Re: Last Call: <draft-ietf-intarea-ipv4-id-update-05.txt> (Updated Specification of the IPv4 ID Field) to Proposed Standard

Joe Touch <touch@isi.edu> Wed, 06 June 2012 00:55 UTC

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Date: Tue, 05 Jun 2012 17:55:07 -0700
From: Joe Touch <touch@isi.edu>
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To: Masataka Ohta <mohta@necom830.hpcl.titech.ac.jp>
Subject: Re: Last Call: <draft-ietf-intarea-ipv4-id-update-05.txt> (Updated Specification of the IPv4 ID Field) to Proposed Standard
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Hi,

On 6/3/2012 12:12 PM, Masataka Ohta wrote:
> C. M. Heard wrote:
> 
>>> Existing routers, which was relying on ID uniqueness of atomic
>>> packets, are now broken when they fragment the atomic packets.
>>
>> Such routers were always broken.  An atomic packet has DF=0 and any
>> router fragmenting such a packet was and is non-compliant with
>> the relevant specifications (RFCs 791, 1122, 1812).
> 
> Thank you. I have overlooked that atomic implied DF=1.
> 
> But, then,
> 
>     >>  Sources emitting non-atomic datagrams MUST NOT repeat IPv4 ID
>     values within one MSL for a given source address/destination
>     address/protocol triple.
> 
> makes most, if not all, IPv4 hosts non compliant if MSL=2min.

This is already noted throughout this document, however there is little
impact to such non-compliance if datagrams don't persist that long.

> Worse, without hard value of MSL, it is a meaningless
> requirement. Note that MSL=2min derived from RFC793 breaks
> 150Mbps TCP.

It breaks at 6.4 Mbps for 1500 byte packets, as is already noted in the doc.

> The proper solution, IMHO, to the ID uniqueness is to request
> a destination host drop fragments from a source host after
> it receives tens (or hundreds) of packets with different IDs
> from the same source host.

That doesn't help ID uniqueness; it helps avoid fragmentation overload.
FWIW, such issues were discussed at length in the INTAREA WG when this
doc was developed.

> A source host, then, is only required to remember the
> previous ID used for each destination.

They don't do anywhere near that right now, but even if they did it
would still be prohibitive in speed (as per above).

Joe