3 ms·
serious question, how would be different from sending packed with no-fragment bit set?
by Szpadel 3y ago
serious question, how would be different from sending packed with no-fragment bit set?
- zamadatix 3y agoThe behavior of DNF is you either forward the whole thing or drop the whole thing.
- deathanatos 3y agoThe DF bit will cause the packet to get dropped & an ICMP response to maybe get generated. If your route looks like this, E_a -- MTU 1500 --> R_1 -- MTU 1200 --> R_2 -- MTU 1000 -> E_b Where E_a & E_b are the endpoints, with two routers. There PMTU is 1000; if you first emit a 1500 B packet with DF, R_1 will drop it, and emit an ICMP with the next hop MTU of 1200. You then have to emit another packet, and get another ICMP response from R_2 with the MTU of 1000, and now you know the PMTU, but it takes two failures to get there. OP's suggestion is a single round trip: you emit a 1500 B packet, E_b receives a truncated 1000 B packet. E_b responds to E_a saying "that got truncated to 1000 B" and now E_a knows the PMTU, in a single shot. In an simple design, we can call the truncated packet "worthless" and force E_a to repeat it with the right PMTU, but it's still only too a single packet from E_a to elicit the right value for the PMTU from the network.