Subject: Re: [core] WGLC draft-ietf-core-coap-09 Reuse of MID

From: Maciej Wasilak <[email-masked]>
Date: Sun, 16 Sep 2012 21:35:35 +0200

Hello,

2012/4/20 Angelo P. Castellani <[email-masked]>
>
>
> In my opinion we need a window smaller than 256 seconds.
>
> A possible solution would be to use an adaptive value, proportional to
> the measured RTT. However there is no easy way to measure RTT,
> especially for a CoAP server.
>
> (based on dreft-ietf-core-coap 11, point 4.5)

I believe for constrained nodes, it is possible to act differently
depending on the type of the message. GET requests are safe and idempotent,
so skipping deduplication is not dangerous. At worst, a node answers
request twice with different value. Observe notifications have a sequence
number, so it's possible to discard duplicate notifications, that have
sequence number smaller than the newest received one (unless you need all
responses). On the other hand POST requests should be deduplicated as
throughly as possible, because every execution of POST can change all
further results.

So if you have only several slots for deduplication, you can save them for
tricky messages, and skip deduplication for safe ones.

Best Regards
Maciej Wasilak

---

From: Carsten Bormann <[email-masked]>
Date: Mon, 17 Sep 2012 00:12:36 +0200

On Sep 16, 2012, at 21:35, Maciej Wasilak <[email-masked]> wrote:

> So if you have only several slots for deduplication, you can save them for tricky messages, and skip deduplication for safe ones.

Indeed, both section 4.5 of draft-ietf-core-coap-11.txt and section 4.4.2.3 of draft-ietf-lwig-guidance-02.txt try to make this point.
If you have suggestions on how to make the text (in particular of the latter) more accessible, I'm all ears.

Grüße, Carsten