[76996] in North American Network Operators' Group
Re: Broken PMTUD for . + TLD servers, was: Re: Smallest Transit MTU
daemon@ATHENA.MIT.EDU (Iljitsch van Beijnum)
Mon Jan 10 11:03:20 2005
In-Reply-To: <Pine.LNX.4.44.0501101124290.31712-100000@server2.tcw.telecomplete.net>
Cc: NANOG list <nanog@merit.edu>
From: Iljitsch van Beijnum <iljitsch@muada.com>
Date: Mon, 10 Jan 2005 17:02:35 +0100
To: "Stephen J. Wilcox" <steve@telecomplete.co.uk>
Errors-To: owner-nanog-outgoing@merit.edu
On 10-jan-05, at 12:26, Stephen J. Wilcox wrote:
> Shifting topic a little.. any idea why DF is used anyway? I've never
> understood what the purpose of not fragmenting is, and if DF didnt
> exist we wouldnt experience the PMTU missing icmp issues
Good question. According to RFC 791:
If the Don't Fragment flag (DF) bit is set, then internet
fragmentation of this datagram is NOT permitted, although it may be
discarded. This can be used to prohibit fragmentation in cases
where the receiving host does not have sufficient resources to
reassemble internet fragments.
One example of use of the Don't Fragment feature is to down line
load a small host. A small host could have a boot strap program
that accepts a datagram stores it in memory and then executes it.
> Windows appears to always set DF, is there a reason why they did that?
<msbash> Greed, what else? </msbash>
Of course I wanted to see this for myself. I used Quicktime to generate
some UDP, but no DFs, either on Win98 or XP.
