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

, , and defined the operation of Bidirectional Forwarding Detection (BFD) protocol between the two systems over IP networks. and set rules for using BFD Asynchronous mode over point-to-point (p2p) Multiprotocol Label Switching (MPLS) Label Switched Path (LSP). These latter standards implicitly assume that the remote BFD system, which is at the egress Label Edge Router (LER), will use the shortest path route regardless of the path the BFD system at the ingress LER uses to send BFD Control packets towards it. Throughourt this document references to ingress LER and egrees LER are used, respectively, as shortened version of "BFD system at the ingress/egress LER". This document defines the use of LSP Ping for Segment Routing networks over MPLS data plane to bootstrap and control path of a BFD session from the egress to ingress LER using Segment Routing tunnel with MPLS data plane (SR-MPLS).

BFD: Bidirectional Forwarding Detection FEC: Forwarding Equivalence Class MPLS: Multiprotocol Label Switching SR-MPLS Segment Routing with MPLS data plane LSP: Label Switched Path LER Label Edge Router p2p Point-to-point p2mp Point-to-multipoint SID Segment Identifier SR Segment Routing

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.

Use of an LSP Ping to bootstrap BFD over MPLS LSP is required, as documented in , to establish an association between a fault detection message, i.e., BFD Control message, and the Forwarding Equivalency Class (FEC) of a single label stack LSP in case of Penultimate Hop Popping or when the egress LER distributes the Explicit NULL label to the penultimate hop router. The Explicit NULL label is not advertised as a Segment Identifier (SID) by an SR node but, as demonstrated in section 3.1 if the operation at the penultimate hop is NEXT; then the egress SR node will receive an IP encapsulated packet. Thus the conclusion is that LSP Ping MUST be used to bootstrap a BFD session in SR-MPLS domain. As demonstrated in , the introduction of Segment Routing network domains with an MPLS data plane requires three new sub-TLVs that MAY be used with Target 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: The initiator, i.e., ingress LER, MUST include FEC(s) corresponding to the destination segment. The initiator MAY include FECs corresponding to some or all of segments imposed in the label stack by the ingress LER 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 multiple BFD sessions for the same <MPLS LSP, FEC> tuple. Because only the ingress LER is aware of the SR-based explicit route, the egress LER can associate the LSP ping with BFD Discriminator TLV with only one of the FECs it advertised for the particular segment. Thus this document clarifies that: When LSP Ping is used to bootstrapping a BFD session for SR-MPLS tunnel the FEC corresponding to the segment to be associated with the BFD session MUST be as the very last sub-TLV in the Target FEC TLV. If the target segment is an anycast prefix segment () the corresponding Anycast SID MUST be included in the Target TLV as the very last sub-TLV. Also, for BFD Control packet the ingress SR node MUST use precisely the same label stack encapsulation, especially Entropy Label (), as for the LSP ping with the BFD Discriminator TLV that bootstrapped the BFD session. Other operational aspects of using BFD to monitor the continuity of the path to the particular Anycast SID, advertised by a group of SR-MPLS capable nodes, will be considered in the future versions of the document. Encapsulation of a BFD Control packet in Segment Routing network with MPLS data plane MUST follow Section 7 when the IP/UDP header used and MUST follow Section 3.4 without IP/UDP header being used.

For BFD over MPLS LSP case, per , egress LER MAY send BFD Control packet to the ingress LER either over IP network or an MPLS LSP. Similarly, for the case of BFD over p2p SR-MPLS tunnel, the egress LER MAY route BFD Control packet over the IP network, as described in , or transmit over a segment tunnel, as described in Section 7 . In some cases, there may be a need to direct egress LER to use a specific path for the reverse direction of the BFD session by using the BFD Reverse Path TLV and following all procedures as defined in .

For the case of MPLS data plane, 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." YANG Data Model for MPLS Static LSPs models outgoing MPLS labels to be imposed as leaf-list , i.e., as array of rt-types:mpls-label . This document defines new optional Non-FEC Path TLV. The format of the Non-FEC Path TLV is presented in

Non-FEC Path TLV Type is two octets in length and has a value of TBD1 (to be assigned by IANA as requested in ). Length field is two octets long and defines the length in octets of the Non-FEC Path field. Non-FEC Path field contains a sub-TLV. Any Non-FEC Path sub-TLV (defined in this document or to be defined in the future) for Non-FEC Path TLV type MAY be used in this field. None or one sub-TLV MAY be included in the Non-FEC Path TLV. If no sub-TLV has been found in the Non-FEC Path TLV, the egress LER MUST revert to using the reverse path selected based on its local policy. If there is more than one sub-TLV, then the Return Code in echo reply MUST be set to value TBD3 "Too Many TLVs Detected" (to be assigned by IANA as requested in ). Non-FEC Path TLV MAY be used to specify the reverse path of the BFD session identified in the BFD Discriminator TLV. If the Non-FEC Path TLV is present in the echo request message the BFD Discriminator TLV MUST be present as well. If the BFD Discriminator TLV is absent when the Non-FEC Path TLV is included, then it MUST be treated as malformed Echo Request, as described in . This document defines Static Routing MPLS Tunnel sub-TLV that MAY be used with the Non-FEC Path TLV. The format of the sub-TLV is presented in .

The Segment Routing MPLS Tunnel sub-TLV Type is two octets in length, and has a value of TBD2 (to be assigned by IANA as requested in ). The egress LER 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. The Non-FEC Path TLV and Segment Routing MPLS Tunnel sub-TLV MAY be used in Reply Path TLV defined in , particularly to ensure end-to-end veryfication of multi-AS SR tunnel, as described in .

When Segment Routed domain with MPLS data plane uses distributed tunnel computation BFD Reverse Path TLV MAY use Target FEC sub-TLVs defined in .

defines how Demand mode of BFD, specified in sections 6.6 and 6.18.4 of , can be used to monitor uni-directional MPLS LSP. Similar procedures can be following in SR-MPLS to monitor uni-directional SR tunnels: an ingress SR node bootstraps BFD session over SR-MPLS in Async BFD mode; once BFD session is Up, the ingress SR node switches the egress LER into the Demand mode by setting D field in BFD Control packet it transmits; if the egress LER detects the failure of the BFD session, it sends its BFD Control packet to the ingress SR node over the IP network with a Poll sequence; if the ingress SR node receives a BFD Control packet from the remote node in a Demand mode with Poll sequence and Diag field indicating the failure, the ingress SR node transmits BFD Control packet with Final over IP and switches the BFD over SR-MPLS back into Async mode, sending BFD Control packets one per second.

defined variants of SR Policy to deliver point-to-multipoint (p2mp) services. For the given P2MP segment can be used if, for example, leaves have an alternative source of the multicast service flow to select. In such a scenario, a leaf may switch to using the alternative flow after p2mp BFD detects the failure in the working multicast path. For scenarios where it is required for the root to monitor the state of the multicast tree can be used. The root may use the detection of the failure of the multicast tree to the particular leaf to restore the path for that leaf or re-instantiate the whole multicast tree. An essential part of using p2mp BFD is the bootstrapping the BFD session at all the leaves. The root, acting as the MultipointHead, MAY use LSP Ping with the BFD Discriminator TLV. Alternatively, extensions to routing protocols, e.g., BGP, or management plane, e.g., PCEP, MAY be used to associate the particular P2MP segment with MultipointHead's Discriminator. Extensions for routing protocols and management plane are for further study.

IANA is requested to assign new TLV type from the from Standards Action range of the registry "Multiprotocol Label Switching Architecture (MPLS) Label Switched Paths (LSPs) Ping Parameters - TLVs" as defined in . Value TLV Name Reference TBD1 Non-FEC Path TLV This document IANA is requested to create new Non-FEC Path sub-TLV registry for the Non-FEC Path TLV, as described in . Range Registration Procedures Note 0-16383 Standards Action This range is for mandatory TLVs or for optional TLVs that require an error message if not recognized. 16384-31743 Specification Required Experimental RFC needed 32768-49161 Standards Action This range is for optional TLVs that can be silently dropped if not recognized. 49162-64511 Specification Required Experimental RFC needed 64512-65535 Private Use IANA is requested to allocate the following values from the Non-FEC Path sub-TLV registry as defined in . Value Description Reference 0 Reserved This document TBD2 Segment Routing MPLS Tunnel sub-TLV This document 65535 Reserved This document

IANA is requested to create Non-FEC Path sub-TLV sub-registry for the new Non-FEC Path TLV and assign a new Return Code value from the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters" registry, "Return Codes" sub-registry, as follows using a Standards Action value. Value Description Reference X TBD3 Too Many TLVs Detected. This document

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

TBD