[183367] in North American Network Operators' Group
Re: PMTUD for IPv4 Multicast - How?
daemon@ATHENA.MIT.EDU (Masataka Ohta)
Mon Aug 31 20:42:24 2015
X-Original-To: nanog@nanog.org
Date: Tue, 01 Sep 2015 09:42:18 +0900
From: Masataka Ohta <mohta@necom830.hpcl.titech.ac.jp>
To: nanog@nanog.org
In-Reply-To: <CAP-guGV2E-4cEHHQGLxyNW32cZz82D-e9+tu8obfF2fMt8qq3A@mail.gmail.com>
Errors-To: nanog-bounces@nanog.org
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
