Egress Validation in Label Switched Path Ping and Traceroute Mechanisms

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Table of Contents

1. Introduction

Segment routing supports the creation of explicit paths using Adj- SIDs, Node-SIDs, and Anycast-SIDs defined in [RFC8402]. In certain usecases, the TE paths are built using mechanisms described in [RFC9256] by stacking the labels that represent the nodes and links in the explicit path. Controllers are often deployed to construct paths across multi-domain networks. In such deployments, the head-end routers may have a database of its own domain and may not be aware of the FEC associated with labels that are used by the controller to build paths across multiple domains. A very useful Operations, Administration, and Maintenance (OAM) requirement is to be able to ping and trace these paths.¶

[RFC8029] describes a simple and efficient mechanism to detect data-plane failures in MPLS Label Switched Paths (LSPs). It defines a probe message called an "MPLS echo request" and a response message called an "MPLS echo reply" for returning the result of the probe. SR related extensions to Echo Request/Echo Reply are specified in [RFC8287]. [RFC8029] provides mechanisms to primarily validate the data plane and secondarily to verify the data plane against the control plane. It also provides the ability to traverse Equal cost Mutiple Paths (ECMP) and validate each of the ECMP paths. The use of Target FEC requires all nodes in the network to have implemented the validation procedures. All intermediate nodes may not have been upgraded to support validation procedures. In such cases, it is useful to have the ability to traverse the paths in ping/traceroute mode without having to obtain the FEC for each label. A simple MPLS Echo Request/Echo Reply mechanism allows for traversing the SR Policy path without validating the control plane state. [RFC8029] supports this mechanism with FECs like Nil FEC and Generic FEC.¶

Generic IPv4 and IPv6 FECs are used when the protocol that is advertising the label is unknown. The information that is carried in Generic FEC is the IPv4 or IPv6 prefix and prefix length. Thus Generic FEC types perform an additional control plane validation. However, the details of Generic FEC and validation procedures are not very detailed in the [RFC8029]. The use-case mostly specifies inter-AS VPNs as the motivation. Certain aspects of SR such as anycast SIDs require clear guidelines on how the validation procedure should work. Also, Generic FEC may not be widely supported and if transit routers are not upgraded to support validation of Generic FEC, traceroute may fail. On other hand, Nil FEC consists of the label and there is no other associated FEC information. Nil FEC is used to traverse the path without validation for cases where the FEC is not defined or routers are not upgraded to support the FECs. Thus, it can be used to check any combination of segments on any data path. The procedures described in [RFC8029] are mostly applicable when the Nil FEC is used where the Nil FEC is an intermediate FEC in the label stack. When all labels in the label-stack are represented using Nil FEC, it poses some challenges.¶

Section 2 discusses the problems associated with using Nil FEC in an MPLS ping/traceroute procedure and Section 3 and Section 4 discuss simple extensions needed to solve the problem.¶

2. Problem with Nil FEC

The purpose of Nil FEC as described in [RFC8029] is to ensure hiding of transit tunnel information and in some cases to avoid false negatives when the FEC information is unknown.¶

This document uses a Nil FEC to represent the complete label stack in an MPLS Echo Request message in ping and traceroute mode. A single Nil FEC is used in the MPLS Echo Request message irrespective of the number of segments in the label-stack. As described in sec 4.4.1 of [RFC8029], "If the outermost FEC of the Target FEC stack is the Nil FEC, then the node MUST skip the Target FEC validation completely." When a router in the label-stack path receives an MPLS Echo Request message, there is no definite way to decide on whether it is the intended egress router since Nil FEC does not carry any information and no validation is performed by the router. So there is high possibility that the packet may be mis-forwarded to an incorrect destination but the MPLS Echo Reply might still return success.¶

To avoid this problem, there is a need to add additional information in the MPLS Echo Request message in ping and treaceroute mode along with Nil FEC to do minimal validation on the egress/destination router and send proper information on success and failure to the ingress router. This additional information should help to report transit router information to the ingress/initiator router that can be used by an offline application to validate the traceroute path.¶

Thus the addition of egress information in the MPLS Echo Request message in ping and traceroute mode will help in validating Nil-FEC on each receiving router on the label-stack path to ensure the correct destination. It can be used to check any combination of segments on any path without upgrading transit nodes. The code point used for Egress TLV is from the range 32768-65535 and can can be silently dropped if not recognised as per [RFC8029] and as per clarifications from [RFC9041]¶

3. Egress TLV

The Egress TLV MAY be included in an MPLS Echo Request message. It is an optional TLV and if it is present it MUST appear before the FEC-stack TLV in the MPLS Echo Request packet. This TLV can only be used in LSP ping/traceroute requests generated by the head-end node of an LSP or SR policy for which verification is performed. In case multiple Nil FECs are present in Target FEC Stack TLV, Egress TLV MUST be added corresponding to the ultimate egress of the label-stack. It can be used for any kind of path with Egress TLV added corresponding to the endpoint of the path. Explicit Path can be created using Node-SID, Adj-SID, Binding-SID etc. Prefix field of Egress TLV MUST be derived from path egress/destination. 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Type = 32771 (Egress TLV)  |          Length             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Prefix (4 or 16 octets)                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type : 32771 (Section 6.1)¶

Length : variable based on IPV4/IPV6 prefix. Length excludes the length of the Type and Length fields. Length will be 4 octets for IPv4 and 16 octets for IPv6.¶

Prefix : This field carries the valid IPv4 prefix of length 4 octets or valid IPv6 Prefix of length 16 octets. It can be obtained from the egress of Nil FEC corresponding to the last label in the label-stack or SR policy endpoint field [I.D-ietf-idr-sr-policy-safi].¶

4. Procedure

This section describes aspects of LSP ping and traceroute operations that require further considerations beyond [RFC8029].¶

                  ----R3----
                 /  (1003)  \
      (1001)    /            \(1005)     (1007)
        R1----R2(1002)        R5----R6----R7(prefix X)
                \            /     (1006)
                 \   (1004) /
                  ----R4----

5. Backward Compatibility

The extensions defined in this document is backward compatible with procedures described in [RFC8029]. A Router that does not support Egress TLV, will ignore it and use current the Nil-FEC procedures described in [RFC8029].¶

When the egress node in the path does not support the extensions defined in this document egress validation will not be done and Best-return-code as 3 ("Replying router is an egress for the FEC at stack-depth") and Best-return- subcode set to stack-depth to will be set in the MPLS Echo Reply message.¶

When the transit node in the path does not support the extensions defined in this document Best-return-code as 8 ("Label switched at stack-depth") and Best-return-subcode as Label-stack-depth to report transit switching will be set in the MPLS Echo Reply message.¶

6. IANA Considerations

The code points in section Section 6.1 and Section 6.2 have been assigned by [IANA] by early allocation on 2023-10-05 and 2021-11-08 respectively.¶

7. Security Considerations

This document defines additional MPLS LSP ping TLVs and follows the mechanisms defined in [RFC8029]. All the security considerations defined in [RFC8287] will be applicable for this document and, in addition, they do not impose any additional security challenges to be considered.¶

9. Acknowledgements

Authors would like to thank Stewart Bryant, Greg Mirsky, Alexander Vainshtein, Sanga Mitra Rajgopal, Adrian Farrel for their careful review and comments.¶

Authors' Addresses