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rfc:rfc8287

Internet Engineering Task Force (IETF) N. Kumar, Ed. Request for Comments: 8287 C. Pignataro, Ed. Category: Standards Track Cisco ISSN: 2070-1721 G. Swallow

                                             Southend Technical Center
                                                              N. Akiya
                                                   Big Switch Networks
                                                               S. Kini
                                                            Individual
                                                               M. Chen
                                                                Huawei
                                                         December 2017
 Label Switched Path (LSP) Ping/Traceroute for Segment Routing (SR)
      IGP-Prefix and IGP-Adjacency Segment Identifiers (SIDs)
                       with MPLS Data Planes

Abstract

 A Segment Routing (SR) architecture leverages source routing and
 tunneling paradigms and can be directly applied to the use of a
 Multiprotocol Label Switching (MPLS) data plane.  A node steers a
 packet through a controlled set of instructions called "segments" by
 prepending the packet with an SR header.
 The segment assignment and forwarding semantic nature of SR raises
 additional considerations for connectivity verification and fault
 isolation for a Label Switched Path (LSP) within an SR architecture.
 This document illustrates the problem and defines extensions to
 perform LSP Ping and Traceroute for Segment Routing IGP-Prefix and
 IGP-Adjacency Segment Identifiers (SIDs) with an MPLS data plane.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8287.

Kumar, et al. Standards Track [Page 1] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

Copyright Notice

 Copyright (c) 2017 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.

Kumar, et al. Standards Track [Page 2] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   1.1.  Coexistence of SR-Capable and Non-SR-Capable Node
         Scenarios . . . . . . . . . . . . . . . . . . . . . . . .   5
 2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   5
 3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
 4.  Challenges with Existing Mechanisms . . . . . . . . . . . . .   5
   4.1.  Path Validation in Segment Routing Networks . . . . . . .   5
 5.  Segment ID Sub-TLV  . . . . . . . . . . . . . . . . . . . . .   7
   5.1.  IPv4 IGP-Prefix Segment ID  . . . . . . . . . . . . . . .   7
   5.2.  IPv6 IGP-Prefix Segment ID  . . . . . . . . . . . . . . .   8
   5.3.  IGP-Adjacency Segment ID  . . . . . . . . . . . . . . . .   9
 6.  Extension to Downstream Detailed Mapping TLV  . . . . . . . .  11
 7.  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   7.1.  FECs in Target FEC Stack TLV  . . . . . . . . . . . . . .  11
   7.2.  FEC Stack Change Sub-TLV  . . . . . . . . . . . . . . . .  12
   7.3.  Segment ID POP Operation  . . . . . . . . . . . . . . . .  13
   7.4.  Segment ID Check  . . . . . . . . . . . . . . . . . . . .  13
   7.5.  TTL Consideration for Traceroute  . . . . . . . . . . . .  19
 8.  Backward Compatibility with Non-SR Devices  . . . . . . . . .  19
 9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   9.1.  New Target FEC Stack Sub-TLVs . . . . . . . . . . . . . .  20
   9.2.  Protocol in the Segment ID Sub-TLV  . . . . . . . . . . .  20
   9.3.  Adjacency Type in the IGP-Adjacency Segment ID  . . . . .  20
   9.4.  Protocol in the Label Stack Sub-TLV of the Downstream
         Detailed Mapping TLV  . . . . . . . . . . . . . . . . . .  21
   9.5.  Return Code . . . . . . . . . . . . . . . . . . . . . . .  21
 10. Security Considerations . . . . . . . . . . . . . . . . . . .  21
 11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
   11.1.  Normative References . . . . . . . . . . . . . . . . . .  22
   11.2.  Informative References . . . . . . . . . . . . . . . . .  22
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  24
 Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

Kumar, et al. Standards Track [Page 3] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

1. Introduction

 "Detecting Multiprotocol Label Switched (MPLS) Data-Plane Failures"
 [RFC8029] defines a simple and efficient mechanism to detect data-
 plane failures in Label Switched Paths (LSPs) 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 [RFC8029] is modeled after the Ping/Traceroute paradigm
 (ICMP echo request [RFC792]) and is typically referred to as "LSP
 Ping" and "LSP Traceroute".  [RFC8029] supports hierarchical and
 stitching LSPs.
 [SR] introduces and describes an SR 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 an SR header.  A detailed definition of
 the SR architecture is available in [SR].
 As described in [SR] and [SR-MPLS], the SR architecture can be
 directly applied to an MPLS data plane, the SID will be 20 bits, and
 the SR header is the label stack.  Consequently, the mechanics of
 data-plane validation of [RFC8029] can be directly applied to SR
 MPLS.
 Unlike LDP or RSVP, which are the other well-known MPLS control plane
 protocols, the basis of Segment ID assignment in SR architecture is
 not always on a hop-by-hop basis.  Depending on the type of Segment
 ID, the assignment can be unique to the node or within a domain.
 This nature of SR raises additional considerations for validation of
 fault detection and isolation in an SR network.  This document
 illustrates the problem and describes a mechanism to perform LSP Ping
 and Traceroute for Segment Routing IGP-Prefix and IGP-Adjacency SIDs
 within an MPLS data plane.

Kumar, et al. Standards Track [Page 4] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

1.1. Coexistence of SR-Capable and Non-SR-Capable Node Scenarios

 [INTEROP] describes how SR operates in a network where SR-capable and
 non-SR-capable nodes coexist.  In such a network, one or more
 SR-based LSPs and non-SR-based LSPs are stitched together to achieve
 an end-to-end LSP.  This is similar to a network where LDP and RSVP
 nodes coexist and the mechanism defined in Section 4.5.2 of [RFC8029]
 is applicable for LSP Ping and Trace.
 Section 8 of this document explains one of the potential gaps that is
 specific to SR-Capable and non-SR-capable node scenarios and explains
 how the existing mechanism defined in [RFC8029] handles it.

2. Requirements Notation

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

3. Terminology

 This document uses the terminology defined in [SR] and [RFC8029];
 readers are expected to be familiar with those terms.

4. Challenges with Existing Mechanisms

 The following example describes the challenges with using the current
 MPLS Operations, Administration, and Maintenance (OAM) mechanisms on
 an SR network.

4.1. Path Validation in Segment Routing Networks

 [RFC8029] defines the MPLS OAM mechanisms that help with fault
 detection and isolation for an MPLS data-plane path by the use of
 various Target Forwarding Equivalence Class (FEC) Stack sub-TLVs 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 for SR, the unique nature of the SR architecture raises the
 need for additional operational considerations for path validation.
 This section discusses the challenges.

Kumar, et al. Standards Track [Page 5] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

                      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, and 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 the 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 the Adjacency
 Segment ID to an incorrect neighbor or over an incorrect link.  The
 exposed Segment ID (of an incorrectly forwarded Adjacency Segment ID)
 might still allow such a packet to reach the intended destination,
 even though the intended strict traversal was broken.
 In the topology above, assume that R1 sends traffic with a 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 packet to R1 or R3, the packet may still be
 delivered to R8 (if the nodes are configured with the same SR Global
 Block (SRGB)) [SR] but not via the expected path.
 MPLS traceroute may help with detecting such a deviation in the
 above-mentioned scenario.  However, in a different example, it may
 not be helpful, for example, if R3 forwards a packet with Adjacency
 Segment ID 9236 via link L1 (due to misprogramming) when it was
 expected to be forwarded over link L2.

Kumar, et al. Standards Track [Page 6] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

5. Segment ID Sub-TLV

 The format of the following Segment ID sub-TLVs follows the
 philosophy of the Target FEC Stack TLV carrying FECs corresponding to
 each label in the label stack.  When operated with the procedures
 defined in [RFC8029], this allows LSP Ping/Traceroute operations to
 function when the Target FEC Stack TLV contains more FECs than
 received label stacks at the responder nodes.
 Three new sub-TLVs are defined for the Target FEC Stack TLV (Type 1),
 the Reverse-Path Target FEC Stack TLV (Type 16), and the Reply Path
 TLV (Type 21).
         Sub-Type    Sub-TLV Name
         --------  ---------------
           34      IPv4 IGP-Prefix Segment ID
           35      IPv6 IGP-Prefix Segment ID
           36      IGP-Adjacency Segment ID
 See Section 9.2 for the registry for the Protocol field specified
 within these sub-TLVs.

5.1. IPv4 IGP-Prefix Segment ID

 The IPv4 IGP-Prefix Segment ID is defined in [SR].  The format is as
 specified 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         IPv4 Prefix                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Prefix Length  |    Protocol   |         Reserved              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 IPv4 Prefix
    This field carries the IPv4 Prefix to which the Segment ID is
    assigned.  In case of an Anycast Segment ID, this field will carry
    the 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).

Kumar, et al. Standards Track [Page 7] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

 Protocol
    This field is set to 1, if the responder MUST perform FEC
    validation using OSPF as the IGP protocol.  Set to 2, if the
    responder MUST perform Egress FEC validation using the
    Intermediate System to Intermediate System (IS-IS) as the IGP
    protocol.  Set to 0, if the responder can use any IGP protocol for
    Egress FEC validation.
 Reserved
    The Reserved field MUST be set to 0 when sent and MUST be ignored
    on receipt.

5.2. IPv6 IGP-Prefix Segment ID

 The IPv6 IGP-Prefix Segment ID is defined in [SR].  The format is as
 specified 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
    This field carries the IPv6 prefix to which the Segment ID is
    assigned.  In case of an Anycast Segment ID, this field will carry
    the 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).

Kumar, et al. Standards Track [Page 8] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

 Protocol
    Set to 1 if the responder MUST perform FEC validation using OSPF
    as the IGP protocol.  Set to 2 if the responder MUST perform
    Egress FEC validation using IS-IS as the IGP protocol.  Set to 0
    if the responder can use any IGP protocol for Egress FEC
    validation.
 Reserved
    MUST be set to 0 on send and MUST be ignored on receipt.

5.3. IGP-Adjacency Segment ID

 This sub-TLV is applicable for any IGP-Adjacency defined in [SR].
 The format is as specified 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 a Parallel Adjacency as
    defined in [SR].  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.
    Set to 0 when the Adjacency Segment is over an unnumbered
    interface.

Kumar, et al. Standards Track [Page 9] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

 Protocol
    Set to 1 if the responder MUST perform FEC validation using OSPF
    as the IGP protocol.  Set to 2 if the responder MUST perform
    Egress FEC validation using IS-IS as the IGP protocol.  Set to 0
    if the responder can use any IGP protocol for Egress FEC
    validation.
 Reserved
    MUST be set to 0 on send and MUST be ignored on receipt.
 Local Interface ID
    An identifier that is assigned by the local Label Switching Router
    (LSR) for a link to which the Adjacency Segment ID is bound.  This
    field is set to a local link address (IPv4 or IPv6).  For IPv4,
    this field is 4 octets; for IPv6, this field is 16 octets.  If
    unnumbered, this field is 4 octets and includes a 32-bit link
    identifier as defined in [RFC4203] and [RFC5307].  If the
    Adjacency Segment ID represents Parallel Adjacencies [SR], this
    field is 4 octets and MUST be set to 4 octets of zeroes.
 Remote Interface ID
    An identifier that is assigned by the remote LSR for a link on
    which the Adjacency Segment ID is bound.  This field is set to the
    remote (downstream neighbor) link address (IPv4 or IPv6).  For
    IPv4, this field is 4 octets; for IPv6, this field is 16 octets.
    If unnumbered, this field is 4 octets and includes a 32-bit link
    identifier as defined in [RFC4203] and [RFC5307].  If the
    Adjacency Segment ID represents Parallel Adjacencies [SR], this
    field is 4 octets and MUST be set to 4 octets of zeroes.
 Advertising Node Identifier
    This specifies the Advertising Node Identifier.  When the Protocol
    field is set to 1, then this field is 4 octets and carries the
    32-bit OSPF Router ID.  If the Protocol field is set to 2, then
    this field is 6 octets and carries the 48-bit IS-IS System ID.  If
    the Protocol field is set to 0, then this field is 4 octets and
    MUST be set to zero.

Kumar, et al. Standards Track [Page 10] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

 Receiving Node Identifier
    This specifies the downstream node identifier.  When the Protocol
    field is set to 1, then this field is 4 octets and carries the
    32-bit OSPF Router ID.  If the Protocol field is set to 2, then
    this field is 6 octets and carries the 48-bit IS-IS System ID.  If
    the Protocol field is set to 0, then this field is 4 octets and
    MUST be set to zero.

6. Extension to Downstream Detailed Mapping TLV

 In an echo reply, the Downstream Detailed Mapping TLV [RFC8029] 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
 Detailed Mapping TLV is used to return the protocol that is used to
 distribute the label carried in the Downstream Label field.  The
 following protocols are defined in [RFC8029]:
    Protocol #        Signaling Protocol
    ----------        ------------------
      0               Unknown
      1               Static
      2               BGP
      3               LDP
      4               RSVP-TE
 With SR, OSPF or IS-IS can be used for label distribution.  This
 document adds two new protocols as follows:
    Protocol #        Signaling Protocol
    ----------        ------------------
      5               OSPF
      6               IS-IS
 See Section 9.4.

7. Procedures

 This section describes aspects of LSP Ping and Traceroute operations
 that require further considerations beyond [RFC8029].

7.1. FECs in Target FEC Stack TLV

 When LSP echo request packets are generated by an initiator, FECs
 carried in the Target FEC Stack TLV may need to differ to support an
 SR architecture.  The following defines the Target FEC Stack TLV
 construction mechanics by an initiator for SR scenarios.

Kumar, et al. Standards Track [Page 11] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

    Ping
       The initiator MUST include FEC(s) corresponding to the
       destination segment.
       The initiator MAY include FECs corresponding to some or all of
       the segments imposed in the label stack by the initiator to
       communicate the segments traversed.
    Traceroute
       The initiator MUST initially include FECs corresponding to all
       segments imposed in the label stack.
       When a received echo reply contains the FEC Stack Change TLV
       with one or more of the original segments being popped, the
       initiator MAY remove a corresponding FEC(s) from the Target FEC
       Stack TLV in the next (TTL+1) traceroute request, as defined in
       Section 4.6 of [RFC8029].
       When a received echo reply does not contain the FEC Stack
       Change TLV, the initiator MUST NOT attempt to remove any FECs
       from the Target FEC Stack TLV in the next (TTL+1) traceroute
       request.
 As defined in [SR-OSPF] and [SR-IS-IS], the Prefix SID can be
 advertised as an absolute value, an index, or as a range.  In any of
 these cases, the initiator MUST derive the Prefix mapped to the
 Prefix SID and use it in the IGP-Prefix Segment ID defined in
 Sections 5.1 and 5.2.  How the responder uses the details in the
 SR-FEC sub-TLV to perform the validation is a local implementation
 matter.

7.2. FEC Stack Change Sub-TLV

 [RFC8029] defines a FEC Stack Change sub-TLV that a router must
 include when the FEC stack changes.
 The network node that advertised the Node Segment ID is responsible
 for generating a FEC Stack Change sub-TLV with the Post Office
 Protocol (POP) operation type for the Node Segment ID, regardless of
 whether or not Penultimate Hop Popping (PHP) is enabled.
 The network node that is immediately downstream of the node that
 advertised the Adjacency Segment ID is responsible for generating the
 FEC Stack Change sub-TLV for POP operation for the Adjacency Segment
 ID.

Kumar, et al. Standards Track [Page 12] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

7.3. Segment ID POP Operation

 The forwarding semantic of the Node Segment ID with the PHP flag is
 equivalent to usage of Implicit Null in MPLS protocols.  The
 Adjacency Segment ID is also similar in a sense that it can be
 thought of as a locally allocated segment that has PHP enabled when
 destined for the next-hop IGP Adjacency Node.  Procedures described
 in Section 4.4 of [RFC8029] rely on the Stack-D and Stack-R
 explicitly having the Implicit Null value.  Implementations SHOULD
 use the Implicit Null for the Node Segment ID PHP and Adjacency
 Segment ID PHP cases.

7.4. Segment ID Check

 This section modifies the procedure defined in Section 4.4.1 of
 [RFC8029].  Step 4 defined in Section 4.4.1 of [RFC8029] is modified
 as below:
      4. If the label mapping for FEC is Implicit Null, set the
         FEC-status to 2 and proceed to step 4a.  Otherwise,
         if the label mapping for FEC is Label-L, proceed to step 4a.
         Otherwise, set the FEC-return-code to 10 ("Mapping for this
         FEC is not the given label at stack-depth"), set the
         FEC-status to 1, and return.
     4a. Segment Routing IGP-Prefix and IGP-Adjacency SID Validation:
       If the Label-stack-depth is 0 and the Target FEC Stack sub-TLV
       at FEC-stack-depth is 34 (IPv4 IGP-Prefix Segment ID), {
          Set the 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.*/
  1. Validate that the Node Segment ID is advertised for the

IPv4 Prefix by IGP Protocol {

             o  When the Protocol field in the received IPv4 IGP-
                Prefix Segment ID sub-TLV is 0, use any locally
                enabled IGP protocol.

Kumar, et al. Standards Track [Page 13] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

             o  When the Protocol field in the received IPv4 IGP-
                Prefix Segment ID sub-TLV is 1, use OSPF as the IGP
                protocol.
             o  When the Protocol field in the received IPv4 IGP-
                Prefix Segment ID sub-TLV is 2, use IS-IS as the IGP
                protocol.
             o  When the Protocol field in the received IPv4 IGP-
                Prefix Segment ID sub-TLV is an unrecognized value, it
                MUST be treated as a Protocol value of 0.
             }
  1. Validate that the Node Segment ID is advertised with the

No-PHP flag. {

             o  When the Protocol is OSPF, the NP-Flag defined in
                Section 5 of [SR-OSPF] MUST be set to 0.
             o  When the Protocol is IS-IS, the P-Flag defined in
                Section 6.1 of [SR-IS-IS] MUST be set to 0.
             }
          If it can be determined that no protocol associated with the
          Interface-I would have advertised the FEC-Type at FEC-stack-
          depth, set the Best-return-code to 12, "Protocol not
          associated with interface at FEC-stack-depth" and return.
          Set FEC-Status to 1 and return.
       }
       Else, if the Label-stack-depth is greater than 0 and the Target
       FEC Stack sub-TLV at FEC-stack-depth is 34 (IPv4 IGP-Prefix
       Segment ID), {
          Set the Best-return-code to 10 if any below conditions fail:
  1. Validate that the Node Segment ID is advertised for the

IPv4 Prefix by the IGP protocol {

             o  When the Protocol field in the received IPv4 IGP-
                Prefix Segment ID sub-TLV is 0, use any locally
                enabled IGP protocol.

Kumar, et al. Standards Track [Page 14] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

             o  When the Protocol field in the received IPv4 IGP-
                Prefix Segment ID sub-TLV is 1, use OSPF as the IGP
                protocol.
             o  When the Protocol field in the received IPv4 IGP-
                Prefix Segment ID sub-TLV is 2, use IS-IS as the IGP
                protocol.
             o  When the Protocol field in the received IPv4 IGP-
                Prefix Segment ID sub-TLV is an unrecognized value, it
                MUST be treated as a Protocol value of 0.
             }
          If it can be determined that no protocol associated with
          Interface-I would have advertised the FEC-Type at FEC-stack-
          depth, set the Best-return-code to 12, "Protocol not
          associated with interface at FEC stack-depth" and return.
          Set FEC-Status to 1 and return.
       }
       Else, if the Label-stack-depth is 0 and the Target FEC sub-TLV
       at FEC-stack-depth is 35 (IPv6 IGP-Prefix Segment ID), {
          Set the Best-return-code to 10 if any of the below
          conditions fail:
          /* The LSR needs to check if it is being a tail-end for the
          LSP and have the prefix advertised with the PHP bit set*/
  1. Validate that the Node Segment ID is advertised for the

IPv6 Prefix by the IGP protocol {

             o  When the Protocol field in the received IPv6 IGP-
                Prefix Segment ID sub-TLV is 0, use any locally
                enabled IGP protocol.
             o  When the Protocol field in the received IPv6 IGP-
                Prefix Segment ID sub-TLV is 1, use OSPF as the IGP
                protocol.
             o  When the Protocol field in the received IPv6 IGP-
                Prefix Segment ID sub-TLV is 2, use IS-IS as the IGP
                protocol.

Kumar, et al. Standards Track [Page 15] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

             o  When the Protocol field in the received IPv6 IGP-
                Prefix Segment ID sub-TLV is an unrecognized value, it
                MUST be treated as a Protocol value of 0.
             }
  1. Validate that the Node Segment ID is advertised with the

No-PHP flag. {

             o  When the Protocol is OSPF, the NP-flag defined in
                Section 5 of [SR-OSPFV3] MUST be set to 0.
             o  When the Protocol is IS-IS, the P-Flag defined in
                Section 6.1 of [SR-IS-IS] MUST be set to 0.
             }
          If it can be determined that no protocol associated with
          Interface-I would have advertised the FEC-Type at FEC-stack-
          depth, set the Best-return-code to 12, "Protocol not
          associated with interface at FEC stack-depth" and return.
          Set the FEC-Status to 1 and return.
       }
       Else, if the Label-stack-depth is greater than 0 and the Target
       FEC sub-TLV at FEC-stack-depth is 35 (IPv6 IGP-Prefix Segment
       ID), {
          Set the Best-return-code to 10 if any below conditions fail:
  1. Validate that the Node Segment ID is advertised for the

IPv4 Prefix by the IGP protocol {

             o  When the Protocol field in the received IPv6 IGP-
                Prefix Segment ID sub-TLV is 0, use any locally
                enabled IGP protocol.
             o  When the Protocol field in the received IPv6 IGP-
                Prefix Segment ID sub-TLV is 1, use OSPF as the IGP
                protocol.
             o  When the Protocol field in the received IPv6 IGP-
                Prefix Segment ID sub-TLV is 2, use IS-IS as the IGP
                protocol.

Kumar, et al. Standards Track [Page 16] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

             o  When the Protocol field in the received IPv6 IGP-
                Prefix Segment ID sub-TLV is an unrecognized value, it
                MUST be treated as a Protocol value of 0.
             }
          If it can be determined that no protocol associated with
          Interface-I would have advertised the FEC-Type at FEC-stack-
          depth, set the Best-return-code to 12, "Protocol not
          associated with interface at FEC stack-depth" and return.
          Set the FEC-Status to 1 and return.
       }
       Else, if the Target FEC sub-TLV at FEC-stack-depth is 36
       (IGP-Adjacency Segment ID), {
          Set the Best-return-code to 35 (Section 9.5) if any below
          conditions fail:
             When the Adj. Type is 1 (Parallel Adjacency):
             o  Validate that the Receiving Node Identifier is the
                local IGP identifier.
             o  Validate that the IGP-Adjacency Segment ID is
                advertised by the Advertising Node Identifier of the
                Protocol in the local IGP database {
  • When the Protocol field in the received IGP-

Adjacency Segment ID sub-TLV is 0, use any locally

                   enabled IGP protocol.
  • When the Protocol field in the received IGP-

Adjacency Segment ID sub-TLV is 1, use OSPF as the

                   IGP protocol.
  • When the Protocol field in the received IGP-

Adjacency Segment ID sub-TLV is 2, use IS-IS as the

                   IGP protocol.
  • When the Protocol field in the received IGP-

Adjacency Segment ID sub-TLV is an unrecognized

                   value, it MUST be treated as a Protocol value of 0.
                }

Kumar, et al. Standards Track [Page 17] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

             When the Adj. Type is 4 or 6 (IGP Adjacency or LAN
             Adjacency):
             o  Validate that the Remote Interface ID matches the
                local identifier of the interface (Interface-I) on
                which the packet was received.
             o  Validate that the Receiving Node Identifier is the
                local IGP identifier.
             o  Validate that the IGP-Adjacency Segment ID is
                advertised by the Advertising Node Identifier of
                Protocol in the local IGP database {
  • When the Protocol field in the received IGP-

Adjacency Segment ID sub-TLV is 0, use any locally

                   enabled IGP protocol.
  • When the Protocol field in the received IGP-

Adjacency Segment ID sub-TLV is 1, use OSPF as the

                   IGP protocol.
  • When the Protocol field in the received IGP-

Adjacency Segment ID sub-TLV is 2, use IS-IS as the

                   IGP protocol.
  • When the Protocol field in the received IGP-

Adjacency Segment ID sub-TLV is an unrecognized

                   value, it MUST be treated as a Protocol value of 0.
                }
          Set the FEC-Status to 1 and return.
       }

Kumar, et al. Standards Track [Page 18] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

7.5. TTL Consideration for Traceroute

 The LSP Traceroute operation can properly traverse every hop of the
 SR network for the Uniform Model as described in [RFC3443].  If one
 or more LSRs employ a Short Pipe Model, as described in [RFC3443],
 then the LSP Traceroute may not be able to properly traverse every
 hop of the SR network due to the absence of TTL copy operation when
 the outer label is popped.  The Short Pipe is one of the most
 commonly used models.  The following TTL manipulation technique MAY
 be used when the Short Pipe Model is used.
 When tracing an LSP according to the procedures in [RFC8029], 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 a TTL of 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 [RFC8029].  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
 [RFC8029] can be recursively applied to traceroute a source-routed
 LSP.

8. Backward Compatibility with Non-SR Devices

 [INTEROP] describes how SR operates in a 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 interoperation 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 an SR-capable node assigns Adj-SID for a non-SR-
 capable node, the LSP traceroute may fail as the non-SR-capable node
 is not aware of the "IGP Adjacency Segment ID" sub-TLV and may not
 reply with the FEC Stack Change sub-TLVs.  This may result in any
 further downstream nodes replying back with a Return Code of 4,
 "Replying router has no mapping for the FEC at stack-depth".

Kumar, et al. Standards Track [Page 19] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

9. IANA Considerations

9.1. New Target FEC Stack Sub-TLVs

 IANA has assigned three new sub-TLVs from the "sub-TLVs for TLV Types
 1, 16, and 21" subregistry of the "Multi-Protocol Label Switching
 (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry [IANA].
 Sub-Type    Sub-TLV Name                 Reference
 --------    -----------------            ------------
   34        IPv4 IGP-Prefix Segment ID   Section 5.1
   35        IPv6 IGP-Prefix Segment ID   Section 5.2
   36        IGP-Adjacency Segment ID     Section 5.3

9.2. Protocol in the Segment ID Sub-TLV

 IANA has created a new "Protocol in the Segment ID sub-TLV" (see
 Section 5) registry under the "Multi-Protocol Label Switching (MPLS)
 Label Switched Paths (LSPs) Ping Parameters" registry.  Code points
 in the range of 0-250 will be assigned by Standards Action [RFC8126].
 The range of 251-254 is reserved for experimental use and will not be
 assigned.  The value of 255 is marked "Reserved".  The initial
 entries into the registry are:
   Value           Meaning              Reference
 ----------        ----------------     ------------
   0               Any IGP protocol     This document
   1               OSPF                 This document
   2               IS-IS                This document

9.3. Adjacency Type in the IGP-Adjacency Segment ID

 IANA has created a new "Adjacency Type in the IGP-Adjacency Segment
 ID" registry (see Section 5.3) under the "Multi-Protocol Label
 Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
 registry.  Code points in the range of 0-250 will be assigned by
 Standards Action.  The range of 251-254 is reserved for experimental
 use and will not be assigned.  The value of 255 is marked "Reserved".
 The initial entries into the registry are:
   Value           Meaning
 ----------        ----------------
   0               Unnumbered Interface Adjacency
   1               Parallel Adjacency
   4               IPv4, Non-parallel Adjacency
   6               IPv6, Non-parallel Adjacency

Kumar, et al. Standards Track [Page 20] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

9.4. Protocol in the Label Stack Sub-TLV of the Downstream Detailed

    Mapping TLV
 IANA has created a new "Protocol in the Label Stack sub-TLV of the
 Downstream Detailed Mapping TLV" registry under the "Multi-Protocol
 Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters"
 registry.  Code points in the range of 0-250 will be assigned by
 Standards Action.  The range of 251-254 is reserved for experimental
 use and will not be assigned.  The value of 255 is marked "Reserved".
 The initial entries into the registry are:
   Value        Meaning              Reference
 ----------     ----------------     ------------
   0            Unknown              Section 3.4.1.2 of RFC 8029
   1            Static               Section 3.4.1.2 of RFC 8029
   2            BGP                  Section 3.4.1.2 of RFC 8029
   3            LDP                  Section 3.4.1.2 of RFC 8029
   4            RSVP-TE              Section 3.4.1.2 of RFC 8029
   5            OSPF                 Section 6 of this document
   6            IS-IS                Section 6 of this document
   7-250        Unassigned
   251-254      Reserved for
                Experimental Use     This document
   255          Reserved             This document

9.5. Return Code

 IANA has assigned a new Return Code from the "Multi-Protocol Label
 Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" in the
 0-191 (Standards Action) range from the "Return Codes" subregistry.
   Value     Meaning                                  Reference
 ----------  -----------------                        ------------
   35        Mapping for this FEC is not associated   Section 7.4 of
             with the incoming interface              this document

10. Security Considerations

 This document defines additional MPLS LSP Ping sub-TLVs and follows
 the mechanisms defined in [RFC8029].  All the security considerations
 defined in [RFC8029] will be applicable for this document and, in
 addition, they do not impose any additional security challenges to be
 considered.

Kumar, et al. Standards Track [Page 21] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

11. References

11.1. 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>.
 [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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc4203>.
 [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,
            <https://www.rfc-editor.org/info/rfc5307>.
 [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
            Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
            Switched (MPLS) Data-Plane Failures", RFC 8029,
            DOI 10.17487/RFC8029, March 2017,
            <https://www.rfc-editor.org/info/rfc8029>.
 [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>.

11.2. Informative References

 [IANA]     IANA, "Multi-Protocol Label Switching (MPLS) Label
            Switched Paths (LSPs) Ping Parameters",
            <http://www.iana.org/assignments/
            mpls-lsp-ping-parameters>.
 [INTEROP]  Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., and
            S. Litkowski, "Segment Routing interworking with LDP",
            Work in Progress, draft-ietf-spring-segment-routing-ldp-
            interop-09, September 2017.

Kumar, et al. Standards Track [Page 22] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

 [RFC792]   Postel, J., "Internet Control Message Protocol", STD 5,
            RFC 792, DOI 10.17487/RFC0792, September 1981,
            <https://www.rfc-editor.org/info/rfc792>.
 [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
            Writing an IANA Considerations Section in RFCs", BCP 26,
            RFC 8126, DOI 10.17487/RFC8126, June 2017,
            <https://www.rfc-editor.org/info/rfc8126>.
 [SR]       Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
            Litkowski, S., and R. Shakir, "Segment Routing
            Architecture", Work in Progress, draft-ietf-spring-
            segment-routing-14, December 2017.
 [SR-IS-IS] Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
            Gredler, H., Litkowski, S., Decraene, B., and J. Tantsura,
            "IS-IS Extensions for Segment Routing", Work in Progress,
            draft-ietf-isis-segment-routing-extensions-15, December
            2017.
 [SR-MPLS]  Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
            Litkowski, S., and R. Shakir, "Segment Routing with MPLS
            data plane", Work in Progress, draft-ietf-spring-segment-
            routing-mpls-11, October 2017.
 [SR-OSPF]  Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
            Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
            Extensions for Segment Routing", Work in Progress,
            draft-ietf-ospf-segment-routing-extensions-24, December
            2017.
 [SR-OSPFV3]
            Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
            Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
            Extensions for Segment Routing", Work in Progress,
            draft-ietf-ospf-ospfv3-segment-routing-extensions-10,
            September 2017.

Kumar, et al. Standards Track [Page 23] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

Acknowledgements

 The authors would like to thank Stefano Previdi, Les Ginsberg, Balaji
 Rajagopalan, Harish Sitaraman, Curtis Villamizar, Pranjal Dutta,
 Lizhong Jin, Tom Petch, Victor Ji, Mustapha Aissaoui, Tony
 Przygienda, Alexander Vainshtein, and Deborah Brungard for their
 review and comments.
 The authors would like to thank Loa Andersson for his comments and
 recommendation to merge documents.

Contributors

 The following are key contributors to this document:
    Hannes Gredler, RtBrick, Inc.
    Tarek Saad, Cisco Systems, Inc.
    Siva Sivabalan, Cisco Systems, Inc.
    Balaji Rajagopalan, Juniper Networks
    Faisal Iqbal, Cisco Systems, Inc.

Kumar, et al. Standards Track [Page 24] RFC 8287 LSP Ping/Trace for SR-MPLS December 2017

Authors' Addresses

 Nagendra Kumar (editor)
 Cisco Systems, Inc.
 7200-12 Kit Creek Road
 Research Triangle Park, NC  27709-4987
 United States of America
 Email: naikumar@cisco.com
 Carlos Pignataro (editor)
 Cisco Systems, Inc.
 7200-11 Kit Creek Road
 Research Triangle Park, NC  27709-4987
 United States of America
 Email: cpignata@cisco.com
 George Swallow
 Southend Technical Center
 Email: swallow.ietf@gmail.com
 Nobo Akiya
 Big Switch Networks
 Email: nobo.akiya.dev@gmail.com
 Sriganesh Kini
 Individual
 Email: sriganeshkini@gmail.com
 Mach(Guoyi) Chen
 Huawei
 Email: mach.chen@huawei.com

Kumar, et al. Standards Track [Page 25]

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