fragmenting network packet

[ehsanen77]

Hi, i'm designening a simple router for a customized network. in this network there is a LAN behind my router which wants to send their TCP/IP packets through me. but this special networl doesn't allow sending 1500 bytes sized packets, so i need to fragment network packets. now i have two thoughts in my mind but don't know how these thoughts can be implemented in c++.
1. to fragment packets inside my router which make my router so busy
2. somehow inform computers in the LAN to fragment their packets before sending them to me

is each of these thoughts possible? and if yes how?

Thanks

[Mudgen]

Should be done by the computers. Read RFC4821 on PMTU discovery, which references prior RFCs on the subject also.

[ramin.honary]

So your "customized" network uses TCP/IP, but the packet size is different? I believe all you need to do is set the MTU for the interface that connects to the "customized" network using Ifconfig. For example, if your router has two ethernet interfaces Eth0 at 111.111.111.0, and the "customized" network is Eth1 at 222.222.222.0, then you can do this:

ifconfig eth1 down
ifconfig eth1 mtu 1200 address 222.222.222.0 netmask 255.255.255.0 up

Then set your routing table using Route:

route add -net 222.222.222.0 netmask 255.255.255.0 gw 111.111.111.0

Where 111 is your ordinary network, and 222 is your customized network.

If your "customized" network is does not allow TCP packets, your router must translate all data from the TCP/IP network into the protocol used by the "customized" network. If your router must translate all packets, then it is best to fragment packets in the router as part of the translation phase. TCP sockets can be read as a stream, and it is very easy to create a buffer of the correct fragment size and read bytes from the socket into the buffer. Then, the router can translate the buffer and transmit it to the special network segment using the correct fragment size.

If you wish to encapsulate TCP packets for use in the "customized" network, you will need to retrieve packets before they are presented to the socket, store them in a large buffer, and then read data fragments from the buffer to encapsulate them for the protocol on your special network. The Tcpdump program can capture all incoming TCP packets and store them into a file or fifo. You can then read this file or fifo for encapsulation.

[BrianMicek]

Hi ehsanen77,
For a router, its probably best if you use a Linux kernel to accomplish what you want to do. It sounds like your developing code to work with a TUN or TAP and reading/writing to and from a user application? If you want to tunnel packets, then openvpn is really good. In the later openvpn case, fragmentation and reassembly is handled by the kernels at each end of openvpn and not inside openvpn itself as far as I know. Its really difficult to handle a TCP stack in userspace.

[ehsanen77]

Thank you all my friends, these are good aproaches.