IP fragmentation problem on linux kernel 2.6.34.8

silva_antonio · 02-18-2013, 01:30 PM

Hello all, I am experiencing a problem with IP fragmentation.
I am receiving an udp packet that is larger than the MTU and it is fragmented along the network.
I am receiving the two fragments but they are not being reassembled correclty.

The MTU of the system is 1500 and I cannot increase it, because I am getting this error: SIOCSIFMTU: Numerical result out of range.
Is it necessary to build the kernel with support for jumbo frames?

There's any option a should enable on the kernel (2.6.34.8) to support this feature?

Any hint that may assist in debugging this issue is welcome.

The first packet it is considered (bad) as a complete packet and I am getting an error at the app level.

Click image for larger version

Name: fragment1.png
Views: 109
Size: 87.7 KB
ID: 11885

The second packet is the last part of the fragment which should be ignored completely:

Click image for larger version

Name: fragment2.png
Views: 72
Size: 81.1 KB
ID: 11886

As reference I am sending the same packet to my laptop which is implementing correctly the fragmentation re-assembly:

Click image for larger version

Name: Packet_fragmentation_good.png
Views: 55
Size: 10.1 KB
ID: 11887

hogar.strashni · 02-18-2013, 07:22 PM

I'll do the two things I hate. First, I'll give you a link. that provides an explanation and a solution. I find it is splendid, giving a tribute this way

. For future reference, I'll make a resume.

Quote:

The MTU of an interface can be changed temporarily using the mtu option of the ifconfig command:

Code:

ifconfig eth0 mtu 9000

The new MTU will not persist beyond a reboot.
An error of the form: "SIOCSIFMTU: Invalid argument" indicates that the requested MTU was rejected by the kernel. Typically this would be due to it exceeding the maximum value supported by the interface hardware. In that case you must either reduce the MTU to a value that is supported or obtain more capable hardware.

Based on what I read there, to make the changes permanently you'll have to edit "mtu" values in /etc/network/interfaces or /etc/sysconfig/network-scripts/ depending on your distribution. First path is for Debian/Ubuntu, and the latter is for RHEL/CentOS/Fedora. Then restart your network interface.
If you follow the link you'll find out how to check and/or test your new configuration, and how you might change your network configuration to support higher MTU.

Now the secound thing I hate, and I'll still do: Why? Why are you making software that sends packages bigger than default MTU?

silva_antonio · 02-19-2013, 03:56 AM

Quote:

Originally Posted by hogar.strashni

I'll do the two things I hate. First, I'll give you a link. that provides an explanation and a solution. I find it is splendid, giving a tribute this way

. For future reference, I'll make a resume.

Based on what I read there, to make the changes permanently you'll have to edit "mtu" values in /etc/network/interfaces or /etc/sysconfig/network-scripts/ depending on your distribution. First path is for Debian/Ubuntu, and the latter is for RHEL/CentOS/Fedora. Then restart your network interface.
If you follow the link you'll find out how to check and/or test your new configuration, and how you might change your network configuration to support higher MTU.

Now the secound thing I hate, and I'll still do: Why? Why are you making software that sends packages bigger than default MTU?

Hi, I am not sending packets bigger than the MTU. I am receiving the SDP descriptor from a SIP server.