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Re: [core] Typical TCP segment header size for CoAP over TCP

Carsten Bormann <cabo@tzi.org> Sat, 21 January 2023 14:53 UTC

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From: Carsten Bormann <cabo@tzi.org>
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Date: Sat, 21 Jan 2023 15:53:50 +0100
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To: Achim Kraus <achimkraus@gmx.net>
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Subject: Re: [core] Typical TCP segment header size for CoAP over TCP
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On 2023-01-21, at 15:44, Achim Kraus <achimkraus@gmx.net> wrote:
> 
> So 20 bytes.

RFC 9006 notes that some TCP options (SACK, Timestamps) may not bring their full benefit to constrained environments, but may still be useful.  

E.g. [1]:

   If a device with less severe memory and processing constraints can
   afford advertising a TCP window size of several MSSs, it makes sense
   to support the SACK option to improve performance.  


So the question really is which TCP options are in wide use in constrained environments.

20 bytes of course is the baseline value (for data packets; I’d expect MSS [2] to be in most SYN packets of implementations that consider IPv4).

Grüße, Carsten

[1]: https://www.rfc-editor.org/rfc/rfc9006#name-selective-acknowledgments-s
[2]: https://www.rfc-editor.org/rfc/rfc9006#name-maximum-segment-size-mss

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