Subject: [MMUSIC] RE: [Sipping] Some comments on draft-ietf-sipping-sigcomp-sip-dictionary-00.txt

From: [email-masked]
Date: Wed, 15 May 2002 18:21:23 -0500

Hi,

A few comments. Technical stuff first:

1) Page 3. What does "prioritized position" mean? Close to 
end of dictionary? Needs clarification.

(Lars-Erik, I think by priority, the authors mean "good"
or "bad" position. It's just unclear to me what "good"
means here.)

2) Is it really meaningful to list "INVITE" and "INVITE "
(and similar cases) as separate entries in the table? One 
entry with the latter seems to be enough as that is what
is stored in dictionary. I guess everyone should understand
that a string S in dictionary means you have all the prefix 
of S available to compress against. This also applies to
the paragraph in page 4, right before section 3.

Some editorial comments:

- section 1, 3rd paragraph. Do you mean "... it seems sensible 
to investigate ..."?

- section 1, 4th paragraph. The last sentence is a bit hard
to read. Rewording it as follows? 
"The static SIP/SDP dictionary constitutes a SigComp state that 
can be referenced in the first SIP message that the compressor 
sends out."

BR,
Zhigang

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From: "Miguel A. Garcia" <[email-masked]>
Date: Thu, 16 May 2002 11:19:33 +0300

Hi Zhignang:

See the comments below.

> or "bad" position. It's just unclear to me what "good"

It is clear that the text is rather confusing, so we will
clarify in the next version of the document.

As I said in a previous e-mail to Lars-Erik, we analyzed 
the probability of finding each string in a SIP/SDP message.
For instance, headers like "From: ", "To: " and "Call-ID: "
are mandatory in every SIP message, so we allocated priority 1.
Other strings are rarely used in certain error messages
used by certain extensions. We allocated priority 5 to those
strings.

Then, when we created the dictionary, strings with priority
1 are placed at the bottom of the dictionary. This to optimize
certain compression algorithms that will run better performance
if this is the case.

> the paragraph in page 4, right before section 3.

I think it is. You may notice that in SIP, the string "INVITE "
appears in the request line of the INVITE itself, whereas the
string "INVITE" appears in the Cseq, Allow, and other headers.

The addition of both strings does not increase the size of the
dictionary, because you may see that the offset for both entries
is the same, just the length is different.

> - section 1, 3rd paragraph. Do you mean "... it seems sensible

Yes, we will fix it.

> can be referenced in the first SIP message that the compressor

Good. Thanks,

/Miguel

-- 
Miguel-Angel Garcia                     Oy LM Ericsson AB
                                        Jorvas, Finland
mailto:[email-masked]     Phone:  +358 9 299 3553
mailto:[email-masked]        Mobile: +358 40 5140002

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From: Jonathan Rosenberg <[email-masked]>
Date: Tue, 21 May 2002 04:10:49 -0400

"Miguel A. Garcia" wrote:

> is the same, just the length is different.

One comment I had overall is that you've gone for a fairly fine-grained
listing of entries in the dictionary. That is, the entries in the
dictionary are at the level of header name or parameter name or
parameter value. In many cases, certain sequences of these things will
appear in the message. For example, you have Via, SIP, 2.0 and UDP all
listed separately. However, you will almost always see the following:

Via: SIP/2.0/UDP

all together in a message. You'll clearly get much better compression
performance when the dictinonary represents coarser grained elements, as
above. I was wondering, then, about the choice to only include such
fine-grained components, leaving out obvious coarser grained ones like
the Via header above.

As another comment, I think it would be helpful to give some more
details on the usage of this dictionary with sigcomp, perhaps even a
simple example.

Thanks,
Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.            72 Eagle Rock Avenue
Chief Scientist                         First Floor
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From: "Miguel A. Garcia" <[email-masked]>
Date: Tue, 21 May 2002 14:15:04 +0300

Jonathan:

Thanks for your comments. We will work to include your suggestions
in the next version.

/Miguel