I run a Internet gallery with app. 1300 artists who upload images of art. Most of them can upload images of any size. But a few (mayby 10-20) has the following problem:

Every time they upload a file it stops at app. 64 kb.

I have searched for answers for a very long time. I have found some articles about TCP window scaling, eg:
http://lwn.net/Articles/92727/

I have tried turning windows scaling on and off, but it doesn't change anything.

Has anybody experienced something like this before?

My server is the following:
Red Hat Enterprise Linux Server release 5.2
Kernel 2.6.18-92.el5


Best regards,
Peter.

Seems like it is an issue with the users, and not the server. Have these users tried using a different web browser? Have you been able to replicate the issue?

The users who have the problem can upload files of any size to other web pages (facebook, gmail, etc), but not mine. And it does not help if they try different browser.

One of the users who had the problem have Windows Vista. He could upload large files to other web pages, but not mine. One day he did a Windows update, and the problem disappeared. So for me it looks like the problem is a interplay between my server and the clints OS. But it seems like other web pages does not have this problem, so it should be possible to solve it by changing something on the server.

I have tried to install two different webservers on my Linux server, and the problem exist on both. So I suppose it must be the OS which causes the problem.

The problem only exist for Windows Vista and Windows 7 users. But not all Windows Vista and Windows 7 users. Only some of them. One person told me, the she had the problem with Windows 7, but when she installed Window XP on the same machine the problem disappeared.

For me it sounds very much like this TCP window scaling problem:
http://lwn.net/Articles/92727/

But it did not solve my problem when I changed the tcp_window_scaling setting.

Hi,

i agree with others, that this will be a client issue.

One of the most common problems i detected with this behaviour is a wrong MTU on the client network/router. Check that clients firewall does not block all ICMP traffic.

ICMP transfers fragmentation information to the client (need fragment bit), if this is not received by the client problems like yours can occur, because the clients sends packets which are to large for the transfering network.

Hi

You are completely right!
I investigated the server firewall, and the ICMP was blocked. I changed it so the ICMP is permitted, and now every upload problem to my webpage/server has disappeared.

Thank you very much