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RFC 4906 - Transport of Layer 2 Frames Over MPLS


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Network Working Group                                    L. Martini, Ed.
Request for Comments: 4906                                 E. Rosen, Ed.
Category: Historic                                   Cisco Systems, Inc.
                                                        N. El-Aawar, Ed.
                                             Level 3 Communications, LLC
                                                               June 2007

                 Transport of Layer 2 Frames Over MPLS

Status of This Memo

   This memo defines a Historic Document for the Internet community.  It
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document describes methods for transporting the Protocol Data
   Units (PDUs) of layer 2 protocols such as Frame Relay, Asynchronous
   Transfer Mode (ATM) Adaption Layer 5 (AAL5), and Ethernet, and for
   providing a Synchronized Optical Network (SONET) circuit emulation
   service across an MPLS network.  This document describes the so-
   called "draft-martini" protocol, which has since been superseded by
   the Pseudowire Emulation Edge to Edge Working Group specifications
   described in RFC 4447 and related documents.

Table of Contents

   1. Introduction ....................................................3
   2. Specification of Requirements ...................................3
   3. Special Note ....................................................3
   4. Tunnel Labels and Virtual Circuit (VC) Labels ...................4
   5. Protocol-Specific Details .......................................5
      5.1. Frame Relay ................................................5
      5.2. ATM ........................................................6
           5.2.1. ATM AAL5 VCC Transport ..............................6
           5.2.2. ATM Transparent Cell Transport ......................6
           5.2.3. ATM VCC and VPC Cell Transport ......................6
           5.2.4. OAM Cell Support ....................................6
           5.2.5. ILMI Support ........................................7
      5.3. Ethernet VLAN ..............................................7
      5.4. Ethernet ...................................................8
      5.5. HDLC .......................................................8
      5.6. PPP ........................................................8
   6. LDP .............................................................8
      6.1. Interface Parameters Field ................................10
      6.2. C Bit Handling Procedures .................................12
           6.2.1. VC Types for Which the Control Word is REQUIRED ....12
           6.2.2. VC Types for Which the Control Word is NOT
                  Mandatory ..........................................12
           6.2.3. Status Codes .......................................15
      6.3. LDP Label Withdrawal Procedures ...........................15
      6.4. Sequencing Considerations .................................15
           6.4.1. Label Mapping Advertisements .......................15
           6.4.2. Label Mapping Release ..............................16
   7. IANA Considerations ............................................16
   8. Security Considerations ........................................16
   9. Normative References ...........................................17
   10. Informative References ........................................18
   11. Co-Authors ....................................................18

1.  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
   procedures for accomplishing this using the encapsulation methods in
   [RFC4905].  We restrict discussion to the case of point-to-point
   transport.  Quality of service (QoS)-related issues are not discussed
   in this document.  This document describes methods for transporting a
   number of protocols; in some cases, transporting a particular
   protocol may have several modes of operation.  Each of these
   protocols and/or modes may be implemented independently.

   An accompanying document [CEM] 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) [ANSI.T1.105] / Synchronous
   Digital Hierarchy (SDH) [ITU.G.707].  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).

2.  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 [RFC2119].

3.  Special Note

   This document describes the so-called "draft-martini" protocol, which
   is used in many deployed implementations.  This document and its
   contents have since been superseded by the Pseudowire Emulation Edge
   to Edge Working Group specifications: [RFC4447], [RFC4385],
   [RFC4448], [RFC4717], [RFC4618], [RFC4619], [RFC4553], [RFC4842], and
   related documents.  This document serves as a documentation of
   current implementations, and MUST NOT be used for new
   implementations.  The PWE3 Label Distribution Protocol (LDP) control
   document [RFC4447], which is backward compatible with this document,
   MUST be used for all new implementations of this protocol.

4.  Tunnel Labels and Virtual Circuit (VC) Labels

   Suppose it is desired to transport layer 2 PDUs from ingress Label
   Switching Router (LSR) R1 to egress LSR R2, across an intervening
   MPLS network.  We assume that there is a Label Switched Path (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.  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.

   Note that the tunnel could be a Generic Routing Encapsulation (GRE)-
   encapsulated MPLS tunnel between R1 and R2.  In this case, R1 would
   be adjacent to R2, and only the VC label would be used, and the
   intervening network need only carry IP packets.

   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
   interface and the VPI/VCI (Virtual Path Identifier / Virtual Circuit
   Identifier) 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 (Data Link Connection Identifier) 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 REQUIRED to assign
   the same VC ID, and VC type for a given circuit in both directions.
   The group ID (see below) MUST NOT be required to match in both
   directions.  The transported frame MAY be modified when it reaches
   the egress router.  If the header of the transported layer 2 frame is
   modified, this MUST be done at the egress LSR only.

   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 the VC label 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.  When 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 session be
   created between R1 and R2.  It should be noted that this LDP session
   is not necessarily transported along the same path as the Layer 2
   PDUs [RFC3036].  In addition, when using LDP to distribute the VC
   label, liberal label retention mode SHOULD be used.  However, as
   required in [RFC3036], the label request operation (mainly used by
   conservative label retention mode) MUST be implemented.  VC labels
   MUST be allocated from the per-platform label space.

   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.

   While this document currently defines the emulation of Frame Relay
   and ATM Permanent Virtual Circuit (PVC) services, it specifically
   does not preclude future enhancements to support switched service
   (Switched Virtual Circuit (SVC) and Switched Permanent Virtual
   Circuit (SPVC)) emulation.

5.  Protocol-Specific Details

5.1.  Frame Relay

   The Frame Relay PDUs are encapsulated according to the procedures
   defined in [RFC4905].  The MPLS edge LSR MUST provide Frame Relay PVC
   status signaling to the Frame Relay network.  If the MPLS edge LSR
   detects a service affecting condition, as defined in [Q.933] Annex
   A.5 cited in Implementation Agreement FRF.1.1, it MUST withdraw the
   label that corresponds to the frame relay DLCI.  The egress LSR
   SHOULD generate the corresponding errors and alarms as defined in
   [Q.933] on the egress Frame relay VC.

5.2.  ATM

5.2.1.  ATM AAL5 VCC Transport

   ATM AAL5 Common Part Convergence Sublayer - Service Data Units
   (CPCS-SDUs) are encapsulated according to [RFC4905] ATM AAL5 CPCS-SDU
   mode.  This mode allows the transport of ATM AAL5 CSPS-SDUs traveling
   on a particular ATM PVC across the MPLS network to another ATM PVC.

5.2.2.  ATM Transparent Cell Transport

   This mode is similar to the Ethernet port mode.  Every cell that is
   received at the ingress ATM port on the ingress LSR, R1, is
   encapsulated according to [RFC4905], ATM cell mode, and sent across
   the LSP to the egress LSR, R2.  This mode allows an ATM port to be
   connected to only one other ATM port.  [RFC4905] allows for grouping
   of multiple cells into a single MPLS frame.  Grouping of ATM cells is
   OPTIONAL for transmission at the ingress LSR, R1.  If the Egress LSR
   R2 supports cell concatenation, the ingress LSR, R1, should only
   concatenate cells up to the "Maximum Number of concatenated ATM
   cells" parameter received as part of the FEC element.

5.2.3.  ATM VCC and VPC Cell Transport

   This mode is similar to the ATM AAL5 Virtual Channel Connection (VCC)
   transport except that cells are transported.  Every cell that is
   received on a pre-defined ATM PVC or ATM Permanent Virtual Path
   (PVP), at the ingress ATM port on the ingress LSR, R1, is
   encapsulated according to [RFC4905], ATM cell mode, and sent across
   the LSP to the egress LSR R2.  Grouping of ATM cells is OPTIONAL for
   transmission at the ingress LSR, R1.  If the egress LSR R2 supports
   cell concatenation, the ingress LSR, R1, MUST only concatenate cells
   up to the "Maximum Number of concatenated ATM cells in a frame"
   parameter received as part of the FEC element.

5.2.4.  OAM Cell Support

   Operations and Management (OAM) cells MAY be transported on the VC
   LSP.  When the LSR is operating in AAL5 CPCS-SDU transport mode, if
   it does not support transport of ATM cells, the LSR MUST discard
   incoming MPLS frames on an ATM VC LSP that contain a VC label with
   the T bit set [RFC4905].  When operating in AAL5 SDU transport mode,
   an LSR that supports transport of OAM cells using the T bit defined
   in [RFC4905], or an LSR operating in any of the three cell transport
   modes, MUST follow the procedures outlined in [FAST] Section 8 for
   mode 0 only, in addition to the applicable procedures specified in
   [ITU.G.707].

5.2.4.1.  OAM Cell Emulation Mode

   AN LSR that does not support transport of OAM cells across an LSP MAY
   provide OAM support on ATM PVCs using the following procedures:

   A pair of LSRs MAY emulate a bidirectional ATM VC by two
   unidirectional LSPs.  If an F5 end-to-end OAM cell is received from a
   ATM VC, by either LSR that is transporting this ATM VC, with a
   loopback indication value of 1, and the LSR has a label mapping for
   the ATM VC, then the LSR MUST decrement the loopback indication value
   and loop back the cell on the ATM VC.  Otherwise, the loopback cell
   MUST be discarded by the LSR.

   The ingress LSR, R1, may also optionally be configured to
   periodically generate F5 end-to-end loopback OAM cells on a VC.  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, R1, receives an AIS (Alarm Indication Signal) F5
   OAM cell, or R1 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, the egress LSR, R2, MUST
   generate AIS F5 OAM cells on the VC associated with the withdrawn
   label mapping.  In this mode it is very useful to apply a unique
   group ID to each interface.  In the case where a physical interface
   goes down, a wild card label withdraw can be sent to all LDP
   neighbors, greatly reducing the signaling response time.

5.2.5.  ILMI Support

   An MPLS edge LSR MAY provide an ATM Integrated Local Management
   Interface (ILMI) to the ATM edge switch.  If an ingress LSR receives
   an ILMI message indicating that the ATM edge switch has deleted a VC,
   or if the physical interface goes down, it MUST withdraw the label
   mappings for all VCs associated with the failure.  When a VC label
   mapping is withdrawn, the egress LSR SHOULD notify its client of this
   failure by deleting the VC using ILMI.

5.3.  Ethernet VLAN

   The Ethernet frame will be encapsulated according to the procedures
   in [RFC4905].  It should be noted that if the VLAN identifier is
   modified by the egress LSR, according to the procedures outlined
   above, the Ethernet spanning tree protocol might fail to work
   properly.  If the LSR detects a failure on the Ethernet physical

   port, or the port is administratively disabled, it MUST withdraw the
   label mappings for all VCs associated with the port.

5.4.  Ethernet

   The Ethernet frame will be encapsulated according to the procedures
   in [RFC4905].  If the LSR detects a failure on the Ethernet physical
   port, or the port is administratively disabled, the corresponding VC
   label mapping MUST be withdrawn.

5.5.  HDLC

   HDLC (High-Level Data Link Control) frames are encapsulated according
   to the procedures in [RFC4905].  If the MPLS edge LSR detects that
   the physical link has failed, or the port is administratively
   disabled, it MUST withdraw the label mapping that corresponds to the
   HDLC link.

5.6.  PPP

   PPP frames are encapsulated according to the procedures in [RFC4905].
   If the MPLS edge LSR detects that the physical link has failed, or
   the port is administratively disabled, it MUST withdraw the label
   mapping that corresponds to the PPP link.

6.  LDP

   The VC label bindings are distributed using the LDP downstream
   unsolicited mode described in [RFC3036].  The LSRs will establish an
   LDP session using the Extended Discovery mechanism described in
   sections 2.4.2 and 2.5 of [RFC3036]; for this purpose, a new type of
   FEC element is defined.  The FEC element type is 128.  Only a single
   VC FEC element MUST be advertised per LDP VC label.  The Virtual
   Circuit FEC 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    VC tlv     |C|         VC Type             |VC info Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Group ID                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        VC ID                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Interface parameters                    |
   |                              "                                |
   |                              "                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      - VC Type

        A 15-bit quantity containing a value that represents the type of
        VC.  Assigned values are:

                VC Type  Description

                0x0001   Frame Relay DLCI
                0x0002   ATM AAL5 VCC transport
                0x0003   ATM transparent cell transport
                0x0004   Ethernet VLAN
                0x0005   Ethernet
                0x0006   HDLC
                0x0007   PPP
                0x8008   CEM [CEM]
                0x0009   ATM VCC cell transport
                0x000A   ATM VPC cell transport

      - Control word bit (C)

        The highest order bit (C) of the VC type is used to flag the
        presence of a control word (defined in [RFC4905]) as follows:

                bit 15 = 1 control word present on this VC.
                bit 15 = 0 no control word present on this VC.

        Please see Section 6.2, "C Bit Handling Procedures", for further
        explanation.

      - VC information length

        Length of the VC ID field and the interface parameters field in
        octets.  If this value is 0, then it references all VCs using
        the specified group ID, and there is no VC ID present, nor any
        interface parameters.

      - Group ID

        An arbitrary 32-bit value, which represents a group of VCs that
        is used to create groups in the VC space.  The group ID is
        intended to be used as a port index, or a virtual tunnel index.
        To simplify configuration, a particular VC ID at ingress could
        be part of the virtual tunnel for transport to the egress
        router.  The group ID is very useful to send wild card label
        withdrawals to remote LSRs upon physical port failure.

      - VC ID

        A non-zero 32-bit connection ID that, together with the VC type,
        identifies a particular VC.

      - Interface parameters

        This variable length field is used to provide interface-specific
        parameters, such as interface MTU.

6.1.  Interface Parameters Field

   This field specifies interface-specific parameters.  When applicable,
   it MUST be used to validate that the LSRs, and the ingress and egress
   ports at the edges of the circuit, have the necessary capabilities to
   interoperate with each other.  The field structure 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Parameter ID |    Length     |    Variable Length Value      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Variable Length Value                 |
   |                             "                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The parameter ID is defined as follows:

   Parameter   ID Length    Description

       0x01         4       Interface MTU in octets.
       0x02         4       Maximum Number of concatenated ATM cells.
       0x03   up to 82      Optional Interface Description string.
       0x04         4       CEM [CEM] Payload Bytes.
       0x05         4       CEM options.

   The Length field is defined as the length of the interface parameter
   including the Parameter ID and Length field itself.  Processing of
   the interface parameters should continue when encountering unknown
   interface parameters, and they MUST be silently ignored.

      - Interface MTU

        A 2-octet value indicating the MTU in octets.  This is the
        Maximum Transmission Unit, excluding encapsulation overhead, of
        the egress packet interface that will be transmitting the
        decapsulated PDU that is received from the MPLS network.  This

        parameter is applicable only to VC types 1, 2, 4, 5, 6, and 7,
        and is REQUIRED for these VC types.  If this parameter does not
        match in both directions of a specific VC, that VC MUST NOT be
        enabled.

      - Maximum Number of concatenated ATM cells

        A 2-octet value specifying the maximum number of concatenated
        ATM cells that can be processed as a single PDU by the egress
        LSR.  An ingress LSR transmitting concatenated cells on this VC
        can concatenate a number of cells up to the value of this
        parameter, but MUST NOT exceed it.  This parameter is applicable
        only to VC types 3, 9, and 0x0a, and is REQUIRED for these VC
        types.  This parameter does not need to match in both directions
        of a specific VC.

      - Optional Interface Description string

        This arbitrary, OPTIONAL interface description string can be
        used to send an administrative description text string to the
        remote LSR.  This parameter is OPTIONAL, and is applicable to
        all VC types.  The interface description parameter string length
        is variable, and can be from 0 to 80 octets.

      - Payload Bytes

        A 2-octet value indicating the number of TDM payload octets
        contained in all packets on the CEM stream from 48 to 1,023
        octets.  All of the packets in a given CEM stream have the same
        number of payload bytes.  Note that there is a possibility that
        the packet size may exceed the Synchronous Payload Envelope
        (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 or equal to 783 for
        STS-1 SPEs.

      - CEM Options

        An optional 16-bit value of CEM flags.  See [CEM] for the
        definition of the bit values.

6.2.  C Bit Handling Procedures

6.2.1.  VC Types for Which the Control Word is REQUIRED

   The Label Mapping messages which are sent in order to set up these
   VCs MUST have c=1.  When a Label Mapping message for a VC of one of
   these types is received, and c=0, a Label Release MUST be sent, with
   an "Illegal C-bit" status code.  In this case, the VC will not come
   up.

6.2.2.  VC Types for Which the Control Word is NOT Mandatory

   If a system is capable of sending and receiving the control word on
   VC types for which the control word is not mandatory, then each such
   VC endpoint MUST be configurable with a parameter that specifies
   whether the use of the control word is PREFERRED or NOT PREFERRED.
   For each VC, there MUST be a default value of this parameter.  This
   specification does NOT state what the default value should be.

   If a system is NOT capable of sending and receiving the control word
   on VC types for which the control word is not mandatory, then it
   behaves exactly as if it were configured for the use of the control
   word to be NOT PREFERRED.

   If a Label Mapping message for the VC has already been received, but
   no Label Mapping message for the VC has yet been sent, then the
   procedure is the following:

     -i. If the received Label Mapping message has c=0, send a Label
         Mapping message with c=0, and the control word is not used.

    -ii. If the received Label Mapping message has c=1, and the VC is
         locally configured such that the use of the control word is
         preferred, then send a Label Mapping message with c=1, and the
         control word is used.

   -iii. If the received Label Mapping message has c=1, and the VC is
         locally configured such that the use of the control word is not
         preferred or the control word is not supported, then act as if
         no Label Mapping message for the VC had been received (i.e.,
         proceed to the next paragraph).

   If a Label Mapping message for the VC has not already been received
   (or if the received Label Mapping message had c=1, and either local
   configuration says that the use of the control word is not preferred
   or the control word is not supported), then send a Label Mapping
   message in which the c bit is set to correspond to the locally
   configured preference for use of the control word.  (That is, set c=1

   if locally configured to prefer the control word, set c=0 if locally
   configured to prefer not to use the control word or if the control
   word is not supported).

   The next action depends on what control message is next received for
   that VC.  The possibilities are:

     -i. A Label Mapping message with the same c bit value as specified
         in the Label Mapping message that was sent.  VC setup is now
         complete, and the control word is used if c=1 but not used if
         c=0.

    -ii. A Label Mapping message with c=1, but the Label Mapping message
         that was sent has c=0.  In this case, ignore the received Label
         Mapping message, and continue to wait for the next control
         message for the VC.

   -iii. A Label Mapping message with c=0, but the Label Mapping message
         that was sent has c=1.  In this case, send a Label Withdraw
         message with a "Wrong C-bit" status code, followed by a Label
         Mapping message that has c=0.  VC setup is now complete, and
         the control word is not used.

    -iv. A Label Withdraw message with the "Wrong C-bit" status code.
         Treat as a normal Label Withdraw, but do not respond.  Continue
         to wait for the next control message for the VC.

   If, at any time after a Label Mapping message has been received, a
   corresponding Label Withdraw or Release is received, the action taken
   is the same as for any Label Withdraw or Release that might be
   received at any time.

   If both endpoints prefer the use of the control word, this procedure
   will cause it to be used.  If either endpoint prefers not to use the
   control word, or does not support the control word, this procedure
   will cause it not to be used.  If one endpoint prefers to use the
   control word but the other does not, the one that prefers not to use
   it is has no extra protocol to execute; it just waits for a Label
   Mapping message that has c=0.

   The following diagram illustrates the above procedures:

                   ------------------
               Y   | Received Label |       N
            -------|  Mapping Msg?  |--------------
            |      ------------------             |
            |                                     |
        --------------                            |
        |            |                            |
        |            |                            |
     -------      -------                         |
     | C=0 |      | C=1 |                         |
     -------      -------                         |
        |            |                            |
        |            |                            |
        |    ----------------                     |
        |    | Control Word |     N               |
        |    |    Capable?  |-----------          |
        |    ----------------          |          |
        |          Y |                 |          |
        |            |                 |          |
        |   ----------------           |          |
        |   | Control Word |  N        |          |
        |   |  Preferred?  |----       |          |
        |   ----------------   |       |          |
        |          Y |         |       |          |
        |            |         |       |   ----------------
        |            |         |       |   | Control Word |
        |            |         |       |   |  Preferred?  |
        |            |         |       |   ----------------
        |            |         |       |     N |     Y |
        |            |         |       |       |       |
      Send         Send      Send    Send    Send    Send
       C=0          C=1       C=0     C=0     C=0     C=1
                               |       |       |       |
                            ----------------------------------
                            | If receive the same as sent,   |
                            | VC setup is complete.  If not: |
                            ----------------------------------
                               |       |       |       |
                              ------------------- -----------
                              |     Receive     | | Receive |
                              |       C=1       | |   C=0   |
                              ------------------- -----------
                                       |               |
                                 Wait for the        Send
                                 next message     Wrong C-Bit
                                                       |
                                              Send Label Mapping
                                               Message with C=0

6.2.3.  Status Codes

   RFC 3036 has a range of Status Code values, which are assigned by
   IANA on a First Come, First Served basis.  These are in the range
   0x20000000-0x3effffff.  The following new status codes are defined:

           0x20000001 "Illegal C-Bit"
           0x20000002 "Wrong C-Bit"

6.3.  LDP Label Withdrawal Procedures

   As mentioned above, the Group ID field can be used to withdraw all VC
   labels associated with a particular group ID.  This procedure is
   OPTIONAL, and if it is implemented, the LDP label withdraw message
   should be as follows: the VC information length field is set to 0,
   the VC ID field is not present, and the interface parameters field is
   not present.  For the purpose of this document, this is called the
   "wild card withdraw procedure", and all LSRs implementing this design
   are REQUIRED to accept such a withdraw message, but are not required
   to send it.

   The interface parameters field MUST NOT be present in any LDP VC
   label withdrawal message or release message.  A wild card release
   message MUST include only the group ID.  A Label Release message
   initiated from the imposition router must always include the VC ID.

6.4.  Sequencing Considerations

   In the case where the router considers the sequence number field in
   the control word, it is important to note the following when
   advertising labels.

6.4.1.  Label Mapping Advertisements

   After a label has been withdrawn by the disposition router and/or
   released by the imposition router, care must be taken to not re-
   advertise (reuse) the released label until the disposition router can
   be reasonably certain that old packets containing the released label
   no longer persist in the MPLS network.

   This precaution is required to prevent the imposition router from
   restarting packet forwarding with sequence number of 1 when it
   receives the same label mapping if there are still older packets
   persisting in the network with sequence number between 1 and 32768.
   For example, if there is a packet with sequence number=n where n is
   in the interval[1,32768] traveling through the network, it would be
   possible for the disposition router to receive that packet after it
   re-advertises the label.  Since the label has been released by the

   imposition router, the disposition router SHOULD be expecting the
   next packet to arrive with sequence number of 1.  Receipt of a packet
   with sequence number equal to n will result in n packets potentially
   being rejected by the disposition router until the imposition router
   imposes a sequence number of n+1 into a packet.  Possible methods to
   avoid this are for the disposition router to always advertise a
   different VC label, or for the disposition router to wait for a
   sufficient time before attempting to re-advertise a recently released
   label.  This is only an issue when sequence number processing at the
   disposition router is enabled.

6.4.2.  Label Mapping Release

   In situations where the imposition router wants to restart forwarding
   of packets with sequence number 1, the router shall 1) send a label
   mapping release to the disposition router, and 2) send a label
   mapping request to the disposition router.  When sequencing is
   supported, advertisement of a VC label in response to a label mapping
   request MUST also consider the issues discussed in Section 6.4.1.

7.  IANA Considerations

   As specified in this document, a Virtual Circuit FEC element contains
   the VC Type field.  VC Type value 0 is reserved.  VC Type values 1
   through 10 are defined in this document.  VC Type values 11 through
   63 are to be assigned by IANA using the "IETF Consensus" policy
   defined in RFC 2434.  VC Type values 64 through 127 are to be
   assigned by IANA, using the "First Come First Served" policy defined
   in RFC 2434.  VC Type values 128 through 32767 are vendor-specific,
   and values in this range are not to be assigned by IANA.

   As specified in this document, a Virtual Circuit FEC element contains
   the Interface Parameters field, which is a list of one or more
   parameters, and each parameter is identified by the Parameter ID
   field.  Parameter ID value 0 is reserved.  Parameter ID values 1
   through 5 are defined in this document.  Parameter ID values 6
   through 63 are to be assigned by IANA using the "IETF Consensus"
   policy defined in RFC 2434.  Parameter ID values 64 through 127 are
   to be assigned by IANA, using the "First Come First Served" policy
   defined in RFC 2434.  Parameter ID values 128 through 255 are
   vendor-specific, and values in this range are not to be assigned by
   IANA.

8.  Security Considerations

   This document does not affect the underlying security issues of MPLS,
   described in [RFC3032].  More detailed security considerations are
   also described in Section 8 of [RFC4447].

9.  Normative References

   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4447]     Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T.,
                 and G. Heron, "Pseudowire Setup and Maintenance Using
                 the Label Distribution Protocol (LDP)", RFC 4447, April
                 2006.

   [RFC4385]     Bryant, S., Swallow, G., Martini, L., and D. McPherson,
                 "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word
                 for Use over an MPLS PSN", RFC 4385, February 2006.

   [RFC4842]     Malis, A., Pate, P., Cohen, R., Ed., and D. Zelig,
                 "Synchronous Optical Network/Synchronous Digital
                 Hierarchy (SONET/SDH) Circuit Emulation over Packet
                 (CEP)", RFC 4842, April 2007.

   [RFC4553]     Vainshtein, A., Ed., and YJ. Stein, Ed., "Structure-
                 Agnostic Time Division Multiplexing (TDM) over Packet
                 (SAToP)", RFC 4553, June 2006.

   [RFC4619]     Martini, L., Ed., Kawa, C., Ed., and A. Malis, Ed.,
                 "Encapsulation Methods for Transport of Frame Relay
                 over Multiprotocol Label Switching (MPLS) Networks",
                 RFC 4619, September 2006.

   [RFC4717]     Martini, L., Jayakumar, J., Bocci, M., El-Aawar, N.,
                 Brayley, J., and G. Koleyni, "Encapsulation Methods for
                 Transport of Asynchronous Transfer Mode (ATM) over MPLS
                 Networks", RFC 4717, December 2006.

   [RFC4618]     Martini, L., Rosen, E., Heron, G., and A. Malis,
                 "Encapsulation Methods for Transport of PPP/High-Level
                 Data Link Control (HDLC) over MPLS Networks", RFC 4618,
                 September 2006.

   [RFC4448]     Martini, L., Ed., Rosen, E., El-Aawar, N., and G.
                 Heron, "Encapsulation Methods for Transport of Ethernet
                 over MPLS Networks", RFC 4448, April 2006.

   [RFC3036]     Andersson, L., Doolan, P., Feldman, N., Fredette, A.,
                 and B. Thomas, "LDP Specification", RFC 3036, January
                 2001.

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

   [RFC3032]     Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
                 Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
                 Encoding", RFC 3032, January 2001.

   [ANSI.T1.105] American National Standards Institute, "Synchronous
                 Optical Network Formats," ANSI T1.105-1995.

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

   [RFC4905]     Martini, L., Ed., Rosen, E., Ed., and N. El-Aawar, Ed.,
                 "Encapsulation Methods for Transport of Layer 2 Frames
                 over MPLS Networks", RFC 4905, June 2007.

10.  Informative References

   [CEM]         Malis, A., Brayley, J., Vogelsang., S., Shirron, J.,
                 and L. Martini, "SONET/SDH Circuit Emulation Service
                 Over MPLS (CEM) Encapsulation", Work in Progress, June
                 2007.

   [FAST]        ATM Forum, "Frame Based ATM over SONET/SDH Transport
                 (FAST)", af-fbatm-0151.000, July 2000.

11.  Co-Authors

   Giles Heron
   Tellabs
   Abbey Place
   24-28 Easton Street
   High Wycombe
   Bucks
   HP11 1NT
   UK
   EMail: giles.heron@tellabs.com

   Dimitri Stratton Vlachos
   Mazu Networks, Inc.
   125 Cambridgepark Drive
   Cambridge, MA 02140
   EMail: d@mazunetworks.com

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

   Jayakumar Jayakumar,
   Cisco Systems Inc.
   225, E.Tasman, MS-SJ3/3,
   San Jose, CA 95134
   EMail: jjayakum@cisco.com

   Alex Hamilton,
   Cisco Systems Inc.
   285 W. Tasman, MS-SJCI/3/4,
   San Jose, CA 95134
   EMail: tahamilt@cisco.com

   Steve Vogelsang
   Laurel Networks, Inc.
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   EMail: sjv@laurelnetworks.com

   John Shirron
   Laurel Networks, Inc.
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   EMail: jshirron@laurelnetworks.com

   Toby Smith
   Network Appliance, Inc.
   800 Cranberry Woods Drive
   Suite 300
   Cranberry Township, PA 16066
   EMail: tob@netapp.com

   Andrew G. Malis
   Tellabs
   90 Rio Robles Dr.
   San Jose, CA 95134
   EMail: Andy.Malis@tellabs.com

   Vinai Sirkay
   Reliance Infocomm
   Dhirubai Ambani Knowledge City
   Navi Mumbai 400 709
   India
   EMail: vinai@sirkay.com

   Vasile Radoaca
   Nortel Networks
   600  Technology Park
   Billerica MA 01821
   EMail: vasile@nortelnetworks.com

   Chris Liljenstolpe
   Alcatel
   11600 Sallie Mae Dr.
   9th Floor
   Reston, VA 20193
   EMail: chris.liljenstolpe@alcatel.com

   Dave Cooper
   Global Crossing
   960 Hamlin Court
   Sunnyvale, CA 94089
   EMail: dcooper@gblx.net

   Kireeti Kompella
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA 94089
   EMail: kireeti@juniper.net

Authors' Addresses

   Luca Martini
   Cisco Systems, Inc.
   9155 East Nichols Avenue, Suite 400
   Englewood, CO 80112
   EMail: lmartini@cisco.com

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

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

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