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

Copyright Notice

Copyright © 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.

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

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.

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. 

                        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.

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). 

   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). 

   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. 

   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. 

   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. 

      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.

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:

  1. 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.*/
  • Validate that the Node Segment ID is advertised for the IPv4 Prefix by IGP Protocol {
  • When the Protocol field in the received IPv4 IGP- Prefix Segment ID sub-TLV is 0, use any locally enabled IGP protocol.
  • When the Protocol field in the received IPv4 IGP- Prefix Segment ID sub-TLV is 1, use OSPF as the IGP protocol.
  • When the Protocol field in the received IPv4 IGP- Prefix Segment ID sub-TLV is 2, use IS-IS as the IGP protocol.
  • 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.
               }
  • Validate that the Node Segment ID is advertised with the No-PHP flag. {
  • When the Protocol is OSPF, the NP-Flag defined in Section 5 of [SR-OSPF] MUST be set to 0.
  • 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:

  • Validate that the Node Segment ID is advertised for the IPv4 Prefix by the IGP protocol {
  • When the Protocol field in the received IPv4 IGP- Prefix Segment ID sub-TLV is 0, use any locally enabled IGP protocol.
  • When the Protocol field in the received IPv4 IGP- Prefix Segment ID sub-TLV is 1, use OSPF as the IGP protocol.
  • When the Protocol field in the received IPv4 IGP- Prefix Segment ID sub-TLV is 2, use IS-IS as the IGP protocol.
  • 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*/
  • Validate that the Node Segment ID is advertised for the IPv6 Prefix by the IGP protocol {
  • When the Protocol field in the received IPv6 IGP- Prefix Segment ID sub-TLV is 0, use any locally enabled IGP protocol.
  • When the Protocol field in the received IPv6 IGP- Prefix Segment ID sub-TLV is 1, use OSPF as the IGP protocol.
  • When the Protocol field in the received IPv6 IGP- Prefix Segment ID sub-TLV is 2, use IS-IS as the IGP protocol.
  • 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.
               }
  • Validate that the Node Segment ID is advertised with the No-PHP flag. {
  • When the Protocol is OSPF, the NP-flag defined in Section 5 of [SR-OSPFV3] MUST be set to 0.
  • 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:

  • Validate that the Node Segment ID is advertised for the IPv4 Prefix by the IGP protocol {
  • When the Protocol field in the received IPv6 IGP- Prefix Segment ID sub-TLV is 0, use any locally enabled IGP protocol.
  • When the Protocol field in the received IPv6 IGP- Prefix Segment ID sub-TLV is 1, use OSPF as the IGP protocol.
  • When the Protocol field in the received IPv6 IGP- Prefix Segment ID sub-TLV is 2, use IS-IS as the IGP protocol.
  • 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):

  • Validate that the Receiving Node Identifier is the local IGP identifier.
  • 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.
                  }

When the Adj. Type is 4 or 6 (IGP Adjacency or LAN Adjacency):

  • Validate that the Remote Interface ID matches the local identifier of the interface (Interface-I) on which the packet was received.
  • Validate that the Receiving Node Identifier is the local IGP identifier.
  • 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. 

         }

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".

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

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.

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.
   
   [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.

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.

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