Broken PMTUD for . + TLD servers, was: Re: Smallest Transit MTU

Iljitsch van Beijnum iljitsch at
Mon Jan 10 16:02:35 UTC 2005

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.

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