[ih] TCP RTT Estimator

[Jack Haverty]

A bit more recollection...

One of the reasons that the checksum was considered "adequate" was 
because, at the time, The Internet was in what I've called its "fuzzy 
peach" stage.   The ARPANET was the peach, and the fuzz was all of the 
"local" networks attached to it.   SATNET was the only other long-haul 
component of The Internet.   ARPANET provided a virtual circuit service 
between gateways, which never dropped datagrams, reordered them or 
otherwise behaved in ways that a checksum would detect.  IIRC SATNET 
behavior was similar, but datagrams could be (and were) dropped in 
underpowered gateways.

Most of the focus on checksumming at the time was on dealing with errors 
in circuits.  But there had been experiences in the ARPANET where a 
problem was caused by a hardware failure in an IMP (memory problem).  
There hadn't been much, if any, study or analysis of such failures, and 
we had no idea how the various kinds of underlying networks that might 
be part of The Internet would generate errors.

When there had been more experience with how the actual links between 
gateways behaved, some new checksum algorithm might be created to deal 
with them.  So, ... it was put on the to-do list for the Next Release.  
Perhaps it's still there.

Jack Haverty

On 4/17/25 08:19, David Finnigan via Internet-history wrote:
> On 13 Apr 2025 6:57 am, John Day via Internet-history wrote:
>> Sometime ago, (I think it was Jack Haverty) said that the TCP checksum
>> was a placeholder until they could consult someone to advise them on
>> what to use and it got lost in the shuffle.  ;-)
>
> Even RFC 791 [Sep 1981] states "experimental evidence indicates it 
> [the checksum] is adequate, but it is provisional and may be replaced 
> by a CRC procedure, depending on further experience."
>
> -David Finnigan

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