SONET/SDH Circuit Emulation over Packet (CEP)
draft-malis-pwe3-sonet-03

Versions: 00 01 02 03                                                   
PWE3 Working Group                                     Andrew G. Malis
Internet Draft                                                 Ken Hsu
Expiration Date:   May 2002                      Vivace Networks, Inc.

Tom Johnson                                             Jeremy Brayley
Marlene Drost                                          Steve Vogelsang
Ed Hallman                                                John Shirron
Litchfield Communications, Inc.                  Laurel Networks, Inc.

Jim Boyle                                                 Luca Martini
Protocol Driven Networks, Inc.                             Craig White
                                          Level 3 Communications, LLC.
Ron Cohen
Lycium Networks                                            David Zelig
                                               Corrigent Systems, LTD.
Prayson Pate
Overture Networks, Inc.


                                                        November 2001



             SONET/SDH Circuit Emulation over Packet (CEP)
                       draft-malis-pwe3-sonet-01.txt



Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of section 10 of RFC 2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet-Drafts as reference
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.





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Abstract

Generic requirements for Pseudo Wire Emulation Edge-to-Edge (PWE3) have
been described in [3].  This draft lists SONET specific requirements
and provides encapsulation formats and semantics for connecting SONET
edge networks through a core packet network using IP, L2TP or MPLS.
This basic application of SONET interworking will allow SONET service
providers to take advantage of new technologies in the core in order to
provide SONET services.

Table of Contents

   1    Conventions used in this document...........................2
   2    Introduction................................................2
   3    Scope.......................................................3
   4    CEP Encapsulation Format....................................4
   5    CEP Operation...............................................9
   6    SONET/SDH Maintenance Signals..............................12
   7    SONET/SDH Transport Timing.................................16
   8    SONET/SDH Pointer Management...............................17
   9    CEP Performance Monitors...................................18
   10   Open Issues................................................20
   11   Security Considerations....................................20
   12   Intellectual Property Disclaimer...........................20
   13   References.................................................22
   14   Acknowledgments............................................23
   15   Author's Addresses.........................................23


1  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [2].


2  Introduction

   This document describes a protocol that performs SONET Emulation
   over a variety of Packet-Switched Networks (PSNs) as part of the
   PWE3 Working Group.  The document assumes that the reader is
   familiar with the PWE3 terminology and concepts described in PWE3
   requirements and framework documents [3] and [4].  The protocol is
   titled "Circuit Emulation over Packet" (CEP).

   The transmission system for circuit-oriented TDM signals is the
   Synchronous Optical Network (SONET) [5], [9] / Synchronous Digital
   Hierarchy (SDH) [6]. To support TDM traffic (which includes voice,
   data, and private leased line services) PSNs must emulate the
   circuit characteristics of SONET/SDH payloads.  A circuit identifer


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   and a CEP header are used to encapsulate the SONET/SDH TDM signals
   for transmission over an arbitrary PSN.

   This document also describes an optional extension to CEP called
   Dynamic Bandwidth Allocation (DBA).  This is a method for
   dynamically reducing the bandwidth utilized by emulated SONET/SDH
   circuits in the packet network.  This bandwidth reduction is
   accomplished by not sending the SONET/SDH payload through the packet
   network under certain conditions such as AIS-P or STS SPE
   Unequipped.

   This document is based on a previous document describing a method
   for encapsulating SONET signals for carriage over MPLS networks
   (CEM) [7].  This document is closely related to [8] which describes
   a MIB for controlling and observing CEP services.


3  Scope

   This document describes how to provide CEP for the following digital
   signals:

   1. SONET STS-1 synchronous payload envelope (SPE)/SDH VC-3

   2. STS-Nc SPE (N = 3, 12, 48, or 192)/SDH VC-4, VC-4-4c, VC-4-16c,
      or VC-4-64c

   For the remainder of this document, these constructs will be
   referred to as SONET/SDH channels.

   Although this document currently covers up to OC-192c/VC-4-64c,
   future revision MAY address higher rates.

   Other SONET/SDH signals, such as virtual tributary (VT) structured
   sub-rate mapping, are not explicitly discussed in this document;
   however, it can be extended in the future to support VT and lower
   speed non-SONET/SDH services.
















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4  CEP Encapsulation Format

   In order to transport SONET/SDH SPEs through a packet-oriented
   network, the SPE is broken into fragments.  A Circuit ID Word and
   CEP Header are pre-pended to each fragment.  The basic CEP packet
   appears in Figure 1.

             +-----------------------------------+
             |         Circuit ID Word           |
             +-----------------------------------+
             |           CEP Header              |
             +-----------------------------------+
             |                                   |
             |                                   |
             |        SONET/SDH SPE Fragment     |
             |                                   |
             |                                   |
             +-----------------------------------+

             Figure 1. Basic CEP Packet


   The Circuit ID Word is a 32-bit field that contains an arbitrary
   value that is used to map CEP packets to specific SONET/SDH
   channels.  The circuit ID word is intentionally designed to match
   the format of an MPLS shim.


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Circuit ID                 | Exp |S|       TTL     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


             Figure 2. CEP Circuit ID Word

   Circuit ID:  Matches the Label Position in an MPLS shim.  It SHOULD
   be used to map individual packet streams to SONET channels.

   Exp:    Experimental Use, 3 bits.  SHOULD conform to use of Exp bits
   within an MPLS shim.

   S:      Bottom of Stack, 1 bit.  SHOULD be set to 1 to indicate the
   bottom of an MPLS label stack.

   TTL:    Time to Live, 8 bits.  SHOULD be utilized in a manner
   consistent with the TTL field of an MPLS Shim.



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   The CEP Header supports a basic and extended mode.  The Basic CEP
   Header provides the minimum functionality necessary to accurately
   emulate a TDM SONET over a PSN.  Bit 0 of the first 32-bit CEP
   header indicates whether or not the extended header is present.
   When this bit is 0, then no extended header is present.  When this
   bit is 1, then an extended header is present.  At this time, the
   contents of the extended header are for future study.  However, it
   is expected that this field will provide support for payload
   compression, header protection, enhanced performance monitoring,
   and/or other extensions to the base protocol.

   The Basic CEP header has the following format:

    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 2
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0|R|D|N|P| Structure Pointer[0:12] |  Sequence Number[0:13]    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 3. Basic CEP Header Format

   The Extended CEP header appears 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 2
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |1|R|D|N|P| Structure Pointer[0:12] |  Sequence Number[0:13]    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Reserved                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 4. Extended CEP Header Format



   The above fields are defined as follows:

   R bit: CEP-RDI.  This bit is set to one to signal to the remote CEP
   function that a loss of packet synchronization has occurred.  See
   section 5.4 for details.

   D bit: Signals DBA Mode.  MUST be set to zero for Normal Operation.
   MUST be set to one if CEP is currently in DBA mode.  DBA is an
   optional mode during which trivial SPEs are not transmitted into the
   packet network.  See Table 1 and section 5 for further details.









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   The N and P bits: MAY be used to explicitly relay negative and
   positive pointer adjustment events across the PSN.  They are also
   used to relay SONET/SDH maintenance signals such as AIS-P.  See
   Table 1 and sections 6 and 8 for more details.

         +---+---+---+----------------------------------------------+
         | D | N | P |         Interpretation                       |
         +---+---+---+----------------------------------------------+
         | 0 | 0 | 0 | Normal Mode - No Ptr Adjustment              |
         | 0 | 0 | 1 | Normal Mode - Positive Ptr Adjustment        |
         | 0 | 1 | 0 | Normal Mode - Negative Ptr Adjustment        |
         | 0 | 1 | 1 | Normal Mode - AIS-P                          |
         |   |   |   |                                              |
         | 1 | 0 | 0 | DBA Mode    - STS SPE Unequipped             |
         | 1 | 0 | 1 | DBA Mode    - STS SPE Unequipped Pos Ptr Adj |
         | 1 | 1 | 0 | DBA Mode    - STS SPE Unequipped Neg Ptr Adj |
         | 1 | 1 | 1 | DBA Mode    - AIS-P                          |
         +---+---+---+----------------------------------------------+

         Table 1. Interpretation of D, N, and P bits

   Sequence Number[0:13]:  This is a packet sequence number, which MUST
   continuously cycle from 0 to 0x3FFF.  It SHOULD begin at zero when a
   CEP channel is created.

   Structure Pointer[0:12]: The Structure Pointer MUST contain the
   offset of the J1 byte within the CEP SPE Fragment.  The value is
   from 0 to 0x1FFE, where 0 means the first byte after the CEP header.
   The Structure Pointer MUST be set to 0x1FFF if a packet does not
   carry the J1 byte.  See [5], [6] and [9] for more information on the
   J1 byte and the SONET/SDH payload pointer.  Implementations MUST
   support SPE Fragments of up to 1023 bytes and MAY support SPE
   fragments of up to 8191 bytes.

   Note: CEP packets are fixed in length for all of the packets of a
   particular emulated TDM stream.  This length is statically
   provisioned for each TDM stream.  Therefore, the length of each CEP
   packet does not need to be carried in the CEP header.


4.1 PSN Encapsulation

   In principle, CEP packets can be carried over any packet-oriented
   network.  The following sections describe specifically how CEP
   packets MUST be encapsulated for carriage over MPLS or IP networks.







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4.1.1   MPLS Encapsulation

   To transport a CEP packet over an MPLS network, an MPLS label-stack
   MUST be pushed on top of the CEP packet.



                 +-----------------------------------+
                 |        MPLS Label Stack           |
                 +-----------------------------------+
                 |         Circuit ID Word           |
                 +-----------------------------------+
                 |            CEP Header             |
                 +-----------------------------------+
                 |                                   |
                 |                                   |
                 |       SONET/SDH SPE Fragment      |
                 |                                   |
                 |                                   |
                 +-----------------------------------+

                 Figure 5. Typical MPLS Transport Encapsulation






























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4.1.2   IP Encapsulation

   It is highly desirable to define a single encapsulation format that
   will work for both IP and MPLS.  Furthermore, it is desirable that
   the encapsulation mechanism be as efficient as possible.

   One way to achieve these goals is to map CEP directly onto IP.
   Because the Circuit ID Word is essentially an MPLS Shim, the CEP
   packet may be treated as an MPLS packet.  A mechanism for carrying
   MPLS over IP is described in [10].

   Using this encapsulation scheme would result in the packet format
   illustrated in figure 6.

                 +-----------------------------------+
                 |                                   |
                 |        IPv6/v4 Header [10]         |
                 |                                   |
                 +-----------------------------------+
                 |         Circuit ID Word           |
                 +-----------------------------------+
                 |           CEP Header              |
                 +-----------------------------------+
                 |                                   |
                 |                                   |
                 |       SONET/SDH SPE Fragment      |
                 |                                   |
                 |                                   |
                 +-----------------------------------+

                  Figure 6. MPLS Transport Encapsulation


4.1.3   L2TP Encapsulation

   Encapsulation for L2TP PSNs is for future study.
















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5  CEP Operation

   The following sections describe CEP operation.


5.1 Introduction and Terminology

   CEP MUST support a normal mode of operation and MAY support an
   optional extension called Dynamic Bandwidth Allocation (DBA).
   During normal operation, SONET/SDH payloads are fragmented, pre-
   pended with the CEP Header, the Circuit ID Word, and the PSN header,
   and then transmitted into the packet network.  During DBA mode, only
   the CEP header, the Circuit ID Word, and PSN header are transmitted.
   This is done to conserve bandwidth when meaningful user data is not
   present in the SPE, such as during AIS-P or STS SPE Unequipped.

5.1.1   CEP Packetizer and De-Packetizer

   As with all adaptation functions, CEP has two distinct components:
   adapting TDM SONET/SDH into a CEP packet stream, and converting the
   CEP packet stream back into a TDM SONET/SDH.  The first function
   will be referred to as CEP Packetizer and the second as CEP De-
   Packetizer.  This terminology is illustrated in figure 7.


             +------------+              +---------------+
             |            |              |               |
   SONET --> |    CEP     | --> PSN  --> |      CEP      | --> SONET
    SDH      | Packetizer |              | De-Packetizer |      SDH
             |            |              |               |
             +------------+              +---------------+

                Figure 7. CEP Terminology

   Note: the CEP de-packetizer requires a buffering mechanism to
   account for delay variation in the CEP packet stream.  This
   buffering mechanism will be generically referred to as the CEP
   jitter buffer.

5.1.2   CEP DBA

   DBA is an optional mode of operation that only transmits the CEP
   Header, the Circuit ID Word, and PSN Header into the packet network
   under certain circumstances such as AIS-P or STS Unequipped.

   If DBA is supported by a CEP implementation, the user SHOULD be able
   to configure if DBA will be triggered by AIS-P, STS Unequipped,
   both, or neither on a per channel basis.



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   If DBA is supported, the determination of AIS-P and STS Unequipped
   MUST be based on the state of SONET/SDH Section, Line, and Path
   Overhead bytes.

   During AIS-P, there is no valid payload pointer, so pointer
   adjustments cannot occur.  During STS Unequipped, the SONET/SDH
   payload pointer is valid, and therefore pointer adjustments MUST be
   supported even during DBA.  See Table 1 for details.


5.2 Description of Normal CEP Operation

   During normal operation, the CEP packetizer will receive a fixed
   rate byte stream from a SONET/SDH interface.  When a packets worth
   of data has been received from a SONET/SDH channel, the CEP Header,
   the Circuit ID Word, and PSN Header are pre-pended to the SPE
   fragment and the resulting CEP packet is transmitted into the packet
   network.  Because all CEP packets associated with a specific
   SONET/SDH channel will have the same length, the transmission of CEP
   packets for that channel SHOULD occur at regular intervals.

   At the far end of the packet network, the CEP de-packetizer will
   receive packets into a jitter buffer and then play out the received
   byte stream at a fixed rate onto the corresponding SONET/SDH
   channel.  The jitter buffer SHOULD be adjustable in length to
   account for varying network delay behavior.  The receive packet rate
   from the packet network should be exactly balanced by the
   transmission rate onto the SONET/SDH channel, on average.  The time
   over which this average is taken corresponds to the depth of the
   jitter buffer for a specific CEP channel.

   The CEP sequence numbers provide a mechanism to detect lost and/or
   mis-ordered packets.  The CEP de-packetizer MUST detect lost or mis-
   ordered packets.  The CEP de-packetizer SHOULD play out an all ones
   pattern (AIS) in place of any dropped packets.  The CEP de-
   packetizer MAY re-order packets received out of order.  If the CEP
   de-packetizer does not support re-ordering, it must drop mis-ordered
   packets.


5.3 Description of CEP Operation during DBA

   There are several issues that should be addressed by a workable CEP
   DBA mechanism.  First, when DBA is invoked, there should be a
   substantial savings in bandwidth utilization in the packet network.
   The second issue is that the transition in and out of DBA should be
   tightly coordinated between the local CEP packetizer and CEP de-
   packetizer at the far side of the packet network.  A third is that
   the transition in and out of DBA should be accomplished with minimal
   disruption to the adapted data stream.



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   Another goal is that the reduction of CEP traffic due to DBA should
   not be mistaken for a fault in the packet network or vice-versa.
   Finally, the implementation of DBA should require minimal
   modifications beyond what is necessary for the nominal CEP case.
   The mechanism described below is a reasonable balance of these
   goals.

   During DBA, packets MUST be emitted at exactly the same rate as they
   would be during normal operation.  This SHOULD be accomplished by
   transmitting each DBA packet after a complete packet of data has
   been received from the SONET/SDH channel.  The only change from
   normal operation is that the CEP packets during DBA MUST only carry
   the CEP header, the Circuit ID Word, and the PSN Header.  Because
   some links have a minimum supported packet size, the CEP packetizer
   MAY append a configurable number of bytes immediately after the CEP
   header to pad out the CEP packet to reach the mimumum supported
   packet size.  The D-bit MUST be set to one, to indicate that DBA is
   active.

   The CEP de-packetizer MUST assume that each packet received with the
   D-bit set represents a normal-sized packet containing an AIS-P or
   SPE Unequipped payload as noted by N and P.  See Table 1.  The CEP
   de-packetizer MUST accept DBA packets with or without padding.

   This allows the CEP packetization and de-packetization logic during
   DBA to be similar to the nominal case.  It ensures that the correct
   SONET/SDH indication is reliably transmitted between CEP adaptation
   points.  It minimizes the risk of under or over running the jitter
   buffer during the transition in and out of DBA, since packets are
   continuously transmitted during DBA.  And, it guarantees that faults
   in the packet network are recognized as distinctly different from
   line conditioning on the SONET/SDH interfaces.


5.4 Packet Synchronization

   A key component in declaring the state of a CEP service is whether
   or not the CEP de-packetizer is in or out of packet synchronization.
   The following paragraphs describe how that determination is made.

   As discussed in section 5, a CEP de-packetizer MAY or MAY NOT
   support re-ordering of mis-ordered packets.

   As packets are received from the PSN, they are placed into a jitter
   buffer prior to play out on the SONET interface.  If a CEP de-
   packetizer supports re-ordering, any packet received before it's
   play out time will still be considered valid.

   If a CEP de-packetizer does not support re-ordering, a number of
   approaches may be used to minimize the impact of mis-ordered or lost
   packets on the final re-assembled SONET stream.  For example, AAL1
   [11] uses a simple state-machine to re-order packets in a sub-set of

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   possible cases.  The algorithm for these state-machines is outside
   of the scope of CEP.

   However, the final determination as to whether or not to declare
   acquisition or loss of packet synchronization MUST be based on the
   same criteria regardless of whether an implementation supports or
   does not support re-ordering.

   Therefore, the determination of acquisition or loss of packet
   synchronization is always made at SONET play-out time.  During SONET
   play-out, the CEP de-packetizer will play received CEP packets onto
   the SONET interface.  However, if the jitter buffer is empty or the
   packet to be played out has not been received, the CEP de-packetizer
   will have to play out an empty packet onto the SONET interface in
   place of the unavailable packet.

   The acquisition of packet synch is based on the number of sequential
   CEP packets that are played onto the SONET interface.  While, loss
   of packet synch is based on the number of sequential 'empty' packets
   that are played onto the SONET interface.  Specific details of these
   two cases is described below.


5.4.1   Acquisition of Packet Synchronization

   At startup, a CEP de-packetizer will be out of packet
   synchronization by default.  To declare packet synchronization at
   startup or after a loss of packet synchronization, the CEP de-
   packetizer must play-out a configurable number of CEP packets with
   sequential sequence numbers towards the SONET interface.

5.4.2   Loss of Packet Synchronization

   Once a CEP de-packetizer is in packet sync, it may encounter a set
   of events that will cause it to lose packet synchronization.


   If the CEP de-packetizer encounters more than a configurable number
   of sequentialempty packets, the CEP de-packetizer MUST declare loss
   of packet synchronization.


6  SONET/SDH Maintenance Signals

   There are several issues that must be considered in the mapping of
   maintenance signals between SONET/SDH and a PSN.  A description of
   how these signals and conditions are mapped between the two domains
   is described below.

   For clarity, the mappings are split into two groups: SONET/SDH to
   PSN, and PSN to SONET/SDH.


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6.1 SONET/SDH to PSN
   The following sections describe how SONET/SDH Maintenance Signals
   and Alarm conditions are mapped into a Packet Switched Network.


















































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6.1.1   AIS-P Indication

   In a SONET/SDH network, SONET Path outages are signaled using
   maintenance alarms such as Path AIS (AIS-P).  In particular, AIS-P
   indicates that the SONET/SDH Path is not currently transmitting
   valid end-user data, and the SPE contains all ones.

   It should be noted that nearly every type of service-effecting
   section or line defect will result in an AIS-P condition.

   The SONET/SDH hierarchy is illustrated below.

                               +----------+
                               |   PATH   |
                               +----------+
                                    ^
                                    |
                                  AIS-P
                                    |
                                    |
                               +----------+
                               |   LINE   |
                               + ---------+
                                  ^     ^
                                  |     |
                                AIS-L   +------ LOP
                                  |
                                  |
                               +----------+
                               | SECTION  |
                               +----------+
                                  ^    ^
                                  |    |
                                  |    |
                                 LOS  LOF


                       Figure 8.  SONET/SDH Fault Hierarchy.

   Should the Section Layer detect a Loss of Signal (LOS) or Loss of
   Frame (LOF) condition, it sends AIS-L up to the Line Layer.  If the
   Line Layer detects AIS-L or Loss of Path (LOP), it sends AIS-P to
   the Path Layer.

   In normal mode during AIS-P, CEP packets are generated as usual.
   The N and P bits MUST be set to 11 binary to signal AIS-P explicitly
   through the packet network.  The D-bit MUST be set to zero to
   indicate that the SPE is being carried through the packet network.
   Normal CEP packets with the SPE fragment, CEP Header, the Circuit ID
   Word, and PSN Header MUST be transmitted into the packet network.



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   However, to conserve network bandwidth during AIS-P, DBA MAY be
   employed.  If DBA has been enabled for AIS-P and AIS-P is currently
   occurring, the N and P bits MUST be set to 11 binary to signal AIS,
   and the D-bit MUST be set to one to indicate that the SPE is not
   being carried through the packet network.  Only the CEP header, the
   Circuit ID Word, and the PSN Header MUST be transmitted into the
   packet network.

6.1.2   STS SPE Unequipped Indication

   The declaration of STS SPE unequipped MUST conform to [9].  Quoted
   below:

   "R6-135 [481] STS PTE shall detect an STS Path Unequipped (UNEQ-P)
   defect within 10 ms of the onset of at least five consecutive
   samples (which may or may not be consecutive frames) of unequipped
   STS Signal Labels (C2 byte), as specified in Table 6-2"

   The termination of STS SPE unequipped MUST also conform to [9].

   "R6-137 [485v2] STS PTE shall terminate an UNEQ-P defect within 10
   ms of the onset of at least five consecutive samples (which may or
   may not be consecutive frames) of STS Signal Labels that are not
   unequipped or all-ons, as specified in Table 6-2"

   For normal operation during SPE Unequipped, the N and P bits MUST be
   interpreted as usual.  The SPE MUST be transmitted into the packet
   network along with the CEP Header, the Circuit ID Word, and PSN
   Header, and the D-Bit MUST be set to zero.

   If DBA has been enabled for STS SPE Unequipped and the Unequipped is
   occurring on the SONET/SDH channel, the D-bit MUST be set to one to
   indicate DBA is active.  Only the CEP Header, the Circuit ID Word,
   and PSN Header MUST be transmitted into the packet network.  The N
   and P bits MAY be used to signal pointer adjustments as normal.  See
   Table 1 and section 5 for details.

6.1.3   CEP-RDI

   The CEP function MUST send CEP-RDI towards the packet network during
   loss of packet synchronization.  This MUST be accomplished by
   setting the R bit to one in the CEP header.


6.2 PSN to SONET/SDH

   The following sections discuss how the various conditions on the
   packet network are converted into SONET/SDH indications.

6.2.1   AIS-P Indication



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   There are several conditions in the packet network that will cause
   the CEP de-packetization function to play out an AIS-P indication
   towards a SONET/SDH channel.

   The first of these is the receipt of CEP packets with the N and P
   bits set to one, and the D-bit set to zero.  This is an explicit
   indication of AIS-P being received at the far-end of the packet
   network, with DBA disabled for AIS-P.  The CEP de-packetizer MUST
   play out the received SPE fragment (which will incidentally be
   carrying all ones), and MUST configure the SONET/SDH Overhead to
   signal AIS-P as defined in [5], [6], and [9].

   The second case is the receipt of CEP packets with the N and P bits
   set to one, and the D-bit set to one.  This indicates that AIS-P is
   being received at the far-end of the packet network, with DBA
   enabled for AIS-P.  The CEP de-packetizer MUST play out one packet's
   worth of all ones for each packet received, and MUST configure the
   SONET/SDH Overhead to signal AIS-P as defined in [5], [6], and [9].

   A third case that will cause a CEP de-packetization function to play
   out an AIS-P indication onto a SONET/SDH channel is during loss of
   packet synchronization.  The CEP de-packetizer MUST configure the
   SONET/SDH Overhead to signal AIS-P as defined in [5], [6], and [9].

6.2.2   STS SPE Unequipped Indication

   There are three conditions in the packet network that will cause the
   CEP function to transmit STS SPE Unequipped indications onto the
   SONET/SDH channel.

   The first, which is transparent to CEP, is the receipt of regular
   CEP packets that happen to be carrying an SPE that contains the
   appropriate Path overhead to signal STS SPE unequipped.  This case
   does not require any special processing on the part of the CEP de-
   packetizer.

   The second case is the receipt of CEP packets that have the D-bit
   set to one to indicate DBA active and the N and P bits set to 00
   binary, 01 binary, or 10 binary to indicate SPE Unequipped with or
   without pointer adjustments.  The CEP de-packetizer MUST use this
   information to transmit a packet of all zeros onto the SONET/SDH
   interface, and adjust the payload pointer as necessary.

   The third case when a CEP de-packetizer MUST play out an STS SPE
   Unequipped Indication towards the SONET interface is when the VC-
   label has been withdrawn due to de-provisioning of the circuit.


7  SONET/SDH Transport Timing

   It is assumed that the distribution of SONET/SDH Transport timing
   information is addressed through external mechanisms such as

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   Building Integrated Timing System (BITS), Global Positioning System
   (GPS) or other such methods and is therefore outside of the scope of
   this specification.


8  SONET/SDH Pointer Management

   A pointer management system is defined as part of the definition of
   SONET/SDH. Details on SONET/SDH pointer management can be found in
   [5], [6], and [9].  If there is a frequency offset between the frame
   rate of the transport overhead and that of the SONET/SDH SPE, then
   the alignment of the SPE shall periodically slip back or advance in
   time through positive or negative stuffing.

   The emulation of this aspect of SONET networks may be accomplished
   using a variety of techniques including (but not limited to)
   explicit pointer adjustment relay (EPAR) and adaptive pointer
   management (APM).

   In any case, the handling of the SPE data by the CEP packetizer is
   the same.

   During a negative pointer adjustment event, the CEP packetizer MUST
   incorporate the H3 byte from the SONET/SDH stream into the CEP
   packet payload in order with the rest of the SPE.  During a positive
   pointer adjustment event, the CEP de-packetizer MUST strip the stuff
   byte from the CEP packet payload.

   When playing out a negative pointer adjustment event, the
   appropriate byte of the CEP payload MUST be placed into the H3 byte
   of the SONET/SDH stream.  When playing out a positive pointer
   adjustment, the CEP de-packetizer MUST insert a stuff-byte into the
   appropriate position within the SONET/SDH stream.

   The details regarding the use of the H3 byte and stuff byte during
   positive and negative pointer adjustments can be found in [5], [6],
   and [9].


8.1 Explicit Pointer Adjustment Relay (EPAR)

   CEP provides an OPTIONAL mechanism to explicitly relay pointer
   adjustment events from one side of the PSN to the other.  This
   technique will be referred to as Explicit Pointer Adjustment Relay
   (EPAR).  The mechanics of EPAR are described below.

   The following text only applies to implementations that choose to
   implement EPAR.  Any CEP implementation that does not support EPAR
   MUST either set the N and P bits to zero or utilize them to relay
   AIS-P and STS Unequipped as shown in table 1.



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   If EPAR is being used, the pointer adjustment event MUST be
   transmitted in three consecutive packets by the packetizer. The de-
   packetizer MUST play out the pointer adjustment event when any one
   packet with N/P bit set is received.

   References [5],[6],and [9] specify that pointer adjustment events
   MUST be separated by three SONET/SDH frames without a pointer
   adjustment event.  In order to explicitly relay all legal pointer
   adjustment events, the packet size for a specific circuit SHOULD be
   no larger than (783 * 4 * N)/3, where N is the STS-Nc multiplier.

   However, there are many SONET implementations which will allow
   pointer adjustments to occur in back to back SONET/SDH frames.  In
   order to support this possibility, EPAR implementations SHOULD set
   the packet size for a particular circuit to be no larger than
   (783*N)/3.  Where N is the STS-Nc multiplier.


   Since the minimum value of N is one, EPAR implementations SHOULD
   support a minimum payload length of 783/3 or 261 bytes.

   For EPAR implementations, the CEP de-packetizer MUST utilize the CEP
   sequence numbers to insure that SONET/SDH pointer adjustment events
   are not played any more frequently than once per every three CEP
   packets transmitted by the remote CEP packetizer.

   If both bits are set, then an AIS-P event has occurred (this is
   further discussed in section 6).

   When DBA is invoked (i.e. the D-bit = 1), N and P have additional
   meanings.  See Table 1 and section 5.


8.2 Adaptive Pointer Management (APM)

   Another OPTIONAL method that may be used to emulate SONET pointer
   management is Adaptive Pointer Management (APM).  In basic terms,
   APM uses information about the depth of the CEP jitter buffers to
   introduce pointer adjustments in the reassembled SONET SPE.

   Details about specific APM algorithms is for future study.


9  CEP Performance Monitors

   SONET/SDH as defined in [5], [6], and [9], includes the definition
   of several counters that may be used to monitor the performance of
   SONET/SDH services.  These counters are referred to as Performance
   Monitors.

   In order for CEP to be utilized by traditional SONET/SDH network
   operators, CEP SHOULD provide similar functionality.  To this end,

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   the following sections describe a number of counters that will
   collectively be referred to as CEP Performance Monitors.


9.1 Near-End Performance Monitors

   These performance monitors are maintained by the CEP De-Packetizer
   during reassembly of the SONET stream.

   The performance monitors are based on two types of defects.

   Type 1 defect is defined as: missing or dropped packet.
   Type 2 defect is defined as: buffer under run, buffer over-run,
   LOPS.

   Each second that contain at least one type 1 defect SHALL be
   declared as ES-CEP.

   Each second that contain type 2 defect, or missing packets above
   pre-defined, configurable threshold of missing/dropped packets SHALL
   be declared both SES-CEP and ES-CEP.  Default value for missing
   packet to SES is 3.

   UAS-CES shall be declared after X consecutives SES-CEP, cleared
   after X consecutive seconds without SES-CEP.  Default value of X is
   10 seconds.

   Once unavailability is declared, ES and SES counts shall be
   inhibited up to the point where the unavailability was started. Once
   unavailability is removed, ES that occurred along the X seconds
   clearing period shall be added to the ES counts. An update is
   required even for closed intervals if  necessary.

   LOPS failure is declared after 2.5 +/- 0.5 seconds of LOPS defect,
   and cleared after 10 seconds free of LOPS defect state. The VC is
   considered down as long as LOPS failure is declared.

   FC-CEP is the number of time type 1 or type 2 defect states were
   declared.  The NE shall have thresholding on ES-CEP, SES-CEP and
   UAS-CEP (thresholding mean activate a notification if more than pre-
   defined # of seconds are declared as ES, etc. in 15 minutes
   interval).


9.2 Far-End Performance Monitors

   These performance monitors provide insight into the CEM De-
   packetizer at the far-end of the PSN.

   Far end statistics are based on the RDI-CEP bit. Limited
   functionality is supported compared to [GR-253] for simplicity and
   because it is assumed that all relevant statistics are available

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   from the end point of the PW. CEP-FE defect is declared when CEP-RDI
   is set in the incoming CEP packets.

   CEP-FE failure declared after 2.5 +/- 0.5 seconds of CEP-FE defect,
   and cleared after 10 seconds free of CES-FE defect state.  Sending
   notification to the OS for CEP-FE failure is local policy.

   This draft does not attempt to define SES-CEPFE, UAS-CEPFE and FC-
   CEPFE, but they can be added if to fully emulate GR-253 far end PM
   (thresholding is required too here except for FC-CEPFE). (Note that
   ES-CEPFE is not relevant since CEP does not report back missing
   packets - only LOPS which is SES).

   The definition of additional performance monitors is for future
   study.


10 Open Issues

   This version of the draft does not address payload compression
   within the emulated SONET.  Payload compression is expected to be
   supported by future versions of this draft by utilizing the extended
   CEP header.

   This version of the draft does not tie into PWE3 maintenance
   mechanisms for the setup and tear down of services.  That short-
   coming will be addressed in future revisions of this document.

   Underlying MPLS QoS requirements are not covered by this revision of
   the draft.  Future revisions may discuss underlying QoS
   requirements.

   Support for VT and lower speed non-SONET/SDH services are not
   covered in this revision of the draft.  Future revisions may address
   VT and non-SONET/SDH TDM services.

   An alternate version of DBA has been suggested that would suppress
   transmission of the entire CEP packet stream under certain
   circumstances.  Future versions of this draft may define such a
   mechanism.


11 Security Considerations

   This document does not address or modify security issues within the
   relevant PSNs.



12 Intellectual Property Disclaimer



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   This document is being submitted for use in IETF standards
   discussions.  Vivace Networks, Inc. has filed one or more patent
   applications relating to the CEP technology outlined in this
   document.  Vivace Networks, Inc. will grant free unlimited licenses
   for use of this technology.

















































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13 References

   [1]  Bradner, S., "The Internet Standards Process -- Revision 3",
        BCP 9, RFC 2026, October 1996.

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

   [3]  Xiao et al, " Requirements for Pseudo-Wire Emulation Edge-to-
        Edge (PWE3)", draft-ietf-pwe3-requirements-01.txt, work in
        progress, July 2001

   [4]  Pate et al, "Framework for Pseudo Wire Emulation Edge-to-Edge
        (PWE3)", draft-pate-pwe3-framework-01.txt, work in progress,
        July 13, 2001

   [5]  American National Standards Institute, "Synchronous Optical
        Network (SONET) - Basic Description including Multiplex
        Structure, Rates and Formats," ANSI T1.105-1995.

   [6]  ITU Recommendation G.707, "Network Node Interface For The
        Synchronous Digital Hierarchy", 1996.

   [7]  Malis et al, "SONET/SDH Circuit Emulation Service Over MPLS
        (CEM) Encapsulation", draft-malis-sonet-ces-mpls-05.txt, work
        in progress, July 2001.

   [8]  Danenberg et al, "SONET/SDH Circuit Emulation Service Over PSN
        (CEP) Management Information Base Using SMIv2", draft-
        danenberg-pw-cem-mib-01.txt, work in progress, July 2001.

   [9]  Telcordia Technologies, "Synchronous Optical Network (SONET)
        Transport Systems: Common Generic Criteria", GR-253-CORE, Issue
        3, November 2000.

   [10]  Worster, "MPLS Label Stack Encapsulation in IP", draft-
        worster-mpls-in-ip-05, work in progress, July 2001.

   [11] ITU-T, "Recommendation I.363.1, B-ISDN Adaptation Layer
        Specification: Type AAL1", Appendix III, August 1996.










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14 Acknowledgments

   The authors would like to thank all the members of the PWE3 WG who
   have contributed to this effort.


15 Author's Addresses

   Andrew G. Malis
   Vivace Networks, Inc.
   2730 Orchard Parkway
   San Jose, CA 95134
   Email: Andy.Malis@vivacenetworks.com

   Ken Hsu
   Vivace Networks, Inc.
   2730 Orchard Parkway
   San Jose, CA 95134
   Email: Ken.Hsu@vivacenetworks.com

   Jeremy Brayley
   Laurel Networks, Inc.
   2706 Nicholson Rd.
   Sewickley, PA 15143
   Email: jbrayley@laurelnetworks.com

   Steve Vogelsang
   Laurel Networks, Inc.
   2706 Nicholson Rd.
   Sewickley, PA 15143
   Email: sjv@laurelnetworks.com

   John Shirron
   Laurel Networks, Inc.
   2607 Nicholson Rd.
   Sewickley, PA 15143
   Email: jshirron@laurelnetworks.com

   Luca Martini
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO 80021
   Email: luca@level3.net

   Tom Johnson
   Litchfield Communications, Inc.
   76 Westbury Park Rd.
   Watertown, CT 06795
   Email: tom_johnson@litchfieldcomm.com


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   Ed Hallman
   Litchfield Communications, Inc.
   76 Westbury Park Rd.
   Watertown, CT 06795
   Email: ed_hallman@litchfieldcomm.com

   Marlene Drost
   Litchfield Communications, Inc.
   76 Westbury Park Rd.
   Watertown, CT 06795
   Email: marlene_drost@litchfieldcomm.com

   Jim Boyle
   Protocol Driven Networks, Inc.
   1381 Kildaire Farm #288
   Cary, NC 27511
   Email: jboyle@pdnets.com

   David Zelig
   Corrigent Systems LTD.
   126, Yigal Alon st.
   Tel Aviv, ISRAEL
   Email:  davidz@corrigent.com

   Ron Cohen
   Lycium Networks
   Hamanofim 9, POB 12256
   Herzeliya, Israel 46733
   Email: ronc@lyciumnetworks.com

   Prayson Pate
   Overture Networks
   P. O. Box 14864
   RTP, NC, USA 27709
   Email: prayson.pate@overturenetworks.com

   Craig White
   Level3 Communications, LLC.
   1025 Eldorado Blvd,
   Broomfield CO 80021
   Email: Craig.White@Level3.com













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   Appendix A. SONET/SDH Rates and Formats

   For simplicity, the discussion in this section uses SONET
   terminology, but it applies equally to SDH as well.  SDH-equivalent
   terminology is shown in the tables.

   The basic SONET modular signal is the synchronous transport signal-
   level 1 (STS-1). A number of STS-1s may be multiplexed into higher-
   level signals denoted as STS-N, with N synchronous payload envelopes
   (SPEs). The optical counterpart of the STS-N is the Optical Carrier-
   level N, or OC-N. Table 2 lists standard SONET line rates discussed
   in this document.


     OC Level          OC-1    OC-3    OC-12      OC-48     OC-192
     SDH Term             -   STM-1    STM-4     STM-16     STM-64
     Line Rate(Mb/s) 51.840 155.520  622.080  2,488.320  9,953.280

                    Table 2. Standard SONET Line Rates


   Each SONET frame is 125 us and consists of nine rows. An STS-N frame
   has nine rows and N*90 columns. Of the N*90 columns, the first N*3
   columns are transport overhead and the other N*87 columns are SPEs.
   A number of STS-1s may also be linked together to form a super-rate
   signal with only one SPE. The optical super-rate signal is denoted
   as OC-Nc, which has a higher payload capacity than OC-N.

   The first 9-byte column of each SPE is the path overhead (POH) and
   the remaining columns form the payload capacity with fixed stuff
   (STS-Nc only).  The fixed stuff, which is purely overhead, is N/3-1
   columns for STS-Nc.  Thus, STS-1 and STS-3c do not have any fixed
   stuff, STS-12c has three columns of fixed stuff, and so on.

   The POH of an STS-1 or STS-Nc is always nine bytes in nine rows. The
   payload capacity of an STS-1 is 86 columns (774 bytes) per frame.
   The payload capacity of an STS-Nc is (N*87)-(N/3) columns per frame.
   Thus, the payload capacity of an STS-3c is (3*87 - 1)*9 = 2,340
   bytes per frame. As another example, the payload capacity of an STS-
   192c is 149,760 bytes, which is 64 times the capacity of an STS-3c.

   There are 8,000 SONET frames per second. Therefore, the SPE size,
   (POH plus payload capacity) of an STS-1 is 783*8*8,000 = 50.112
   Mb/s. The SPE size of a concatenated STS-3c is 2,349 bytes per frame
   or 150.336 Mb/s. The payload capacity of an STS-192c is 149,760
   bytes per frame, which is equivalent to 9,584.640 Mb/s. Table 2
   lists the SPE and payload rates supported.







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   SONET STS Level     STS-1   STS-3c  STS-12c    STS-48c   STS-192c
   SDH VC Level            -     VC-4  VC-4-4c   VC-4-16c   VC-4-64c
   Payload Size(Bytes)   774    2,340    9,360     37,440    149,760
   Payload Rate(Mb/s) 49.536  149.760  599.040  2,396.160  9,584.640
   SPE Size(Bytes)       783    2,349    9,396     37,584    150,336
   SPE Rate(Mb/s)     50.112  150.336  601.344  2,405.376  9,621.504

                      Table 2. Payload Size and Rate


   To support circuit emulation, the entire SPE of a SONET STS or SDH
   VC level is encapsulated into packets, using the encapsulation
   defined in section 4, for carriage across packet-switched networks.


Full Copyright Statement

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Acknowledgement

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   Internet Society.






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