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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 RFC 4447

Network Working Group                                       Luca Martini
Internet Draft                                           Nasser El-Aawar
Expiration Date: May 2003                   Level 3 Communications, LLC.

Toby Smith                                                 Eric C. Rosen
Laurel Networks, Inc.                                Cisco Systems, Inc.
Giles Heron
PacketExchange Ltd.

                                                           November 2002


                 Transport of Layer 2 Frames Over MPLS


                draft-ietf-pwe3-control-protocol-01.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
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   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
   http://www.ietf.org/shadow.html.

Abstract

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








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Table of Contents

    1      Specification of Requirements  ..........................   2
    2      Introduction  ...........................................   3
    3      PSN Tunnel Labels and PW Labels  ........................   4
    4      Protocol-Specific Details  ..............................   6
    4.1    Frame Relay  ............................................   6
    4.2    ATM  ....................................................   6
    4.2.1  ATM AAL5 VCC Transport  .................................   6
    4.2.2  ATM Transparent Cell Transport  .........................   6
    4.2.3  ATM VCC and VPC Cell Transport  .........................   6
    4.2.4  OAM Cell Support  .......................................   7
    4.2.5  ILMI Support  ...........................................   8
    4.3    Ethernet VLAN  ..........................................   8
    4.4    Ethernet  ...............................................   8
    4.5    HDLC and PPP  ...........................................   8
    5      LDP  ....................................................   9
    5.1    Interface Parameters Field  .............................  10
    5.1.1  PW types for which the control word is REQUIRED  ........  12
    5.1.2  PW types for which the control word is NOT mandatory  ...  13
    5.1.3  Status codes  ...........................................  14
    5.2    LDP label Withdrawal procedures  ........................  15
    5.3    Sequencing Considerations  ..............................  15
    5.3.1  Label Mapping Advertisements  ...........................  15
    5.3.2  Label Mapping Release  ..................................  16
    6      IANA Considerations  ....................................  16
    7      Security Considerations  ................................  16
    8      References  .............................................  17
    9      Author Information  .....................................  18
   10      Appendix A - C-bit Handling Procedures Diagram  .........  21




1. Specification of Requirements

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










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2. Introduction

   In an MPLS network, it is possible to carry the Protocol Data Units
   (PDUs) of layer 2 protocols by prepending an MPLS label stack to
   these PDUs. This document specifies the necessary label distribution
   procedures for accomplishing this using the encapsulation methods in
   [7],[10],[12]. We restrict discussion to the case of point-to-point
   transport. QoS related issues are not discussed in this draft. This
   document describes methods for transporting a number of protocols; in
   some cases, transporting a particular protocol may have several modes
   of operation, though a default mode of operation which MUST be
   supported is identified for each. Each of these protocols and/or
   modes may be implemented independently.

   An accompanying document [8] also describes a method for transporting
   time division multiplexed (TDM) digital signals (TDM circuit
   emulation) over a packet-oriented MPLS network. The transmission
   system for circuit-oriented TDM signals is the Synchronous Optical
   Network (SONET)[5]/Synchronous Digital Hierarchy (SDH) [6]. To
   support TDM traffic, which includes voice, data, and private leased
   line service, the MPLS network must emulate the circuit
   characteristics of SONET/SDH payloads. MPLS labels and a new circuit
   emulation header are used to encapsulate TDM signals and provide the
   Circuit Emulation Service over MPLS (CEM). The following two figures
   describe the reference models which are derived from [PWE3-FRAME] to
   support the Ethernet PW emulated services.

           Native    |<----- Pseudo Wire ---->|    Native
           Layer2    |                        |    Layer2
          Service    |  |<-- PSN Tunnel -->|  |    Service
             |       V  V                  V  V       |
             |     +----+                  +----+     |
   +----+    |     | PE1|==================| PE2|     |    +----+
   |    |----------|............PW1.............|----------|    |
   | CE1|    |     |    |                  |    |     |    |CE2 |
   |    |----------|............PW2.............|----------|    |
   +----+    |     |    |==================|    |     |    +----+
        ^          +----+                  +----+     |    ^
        |      Provider Edge 1         Provider Edge 2     |
        |                                                  |
        |<-------------- Emulated Service ---------------->|

      Figure 1: PWE3 Ethernet/VLAN Interface Reference Configuration








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   +-------------+                                +-------------+
   |  Layer2     |                                |  Layer2     |
   |  Emulated   |                                |  Emulated   |
   |  Services   |         Emulated Service       |  Services   |
   |             |<==============================>|             |
   +-------------+           Pseudo Wire          +-------------+
   |Demultiplexer|<==============================>|Demultiplexor|
   +-------------+                                +-------------+
   |    PSN      |            PSN Tunnel          |    PSN      |
   |   MPLS      |<==============================>|   MPLS      |
   +-------------+                                +-------------+
   |  Physical   |                                |  Physical   |
   +-----+-------+                                +-----+-------+

      Figure 2: Ethernet PWE3 Protocol Stack Reference Model

   For the purpose of this document, PE1 will be defined as the ingress
   router, and PE2 as the egress router. A layer 2 PDU will be received
   at PE1, encapsulated at PE1, transported, decapsulated at PE2, and
   transmitted out of PE2.



3. PSN Tunnel Labels and PW Labels

   Suppose it is desired to transport layer 2 PDUs from ingress LSR PE1
   to egress LSR PE2, across an intervening MPLS network. We assume that
   there is an LSP ( PSN tunnel ) from PE1 to PE2. That is, we assume
   that PE1 can cause a packet to be delivered to PE2 by pushing some
   label onto the packet and sending the result to one of its
   adjacencies. Call this label the "PSN tunnel label", and the
   corresponding LSP the "PSN tunnel LSP".

   The PSN tunnel LSP merely gets packets from PE1 to PE2, the
   corresponding label doesn't tell PE2 what to do with the payload, and
   in fact if penultimate hop popping is used, PE2 may never even see
   the corresponding label. (If PE1 itself is the penultimate hop, a PSN
   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 PE2,
   that tells PE2 how to treat the received packet. Call this label the
   "PW label".

   So when PE1 sends a layer 2 PDU to PE2, it first pushes a PW label on
   its label stack, and then (if PE1 is not adjacent to PE2) pushes on a
   PSN tunnel label. The PSN tunnel label gets the MPLS packet from PE1
   to PE2; the PW label is not visible until the MPLS packet reaches
   PE2. PE2's disposition of the packet is based on the PW label.




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   Note that the tunnel could be a GRE encapsulated MPLS tunnel between
   PE1 and PE2. In this case PE1 would be adjacent to PE2, and only the
   PW 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 PE2.
   That is, PE2 needs to be able to infer from the PW label the outgoing
   interface and the VPI/VCI value for the AAL5 PDU. If the payload is a
   Frame Relay PDU, then PE2 needs to be able to infer from the PW label
   the outgoing interface and the DLCI value. If the payload is an
   Ethernet frame, then PE2 needs to be able to infer from the PW label
   the outgoing interface, and perhaps the VLAN identifier. This process
   is uni-directional, and will be repeated independently for bi-
   directional operation. It is REQUIRED to assign the same PW ID, and
   PW type for a given circuit in both directions. The group ID (see
   below) MUST NOT be required to match in both directions. The
   transported frame MAY be modified when it reaches the egress router.
   If the header of the transported layer 2 frame is modified, this MUST
   be done at the egress LSR only.

   Note that the PW label must always be at the bottom of the label
   stack, and the PSN tunnel label, if present, must be immediately
   above the PW 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 PE1 itself may push on additional labels above
   the PSN tunnel label. If PE1 and PE2 are directly adjacent LSRs, then
   it may not be necessary to use a PSN tunnel label at all.

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

   This document does specify a method for assigning and distributing
   the PW label. Static label assignment MAY be used, and
   implementations SHOULD provide support for this. When signaling is
   used, the PW label MUST be distributed from PE2 to PE1 using LDP in
   the downstream unsolicited mode; this requires that an LDP session be
   created between PE1 and PE2. It should be noted that this LDP session
   is not necessarily transported along the same path as the Layer 2
   PDUs. [1] In addition, when using LDP to distribute the PW label,
   liberal label retention mode SHOULD be used.

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




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


4. Protocol-Specific Details

4.1. Frame Relay

   The Frame Relay PDUs are encapsulated according to the procedures
   defined in [7]. 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 [2] Q.933 Annex A.5 sited
   in IA FRF1.1, 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 [2] on the egress Frame relay VC.

4.2. ATM

4.2.1. ATM AAL5 VCC Transport

   ATM AAL5 CSPS-SDUs are encapsulated according to [10] 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.


4.2.2. 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, PE1, is
   encapsulated according to [10], ATM cell mode n-to-one, and sent
   across the PW to the egress LSR, PE2. This mode allows an ATM port to
   be connected to only one other ATM port. [10] ATM cell n-to-one mode
   allows for grouping of multiple cells into a single MPLS frame.
   Grouping of ATM cells is OPTIONAL for transmission at the ingress
   LSR, PE1. If the Egress LSR PE2 supports cell concatenation the
   ingress LSR, PE1, should only concatenate cells up to the "Maximum
   Number of concatenated ATM cells" parameter received as part of the
   FEC element.


4.2.3. ATM VCC and VPC Cell Transport

   This mode is similar to the ATM AAL5 VCC transport except that cells
   are transported. Every cell that is received on a pre-defined ATM
   PVC, or ATM PVP, at the ingress ATM port on the ingress LSR, PE1, is
   encapsulated according to [10], ATM cell mode, and sent across the



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   LSP to the egress LSR PE2. Grouping of ATM cells is OPTIONAL for
   transmission at the ingress LSR, PE1. If the Egress LSR PE2 supports
   cell concatenation the ingress LSR, PE1, MUST only concatenate cells
   up to the "Maximum Number of concatenated ATM cells in a frame"
   parameter received as part of the FEC element.


4.2.4. OAM Cell Support

   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 [10]. When operating in
   AAL5 SDU transport mode an LSR that supports transport of OAM cells
   using the T bit defined in [7], or an LSR operating in any of the
   three cell transport modes MUST follow the procedures outlined in [9]
   section 8 for mode 0 only, in addition to the applicable procedures
   specified in [6].


4.2.4.1. 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 bi-directional ATM VC by two uni-
   directional 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, PE1, 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, PE1, 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 PW label mapping is withdrawn,
   the egress LSR, PE2, 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



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   time.


4.2.5. ILMI Support

   An MPLS edge LSR MAY provide an ATM 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 PW label mapping is withdrawn, the egress LSR SHOULD
   notify its client of this failure by deleting the VC using ILMI.


4.3. Ethernet VLAN

   The Ethernet frame will be encapsulated according to the procedures
   in [12] tagged mode. It should be noted that if the VLAN identifier
   is modified by the egress LSR, according to the procedures outlined
   above, the Ethernet spanning tree protocol might fail to work
   properly. If the LSR detects a failure on the Ethernet physical port,
   or the port is administratively disabled, it MUST withdraw the label
   mappings for all PWs associated with the port.


4.4. Ethernet

   The Ethernet frame will be encapsulated according to the procedures
   in [12].  If the LSR detects a failure on the Ethernet physical port,
   or the port is administratively disabled, the corresponding PW label
   mapping MUST be withdrawn.


4.5. HDLC and PPP

   HDLC and PPP frames are encapsulated according to the procedures in
   [11]. 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 or PPP link.













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5. LDP

   The PW label bindings are distributed using the LDP downstream
   unsolicited mode described in [1]. The LSRs will establish an LDP
   session using the Extended Discovery mechanism described in [1,
   section 2.4.2 and 2.5], for this purpose a new type of FEC element is
   defined. The FEC element type is 128. [note1] Only a single PW FEC
   element MUST be advertised per LDP PW label.  The pseudo wire 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|         PW type             |VC info Length |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Group ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           PW ID                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Interface parameters                    |
   |                              "                                |
   |                              "                                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


     - PW type

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

               PW 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 [8]
               0x0009   ATM VCC cell transport
               0x000A   ATM VPC cell transport

     - Control word bit (C)

       The  highest  order bit (C) of the PW type is used to flag the
       presence  of  a  control  word  ( defined in [7] ) as follows:




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

       Please see the section "C-Bit Handling Procedures" for further
       explanation.

     - VC information length

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

     - Group ID

       An arbitrary 32 bit value which represents a group of PWs 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 PW 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.

     - PW ID

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

     - Interface parameters

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


5.1. 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:










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    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 [8] Payload Bytes.
       0x05         2       CEM options.
       0x06         2       Requested VLAN ID

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

     - Interface MTU

       A 2 octet value indicating the MTU in octets. This is the Maximum
       Transmission Unit, excluding encapsulation overhead, of the
       egress packet interface that will be transmitting the
       decapsulated PDU that is received from the MPLS network. This
       parameter is applicable only to PW types 1, 2, 4, 5, 6, and 7,
       and is REQUIRED for these PW types. If this parameter does not
       match in both directions of a specific PW, that PW MUST NOT be
       enabled.

     - Maximum Number of concatenated ATM cells

       A 2 octet value specifying the maximum number of concatenated ATM
       cells that can be processed as a single PDU by the egress LSR. An
       ingress LSR transmitting concatenated cells on this VC can
       concatenate a number of cells up to the value of this parameter,
       but MUST NOT exceed it. This parameter is applicable only to PW
       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.





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     - Optional Interface Description string

       This arbitrary, OPTIONAL, interface description string is used to
       send a human-readable administrative string describing the
       interface to the remote. This parameter is OPTIONAL, and is
       applicable to all PW types.  The interface description parameter
       string length is variable, and can be from 0 to 80 octets.
       Human-readable text MUST be provided in the UTF-8 charset using
       the Default Language [RFC2277].

     - 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 SPE size in the case of an STS-1
       SPE, which could cause two pointers to be needed in the CEM
       header, since the payload may contain two J1 bytes for
       consecutive SPEs. For this reason, the number of payload bytes
       must be less than or equal to 783 for STS-1 SPEs.

     - CEM Options.  An optional 16 Bit value of CEM Flags. See [8] for
       the definition of the bit values.

     - Requested VLAN ID.  An Optional 16 bit value indicating the
       requested VLAN ID. This parameter MAY be used by an LSR that is
       incapable of rewriting the 802.1Q ethernet VLAN tag on output. If
       the ingress LSR receives this request it MAY rewrite the VLAN ID
       tag in input to match the requested VLAN ID. If this is not
       possible, and the VLAN ID does not already match configured
       ingress VLAN ID the PW should not be enabled.This parameter is
       applicable only to PW type 4.


5.1.1. PW types for which the control word is REQUIRED

   The Label Mapping messages which are sent in order to set up these
   PWs 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.









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5.1.2. PW types for which the control word is NOT mandatory

   If a system is capable of sending and receiving the control word on
   PW types for which the control word is not mandatory, then each such
   PW endpoint MUST be configurable with a parameter that specifies
   whether the use of the control word is PREFERRED or NOT PREFERRED.
   For each PW, 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 as exactly as if it were configured for the use of the
   control word to be NOT PREFERRED.

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

        -i. If the received Label Mapping message has c=0, send a Label
            Mapping message with c=0, and the control word is not used.
       -ii. If the received Label Mapping message has c=1, and the PW is
            locally configured such that the use of the control word is
            preferred, then send a Label Mapping message with c=1, and
            the control word is used.
      -iii. If the received Label Mapping message has c=1, and the PW 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 PW had been received
            (i.e., proceed to the next paragraph).

   If a Label Mapping message for the PW 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.  (I.e., set c=1 if
   locally configured to prefer the control word, set c=0 if locally
   configured to prefer not to use the control word or if the control
   word is not supported).

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

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




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       -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 PW.
      -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. PW setup is now
            complete, and the control word is not used.
       -iv. A Label Withdraw message with the "Wrong c-bit" status code.
            Treat as a normal Label Withdraw, but do not respond.
            Continue to wait for the next control message for the PW.

   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. Note that receiving a Label Withdraw should not
   cause a corresponding Label Release to be sent.

   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 diagram in Appendix A illustrates the above procedure.


5.1.3. 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 [note2]. The following new status codes are
   defined:

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












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5.2. LDP label Withdrawal procedures

   As mentioned above the Group ID field can be used to withdraw all PW
   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 PW information length field is set to 0,
   the PW 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 PW
   label withdrawal message or release message. A wildcard release
   message MUST include only the group ID. A Label Release message
   initiated from the imposition router must always include the PW ID.


5.3. 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


5.3.1. 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 (re-use) the released label until the disposition router
   can be reasonably certain that old packets containing the released
   label no longer persist in the MPLS network.

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



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   to wait for a sufficient time before attempting to re-advertised a
   recently released label. This is only an issue when sequence number
   processing at the disposition router is enabled.


5.3.2. Label Mapping Release

   In situations where the imposition router wants to restart forwarding
   of packets with sequence number 1, the router shall 1) Send to
   disposition router a label mapping release, and 2) Send to
   disposition router a label mapping request. When sequencing is
   supported, advertisement of a PW label in response to a label mapping
   request MUST also consider the issues discussed in the section on
   Label Mapping Advertisements.


6. IANA Considerations

   As specified in this document, a Virtual Circuit FEC element contains
   the PW Type field. PW type value 0 is reserved. PW type values 1
   through 10 are defined in this document. PW type values 11 through 63
   are to be assigned by IANA using the "IETF Consensus" policy defined
   in [RFC2434]. PW type values 64 through 127 are to be assigned by
   IANA, using the "First Come First Served" policy defined in [RFC
   2434]. PW 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 Pseudo Wire 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 6 are defined in this document.  Parameter ID values 7
   through 63 are to be assigned by IANA using the "IETF Consensus"
   policy defined in RFC2434. Parameter ID values 64 through 127 are to
   be assigned by IANA, using the "First Come First Served" policy
   defined in RFC2434. Parameter ID values 128 through 255 are vendor-
   specific, and values in this range are not to be assigned by IANA.


7. Security Considerations

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









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8. References

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

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

   [3] "MPLS Label Stack Encoding", E. Rosen, Y. Rekhter, D. Tappan, G.
        Fedorkow, D. Farinacci, T. Li, A. Conta. RFC3032

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

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

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

   [7] "Encapsulation Methods for Transport of Frame-Relay Over IP and
   MPLS Networks", draft-ietf-pwe3-frame-encap-01.txt. ( work in
   progress )

   [8] "SONET/SDH Circuit Emulation Service Over MPLS (CEM)
   Encapsulation", draft-malis-sonet-ces-mpls-05.txt ( Work in progress
   )

   [9] ATM Forum Specification fb-fbatm-0151.000 (2000) ,Frame Based ATM
   over SONET/SDH Transport (FAST)

   [10] "Encapsulation Methods for Transport of ATM Cells/Frame Over IP
   and MPLS Networks", draft-ietf-pwe3-atm-encap-00.txt ( work in
   progress )

   [11] "Encapsulation Methods for Transport of PPP/HDLC Frames Over IP
   and MPLS Networks", draft-martini-ppp-hdlc-encap-mpls-00.txt. ( work
   in progress )

   [12] "Encapsulation Methods for Transport of Ethernet Frames Over
   IP/MPLS Networks", draft-ietf-pwe3-ethernet-encap-01.txt. ( work in
   progress )

   [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
   IANA Considerations section in RFCs", BCP 26, RFC 2434, October 1998.

   [RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
   Languages", BCP 18, RFC 2277, January 1998.




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

   [note2] Status codes assigment is pending IANA approval.


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
   PacketExchange Ltd.
   The Truman Brewery
   91 Brick Lane
   LONDON E1 6QL
   United Kingdom
   e-mail: giles@packetexchange.net


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


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








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


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


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


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


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


   Toby Smith
   Omega Corporate Center
   1300 Omega Drive
   Pittsburgh, PA 15205
   Laurel Networks, Inc.
   e-mail: tob@laurelnetworks.com







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


   Vinai Sirkay
   Vivace Networks, Inc.
   2730 Orchard Parkway
   San Jose, CA 95134
   e-mail: sirkay@technologist.com


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


   Chris Liljenstolpe
   Cable & Wireless
   11700 Plaza America Drive
   Reston, VA 20190
   e-mail: chris@cw.net


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


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









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10. Appendix A - C-bit Handling Procedures Diagram

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




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