Bidirectional Forwarding Detection (BFD) is a network protocol used to detect faults between two forwarding engines. It provides low-overhead, low-latency detection of faults even on physical media that don't support failure detection of any kind, such as ethernet, virtual circuits, tunnels and MPLS LSPs. It has been suggested that IEEE 802. ... A virtual circuit (VC) is a communications arrangement in which data from a source user may be passed to a destination user over more than one real circuit configuration during a single period of communication, but the switching is hidden from the users. ... A tunneling protocol is a network protocol which encapsulates one protocol or session inside another. ... In computer networking and telecommunications, Multiprotocol Label Switching (MPLS) is a data-carrying mechanism, operating at a layer Layer3 below and in parallel to IP. It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching clients which provide a datagram service model. ... In MPLS networking, a Label Switched Path (LSP) is a path through an MPLS network, set up by a signaling protocol such as LDP, RSVP-TE, or CR-LDP. The path is set up based on criteria in the forwarding equivalence class (FEC). ...

BFD establishes a session between two endpoints over a particular link. If more than one link exists between two systems, multiple BFD sessions may be established to monitor each one of them. The session is established with a three-way handshake, and is torn down the same way. Authentication may be enabled on the session. A choice of simple password, MD5 or SHA1 authentication is available.

BFD does not have a discovery mechanism; sessions must be explicitly configured between endpoints. BFD may be used on many different underlying transport mechanisms and layers, and operates independently of all of these. Therefore, it needs to be encapsulated by whatever transport it uses. For example, monitoring MPLS LSPs involves piggybacking session establishment on LSP-Ping packets. Protocols that support some form of adjacency setup, such as OSPF or IS-IS, may also be used to bootstrap a BFD session. These protocols may then use BFD to receive faster notification of failing links than would normally be possible using the protocol's own keepalive mechanism. Open Shortest Path First (OSPF) is a link-state, hierarchical Interior Gateway Protocol (IGP) routing protocol. ... Intermediate system to intermediate system (IS-IS), is an IGP routing protocol originally designed for CLNS as part of the OSI protocol stack and described in ISO 10589 . ... Keepalive is a message sent by one device to another to check that the link between the two is operating. ...

A session may operate in one of two modes: asynchronous mode and demand mode. In asynchronous mode, both endpoints periodically send Hello packets to each other. If a number of those packets are not received, the session is considered down.

In demand mode, no Hello packets are exchanged after the session is established; it is assumed that the endpoints have another way to verify connectivity to each other, perhaps on the underlying physical layer. However, either host may still send Hello packets if needed.

Regardless of which mode is in use, either endpoint may also initiate an Echo function. When this function is active, a stream of Echo packets is sent, and the other endpoint then sends these back to the sender via its forwarding plane. This is used to test the forwarding path on the remote system.

Standards

The BFD protocol is standardized in an IETF working group. As of January 2006, Internet Drafts have been produced defining the BFD protocol, detecting MPLS LSP failure, using BFD to monitor connectivity across multiple network hops, and using BFD for IPv4 and IPv6. This last draft also defines using BFD in conjunction with OSPF and IS-IS. Wikipedia does not have an article with this exact name. ... Internet Drafts (IDs) are a series of documents published by the IETF. They are drafts for RFCs. ... IPv4 is version 4 of the Internet Protocol (IP). ... Internet Protocol version 6 (IPv6) is a network layer standard used by electronic devices to exchange data across a packet-switched internetwork. ... Open Shortest Path First (OSPF) is a link-state, hierarchical Interior Gateway Protocol (IGP) routing protocol. ... Intermediate system to intermediate system (IS-IS), is an IGP routing protocol originally designed for CLNS as part of the OSI protocol stack and described in ISO 10589 . ...

These drafts are expected to be submitted to the IESG, and subsequently released as RFCs. In internetworking and computer network engineering, Request for Comments (RFC) documents are a series of memoranda encompassing new research, innovations, and methodologies applicable to Internet technologies. ...

External links

• IETF BFD Working Group
• BFD presentation by Juniper Networks