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Versions: 00

Routing area                                                    S. Hegde
Internet-Draft                                                 C. Bowers
Intended status: Informational                     Juniper Networks Inc.
Expires: April 3, 2021                                      S. Litkowski
                                                           Cisco Systems
                                                                   X. Xu
                                                            Alibaba Inc.
                                                                   F. Xu
                                                                 Tencent
                                                      September 30, 2020


                   Segment Protection for SR-TE Paths
          draft-ietf-spring-segment-protection-sr-te-paths-00

Abstract

   Segment routing supports the creation of explicit paths using Adj-
   Segment-ID (SID), Node-SIDs, and BSIDs.  It is important to provide
   fast reroute (FRR) mechanisms to respond to failures of links and
   nodes in the Segment-Routed Traffic-Engineered(SR-TE) path.  A point
   of local repair (PLR) can provide FRR protection against the failure
   of a link in an SR-TE path by examining only the first (top) label in
   the SR label stack.  In order to protect against the failure of a
   node, a PLR may need to examine the second label in the stack as
   well, in order to determine SR-TE path beyond the failed node.  This
   document specifies how a PLR can use the first and second label in
   the SR-MPLS label stack describing an SR-TE path to provide
   protection against node failures.

Status of This Memo

   This Internet-Draft is submitted 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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 3, 2021.





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Copyright Notice

   Copyright (c) 2020 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
   Provisions Relating to IETF Documents
   (https://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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Node Failures Along SR-TE Paths . . . . . . . . . . . . . . .   3
     2.1.  Segment protection for explicit paths with Node-SIDs  . .   4
     2.2.  Segment Protection for Anycast-SIDs . . . . . . . . . . .   4
     2.3.  Segment protection for explicit paths with Adj-SIDs . . .   5
   3.  Detailed Solution using Context Tables  . . . . . . . . . . .   7
     3.1.  Building Context Tables . . . . . . . . . . . . . . . . .   7
     3.2.  Segment protection for Node-SIDs  . . . . . . . . . . . .   8
     3.3.  Segment protection for Adj-SIDs . . . . . . . . . . . . .   9
     3.4.  Segment protection for edge nodes . . . . . . . . . . . .  10
       3.4.1.  Detailed Example for Segment protection for edge
               nodes . . . . . . . . . . . . . . . . . . . . . . . .  11
   4.  Determining node can be bypassed  . . . . . . . . . . . . . .  12
   5.  Hold timers for Node-SID/Prefix-SIDs and Adj-SIDs . . . . . .  13
     5.1.  Interaction with micro-loop avoidance . . . . . . . . . .  14
   6.  Optimization Considerations . . . . . . . . . . . . . . . . .  14
     6.1.  Segment Protection Example with Common SRGB . . . . . . .  15
   7.  Operational Considerations  . . . . . . . . . . . . . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     11.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   It is possible for a routing device to completely go out of service
   abruptly due to power failure, hardware failure or software crashes.
   Node protection is an important property of the Fast Reroute



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   mechanism.  It provides protection against a node failure by
   rerouting traffic around the failed node.  For example, the
   mechanisms described in Loop Free Alternates ([RFC5286]), Remote Loop
   Free Alternates ([RFC8102]), and
   [I-D.ietf-rtgwg-segment-routing-ti-lfa] can be used to provide node
   protection to ensure minimal traffic loss after a node failure.

   Section 2 describes problems with SR-TE paths and the need for a
   specialized mechanism to provide node protection for SR-TE paths.
   Section 3 describes the solution applied to paths built using Adj-
   SIDs and Node-SIDs.  In order to distinguish the node failures of the
   segment endpoints (mid points) in an SR-TE path from the usual node
   protection mechanisms described in various LFA mechansims, this
   document uses the term Segment Protection.

2.  Node Failures Along SR-TE Paths

   The topology shown in Figure 1. illustrates a example network
   topology with Segment Routing enabled on each node.

      Node          Node          Node          Node          Node
      SID:1         SID:2         SID:3         SID:4         SID:5
      +----+   10   +----+   10   +----+   10   +----+   10   +----+
      | R1 |--------| R2 |--------| R3 |--------| R4 |--------| R5 |
      +----+        +----+        +----+        +----+        +----+
          \                           \          /
           \ 10                        \ 100    / 60
            \                           \      /
             \   +----+                  +----+
              +--| R7 |------------------| R8 |
                 +----+    30            +----+
                / Node                   Node             Label stack:
               /  SID:7                  SID:8            +------------+
         +----+                          SRGB:            |  1008 (top)|
         | R6 |                          3000-4000        +------------+
         +----+                                           |  3005      |
         Node                                             +------------+
         SID:6


             * Numbers on the links represent the symmetric link cost

   Figure 1: Example topology.  The segment index for each node is shown
     in the diagram.  All nodes have SRGB = [1000-2000], except for R8
   which has SRGB = [3000-4000].  A label stack that represents the path
                   R1->R7->R8->R4->R5 is shown as well.





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2.1.  Segment protection for explicit paths with Node-SIDs

   Consider an explicit path in the topology in Figure 1 from R1->R5 via
   R1->R7->R8->R4->R5.  This path can be built using the shortest paths
   from R1-to-R8 and R8-to-R5.  The label stack to instantiate this path
   contains two Node-SIDs 1008 and 3005.  The 1008 label will take the
   packet from R1 to R8 via R7 and get popped.  The next label in the
   stack 3005 will take the packet from R8 to the destination R5 via R4.
   If the node R8 goes down, it is not possible for R7 to perform FRR
   without examining the second label in the incoming label stack
   (3005).

   Note that in the absence of a failure, R7 does not need to understand
   the meaning of the second label (3005) in order to perform normal
   forwarding.  However, in order to support segment protection, R7 will
   need to understand the meaning of label 3005 in order to determine
   where the packet is headed after R8.

   The mechanisms used to detect whether a node failed or a link failed,
   is outside the scope of this document.  The possible options for node
   failure detection capabilities of a device and resultant forwarding
   state is described in section 5.2 in [RFC8679] are applicable to this
   draft as well.

2.2.  Segment Protection for Anycast-SIDs

   A prefix segment advertised as a Node-SID may only be advertised by
   one node in the network.  Instead, an anycast prefix segment may be
   advertised by more than one node.  In some situations, one can use
   Anycast-SIDs to construct SR-TE paths that are protected against node
   failure, without the need for the mechanism described in this
   document.



















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      +----+   10   +----+   10   +----+   10   +----+   10   +----+
      | R1 |--------| R2 |--------| R3 |--------| R4 |--------| R5 |
      +----+        +----+        +----+        +----+        +----+
          \                           \          / |
           \ 10                        \100   60/  |
            \                           \      /   |
             \   +----+    30            +----+    |
              +--| R7 |------------------| R8 |    |
                 +----+                  +----+    |
                /    \                  Anycast    +
               /      \                 SID:100   /
         +----+        \                         /
         | R6 |         \    40          +----+ /60
         +----+          +---------------| R9 |+          Label stack:
                                         +----+           +------------+
                                        Anycast           |  1100 (top)|
                                        SID:100           +------------+
                                                          |  1005      |
                                                          +------------+
           * Numbers on the links represent the symmetric link cost

      Figure 2: Topology illustrating use of Anycast-SIDs to protect
        against node failures.  All nodes have SRGB = [1000-2000].

   An example of this is shown in Figure 2.  In this example, R8 and R9
   advertise an Anycast-SID of 100.  The label stack in this example =
   [1100, 1005];. The top label (1100) corresponds to the Anycast-SID
   advertised by both R8 and R9.  In the absence of a failure, the
   packet sent by R1 with this label stack will follow the path from
   R1->R5 along R1->R7->R8->R4->R5.

   If R7 is performing a per-prefix LFA calculation [RFC5286], then R7
   will install a backup next-hop to R9 for this Anycast-SID, protecting
   against the failure of the primary next-hop to R8.  This backup path
   does not pass through R8, so it is would not be affected by a
   complete failure of node R8.  As illustrated by this example, for
   some topologies segment-protecting SR-TE paths can be constructed
   through the use of Anycast-SIDs, as opposed to the mechanism
   described in this document.

2.3.  Segment protection for explicit paths with Adj-SIDs










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                                  Adj-SID:
                                  R3-R8:9044

      Node-         Node          Node          Node          Node
      SID:1         SID:2         SID:3         SID:4         SID:5
      +----+   10   +----+   10   +----+   10   +----+   10   +----+
      | R1 |--------| R2 |--------| R3 |--------| R4 |--------| R5 |
      +----+        +----+        +----+        +----+        +----+
          \                           \          /              |
           \ 10                        \ 100    / 60            | 10
            \                           \      /                |
             \   +----+                  +----+               +----+
              +--| R7 |------------------| R8 |---------------| R9 |
                 +----+    30            +----+      10       +----+
                / Node                   Node                 Node
               /  SID:7                  SID:8                SID:9
         +----+                          SRGB:
         | R6 |                          3000-4000        Label stack:
         +----+                                           +------------+
         Node                            Adj-SIDs:        |  1003 (top)|
         SID:6                           R8-R4:9054       +------------+
                                                          |  9044      |
                                                          +------------+
                                                          |  9054      |
                                                          +------------+
                                                          |  1005      |
                                                          +------------+
         * Numbers on the links represent the symmetric link cost

     Figure 3: Explicit path using an Adj-SID.  All nodes have SRGB =
         [1000-2000], except for R8 which has SRGB = [3000-4000].

   Consider an explicit path from R1->R5 via R1->R2->R3->R8->R4->R5.
   This path can be built using a combination of Node-SIDs and Adj-SIDs,
   as shown in Figure 3.  The diagram shows the label stack needed to
   instantiate this path, as well as several Adj-SIDs advertised by
   nodes involved in this path.  When a packet leaving R1 with this
   label stack reaches R3, the top label is 9044, which will take the
   packet to R8.  The next-next-hop in the path is R4.  To provide
   protection for the failure of node R8, R3 would need to send the the
   packet to R4 without going through R8.  However, the only way R3 can
   learn that the packet needs to go to the R4 is to examine the next
   label in the stack, label 9054.  Since R3 knows that R8 has
   advertised label 9054 as the adjacency segment for the link from R8
   to R4, R3 knows that a backup path can merge back into the original
   explicit path at R4.





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3.  Detailed Solution using Context Tables

   This section provides a detailed description of how to construct
   node-protecting backup paths for SR-TE paths using context tables.
   The end result of this description is externally visible forwarding
   behavior that can be specified as a packet arriving at a PLR with a
   particular incoming label stack and leaving the PLR on a particular
   outgoing interface with a particular outgoing label stack.  There may
   be other methods of arriving at the same externally visible
   forwarding behavior as described in draft
   [I-D.ietf-rtgwg-segment-routing-ti-lfa]section 6.2.  It is not the
   intent of this document to exclude other methods, as long as the
   externally visible forwarding behavior is the same as produced by
   this method.

3.1.  Building Context Tables

   [RFC5331] introduced the concept of Context Specific Label Spaces and
   there are various applications making use of this concept.A context
   label table on a router represents the Label Forwarding Information
   Base (LFIB) from the point of view of a particular neighbor . Context
   tables are built by constructing incoming label mappings advertised
   by the neighbor and the actions corresponding to those labels.  The
   labels advertised by each node are local to the node and may not be
   unique across the segment routing domain.  The context tables are
   separate tables built on a per-neighbor basis on every node to ensure
   they represent LFIBs of a particular neighbor.

   When a PLR needs to protect an SR-TE path against the failure of a
   neighbor N, it creates a context table associated with N.  This
   context table is populated with the following segment routing
   forwarding entries:

      - All the Prefix-SIDs of the network.  The programmed incoming
      label map uses the SRGB of N to compute the input label value.
      The NHLFE (Next Hop Label Forwarding Entry) is then constructed by
      looking into all the nexthops for the Prefix-SID and choosing a
      loop-free path as explained in Section 3.2

      - All the Adj-SIDs advertised by N.  The NHLFE is constructed as
      explained in Section 3.3

   The following section illustrates how the context table is
   constructed to allow the PLR to provide node-protecting paths for the
   next-next hops in the topology shown in Figure 1 and Figure 3.






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3.2.  Segment protection for Node-SIDs

   Figure 4 shows the routing table entries on R7 corresponding to the
   Node-SIDs to reach R1 and R8 for the topology in Figure 1.  In the
   absence of a failure, a packet with a label stack whose top label is
   1008 will have its top label popped by R7 (assuming PHP behavior),
   and R7 will forward the packet to R8.  When the interface to R8 is
   down, the backup next-hop entry is used.  R7 will pop the top label
   of 1008, and use the context table that R7 computed for R8 to
   evaluate the next label on the stack.

       R7's Routing Table (partial)
       Transits routes for Node-SIDs for R1 and R8
      +=============+=============================================+
      | In label    | Outgoing label action                       |
      +=============+=============================================+
      | 1001        | Primary: pop, fwd to R1                     |
      |             | Backup: pop, lookup context.r1              |
      +-------------+---------------------------------------------+
      | 1008        | Primary: pop, fwd to R8                     |
      |             | Backup: pop, lookup context.r8              |
      +-------------+---------------------------------------------+

       R7's Context Table for R8 (context.r8, partial)
      +=============+=============================================+
      | In label    | Outgoing label action                       |
      +=============+=============================================+
      | 3004        | swap 1004, fwd to R1                        |
      +-------------+---------------------------------------------+
      | 3005        | swap 1005, fwd to R1                        |
      +-------------+---------------------------------------------+
      | 3008        | drop                                        |
      +-------------+---------------------------------------------+

      Figure 4: Building node-protecting backup paths for SR-TE paths
                            involving Node-SIDs

   R7 builds context table for R8 using the following process.  R7
   computes the mapping of incoming label to Node-SID that R8 expects to
   see based on the SRGB advertised by R8.  In the example in Figure 1,
   R7 can determine that R8 interprets in incoming label of 3005 as
   mapping to the the Node-SID for R5.

   R7 then computes a loop-free backup path to reach R5 which is node-
   protecting with respect to the failure of R8.  In this example, the
   backup path computed by R7 to reach R5 without passing through R8 can
   be achieved forwarding the packet to R1 with a top label of 1005,
   corresponding to the Node-SID for R5 in the context of R1's SRGB.



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   The loop-free path computation may be based on a mechanism such as
   LFA, R-LFA, TI-LFA, or constraint based SPF avoiding failure.  To
   populate the context table for R8, R7 maps the out label actions
   corresponding to the backup path to R5 to the incoming label 3005.
   This results in the entry for label 3005 shown in context.r8 in
   Figure 4.

   Therefore, when a packet arrives at R7 with label stack = [1008,
   3005], and the link from R7 to R8 has recently failed, R7 will use
   backup next-hop entry for label 1008 in its main routing table.
   Based on this entry, R7 will pop label 1008, and use context.r8 to
   lookup the new top label = 3005.  R7 will swap label 3005 for 1005
   and forward the packet to R1.  This will get the packet to R5 on a
   node protecting backup path.

   Note that R7 activates the node-protecting backup path when it
   detects that the link to R8 has failed.  R7 does not know that node
   R8 has actually failed.  However, the node-protecting backup path is
   computed assuming that the failure of the link to R8 implies that R8
   has failed.

3.3.  Segment protection for Adj-SIDs

   This section gives an example of how to constuct node-protecting
   backup paths when the SR-TE path uses Adj-SIDs.  Figure 5 shows some
   of the routing table entries for R3 corresponding to the sample
   network shown in Figure 3.  When the top label of the label stack is
   an Adj-SID, the PLR needs to recognize that in order to provide a
   node-protecting backup path, it needs to pop the top label and
   examine the next label in the context of the next-hop router
   identified by the top label Adj-SID.  In this example, when R3 is
   constructing its routing table, it recognizes that label 9044
   corresponds to a next-hop of R8, so it installs a backup entry,
   corresponding to the failure of the link to R8, when pops label 9044,
   and then examines the new top label in the context of R8.
















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       R3's Routing Table (partial)
       Transit route for Adj-SID
      +=============+=============================================+
      | In label    | Outgoing label action                       |
      +=============+=============================================+
      | 9044        | Primary: pop, fwd to R8                     |
      |             | Backup: pop, lookup context.r8              |
      +-------------+---------------------------------------------+


       R3's Context Table for R8 (context.r8, partial)
      +=============+=============================================+
      | In label    | Outgoing label action                       |
      +=============+=============================================+
      | 3005        | swap 1005, fwd to R4                        |
      +-------------+---------------------------------------------+
      | 9054        | pop, fwd to R4                              |
      +-------------+---------------------------------------------+

      Figure 5: Building node-protecting backup paths for SR-TE paths
                            involving Adj-SIDs

   R3 constructs its context table for R8 by determining which labels R8
   expects to receive to accomplish different forwarding actions.  The
   entry for incoming label 3005 in context.r8 in Figure 5 corresponds
   to a Node-SID This entry is computed using the methods described in
   Section 3.2

   The entry for incoming label 9054 in context.r8 corresponds to an
   Adj-SID.  R3 recognizes that R8 has advertised this Adj-SID for the
   link from R8 to R4 in Figure 3.  So R3 determines the outgoing label
   action needed to reach R4 without passing through R8.  This can be
   accomplished by popping the label 9054, and forwarding the packet
   directly on the link from R3 to R4.

3.4.  Segment protection for edge nodes

   The segment protection mechanism described in the previous sections
   depends on the assumption that the label immediately below the top
   label in the label stack is understood in the IGP domain.When the
   provider edge routers exchange service labels via BGP or some other
   non-IGP mechanism the bottom label is not understood in the IGP
   domain.

   The EPE-SIDs as described in [I-D.ietf-idr-bgpls-segment-routing-epe]
   are used to choose egress interface among a set of egress paths.
   EPE-SID can be a bottom-most label in a SR-TE path.  EPE-SIDs are not
   understood in the IGP domain.  In order to support the procedures



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   described in this document, EPE-SIDs should always be added after
   Anycast-SID for the nodes that advertised the EPE-SIDs.  Same EPE-SID
   should be configured on all these Anycast nodes so that in case of
   node failure, the traffic is correctly forwarded by the other
   protector nodes.  If a Node-SID is used instead of an Anycast SID,
   above the EPE-SID in the label stack, if procedures in this document
   are in use, it may cause packets to be dropped.

   The egress node protection mechanisms described in the draft
   [RFC8679] is applicable to this usecase and no additional changes
   will be required for SR based networks

3.4.1.  Detailed Example for Segment protection for edge nodes

        sid:1    sid:2     sid:3       sid:4      sid:5
 1000-2000   1000-2000 1000-2000   1000-2000  1000-2000
   R2:1024    R3:1034   R8:1044     R5:1064
       R4:2014 =========================
   +----+ 10 +----+ 10 +----+  10   +----+ 10 +----+ Primary
   | PE1|----| R2 |----| R3 |-------| R4  |-- | PE2| context 1.1.1.1: sid 10
   +----+    +----+    +----+       +----+    +----+\
       \                  \          /               \+-----+
        \ 10               \ 100    / 60             /| CE1 |
         \                  \      /               /  +-----+
          \   +----+         +----+ R4:1054 +-----+
           +--| R7 |---------| R8 | --------| PE3 |context 1.1.1.1 sid 10
              +----+    30   +----+         +-----+ Protector mirror SID 100
               /   sid:7       sid:8         sid:9
              /    1000-2000   3000-4000     1000-2000
             / 10
          +----+
          | R6 |
          +----+
          sid:6
          1000-2000

                  R4's Context Table for PE2 (context.PE2, partial)
   +=============+=============================================+
   | In label    | Outgoing label action                       |
   +=============+=============================================+
   | 1010        | swap 1100(mirror sid), push 1010 fwd to R8  |
   +-------------+---------------------------------------------+

    * Numbers on the links represent the symmetric link cost


             Figure 6: Node protection for edge nodes Adj-SIDs




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   The segment protection mechanisms that are described in previous
   sections depend on the assumption that the label below the top label
   in the label stack are understood in the IGP domain.  If the edge
   node goes down, the label below the top label representing the edge
   node could be BGP service label or labels representing other
   applications.  Service mirroring use case is described in [RFC8402]
   section 5.1.  The Customer edges are multi-homed to provider edges
   and one of the PE's acts in primary role and the other in protector
   role.  The two PEs advertise a context ip address for each customer
   site and attaches a Anycast-SID to the context.  The protector PE
   advertises a binding sid with M bit set (Mirror-SID)which implies
   mirroring capability for the context.  Protector PE builds the
   context table for the BGP service labels advertised by the primary PE
   for the same context.  The BGP service resolves on a transport that
   has stack of labels with context-sid at the bottom of the label
   stack.  Any penultimate node of PE2 builds a context table for PE2 as
   explained in the section Section 3.1.  This context table contains
   the sid for the context-id and output action is to pop the top label
   and replace with the Mirror-SID that the protector PE advertised for
   the context 1.1.1.1.  As shown in the example Section 3.4.1 the SID
   10 attached to context-id 1.1.1.1 has been programmed in the
   context.PE2 on the penultimate router R4.  The action is to swap 1010
   with Mirror-SID 1100 and push 1010 which is PE2's context SID.  When
   packet reaches PE2, it has top label of 1100 which is a Mirror-
   SID(context label)on PE2 and directs the protector PE to lookup the
   context table of Primary PE for the BGP service labels.

4.  Determining node can be bypassed

   In certain scenarios, the node in the label stack may represent an
   important function such as firewall filter which must be performed.
   Bypassing such a functionality may cause major security issues.  When
   segment protection mechanisms described in this document are applied,
   it's possible that if the firewall goes down, traffic is re-routed
   via the next label in the stack.  There are multiple ways this
   problem could be solved.

   The procedures described in this document should be optional and
   should be enabled when devices are configured to apply the procedures
   and examine next label in the stack.  The feature should be
   controllable on a per neighbor granularity.  When certain devices
   offer a critical function, the neighbors of the devices may disable
   the segment protection for this particular neighbor providing
   critical functions.

   IGP protocol extensions are proposed in
   [I-D.li-rtgwg-enhanced-ti-lfa] which define a "no bypass" flag for
   the SIDs.  The nodes that indicate critical functions may advertise



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   SIDs with "NB" bit set.  Segment protection procedures described in
   this document should not be applied on these SIDs and in case of
   failure either link protecting backup paths can be programmed or
   packet can be dropped with no protection.

5.  Hold timers for Node-SID/Prefix-SIDs and Adj-SIDs

   SR-TE paths may be computed by a controller or by the head-end
   router.  When there is a node failure in the network, the controller
   or head-end router has to learn about the failure, recompute the
   label stacks of any affected SR-TE paths, and get the new label
   stacks programmed into the forwarding plane of the head-end router.
   This process may be slow compared to the speed with which routers in
   the network react to the event.  After learning about a node failure,
   the non-PLR routers in the network will no longer be able to compute
   a path to reach the failed node.  If no special precautions are
   taken, these non-PLR routers will remove the forwarding entries
   corresponding the Node-SID and Prefix-SIDs advertised by the failed
   node.  If the head-end router is still sending traffic with that
   Node-SID/Prefix-SID in the stack, traffic can be blackholed at a non-
   PLR router.  In this case, the node-protection FRR mechanisms do not
   bring full benefit.

   In order to solve the above problem, hold timers are recommended.
   The hold-timer corresponds to the maximum time that a combination of
   controller and head-end router or a head-end router alone takes to
   compute and install label stacks corresponding to a new SR-TE paths
   in the event of a node failure.  The hold times should be applied to
   forwarding entries for Node-SIDs and Prefix-SIDs that are advertised
   by single node in the network.  If the Node-SID or Prefix-SID becomes
   unreachable, the event and resulting forwarding changes should not
   communicated to the forwarding planes on all configured routers
   (including PLRs for the failed node) until the hold-timer expires.
   The traffic will continue to follow the previous path and get FRR
   protection on the PLR.

   A route corresponding to a global Adj-SID advertised by a node that
   becomes unreachable should also be left in the forwarding table for
   the duration of the hold-timer.

   The node-protecting backup forwarding entry on the PLR corresponding
   to the local Adj-SID from the PLR to the failed node should also be
   left in the forwarding table for the duration of the hold-timer.








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5.1.  Interaction with micro-loop avoidance

   During network convergence, the micro-loop avoidance mechansims as
   described in [I-D.bashandy-rtgwg-segment-routing-uloop] may be
   applied.For the failed node, all the nodes in the network should
   consistently detect the failure and maintain the pre-failure shortest
   path in the forwarding plane so that the traffic can follow pre-
   failure shortest path and take the node-protecting backup path at the
   PLR of the failed node.

6.  Optimization Considerations

   The solution described in this document requires that a PLR build a
   context table for each neighbor for which node-protection is desired.
   The context table for each protected neighbor needs to contain route
   entries for all of the Prefix-SIDs in the network, as well as the
   route entries corresponding to the Adj-SIDs advertised by the
   protected neighbor.  Although the scale of IGP domain is limited,
   this may result in considerable additional memory consumption on the
   routers.  It is possible to take advantage of an optimization that
   allows the PLR to avoid creating context-tables when all of the nodes
   in the network advertise the same Segment Routing Global Block (SRGB)
   and all Adj-SIDs in the network are advertised as global Adj-SIDs.
   In this case, all labels in the stack representing an SR-TE path are
   globally unique.Protection for node failure cases in such a
   deployment can be achieved by doing a lookup of the first label and
   potentially a second lookup of the second label using a common route
   table with primary and backup entries for all Prefix-SIDs as well as
   for all of the global Adj-SIDs.

   The primary route entries for global Adj-SIDs not advertised by the
   PLR will be the shortest path to the node advertising the global Adj-
   SID.  The backup route entries for these global Adj-SIDs will
   generally correspond to the node-protecting backup path to the node
   advertising the global Adj-SID.  However, for a global Adj-SID
   advertised by the direct neighbor of the PLR the node-protecting
   backup route entry will correspond to the backup path to the node on
   the far end of the Adj-SID.

   With the common route table constructed in this manner, when the PLR
   receives a packet whose first label is a global Adj-SID advertised by
   the failed neighbor of the PLR, the lookup of the first label will
   produce the correct backup path directly.  When the PLR receives a
   packet whose first label is the Node-SID of the failed neighbor,or an
   Adj-SID advertised by the PLR corresponding to the failed neighbor,
   the route entry will instruct the PLR to lookup the second label
   using the common route table.  Finally, when the PLR receives a
   packet whose first label is a global Adj-SID or a Node-SID advertised



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   by a node which is neither the PLR nor the failed neighbor, then the
   usual link-protecting backup path will be produced based on a lookup
   of the first label only.

6.1.  Segment Protection Example with Common SRGB



Node               Node               Node               Node             Node
sid:1000           sid:1001           sid:1002           sid:1003      sid:1004
+----+2001 1 2100+----+2102  1  2201+----+2203  1  2302+----+2304  1 2403+----+
| R0 |-----------| R1 |-------------| R2 |-------------| R3 |------------| R4 |
+----+           +----+             +----+             +----+            +----+
    \ 2005                              \ 2206         / 2306          2407 |
     \                                   \            /                     |
      \ 1                                 \ 10       / 6                  1 |
       \                                   \        /                       |
        \                              2602 \      / 2603              2704 |
         \ 2500+----+ 2506               2605+----+2607              2706+----+
          +----| R5 |------------------------| R6 |----------------------| R7 |
               +----+           3            +----+            1         +----+
               Node                          Node                          Node
               sid:1005                      sid:1006                  sid:1007


      * Numbers on the links represent the symmetric link cost
          * All nodes have SRGB = [400000-405000] size 5000

            R2's Routing Table (partial)

   +=============+=============================================+
   | In label    | Outgoing label action                       |
   +=============+=============================================+
   | 4001003     | Primary: pop, fwd to R3                     |
   |             | Backup: pop, lookup ilm table or ip table   |
   |             |              based on BOS bit               |
   +-------------+---------------------------------------------+
   | 4001007     | Primary: swap 401007, fwd to R6             |
   |             | Backup: Swap 401007, Push 401005(top),fwd R1|
   +-------------+---------------------------------------------+
   | 4002203     | Primary: pop, fwd to R3                     |
   |             | Backup: pop, lookup ilm table or ip table   |
   |             |              based on BOS bit               |
   +-------------+---------------------------------------------+

Label Stack 1:
   +-------------+
   |4001003 (top)|



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   +-------------+
   |   4001007   |
   +-------------+                                Label Stack 2:
                                                       +-------------+
                                                       |4001003 (top)|
                                                       +-------------+
                                                       |   4001007   |
                                                       +-------------+



                           Figure 7: Common SRGB

   The diagram Figure 7 shows an example where optimized Segment
   Protection mechanism is deployed.  All the nodes have a common SRGB
   of 400000 to 4005000.  The Node-SIDs are in the range 1001, 1002 etc
   and the global Adj-SIDs are in the range 2001, 2005 and so on.  R2's
   partial ILM table consisting of primary and backup nexthops is also
   shown in the diagram.  Node-SID of R3 which is represented by label
   4001003 has a primary nexthop pointing to R3 and backup nexthop which
   pops the label and looks up ILM table with next label in the packet.
   For Example consider a path from R0 to R7 with a label stack
   consisting of 4001003 and 4001007.  When the node R3 fails, R2 which
   is the PLR, will pop the label 4001003 and lookup for next label in
   the same table.  Next label in this example is 4001007.  Based on the
   primary nexthop for 4001007, traffic is forwarded to R6.  Another
   example label stack consists of global Adj-SID of 4002203 (Adj-SID
   from R2->R3).  As shown in the partial ILM table on R2, 4002203 also
   has a backup nexthop which pops the label and looks-up next label in
   the packet.On R3's failure, traffic will get forwarded via R6.

7.  Operational Considerations

   The procedures described in this document should be configurable and
   applied only when enabled explicitly.  In order to satisfy scenarios
   described in Section 4, the feature should be controllable on the per
   neighbor basis.  The optimisation procedures described in Section 6,
   should be applied only when the entire network has a common SRGB and
   all nodes advertise global Adj-SIDs.  This optimization should be
   applied based on explicit configuration.

8.  Security Considerations

   The procedures described in this document will in most common cases
   be deployed inside a single ownership IGP domain.  No new security
   risks are exposed due to the procedures described in this document.
   The security considerations for SR-MPLS with label stacking is
   described in detail in [RFC8402] are applicable for this document as



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   well.  This document introduces the context table lookup for the
   labels in the label stack.  As described in [RFC8402] MPLS packet
   filtering at the boundaries ensures the operations on the MPLS labels
   inside the domain is safe includingcontext table lookup operation.
   The security procedures applicable to IGP protocols are also
   applicable to segment routing extensions as described in [RFC8667]
   and [RFC8665] and ensure required protection for the segment
   protection procedures described in this document.

9.  IANA Considerations

10.  Acknowledgments

   The authors would like to thank Peter Psenak, Bruno Decraene,
   Alexander Vainshtein and Huzibo, Dhruv Dhody Ketan Talaulikar for
   their review and suggestions.

11.  References

11.1.  Normative References

   [RFC5286]  Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
              IP Fast Reroute: Loop-Free Alternates", RFC 5286,
              DOI 10.17487/RFC5286, September 2008,
              <https://www.rfc-editor.org/info/rfc5286>.

   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
              Label Assignment and Context-Specific Label Space",
              RFC 5331, DOI 10.17487/RFC5331, August 2008,
              <https://www.rfc-editor.org/info/rfc5331>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

11.2.  Informative References

   [I-D.bashandy-rtgwg-segment-routing-uloop]
              Bashandy, A., Filsfils, C., Litkowski, S., Decraene, B.,
              Francois, P., and P. Psenak, "Loop avoidance using Segment
              Routing", draft-bashandy-rtgwg-segment-routing-uloop-09
              (work in progress), June 2020.








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   [I-D.ietf-idr-bgpls-segment-routing-epe]
              Previdi, S., Talaulikar, K., Filsfils, C., Patel, K., Ray,
              S., and J. Dong, "BGP-LS extensions for Segment Routing
              BGP Egress Peer Engineering", draft-ietf-idr-bgpls-
              segment-routing-epe-19 (work in progress), May 2019.

   [I-D.ietf-rtgwg-segment-routing-ti-lfa]
              Litkowski, S., Bashandy, A., Filsfils, C., Decraene, B.,
              Francois, P., Voyer, D., Clad, F., and P. Camarillo,
              "Topology Independent Fast Reroute using Segment Routing",
              draft-ietf-rtgwg-segment-routing-ti-lfa-04 (work in
              progress), August 2020.

   [I-D.li-rtgwg-enhanced-ti-lfa]
              Li, C. and Z. Hu, "Enhanced Topology Independent Loop-free
              Alternate Fast Re-route", draft-li-rtgwg-enhanced-ti-
              lfa-02 (work in progress), August 2020.

   [RFC8102]  Sarkar, P., Ed., Hegde, S., Bowers, C., Gredler, H., and
              S. Litkowski, "Remote-LFA Node Protection and
              Manageability", RFC 8102, DOI 10.17487/RFC8102, March
              2017, <https://www.rfc-editor.org/info/rfc8102>.

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

   [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>.

   [RFC8679]  Shen, Y., Jeganathan, M., Decraene, B., Gredler, H.,
              Michel, C., and H. Chen, "MPLS Egress Protection
              Framework", RFC 8679, DOI 10.17487/RFC8679, December 2019,
              <https://www.rfc-editor.org/info/rfc8679>.

Authors' Addresses










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   Shraddha Hegde
   Juniper Networks Inc.
   Exora Business Park
   Bangalore, KA  560103
   India

   Email: shraddha@juniper.net


   Chris Bowers
   Juniper Networks Inc.

   Email: cbowers@juniper.net


   Stephane Litkowski
   Cisco Systems

   Email: slitkows.ietf@gmail.com


   Xiaohu Xu
   Alibaba Inc.
   Beijing
   China

   Email: xiaohu.xxh@alibaba-inc.com


   Feng Xu
   Tencent
   China

   Email: oliverxu@tencent.com

















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