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rfc:rfc4905

Network Working Group L. Martini, Ed. Request for Comments: 4905 E. Rosen, Ed. Category: Historic Cisco Systems, Inc.

                                                      N. El-Aawar, Ed.
                                           Level 3 Communications, LLC
                                                             June 2007
              Encapsulation Methods for Transport of
                 Layer 2 Frames over MPLS Networks

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 encapsulating the Protocol Data
 Units (PDUs) of layer 2 protocols such as Frame Relay, Asynchronous
 Transfer Mode (ATM), or Ethernet for transport 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 protocol, which has since been superseded by the Pseudowire Emulation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 4905 Encapsulation for L2 Frames over MPLS June 2007

Table of Contents

 1. Introduction ....................................................3
 2. Specification of Requirements ...................................3
 3. Special Note ....................................................4
 4. General Encapsulation Method ....................................4
    4.1. The Control Word ...........................................4
         4.1.1. Setting the Sequence Number .........................5
         4.1.2. Processing the Sequence Number ......................6
    4.2. MTU Requirements ...........................................6
 5. Protocol-Specific Details .......................................7
    5.1. Frame Relay ................................................7
    5.2. ATM ........................................................8
         5.2.1. ATM AAL5 CPCS-SDU Mode ..............................9
         5.2.2. ATM Cell Mode ......................................10
         5.2.3. OAM Cell Support ...................................12
         5.2.4. CLP bit to Quality of Service Mapping ..............12
    5.3. Ethernet VLAN .............................................12
    5.4. Ethernet ..................................................12
    5.5. High-Level Data Link Control (HDLC) .......................13
    5.6. PPP .......................................................13
 6. Using an MPLS Label as the Demultiplexer Field .................13
    6.1. MPLS Shim EXP Bit Values ..................................14
    6.2. MPLS Shim S Bit Value .....................................14
    6.3. MPLS Shim TTL Values ......................................14
 7. Security Considerations ........................................14
 8. Normative References ...........................................14
 9. Informative References .........................................16
 10. Co-Authors ....................................................16

Martini, et al. Historic [Page 2] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

1. Introduction

 In an MPLS network, it is possible to use control protocols such as
 those specified in [RFC4906] to set up "emulated virtual circuits"
 that carry the Protocol Data Units of layer 2 protocols across the
 network.  A number of these emulated virtual circuits (VCs) may be
 carried in a single tunnel.  This requires, of course, that the layer
 2 PDUs be encapsulated.  We can distinguish three layers of this
 encapsulation:
  1. the "tunnel header", which contains the information needed to

transport the PDU across the MPLS network; this header belongs

      to the tunneling protocol, e.g., MPLS, Generic Routing
      Encapsulation (GRE), and Layer 2 Tunneling Protocol (L2TP).
  1. the "demultiplexer field", which is used to distinguish

individual emulated virtual circuits within a single tunnel;

      this field must be understood by the tunneling protocol as well;
      it may be, e.g., an MPLS label or a GRE key field.
  1. the "emulated VC encapsulation", which contains the information

about the enclosed layer 2 PDU that is necessary in order to

      properly emulate the corresponding layer 2 protocol.
 This document specifies the emulated VC encapsulation for a number of
 layer 2 protocols.  Although different layer 2 protocols require
 different information to be carried in this encapsulation, an attempt
 has been made to make the encapsulation as common as possible for all
 layer 2 protocols.
 This document also specifies the way in which the demultiplexer field
 is added to the emulated VC encapsulation when an MPLS label is used
 as the demultiplexer field.
 Quality of service (QoS)-related issues are not discussed in this
 document.
 For the purpose of this document, R1 will be defined as the ingress
 router, and R2 as the egress router.  A layer 2 PDU will be received
 at R1, encapsulated at R1, transported, decapsulated at R2, and
 transmitted out of R2.

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].

Martini, et al. Historic [Page 3] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

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 documentation of current
 implementations, and MUST NOT be used for new implementations.  The
 PWE3 Label Distribution Protocol control protocol document [RFC4447],
 which is backward compatible with this document, MUST be used for all
 new implementations of this protocol.

4. General Encapsulation Method

 In most cases, it is not necessary to transport the layer 2
 encapsulation across the network; rather, the layer 2 header can be
 stripped at R1 and reproduced at R2.  This is done using information
 carried in the control word (see below), as well as information that
 may already have been signaled from R1 to R2.

4.1. The Control Word

 There are three requirements that may need to be satisfied when
 transporting layer 2 protocols over an MPLS backbone:
  1. i. Sequentiality may need to be preserved.
  1. ii. Small packets may need to be padded in order to be transmitted

on a medium where the minimum transport unit is larger than the

       actual packet size.
  1. iii. Control bits carried in the header of the layer 2 frame may

need to be transported.

 The control word defined here addresses all three of these
 requirements.  For some protocols, this word is REQUIRED, and for
 others OPTIONAL.  For protocols where the control word is OPTIONAL,
 implementations MUST support sending no control word, and MAY support
 sending a control word.
 In all cases, the egress router must be aware of whether the ingress
 router will send a control word over a specific virtual circuit.
 This may be achieved by configuration of the routers or by signaling,
 for example, as defined in [RFC4906].

Martini, et al. Historic [Page 4] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

 The 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Rsvd  | Flags |0 0|   Length  |     Sequence Number           |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 In the above diagram, the first 4 bits are reserved for future use.
 They MUST be set to 0 when transmitting, and MUST be ignored upon
 receipt.
 The next 4 bits provide space for carrying protocol-specific flags.
 These are defined in the protocol-specific details below.
 The next 2 bits MUST be set to 0 when transmitting.
 The next 6 bits provide a length field, which is used as follows: If
 the packet's length (defined as the length of the layer 2 payload
 plus the length of the control word) is less than 64 bytes, the
 length field MUST be set to the packet's length.  Otherwise, the
 length field MUST be set to 0.  The value of the length field, if
 non-zero, can be used to remove any padding.  When the packet reaches
 the service provider's egress router, it may be desirable to remove
 the padding before forwarding the packet.
 The next 16 bits provide a sequence number that can be used to
 guarantee ordered packet delivery.  The processing of the sequence
 number field is OPTIONAL.
 The sequence number space is a 16-bit, unsigned circular space.  The
 sequence number value 0 is used to indicate an unsequenced packet.

4.1.1. Setting the Sequence Number

 For a given emulated VC, and a pair of routers R1 and R2, if R1
 supports packet sequencing, then the following procedures should be
 used:
  1. The initial packet transmitted on the emulated VC MUST use

sequence number 1.

  1. Subsequent packets MUST increment the sequence number by 1 for

each packet.

  1. When the transmit sequence number reaches the maximum 16 bit

value (65535), the sequence number MUST wrap to 1.

Martini, et al. Historic [Page 5] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

 If the transmitting router R1 does not support sequence number
 processing, then the sequence number field in the control word MUST
 be set to 0.

4.1.2. Processing the Sequence Number

 If a router R2 supports receive sequence number processing, then the
 following procedures should be used:
 When an emulated VC is initially set up, the "expected sequence
 number" associated with it MUST be initialized to 1.
 When a packet is received on that emulated VC, the sequence number
 should be processed as follows:
  1. If the sequence number on the packet is 0, then the packet

passes the sequence number check.

  1. Else if the packet sequence number >= the expected sequence

number and the packet sequence number - the expected sequence

      number < 32768, then the packet is in order.
  1. Else if the packet sequence number < the expected sequence

number and the expected sequence number - the packet sequence

      number >= 32768, then the packet is in order.
  1. Otherwise, the packet is out of order.
 If a packet passes the sequence number check or is in order, then it
 can be delivered immediately.  If the packet is in order, then the
 expected sequence number should be set using the algorithm:
 expected_sequence_number := packet_sequence_number + 1 mod 2**16
 if (expected_sequence_number = 0) then expected_sequence_number := 1;
 Packets that are received out of order MAY be dropped or reordered at
 the discretion of the receiver.
 If a router R2 does not support receive sequence number processing,
 then the sequence number field MAY be ignored.

4.2. MTU Requirements

 The network MUST be configured with an MTU that is sufficient to
 transport the largest encapsulation frames.  If MPLS is used as the
 tunneling protocol, for example, this is likely to be 12 or more
 bytes greater than the largest frame size.  Other tunneling protocols
 may have longer headers and require larger MTUs.  If the ingress

Martini, et al. Historic [Page 6] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

 router determines that an encapsulated layer 2 PDU exceeds the MTU of
 the tunnel through which it must be sent, the PDU MUST be dropped.
 If an egress router receives an encapsulated layer 2 PDU whose
 payload length (i.e., the length of the PDU itself without any of the
 encapsulation headers) exceeds the MTU of the destination layer 2
 interface, the PDU MUST be dropped.

5. Protocol-Specific Details

5.1. Frame Relay

 A Frame Relay PDU is transported without the Frame Relay header or
 the Frame Check Sequence (FCS).  The control word is REQUIRED;
 however, its use is optional, although desirable.  Use of the control
 word means that the ingress and egress Label Switching Routers (LSRs)
 follow the procedures below.  If an ingress LSR chooses not to use
 the control word, it MUST set the flags in the control word to 0; if
 an egress LSR chooses to ignore the control word, it MUST set the
 Frame Relay control bits to 0.
 The BECN (Backward Explicit Congestion Notification), FECN (Forward
 Explicit Congestion Notification), DE (Discard Eligibility), and C/R
 (Command/Response) bits are carried across the network in the control
 word.  The edge routers that implement this document MAY, when either
 adding or removing the encapsulation described herein, change the
 BECN and/or FECN bits from 0 to 1 in order to reflect congestion in
 the network that is known to the edge routers, and the D/E bit from 0
 to 1 to reflect marking from edge policing of the Frame Relay
 Committed Information Rate.  The BECN, FECN, and D/E bits SHOULD NOT
 be changed from 1 to 0.
 The following is an example of a Frame Relay packet:
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Rsvd  |B|F|D|C|    Length     |        Sequence Number        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                      Frame Relay PDU                          |
 |                             "                                 |
 |                             "                                 |
 |                             "                                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Martini, et al. Historic [Page 7] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

  • B ( BECN ) Bit
      The ingress router, R1, SHOULD copy the BECN field from the
      incoming Frame Relay header into this field.  The egress router,
      R2, MUST generate a new BECN field based on the value of the B
      bit.
  • F ( FECN ) Bit
      The ingress router, R1, SHOULD copy the FECN field from the
      incoming Frame Relay header into this field.  The egress router,
      R2, MUST generate a new FECN field based on the value of the F
      bit.
  • D ( DE ) Bit
      The ingress router, R1, SHOULD copy the DE field from the
      incoming Frame Relay header into this field.  The egress router,
      R2, MUST generate a new DE field based on the value of the D
      bit.
      If the tunneling protocol provides a field that can be set to
      specify a Quality of Service, the ingress router, R1, MAY
      consider the DE bit of the Frame Relay header when determining
      the value of that field.  The egress router MAY then consider
      the value of this field when queuing the layer 2 PDU for egress.
      Note however that frames from the same VC MUST NOT be reordered.
  • C ( C/R ) Bit
      The ingress router, R1, SHOULD copy the C/R bit from the
      received Frame Relay PDU to the C bit of the control word.  The
      egress router, R2, MUST copy the C bit into the output frame.

5.2. ATM

 Two encapsulations are supported for ATM transport: one for ATM
 Adaption Layer 5 (AAL5) and another for ATM cells.
 The AAL5 Common Part Convergence Sublayer - Service Data Unit
 (CPCS-SDU) encapsulation consists of the REQUIRED control word and
 the AAL5 CPCS-SDU.  The ATM cell encapsulation consists of an
 OPTIONAL control word, a 4-byte ATM cell header, and the ATM cell
 payload.

Martini, et al. Historic [Page 8] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

5.2.1. ATM AAL5 CPCS-SDU Mode

 In ATM AAL5 mode, the ingress router is required to reassemble AAL5
 CPCS-SDUs from the incoming VC and transport each CPCS-SDU as a
 single packet.  No AAL5 trailer is transported.  The control word is
 REQUIRED; its use, however, is optional, although desirable.  Use of
 the control word means that the ingress and egress LSRs follow the
 procedures below.  If an ingress LSR chooses not to use the control
 word, it MUST set the flags in the control word to 0; if an egress
 LSR chooses to ignore the control word, it MUST set the ATM control
 bits to 0.
 The EFCI (Explicit Forward Congestion Indication) and CLP (Cell Loss
 Priority) bits are carried across the network in the control word.
 The edge routers that implement this document MAY, when either adding
 or removing the encapsulation described herein, change the EFCI bit
 from 0 to 1 in order to reflect congestion in the network that is
 known to the edge routers, and the CLP bit from 0 to 1 to reflect
 marking from edge policing of the ATM Sustained Cell Rate.  The EFCI
 and CLP bits MUST NOT be changed from 1 to 0.
 The AAL5 CPCS-SDU is prepended by the following header:
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Rsvd  |T|E|L|C|    Length     |        Sequence Number        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                     ATM  AAL5 CPCS-SDU                        |
 |                             "                                 |
 |                             "                                 |
 |                             "                                 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  • T (transport type) bit
      Bit (T) of the control word indicates whether the packet
      contains an ATM cell or an AAL5 CPCS-SDU.  If set, the packet
      contains an ATM cell, encapsulated according to the ATM cell
      mode section below; otherwise, it contains an AAL5 CPCS-SDU.
      The ability to transport an ATM cell in the AAL5 mode is
      intended to provide a means of enabling Operations and
      Management (OAM) functionality over the AAL5 VC.

Martini, et al. Historic [Page 9] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

  • E ( EFCI ) Bit
      The ingress router, R1, SHOULD set this bit to 1 if the EFCI bit
      of the final cell of those that transported the AAL5 CPCS-SDU is
      set to 1, or if the EFCI bit of the single ATM cell to be
      transported in the packet is set to 1.  Otherwise, this bit
      SHOULD be set to 0.  The egress router, R2, SHOULD set the EFCI
      bit of all cells that transport the AAL5 CPCS-SDU to the value
      contained in this field.
  • L ( CLP ) Bit
      The ingress router, R1, SHOULD set this bit to 1 if the CLP bit
      of any of the ATM cells that transported the AAL5 CPCS-SDU is
      set to 1, or if the CLP bit of the single ATM cell to be
      transported in the packet is set to 1.  Otherwise, this bit
      SHOULD be set to 0.  The egress router, R2, SHOULD set the CLP
      bit of all cells that transport the AAL5 CPCS-SDU to the value
      contained in this field.
  • C ( Command / Response Field ) Bit
      When FRF.8.1 Frame Relay / ATM PVC Service Interworking
      [FRF.8.1] traffic is being transported, the CPCS-UU Least
      Significant Bit (LSB) of the AAL5 CPCS-SDU may contain the Frame
      Relay C/R bit.  The ingress router, R1, SHOULD copy this bit to
      the C bit of the control word.  The egress router, R2, SHOULD
      copy the C bit to the CPCS-UU Least Significant Bit (LSB) of the
      AAL5 CPCS PDU.

5.2.2. ATM Cell Mode

 In this encapsulation mode, ATM cells are transported individually
 without a Segmentation and Reassembly (SAR) process.  The ATM cell
 encapsulation consists of an OPTIONAL control word, and one or more
 ATM cells - each consisting of a 4-byte ATM cell header and the 48-
 byte ATM cell payload.  This ATM cell header is defined in the FAST
 encapsulation [FAST] section 3.1.1, but without the trailer byte.
 The length of each frame, without the encapsulation headers, is a
 multiple of 52 bytes long.  The maximum number of ATM cells that can
 be fitted in a frame, in this fashion, is limited only by the network
 MTU and by the ability of the egress router to process them.  The
 ingress router MUST NOT send more cells than the egress router is
 willing to receive.  The number of cells that the egress router is
 willing to receive may either be configured in the ingress router or
 may be signaled, for example, using the methods described in
 [RFC4906].  The number of cells encapsulated in a particular frame
 can be inferred by the frame length.  The control word is OPTIONAL.

Martini, et al. Historic [Page 10] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

 If the control word is used, then the flag bits in the control word
 are not used, and MUST be set to 0 when transmitting, and MUST be
 ignored upon receipt.
 The EFCI and CLP bits are carried across the network in the ATM cell
 header.  The edge routers that implement this document MAY, when
 either adding or removing the encapsulation described herein, change
 the EFCI bit from 0 to 1 in order to reflect congestion in the
 network that is known to the edge router, and the CLP bit from 0 to 1
 to reflect marking from edge policing of the ATM Sustained Cell Rate.
 The EFCI and CLP bits SHOULD NOT be changed from 1 to 0.
 This diagram illustrates an encapsulation of two ATM cells:
  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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  Control word ( Optional )                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          VPI          |              VCI              | PTI |C|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  ATM Payload ( 48 bytes )                     |
 |                          "                                    |
 |                          "                                    |
 |                          "                                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |          VPI          |              VCI              | PTI |C|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                  ATM Payload ( 48 bytes )                     |
 |                          "                                    |
 |                          "                                    |
 |                          "                                    |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  • VPI (Virtual Path Identifier)
      The ingress router MUST copy the VPI field from the incoming
      cell into this field.  For particular emulated VCs, the egress
      router MAY generate a new VPI and ignore the VPI contained in
      this field.
  • VCI (Virtual Circuit Identifier)
      The ingress router MUST copy the VCI field from the incoming ATM
      cell header into this field.  For particular emulated VCs, the
      egress router MAY generate a new VCI.

Martini, et al. Historic [Page 11] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

  • PTI (Payload Type Identifier) & CLP ( C bit )
      The PTI and CLP fields are the PTI and CLP fields of the
      incoming ATM cells.  The cell headers of the cells within the
      packet are the ATM headers (without HEC) of the incoming cell.

5.2.3. OAM Cell Support

 OAM cells MAY be transported on the VC LSP.  An egress router that
 does not support transport of OAM cells MUST discard frames that
 contain an ATM cell with the high-order bit of the PTI field set to
 1.  A router that supports transport of OAM cells MUST follow the
 procedures outlined in [FAST] section 8 for mode 0 only, in addition
 to the applicable procedures specified in [RFC4906].

5.2.4. CLP bit to Quality of Service Mapping

 The ingress router MAY consider the CLP bit when determining the
 value to be placed in the Quality of Service fields (e.g., the EXP
 fields of the MPLS label stack) of the encapsulating protocol.  This
 gives the network visibility of the CLP bit.  Note however that cells
 from the same VC MUST NOT be reordered.

5.3. Ethernet VLAN

 For an Ethernet 802.1q VLAN, the entire Ethernet frame without the
 preamble or FCS is transported as a single packet.  The control word
 is OPTIONAL.  If the control word is used, then the flag bits in the
 control word are not used, and MUST be set to 0 when transmitting,
 and MUST be ignored upon receipt.  The 4-byte VLAN tag is transported
 as is, and MAY be overwritten by the egress router.
 The ingress router MAY consider the user priority field [IEEE802.3ac]
 of the VLAN tag header when determining the value to be placed in the
 Quality of Service field of the encapsulating protocol (e.g., the EXP
 fields of the MPLS label stack).  In a similar way, the egress router
 MAY consider the Quality of Service field of the encapsulating
 protocol when queuing the packet for egress.  Ethernet packets
 containing hardware-level Cyclic Redundancy Check (CRC) errors,
 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
 control word is OPTIONAL.  If the control word is used, then the flag
 bits in the control word are not used, and MUST be set to 0 when
 transmitting, and MUST be ignored upon receipt.  As in the Ethernet

Martini, et al. Historic [Page 12] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

 VLAN case, Ethernet packets with hardware-level CRC errors, framing
 errors, and runt packets MUST be discarded on input.

5.5. High-Level Data Link Control (HDLC)

 HDLC mode provides port to port transport of HDLC-encapsulated
 traffic.  The HDLC PDU is transported in its entirety, including the
 HDLC address, control, and protocol fields, but excluding HDLC flags
 and the FCS.  Bit/byte stuffing is undone.  The control word is
 OPTIONAL.  If the control word is used, then the flag bits in the
 control word are not used, and MUST be set to 0 when transmitting,
 and MUST be ignored upon receipt.
 The HDLC mode is suitable for port to port transport of Frame Relay
 User-Network Interface (UNI) or Network-Network Interface (NNI)
 traffic.  It must be noted, however, that this mode is transparent to
 the FECN, BECN, and DE bits.

5.6. PPP

 PPP mode provides point to point transport of PPP-encapsulated
 traffic, as specified in [RFC1661].  The PPP PDU is transported in
 its entirety, including the protocol field (whether compressed using
 PFC or not), but excluding any media-specific framing information,
 such as HDLC address and control fields or FCS.  Since media-specific
 framing is not carried, the following options will not operate
 correctly if the PPP peers attempt to negotiate them:
  1. Frame Check Sequence (FCS) Alternatives
  2. Address-and-Control-Field-Compression (ACFC)
  3. Asynchronous-Control-Character-Map (ACCM)
 Note also that VC LSP Interface MTU negotiation as specified in
 [RFC4906] is not affected by PPP Maximum Receive Unit (MRU)
 advertisement.  Thus, if a PPP peer sends a PDU with a length in
 excess of that negotiated for the VC LSP, that PDU will be discarded
 by the ingress router.
 The control word is OPTIONAL.  If the control word is used, then the
 flag bits in the control word are not used, and MUST be set to 0 when
 transmitting, and MUST be ignored upon receipt.

6. Using an MPLS Label as the Demultiplexer Field

 To use an MPLS label as the demultiplexer field, a 32-bit label stack
 entry [RFC3032] is simply prepended to the emulated VC encapsulation,
 and hence will appear as the bottom label of an MPLS label stack.
 This label may be called the "VC label".  The particular emulated VC

Martini, et al. Historic [Page 13] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

 identified by a particular label value must be agreed by the ingress
 and egress LSRs, either by signaling (e.g., via the methods of
 [RFC4906]) or by configuration.  Other fields of the label stack
 entry are set as follows.

6.1. MPLS Shim EXP Bit Values

 If it is desired to carry Quality of Service information, the Quality
 of Service information SHOULD be represented in the EXP field of the
 VC label.  If more than one MPLS label is imposed by the ingress LSR,
 the EXP field of any labels higher in the stack SHOULD also carry the
 same value.

6.2. MPLS Shim S Bit Value

 The ingress LSR, R1, MUST set the S bit of the VC label to a value of
 1 to denote that the VC label is at the bottom of the stack.

6.3. MPLS Shim TTL Values

 The ingress LSR, R1, SHOULD set the TTL field of the VC label to a
 value of 2.

7. Security Considerations

 This document specifies only encapsulations, and not the protocols,
 used to carry the encapsulated packets across the network.  Each such
 protocol may have its own set of security issues, but those issues
 are not affected by the encapsulations specified herein.  More
 detailed security considerations are also described in Section 8 of
 [RFC4447].

8. 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.

Martini, et al. Historic [Page 14] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

 [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.
 [RFC4906]     Martini, L., Ed., Rosen, E., Ed., and N. El-Aawar, Ed.,
               "Transport of Layer 2 Frames Over MPLS", RFC 4906, 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.
 [FRF.8.1]     Frame Relay Forum, "Frame Relay / ATM PVC Service
               Interworking Implementation Agreement", February 2000.
 [FAST]        ATM Forum, "Frame Based ATM over SONET/SDH Transport
               (FAST)", af-fbatm-0151.000, July 2000.

Martini, et al. Historic [Page 15] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

 [IEEE802.3ac] IEEE 802.3ac-1998, "Information technology -
               Telecommunications and information exchange between
               systems - Local and metropolitan area networks -
               Specific requirements Part 3: Carrier sense multiple
               access with collision detection (CSMA/CD) frame
               extensions for Virtual Bridged Local Area Networks
               (VLAN) tagging on 802.3 networks".

9. Informative References

 [RFC1661]     Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",
               STD 51, RFC 1661, July 1994.

10. 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.
 1414 Massachusetts Avenue
 Boxborough, MA 01719
 EMail: tappan@cisco.com
 Jayakumar Jayakumar
 Cisco Systems Inc.
 225, E.Tasman, MS-SJ3/3,
 San Jose, CA 95134
 EMail: jjayakum@cisco.com

Martini, et al. Historic [Page 16] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

 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
 Redback Networks
 300 Holger Way
 San Jose, CA 95134
 EMail: vsirkay@redback.com

Martini, et al. Historic [Page 17] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

 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 18] RFC 4905 Encapsulation for L2 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.
 1414 Massachusetts Avenue
 Boxborough, MA 01719
 EMail: erosen@cisco.com

Martini, et al. Historic [Page 19] RFC 4905 Encapsulation for L2 Frames over MPLS June 2007

Full Copyright Statement

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 contained in BCP 78, and except as set forth therein, the authors
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 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
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 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

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 Internet Society.

Martini, et al. Historic [Page 20]

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