Subject: [core] Message type across proxies (was: CoAP over WebSockets version -00 published)

From: Klaus Hartke <[email-masked]>
Date: Thu, 18 Jul 2013 19:36:07 +0300

Hi Floris,

> One remark I have is that - in the CoAP proxy use-case - the WS Client can't
> inform the CoAP proxy which message type (NON/CON) it should use to contact
> the CoAP UDP Server. When talking CoAP over UDP on both ends of the proxy I
> would imagine that a proxy would just re-use the CoAP message-type of the
> incoming request? This remark isn't specific to the WebSockets transport
> though, it applies to all transports that provide reliability. Do the
> authors (or the list) know of any guidelines for this remark (or should we
> help define these)?

This is indeed not specific to the WebSockets transport.

'Confirmable' solely means that the recipient is required to return an
acknowledgement for the message. It's not an requirement that the
message is forwarded by an intermediary as confirmable as well. The
communication between each pair of hops is independent. An
intermediary is free to satisfy a request as it sees fit.

The text in coap-18 suggests that the response to a confirmable
request should be piggy-backed or separate, and the response to a
non-confirmable request should be non-confirmable. But there are
indeed no explicit guidelines on the message type used for forwarding
messages by intermediaries.

My implementation currently uses confirmable messages for all requests
it sends. If there is a need for an end-to-end quality-of-service
indication (like "I really want a response" or "I don't care if the
request or response is lost on the way") then I'd say we need a new
CoAP option.

Klaus