Re: [TERNLI] Forwarding corrupt packets

Joe Touch <touch@ISI.EDU> Fri, 01 September 2006 14:07 UTC

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Date: Fri, 01 Sep 2006 07:06:44 -0700
From: Joe Touch <touch@ISI.EDU>
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To: Michael Welzl <michael.welzl@uibk.ac.at>
Subject: Re: [TERNLI] Forwarding corrupt packets
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Michael Welzl wrote:
> Hi all,
> 
> Here's an idea for a potentially useful message that
> could be exchanged between end systems and the inner
> network:
> 
>>From transport end point to network:
> "Corruption Acceptable (CA)" (meaning that it would be
> preferrable to forward packets that are corrupt rather
> than drop them)
> 
>>From network to transport end point: "Corruption
> Forwarding supported (CF)"
> 
> Purpose: help the end system decide whether to use
> UDP-Lite, or partial checksums in DCCP, or the
> Data Checksum option in DCCP.

Why does the _network_ need to know about these? The network doesn't
check (or shouldn't check) transport checksums.

The only reason the network would think a packet is corrupt:

1) bad net checksum (e.g., IPv4)

2) bad link checksum

In both cases, the destination address is not trusted anymore, so you're
potentially sending the corrupt packet to the wrong _place_. If you
can't send it the right place, then why are you sending it?

This isn't a new issue; it's one of the reasons for the partial checksum
in lite/DCCP - but also why it should be only over the _data_ portion.

In those conditions, you might end up with one e2e-pair causing a
separate endpoint to throttle-back thinking its packets are corrupted.
That cross-contamination seems like a sufficient reason not to do this.

IMO, partial transport checksums are useful only where the header
checksum is still valid; otherwise, there's no point in interpreting the
header at all.



Joe