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rfc:rfc5086

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).

Vainshtein, et al. Informational [Page 1] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

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

Vainshtein, et al. Informational [Page 2] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

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.

Vainshtein, et al. Informational [Page 3] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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

Vainshtein, et al. Informational [Page 4] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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.

Vainshtein, et al. Informational [Page 5] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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]).

Vainshtein, et al. Informational [Page 6] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

      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

Vainshtein, et al. Informational [Page 7] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

     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.

Vainshtein, et al. Informational [Page 8] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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.

Vainshtein, et al. Informational [Page 9] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

|=================================================================|
| 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.

Vainshtein, et al. Informational [Page 10] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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.

Vainshtein, et al. Informational [Page 11] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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:

Vainshtein, et al. Informational [Page 12] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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.

Vainshtein, et al. Informational [Page 13] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

                       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)

Vainshtein, et al. Informational [Page 14] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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.

Vainshtein, et al. Informational [Page 15] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

     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.

Vainshtein, et al. Informational [Page 16] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

   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

Vainshtein, et al. Informational [Page 17] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

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.

Vainshtein, et al. Informational [Page 18] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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

Vainshtein, et al. Informational [Page 19] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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.

Vainshtein, et al. Informational [Page 20] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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,

Vainshtein, et al. Informational [Page 21] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

     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:

Vainshtein, et al. Informational [Page 22] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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.

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 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.

Vainshtein, et al. Informational [Page 24] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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

Vainshtein, et al. Informational [Page 25] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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.

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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.

Vainshtein, et al. Informational [Page 27] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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.

Vainshtein, et al. Informational [Page 28] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 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.

Vainshtein, et al. Informational [Page 29] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

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.

Vainshtein, et al. Informational [Page 30] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 [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.

Vainshtein, et al. Informational [Page 31] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

 [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.

Vainshtein, et al. Informational [Page 32] RFC 5086 TDM Circuit Emulation Service over PSN 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.

Vainshtein, et al. Informational [Page 33] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

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

Vainshtein, et al. Informational [Page 34] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

       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.

Vainshtein, et al. Informational [Page 35] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

             +------------------------------------------+
             |                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.

Vainshtein, et al. Informational [Page 36] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

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

Vainshtein, et al. Informational [Page 37] RFC 5086 TDM Circuit Emulation Service over PSN December 2007

Full Copyright Statement

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Vainshtein, et al. Informational [Page 38]

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