Subject: [core] CoAP over TCP Status Update

From: Hannes Tschofenig <[email-masked]>
Date: Fri, 25 Mar 2016 17:59:09 +0100

Hi all,

at the T2TRG meeting, which took place the two days before the IAB IOTSI
workshop in Santa Clara, we met with various OIC/OCF guys and talked
about ongoing IoT standardization work in the IETF/IRTF as well as in
the OIC/OCF.

We learned that they have selected a different solution approach for
CoAP over TCP: we have selected option L1 from Section 4 of
https://tools.ietf.org/html/draft-tschofenig-core-coap-tcp-tls-04
and they have selected option L3.

In an off-list mail exchange we also received information about the
performance investigations they have done. Here is what we got:

> For 4MB data, the transmission time is
>
>     1. CoAP over TCP L1 + BWT : 70 s
>
>     2. CoAP over TCP L3       : 1.7 s

(BWT = Block-Wise Transfer)

We have not taken the performance aspect of larger file transfers (e.g.,
transfer of a 4MB firmware update) into account when we made the
solution decision in Yokohama. The reason why the OIC/OCF take such
large file transfers into account is because of their desire to use CoAP
also for non-constrained devices.

I believe it would be good to verify / analyse the performance analysis
of the OIC/OCF guys.

In particular, it would be reasonable to compare the performance of
option L1 and option L3 from Section 4 of
https://tools.ietf.org/html/draft-tschofenig-core-coap-tcp-tls-04  with
a CoAP protocol stack, such as Californium and the mbed CoAP library.
Carsten pointed out that L1 is limited to 64 KiB (which is further
reduced by Block's current largest block size of 1 KiB in the analysis
above), while L3 allows for larger payloads. Of course, we may not need
to use Block Transfer in CoAP when we already use CoAP over TCP.

We are trying to find some folks who have time to play around with code
during the next week to have a more informed discussion at the upcoming
IETF meeting. If you are interested to help please get in touch with us
(Carsten, Simon, and myself).

Besides this issue there are also some open issues in the tracker:
https://tools.ietf.org/wg/core/trac/query?component=coap-tcp-tls

We will drop some mails about these open issues to the list in the hope
to get some feedback from you before the IETF meeting.

Ciao
Hannes