[183371] in North American Network Operators' Group
Re: PMTUD for IPv4 Multicast - How?
daemon@ATHENA.MIT.EDU (Mark Andrews)
Mon Aug 31 21:05:41 2015
X-Original-To: nanog@nanog.org
To: Masataka Ohta <mohta@necom830.hpcl.titech.ac.jp>
From: Mark Andrews <marka@isc.org>
In-reply-to: Your message of "Tue, 01 Sep 2015 09:49:47 +0900."
<55E4F62B.6060300@necom830.hpcl.titech.ac.jp>
Date: Tue, 01 Sep 2015 11:05:33 +1000
Cc: "nanog@nanog.org" <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
In message <55E4F62B.6060300@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
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742 INTERNET: marka@isc.org
