Hi! On 01:47 Sun 13 Jul , Sudharsan Vijayaraghavan wrote:
Hi,
i find that even when /proc/sys/net/ipv4/ip_no_pmtu_disc is 0 (default value), that is path mtu is enabled, IP layer fragments the packets if it exceeds MTU
For example MTU is 1500, i do ping -s 15000 <ip addr>, IP layer nicely fragments the packets, how is that possible?
Path mtu discovery basically means that "fragmentation" is done on layer 4 (e.g. TCP) and not on layer 3 (IP). If it is enabled the "do not fragment" bit in the IP header is set. If such a packet is about to be sent via a link with a small MTU instead of being fragmented an "fragmentation needed" header is sent back to the sender. The sender will then sent smaller packets. Generating packets with the right size at the sender is supposed to be faster than fragmenting them in the middle of a route. Protocols like ICMP or UDP cannot do "fragmentation" themself. This is why path mtu discovery does not have any effect on them. -Michi -- programing a layer 3+4 network protocol for mesh networks see http://michaelblizek.twilightparadox.com