[ih] TCP RTT Estimator

[touch at strayalpha.com]

AFAICT, the Internet checksum was basically a trade-off between an additional E2E check and the cost of implementing it in software.

Link devices already had CRCs, but implemented in hardware, where bit-wise mixing is very fast. In software, the same function can require multiple opcodes per bit; the Internet checksum is one opcode per CPU word (1 per byte in the early days, fewer now), i.e., runs around 16-30x faster in software. Because the Internet assumes link layers can vary, there’s no easy way to assume all net devices implement the same CRC in hardware.

This kind of speed difference gave rise to the misconception that *every* SW crypto/check alg was 10-50x faster in HW, which notably isn’t true for some hashes such as MD5.

Joe

—
Dr. Joe Touch, temporal epistemologist
www.strayalpha.com

> On Apr 17, 2025, at 11:34 AM, Leonard Kleinrock via Internet-history <internet-history at elists.isoc.org> wrote:
> 
> Hi Dave,,
> As I understood it, the CRC code was especially easy and powerful for detecting errors, but extremely complicated for correcting errors. I would’ve guessed that the error detection was indeed CRC (the BCH code). 
> Len
> 
> Sent from my iPhone
> 
>> On Apr 17, 2025, at 8:19 AM, David Finnigan via Internet-history <internet-history at elists.isoc.org> 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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