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

                                                      Steve Vogelsang
                                                        John Shirron
                                                Laurel Networks, Inc.

                                                         Luca Martini
                                         Level 3 Communications, LLC.

                                                        February 2001

   SONET/SDH Circuit Emulation Service Over MPLS (CEM) Encapsulation

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-

   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

   The list of Internet-Draft Shadow Directories can be accessed at

1. Abstract

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

2. Conventions used in this document

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

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3. Introduction

   This document describes a method for encapsulating time division
   multiplexed (TDM) digital signals (TDM circuit emulation) for
   transmission over a packet-oriented MPLS network. The transmission
   system for circuit-oriented TDM signals is the Synchronous Optical
   Network (SONET)[3]/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 is closely related to references [5], which describes
   the control protocol methods used to signal the usage of CEM, and
   [6], 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

   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

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

   A TDM data stream is segmented into packets and encapsulated in MPLS
   packets. Each packet has one or more MPLS labels, followed by a 32-
   bit CEM header to associate the packet with the TDM stream.

   The outside label is used to identify the MPLS LSP used to tunnel
   the TDM packets through the MPLS network (the tunnel LSP).  The
   interior label is used to multiplex multiple TDM connections within
   the same tunnel.  This is similar to the label stack usage defined
   in [5] and [6].

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   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
      | Rsvd  |   Sequence Num    | Structure Pointer |N|P|   ECC-6   |

                        Figure 1. CEM Header Format

   The above fields are defined as follows:

   Reserved: These bits are reserved for future use.

   Sequence Number:  This is a packet sequence number, which
   continuously cycles from 0 to 1023.  It begins at 0 when a TDM LSP
   is created.

   Structure Pointer: The pointer points to the J1 byte in the payload
   area. The value is from 0 to 1,022, where 0 means the first byte
   after the CEM header. The pointer is set to 0x3FF (1,023) if a
   packet does not carry the J1 byte.  See [3] and [4] for more
   information on the J1 byte and the structure pointer.

   The N and P bits: See sections 6 and 7 for their definition.

   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.

6. Clocking Mode

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

6.1 Synchronous

   When synchronous SONET timing is available at both ends of the
   circuit, the N(JE) and P(JE) bits are set for negative or positive
   justification events. The event is carried in five consecutive
   packets at the transmitter. The receiver plays out the event when
   three out of five packets with NJE/PJE bit set are received. If both
   bits are set, then path AIS event has occurred (this is further
   discussed in section 7).  If there is a frequency offset between the
   frame rate of the transport overhead and that of the STS SPE, then
   the alignment of the SPE shall periodically slip back or advance in
   time through positive or negative stuffing. The N(JE) and P(JE) bits
   are used to replay the stuff indicators and eliminate transport

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6.2 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 [7].

7. Circuit Outages

   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 Path is not currently transmitting valid
   end-user data, and the SPE contains all one bits.  To conserve
   network bandwidth, the CEM header is used to indicate that the
   emulated SONET Path is signaling AIS-P, and the actual one bits are
   not transmitted.

   In the CEM header, both the N and P bits are set to signal AIS-P.
   When a CEM header is received with both bits set, the CEM receiver
   transmits the AIS-P alarm out the associated TDM interface.

   Note that the return RDI-P indication is contained, as usual, in the
   G1 octet in the SONET header.

   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.

8. 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.

9. Open Issues

   Future revisions of this draft will discuss underlying MPLS QoS
   requirements and mechanisms for CEM, support for VT and lower speed

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   non-SONET services, and sending additional maintenance alarms in
   addition to AIS-P.

10. Security Considerations

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

11. 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.

12. 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

   [6]  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.

   [7]  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, and Jeremy Brayley of Laurel Networks, for their
   comments and suggestions.

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14. Authors' 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

   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

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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 1 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 1. 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

   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 2. 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
   2.  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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   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 2.

   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
   2.  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.

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