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Versions: (draft-kumar-mpls-spring-lsp-ping) 00 01 02 03 04 05 06 draft-ietf-mpls-spring-lsp-ping

Network Work group                                              N. Kumar
Internet-Draft                                                G. Swallow
Intended status: Standards Track                            C. Pignataro
Expires: September 22, 2016                          Cisco Systems, Inc.
                                                                N. Akiya
                                                     Big Switch Networks
                                                                 S. Kini
                                                              Individual
                                                              H. Gredler
                                                        Juniper Networks
                                                                 M. Chen
                                                                  Huawei
                                                          March 21, 2016


Label Switched Path (LSP) Ping/Trace for Segment Routing Networks Using
                             MPLS Dataplane
                draft-kumarkini-mpls-spring-lsp-ping-06

Abstract

   Segment Routing architecture leverages the source routing and
   tunneling paradigms and can be directly applied to MPLS data plane.
   A node steers a packet through a controlled set of instructions
   called segments, by prepending the packet with a Segment Routing
   header.

   The segment assignment and forwarding semantic nature of Segment
   Routing raises additional consideration for connectivity verification
   and fault isolation in LSP with Segment Routing architecture.  This
   document illustrates the problem and describe a mechanism to perform
   LSP Ping and Traceroute on Segment Routing network over MPLS data
   plane.

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 http://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."



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   This Internet-Draft will expire on September 22, 2016.

Copyright Notice

   Copyright (c) 2016 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements notation . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Challenges with Existing mechanism  . . . . . . . . . . . . .   4
     4.1.  Path validation in Segment Routing networks . . . . . . .   4
     4.2.  Service Label . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Segment ID sub-TLV  . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  IPv4 IGP-Prefix Segment ID  . . . . . . . . . . . . . . .   5
     5.2.  IPv6 IGP-Prefix Segment ID  . . . . . . . . . . . . . . .   6
     5.3.  IGP-Adjacency Segment ID  . . . . . . . . . . . . . . . .   7
   6.  Extension to Downstream Mapping TLV . . . . . . . . . . . . .   8
   7.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  FECs in Target FEC Stack TLV  . . . . . . . . . . . . . .   9
     7.2.  FEC Stack Change sub-TLV  . . . . . . . . . . . . . . . .  10
     7.3.  Segment ID POP Operation  . . . . . . . . . . . . . . . .  10
     7.4.  Segment ID Check  . . . . . . . . . . . . . . . . . . . .  10
     7.5.  TTL Consideration for traceroute  . . . . . . . . . . . .  12
   8.  Issues with non-forwarding labels . . . . . . . . . . . . . .  13
   9.  Backward Compatibility with non Segment Routing devices . . .  13
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  New Target FEC Stack Sub-TLVs  . . . . . . . . . . . . .  13
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   12. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  14
   13. Contributing Authors  . . . . . . . . . . . . . . . . . . . .  14
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     14.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16




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1.  Introduction

   [I-D.ietf-spring-segment-routing] introduces and explains Segment
   Routing architecture that leverages the source routing and tunneling
   paradigms.  A node steers a packet through a controlled set of
   instructions called segments, by prepending the packet with Segment
   Routing header.  A detailed definition about Segment Routing
   architecture is available in [I-D.ietf-spring-segment-routing]

   As defined in [I-D.ietf-spring-segment-routing-mpls], the Segment
   Routing architecture can be directly applied to MPLS data plane in a
   way that, the Segment identifier (Segment ID) will be of 20-bits size
   and Segment Routing header is the label stack.

   "Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures"
   [RFC4379] defines a simple and efficient mechanism to detect data
   plane failures in Label Switched Paths (LSP) by specifying
   information to be carried in an MPLS "echo request" and "echo reply"
   for the purposes of fault detection and isolation.  Mechanisms for
   reliably sending the echo reply are defined.  The functionality
   defined in [RFC4379]is modeled after the ping/traceroute paradigm
   (ICMP echo request [RFC0792]) and is typically referred to as LSP
   ping and LSP traceroute.  [RFC6424] updates [RFC4379] to support
   hierarchal and stitching LSPs.

   Unlike LDP or RSVP which are the other well-known MPLS control plane
   protocols, segment assignment in Segment Routing architecture is not
   hop-by-hop basis.

   This nature of Segment Routing raises additional consideration for
   fault detection and isolation in Segment Routing network.  This
   document illustrates the problem and describe a mechanism to perform
   LSP Ping and Traceroute on Segment Routing network over MPLS data
   plane.

2.  Requirements notation

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

3.  Terminology

   This document uses the terminologies defined in
   [I-D.ietf-spring-segment-routing], [RFC4379], and so the readers are
   expected to be familiar with the same.





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4.  Challenges with Existing mechanism

   This document defines sub-TLVs for the Target FEC Stack TLV and
   explains how they can be used to tackle below challenges.

4.1.  Path validation in Segment Routing networks

   [RFC4379] defines the OAM machinery that helps with fault detection
   and isolation in MPLS dataplane path with the use of various Target
   FEC Stack Sub-TLV that are carried in MPLS Echo Request packets and
   used by the responder for FEC validation.  While it is obvious that
   new Sub-TLVs need to be assigned, the unique nature of Segment
   Routing architecture raises a need for additional machinery for path
   validation.  This section discuss the challenges as below:


                      L1
                  +--------+
                  |   L2   |
                  R3-------R6
                 /           \
                /             \
        R1----R2               R7----R8
                \             /
                 \           /
                  R4-------R5

          Figure 1: Segment Routing network

 The Node segment IDs for R1, R2, R3, R4, R5, R6, R7 and R8 are 5001, 5002, 5003, 5004, 5005, 5006, 5007, 5008 respectively.

    9136 --> Adjacency Segment ID from R3 to R6 over link L1.
    9236 --> Adjacency Segment ID from R3 to R6 over link L2.
    9124 --> Adjacency segment ID from R2 to R4.
    9123 --> Adjacency Segment ID from R2 to R3.


   The forwarding semantic of Adjacency Segment ID is to pop the segment
   ID and send the packet to a specific neighbor over a specific link.
   A malfunctioning node may forward packets using Adjacency Segment ID
   to incorrect neighbor or over incorrect link.  Exposed segment ID
   (after incorrectly forwarded Adjacency Segment ID) might still allow
   such packet to reach the intended destination, although the intended
   strict traversal has been broken.

   Assume in above topology, R1 sends traffic with segment stack as
   {9124, 5008} so that the path taken will be R1-R2-R4-R5-R7-R8.  If
   the Adjacency Segment ID 9124 is misprogrammed in R2 to send the



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   packet to R1 or R3, it will still be delivered to R8 but is not via
   the expected path.

   MPLS traceroute may help with detecting such deviation in above
   mentioned scenario.  However, in a different example, it may not be
   helpful.  For example if R3, due to misprogramming, forwards packet
   with Adjacency Segment ID 9236 via link L1 while it is expected to be
   forwarded over Link L2.

4.2.  Service Label

   A Segment ID can represent a service based instruction.  An Segment
   Routing header can have label stack entries where the label
   represents a service to be applied along the path.  Since these
   labels are part of the label stack, they can influence the path taken
   by a packet and consequently have implications on MPLS OAM.  Service
   Label is left for future study.

5.  Segment ID sub-TLV

   The format of the following Segment ID sub-TLVs follows the
   philosophy of Target FEC Stack TLV carrying FECs corresponding to
   each label in the label stack.  When operated with the procedures
   defined in [RFC4379], this allows LSP ping/traceroute operations to
   function when Target FEC Stack TLV contains more FECs than received
   label stack at responder nodes.

   Three new sub-TLVs are defined for Target FEC Stack TLVs (Type 1),
   Reverse-Path Target FEC Stack TLV (Type 16) and Reply Path TLV (Type
   21).

           sub-Type    Value Field
           --------  ---------------
           TBD1      IPv4 IGP-Prefix Segment ID
           TBD2      IPv6 IGP-Prefix Segment ID
           TBD3      IGP-Adjacency Segment ID


   Service Segments and FRR will be considered in future version.

5.1.  IPv4 IGP-Prefix Segment ID

   The format is as below:








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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         IPv4 Prefix                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Prefix Length  |    Protocol   |         Reserved              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   IPv4 Prefix

      This field carries the IPv4 prefix to which the Segment ID is
      assigned.  In case of Anycast Segment ID, this field will carry
      IPv4 Anycast address.  If the prefix is shorter than 32 bits,
      trailing bits SHOULD be set to zero.

   Prefix Length

      The Prefix Length field is one octet, it gives the length of the
      prefix in bits (values can be 1 - 32).

   Protocol

      Set to 1 if the IGP protocol is OSPF and 2 if IGP protocol is
      ISIS.

5.2.  IPv6 IGP-Prefix Segment ID

   The format is as below:


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                         IPv6 Prefix                           |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Prefix Length  |    Protocol   |              Reserved         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



   IPv6 Prefix





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      This field carries the IPv6 prefix to which the Segment ID is
      assigned.  In case of Anycast Segment ID, this field will carry
      IPv4 Anycast address.  If the prefix is shorter than 128 bits,
      trailing bits SHOULD be set to zero.

   Prefix Length

      The Prefix Length field is one octet, it gives the length of the
      prefix in bits (values can be 1 - 128).

   Protocol

      Set to 1 if the IGP protocol is OSPF and 2 if IGP protocol is
      ISIS.

5.3.  IGP-Adjacency Segment ID

   The format is as below:


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Adj. Type   |    Protocol   |          Reserved             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                Local Interface ID (4 or 16 octets)            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                Remote Interface ID (4 or 16 octets)           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                                                               ~
     |          Advertising Node Identifier (4 or 6 octets)          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                                                               ~
     |             Receiving Node Identifier (4 or 6 octets)         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Adj. Type (Adjacency Type)

      Set to 1, when the Adjacency Segment is Parallel Adjacency as
      defined in section 3.5.1 of [I-D.ietf-spring-segment-routing].
      Set to 4, when the Adjacency segment is IPv4 based and is not a
      parallel adjacency.  Set to 6, when the Adjacency segment is IPv6
      based and is not a parallel adjacency.

   Protocol

      Set to 1 if the IGP protocol is OSPF and 2 if IGP protocol is ISIS



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   Local Interface ID

      An identifier that is assigned by local LSR for a link on which
      Adjacency Segment ID is bound.  This field is set to local link
      address (IPv4 or IPv6).  Incase of unnumbered, 32 bit link
      identifier defined in [RFC4203], [RFC5307] is used.  If the
      Adjacency Segment ID represents parallel adjacencies
      (Section 3.5.1 of [I-D.ietf-spring-segment-routing]) this field
      MUST be set to zero.

   Remote Interface ID

      An identifier that is assigned by remote LSR for a link on which
      Adjacency Segment ID is bound.  This field is set to remote
      (downstream neighbor) link address (IPv4 or IPv6).  In case of
      unnumbered, 32 bit link identifier defined in [RFC4203], [RFC5307]
      is used.  If the Adjacency Segment ID represents parallel
      adjacencies (Section 3.5.1 of [I-D.ietf-spring-segment-routing])
      this field MUST be set to zero.

   Advertising Node Identifier

      Specifies the advertising node identifier.  When Protocol is set
      to 1, then the 32 rightmost bits represent OSPF Router ID and if
      protocol is set to 2, this field carries 48 bit ISIS System ID.

   Receiving Node Identifier

      Specifies the downstream node identifier.  When Protocol is set to
      1, then the 32 rightmost bits represent OSPF Router ID and if
      protocol is set to 2, this field carries 48 bit ISIS System ID.

6.  Extension to Downstream Mapping TLV

   In an echo reply, the Downstream Mapping TLV [RFC4379] is used to
   report for each interface over which a FEC could be forwarded.  For a
   FEC, there are multiple protocols that may be used to distribute
   label mapping.  The "Protocol" field of the Downstream Mapping TLV is
   used to return the protocol that is used to distribute a specific a
   label.  The following protocols are defined in section 3.2 of
   [RFC4379]:










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      Protocol #        Signaling Protocol
      ----------        ------------------
               0        Unknown
               1        Static
               2        BGP
               3        LDP
               4        RSVP-TE

   With segment routing, OSPF or ISIS can be used for label
   distribution, this document adds two new protocols as follows:

      Protocol #        Signaling Protocol
      ----------        ------------------
          TBD5                  OSPF
          TBD6                  ISIS

7.  Procedures

   This section describes aspects of LSP Ping and traceroute operations
   that require further considerations beyond [RFC4379].

7.1.  FECs in Target FEC Stack TLV

   When LSP echo request packets are generated by an initiator, FECs
   carried in Target FEC Stack TLV may need to have deviating contents.
   This document outlines expected Target FEC Stack TLV construction
   mechanics by initiator for known scenarios.

      Ping

         Initiator MUST include FEC(s) corresponding to the destination
         segment.

         Initiator MAY include FECs corresponding to some or all of
         segments imposed in the label stack by the initiator to
         communicate the segments traversed.

      Traceroute

         Initiator MUST initially include FECs corresponding to all of
         segments imposed in the label stack.

         When a received echo reply contains FEC Stack Change TLV with
         one or more of original segment(s) being popped, initiator MAY
         remove corresponding FEC(s) from Target FEC Stack TLV in the
         next (TTL+1) traceroute request as defined in section 4.3.1.2
         of [RFC6424].




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         When a received echo reply does not contain FEC Stack Change
         TLV, initiator MUST NOT attempt to remove FEC(s) from Target
         FEC Stack TLV in the next (TTL+1) traceroute request.

7.2.  FEC Stack Change sub-TLV

   Section 3.3.1.3 of [RFC6424] defines FEC Stack Change sub-TLV that a
   router must include when the FEC stack changes.

   The network node which advertised the Node Segment ID is responsible
   for generating FEC Stack Change sub-TLV of &pop& operation for Node
   Segment ID, regardless of if PHP is enabled or not.

   The network node that is immediate downstream of the node which
   advertised the Adjacency Segment ID is responsible for generating FEC
   Stack Change sub-TLV of &pop& operation for Adjacency Segment ID.

7.3.  Segment ID POP Operation

   The forwarding semantic of Node Segment ID with PHP flag is
   equivalent to usage of implicit Null in MPLS protocols.  Adjacency
   Segment ID is also similar in a sense that it can be thought as next
   hop destined locally allocated segment that has PHP enabled.
   Procedures described in Section 4.4 of [RFC4379] relies on Stack-D
   and Stack-R explicitly having Implicit Null value.  It may simplify
   implementations to reuse Implicit Null for Node Segment ID PHP and
   Adjacency Segment ID cases.

7.4.  Segment ID Check

   This section updates the procedure defined in Step 6 of section 4.4.
   of [RFC4379]

      If the Label-stack-depth is 0 and Target FEC Stack Sub-TLV at FEC-
      stack-depth is TBD1 (IPv4 IGP-Prefix Segment ID), the responder
      should set Best return code to 10, "Mapping for this FEC is not
      the given label at stack-depth <RSC>" if any below conditions
      fail:

      /* The responder LSR is to check if it is the egress of the IPv4
      IGP-Prefix Segment ID described in the Target FEC Stack Sub-TLV,
      and if the FEC was advertised with the PHP bit set.*/



      *  Validate that Node Segment ID is advertised for IPv4 Prefix.

      *  Validate that Node Segment ID is advertised with No-PHP flag {



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         +  When Protocol is OSPF, NP-flag defined in Section 5 of
            [I-D.ietf-ospf-segment-routing-extensions] should be set to
            0.

         +  When Protocol is ISIS, P-Flag defined in Section 2.1 of
            [I-D.ietf-isis-segment-routing-extensions] should be set to
            0.

      *  }

      If the Label-stack-depth is more than 0 and Target FEC Stack Sub-
      TLV at FEC-stack-depth is TBD1 (IPv4 IGP-Prefix Segment ID), the
      responder is to set Best return code to 10 if any below conditions
      fail:

      *  Validate that Node Segment ID is advertised for IPv4 Prefix.

      If the Label-stack-depth is 0 and Target FEC Sub-TLV at FEC-stack-
      depth is TBD2 (IPv6 IGP-Prefix Segment ID), set Best return code
      to 10 if any below conditions fail:

      /* The LSR needs to check if its being a tail-end for the LSP and
      have the prefix advertised with PHP bit set*/



      *  Validate that Node Segment ID is advertised for IPv6 Prefix.

      *  Validate that Node Segment ID is advertised of PHP bit.

      If the Label-stack-depth is more than 0 and Target FEC Sub-TLV at
      FEC-stack-depth is TBD2 (IPv6 IGP-Prefix Segment ID), set Best
      return code to 10 if any below conditions fail:

      *  Validate that Node Segment ID is advertised for IPv6 Prefix.

      If the Label-stack-depth is 0 and Target FEC sub-TLV at FEC-stack-
      depth is TBD3 (Adjacency Segment ID), set Best return code to
      (error code TBD) if any below conditions fail:

         When the Adj. Type is 1 (Parallel Adjacency):

         +  Validate that Receiving Node Identifier is local IGP
            identifier.

         +  Validate that Adjacency Segment ID is advertised by
            Advertising Node Identifier of Protocol in local IGP
            database.



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         When the Adj. Type is 4 or 6:

         +  Validate that Remote Interface ID matches the local
            identifier of the interface (Interface-I) on which the
            packet was received.

         +  Validate that Receiving Node Identifier is local IGP
            identifier.

         +  Validate that IGP-Adjacency Segment ID is advertised by
            Advertising Node Identifier of Protocol in local IGP
            database.

7.5.  TTL Consideration for traceroute

   LSP Traceroute operation can properly traverse every hop of Segment
   Routing network in Uniform Model described in [RFC3443].  If one or
   more LSRs employ Short Pipe Model described in [RFC3443], then LSP
   Traceroute may not be able to properly traverse every hop of Segment
   Routing network due to absence of TTL copy operation when outer label
   is popped.  Short Pipe being the most commonly used model.  The
   following TTL manipulation technique MAY be used when Short Pipe
   model is used.

   When tracing a LSP according to the procedures in [RFC4379] the TTL
   is incremented by one in order to trace the path sequentially along
   the LSP.  However when a source routed LSP has to be traced there are
   as many TTLs as there are labels in the stack.  The LSR that
   initiates the traceroute SHOULD start by setting the TTL to 1 for the
   tunnel in the LSP's label stack it wants to start the tracing from,
   the TTL of all outer labels in the stack to the max value, and the
   TTL of all the inner labels in the stack to zero.  Thus a typical
   start to the traceroute would have a TTL of 1 for the outermost label
   and all the inner labels would have TTL 0.  If the FEC Stack TLV is
   included it should contain only those for the inner stacked tunnels.
   The Return Code/Subcode and FEC Stack Change TLV should be used to
   diagnose the tunnel as described in [RFC4379] and [RFC6424].  When
   the tracing of a tunnel in the stack is complete, then the next
   tunnel in the stack should be traced.  The end of a tunnel can be
   detected from the "Return Code" when it indicates that the responding
   LSR is an egress for the stack at depth 1.  Thus the traceroute
   procedures in [RFC4379] can be recursively applied to traceroute a
   source routed LSP.








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8.  Issues with non-forwarding labels

   Source stacking can be optionally used to apply services on the
   packet at a LSR along the path, where a label in the stack is used to
   trigger service application.  A data plane failure detection and
   isolation mechanism should provide its functionality without applying
   these services.  This is mandatory for services that are stateful,
   though for stateless services [RFC4379] could be used as-is.  It MAY
   also provide a mechanism to detect and isolate faults within the
   service function itself.

   How a node treats Service label is outside the scope of this document
   and will be included in this or a different document later.

9.  Backward Compatibility with non Segment Routing devices

   [I-D.ietf-spring-segment-routing-ldp-interop] describes how Segment
   Routing operates in network where SR-capable and non-SR-capable nodes
   coexist.  In such networks, there may not be any FEC mapping in the
   responder when the Initiator is SR-capable while the responder is not
   (or vice-versa).  But this is not different from RSVP and LDP interop
   scenarios.  When LSP Ping is triggered, the responder will set the
   FEC-return-code to Return 4, "Replying router has no mapping for the
   FEC at stack-depth".

   Similarly when SR-capable node assigns Adj-SID for non-SR-capable
   node, LSP trace may fail as the non-SR-capable node is not aware of
   "IGP Adjacency Segment ID" sub-TLV and may not reply with FEC Stack
   change.  This may result in any further downstream nodes to reply
   back with Return-code as 4, "Replying router has no mapping for the
   FEC at stack-depth".

10.  IANA Considerations

10.1.  New Target FEC Stack Sub-TLVs

   IANA is requested to assign 3 new Sub-TLVs from "Sub-TLVs for TLV
   Types 1, 16 and 21" sub-registry.

     Sub-Type          Sub-TLV Name             Reference
   ----------        -----------------         ------------
      TBD1        IPv4 IGP-Prefix Segment ID  Section 4.1 (this document)
      TBD2        IPv6 IGP-Prefix Segment ID  Section 4.2 (this document)
      TBD3        IGP-Adjacency Segment ID    Section 4.3 (this document)







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11.  Security Considerations

   This document defines additional Sub-TLVs and follows the mechanism
   defined in [RFC4379].  So all the security consideration defined in
   [RFC4379] will be applicable for this document and in addition it
   does not impose any security challenges to be considered.

12.  Acknowledgement

   The authors would like to thank Stefano Previdi, Les Ginsberg, Balaji
   Rajagopalan, Harish Sitaraman, Curtis Villamizar, Pranjal Dutta and
   Lizhong Jin for their review and comments.

   The authors wold like to thank Loa Andersson for his comments and
   recommendation to merge drafts.

13.  Contributing Authors

   Tarek Saad
   Cisco Systems
   Email: tsaad@cisco.com

   Siva Sivabalan
   Cisco Systems
   Email: msiva@cisco.com

   Balaji Rajagopalan
   Juniper Networks
   Email: balajir@juniper.net

   Faisal Iqbal
   Cisco Systems
   Email: faiqbal@cisco.com

14.  References

14.1.  Normative References

   [I-D.ietf-isis-segment-routing-extensions]
              Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
              Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS
              Extensions for Segment Routing", draft-ietf-isis-segment-
              routing-extensions-06 (work in progress), December 2015.








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   [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-07 (work in progress), March 2016.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
              and R. Shakir, "Segment Routing Architecture", draft-ietf-
              spring-segment-routing-07 (work in progress), December
              2015.

   [I-D.ietf-spring-segment-routing-ldp-interop]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., and
              S. Litkowski, "Segment Routing interoperability with LDP",
              draft-ietf-spring-segment-routing-ldp-interop-00 (work in
              progress), October 2015.

   [I-D.ietf-spring-segment-routing-mpls]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Shakir, R., Tantsura, J.,
              and E. Crabbe, "Segment Routing with MPLS data plane",
              draft-ietf-spring-segment-routing-mpls-04 (work in
              progress), March 2016.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <http://www.rfc-editor.org/info/rfc792>.

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

   [RFC3443]  Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
              in Multi-Protocol Label Switching (MPLS) Networks",
              RFC 3443, DOI 10.17487/RFC3443, January 2003,
              <http://www.rfc-editor.org/info/rfc3443>.

   [RFC4203]  Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in
              Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005,
              <http://www.rfc-editor.org/info/rfc4203>.

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              DOI 10.17487/RFC4379, February 2006,
              <http://www.rfc-editor.org/info/rfc4379>.



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   [RFC5307]  Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
              <http://www.rfc-editor.org/info/rfc5307>.

   [RFC6424]  Bahadur, N., Kompella, K., and G. Swallow, "Mechanism for
              Performing Label Switched Path Ping (LSP Ping) over MPLS
              Tunnels", RFC 6424, DOI 10.17487/RFC6424, November 2011,
              <http://www.rfc-editor.org/info/rfc6424>.

14.2.  Informative References

   [RFC6291]  Andersson, L., van Helvoort, H., Bonica, R., Romascanu,
              D., and S. Mansfield, "Guidelines for the Use of the "OAM"
              Acronym in the IETF", BCP 161, RFC 6291,
              DOI 10.17487/RFC6291, June 2011,
              <http://www.rfc-editor.org/info/rfc6291>.

   [RFC6425]  Saxena, S., Ed., Swallow, G., Ali, Z., Farrel, A.,
              Yasukawa, S., and T. Nadeau, "Detecting Data-Plane
              Failures in Point-to-Multipoint MPLS - Extensions to LSP
              Ping", RFC 6425, DOI 10.17487/RFC6425, November 2011,
              <http://www.rfc-editor.org/info/rfc6425>.

Authors' Addresses

   Nagendra Kumar
   Cisco Systems, Inc.
   7200 Kit Creek Road
   Research Triangle Park, NC  27709
   US

   Email: naikumar@cisco.com


   George Swallow
   Cisco Systems, Inc.
   1414 Massachusetts Ave
   Boxborough, MA  01719
   US

   Email: swallow@cisco.com









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   Carlos Pignataro
   Cisco Systems, Inc.
   7200 Kit Creek Road
   Research Triangle Park, NC  27709-4987
   US

   Email: cpignata@cisco.com


   Nobo Akiya
   Big Switch Networks

   Email: nobo.akiya.dev@gmail.com


   Sriganesh Kini
   Individual

   Email: sriganeshkini@gmail.com


   Hannes Gredler
   Juniper Networks

   Email: hannes@juniper.net


   Mach(Guoyi) Chen
   Huawei

   Email: mach.chen@huawei.com




















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