BIRD drops specific IPv6 session for no reason

[Alarig Le Lay]

Hi Stavros,

On ven. 28 févr. 12:41:24 2020, Stavros Konstantaras wrote:
> Hi Bird community,
> 
> We are investigating a weird customer issue regarding our Bird Route
> Servers (version 1.6.3) and a specific IPv6 session. Customer reports
> a sudden drop of his IPv6 session and -until now- we could not relate
> those drops with any issue or instability. Everything seems normal and
> no other customer complained at the moment of the incident. 
> 
> 
> 
> After some packet capturing at the moment of the event, we discovered
> that BIRD does not send a response messages to the customer’s BGP
> keepalive messages (see attached picture), which result to the BGP
> hold timer to expire and the sessions to be dropped. We observed this
> anomaly with both RSs but at different time slots and the tcpdump
> capture was running at the Interface were Bird is sending all BGP
> traffic for customers. At the moment of the event, we didn’t do any
> maintenance or other RS related work.
> 
> Has any of you experienced this in the past? If yes, how did you solve
> this?
> Any related feedback is welcomed. 

Do you have the MSS used to establish the session? I had an issue about
a session flapping with edgecast (verizonmedia) flapping on AMS-IX
because both were having a MTU at 9216 on our port. But some switch
didn’t like it well and sometime a packet is loss. If it’s the one
containing the keepalive, the session goes down.

I resolved it by setting a MTU of 1514 on my side (which should have
been since always).

Also, note that I’m not directly connected to the IXP, I’m using a
reseller.

Regards,
-- 
Alarig