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RTG mpls OAM EPE BGP-LS BGP SPRING SDN SID Egress Peer Engineering (EPE) is an application of Segment Routing (SR) that solves the problem of egress peer selection. The SR-based BGP-EPE solution allows a centralized controller, e.g., a Software-Defined Network (SDN) controller, to program any egress peer. The EPE solution requires the node or the SDN controller to program 1) the PeerNode Segment Identifier (SID) describing a session between two nodes, 2) the PeerAdj SID describing the link or links that are used by the sessions between peer nodes, and 3) the PeerSet SID describing any connected interface to any peer in the related group. This document provides new sub-TLVs for EPE-SIDs that are used in the MPLS Target stack TLV (Type 1) in MPLS Ping and Traceroute procedures.

Introduction Egress Peer Engineering (EPE), as defined in , 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 to choose exit points where the remote AS has a completely different ownership. This scenario is out of scope for this document.

Reference Diagram

In , EPE-SIDs are configured on AS1 towards AS2 and AS3 and advertised in the Border Gateway Protocol - Link State (BGP-LS) . 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 . 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 , 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 and clarified in , are applicable for EPE-SIDs as well.

Theory of Operation provides mechanisms to advertise the EPE-SIDs in BGP-LS. These EPE-SIDs may be used to build SR paths as described in or using Path Computation Element Protocol (PCEP) extensions as defined in . 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 . The extensions in and 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 . 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. 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 may be in force. In these scenarios, if the local and remote AS fields in the FEC (as described in ) carry the globally configured Access Service Network (ASN) and not the "local AS" (as defined in ), then the FEC validation procedures may fail. As described in , this document defines FEC stack TLVs for EPE-SIDs that can be used in detecting MPLS data plane failures . 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.

Requirements Language 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, , when, and only when, they appear in all capitals, as shown here.

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). New Sub-TLV Types

Sub-Type	Sub-TLV Name
38	PeerAdj SID
39	PeerNode SID
40	PeerSet SID

PeerNode SID Sub-TLV

PeerNode SID Sub-TLV

Type:

2 octets

Value:

39

Length:

2 octets

Value:

16

Local AS Number:

4 octets. The unsigned integer representing the AS number of the AS to which the PeerNode SID advertising node belongs. If Confederations are in use, and if the remote node is a member of a different Member-AS within the local Confederation, this is the Member-AS Number inside the Confederation and not the Confederation Identifier.

Remote AS Number:

4 octets. The unsigned integer representing the AS number of the AS of the remote node for which the PeerNode SID is advertised. If Confederations are in use, and if the remote node is a member of a different Member-AS within the local Confederation, this is the Member-AS Number inside the Confederation and not the Confederation Identifier.

Local BGP Router ID:

4 octets. Unsigned integer representing the BGP Identifier of the PeerNode SID advertising node as defined in and .

Remote BGP Router ID:

4 octets. Unsigned integer representing the BGP Identifier of the remote node as defined in and .

When there is a multi-hop EBGP session between two ASBRs, a PeerNode SID is advertised for this session, and traffic can be load-balanced across these interfaces. An EPE controller that performs bandwidth management for these links should be aware of the links on which the traffic will be load-balanced. As per , the node advertising the EPE-SIDs will send a Downstream Detailed Mapping (DDMAP) TLV specifying the details of the next-hop interfaces, e.g, when the OAM packet will be sent out. Based on this information, the controller MAY choose to verify the actual forwarding state with the topology information that the controller has. On the router, the validation procedures will include the received DDMAP validation, as specified in , to verify the control state and the forwarding state synchronization on the two routers. Any discrepancies between the controller's state and the forwarding state will not be detected by the procedures described in this document.

PeerAdj SID Sub-TLV

PeerAdj SID Sub-TLV

Type:

2 octets

Value:

38

Length:

2 octets

Value:

Variable based on the IPv4/IPv6 interface address. Length excludes the length of the Type and Length fields. For IPv4 interface addresses, the length will be 28 octets. In the case of an IPv6 address, the length will be 52 octets.

Adj-Type:

1 octet

Value:

Set to 1 when the Adjacency Segment is IPv4. Set to 2 when the Adjacency Segment is IPv6.

RESERVED:

3 octets. MUST be zero when sending and ignored on receiving.

Local AS Number:

4 octets. The unsigned integer representing the AS number of the AS to which the PeerAdj SID advertising node belongs. If Confederations are in use, and if the remote node is a member of a different Member-AS within the local Confederation, this is the Member-AS Number inside the Confederation and not the Confederation Identifier.

Remote AS Number:

4 octets. The unsigned integer representing the AS number of the remote node's AS for which the PeerAdj SID is advertised. If Confederations are in use, and if the remote node is a member of a different Member-AS within the local Confederation, this is the Member-AS Number inside the Confederation and not the Confederation Identifier.

Local BGP Router ID:

4 octets. The unsigned integer representing the BGP Identifier of the PeerAdj SID advertising node as defined in and .

Remote BGP Router ID:

4 octets. Unsigned integer representing the BGP Identifier of the remote node as defined in and .

Local Interface Address:

4 octets or 16 octets. In the case of PeerAdj SID, the Local interface address corresponding to the PeerAdj SID should be specified in this field. For IPv4, this field is 4 octets; for IPv6, this field is 16 octets. Link-local IPv6 addresses are not in the scope of this document.

Remote Interface Address:

4 octets or 16 octets. In the case of PeerAdj SID, the Remote interface address corresponding to the PeerAdj SID should be specified in this field. For IPv4, this field is 4 octets; for IPv6, this field is 16 octets. Link-local IPv6 addresses are not in the scope of this document.

mandates sending a local interface ID and remote interface ID in the Link Descriptors and allows a value of 0 in the remote descriptors. It is useful to validate the incoming interface for an OAM packet, but if the remote descriptor is 0, this validation is not possible. Optional link descriptors of local and remote interface addresses are allowed as described in . In this document, it is RECOMMENDED to send these optional descriptors and use them to validate incoming interfaces. When these local and remote interface addresses are not available, an ingress node can send 0 in the local and/or remote interface address field. The receiver SHOULD skip the validation for the incoming interface if the address field contains 0.

PeerSet SID Sub-TLV

PeerSet SID Sub-TLV

Type:

2 octets

Value:

40

Length:

2 octets

Value:

Expressed in octets and variable based on the number of elements in the set. The length field does not include the length of Type and Length fields.

Local AS Number:

4 octets. The unsigned integer representing the AS number of the AS to which the PeerSet SID advertising node belongs. If Confederations are in use, and if the remote node is a member of a different Member-AS within the local Confederation, this is the Member-AS Number inside the Confederation and not the Confederation Identifier.

Local BGP Router ID:

4 octets. The unsigned integer representing the BGP Identifier of the PeerSet SID advertising node, as defined in and .

No.of elements in set:

2 octets. The number of remote ASes over which the set SID performs load-balancing.

Reserved:

2 octets. MUST be zero when sent and ignored when received.

Remote AS Number:

4 octets. The unsigned integer representing the AS number of the remote node's AS for which the PeerSet SID is advertised. If Confederations are in use, and if the remote node is a member of a different Member-AS within the local Confederation, this is the Member-AS Number inside the Confederation and not the Confederation Identifier.

Remote BGP Router ID:

4 octets. The unsigned integer representing the BGP Identifier of the remote node as defined in and .

PeerSet SID may be associated with a number of PeerNode SIDs and PeerAdj SIDs. The remote AS number and the Router ID of each of these PeerNode SIDs and PeerAdj SIDs MUST be included in the FEC.

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.

Length Validation

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

EPE-SID FEC Validation Segment Routing IGP-Prefix, IGP-Adjacency SID, and EPE-SID Validation: Receiving node term used in this section implies the node that receives OAM message with the FEC stack TLV.

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. Sub-TLVs for TLV Types 1, 16, and 21 Registry

Sub-Type	Sub-TLV Name
38	PeerAdj SID
39	PeerNode SID
40	PeerSet SID

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. 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.

References Normative References Informative References

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 . 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

Acknowledgments Thanks to , , , , , and for careful reviews and comments. Thanks to for providing the example described in .