Subject: [lp-wan] Ticket #12 Padding Place

From: Ana Minaburo <[email-masked]>
Date: Tue, 3 Apr 2018 15:41:50 +0200

Is it possible to put padding between SCHC fragment header and payload to
align the byte boundary so to easily put and take the payload?

Non, the padding is put after the payload, because as the payload is also
not aligned is better to align once and not twice.
comment:1 <https://trac.ietf.org/trac/lpwan/ticket/12#comment:1>Changed 5
weeks ago
<https://trac.ietf.org/trac/lpwan/timeline?from=2018-02-26T13%3A04%3A21-08%3A00&precision=second>
 by ana@…
Last edited 5 weeks ago
<https://trac.ietf.org/trac/lpwan/timeline?from=2018-02-26T13%3A06%3A24-08%3A00&precision=second>
 by ana@… (previous
<https://trac.ietf.org/trac/lpwan/ticket/12?cversion=0&cnum_hist=1#comment:1>)
(diff
<https://trac.ietf.org/trac/lpwan/ticket/12?action=comment-diff&cnum=1&version=1>
)
2      Edgar: comment about the whole padding thing. NB-IOT does not need
byte alignement of the payload. There is padding a Layer 2 anyway. Does
padding need to be here, or should be left to the technology dcument?
3      JCZ: agree to remove it from the general SCHC document, and leave it
to the technology documents.
4      Ana: several months ago, decided that padding was best put at the
end.
5      Edgar: padding here is to achieve byte alignement. Also filling a
transport block. leave the latter to the L2. The former may not be needed
on some technologies.
6      Carles: the SCHC doc could mention the two options (padding between
header and payload or at the end). Leave to the tech document to select.
7      Edgar: rather have only one specified, and activate it or not.
Otherwise interop will be hard, especially multi-RAT networks.
8      Laurent: in LoRa, we (Acklio?) use the FPort byte to carry the Rule
ID, in one full byte. Some form of padding!
9      Laurent:
10      Ana: suggests to have a DEFAULT specification in the SCHC document,
and leave it to the technology document to specify differently if it wants!
comment:2 <https://trac.ietf.org/trac/lpwan/ticket/12#comment:2>Changed 1
second ago
<https://trac.ietf.org/trac/lpwan/timeline?from=2018-04-03T06%3A35%3A50-07%3A00&precision=second>
 by ana@…
------------------------------

IETF101

The padding is not mandatory and is optional to the technology-specific
document to give a different solution, in the document we have given some
inputs on how to manage the padding.

In version 10 we have created a new section padding:

Padding is not mandatory and depends on the L2 technology

   - On the way forward, padding is done at most twice:

– After SCHC compression
– Last fragment (All-1) of fragmentation
– Padding length is implicit, not transmitted
– Overall, at most 14 superfluous bits are transmitted

   - On the return path, ACKs may be padded, too.