SR-TE Path Midpoint Protection
draft-hu-spring-segment-routing-proxy-forwarding-04

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Network Working Group                                              Z. Hu
Internet-Draft                                       Huawei Technologies
Intended status: Standards Track                                 H. Chen
Expires: January 9, 2020                                       Futurewei
                                                                  J. Yao
                                                     Huawei Technologies
                                                               C. Bowers
                                                        Juniper Networks
                                                                  Y. Zhu
                                                           China Telecom
                                                            July 8, 2019

                     SR-TE Path Midpoint Protection
          draft-hu-spring-segment-routing-proxy-forwarding-04

Abstract

   Segment Routing Traffic Engineering (SR-TE) supports the creation of
   explicit paths using segment lists containing adjacency-sids, node-
   sids, anycast-sids, and binding-sids.  When the segment list defining
   an SR-TE path contains a node-sid, and the node fails, the network
   may no longer be able to properly forward traffic on that SR-TE path.
   [I-D.bashandy-rtgwg-segment-routing-ti-lfa] and
   [I-D.hegde-spring-node-protection-for-sr-te-paths] describe a
   mechanism that allows local repair actions on the direct neighbors of
   the failed node to temporarily route traffic to the node immediately
   following the failed node on the SR-TE path segment list.  However,
   once the IGP shortest paths have converged, the local repair
   mechanism is no longer sufficient to continue forwarding traffic
   using the original segment list of the SR-TE path, since the non-
   neighbors of the failed node will no longer have a route to reach the
   failed node.  This document describes a mechanism that allows traffic
   to continue to be forwarded on an SR-TE path for an extended period
   of time after the failure of a node used in the path's segment list.

Requirements Language

   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 [RFC2119].

Status of This Memo

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Extensions to IGP for Proxy Forwarding  . . . . . . . . . . .   4
     2.1.  Extensions to OSPF  . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Advertising Proxy Forwarding  . . . . . . . . . . . .   4
       2.1.2.  Advertising Binding Segment . . . . . . . . . . . . .   7
     2.2.  Extensions to IS-IS . . . . . . . . . . . . . . . . . . .   9
       2.2.1.  Advertising Proxy Forwarding  . . . . . . . . . . . .   9
       2.2.2.  Advertising Binding Segment . . . . . . . . . . . . .  11
   3.  Building Proxy Forwarding Table . . . . . . . . . . . . . . .  13
     3.1.  Advertising Proxy Forwarding  . . . . . . . . . . . . . .  15
     3.2.  Building Proxy Forwarding Table . . . . . . . . . . . . .  15
   4.  Node Protection for Segment List  . . . . . . . . . . . . . .  15
     4.1.  Next Segment is an Adjacency Segment  . . . . . . . . . .  16
     4.2.  Next Segment is a Node Segment  . . . . . . . . . . . . .  16
     4.3.  Next Segment is a Binding Segment . . . . . . . . . . . .  17
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18

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     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   Segment Routing Traffic Engineering (SR-TE) is a technology that
   implements traffic engineering using Segment Routing.  SR-TE supports
   the creation of explicit paths using adjacency-sids, node-sids,
   anycast-sids, and binding-sids.  A node-sid in the segment list
   defining an SR-TE path indicates a loose hop that the SR-TE path
   should pass through.  When a particular node fails, it would be
   useful to be able to continue to send traffic on an SR-TE path that
   uses the node-sid of the failed node for an extended period of time,
   without having to immediately modify the segment list used at the
   ingress to the SR-TE path.

   The first step to achieve this objective is to make the rest of the
   routers in the network continue to forward traffic using the node-sid
   of the failed node.  If we don't do anything special, once the IGP
   converges to take into account the failed node, a given router will
   no longer maintain a route corresponding to the node-sid.  Any
   traffic that arrives at the router with the node-sid of the failed
   node as the active segment will be dropped.  This document addresses
   this problem by having each neighbor of the failed node advertise its
   SR proxy forwarding capability.  This indicates that the neighbor
   (the Proxy Forwarder) will forward traffic on behalf of the failed
   node.  A router receiving the SR Proxy Forwarding capability from
   neighbors of a failed node will send traffic using the node-sid of
   the failed node to the nearest Proxy Forwarder.

   Once the affected traffic reaches a Proxy Forwarder, the Proxy
   Forwarder sends the traffic on the post-failure shortest path to the
   node immediately following the failed node in the segment list.
   [I-D.bashandy-rtgwg-segment-routing-ti-lfa] and
   [I-D.hegde-spring-node-protection-for-sr-te-paths] describe how the
   immediate neighbors of a failed node can accomplish this by
   forwarding based on the first two segments in the segment list.  The
   forwarding described in these drafts was originally intended to be
   used for only a short period of time, to provide fast-reroute
   protection until the IGP converges.  The current document proposes to
   extend this behavior on the Proxy Forwarder until well after the IGP
   has converged.

   If the faulty node is a label adhesion node, the Binding-sids cannot
   be exchanged to the label stack for its identity, and the traffic
   will be lost before it reaches the faulty node.

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   In this document, the proxy mechanism is provided in the neighbor
   node of the faulty node of the forwarding path to implement traffic
   forwarding after the node with the label adhesion fails on the SR-TE
   loose path.

2.  Extensions to IGP for Proxy Forwarding

   When a node has segment routing proxy forwarding capability, it
   advertises this capability.  The capability indicates that the node
   has the ability to proxy forward the global sid of each of its
   neighbors.  When an neighbor who advertises its global sid fails, the
   traffic can be forwarded to the proxy node.

2.1.  Extensions to OSPF

2.1.1.  Advertising Proxy Forwarding

   When a node P has the capability to do a SR proxy forwarding for all
   its neighboring nodes for protecting the failures of these nodes,
   node P advertises its SR proxy forwarding capability in its router
   information opaque LSA, which contains a Router Functional
   Capabilities TLV of the format as shown in Figure 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |              Type             |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Functional Capabilities                   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 1: Router Functional Capabilities TLV

   One bit (called PF bit) in the Functional Capabilities field of the
   TLV is used to indicate node P's SR proxy forwarding capability.
   When this bit is set to one by node P, it indicates that node P is
   capable of doing a SR proxy forwarding for its neighboring nodes.

   For a node X in the network, it learns the prefix/node SID of node N,
   which is originated and advertised by node N.  It creates a proxy
   prefix/node SID of node N for node P if node P is capable of doing SR
   proxy forwarding for node N.  The proxy prefix/node SID of node N for
   node P is a copy of the prefix/node SID of node N originated by node
   N, but stored under (or say, associated with) node P.

   In normal operations, node X prefers to use the prefix/node SID of
   node N.  When node N fails, node X prefers to use the proxy prefix/
   node SID of node N.  Thus node X will forward the traffic targeting

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   to node N to node P when node N fails, and node P will do a SR proxy
   forwarding for node N and forwarding the traffic to its destination
   without going through node N.  After node N fails, node X will keep
   the proxy prefix/node SID of node N for a given period of time.

   If node P can not do a SR proxy forwarding for all its neighboring
   nodes, but for some of them, then it advertises the node SID of each
   of the nodes as a proxy node SID, indicating that it is able to do
   proxy forwarding for the node SID.

   A new TLV, called Proxy Node SIDs TLV, is defined for node P to
   advertise the node SIDs of some of its neighboring nodes.  It has the
   format as shown in Figure 2.

     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 (TBD1)           |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Node SID Sub-TLVs                      |
    :                                                               :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 2: OSPF Proxy Node SIDs TLV

   The Type (TBD1) is to be assigned by IANA.  The TLV contains a number
   of Node SID Sub-TLVs.  The Length is the total size of the Node SID
   Sub-TLVs included in the TLV.  A Node SID Sub-TLV is the Prefix SID
   Sub-TLV defined in [I-D.ietf-ospf-segment-routing-extensions].

   A proxy forwarding node P originates an Extended Prefix Opaque LSA
   containing this new TLV.  The TLV includes the Node SID Sub-TLVs for
   the node SIDs of some of P's neighboring nodes.  For each of some of
   P's neighboring nodes, the Node SID Sub-TLV for its prefix/node SID
   is included the TLV.  This prefix/node SID is called a proxy prefix/
   node SID.

   A proxy forwarding node will originate an Extended Prefix Opaque LSA,
   which includes a Proxy Node SIDs TLV.  The format of the LSA is shown
   in Figure 3.

   For a proxy forwarding node P, having a number of neighboring nodes,
   P originates and maintains an Extended Prefix Opaque LSA, which
   includes a Proxy Node SIDs TLV.  The TLV contains the Prefix/Node SID
   Sub-TLV for each of some of the neighboring nodes after node P
   creates the corresponding proxy forwarding entries for protecting the
   failure of some of the neighboring nodes.

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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            LS age             |     Options   |   LS Type     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Opaque Type(7)|                 Opaque ID                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Advertising Router                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     LS sequence number                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         LS checksum           |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    :                             TLVs                              :
    :                (including Proxy Node SIDs TLV)                :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: OSPFv2 Extended Prefix Opaque LSA

   When an neighboring node fails, P maintains the LSA with the TLV
   containing the Prefix/Node SID Sub-TLV for the neighboring node for a
   given period of time.  After the given period of time, the Prefix/
   Node SID Sub-TLV for the neighboring node is removed from the TLV in
   the LSA and then after a given time the corresponding proxy
   forwarding entries for protecting the failure of the neighboring node
   is removed.

   For a node X in the network, it learns the prefix/node SID of node N
   and the proxy prefix/node SID of node N.  The former is originated
   and advertised by node N, and the latter is originated and advertised
   by the proxy forwarding node P of node N.  Note that the proxy
   Prefix/Node SID Sub-TLV for node N does not contain a prefix of node
   N, and the prefix is the prefix associated with the prefix/node SID
   of node N originated by node N.

   In normal operations, node X prefers to use the prefix/node SID of
   node N.  When node N fails, node X prefers to use the proxy prefix/
   node SID of node N.  Thus node X will forward the traffic targeting
   to node N to node P when node N fails, and node P will do a proxy
   forwarding for node N and forwarding the traffic to its destination
   without going through node N.

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2.1.2.  Advertising Binding Segment

   For a binding segment (or binding for short) on a node A, which
   consists of a binding SID and a list of segments, node A advertises
   an LSA containing the binding (i.e., the binding SID and the list of
   the segments).  The LSA is advertised only to each of the node A's
   neighboring nodes.  For OSPFv2, the LSA is a opaque LSA of LS type 9
   (i.e., a link local scope LSA).

   A binding segment is represented by binding segment TLV of the format
   as shown in Figure 4.

     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 (TBD2)          |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Reserved            |BindingSID Type|   SIDs Type   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                   Binding SID Sub-TLV/value                   ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    ~                       SID Sub-TLVs/values                     ~
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: OSPF Binding Segment TLV

   It comprises a binding SID and a list of segments (SIDs).  The fields
   of this TLV are defined as follows:

   Type: 2 octets, its value (TBD2) is to be assigned by IANA.

   Length: 2 octets, its value is (4 + length of Sub-TLVs/values).

   Binding SID Type (BT): 1 octet indicates whether the binding SID is
   represented by a Sub-TLV or a value included in the TLV.  For the
   binding SID represented by a value, it indicates the type of binding
   SID.  The following BT values are defined:

   o BT = 0: The binding SID is represented by a Sub-TLV (i.e., Binding
   SID Sub-TLV) in the TLV.  A binding SID Sub-TLV is a SID/Label Sub-
   TLV defined in [I-D.ietf-ospf-segment-routing-extensions].  BT != 0
   indicates that the binding SID is represented by a value.

   o BT = 1: The binding SID value is a label, which is represented by
   the 20 rightmost bits.  The length of the value is 3 octets.

   o BT = 2: The binding SID value is a 32-bit SID.  The length of the
   value is 4 octets.

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   SIDs Type (ST): 1 octet indicates whether the list of segments (SIDs)
   are represented by Sub-TLVs or values included in the TLV.  For the
   SIDs represented by values, it indicates the type of SIDs.  The
   following ST values are defined:

   o ST = 0: The SIDs are represented by Sub-TLVs (i.e., SID Sub-TLVs)
   in the TLV.  A SID Sub-TLV is an Adj-SID Sub-TLV, a Prefix-SID Sub-
   TLV or a SID/Label Sub-TLV defined in
   [I-D.ietf-ospf-segment-routing-extensions].  ST != 0 indicates that
   the SIDs are represented by values.

   o ST = 1: Each of the SID values is a label, which is represented by
   the 20 rightmost bits.  The length of the value is 3 octets.

   o ST = 2: Each of the SID values is a 32-bit SID.  The length of the
   value is 4 octets.

   The opaque LSA of LS Type 9 containing the binding segment (i.e., the
   binding SID and the list of the segments) has the format as shown in
   Figure 5.  It may have Opaque Type of x (the exact type is to be
   assigned by IANA) for Binding Segment Opaque LSA.

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            LS age             |     Options   |  LS Type (9)  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Opaque Type(x)|                 Opaque ID                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Advertising Router                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     LS sequence number                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         LS checksum           |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    :                      Binding Segment TLVs                     :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 5: OSPFv2 Binding Segment Opaque LSA

   For every binding on a node A, the LSA originated by A contains a
   binding segment TLV for it.

   For node A running OSPFv3, it originates a link-local scoping LSA of
   a new LSA function code (TBD3) containing binding segment TLVs for

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   the bindings on it.  The format of the LSA is illustrated in
   Figure 6.

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            LS age             |0|0|0|       BS-LSA (TBD3)     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Link State ID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Advertising Router                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      LS Sequence Number                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         LS checksum           |           Length              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    :                      Binding Segment TLVs                     :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 6: OSPFv3 Binding Segment Opaque LSA

   The U-bit is set to 0, and the scope is set to 00 for link-local
   scoping.

2.2.  Extensions to IS-IS

2.2.1.  Advertising Proxy Forwarding

   When a node P has the capability to do a SR proxy forwarding for its
   neighboring nodes for protecting the failures of them, node P
   advertises its SR proxy forwarding capability in its LSP, which
   contains a Router Capability TLV of Type 242 including a SR
   capabilities sub-TLV of sub-Type 2.

   One bit (called PF bit as shown in Figure 7) in the Flags field of
   the SR capabilities sub-TLV is defined to indicate node P's SR proxy
   forwarding capability.  When this bit is set to one by node P, it
   indicates that node P is capable of doing a SR proxy forwarding for
   its neighboring nodes.

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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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Type (2)    |     Length    |    Flags      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Range                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                SID/Label Sub-TLV (variable)                 //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                0  1  2  3  4  5  6  7
                               +--+--+--+--+--+--+--+--+
                               | I| V|PF|              |
                               +--+--+--+--+--+--+--+--+
                                         Flags

                     Figure 7: SR Capabilities sub-TLV

   If node P can not do a SR proxy forwarding for all its neighboring
   nodes, but for some of them, then it advertises the node SID of each
   of the nodes as a proxy node SID, indicating that it is able to do
   proxy forwarding for the node SID.

   The IS-IS SID/Label Binding TLV (suggested value 149) is defined in
   [I-D.ietf-isis-segment-routing-extensions].  A Proxy Forwarder uses
   the SID/Label Binding TLV to advertise the node Sid of its
   neighboring node.  The Flags field of the SID/Label Binding TLV is
   extended to include a P flag as shown in Figure 8.  The prefix/node
   SID in prefix/node Sid Sub-TLV included in SID/Label Binding TLV is
   identified as a proxy forwarding prefix/node SID.

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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Type     |     Length    |     Flags     |     RESERVED  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Range              | Prefix Length |     Prefix    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      //               Prefix (continued, variable)                  //
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    SubTLVs (variable)                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                       0 1 2 3 4 5 6 7
                                      +-+-+-+-+-+-+-+-+
                                      |F|M|S|D|A|P|   |
                                      +-+-+-+-+-+-+-+-+
                                            Flags

                      Figure 8: SID/Label Binding TLV

   Where:

   P-Flag: Proxy forwarding flag.  If set, this prefix/node Sid is
   advertised by the proxy node.  This TLV is used to announce that the
   node has the ability to proxy forward the prefix/node Sid.

   When the P-flag is set in the SID/Label Binding TLV, the following
   usage rules apply.

   The Range, Prefix Length and Prefix field are not used.  They should
   be set to zero on transmission and ignored on receipt.

   SID/Label Binding TLV contains a number of prefix/node SID Sub-TLVs.
   The TLV advertised by a proxy forwarding node P contains prefix/node
   SID Sub-TLVs for the node SIDs of P's neighbor nodes.  Each of the
   Sub-TLVs is a prefix/node SID Sub-TLV defined in
   [I-D.ietf-isis-segment-routing-extensions].  From the SID in a
   prefix/node SID Sub-TLV advertised by the Proxy Forwarding node, its
   prefix can be obtained through matching corresponding prefix/node SID
   advertised by the neighbor/protected node using TLV-135 (or 235, 236,
   or 237) together with the prefix/node SID Sub-TLV.

2.2.2.  Advertising Binding Segment

   [I-D.ietf-spring-segment-routing-policy] has defined the usage of
   binding-sid.  For supporting binding sid proxy forwarding, a new IS-
   IS TLV, called Binding Segment TLV, is defined.  It contains a
   binding SID and a list of segments (SIDs).  This TLV may be

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   advertised in IS-IS Hello (IIH) PDUs, LSPs, or in Circuit Scoped Link
   State PDUs (CS-LSP) [RFC7356].  Its format is shown in Figure 9.

      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     |     Length    |BindingSID Type|   SIDs Type   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                   Binding SID value/Sub-TLV                   ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                      SID values/Sub-TLVs                      ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 9: IS-IS Binding Segment TLV

   The fields of this TLV are defined as follows:

   Type: 1 octet Suggested value 152 (to be assigned by IANA)

   Length: 1 octet (2 + length of Sub-TLVs/values).

   Binding SID Type (BT): 1 octet indicates whether the binding SID is
   represented by a Sub-TLV or a value included in the TLV.  For the
   binding SID represented by a value, it indicates the type of binding
   SID.  The following BT values are defined:

   o BT = 0: The binding SID is represented by a Sub-TLV (i.e., binding
   SID Sub-TLV) in the TLV.  A binding SID Sub-TLV is a SID/Label Sub-
   TLV defined in [I-D.ietf-isis-segment-routing-extensions].  BT != 0
   indicates that the binding SID is represented by a value.

   o BT = 1: The binding SID value is a label, which is represented by
   the 20 rightmost bits.  The length of the value is 3 octets.

   o BT = 2: The binding SID value is a 32-bit SID.  The length of the
   value is 4 octets.

   SIDs Type (ST): 1 octet indicates whether the SIDs are represented by
   Sub-TLVs or values included in the TLV.  For the SIDs represented by
   values, it indicates the type of SIDs.  The following ST values are
   defined:

   o ST = 0: The SIDs are represented by Sub-TLVs (i.e., SID Sub-TLVs)
   in the TLV.  A SID Sub-TLV is an Adj-SID Sub-TLV, a Prefix-SID Sub-
   TLV or a SID/Label Sub-TLV defined in
   [I-D.ietf-isis-segment-routing-extensions].  ST != 0 indicates that
   the SIDs are represented by values.

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   o ST = 1: Each of the SID values is a label, which is represented by
   the 20 rightmost bits.  The length of the value is 3 octets.

   o ST = 2: Each of the SID values is a 32-bit SID.  The length of the
   value is 4 octets.

3.  Building Proxy Forwarding Table

   Figure 10 is used to illustrate the SR proxy forwarding approach.
   Each node N has SRGB = [N000-N999].  RT1 is an ingress node of SR
   domain.  RT3 is a failure node.  RT2 is a Point of Local Repair (PLR)
   node, i.e., a proxy forwarding node.  Three label stacks are shown in
   the figure.  Label Stack 1 uses only adjacency-SIDs and represents
   the path RT1->RT2->RT3->RT4->RT5.  Label Stack 2 uses only node-SIDs
   and represents the ECMP-aware path RT1->RT3->RT4->RT5.  Label Stack 3
   uses a node-SID and a binding SID.  The Binding-SID with label=100 at
   RT3 represents the ECMP-aware path RT3->RT4->RT5.  So Label Stack 3,
   which consists of the node-SID for RT3 following by Binding-SID=100,
   represents the ECMP-aware path RT1->RT3->RT4->RT5.

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             Node Sid:2      Node Sid:3
             +-----+          +-----+
             |     |----------+     |
           / |RT2  |          | RT3 |\
          /  +-----+          +-----+ \
         /      | \             /|     \
        /       |  \           / |      \
       /        |   \         /  |       \
      /         |    \       /   |        \
     /          |     \     /    |         \
 Node Sid:1     |      \   /     |          \Node Sid:4    Node Sid:5
+-----+         |       \ /      |           +-----+       +-----+
|     |         |        X       |           |     |-------|     |
| RT1 |         |       / \      |           | RT4 |       | RT5 |
+-----+         |      /   \     |           +-----+       +-----+
   \            |     /     \    |           /
    \           |    /       \   |          /
     \          |   /         \  |         /
      \         |  /           \ |        /
       \        | /             \|       /
        \       |/               |      /
         \   +-----+           +-----+ /
          \  |     |           |     |/
           \ | RT6 |-----------| RT7 |
             +-----+           +-----+
             Node Sid:6        Node Sid:7

+-----------------+  +--------------+
|    Node SRGB    |  | Adj-Sid      |  +-------+  +-------+  +-------+
+-----------------+  +--------------+  |Label  |  |Label  |  |Label  |
| RT1:[1000-1999] |  |RT1->RT2:10012|  |Stack 1|  |Stack 2|  |Stack 3|
+-----------------+  +--------------+  +-------+  +-------+  +-------+
| RT2:[2000-2999] |  |RT2->RT3:20023|  | 10012 |  | 1003  |  | 1003  |
+-----------------+  +--------------+  +-------+  +-------+  +-------+
| RT3:[3000-3999] |  |RT3->RT6:30036|  | 20023 |  | 3004  |  | 100   |
+-----------------+  +--------------+  +-------+  +-------+  +-------+
| RT4:[4000=4999] |  |RT3->RT7:30037|  | 30034 |  | 4005  |   100 is
+-----------------+  +--------------+  +-------+  +-------+  binding SID
| RT5:[5000-5999] |  |RT3->RT4:30034|  | 40045 |             to
+-----------------+  +--------------+  +-------+            {30034,40045}
| RT6:[6000-6999] |  |RT7->RT4:70074|
+-----------------+  +--------------+
| RT7:[7000-7999] |  |RT4->RT5:40045|
+-----------------+  +--------------+

                     Figure 10: Topology of SR-TE Path

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3.1.  Advertising Proxy Forwarding

   If the Point of Local Repair (PLR), for example, RT2, has the
   capability to do a SR proxy forwarding for all its neighboring nodes,
   it must advertises this capability.  If the PLR can not do a SR proxy
   forwarding for all its neighboring nodes, but for some of them, for
   example, RT3, then it uses proxy Node SIDs TLV to advertise the
   prefix-sid learned from RT3.  The TLV contains the Sub-TLV/value for
   the prefix/node sid of RT3 as a proxy SID.  When RT3 fails, RT2 needs
   to maintain the Sub-TLV/value for a period of time.  When the proxy
   forwarding table corresponding to the fault node is deleted (see
   section 3.2), the Sub-TLV/value is withdrawn.  The nodes in the
   network (for example, RT1) learn the prefix/node Sid TLV advertised
   by RT3 and the proxy Node SIDs TLV advertised by RT2.  When RT3 is
   normal, the nodes prefer prefix/node Sid TLV.  When the RT3 fails,
   the proxy prefix/node SIDs TLV advertised by RT2 is preferred.

3.2.  Building Proxy Forwarding Table

   A SR proxy node P needs to build an independent proxy forwarding
   table for each neighbor N.  The proxy forwarding table for node N
   contains the following information:

   1: Node N's SRGB range and the difference between the SRGB start
   value of node P and that of node N;

   2: All adjacency-SID of N and Node-SID of the node pointed to by node
   N's adjacency-SID.

   3: The binding-SID of N and the label stack associated with the
   binding-SID.

   Node P (PLR) uses a proxy forwarding table based on the next segment
   to find a node N as a backup forwarding entry to the adj-SID and
   Node-SID of node N.  When node N fails, the proxy forwarding table
   needs to be maintained for a period of time, which is recommended for
   30 minutes.

   Node RT3 in the topology of Figure 1 is node N, and node RT2 is node
   P (PLR).  RT2 builds the proxy forwarding table for RT3.  The
   structure of the table and how to build the table is a local
   implementation issue.

4.  Node Protection for Segment List

   Segment Routing Traffic Engineering supports the creation of explicit
   paths using adjacency-sids, node-sids, and binding-sids.  The label
   stack is a combination of one or more of adjacency-sids, node-sids,

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   and binding-sids.  This Section shows how a proxy node uses the SR
   proxy forwarding mechanism to protect traffic to the destination node
   when the next segment of label stack is adjacency-sids, node-sids, or
   binding-sids, respectively.

4.1.  Next Segment is an Adjacency Segment

   As shown in Figure 1, Label Stack 1 {10012, 20023, 30034, 40045}
   represents SR-TE strict explicit path RT1->RT2->RT3->RT4->RT5.  When
   RT3 fails, node RT2 acts as a PLR, and uses next adj-SID (30034) of
   the label stack to lookup the proxy forwarding table built by RT2
   locally for RT3.  The path returned is the label forwarding path to
   RT3's next hop node RT4, which bypasses RT3.  The specific steps are
   as follows:

   a.  RT1 pops top adj-SID 10012, and forwards the packet to RT2;

   b.  RT2 uses the label 20023 to identify the next hop node RT3, which
   has failed.  RT2 pops label 20023 and queries the Proxy Forwarding
   Table corresponding to RT3 with label 30034.  The Proxy Forwarding
   Table corresponding to RT3 returns an outgoing interface and label
   stack representing a path to RT4 that does not pass through RT3.  In
   this case, outgoing interface to RT7 with label stack 7004, satisfies
   this requirement.

   c.  So the packet leaves RT2 out the interface to RT7 with label
   stack {7004, 40045}. RT4 forwards it to RT4, where the original path
   is rejoined.

   d.  RT2 forwards packets to RT7.  RT7 queries the local routing table
   to forward the packet to RT4.

4.2.  Next Segment is a Node Segment

   As shown in Figure 1, Label Stack 2 {1003, 3004, 4005} represents SR-
   TE loose path RT1->RT3->RT4->RT5, where 1003 is the node SID of RT3.

   When the node RT3 fails, the proxy forwarding TLV advertised by the
   RT2 is preferred to direct the traffic of the RT1 to the PLR node
   RT2.  Node RT2 acts as a PLR node and queries the proxy forwarding
   table locally built for RT3.  The path returned is the label
   forwarding path to RT3's next hop node RT4, which bypasses RT3.  The
   specific steps are as follows:

   a.  RT1 swaps label 1003 to out-label 2003 to RT3.

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   b.  RT2 receives the label forwarding packet whose top label of label
   stack is 2003, and searches for the local Routing Table, the behavior
   found is to lookup Proxy Forwarding table due to RT3 failure.

   c.  RT2 uses 2003 as the in-label to lookup Proxy Forwarding table,
   and the query result is forwarding the packet to RT4.

   d.  Then RT2 querries the Routing Table to RT4, using the primary or
   backup path to RT4.  The next hop is RT7.

   e.  RT2 forwards the packet to RT7.  RT7 queries the local routing
   table to forward the packet to RT4.

   f.  After RT1 convergences, node SID 1003 is preferred to the proxy
   SID implied/advertised by RT2.

4.3.  Next Segment is a Binding Segment

   As shown in Figure 1, Label Stack 3 {1003, 100} represents SR-TE
   loose path RT1->RT3->RT4->RT5, where 100 is a Binding-Sid, which
   represents segment list {30034, 40045}.

   When the node RT3 fails, the proxy forwarding SID implied or
   advertised by the RT2 is preferred to forward the traffic of the RT1
   to the PLR node RT2.  Node RT2 acts as a PLR node and uses Binding-
   SID to query the proxy forwarding table locally built for RT3.  The
   path returned is the label forwarding path to RT3's next hop node
   (RT4), which bypasses RT3.  The specific steps are as follows:

   a.  RT1 swaps label 1003 to out-label 2003 to RT3.

   b.  RT2 receives the label forwarding packet whose top label of label
   stack is 2003, and searches for the local Routing Table, the behavior
   found is to lookup Proxy Forwarding table due to RT3 failure.

   c.  RT2 uses Binding-sid:100 (label 2003 has pop) as the in-label to
   lookup the Next Label record of the Proxy Forwarding Table, the
   behavior found is to swap to Segment list {30034, 40045}.

   d.  RT2 swaps Binding-sid:100 to Segment list {30034, 40045}, and
   uses the 3034 to lookup the Next Label record of the Proxy Forwarding
   table again.  The behavior found is to forward the packet to RT4.

   e.  RT2 queries the Routing Table to RT4, using primary or backup
   path to RT4.  The next hop is RT7.

   f.  RT2 forwards packets to RT7.  RT7 queries the local routing table
   to forward the packet to RT4.

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5.  Security Considerations

   TBD

6.  IANA Considerations

   TBD

7.  Acknowledgements

   The authors would like to thank Peter Psenak, Acee Lindem and Les
   Ginsberg for their comments to this work.

8.  References

8.1.  Normative References

   [I-D.ietf-isis-segment-routing-extensions]
              Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
              Gredler, H., and B. Decraene, "IS-IS Extensions for
              Segment Routing", draft-ietf-isis-segment-routing-
              extensions-25 (work in progress), May 2019.

   [I-D.ietf-ospf-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", draft-ietf-ospf-segment-
              routing-extensions-27 (work in progress), December 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7356]  Ginsberg, L., Previdi, S., and Y. Yang, "IS-IS Flooding
              Scope Link State PDUs (LSPs)", RFC 7356,
              DOI 10.17487/RFC7356, September 2014,
              <https://www.rfc-editor.org/info/rfc7356>.

   [RFC7770]  Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
              S. Shaffer, "Extensions to OSPF for Advertising Optional
              Router Capabilities", RFC 7770, DOI 10.17487/RFC7770,
              February 2016, <https://www.rfc-editor.org/info/rfc7770>.

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8.2.  Informative References

   [I-D.bashandy-rtgwg-segment-routing-ti-lfa]
              Bashandy, A., Filsfils, C., Decraene, B., Litkowski, S.,
              Francois, P., daniel.voyer@bell.ca, d., Clad, F., and P.
              Camarillo, "Topology Independent Fast Reroute using
              Segment Routing", draft-bashandy-rtgwg-segment-routing-ti-
              lfa-05 (work in progress), October 2018.

   [I-D.hegde-spring-node-protection-for-sr-te-paths]
              Hegde, S., Bowers, C., Litkowski, S., Xu, X., and F. Xu,
              "Node Protection for SR-TE Paths", draft-hegde-spring-
              node-protection-for-sr-te-paths-05 (work in progress),
              July 2019.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., daniel.voyer@bell.ca, d.,
              bogdanov@google.com, b., and P. Mattes, "Segment Routing
              Policy Architecture", draft-ietf-spring-segment-routing-
              policy-03 (work in progress), May 2019.

   [I-D.sivabalan-pce-binding-label-sid]
              Sivabalan, S., Filsfils, C., Tantsura, J., Hardwick, J.,
              Previdi, S., and C. Li, "Carrying Binding Label/Segment-ID
              in PCE-based Networks.", draft-sivabalan-pce-binding-
              label-sid-07 (work in progress), July 2019.

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
              2009, <https://www.rfc-editor.org/info/rfc5462>.

Authors' Addresses

   Zhibo Hu
   Huawei Technologies
   Huawei Bld., No.156 Beiqing Rd.
   Beijing  100095
   China

   Email: huzhibo@huawei.com

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   Huaimo Chen
   Futurewei
   Boston, MA
   USA

   Email: Huaimo.chen@futurewei.com

   Junda Yao
   Huawei Technologies
   Huawei Bld., No.156 Beiqing Rd.
   Beijing  100095
   China

   Email: yaojunda@huawei.com

   Chris Bowers
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   USA

   Email: cbowers@juniper.net

   Yongqing
   China Telecom
   109, West Zhongshan Road, Tianhe District
   Guangzhou  510000
   China

   Email: zhuyq@gsta.com

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