Subject: (IPng 6512) Re: Host Fragmentation

From: Gunnar Lindberg <[email-masked]>
Date: Thu, 24 Sep 1998 11:42:06 +0200 (MET DST)

>From: [email-masked]
>Date: Wed, 23 Sep 1998 19:38:38 EDT
>Message-ID: <[email-masked]>
>To: [email-masked], [email-masked]
>Cc: [email-masked]
>Subject: (IPng 6511) Re:  Host Fragmentation

> ...

>The document is practically useless as an input into the design of the
>protocol of the future. None of the analysis actually indicates any
>real flaws with fragmentation as a procedure when it is used
>properly. Almost all the problems identified in this paper are
>problems associated with 1987 software and hardware including without
>limitation:

There is at least one case where fragmentation can cause severe
problems (page 4 in "Fragmentation Considered Harmful"). If you
fragment and one of the fragments gets dropped:

    a)	The entire IP-packet is lost.
	Maybe no big deal, that's life on the Internet.

    b)	The other fragments may still have been transmitted,
	using up bandwidth and other (network) resources.
	Now, that *is* bad.

That's reality. If that requires another paradigm is beyond me.

		Gunnar Lindberg
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From: James Carlson <[email-masked]>
Date: 24 Sep 1998 07:53:10 -0400

> 	using up bandwidth and other (network) resources.

Quite true.  To avoid that, we'd need some kind of horrible ATM-like
PPD/EPD trick, which implies per-flow state.

This congestion problem with IP fragmentation also gets noticeably
worse as the drop rate goes up.  You very quickly into a situation
where the only things that are sent are worthless shards of broken
packets because IP, unlike TCP, is not congestion-sensitive.  And all
of those fragments with offset==0 cause stale reassembly buffers to
time out in the affected hosts ...

(Wonderful congestion-avoidance tricks like RED would seem to make the
problem worse still, since they cause statistical rather than bursty
packet loss, which is exactly the situation in which IP fragments fail
to reassemble at high rates.)

-- 
James Carlson, Consulting S/W Engineer  <[email-masked]>
IronBridge Networks / 55 Hayden Avenue  71.246W    Vox:  +1 781 372 8132
Lexington MA  02421-7996 / USA          42.423N    Fax:  +1 781 372 8190
"PPP Design and Debugging" --- http://people.ne.mediaone.net/carlson/ppp
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