[core] CoAP over TCP: Clarifying text for "Max-Message-Size Capability Option"

[Hannes Tschofenig <[e-mail]>]

Hi all,

I provided some additional clarifying text to the Max-Message-Size Capability Option, see
https://core-wg.github.io/coap-tcp-tls/hannestschofenig-max-message-size/draft-ietf-core-coap-tcp-tls.html#max-message-size

Here is the relevant section:

5.3.1. Max-Message-Size Capability Option

The sender can use the elective Max-Message-Size Option to indicate the maximum size of a message in bytes that it can receive. The message size indicated includes the entire message, starting from the first byte of the message header and ending at the end of the message payload.

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Note that there is no relationship of the message size to the overall request or response body size that may be achievable in block-wise transfer. For example, consider the GET with BERT block-wise transfer in Figure 13. If the CoAP client indicates a value of 6000 bytes using the Max-Message-Size option then the exchange will succeed since none of the three transfers will be above the indicated maximum size limit (ignoring the size of the CoAP header and options, which are not shown in the message flow). Consider a second case where the same CoAP client indicates a value of 4000 bytes using the Max-Message-Size option then the CoAP stack on the server will refuse the transmission (or adjust the transmission size) since the payload exceeds the limit indicated by the client.

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Ciao
Hannes


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