Subject: [Int-area] ipv4-id-update question on routers setting DF bit

From: Joe Touch <[email-masked]>
Date: Fri, 01 Apr 2011 02:51:28 -0700

Hi, all,

A separate discussion on possible reuse of the ID and frag_offset fields 
(the latter which I hope has been put to rest) raised an issue which the 
current ipv4-id-update doc does not address:

* Can intermediate devices (e.g., routers) set the DF bit?

It's clear they MUST NOT clear the bit (DF=1 -> 0) as this would 
interfere with the purpose of setting the bit.

However, there doesn't appear to be any explicit advice on setting the 
bit (DF=0 -> 1). I'm still digging around for that, and if anyone knows 
more, please post.

Regardless, it seems that setting the bit is even more dangerous, 
because the returning ICMP too-big messages might not arrive at the 
device at the router which sets it, due to legitimate (and even common) 
path asymmetries.

So I'm wondering whether the doc should also include one of the following:

	- intermediate devices MUST NOT set the DF bit

	- intermediate devices SHOULD NOT set the DF bit
	(this case would require a description of a legitimate case, but
	there does not appear to be any such case which is safe)

Comments would be appreciated.

Joe