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RFC 5086 - Structure-Aware Time Division Multiplexed (TDM) Circu


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Network Working Group                                 A. Vainshtein, Ed.
Request for Comments: 5086                                     I. Sasson
Category: Informational                                  Axerra Networks
                                                                 E. Metz
                                                                     KPN
                                                                T. Frost
                                                   Zarlink Semiconductor
                                                                 P. Pate
                                                       Overture Networks
                                                           December 2007

   Structure-Aware Time Division Multiplexed (TDM) Circuit Emulation
            Service over Packet Switched Network (CESoPSN)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   This document describes a method for encapsulating structured (NxDS0)
   Time Division Multiplexed (TDM) signals as pseudowires over packet-
   switching networks (PSNs).  In this regard, it complements similar
   work for structure-agnostic emulation of TDM bit-streams (see RFC
   4553).

 Table of Contents

   1. Introduction ....................................................3
   2. Terminology and Reference Models ................................3
      2.1. Terminology ................................................3
      2.2. Reference Models ...........................................4
      2.3. Requirements and Design Constraint .........................4
   3. Emulated Services ...............................................5
   4. CESoPSN Encapsulation Layer .....................................6
      4.1. CESoPSN Packet Format ......................................6
      4.2. PSN and Multiplexing Layer Headers .........................8
      4.3. CESoPSN Control Word .......................................9
      4.4. Usage of the RTP Header ...................................11
   5. CESoPSN Payload Layer ..........................................12
      5.1. Common Payload Format Considerations ......................12
      5.2. Basic NxDS0 Services ......................................13
      5.3. Extending Basic NxDS0 Services with CE Application
           Signaling .................................................15
      5.4. Trunk-Specific NxDS0 Services with CAS ....................18
   6. CESoPSN Operation ..............................................20
      6.1. Common Considerations .....................................20
      6.2. IWF Operation .............................................20
           6.2.1. PSN-Bound Direction ................................20
           6.2.2. CE-Bound Direction .................................20
      6.3. CESoPSN Defects ...........................................23
      6.4. CESoPSN PW Performance Monitoring .........................24
   7. QoS Issues .....................................................25
   8. Congestion Control .............................................25
   9. Security Considerations ........................................27
   10. IANA Considerations ...........................................27
   11. Applicability Statement .......................................27
   12. Acknowledgements ..............................................29
   13. Normative References ..........................................30
   14. Informative References ........................................31
   Appendix A. A Common CE Application State Signaling Mechanism .....33
   Appendix B. Reference PE Architecture for Emulation of NxDS0
       Services ......................................................34
   Appendix C. Old Mode of CESoPSN Encapsulation Over L2TPV3 .........36

1.  Introduction

   This document describes a method for encapsulating structured (NxDS0)
   Time Division Multiplexed (TDM) signals as pseudowires over packet-
   switching networks (PSN).  In this regard, it complements similar
   work for structure-agnostic emulation of TDM bit-streams [RFC4553].

   Emulation of NxDS0 circuits provides for saving PSN bandwidth, and
   supports DS0-level grooming and distributed cross-connect
   applications.  It also enhances resilience of CE devices to effects
   of loss of packets in the PSN.

   The CESoPSN solution presented in this document fits the Pseudowire
   Emulation Edge-to-Edge (PWE3) architecture described in [RFC3985],
   satisfies the general requirements put forth in [RFC3916], and
   specific requirements for structured TDM emulation put forth in
   [RFC4197].

2.  Terminology and Reference Models

2.1.  Terminology

   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].

   The terms defined in [RFC3985], Section 1.4, and in [RFC4197],
   Section 3, are consistently used without additional explanations.

   This document uses some terms and acronyms that are commonly used in
   conjunction with TDM services.  In particular:

   o  Loss of Signal (LOS) is a common term denoting a condition where a
      valid TDM signal cannot be extracted from the physical layer of
      the trunk.  Actual criteria for detecting and clearing LOS are
      described in [G.775].

   o  Frame Alignment Signal (FAS) is a common term denoting a special
      periodic pattern that is used to impose TDM structures on E1 and
      T1 circuits.  These patterns are described in [G.704].

   o  Out of Frame Synchronization (OOF) is a common term denoting the
      state of the receiver of a TDM signal when it failed to find valid
      FAS.  Actual criteria for declaring and clearing OOF are described
      in [G.706].  Handling of this condition includes invalidation of
      the TDM data.

   o  Alarm Indication Signal (AIS) is a common term denoting a special
      bit pattern in the TDM bit stream that indicates presence of an
      upstream circuit outage.  Actual criteria for declaring and
      clearing the AIS condition in a TDM stream are defined in [G.775].

   o  Remote Alarm Indication (RAI) and Remote Defect Indication (RDI)
      are common terms (often used as synonyms) denoting a special
      pattern in the framing of a TDM service that is sent back by the
      receiver that experiences an AIS condition.  This condition cannot
      be detected while an LOS, OOF, or AIS condition is detected.
      Specific rules for encoding this pattern in the TDM framing are
      discussed in [G.775].

   We also use the term Interworking Function (IWF) to describe the
   functional block that segments and encapsulates TDM into CESoPSN
   packets and, in the reverse direction, decapsulates CESoPSN packets
   and reconstitutes TDM.

2.2.  Reference Models

   Generic models that have been defined in Sections 4.1, 4.2, and 4.4
   of [RFC3985] are fully applicable for the purposes of this document
   without any modifications.

   The Network Synchronization reference model and deployment scenarios
   for emulation of TDM services have been described in [RFC4197],
   Section 4.3.

   Structured services considered in this document represent special
   cases of the "Structured bit stream" payload type defined in Section
   3.3.4 of [RFC3985].  In each specific case, the basic service
   structures that are preserved by a CESoPSN PW are explicitly
   specified (see Section 3 below).

   In accordance with the principle of minimum intervention ([RFC3985],
   Section 3.3.5), the TDM payload is encapsulated without any changes.

2.3.  Requirements and Design Constraints

   The CESoPSN protocol has been designed in order to meet the following
   design constraints:

   1.  Fixed amount of TDM data per packet: All the packets belonging to
       a given CESoPSN PW MUST carry the same amount of TDM data.  This
       approach simplifies compensation of a lost PW packet with a
       packet carrying exactly the same amount of "replacement" TDM data

   2.  Fixed end-to-end delay: CESoPSN implementations SHOULD provide
       the same end-to-end delay between a given pair of CEs regardless
       of the bit rate of the emulated service.

   3.  Packetization latency range: a) All the implementations of
       CESoPSN SHOULD support packetization latencies in the range 1 to
       5 milliseconds. b) CESoPSN implementations that support
       configurable packetization latency MUST allow configuration of
       this parameter with the granularity, which is a multiple of 125
       microseconds.

   4.  Common data path for services with and without CE application
       signaling (e.g., Channel-Associated Signaling (CAS)-- see
       [RFC4197]): If, in addition to TDM data, CE signaling must be
       transferred between a pair of CE devices for the normal operation
       of the emulated service, this signaling is passed in dedicated
       signaling packets specific for the signaling protocol while
       format and processing of the packets carrying TDM data remain
       unchanged.

3.  Emulated Services

   In accordance with [RFC4197], structured services considered in this
   specification are NxDS0 services, with and without CAS.

   NxDS0 services are usually carried within appropriate physical
   trunks, and Provider Edges (PEs) providing their emulation include
   appropriate Native Service Processing (NSP) blocks, commonly referred
   to as Framers.

   The NSPs may also act as digital cross-connects, creating structured
   TDM services from multiple synchronous trunks.  As a consequence, the
   service may contain more timeslots that could be carried over any
   single trunk, or the timeslots may not originate from any single
   trunk.

   The reference PE architecture supporting these services is described
   in Appendix B.

   This document defines a single format for packets carrying TDM data
   regardless of the need to carry CAS or any other CE application
   signaling.  The resulting "basic NxDS0 service" can be extended to
   carry CE application signaling (e.g., CAS) using separate signaling
   packets.  Signaling packets MAY be carried in the same PW as the
   packets carrying TDM data or in a separate dedicated PW.

   In addition, this document also defines dedicated formats for
   carrying NxDS0 services with CAS in signaling sub-structures in some
   of the packets.  These formats effectively differ for NxDS0 services
   that originated in different trunks so that their usage results in
   emulating trunk-specific NxDS0 services with CAS.

4.  CESoPSN Encapsulation Layer

4.1.  CESoPSN Packet Format

   The CESoPSN header MUST contain the CESoPSN Control Word (4 bytes)
   and MAY also contain a fixed RTP header [RFC3550].  If the RTP header
   is included in the CESoPSN header, it MUST immediately follow the
   CESoPSN control word in all cases except UDP demultiplexing, where it
   MUST precede it (see Figures 1a, 1b, and 1c below).

   Note: The difference in the CESoPSN packet formats for IP PSN using
   UDP-based demultiplexing and the rest of the PSN and demultiplexing
   combinations, is based on the following considerations:

   1.  Compliance with the existing header compression mechanisms for
       IPv4/IPv6 PSNs with UDP demultiplexing requires placing the RTP
       header immediately after the UDP header.

   2.  Compliance with the common PWE3 mechanisms for keeping PWs Equal
       Cost Multipath (ECMP)-safe for the MPLS PSN by providing for PW-
       IP packet discrimination (see [RFC3985], Section 5.4.3).  This
       requires placing the PWE3 control word immediately after the PW
       label.

   3.  Commonality of the CESoPSN packet formats for MPLS networks and
       IPv4/IPv6 networks with Layer 2 Tunneling Protocol Version 3
       (L2TPv3) demultiplexing facilitates smooth stitching of L2TPv3-
       based and MPLS-based segments of CESoPSN 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 (demultiplexing layer) headers       |
       |                           ...                                 |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                       OPTIONAL                                |
       +--                                                           --+
       |                                                               |
       +--                                                           --+
       |                 Fixed RTP Header (see [RFC3550])              |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                  CESoPSN Control Word                         |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                Packetized TDM data (Payload)                  |
       |                            ...                                |
       |                            ...                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 1a.  CESoPSN Packet Format for an IPv4/IPv6 PSN with
                              UDP 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                           |
       |                           ...                                 |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                  CESoPSN Control Word                         |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                       OPTIONAL                                |
       +--                                                           --+
       |                                                               |
       +--                                                           --+
       |                 Fixed RTP Header (see [RFC3550])              |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                  Packetized TDM data (Payload)                |
       |                            ...                                |
       |                            ...                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 1b.  CESoPSN Packet Format for an MPLS PSN

       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 (demultiplexing layer) headers   |
       |                           ...                                 |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                  CESoPSN Control Word                         |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                       OPTIONAL                                |
       +--                                                           --+
       |                                                               |
       +--                                                           --+
       |                 Fixed RTP Header (see [RFC3550])              |
       +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
       |                   Packetized TDM data (Payload)               |
       |                            ...                                |
       |                            ...                                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 1c.  CESoPSN Packet Format for an IPv4/IPv6 PSN with
                            L2TPv3 Demultiplexing

4.2.  PSN and Multiplexing Layer Headers

   The total size of a CESoPSN packet for a specific PW MUST NOT exceed
   path MTU between the pair of PEs terminating this PW.

   CESoPSN implementations working with IPv4 PSN MUST set the "Don't
   Fragment" flag in IP headers of the packets they generate.

   Usage of MPLS and L2TPv3 as demultiplexing layers is explained in
   [RFC3985] and [RFC3931], respectively.

   Setup and maintenance of CESoPSN PWs over MPLS PSN is described in
   [PWE3-TDM-CONTROL].

   Setup and maintenance of CESoPSN PWs over IPv4/IPv6 using L2TPv3
   demultiplexing is defined in [L2TPEXT-TDM].

   The destination UDP port MUST be used to multiplex and demultiplex
   individual PWs between nodes.  Architecturally (see [RFC3985]) this
   makes the destination UDP port act as the PW Label.

   UDP ports MUST be manually configured by both endpoints of the PW.
   The configured destination port together with both the source and
   destination IP addresses uniquely identifies the PW for the receiver.
   All UDP port values that function as PW labels SHOULD be in the range
   of dynamically allocated UDP port numbers (49152 through 65535).

   While many UDP-based protocols are able to traverse middleboxes
   without dire consequences, the use of UDP ports as PW labels makes
   middlebox traversal more difficult.  Hence, it is NOT RECOMMENDED to
   use UDP-based PWs where port-translating middleboxes are present
   between PW endpoints.

4.3.  CESoPSN Control Word

   The structure of the CESoPSN Control Word that MUST be used with all
   combinations of the PSN and demultiplexing mechanisms described in
   the previous section is shown in Figure 2 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| M |FRG|   LEN     |       Sequence number         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 2.  Structure of the CESoPSN 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 CESoPSN PW is being monitored using Virtual Circuit Connectivity
   Verification [RFC5085].

   L - if set, indicates some abnormal condition of the attachment
       circuit.

   M - a 2-bit modifier field.  In case of L cleared, this field allows
       discrimination of signaling packets and carrying RDI of the
       attachment circuit across the PSN.  In case of L set, only the
       '00' value is currently defined; other values are reserved for
       future extensions.  L and M bits can be treated as a 3-bit code
       point space that is described in detail in Table 1 below.

   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 pre-configured
       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 pre-configured number of
       consecutive packets.

  |=================================================================|
  | L |  M  |               Code Point Interpretation               |
  |===|=====|=======================================================|
  | 0 | 00  | CESoPSN data packet - normal situation.  All CESoPSN  |
  |   |     | implementations MUST recognize this code point.       |
  |   |     | Payload MUST be played out "as received".             |
  |---|-----|-------------------------------------------------------|
  | 0 | 01  | Reserved for future extensions.                       |
  |---|-----|-------------------------------------------------------|
  | 0 | 10  | CESoPSN data packet, RDI condition of the AC.  All    |
  |   |     | CESoPSN implementations MUST support this codepoint:  |
  |   |     | payload MUST be played out "as received", and, if      |
  |   |     | so configured, the receiving CESoPSN IWF instance     |
  |   |     | SHOULD be able to command the NSP to force the RDI    |
  |   |     | condition on the outgoing TDM trunk.                  |
  |---|-----|-------------------------------------------------------|
  | 0 | 11  | Reserved for CESoPSN signaling packets.               |
  |---|-----|-------------------------------------------------------|
  | 1 | 00  | TDM data is invalid; payload MAY be omitted.  All     |
  |   |     | implementations MUST recognize this code point and    |
  |   |     | insert appropriate amount of the configured "idle     |
  |   |     | code" in the outgoing attachment circuit. In addition,|
  |   |     | if so configured, the receiving CESoPSN IWF instance  |
  |   |     | SHOULD be able to force the AIS condition on the      |
  |   |     | outgoing TDM trunk.                                   |
  |---|-----|-------------------------------------------------------|
  | 1 | 01  | Reserved for future extensions                        |
  |---|-----|-------------------------------------------------------|
  | 1 | 10  | Reserved for future extensions                        |
  |---|-----|-------------------------------------------------------|
  | 1 | 11  | Reserved for future extensions                        |
  |=================================================================|

       Table 1.  Interpretation of bits L and M in the CESoPSN CW

   Notes:

   1.  Bits in the M field are shown in the same order as in Figure 2
       (i.e., bit 6 of the CW followed by bit 7 of the CW).

   2.  Implementations that do not support the reserved code points MUST
       silently discard the corresponding packets upon reception.

   The FRG bits in the CESoPSN control word MUST be cleared for all
   services, excluding trunk-specific NxDS0 with CAS.  In case of these
   services, they MAY be used to denote fragmentation of the multiframe
   structures between CESoPSN packets as described in [RFC4623]; see
   Section 5.4 below.

   LEN (bits (10 to 15) MAY be used to carry the length of the CESoPSN
   packet (defined as the size of the CESoPSN header + the payload size)
   if it is less than 64 bytes, and MUST be set to zero otherwise.
   Note:  If fixed RTP header is used in the encapsulation, it is
   considered part of the CESoPSN header.

   The sequence number is 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, i.e.:

   o Its space is a 16-bit unsigned circular space

   o Its initial value SHOULD be random (unpredictable)

   o It MUST be incremented with each CESoPSN data packet sent in the
     specific PW.

4.4.  Usage of the RTP Header

   Although CESoPSN MAY employ an RTP header when explicit transfer of
   timing information is required, this is purely formal reuse of the
   header format.  RTP mechanisms, such as header extensions,
   contributing source (CSRC) list, padding, RTP Control Protocol
   (RTCP), RTP header compression, Secure RTP (SRTP), etc., are not
   applicable to CESoPSN pseudowires.

   When a fixed RTP header (see [RFC3550], Section 5.1) is used with
   CESoPSN, its fields are used in the following way:

   1.  V (version) is always set to 2.

   2.  P (padding), X (header extension), CC (CSRC count), and M
       (marker) are always set to 0.

   3.  PT (payload type) is used as following:

       a) 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.

       b) The PE at the PW ingress MUST set the PT field in the RTP
          header to the allocated value.

       c) The PE at the PW egress MAY use the received value to detect
          malformed packets.

   4.  Sequence number in the RTP header MUST be equal to the sequence
       number in the CESoPSN CW.

   5.  Timestamps are used for carrying timing information over the
       network:

       a) Their values are generated in accordance with the rules
          established in [RFC3550].

       b) Frequency of the clock used for generating timestamps MUST be
          an integer multiple of 8 kHz.  All implementations of CESoPSN
          MUST support the 8 kHz clock.  Other frequencies that are
          integer multiples of 8 kHz MAY be used if both sides agree to
          that.

       c) Possible modes of timestamp generation are discussed below.

   6.  The SSRC (synchronization source) value in the RTP header MAY be
       used for detection of misconnections.

   The RTP header in CESoPSN can be used in conjunction with at least
   the following modes of timestamp generation:

   1.  Absolute mode: the ingress PE sets timestamps using the clock
       recovered from the incoming TDM circuit.  As a consequence, the
       timestamps are closely correlated with the sequence numbers.  All
       CESoPSN implementations MUST support this mode.

   2.  Differential mode: PE devices connected by the PW have access to
       the same high-quality synchronization source, and this
       synchronization source is used for timestamp generation.  As a
       consequence, the second derivative of the timestamp series
       represents the difference between the common timing source and
       the clock of the incoming TDM circuit.  Support of this mode is
       OPTIONAL.

5.  CESoPSN Payload Layer

5.1.  Common Payload Format Considerations

   All the services considered in this document are treated as sequences
   of "basic structures" (see Section 3 above).  The payload of a
   CESoPSN packet always consists of a fixed number of octets filled,
   octet by octet, with the data contained in the corresponding
   consequent basic structures that preserve octet alignment between
   these structures and the packet payload boundaries, in accordance
   with the following rules:

   1.  The order of the payload octets corresponds to their order on the
       TDM AC.

   2.  Consecutive bits coming from the TDM AC fill each payload octet,
       starting from its most significant bit to the least significant
       one.

   3.  All the CESoPSN packets MUST carry the same amount of valid TDM
       data in both directions of the PW.  In other words, the time that
       is required to fill a CESoPSN packet with the TDM data must be
       constant.  The PE devices terminating a CESoPSN PW MUST agree on
       the number of TDM payload octets in the PW packets for both
       directions of the PW at the time of the PW setup.

   Notes:

   1.  CESoPSN packets MAY omit invalid TDM data in order to save the
       PSN bandwidth.  If the CESoPSN packet payload is omitted, the L
       bit in the CESoPSN control word MUST be set.

   2.  CESoPSN PWs MAY carry CE signaling information either in separate
       packets or appended to packets carrying valid TDM data.  If
       signaling information and valid TDM data are carried in the same
       CESoPSN packet, the amount of the former does not affect the
       amount of the latter.

5.2.  Basic NxDS0 Services

   As mentioned above, the basic structure preserved across the PSN for
   this service consists of N octets filled with the data of the
   corresponding NxDS0 channels belonging to the same frame of the
   originating trunk(s), and the service generates 8000 such structures
   per second.

   CESoPSN MUST use alignment of the basic structures with the packet
   payload boundaries in order to carry the structures across the PSN.
   This means that:

   1.  The amount of TDM data in a CESoPSN packet MUST be an integer
       multiple of the basic structure size

   2.  The first structure in the packet MUST start immediately at the
       beginning of the packet payload.

   The resulting payload format is shown in Figure 3 below.

                         0 1 2 3 4 5 6 7
                    --- +-+-+-+-+-+-+-+-+
                        |   Timeslot 1  |
                        +-+-+-+-+-+-+-+-+
                        |   Timeslot 2  |
           Frame #1     |      ...      |
                        |   Timeslot N  |
                    --- +-+-+-+-+-+-+-+-+
                        |   Timeslot 1  |
                        +-+-+-+-+-+-+-+-+
                        |   Timeslot 2  |
           Frame #2     |      ...      |
                        |   Timeslot N  |
                    --- +-+-+-+-+-+-+-+-+
           ...          |    ...        |
                    --- +-+-+-+-+-+-+-+-+
                        |   Timeslot 1  |
                        +-+-+-+-+-+-+-+-+
                        |   Timeslot 2  |
           Frame #m     |      ...      |
                        |   Timeslot N  |
                    --- +-+-+-+-+-+-+-+-+

           Figure 3.  The CESoPSN Packet Payload Format for the
                           Basic NxDS0 Service

   This mode of operation complies with the recommendation in [RFC3985]
   to use similar encapsulations for structured bit stream and cell
   generic payload types.

   Packetization latency, number of timeslots, and payload size are
   linked by the following obvious relationship:

   L = 8*N*D

   where:

   o  D is packetization latency, milliseconds

   o  L is packet payload size, octets

   o  N is number of DS0 channels.

   CESoPSN implementations supporting NxDS0 services MUST support the
   following set of configurable packetization latency values:

   o  For N = 1: 8 milliseconds (with the corresponding packet payload
      size of 64 bytes)

   o  For 2 <=N <= 4: 4 millisecond (with the corresponding packet
      payload size of 32*N bytes)

   o  For N >= 5: 1 millisecond (with the corresponding packet payload
      size of 8*N octets).

   Support of 5 ms packetization latency for N = 1 is RECOMMENDED.

   Usage of any other packetization latency (packet payload size) that
   is compatible with the restrictions described above is OPTIONAL.

5.3.  Extending Basic NxDS0 Services with CE Application Signaling

   Implementations that have chosen to extend the basic NxDS0 service to
   support CE application state signaling carry-encoded CE application
   state signals in separate signaling packets.

   The format of the CESoPSN signaling packets over both IPv4/IPv6 and
   MPLS PSNs for the case when the CE maintains a separate application
   state per DS0 channel (e.g., CAS for the telephony applications) is
   shown in Figures 4a and 4b below, respectively.

   Signaling packets SHOULD be carried in a separate dedicated PW.
   However, implementations MAY carry them in the same PW as the TDM
   data packets for the basic NxDS0 service.  The methods of "pairing"
   the PWs carrying TDM data and signaling packets for the same extended
   NxDS0 service are out of scope of this document.

   Regardless of the way signaling packets are carried across the PSN,
   the following rules apply:

   1.  The CESoPSN signaling packets MUST:

       a) Use their own sequence numbers in the control word

       b) Set the flags in the control word like following:

          i)   L = 0

          ii)  M = '11'

          iii) R = 0

   2.  If an RTP header is used in the data packets, it MUST be also
       used in the signaling packets with the following restrictions:

       a) An additional RTP payload type (from the range of dynamically
          allocated types) MUST be allocated for the signaling packets.

       b) In addition, the signaling packets MUST use their own SSRC
          value.

   The protocol used to assure reliable delivery of signaling packets is
   discussed in Appendix A.

   Encoding of CE application state for telephony applications using CAS
   follows [RFC2833] (which has since been obsoleted by [RFC4733] and
   [RFC4734], but they do not affect the relevant text).

   Encoding of CE application state for telephony application using CCS
   will be considered in a separate document.

     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 multiplexing layer headers         |
    |                           ...                                 |
    +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    |                OPTIONAL Fixed                                 |
    +--                                                           --+
    |                        RTP                                    |
    +--                                                           --+
    |                  Header (see [RFC3550])                       |
    +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    |                  CESoPSN Control Word                         |
    +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    | Encoded CE application state entry for the DS0 channel #1     |
    +--                                                           --+
    |                         ...                                   |
    +--                                                           --+
    | Encoded CE application state entry for the DS0 channel #N     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4a.  CESoPSN Signaling Packet Format over an IPv4/IPv6 PSN

     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                       |
    |                           ...                                 |
    +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    |                  CESoPSN Control Word                         |
    +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    |                    OPTIONAL Fixed                             |
    +--                                                           --+
    |                        RTP                                    |
    +--                                                           --+
    |                  Header (see [RFC3550])                       |
    +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
    | Encoded CE application state entry for the DS0 channel #1     |
    +--                                                           --+
    |                         ...                                   |
    +--                                                           --+
    | Encoded CE application state entry for the DS0 channel #N     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 4b.  CESoPSN Signaling Packet Format over an MPLS PSN

5.4.  Trunk-Specific NxDS0 Services with CAS

   The structure preserved by CESoPSN for this group of services is the
   trunk multiframe sub-divided into the trunk frames, and signaling
   information is carried appended to the TDM data using the signaling
   substructures defined in [ATM-CES].  These substructures comprise N
   consecutive nibbles, so that the i-th nibble carries CAS bits for the
   i-th DS0 channel, and are padded with a dummy nibble for odd values
   of N.

   CESoPSN implementations supporting trunk-specific NxDS0 services with
   CAS MUST NOT carry more TDM data per packet than is contained in a
   single trunk multiframe.

   All CESoPSN implementations supporting trunk-specific NxDS0 with CAS
   MUST support the default mode, where a single CESoPSN packet carries
   exactly the amount of TDM data contained in exactly one trunk
   multiframe and appended with the signaling sub-structure.  The TDM
   data is aligned with the packet payload.  In this case:

   1.  Packetization latency is:

       a) 2 milliseconds for E1 NxDS0

       b) 3 milliseconds for T1 NxDS0

   2.  The packet payload size is:

       a) 16*N + floor((N+1)/2) for E1-NxDS0

       b) 24*N + floor((N+1)/2) for T1/ESF-NxDS0 and T1/SF- NxDS0

   3.  The packet payload format coincides with the multiframe structure
       described in [ATM-CES] (Section 2.3.1.2).

   In order to provide lower packetization latency, CESoPSN
   implementations for trunk-specific NxDS0 with CAS SHOULD support
   fragmentation of multiframe structures between multiple CESoPSN
   packets. In this case:

   1.  The FRG bits MUST be used to indicate first, intermediate, and
       last fragment of a multiframe as described in [RFC4623].

   2.  The amount of the TDM data per CESoPSN packet must be constant.

   3.  Each multiframe fragment MUST comprise an integer multiple of the
       trunk frames.

   4.  The signaling substructure MUST be appended to the last fragment
       of each multiframe.

   Format of CESoPSN packets carrying trunk-specific NxDS0 service with
   CAS that do and do not contain signaling substructures is shown in
   Figures 5 (a) and (b), respectively.  In these figures, the number of
   the trunk frames per multiframe fragment ("m") MUST be an integer
   divisor of the number of frames per trunk multiframe.

                  0 1 2 3 4 5 6 7                   0 1 2 3 4 5 6 7
             --- +-+-+-+-+-+-+-+-+             --- +-+-+-+-+-+-+-+-+
                 |   Timeslot 1  |                 |   Timeslot 1  |
                 +-+-+-+-+-+-+-+-+                 +-+-+-+-+-+-+-+-+
                 |   Timeslot 2  |                 |   Timeslot 2  |
    Frame #1     |      ...      |       Frame #1  |      ...      |
                 |   Timeslot N  |                 |   Timeslot N  |
             --- +-+-+-+-+-+-+-+-+             --- +-+-+-+-+-+-+-+-+
                 |   Timeslot 1  |                 |   Timeslot 1  |
                 +-+-+-+-+-+-+-+-+                 +-+-+-+-+-+-+-+-+
                 |   Timeslot 2  |       Frame #2  |   Timeslot 2  |
    Frame #2     |      ...      |                 |      ...      |
                 |   Timeslot N  |                 |   Timeslot N  |
             --- +-+-+-+-+-+-+-+-+             --- +-+-+-+-+-+-+-+-+
    ...          |    ...        |                 |     ...       |
             --- +-+-+-+-+-+-+-+-+             --- +-+-+-+-+-+-+-+-+
                 |   Timeslot 1  |                 |   Timeslot 1  |
                 +-+-+-+-+-+-+-+-+                 +-+-+-+-+-+-+-+-+
                 |   Timeslot 2  |                 |   Timeslot 2  |
    Frame #m     |      ...      |        Frame #m |      ...      |
                 |   Timeslot N  |                 |   Timeslot N  |
             --- +-+-+-+-+-+-+-+-+             --- +-+-+-+-+-+-+-+-+
    Nibbles 1,2  |A B C D|A B C D|
                 +-+-+-+-+-+-+-+-+
    Nibbles 3,4  |A B C D|A B C D|
                 +-+-+-+-+-+-+-+-+
    Nibble n     |A B C D| (pad) |
    (odd) & pad  +-+-+-+-+-+-+-+-+

                (a) The packet with             (b) The packet without
                the signaling structure         the signaling structure
                (the last fragment of           (not the last fragment
                the multiframe)                  of the multiframe)

            Figure 5.  The CESoPSN Packet Payload Format for
                      Trunk-Specific NxDS0 with CAS

   Notes:

   1.  In case of T1-NxDS0 with CAS, the signaling bits are carried in
       the TDM data, as well as in the signaling substructure.  However,
       the receiver MUST use the CAS bits as carried in the signaling
       substructures.

   2.  In case of trunk-specific NxDS0 with CAS originating in a T1-SF
       trunk, each nibble of the signaling substructure contains A and B
       bits from two consecutive trunk multiframes as described in
       [ATM-CES].

6.  CESoPSN Operation

6.1.  Common Considerations

   Edge-to-edge emulation of a TDM service using CESoPSN is only
   possible when the two PW attachment circuits are of the same type
   (basic NxDS0 or one of the trunk-specific NxDS0 with CAS) and bit
   rate.  The service type and bit rate are 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 CESoPSN IWF operates as follows:

   TDM data is packetized using the configured number of payload bytes
   per packet.

   Sequence numbers, flags, and timestamps (if the RTP header is used)
   are inserted in the CESoPSN headers and, for trunk-specific NxDS0
   with CAS, signaling substructures are appended to the packets
   carrying the last fragment of a multiframe.

   CESoPSN, multiplexing 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 CESoPSN IWF SHOULD include a jitter buffer where payload
   of the received CESoPSN 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 CESoPSN IWF MUST detect lost and misordered packets.  It
   SHOULD use the sequence number in the control word for these purposes
   but, if the RTP header is used, the RTP sequence number MAY be used
   instead.

   The CE-bound CESoPSN IWF MAY reorder misordered packets.  Misordered
   packets that cannot be reordered MUST be discarded and treated as
   lost.

   The payload of the received CESoPSN data packets marked with the L
   bit set SHOULD be replaced by the equivalent amount of some locally
   configured "idle" bit pattern even if it has not been omitted.  In
   addition, the CE-bound CESoPSN IWF will be locally configured to
   command its local NSP to perform one of the following actions:

   o  None (MUST be supported by all the implementations)

   o  Transmit the AIS pattern towards the local CE on the E1 or T1
      trunk carrying the local attachment circuit (support of this
      action is RECOMMENDED)

   o  Send the "Channel Idle" signal to the local CE for all the DS0
      channels comprising the local attachment circuit (support of this
      action is OPTIONAL).

   If the data packets received are marked with L bit cleared and M bits
   set to '10' or with R bit set, the CE-bound CESoPSN IWF will be
   locally configured to command its local NSP to perform one of the
   following actions:

   o  None (MUST be supported by all the implementations)

   o  Transmit the RAI pattern towards the local CE on the E1 or T1
      trunk carrying the local attachment circuit (support of this
      action is RECOMMENDED)

   o  Send the "Channel Idle" signal to the local CE for all the DS0
      channels comprising the local attachment circuit (support of this
      action is OPTIONAL and requires also that the CE-bound CES IWF
      replaces the actually received payload with the equivalent amount
      of the locally configured "idle" bit pattern.

   Notes:

   1.  If the pair of IWFs at the two ends of the PW have been
       configured to force the TDM trunks carrying their ACs to transmit
       AIS upon reception of data packets with the L bit set and to
       transmit RAI upon reception of data packets with the R bit set,

       or with the L bit cleared and M bits set to '10', this PW
       provides a bandwidth-saving emulation of a fractional E1 or T1
       service between the pair of CE devices.

   2.  If the pair of IWFs at the two ends of the PW have been
       configured to signal "Channel Idle" CE application state to its
       local CE upon reception of packets marked with L bit set, R bit
       set, or (L,M) set to '010', and to replace the actually received
       payload with the locally configured "idle" bit pattern, the
       resulting PW will comply with the requirements for Downstream
       Trunk conditioning as defined in [TR-NWT-170].

   3.  Usage of bits R, L, and M described above additionally provides
       the tools for "single-ended" management of the CESoPSN
       pseudowires with ability to distinguish between the problems in
       the PSN and in the TDM attachment circuits.

   The payload of each lost CESoPSN data 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 CESoPSN implementations MUST support
   generation of the locally configurable "idle" 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 locally configurable "idle" pattern to its TDM
   attachment circuit.

   Once the PW has been set up, the CE-bound IWF begins to receive
   CESoPSN packets and to store their payload in the jitter buffer, but
   continues to play out the locally configurable "idle" pattern to its
   TDM attachment circuit.  This intermediate state persists until a
   pre-configured amount of TDM data (usually half of the jitter buffer)
   has been received in consecutive CESoPSN packets, or until a pre-
   configured intermediate state timer expires.

   Once the pre-configured amount of the TDM data has been received, the
   CE-bound CESoPSN IWF enters its normal operation state, where it
   continues to receive CESoPSN packets and 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 CESoPSN IWF detects loss of a pre-configured 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:

   o  The locally configurable "idle" pattern SHOULD be played out to
      the TDM attachment circuit.

   o  The local PSN-bound CESoPSN IWF SHOULD mark every packet it
      transmits with the R bit set.

   The CE-bound CESoPSN IWF leaves this state and transits to the normal
   one once a pre-configured number of consecutive CESoPSN packets have
   been received.

6.3.  CESoPSN Defects

   In addition to the packet loss state of the CE-bound CESoPSN 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 pre-
   configured amount of time (typically 2.5 seconds) and MUST be cleared
   after the corresponding defect is undetected for a second pre-
   configured 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 multiplexing 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 MAY be 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 multiplexing 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.  Other methods of detecting malformed packets are
   implementation-specific.  Malformed in-order packets MUST be
   discarded by the CE-bound IWF and replacement data generated as for
   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 pre-configured threshold.

   Buffer overrun is detected in the normal operation state when the
   jitter buffer of the CE-bound IWF cannot accommodate newly arrived
   CESoPSN packets.

   Remote packet loss is indicated by reception of packets with their R
   bit set.

6.4.  CESoPSN PW Performance Monitoring

   Performance monitoring (PM) parameters are routinely collected for
   TDM services and provide an important maintenance mechanism in TDM
   networks.  Ability to collect compatible PM parameters for CESoPSN
   PWs enhances their maintenance capabilities.

   Collection of the CESoPSN PW performance monitoring parameters is
   OPTIONAL and, if implemented, is only performed after the CE-bound
   IWF has exited its intermediate state.

   CESoPSN defines error events, errored blocks, and defects as follows:

   o  A CESoPSN error event is defined as insertion of a single
      replacement packet into the jitter buffer (replacement of payload
      of CESoPSN packets with the L bit set is not considered as
      insertion of a replacement packet).

   o  A CESoPSN errored data block is defined as a block of data played
      out to the TDM attachment circuit and of size defined in
      accordance with the [G.826] rules for the corresponding TDM
      service that has experienced at least one CESoPSN error event.

   o  A CESoPSN defect is defined as the packet loss state of the CE-
      bound CESoPSN IWF.

   The CESoPSN PW PM parameters (Errored, Severely Errored, and
   Unavailable Seconds) are derived from these definitions, in
   accordance with [G.826].

7.  QoS Issues

   If the PSN providing connectivity between PE devices is Diffserv-
   enabled and provides a per-domain behavior (PDB) [RFC3086] that
   guarantees low-jitter and low-loss, the CESoPSN 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).

8.  Congestion Control

   As explained in [RFC3985], the PSN carrying the PW may be subject to
   congestion.  CESoPSN 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 CESoPSN PWs can contribute to network congestion that
   may impact network control protocols.

   Whenever possible, CESoPSN 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 CESoPSN PWs on the neighboring streams.  In order
   to facilitate boundary traffic conditioning of CESoPSN traffic over
   IP PSNs, the CESoPSN IP packets SHOULD NOT use the Diffserv Code
   Point (DSCP) value reserved for the Default PHB [RFC2474].

   If CESoPSN 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 CESoPSN PW SHOULD shut down
   bidirectionally for some period of time as described in Section 6.5
   of [RFC3985].

   Note that:

   1.  The CESoPSN IWF can inherently provide packet loss measurement,
       since the expected rate of arrival of CESoPSN packets is fixed
       and known

   2.  The results of the CESoPSN packet loss measurement may not be a
       reliable indication of presence or absence of severe congestion
       if the PSN provides enhanced delivery, e.g.,:

       a) If CESoPSN traffic takes precedence over non-CESoPSN traffic,
          severe congestion can develop without significant CESoPSN
          packet loss.

       b) If non-CESoPSN traffic takes precedence over CESoPSN traffic,
          CESoPSN may experience substantial packet loss due to a
          short-term burst of high-priority traffic.

   3.  The TDM services emulated by the CESoPSN PWs have high
       availability objectives (see [G.826]) that MUST be taken into
       account when deciding on temporary shutdown of CESoPSN PWs.

   This specification does not define the exact criteria for detecting
   "severe congestion" using the CESoPSN packet loss rate, or the
   specific methods for bidirectional shutdown that the CESoPSN PWs
   (when such severe congestion has been detected) and their consequent
   restart after a suitable delay.  This is left for further study.
   However, the following considerations may be used as guidelines for
   implementing the CESoPSN severe congestion shutdown mechanism:

   1.  CESoPSN Performance Monitoring techniques (see Section 6.4)
       provide entry and exit criteria for the CESoPSN 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 CESoPSN PW while the emulated TDM
       circuit is still considered available by the CE.

   2.  If the CESoPSN 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 CESoPSN 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 bidirectional PW shutdown.

9.  Security Considerations

   CESoPSN 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.

   CESoPSN PWs share susceptibility to a number of pseudowire-layer
   attacks, and will use whatever mechanisms for confidentiality,
   integrity, and authentication that are developed for general PWs.
   These methods are beyond the scope of this document.

   Although CESoPSN PWs MAY employ an RTP header when explicit transfer
   of timing information is required, it is not possible to use SRTP
   (see [RFC3711]) mechanisms as a substitute for PW layer security.

   Misconnection detection capabilities of CESoPSN increase its
   resilience to misconfiguration and some types of DoS attacks.

   Random initialization of sequence numbers, in both the control word
   and the optional RTP header, makes known-plaintext attacks on
   encrypted CESoPSN PWs more difficult.  Encryption of PWs is beyond
   the scope of this document.

10.  IANA Considerations

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

11.  Applicability Statement

   CESoPSN is an encapsulation layer intended for carrying NxDS0
   services with or without CAS over PSN.

   CESoPSN allows emulation of certain end-to-end delay properties of
   TDM networks.  In particular, the end-to-end delay of a TDM circuit
   emulated by a CESoPSN PW does not depend upon the bit rate of the
   service.

   CESoPSN fully complies with the principle of minimal intervention,
   minimizing overhead, and computational power required for
   encapsulation.

   CESoPSN can be used in conjunction with various clock recovery
   techniques and does not presume availability of a global synchronous
   clock at the ends of a PW.  However, if the global synchronous clock
   is available at both ends of a CESoPSN PW, using RTP and differential
   mode of timestamp generation improves the quality of the recovered
   clock.

   CESoPSN allows carrying CE application state signaling that requires
   synchronization with data in-band in separate signaling packets.  A
   special combination of flags in the CESoPSN control word is used to
   distinguish between data and signaling packets, while the Timestamp
   field in the RTP headers is used for synchronization.  This makes
   CESoPSN extendable to support different types of CE signaling without
   affecting the data path in the PE devices.

   CESoPSN also allows emulation of NxDS0 services with CAS carrying the
   signaling information appended to (some of) the packets carrying TDM
   data.

   CESoPSN allows the PSN bandwidth conservation by carrying only AIS
   and/or Idle Code indications instead of data.

   CESoPSN allows deployment of bandwidth-saving Fractional point-to-
   point E1/T1 applications.  These applications can be described as the
   following:

   o  The pair of CE devices operates as if it was connected by an
      emulated E1 or T1 circuit.  In particular, it reacts to AIS and
      RAI states of its local ACs in the standard way.

   o  The PSN carries only an NxDS0 service, where N is the number of
      actually used timeslots in the circuit connecting the pair of CE
      devices, thus saving the bandwidth.

   Being a constant bit rate (CBR) service, CESoPSN cannot provide TCP-
   friendly behavior under network congestion.  If the service
   encounters congestion, it SHOULD be temporarily shut down.

   CESoPSN allows collection of TDM-like faults and performance
   monitoring parameters; hence, emulating 'classic' carrier services of
   TDM circuits (e.g., SONET/SDH).  Similarity with these services is
   increased by the CESoPSN ability to carry 'far end error'
   indications.

   CESoPSN 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.

   CESoPSN provides for detection of lost packets and allows using
   various techniques for generation of "replacement packets".

   CESoPSN carries indications of outages of incoming attachment circuit
   across the PSN, thus, providing for effective fault isolation.

   Faithfulness of a CESoPSN PW may be increased if the carrying PSN is
   Diffserv-enabled and implements a PDB that guarantees low loss and
   low jitter.

   CESoPSN does not provide any mechanisms for protection against PSN
   outages.  As a consequence, resilience of the emulated service to
   such outages is defined by the PSN behavior.  On the other hand:

   o  The jitter buffer and packets' reordering mechanisms associated
      with CESoPSN increase resilience of the emulated service to fast
      PSN re-convergence events

   o  Remote indication of lost packets is carried backward across the
      PSN from the receiver (that has detected loss of packets) to
      transmitter.  Such an indication MAY be used as a trigger for
      activation of proprietary, service-specific protection mechanisms.

   Security of TDM services provided by CESoPSN across a shared PSN may
   be below the level of security traditionally associated with TDM
   services carried across TDM networks.

12.  Acknowledgements

   Akiva Sadovski has been an active participant of the team that co-
   authored early versions of this document.

   We express deep gratitude to Stephen Casner, who reviewed an early
   version of this document in detail, corrected some serious errors,
   and provided many valuable inputs.

   The present version of the text of the QoS section has been suggested
   by Kathleen Nichols.

   We thank Maximilian Riegel, Sim Narasimha, Tom Johnson, Ron Cohen,
   and Yaron Raz for valuable feedback.

   We thank Alik Shimelmits for many fruitful discussions.

13.  Normative References

   [ATM-CES]    The ATM Forum Technical Committee. Circuit Emulation
                Service Interoperability Specification version 2.0
                af-vtoa-0078.000, January 1997.

   [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.706]      ITU-T Recommendation G.706 (04/91) - Frame Alignment and
                Cyclic Redundancy Check (CRC) Procedures Relating to
                Basic Frame Structured Defined in Recommendation G.704

   [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.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

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

   [RFC2833]    Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF
                Digits, Telephony Tones and Telephony Signals", RFC
                2833, May 2000.

   [RFC2914]    Floyd, S., "Congestion Control Principles", BCP 41, RFC
                2914, September 2000.

   [RFC3086]    Nichols, K. and B. Carpenter, "Definition of
                Differentiated Services Per Domain Behaviors and Rules
                for their Specification", RFC 3086, April 2001.

   [RFC3916]    Xiao, X., McPherson, D., and P. Pate, "Requirements for
                Pseudo-Wire Emulation Edge-to-Edge (PWE3)", RFC 3916,
                September 2004.

   [RFC4197]    Riegel, M., "Requirements for Edge-to-Edge Emulation of
                Time Division Multiplexed (TDM) Circuits over Packet
                Switching Networks", RFC 4197, October 2005.

   [RFC3985]    Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-
                Edge (PWE3) Architecture", RFC 3985, March 2005.

   [RFC3550]    Schulzrinne, H., Casner, S., Frederick, R., and V.
                Jacobson, "RTP: A Transport Protocol for Real-Time
                Applications", STD 64, RFC 3550, July 2003.

   [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.

   [RFC4447]    Martini L. et al, Pseudowire Setup and Maintenance Using
                the Label Distribution Protocol (LDP), RFC 4447, April
                2006

   [RFC4623]    Malis, A. and M. Townsley, "Pseudowire Emulation Edge-
                to-Edge (PWE3) Fragmentation and Reassembly", RFC 4623,
                August 2006.

   [RTP-TYPES]  RTP PARAMETERS, <http://www.iana.org/assignments/rtp-
                parameters>.

   [TR-NWT-170] Digital Cross Connect Systems - Generic Requirements and
                Objectives, Bellcore, TR-NWT-170, January 1993

14.  Informative References

   [L2TPEXT-TDM]
                Vainshtein, A. and S. Galtsur, "Layer Two Tunneling
                Protocol - Setup of TDM Pseudowires", Work in Progress,
                February 2007.

   [PWE3-MS]    Martini, L., Metz, C., Nadeau, T., and M. Duckett,
                "Segmented Pseudo Wire", Work in Progress, November
                2007.

   [PWE3-TDM-CONTROL]
                Vainshtein, A. and Y. Stein, "Control Protocol
                Extensions for Setup of TDM Pseudowires in MPLS
                Networks", Work in Progress, November 2007.

   [RFC2212]    Shenker, S., Partridge, C., and R. Guerin,
                "Specification of Guaranteed Quality of Service", RFC
                2212, September 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.

   [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.

   [RFC3711]    Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
                K. Norrman, "The Secure Real-time Transport Protocol
                (SRTP)", RFC 3711, March 2004.

   [RFC3931]    Lau, J., Townsley, M., and I. Goyret, "Layer Two
                Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931,
                March 2005.

   [RFC4446]    Martini, L., "IANA Allocations for Pseudowire Edge to
                Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.

   [RFC4553]    Vainshtein, A. and YJ. Stein, "Structure-Agnostic Time
                Division Multiplexing (TDM) over Packet (SAToP)", RFC
                4553, June 2006.

   [RFC4733]    Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
                Digits, Telephony Tones, and Telephony Signals", RFC
                4733, December 2006.

   [RFC4734]    Schulzrinne, H. and T. Taylor, "Definition of Events for
                Modem, Fax, and Text Telephony Signals", RFC 4734,
                December 2006.

   [RFC5085]    Nadeau, T., Ed., and C. Pignataro, Ed., "Pseudowire
                Virtual Circuit Connectivity Verification (VCCV): A
                Control Channel for Pseudowires", Work in Progress, RFC
                5085, December 2007.

Appendix A.  A Common CE Application State Signaling Mechanism

   Format of the CESoPSN signaling packets is discussed in Section 5.3
   above.

   The sequence number in the CESoPSN control word for the signaling
   packets is generated according to the same rules as for the TDM data
   packets.

   If the RTP header is used in the CESoPSN signaling packets, the
   timestamp in this header represents the time when the CE application
   state has been collected.

   Signaling packets are generated by the ingress PE, in accordance with
   the following logic (adapted from [RFC2833]):

   1.  The CESoPSN signaling packet with the same information (including
       the timestamp in the case RTP header is used) is sent 3 times at
       an interval of 5 ms under one of the following conditions:

       a) The CESoPSN PW has been set up

       b) A change in the CE application state has been detected.  If
          another change of the CE application state has been detected
          during the 10 ms period (i.e., before all 3 signaling packets
          reporting the previous change have been sent), this process is
          re-started, i.e.:

         i)   The unsent signaling packet(s) with the previous CE
              application state are discarded

         ii)  Triple send of packets with the new CE application state
              begins.

       c) Loss of packets defect has been cleared

       d) Remote Loss of Packets indication has been cleared (after
          previously being set)

   2.  Otherwise, the CESoPSN signaling packet with the current CE
       application state information is sent every 5 seconds.

   These rules allow fast probabilistic recovery after loss of a single
   signaling packet, as well as deterministic (but possibly slow)
   recovery following PW setup and PSN outages.

Appendix B.  Reference PE Architecture for Emulation of NxDS0 Services

   Structured TDM services do not exist as physical circuits.  They are
   always carried within appropriate physical attachment circuits (AC),
   and the PE providing their emulation always includes a Native Service
   Processing Block (NSP), commonly referred to as Framer.  As a
   consequence, the architecture of a PE device providing edge-to-edge
   emulation for these services includes the Framer and Forwarder
   blocks.

   In case of NxDS0 services (the only type of structured services
   considered in this document), the AC is either an E1 or a T1 trunk,
   and bundles of NxDS0 are cut out of it using one of the framing
   methods described in [G.704].

   In addition to detecting the FAS and imposing associated structure on
   the "trunk" AC, E1, and T1, framers commonly support some additional
   functionality, including:

   1.  Detection of special states of the incoming AC (e.g., AIS, OOF,
       or RAI)

   2.  Forcing special states (e.g., AIS and RAI) on the outgoing AC
       upon explicit request

   3.  Extraction and insertion of CE application signals that may
       accompany specific DS0 channel(s).

   The resulting PE architecture for NxDS0 services is shown in Figure
   B.1 below.  In this diagram:

   1.  In the PSN-bound direction:

       a) The Framer:

         i)  Detects frame alignment signal (FAS) and splits the
              incoming ACs into separate DS0 channels

         ii)  Detects special AC states

         iii) If necessary, extracts CE application signals accompanying
              each of the separate DS0 services

       b) The Forwarder:

         i)   Creates one or more NxDS0 bundles

         ii)  Sends the data received in each such bundle to the PSN-
              bound direction of a respective CESoPSN IWF instance

         iii) If necessary, sends the current CE application state data
              of the DS0 services in the bundle to the PSN-bound
              direction of the respective CESoPSN IWF instance

         iv)  If necessary, sends the AC state indications to the PSN-
              bound directions of all the CESoPSN instances associated
              with the given AC

       c) Each PSN-bound PW IWF instance encapsulates the received data,
          application state signal, and the AC state into PW PDUs, and
          sends the resulting packets to the PSN

   2.  In the CE-bound direction:

       a) Each CE-bound instance of the CESoPSN IWF receives the PW PDUs
          from the PSN, extracts the TDM data, AC state, and CE
          application state signals, and sends them

       b) The Forwarder sends the TDM data, application state signals
          and, if necessary, a single command representing the desired
          AC state, to the Framer

       c) The Framer accepts all the data of one or more NxDS0 bundles
          possibly accompanied by the associated CE application state,
          and commands referring to the desired AC state, and generates
          a single AC accordingly with correct FAS.

   Notes: This model is asymmetric:

   o  AC state indication can be forwarded from the framer to multiple
      instances of the CESoPSN IWF

   o  No more than one CESoPSN IWF instance should forward AC state-
      affecting commands to the framer.

               +------------------------------------------+
               |                PE Device                 |
               +------------------------------------------+
               |     | Forwarder           |              |
               |     |---------------------|              |
               |     |                     |              |
               |     +<-- AC State---->-   |              |
               |     |                 |   |              |
               |     |                 |   |              |
      E1 or T1 |     |                 |   |              |
         AC    |     |                 |   |              |
      <=======>|     |-----------------+---|--------------|
               |     |                 |   | At most, one |
               |     |                 |-->+ PW IWF       |
               |     |                     | instance     |
         ...   |     +<---NxDS0 TDM Data-->+ imposing     | PW Instance
               |  F  |                     | state        X<===========>
               |     +<---CE App State --->+ on the       |
      E1 or T1 |  R  |                     | outgoing AC  |
         AC    |     +<--AC Command -------+              |
      <=======>o  A  |---------------------|--------------|
               |     |      ...        |        ...       | ...
               |  M  |-----------------+---|--------------|
               |     |                 |   | Zero, one or |
               |  E  |                 |-->+ more PW IWF  |
               |     |                     | instances    |
               |  R  +<---NxDS0 TDM Data-->+ that do not  | PW Instance
               |     |                     | impose state X<===========>
               |     +<---CE App State --->+ on the out-  |
               |     |                     | going AC     |
               +------------------------------------------+

          Figure B.1.  Reference PE Architecture for NxDS0 Services

Appendix C.  Old Mode of CESoPSN Encapsulation Over L2TPV3

   Previous versions of this specification defined a CESoPSN PW
   encapsulation over L2TPv3, which differs from one described in
   Section 4.1 and Figure 1c.  In these versions, the RTP header, if
   used, precedes the CESoPSN control word.

   Existing implementations of the old encapsulation mode MUST be
   distinguished from the encapsulations conforming to this
   specification via the CESoPSN PW setup.

Authors' Addresses

   Alexander ("Sasha") Vainshtein
   Axerra Networks
   24 Raoul Wallenberg St.,
   Tel Aviv 69719, Israel
   EMail: sasha@axerra.com, vainshtein.alex@gmail.com

   Israel Sasson
   Axerra Networks
   24 Raoul Wallenberg St.,
   Tel Aviv 69719, Israel
   EMail: israel@axerra.com

   Eduard Metz
   KPN
   Regulusweg 1
   2316 AC The Hague
   Netherlands
   EMail: eduard.metz@kpn.com

   Tim Frost
   Symmetricom, Inc.
   Tamerton Road
   Roborough, Plymouth
   PL6 7BQ, UK
   EMail: tfrost@symmetricom.com

   Prayson Pate
   Overture Networks
   507 Airport Boulevard
   Building 111
   Morrisville, North Carolina 27560  USA
   EMail: prayson.pate@overturenetworks.com

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