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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 4905

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

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

                                                         Steve Vogelsang
                                                            John Shirron
                                                   Laurel Networks, Inc.

                                                         Andrew G. Malis
                                                   Vivace Networks, Inc.

                                                           November 2000


    Encapsulation Methods for Transport of Layer 2 Frames Over MPLS


               draft-martini-l2circuit-encap-mpls-00.txt

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
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Martini, et al.                                                 [Page 1]

Internet Draft draft-martini-l2circuit-encap-mpls-00.txt   November 2000


Abstract

   This document describes methods for encapsulating the Protocol Data
   Units (PDUs) of layer 2 protocols such as Frame Relay, ATM AAL5,
   Ethernet for transport across an MPLS network.


Table of Contents

    1      Specification of Requirements  ..........................   2
    2      Introduction  ...........................................   2
    3      Optional Sequencing and/or Padding  .....................   3
    4      MTU Requirements  .......................................   4
    5      Protocol-Specific Issues  ...............................   4
    5.1    Frame Relay  ............................................   4
    5.2    ATM  ....................................................   4
    5.2.1  OAM Cell Support  .......................................   6
    5.2.2  CLP Bit to EXP Bit Mapping  .............................   7
    5.3    Ethernet VLAN  ..........................................   7
    5.4    Ethernet  ...............................................   7
    5.5    HDLC ( Cisco )  .........................................   7
    5.6    PPP  ....................................................   8
    6      Security Considerations  ................................   8
    7      Intellectual Property Disclaimer  .......................   8
    8      References  .............................................   8
    9      Author Information  .....................................   9





1. Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119


2. Introduction

   In an MPLS network, it is possible to carry the Protocol Data Units
   (PDUs) of layer 2 protocols by prepending an MPLS label stack to
   these PDUs. This document specifies the necessary encapsulation
   procedures for accomplishing this. The control protocol methods are
   described in [5]. QoS related issues are not discussed in this draft.






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Internet Draft draft-martini-l2circuit-encap-mpls-00.txt   November 2000


3. Optional Sequencing and/or Padding

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

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

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

   The generic control word is defined as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Reserved |T|     Length    |         Sequence Number       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   In the above diagram the first 7 bits are reserved for future use.
   They MUST be set to 0 when transmitting, and MUST be ignored upon
   receipt. The T bit is used in ATM encapsulations only, and MUST be
   set to zero for other encapsulations. The length byte is set as
   specified above.

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

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



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Internet Draft draft-martini-l2circuit-encap-mpls-00.txt   November 2000


4. MTU Requirements

   The MPLS network should be configured with an MTU that is at least 12
   bytes larger then the largest frame size that will be transported in
   the LSPs. If a packet length, once it has been encapsulated on the
   ingress LSR, exceeds the LSP MTU, it MUST be dropped. If an egress
   LSR receives a packet on a VC LSP with a length, once the label stack
   and sequencing control word have been popped, that exceeds the MTU of
   the destination layer 2 interface it MUST be dropped.



5. Protocol-Specific Issues

5.1. Frame Relay

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

   The BECN and FECN signals are carried unchanged across the network in
   the Frame Relay header. These signals do not appear in the MPLS
   header, and are unseen by the MPLS network. The Label Edge Routers
   that implement this document MAY, when either adding or removing the
   encapsulation described herein, change a zero to a one in either or
   both of these bits in order to reflect congestion in the MPLS network
   that is known to the LERs. The BECN and FECN bits MUST NEVER be
   changed from one to zero.

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


5.2. ATM

   Two encapsulations are supported for ATM transport: one for AAL5
   CPCS-PDUs and another for ATM cells. The AAL5 CPCS-PDU encapsulation
   consists of the MPLS label stack, an optional sequencing control
   word, and the AAL5 CPCS-PDU. The ATM cell encapsulation consists of
   an MPLS label stack, a required generic sequencing control word, a 4
   byte ATM cell header, and the ATM cell payload as shown below:









Martini, et al.                                                 [Page 4]

Internet Draft draft-martini-l2circuit-encap-mpls-00.txt   November 2000


    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Reserved   |T|     Length    |         Sequence Number       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          VPI          |              VCI              | PTI |C|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Two transport modes are supported for ATM transport, VCC transport
   and VPC transport.

   VCC transport mode may be used to transport ATM Adaptation Layer 5
   (AAL5) CPCS-PDUs, ATM cells, or both, across the VC LSP. The
   sequencing control word is optional for VCC transport if only AAL5
   CPCS-PDUs are to be transported. If ATM cells, or a combination of
   ATM cells and AAL5 CPCS-PDUs, are to be transported the sequencing
   control word is required.

   VPC transport mode may only be used to transport ATM cells.  The
   sequencing control word is optional in VPC transport mode.

     * T (transport type) bit

       In VCC transport mode, bit (T) of the sequencing control word
       indicates whether the MPLS packet contains an ATM cell or an AAL5
       CPCS-PDU. If set the packet contains an ATM cell, otherwise it
       contains an AAL5 CPCS-PDU. In VPC transport mode this bit MUST be
       set to 1.

     * VPI

       In both modes the ingress router MAY copy the VPI field from the
       incoming cell into this field. The egress router MAY generate a
       new VPI based on the value of the VC label and ignore the VPI
       contained in this field.

     * VCI

       In VCC transport mode the ingress router MAY copy the VCI field
       from the incoming ATM cell header into this field and the egress
       router MAY generate a new VCI based on the value of the VC label.

       When in VPC transport mode the ingress LSR MUST copy the VCI
       field from the incoming cell into this field and the egress LSR
       MUST copy the VCI from this field into the outgoing ATM cell
       header.




Martini, et al.                                                 [Page 5]

Internet Draft draft-martini-l2circuit-encap-mpls-00.txt   November 2000


     * PTI & CLP

       When present the ingress router SHOULD copy the PTI and CLP
       fields of the outgoing frame from the ATM cell header and the
       egress router SHOULD set the left-most and EFCI bits of the PTI
       in all outgoing cells to that contained in the PTI field of the
       FAST header and set the CLP bit of outgoing cells to the CLP bit
       contained in the FAST header. [7]


5.2.1. OAM Cell Support

   OAM cells MAY be transported on the VC LSP. A router that does not
   support transport of OAM cells MUST discard incoming MPLS frames on
   an ATM VC LSP 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 [7] section 8 for mode 0 only,
   in addition to the applicable procedures specified in [5].

   A router that does not support transport of OAM cells across an LSP
   may provide OAM support on ATM PVCs using the following procedures:

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

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

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








Martini, et al.                                                 [Page 6]

Internet Draft draft-martini-l2circuit-encap-mpls-00.txt   November 2000


5.2.2. CLP Bit to EXP Bit Mapping

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

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


5.3. Ethernet VLAN

   For an ethernet 802.1q VLAN the entire ethernet frame without the
   preamble or FCS is transported as a single packet. The sequencing
   control word is OPTIONAL. If a packet is received out of sequence it
   MUST be dropped. The 4 byte VLAN tag is transported as is, and MAY be
   overwritten by the egress LSR.

   The ingress LSR MAY consider the user priority field [4] of the VLAN
   tag header when determining the value to be placed in the EXP fields
   of the MPLS label stack. In a similar way, the egress LSR MAY
   consider the EXP field of the VC label when queuing the packet for
   egress. Ethernet packets containing hardware level CRC, Framing
   errors, or runt packets MUST be discarded on input.


5.4. Ethernet

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



5.5. HDLC ( Cisco )

   HDLC (Cisco) mode provides port to port transport of Cisco 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 stuffing is undone. The
   sequencing control word is OPTIONAL.








Martini, et al.                                                 [Page 7]

Internet Draft draft-martini-l2circuit-encap-mpls-00.txt   November 2000


5.6. PPP

   PPP mode provides point to point transport of PPP encapsulated
   traffic, as specified in [8]. The PPP PDU is transported in its
   entirety, including the protocol field, but excluding any media-
   specific framing information, such as HDLC address and control fields
   or FCS. The sequencing control word is OPTIONAL.


6. Security Considerations

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


7. Intellectual Property Disclaimer

   This document is being submitted for use in IETF standards
   discussions.


8. References

   [1] "LDP Specification", draft-ietf-mpls-ldp-11.txt ( work in
   progress )

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

   [3] "MPLS Label Stack Encoding", draft-ietf-mpls-label-encaps-08.txt
   ( work in progress )

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

   [5] "Transport of Layer 2 Frames Over MPLS", draft-martini-
   l2circuit-trans-mpls-04.txt. ( work in progress )

   [6] ITU Recommendation I.610, "B-ISDN operation and maintenance
   principles and functions", 1999.

   [7] "Frame Based ATM over SONET/SDH Transport (FAST)," 2000.

   [8] "The Point-to-Point Protocol (PPP)", RFC 1661.

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







Martini, et al.                                                 [Page 8]

Internet Draft draft-martini-l2circuit-encap-mpls-00.txt   November 2000


9. Author Information


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


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


   Giles Heron
   Level 3 Communications
   66 Prescot Street
   London
   E1 8HG
   United Kingdom
   e-mail: giles@level3.net


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


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


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






Martini, et al.                                                 [Page 9]

Internet Draft draft-martini-l2circuit-encap-mpls-00.txt   November 2000



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


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


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






























Martini, et al.                                                [Page 10]


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