Internet Draft






Network Working Group                                       Luca Martini
Internet Draft                                           Nasser El-Aawar
Expiration Date: March 2001                 Level 3 Communications, LLC.

                                                Dimitri Stratton Vlachos
                                                           Daniel Tappan
                                                           Eric C. Rosen
                                                     Cisco Systems, Inc.

                                                         Steve Vogelsang
                                                            John Shirron
                                                   Laurel Networks, Inc.

                                                         Andrew G. Malis
                                                                 Ken Hsu
                                                   Vivace Networks, Inc.

                                                          September 2000


                 Transport of Layer 2 Frames Over MPLS


               draft-martini-l2circuit-trans-mpls-03.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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Abstract

   This document describes methods for transporting the Protocol Data
   Units (PDUs) of layer 2 protocols such as Frame Relay, ATM AAL5,
   Ethernet, and providing a SONET circuit emulation service across an
   MPLS network.


Table of Contents

    1      Specification of Requirements  ..........................   2
    2      Introduction  ...........................................   3
    3      Tunnel Labels and VC Labels  ............................   3
    4      Optional Sequencing and/or Padding  .....................   4
    5      Protocol-Specific Issues  ...............................   5
    5.1    Frame Relay  ............................................   5
    5.2    ATM  ....................................................   6
    5.2.1  F5 OAM Cell Support  ....................................   6
    5.2.2  CLP Bit  ................................................   7
    5.2.3  PTI Field in ATM Cell Mode  .............................   7
    5.3    Ethernet VLAN  ..........................................   7
    5.4    Ethernet  ...............................................   7
    5.5    Circuit Emulation Service over MPLS (CEM)  ..............   8
    5.5.1  CEM Encapsulation Format  ...............................   8
    5.5.2  Clocking Mode  ..........................................   9
    5.5.3  Synchronous  ............................................   9
    5.5.4  Asynchronous  ...........................................  10
    6      LDP  ....................................................  10
    7      Security Considerations  ................................  13
    8      Open Issues  ............................................  13
    9      Intellectual  ...........................................  13
   10      References  .............................................  13
   11      Author Information  .....................................  14
   12      Appendix A: SONET/SDH Rates and Formats  ................  15





1. Specification of Requirements

   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.







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2. Introduction

   In an MPLS network, it is possible to carry the Protocol Data Units
   (PDUs) of layer 2 protocols by prepending an MPLS label stack to
   these PDUs.  This document specifies the necessary label distribution
   and encapsulation procedures for accomplishing this.  We restrict
   discussion to the case of point-to-point transport.  QoS related
   issues are not discussed in this draft.

   This document also describes a method for transporting time division
   multiplexed (TDM) digital signals (TDM circuit emulation) over a
   packet-oriented MPLS network. The transmission system for circuit-
   oriented TDM signals is the Synchronous Optical Network
   (SONET)[5]/Synchronous Digital Hierarchy (SDH) [6]. 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).


3. Tunnel Labels and VC Labels

   Suppose it is desired to transport layer 2 PDUs from ingress LSR R1
   to egress LSR R2, across an intervening MPLS network.  We assume that
   there is an LSP from R1 to R2.  That is, we assume that R1 can cause
   a packet to be delivered to R2 by pushing some label onto the packet
   and sending the result to one of its adjacencies.  Call this label
   the "tunnel label", and the corresponding LSP the "tunnel LSP".

   The tunnel LSP merely gets packets from R1 to R2, the corresponding
   label doesn't tell R2 what to do with the payload, and in fact if
   penultimate hop popping is used, R2 may never even see the
   corresponding label.  (If R1 itself is the penultimate hop, a tunnel
   label may not even get pushed on.)  Thus if the payload is not an IP
   packet, there must be a label, which becomes visible to R2, that
   tells R2 how to treat the received packet.  Call this label the "VC
   label".

   So when R1 sends a layer 2 PDU to R2, it first pushes a VC label on
   its label stack, and then (if R1 is not adjacent to R2) pushes on a
   tunnel label.  The tunnel label gets the MPLS packet from R1 to R2;
   the VC label is not visible until the MPLS packet reaches R2.  R2's
   disposition of the packet is based on the VC label.

   If the payload of the MPLS packet is, for example, an ATM AAL5 PDU,
   the VC label will generally correspond to a particular ATM VC at R2.
   That is, R2 needs to be able to infer from the VC label the outgoing



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   interface and the VPI/VCI value for the AAL5 PDU.  If the payload is
   a Frame Relay PDU, then R2 needs to be able to infer from the VC
   label the outgoing interface and the DLCI value.  If the payload is
   an ethernet frame, then R2 needs to be able to infer from the VC
   label the outgoing interface, and perhaps the VLAN identifier. This
   process is unidirectional, and will be repeated independently for
   bidirectional operation. It is desirable, but not required, to assign
   the same VC, and Group ID for a given circuit in both directions.
   Note that the VC label must always be at the bottom of the label
   stack, and the tunnel label, if present, must be immediately above
   the VC label.  Of course, as the packet is transported across the
   MPLS network, additional labels may be pushed on (and then popped
   off) as needed.  Even R1 itself may push on additional labels above
   the tunnel label.  If R1 and R2 are directly adjacent LSRs, then it
   may not be necessary to use a tunnel label at all.

   This document does not specify a method for distributing the tunnel
   label or any other labels that may appear above it on the stack.  Any
   acceptable method of MPLS label distribution will do.

   This document does specify a method for assigning and distributing
   the VC label. Static label assignment MAY be used, and
   implementations SHOULD provide support for this.  If signaling is
   used, the VC label MUST be distributed from R2 to R1 using LDP in the
   downstream unsolicited mode; this requires that an LDP connection be
   created between R1 and R2.

   Note that this technique allows an unbounded number of layer 2 "VCs"
   to be carried together in a single "tunnel".  Thus it scales quite
   well in the network backbone.

   The MPLS network should be configured with a MTU that is at least 12
   bytes larger then the largest packet size that will be transported in
   the LSPs.  If a packet, once it has been encapsulated, exceeds the
   LSP MTU, it MUST be dropped.


4. Optional Sequencing and/or Padding

   Sometimes it is important to guarantee that sequentiality is
   preserved on a layer 2 virtual circuit.  To accommodate this
   requirement, we provide an optional control word which may appear
   immediately after the label stack and immediately before the layer 2
   PDU.  This control word contains a sequence number. R1 and R2 both
   need to be configured with the knowledge of whether a control word
   will be used for a specific virtual circuit.

   Sometimes it is necessary to transmit a small packet on a medium



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   where there is a minimum transport unit larger than the actual packet
   size.  In this case, padding is appended to the packet. When the VC
   label is popped, it may be desirable to remove the padding before
   forwarding the packet.

   To facilitate this, the control word has a length field.  If the
   packet's length (without any padding) is less than 256 bytes, the
   length field MUST be set to the packet's length (without padding).
   Otherwise the length field MUST be set to zero.  The value of the
   length field, if non-zero, can be used to remove any padding.

   The generic control word is defined as follows:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Reserved   |     Length    |         Sequence Number       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The first 8 bits are reserved for future use.  They MUST be set to 0
   when transmitting, and MUST be ignored upon receipt. The length byte
   is set as specified above.

   The next 16 bits are the sequence number that is used to guarantee
   ordered packet delivery.  For a given VC label, and a given pair of
   LSRs, R1 and R2, where R2 has distributed that VC label to R1, the
   sequence number is initialized to 0, and is incremented by one for
   each successive packet carrying that VC label which R1 transmits to
   R2.

   The sequence number space is a 16 bit unsigned circular space.  PDUs
   carrying the control word MUST NOT be delivered out of order.   They
   may be discarded or reordered.


5. Protocol-Specific Issues

5.1. Frame Relay

   A Frame Relay PDU is transported in its entirety, including the Frame
   Relay Header.  The sequencing control word is OPTIONAL.

   The BECN and FECN signals are carried unchanged across the network in
   the frame relay header.  These signals do not appear in the MPLS
   header, and are unseen by the MPLS network.

   If the MPLS edge LSR detects a service affecting condition as defined



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   in [2] Q.933 Annex A.5 sited in IA FRF1.1, it will withdraw the label
   that corresponds to the frame relay DLCI. The Egress side should
   generate the corresponding errors and alarms as defined in [2] on the
   Frame relay VC.

   The ingress LSR MAY consider the DE bit of the Frame Relay header
   when determining the value to be placed in the EXP fields of the MPLS
   label stack. In a similar way, the egress LSR MAY consider the EXP
   field of the VC label when queuing the packet for egress.


5.2. ATM

   Two modes are supported for ATM transport, ATM Adaptation Layer 5
   (AAL5) and ATM cell.

   In ATM AAL5 mode the ingress LSR is required to reassemble AAL5
   CPCS-PDUs from the incoming VC and transport each CPCS-PDU as a
   single packet. No AAL5 trailer is transported. The sequencing control
   word is OPTIONAL.

   In ATM cell mode the ingress LSR transports each ATM cell payload as
   a single packet. No ATM cell header is transported. The sequencing
   control word is OPTIONAL.


5.2.1. F5 OAM Cell Support

   F5 OAM cells are not transported on the VC LSP.

   If an F5 end-to-end OAM cell is received from a VC by a LSR with a
   loopback indication value of 1 and the LSR has a label mapping for
   the VC, the LSR must decrement the loopback indication value and loop
   back the cell on the VC. Otherwise the loopback cell must be silently
   discarded by the LSR.

   A LSR may optionally be configured to periodically generate F5 end-
   to-end loopback OAM cells on a VC. In this case, the LSR must only
   generate F5 end-to-end loopback cells while a label mapping exists
   for the VC. If the VC label mapping is withdrawn the LSR must cease
   generation of F5 end-to-end loopback OAM cells. If the LSR fails to
   receive a response to an F5 end-to-end loopback OAM cell for a pre-
   defined period of time it must withdraw the label mapping for the VC.

   If an ingress LSR receives an AIS F5 OAM cell, fails to receive a
   pre-defined number of the End-to-End loop OAM cells, or a physical
   interface goes down, it must withdraw the label mappings for all VCs
   associated with the failure. When a VC label mapping is withdrawn,



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   the egress LSR must generate AIS F5 OAM cells on the VC associated
   with the withdrawn label mapping.


5.2.2. CLP Bit

   The ingress LSR MAY consider the CLP bit when determining the value
   to be placed in the EXP fields of the MPLS label stack.

   The egress LSR MAY consider the value of the EXP field of the VC
   label when determining the value of the ATM CLP bit.


5.2.3. PTI Field in ATM Cell Mode

   ATM cell mode is intended for transporting non-AAL5 traffic only. The
   ingress LSR must transport cells with a PTI of 0. Cells with a PTI
   other than 0 are not transported on the LSP. The egress LSR must set
   the PTI to 0 for cells switched from a VC LSP to an outgoing VC.


5.3. Ethernet VLAN

   For and ethernet 802.1q VLAN the entire ethernet frame without the
   preamble or FCS is transported as a single packet. The sequencing
   control word is OPTIONAL. If a packet is received out of sequence it
   MUST be dropped. The VLAN 4 byte tag is transported as is, and MAY be
   overwritten by the egress LSR.  The ingress LSR MAY consider the user
   priority field [4] of the VLAN tag header when determining the value
   to be placed in the EXP fields of the MPLS label stack. In a similar
   way, the egress LSR MAY consider the EXP field of the VC label when
   queuing the packet for egress. Ethernet packets containing hardware
   level CRC, Framing errors, or runt packets MUST be discarded on
   input.


5.4. Ethernet

   For simple ethernet port to port transport,the entire ethernet frame
   without the preamble or FCS is transported as a single packet. The
   sequencing control word is OPTIONAL. If a packet is received out of
   sequence it MUST be dropped. As in the Ethernet VLAN case, ethernet
   packets with hardware level CRC, framing, and runt errors are
   discarded.







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5.5. Circuit Emulation Service over MPLS (CEM)

   This section 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

   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.


5.5.1. 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 TDM header to associate the packet with the TDM stream. Note that
   the CEM control word is used instead of the generic control word in
   section 4.

   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.

   The 32-bit CEM control word 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Reserved   |  Sequnce Num  | Structure Pointer |N|P| BIP-4 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The above fields are defined as follows:

     - Reserved

       These eight bits are reserved for future use, such as alarm
       signaling or OAM.





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     - 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
       control word. The pointer is set to 0x3FF (1,023) if a packet
       does not carry the J1 byte. See [5] and [6] for more information
       on the J1 byte and the structure pointer.

     - The N and P bits

       See Section 5.4.2 below for their definition.

     - Seq Num

       This is a packet sequence number, which continuously cycles from
       0 to 255.  It begins at 0 when a TDM LSP is created.

     - BIP-4

       The bit interleaved even parity over the first 28 header bits.


5.5.2. 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.


5.5.3. 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 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.









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5.5.4. 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].


6. LDP

   The VC label bindings are distributed using the LDP downstream
   unsolicited mode described in [1].  The LSRs will establish an LDP
   session using the Extended Discovery mechanism described in [1,
   section 2.4-2.5], for this purpose a new type of FEC TLV element is
   defined.  The FEC element type in 128. [note1]

   The Virtual Circuit FEC TLV element, is defined as follows:

   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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    VC tlv     |C|         VC Type             |  VC ID len    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Group ID                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                          VC ID                                |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     - VC Type

       A 15 bit quantity containing a value which represents the type of
       VC. Assigned Values are:

               VC Type  Description

               0x0001   Frame Relay DLCI
               0x0002   ATM AAL5 PVC
               0x0003   ATM Cell
               0x0004   Ethernet VLAN
               0x0005   Ethernet
               0x0006   HDLC ( Cisco )
               0x0007   PPP
               0x8008   CEM




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       The highest order bit is used to flag the presence of a control word as
       follows:
         bit 15 = 1 control word present on this VC.
         bit 15 = 0 no control word present on this VC.

     - VC ID length

       Length of the VC ID field in octets. If this value is 0, then it
       references all VCs using the specified group ID

     - Group ID

       An arbitrary 32 bit value which represents a group of VCs that is
       used to augment the VC space. This value MUST be user
       configurable.  The group ID is intended to be used as either a
       port index , or a virtual tunnel index. In the latter case a
       switching function at ingress will map a particular circuit from
       a port to a circuit in the virtual tunnel for transport to the
       egress router.

     - VC ID

       Identifies a particular VC.  The interpretation of the identifier
       depends on the VC type:

         * Frame Relay

           A 32-bit value representing a 16-bit DLCI value as follows:

           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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |          Reserved             |             DLCI              |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


         * ATM AAL5 PVC

           A 32-bit value representing a 16-bit VPI, and a 16-bit VCI as
           follows:

           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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |             VPI               |              VCI              |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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         * ATM Cell

           A 32-bit value representing a 16-bit VPI, and a 16-bit VCI as
           follows:

           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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |             VPI               |              VCI              |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


         * Ethernet VLAN

           A 32 bit value representing 16bit vlan identifier as follows:

           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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |           Reserved            |            VLAN ID            |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


         * Ethernet

           A 32 bit port identifier.

         * HDLC ( Cisco )

           A 32-bit port identifier (details TBD).

         * PPP

           A 32-bit port identifier (details TBD).

         * CEM

           A 32-bit value used follows:
           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
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |   Reserved    |        Circuit ID         |   Payload Bytes   |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


           Circuit ID:  An assigned number for the SONET circuit being
           transported.




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           Payload Bytes(N):  the number of TDM payload bytes contained
           in all packets on the CES stream, 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.


           The reserved fields in the above specifications MUST be set
           to 0 in the FEC TLV, and ignored when received.


7. Security Considerations

   This document does not affect the underlying security issues of MPLS.


8. Open Issues

   Future revisions of this draft will discuss QoS requirements 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. Intellectual

   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.


10. References

   [1] "LDP Specification", draft-ietf-mpls-ldp-07.txt ( work in
   progress )

   [2] ITU-T Recommendation Q.933, and Q.922 Specification for Frame
   Mode Basic call control, ITU Geneva 1995

   [3] "MPLS Label Stack Encoding", draft-ietf-mpls-label-encaps-07.txt
   ( work in progress )




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   [4] "IEEE 802.3ac-1998" IEEE standard specification.

   [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.

   [note1] FEC element type 128 is pending IANA approval.



11. Author Information


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


   Nasser El-Aawar
   Level 3 Communications, LLC.
   1025 Eldorado Blvd.
   Broomfield, CO, 80021
   e-mail: nna@level3.net


   Dimitri Stratton Vlachos
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   e-mail: dvlachos@cisco.com


   Dan Tappan
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   e-mail: tappan@cisco.com









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   Eric Rosen
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   e-mail: erosen@cisco.com


   Steve Vogelsang
   Laurel Networks, Inc.
   2607 Nicholson Rd.
   Sewickley, PA 15143
   e-mail: sjv@laurelnetworks.com


   John Shirron
   Laurel Networks, Inc.
   2607 Nicholson Rd.
   Sewickley, PA 15143
   e-mail: sjv@laurelnetworks.com


   Andrew G. Malis
   Vivace Networks, Inc.
   2730 Orchard Parkway
   San Jose, CA 95134
   Phone: +1 408 383 7223
   Email: Andy.Malis@vivacenetworks.com


   Ken Hsu
   Vivace Networks, Inc.
   2730 Orchard Parkway
   San Jose, CA 95134 CA
   Phone: +1 408 432 7772
   Email: Ken.Hsu@vivacenetworks.com



12. 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



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   (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 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 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.

   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




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   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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