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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 5143

Network Working Group                                 Andrew G. Malis
Internet Draft                                                Ken Hsu
Expiration Date: October 2001                   Vivace Networks, Inc.

                                                       Jeremy Brayley
                                                      Steve Vogelsang
                                                         John Shirron
                                                Laurel Networks, Inc.

                                                         Luca Martini
                                         Level 3 Communications, LLC.

                                                          Tom Johnson
                                                        Marlene Drost
                                                           Ed Hallman
                                            Litchfield Communications

                                                           April 2001


   SONET/SDH Circuit Emulation Service Over MPLS (CEM) Encapsulation
                     draft-malis-sonet-ces-mpls-04.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
   material or to cite them other than as "work in progress."

   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.


1. Abstract

This document describes a method for encapsulating SONET/SDH Path
signals for transport across an MPLS network.





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2. 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].



3. Introduction

   This document describes a method for encapsulating time division
   multiplexed (TDM) digital signals for transmission over a packet-
   oriented MPLS network.  The transmission system for circuit-oriented
   TDM signals is the Synchronous Optical Network (SONET) [3], [6] /
   Synchronous Digital Hierarchy (SDH) [4]. To support TDM traffic,
   which includes voice, data, and private leased line service, the
   MPLS network must emulate the circuit characteristics of SONET/SDH
   payloads.  MPLS labels and a new circuit emulation header are used
   to encapsulate TDM signals and provide the Circuit Emulation Service
   over MPLS (CEM).

   This document also describes an optional extension to CEM 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 closely related to references [5], which describes
   the control protocol methods used to signal the usage of CEM, and
   [7], which describes a related method of encapsulating Layer 2
   frames over MPLS and which shares the same signaling.


4. Scope

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

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

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

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

   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. OC-192c SPE/VC-4-64c are also not
   included at this point, since most MPLS networks use OC-192c or

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   slower trunks, and thus would not have sufficient capacity.  As
   trunk capacities increase in the future, the scope of this document
   can be accordingly extended.

5. CEM Encapsulation Format

   In order to transport SONET/SDH SPEs through a packet-oriented
   network, the SPE is broken into fragments.  A 32-bit CEM Header is
   pre-pended to each fragment.  The Basic CEM packet appears in Figure
   1.

             +-----------------------------------+
             |            CEM Header             |
             +-----------------------------------+
             |                                   |
             |                                   |
             |        SONET/SDH SPE Fragment     |
             |                                   |
             |                                   |
             +-----------------------------------+

             Figure 1. Basic CEM Packet


   The 32-bit CEM 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |D|Resvd|   Sequence Num    | Structure Pointer |N|P|   ECC-6   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 2. CEM Header Format


   The above fields are defined as follows:

   D bit: Signals DBA Mode.  MUST be set to zero for Normal Operation.
   MUST be set to one if CEM 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 sections 7 and 8 for further
   details.

   Reserved: These bits are reserved for future use, and MUST be set to
   zero.

   Sequence Number:  This is a packet sequence number, which MUST
   continuously cycle from 0 to 1023.  It SHOULD begin at zero when a
   CEM LSP is created.

   Structure Pointer: The Structure Pointer MUST contain the offset of
   the J1 byte within the CEM payload. The value is from 0 to 1,022,
   where 0 means the first byte after the CEM header. The Structure

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   Pointer MUST be set to 0x3FF (1,023) if a packet does not carry the
   J1 byte.  See [3], [4] and [6] for more information on the J1 byte
   and the SONET/SDH payload pointer.

   The N and P bits: Indicate negative and positive pointer adjustment
   events.  They are also used to relay SONET/SDH maintenance signals
   such as AIS-P.  See Table 1 and sections 7 and 8 for more details.

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

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

   ECC-6: An Error Correction Code to protect the CEM header.  This
   offers the ability to correct single bit errors and detect up to two
   bit errors.  The ECC algorithm is described in Appendix B.

   Note: CEM packets are fixed in length for all of the packets of a
   particular emulated TDM stream.  This length is signaled using the
   CEM Payload Bytes parameter defined in [5], or is statically
   provisioned for each TDM stream.  Therefore, the length of each CEM
   packet does not need to be carried in the CEM header.

5.1 Transport Encapsulation

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

5.1.1 MPLS Transport

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

   The last two labels prior to the CEM header are referred to as the
   Tunnel and VC labels.

   The VC label is required, and is the last label prior to the CEM
   Header.  The VC label MUST be used to identify the CEM connection
   within the MPLS tunnel.


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   The optional tunnel label is immediately above the VC label on the
   label stack.  If present, the tunnel label MUST be used to identify
   the MPLS LSP used to tunnel the TDM packets through the MPLS network
   (the tunnel LSP).

   This is similar to the label stack usage defined in [5] and [7].


                 +-----------------------------------+
                 | Additional MPLS Labels (Optional) |
                 +-----------------------------------+
                 |       Tunnel Label (Optional)     |
                 +-----------------------------------+
                 |             VC Label              |
                 +-----------------------------------+
                 |            CEM Header             |
                 +-----------------------------------+
                 |                                   |
                 |                                   |
                 |       SONET/SDH SPE Fragment      |
                 |                                   |
                 |                                   |
                 +-----------------------------------+

                 Figure 3. Typical MPLS Transport Encapsulation


5.1.1 IP Transport

   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 CEM directly onto IP by
   mapping the previously described MPLS packets onto IP.

   A mechanism for carrying MPLS over IP is described in [8].

















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   Using this encapsulation scheme would result in the packet format
   illustrated in figure 4.



                 +-----------------------------------+
                 |                                   |
                 |        IPv6/v4 Header [8]         |
                 |                                   |
                 +-----------------------------------+
                 |      Tunnel Label (Optional)      |
                 +-----------------------------------+
                 |             VC Label              |
                 +-----------------------------------+
                 |            CEM Header             |
                 +-----------------------------------+
                 |                                   |
                 |                                   |
                 |       SONET/SDH SPE Fragment      |
                 |                                   |
                 |                                   |
                 +-----------------------------------+

                  Figure 4. MPLS Transport Encapsulation






























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6. CEM Operation

   The following sections describe CEM operation.

6.1 Introduction and Terminology

   CEM 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 CEM Header and the MPLS label-stack, and then
   transmitted into the packet network.  During DBA mode, only the CEM
   header and MPLS label stack 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.

6.1.1 CEM Packetizer and De-Packetizer

   As with all adaptation functions, CEM has two distinct components:
   adapting TDM SONET/SDH into a CEM packet stream, and converting the
   CEM packet stream back into a TDM SONET/SDH.  The first function
   will be referred to as CEM Packetizer and the second as CEM De-
   Packetizer.  This terminology is illustrated in figure 5.


             +------------+              +---------------+
             |            |              |               |
   SONET --> |    CEM     | --> MPLS --> |      CEM      | --> SONET
    SDH      | Packetizer |              | De-Packetizer |      SDH
             |            |              |               |
             +------------+              +---------------+

   Figure 5. CEM Terminology

   Note: the CEM receive function requires a buffering mechanism to
   account for delay variation in the CEM packet stream.  This
   buffering mechanism will be generically referred to as the CEM
   jitter buffer.

6.1.2 CEM DBA

   CEM DBA is an optional mode of operation that only transmits the CEM
   Header and MPLS Label Stack into the packet network under certain
   circumstances such as AIS-P or STS Unequipped.

   If DBA is supported by a CEM 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.

   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.  DBA based on pattern detection within the SPE (i.e.
   all zeros, 7Es, or ATM idle cells) is for further study.

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   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.

6.2 Description of Normal CEM Operation

   During normal operation, the CEM 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 CEM Header
   and MPLS label stack are pre-pended to the SPE fragment and the
   resulting CEM packet is transmitted into the MPLS network.  Because
   all CEM packets associated with a specific SONET/SDH channel will
   have the same length, the transmission of CEM packets for that
   channel SHOULD occur at regular intervals.

   At the far end of the packet network, the CEM 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 CEM channel.

6.3 Description of CEM Operation during DBA

   There are several issues that should be addressed by a workable CEM
   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 CEM packetizer and CEM 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.

   Another goal is that the reduction of CEM 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 CEM 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 CEM packets during DBA MUST only carry
   the CEM header and the MPLS label stack.  The D-bit MUST be set to
   one, to indicate that DBA is active.


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   The CEM 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.

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

7. SONET/SDH Maintenance Signals

   There are several issues that must be considered in the mapping of
   maintenance signals between SONET/SDH and MPLS.  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
   MPLS, and MPLS to SONET/SDH.

7.1 SONET/SDH to MPLS
   The following sections describe how SONET/SDH Maintenance Signals
   and Alarm conditions are mapped into MPLS.

7.1.1 AIS-P Indication

   In a SONET/SDH network, circuit 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.

















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


                       Figure 6.  SONET/SDH Fault Hierarchy.

   Should the Section Layer detect a Loss of Section (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, CEM 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 CEM packets with the SPE fragment, CEM Header, and MPLS label
   stack MUST be transmitted into the packet network.

   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 CEM header and
   the MPLS label stack MUST be transmitted into the packet network.

   Also note that this differs from the outage mechanism in [5], which
   withdraws labels as a result of an endpoint outage.  TDM circuit
   emulation requires the ability to distinguish between the de-
   provisioning of a circuit, which would cause the labels to be
   withdrawn, and temporary outages, which are signaled using AIS-P.


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7.1.2 STS SPE Unequipped Indication

   The STS SPE Unequipped Indication is a slightly different case than
   AIS-P.  When byte C2 of the Path Overhead (STS path signal label) is
   00h and Byte B3 (STS Path BIP-8) is valid, it indicates that the SPE
   is unequipped.  Note: this is typically signaled by setting the
   entire SPE to zeros.

   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 CEM Header and MPLS label stack, 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 CEM Header and MPLS Label Stack
   must be transmitted into the packet network.  The N and P bits MUST
   be used to signal pointer adjustments as normal.  See Table 1 and
   section 8 for details.

7.1.3 RDI-P Indication

   The CEM function MUST send RDI-P towards the packet network under a
   variety of network errors such as loss of packet synchronization.
   This MUST be accomplished by modifying the SONET/SDH Path Overhead
   within the CEM packets.  Specifically the G1 byte must be updated to
   signal RDI-P and the B3 (Path BIP-8) must be re-computed.  See [3],
   [4], and [6] for details.

7.2 MPLS to SONET/SDH

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

7.2.1 AIS-P Indication

   There are several conditions in the packet network that will cause
   the CEM de-packetization function to send an AIS-P indication onto a
   SONET/SDH channel.

   The first of these is the receipt of CEM 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 CEM 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 [3], [4], and [6].

   The second case is the receipt of CEM packets with the N and P bits
   set to one, and the D-bit set to one.  This is an explicit
   indication of AIS-P being received at the far-end of the packet
   network, with DBA enabled for AIS-P.  The CEM de-packetizer MUST
   play out one packetÆs worth of all ones for each packet received,

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   and MUST configure the SONET/SDH Overhead to signal AIS-P as defined
   in [3], [4], and [6].

   Additional conditions that SHOULD trigger the transmission of AIS-P
   onto a SONET/SDH channel include loss of packet synchronization and
   jitter buffer under-run.  The definition of these conditions are
   under investigation and will be clarified in a subsequent revision
   of this draft.

7.2.2 STS SPE Unequipped Indication

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

   The first, which is transparent to CEM, is the receipt of regular
   CEM 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 CEM de-
   packetizer.

   The second case is the receipt of CEM 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 CEM 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.

7.2.3 RDI-P Indication

   The CEM function MUST send an RDI-P towards a SONET/SDH channel
   under the conditions defined for SONET/SDH Line Terminating
   equipment in [3], [4], and [6].

8. Clocking Modes

   It is necessary to be able to regenerate the input service clock at
   the output interface.  Two clocking modes are supported: synchronous
   and asynchronous.

8.1 Synchronous

   When synchronous SONET/SDH timing is available at both ends of the
   circuit, the N and P bits are used to signal negative or positive
   pointer justification events.

   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 N and P bits are used to replay
   the stuff indicators and eliminate transport jitter.



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   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.

   Furthermore, it is possible for pointer adjustments to occur in back
   to back SONET/SDH frames.  In order to support this possibility, the
   packet size for a particular circuit MUST be no larger than
   (783*N)/3.  Where N is the STS-Nc multiplier.

   Since the minimum value of N is one, all CEM implementations MUST
   support a minimum payload length of 783/3 or 261 bytes.  Smaller
   payload lengths MAY be supported as an option.

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

   If both bits are set, then an AIS-P event has occurred (this is
   further discussed in section 7).

   When DBA is invoked (i.e. the D-bit = 1), N and P have additional
   meanings.  See Table 1 and section 7.

8.2 Asynchronous

   If synchronous timing is not available, the N and P bits are not
   used for frequency justification and adaptive methods are used to
   recover the timing. The N and P bits are only used for the
   occurrence of a path AIS event. An example adaptive method can be
   found in section 3.4.2 of [9].


9. CEM LSP Signaling

   For maximum network scaling, CEM LSP signaling may be performed
   using the LDP Extended Discovery mechanism as augmented by the VC
   FEC Element defined in [5].  MPLS traffic tunnels may be dedicated
   to CEM, or shared with other MPLS-based services.  The value 8008 is
   used for the VC Type in the VC FEC Element in order to signify that
   the LSP being signaled is to carry CEM.  Note that the generic
   control word defined in [6] is not used, as its functionality is
   included in the CEM encapsulation header.

   Alternatively, static label assignment may be used, or a dedicated
   traffic engineered LSP may be used for each CEM circuit.

   CEM packets are fixed in length for all of the packets of a
   particular emulated TDM stream.  This length is signaled using the
   CEM Payload Bytes parameter defined in [5], or is statically
   provisioned for each TDM stream.


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   The use of DBA is signaled by the use of the CEM Options parameter
   defined in [5], or is statically provisioned for each TDM stream.

10. Open Issues

   Future revisions of this document may discuss the following items.

   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.

   The current draft only considers DBA based on SONET/SDH Overhead.
   It would be very desirable to extending DBA to include pattern-based
   suppression such as long runs of HDLC flags (i.e. 0x7E).  One issue
   that complicates pattern-based DBA is that the path overhead appears
   every Nx87 bytes within the SPE.  One solution may be to have a
   special mode of DBA, where the Path Overhead is explicitly
   transported within the packet along with the specific pattern (e.g.
   7E).

   This revision of the draft does not provide a definition for æloss
   of packet synchronizationÆ or æjitter buffer under-runÆ.  Details
   for declaring these conditions at the de-packetizer will be
   addressed in future revisions.

11. Security Considerations

   As with [5], this document does not affect the underlying security
   issues of MPLS.





















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                SONET/SDH Circuit Emulation Over MPLS      April 2001


12. Intellectual Property Disclaimer

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


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, March 1997

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

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

   [5]  Martini et al, "Transport of Layer 2 Frames Over MPLS", draft-
        martini-l2circuit-trans-mpls-05.txt, work in progress, February
        2001.

   [6]  Telcordia Technologies, ôSynchronous Optical Network (SONET)
        Transport Systems: Common Generic Criteriaö, GR-253-CORE, Issue
        3, September 2000.

   [7]  Martini et al, "Encapsulation Methods for Transport of Layer 2
        Frames Over MPLS", draft-martini-l2circuit-encap-mpls-01.txt,
        work in progress, February 2001.

   [8]  Worster, ôMPLS Label Stack Encapsulation in IPö, draft-worster-
        mpls-in-ip-04, work in progress, Expires August 2001.

   [9]  ATM Forum, "Circuit Emulation Service Interoperability
        Specification Version 2.0", af-vtoa-0078.000, January 1997.


13. Acknowledgments

   The authors would like to thank Mitri Halabi and Bob Colvin, both of
   Vivace Networks, for their comments and suggestions.


14. Authors' Addresses

   Andrew G. Malis
   Vivace Networks, Inc.

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                SONET/SDH Circuit Emulation Over MPLS      April 2001


   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

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

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

   Marlene Drost
   Litchfield Communications
   76 Westbury Park Rd.
   Watertown, CT 06795
   Email: marlene_drost@litchfieldcomm.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 ´s 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 exactly 64 times larger than the
   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 5, for carriage across MPLS networks.


Appendix B. ECC-6 Definition

   ECC-6 is an Error Correction Code to protect the CEM header.  This
   provides single bit correction and the ability to detect up to two
   bit errors.


   Error Correction Code:


   |---------------Header bits 0-25 -------------------| ECC-6 code|
   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 1 1 1 1 0 0 0 1 0 0 0 1 1 1 1 1 0 1 0 0 0 1 0 1 1|1 0 0 0 0 0|
   |1 1 1 1 0 1 0 0 0 1 0 0 1 0 0 0 0 1 0 1 1 1 1 1 1 1|0 1 0 0 0 0|
   |1 0 0 0 1 1 1 1 0 0 1 0 1 1 1 0 0 0 1 1 1 1 0 0 1 1|0 0 1 0 0 0|
   |0 1 0 0 1 1 1 1 0 0 0 1 1 0 0 1 1 1 1 1 0 0 1 1 0 1|0 0 0 1 0 0|
   |0 0 1 0 0 0 1 0 1 1 1 1 1 1 0 0 1 1 1 1 1 0 1 0 1 0|0 0 0 0 1 0|
   |0 0 0 1 0 0 0 1 1 1 1 1 0 0 1 1 0 0 1 1 0 1 1 1 1 1|0 0 0 0 0 1|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 7. ECC-6 Check Matrix X


   The ECC-6 code protects the 32 bit CEM header as follows:

   The encoder generates the 6 bit ECC using the matrix shown in Figure
   7.  In brief, the encoder builds another 26 column by 6 row matrix
   and calculates even parity over the rows.  The matrix columns
   represent CEM header bits 0 through 25.

   Denote each column of the ECC-6 check matrix by X[], and each column
   of the intermediate encoder matrix as Y[].  CEM[] denotes the CEM
   header and ECC[] is the error correction code that is inserted into
   CEM header bits 26 through 31.



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                SONET/SDH Circuit Emulation Over MPLS      April 2001


   for i = 0 to 25 {
        if CEM[i] = 0 {
                Y[i] = 0;
        } else {
                Y[i] = X[i];
        }
   }

   In other words, for each CEM header bit (i) set to 1, set the
   resulting matrix column Y[i] according to Figure 7.

   The final ECC-6 code is calculated as even parity of each row in Y
   (i.e. ECC[k]=CEM[25+k]=even parity of row k).

   The receiver also uses matrix X to calculate an intermediate matrix
   YÆ based on all 32 bits of the CEM header.  Therefore YÆ is 32
   columns wide and includes the ECC-6 code.

   for i = 0 to 31 {
        if CEM[i] = 0 {
                YÆ[i] = 0;
        } else {
                YÆ[i] = X[i];
        }
   }

   The receiver then appends the incoming ECC-6 code to Y as column 32
   (ECC[0] should align with row 0) and calculates even parity for each
   row.  The result is a single 6 bit column Z.  If all 6 bits are 0,
   there are no bit errors (or at least no detectable errors).
   Otherwise, it uses Z to perform a reverse lookup on X[] from Figure
   7.  If Z matches column X[i], then there is a single bit error.  The
   receiver should invert bit CEM[i] to correct the header.  If Z fails
   to match any column of X, then the CEM header contains more than one
   bit error and the CEM packet MUST be discarded.

   Note that the ECC-6 code provides single bit correction and 2-bit
   detection of errors within the received ECC-6 code itself


Full Copyright Statement

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   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for

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                SONET/SDH Circuit Emulation Over MPLS      April 2001


   copyrights defined in the Internet Standards process must be
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Acknowledgement

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




































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