| Internet Engineering Task Force | D. Katz |
| Internet-Draft | Juniper Networks |
| Intended status: Standards Track | D. Ward |
| Expires: June 14, 2018 | Cisco Systems |
| S. Pallagatti, Ed. | |
| Individual contributor | |
| December 11, 2017 |
BFD for Multipoint Networks
draft-ietf-bfd-multipoint-11
This document describes extensions to the Bidirectional Forwarding Detection (BFD) protocol for its use in multipoint and multicast networks. Comments on this draft should be directed to rtg-bfd@ietf.org.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on June 14, 2018.
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The Bidirectional Forwarding Detection protocol [RFC5880] specifies a method for verifying unicast connectivity between a pair of systems. This document defines a method for using BFD to provide verification of multipoint or multicast connectivity between a multipoint sender (the "head") and a set of one or more multipoint receivers (the "tails").
As multipoint transmissions are inherently unidirectional, this mechanism purports only to verify this unidirectional connectivity. Although this seems in conflict with the "Bidirectional" in BFD, it is a natural fit for that protocol.
This application of BFD allows for the tails to detect a lack of connectivity from the head. Due to unidirectional nature, virtually all options and timing parameters are controlled by the head.
As an option, the tail may notify the head of the lack of multipoint connectivity. Details of tail notification to head are outside the scope of this document.
Throughout this document, the term "multipoint" is defined as a mechanism by which one or more systems receive packets sent by a single sender. This specifically includes such things as IP multicast and point-to-multipoint MPLS.
This document effectively modifies and adds to the base BFD specification. It is the intention of the authors to fold these extensions into the base specification at the appropriate time.
The primary goal of this mechanism is to allow tails to rapidly detect the fact that multipoint connectivity from the head has failed.
Another goal is for the mechanism to work on any multicast or multipoint medium.
A further goal is to support multiple, overlapping multipoint paths, as well as multipoint paths with multiple heads, and to allow point-to-point BFD sessions to operate simultaneously among the systems participating in Multipoint BFD.
A final goal is to integrate multipoint operation into the base specification in such a way as to make it relatively easy to support both multipoint and point-to-point operation in a single implementation.
It is a non-goal for this protocol to verify point-to-point connectivity between the head and any tails. This can be done independently (and with no penalty in protocol overhead) by using point-to-point BFD.
The heart of this protocol is the periodic transmission of BFD Control packets along a multipoint path, from the head to all tails on the tree. The contents of the BFD packets provide the means for the tails to calculate the detection time for path failure. If no BFD Control packets are received by a tail for a detection time, the tail declares the path to have failed. For some applications this is the only mechanism necessary; the head can remain ignorant of the tails.
Head may wish to be alerted to the tails' connectivity (or lack thereof). Details of how head keeps track of tails and how tails alert it's connectivity to head are outside scope of this document.
Although this document describes a single head and a set of tails spanned by a single multipoint path, the protocol is capable of supporting (and discriminating between) more than one multipoint path at both heads and tails. Furthermore, the same head and tail may share multiple multipoint paths, and a multipoint path may have multiple heads.
This section describes the operation of Multipoint BFD in detail.
Multipoint BFD Control packets (packets sent by the head over a multipoint path) are explicitly marked as such, via the setting of the M bit. This means that Multipoint BFD does not depend on the recipient of a packet to know whether the packet was received over a multipoint path. This can be useful in scenarios where this information may not be available to the recipient.
Multipoint BFD is modeled as a set of sessions of different types. The elements of procedure differ slightly for each type.
Point-to-point sessions, as described in [BFD], are of type PointToPoint.
The head has a session of type MultipointHead that is bound to a multipoint path. Multipoint BFD Control packets are sent by this session over the multipoint path, and no BFD Control packets are received by it.
Each tail has a session of type MultipointTail associated with a multipoint path. These sessions receive BFD Control packets from the head over multipoint path.
The semantics of session failure are subtle enough to warrant further explanation.
MultipointHead sessions cannot fail (since they are controlled administratively.)
If a MultipointTail session fails, it means that the tail definitely has lost contact with the head (or the head has been administratively disabled) and the tail should take appropriate action.
Multipoint BFD introduces some new state variables, and modifies the usage of a few existing ones.
A number of state variables are added to the base specification in support of Multipoint BFD.
Some state variables defined in section 6.8.1 of the [RFC5880] needs to be initialized or manipulated differently depending on the session type.
The BFD state machine works slightly differently in the multipoint application. In particular, since there is a many-to-one mapping, three-way handshakes for session establishment and teardown are neither possible nor appropriate. As such there is no Init state. Session of type MultipointHead MUST NOT send BFD control packets with the State field being set to INIT, and must be ignored on receipt.
The following diagram provides an overview of the state machine for session type MultipointTail. The notation on each arc represents the state of the remote system (as received in the State field in the BFD Control packet) or indicates the expiration of the Detection Timer.
DOWN, ADMIN DOWN,
+------+ TIMER +------+
+----| |<---------------------| |----+
DOWN,| | DOWN | | UP | |UP
ADMIN DOWN,+--->| |--------------------->| |<---+
TIMER +------+ UP +------+
Unlike Point-to-point BFD, Multipoint BFD provides a form of discovery mechanism for tails to discover the head. The minimum amount of a priori information required both on the head and tails is the binding to the multipoint path over which BFD is running. The head transmits Multipoint BFD packets on that tree, and the tails listen for BFD packets on that tree. All other information MAY be determined dynamically.
A session of type MultipointHead is created for each multipoint path over which the head wishes to run BFD. This session runs in the Active role. Except when terminating BFD service, this session is always in state Up and always operates in Demand mode. No received packets are ever demultiplexed to the MultipointHead session. In this sense it is a degenerate form of a session.
Sessions on the tail MAY be established dynamically, based on the receipt of a Multipoint BFD Control packet from the head, and are of type MultipointTail. Tail sessions always take the Passive role.
The use of Discriminators is somewhat different in Multipoint BFD than in Point-to-point BFD.
The head sends Multipoint BFD Control packets over the MultipointHead session with My Discr set to a value bound to the multipoint path, and with Your Discr set to zero.
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 tree which the Multipoint BFD Control packet was received from. Together they uniquely identify the head of the multipoint path. Bootstrapping BFD session to a multipoint LSP in case of penultimate hop popping is outside the scope of this document.
Note that, unlike PointToPoint sessions, the discriminator values on all multipoint session types MUST NOT be changed during the life of a session. This is a side effect of the more complex demultiplexing scheme.
BFD packets received on tails for a multicast group MUST be consumed by tails and MUST not be forwarded to receivers. Session of type MultipointTail MUST identify the packet as BFD with the help of destination UDP port number "3784" on IP multipoint path. For multipoint LSP, MultipointTail MUST use destination UDP port "3784" and IP "127.0.0.0/8" range. Packet identified as BFD packet MUST be consumed by MultipointTail and demultiplex as described in Section 4.13.2
Because there is no three-way handshake in Multipoint BFD, a newly started head (that does not have any previous state information available) SHOULD start with bfd.SessionState set to Down and with bfd.RequiredMinRxInterval set to zero in the MultipointHead session. The session SHOULD remain in this state for a time equal to (bfd.DesiredMinTxInterval * bfd.DetectMult). This will ensure that all MultipointTail sessions are reset (so long as the restarted head is using the same or larger value of bfd.DesiredMinTxInterval than it did previously.)
Multipoint BFD service is brought up by administratively setting bfd.SessionState to Up in the MultipointHead session.
A head may wish to shut down its BFD service in a controlled fashion. This is desirable because the tails need not wait a detection time prior to declaring the multipoint session to be down (and taking whatever action is necessary in that case.)
To shut down a multipoint session a 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 send BFD Control packets in this state for a period equal to (bfd.DesiredMinTxInterval * bfd.DetectMult).
The semantic difference between Down and AdminDown state is for further discussion.
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 change in packet MultipointHead session MUST send packet with P bit set. MultipointTail session MUST NOT reply if packet has M, P bit set and bfd.RequiredMinRxInterval set to 0.
The MultipointHead MUST send bfd.DetectMult packets with P bit set at the old transmit interval before using the higher value in order to avoid false detection timeouts at the tails. MultipointHead May also wait some amount of time before making the changes to the transmit interval (through configuration).
Change in the value of bfd.RequiredMinRxInterval is outside the scope of this document.
Multipoint BFD is inherently asymmetric. As such, each session type has a different approach to detection times.
Since the MultipointHead session never receives packets, it does not calculate a detection time.
MultipointTail sessions cannot influence the transmission rate of the MultipointHead session using the Required Min Rx Interval field because of its one-to-many nature. As such, the Detection Time calculation for a MultipointTail session does not use bfd.RequiredMinRxInterval in the calculation. The detection time is calculated as 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.
The length of time session state is kept after the session goes down determines how long the session will continue to send BFD Control packets (since no packets can be sent after the session is destroyed.)
When a MultipointHead session transitions to states Down or AdminDown, the state SHOULD be maintained for a period equal to (bfd.DesiredMinTxInterval * bfd.DetectMult) to ensure that the tails more quickly detect the session going down (by continuing to transmit BFD Control packets with the new state.)
MultipointTail sessions MAY be destroyed immediately upon leaving Up state, since tail will transmit no packets.
Otherwise, MultipointTail sessions SHOULD be maintained as long as BFD Control packets are being received by it (which by definition will indicate that the head is not Up.)
The following sections are meant to replace the corresponding sections in the base specification.
The following procedure replaces section 6.8.6 of [RFC5880].
When a BFD Control packet is received, the following procedure MUST be followed, in the order specified. If the packet is discarded according to these rules, processing of the packet MUST cease at that point.
This section is part of the replacement for [RFC5880] section 6.8.6, separated for clarity.
The following procedure replaces section 6.8.7 of [RFC5880].
BFD Control packets MUST be transmitted periodically at the rate determined according to [RFC5880] section 6.8.2, except as specified in this section.
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.SilentTail is 1.
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, as 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 is equal to the greater of those two values. See [BFD] sections 6.8.2 and 6.8.3 for details on transmit timers.
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 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 the contents of that packet would differ from that in the previously transmitted packet (other than the Poll and Final bits) in order to more rapidly communicate a change in state.
The contents of transmitted BFD Control packets MUST be set as follows:
If authentication is in use, all tails must be configured to have a common authentication key in order to receive the multipoint BFD Control packets.
This document has no actions for IANA.
Implementations that creates MultpointTail sessions dynamically upon receipt of Multipoint BFD Control packets MUST implement protective measures to prevent infinite number of MultipointTail session being created. Below lists some points to be considered in such implementations.
Rahul Aggarwal of Juniper Networks and George Swallow of Cisco Systems provided the initial idea for this specification and contributed to its development.
Authors would also like to thank Nobo Akiya, Vengada Prasad Govindan, Jeff Haas, Wim Henderickx, Gregory Mirsky and Mingui Zhang who have greatly contributed to this document.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC5880] | Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010. |