Subject: [core] Reordered notifications (was Re: Comments on draft-ietf-core-observe-00)

From: Klaus Hartke <[email-masked]>
Date: Tue, 2 Nov 2010 18:25:39 +0100

Zach Shelby wrote:
>>> Also, the remaining lifetime can be used to detect if notification
>>> messages got reordered, although this hasn't been formalized yet.
>>
> Klaus has an interesting point, although that might not be the most reliable way to do it (what if your CoAP implementation automatically updates the subscription lifetime, but some application doesn't realize it?). I usually recommend the payload data to include a sequence number or timestamp if this is important.

I'd prefer if the CoAP layer would weed out notifications that arrive
later than more recent notifications.

The goal is that the state observed by the application running on the
client becomes eventually consistent with the actual state of the
resource. If the sequence number or time-stamp is in the payload, it
is the responsibility of the application to determine the latest known
state of a resource. This is something that can be easily done by the
CoAP layer, with the same result. It can simply abstract the
possibility of notifications being reordered away, so applications do
not have to worry about it.

Klaus