Subject: [lp-wan] Outline, SCHC Design Team meeting, Februrary 8th

From: Alexander Pelov <[email-masked]>
Date: Thu, 9 Feb 2017 21:39:27 +0100

Dear all,

You’ll find in this mail the outline of the discussions of yesterday's meeting, part of the weekly calls on SCHC. Today’s discussions were primarily on the CoAP draft, with some minor discussions on the IP/UDP draft. Next week is the finalization of the IP/UDP draft and its publishing of -01 version + discussion on the ML.

Best,
Alexander

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Present: Ana, Laurent, Carles, Alexander, Pascal

Discussion topic: IP/UDP SCHC ( draft-ietf-lpwan-ipv6-static-context-hc )
- IP/UDP + Fragmentation
 - Question on fragmentation window size (and related values), which depend on the underlaying technology - how are values put in the draft. The idea is to have parameters (e.g. K+M+N), with recommended values per technology in a separate table in the draft.

Discussion topic: CoAP SCHC ( draft-ietf-lpwan-coap-static-context-hc-00 )
- CoAP
 - The major difference between CoAP and IP/UDP is that it is asymmetric (e.g. REQ/RESP)
   - This leads to questions on how to compress this asymmetric trafic (different rules / marking directions in existing rules)
   - The chosen design is to add a field "direction" - bidirectional / uplink / downlink. 
   - IP/UDP rules being symmetrical will be all bidirectional. CoAP rules will depend.

 - Compressing variable-length fields is less trivial, as the length is not known ahead of time. A new compression function was added for these fields. 

 - A request will be made for the CoMI draft to relax the requirement of MUST confirming a message to SHOULD. There are several reasons behind this request, one of the major ones being that the CON is redundant with an application-level ACK, which taken along with the timers on a different time-scale than usual may lead to unnecessary trafic.
   - For removing the "MUST" constraint for CoAP Observe (and more generic CoAP) use over LPWAN we might consider following the way it was done for CoAP over TCP and add an appendix, which specifies the modification.

Chairs' note: Generally both drafts are advancing pretty well. IP/UDP should be ready for general discussion next week. The design team has taken a pragmatic approach with the CoAP SCHC, which will also ensure timely delivery of the draft, as well as simplified operation. Advanced scenarios may be discussed later on the ML.