tcp retransmission

desrochj1 · ‎09-28-2010

Hello,

i have a servers located on a dmz (ASA 5520) wich we communicate in the

inside network with ssh.

Uploading files with ssh to this server is ok and wireshark traces (tcp communications) are always fine.

(upload files with winscp from inside to that server located in dmz is ok)

But everytime we try to download a file from this server to a pc on the inside network. (download with scp)

network performance is decreased and it takes forever to download. I can see tons of tcp retransmissions

with wireshark.

I checked network interfaces and got no crc errors, no input / output errors on switch interface.

can you help me on this ?

Ashley Georgeson · ‎09-28-2010

Hi desrochj1

Interesting problem. Do these retransmissions correlate against any tcp window sizes?

TCP retransmissions would effectively be requested by the receiver through use of selective acks etc, so it implies that the segmentsd are either not reaching the destination or the acknowledgements are not reaching the sender properly.

If you have the time, I would set up a small file transfer first using ssh then ftp etc and capture at each interface in the path.

Time consuming, but you could then compare the tcp streams to isolate at which point the problem occurs.

There may be far simpler and helpful troubleshooting methods than mine that are posted however!

Regards, Ash.