Internet Engineering Task Force (IETF) N. Akiya
Request for Comments: 7419 M. Binderberger
Updates: 5880 Cisco Systems
Category: Informational G. Mirsky
ISSN: 2070-1721 Ericsson
December 2014 Common Interval Support in Bidirectional Forwarding Detection
Bidirectional Forwarding Detection (BFD) requires that messages be
transmitted at regular intervals and provides a way to negotiate the
interval used by BFD peers. Some BFD implementations may be
restricted to only support several interval values. When such BFD
implementations speak to each other, there is a possibility of two
sides not being able to find a common value for the interval to run
This document updates RFC 5880 by defining a small set of interval
values for BFD that we call "Common Intervals" and recommends
implementations to support the defined intervals. This solves the
problem of finding an interval value that both BFD speakers can
support while allowing a simplified implementation as seen for
hardware-based BFD. It does not restrict an implementation from
supporting more intervals in addition to the Common Intervals.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 22. The Problem with Few Supported Intervals . . . . . . . . . . 33. Well-Defined, Common Intervals . . . . . . . . . . . . . . . 44. Security Considerations . . . . . . . . . . . . . . . . . . . 45. References . . . . . . . . . . . . . . . . . . . . . . . . . 55.1. Normative References . . . . . . . . . . . . . . . . . . 55.2. Informative References . . . . . . . . . . . . . . . . . 5Appendix A. Why Some Values Are in the Common Interval Set . . . 6Appendix B. Timer Adjustment with Non-identical Interval Sets . 6
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 81. Introduction
The Bidirectional Forwarding Detection (BFD) standard [RFC5880]
describes how to calculate the transmission interval and the
detection time. However, it does not make any statement about how to
solve a situation where one BFD speaker cannot support the calculated
value. In practice, this may not have been a problem as long as
software-implemented timers were used and as long as the granularity
of such timers was small compared to the interval values being
supported, i.e. as long as the error in the timer interval was small
compared to 25 percent jitter.
In the meantime, requests exist for very fast interval values, down
to 3.3 msec for the MPLS Transport Profile (MPLS-TP). At the same
time, the requested scale for the number of BFD sessions is
increasing. Both requirements have driven vendors to use Network
Processors (NP), Field Programmable Gate Arrays (FPGAs), or other
hardware-based solutions to offload the periodic packet transmission
and the timeout detection in the receive direction. A potential
problem with this hardware-based BFD is the granularity of the
interval timers. Depending on the implementation, only a few
intervals may be supported, which can cause interoperability
problems. This document proposes a set of interval values that
should be supported by all implementations. Details are laid out in
the following sections.
2. The Problem with Few Supported Intervals
Let's assume vendor "A" supports 10 msec, 100 msec, and 1 sec
interval timers in hardware, and vendor "B" supports every value from
20 msec onward, with a granularity of 1 msec. For a BFD session, "A"
tries to set up the session with 10 msec while "B" uses 20 msec as
the value for RequiredMinRxInterval and DesiredMinTxInterval. Rx and
Tx are negotiated as described in [RFC5880], which is 20 msec in this
case. However, system "A" is not able to support the 20 msec
interval timer. Multiple ways exist to resolve the dilemma, but none
of them is without problems.
a. Realizing that it cannot support 20 msec, system "A" sends out a
new BFD packet advertising the next larger interval of 100 msec
with RequiredMinRxInterval and DesiredMinTxInterval. The new
negotiated interval between "A" and "B" is then 100 msec, which
is supported by both systems. However, the problem is that we
moved from the 10/20 msec range to 100 msec, which has far
deviated from operator expectations.
b. System "A" could violate [RFC5880] and use the 10 msec interval
for the Tx direction. In the receive direction, it could use an
adjusted multiplier value M' = 2 * M to match the correct
detection time. Now, in addition to the fact that we explicitly
violate [RFC5880], there may be the problem that system "B" drops
up to 50% of the packets; this could be the case when "B" uses an
ingress rate policer to protect itself and the policer would be
programmed with an expectation of 20 msec receive intervals.
The example above could be worse when we assume that system "B" can
only support a few timer values itself. Let's assume "B" supports 20
msec, 300 msec, and 1 sec. If both systems would adjust their
advertised intervals, then the adjustment ends at 1 sec. The example
above could even be worse when we assume that system "B" can only
support 50 msec, 500 msec, and 2 sec. Even if both systems walk
through all of their supported intervals, the two systems will never
be able to agree on an interval to run any BFD sessions.
3. Well-Defined, Common Intervals
The problem can be reduced by defining interval values that are
supported by all implementations. Then, the adjustment mechanism
could find a commonly supported interval without deviating too much
from the original request.
In technical terms, the requirement is as follows: a BFD
implementation should support all values in the set of Common
Interval values that are equal to or larger than the fastest (i.e.,
lowest) interval the particular BFD implementation supports.
This document defines the set of Common Interval values to be: 3.3
msec, 10 msec, 20 msec, 50 msec, 100 msec, and 1 sec.
In addition, both a 10 sec interval and multiplier values up to 255
are recommended to support graceful restart.
The adjustment is always towards larger (i.e., slower) interval
values when the initial interval proposed by the peer is not
This document is not adding new requirements with respect to the
precision with which a timer value must be implemented. Supporting
an interval value means advertising this value in the
DesiredMinTxInterval and/or RequiredMinRxInterval field of the BFD
packets and providing timers that are reasonably close. [RFC5880]
defines safety margins for the timers by defining a jitter range.
How is the Common Interval set used exactly? In the example above,
vendor "A" has a fastest interval of 10 msec and thus would be
required to support all intervals in the Common Interval set that are
equal or larger than 10 msec, i.e., it would support 10 msec, 20
msec, 50 msec, 100 msec, and 1 sec. Vendor "B" has a fastest
interval of 20 msec and thus would need to support 20 msec, 50 msec,
100 msec, and 1 sec. As long as this requirement is met for the
common set of values, then both vendor "A" and "B" are free to
support additional values outside of the Common Interval set.
4. Security Considerations
This document does not introduce any additional security concerns.
The security considerations described in the BFD documents, [RFC5880]
and others, apply to devices implementing the BFD protocol,
regardless of whether or not the Common Interval set is implemented.
Appendix A. Why Some Values Are in the Common Interval Set
The list of Common Interval values is trying to balance various
objectives. The list should not contain too many values, as more
timers may increase the implementation costs. On the other hand,
fewer values produces larger gaps and adjustment jumps. More values
in the lower interval range are thus seen as critical to support
customer needs for fast detection in setups with multiple vendors.
o 3.3 msec: required by MPLS-TP, to support the defect detection
time of 10 msec from [GR-253-CORE].
o 10 msec: general consensus is to support 10 msec. Multiple
vendors plan to or do already implement 10 msec.
o 20 msec: basically avoids a larger gap in this critical interval
region. Still allows 50-60 msec detect and restore (with
multiplier of 2) and covers existing software-based
o 50 msec: widely deployed interval. Supporting this value reflects
the reality of many BFD implementations today.
o 100 msec: similar to 10 msec, this value allows the reuse of
[G.8013_Y.1731] implementations, especially hardware. It supports
a large number of 100 msec sessions with multiplier 9 (9 x 100
msec), which could be replacing of 3 x 300 msec configurations
used by customers to have a detection time slightly below 1 sec
for VoIP setups.
o 1 sec: as mentioned in [RFC5880]. While the interval for Down
packets can be 1 sec or larger, this document recommends use of
exactly 1 sec to avoid interoperability issues.
The recommended value for large intervals is 10 sec, allowing for a
timeout of 42.5 minutes with a multiplier of 255. This value is kept
outside the Common Interval set, as it is not required for normal BFD
operations that occur in the sub-second range. Instead, the expected
usage is for graceful restart, if needed.
Appendix B. Timer Adjustment with Non-identical Interval Sets
[RFC5880] implicitly assumes that a BFD implementation can support
any timer value equal to or above the advertised value. When a BFD
speaker starts a Poll Sequence, then the peer must reply with the
Final (F) bit set and adjust the transmit and detection timers
accordingly. With contiguous software-based timers, this is a valid
assumption. Even in the case of a small number of supported interval
values, this assumption holds when both BFD speakers support exactly
the same interval values.
But what happens when both speakers support intervals that are not
supported by the peer? An example is router "A" supporting the
Common Interval set plus 200 msec, while router "B" supports the
Common Intervals plus 300 msec. Assume both routers are configured
and run at 50 msec. Now, router A is configured for 200 msec. We
know the result must be that both BFD speakers use 1 sec timers, but
how do they reach this endpoint?
First, router A sends a packet with 200 msec. The P bit is set
according to [RFC5880]. The Tx timer stays at 50 msec, the detection
timer is 3 * 200 msec:
(A) DesiredTx: 200 msec, MinimumRx: 200 msec, P-bit
Tx: 50 msec, Detect: 3 * 200 msec
Router B now must reply with an F bit. The problem is B is
confirming timer values that it cannot support. The only setting to
avoid a session flap would be
(B) DesiredTx: 300 msec, MinimumRx: 300 msec, F-bit
Tx: 50 msec, Detect: 3 * 300 msec
immediately followed by a P-bit packet, as the advertised timer
values have been changed:
(B) DesiredTx: 300 msec, MinimumRx: 300 msec, P-bit
Tx: 50 msec, Detect: 3 * 300 msec
This is not exactly what Section 6.8.7 of [RFC5880] states about the
transmission rate. On the other hand, as we will see, this state
does not last for long. Router A would adjust its timers based on
the received Final bit:
(A) Tx: 200 msec, Detect: 3 * 1 sec
Router A is not supporting the proposed 300 msec and would use 1 sec
instead for the detection time. It would then respond to the
received Poll Sequence from router B using 1 sec, as router A does
not support the Max(200 msec, 300 msec):
(A) DesiredTx: 1 sec, MinimumRx: 1 sec, F-bit
Tx: 200 msec, Detect: 3 * 1 sec
followed by its own Poll Sequence, as the advertised timer values
have been changed:
(A) DesiredTx: 1 sec, MinimumRx: 1 sec, P-bit
Tx: 200 msec, Detect: 3 * 1 sec
Router B would adjust its timers based on the received Final bit
(B) Tx: 300 msec , Detect: 3 * 1 sec
and would then reply to the Poll Sequence from router A:
(B) DesiredTx: 300 msec, MinimumRx: 300 msec, F-bit
Tx: 1 sec, Detect: 3 * 1 sec
which finally makes router A adjust its timers:
(A) Tx: 1 sec, Detect: 3 * 1 sec
In other words, router A and B go through multiple Poll Sequences
until they reach a commonly supported interval value. Reaching such
a value is guaranteed by this document.
We would like to thank Sylvain Masse and Anca Zamfir for bringing up
the discussion about the Poll Sequence, and Jeffrey Haas for helping
find the fine line between "exact" and "pedantic".