Re: tcp hang when socket fills up ?

[Dominique Martinet <asmadeus@codewreck.org>]

Michal Kubecek wrote on Tue, Apr 17, 2018:
> Data (21 bytes) packet in reply direction. And somewhere between the
> first and second debugging print, we ended up with sender scale=0 and
> that value is then preserved from now on.
> 
> The only place between the two debug prints where we could change only
> one of the td_sender values are the two calls to tcp_options() but
> neither should be called now unless I missed something. I'll try to
> think about it some more.

Could it have something to do with the way I setup the connection?
I don't think the "both remotes call connect() with carefully selected
source/dest port" is a very common case..

If you look at the tcpdump outputs I attached the sequence usually is
something like
 server > client SYN
 client > server SYN
 server > client SYNACK
 client > server ACK

ultimately it IS a connection, but with an extra SYN packet in front of
it (that first SYN opens up the conntrack of the nat so that the
client's syn can come in, the client's conntrack will be that of a
normal connection since its first SYN goes in directly after the
server's (it didn't see the server's SYN))


Looking at my logs again, I'm seeing the same as you:

This looks like the actual SYN/SYN/SYNACK/ACK:
 - 14.364090 seq=505004283 likely SYN coming out of server
 - 14.661731 seq=1913287797 on next line it says receiver
end=505004284 so likely the matching SYN from client
Which this time gets a proper SYNACK from server:
14.662020 seq=505004283 ack=1913287798
And following final dataless ACK:
14.687570 seq=1913287798 ack=505004284

Then as you point out some data ACK, where the scale poofs:
14.688762 seq=1913287798 ack=505004284+(0) sack=505004284+(0) win=229 end=1913287819
14.688793 tcp_in_window: sender end=1913287798 maxend=1913316998 maxwin=29312 scale=7 receiver end=505004284 maxend=505033596 maxwin=29200 scale=7
14.688824 tcp_in_window: 
14.688852 seq=1913287798 ack=505004284+(0) sack=505004284+(0) win=229 end=1913287819
14.688882 tcp_in_window: sender end=1913287819 maxend=1913287819 maxwin=229 scale=0 receiver end=505004284 maxend=505033596 maxwin=29200 scale=7

As you say, only tcp_options() will clear only on side of the scales.
We don't have sender->td_maxwin == 0 (printed) so I see no other way
than we are in the last else if:
 - we have after(end, sender->td_end) (end=1913287819 > sender
end=1913287798)
 - I assume the tcp state machine must be confused because of the
SYN/SYN/SYNACK/ACK pattern and we probably enter the next check, 
but since this is a data packet it doesn't have the tcp option for scale
thus scale resets.


At least peeling the logs myself helped me follow the process, I'll
sprinkle some carefully crafted logs tomorrow to check if this is true
and will let you figure what is best of trying to preserve scale if it
was set before, setting a default to 14 or something else.

Thanks!
-- 
Dominique Martinet | Asmadeus