James Nichols a écrit :
Well... please dont start a flame war :(
Back to your SYN_SENT problem, I suppose the remote IP is known, so you
probably could post here the result of a tcdpump ?
tcpdump -p -n -s 1600 host IP_of_problematic_peer -c 500
Most probably remote peer received too many attempts from you, and a
anti DOS mechanism is droping all SYN packets.
Ah well... I remember now that you mentioned tcp_sack setting had an
effect, so forget the "Most probably..." and give some tcpdump traces :)
I've run tcpdump for all IPs during this problem. I haven't tried
doing it for a single explicit IP address- due to the nature of the
workload it's very difficult to know which IPs will be hit at any
given moment. What I did see in the full IP captures is that the
returning ACKs don't show up in the packet capture. Unfortunately,
tcpdump reported that some packets were dropped during the capture.
Is it possible that the kernel was dropping the packets before they
could be captured by tcpdump?
Yes it can happens, because an active sniffer makes the stack using more
cpu cycles (timestamping for example).
So you see outgoing SYN packets, but no SYN replies coming from the remote
peer ? (you mention ACKS, but the first packet received from the remote
peer should be a SYN+ACK),
client->server SYN
server->client SYN+ACK
client->server ACK
Also, I have some doubts about it being the end points or an
intermediate router, please let me know if these are unreasonable:
1) We've completely replaced our routing equipment several times in
the past 4 years... totally different colos, router vendors, firewall
vendors, firewall rules, etc.
2) It occurs across all remote end points at the exact same time.
The endpoints are hetrogenous, run brain-dead OS's that don't do any
DOS detection, reboot at random times of the day, are geographically
distributed, are on different ISPs, etc. etc.
3) Turning of tcp_sack instantaneously makes the problem go away. If
it were endpoints or a router, it seems like a stretch that removing a
single TCP option would make the problem instantly resolve itself in
so many places other than the originating host.
CC to netdev where linux network guys might have an idea.
When the problem comes, instead of restarting the application, please take a
tcpdump of say 10.000 packets.
Then turn off tcp_sack and take a 2nd tcpdump sample, and make both samples
available to us.
If turning off tcp_sack makes the problem go away, why dont you turn it off
all the time ?
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