Exclude Routes - Extension to Resource ReserVation Protocol-Traffic Engineering (RSVP-TE)

Network Working Group                                            CY. Lee
Request for Comments: 4874                                     A. Farrel
Updates: 3209, 3473                                   Old Dog Consulting
Category: Standards Track                                  S. De Cnodder
                                                          Alcatel-Lucent
                                                              April 2007


                     Exclude Routes - Extension to
      Resource ReserVation Protocol-Traffic Engineering (RSVP-TE)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document specifies ways to communicate route exclusions during
   path setup using Resource ReserVation Protocol-Traffic Engineering
   (RSVP-TE).

   The RSVP-TE specification, "RSVP-TE: Extensions to RSVP for LSP
   Tunnels" (RFC 3209) and GMPLS extensions to RSVP-TE, "Generalized
   Multi-Protocol Label Switching (GMPLS) Signaling Resource ReserVation
   Protocol-Traffic Engineering (RSVP-TE) Extensions" (RFC 3473) allow
   abstract nodes and resources to be explicitly included in a path
   setup, but not to be explicitly excluded.

   In some networks where precise explicit paths are not computed at the
   head end, it may be useful to specify and signal abstract nodes and
   resources that are to be explicitly excluded from routes.  These
   exclusions may apply to the whole path, or to parts of a path between
   two abstract nodes specified in an explicit path.  How Shared Risk
   Link Groups (SRLGs) can be excluded is also specified in this
   document.

Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements Notation ......................................4
      1.2. Scope of Exclude Routes ....................................4
      1.3. Relationship to MPLS TE MIB ................................5
   2. Shared Risk Link Groups .........................................6
      2.1. SRLG Subobject .............................................6
   3. Exclude Route List ..............................................7
      3.1. EXCLUDE_ROUTE Object (XRO) .................................7
           3.1.1. IPv4 Prefix Subobject ...............................8
           3.1.2. IPv6 Prefix Subobject ...............................9
           3.1.3. Unnumbered Interface ID Subobject ..................10
           3.1.4. Autonomous System Number Subobject .................10
           3.1.5. SRLG Subobject .....................................11
      3.2. Processing Rules for the EXCLUDE_ROUTE Object (XRO) .......11
   4. Explicit Exclusion Route .......................................13
      4.1. Explicit Exclusion Route Subobject (EXRS) .................13
      4.2. Processing Rules for the Explicit Exclusion Route
           Subobject (EXRS) ..........................................15
   5. Processing of XRO together with EXRS ...........................16
   6. Minimum Compliance .............................................16
   7. Security Considerations ........................................16
   8. IANA Considerations ............................................17
      8.1. New ERO Subobject Type ....................................17
      8.2. New RSVP-TE Class Numbers .................................18
      8.3. New Error Codes ...........................................18
   9. Acknowledgments ................................................19
   10. References ....................................................19
      10.1. Normative References .....................................19
      10.2. Informative References ...................................19
   Appendix A. Applications ..........................................21
      A.1. Inter-Area LSP Protection .................................21
      A.2. Inter-AS LSP Protection ...................................22
      A.3. Protection in the GMPLS Overlay Model .....................24
      A.4. LSP Protection inside a Single Area .......................25

1.  Introduction

   The RSVP-TE specification [RFC3209] and GMPLS extensions [RFC3473]
   allow abstract nodes and resources to be explicitly included in a
   path setup, using the Explicit Route Object (ERO).

   In some systems, it may be useful to specify and signal abstract
   nodes and resources that are to be explicitly excluded from routes.
   This may be because loose hops or abstract nodes need to be prevented
   from selecting a route through a specific resource.  This is a
   special case of distributed path calculation in the network.

   For example, route exclusion could be used in the case where two
   non-overlapping Label Switched Paths (LSPs) are required.  In this
   case, one option might be to set up one path and collect its route
   using route recording, and then to exclude the routers on that first
   path from the setup for the second path.  Another option might be to
   set up two parallel backbones, dual home the provider edge (PE)
   routers to both backbones, and then exclude the local router on
   backbone A the first time that you set up an LSP (to a particular
   distant PE), and exclude the local router on backbone B the second
   time that you set up an LSP.

   Two types of exclusions are required:

   1. Exclusion of certain abstract nodes or resources on the whole
      path.  This set of abstract nodes is referred to as the Exclude
      Route list.

   2. Exclusion of certain abstract nodes or resources between a
      specific pair of abstract nodes present in an ERO.  Such specific
      exclusions are referred to as Explicit Exclusion Route.

   To convey these constructs within the signaling protocol, a new RSVP
   object and a new ERO subobject are introduced respectively.

   - A new RSVP-TE object is introduced to convey the Exclude Route
     list.  This object is the EXCLUDE_ROUTE object (XRO).

   - The second type of exclusion is achieved through a modification to
     the existing ERO.  A new ERO subobject type the Explicit Exclusion
     Route Subobject (EXRS) is introduced to indicate an exclusion
     between a pair of included abstract nodes.

   The knowledge of SRLGs, as defined in [RFC4216], may be used to
   compute diverse paths that can be used for protection.  In systems
   where it is useful to signal exclusions, it may be useful to signal
   SRLGs to indicate groups of resources that should be excluded on the
   whole path or between two abstract nodes specified in an explicit
   path.

   This document introduces a subobject to indicate an SRLG to be
   signaled in either of the two exclusion methods described above.
   This document does not assume or preclude any other usage for this
   subobject.  This subobject might also be appropriate for use within
   an Explicit Route object (ERO) or Record Route object (RRO), but this
   is outside the scope of this document.

1.1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

1.2.  Scope of Exclude Routes

   This document does not preclude a route exclusion from listing
   arbitrary nodes or network elements to avoid.  The intent is,
   however, to indicate only the minimal number of subobjects to be
   explicitly avoided.  For instance, it may be necessary to signal only
   the SRLGs (or Shared Risk Link Groups) to avoid.  That is, the route
   exclusion is not intended to define the actual route by listing all
   of the choices to exclude at each hop, but rather to constrain the
   normal route selection process where loose hops or abstract nodes are
   to be expanded by listing certain elements to be avoided.

   It is envisaged that most of the conventional inclusion subobjects
   are specified in the signaled ERO only for the area where they are
   pertinent.  The number of subobjects to be avoided, specified in the
   signaled XRO, may be constant throughout the whole path setup, or the
   subobjects to be avoided may be removed from the XRO as they become
   irrelevant in the subsequent hops of the path setup.

   For example, consider an LSP that traverses multiple computation
   domains.  A computation domain may be an area in the administrative
   or IGP sense, or may be an arbitrary division of the network for
   active management and path computational purposes.  Let the primary
   path be (Ingress, A1, A2, AB1, B1, B2, BC1, C1, C2, Egress) where:

   - Xn denotes a node in domain X, and

   - XYn denotes a node on the border of domain X and domain Y.

   Note that Ingress is a node in domain A, and Egress is a node in
   domain C.  This is shown in Figure 1 where the domains correspond
   with areas.

           area A               area B              area C
    <-------------------> <----------------> <------------------>

   Ingress-----A1----A2----AB1----B1----B2----BC1----C1----C2----Egress
   ^  \                / | \              / | \                /
   |   \              /  |  \            /  |  \              /
   |    A3----------A4--AB2--B3--------B4--BC2--C3----------C4
   |                     ^                  ^
   |                     |                  |
   |                     |                  |
   |                     |              ERO: (C3-strict, C4-strict,
   |                     |                    Egress-strict)
   |                     |              XRO: Not needed
   |                     |
   |               ERO: (B3-strict, B4-strict, BC2-strict, Egress-loose)
   |               XRO: (BC1, C1, C2)
   |
   ERO: (A3-strict, A4-strict, AB2-strict, Egress-loose)
   XRO: (AB1, B1, B2, BC1, C1, C2, Egress)

           Figure 1: Domains Corresponding to IGP Areas

   Consider the establishment of a node-diverse protection path in the
   example above.  The protection path must avoid all nodes on the
   primary path.  The exclusions for area A are handled during
   Constrained Shortest Path First (CSPF) computation at Ingress, so the
   ERO and XRO signaled at Ingress could be (A3-strict, A4-strict,
   AB2-strict, Egress-loose) and (AB1, B1, B2, BC1, C1, C2),
   respectively.  At AB2, the ERO and XRO could be (B3-strict, B4-
   strict, BC2-strict, Egress-loose) and (BC1, C1, C2), respectively.
   At BC2, the ERO could be (C3-strict, C4-strict, Egress-strict) and an
   XRO is not needed from BC2 onwards.

   In general, consideration SHOULD be given (as with explicit route) to
   the size of signaled data and the impact on the signaling protocol.

1.3.  Relationship to MPLS TE MIB

   [RFC3812] defines managed objects for managing and modeling MPLS-
   based traffic engineering.  Included in [RFC3812] is a means to
   configure explicit routes for use on specific LSPs.  This
   configuration allows the exclusion of certain resources.

   In systems where the full explicit path is not computed at the
   ingress (or at a path computation site for use at the ingress), it
   may be necessary to signal those exclusions.  This document offers a
   means of doing this signaling.

2.  Shared Risk Link Groups

   The identifier of an SRLG is defined as a 32-bit quantity in
   [RFC4202].  An SRLG subobject is introduced such that it can be used
   in the exclusion methods as described in the following sections.
   This document does not assume or preclude any other usage for this
   subobject.  This subobject might also be appropriate for use within
   Explicit Route object (ERO) or Record Route object (RRO), but this is
   outside the scope of this document.

2.1.  SRLG Subobject

   The new SRLG subobject is defined by this document as follows.  Its
   format is modeled on the ERO subobjects defined in [RFC3209].

    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    |       SRLG Id (4 bytes)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      SRLG Id (continued)      |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

         For exclusions (as used by XRO and EXRS defined in this
         document), the L bit SHOULD be set to zero and ignored.

      Type
         The type of the subobject (34)

      Length
         The Length contains the total length of the subobject in bytes,
         including the Type and Length fields.  The Length is always 8.

      SRLG Id
         The 32-bit identifier of the SRLG.

      Reserved
         This field is reserved.  It SHOULD be set to zero on
         transmission and MUST be ignored on receipt.

3.  Exclude Route List

   The exclude route identifies a list of abstract nodes that should not
   be traversed along the path of the LSP being established.  It is
   RECOMMENDED that the size of the exclude route list be limited to a
   value local to the node originating the exclude route list.

3.1.  EXCLUDE_ROUTE Object (XRO)

   Abstract nodes to be excluded from the path are specified via the
   EXCLUDE_ROUTE object (XRO).

   Currently, one C_Type is defined, Type 1 EXCLUDE_ROUTE.  The
   EXCLUDE_ROUTE object has the following format:

         Class = 232, C_Type = 1

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       //                        (Subobjects)                         //
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The contents of an EXCLUDE_ROUTE object are a series of variable-
   length data items called subobjects.  This specification adapts ERO
   subobjects as defined in [RFC3209], [RFC3473], and [RFC3477] for use
   in route exclusions.  The SRLG subobject as defined in Section 2 of
   this document has not been defined before.  The SRLG subobject is
   defined here for use with route exclusions.

   The following subobject types are supported.

        Type           Subobject
        -------------+-------------------------------
        1              IPv4 prefix
        2              IPv6 prefix
        4              Unnumbered Interface ID
        32             Autonomous system number
        34             SRLG

   The defined values for Type above are specified in [RFC3209] and in
   this document.

   The concept of loose or strict hops has no meaning in route
   exclusion.  The L bit, defined for ERO subobjects in [RFC3209], is
   reused here to indicate that an abstract node MUST be excluded (value
   0) or SHOULD be avoided (value 1).  The distinction is that the path
   of an LSP must not traverse an abstract node listed in the XRO with
   the L bit clear, but may traverse one with the L bit set.  A node
   responsible for routing an LSP (for example, for expanding a loose
   hop) should attempt to minimize the number of abstract nodes listed
   in the XRO with the L bit set that are traversed by the LSP according
   to local policy.  A node generating XRO subobjects with the L bit set
   must be prepared to accept an LSP that traverses one, some, or all of
   the corresponding abstract nodes.

   Subobjects 1, 2, and 4 refer to an interface or a set of interfaces.
   An Attribute octet is introduced in these subobjects to indicate the
   attribute (e.g., interface, node, SRLG) associated with the
   interfaces that should be excluded from the path.  For instance, the
   attribute node allows a whole node to be excluded from the path by
   specifying an interface of that node in the XRO subobject, in
   contrast to the attribute interface, which allows a specific
   interface (or multiple interfaces) to be excluded from the path
   without excluding the whole node.  The attribute SRLG allows all
   SRLGs associated with an interface to be excluded from the path.

3.1.1.  IPv4 Prefix Subobject

    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 Length |   Attribute   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      L
         0 indicates that the attribute specified MUST be excluded.
         1 indicates that the attribute specified SHOULD be avoided.

      Attribute

         Interface attribute values
            0 indicates that the interface or set of interfaces
            associated with the IPv4 prefix should be excluded or
            avoided.

         Node attribute value
            1 indicates that the node or set of nodes associated with
            the IPv4 prefix should be excluded or avoided.

         SRLG attribute values
            2 indicates that all the SRLGs associated with the IPv4
            prefix should be excluded or avoided.

   The rest of the fields are as defined in [RFC3209].

3.1.2.  IPv6 Prefix Subobject

    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 Length |   Attribute   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      L
         0 indicates that the attribute specified MUST be excluded.
         1 indicates that the attribute specified SHOULD be avoided.

      Attribute

         Interface attribute value
            0 indicates that the interface or set of interfaces
            associated with the IPv6 prefix should be excluded or
            avoided.

         Node attribute value
            1 indicates that the node or set of nodes associated with
            the IPv6 prefix should be excluded or avoided.

         SRLG attribute value
            2 indicates that all the SRLGs associated with the IPv6
            prefix should be excluded or avoided.

   The rest of the fields are as defined in [RFC3209].

3.1.3.  Unnumbered Interface ID Subobject

    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    |    Reserved   |  Attribute    |
   | |             |               |(must be zero) |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        TE Router ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Interface ID (32 bits)                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      L
         0 indicates that the attribute specified MUST be excluded.
         1 indicates that the attribute specified SHOULD be avoided.

      Attribute

         Interface attribute value
            0 indicates that the Interface ID specified should be
            excluded or avoided.

         Node attribute value
            1 indicates that the node with the Router ID should be
            excluded or avoided (this can be achieved using an IPv4/v6
            subobject as well, but is included here because it may be
            convenient to use information from subobjects of an RRO, as
            defined in [RFC3477], in specifying the exclusions).

         SRLG attribute value
            2 indicates that all the SRLGs associated with the interface
            should be excluded or avoided.

      Reserved
         This field is reserved.  It SHOULD be set to zero on
         transmission and MUST be ignored on receipt.

   The rest of the fields are as defined in [RFC3477].

3.1.4.  Autonomous System Number Subobject

   The meaning of the L bit is as follows:
      0 indicates that the abstract node specified MUST be excluded.
      1 indicates that the abstract node specified SHOULD be avoided.

   The rest of the fields are as defined in [RFC3209].  There is no
   Attribute octet defined.

3.1.5.  SRLG Subobject

   The meaning of the L bit is as follows:
      0 indicates that the SRLG specified MUST be excluded
      1 indicates that the SRLG specified SHOULD be avoided

   The Attribute octet is not present.  The rest of the fields are as
   defined in the "SRLG Subobject" section of this document.

3.2.  Processing Rules for the EXCLUDE_ROUTE Object (XRO)

   The exclude route list is encoded as a series of subobjects contained
   in an EXCLUDE_ROUTE object.  Each subobject identifies an abstract
   node in the exclude route list.

   Each abstract node may be a precisely specified IP address belonging
   to a node, or an IP address with prefix identifying interfaces of a
   group of nodes, an Autonomous System, or an SRLG.

   The Explicit Route and routing processing is unchanged from the
   description in [RFC3209] with the following additions:

   1. When a Path message is received at a node, the node MUST check
      that it is not a member of any of the abstract nodes in the XRO if
      it is present in the Path message.  If the node is a member of any
      of the abstract nodes in the XRO with the L-flag set to "exclude",
      it SHOULD return a PathErr with the error code "Routing Problem"
      and error value of "Local node in Exclude Route".  If there are
      SRLGs in the XRO, the node SHOULD check that the resources the
      node uses are not part of any SRLG with the L-flag set to
      "exclude" that is specified in the XRO.  If it is, it SHOULD
      return a PathErr with error code "Routing Problem" and error value
      of "Local node in Exclude Route".

   2. Each subobject MUST be consistent.  If a subobject is not
      consistent then the node SHOULD return a PathErr with error code
      "Routing Problem" and error value "Inconsistent Subobject".  An
      example of an inconsistent subobject is an IPv4 Prefix subobject
      containing the IP address of a node and the attribute field is set
      to "interface" or "SRLG".

   3. The subobjects in the ERO and XRO SHOULD NOT contradict each
      other.  If a Path message is received that contains contradicting
      ERO and XRO subobjects, then:

      - Subobjects in the XRO with the L flag not set (zero) MUST take
        precedence over the subobjects in the ERO -- that is, a
        mandatory exclusion expressed in the XRO MUST be honored and an
        implementation MUST reject such a Path message.  This means that
        a PathErr with error code "Routing Problem" and error value of
        "Route blocked by Exclude Route" is returned.

      - Subobjects in the XRO with the L flag set do not take precedence
        over ERO subobjects -- that is, an implementation MAY choose to
        reject a Path message because of such a contradiction, but MAY
        continue and set up the LSP (ignoring the XRO subobjects that
        contradict the ERO subobjects).

   4. When choosing a next hop or expanding an explicit route to include
      additional subobjects, a node:

      a. MUST NOT introduce an explicit node or an abstract node that
         equals or is a member of any abstract node that is specified in
         the EXCLUDE_ROUTE object with the L-flag set to "exclude".  The
         number of introduced explicit nodes or abstract nodes with the
         L flag set to "avoid", which indicates that it is not mandatory
         to be excluded but that it is less preferred, SHOULD be
         minimized in the computed path.

      b. MUST NOT introduce links, nodes, or resources identified by the
         SRLG Id specified in the SRLG subobjects(s).  The number of
         introduced SRLGs with the L flag set to "avoid" SHOULD be
         minimized.

      If these rules preclude further forwarding of the Path message,
      the node SHOULD return a PathErr with the error code "Routing
      Problem" and error value of "Route blocked by Exclude Route".

      Note that the subobjects in the XRO is an unordered list of
      subobjects.

   A node receiving a Path message carrying an XRO MAY reject the
   message if the XRO is too large or complicated for the local
   implementation or the rules of local policy.  In this case, the node
   MUST send a PathErr message with the error code "Routing Error" and
   error value "XRO Too Complex".  An ingress LSR receiving this error
   code/value combination MAY reduce the complexity of the XRO or route
   around the node that rejected the XRO.

   The XRO Class-Num is of the form 11bbbbbb so that nodes that do not
   support the XRO forward it uninspected and do not apply the
   extensions to ERO processing described above.  This approach is
   chosen to allow route exclusion to traverse parts of the network that
   are not capable of parsing or handling the new function.  Note that
   Record Route may be used to allow computing nodes to observe
   violations of route exclusion and attempt to re-route the LSP
   accordingly.

   If a node supports the XRO, but not a particular subobject or part of
   that subobject, then that particular subobject is ignored.  Examples
   of a part of a subobject that can be supported are: (1) only prefix
   32 of the IPv4 prefix subobject could be supported, or (2) a
   particular subobject is supported but not the particular attribute
   field.

   When a node forwards a Path message, it can do the following three
   operations related to XRO besides the processing rules mentioned
   above:

   1. If no XRO was present, an XRO may be included.

   2. If an XRO was present, it may remove the XRO if it is sure that
      the next nodes do not need this information anymore.  An example
      is where a node can expand the ERO to a full strict path towards
      the destination.  See Figure 1 where BC2 is removing the XRO from
      the Path message.

   3. If an XRO was present, the content of the XRO can be modified.
      Subobjects can be added or removed.  See Figure 1 for an example
      where AB2 is stripping off some subobjects.

   In any case, a node MUST NOT introduce any explicit or abstract node
   in the XRO (irrespective of the value of the L flag) that it also has
   introduced in the ERO.

4.  Explicit Exclusion Route

   The Explicit Exclusion Route defines abstract nodes or resources
   (such as links, unnumbered interfaces, or labels) that must not or
   should not be used on the path between two inclusive abstract nodes
   or resources in the explicit route.

4.1.  Explicit Exclusion Route Subobject (EXRS)

   A new ERO subobject type is defined.  The Explicit Exclusion Route
   Subobject (EXRS) has type 33.  Although the EXRS is an ERO subobject
   and the XRO is reusing the ERO subobject, an EXRS MUST NOT be present
   in an XRO.  An EXRS is an ERO subobject that contains one or more
   subobjects of its own, called EXRS subobjects.

   The format of the EXRS is 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |L|    Type     |     Length    |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                one or more EXRS subobjects                  //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      L
         It MUST be set to zero on transmission and MUST be ignored on
         receipt.  (Note: The L bit in an EXRS subobject is as defined
         for the XRO subobjects.)

      Type
         The type of the subobject (33).

      Reserved
         This field is reserved.  It SHOULD be set to zero on
         transmission and MUST be ignored on receipt.

      EXRS subobjects
         An EXRS subobject indicates the abstract node or resource to be
         excluded.  The format of an EXRS subobject is exactly the same
         as the format of a subobject in the XRO.  An EXRS may include
         all subobjects defined in this document for the XRO.

   Thus, an EXRS for an IP hop may look 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |L|    Type     |     Length    |           Reserved            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |L|    Type     |     Length    | IPv4 address (4 bytes)        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | IPv4 address (continued)      | Prefix Length |   Attribute   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.2.  Processing Rules for the Explicit Exclusion Route Subobject (EXRS)

   Each EXRS may carry multiple exclusions.  The exclusion is encoded
   exactly as for XRO subobjects and prefixed by an additional Type and
   Length.

   The scope of the exclusion is the step between the previous ERO
   subobject that identifies an abstract node, and the subsequent ERO
   subobject that identifies an abstract node.  The processing rules of
   the EXRS are the same as the processing rule of the XRO within this
   scope.  Multiple exclusions may be present between any pair of
   abstract nodes.

   Exclusions may indicate explicit nodes, abstract nodes, or Autonomous
   Systems that must not be traversed on the path to the next abstract
   node indicated in the ERO.

   Exclusions may also indicate resources (such as unnumbered
   interfaces, link ids, and labels) that must not be used on the path
   to the next abstract node indicated in the ERO.

   SRLGs may also be indicated for exclusion from the path to the next
   abstract node in the ERO by the inclusion of an EXRS containing an
   SRLG subobject.  If the L bit in the SRLG subobject is zero, the
   resources (nodes, links, etc.) identified by the SRLG MUST NOT be
   used on the path to the next abstract node indicated in the ERO.  If
   the L bit is set, the resources identified by the SRLG SHOULD be
   avoided.

   If a node is called upon to process an EXRS and does not support
   handling of exclusions it will behave as described in [RFC3209] when
   an unrecognized ERO subobject is encountered.  This means that this
   node will return a PathErr with error code "Routing Error" and error
   value "Bad EXPLICIT_ROUTE object" with the EXPLICIT_ROUTE object
   included, truncated (on the left) to the offending EXRS.

   If the presence of EXRS precludes further forwarding of the Path
   message, the node SHOULD return a PathErr with the error code
   "Routing Problem" and error value "Route Blocked by Exclude Route".

   A node MAY reject a Path message if the EXRS is too large or
   complicated for the local implementation or as governed by local
   policy.  In this case, the node MUST send a PathErr message with the
   error code "Routing Error" and error value "EXRS Too Complex".  An
   ingress LSR receiving this error code/value combination MAY reduce
   the complexity of the EXRS or route around the node that rejected the
   EXRS.

5.  Processing of XRO together with EXRS

   When an LSR performs ERO expansion and finds both the XRO in the Path
   message and EXRS in the ERO, it MUST exclude all the SRLGs, nodes,
   links, and resources listed in both places.  Where some elements
   appear in both lists, it MUST be handled according to the stricter
   exclusion request.  That is, if one list says that an SRLG, node,
   link, or resource must be excluded, and the other says only that it
   should be avoided, then the element MUST be excluded.

6.  Minimum Compliance

   An implementation MUST be at least compliant with the following:

   1. The XRO MUST be supported with the following restrictions:

      - The IPv4 Prefix subobject MUST be supported with a prefix length
        of 32, and an attribute value of "interface" and "node".  Other
        prefix values and attribute values MAY be supported.

      - The IPv6 Prefix subobject MUST be supported with a prefix length
        of 128, and an attribute value of "interface" and "node".  Other
        prefix values and attribute values MAY be supported.

   2. The EXRS MAY be supported.  If supported, the same restrictions as
      for the XRO apply.  If not supported, an EXRS encountered during
      normal ERO processing MUST be rejected as an unknown ERO subobject
      as described in Section 4.2.  Note that a node SHOULD NOT parse
      ahead into an ERO, and if it does, it MUST NOT reject the ERO if
      it discovers an EXRS that applies to another node.

   3. If XRO or EXRS are supported, the implementation MUST be compliant
      with the processing rules of the supported, not supported, or
      partially supported subobjects as specified within this document.

7.  Security Considerations

   Security considerations for MPLS-TE and GMPLS signaling are covered
   in [RFC3209] and [RFC3473].  This document does not introduce any new
   messages or any substantive new processing, and so those security
   considerations continue to apply.

   Note that any security concerns that exist with explicit routes
   should be considered with regard to route exclusions.  For example,
   some administrative boundaries may consider explicit routes to be
   security violations and may strip EROs from the Path messages that
   they process.  In this case, the XRO should also be considered for
   removal from the Path message.

   It is possible that an arbitrarily complex XRO or EXRS sequence could
   be introduced as a form of denial-of-service attack since its
   presence will potentially cause additional processing at each node on
   the path of the LSP.  It should be noted that such an attack assumes
   that an otherwise trusted LSR (i.e., one that has been authenticated
   by its neighbors) is misbehaving.  A node that receives an XRO or
   EXRS sequence that it considers too complex according to its local
   policy may respond with a PathErr message carrying the error code
   "Routing Error" and error value "XRO Too Complex" or "EXRS Too
   Complex".

8.  IANA Considerations

   It might be considered that an alternative approach would be to
   assign one of the bits of the ERO subobject type field (perhaps the
   top bit) to identify that a subobject is intended for inclusion
   rather than exclusion.  However, [RFC3209] states that the type field
   (seven bits) should be assigned as 0 - 63 through IETF consensus
   action, 64 - 95 as first come first served, and 96 - 127 are reserved
   for private use.  It would not be acceptable to disrupt existing
   implementations, so the only option would be to split the IETF
   consensus range leaving only 32 subobject types.  It is felt that 32
   would be an unacceptably small number for future expansion of the
   protocol.

8.1.  New ERO Subobject Type

   IANA registry: RSVP PARAMETERS
   Subsection: Class Names, Class Numbers, and Class Types

   A new subobject has been added to the existing entry for:

   20  EXPLICIT_ROUTE

   The registry reads:

               33  Explicit Exclusion Route subobject (EXRS)

   The Explicit Exclusion Route subobject (EXRS) is defined in Section
   4.1, "Explicit Exclusion Route Subobject (EXRS)".  This subobject may
   be present in the Explicit Route Object, but not in the Route Record
   Object or in the new EXCLUDE_ROUTE object, and it should not be
   listed among the subobjects for those objects.

8.2.  New RSVP-TE Class Numbers

   IANA registry: RSVP PARAMETERS
   Subsection: Class Names, Class Numbers, and Class Types

   A new class number has been added for EXCLUDE_ROUTE object (XRO) as
   defined in Section 3.1, "EXCLUDE_ROUTE Object (XRO)".

   EXCLUDE_ROUTE
   Class-Num of type 11bbbbbb
   Value: 232
   Defined CType: 1 (EXCLUDE_ROUTE)

   Subobjects 1, 2, 4, and 32 are as defined for Explicit Route Object.
   An additional subobject has been registered as requested in Section
   8.1, "New ERO Subobject Type".  The text should appear as:

   Sub-object type
                1   IPv4 address              [RFC3209]
                2   IPv6 address              [RFC3209]
                4   Unnumbered Interface ID   [RFC3477]
               32   Autonomous system number  [RFC3209]
               33   Explicit Exclusion Route subobject (EXRS) [RFC4874]
               34   SRLG                      [RFC4874]

   The SRLG subobject is defined in Section 3.1.5, "SRLG Subobject".
   The value 34 has been assigned.

8.3.  New Error Codes

   IANA registry: RSVP PARAMETERS
   Subsection: Error Codes and Globally-Defined Error Value Sub-Codes

   New Error Values sub-codes have been registered for the Error Code
   'Routing Problem' (24).

     64 = Unsupported Exclude Route Subobject Type
     65 = Inconsistent Subobject
     66 = Local Node in Exclude Route
     67 = Route Blocked by Exclude Route
     68 = XRO Too Complex
     69 = EXRS Too Complex

9.  Acknowledgments

   This document reuses text from [RFC3209] for the description of
   EXCLUDE_ROUTE.

   The authors would like to express their thanks to Lou Berger, Steffen
   Brockmann, Igor Bryskin, Dimitri Papadimitriou, Cristel Pelsser, and
   Richard Rabbat for their considered opinions on this document.  Also
   thanks to Yakov Rekhter for reminding us about SRLGs!

   Thanks to Eric Gray for providing GenArt review and to Ross Callon
   for his comments.

10.  References

10.1.  Normative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3209]   Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
               and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
               Tunnels", RFC 3209, December 2001.

   [RFC3473]   Berger, L., "Generalized Multi-Protocol Label Switching
               (GMPLS) Signaling Resource ReserVation Protocol-Traffic
               Engineering (RSVP-TE) Extensions", RFC 3473, January
               2003.

   [RFC3477]   Kompella, K. and Y. Rekhter, "Signalling Unnumbered Links
               in Resource ReSerVation Protocol - Traffic Engineering
               (RSVP-TE)", RFC 3477, January 2003.

   [RFC4202]   Kompella, K. and Y. Rekhter, "Routing Extensions in
               Support of Generalized Multi-Protocol Label Switching
               (GMPLS)", RFC 4202, October 2005.

10.2.  Informative References

   [CRANKBACK] Farrel, A., Satyanarayana, A., Iwata, A., Ash, G., and S.
               Marshall-Unitt, "Crankback Signaling Extensions for MPLS
               Signaling", Work in Progress, January 2007.

   [RFC3630]   Katz, D., Kompella, K., and D. Yeung, "Traffic
               Engineering (TE) Extensions to OSPF Version 2", RFC 3630,
               September 2003.

   [RFC3784]   Smit, H. and T. Li, "Intermediate System to Intermediate
               System (IS-IS) Extensions for Traffic Engineering (TE)",
               RFC 3784, June 2004.

   [RFC3812]   Srinivasan, C., Viswanathan, A., and T. Nadeau,
               "Multiprotocol Label Switching (MPLS) Traffic Engineering
               (TE) Management Information Base (MIB)", RFC 3812, June
               2004.

   [RFC4208]   Swallow, G., Drake, J., Ishimatsu, H., and Y. Rekhter,
               "Generalized Multiprotocol Label Switching (GMPLS) User-
               Network Interface (UNI): Resource ReserVation Protocol-
               Traffic Engineering (RSVP-TE) Support for the Overlay
               Model", RFC 4208, October 2005.

   [RFC4216]   Zhang, R. and JP. Vasseur, "MPLS Inter-Autonomous System
               (AS) Traffic Engineering (TE) Requirements", RFC 4216,
               November 2005.

Appendix A.  Applications

   This section describes some applications that can make use of the
   XRO.  The intention is to show that the XRO is not an application-
   specific object, but that it can be used for multiple purposes.  In a
   few examples, other solutions might be possible for that particular
   case, but the intention is to show that a single object can be used
   for all the examples, hence making the XRO a rather generic object
   without having to define a solution and new objects for each new
   application.

A.1.  Inter-Area LSP Protection

   One method to establish an inter-area LSP is where the ingress router
   selects an ABR, and then the ingress router computes a path towards
   this selected ABR such that the configured constraints of the LSP are
   fulfilled.  In the example of Figure A.1, an LSP has to be
   established from node A in area 1 to node C in area 2.  If no loose
   hops are configured, then the computed ERO at A could look as
   follows: (A1-strict, A2-strict, ABR1-strict, C-loose).  When the Path
   message arrives at ABR1, then the ERO is (ABR1-strict, C-loose), and
   it can be expanded by ABR1 to (B1-strict, ABR3-strict, C-loose).
   Similarly, at ABR3 the received ERO is (ABR3-strict, C-loose), and it
   can be expanded to (C1-strict, C2-strict, C-strict).  If a backup LSP
   also has to be established, then A takes another ABR (ABR2 in this
   case) and computes a path towards this ABR that fulfills the
   constraints of the LSP and that is disjoint from the path of the
   primary LSP.  The ERO generated by A looks as follows for this
   example: (A3-strict, A4-strict, ABR2-strict, C-loose).

   In order to let ABR2 expand the ERO, it also needs to know the path
   of the primary LSP so that the ERO expansion is disjoint from the
   path of the primary LSP.  Therefore, A also includes an XRO that at
   least contains (ABR1, B1, ABR3, C1, C2).  Based on these constraints,
   ABR2 can expand the ERO such that it is disjoint from the primary
   LSP.  In this example, the ERO computed by ABR2 would be (B2-strict,
   ABR4-strict, C-loose), and the XRO generated by B contains at least
   (ABR3, C1, C2).  The latter information is needed for ABR4 to expand
   the ERO so that the path is disjoint from the primary LSP in area 2.

            Area 1           Area 0          Area 2
       <---------------><--------------><--------------->

       +---A1---A2----ABR1-----B1-----ABR3----C1---C2---+
       |        |              |              |         |
       |        |              |              |         |
       A        |              |              |         C
       |        |              |              |         |
       |        |              |              |         |
       +---A3---A4----ABR2-----B2-----ABR4----C3---C4---+

                 Figure A.1: Inter-area LSPs

   In this example, a node performing the path computation first selects
   an ABR and then computes a strict path towards this ABR.  For the
   backup LSP, all nodes of the primary LSP in the next areas have to be
   put in the XRO (with the exception of the destination node if node
   protection and no link protection is required).  When an ABR computes
   the next path segment, i.e., the path over the next area, it may
   remove the nodes from the XRO that are located in that area with the
   exception of the ABR where the primary LSP is exiting the area.  The
   latter information is still required because when the selected ABR
   (ABR4 in this example) further expands the ERO, it has to exclude the
   ABR on which the primary LSP is entering that area (ABR3 in this
   example).  This means that when ABR2 generates an XRO, it may remove
   the nodes in area 0 from the XRO but not ABR3.  Note that not doing
   this would not cause harm in this example because there is no path
   from ABR4 to C via ABR3 in area 2.  If there is a link between ABR4-
   ABR3 and ABR3-C, then it is required to have ABR3 in the XRO
   generated by ABR2.

   Discussion on the length of the XRO: When link or node protection is
   requested, the length of the XRO is bounded by the length of the RRO
   of the primary LSP.  It can be made shorter by removing nodes by the
   ingress node and the ABRs.  In the example above, the RRO of the
   primary LSP contains 8 subobjects, while the maximum XRO length can
   be bounded by 6 subobjects (nodes A1 and A2 do not have to be in the
   XRO).  For SRLG protection, the XRO has to list all SRLGs that are
   crossed by the primary LSP.

A.2.  Inter-AS LSP Protection

   When an inter-AS LSP (which has to be protected by a backup LSP to
   provide link or node protection) is established, the same method as
   for the inter-area LSP case can be used.  The difference is when the
   backup LSP is not following the same AS-path as the primary LSP
   because then the XRO should always contain the full path of the
   primary LSP.  In case the backup LSP is following the same AS-path
   (but with different ASBRs -- at least in case of node protection), it
   is similar to the inter-area case: ASBRs expanding the ERO over the
   next AS may remove the XRO subobjects located in that AS.  Note that
   this can only be done by an ingress ASBR (the ASBR where the LSP is
   entering the AS).

   Discussion on the length of the XRO: the XRO is bounded by the length
   of the RRO of the primary LSP.

   Suppose that SRLG protection is required, and the ASs crossed by the
   main LSP use a consistent way of allocating SRLG-ids to the links
   (i.e., the ASs use a single SRLG space).  In this case, the SRLG-ids
   of each link used by the main LSP can be recorded by means of the
   RRO; the SRLG-ids are then used by the XRO.  If the SRLG-ids are only
   meaningful when local to the AS, putting SRLG-ids in the XRO crossing
   many ASs makes no sense.  To provide SRLG protection for inter-AS
   LSPs the link IP address of the inter-AS link used by the primary LSP
   can be put into the XRO of the Path message of the detour LSP or
   bypass tunnel.  The ASBR where the detour LSP or bypass tunnel is
   entering the AS can translate this into the list of SRLG-ids known to
   the local AS.

   Discussion on the length of the XRO: the XRO only contains 1
   subobject, which contains the IP address of the inter-AS link
   traversed by the primary LSP (assuming that the primary LSP and
   detour LSP or bypass tunnel are leaving the AS in the same area, and
   that they are also entering the next AS in the same area).

A.3.  Protection in the GMPLS Overlay Model

   When an edge-node wants to establish an LSP towards another edge-node
   over an optical core network as described in [RFC4208] (see Figure
   A.2), the XRO can be used for multiple purposes.

     Overlay                                                  Overlay
     Network        +--------------------------------+        Network
   +----------+     |                                |     +----------+
   |   +----+ |     |  +-----+   +-----+   +-----+   |     | +----+   |
   |   |    | |     |  |     |   |     |   |     |   |     | |    |   |
   | --+ EN1+-+-----+--+ CN1 +---+ CN2 +---+ CN3 +---+-----+-+ EN3+-- |
   |   |    | |  +--+--+     |   |     |   |     +---+--+  | |    |   |
   |   +----+ |  |  |  +--+--+   +--+--+   +--+--+   |  |  | +----+   |
   |          |  |  |     |         |         |      |  |  |          |
   +----------+  |  |     |         |         |      |  |  +----------+
                 |  |     |         |         |      |  |
   +----------+  |  |     |         |         |      |  |  +----------+
   |          |  |  |  +--+--+      |      +--+--+   |  |  |          |
   |   +----+ |  |  |  |     |      +------+     |   |  |  | +----+   |
   |   |    +-+--+  |  | CN4 +-------------+ CN5 |   |  +--+-+    |   |
   | --+ EN2+-+-----+--+     |             |     +---+-----+-+ EN4+-- |
   |   |    | |     |  +-----+             +-----+   |     | |    |   |
   |   +----+ |     |                                |     | +----+   |
   |          |     +--------------------------------+     |          |
   +----------+                 Core Network               +----------+

        Overlay                                                 Overlay
        Network                                                 Network

    Legend:
         EN - Edge-Node
         CN - Core-Node

                                 Figure A.2

   A first application is where an edge-node wants to establish multiple
   LSPs towards the same destination edge-node, and these LSPs need to
   have few or no SRLGs in common.  In this case EN1 could establish an
   LSP towards EN3, and then it can establish a second LSP listing all
   links used by the first LSP with the indication to avoid the SRLGs of
   these links.  This information can be used by CN1 to compute a path
   for the second LSP.  If the core network consists of multiple areas,
   then the SRLG-ids have to be listed in the XRO.  The same example
   applies to nodes and links.

   Another application is where the edge-node wants to set up a backup
   LSP that is also protecting the links between the edge-nodes and
   core-nodes.  For instance, when EN2 establishes an LSP to EN4, it
   sends a Path message to CN4, which computes a path towards EN4 over
   (for instance) CN5.  When EN2 gets back the RRO of that LSP, it can
   signal a new LSP to CN1 with EN4 as the destination and the XRO
   computed based on the RRO of the first LSP.  Based on this
   information, CN1 can compute a path that has the requested diversity
   properties (e.g., a path going over CN2 and CN3, and then to EN4).

   It is clear that in these examples, the core-node may not alter the
   RRO in a Resv message to make its only contents be the subobjects
   from the egress core-node through the egress edge-node.

A.4.  LSP Protection inside a Single Area

   The XRO can also be used inside a single area.  Take for instance a
   network where the TE extensions of the IGPs as described in [RFC3630]
   and [RFC3784] are not used.  Hence, each node has to select a next-
   hop and possibly crankback [CRANKBACK] has to be used when there is
   no viable next-hop.  In this case, when signaling a backup LSP, the
   XRO can be put in the Path message to exclude the links, nodes, or
   SRLGs of the primary LSP.  An alternative way to provide this
   functionality would be to indicate the following in the Path message
   of the backup LSP: the primary LSP and which type of protection is
   required.  This latter solution would work for link and node
   protection, but not for SRLG protection.

   When link or node protection is requested, the XRO is of the same
   length as the RRO of the primary LSP.  For SRLG protection, the XRO
   has to list all SRLGs that are crossed by the primary LSP.  Note that
   for SRLG protection, the link IP address to reference the SRLGs of
   that link cannot be used since the TE extensions of the IGPs are not
   used in this example.  Hence, a node cannot translate any link IP
   address located in that area to its SRLGs.

Authors' Addresses

   Cheng-Yin Lee
   EMail: c.yin.lee@gmail.com


   Adrian Farrel
   Old Dog Consulting
   Phone: +44 (0) 1978 860944
   EMail: adrian@olddog.co.uk


   Stefaan De Cnodder
   Alcatel-Lucent
   Copernicuslaan 50
   B-2018 Antwerp
   Belgium
   Phone: +32 3 240 85 15
   EMail: stefaan.de_cnodder@alcatel-lucent.be

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