attachment
       circuit wherein the incoming signal cannot be detected.
       Criteria for entering and leaving the LOS condition can be
       found in [G.775].
    o  Alarm Indication Signal (AIS) - a special bit pattern (e.g., as
       described in [G.775]) in the TDM bit stream that indicates
       presence of an upstream circuit outage.  For E1, T1, and E3
       circuits, the AIS pattern is a sequence of binary "1" values of
       appropriate duration (the "all ones" pattern), and hence it can
       be detected and generated by structure-agnostic means.  The T3
       AIS pattern requires T3 framing (see [G.704], Section
       2.5.3.6.1) and hence can only be handled by a structure-aware
       NSP.
 We also use the term Interworking Function (IWF) to describe the
 functional block that segments and encapsulates TDM into SAToP
 packets and that in the reverse direction decapsulates SAToP packets
 and reconstitutes TDM.

2.2. Reference Models

 The generic models defined in Sections 4.1, 4.2, and 4.4 of [RFC3985]
 fully apply to SAToP.
 The native service addressed in this document is a special case of
 the bit stream payload type defined in Section 3.3.3 of [RFC3985].
 The Network Synchronization reference model and deployment scenarios
 for emulation of TDM services are described in [RFC4197], Section
 4.3.

3. Emulated Services

 This specification describes edge-to-edge emulation of the following
 TDM services described in [G.702]:
    1. E1  (2048 kbit/s)
    2. T1  (1544 kbit/s); this service is also known as DS1
    3. E3 (34368 kbit/s)
    4. T3 (44736 kbit/s); this service is also known as DS3
 The protocol used for emulation of these services does not depend on
 the method in which attachment circuits are delivered to the PEs.
 For example, a T1 attachment circuit is treated in the same way

Vainshtein & Stein Standards Track [Page 4] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 regardless of whether it is delivered to the PE on copper [G.703],
 multiplexed in a T3 circuit [T1.107], mapped into a virtual tributary
 of a SONET/SDH circuit [G.707], or carried over an ATM network using
 unstructured ATM Circuit Emulation Service (CES) [ATM-CES].
 Termination of any specific "carrier layers" used between the PE and
 CE is performed by an appropriate NSP.

4. SAToP Encapsulation Layer

4.1. SAToP Packet Format

 The basic format of SAToP packets is shown in Figure 1 below.
  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 and PW demultiplexing layer headers          |
 |                             ...                               |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                             ...                               |
 +--                                                           --+
 |                   SAToP Encapsulation Header                  |
 +--                                                           --+
 |                             ...                               |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                             ...                               |
 |                        TDM data (Payload)                     |
 |                             ...                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                 Figure 1.  Basic SAToP Packet Format

4.2. PSN and PW Demultiplexing Layer Headers

 Both UDP and L2TPv3 [RFC3931] can provide the PW demultiplexing
 mechanisms for SAToP PWs over an IPv4/IPv6 PSN.  The PW label
 provides the demultiplexing function for an MPLS PSN as described in
 Section 5.4.2 of [RFC3985].
 The total size of a SAToP packet for a specific PW MUST NOT exceed
 path MTU between the pair of PEs terminating this PW.  SAToP
 implementations using IPv4 PSN MUST mark the IPv4 datagrams they
 generate as "Don't Fragment" [RFC791] (see also [PWE3-FRAG]).

Vainshtein & Stein Standards Track [Page 5] RFC 4553 Structure-Agnostic TDM over Packet June 2006

4.3. SAToP Header

 The SAToP header MUST contain the SAToP Control Word (4 bytes) and
 MAY also contain a fixed RTP header [RFC3550].  If the RTP header is
 included in the SAToP header, it MUST immediately follow the SAToP
 control word in all cases except UDP multiplexing, where it MUST
 precede it (see Figures 2a, 2b, and 2c below).
 Note: Such an arrangement complies with the traditional usage of RTP
 for the IPv4/IPv6 PSN with UDP multiplexing while making SAToP PWs
 Equal Cost Multi-Path (ECMP)-safe for the MPLS PSN by providing for
 PW-IP packet discrimination (see [RFC3985], Section 5.4.3).
 Furthermore, it facilitates seamless stitching of L2TPv3-based and
 MPLS-based segments of SAToP PWs (see [PWE3-MS]).
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             ...                               |
 |       IPv4/IPv6 and UDP (PW demultiplexing layer) headers     |
 |                             ...                               |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                                                               |
 +--                     OPTIONAL                              --+
 |                                                               |
 +--               Fixed RTP Header (see [RFC3550])            --+
 |                                                               |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                  SAToP Control Word                           |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                            ...                                |
 |                      TDM data (Payload)                       |
 |                            ...                                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      Figure 2a.  SAToP Packet Format for an IPv4/IPv6 PSN with
                           UDP PW Demultiplexing

Vainshtein & Stein Standards Track [Page 6] RFC 4553 Structure-Agnostic TDM over Packet June 2006

  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             ...                               |
 |     IPv4/IPv6 and L2TPv3 (PW demultiplexing layer) headers    |
 |                             ...                               |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                  SAToP Control Word                           |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                                                               |
 +--                     OPTIONAL                              --+
 |                                                               |
 +--               Fixed RTP Header (see [RFC3550])            --+
 |                                                               |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                             ...                               |
 |                       TDM data (Payload)                      |
 |                             ...                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      Figure 2b.  SAToP Packet Format for an IPv4/IPv6 PSN with
                        L2TPv3 PW Demultiplexing
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                             ...                               |
 |                      MPLS Label Stack                         |
 |                             ...                               |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                  SAToP Control Word                           |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                                                               |
 +--                     OPTIONAL                              --+
 |                                                               |
 +--               Fixed RTP Header (see [RFC3550])            --+
 |                                                               |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |                             ...                               |
 |                       TDM data (Payload)                      |
 |                             ...                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           Figure 2c.  SAToP Packet Format for an MPLS PSN

Vainshtein & Stein Standards Track [Page 7] RFC 4553 Structure-Agnostic TDM over Packet June 2006

4.3.1. Usage and Structure of the Control Word

 Usage of the SAToP control word allows:
    1. Detection of packet loss or misordering
    2. Differentiation between the PSN and attachment circuit problems
       as causes for the outage of the emulated service
    3. PSN bandwidth conservation by not transferring invalid data
       (AIS)
    4. Signaling of faults detected at the PW egress to the PW
       ingress.
 The structure of the SAToP Control Word is shown in Figure 3 below.
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0 0 0 0|L|R|RSV|FRG|   LEN     |       Sequence number         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 3.  Structure of the SAToP Control Word
 The use of Bits 0 to 3 is described in [RFC4385].  These bits MUST be
 set to zero unless they are being used to indicate the start of an
 Associated Channel Header (ACH).  An ACH is needed if the state of
 the SAToP PW is being monitored using Virtual Circuit Connectivity
 Verification [PWE3-VCCV].
 L - If set, indicates that TDM data carried in the payload is invalid
     due to an attachment circuit fault.  When the L bit is set the
     payload MAY be omitted in order to conserve bandwidth.  The CE-
     bound IWF MUST play out an appropriate amount of filler data
     regardless of the payload size.  Once set, if the fault is
     rectified, the L bit MUST be cleared.
 Note: This document does not specify which TDM fault conditions are
 treated as invalidating the data carried in the SAToP packets.
 Possible examples include, but are not limited to LOS and AIS.
 R - If set by the PSN-bound IWF, indicates that its local CE-bound
     IWF is in the packet loss state, i.e., has lost a preconfigured
     number of consecutive packets.  The R bit MUST be cleared by the
     PSN-bound IWF once its local CE-bound IWF has exited the packet
     loss state, i.e., has received a preconfigured number of
     consecutive packets.

Vainshtein & Stein Standards Track [Page 8] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 RSV and FRG (bits 6 to 9) - MUST be set to 0 by the PSN-bound IWF and
     MUST be ignored by the CE-bound IWF.  RSV is reserved.  FRG is
     fragmentation; see [PWE3-FRAG].
 LEN (bits 10 to 15) - MAY be used to carry the length of the SAToP
     packet (defined as the size of the SAToP header + the payload
     size) if it is less than 64 bytes, and MUST be set to zero
     otherwise.  When the LEN field is set to 0, the preconfigured
     size of the SAToP packet payload MUST be assumed to be as
     described in Section 5.1, and if the actual packet size is
     inconsistent with this length, the packet MUST be considered
     malformed.
 Sequence number - used to provide the common PW sequencing function
     as well as detection of lost packets.  It MUST be generated in
     accordance with the rules defined in Section 5.1 of [RFC3550] for
     the RTP sequence number:
       o Its space is a 16-bit unsigned circular space
       o Its initial value SHOULD be random (unpredictable).
     It MUST be incremented with each SAToP data packet sent in the
     specific PW.

4.3.2. Usage of RTP Header

 When RTP is used, the following fields of the fixed RTP header (see
 [RFC3550], Section 5.1) MUST be set to zero: P (padding), X (header
 extension), CC (CSRC count), and M (marker).
 The PT (payload type) field is used as follows:
    1. One PT value MUST be allocated from the range of dynamic values
       (see [RTP-TYPES]) for each direction of the PW.  The same PT
       value MAY be reused for both directions of the PW and also
       reused between different PWs.
    2. The PSN-bound IWF MUST set the PT field in the RTP header to
       the allocated value.
    3. The CE-bound IWF MAY use the received value to detect malformed
       packets.
 The sequence number MUST be the same as the sequence number in the
 SAToP control word.

Vainshtein & Stein Standards Track [Page 9] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 The RTP timestamps are used for carrying timing information over the
 network.  Their values are generated in accordance with the rules
 established in [RFC3550].
 The frequency of the clock used for generating timestamps MUST be an
 integer multiple of 8 kHz.  All implementations of SAToP MUST support
 the 8 kHz clock.  Other multiples of 8 kHz MAY be used.
 The SSRC (synchronization source) value in the RTP header MAY be used
 for detection of misconnections, i.e., incorrect interconnection of
 attachment circuits.
 Timestamp generation MAY be used in the following modes:
    1. Absolute mode: The PSN-bound IWF sets timestamps using the
       clock recovered from the incoming TDM attachment circuit.  As a
       consequence, the timestamps are closely correlated with the
       sequence numbers.  All SAToP implementations that support usage
       of the RTP header MUST support this mode.
    2. Differential mode: Both IWFs have access to a common high-
       quality timing source, and this source is used for timestamp
       generation.  Support of this mode is OPTIONAL.
 Usage of the fixed RTP header in a SAToP PW and all the options
 associated with its usage (the timestamping clock frequency, the
 timestamping mode, selected PT and SSRC values) MUST be agreed upon
 between the two SAToP IWFs during PW setup as described in
 [TDM-CONTROL].  Other, RTP-specific methods (e.g., see [RFC3551])
 MUST NOT be used.

5. SAToP Payload Layer

5.1. General Payloads

 In order to facilitate handling of packet loss in the PSN, all
 packets belonging to a given SAToP PW are REQUIRED to carry a fixed
 number of bytes filled with TDM data received from the attachment
 circuit.  The packet payload size MUST be defined during the PW
 setup, MUST be the same for both directions of the PW, and MUST
 remain unchanged for the lifetime of the PW.
 The CE-bound and PSN-bound IWFs MUST agree on SAToP packet payload
 size during PW setup (default payload size values defined below
 guarantee that such an agreement is always possible).  The SAToP
 packet payload size can be exchanged over the PWE3 control protocol
 ([TDM-CONTROL]) by using the Circuit Emulation over Packet (CEP)/TDM
 Payload Bytes sub-TLV of the Interface Parameters TLV ([RFC4446]).

Vainshtein & Stein Standards Track [Page 10] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 SAToP uses the following ordering for packetization of the TDM data:
    o  The order of the payload bytes corresponds to their order on
       the attachment circuit.
    o  Consecutive bits coming from the attachment circuit fill each
       payload byte starting from most significant bit to least
       significant.
 All SAToP implementations MUST be capable of supporting the following
 payload sizes:
    o  E1 - 256 bytes
    o  T1 - 192 bytes
    o  E3 and T3 - 1024 bytes.
 Notes:
    1. Whatever the selected payload size, SAToP does not assume
       alignment to any underlying structure imposed by TDM framing
       (byte, frame, or multiframe alignment).
    2. When the L bit in the SAToP control word is set, SAToP packets
       MAY omit invalid TDM data in order to conserve PSN bandwidth.
    3. Payload sizes that are multiples of 47 bytes MAY be used in
       conjunction with unstructured ATM-CES [ATM-CES].

5.2. Octet-Aligned T1

 An unstructured T1 attachment circuit is sometimes provided already
 padded to an integer number of bytes, as described in Annex B of
 [G.802].  This occurs when the T1 is de-mapped from a SONET/SDH
 virtual tributary/container, or when it is de-framed by a dual-mode
 E1/T1 framer.
 In order to facilitate operation in such cases, SAToP defines a
 special "octet-aligned T1" transport mode.  In this mode, the SAToP
 payload consists of a number of 25-byte subframes, each subframe
 carrying 193 bits of TDM data and 7 bits of padding.  This mode is
 depicted in Figure 4 below.

Vainshtein & Stein Standards Track [Page 11] RFC 4553 Structure-Agnostic TDM over Packet June 2006

    |     1         |        2      | ...   |      25       |
    |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7| ...   |0 1 2 3 4 5 6 7|
    |=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    |           TDM Data                      |  padding    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            .................................          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           TDM Data                      |  padding    |
    +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    Figure 4.  SAToP Payload Format for Octet-Aligned T1 Transport
 Notes:
 1. No alignment with the framing structure that may be imposed on the
    T1 bit-stream is implied.
 2. An additional advantage of the octet-aligned T1 transport mode is
    the ability to select the SAToP packetization latency as an
    arbitrary integer multiple of 125 microseconds.
 Support of the octet-aligned T1 transport mode is OPTIONAL.  An
 octet-aligned T1 SAToP PW is not interoperable with a T1 SAToP PW
 that carries a non-aligned bit-stream, as described in the previous
 section.
 Implementations supporting octet-aligned T1 transport mode MUST be
 capable of supporting a payload size of 200 bytes (i.e., a payload of
 eight 25-byte subframes) corresponding to precisely 1 millisecond of
 TDM data.

6. SAToP Operation

6.1. Common Considerations

 Edge-to-edge emulation of a TDM service using SAToP is only possible
 when the two PW attachment circuits are of the same type (T1, E1, T3,
 E3).  The service type is exchanged at PW setup as described in
 [RFC4447].

6.2. IWF Operation

6.2.1. PSN-Bound Direction

 Once the PW is set up, the PSN-bound SAToP IWF operates as follows:
 TDM data is packetized using the configured number of payload bytes
 per packet.

Vainshtein & Stein Standards Track [Page 12] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 Sequence numbers, flags, and timestamps (if the RTP header is used)
 are inserted in the SAToP headers.
 SAToP, PW demultiplexing layer, and PSN headers are prepended to the
 packetized service data.
 The resulting packets are transmitted over the PSN.

6.2.2. CE-Bound Direction

 The CE-bound SAToP IWF SHOULD include a jitter buffer where the
 payload of the received SAToP packets is stored prior to play-out to
 the local TDM attachment circuit.  The size of this buffer SHOULD be
 locally configurable to allow accommodation to the PSN-specific
 packet delay variation.
 The CE-bound SAToP IWF SHOULD use the sequence number in the control
 word for detection of lost and misordered packets.  If the RTP header
 is used, the RTP sequence numbers MAY be used for the same purposes.
 Note: With SAToP, a valid sequence number can be always found in bits
 16 - 31 of the first 32-bit word immediately following the PW
 demultiplexing header regardless of the specific PSN type,
 multiplexing method, usage or non-usage of the RTP header, etc.  This
 approach simplifies implementations supporting multiple encapsulation
 types as well as implementation of multi-segment (MS) PWs using
 different encapsulation types in different segments.
 The CE-bound SAToP IWF MAY reorder misordered packets.  Misordered
 packets that cannot be reordered MUST be discarded and treated as
 lost.
 The payload of the received SAToP packets marked with the L bit set
 SHOULD be replaced by the equivalent amount of the "all ones" pattern
 even if it has not been omitted.
 The payload of each lost SAToP packet MUST be replaced with the
 equivalent amount of the replacement data.  The contents of the
 replacement data are implementation-specific and MAY be locally
 configurable.  By default, all SAToP implementations MUST support
 generation of the "all ones" pattern as the replacement data.  Before
 a PW has been set up and after a PW has been torn down, the IWF MUST
 play out the "all ones" pattern to its TDM attachment circuit.
 Once the PW has been set up, the CE-bound IWF begins to receive SAToP
 packets and to store their payload in the jitter buffer but continues
 to play out the "all ones" pattern to its TDM attachment circuit.
 This intermediate state persists until a preconfigured amount of TDM

Vainshtein & Stein Standards Track [Page 13] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 data (usually half of the jitter buffer) has been received in
 consecutive SAToP packets or until a preconfigured intermediate state
 timer (started when the PW setup is completed) expires.
 Once the preconfigured amount of the TDM data has been received, the
 CE-bound SAToP IWF enters its normal operation state where it
 continues to receive SAToP packets and to store their payload in the
 jitter buffer while playing out the contents of the jitter buffer in
 accordance with the required clock.  In this state, the CE-bound IWF
 performs clock recovery, MAY monitor PW defects, and MAY collect PW
 performance monitoring data.
 If the CE-bound SAToP IWF detects loss of a preconfigured number of
 consecutive packets or if the intermediate state timer expires before
 the required amount of TDM data has been received, it enters its
 packet loss state.  While in this state, the local PSN-bound SAToP
 IWF SHOULD mark every packet it transmits with the R bit set.  The
 CE-bound SAToP IWF leaves this state and transitions to the normal
 one once a preconfigured number of consecutive valid SAToP packets
 have been received.  (Successfully reordered packets contribute to
 the count of consecutive packets.)
 The CE-bound SAToP IWF MUST provide an indication of TDM data
 validity to the CE.  This can be done by transporting or by
 generating the native AIS indication.  As mentioned above, T3 AIS
 cannot be detected or generated by structure-agnostic means, and
 hence a structure-aware NSP MUST be used when generating a valid AIS
 pattern.

6.3. SAToP Defects

 In addition to the packet loss state of the CE-bound SAToP IWF
 defined above, it MAY detect the following defects:
    o  Stray packets
    o  Malformed packets
    o  Excessive packet loss rate
    o  Buffer overrun
    o  Remote packet loss
 Corresponding to each defect is a defect state of the IWF, a
 detection criterion that triggers transition from the normal
 operation state to the appropriate defect state, and an alarm that
 MAY be reported to the management system and thereafter cleared.
 Alarms are only reported when the defect state persists for a
 preconfigured amount of time (typically 2.5 seconds) and MUST be

Vainshtein & Stein Standards Track [Page 14] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 cleared after the corresponding defect is undetected for a second
 preconfigured amount of time (typically 10 seconds).  The trigger and
 release times for the various alarms may be independent.
 Stray packets MAY be detected by the PSN and PW demultiplexing
 layers.  When RTP is used, the SSRC field in the RTP header MAY be
 used for this purpose as well.  Stray packets MUST be discarded by
 the CE-bound IWF, and their detection MUST NOT affect mechanisms for
 detection of packet loss.
 Malformed packets are detected by mismatch between the expected
 packet size (taking the value of the L bit into account) and the
 actual packet size inferred from the PSN and PW demultiplexing
 layers.  When RTP is used, lack of correspondence between the PT
 value and that allocated for this direction of the PW MAY also be
 used for this purpose.  Malformed in-order packets MUST be discarded
 by the CE-bound IWF and replacement data generated as with lost
 packets.
 Excessive packet loss rate is detected by computing the average
 packet loss rate over a configurable amount of times and comparing it
 with a preconfigured threshold.
 Buffer overrun is detected in the normal operation state when the
 jitter buffer of the CE-bound IWF cannot accommodate newly arrived
 SAToP packets.
 Remote packet loss is indicated by reception of packets with their R
 bit set.

6.4. SAToP PW Performance Monitoring

 Performance monitoring (PM) parameters are routinely collected for
 TDM services and provide an important maintenance mechanism in TDM
 networks.  The ability to collect compatible PM parameters for SAToP
 PWs enhances their maintenance capabilities.
 Collection of the SAToP PW performance monitoring parameters is
 OPTIONAL and, if implemented, is only performed after the CE-bound
 IWF has exited its intermediate state.
 SAToP defines error events, errored blocks, and defects as follows:
    o  A SAToP error event is defined as insertion of a single
       replacement packet into the jitter buffer (replacement of
       payload of SAToP packets with the L bit set is not considered
       insertion of a replacement packet).

Vainshtein & Stein Standards Track [Page 15] RFC 4553 Structure-Agnostic TDM over Packet June 2006

    o  A SAToP errored data block is defined as a block of data played
       out to the TDM attachment circuit and of a size defined in
       accordance with the [G.826] rules for the corresponding TDM
       service that has experienced at least one SAToP error event.
    o  A SAToP defect is defined as the packet loss state of the
       CE-bound SAToP IWF.
 The SAToP PW PM parameters (Errored, Severely Errored, and
 Unavailable Seconds) are derived from these definitions in accordance
 with [G.826].

7. Quality of Service (QoS) Issues

 SAToP SHOULD employ existing QoS capabilities of the underlying PSN.
 If the PSN providing connectivity between PE devices is Diffserv-
 enabled and provides a PDB [RFC3086] that guarantees low jitter and
 low loss, the SAToP PW SHOULD use this PDB in compliance with the
 admission and allocation rules the PSN has put in place for that PDB
 (e.g., marking packets as directed by the PSN).
 If the PSN is Intserv-enabled, then GS (Guaranteed Service) [RFC2212]
 with the appropriate bandwidth reservation SHOULD be used in order to
 provide a bandwidth guarantee equal or greater than that of the
 aggregate TDM traffic.

8. Congestion Control

 As explained in [RFC3985], the PSN carrying the PW may be subject to
 congestion.  SAToP PWs represent inelastic constant bit-rate (CBR)
 flows and cannot respond to congestion in a TCP-friendly manner
 prescribed by [RFC2914], although the percentage of total bandwidth
 they consume remains constant.
 Unless appropriate precautions are taken, undiminished demand of
 bandwidth by SAToP PWs can contribute to network congestion that may
 impact network control protocols.
 Whenever possible, SAToP PWs SHOULD be carried across traffic-
 engineered PSNs that provide either bandwidth reservation and
 admission control or forwarding prioritization and boundary traffic
 conditioning mechanisms.  IntServ-enabled domains supporting
 Guaranteed Service (GS) [RFC2212] and DiffServ-enabled domains
 [RFC2475] supporting Expedited Forwarding (EF) [RFC3246] provide
 examples of such PSNs.  Such mechanisms will negate, to some degree,
 the effect of the SAToP PWs on the neighboring streams.  In order to
 facilitate boundary traffic conditioning of SAToP traffic over IP

Vainshtein & Stein Standards Track [Page 16] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 PSNs, the SAToP IP packets SHOULD NOT use the DiffServ Code Point
 (DSCP) value reserved for the Default Per-Hop Behavior (PHB)
 [RFC2474].
 If SAToP PWs run over a PSN providing best-effort service, they
 SHOULD monitor packet loss in order to detect "severe congestion".
 If such a condition is detected, a SAToP PW SHOULD shut down bi-
 directionally for some period of time as described in Section 6.5 of
 [RFC3985].
 Note that:
 1. The SAToP IWF can inherently provide packet loss measurement since
    the expected rate of arrival of SAToP packets is fixed and known
 2. The results of the SAToP packet loss measurement may not be a
    reliable indication of presence or absence of severe congestion if
    the PSN provides enhanced delivery.  For example:
    a) If SAToP traffic takes precedence over non-SAToP traffic,
       severe congestion can develop without significant SAToP packet
       loss.
    b) If non-SAToP traffic takes precedence over SAToP traffic, SAToP
       may experience substantial packet loss due to a short-term
       burst of high-priority traffic.
 3. The TDM services emulated by the SAToP PWs have high availability
    objectives (see [G.826]) that MUST be taken into account when
    deciding on temporary shutdown of SAToP PWs.
 This specification does not define the exact criteria for detecting
 "severe congestion" using the SAToP packet loss rate or the specific
 methods for bi-directional shutdown the SAToP PWs (when such severe
 congestion has been detected) and their subsequent re-start after a
 suitable delay.  This is left for further study.  However, the
 following considerations may be used as guidelines for implementing
 the SAToP severe congestion shutdown mechanism:
 1. SAToP Performance Monitoring techniques (see Section 6.4) provide
    entry and exit criteria for the SAToP PW "Unavailable" state that
    make it closely correlated with the "Unavailable" state of the
    emulated TDM circuit as specified in [G.826].  Using the same
    criteria for "severe congestion" detection may decrease the risk
    of shutting down the SAToP PW while the emulated TDM circuit is
    still considered available by the CE.

Vainshtein & Stein Standards Track [Page 17] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 2. If the SAToP PW has been set up using either PWE3 control protocol
    [RFC4447] or L2TPv3 [RFC3931], the regular PW teardown procedures
    of these protocols SHOULD be used.
 3. If one of the SAToP PW end points stops transmission of packets
    for a sufficiently long period, its peer (observing 100% packet
    loss) will necessarily detect "severe congestion" and also stop
    transmission, thus achieving bi-directional PW shutdown.

9. Security Considerations

 SAToP does not enhance or detract from the security performance of
 the underlying PSN; rather, it relies upon the PSN mechanisms for
 encryption, integrity, and authentication whenever required.
 SAToP PWs share susceptibility to a number of pseudowire-layer
 attacks and will use whatever mechanisms for confidentiality,
 integrity, and authentication are developed for general PWs.  These
 methods are beyond the scope of this document.
 Although SAToP PWs MAY employ an RTP header when explicit transfer of
 timing information is required, SRTP (see [RFC3711]) mechanisms are
 NOT RECOMMENDED as a substitute for PW layer security.
 Misconnection detection capabilities of SAToP increase its resilience
 to misconfiguration and some types of denial-of-service (DoS)
 attacks.
 Random initialization of sequence numbers, in both the control word
 and the optional RTP header, makes known-plaintext attacks on
 encrypted SAToP PWs more difficult.  Encryption of PWs is beyond the
 scope of this document.

10. Applicability Statement

 SAToP is an encapsulation layer intended for carrying TDM circuits
 (E1/T1/E3/T3) over PSN in a structure-agnostic fashion.
 SAToP fully complies with the principle of minimal intervention, thus
 minimizing overhead and computational power required for
 encapsulation.
 SAToP provides sequencing and synchronization functions needed for
 emulation of TDM bit-streams, including detection of lost or
 misordered packets and appropriate compensation.

Vainshtein & Stein Standards Track [Page 18] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 TDM bit-streams carried over SAToP PWs may experience delays
 exceeding those typical of native TDM networks.  These delays include
 the SAToP packetization delay, edge-to-edge delay of the underlying
 PSN, and the delay added by the jitter buffer.  It is recommended to
 estimate both delay and delay variation prior to setup of a SAToP PW.
 SAToP carries TDM streams over PSN in their entirety, including any
 TDM signaling contained within the data.  Consequently, the emulated
 TDM services are sensitive to the PSN packet loss.  Appropriate
 generation of replacement data can be used to prevent shutting down
 the CE TDM interface due to occasional packet loss.  Other effects of
 packet loss on this interface (e.g., errored blocks) cannot be
 prevented.
 Note: Structure-aware TDM emulation (see [CESoPSN] or [TDMoIP])
 completely hides effects of the PSN packet loss on the CE TDM
 interface (because framing and Cyclic Redundancy Checks (CRCs) are
 generated locally) and allows usage of application-specific packet
 loss concealment methods to minimize effects on the applications
 using the emulated TDM service.
 SAToP can be used in conjunction with various network synchronization
 scenarios (see [RFC4197]) and clock recovery techniques.  The quality
 of the TDM clock recovered by the SAToP IWF may be implementation-
 specific.  The quality may be improved by using RTP if a common clock
 is available at both ends of the SAToP PW.
 SAToP provides for effective fault isolation by carrying the local
 attachment circuit failure indications.
 The option not to carry invalid TDM data enables PSN bandwidth
 conservation.
 SAToP allows collection of TDM-like faults and performance monitoring
 parameters and hence emulates 'classic' carrier services of TDM.
 SAToP provides for a carrier-independent ability to detect
 misconnections and malformed packets.  This feature increases
 resilience of the emulated service to misconfiguration and DoS
 attacks.
 Being a constant bit rate (CBR) service, SAToP cannot provide TCP-
 friendly behavior under network congestion.
 Faithfulness of a SAToP PW may be increased by exploiting QoS
 features of the underlying PSN.

Vainshtein & Stein Standards Track [Page 19] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 SAToP does not provide any mechanisms for protection against PSN
 outages, and hence its resilience to such outages is limited.
 However, lost-packet replacement and packet reordering mechanisms
 increase resilience of the emulated service to fast PSN rerouting
 events.

11. IANA Considerations

 Allocation of PW Types for the corresponding SAToP PWs is defined in
 [RFC4446].

12. Acknowledgements

 We acknowledge the work of Gil Biran and Hugo Silberman who
 implemented TDM transport over IP in 1998.
 We would like to thank Alik Shimelmits for many productive
 discussions and Ron Insler for his assistance in deploying TDM over
 PSN.
 We express deep gratitude to Stephen Casner who has reviewed in
 detail one of the predecessors of this document and provided valuable
 feedback regarding various aspects of RTP usage, and to Kathleen
 Nichols who has provided the current text of the QoS section
 considering Diffserv-enabled PSN.
 We thank William Bartholomay, Robert Biksner, Stewart Bryant, Rao
 Cherukuri, Ron Cohen, Alex Conta, Shahram Davari, Tom Johnson, Sim
 Narasimha, Yaron Raz, and Maximilian Riegel for their valuable
 feedback.

13. Co-Authors

 The following are co-authors of this document:
 Motty Anavi                 RAD Data Communications
 Tim Frost                   Zarlink Semiconductors
 Eduard Metz                 TNO Telecom
 Prayson Pate                Overture Networks
 Akiva Sadovski
 Israel Sasson               Axerra Networks
 Ronen Shashoua              RAD Data Communications

Vainshtein & Stein Standards Track [Page 20] RFC 4553 Structure-Agnostic TDM over Packet June 2006

14. Normative References

 [G.702]        ITU-T Recommendation G.702 (11/88) - Digital Hierarchy
                Bit Rates.
 [G.703]        ITU-T Recommendation G.703 (10/98) -
                Physical/Electrical Characteristics of Hierarchical
                Digital Interfaces.
 [G.704]        ITU-T Recommendation G.704 (10/98) - Synchronous frame
                structures used at 1544, 6312, 2048, 8448 and 44 736
                Kbit/s hierarchical levels.
 [G.707]        ITU-T Recommendation G.707 (03/96) - Network Node
                Interface for the Synchronous Digital Hierarchy (SDH).
 [G.775]        ITU-T Recommendation G.775 (10/98) - Loss of Signal
                (LOS), Alarm Indication Signal (AIS) and Remote Defect
                Indication (RDI) Defect Detection and Clearance
                Criteria for PDH Signals.
 [G.802]        ITU-T Recommendation G.802 (11/88) - Interworking
                between Networks Based on Different Digital
                Hierarchies and Speech Encoding Laws.
 [G.826]        ITU-T Recommendation G.826 (02/99) - Error performance
                parameters and objectives for international, constant
                bit rate digital paths at or above the primary rate.
 [RFC791]       Postel, J., "Internet Protocol", STD 5, RFC 791,
                September 1981.
 [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2474]      Nichols, K., Blake, S., Baker, F., and D. Black,
                "Definition of the Differentiated Services Field (DS
                Field) in the IPv4 and IPv6 Headers", RFC 2474,
                December 1998.
 [RFC2475]      Blake, S., Black, D., Carlson, M., Davies, E., Wang,
                Z., and W. Weiss, "An Architecture for Differentiated
                Service", RFC 2475, December 1998.
 [RFC2914]      Floyd, S., "Congestion Control Principles", BCP 41,
                RFC 2914, September 2000.

Vainshtein & Stein Standards Track [Page 21] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 [RFC3086]      Nichols, K. and B. Carpenter, "Definition of
                Differentiated Services Per Domain Behaviors and Rules
                for their Specification", RFC 3086, April 2001.
 [RFC3550]      Schulzrinne, H., Casner, S., Frederick, R., and V.
                Jacobson, "RTP: A Transport Protocol for Real-Time
                Applications", STD 64, RFC 3550, July 2003.
 [RFC3931]      Lau, J., Townsley, M., and I. Goyret, "Layer Two
                Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931,
                March 2005.
 [RFC4385]      Bryant, S., Swallow, G., Martini, L., and D.
                McPherson, "Pseudowire Emulation Edge-to-Edge (PWE3)
                Control Word for Use over an MPLS PSN", RFC 4385,
                February 2006.
 [RFC4446]      Martini, L., "IANA Allocations for Pseudowire Edge to
                Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
 [RFC4447]      Martini, L., Rosen, E., El-Aawar, N., Smith, T., and
                G. Heron, "Pseudowire Setup and Maintenance Using the
                Label Distribution Protocol (LDP)", RFC 4447, April
                2006.
 [RTP-TYPES]    RTP PARAMETERS, <http://www.iana.org/assignments/rtp-
                parameters>.
 [T1.107]       American National Standard for Telecommunications -
                Digital Hierarchy - Format Specifications, ANSI
                T1.107-1988.

15. Informative References

 [ATM-CES]      ATM forum specification af-vtoa-0078 (CES 2.0) Circuit
                Emulation Service Interoperability Specification Ver.
                2.0.
 [CESoPSN]      Vainshtein, A., Ed., Sasson, I., Metz, E., Frost, T.,
                and P. Pate, "TDM Circuit Emulation Service over
                Packet Switched Network (CESoPSN)", Work in Progress,
                November 2005.
 [PWE3-MS]      Martini, L., Metz, C., Nadeau, T., Duckett, M., and F.
                Balus, "Segmented Pseudo Wire", Work in Progress,
                March 2006.

Vainshtein & Stein Standards Track [Page 22] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 [PWE3-FRAG]    Malis, A. and M. Townsley, "PWE3 Fragmentation and
                Reassembly", Work in Progress, November 2005.
 [PWE3-VCCV]    Nadeau, T. and R. Aggarwal, "Pseudo Wire Virtual
                Circuit Connectivity", Work in Progress, August 2005.
 [RFC2212]      Shenker, S., Partridge, C., and R. Guerin,
                "Specification of Guaranteed Quality of Service", RFC
                2212, September 1997.
 [RFC3246]      Davie, B., Charny, A., Bennet, J.C., Benson, K., Le
                Boudec, J., Courtney, W., Davari, S., Firoiu, V., and
                D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop
                Behavior)", RFC 3246, March 2002.
 [RFC3551]      Schulzrinne, H. and S. Casner, "RTP Profile for Audio
                and Video Conferences with Minimal Control", STD 65,
                RFC 3551, July 2003.
 [RFC3711]      Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
                K. Norrman, "The Secure Real-time Transport Protocol
                (SRTP)", RFC 3711, March 2004.
 [RFC3916]      Xiao, X., McPherson, D., and P. Pate, "Requirements
                for Pseudo-Wire Emulation Edge-to-Edge (PWE3)", RFC
                3916, September 2004.
 [RFC3985]      Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-
                to-Edge (PWE3) Architecture", RFC 3985, March 2005.
 [RFC4197]      Riegel, M., "Requirements for Edge-to-Edge Emulation
                of Time Division Multiplexed (TDM) Circuits over
                Packet Switching Networks", RFC 4197, October 2005.
 [TDM-CONTROL]  Vainshtein, A. and Y. Stein, "Control Protocol
                Extensions for Setup of TDM Pseudowires", Work in
                Progress, July 2005.
 [TDMoIP]       Stein, Y., "TDMoIP", Work in Progress, February 2005.

Vainshtein & Stein Standards Track [Page 23] RFC 4553 Structure-Agnostic TDM over Packet June 2006

Appendix A: Old Mode of SAToP Encapsulation over L2TPv3

 Previous versions of this specification defined a SAToP PW
 encapsulation over L2TPv3, which differs from that described in
 Section 4.3 and Figure 2b.  In these versions, the RTP header, if
 used, precedes the SAToP control word.
 Existing implementations of the old encapsulation mode MUST be
 distinguished from the encapsulations conforming to this
 specification via the SAToP PW setup.

Appendix B: Parameters That MUST Be Agreed upon during the PW Setup

 The following parameters of the SAToP IWF MUST be agreed upon between
 the peer IWFs during the PW setup.  Such an agreement can be reached
 via manual configuration or via one of the PW setup protocols:
 1. Type of the Attachment Circuit (AC)
    As mentioned in Section 3, SAToP supports the following AC types:
       i)   E1  (2048 kbit/s)
       ii)  T1  (1544 kbit/s); this service is also known as DS1
       iii) E3 (34368 kbit/s)
       iv)  T3 (44736 kbit/s); this service is also known as DS3
    SAToP PWs cannot be established between ACs of different types.
 2. Usage of octet-aligned mode for T1
    a) This OPTIONAL mode of emulating T1 bit-streams with SAToP PWs
       is described in Section 5.2.
    b) Both sides MUST agree on using this mode for a SAToP PW to be
       operational.
 3. Payload size, i.e., the amount of valid TDM data in a SAToP packet
    a) As mentioned in Section 5.1:
       i)  The same payload size MUST be used in both directions of
           the SAToP PW.
       ii) The payload size cannot be changed once the PW has been set
           up.
    b) In most cases, any mutually agreed upon value can be used.
       However, if octet-aligned T1 encapsulation mode is used, the
       payload size MUST be an integral multiple of 25, and it
       expresses the amount of valid TDM data including padding.

Vainshtein & Stein Standards Track [Page 24] RFC 4553 Structure-Agnostic TDM over Packet June 2006

 4. Usage of the RTP header in the encapsulation
    a) Both sides MUST agree on using RTP header in the SAToP PW.
    b) In the case of a SAToP PW over L2TPv3 using the RTP header,
       both sides MUST agree on usage of the "old mode" described in
       Appendix A.
 5. RTP-dependent parameters.  The following parameters MUST be agreed
    upon if usage of the RTP header for the SAToP PW has been agreed
    upon.
    a) Timestamping mode (absolute or differential); this mode MAY be
       different for the two directions of the PW, but the receiver
       and transmitter MUST agree on the timestamping mode for each
       direction of the PW
    b) Timestamping clock frequency:
       i)  The timestamping frequency MUST be a integral multiple of 8
           kHz.
       ii) The timestamping frequency MAY be different for the two
           directions of the PW, but the receiver and transmitter MUST
           agree on the timestamping mode for each direction of the
           PW.
    c) RTP Payload Type (PT) value; any dynamically assigned value can
       be used with SAToP PWs.
    d) Synchronization Source (SSRC) value; the transmitter MUST agree
       to send the SSRC value requested by the receiver.

Vainshtein & Stein Standards Track [Page 25] RFC 4553 Structure-Agnostic TDM over Packet June 2006

Editors' Addresses

 Alexander ("Sasha") Vainshtein
 Axerra Networks
 24 Raoul Wallenberg St.,
 Tel Aviv 69719, Israel
 EMail: sasha@axerra.com
 Yaakov (Jonathan) Stein
 RAD Data Communications
 24 Raoul Wallenberg St., Bldg C
 Tel Aviv 69719, Israel
 EMail: yaakov_s@rad.com

Vainshtein & Stein Standards Track [Page 26] RFC 4553 Structure-Agnostic TDM over Packet June 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 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.
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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Acknowledgement

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Vainshtein & Stein Standards Track [Page 27]

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