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Internet Engineering Task Force                                  H. Chen
Internet-Draft                                       Huawei Technologies
Intended status: Standards Track                          April 30, 2018
Expires: November 1, 2018


Extensions to the Path Computation Element Communication Protocol (PCEP)
     for Backup Egress of a Traffic Engineering Label Switched Path
              draft-chen-pce-compute-backup-egress-13.txt

Abstract

   This document presents extensions to the Path Computation Element
   Communication Protocol (PCEP) for a PCC to send a request for
   computing a backup egress for an MPLS TE P2MP LSP or an MPLS TE P2P
   LSP to a PCE and for a PCE to compute the backup egress and reply to
   the PCC with a computation result for the backup egress.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 1, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as



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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Conventions Used in This Document  . . . . . . . . . . . . . .  3
   4.  Extensions to PCEP . . . . . . . . . . . . . . . . . . . . . .  3
     4.1.  Backup Egress Capability Advertisement . . . . . . . . . .  4
       4.1.1.  Capability TLV in Existing PCE Discovery Protocol  . .  4
       4.1.2.  Open Message Extension . . . . . . . . . . . . . . . .  5
     4.2.  Request and Reply Message Extension  . . . . . . . . . . .  6
       4.2.1.  RP Object Extension  . . . . . . . . . . . . . . . . .  6
       4.2.2.  External Destination Nodes . . . . . . . . . . . . . .  6
       4.2.3.  Constraints between Egress and Backup Egress . . . . . 11
       4.2.4.  Constraints for Backup Path  . . . . . . . . . . . . . 11
       4.2.5.  Backup Egress Node . . . . . . . . . . . . . . . . . . 11
       4.2.6.  Backup Egress PCEP Error Objects and Types . . . . . . 12
       4.2.7.  Request Message Format . . . . . . . . . . . . . . . . 12
       4.2.8.  Reply Message Format . . . . . . . . . . . . . . . . . 12
   5.  Security  Considerations . . . . . . . . . . . . . . . . . . . 13
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
     6.1.  Backup Egress Capability Flag  . . . . . . . . . . . . . . 13
     6.2.  Backup Egress Capability TLV . . . . . . . . . . . . . . . 14
     6.3.  Request Parameter Bit Flags  . . . . . . . . . . . . . . . 14
     6.4.  PCEP Objects . . . . . . . . . . . . . . . . . . . . . . . 14
   7.  Acknowledgement  . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15



















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1.  Introduction

   RFC 4655 "A Path Computation Element-(PCE) Based Architecture"
   describes a set of building blocks for constructing solutions to
   compute Point-to-Point (P2P) Traffic Engineering (TE) label switched
   paths across multiple areas or Autonomous System (AS) domains.  A
   typical PCE-based system comprises one or more path computation
   servers, traffic engineering databases (TED), and a number of path
   computation clients (PCC).  A routing protocol is used to exchange
   traffic engineering information from which the TED is constructed.  A
   PCC sends a Point-to-Point traffic engineering Label Switched Path
   (LSP) computation request to the path computation server, which uses
   the TED to compute the path and responses to the PCC with the
   computed path.  A path computation server is named as a PCE.  The
   communications between a PCE and a PCC for Point-to-Point label
   switched path computations follow the PCE communication protocol
   (PCEP).

   RFC6006 "Extensions to PCEP for Point-to-Multipoint Traffic
   Engineering Label Switched Paths" describes extensions to PCEP to
   handle requests and responses for the computation of paths for P2MP
   TE LSPs.

   This document defines extensions to the Path Computation Element
   Communication Protocol (PCEP) for a PCC to send a request for
   computing a backup egress node for an MPLS TE P2MP LSP or an MPLS TE
   P2P LSP to a PCE and for a PCE to compute the backup egress node and
   reply to the PCC with a computation result for the backup egress
   node.


2.  Terminology

   This document uses terminologies defined in RFC5440, and RFC4875.


3.  Conventions Used in This Document

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


4.  Extensions to PCEP

   This section describes the extensions to PCEP for computing a backup
   egress of an MPLS TE P2MP LSP and an MPLS TE P2P LSP.




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4.1.  Backup Egress Capability Advertisement

4.1.1.  Capability TLV in Existing PCE Discovery Protocol

   An option for advertising a PCE capability for computing a backup
   egress for an MPLS TE P2MP LSP or an MPLS TE P2P LSP is to define two
   new flags.  One new flag in the OSPF and IS-IS PCE Capability Flags
   indicates the capability that a PCE is capable to compute a backup
   egress for an MPLS TE P2MP LSP; and another new flag in the OSPF and
   IS-IS PCE Capability Flags indicates the capability that a PCE is
   capable to compute a backup egress for an MPLS TE P2P LSP.

   The format of the PCE-CAP-FLAGS sub-TLV 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              Type = 5         |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~                 PCE Capability Flags                          ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Type:     5
         Length:   Multiple of 4 octets
         Value:    This contains an array of units of 32-bit flags
                   numbered from the most significant as bit zero, where
                   each bit represents one PCE capability.

   The following capability bits have been assigned by IANA:

         Bit       Capabilities
          0        Path computation with GMPLS link constraints
          1        Bidirectional path computation
          2        Diverse path computation
          3        Load-balanced path computation
          4        Synchronized path computation
          5        Support for multiple objective functions
          6        Support for additive path constraints
                   (max hop count, etc.)
          7        Support for request prioritization
          8        Support for multiple requests per message
          9        Global Concurrent Optimization (GCO)
          10       P2MP path computation
         11-31     Reserved for future assignments by IANA.


   Reserved bits SHOULD be set to zero on transmission and MUST be



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   ignored on receipt.

   For the backup egress capabilities, one bit such as bit 13 may be
   assigned to indicate that a PCE is capable to compute a backup egress
   for an MPLS TE P2MP LSP and another bit such as bit 14 may be
   assigned to indicate that a PCE is capable to compute a backup egress
   for an MPLS TE P2P LSP as follows.

         Bit       Capabilities
          13       Backup egress computation for P2MP LSP
          14       Backup egress computation for P2P LSP
         15-31     Reserved for future assignments by IANA.


4.1.2.  Open Message Extension

   If a PCE does not advertise its backup egress compution capability
   during discovery, PCEP should be used to allow a PCC to discover,
   during the Open Message Exchange, which PCEs are capable of
   supporting backup egress computation.

   To achieve this, we extend the PCEP OPEN object by defining a new
   optional TLV to indicate the PCE's capability to perform backup
   egress computation for an MPLS TE P2MP LSP and an MPLS TE P2P LSP.

   We request IANA to allocate a value such as 8 from the "PCEP TLV Type
   Indicators" subregistry, as documented in Section below ("Backup
   Egress Capability TLV").  The description is "backup egress capable",
   and the length value is 2 bytes.  The value field is set to indicate
   the capability of a PCE for backup egress computation for an MPLS TE
   LSP in details.

   We can use flag bits in the value field in the same way as the PCE
   Capability Flags described in the previous section.

   The inclusion of this TLV in an OPEN object indicates that the sender
   can perform backup egress computation for an MPLS TE P2MP LSP or an
   MPLS TE P2P LSP.

   The capability TLV is meaningful only for a PCE, so it will typically
   appear only in one of the two Open messages during PCE session
   establishment.  However, in case of PCE cooperation (e.g., inter-
   domain), when a PCE behaving as a PCC initiates a PCE session it
   SHOULD also indicate its path computation capabilities.







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4.2.  Request and Reply Message Extension

   This section describes extensions to the existing RP (Request
   Parameters) object to allow a PCC to request a PCE for computing a
   backup egress of an MPLS TE P2MP LSP or an MPLS TE P2P LSP when the
   PCE receives the PCEP request.

4.2.1.  RP Object Extension

   The following flags are added into the RP Object:

   The T bit is added in the flag bits field of the RP object to tell
   the receiver of the message that the request/reply is for computing a
   bcakup egress of an MPLS TE P2MP LSP and an MPLS TE P2P LSP.

       o T ( Backup Egress bit - 1 bit):
           0: This indicates that this is not PCReq/PCRep
              for backup egress.
           1: This indicates that this is PCReq or PCRep message
              for backup egress.


   The IANA request is referenced in Section below (Request Parameter
   Bit Flags) of this document.

   This T bit with the N bit defined in RFC 6006 can indicate whether a
   request/reply is for a bcakup egress of an MPLS TE P2MP LSP or an
   MPLS TE P2P LSP.

       o T = 1 and N = 1: This indicates that this is a PCReq/PCRep
                          message for backup egress of an MPLS TE
                          P2MP LSP.
       o T = 1 and N = 0: This indicates that this is a PCReq/PCRep
                          message for backup egress of an MPLS TE
                          P2P LSP.


4.2.2.  External Destination Nodes

   In addition to the information about the path that an MPLS TE P2MP
   LSP or an MPLS TE P2P LSP traverses, a request message may comprise
   other information that may be used for computing the backup egress
   for the P2MP LSP or P2P LSP.  For example, the information about an
   external destination node, to which data traffic is delivered from an
   egress node of the P2MP LSP or P2P LSP, is useful for computing a
   backup egress node.





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4.2.2.1.  External Destination Nodes Object

   The PCC can specify an external destination nodes (EDN) Object.  In
   order to represent the external destination nodes efficiently, we
   define two types of encodes for the external destination nodes in the
   object.

   One encode indicates that the EDN object contains an external
   destination node for every egress node of an MPLS TE P2MP LSP or an
   MPLS TE P2P LSP.  The order of the external destination nodes in the
   object is the same as the egress node(s) of the P2MP LSP or P2P LSP
   contained in the PCE messages.

   Another encode indicates that the EDN object contains a list of
   egress node and external destination node pairs.  For an egress node
   and external destination node pair, the data traffic is delivered to
   the external destination node from the egress node of the LSP.

   The format of the external destination nodes (EDN) object boby for
   IPv4 with the first type of encodes is illustrated 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Encode of External Destination Nodes (1)            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              External  Destination  IPv4 address              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              External  Destination  IPv4 address              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           ...                                 ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              External  Destination  IPv4 address              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The format of the external destination nodes (EDN) object boby for
   IPv4 with the second type of encodes is illustrated below:













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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Encode of External Destination Nodes (2)            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Egress IPv4 address                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               External Destination IPv4 address               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Egress IPv4 address                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               External Destination IPv4 address               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           ...                                 ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Egress IPv4 address                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               External Destination IPv4 address               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The format of the external destination nodes (EDN) object boby for
   IPv6 with the first type of encodes is illustrated 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Encode of External Destination Nodes (1)            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |          External Destination IPv6 address (16 bytes)         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |          External Destination IPv6 address (16 bytes)         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           ...                                 ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |          External Destination IPv6 address (16 bytes)         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The format of the external destination nodes (EDN) object boby for
   IPv6 with the second type of encodes is illustrated below:




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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Encode of External Destination Nodes (2)            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                        Egress IPv6 address                    |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |          External Destination IPv6 address (16 bytes)         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                        Egress IPv6 address                    |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |          External Destination IPv6 address (16 bytes)         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           ...                                 ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |                        Egress IPv6 address                    |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       |          External Destination IPv6 address (16 bytes)         |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The object can only be carried in a PCReq message.  A Path Request
   may carry at most one external destination nodes Object.

   The Object-Class and Object-types will need to be allocated by IANA.
   The IANA request is documented in Section below (PCEP Objects).

4.2.2.2.  New Type of END-POINTS Objects

   Alternatively, we may use END-POINTS to represent external
   destination nodes in a request message for computing backup egress
   nodes of MPLS LSP.

   The format of the external destination nodes (EDN) END-POINTS object
   boby for IPv4 with the first type of encodes is illustrated as
   follows:



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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Compact External Destination Nodes Type (12)        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              External  Destination  IPv4 address              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              External  Destination  IPv4 address              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           ...                                 ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              External  Destination  IPv4 address              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The new type of END-POINTS is Compact External Destination Nodes Type
   (12).  The final value for the type will be assigned by IANA.  The
   EDN END-POINTS object of type 12 contains an external destination
   node for every egress node of an MPLS TE P2MP LSP or an MPLS TE P2P
   LSP.  The order of the external destination nodes in the object is
   the same as the egress node(s) of the P2MP LSP or P2P LSP contained
   in the PCE messages.

   The format of the external destination nodes END-POINTS object boby
   for IPv4 with the second type of encodes is illustrated below:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |              External Destination Nodes Type (13)             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Egress IPv4 address                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               External Destination IPv4 address               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Egress IPv4 address                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               External Destination IPv4 address               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       ~                           ...                                 ~
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Egress IPv4 address                    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |               External Destination IPv4 address               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The new type of END-POINTS is External Destination Nodes Type (13).



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   The final value for the type will be assigned by IANA.  The EDN END-
   POINTS object of type 13 contains a list of egress node and external
   destination node pairs.  For an egress node and external destination
   node pair, the data traffic is delivered to the external destination
   node from the egress node of the LSP.

4.2.3.  Constraints between Egress and Backup Egress

   A request message sent to a PCE from a PCC for computing a backup
   egress of an MPLS TE P2MP LSP or an MPLS TE P2P LSP may comprise a
   constraint indicating that there must be a path from the backup
   egress node to be computed to the egress node of the P2MP LSP or P2P
   LSP and that the length of the path is within a given hop limit such
   as one hop.

   This constraint can be considered as default by a PCE or explicitly
   sent to the PCE by a PCC [TBD].

4.2.4.  Constraints for Backup Path

   A request message sent to a PCE from a PCC for computing a backup
   egress of a P2MP LSP or P2P LSP may comprise a constraint indicating
   that the backup egress node to be computed may not be a node on the
   P2MP LSP or P2P LSP.  In addition, the request message may comprise a
   list of nodes, each of which is a candidate for the backup egress
   node.

   A request message sent to a PCE from a PCC for computing a backup
   egress of a P2MP LSP or P2P LSP may comprise a constraint indicating
   that there must be a path from the previous hop node of the egress
   node of the P2MP LSP or P2P LSP to the backup egress node to be
   computed and that there is not an internal node of the path from the
   previous hop node of the egress node of the P2MP LSP or P2P LSP to
   the backup egress that is on the path of the P2MP LSP or P2P LSP.

   Most of these constraints for the backup path can be considered as
   default by a PCE.  The constraints for the backup path may be
   explicitly sent to the PCE by a PCC [TBD].

4.2.5.  Backup Egress Node

   The PCE may send a reply message to the PCC in return to the request
   message for computing a new backup egress node or a number of backup
   egress nodes.  The reply message may comprise information about the
   computed backup egress node(s), which is contained in the path(s)
   from the previous-hop node of the egress node of the P2MP LSP or P2P
   LSP to the backup egress node(s) computed.




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4.2.6.  Backup Egress PCEP Error Objects and Types

   In some cases, the PCE may not complete the backup egress computation
   as requested, for example based on a set of constraints.  As such,
   the PCE may send a reply message to the PCC that indicates an
   unsuccessful backup egress computation attempt.  The reply message
   may comprise a PCEP-error object, which may comprise an error-value,
   error-type and some detail information.

4.2.7.  Request Message Format

   The PCReq message is encoded as follows using RBNF as defined in
   [RFC5511].

   Below is the message format for a request message:

               <PCReq Message>::= <Common Header>
                                  [<svec-list>]
                                  <request>
               <request>::= <RP> <end-point-rro-pair-list>
                            [<OF>] [<LSPA>] [<BANDWIDTH>]
                            [<metric-list>]
                            [<EDNO>]
                            [<IRO>]
                            [<LOAD-BALANCING>]
           where:
                  <EDNO> is an external destination nodes object.


   The definitions for svec-list, RP, end-point-rro-pair-list, OF, LSPA,
   BANDWIDTH, metric-list, IRO, and LOAD-BALANCING are described in
   RFC5440 and RFC6006.

4.2.8.  Reply Message Format

   The PCRep message is encoded as follows using RBNF as defined in
   [RFC5511].

   Below is the message format for a reply message:












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               <PCRep Message>::= <Common Header>
                                  <response>
               <response>::= <RP>
                             <end-point-path-pair-list>
                             [<NO-PATH>]
                             [<attribute-list>]
        where:
              <end-point-path-pair-list>::=
                      [<END-POINTS>]<path>[<end-point-path-pair-list>]

              <path> ::= (<ERO>|<SERO>) [<path>]

              <attribute-list>::= [<OF>]
                                  [<LSPA>]
                                  [<BANDWIDTH>]
                                  [<metric-list>]
                                  [<IRO>]


   The definitions for RP, NO-PATH, END-POINTS, OF, LSPA, BANDWIDTH,
   metric-list, IRO, and SERO are described in RFC5440, RFC6006 and
   RFC4875.


5.  Security  Considerations

   The mechanism described in this document does not raise any new
   security issues for the PCEP, OSPF or IS-IS protocols.


6.  IANA Considerations

   This section specifies requests for IANA allocation.

6.1.  Backup Egress Capability Flag

   Two new OSPF Capability Flags are defined in this document to
   indicate the capabilities for computing a backup egress for an MPLS
   TE P2MP LSP and an MPLS TE P2P LSP.  IANA is requested to make the
   assignment from the "OSPF Parameters Path Computation Element (PCE)
   Capability Flags" registry:

         Bit       Description                         Reference
          13       Backup egress for P2MP LSP          This I-D
          14       Backup egress for P2P LSP           This I-D






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6.2.  Backup Egress Capability TLV

   A new backup egress capability TLV is defined in this document to
   allow a PCE to advertize its backup egress computation capability.
   IANA is requested to make the following allocation from the "PCEP TLV
   Type Indicators" sub-registry.

         Value       Description                      Reference
          8          Backup egress capable            This I-D


6.3.  Request Parameter Bit Flags

   A new RP Object Flag has been defined in this document.  IANA is
   requested to make the following allocation from the "PCEP RP Object
   Flag Field" Sub-Registry:

         Bit       Description                         Reference
          15       Backup egress (T-bit)               This I-D


6.4.  PCEP Objects

   An External Destination Nodes Object-Type is defined in this
   document.  IANA is requested to make the following Object-Type
   allocation from the "PCEP Objects" sub-registry:

           Object-Class Value     34
           Name                   External Destination Nodes
           Object-Type            1: IPv4
                                  2: IPv6
                                  3-15: Unassigned
           Reference              This I-D



7.  Acknowledgement

   The author would like to thank Ramon Casellas, Dhruv Dhody and
   Quintin Zhao for their valuable comments on this draft.


8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/



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              RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [RFC4090]  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
              Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              DOI 10.17487/RFC4090, May 2005,
              <https://www.rfc-editor.org/info/rfc4090>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC4875]  Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
              Yasukawa, Ed., "Extensions to Resource Reservation
              Protocol - Traffic Engineering (RSVP-TE) for Point-to-
              Multipoint TE Label Switched Paths (LSPs)", RFC 4875,
              DOI 10.17487/RFC4875, May 2007,
              <https://www.rfc-editor.org/info/rfc4875>.

   [RFC6006]  Zhao, Q., Ed., King, D., Ed., Verhaeghe, F., Takeda, T.,
              Ali, Z., and J. Meuric, "Extensions to the Path
              Computation Element Communication Protocol (PCEP) for
              Point-to-Multipoint Traffic Engineering Label Switched
              Paths", RFC 6006, DOI 10.17487/RFC6006, September 2010,
              <https://www.rfc-editor.org/info/rfc6006>.

8.2.  Informative References

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655, DOI 10.17487/
              RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC5862]  Yasukawa, S. and A. Farrel, "Path Computation Clients
              (PCC) - Path Computation Element (PCE) Requirements for
              Point-to-Multipoint MPLS-TE", RFC 5862, DOI 10.17487/
              RFC5862, June 2010,
              <https://www.rfc-editor.org/info/rfc5862>.







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Author's Address

   Huaimo Chen
   Huawei Technologies
   Boston, MA
   USA

   Email: Huaimo.chen@huawei.com











































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