PMTUD for IPv4 Multicast - How?

Tue Sep 1 00:42:18 UTC 2015

William Herrin wrote:

> It'd make more sense to truncate the
> packet, set a flag, and then let layer 4 at the recipient deal with
> negotiating a new size with the sender.

For routers, truncating the packet and setting a flag is as
burdensome as fragmentation or ICMP generation.

Moreover, just with plain fragmentation enabled IPv4 packets, layer
4 can deal similarly.

> You know, end to end principle and all.

PMTUD requires "knowledge and help" (quote from the end to end
argument) of all the intermediate routers. That is, you apply the
end to end argument completely wrongly.

> That'd eliminate the problems with firewall-blocked protocols
> and routers using private IP addresses, the usual culprits for pmtud
> breakage.

With your approach, you will find firewalls dropping truncated packets.

						Masataka Ohta