Subject: [lp-wan] SCHC, single padding proposal

From: <[email-masked]>
Date: Wed, 30 May 2018 17:05:59 +0000

Dear all,

In the current revision of the SCHC draft, draft-ietf-lpwan-ipv6-static-context-hc-13<https://tools.ietf.org/html/draft-ietf-lpwan-ipv6-static-context-hc-13>, the generic SCHC mechanism does padding at most twice for each datagram:
- A first time after header compression.
- A second time after fragmentation (on the last fragment only).
This results in at most 14 superfluous bits be transmitted for each datagram (when padding to bytes). See Section 8<https://tools.ietf.org/html/draft-ietf-lpwan-ipv6-static-context-hc-13#section-8>.

At today's interim meeting, I presented a proposal to do padding at most once. This result in a most 7 superfluous bits per datagram. The proposal also better compresses the SCHC ACK bitmaps.
See my slides at https://datatracker.ietf.org/meeting/interim-2018-lpwan-05/materials/slides-interim-2018-lpwan-05-sessa-aggregated-slides, pages 7  to 21.
At the interim meeting, the attendees agreed with the proposed change.
The implementers with Acklio agreed that the code change is small.
The draft text is already on GitHub, in a separate branch, and can be merged promptly.
See the minutes of the interim meeting https://datatracker.ietf.org/meeting/interim-2018-lpwan-05/materials/minutes-interim-2018-lpwan-05-201805301700
The proposed changes to the draft can be seen at https://github.com/lp-wan/ip-compression/pull/20/files

If anybody was not at today's interim meeting and cares about padding, please express your opinion within the next few days so that the WG can make a decision on this change.
Thanks,

Dominique

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