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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: Informational                             Cisco Systems
Expires: May 17, 2014                                  November 13, 2013


                  Standardized interval support in BFD
                      draft-akiya-bfd-intervals-04

Abstract

   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.  It does not restrict an implementation
   from supporting more intervals in addition to the Standard intervals.

Requirements Language

   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 [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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 17, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the



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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
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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 . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Appendix A.  Why some intervals are in the standard set . . . . . . 5
   Appendix B.  Timer adjustment with non-identical interval sets  . . 6
   Appendix C.  Open/upcoming topics . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 8

























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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 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 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 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%



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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 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 MUST support all values in the set of Standard
   interval values which are equal to or larger than the fastest, i.e.
   lowest, interval the particular BFD implementation supports.

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

   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 how
   exact 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 "Standard 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 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.  As long as
   this requirement is met for the standard set of values, then both
   vendor "A" and "B" are free to support additional values outside of
   the standard set.



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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.


6.  Acknowledgements

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


7.  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



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      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.

   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.

   Candidates for larger intervals to be part of the Standard interval
   set would be 10sec, 1min and 10min.  Such larger BFD intervals may be
   used for BFD graceful restarts.


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
   standard interval set plus 100msec while router "B" support the
   standard intervals plus 200msec.  Assume both routers are configured
   and run at 50msec.  Now router A is configured for 100msec.  We know
   the result must be that both BFD speaker use 300msec timers but how
   do they reach this endpoint?

   First router A is sending a packet with 100msec.  The P bit is set
   according to RFC5880.  The Tx timer stays at 50msec, the detection
   timer is 3 * 100msec:

      (A) DesiredTx: 100msec, MinimumRx: 100msec, P-bit
      Tx: 50msec , Detect: 3 * 100msec

   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

      (B) DesiredTx: 200msec, MinimumRx: 200msec, F-bit
      Tx: 50msec , Detect: 3 * 200msec




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

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

   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 it's timers based on the
   received Final bit

      (A) Tx: 100msec , Detect: 3 * 300msec

   Router A is not supporting the proposed 200msec and would use 300msec
   instead for the detection time.  It would then respond to the
   received Poll sequence from router B, using 300msec as router A does
   not support the Max(100msec, 200msec):

      (A) DesiredTx: 300msec, MinimumRx: 300msec, F-bit
      Tx: 100msec , Detect: 3 * 300msec

   followed by it's own Poll sequence as the advertised timer values
   have been changed:

      (A) DesiredTx: 300msec, MinimumRx: 300msec, P-bit
      Tx: 100msec , Detect: 3 * 300msec

   Router B would adjust it's timers based on the received Final

      (B) Tx: 200msec , Detect: 3 * 300msec

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

      (B) DesiredTx: 200msec, MinimumRx: 200msec, F-bit
      Tx: 300msec , Detect: 3 * 300msec

   which finally makes router A adjusting it's timers:

      (A) Tx: 300msec , Detect: 3 * 100msec

   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.


Appendix C.  Open/upcoming topics

   As part of the ongoing BFD workgroup effort the following topics may



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   require more discussion and investigation:

   Alignment of BFD intervals with Y.1731 intervals.  Some contradicting
   requirements.  Ethernet OAM intervals do not fit intervals of
   existing BFD implementations while adding more intervals to the
   standards set would complicate hardware implementations.

   Larger interval values.  To simplify hardware implementation a
   question would be if we want to make all the intervals above 1sec
   part of the standard interval set.

   Jitter.  For the very fast interval of 3.3msec, do we want jitter.
   Question comes up from hardware teams.


Authors' Addresses

   Nobo Akiya
   Cisco Systems

   Email: nobo@cisco.com


   Marc Binderberger
   Cisco Systems

   Email: mbinderb@cisco.com
























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