Subject: [2.6.24.3][net] bug: TCP 3rd handshake abnormal timeouts

From: Gabriel Barazer <[email-masked]>
Date: Sat, 15 Mar 2008 09:53:27 +0100

[resent because of a typo in the netdev mailing list address]

Hi,

I am experiencing a very annoying bug which I think is a kernel bug,
related to how a client establishes a TCP connection to a server (both
linux, same  vanilla 2.6.24.3 kernels but the problem happened also in
the previous 2.6.23.14 and kernel installed).

The case is about a bunch of web servers accessing a MySQL database
server via TCP and non-persistent connections and all application level
errors have been excluded.
"Sometimes" when establishing a TCP connection to the server, we are
seeing a 3000ms delay before the connection if effectively made. This
has been confirmed first by some strace-d telnet tests, then by tcpdump
captures on both side at the same time.

Here is a simplified version of what _both_ the server and the client
see (it's important because this means the packets are not lost and the
bug not caused by a lossy network). I can send the original pcap file if
necessary.

- The client send a TCP packet with the SYN bit
- The server replies by a TCP packet with the SYN,ACK bits (connection
appears in netstat -a as in the SYN_RECV state)
- Here the client should have replied with a TCP ACK packet, but for
some reason wait for exactly 3 seconds instead (the delay for
retransmitting a SYN packet).
- The client re-send a TCP SYN packet
- The server replies by its TCP SYN,ACK packet
- And finally the client send its TCP ACK packet, completing the 3-way
handshake and beginning communication with the MySQL server.

During the time when the bug happens (~1 minute), I can log into the
server, and confirm this 3 second delay when strace-ing a telnet session
to the mysql port.

I believe the problem is on the client side because when this abnormal
delay happens, all the other mysql connections from the other web
servers are doing fine (the MySQL server serves other host's queries
without any connection delay), and I think I can rule out the userland
processes as well because they are not aware of this TCP handshake
failing and retrying, only the connect() syscall does.

I have reports of other people having this problem on other networks
with different hardware specs, and on different networks (I run this on
a local network).

I have no syncookies enabled, and have the netfilter connection tracking
enabled but the tracked connections count is very stable, not
overflowing and not even increasing when the problem happens.

Obviously, no message in the syslog or the kernel buffer ring.

Has anyone any idea on where to start to find and fix this bug? the
situation is becoming critical as this affects all of our installations
and I can't find a way to solve or even workaround this.

Any comment or idea would be very appreciated.

Gabriel