Problem with MTU size and don't fragement switch (General questions)

My application has been working perfectly in a WAN and LAN environment, but we have now encountered a problem in a wireless broadband environment which requires a MTU size 1352. On transferring a file we always get error 30027 as a result of a timeout in sending the data. My network support has traced the packets and found 1) the sending PC appears to have ignored the target linux system MTU size and is using too large a MTU and 2) the Don't Fragment (DF) is on. The result is the wireless network throws away the packet. We believe the problem can be solved in two ways - A) use the correct MTU (I know this works since when both transmitter and receiver are on the same wireless network the correct MTU size is used), and B) do not set the (DF) flag. Has anyone any experience with these issues? Who sets the (DF) flag - wodsftp or is it forced by Windows (2K and XP)? How is the MTU size for the connection determined?

Hi John,

This is fist time that someone has that kind of problem.

Please try with some other SFTP client. If you have same problem than wodSFTP is not responsible for that problem.

Perhaps problem could comes from your router setting or as you mention windows maybe .

Let us know how it goes.

Regards,
Drazen

John,

hi. We cannot handle this. You talk about layer which is not to be used by applications. You can define MTU on your system, not MTU for specific socket - so it's below our reach actually.

You can, if you wish, change BufferSize to lower down size of our packets - but please do not mix our 'packets' with TCP or IP packets - this is something completely different.

Kreso

Gentlemen thank you for your help - the problem is resolved. When a new connection is established TCP/IP allows the various nodes on the route to reduce the size of the MIU if they cannot handle the proposed size. It appears that some wireless broadband networks do not do this. The result - error 30027 when sending data via SFTP, and guess who gets the blame.... your application does not work on wireless says the customer. We have solved the problem by reducing the MIU size on the Linux definitions from 1500 to 1352 with the result that the proposed MIU size is now acceptable to the wireless network.