Message-ID: <DB30737F060741FFACB69142C65FF035@uwe>
Reply-To: "Uwe Appelt" <appelt AT rainbow-it DOT de>
From: "Uwe Appelt" <appelt AT rainbow-it DOT de>
To: <cygwin AT cygwin DOT com>
Subject: WinSock: Connections/min & MaxUserPort=5000
Date: Sun, 8 Feb 2009 14:38:28 +0100
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Dear Cygwin-guys,

i'm trying to run a bioinformatic tools suite from UCSC
(http://genome.ucsc.edu/) under cygwin and unfortunately there seems to be a
incompatibilty with regard to the standard TcpStack-settings in *ix and
windows. I hope, this is the right place to discuss this.

This is my environment:
1) Windows Server 2003 R2 x64 Enterprise ServicePack2 (can also be
reproduced Win2003 x86 and the WinXP pendants)
2) Cygwin 1.7 (can also be reproduced with current 1.5.25-15 release)
3) blatSuite - official sources:
http://www.soe.ucsc.edu/~kent/src/blatSrc34.zip

What the UCSC-blatsuite does:
The parts i'm using consist of a server app (1), that binds to a
preconfigured tcp-port to accept client (2) inquiries that are sent via
socket connections. The client opens up a new connection per inquiry, sends
the query details to the server, receives the server answer within the same
connection and finally closes the connection. Due to the nature of the
biological context, it is a common task, to use the client to make the
server answer several thousand inquiries per minute. Openeing so many
connections within such a short period of time seems to be a problem for
windows, because after a certain amount of inquies the client aborts the job
(collection of inquiries) and claims the server wouln'd be available.
Browsing the sources reveals, that the client was not able to open a new
connection in such a case.

TCP-problem?
In fact netstat on *ix shows, that there are dozens of connections, even if
the client has already terminated. This is the same on windows with one
exception - the maximum port number, the client uses for outgoing
connections seems to be 5000. Googling this told me things about
connections, held in TIME_WAIT status (a certain period of time, the OS
keeps ports opened/blocked, after they've been closed by the application
code). And it told me, that there is indeed a windows port number limit for
temporary connections, namely 5000.
I guess, the OS blocks ports of recently closed connections for a certain
period of time to avoid that for example a TCP_ACK of a former connection
disturbs the new connection. This clearly leads to a lack of
available/usable ports, as soon as an application opens a certain amount of
connections per second.
It seems, i'm having different chances to solve this. Raising the
MaxUserPort and lowering the TCPTimedWaitDelay settings in
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters]
seems to solve my problem. I think doing so ensures, that the OS has always
enough ports left, that can be used to open new client connections, while
recently closed ports can be blocked in the TIME_WAIT status. However i
feel, that this a weak workaround, since its efficiency only a question of
cpu-performance. Assume an enironment, thats capable to answer a 100 times
more queries (just because of faster cpus), the client app will again fail
due to the higher new connections per second rate.

Has anyone had the same problem (i think, it would be same with a heavily
loaded webserver) and what is appropriate way to circumwent this?

Regards
Uwe Appelt

ps: retrospectively, this post doesn't look like a cygwin question. On the
other hand cygwin provides many tricky things, that *ix apps expect,
especially in network fields.


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