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Common Interval Support in BFD

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7419.
Authors Nobo Akiya , Marc Binderberger , Greg Mirsky
Last updated 2014-09-01 (Latest revision 2014-08-16)
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Jeffrey Haas
Shepherd write-up Show Last changed 2014-07-28
IESG IESG state Became RFC 7419 (Informational)
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Adrian Farrel
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IANA IANA review state IANA OK - No Actions Needed
Internet Engineering Task Force                                 N. Akiya
Internet-Draft                                           M. Binderberger
Intended status: Informational                             Cisco Systems
Expires: February 17, 2015                                     G. Mirsky
                                                         August 16, 2014

                     Common Interval Support in BFD


   BFD requires that messages are 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 interval
   value to run BFD sessions.

   This document defines 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.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

   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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on February 17, 2015.

Copyright Notice

   Copyright (c) 2014 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
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The problem with few supported intervals  . . . . . . . . . .   3
   3.  Well-defined, common intervals  . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Appendix A.  Why some intervals are in the common set . . . . . .   5
   Appendix B.  Timer adjustment with non-identical interval sets  .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The standard [RFC5880] describes how to calculate the transmission
   interval and the detection time.  It does not make any statement
   though how to solve a situation where one BFD speaker cannot support
   the calculated value.  In practice this may not been a problem as
   long as software-implemented timers have been 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
   3.3msec for 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), FPGAs or other hardware-based
   solutions to offload the periodic packet transmission and the timeout

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   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 10msec, 100msec and 1sec interval
   timers in hardware.  Vendor "B" supports every value from 20msec
   onward, with a granularity of 1msec.  For a BFD session "A" tries to
   set up the session with 10msec while "B" uses 20msec as the value for
   RequiredMinRxInterval and DesiredMinTxInterval.  [RFC5880] describes
   that the negotiated value for Rx and Tx is 20msec.  But system "A" is
   not able to support this.  Multiple ways exist to resolve the dilemma
   but none of them is without problems.

   a.  Realizing that it cannot support 20msec, system "A" sends out a
       new BFD packet, advertising the next larger interval of 100msec
       with RequiredMinRxInterval and DesiredMinTxInterval.  The new
       negotiated interval between "A" and "B" then is 100msec, which is
       supported by both systems.  The problem though is that we moved
       from the 10/20msec range to 100msec, which has far deviated from
       operator expectations.

   b.  System "A" could violate [RFC5880] and use the 10msec 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 beside 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 20msec 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
   "20msec", "300msec" and "1sec".  If both systems would adjust their
   advertised intervals, then the adjustment ends at 1sec.  The example
   above could even be worse when we assume that system "B" can only
   support "50msec", "500msec" and "2sec".  Even if both systems walk
   their supported intervals, the two systems will never be able to
   agree on a interval to run any BFD sessions.

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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 which are equal to or larger than the fastest, i.e.
   lowest, interval the particular BFD implementation supports.

   The proposed set of Common interval values is: 3.3msec, 10msec,
   20msec, 50msec, 100msec and 1sec.

   In addition support for 10sec interval together with multiplier
   values up to 255 is 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 to advertise this value in the
   DesiredMinTxInterval and/or RequiredMinRxInterval field of the BFD
   packets and to provide 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 10msec and thus would be
   required to support all intervals in the common set that are equal or
   larger than 10msec, i.e. it would support 10msec, 20msec, 50msec,
   100msec, 1sec.  Vendor "B" has a fastest interval of 20msec and thus
   would need to support 20msec, 50msec, 100msec and 1sec.  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 set.

4.  IANA Considerations

   No request to IANA.

5.  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.

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6.  Acknowledgements

   We would like to thank Sylvain Masse and Anca Zamfir for bringing up
   the discussion about the Poll sequence, and Jeffrey Haas helped
   finding the fine line between "exact" and "pedantic".

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

7.2.  Informative References

              ITU-T G.8013/Y.1731, "ITU-T OAM functions and mechanisms
              for Ethernet based network", November 2013.

              Telcordia Technologies, Inc., "Synchronous Optical Network
              (SONET) Transport Systems: Common Generic Criteria",
              October 2009.

Appendix A.  Why some intervals are in the common 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 less
   values produces larger gaps and adjustment jumps.  More values in the
   lower interval range is thus seen as critical to support customer
   needs for fast detection in setups with multiple vendors.

   o  3.3msec: required by MPLS-TP, adopting the detection time of
      10msec from [GR-253-CORE].

   o  10msec: general consensus is to support 10msec.  Multiple vendors
      plan to or do already implement 10msec.

   o  20msec: basically avoids a larger gap in this critical interval
      region.  Still allows 50-60msec detect and restore (with
      multiplier of 2) and covers existing software-based

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   o  50msec: widely deployed interval.  Supporting this value reflects
      reality of many BFD implementations today.

   o  100msec: similar to 10msec this value allows the reuse of
      [G.8013_Y.1731] implementations, especially hardware.  It allows
      to support large scale of 9 x 100msec setups and would be a
      replacement for 3 x 300msec configurations used by customers to
      have a detection time slightly below 1sec for VoIP setups.

   o  1sec: as mentioned in [RFC5880].  While the interval for Down
      packets can be 1sec or larger this draft proposes to use exactly
      1sec to avoid interoperability issues.

   The proposed value for large intervals is 10sec, 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, which 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 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 200msec while router "B" support the common
   intervals plus 300msec.  Assume both routers are configured and run
   at 50msec.  Now router A is configured for 200msec.  We know the
   result must be that both BFD speaker use 1sec timers but how do they
   reach this endpoint?

   First router A is sending a packet with 200msec.  The P bit is set
   according to [RFC5880].  The Tx timer stays at 50msec, the detection
   timer is 3 * 200msec:

      (A) DesiredTx: 200msec, MinimumRx: 200msec, P-bit
      Tx: 50msec , Detect: 3 * 200msec

   Router B now must reply with an F bit.  The problem is B is
   confirming timer values which it cannot support.  The only setting to
   avoid a session flap would be

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      (B) DesiredTx: 300msec, MinimumRx: 300msec, F-bit
      Tx: 50msec , Detect: 3 * 300msec

   immediately followed by a P-bit packet as the advertised timer values
   have been changed:

      (B) DesiredTx: 300msec, MinimumRx: 300msec, P-bit
      Tx: 50msec , Detect: 3 * 300msec

   This is not exactly what [RFC5880] states in section 6.8.7 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: 200msec , Detect: 3 * 1sec

   Router A is not supporting the proposed 300msec and would use 1sec
   instead for the detection time.  It would then respond to the
   received Poll sequence from router B, using 1sec as router A does not
   support the Max(200msec, 300msec):

      (A) DesiredTx: 1sec, MinimumRx: 1sec, F-bit
      Tx: 200msec , Detect: 3 * 1sec

   followed by its own Poll sequence as the advertised timer values have
   been changed:

      (A) DesiredTx: 1sec, MinimumRx: 1sec, P-bit
      Tx: 200msec , Detect: 3 * 1sec

   Router B would adjust its timers based on the received Final

      (B) Tx: 300msec , Detect: 3 * 1sec

   and would then reply to the Poll sequence from router A:

      (B) DesiredTx: 300msec, MinimumRx: 300msec, F-bit
      Tx: 1sec , Detect: 3 * 1sec

   which finally makes router A adjusting its timers:

      (A) Tx: 1sec , Detect: 3 * 1sec

   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 draft.

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Authors' Addresses

   Nobo Akiya
   Cisco Systems


   Marc Binderberger
   Cisco Systems


   Greg Mirsky


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