Subject: RE: [Sip] RE: [rohc] New I-D on compression of SIP

From: Romeo Zwart <[email-masked]>
Date: Fri, 20 Jul 2001 13:38:02 +0200

At 09:28 PM 7/19/2001, [email-masked] wrote:
>> The draft talks about the encoder that learns "in some way" 
>> that the message was received at the other end. Magic I can 
>> think of here would either look very much like an (implicit) 
>> acknowledgement mechanism, where the decoder and encoder on 
>> one side of the link communicate to confirm  message reception. 
>> Alternatively, message confirmation might depend on  direct 
>> signalling from the application layer to the compression layer. 
>> However, since the SIP layer is not aware of the encoder, I 
>> don't think that's a valid approach. I'd be interested to 
>> how this is perceived to work.

>Or explicit acknowledgement, which can be either stand-lone or
>piggybacked on the compressed message from the other direction.
>This makes the compression layer independent of the upper layer
>(e.g. SIP in this case). 

The 3-way SCRIBE acknowledgement seems rather elaborate for 
what it needs to do. The ROGER draft section 5.6 shows that 
at least the third leg of the acknowledgement can be done
implicitly too.

>Of course, if there is some interface
>between SIP and compression layer, we should not exclude the
>use of such interface.

My understanding of the SIP compression draft is that the
interface is left unspecified. However, defining an internal
mechanism to allow the application layer to keep track of the
messages sent at the compression layer seems more complex than
defining a simple acknowledgement scheme.  

>> By the way, I am not so much concerned with the pathological 
>> case you've given above. If lower layers are dropping *all* 
>> requests, a non-updated codebook is probably not my main worry. 
>> But I'd agree that in the case of deteriorating network behavior, 
>> e.g. ramping up towards an overload situation, non-updated 
>> codebooks are going to worsen the problem.
>
>This problem can be alleviated by the static and user-specific 
>dictionary or codebook (see 
>http://www.ietf.org/internet-drafts/draft-liu-rohc-scribe-00.txt,
>a newer version has been submitted to ROHC WG and should appear
>soon). That also allows good compression efficiency for the
>first few messages in a session.

I agree a partially static/profile dependent dictionary would 
be expected to show better results for the initial messages in 
a series of messages. In the case of mid-session network 
deterioration, a dynamic mechanism would add some additional 
overhead trying to update the codebook, whereas a static 
mechanism obviously would not, but I don't think that this 
would push the overall scale to favour static dictionaries.

The version -00 SCRIBE draft does not show any examples or 
compression ratio assessments. Both the SIP compression draft 
and the ROGER draft have some very rough and hard to compare 
examples. Is there a draft that shows assessments of the
achievable compression results with SCRIBE? 

Romeo

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---

From: "Dr. Carsten Bormann" <[email-masked]>
Date: Fri, 20 Jul 2001 13:45:15 +0200

> achievable compression results with SCRIBE?

I have asked Hans Hannu to make his benchmarks available to the WG -- or if
anybody else has some, having them available would help.

Gruesse, Carsten