Re: [Idr] TCP & BGP: Some don't send terminate BGP when holdtimer expired, because TCP recv window is 0

Claudio Jeker <cjeker@diehard.n-r-g.com> Thu, 17 December 2020 11:50 UTC

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Date: Thu, 17 Dec 2020 12:49:13 +0100
From: Claudio Jeker <cjeker@diehard.n-r-g.com>
To: Enke Chen <enchen@paloaltonetworks.com>
Cc: Job Snijders <job@sobornost.net>, idr@ietf.org
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Subject: Re: [Idr] TCP & BGP: Some don't send terminate BGP when holdtimer expired, because TCP recv window is 0
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On Wed, Dec 16, 2020 at 06:40:01PM -0800, Enke Chen wrote:
> Hi, Folks:
> 
> Regarding the patch for openBGPD pointed out by Job, I do not think it
> would work. When the TCP rcv window from the remote is 0, the BGP keepalive
> can still be queued to the socket buffer. It can take a long time for the
> socket buffer to be filled up by BGP keepalives.

I agree that it can take time for the socket buffers to fill up. This
affects both send and recv socket buffers. If a client stops to recv(2) any
data then it will take a long time for an idle session to fill up all
these buffers with 19 byte KEEPALIVEs. At the same time an idle session
will not result in stale RIB entries.
In the DFZ there is enough update chatter to reduce the time significantly.

In the end the goal is to reduce the current state of keeping the
connection open forever to something more reasonable. As mentioned to 
fill up the receive buffer and closing the window to 0 already takes time.
This trigger will not be precise but limiting the socket buffer size can
help to reduce the time.
 
> It seems that the TCP_USER_TIMEOUT option can be used for the persistent
> zero-size window issue.  The timeout value could be multiples of the
> holdtimer (with min and max adjustments), perhaps somewhere around 5 or 6
> minutes.

Not all systems implement TCP_USER_TIMEOUT. It is one way to detect send
side problems. With a large timeout it is probably a good mechanism.

-- 
:wq Claudio