Supporting Flexible Algorithm Prefix SIDs in LSP Ping/Traceroute
draft-iqbal-spring-mpls-ping-algo-00

Abstract

RFC8287 defines the extensions to MPLS LSP Ping and Traceroute for Segment Routing IGP-Prefix and IGP-Adjacency Segment Identifier (SIDs) with an MPLS data plane. [I-D.ietf-lsr-flex-algo] proposes a mechanism to allow IGPs to compute constraint based path over network and use Segment Routing Prefix-SIDs to steer packets along the constraint-based paths. All Prefix-SIDs associated with the Flexible Algorithm are assigned to the same IPv4/IPv6 Prefix. Any Segment Routing network that uses Flexible Algorithm based path computation needs additional details to be carried in the FEC Stack sub-TLV for FEC validaiton.

This document updates [RFC8287] by modifying IPv4 and IPv6 IGP-Prefix Segment ID FEC sub-TLVs to also include algorithm identification.

Status of This Memo

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Table of Contents

• 1. Introduction
• 1.1. Conventions
• 2. Motivation
• 3. Algorithm Identification for IGP-Prefix SID Sub-TLVs
• 3.1. IPv4 IGP-Prefix Segment ID Sub-TLV
• 3.2. IPv6 IGP-Prefix Segment ID Sub-TLV
• 4. Procedures
• 4.1. Initiator Node Procedures
• 4.2. Responder Node Procedures
• 5. IANA Considerations
• 6. Security Considerations
• 7. Acknowledgements
• 8. Contributors
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Authors' Addresses

1. Introduction

[RFC8287] defines the extensions to MPLS LSP Ping and Traceroute for Segment Routing IGP-Prefix SID and IGP-Adjacency SID with an MPLS data plane. [RFC8287] proposes 3 Target FEC Stack Sub-TLVs to carry this information. [I-D.ietf-lsr-flex-algo] introduces the concept of Flexible Algorithm that allows IGPs (ISIS, OSPFv2 and OSPFv3) to compute constraint-based path over an MPLS network. The constraint-based paths enables the IGP of a router to associate one or more Segment Routing Prefix-SID with a particular Flexible Algorithm. Multiple Flexible Algorithms are assigned to the same IPv4/IPv6 Prefix while each utilizing a different MPLS Prefix SID label.

Existing MPLS Ping/Traceroute machinery for SR Prefix SIDs, defined in [RFC8287], carries prefix, prefix length, and IGP protocol. To correctly identify and validate a Flexible Algorithm Prefix-SID, the validating device also requires algorithm identification to be supplied in the FEC Stack sub-TLV. This document extends SR-IGP IPv4 and IPv6 Prefix SID FECs to validate a particular Flexible Algorithm, while maintaining backwards compatibility with existing implementations of [RFC8287].

1.1. Conventions

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 RFC 2119 [RFC2119].

The term "Must Be Zero" (MBZ) is used in object descriptions for reserved fields. These fields MUST be set to zero when sent and ignored on receipt.

Since this document refers to the MPLS Time to Live (TTL) far more frequently than the IP TTL, the authors have chosen the convention of using the unqualified "TTL" to mean "MPLS TTL" and using "IP TTL" for the TTL value in the IP header.

2. Motivation

In presence of Flexible Algorithms, a single IGP Prefix may be associated with zero or more IGP Prefix SIDs in addition to the default (Shortest Path First) Prefix SID. Each Prefix SID will have a distinct Prefix SID label and may possibly have a distinct set of next-hops based on associated constraint-based path calculation criteria. This means that to reach the same destination, Flexible Algorithm based IGP-Prefix SID may take a different path than default IGP Prefix SID algorithm.

          R3------R6
         /          \
        /            \
R1----R2              R7----R8
        \            /
         \          /
          R4------R5
     

Figure above, which is a simplification of the diagram used in [RFC8287] illustrates this point through an example. Node Segment IDs for R1, R2, R3, R4, R5, R6, R7, and R8 for the default algorithm are 5001, 5002, 5003, 5004, 5005, 5006, 5007, and 5008, respectively. Nodes R1, R2, R4, R5, R7, and R8 also participate in Flexible Algorithm 128. Their corresponding Node Segment IDs for the algorithm are 5801, 5802, 5804, 5805, 5807, and 5808, respectively.

Now consider an MPLS LSP Traceroute request to validate the path to reach node R8 through Flexible Algorithm 128. The TTL of the first echo request packet expires at node R2 with incoming label 5808. Node R2 attempts to validate IGP-Prefix SID Target FEC stack sub-TLV from the echo request. However, this TFS sub-TLV does not contain information identifying the algorithm. As a result, R2 will attempt validation with default algorithm which expects the echo packet to arrive with Prefix SID label 5008. The validation fails, and node R2 responds with error code 10 resulting in a false negative.

Carrying algorithm identification in the Target FEC Stack sub-TLV of MPLS echo request will help avoid such false negatives. It will also help detect forwarding deviations such as when the packet for a particular destination is incorrectly forwarded to a device that is participating in the default algo but does not participate in a given Flexible Algorithm.

3. Algorithm Identification for IGP-Prefix SID Sub-TLVs

Section 5 of [RFC8287] defines 3 different Segment ID Sub-TLVs that will be included in Target FEC Stack TLV defined in [RFC8029]. This section updates IPv4 IGP-Prefix Segment ID Sub-TLV and IPv6 IGP-Prefix Segment ID Sub-TLV to also include an additional field identifying the algorithm.

3.1. IPv4 IGP-Prefix Segment ID Sub-TLV

The Sub-TLV format for IPv4 IGP-Prefix Segment ID MUST be set as shown in the below TLV format:

    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   |      Algo     |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     

Algo field must be set to 0 if the default algorithm is used. Algo field is set to 1 if Strict Shortest Path First (Strict-SPF) algorithm is used. For Flex-Algo, the Algo field must be set with the algorithm value (values can be 128-255).

3.2. IPv6 IGP-Prefix Segment ID Sub-TLV

The Sub-TLV format for IPv6 IGP-Prefix Segment ID MUST be set as shown in the below TLV format:

    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   |      Algo     |   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     

Algo field must be set to 0 if the default algorithm is used. Algo field is set to 1 if Strict Shortest Path First (Strict-SPF) algorithm is used. For Flex-Algo, the Algo field must be set with the algorithm value (values can be 128-255).

4. Procedures

4.1. Initiator Node Procedures

A node initiating LSP echo request packet for the Node Segment ID MUST identify and include the algorithm associated with the IGP Prefix SID in the Target FEC Stack sub-TLV. If the initiating node is not aware of the algorithm, the default algorithm (id 0) of Shortest Path First is assumed.

4.2. Responder Node Procedures

This section updates the procedures defined in Section 7.4 of [RFC8287] for IPv4/IPv6 IGP Prefix SID FEC. If the algorithm is 0, the procedures from [RFC8287] do not require any change. For any other algorithm value, if the responding node is validating the FEC stack, it MUST also validate the IGP Prefix SID advertisement for the algorithm defined in Algo field.

If the responding node is including IGP Prefix SID FEC in the FEC stack due to FEC Stack Change operation, it MUST also include algorithm associated with the Prefix SID.

5. IANA Considerations

This document does not introduce any IANA considerations.

6. Security Considerations

This document updates [RFC8287] and does not introduce any security considerations.

7. Acknowledgements

TBA.

8. Contributors

TBA

9. References

9.1. Normative References

9.2. Informative References

Authors' Addresses