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

                                                      Steve Vogelsang
                                                         John Shirron
                                                Laurel Networks, Inc.

                                                         Luca Martini
                                         Level 3 Communications, LLC.

                                                        November 2000

   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 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 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 TDM 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
      |   Payload Bytes   |   Struct Pointer  |N|P|  Seq num  | BIP-4 |

                        Figure 1. TDM Header Format

   The above fields are defined as follows:

   Payload Bytes(N): the number of TDM payload bytes contained in this
   packet,  from 48 to 1,023 bytes.  All of the packets in a given CES
   stream have the same number of payload bytes.  Note that there is a
   possibility that the packet size may exceed the SPE size in the case
   of an STS-1 SPE, which could cause two pointers to be needed in the
   CEM header, since the payload may contain two J1 bytes for
   consecutive SPEs.  For this reason, the number of payload bytes must
   be less than 783 for STS-1 SPEs.

   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 TDM 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 Section 7 below for their definition.

   Seq Num:  This is a packet sequence number, which continuously
   cycles from 0 to 63.  It begins at 0 when a TDM LSP is created.

   BIP-4: The bit interleaved even parity is over the first 28 header

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.  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

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   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 jitter.

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 checked for the
   occurrence of a path AIS event. An example adaptive method can be
   found in Section 3.4.2 of [7].

7. CEM LSP Signaling

   For maximum network scaling, CEM LSP signaling may be performed
   using the LDP Extended Discovery mechanism as described 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 defined in [5] in order to signify that the LSP being
   signaled is to carry CEM.  Note that the sequencing control word in
   [6] is not used, as its functionality is included in the CEM

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

8.  Open Issues

   Future revisions of this draft will discuss QoS requirements and
   mechanisms for CEM, methods to provide (or simulate) bi-directional
   LSPs (perhaps using the Group ID from [5]), signaling for the number
   of payload bytes, and sending additional end-to-end alarm
   information in addition to AIS.

9. Security Considerations

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

10. 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.

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11. 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-04.txt, work in progress, November

   [6]  Martini et al, "Encapsulation Methods for Transport of Layer 2
        Frames Over MPLS", draft-martini-l2circuit-encap-mpls-00.txt,
        work in progress, November 2000.

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

12. Acknowledgments

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

13. 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

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                SONET/SDH Circuit Emulation Over MPLS       July 2000

   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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                SONET/SDH Circuit Emulation Over MPLS       July 2000

   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)û(N/3) columns per frame.
   Thus, the payload capacity of an STS-3c is (3*87 û 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 the next section, for carriage across MPLS networks.

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