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rfc:rfc4906

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 called "draft-martini" protocol, which has since been superseded by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.

Martini, et al. Historic [Page 1] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

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

Martini, et al. Historic [Page 2] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

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.

Martini, et al. Historic [Page 3] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

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.

Martini, et al. Historic [Page 4] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

 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.

Martini, et al. Historic [Page 5] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

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].

Martini, et al. Historic [Page 6] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

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

Martini, et al. Historic [Page 7] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

 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                    |
 |                              "                                |
 |                              "                                |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Martini, et al. Historic [Page 8] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

  1. 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
  1. 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.
  1. 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.
  1. 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.

Martini, et al. Historic [Page 9] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

  1. VC ID
      A non-zero 32-bit connection ID that, together with the VC type,
      identifies a particular VC.
  1. 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.
  1. 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

Martini, et al. Historic [Page 10] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

      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.
  1. 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.
  1. 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.
  1. 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.
  1. CEM Options
      An optional 16-bit value of CEM flags.  See [CEM] for the
      definition of the bit values.

Martini, et al. Historic [Page 11] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

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:
  1. 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.

  1. 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.
  1. 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

Martini, et al. Historic [Page 12] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

 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:
  1. 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.
  1. 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.
  1. 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.
  1. 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:

Martini, et al. Historic [Page 13] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

  1. —————–

Y | Received Label | N

  1. ——| 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

Martini, et al. Historic [Page 14] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

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

Martini, et al. Historic [Page 15] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

 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].

Martini, et al. Historic [Page 16] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

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.

Martini, et al. Historic [Page 17] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

 [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

Martini, et al. Historic [Page 18] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

 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

Martini, et al. Historic [Page 19] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

 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

Martini, et al. Historic [Page 20] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

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

Martini, et al. Historic [Page 21] RFC 4906 Transport of Layer 2 Frames Over MPLS June 2007

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 contained in BCP 78, and except as set forth therein, the authors
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Martini, et al. Historic [Page 22]

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