Draft minutes of the "Route Flap Dampening BOF" @ RIPE26

["Christian Panigl, ACOnet/UniVie" <]

    Dear Routing WG,
    
    thanks to Joachim who took notes and provided me with very elaborated
    draft minutes of our "Route Flap Dampening BOF" !  I did some additions
    and I'm now asking all participants to come back with comments.
    
    And I'd like to remind you to send me your pointers to similar
    discussions and related recommendations (IETF, NANOG, ...) since I
    wasn't able to attend other forums than RIPE yet.
    
    Regards
    Christian
    
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===================================================
 Route Flap Dampening BOF, RIPE 26, 22.1.97, 14:00
 Chairman:  Christian Panigl (CP)
 Scribe:    Joachim Schmitz (JS)
 Attendees: approx 30

 In the Routing WG session Christian Panigl asked whether people are
 interested to participate in a BOF on route flap dampening. The BOF
 session was held after the plenary session of the RIPE meeting on
 Wednesday.

 CP experienced quite severe reachability problems of customer networks
 because route flap dampening became active at various AS borders
 following scheduled maintenance actions on a core router.  If the default
 dampening parameters were used everywhere, it wouldn't have hurt that much,
 since dampening would have lasted for ~20-30 minutes only for all prefixes.  
 
 Some backbone ISPs, however, have started to implement "progressive route
 flap dampening" typically using different parameters.  The common effect is
 that longer prefixes are dampened more aggressively than shorter prefixes.
 
 In the observed case all /24 customer networks were cut off from parts of
 the Internet for more than 2 hours and were no longer able to reach for
 instance the root nameservers.  By the way, many, even top- and
 second-level nameservers are sitting in /24 (192/TWD) prefixes themselves
 and could easily be "victims" of such a progressive dampening policy !
 
 CP wasn't branding route flap dampening itself, but the aggressiveness of
 some of the implemented "progressive" parameters and was questioning the
 real usefulness of progressive dampening at all.
 
 Following CP's introduction a vivid discussion on route flap dampening
 came off:

 * Does flapping really depend on the prefix length?

   - To the knowledge of people attending the BOF session no measurements
     exist. Although several items were already measured by Merit on the
     stability of routes (as seen in the presentation by G.Winters in the
     Routing WG) they did not include a stability analysis with regard
     to the prefix length. If flapping does not necessarily depend on the
     prefix length longer prefixes should not be punished by more aggressive
     dampening.

   - However, the number of longer prefixes in the routing tables is much
     bigger than the number of shorter ones.  As a consequence, if the
     percentage of flapping routes is the same for all prefix lengths the
     absolute number of flaps will be definitely higher for longer prefixes.
     As each flap consumes the same performance on the router (regardless of
     the prefix length) and to get the the best CPU saving factor, longer
     prefixes should be dampened more aggressively.

   - Further justification for the latter was primarily based on the
     assumption that longer prefixes are serving less users, which of course
     didn't stay uncontradicted (think of important servers sitting in a
     /24).

* Which networks or prefixes are "important"?

   - Stating that shorter prefixes are more important because they cover
     more users doesn't hold in general.  On the one hand this may be 
     valuable and motivate ISPs to CIDRize and customers to renumber, on the 
     other hand it may lead to the situation that organisations try to get
     (or keep, think of Class A/B recycling) as short a prefix as possible,
     wasting address space without having to care for stability. In this
     case instability would be moved to shorter prefixes which is far from
     desirable.

   - Long prefixes need not be instable. There are discussions to use long
     prefix routes ("golden networks") for root nameservers or for other
     Internet structure servers (even for application servers as news, etc).
     It can be well assumed that these routes are more stable than others
     and they must not be dampened too aggressively in order not to tackle
     the functionality of the Internet itself.

 During all the discussion the general consensus was clear: for routers
 with large BGP tables (notably with full routing) the CPU load would kill
 any existing router.  To survive instabilities route flap dampening should
 be applied by *everybody*.  However, it was obvious that dampening 
 parameters need to be coordinated throughout the Internet in order to

 - allow efficient dampening and easy clearing after repair

 - dampen flaps at their source by keeping them from spreading in the network

 This will significantly increase the overall stability and the
 manageability.  The broader (soft and default) dampening is deployed
 allover the Internet, the less the need for aggressive paramaters will
 be.
 
 The group was forming into two major camps with regard to how dampening
 should be done:
 
    - progressive dampening:  needs to be accompanied by means to
      explicitely exclude "golden networks" from "hostility acts"
      
    - flat (default) dampening:  because it's very hard to make a 
      distinction between less and more "important", not to say "golden"
      networks, all prefixes should be treated equally.  Efforts should be
      focussed on the propagation of dampening throughout the Internet.

 The default values for dampening parameters as they are found in Cisco
 routers are based upon some experiments approx one year ago.  These
 experiments lead to recommendations by the IETF last year.  
 
 Nevertheless, many ISPs have moved away from the default values and are
 using their own parameters.  Because of the urgent need of coordination
 of these values CP will try to collect related recommendations and the
 outcome of similar discussions.  This is an activity of the RIPE Routing
 WG, therefore everybody who is aware of related efforts (IETF, NANOG, ...)
 should come back to the Routing WG list with hints and pointers !

 New Action 26.R4 on Chrisian Panigl
 To collect reasonable route flap dampening parameter values and to present
 them at the next RIPE meeting in the Routing WG.

 Further reading:
 ftp://ftp.ripe.net/ripe/minutes/ripe-m-24.ps
 ftp://ftp.ripe.net/ripe/minutes/ripe-m-25.ps
 http://www.ripe.net/wg/routing/r25-routing.html
 ftp://ftp.ripe.net/ripe/presentations/ripe-m25-tbarber-bgp-damp.html