[core] CoAP over TCP

[Guido Moritz <[e-mail]>]

Dear WG,

as stated in the current CoAP draft TCP as transport is out of scope.
Nevertheless, has anybody ever tried to use CoAP over TCP? We would be very
interested in the experiences and problems!

We have to use CoAP over TCP, because TCP and TLS are required by a specific
security profile we are implementing. The security profile is independent
from CoAP itself, but TCP and TLS are mandatory. So what we are trying to
realize is a persistent TCP connection to have the handshakes etc. only one
time at the startup of the connection. 

At a first glance, the major issue we identified is the missing length
information for the packet size when using TCP instead of UDP. The solutions
we came up with are either (1) a new length option for CoAP similar to HTTP
or (2) use blocking to indicate if for this request more payload will be
send or if this is the last packet. At (2) we are not sure if a sender can
indicate for small payload which fits into one packet that the first block
is also the last.

Has anybody on the list comments or further experiences. It might already
have been discussed somewhere, but then I have missed this discussion.

Regards,
Guido