Re: [core] #388 (coap-tcp-tls): Multiple versions over the same connection

[Carsten Bormann <[e-mail]>]

Hi Christian,

> Is there any assumption that new CoAP versions need to be backward
> compatible? I didn't see anything mentioned in RFC7252.

UDP CoAP packets have a "version number" because there were two bits
free in the header and we wanted to have a specific bit pattern in there
so CoAP packets multiplex nicely with DTLS and STUN packets on the same
port.

Nobody had or has any idea what a future version of CoAP would look
like; no need for one has come up*).  A different version number would
only be used to ensure that current CoAP implementations never process
messages in a future new format; CoAP's protocol evolution method is to
use options.

In a reliable stream (TCP/TLS/Websockets), there is no need to indicate
a protocol version per-message; if version negotiation is ever required,
this can be done at the start of the stream.

Grüße, Carsten

PS.: I'm not sure people have tried using different HTTP versions (1.0
vs. 1.1) within the same TCP connection; not that HTTP versions ever
meant that much...   (See the note in Section 3.5 of RFC 7540 for the
HTTP/2 view on that.)

*)  OK, maybe we want to have a longer message ID in a future high-rate
variant of UDP CoAP.  No message-IDs are needed in TCP CoAP...