Egress TLV for Nil FEC 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 adjacency- sids, node-sids, and anycast-sids. 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. When the SR-TE paths are built by the controller, the head-end routers may not have the complete database of the network and may not be aware of the FEC associated with labels that are used in the label stack. A very useful Operations And Maintenance (OAM) requirement is to be able to ping and trace these paths. A simple mpls ping and traceroute mechanism comprises of ability to traverse the SR-TE path without having to validate the control plane state.¶

MPLS ping and traceroute mechanism as described in [RFC8029] and related extensions for SR as defined in [RFC8287] is very useful to precisely validate the control plane and data plane synchronization. It also provides ability to traverse multiple ECMP paths and validate each of the ECMP paths. 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 ability to traverse the paths using ping and traceroute without having to obtain the Forwarding Equivalence Class (FEC) for each label.[RFC8029] supports this mechanism with FECs like Nil FEC and Generic FEC.¶

Generic IPv4 and IPv6 FEC 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. But 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 Segment Routing such as anycast SIDs requires 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 label-stack are represented using single Nil FEC, it poses some challenges.¶

Section 2 discusses the problems associated with using single Nil FEC in a MPLS ping/traceroute procedure and Section 3 and Section 4 discusses 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 draft uses single Nil FEC to represent complete label stack in MPLS ping/traceroute packet irrespective of number of segments in the label-stack. When router in the label-stack path receives MPLS ping/traceroute packets, there is no definite way to decide on whether it is the intended egress router since Nil FEC does not carry any information. So there is high possibility that the packet may be mis-forwarded to incorrect destination but the ping/traceroute might still return success.¶

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

Thus addition of egress information in ping/traceroute packet will help in validating Nil-FEC on each receiving router on 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.¶

3. Egress TLV

The Egress object is a TLV that MAY be included in an MPLS Echo Request message. Its an optional TLV and should appear before FEC-stack TLV in the MPLS Echo Request packet. In case multiple Nil FEC is present in Target FEC Stack TLV, Egress TLV should be added corresponding to the ultimate egress of the label-stack. It can be use for any kind of path with Egress TLV added corresponding to the end-point of the path. Explicit Path can be created using node-sid, adj-sid, binding-sid etc, EGRESS TLV prefix will be derived from path egress/destination and not based on labels used in the path to reach the 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 = 28 (EGRESS TLV)  |          Length                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Prefix (4 or 16 octets)                  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type : 28 (Section 6.1)¶

Length : variable based on IPV4/IPV6 prefix. Length excludes the length of Type and length field. 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 egress of Nil FEC corresponding to last label in the label-stack or SR-TE policy endpoint field [I.D-ietf-idr-segment-routing-te-policy].¶

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 extension proposed in this document is backward compatible with procedures described in [RFC8029]. Router that does not support EGRESS-TLV, will ignore it and use current Nil-FEC procedures described in [RFC8029].¶

When the egress node in the path does not support the extensions proposed in this draft 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 as 1 to report egress ok will be set in MPLS Echo Reply message.¶

When the transit node in the path does not support the extensions proposed in this draft 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 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 2021-11-08.¶

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.¶

8. Acknowledgements

Authors' Addresses