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Versions: 00 01 02 03 04 draft-ietf-bfd-intervals

Internet Engineering Task Force                                 N. Akiya
Internet-Draft                                           M. Binderberger
Intended status: Standards Track                           Cisco Systems
Expires: October 27, 2012                                 April 25, 2012

                  Standardized interval support in BFD


   This document defines a set of interval values that we call "Standard
   intervals".  Values of this set must be supported for transmitting
   BFD control packets and for calculating the detection time in the
   receive direction when the value is equal or larger than the fastest,
   i.e. lowest, interval a particular BFD implementation supports.

   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.

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
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   This Internet-Draft will expire on October 27, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  The problem with few supported intervals  . . . . . . . . . . . 3
   3.  Well-defined, standardized intervals  . . . . . . . . . . . . . 4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Appendix A.  Why some intervals are in the standard set . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 5

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1.  Introduction

   The standard [RFC5880] describes how to calculate the transmission
   interval and the detection time.  It does not make any statement
   though how so solve a situation where one BFD speaker cannot support
   the calculated value.  In practise 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 in 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
   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 must be supported by all
   implementations.  Details are laid out in the following sections.

2.  The problem with few supported intervals

   Lets 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 likely is not fitting
       the requirements.

   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%

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       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.  Lets 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.

3.  Well-defined, standardized intervals

   The problem can be reduced by defining interval values that must be
   supported by all implementations.  Then the adjustment mechanism
   could find a commonly supported interval without deviating too much
   from the original request.

   The proposed set of Standard interval values is: 3.3msec, 10msec,
   20msec, 50msec, 300msec and 1sec.

   What does "Standard interval set" mean exactly?  In the example
   above, vendor "A" has a fastest interval of 10msec and thus would be
   required to support all intervals in the standard set that are equal
   or larger than 10msec, i.e. it would support 10msec, 20msec, 50msec,
   300msec, 1sec.  Vendor "B" has a fastest interval of 20msec and thus
   would need to support 20msec, 50msec, 300msec and 1sec.  This does
   not restrict both vendors from implementing additional values outside
   of the standard set.  As long as requirements are met for the
   standard set of values, then both vendor "A" and "B" are free to
   support additional values outside of the standard set.

4.  IANA Considerations

   No request to IANA.

5.  Security Considerations

   This document does not introduce any additional security issues and
   the security mechanisms defined in [RFC5880] apply in this document.

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6.  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.

Appendix A.  Why some intervals are in the standard set

   The list of standard 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.  Larger values in
   the lower interval range may be easier to support, potentially even
   in software instead of hardware.

   o  3.3msec: required by MPLS-TP

   o  10msec and 20msec: both values allow to detect faster than 50msec,
      when used with a multiplier of 2 or 3 (for 10msec).  A compromise
      could be a single interval of 15msec.

   o  50msec: this seems an interval often supported by software
      implementations, so the assumption here is that for convenience
      this value should be supported.

   o  300msec: this would support large scale of 3 x 300msec setups used
      by customers to have a detection time slightly below 1sec for VoIP

   o  1sec: as mentioned in RFC5880

   With that stated, one of the primary intention of this first draft is
   to seek feedback on the number of interval values in the standard set
   as well as each value.

Authors' Addresses

   Nobo Akiya
   Cisco Systems

   Email: nobo@cisco.com

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   Marc Binderberger
   Cisco Systems

   Email: mbinderb@cisco.com

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