RFC 8562

Bidirectional Forwarding Detection (BFD) for Multipoint Networks

📅 published April 2019, from ietf

Bidirectional Forwarding Detection (BFD) for Multipoint Networks 🦋

  • RFC: 8562
  • Stream: Internet Engineering Task Force (IETF)
  • Category: Standards Track
  • Authors: D. Katz (Juniper Networks), D. Ward (Cisco Systems), S. Pallagatti, Ed. (VMware), G. Mirsky, Ed. (ZTE Corp.)
  • Date: April 2019
  • ISSN: 2070-1721
  • Updates: RFC 5880

Abstract ✨

okay so this doc extends the Bidirectional Forwarding Detection (BFD) protocol so it can be used in multipoint and multicast networks 🌸 It also updates RFC 5880, the base BFD spec.

Status of This Memo 📜

This is an Internet Standards Track document 💅

It's a product of the Internet Engineering Task Force (IETF), so it reflects the consensus of the IETF community. It got public review and was approved for publication by the Internet Engineering Steering Group (IESG). If you want more on Internet Standards, check Section 2 of RFC 7841.

For the current status, any errata, or to send feedback, go to https://www.rfc-editor.org/info/rfc8562 🔗

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) as they stand on the publication date. Please actually read these, because they spell out your rights and restrictions for this document. Code Components pulled out of this document must include the Simplified BSD License text, as described in Section 4.e of the Trust Legal Provisions, and they come without warranty, as the Simplified BSD License describes.

Table of Contents 🗺️

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
2.  Keywords  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
3.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
4.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
5.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .   5
  5.1.  Multipoint BFD Control Packets  . . . . . . . . . . . . .   6
  5.2.  Session Model . . . . . . . . . . . . . . . . . . . . . .   6
  5.3.  Session-Failure Semantics . . . . . . . . . . . . . . . .   6
  5.4.  State Variables . . . . . . . . . . . . . . . . . . . . .   6
    5.4.1.  New State Variable Values . . . . . . . . . . . . . .   6
    5.4.2.  State Variable Initialization and Maintenance . . . .   7
  5.5.  State Machine . . . . . . . . . . . . . . . . . . . . . .   7
  5.6.  Session Establishment . . . . . . . . . . . . . . . . . .   8
  5.7.  Discriminators and Packet Demultiplexing  . . . . . . . .   8
  5.8.  Packet Consumption on Tails . . . . . . . . . . . . . . .   9
  5.9.  Bringing Up and Shutting Down Multipoint BFD Service  . .   9
  5.10. Timer Manipulation  . . . . . . . . . . . . . . . . . . .  10
  5.11. Detection Times . . . . . . . . . . . . . . . . . . . . .  10
  5.12. State Maintenance for Down/AdminDown Sessions . . . . . .  11
    5.12.1.  MultipointHead Sessions  . . . . . . . . . . . . . .  11
    5.12.2.  MultipointTail Sessions  . . . . . . . . . . . . . .  11
  5.13. Base Specification Text Replacement . . . . . . . . . . .  11
    5.13.1.  Reception of BFD Control Packets . . . . . . . . . .  12
    5.13.2.  Demultiplexing BFD Control Packets . . . . . . . . .  15
    5.13.3.  Transmitting BFD Control Packets . . . . . . . . . .  16
6.  Congestion Considerations . . . . . . . . . . . . . . . . . .  19
7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
8.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
  9.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
  9.2.  Informative References  . . . . . . . . . . . . . . . . .  22
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  22
Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  22
Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1. Introduction ✨

BFD [RFC5880] is the protocol that checks whether a path is still working between two systems, and it does that by verifying unicast connectivity between a pair of them 💖 This doc updates [RFC5880] with a brand-new way of using BFD. This version gives you multipoint or multicast connectivity checks between a multipoint sender (the "head") and one or more multipoint receivers (the "tails") 🧚

Here's the twist 👀 Multipoint transmissions are inherently one-way, so this mechanism only verifies that one-way connectivity. Yes, that sounds like it clashes with the "Bidirectional" in BFD, but the protocol can totally handle this use case 💅 Running BFD in Demand mode lets a tail keep an eye on a multipoint path even when there's no return path to the head. And if a return path from a tail to the head does exist, the tail can optionally tell the head that multipoint connectivity is gone. The details of that tail-to-head notification are out of scope here and live in [RFC8563] 🦋

This is how tails find out they've lost connectivity from the head. Some apps really need the tail to catch the failure, like using multipoint BFD for fast failure detection and faster failover in multicast VPN, as described in [MVPN-FAILOVER] 🚀 Because the whole thing is one-way, pretty much all the options and timing parameters are set by the head 👑

Quick vocab check: in this doc, "multipoint" means any mechanism where one or more systems receive packets sent by a single sender. That includes stuff like IP multicast and point-to-multipoint MPLS 🌈

And "connectivity" here doesn't mean connectivity verification in a transport network. It's used as a stand-in for "continuity", which just means a forwarding path exists between sender and receiver 🌸

Bottom line: this doc effectively updates and extends the base BFD spec [RFC5880] 💜

2. Keywords 🔑

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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here 🎀

3. Goals 🎯

The main goal is to let tails rapidly detect that multipoint connectivity from the head has failed ⚡

Another goal is for the mechanism to work on any multicast technology 🌍

It also aims to support multiple, overlapping point-to-multipoint paths, plus multipoint-to-multipoint paths. It should let point-to-point BFD sessions run at the same time among the systems taking part in multipoint BFD 🧩

What it isn't trying to do is verify point-to-point bidirectional connectivity between the head and any tail. You can do that separately, with no extra protocol overhead, by using point-to-point BFD 🙅‍♀️

4. Overview 🌊

The heart of this protocol is the periodic sending of BFD Control packets along a multipoint path, from the head out to all the tails on that path 💓 The contents of those packets give the tails what they need to work out the Detection Time for path failure. If a tail gets no BFD Control packets for a whole Detection Time, it declares the path failed 💀 For some apps, that's the only mechanism needed, and the head can stay totally unaware of connectivity to the tails 🙈

A head running a multipoint BFD session might want to know whether its tails are connected (or not). How the head tracks its tails, and how tails report their connectivity back, is out of scope for this doc and is covered in [RFC8563] 📬

This document describes a single head and a set of tails spanned by one multipoint path. But the protocol can support, and tell apart, more than one multipoint path at both heads and tails, as described in Sections 5.7 and 5.13.2 🦄 A head and a tail can also share multiple multipoint paths, and one multipoint path can have multiple heads 🫶

5. Protocol Details 🧁

This section describes how Multipoint BFD works, in detail 🔍

5.1. Multipoint BFD Control Packets 📦

Multipoint BFD Control packets (the ones the head sends over a multipoint path) are explicitly marked as such, by setting the Multipoint (M) bit [RFC5880]. So multipoint BFD doesn't need the recipient to figure out whether a packet came over a multipoint path. That's handy in situations where the recipient can't know that otherwise 🧠

5.2. Session Model 🪞

Multipoint BFD is modeled as a set of sessions, each of a different type. The elements of procedure vary a little for each type 🎭

The head has a session of type MultipointHead, defined in Section 5.4.1, bound to a multipoint path. This session sends Multipoint BFD Control packets over that path, and it never receives any BFD Control packets 📤

Each tail has a session of type MultipointTail, defined in Section 5.4.1, associated with a multipoint path. These sessions receive BFD Control packets from the head over that multipoint path 📥

5.3. Session-Failure Semantics 🧩

The semantics of session failure are subtle enough to deserve a proper explanation 🤓

MultipointHead sessions can't fail, since they're controlled administratively 🫡

If a MultipointTail session fails, it means the tail has definitely lost contact with the head (or the head has been administratively disabled). The tail may then use mechanisms other than BFD, such as logging or NETCONF [RFC6241], to send a notification to the user 📣

5.4. State Variables 🧪

Multipoint BFD introduces some new state variables and changes how a few existing ones are used 🔧

5.4.1. New State Variable Values 🌈

A bunch of new values for the state variable bfd.SessionType get added to the base BFD [RFC5880] and base Seamless Bidirectional Forwarding Detection (S-BFD) [RFC7880] specs to support multipoint BFD 💜

The state variable is bfd.SessionType. Its description says it's the type of this session as defined in [RFC7880]. The newly added values are:

  • PointToPoint: classic point-to-point BFD, as described in [RFC5880] 💫
  • MultipointHead: a session on the head that's responsible for periodically sending multipoint BFD Control packets along the multipoint path 🚀
  • MultipointTail: a multipoint session on a tail 🐚

This variable MUST be initialized to the appropriate type when the session is created 🌟

5.4.2. State Variable Initialization and Maintenance 🎀

okay so some of the state variables from Section 6.8.1 of [RFC5880] need different handling depending on what kind of session you're running 💅

  • bfd.RequiredMinRxInterval MUST be initialized to zero for session type MultipointHead. 🍓
  • bfd.DemandMode MUST be initialized to 1 for session type MultipointHead and MUST be initialized to zero for session type MultipointTail. ✨

5.5. State Machine 🌸

the multipoint state machine is giving slightly different vibes than regular BFD 🦋 since there's a many-to-one mapping, three-way handshakes for setting up and tearing down sessions are neither possible nor appropriate 🙅‍♀️ so there is no Init state. sessions of type MultipointHead MUST NOT send BFD Control packets with the State field set to INIT, and those packets MUST be ignored on receipt 💀

here's a little diagram of the state machine for session type MultipointTail. the notation on each arc is the state of the remote system (as received in the State field in the BFD Control packet) or the expiration of the Detection Timer 🧚

                     DOWN, ADMIN DOWN,
                   +------+  TIMER               +------+
              +----|      |<---------------------|      |----+
         DOWN,|    | DOWN |                      |  UP  |    |UP
   ADMIN DOWN,+--->|      |--------------------->|      |<---+
        TIMER      +------+          UP          +------+

sessions of type MultipointHead never receive packets and have no Detection Timer, so all state transitions are administratively driven 👑

5.6. Session Establishment 💜

unlike point-to-point BFD, multipoint BFD has a discovery mechanism that lets tails find the head 🕵️‍♀️ the bare minimum info needed on both the head and the tails is the binding to the multipoint path BFD is running over. the head transmits multipoint BFD packets on that path, and the tails listen for BFD packets on it. everything else can be figured out dynamically 🫶

a session of type MultipointHead gets created for each multipoint path the head wants to run BFD over. it runs in the Active role, per Section 6.1 of [RFC5880]. except when BFD service is being administratively terminated, this session is always in state Up and always operates in Demand mode. no received packets ever get demultiplexed to the MultipointHead session, so it's a degenerate form of a session, and honestly iconic in its own way 🌈

sessions on the tail MAY be established dynamically, based on receiving a multipoint BFD Control packet from the head, and they're of type MultipointTail. tail sessions always take the Passive role, per Section 6.1 of [RFC5880] ✨

5.7. Discriminators and Packet Demultiplexing 🧩

discriminators work a little differently in multipoint BFD than in point-to-point BFD, so pay attention bestie 👀

the head sends multipoint BFD Control packets over the multipoint path via the MultipointHead session, with My Discriminator set to a value bound to the multipoint path and Your Discriminator set to 0 📮

IP and MPLS multipoint tails MUST demultiplex BFD packets based on a combination of the source address, My Discriminator, and the identity of the multipoint path the multipoint BFD Control packet came in on. together, those uniquely identify the head of the multipoint path 🔍 bootstrapping a BFD session to a multipoint MPLS Label Switched Path (LSP) may use the control plane, e.g., as described in [MVPN-FAILOVER], and that is outside the scope of this document 🙈

heads up: unlike point-to-point sessions, the My Discriminator value on the MultipointHead session MUST NOT be changed during the life of a session. this is a side effect of the more complex demultiplexing scheme, so no changing it mid-session, periodt 🚫💅

5.8. Packet Consumption on Tails 🛑

BFD packets received on tails for an IP multicast group MUST be consumed by tails and MUST NOT be forwarded to receivers 🙅‍♀️ nodes with a BFD session of type MultipointTail MUST identify packets received on an IP multipoint path as a BFD Control packet if the destination UDP port value equals 3784.

for multipoint LSPs, when IP/UDP encapsulation of BFD Control packets is used, MultipointTail MUST expect destination UDP port 3784 🎯 the destination IP address of a BFD Control packet MUST be in the 127.0.0.0/8 range for IPv4 or in the 0:0:0:0:0:FFFF:7F00:0/104 range for IPv6. these destination addresses are consistent with the explanations and usage in [RFC8029], so no surprises there 🦋 packets identified as BFD packets MUST be consumed by MultipointTail and demultiplexed as described in Section 5.13.2. using any other kind of encapsulation for the BFD control message over a multipoint LSP is outside the scope of this document 🌸

5.9. Bringing Up and Shutting Down Multipoint BFD Service 🚦

there's no three-way handshake in multipoint BFD, so a newly started head (one that doesn't have any previous state info available) SHOULD start with bfd.SessionState set to Down, and bfd.RequiredMinRxInterval MUST be set to zero in the MultipointHead session 🌅 the session SHOULD stay in this state for a time equal to (bfd.DesiredMinTxInterval * bfd.DetectMult). this makes sure all the MultipointTail sessions get reset (as long as the restarted head is using the same or a larger value of bfd.DesiredMinTxInterval than it did before) 🔄

multipoint BFD service comes up by administratively setting bfd.SessionState to Up in the MultipointHead session 🎉

the head of a multipoint BFD session may want to shut its BFD service down in a controlled way. this is desirable because the tails don't need to wait for a Detection Time before declaring the multipoint session down (and taking whatever action that calls for) 🫠

to shut a multipoint session down in a controlled way, the head MUST administratively set bfd.SessionState in the MultipointHead session to either Down or AdminDown, and SHOULD set bfd.RequiredMinRxInterval to zero. the session SHOULD keep sending BFD Control packets in this state for a period equal to (bfd.DesiredMinTxInterval * bfd.DetectMult) 📤 alternatively, the head MAY stop transmitting BFD Control packets and not send any more BFD Control packets with the new state (Down or AdminDown). tails will only declare the multipoint session down after the Detection Time interval runs out ⏳

5.10. Timer Manipulation ⏲️

because of the one-to-many mapping, a session of type MultipointHead SHOULD NOT initiate a Poll Sequence in conjunction with timer value changes 🙅‍♀️ however, to indicate a change in the packets, a MultipointHead session MUST send packets with the P bit set 🚩 a MultipointTail session MUST NOT reply if the packet has the M and P bits set and bfd.RequiredMinRxInterval set to zero. since the Poll Sequence isn't used, the tail can't negotiate down MultpointHead's transmit interval 😭 if the value of Desired Min TX Interval in the BFD Control packet received by MultipointTail is too high (that determination may change over time based on the current environment), it must be handled by the implementation and may be controlled by local policy, e.g., close the MultipointTail session 🧯

the MultipointHead, when changing the transmit interval to a higher value, MUST send BFD Control packets with the P bit set at the old transmit interval before using the higher value, so the tails don't get false detection timeouts 💫 a MultipointHead session MAY also wait some amount of time before making changes to the transmit interval (through configuration) ⏰

changing the value of bfd.RequiredMinRxInterval is outside the scope of this document and is discussed in [RFC8563] 📚

5.11. Detection Times ⏱️

multipoint BFD is inherently asymmetric, so each session type has its own approach to Detection Times 🔀

since MultipointHead sessions never receive packets, they don't calculate a Detection Time 🫥

MultipointTail sessions can't influence the transmission rate of the MultipointHead session using the Required Min Rx Interval field, because of its one-to-many nature 🙃 so the Detection Time calculation for a MultipointTail session doesn't use bfd.RequiredMinRxInterval. the Detection Time is the product of the last received values of Desired Min TX Interval and Detect Mult 🧮

the value of bfd.DetectMult may be changed at any time on any session type 🔁

5.12. State Maintenance for Down/AdminDown Sessions 🕰️

how long the session state is kept after the session goes down decides how long the session keeps sending BFD Control packets (since no packets can be sent after the session is destroyed) 💅

5.12.1. MultipointHead Sessions 👑

when a MultipointHead session transitions to Down or AdminDown, the state SHOULD be kept for a period equal to (bfd.DesiredMinTxInterval * bfd.DetectMult) so the tails detect the session going down more quickly (by continuing to transmit BFD Control packets with the new state) 🏃‍♀️💨

5.12.2. MultipointTail Sessions 🧚

MultipointTail sessions MAY be destroyed immediately upon leaving Up state, since the tail transmits no packets 🫧 otherwise, MultipointTail sessions SHOULD be kept around as long as BFD Control packets are being received (which by definition means the head isn't Up) 🌷

5.13. Base Specification Text Replacement 🦋

the following sections are meant to replace the corresponding sections in the base specification [RFC5880], so BFD can support multipoint networks without changing how point-to-point BFD gets processed 🎀✨

5.13.1. Reception of BFD Control Packets 🦋

Okay bestie, this is the main event 💅 This whole procedure completely replaces Section 6.8.6 of [RFC5880], so it takes over from there. Whenever a BFD Control packet shows up, the following procedure MUST be followed, in the order specified. If the packet gets thrown out by any of these rules, processing of the packet MUST cease right there 💀

  1. If the version number is not correct (1), the packet MUST be discarded.
  2. If the Length field is less than the minimum correct value (24 if the A bit is clear, or 26 if the A bit is set), the packet MUST be discarded.
  3. If the Length field is greater than the payload of the encapsulating protocol, the packet MUST be discarded.
  4. If the Detect Mult field is zero, the packet MUST be discarded.
  5. If the My Discriminator field is zero, the packet MUST be discarded.
  6. Demultiplex the packet to a session according to Section 5.13.2. The result is either a session of the proper type, or the packet is discarded (and packet processing MUST cease).
  7. If the A bit is set and no authentication is in use (bfd.AuthType is zero), the packet MUST be discarded.
  8. If the A bit is clear and authentication is in use (bfd.AuthType is nonzero), the packet MUST be discarded.
  9. If the A bit is set, the packet MUST be authenticated under the rules of Section 6.7 of [RFC5880], based on the authentication type in use (bfd.AuthType). This can end with the packet getting discarded.
  10. Set bfd.RemoteDiscr to the value of My Discriminator.
  11. Set bfd.RemoteState to the value of the State (Sta) field.
  12. Set bfd.RemoteDemandMode to the value of the Demand (D) bit.
  13. Set bfd.RemoteMinRxInterval to the value of Required Min RX Interval.
  14. If the Required Min Echo RX Interval field is zero, the transmission of Echo packets, if any, MUST cease.
  15. If a Poll Sequence is being transmitted by the local system and the Final (F) bit in the received packet is set, the Poll Sequence MUST be terminated.
  16. If bfd.SessionType is PointToPoint, update the transmit interval as described in Section 6.8.2 of [RFC5880].
  17. If bfd.SessionType is PointToPoint, update the Detection Time as described in Section 6.8.4 of [RFC5880].
  18. Else:
    1. If bfd.SessionType is MultipointTail, then update the Detection Time as the product of the last received values of Desired Min TX Interval and Detect Mult, as described in Section 5.11 of this specification.
  19. If bfd.SessionState is AdminDown:
    1. Discard the packet.
  20. If the received State is AdminDown:
    1. If bfd.SessionState is not Down:
      1. Set bfd.LocalDiag to 3 (Neighbor signaled session down).
      2. Set bfd.SessionState to Down.
  21. Else:
    1. If bfd.SessionState is Down:
      1. If bfd.SessionType is PointToPoint:
        1. If received State is Down, set bfd.SessionState to Init.
        2. Else if received State is Init, set bfd.SessionState to Up.
      2. Else (bfd.SessionType is not PointToPoint):
        1. If received State is Up, set bfd.SessionState to Up.
    2. Else if bfd.SessionState is Init:
      1. If received State is Init or Up, set bfd.SessionState to Up.
    3. Else (bfd.SessionState is Up):
      1. If received State is Down:
        1. Set bfd.LocalDiag to 3 (Neighbor signaled session down).
        2. Set bfd.SessionState to Down.
  22. Check to see if Demand mode should become active or not (see [RFC5880], Section 6.6).
  23. If bfd.RemoteDemandMode is 1, bfd.SessionState is Up, and bfd.RemoteSessionState is Up, Demand mode is active on the remote system and the local system MUST cease the periodic transmission of BFD Control packets (see Section 5.13.3).
  24. If bfd.RemoteDemandMode is zero, bfd.SessionState is not Up, or bfd.RemoteSessionState is not Up, Demand mode is not active on the remote system and the local system MUST send periodic BFD Control packets (see Section 5.13.3).
  25. If the Poll (P) bit is set, and bfd.SessionType is PointToPoint, send a BFD Control packet to the remote system with the Poll (P) bit clear, and the Final (F) bit set (see Section 5.13.3).
  26. If the packet was not discarded, it has been received for purposes of the Detection Time expiration rules in Section 6.8.4 of [RFC5880].

5.13.2. Demultiplexing BFD Control Packets ✨

This section is part of the replacement for Section 6.8.6 of [RFC5880]. It's split out here just for clarity, because it's a lot to hold in your head at once 🧚

  1. If the Multipoint (M) bit is set:
    1. If the Your Discriminator field is nonzero, the packet MUST be discarded.
    2. Select a session based on the source address, My Discriminator, and the identity of the multipoint path on which the multipoint BFD Control packet was received.
    3. If a session is found, and bfd.SessionType is not MultipointTail, the packet MUST be discarded.
    4. Else:
      1. If a session is not found, a new session of type MultipointTail MAY be created, or the packet MAY be discarded. This choice can be controlled by the local policy, e.g., by setting a maximum number of MultipointTail sessions. Use of the local policy and the exact mechanism of it are outside the scope of this specification.
  2. Else (Multipoint (M) bit is clear):
    1. If the Your Discriminator field is nonzero:
      1. Select a session based on the value of Your Discriminator. If no session is found, the packet MUST be discarded.
    2. Else (Your Discriminator is zero):
      1. If the State field is not Down or AdminDown, the packet MUST be discarded.
      2. Otherwise, the session MUST be selected based on some combination of other fields, possibly including source addressing information, the My Discriminator field, and the interface over which the packet was received. The exact method of selection is application specific and is thus outside the scope of this specification.
      3. If a matching session is found, and bfd.SessionType is not PointToPoint, the packet MUST be discarded.
      4. If a matching session is not found, a new session of type PointToPoint MAY be created, or the packet MAY be discarded. This choice MAY be controlled by a local policy and is outside the scope of this specification.
    3. If the State field is Init and bfd.SessionType is not PointToPoint, the packet MUST be discarded.

Phew, that's the whole replacement, bestie 🫶

5.13.3. Transmitting BFD Control Packets ✨

Okay bestie, this bit replaces Section 6.8.7 of [RFC5880] entirely 💅. With the exceptions listed in the rest of this section, a system MUST NOT transmit BFD Control packets at an interval less than the larger of bfd.DesiredMinTxInterval and bfd.RemoteMinRxInterval, less applied jitter (see below). So basically, the system reporting the slower rate is the one that sets the transmission rate 🐢

The periodic transmission of BFD Control packets MUST be jittered on a per-packet basis by up to 25%. That means the interval MUST be reduced by a random value of 0 to 25%, so systems on the same subnetwork don't accidentally sync up with each other 🌀 The average interval between packets ends up roughly 12.5% less than what was negotiated.

If bfd.DetectMult is equal to 1, the interval between transmitted BFD Control packets MUST be no more than 90% of the negotiated transmission interval and MUST be no less than 75% of the negotiated transmission interval. This is so the remote system's calculated Detection Time doesn't run out before the next BFD Control packet shows up 😭⏰

A system MUST NOT transmit any BFD Control packets if bfd.RemoteDiscr is zero and the system is taking the Passive role 🙈

A system MUST NOT transmit any BFD Control packets if bfd.SessionType is MultipointTail 🚫

A system MUST NOT periodically transmit BFD Control packets if Demand mode is active on the remote system (bfd.RemoteDemandMode is 1, bfd.SessionState is Up, and bfd.RemoteSessionState is Up), and a Poll Sequence is not being transmitted 💤

A system MUST NOT periodically transmit BFD Control packets if bfd.RemoteMinRxInterval is zero 🫠

If bfd.SessionType is MultipointHead, the transmit interval MUST be set to bfd.DesiredMinTxInterval. This should happen automatically, since bfd.RemoteMinRxInterval will be zero ✨

If bfd.SessionType is not MultipointHead, the transmit interval MUST be recalculated whenever bfd.DesiredMinTxInterval changes, or whenever bfd.RemoteMinRxInterval changes, and it is equal to the greater of those two values. For the details on transmit timers, see Sections 6.8.2 and 6.8.3 of [RFC5880] 🕰️

A system MUST NOT set the Demand (D) bit if bfd.SessionType is MultipointTail 🙅‍♀️

A system MUST NOT set the Demand (D) bit if bfd.SessionType is PointToPoint unless bfd.DemandMode is 1, bfd.SessionState is Up, and bfd.RemoteSessionState is Up 🎀

If bfd.SessionType is PointToPoint or MultipointHead, a BFD Control packet SHOULD be transmitted during the interval between periodic Control packet transmissions when its contents would differ from the previously transmitted packet (other than the Poll (P) and Final (F) bits). That's how you quickly tell the other side about a change in state 🚀

The contents of transmitted BFD Control packets MUST be set as follows 📦

  • Version: Set to the current version number (1).
  • Diagnostic (Diag): Set to bfd.LocalDiag.
  • State (Sta): Set to the value indicated by bfd.SessionState.
  • Poll (P): Set to 1 if the local system is sending a Poll Sequence or is a session of type MultipointHead soliciting the identities of the tails, or zero if not.
  • Final (F): Set to 1 if the local system is responding to a BFD Control packet received with the Poll (P) bit set, or zero if not.
  • Control Plane Independent (C): Set to 1 if the local system's BFD implementation is independent of the control plane (it can keep functioning through a disruption of the control plane) 💜
  • Authentication Present (A): Set to 1 if authentication is in use in this session (bfd.AuthType is nonzero), or zero if not.
  • Demand (D): Set to bfd.DemandMode if bfd.SessionState is Up and bfd.RemoteSessionState is Up. Set to 1 if bfd.SessionType is MultipointHead. Otherwise, it is set to zero.
  • Multipoint (M): Set to 1 if bfd.SessionType is MultipointHead. Otherwise, it is set to zero.
  • Detect Mult: Set to bfd.DetectMult.
  • Length: Set to the appropriate length, based on the fixed header length (24) plus any Authentication Section.
  • My Discriminator: Set to bfd.LocalDiscr.
  • Your Discriminator: Set to bfd.RemoteDiscr.
  • Desired Min TX Interval: Set to bfd.DesiredMinTxInterval.
  • Required Min RX Interval: Set to bfd.RequiredMinRxInterval.
  • Required Min Echo RX Interval: Set to zero if bfd.SessionType is MultipointHead or MultipointTail. Otherwise, set to the minimum required Echo packet receive interval for this session. If this field is set to zero, the local system is unwilling or unable to loop back BFD Echo packets to the remote system, and the remote system will not send Echo packets 🔁
  • Authentication Section: Included and set according to the rules in Section 6.7 of [RFC5880] if authentication is in use (bfd.AuthType is nonzero). Otherwise, this section is not present 🔐

6. Congestion Considerations ✨

Okay so first things first, congestion can happen for a bunch of reasons, but high transmission rates can cause it all by themselves, either along the path or at the tail end(s) 🚦 As stated in [RFC5883]:

it is required that the operator correctly provision the rates at which BFD is transmitted to avoid congestion (e.g link, I/O, CPU) and false failure detection.

Using BFD in multipoint networks, as specified in this document, over multiple hops means you have to think about how to react to network congestion 🌊 The requirements in Section 7 of the BFD base specification [RFC5880] apply to BFD in multipoint networks too, and they're repeated here:

When BFD is used across multiple hops, a congestion control mechanism MUST be implemented, and when congestion is detected, the BFD implementation MUST reduce the amount of traffic it generates.

The mechanism that controls the load of BFD traffic MAY use BFD's configuration interface to control the BFD state variable bfd.DesiredMinTxInterval. However, that control loop isn't part of the BFD protocol itself, so its specification is outside the scope of this document 🎛️

There are extra considerations for BFD in multipoint networks as specified in this document 👀 Because a tail doesn't send any BFD Control packets to the head of the BFD session, the head node has no BFD-based mechanism and isn't aware of the session's state at the tail. Without any other mechanism, the head could keep sending packets toward the tail(s) even after a link failure has happened 😱 If the head of the session needs to know the state of the tail, it is RECOMMENDED to implement the extension described in [RFC8563] 🧠

7. IANA Considerations 🌸

This document has no IANA actions.

8. Security Considerations 💀

The same security considerations as those described in [RFC5880] apply to this document. On top of that, implementations that create MultpointTail sessions dynamically when they receive multipoint BFD Control packets MUST implement protective measures to prevent an infinite number of MultipointTail sessions from being created. Here are some points to think about for those implementations 🚨

  • If a multipoint BFD Control packet did not arrive on a multicast path (e.g., on the expected interface, with the expected MPLS label, etc.), a MultipointTail session should not be created.
  • If redundant streams are expected for a given multicast stream, the implementations should not create more MultipointTail sessions than the number of streams. Additionally, when the number of MultipointTail sessions exceeds the number of expected streams, the implementation should generate an alarm to users to indicate the anomaly.
  • The implementation should have a reasonable upper bound on the number of MultipointHead sessions that can be created, with the upper bound potentially being computed based on the load these would generate.
  • The implementation should have a reasonable upper bound on the number of MultipointTail sessions that can be created, with the upper bound potentially being computed based on the number of multicast streams that the system is expecting.

If authentication is in use, the head and all tails may be configured to have a common authentication key so the tails can validate multipoint BFD Control packets 🔑

Shared keys in multipoint scenarios let any tail spoof the head from the viewpoint of any other tail. For this reason, using shared keys to authenticate BFD Control packets in multipoint scenarios is a significant security exposure, unless all tails can be trusted not to spoof the head 😱 Otherwise, asymmetric message authentication would be needed, e.g., protocols that use Timed Efficient Stream Loss-Tolerant Authentication (TESLA) as described in [RFC4082]. Applying asymmetric message authentication to BFD for multipoint networks is outside the scope of this specification and is for further study 🔮

9. References ✨

9.1. Normative References

9.2. Informative References 📚

okay so these are the extra reading picks, bestie. they're not required to implement the spec, but they're the sources the spec points to for more context 🧚

[MVPN-FAILOVER]
           Morin, T., Ed., Kebler, R., Ed., and G. Mirsky, Ed.,
           "Multicast VPN fast upstream failover", Work in Progress,
           draft-ietf-bess-mvpn-fast-failover-05, February 2019.

[RFC4082]  Perrig, A., Song, D., Canetti, R., Tygar, J., and
           B. Briscoe, "Timed Efficient Stream Loss-Tolerant
           Authentication (TESLA): Multicast Source Authentication
           Transform Introduction", RFC 4082, DOI 10.17487/RFC4082,
           June 2005, <https://www.rfc-editor.org/info/rfc4082>.

[RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
           (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
           June 2010, <https://www.rfc-editor.org/info/rfc5883>.

[RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
           and A. Bierman, Ed., "Network Configuration Protocol
           (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
           <https://www.rfc-editor.org/info/rfc6241>.

[RFC8563]  Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky,
           Ed., "Bidirectional Forwarding Detection (BFD) Multipoint
           Active Tails", RFC 8563, DOI 10.17487/RFC8563, April 2019,
           <https://www.rfc-editor.org/info/rfc8563>.

Acknowledgments 🫶

huge thank-you hugs to Nobo Akiya, Vengada Prasad Govindan, Jeff Haas, Wim Henderickx, Gregory Mirsky, and Mingui Zhang 💖 they all seriously levelled up this document, no cap ✨

Contributors 🌸

Rahul Aggarwal of Juniper Networks and George Swallow of Cisco Systems came up with the initial idea for this spec and helped build it out 👑

Authors' Addresses 🎀

the people who wrote this, with their contact info, lowkey iconic 💅

Dave Katz
Juniper Networks
1194 N. Mathilda Ave.
Sunnyvale, California  94089-1206
United States of America

Email: dkatz@juniper.net


Dave Ward
Cisco Systems
170 West Tasman Dr.
San Jose, California  95134
United States of America

Email: wardd@cisco.com


Santosh Pallagatti (editor)
VMware

Email: santosh.pallagatti@gmail.com


Greg Mirsky (editor)
ZTE Corp.

Email: gregimirsky@gmail.com

🦋 and that's the whole thing, bestie. you did it 🎉

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