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/rfc9703.¶

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Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶

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Table of Contents

1. Introduction

Egress Peer Engineering (EPE), as defined in [RFC9087], is an effective mechanism that is used to select the egress peer link based on different criteria. In this scenario, egress peers may belong to a completely different ownership. The EPE-SIDs provide the means to represent egress peer nodes, links, sets of links, and sets of nodes. Many network deployments have built their networks consisting of multiple Autonomous Systems (ASes) either for the ease of operations or as a result of network mergers and acquisitions. The inter-AS links connecting any two ASes could be traffic-engineered using EPE-SIDs in this case, where there is single ownership but different AS numbers. It is important to validate the control plane to forwarding plane synchronization for these SIDs so that any anomaly can be easily detected by the network operator. EPE-SIDs may also be used in an ingress Segment Routing (SR) policy [RFC9256] to choose exit points where the remote AS has a completely different ownership. This scenario is out of scope for this document.¶

   +---------+      +------+
   |         |      |      |
   |    H    B------D      G
   |         | +---/| AS2  |\  +------+
   |         |/     +------+ \ |      |---L/8
   A   AS1   C---+            \|      |
   |         |\\  \  +------+ /| AS4  |---M/8
   |         | \\  +-E      |/ +------+
   |    X    |  \\   |      K
   |         |   +===F AS3  |
   +---------+       +------+

In Figure 1, EPE-SIDs are configured on AS1 towards AS2 and AS3 and advertised in the Border Gateway Protocol - Link State (BGP-LS) [RFC9086]. In certain cases, the EPE-SIDs advertised by the control plane may not be in synchronization with the label programmed in the data plane. For example, on C, a PeerAdj SID could be advertised to indicate it is for the link C->D. Due to some software anomaly, the actual data forwarding on this PeerAdj SID could be happening over the C->E link. If E had relevant data paths for further forwarding the packet, this kind of anomaly would go unnoticed by the network operator. A detailed example of a correctly programmed state and an incorrectly programmed state along with a description of how the incorrect state can be detected is described in Appendix A. A Forwarding Equivalence Class (FEC) definition for the EPE-SIDs will detail the control plane association of the SID. The data plane validation of the SID will be done during the MPLS traceroute procedure. When there is a multi-hop External BGP (EBGP) session between the ASBRs, a PeerNode SID is advertised, and the traffic MAY be load-balanced between the interfaces connecting the two nodes. In Figure 1, C and F could have a PeerNode SID advertised. When the Operations, Administration, and Maintenance (OAM) packet is received on F, it needs to be validated that the packet came from one of the two interfaces connected to C.¶

This document provides Target Forwarding Equivalence Class (FEC) Stack TLV definitions for EPE-SIDs. This solution requires the node constructing the target FEC stack to determine the types of SIDs along the path of the LSP. Other procedures for MPLS Ping and Traceroute, as defined in Section 7 of [RFC8287] and clarified in [RFC8690], are applicable for EPE-SIDs as well.¶

2. Theory of Operation

[RFC9086] provides mechanisms to advertise the EPE-SIDs in BGP-LS. These EPE-SIDs may be used to build SR paths as described in [SR-TE-POLICY] or using Path Computation Element Protocol (PCEP) extensions as defined in [RFC8664]. Data plane monitoring for such paths that consist of EPE-SIDs will use extensions defined in this document to build the Target FEC stack TLV. The MPLS Ping and Traceroute procedures MAY be initiated by the head-end of the SR path or a centralized topology-aware data plane monitoring system, as described in [RFC8403]. The extensions in [SR-TE-POLICY] and [RFC8664] do not define how to carry the details of the SID that can be used to construct the FEC. Such extensions are out of scope for this document. The node initiating the data plane monitoring may acquire the details of EPE-SIDs through BGP-LS advertisements, as described in [RFC9086]. There may be other possible mechanisms that can be used to learn the definition of the SID from the controller. Details of such mechanisms are out of scope for this document.¶

The EPE-SIDs are advertised for inter-AS links that run EBGP sessions. [RFC9086] does not define the detailed procedures of how to operate EBGP sessions in a scenario with unnumbered interfaces. Therefore, these scenarios are out of scope for this document. Anycast and multicast addresses are not in the scope of this document. During the AS migration scenario, procedures described in [RFC7705] may be in force. In these scenarios, if the local and remote AS fields in the FEC (as described in Section 4) carry the globally configured Access Service Network (ASN) and not the "local AS" (as defined in [RFC7705]), then the FEC validation procedures may fail.¶

As described in Section 1, this document defines FEC stack TLVs for EPE-SIDs that can be used in detecting MPLS data plane failures [RFC8029]. This mechanism applies to paths created across ASes of cooperating administrations. If the ping or traceroute packet enters a non-cooperating AS domain, it might be dropped by the routers in the non-cooperating domain. Although a complete path validation cannot be done across non-cooperating domains, it still provides useful information that the ping or traceroute packet entered a non-cooperating domain.¶

4. FEC Definitions

In this document, 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).¶

 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 = 39                      |          Length               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Local AS Number (4 octets)                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Remote AS Number (4 octets)                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Local BGP Router ID (4 octets)                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Remote BGP Router ID (4 octets)                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 38                      |          Length               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Adj-Type      |            RESERVED                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Local AS Number (4 octets)                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Remote AS Number (4 octets)                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Local BGP Router ID (4 octets)                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Remote BGP Router ID (4 octets)                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Local Interface Address (4/16 octets)            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Remote Interface Address (4/16 octets)           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 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 = 40                     |          Length               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Local AS Number (4 octets)                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Local BGP Router ID (4 octets)                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    No.of elements in set      |          Reserved             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Remote AS Number (4 octets)                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Remote BGP Router ID (4 octets)                  |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++


 One element in set consists of the details below
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Remote AS Number (4 octets)                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|              Remote BGP Router ID (4 octets)                  |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-++

5. EPE-SID FEC Validation

When a remote ASBR of the EPE-SID advertisement receives the MPLS OAM packet with the top FEC being the EPE-SID, it MUST perform validity checks on the content of the EPE-SID FEC sub-TLV. The basic length check should be performed on the received FEC.¶

 PeerAdj SID
 -----------
 If Adj type = 1, Length should be 28 octets
 If Adj type = 2, Length should be 52 octets

 PeerNode SID
 -------------
 Length = (20 + No.of IPv4 interface pairs * 8  +
           No.of IPv6 interface pairs * 32) octets

 PeerSet SID
 -----------
 Length = (9 + No.of elements in the set *
          (8 + No.of IPv4 interface pairs * 8 +
           No.of IPv6 interface pairs * 32) octets

If a malformed FEC sub-TLV is received, then a return code of 1, "Malformed echo request received", as defined in [RFC8029] MUST be sent. The section below is appended to the procedure given in step 4a of Section 7.4 of [RFC8287].¶

Else, if the Label-stack-depth is 0 and the Target FEC Stack sub-TLV
at FEC-stack-depth is 38 (PeerAdj SID sub-TLV), {

    Set the Best-return-code to 10, "Mapping for this FEC is not
    the given label at stack-depth".  If any below conditions fail:

           -  Validate that the receiving node's BGP Local AS matches
              with the remote AS field in the received PeerAdj SID
              FEC sub-TLV.

           -  Validate that the receiving node's BGP Router-ID
              matches with the Remote Router ID field in the
              received PeerAdj SID FEC.

           -  Validate that there is an EBGP session with a peer
              having a local AS number and BGP Router-ID as
              specified in the Local AS number and Local Router-ID
              field in the received PeerAdj SID FEC sub-TLV.

    If the Remote interface address is not zero, validate the
    incoming interface.  Set the Best-return-code to 35,
    "Mapping for this FEC is not associated with the incoming
    interface" [RFC8287].  If any below conditions fail:

           -  Validate that the incoming interface on which the
              OAM packet was received matches with the remote
              interface specified in the PeerAdj SID FEC sub-TLV.

    If all above validations have passed, set the return code to 3,
    "Replying router is an egress for the FEC at stack-depth".
    }

Else, if the Target FEC sub-TLV at FEC-stack-depth is 39
     (PeerNode SID sub-TLV), {

    Set the Best-return-code to 10, "Mapping for this FEC is not
    the given label at stack-depth".  If any below conditions
    fail:

       -  Validate that the receiving node's BGP Local AS matches
          with the remote AS field in the received PeerNode SID
          FEC sub-TLV.

       -  Validate that the receiving node's BGP Router-ID matches
          with the Remote Router ID field in the received
          PeerNode SID FEC.

       -  Validate that there is an EBGP session with a peer
          having a local AS number and BGP Router-ID as
          specified in the Local AS number and Local Router-ID
          field in the received PeerNode SID FEC sub-TLV.

    If all above validations have passed, set the return code to 3,
    "Replying router is an egress for the FEC at stack-depth".
    }
Else, if the Target FEC sub-TLV at FEC-stack-depth is 40
     (PeerSet SID sub-TLV), {

    Set the Best-return-code to 10, "Mapping for this FEC is not
    the given label at stack-depth".  If any below conditions
    fail:

       -  Validate that the Receiving Node BGP Local AS matches
          with one of the remote AS fields in the received
          PeerSet SID FEC sub-TLV.

       -  Validate that the Receiving Node BGP Router-ID matches
          with one of the Remote Router ID fields in the
          received PeerSet SID FEC sub-TLV.

       -  Validate that there is an EBGP session with a peer having
          a local AS number and BGP Router-ID as specified in the
          Local AS number and Local Router-ID fields in the received
          PeerSet SID FEC sub-TLV.

    If all above validations have passed, set the return code to 3,
    "Replying router is an egress for the FEC at stack-depth".
    }

6. IANA Considerations

IANA has allocated three new Target FEC stack sub-TLVs in the "Sub-TLVs for TLV Types 1, 16, and 21" registry within the "TLVs" registry of the "Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry group.¶

7. Security Considerations

The EPE-SIDs are advertised for egress links for EPE purposes or for inter-AS links between cooperating ASes. When cooperating domains are involved, they can allow the packets arriving on trusted interfaces to reach the control plane and be processed.¶

When EPE-SIDs are created for egress TE links where the neighbor AS is an independent entity, it may not allow the packets arriving from the external world to reach the control plane. In such deployments, the MPLS OAM packets will be dropped by the neighboring AS that receives the MPLS OAM packet.¶

In MPLS traceroute applications, when the AS boundary is crossed with the EPE-SIDs, the FEC stack is changed. [RFC8287] does not mandate that the initiator, upon receiving an MPLS Echo Reply message that includes the FEC Stack Change TLV with one or more of the original segments being popped, remove the corresponding FEC(s) from the Target FEC Stack TLV in the next (TTL+1) traceroute request.¶

If an initiator does not remove the FECs belonging to the previous AS that has traversed, it may expose the internal AS information to the following AS being traversed in the traceroute.¶

Appendix A. Appendix

This section describes examples of both a correctly and an incorrectly programmed state and provides details on how the new sub-TLVs described in this document can be used to validate the correctness. Consider the diagram from Figure 1.¶

Correctly programmed state:¶

• C assigns label 16001 and binds it to adjacency C->E¶

• C signals that label 16001 is bound to adjacency C->E (e.g., via BGP-LS)¶

• The controller/ingress programs an SR path that has SID/label 16001 to steer the packet on the exit point from C onto adjacency C->E¶

• Using MPLS trace procedures defined in this document, the PeerAdj SID Sub-TLV is populated with entities to be validated by C when the OAM packet reaches it¶

• C receives the OAM packet and validates that the top label (16001) is indeed corresponding to the entities populated in the PeerAdj SID Sub-TLV¶

Incorrectly programmed state:¶

• C assigns label 16001 and binds it to adjacency C->D¶

• The controller learns that PeerAdj SID label 16001 is bound to adjacency C->E (e.g., via BGP-LS) -- this could be a software bug on C or on the controller¶

• The controller/ingress programs an SR path that has SID/label 16001 to steer the packet on the exit point from C onto adjacency C->E¶

• Using MPLS trace procedures defined in this document, the PeerAdj SID Sub-TLV is populated with entities to be validated by C (including a local/remote interface address of C->E) when the OAM packet reaches it¶

• C receives the OAM packet and validates that the top label (16001) is NOT bound to C->E as populated in the PeerAdj SID Sub-TLV and then responds with the respective error code¶

Authors' Addresses