Bidirectional Forwarding Detection (BFD) Directed Return Path

RFC 5880 , RFC 5881 , and RFC 5883 established the BFD protocol for IP networks and RFC 5884 set rules of using BFD asynchronous mode over IP/MPLS LSPs. These four standards implicitly assume that the egress BFD peer will use the shortest path route regardless of route being used to send BFD control packets towards it. For the case where an LSP is explicitly routed, if it is desired that BFD control packets follow the same path in the reverse direction (for support of common fault detection for explicitly routed bidirectional co-routed LSPs, for example), it is likely that the shortest return path to the ingress BFD peer may not follow the same path as the LSP in the forward direction. The fact that BFD control packets are not guaranteed to cross the same links and nodes in both forward and reverse directions is a significant factor in producing false positive defect notifications, i.e. false alarms, if used by the ingress BFD peer to deduce the state of the forward direction. This document defines the BFD Reverse Path TLV as an extension to LSP Ping and proposes that it to be used to instruct the egress BFD peer to use explicit path for its BFD control packets associated with the particular BFD session. The TLV will be allocated from the TLV and sub-TLV registry defined by RFC 4379 . As a special case, forward and reverse directions of the BFD session can form a bi-directional co-routed associated channel.

BFD: Bidirectional Forwarding Detection MPLS: Multiprotocol Label Switching LSP: Label Switching Path LoC: Loss of Continuity

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 .

BFD is best suited to monitor bi-directional co-routed paths. In most cases, given stable environments, the forward and reverse directions between two nodes are likely to be co-routed. If BFD is used to monitor unidirectional explicitly routed path, e.g. MPLS-TE LSP, BFD control packets in forward direction would be in-band using the mechanism defined in and . But the reverse direction of the BFD session would still follow the shortest path route and that might lead to the following problem in detecting failures on a unidirectional explicit path: a failure detection by ingress node on the reverse path cannot be interpreted as bi-directional failure with all the certainty and thus trigger, for example, protection switchover of the forward direction without possibility of being a false positive defect notification. To address this scenario the egress BFD peer should be instructed to use a specific path for BFD control packets.

LSP ping, defined in , uses BFD Discriminator TLV to bootstrap a BFD session over an MPLS LSP. This document defines a new TLV, BFD Reverse Path TLV, that MUST contain a single sub-TLV that can be used to carry information about the reverse path for the BFD session that is specified by value in BFD Discriminator TLV.

The BFD Reverse Path TLV is an optional TLV within the LSP ping protocol. However, if used, the BFD Discriminator TLV MUST be included in an Echo Request message as well. If the BFD Discriminator TLV is not present when the BFD Reverse Path TLV is included, then it MUST be treated as malformed Echo Request, as described in . The BFD Reverse Path TLV carries information about the path onto which the egress BFD peer of the BFD session referenced by the BFD Discriminator TLV MUST transmit BFD control packets. The format of the BFD Reverse Path TLV is as presented in .

BFD Reverse Path TLV Type is 2 octets in length and has a value of TB1 (to be assigned by IANA as requested in ). Length field is 2 octets long and defines the length in octets of the Reverse Path field. Reverse Path field contains a sub-TLV. Any Target FEC sub-TLV (already defined, or to be defined in the future) for TLV Types 1, 16, and 21 of MPLS LSP Ping Parameters registry MAY be used in this field. Exactly one sub-TLV MUST be included in the Reverse Path TLV. If more than one 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" . If the egress LSR cannot find the path specified in the Reverse Path TLV it MUST send Echo Reply with the received Reverse Path TLV and set the Return Code to "Failed to establish the BFD session. The specified reverse path was not found" . The egress BFD peer MAY establish the BFD session over IP network as defined in .

When an explicit path on an MPLS data plane is set either as Static or RSVP-TE LSP respective sub-TLVs defined in MAY be used to identify the explicit reverse path for the BFD session.

In addition to Static and RSVP-TE, Segment Routing with MPLS data plane can be used to set an explicit path. In this case a new sub-TLV is defined in this document as presented in .

The Segment Routing Tunnel sub-TLV Type is two octets in length, and has a value of TB2 (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. The Segment Routing Tunnel sub-TLV MAY be used in Reply Path TLV defined 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 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. " When LSP ping is used to bootstrap BFD session this document updates the statement and defines that LSP Ping MUST include the 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.

This document defines the following Return Codes for MPLS LSP Echo Reply: "Too Many TLVs Detected", (TBD3). When more than one Reverse Path TLV found in the received Echo Request by the egress BFD peer, an Echo Reply with the return code set to "Too Many TLVs Detected" MUST be sent to the ingress BFD peer . "Failed to establish the BFD session. The specified reverse path was not found", (TBD4). When a specified reverse path is not available at the egress BFD peer, an Echo Reply with the return code set to "Failed to establish the BFD session. The specified reverse path was not found" MUST be sent back to the ingress BFD peer .

In the network presented in node A monitors two tunnels to node H: A-B-C-D-G-H and A-B-E-F-G-H. To bootstrap a BFD session to monitor the first tunnel, node A MUST include a BFD Discriminator TLV with Discriminator value (e.g. foobar-1) and MAY include a BFD Reverse Path TLV that references H-G-D-C-B-A tunnel. To bootstrap a BFD session to monitor the second tunnel, node A MUST include a BFD Discriminator TLV with a different Discriminator value (e.g. foobar-2) and MAY include a BFD Reverse Path TLV that references H-G-F-E-B-A tunnel.

If an operator needs node H to monitor a path to node A, e.g. H-G-D-C-B-A tunnel, then by looking up list of known Reverse Paths it MAY find and use the existing BFD session.

The IANA is requested to assign a new value for BFD Reverse Path TLV from the "Multiprotocol Label Switching Architecture (MPLS) Label Switched Paths (LSPs) Ping Parameters - TLVs" registry, "TLVs and sub-TLVs" sub-registry. Value Description Reference X (TBD1) BFD Reverse Path TLV This document

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 (TBD2) Segment Routing MPLS Tunnel sub-TLV This document

The IANA is requested to 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 X (TBD4) Failed to establish the BFD session. The specified reverse path was not found. This document

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

Authors greatly appreciate thorough review and the most helpful comments from Eric Gray.