#5501 HIGH Update.: XOs vanishing/reappearing/flashing in the mesh view is related to avahi losing its cache of hosts

[Zarro Boogs per Child]

#5501: XOs vanishing/reappearing/flashing in the mesh view is related to avahi
losing its cache of hosts
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  Reporter:  yani             |       Owner:  sjoerd  
      Type:  defect           |      Status:  new     
  Priority:  high             |   Milestone:  Update.1
 Component:  telepathy-salut  |     Version:          
Resolution:                   |    Keywords:          
  Verified:  0                |  
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Changes (by sjoerd):

 * cc: daf (added)


Comment:

 the MDNS spec describes a technique called ``Passive Observation of
 Failures''. This allows failures to be detected before the ttl of a record
 runs out. The way this is done is by observer queries of others, if there
 are no reponses observed after various queries. The record can be assumed
 to be to have failed.

 The way this was implemented in avahi is that a record is expunged from
 the case if there isn't a response within a second of the _second_ query
 for it then it's assumed to have failed.. When you introduce new nodes
 running salut in a network (or as in your example let them reconnect),
 they query the _presence._tcp PTR record. Which can easily trigger
 othernodes to observer certain records as failure. Which in turn explains
 why you see the nodes jump as soon as you introduce nodes in the network

 I'll attach a patch for avahi that makes the code a bit more demanding in
 what it needs to see before it assumes it observed a failure. Instead of
 two queries. There need to be at least 4 queries and each query has to be
 at least one second apart. This should be enough to prevent most false-
 positives.. Please see if this helps on the OLPC network too

 Daf is currently working on getting the patch into the joyride avahi
 package.

-- 
Ticket URL: <http://dev.laptop.org/ticket/5501#comment:7>
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