BFD common intervals: Common Interval Support in Bidirectional Forwarding Detection

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

Abstract

   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
   BFD sessions.

   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
   http://www.rfc-editor.org/info/rfc7419.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The Problem with Few Supported Intervals  . . . . . . . . . .   3
   3.  Well-Defined, Common Intervals  . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Why Some Values Are in the Common Interval Set . . .   6
   Appendix B.  Timer Adjustment with Non-identical Interval Sets  .   6
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  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
   supported.

   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.

5.  References

5.1.  Normative References

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010,
              <http://www.rfc-editor.org/info/rfc5880>.

5.2.  Informative References

   [G.8013_Y.1731]
              International Telecommunications Union, "OAM functions and
              mechanisms for Ethernet based networks", ITU-T
              Recommendation G.8013/Y.1731, November 2013.

   [GR-253-CORE]
              Telcordia Technologies, Inc., "Synchronous Optical Network
              (SONET) Transport Systems: Common Generic Criteria",
              GR-253-CORE Issue 05, October 2009.

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
      implementations.

   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.

Acknowledgments

   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".

Authors' Addresses

   Nobo Akiya
   Cisco Systems

   EMail: nobo@cisco.com


   Marc Binderberger
   Cisco Systems

   EMail: mbinderb@cisco.com


   Greg Mirsky
   Ericsson

   EMail: gregory.mirsky@ericsson.com