Subject: Re: [core] Message ID sliding window

From: Jari Arkko <[email-masked]>
Date: Thu, 30 Jun 2011 01:46:44 +0200

I actually like the freedom to use any approach to assigning message 
IDs. I can imagine a few reasons why it might be easier in some cases to 
have that freedom. As a hypothetical example, maybe I want to store only 
one counter but send messages to multiple other nodes. Also, as Carsten 
pointed out, sequential assignment is not as trivial as it sounds. What 
do you or the receivers do after a reboot?

That being said, if you want your sender to use sequential message IDs, 
it can just go ahead and do it. On the receiver side the issue is more 
complex. COAP has acknowledgments to deal with those cases where you 
absolutely have to sequence some operations, and relying merely on 
sequence numbers would appear to not work in all cases, e.g., when a 
message is lost. Given the reboot problem, a simple sliding window 
approach might not be sufficient to keep track of which messages you 
have received.

As a side note, I scanned the COAP draft today and did not find anything 
about the scope of message IDs. Presumably message ID duplicate 
detection etc should only apply for a given IP source and destination 
pair. Otherwise there will be too many collisions. Is this specified 
somewhere?

Jari