Re: [Lksctp-developers] SCTP dropping chunks in multi-threaded implementation

[Vlad Y. <vla...@hp...>]

John Gilmore wrote:
> Hi,
> 
> I am working on a multi-threaded implementation of SCTP in which each SCTP
> association is managed by one reader thread and one writer thread.  Whenever
> I run more than one association from my process, SCTP seems to periodically
> drop data chunks.

I am assuming you are actually this on a socket bases in 1-to-1 sockets, right?
I say this because there is really no way to receive a packet for a specific
association.  You can do sends on an association, but not receives.

> 
> I am running tcpdump on the server on which my process is running and I see
> that my associations are sending gap acks for packets that were received in
> the tcpdump.  I am trying to run at the relatively low rate of 1500
> bi-directional transactions per second each for the two associations.

They may have been dropped due to lack of receive space.

Some questions:
Which kernel version are you running?
What is the value in /proc/sys/net/sctp/sctp_rmem?

Does the behavior change if you do:
	echo 1 > /proc/sys/net/sctp/rcvbuf_policy

> 
> My network is very clean (5-6ms RTT) with no packet loss.  This problem
> never occurs if I run only one association from my process, even if I run
> higher rates (e.g. 4000 bi-directional transactions per second).
> 
> I have tcpdump captures that show that all packets my association requests
> retransmission for were actually received on the box.  This problem is easy
> to reproduce.  I suspect that the SCTP code has some problem with thread
> safety.

All of the SCTP code is in the kernel and really could care less about user
level threading.

In your wireshark traces, what does the a_rwnd look like in these SACKs that
are reporting gaps.  Also, can you post the output of:
  cat /proc/net/sctp/assocs

that would contain some useful information.

Thanks
-vlad

> 
> I am looking for some help with finding and correcting the problem.  I can
> provide more information as needed if I know what will be most helpful.
> 
> Thanks!
> 
> John
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