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Versions: (draft-jamoussi-mpls-cr-ldp) 00 01 02 03 04 05 RFC 3212

MPLS Working Group              L. Andersson, A. Fredette, B. Jamoussi
Internet Draft                                         Nortel Networks
Expiration Date: July 1999
                                                             R. Callon
                                                   IronBridge Networks

                                                             P. Doolan
                                                     Ennovate Networks

                                                            N. Feldman
                                                              IBM Corp

                                                               E. Gray
                                                   Lucent Technologies

                                                            J. Halpern
                                                    Newbridge Networks

                                                           J. Heinanen
                                                         Telia Finland

                                                           T. E. Kilty
                                            Northchurch Communications

                                                           A. G. Malis
                                           Ascend Communications, Inc.

                                                             M. Girish
                                        SBC Technology Resources, Inc.

                                                            K. Sundell
                                                              Ericsson

                                                           P. Vaananen
                                              Nokia Telecommunications

                                                            T. Worster
                                                General DataComm, Inc.

                                                       L. Wu, R. Dantu
                                                               Alcatel

                                                          January 1998

                  Constraint-Based LSP Setup using LDP

                     draft-ietf-mpls-cr-ldp-00.txt

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working



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   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."

   To learn the current status of any Internet-Draft, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or
   ftp.isi.edu (US West Coast).

Abstract

   Label Distribution Protocol (LDP) is defined in [LDP] for
   distribution of labels inside one MPLS domain.  One of the most
   important services that may be offered using MPLS in general and LDP
   in particular is support for constraint-based routing of traffic
   across the routed network. Constraint-based routing offers the
   opportunity to extend the information used to setup paths beyond what
   is available for the routing protocol. For instance, an LSP can be
   setup based on an explicit route constraint, a Service Class (SC)
   constraint, or both. Constraint-based routing (CR) and Traffic
   Engineering requirements  have been proposed by [FRAME], [ARCH] and
   [TER]. These requirements may be met by extending LDP for support of
   constraint-based routed label switched paths (CRLSPs). Other uses
   exist for CRLSPs as well ([VPN1] and [VPN2]).

   This draft specifies mechanisms and TLVs for support of CRLSPs using
   LDP. The Explicit Route object and procedures are extracted from
   [ER].

1. Introduction

   The need for constraint-based routing (CR) in MPLS has been explored
   elsewhere [ARCH], [FRAME], and [TER].  Explicit routing is a subset
   of the more general constraint-based routing function. At the MPLS WG
   meeting held during the Washington IETF there was consensus that LDP
   should support explicit routing of LSPs with provision for indication
   of associated (forwarding) priority.  In the Chicago meeting, the
   decision was made that support for explicit path setup in LDP will be
   moved to a separate document. This document provides that support. We
   propose an end-to-end setup mechanism of a constraint-based routed
   LSP (CRLSP) initiated by the ingress LSR. We also specify mechanisms
   to provide means for reservation of resources for the explicitly
   routed LSP.

   We introduce TLVs and procedures that provide support for:



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    - Strict and Loose Explicit Routing
    - Specification of Service Class
    - Specification of Traffic Parameters
    - Route Pinning
    - CRLSP bumping though setup/holding priority
    - Handling Failures

2. CRLSP Overview

   CRLSP over LDP Specification is designed with several goals in mind:

      1. Meet the requirements outlined in [TER] for performing traffic
      engineering and provide a solid foundation for performing more
      general constrain-based routing.

      2. Build on already specified functionality that meets the
      requirements whenever possible. Hence, this specifications is
      based on [LDP] and the Explicit Route object and procedures
      defined in [ER].

      3. Keep the solution simple and tractable.

   In this document, support for unidirectional point-to-point CRLSPs is
   specified. Support for point-to-multipoint, multipoint-to-point, is
   for further study (FFS).

   Support for explicitly routed LSPs in this specification depends on
   the following minimal LDP behaviors as specified in [LDP]:

      - Basic and/or Extended Discovery Mechanisms.

      - Use the Label Request Message defined in [LDP] in downstream on
      demand label advertisement mode with ordered control.

      - Use the Label Mapping Message defined in [LDP] in downstream on
      demand mode with ordered control.

      - Use the Notification Message defined in [LDP].

      - Use the Withdraw and Release Messages defined in [LDP].

      - Loop detection (in the case of loosely routed segments of a
      CRLSP) mechanisms.

   In addition, the following functionality is added to what's defined
   in [LDP]:

      - The Label Request Message used to setup a CRLSP includes a CR-
      TLV based on the path vector defined in [ER] and specified in
      Section 4 of this document.




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      - An LSR implicitly infers ordered control from the existence of a
      CR-TLV in the Label Request Message. This means that the LSR can
      still be configured for independent control for LSPs established
      as a result of dynamic routing. However, when a Label Request
      Message includes a CR TLV, then ordered control is used to setup
      the CRLSP. Note that this is also true for the loosely routed
      parts of a CRLSP.

      - Traffic Parameters TLVs may optionally be carried in the Label
      Request Message to specify the CRLSP traffic characteristics.

      - New status codes are defined to handle error notification for
      failure of established paths specified in the CR-TLV.

   Examples of CRLSP establishment are given in Appendix A to illustrate
   how the mechanisms described in this draft work.

3. Required Messages and TLVs

   Any Messages, TLVs, and procedures not defined explicitly in this
   document are defined in the [LDP] Specification. The following
   subsections are meant as a cross reference to the [LDP] document and
   indication of additional functionality beyond what's defined in [LDP]
   where necessary.

3.1 Label Request Message

   The Label Request Message is as defined in 3.5.8 of [LDP] with the
   following modifications (required only if the CR-TLV is included in
   the Label Request Message):

      - Only a single FEC-TLV may be included in the Label Request
      Message.

      - The Optional Parameters TLV includes the definition of the
      Constraint-based TLV specified in Section 4 and the Traffic
      Parameters TLV specified in Section 5.

      - The Procedures to handle the Label Request are augmented by the
      procedures for processing of the CR-TLV as defined in Section 4.

      - The Procedures to handle Service Classes are defined in Section
      5.

3.2 Label Mapping Message

   The Label Mapping Message is as defined in 3.5.7 of [LDP] with the
   following modifications:

      - Only a single Label-TLV may be included in the Label Mapping
      Message.



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      - The FEC-Label Mapping TLV does not include any of the optional
      TLVs.

      - The Label Mapping Message Procedures are limited to downstream
      on demand ordered control mode of mapping.

   A Mapping message is transmitted by a downstream LSR to an upstream
   LSR under one of the following conditions:

      1. The LSR is the egress end of the CRLSP and an upstream mapping
      has been requested.

      2. The LSR received a mapping from its downstream next hop LSR for
      an CRLSP for which an upstream request is still pending.

3.3. Notification Message

   The Notification message is as defined in Section 3.5.1 of [LDP] and
   the Status TLV encoding is as defined in Section 3.4.7 of [LDP].

   Establishment of an Explicitly Routed LSP may fail for a variety of
   reasons.  All such failures are considered advisory conditions and
   they are signaled by the Notification Message.

   Notification messages carry Status TLVs to specify events being
   signaled. New status codes are defined in Section 4.8.3 to signal
   error notifications associated with the establishment of a CRLSP and
   the processing of the CR-TLV.

4. Constraint-based Routing TLV

   Label Request Messages defined in [LDP] optionally carry the
   Constraint-based Routing TLV (CR-TLV) based on the path vector
   defined in [ER] and described in this section of the specification.
   The inclusion of the CR TLV in the Label Request Message indicates
   the path to be taken in the network even if normal routing indicates
   otherwise.

   The format of the CR-TLV is described below.

4.1 CR-TLV

   The CR-TLV is an object that specifies the path to be taken by the
   LSP being established. In addition, the CR-TLV may also include the
   the Service Class (SC) constraints associated with the LSP, a setup
   and a holding priority used for path bumping, and an LSP pinning
   request flag.  Reserved bits in the CR-TLV allow for the
   specification of other LSP attributes in the future. If the reserved
   bits are exhausted, additional TLVs may be specified to allow for the
   indication of other LSP attributes during the CRLSP setup.




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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |U|F|         CR-TLV  (0x0800)  |      Length                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Reserved              | Reserved  |  SC |P| Hp  | Sp  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          ER-Hop TLV 1                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          ER-Hop TLV 2                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                          ............                         ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          ER-Hop TLV n                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

U bit

   Unknown TLV bit.  Upon receipt of an unknown TLV, if clear (=0), a
   notification must be returned to the message originator and the
   entire message must be ignored; if set (=1), the unknown TLV is
   silently ignored and the rest of the message is processed as if the
   unknown TLV did not exist.

F bit

   Forward unknown TLV bit.  This bit only applies when the U bit is set
   and the LDP message containing the unknown TLV is to be forwarded.
   If clear (=0), the unknown TLV is not forwarded with the containing
   message; if set (=1), the unknown TLV is forwarded with the
   containing message.

Type

   A two byte field carrying the value of the CR-TLV type which is
   0x800.

Length

   Specifies the length of the value field in bytes.

Reserved

   This field is reserved.  It must be set to zero on transmission and
   must be ignored on receipt. We expect to use these fields for
   carrying information that support other constrain-based routing
   information.

P bit




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   When set indicates that the loosely routed segments must remain
   pinned-down.  CRLSP must be rerouted only when adjacency is lost
   along the segment.  When not set, it indicates that the loose segment
   is not pinned down and must be changed to match the underlying hop-
   by-hop path.

SC

   The SC Field is used to specify the Service Class of the CRLSP. This
   field allows for the definition of up to 8 different Service Classes.
   Currently, Three Service Classes are defined: Best Effort (0),
   Throughput Sensitive (1), and Delay Sensitive (2) Service Classes.
   These SCs are further defined in Section 5.

Sp

   A SetupPriority of value zero (0) is the priority assigned to the
   most important path. It is referred to as the highest priority. Four
   (4) is the priority for the least important path. The higher the
   setup priority, the more paths CR-LDP can bump to set up the path.
   The default value is 2. Values 5, 6, and 7 are reserved.

Hp

   A HoldingPriority of value zero (0) is the priority assigned to the
   most important path. It is referred to as the highest priority. Four
   (4) is the priority for the least important path. The higher the
   holding priority, the less likely it is for CR-LDP to reallocate its
   bandwidth to a new path.  The default value is 2. Values 5, 6, and 7
   are reserved.

4.1.1 Setup and holding priorities

   CR-LDP signals the resources required by a path on each hop of the
   route. If a route with sufficient resources can not be found,
   existing paths may be rerouted to reallocate resources to the new
   path. This is the process of bumping paths. Setup and holding
   priorities are used to rank existing paths (holding priority) and the
   new path (setup priority) to determine if the new path can bump an
   existing path.

   The setupPriority of a new CRLSP and the holdingPriority attributes
   of the existing CRLSP are used to specify these priorities. The
   higher the holding priority, the less likely it is for CR-LDP to
   reallocate its bandwidth to a new path. Similarly, the higher the
   setup priority, the more paths CR-LDP can bump to set up the path.

   The setup and holding priority values range from zero (0) to four
   (4). The value zero (0) is the priority assigned to the most
   important path. It is referred to as the highest priority. Four (4)
   is the priority for the least important path. The default values for



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   both setup and holding priority should be 2. By setting the default
   value of both setup and holding priorities at the middle of the
   range, all connections are initially treated the same. However, when
   network operators see a need for the use of path bumping, the values
   of setup and holding priorities can be gracefully adjusted up or down
   from the middle of the range.

   An existing path can be bumped if and only if the setupPriority of
   the new path is numerically less than the holdingPriority of the
   existing path.

   To illustrate the use of the setup and holding priority, consider a
   network which supports two service types (e.g., video and data
   services).  The video traffic is given a low setup priority because
   new video paths can use an alternate public network if the primary
   network cannot accommodate the new path. However, the video traffic
   is given a high holding priority since it is undesirable for the path
   to be rerouted during an active LSP. For data traffic, high setup and
   holding priorities are desirable since data paths cannot be
   established on an alternate network.

   The setup and holding priorities can be different to allow setup at
   one priority and holding at an independent priority. This would allow
   some calls not to invoke bumping and not to be bumped at the same
   time.

   The setupPriority of a CRLSP should not be higher (numerically less)
   than its holdingPriority since it might bump an LSP and be bumped by
   next "equivalent" request.

   Bumping by default only happens as a last resort when there are no
   routes available for a given path.

   During the instantiation of a path that must bump other paths, lower
   holding priority paths are bumped before higher priority paths. The
   decision as to which of the available paths are bumped at each
   intermediate node by the new path is arbitrary.

4.2 ER-Hop TLV

   The contents of a constraint-based route TLV are a series of variable
   length ER-Hop TLVs. Each ER-Hop TLV has the form:

         0                   1
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--------//--------------+
        |L|    Type     |  Length       |       Contents         |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--------//--------------+

L




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   The L bit is an attribute of the ER-Hop.  The L bit is set if the
   ER-Hop represents a loose hop in the explicit route.  If the bit is
   not set, the ER-Hop represents a strict hop in the explicit route.

Type

   A seven-bit field indicating the type of contents of the ER-Hop.
   Currently defined values are:


             Value                   Type
             -----                   ------------------------
             0                       Reserved
             1                       IPv4 prefix
             2                       IPv6 prefix
             32                      Autonomous system number


Length

   The Length field contains the total length of the ER-Hop in bytes. It
   includes the L bit, Type and Length fields. The length must always be
   a multiple of 4, and at least 4.

Contents

   A variable length field containing the node or abstract node that is
   the consecutive nodes that make up the explicit routed LSP.

4.3 Applicability

   The CR-TLV in this version of the specification is intended for
   unicast only. CRLSPs for multicast are FFS.

4.4 Semantics of the CR-TLV

   Like any other LSP an CRLSP is a path through a network. The
   difference is that while other paths are setup solely based on
   information in routing tables or from a management system, the
   constraint-based route is calculated at one point at the edge of
   network based on criteria, including but not limited to routing
   information. The intention is that this functionality shall give
   desired special characteristics to the LSP in order to better support
   the traffic sent over the LSP. The reason for setting up CRLSPs,
   might be that one wants to assign certain bandwidth or other Service
   Class characteristics to the LSP, or that one wants to make sure that
   alternative routes use physically separate paths through the network.

   A CRLSP is represented in a Label Request Message  as a list of nodes
   or groups of nodes along the constraint-based route. When the CRLSP
   is established, all or a subset of the nodes in a group may be



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   traversed by the LSP.  Certain operations to be performed along the
   path can also be encoded in the constraint-based route.

   The capability to specify, in addition to specified nodes, groups of
   nodes, of which a subset will be traversed by the CRLSP, allows the
   system a significant amount of local flexibility in fulfilling a
   request for a constraint-based route.  This allows the generator of
   the constraint-based route to have some degree of imperfect
   information about the details of the path.

   The constraint-based route is encoded as a series of ER-Hops
   contained in a constraint-based route TLV.  Each ER-Hop may identify
   a group of nodes in the constraint-based route. A constraint-based
   route is then a path including all of the identified groups of nodes.

   To simplify the discussion, we call each group of nodes an abstract
   node.  Thus, we can also say that a constraint-based route is a path
   including all of the abstract nodes, with the specified operations
   occurring along that path.

4.5 Strict and Loose ER-Hops

   The L bit in the ER-Hop is a one-bit attribute.  If the L bit is set,
   then the value of the attribute is "loose."  Otherwise, the value of
   the attribute is "strict."  For brevity, we say that if the value of
   the ER-Hop attribute is loose then it is a "loose ER-Hop."
   Otherwise, it's a "strict ER-Hop."  Further, we say that the abstract
   node of a strict or loose ER-Hop is a strict or a loose node,
   respectively.  Loose and strict nodes are always interpreted relative
   to their prior abstract nodes.

   The path between a strict node and its prior node MUST include only
   network nodes from the strict node and its prior abstract node.

   The path between a loose node and its prior node MAY include other
   network nodes which are not part of the strict node or its prior
   abstract node.

4.6 Loops

   While the constraint-based route TLV is of finite length, the
   existence of loose nodes implies that it is possible to construct
   forwarding loops during transients in the underlying routing
   protocol.  This may be detected by the originator of the constraint-
   based route through the use a path vector object as defined in [LDP].

4.7 ER-Hop semantics

4.7.1. ER-Hop 1:  The IPv4 prefix

   The contents of an IPv4 prefix ER-Hop are a 4 byte IPv4 address, 1



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   byte of prefix length, and 1 byte of padding.  The abstract node
   represented by this ER-Hop is the set of nodes which have an IP
   address which lies within this prefix.  Note that a prefix length of
   32 indicates a single IPv4 node.

   The length of the IPv4 prefix ER-Hop is 8 bytes.  The contents of the
   1 byte of padding must be zero on transmission and must not be
   checked on receipt.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |L|    Type     |     Length    | IPv4 Address (4 bytes)        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      IPv4 Address (Continued) |   Prefix      |0 0 0 0 0 0 0 0|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

   IPv4 Address 0x01

Length

   A one byte field indicating the total length of the TLV in bytes. It
   includes the L-bit, the Type, Length, the IP Address, and the Prefix
   fields. The length is always 8 bytes.

IP Address

   A four byte field indicating the IP Address.

Prefix Length

   1-32

Padding

   Zero on transmission.  Ignored on receipt.

4.7.2. ER-Hop 2:  The IPv6 address














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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |L|    Type     |     Length    |   IPV6 address (16 bytes)     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | IPV6 address (continued)                                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | IPV6 address (continued)                                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | IPV6 address (continued)                                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | IPV6 address (continued)      |   Prefix      |0 0 0 0 0 0 0 0|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

   0x02  IPv6 address

Length

   The Length contains the total length of the ER-Hop TLV in bytes,
   including the Type and Length fields.  The Length is always 20.

IPv6 address

   A 128-bit unicast host address.

Prefix Length

   1-128

Padding

   Zero on transmission.  Ignored on receipt.

4.7.3. ER-Hop 32:  The autonomous system number

   The contents of an autonomous system (AS) number ER-Hop are a 2 byte
   autonomous system number.  The abstract node represented by this ER-
   Hop is the set of nodes belonging to the autonomous system.

   The length of the AS number ER-Hop is 4 bytes.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |L|    Type     |     Length    | Autonomous System number      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type



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   AS Number 0x20

Length

   A one byte field indicating the total length of the TLV in bytes. It
   includes the L-bit, the Type, and Length, and the AS number fields.
   The length is always 4 bytes.

AS number

   A two byte field indicating the AS number.

4.8. Processing of the Constraint-Based Route TLV

4.8.1. Selection of the next hop

   A Label Request message containing a constraint-based route TLV must
   determine the next hop for this path.  Selection of this next hop may
   involve a selection from a set of possible alternatives.  The
   mechanism for making a selection from this set is implementation
   dependent and is outside of the scope of this specification.
   Selection of particular paths is also outside of the scope of this
   specification, but it is assumed that each node will make a best
   effort attempt to determine a loop-free path.  Note that such best
   efforts may be overridden by local policy.

   To determine the next hop for the path, a node performs the following
   steps:

      1) The node receiving the Label Request message must first
      evaluate the first ER-Hop. If the L bit is not set in the first
      ER-Hop and if the node is not part of the abstract node described
      by the first ER-Hop, it has received the message in error, and
      should return a "Bad initial ER-Hop" error. If the L bit is set
      and the local node is not part of the abstract node described by
      the first ER-Hop, the node selects a next hop that is along the
      path to the abstract node described by the first ER-Hop. If there
      is no first ER-Hop, the message is also in error and the system
      should return a "Bad Constraint-Based Routing TLV" error.

      2) If there is no second ER-Hop, this indicates the end of the
      constraint-based route. The constraint-based route TLV should be
      removed from the Label Request message.  This node may or may not
      be the end of the LSP.  Processing continues with section 4.8.2,
      where a new constraint-based route TLV may be added to the Label
      Request message.

      3) If the node is also a part of the abstract node described by
      the second ER-Hop, then the node deletes the first ER-Hop and
      continues processing with step 2, above.  Note that this makes the
      second ER-Hop into the first ER-Hop of the next iteration.



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      4) The node determines if it is topologically adjacent to the
      abstract node described by the second ER-Hop.  If so, the node
      selects a particular next hop which is a member of the abstract
      node.  The node then deletes the first ER-Hop and continues
      processing with section 4.8.2.

      5) Next, the node selects a next hop within the abstract node of
      the first ER-Hop that is along the path to the abstract node of
      the second ER-Hop.  If no such path exists then there are two
      cases:

      5a) If the second ER-Hop is a strict ER-Hop, then there is an
      error and the node should return a "Bad strict node" error.

      5b) Otherwise, if the second ER-Hop is a loose ER-Hop, then the
      node selects any next hop that is along the path to the next
      abstract node.  If no path exists, then there is an error, and the
      node should return a "Bad loose node" error.

      6) Finally, the node replaces the first ER-Hop with any ER-Hop
      that denotes an abstract node containing the next hop.  This is
      necessary so that when the constraint-based route is received by
      the next hop, it will be accepted.

      7) Progress the Label Request Message to the next hop.

4.8.2. Adding ER-Hops to the constraint-based route TLV

   After selecting a next hop, the node may alter the constraint-based
   route in the following ways.

   If, as part of executing the algorithm in section 4.8.1, the
   constraint-based route TLV is removed, the node may add a new
   constraint-based route TLV.

   Otherwise, if the node is a member of the abstract node for the first
   ER-Hop, then a series of ER-Hops may be inserted before the first
   ER-Hop or may replace the first ER-Hop.  Each ER-Hop in this series
   must denote an abstract node that is a subset of the current abstract
   node.

   Alternately, if the first ER-Hop is a loose ER-Hop, an arbitrary
   series of ER-Hops may be inserted prior to the first ER-Hop.

4.8.3. Error subcodes

   In the processing described above, certain errors need to be reported
   as part of the Notification message.  This section defines the status
   codes for the errors described above.





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CR-LDP Specification             - 15 -                    Exp. Apr 1999



      Status Code                                    Type
      --------------------------------------         ----------
      Bad Constraint-Based Routing TLV Error         0x04000001
      Bad Strict Node Error                          0x04000002
      Bad Loose  Node Error                          0x04000003
      Bad Initial ER-Hop Error                       0x04000004
      Resource Unavailable                           0x04000005
      Service Class Unavailable                      0x04000006
      Traffic Parameters Unavailable                 0x04000007


5.0 CRLSP Service Classes and Traffic Parameters

   The following sections describe the CRLSP Service Classes (SCs), and
   their associated traffic parameters.

   The CRLSP Service Class is signaled in the SC Field of the CR-TLV
   defined in Section 4.1.

   Three Service Classes are currently supported by CR-LDP:

            Service Class                                  Value
            --------------------------                     -----
            Best Effort           (BE)                     0x0
            Throughput Sensitive  (TS)                     0x1
            Delay Sensitive       (DS)                     0x2

   These service classes are specified in the following sections.

5.1 Best Effort (BE)

   The request of the BE SC implies that there are no expected service
   guarantees from the network. The service provided by the network is
   the familiar best effort service.

   The Peak Date Rate (PDR) is the only traffic parameter that may be
   specified with the BE SC. The specification of the PDR allows the
   network to perform traffic shaping and policing functions.

5.2 Throughput Sensitive (TS)

   In the service model for the Throughput Sensitive SC, the network
   commits to deliver with high probability user datagrams at a rate of
   at least CDR (Committed Data Rate).  The user may transmit at a rate
   higher than CDR but datagrams in excess of CDR would have a lower
   probability of being delivered. If the user sends at a rate of CDR or
   lower the network commits to deliver with high probability all the
   user datagrams.

   The TS SC has an associated tolerance to the burstiness of arriving



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CR-LDP Specification             - 16 -                    Exp. Apr 1999


   user datagrams. This tolerance is defined by the traffic parameter
   Committed Burst Tolerance (CBT).

   Ideally, a TS CRLSP request carries with it a rich set of three
   traffic parameters (PDR, CDR, and CBT) that accurately describe its
   traffic characteristics. This allows the network to perform resource
   reservation, traffic shaping, and traffic policing.

   However, for the sake of simplicity of the service definition, the
   CDR is the only parameter that MUST always be specified for a TS
   CRLSP.  A peak data rate parameter (PDR) and a CBT are optional
   traffic parameters for the TS SC.

   The network should make every effort to preserve ordering of the
   delivered datagrams of a TS CRLSP.

   Network traffic that requires a low packet loss ratio at a given CDR
   but is not particularly sensitive to delay and jitter (e.g., network
   control traffic) is suited to the TS SC. The selection of the TS SC
   is used to signal to the various nodes along the path that the
   queuing and scheduling mechanisms used to handle the CRLSP should
   provide a low packet loss ratio.

5.3 Delay Sensitive (DS)

   In the service model for the Delay Sensitive SC, the network commits
   to deliver with high probability user datagrams at a rate of CDR
   (Committed Data Rate) with minimum delay and delay variation. The
   user MUST transmit data at a rate of CDR or lower in order to be
   eligible for DS service. Datagrams in excess of CDR may be discarded
   by the network. If the user sends at a rate of CDR or lower the
   network commits to deliver with high probability all user datagrams
   with low delay and delay variation. If the user sends at a rate
   higher than CDR the network does not provide any guarantees on the
   excess traffic.

   The Delay Sensitive SC has an associated tolerance to the burstiness
   of arriving user datagrams. This tolerance is defined by the traffic
   parameter Committed Burst Tolerance (CBT).

   Ideally, a DS CRLSP request carries with it a rich set of three
   traffic parameters (PDR, CDR, and CBT) that accurately describe its
   traffic characteristics. This allows the network to perform resource
   reservation, traffic shaping and policing.

   However, for the sake of simplicity of the service definition, the
   CDR is the only parameter that MUST always be specified for a DS
   CRLSP.  A peak data rate parameter (PDR) and a CBT are optional
   traffic parameters for the DS SC.

   The network should make every effort to preserve ordering of the



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CR-LDP Specification             - 17 -                    Exp. Apr 1999


   delivered datagrams of a DS CRLSP.

   Network traffic that requires a low delay and delay variation at a
   given CDR (e.g., voice traffic) is suited to the DS SC. The selection
   of the DS SC is used to signal to the various nodes along the path
   that the queuing and scheduling mechanisms used to handle the CRLSP
   should provide low delay and delay variation.

5.4  Traffic Parameters

   The CRLSP traffic parameters are defined in this section.

   The traffic parameters CDR, CBT and PDR are defined in terms of a
   TOKEN_BUCKET_TSPEC as specified in [RFC2215]. The following mapping
   of parameters in the TOKEN_BUCKET_TSPEC is used:

                  Token rate,                    r = CDR
                  Bucket depth,                  b = CBT
                  Peak traffic rate,             p = PDR
                  Minimum policed unit,          m = 1
                  Maximum packet size,           M = MTU

   The Traffic Parameters TLV is used to signal the traffic
   characteristics of the CRLSP. These traffic parameters are used to
   perform functions such as resource reservation, Shaping, and
   Policing. See [SIN] for more details. The encoding for the Traffic
   Parameters TLV is:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |U|F|  Traffic   TLV  (0x0810)  |      Length                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            PDR TLV                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            CDR TLV                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            CBT TLV                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.4.1  Peak data rate (PDR) TLV

   The value of traffic parameter PDR is given as a positive integer in
   bytes per second. Zero is not a valid value of PDR.

   The user may specify the value of PDR depending the SC of the CRLSP.
   Specifying the PDR allows the network to use traffic management
   functions such as shaping.






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CR-LDP Specification             - 18 -                    Exp. Apr 1999



      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |U|F|  PDR TLV  (0x0811)        |      Length                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       PDR in Bytes/sec                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.4.2. Committed Data Rate (CDR)

   The value of traffic parameter CDR is given as a positive integer in
   bytes per second. Zero is not a valid  value of CDR.

   The user may provide a requested value of CDR in the CRLSP request
   depending on the SC  of the CRLSP.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |U|F|  CDR TLV  (0x0812)        |      Length                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       CDR in Bytes/sec                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.4.3. Committed Burst Tolerance (CBT)

   The value of traffic parameter CBT is given in bytes.  Zero is not a
   valid  value of CBT.

   The requested value of CBT MUST be no smaller than the MTU of the
   originating interface.

   The user may provide a requested value of CBT in the CRLSP request.
   If the user chooses not to specify a requested value of CBT and the
   network is policing the traffic, then any excess traffic will be
   dropped by the network.

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |U|F|  CBT TLV  (0x0813)        |      Length                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           CBT in Bytes                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


6. Open Issues

   This section captures the issues that need further study.




Jamoussi, et. al.           January 26, 1999                   [Page 18]


CR-LDP Specification             - 19 -                    Exp. Apr 1999


   1) Review the FSM described in Appendix B and extend it by the CR-TLV
   processing defined in Sections 4.8.1 and 4.8.2.

   2) Consider if all three traffic parameters have to be signaled at
   all times and if the network should supply default values for the
   missing parameters.

   3) Consider the following extensions to the CR-TLV:

      3.1) Changing the 'P' bit to "next hop flag" and making it a 2-bit
      wide field with the following values:

         - 00 "local repair", which means if it belongs to a loosely
         routed segment, and the LSR detects a next hop change, the LSR
         will try to establish a new LSP from this point on and switch
         it over to the new LSP when it is setup.

         - 01 "global repair", which means when the LSR detects a next
         hop change, the LSR will tear down the LSP, the ingress LSR
         will try to reestablish another LSP through the new path.

         - 10 "pinned", which means that the loosely routed segments
         must remain pinned down.

         - 11 Reserved.

      3.2) Adding one more field "LSPID" before ER-Hop TLV.  LSPID can
      be used to identify a network wide unique CRLSP.

         - The first 4 bytes carrying the ingress LSR IP address

         - The second 4 bytes carrying the unique ID value assigned by
         the ingress LSR.

   4) Consider the following extension to the ER-Hop TLV:

      For Type field, add one more type, LSPID, which means the current
      CRLSP will go through another CRLSP which is identified with this
      LSPID value:

       Value   Type
       -----   -----
       4       LSPID

      Extend processing the LSPID ER-Hop as follows: If the type of ER-
      Hop is LSPID, and the other end of this CRLSP is not part of the
      constraint-based route TLV, add it to the constraint-based TLV
      with L bit turned off.

   5) Consider traffic parameter negotiation and the ability to change
   the traffic parameters associated with an already established path



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CR-LDP Specification             - 20 -                    Exp. Apr 1999


   without tearing the old path down.

7. Security

   No security issues are discussed in this version of the draft.

8. Acknowledgments

   The messages used to signal the CRLSP setup are based on the work
   done by the [LDP] team. The Explicit Route object and procedures used
   in this specification are based on [ER].

   The authors would also like to acknowledge the careful review and
   comments of Osama Aboul-Magd, Ken Hayward, Greg Wright, Geetha Brown,
   Brian Williams, Peter Ashwood-smith, Paul Beaubien, Matthew Yuen,
   Liam Casey, and Ankur Anand.

9. References

   [FRAME] Callon et al, "Framework for Multiprotocol Label Switching",
   work in progress (draft-ietf-mpls-framework-02), November 1997.

   [ARCH] Rosen et al, "Multiprotocol Label Switching Architecture",
   work in progress (draft-ietf-mpls-arch-02), July 1998.

   [LDP] Andersson et al, "Label Distribution Protocol Specification"
   work in progress (draft-ietf-mpls-ldp-02.txt), November 1998.

   [ER] Guerin et al, "Setting up Reservations on Explicit Paths using
   RSVP", work in progress (draft-guerin-expl-path-rsvp-01.txt, November
   1997.

   [TER] Awduche et al, "Requirements for Traffic Engineering Over
   MPLS", work in progress (draft-awduche-mpls-traffic-eng-00), April
   1998.

   [VPN1] Heinanen et al, "MPLS Mappings of Generic VPN Mechanisms",
   work in progress (draft-heinanen-generic-vpn-mpls-00), August 1998.

   [VPN2] Jamieson et al, "MPLS VPN Architecture" work in progress
   (draft-jamieson-mpls-vpn-00), August 1998.

   [RFC2215] S. Shenker and J. Wroclawski, General Characterization
   Parameters for Integrated Service Network Elements, RFC 2215, Sep
   1997.

   [SIN] B. Jamoussi, N. Feldman, and L. Andersson, "MPLS Ships in the
   Night with ATM", (draft-jamoussi-mpls-sin-00.txt), August 1998.






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CR-LDP Specification             - 21 -                    Exp. Apr 1999


10. Author Information

   Loa Andersson
   Director Bay Architecture Lab, EMEA
   Kungsgatan 34, PO Box 1788
   111 97 Stockholm, Sweden
   phone: +46 8 441 78 34
   mobile +46 70 522 78 34
   e-mail: loa_andersson@baynetworks.com

   Ross Callon
   IronBridge Networks
   55 Hayden Avenue,
   Lexington, MA  02173
   Phone: +1-781-402-8017
   Email: rcallon@ironbridgenetworks.com

   Ram Dantu
   Alcatel USA Inc.
   IP Competence Center
   1201 E. Campbell Road.,446-315
   Richadson, TX USA., 75081-2206
   Phone: 972 996 2938
   Fax:   972 996 5902
   Email: ram.dantu@aud.alcatel.com

   Paul Doolan
   Ennovate Networks
   330 Codman Hill Rd
   Marlborough MA 01719
   Phone: 978-263-2002
   email: pdoolan@ennovatenetworks.com

   Nancy Feldman
   IBM Corp.
   17 Skyline Drive
   Hawthorne NY 10532
   Phone:  914-784-3254
   email: nkf@us.ibm.com

   Andre Fredette
   Nortel Networks
   3 Federal Street
   Billerica, MA 01821
   email: fredette@baynetworks.com

   Eric Gray
   Lucent Technologies, Inc
   1600 Osgood St.
   North Andover, MA  01847
   email: ewgray@lucent.com



Jamoussi, et. al.           January 26, 1999                   [Page 21]


CR-LDP Specification             - 22 -                    Exp. Apr 1999


   Joel M. Halpern
   Newbridge Networks Inc.
   593 Herndon Parkway
   Herndon, VA 20170
   email: jhalpern@newbridge.com
   phone: 1-703-736-5954
   fax:   1-703-736-5959

   Juha Heinanen
   Telia Finland, Inc.
   Myyrmaentie 2
   01600 VANTAA
   Finland
   Tel: +358 303 944 808
   Email: jh@telia.fi

   Bilel Jamoussi
   Nortel Networks
   P O Box 3511 Station C
   Ottawa, ON K1Y 4H7
   Canada
   phone: +1 613 765-4814
   email: jamoussi@NortelNetworks.com

   Timothy E. Kilty
   Northchurch Communications
   5 Corporate Drive,
   Andover, MA 018110
   phone: 978 691-4656
   Email: tkilty@northc.com

   Andrew G. Malis
   Ascend Communications, Inc.
   1 Robbins Road
   Westford, MA 01886
   phone: 978 952-7414
   fax:   978 392-2074
   Email: malis@ascend.com

   Muckai K Girish
   SBC Technology Resources, Inc.
   4698 Willow Road
   Pleasanton, CA 94588
   Phone: (925) 598-1263
   Fax:   (925) 598-1321
   Email: mgirish@tri.sbc.com

   Kenneth Sundell
   Ericsson
   SE-126 25 Stockholm
   Sweden



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CR-LDP Specification             - 23 -                    Exp. Apr 1999


   email: kenneth.sundell@etx.ericsson.se

   Pasi Vaananen
   Nokia Telecommunications
   3 Burlington Woods Drive, Suite 250
   Burlington, MA 01803
   Phone: +1-781-238-4981
   Email: pasi.vaananen@ntc.nokia.com

   Tom Worster
   General DataComm, Inc.
   5 Mount Royal Ave.
   Marlboro MA 01752
   Email: tom.worster@gdc.com

   Liwen Wu
   Alcatel U.S.A
   44983 Knoll Square
   Ashburn, Va. 20147
   USA
   Phone: (703) 724-2619
   FAX:   (703) 724-2005
   Inet:  liwen.wu@adn.alcatel.com

Appendix A: CRLSP Establishment Examples

A.1 Strict Constraint-Based Route Example

   This appendix provides an example for the setup of a strictly  routed
   CRLSP.  In  this  example,  each  abstract  node  is represented by a
   specific node.

   The sample network used here is a four node  network  with  two  edge
   LSRs and two core LSRs as follows:

                             a         b         c
                    LSR1------LSR2------LSR3------LSR4

   LSR1 generates a Label Request Message as described in Section 3.1 of
   this draft and sends it to LSR2. This message includes the CR-TLV.

   The CR-TLV is composed by a vector of three ER-Hop TLVs <a, b, c>.
   The ER-Hop TLVs used in this example are of type 0x01 (IPv4 prefix)
   with a prefix length of 32. Hence, each ER-Hop TLV identifies a
   specific node as opposed to a group of nodes.

   At LSR2, the following processing of the CR-TLV per Section 4.8.1 of
   this draft takes place:

      1) The first hop <a> is part of the abstract node LSR2. Therefore,
      the first step passes the test. Go to step 2.



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CR-LDP Specification             - 24 -                    Exp. Apr 1999


      2) There is a second ER-Hop, <b>. Go to step 3.

      3) LSR2 is not part of the abstract node described by the second
      ER-Hop <b>. Go to Step 4.

      4) LSR2 determines that it is topologically adjacent to the
      abstract node described by the second ER-Hop <b>. LSR2 selects a
      next hop (LSR3) which is the abstract node. LSR2 deletes the first
      ER-Hop <a> from the CR-TLV which now becomes <b , c>. Go to
      Section 4.8.2.

   At LSR2, the following processing of Section 4.8.2 takes place:

      Executing algorithm 4.8.1 did not result in the removal of the
      CR-TLV.

      Also, LSR2 is not a member of the abstract node described by the
      first ER-Hop <b>.

      Finally, the first ER-Hop <b> is a strict hop.

      Therefore, processing section 4.8.2 does not result in the
      insertion of new ER-Hops. The selection of the next hop has been
      already done is step 4 of Section 4.8.1 and the processing of the
      CR-TLV is completed at LSR2. In this case, the Label Request
      Message including the CR-TLV <b, c> is progressed by LSR2 to LSR3.

   At LSR3, a similar processing to the CR-TLV takes place except that
   the incoming CR-TLV = <b, c> and the outgoing CR-TLV is <c>.

   At LSR4, the following processing of section 4.8.1 takes place:

      1) The first hop <c> is part of the abstract node LSR4. Therefore,
      the first step passes the test. Go to step 2.

      2) There is no second ER-Hop, this indicates the end of the CRLSP.
      The CR-TLV is removed from the Label Request Message. Processing
      continues with Section 4.8.2.

   At LSR4, the following processing of Section 4.8.2 takes place:

      Executing algorithm 4.8.1 resulted in the removal of the CR-TLV.
      LSR4 does not add a new CR-TLV.

      Therefore, processing section 4.8.2 does not result in the
      insertion of new ER-Hops. This indicates the end of the CRLSP and
      the processing of the CR-TLV is completed at LSR4.

   At LSR4, processing of Section 3.2 is invoked. The first condition is
   satisfied (LSR4 is the egress end of the CRLSP and upstream mapping
   has been requested). Therefore, a Label Mapping Message is generated



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CR-LDP Specification             - 25 -                    Exp. Apr 1999


   by LSR4 and sent to LSR3.

   At LSR3, the processing of Section 3.2 is invoked. The second
   condition is satisfied (LSR3 received a mapping from its downstream
   next hop LSR4 for a CRLSP for which an upstream request is still
   pending). Therefore, a Label Mapping Message is generated by LSR3 and
   sent to LSR2.

   At LSR2, a similar processing to LSR 3 takes place and a Label
   Mapping Message is sent back to LSR1 which completes the end-to-end
   CRLSP setup.


A.2. Node Groups and Specific Nodes Example

   A request at an ingress LSR to setup a CRLSP might originate from a
   management system or an application, the details are implementation
   specific.

   The ingress LSR uses information provided by the management system or
   the application and possibly also information from the routing
   database to calculated the constraint-based route and to create the
   Label Request Message.

   The Label request message carries together with other necessary
   information a CR-TLV defining the constraint-based routed path. In
   our example the list of hops in the ER-Hop TLV is supposed to contain
   an abstract node representing a group of nodes, an abstract node
   representing a specific node, another abstract node representing a
   group of nodes, and an abstract node representing a specific egress
   point.

      In--{Group 1}--{Specific A}--{Group 2}--{Specific Out: B}

   The CR-TLV contains four ER-Hop TLVs:

      1. An ER-Hop TLV that specifies a group of LSR valid for the first
      abstract node representing a group of nodes (Group 1).

      2. An ER-Hop TLV that indicates the specific node (Node A).

      3. An ER-Hop TLV that specifies a group of LSRs valid for the
      second abstract node representing a group of nodes (Group 2).

      4. An ER-Hop TLV that indicates the specific egress point for the
      CRLSP (Node B).

   All the ER-Hop TLVs are strictly routed nodes.

   The setup procedure for this CRLSP works as follows:




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CR-LDP Specification             - 26 -                    Exp. Apr 1999


      1. The ingress node sends the Label Request to a node that is a
      member the group of nodes indicated in the first ER-Hop TLV,
      following normal routing for the specific node (A).

      2. The node that receives the message identifies itself as part of
      the group indicated in the first ER-Hop TLV, and that it is not
      the specific node (A) in the second. Further it realizes that the
      specific node (A) is not one of its next hops.

      3. It keeps the ER-Hop TLVs intact and sends a Label Request
      Message to a node that is part of the group indicated in the first
      ER-Hop TLV (Group 1), following normal routing for the specific
      node (A).

      4. The node that receives the message identifies itself as part of
      the group indicated in the first ER-Hop TLV, and that it is not
      the specific node (A) in the second ER-Hop TLV. Further it
      realizes that the specific node (A) is one of its next hops.

      5. It removes the first ER-Hop TLVs and sends a Label Request
      Message to the specific node (A).

      6. The specific node (A) recognizes itself in the first ER-Hop
      TLV. Removes the specific ER-Hop TLV.

      7. It sends a Label Request message to a node that is a member of
      the group (Group 2) indicated in the ER-Hop TLV.

      8. The node that receives the message identifies itself as part of
      the group indicated in the first ER-Hop TLV, further it realizes
      that the specific egress node (B) is one of its next hops.

      9. It sends a Label Request message to the specific egress node
      (B).

      10. The specific egress node (B) recognizes itself as the egress
      for the CRLSP, it returns a Label Mapping Message, that will
      traverse the same path as the Label Request Message in the
      opposite direction.















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CR-LDP Specification             - 27 -                    Exp. Apr 1999


Appendix B. CR-LDP Finite State Machine

      In this description of the CR-LDP FSM, behavior  relating  to  the
      state  of  LDP  messages  is  assumed to be defined (implicitly or
      explicitly) in [LDP].  In particular, LDP  is  assumed  to  retain
      state  information  relating  a Label Request made of a downstream
      neighbor to the Label Request  message(s)  of  upstream  neighbors
      (downstream-on-demand  mode)  which the (downstream) Label Request
      is meant  to  satisfy.   This  will  be  true  of  many  potential
      applications  of  LDP,  of which CR-LDP is an example.  Minimally,
      this state should include message IDs of Label Requests (both sent
      and  received)  and the LSR(s) from which pending Label Request(s)
      were received.

      The FSM describes CR-LDP behavior in the following operations:

      - Start of CRLSP setup (in which a Label Request is sent);

      - Processing the CR-TLV portion of Label Requests;

      - Completion of CRLSP setup (via Label Mapping messages);

      - Notification of originator when:

         - a loop is detected in a loose constraint-based route segment,

         - an ER-Hop is not reachable from a previous ER-Hop,

         - a next ER-Hop is strict and not  directly  connected  to  the
         current LSR or

         - the current LSR is strict and is not (part  of  the  abstract
         node in) the first ER-Hop in the CR-TLV;

      - Withdrawing a CRLSP.

   For the description, the following pictorial representations  may  be
   used as an aid to understanding:

            LSR 1              LSR 2          ...          LSR n

           .-----.            .-----.                     .-----.
           | ER  |            | ER  |                     | ER  |
           `-----'            `-----'                     `-----'
               | CR-TLV  CR-TLV ^ | CR-TLV           CR-TLV ^
               |  Next          | |  Next                   |
               |  Hop           | |  Hop                    |
               V                | V                         |
           .-----. Label      .-----. Label       Label   .-----.
           | LDP |----------->| LDP |-------> ... ------->| LDP |
           `-----' Request    `-----' Request     Request `-----'



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CR-LDP Specification             - 28 -                    Exp. Apr 1999


                          CRLSP Setup propagation



               LSR 1              LSR 2          ...          LSR n

              .-----.            .-----.                     .-----.
              | ER  |            | ER  |                     | ER  |
              `-----'            `-----'                     `-----'
                ^ Status                                 Status  |
                |                                       Previous |
                |                                         Hop    |
                |                                                V
              .-----. Label      .-----. Label       Label   .-----.
              | LDP |<-----------| LDP |<------- ... <-------| LDP |
              `-----' Mapping    `-----' Mapping     Mapping `-----'

                             CRLSP Status propagation



                        .---------------.
                        | ER            |       .---------------.
                        |     Link/Call |       | LDP           |
                        |     Admission |       |               |
                        |      Control  |       |      Label    |
                        `---------------'       |    Allocation |
                                                `---------------'

                                     Related Tasks


B.1. CR-LDP Primitives

      The following sections describe the logical interactions between
      Constrain-based Route and LDP state machines in terms of
      primitives that describe the minimal information exchange
      required.  These assume an asynchronous exchange model involving
      locally significant IDs that is used to tie status of a request to
      the initial setup and to allow LDP to relate incoming/outgoing
      Label Request messages.  A synchronous model - possibly based on
      multiple threads - is also possible and would eliminate the need
      for IDs.

B.1.1. CR to LDP Primitives

      LDP_SEND_REQ( TLV_List, To_LSR, Identifier )

        TLV_List

          TLVs to be sent to a neighboring LSR; includes at least an



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          CR-TLV and may contain additional TLVs (i.e. QoS TLVs).

        To_LSR

          The neighbor LSR to which a Label Request is to be sent.

        Identifier

          Locally significant unique identifier.  May be used to
          associate the Label Request to be sent either with a Label
          Request that was previously received (e.g. - LSR 2 above)
          or a subsequent CRLSP Status (e.g. - LSR 1 above).

      LDP_SEND_RSP( Status, Identifier )

        Status

          Status of a specific CRLSP Setup Request.  A Status of zero
          indicates success; other Status values are given in Error
          Subcodes section.  This Status is carried in Label Mapping or
          Notification messages to the originator of the CRLSP setup.

        Identifier

          Locally significant unique identifier used to associate the
          Label Mapping to be sent with a Label Request received (e.g.
          LSR n above).

B.1.2. LDP to CR Primitives

      CR_RECEIVED_REQ( TLV_List, Identifier )

        TLV_List

          TLVs to be processed by the local constraint-based route
      function.

        Identifier

         Locally significant unique identifier used to associate the
         received request either with a subsequent further request
         or a response.  For example, the identifier provided here
         would be used in a subsequent LDP_SEND_REQ or LDP_SEND_RSP.

      CR_LSP_STATUS( Status, Identifier )

        Status

          Status of a specific CRLSP Setup Request.  A Status of zero
          indicates success; other Status values are given in section
          Error Subcodes.  This Status originated at the remote LSR



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          which either completed the CRLSP setup or determined that
          CRLSP setup could not be done.

        Identifier

          Locally significant unique identifier used to associate the
          received response with the original request.  For example,
          this identifier would be the same as was used in the initial
          LDP_SEND_REQ.

B.2. CR-LDP States

      This document defines 3 states relative to any one specific CRLSP.
      They are:

         CR_Non_Existant - no state information exists relative to this
         CRLSP;

         CR_In_Progress - LDP_SEND_REQ has been called in result
         of external input (e.g. - management);

         CR_Established - a successful status has been received from
         an earlier setup.

      These states are defined such that no additional state is required
      to support CRLSPs using LDP at intermediate LSRs than is already
      required in LDP.

B.3. CR-LDP Events

      This document defines 4 events impacting any one specific CRLSP.
      They are:

         CR_Start - a CRLSP is required based on an external stimulus
         (e.g. - management);

         CR_Req_Received - further CRLSP setup processing is required
         based on CR_RECEIVED_REQ (i.e. - from an upstream LSR's CRLSP
         Label Request);

         CR_Setup_Complete - CRLSP setup has been successfully completed
         based on CR_LSP_STATUS (with success status);

         CR_LSP_Failure - Either a CRLSP could not be established as
         requested, or a setup CRLSP has dropped; based on CR_LSP_STATUS
         (with error status).

B.4. CR-LDP Transitions

      State transitions are defined as follows:




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      State                 Event              Action  New State
      ====================  =================  ======  ===============
      CR_Non_Existant       CR_Start            1      CR_In_Progress
      CR_Non_Existant       CR_Req_Rec          2      CR_Non_Existant
      CR_In_Progress        CR_Setup_Complete          CR_Established
      CR_In_Progress        CR_LSP_Failure      3      CR_Non_Existant
      CR_Established        CR_LSP_Failure      3      CR_Non_Existant


      Actions:

         1) Establish CRLSP state, create CR-TLV information,
      LDP_SEND_REQ.
         2) Process CR-TLV (as described in "Processing of
            the Constraint-Based Route TLV" section) and either
            LDP_SEND_REQ or LDP_SEND_RSP.
         3) Remove state information relative to this CRLSP (may notify
            management, other external source initially requiring
      setup).

      For the purposes of this transition table, illegal transitions
      (not included in the table) are ignored.































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