OutLook client freezes from time to time on port 1026

OutLook client freezes from time to time on port 1026
Hello,

Our OutLook clients using a central Exchange server freeze from time
to time for some 30 seconds
or even minutes. I have looked deeper into the communication of a PC
with the Exchange server
and it turned out that in such a situation the Exchange server is
contacted from the PC on
port 1026 but not responding, not even with ACK packages; the client
is re-sending the IP package
to port 1026 until giving up and establishing a new connection to port
1026 which works fine; here is
the trace of such a situation (xxx.xxx.xxx.xxx is the IP of the
Exchange server):

the communication is fine until 10:15:26.545503:

...
10:15:26.253736 IP xxx.xxx.xxx.xxx.1332 > 10.0.1.101.1260: P
249:361(112) ack 352 win 65535
10:15:26.255094 IP 10.0.1.101.1257 > xxx.xxx.xxx.xxx.1026: P
686150432:686150608(176) ack 598268727 win 64727
10:15:26.545503 IP 10.0.1.101.1260 > xxx.xxx.xxx.xxx.1332: . ack 361
win 65423

at 10:15:26.748741 the PC tries to send some 176 bytes to the Exchange
server to
port 1026 and re-send this again and again because of no ACK from
Exchange:

10:15:26.748741 IP 10.0.1.101.1257 > xxx.xxx.xxx.xxx.1026: P
0:176(176) ack 1 win 64727
10:15:27.842432 IP 10.0.1.101.1257 > xxx.xxx.xxx.xxx.1026: P
0:176(176) ack 1 win 64727
10:15:29.920447 IP 10.0.1.101.1257 > xxx.xxx.xxx.xxx.1026: P
0:176(176) ack 1 win 64727
10:15:33.857730 IP 10.0.1.101.1257 > xxx.xxx.xxx.xxx.1026: P
0:176(176) ack 1 win 64727
10:15:41.951011 IP 10.0.1.101.1257 > xxx.xxx.xxx.xxx.1026: P
0:176(176) ack 1 win 64727

it gives up this dead connection to port 1026 and establishes a new
one at 10:15:58.037584:

10:15:58.037584 IP 10.0.1.101.1262 > xxx.xxx.xxx.xxx.1026: S
1448428914:1448428914(0) win 65535 <mss 1460,nop,nop,sackOK>
10:15:58.061976 IP xxx.xxx.xxx.xxx.1026 > 10.0.1.101.1262: S
2518217535:2518217535(0) ack 1448428915 win 16384 <mss 1460,nop,nop,sa
ckOK>
10:15:58.062064 IP 10.0.1.101.1262 > xxx.xxx.xxx.xxx.1026: . ack 1 win
65535

between 10:15:26.748741 and 10:15:58.037584 (around 32 secs) the
OutLook is
frozen for the user;

the port 1026 is explained in the Microsoft document
http://support.microsoft.com/default.aspx?scid=kb;en-us;280132
as the port for:

    ...
    One port for the Active Directory logon and directory replication
    interface (universally unique identifiers [UUIDs]
    12345678-1234-abcd-ef00-01234567cffb and
    e3514235-4b06-11d1-ab04-00c04fc2dcd2).

    This is typically assigned port 1025 or 1026 during startup.

There is not network trouble between 10.0.1.101 and the
xxx.xxx.xxx.xxx because at
the same time while the PC is trying to resend that IP package to port
1026 a cyclic PING
from the PC to the server xxx.xxx.xxx.xxx is working (responding) fine
(the data is not
included in the above trace, but I've checked this to make it sure);
also the problemless
re-esstablishing of a new connection to the port 1026 makes clear that
the network itself
is working fine;

The main question is: why the communication to 1026 freezes?

Any ideas? Thanks in advance

   Matthias

--
Matthias Apitz
Manager Technical Support - OCLC PICA GmbH
Gruenwalder Weg 28g - 82041 Oberhaching - Germany
t +49-89-61308 351 - f +49-89-61308 399 - m +49-170-4527211
e  - w http://www.oclcpica.org/ http://www.UnixArea.de/
b http://gurucubano.blogspot.com/
date: Thu, 06 Sep 2007 02:15:54 -0700 author: unknown