SR-TE Path Midpoint Restoration
draft-hu-spring-segment-routing-proxy-forwarding-16

Network Working Group                                              Z. Hu
Internet-Draft                                       Huawei Technologies
Intended status: Standards Track                                 H. Chen
Expires: 4 July 2022                                           Futurewei
                                                                  J. Yao
                                                     Huawei Technologies
                                                               C. Bowers
                                                        Juniper Networks
                                                                  Y. Zhu
                                                           China Telecom
                                                                  Y. Liu
                                                            China Mobile
                                                        31 December 2021

                    SR-TE Path Midpoint Restoration
          draft-hu-spring-segment-routing-proxy-forwarding-16

Abstract

   Segment Routing Traffic Engineering (SR-TE) supports explicit paths
   using segment lists containing adjacency-SIDs, node-SIDs and binding-
   SIDs.  The current SR FRR such as TI-LFA provides fast re-route
   protection for the failure of a node along a SR-TE path by the direct
   neighbor or say point of local repair (PLR) to the failure.  However,
   once the IGP converges, the SR FRR is no longer sufficient to forward
   traffic of the path around the failure, since the non-neighbors of
   the failure will no longer have a route to the failed node.  This
   document describes a mechanism for the restoration of the routes to
   the failure of a SR-MPLS TE path after the IGP converges.  It
   provides the restoration of the routes to an adjacency segment, a
   node segment and a binding segment of the path.  With the restoration
   of the routes to the failure, the traffic is continuously sent to the
   neighbor of the failure after the IGP converges.  The neighbor as a
   PLR fast re-routes the traffic around the failure.

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 [RFC2119] [RFC8174]
   when, and only when, they appear in all capitals, as shown here.

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.  Proxy Forwarding  . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Extensions to IGP for Proxy Forwarding  . . . . . . . . . . .   4
     3.1.  Extensions to OSPF  . . . . . . . . . . . . . . . . . . .   4
       3.1.1.  Advertising Proxy Forwarding  . . . . . . . . . . . .   4
       3.1.2.  Advertising Binding Segment . . . . . . . . . . . . .   6
     3.2.  Extensions to IS-IS . . . . . . . . . . . . . . . . . . .   6
       3.2.1.  Advertising Proxy Forwarding  . . . . . . . . . . . .   6
       3.2.2.  Advertising Binding Segment . . . . . . . . . . . . .   6
   4.  Proxy Forwarding Example  . . . . . . . . . . . . . . . . . .   6
     4.1.  Advertising Proxy Forwarding  . . . . . . . . . . . . . .   8
     4.2.  Building Proxy Forwarding Table . . . . . . . . . . . . .   8
     4.3.  Proxy Forwarding for Binding Segment  . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Proxy Forwarding for Adjacency and Node Segment  . .  11

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     A.1.  Next Segment is an Adjacency Segment  . . . . . . . . . .  11
     A.2.  Next Segment is a Node Segment  . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Segment Routing Traffic Engineering (SR-TE) is a technology that
   implements traffic engineering using a segment list.  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 the node fails, the network may no longer
   be able to properly forward traffic on that SR-TE path.

   [I-D.ietf-rtgwg-segment-routing-ti-lfa] describes an SR FRR mechanism
   that provides fast re-route protection for the failure of a node on a
   SR-TE path by the direct neighbor or say point of local repair (PLR)
   to the failure.  However, once the IGP converges, the SR FRR is no
   longer sufficient to forward traffic of the path around the failure,
   since the non-neighbors of the failure will no longer have a route to
   the failed node and drop the traffic.

   To solve this problem,
   [I-D.ietf-spring-segment-protection-sr-te-paths] proposes that a hold
   timer should be configured on every router in a network.  After the
   IGP converges on the event of a node failure, if the node-SID of the
   failed node becomes unreachable, the forwarding changes should not be
   communicated to the forwarding planes on all configured routers
   (including PLRs for the failed node) until the hold timer expires.
   This solution may not work for some cases such as some of nodes in
   the network not supporting this solution.

   This document describes a proxy forwarding mechanism for the
   restoration of the routes to the failure of a SR-MPLS TE path after
   the IGP converges.  It provides the restoration of the routes to an
   adjacency segment, a node segment and a binding segment on a failed
   node along the path.  With the restoration of the routes to the
   failure, the traffic for the SR-MPLS TE path is continuously sent to
   the neighbor of the failure after the IGP converges.  The neighbor as
   a PLR fast re-routes the traffic around the failure.

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2.  Proxy Forwarding

   In the proxy forwarding mechanism, each neighbor of a possible failed
   node advertises its SR proxy forwarding capability in its network
   domain when it has the capability.  This capability 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 the neighbors of a failed node will send traffic
   using the node-SID of the failed node to the nearest Proxy Forwarder
   after the IGP converges on the event of the failure.

   Once the affected traffic reaches a Proxy Forwarder, it sends the
   traffic on the post-failure shortest path to the node immediately
   following the failed node in the segment list.

   For a binding segment of a possible failed node, the node advertises
   the information about the binding segment, including the binding SID
   and the list of SIDs associated with the binding SID, to its direct
   neighbors only.  Note that the information is not advertised in the
   network domain.

   After the node fails and the IGP converges on the failure, the
   traffic with the binding SID of the failed node will reach its
   neighbor having SR Proxy Forwarding capability.  Once receiving the
   traffic, the neighbor swaps the binding SID with the list of SIDs
   associated with the binding SID and sends the traffic along the post-
   failure shortest path to the first node in the segment list.

3.  Extensions to IGP for Proxy Forwarding

   This section describes the semantic of extensions to IGP for
   advertising the SR proxy forwarding capability of a node in a network
   domain and the information about each binding segment (including its
   binding SID and the list of SIDs associated) of a node to its direct
   neighbors.

3.1.  Extensions to OSPF

3.1.1.  Advertising Proxy Forwarding

   When a node P is able to do SR proxy forwarding for all its
   neighboring nodes for protecting the failures of these nodes, P
   advertises its SR proxy forwarding capability in its router
   information opaque LSA.  The LSA contains a Router Functional
   Capabilities TLV with one bit (called PF bit) set to one indicating
   that P is capable of doing SR proxy forwarding for all its neighbors.

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   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.  The route to
   the proxy prefix/node SID is through proxy forwarding capable nodes.

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

   Note that the behaviors of normal IP forwarding and routing
   convergences in a network are not changed at all by the SR proxy
   forwarding.  For example, the next hop used by BGP is an IP address
   (or prefix).  The IGP and BGP converge in normal ways for changes in
   the network.  The packet with its IP destination to this next hop is
   forwarded according to the IP forwarding table (FIB) derived from IGP
   and BGP routes.

   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.  P
   originates an Extended Prefix Opaque LSA containing this new TLV.

   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.

   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.

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3.1.2.  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) in a binding segment TLV.  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).

3.2.  Extensions to IS-IS

3.2.1.  Advertising Proxy Forwarding

   When a node P has the capability to do SR proxy forwarding for all
   its neighbors for protecting the failures of them, P advertises its
   SR proxy forwarding capability in its LSP.  The LSP contains a Router
   Capability TLV including a SR capabilities sub-TLV.  One bit (called
   PF bit) in the Flags field of the sub-TLV is set to one indicating
   that P is capable of doing SR proxy forwarding for all its neighbors.

   If node P can not do SR proxy forwarding for all its neighbors, but
   for some of them, then it advertises the node SID of each of the
   (some) neighbors as a proxy node SID, indicating that P is able to do
   proxy forwarding for the node SID.  P uses the SID/Label Binding TLV
   defined in [RFC8667] to advertise the node SID of its neighbor.

3.2.2.  Advertising Binding Segment

   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 advertised in IS-IS Hello
   (IIH) PDUs, LSPs, or in Circuit Scoped Link State PDUs (CS-LSP)
   [RFC7356].

4.  Proxy Forwarding Example

   This section illustrates the proxy forwarding for a binding SID
   through an example.  The proxy forwarding for a node SID and an
   adjacency SID can refer to
   [I-D.ietf-spring-segment-protection-sr-te-paths] or Appendix.
   Figure 1 is an example network topology used to illustrate the proxy
   forwarding mechanism for a binding SID.  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.  Label Stack 1 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 1, which consists of the node-SID for
   RT3 following by Binding-SID=100, represents the ECMP-aware path

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   RT1->RT3->RT4->RT5.

             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 3|  |Stack 2|  |Stack 1|
+-----------------+  +--------------+  +-------+  +-------+  +-------+
| 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 1: Topology of SR-TE Path

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4.1.  Advertising Proxy Forwarding

   If the Point of Local Repair (PLR), for example, RT2, has the
   capability to do SR proxy forwarding for all its neighboring nodes,
   it must advertise this capability.  If the PLR can not do 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, 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.

   For binding-SID 100, which is associated with segment list {30034,
   40045}, RT3 advertises the binding (i.e., 100 bond to {30034, 40045})
   to its neighbors RT2, RT4 and RT7.  RT2 as PLR uses the binding to
   build an entry for proxy forwarding for binding-SID 100 in its Proxy
   Forwarding Table for RT3.  The entry is used when RT3 fails.

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

   3: Every 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 adjacency-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 Figure 1 is node N, and node RT2 is node P (PLR).  RT2
   builds the proxy forwarding table for RT3.  RT2 calculates the proxy
   forwarding table for RT3, as shown in Figure 2.

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  +==========+===============+============+=============+==============+
  | In-label | SRGBDiffValue | Next Label |   Action    |   Map Label  |
  +==========+===============+============+=============+==============+
  | 2003     |    -1000      |    30034   |  Fwd to RT4 |    2004      |
  +----------+---------------+------------+-------------+--------------+
                             |    30036   |  Fwd to RT6 |    2006      |
                             +------------+-------------+--------------+
                             |    30037   |  Fwd to RT7 |    2007      |
                             +------------+-------------+--------------+
                             |    100     |  Swap to { 30034, 40045 }  |
                             +------------+-------------+--------------+

              Figure 2: RT2's Proxy Forwarding Table for RT3

4.3.  Proxy Forwarding for Binding Segment

   This Section shows through example how a proxy node uses the SR proxy
   forwarding mechanism to forward traffic to the destination node when
   a node fails and the next segment of label stack is a binding-SID.

   As shown in Figure 1, Label Stack 1 {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 30034 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.

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

5.  Security Considerations

   The extensions to OSPF and IS-IS described in this document result in
   two types of behaviors in data plane when a node in a network fails.
   One is that for a node, which is a upstream (except for the direct
   upstream) node of the failed node along a SR-TE path, it continues to
   send the traffic to the failed node along the SR-TE path for an
   extended period of time.  The other is that for a node, which is the
   direct upstream node of the failed node, it fast re-routes the
   traffic around the failed node to the direct downstream node of the
   failed node along the SR-TE path.  These behaviors are internal to a
   network and should not cause extra security issues.

6.  IANA Considerations

7.  Acknowledgements

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

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/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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

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8.2.  Informative References

   [I-D.ietf-rtgwg-segment-routing-ti-lfa]
              Litkowski, S., Bashandy, A., Filsfils, C., Francois, P.,
              Decraene, B., and D. Voyer, "Topology Independent Fast
              Reroute using Segment Routing", Work in Progress,
              Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
              07, 29 June 2021, <https://www.ietf.org/archive/id/draft-
              ietf-rtgwg-segment-routing-ti-lfa-07.txt>.

   [I-D.ietf-spring-segment-protection-sr-te-paths]
              Hegde, S., Bowers, C., Litkowski, S., Xu, X., and F. Xu,
              "Segment Protection for SR-TE Paths", Work in Progress,
              Internet-Draft, draft-ietf-spring-segment-protection-sr-
              te-paths-01, 11 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-spring-
              segment-protection-sr-te-paths-01.txt>.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", Work in
              Progress, Internet-Draft, draft-ietf-spring-segment-
              routing-policy-14, 25 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-spring-
              segment-routing-policy-14.txt>.

Appendix A.  Proxy Forwarding for Adjacency and Node Segment

   This Section shows through example how a proxy node forward traffic
   to the destination node when a node fails and the next segment of
   label stack is an adjacency-SID or node-SID.

A.1.  Next Segment is an Adjacency Segment

   As shown in Figure 1, Label Stack 3 {10012, 20023, 30034, 40045} uses
   only adjacency-SIDs and represents the SR-TE strict explicit path
   RT1->RT2->RT3->RT4->RT5.  When RT3 fails, node RT2 acts as a PLR, and
   uses next adjacency-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 adjacency-SID 10012, and forwards the packet to RT2;

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   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 query result is
   2004.  RT2 uses 2004 as the incoming label to query the label
   forwarding table.  The next hop is RT7, and the incoming label is
   changed to 7004.

   c.  So the packet leaves RT2 out the interface to RT7 with label
   stack {7004, 40045}. RT7 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.

A.2.  Next Segment is a Node Segment

   As shown in Figure 1, Label Stack 2 {1003, 3004, 4005} uses only
   node-SIDs and represents the ECMP-aware 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.

   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, RT2
   pops label 2003.

   c.  RT2 uses 3004 as the in-label to lookup Proxy Forwarding table,
   The value of Map Label calculated based on SRGBDiffValue is 2004.
   and the query result is forwarding the packet to RT4.

   d.  Then RT2 queries 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.

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Authors' Addresses

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

   Email: huzhibo@huawei.com

   Huaimo Chen
   Futurewei
   Boston, MA,
   United States of America

   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
   United States of America

   Email: cbowers@juniper.net

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

   Email: zhuyq8@chinatelecom.cn

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   Yisong
   China Mobile
   510000
   China

   Email: liuyisong@chinamobile.com

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