PMTUD for IPv4 Multicast - How?

Tue Sep 1 01:05:33 UTC 2015

In message <55E4F62B.6060300 at necom830.hpcl.titech.ac.jp>, Masataka Ohta writes:
> William Herrin wrote:
> 
> >> for routers, generating ICMP PTB is as burdensome
> >> as generating fragments?
> > 
> > No, it isn't.
> 
> Yes, it is. Generating an ICMP PTB is as burdensome as
> fragmenting a packet.

Well it could be done at wire speed.  It just requires more complicated
hardware.  Routers usually punt it to the cpu but there is no real
reason that they have to do that.  There is no theoretical reason
why it has to be more burdensome than forwarding a packet.  It's a
implementation choice.

> > When a router fragments a packet, it has to fragment the next and the
> > next and the next. Maybe tens or hundreds of thousands of packets
> > before the end of that one user's session.
> 
> Not necessarily, because transport layer can react against fragmented
> packets.
> 
> > When a router generates a PTB, there is no next. PTB is a soft
> > failure. The origin must correct the error (by reducing packet size)
> 
> What if, the origin does not reduce packet size?

The communiction fails.  Additionally routers normally rate limit
PTB generation thereby reducing cpu loads to a acceptable level
which is the whole point of moving the fragmentation to the originating
node.

> 							Masataka Ohta
> 
-- 
Mark Andrews, ISC
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