Re: [tcpm] comments on draft-ietf-tcpm-icmp-attacks-05

Fernando Gont <fernando@gont.com.ar> Mon, 15 June 2009 02:28 UTC

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Date: Sun, 14 Jun 2009 23:28:26 -0300
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To: Florian Weimer <fw@deneb.enyo.de>
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Cc: "tcpm@ietf.org" <tcpm@ietf.org>, Fernando Gont <fernando.gont@gmail.com>
Subject: Re: [tcpm] comments on draft-ietf-tcpm-icmp-attacks-05
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Florian Weimer wrote:

>>> For instance, I'm not certain that setting the DF bit is only
>>> possible for hosts that support PMTUD ... is there a reference for
>>> that?
>> What's the reason for setting the DF flag for IP packets carrying TCP
>> segments if you don't implement PMTUD?
> 
> You don't have to put randomness into the IP ID field (at least in
> theory; in practice, DF=1 packets get fragmented, too).

Yes, in theory. For instance, IIRC Linux used to zero the IP ID field
when DF was set, but then backed-out this change.

Thanks,
-- 
Fernando Gont
e-mail: fernando@gont.com.ar || fgont@acm.org
PGP Fingerprint: 7809 84F5 322E 45C7 F1C9 3945 96EE A9EF D076 FFF1