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diff --git a/doc/developer/sbfd.rst b/doc/developer/sbfd.rst new file mode 100644 index 0000000000..7bbd2428dd --- /dev/null +++ b/doc/developer/sbfd.rst @@ -0,0 +1,140 @@ +.. _sbfd: + +**** +SBFD +**** + +:abbr:`SBFD (Seamless Bidirectional Forwarding Detection)` is: + + Seamless Bidirectional Forwarding Detection, a simplified mechanism for using BFD with a large + proportion of negotiation aspects eliminated, thus providing benefits + such as quick provisioning, as well as improved control and + flexibility for network nodes initiating path monitoring. + + -- :rfc:`7880` + +It is described and extended by the following RFCs: + +* :rfc:`7880` +* :rfc:`7881` + +.. _sbfd-sate-machine: + +SBFD state machine +================== + +SBFD takes the same data packet format as BFD, but with a much simpler state machine. +According to RFC7880, SBFD has a stateless SBFDReflector and a stateful SBFDInitiator with the state machine as below: + +:: + + +--+ + ADMIN DOWN, | | + TIMER | V + +------+ UP +------+ + | |-------------------->| |----+ + | DOWN | | UP | | UP + | |<--------------------| |<---+ + +------+ ADMIN DOWN, +------+ + TIMER + + Figure 1: SBFDInitiator Finite State Machine + +.. _sbfd-extention: + +SBFD extension - SRv6 encapsulation +=================================== + +SBFDInitiator periodically send packets to monitor the connection to SBFDReflector. We set up an SBFD connection between the source and the destination node of a path, +with the source node serving as Initiator and the destination node as Reflector. The communicated SBFD packets should also follow every exact hop in the path, +from the source to the destination, which could be achieved by segment routing. This requirement extends the node verification to the path verification. + +.. _sbfd-implement: + +implementation +=============== + +Some considerations when implementing sbfd. + + + +.. _sbfd-implement-coexist: + +SBFD Co-exist with BFD +-------------------------- + +Both SBFD and Classical BFD have their unique discriminator, SBFD can co-exist with BFD since they sharing a same discriminator pool in bfdd. +Also in bfdd SBFD and BFD can share most code logic, SBFD packet and BFD packet are demultiplexed by different discriminators. + + +.. _sbfd-implement-bfdname: + +SBFD name +--------- + +We introduced a bfd-name for every sbfd session. A unique bfd-name can be used to identify a sbfd session quickly. This is quite useful in our Srv6 deployment for path protection case. +A bfd-name is always associated with a TE path, for example if we use the sbfd session to protect the path A-B-D, we would assign the name 'path-a-b-d' or 'a-b-d' to the session. + +Meanwhile bfdd will notify the sbfd status to the Pathd, we should add the bfd-name field in PTM bfd notify message ZEBRA_BFD_DEST_REPLAY: + +:: + * Message format: + * - header: command, vrf + * - l: interface index + * - c: family + * - AF_INET: + * - 4 bytes: ipv4 + * - AF_INET6: + * - 16 bytes: ipv6 + * - c: prefix length + * - l: bfd status + * - c: family + * - AF_INET: + * - 4 bytes: ipv4 + * - AF_INET6: + * - 16 bytes: ipv6 + * - c: prefix length + * - c: cbit + * - c: bfd name len <---- new field + * - Xbytes: bfd name <---- new field + * + * Commands: ZEBRA_BFD_DEST_REPLAY + * + * q(64), l(32), w(16), c(8) + + + +.. _sbfd-implement-port: + +SBFD UDP port +------------- + +According to RFC7881, SBFD Control packet dst port should be 7784, src port can be any but NOT 7784. In our implementation, the UDP ports in packet are set as: + + +:: + UDP(sport=4784, dport=7784)/BFD() or UDP(sport=3784, dport=7784)/BFD() + +if "multihop" is specified for sbfd initiator we choose the 4784 as the source port, so the reflected packet will take 4784 as the dst port, this is a local BFD_MULTI_HOP_PORT so the reflected packet can be handled by the existing bfd_recv_cb function. +if "multihop" is not specified for sbfd initiator we choose the 3784 as the source port, this is a local BFD_DEFDESTPORT so the reflected packet can be handled by the existing bfd_recv_cb function. + + +For echo SBFD with SRv6 encapsulation case, we re-use the BFD Echo port, the UDP ports in packet are set as: + +:: + UDP(sport=3785, dport=3785)/BFD() + + +we choose the 3785 as the source port, so the echo back packet will take 3785 as the dst port, this is a local BFD_DEF_ECHO_PORT so the packet can be handled by the existing bfd_recv_cb function. + + +.. _sbfd-not-implemented: + +Todo list for SBFD +------------------ + + Currently some features are not yet implemented for SBFD, will add it in future: + 1) SBFD in IPv4 only packet + 2) The ADMIN DOWN logic + 3) SBFD echo function in a initiator session + 4) SBFD over MPLS |