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Re: [6lo] GHC: Comments regarding the initial dictionary.

Ulrich Herberg <ulrich@herberg.name> Thu, 19 June 2014 17:03 UTC

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Date: Thu, 19 Jun 2014 10:03:40 -0700
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From: Ulrich Herberg <ulrich@herberg.name>
To: Carsten Bormann <cabo@tzi.org>
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Cc: Thomas Björklund <thomas.bjorklund@effnet.com>, "6lo@ietf.org" <6lo@ietf.org>
Subject: Re: [6lo] GHC: Comments regarding the initial dictionary.
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Thanks Carsten for submitting this new revision, addressing Thomas'
review and my editorial suggestions. Thomas, could you please review
this new update?

Minor nits: When you submit an updated version, change

OLD
       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
       NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       RFC 2119.
NEW
       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
       NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
       RECOMMENDED",  "MAY", and "OPTIONAL" in this document are to
       be interpreted as described in RFC 2119.

(as per the RFC2119 errata)

Also, please update the following reference:
draft-bormann-6lowpan-roadmap-04 -->
draft-bormann-6lo-6lowpan-roadmap-00  (replaced-by)


Regards
Ulrich

On Wed, Jun 18, 2014 at 11:05 PM, Carsten Bormann <cabo@tzi.org> wrote:
> Hi Thomas,
>
> these are very good comments, and I apologize for only getting to them now.
>
>> In my opinion the Upper-Layer Packet Length shall not be a part of the
>> initial dictionary.
>
> I agree that this might complicate the decompressor beyond what is acceptable for a very constrained device.
>
> It turns out that both DTLS and PANA benefit from the embedded length as defined now, so taking this out makes the compression slightly worse for these formats.  But having a perfect compression ratio never was the intention of GHC.
>
> In other cases, reducing the length of the predefined dictionary actually is a boon, as it allows shorter references to the juicy material in there.
>
>> […] Since sequences of zeros can be efficiently compressed
>> using the 1000nnnn code byte my gut feeling is that a carefully chosen
>> initial dictionary with fewer zeros could result in higher compression
>> efficiency.
>
> Indeed.  I would not zero out that part, but simply remove it (see above).
> So GHC would no longer reference the pseudo-header concept from RFC 2460, but would simply say the IP source and destination addresses of the encapsulating header are used as the initial 32 bytes of the predefined dictionary, followed by the static part.
>
>> [..]. In RFC 6282 the
>> term "encapsulating header" is used
>
> Aligning the terminology is good!
>
> I’ll play a bit more with the static part of the predefined dictionary (the change to the dynamic part may throw off something there) and then submit an updated version.
>
> Grüße, Carsten
>
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