Subject: [Sipping] RE: SIP/SDP dictionary for SigComp

From: Adam Roach <[email-masked]>
Date: Mon, 28 Oct 2002 18:21:41 -0600

> If somebody thinks that this dictionary should be optional rather than
> mandadory, speak up now and state your reasons. We would like to have
> this work finished pretty soon.

It is demonstratably not necessary, and has caused actual deployment
issues. I say this as one of a very few implementors (if not the
only implementor) who has actually put SigComp code on an low-end
3G handset.

- SigComp has methods for discovery of offered states, and they are
  rather trivial to implement. They can be used to advertise
  availability of the SIP dictionary.

- I have personally run into embedded environments in which the
  additional 8kb required for the SigComp SIP dictionary is the
  make-or-break decision between going with SigComp or going with
  a proprietary solution. I don't want a proprietary solution to
  "win" just because of a single *unnecessary* "MUST" strength
  requirement in this document. I suspect you don't either.

If you have operational experience that you beleive mitigates or
refutes either of these statements, I'd be interested to know
about it.

/a
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---

From: Gonzalo Camarillo <[email-masked]>
Date: Tue, 29 Oct 2002 11:09:55 +0200

Adam,

>   availability of the SIP dictionary.

I was wondering what happens with the first message that two entities
exchange. Since they cannot know whether or not the dictionary is
supported at the other end, they would not be able to use it, right? 

>   requirement in this document. I suspect you don't either.

I guess they are concerned about ROM memory, because the use of
Compression saves quite a lot of RAM memory in buffers.

> refutes either of these statements, I'd be interested to know

No, I do not have any operational experience. I am only trying to reach
consensus. That is why I wanted to hear the opinion of people working on
this particular draft.

I understood that this was an open issue, so I expect to get more
opinions on this... and if we don't, it would mean that the issue was
not open at all and that we can progress the draft making its support
optional.

Further comments?

Gonzalo

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From: Carsten Bormann <[email-masked]>
Date: Tue, 29 Oct 2002 14:16:21 +0100

In my view the arguments are:

Pro mandatory:
-- this is the right choice for the general Internet.
    Here, often, SIP entites have not talked to each other before 
exchanging invites.
    Having a known good compression dictionary actually saves bytes on 
this initial exchange.
-- the dictionary is highly compressible itself.  
    Since any sigcomp implementation ostensibly already has a UDVM and a 
decompressor it can send to the other side, the dictionary can be 
stored in compressed form, reducing flash size.  With some 
application-specific compression added, the LZ78 part becomes very 
small.
-- in a tight device, the dictionary can be shared with the string 
table the SIP implementation needs anyway.

Pro optional:
-- this is the right choice for application-specific SIP, where a 
device already has met its proxy/CSCF/whatever before invites are 
exchanged.
-- even the compressed/shared dictionary is still large enough to cause 
headaches.

I don't quite buy the latter (red herring alert), but then I have not 
been there.  Adam has.

Gruesse, Carsten