Where is the Don't Fragment Bit of the IP Flags used?

Question

I am curious to know where the \"Don\'t Fragment\" [DF] Bit of the IP Flags is used. As fragmentation is invisible to higher layers and they don\'t care too.

I am also l

Answer 1

Do note that there is no standard way to set DF in C. On Linux, this code works:

result = setsockopt(mysocket, IPPROTO_IP, 
                IP_MTU_DISCOVER, IP_PMTUDISC_DO, sizeof(int));

but it does not on FreeBSD 6

Also, Path MTU discovery is extremely unreliable on the real Internet. Too many broken firewalls and middleboxes filter out ICMP "Packet too big" messages (here is a good way to test a candidate network administrator during an interview: ask him/her to stop ping and he/she will probably block completely ICMP.) See RFC 2923: "TCP Problems with Path MTU Discovery"

That's the reason why the IETF now suggest a new way to test the MTU, without relying on Path MTU Discovery: RFC 4821: "Packetization Layer Path MTU Discovery"