Bidirectional Forwarding Detection (BFD) in Segment Routing Networks Using MPLS Dataplane

, , and established the Bidirectional Forwarding Detection (BFD) protocol for IP networks. and set rules of using BFD Asynchronous mode over Multiprotocol Label Switching (MPLS) Label Switched Path (LSP). These latter standards implicitly assume that the egress BFD peer, which is the egress Label Edge Router (LER), will use the shortest path route regardless of the path the ingress LER uses to send BFD control packets towards it. This document defines use of LSP Ping for Segment Routing networks over MPLS dataplane to bootstrap and control path of a BFD session from the egress to ingress LER.

BFD: Bidirectional Forwarding Detection FEC: Forwarding Equivalence Class MPLS: Multiprotocol Label Switching LSP: Label Switching Path LER: Label Edge Router

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 .

As discussed in introduction of Segment Routing network domains with an MPLS data plane adds three new sub-TLVs that MAY be used with Target Forwarding Equivalence Class (FEC) TLV. Section 6.1 addresses use of the new sub-TLVs in Target FEC TLV in LSP ping and LSP traceroute. For the case of LSP ping the states that: Initiator MUST include FEC(s) corresponding to the destination segment. Initiator, i.e. ingress LSR, MAY include FECs corresponding to some or all of segments imposed in the label stack by the ingress LSR to communicate the segments traversed. It has been noted in that a BFD session monitors for defects particular <MPLS LSP, FEC> tuple. clarified how to establish and operate mutiple BFD sessions for the same <MPLS LSP, FEC> tuple. Because only ingress edge router is aware of the SR-based explicit route egress edge router can associate the LSP ping with BFD Discriminator TLV with only one of the FECs it advertised for the particular segment. Thus this document defines that: When LSP ping is used to bootstrap a BFD session this document updates the statement and defines that: When LSP Ping is used to bootstrap a BFD session it MUST include only one FEC corresponding to the destination segment and SHOULD NOT include FECs corresponding to some or all of other segments imposed by the ingress LSR. Operationally such restriction would not cause any problem or uncertainty as LSP ping with FECs corresponding to some or all segments or traceroute that validate the segment route MAY precede the LSP ping that bootstraps the BFD session. Encapsulation of a BFD Control packet in Segment Routing network with MPLS dataplane MUST follow Section 7 when IP/UDP header used and MUST follow Section 3.4 without IP/UDP header being used.

When a BFD session is used to monitor a source routed unidirectional path there may be a need to direct egress BFD peer to use specific path for the reverse direction of the BFD session by using the BFD Reverse Path TLV . For the case of MPLS dataplane, Segment Routing Architecture explains that "a segment is encoded as an MPLS label. An ordered list of segments is encoded as a stack of labels." Following on that this document defines Segment Routing with MPLS dataplane sub-TLV that MAY be used with the BFD Reverse Path TLV . The format of the sub-TLV is presented in .

The Segment Routing Tunnel sub-TLV Type is two octets in length, and has a value of TBD (to be assigned by IANA as requested in ). The egress LSR MUST use the Value field as label stack for BFD control packets for the BFD session identified by the source IP address of the MPLS LSP Ping packet and the value in the BFD Discriminator TLV. Label Entries MUST be in network order. Exactly one Segment Routing Tunnel sub-TLV MUST be included in the Reverse Path TLV. If more than one Segment Routing Tunnel sub-TLV is present in the Reverse Path TLV, then, in order to avoid ambiguity of which of TLVs to use, the egress BFD peer MUST send Echo Reply with the received Reverse Path TLVs and set the Return Code to "Too Many TLVs Detected" The Segment Routing Tunnel sub-TLV MAY be used in Reply Path TLV defined in

The IANA is requested to assign new sub-TLV type from "Multiprotocol Label Switching Architecture (MPLS) Label Switched Paths (LSPs) Ping Parameters - TLVs" registry, "Sub-TLVs for TLV Types 1, 16, and 21" sub-registry. Value Description Reference X (TBD) Segment Routing MPLS Tunnel sub-TLV This document

Security considerations discussed in , , , and apply to this document.

TBD