[core] ack message format in confirmable notifications

[Matthieu Vial <[e-mail]>]

Hi all,

There is no example of confirmable notification in RFC7641 and I would like
to clarify the format of the ack message sent by the coap client.

Basically, an ack message to a confirmable notification has no content and
no REST semantics so the code is 0.00.

According to https://tools.ietf.org/html/rfc7252#section-3:
"As a special case, Code 0.00 indicates an Empty message."

So this message should be an empty message.

According to https://tools.ietf.org/html/rfc7252#section-4.1 an empty
message is built as :

"An Empty message has the Code field set to 0.00.  The Token Length
   field MUST be set to 0 and bytes of data MUST NOT be present after
   the Message ID field.  If there are any bytes, they MUST be processed
   as a message format error."

If my understanding of the RFC is right, as a CoAP implementer, do you
enforce 0-length token on server side. Is it something you check in
plugtests ? Is it something we want to enforce in an implementation ?

Thanks,
Matthieu