[Ecn-sane] cautionary tcp tale

[David P. Reed]

This is the stupidity of NAT, which encouraged modifying TCP checksums, which are supposed to be not modified in the network at all.

The whole point of an "end-to-end" checksum is that the middle shouldn't touch it!

Now I get why NAT became popular, though at the time many of us pretty much vomited all over the idea as completely bogus.

I do worry that now there is TCP checksum offloading in NICs, that middleboxes are feeling freer just to throw away and recalculate checksums as they pass through, and not bother to forward non-TCP, nonUDP, nonICMP IP packets at all.

This would be the kind of thing that Cisco, for example, might just do, since they have long thought that they owned the Internet design as a corporate entity.


On Monday, August 12, 2019 7:37pm, "Jonathan Morton" <chromatix99 at gmail.com> said:

>> On 13 Aug, 2019, at 12:30 am, Dave Taht <dave.taht at gmail.com> wrote:
>>
>> https://www.snellman.net/blog/archive/2017-07-20-s3-mystery/
> 
> TL;DR summary:
> 
> Buggy checksum recalculation in a cable modem caused minor packet loss (through
> rejection at the receiver).  In the absence of TCP Timestamps, the retransmissions
> of these lost packets were identical and triggered the same bug.  Result,
> connections to certain particular servers which had the unusual property of
> disabling TCP Timestamps would quickly stall.
> 
> I think a tool could be made to watch a sample of received traffic for this
> pattern: incorrect checksums where the correct checksum is the same each time
> (though different per deployment).  How much network equipment exhibits this bug?
> 
>  - Jonathan Morton
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