FW: [rohc] RE: [Sipping] SIGCOMP and large binary content SIP mes sages

["Mark Watson" <[e-mail]>]

Resent to rohc as first try didn't work...

-----Original Message-----
From: Watson, Mark [MOP:EP10:EXCH] 
Sent: 03 April 2003 15:35
To: 'Price, Richard'; '[e-mail]'; [e-mail];
[e-mail]
Cc: [e-mail]; [e-mail]
Subject: RE: [rohc] RE: [Sipping] SIGCOMP and large binary content SIP
mes sages


Richard, all,

The issue I would like to see addressed is as follows:

The text part of SIP/SDP is obviously highly compressible using very simple
algorithms which refer back to previous parts of the message/earlier
messages. In fact, subsequent messages in a SIP dialog seem to consist
almost entirely of repeated text from previous messages.

So, it seems important to me that large binary content in SIP messages does
not destroy state that will be useful in compressing subsequent messages.

In a wireless environment, the principle advantage of compression is
reduction in the time taken to actually send the message over the wireless
link (this may even be more important than the overall reduction in
signalling bandwidth).

Now if the message contains large binary content, then of course we are 'out
of luck' on the message transfer time stakes, but this shouldn't affect
subsequent messages.

I believe we should document recommendations to SIGCOMP compressor designers
to ensure this aspect is not overlooked (although it is not immediately
clear to me how the compressor (being unaware of the SIP message structure)
might achieve this.)

Is this something that would be appropriate to item 4 below ?

Regards...Mark

> -----Original Message-----
> From: Price, Richard [mailto:[e-mail]]
> Sent: 26 March 2003 13:58
> To: '[e-mail]'; [e-mail];
> [e-mail]
> Cc: [e-mail]; [e-mail]
> Subject: RE: [rohc] RE: [Sipping] SIGCOMP and large binary content SIP
> mes sages
> 
> 
> Hi,
> 
> I think that the issue with SigComp and large binary
> content SIP messages is part of a more general issue -
> namely that before using SigComp to compress a particular
> application we need to standardise the following:
> 
> 1. Mechanism for SigComp discovery
> 2. Resources offered by SigComp
> 3. How to delimit messages
> 4. When to save state
> 
> Currently the SIPPING WG has provided RFCs for 1 and 2,
> but not for 3 and 4. It would be useful to get a draft
> out for these ASAP, as it would clarify several issues on
> how to get SigComp working with SIP (including the issue
> with large binary content SIP messages).
> 
> > Unless someone objects, I'll submit it in ROHC.
> 
> I think that any WG draft should be a product of the
> SIPPING WG, because the proposed solution for delimiting
> SigComp messages will need to take into account the
> behaviour of SIP itself (or we won't be able to multiplex
> SigComp and SIP messages on the same port). However, it
> would be good to get the ROHC WG involved in generating
> the draft as well!
> 
> Regards,
> 
> Richard
> 
> P.S. For anyone who's interested, the mechanisms that
> particular applications need to provide before they can
> use SigComp are as follows...
> 
> ------------------------------------------------------
> 
> 1. Mechanism for SigComp discovery
> 
>    - How does the application know when to use SigComp?
> 
> For SIP this issue is already sorted (RFC 3486).
> 
> ------------------------------------------------------
> 
> 2. Resources offered by SigComp
> 
>    - Does the application need to offer more than the
>      minimum amount of resources?
> 
>    - Does the application need any extra state (e.g.
>      a static dictionary)?
> 
> For SIP we've already got a static dictionary (RFC 3485).
> However, we haven't currently discussed how much memory
> should be provided in a SigComp implementation for SIP.
> If we don't specify a value then SigComp defaults to the
> minimum possible (2K), which is rather tight for some of
> the more generously sized SIP messages!
> 
> ------------------------------------------------------
> 
> 3. How to delimit messages
> 
>    - How does the application distinguish between
>      SigComp messages and uncompressed messages?
> 
>    - Is the application allowed to use "continuous mode"?
> 
> This is the issue that's been raised for large binary
> content SIP messages. If we want to send a mix of
> uncompressed and SigComp messages within the same TCP
> connection, how do we delimit the end of one message and
> the start of another?
> 
> A related issue is how to handle very large SIP messages
> (> 64K). SigComp is able to compress large SIP messages
> by using "continuous mode", where the message boundaries
> are ignored completely and the application data is just
> transmitted as a stream. However, this mode of operation
> raises some additional security issues, so it's up to the
> SIPPING WG to decide whether or not to use it in SigComp
> for SIP.
> 
> ------------------------------------------------------
> 
> 4. When to save state
> 
>    - How much security is needed in order to save state?
> 
>    - Which messages belong to each state "compartment"?
> 
>    - How long should the application keep state?
> 
> This is quite an important issue that doesn't seem to
> have been discussed in the SIPPING WG. It's up to the
> application whether or not to let SigComp save state,
> and for interoperability it's important to have at
> least a general idea of how long to keep saved state,
> when to reject state due to insufficient security etc.
> 
> ------------------------------------------------------
> 
> > -----Original Message-----
> > From: [e-mail] [mailto:[e-mail]]
> > Sent: Tuesday, March 25, 2003 6:00 PM
> > To: [e-mail]; [e-mail]
> > Cc: [e-mail]; [e-mail]
> > Subject: RE: [rohc] RE: [Sipping] SIGCOMP and large binary 
> content SIP
> > messages
> > 
> > 
> > Hi Carsten and Lars-Erik,
> > 
> > I've volunteered to submit a personal draft on this issue. From
> > ROHC WG chairs' point of view, do you think this is the way 
> > to proceed?
> > 
> > Of course, I understood during the IETF56 that Carsten had some
> > doubts on whether this is indeed an issue. My email below explained
> > the issue a bit. I don't know if it's clear. Either way, we 
> > can continue discuss this in ROHC mailing list
> > 
> http://www1.ietf.org/mail-archive/working-groups/sipping/curre
> nt/msg04115.html
> > 
> > BR,
> > Zhigang
> > 
> > > -----Original Message-----
> > > From: ext [e-mail] [mailto:[e-mail]]
> > > Sent: March 13, 2003 9:55 AM
> > > To: [e-mail]
> > > Cc: [e-mail]; Isomaki Markus (NRC/Helsinki);
> > > [e-mail]; [e-mail]; [e-mail]
> > > Subject: [rohc] RE: [Sipping] SIGCOMP and large binary content SIP
> > > messages
> > > 
> > > 
> > > I agree. How to handling binary content is a generic issue for any
> > > application messages. What I have in mind is to give generic 
> > > descriptions/requirements (mainly on the SigComp receiver 
> parsing part),
> > > then touch on SIP (perhaps RTSP too) as a particular example.
> > > 
> > > Unless someone objects, I'll submit it in ROHC.
> > > 
> > > Zhigang
> > > 
> > > 
> > > > -----Original Message-----
> > > > From: ext Gonzalo Camarillo 
> [mailto:[e-mail]]
> > > > Sent: March 13, 2003 5:36 AM
> > > > To: Liu Zhigang.C (NRC/Dallas)
> > > > Cc: [e-mail]; Isomaki Markus (NRC/Helsinki);
> > > > [e-mail]; [e-mail]
> > > > Subject: Re: [Sipping] SIGCOMP and large binary content 
> SIP messages
> > > > 
> > > > 
> > > > Zhigang,
> > > > 
> > > > you can submit that draft to ROHC. SIPPING can review 
> it afterwards.
> > > > IMO, the competence needed to generate this draft can 
> be found more
> > > > easily in ROHC than in SIPPING.
> > > > 
> > > > Besides, you could use SigComp to compress RTSP 
> messages, for instance
> > > > And RTSP is not handle by SIPPING either.
> > > > 
> > > > Gonzalo
> > > > 
> > > > [e-mail] wrote:
> > > > > 
> > > > > Yes, that is also what I had in mind for SIP. But my 
> point is that
> > > > > a SigComp receiver now needs to have some knowledge 
> about application
> > > > > message format and do parsing to determine the end of 
> an uncompressed
> > > > > message. This is something new (or at least 
> unspecified) in RFC 3320
> > > > > and needs to be spelled out in a new (standard track) 
> RFC. The document
> > > > > will be short, but I think it's needed for interoperability.
> > > > > 
> > > > > I'll write up an I-D and submit it after IETF 56. But 
> I'd like to know
> > > > > if folks in SIPPING think it should be done here or in ROHC.
> > > > > 
> > > > > Zhigang
> > > > > 
> > > > > > -----Original Message-----
> > > > > > From: ext Adam Roach [mailto:[e-mail]]
> > > > > > Sent: March 11, 2003 9:03 PM
> > > > > > To: Liu Zhigang.C (NRC/Dallas); Adam Roach; Isomaki Markus
> > > > > > (NRC/Helsinki); [e-mail]; [e-mail]
> > > > > > Subject: RE: [Sipping] SIGCOMP and large binary 
> content SIP messages
> > > > > >
> > > > > >
> > > > > > > -----Original Message-----
> > > > > > > From: [e-mail] 
> [mailto:[e-mail]]
> > > > > > >
> > > > > > > Having binary content in SIP messages may have 
> two impacts on
> > > > > > > SigComp:
> > > > > > >
> > > > > > > a) multiplexing of uncompressed messages with 
> SigComp messages on the
> > > > > > > same port.
> > > > > > ...
> > > > > > > For a), it is only an issue for SigComp/TCP, not 
> SigComp/UDP (where one
> > > > > > > SigComp message maps to one UDP packet). Adam's 
> point 1 and 2 are related
> > > > > > > to this. As to point 1, I think the SigComp 
> receiver still needs to know
> > > > > > > (somehow) the end of an uncompressed message, or 
> at least the end of the
> > > > > > > binary part of each SIP messages. Otherwise, a 
> bit pattern 11111 in
> > > > > > > the binary part will be mistaken by SigComp as 
> the beginning of a SigComp
> > > > > > > message. As to point 2, I would add that the 
> delimiter 0xFFFF can appear
> > > > > > > at the both the beginning and the end of a 
> SigComp message. So, the
> > > > > > > receiver needs to have the same parsing logic 
> just mentioned for bit pattern
> > > > > > > 11111.
> > > > > >
> > > > > > This isn't any more of an issue than it is without 
> SigComp. SIP
> > > > > > messages provide their own framing, and SigComp 
> messages do the
> > > > > > same. They can be trivially mixed on the same stream:
> > > > > >
> > > > > > 1. Peek at the first byte waiting in the stream.
> > > > > >
> > > > > > 2. If it is 0xf8 or higher, the next message in the 
> stream is
> > > > > >    a SigComp message. The message continues until a 
> 0xFFFF is
> > > > > >    encountered in the stream.
> > > > > >
> > > > > > 3. Otherwise, the next message in the stream is a 
> SIP message.
> > > > > >    Read the stream until a CR/LF/CR/LF is encountered. Parse
> > > > > >    the headers, and then read the number of bytes specified
> > > > > >    in the "Content-Length" header.
> > > > > >
> > > > > > 4. Return to step 1.
> > > > > >
> > > > > > /a
> > > > > >
> > > > > _______________________________________________
> > > > > Sipping mailing list 
> https://www1.ietf.org/mailman/listinfo/sipping
> > > > This list is for NEW development of the application of SIP
> > > > Use [e-mail] for questions on 
> current sip
> > > > Use [e-mail] for new developments of core SIP
> > > 
> > > -- 
> > > Gonzalo Camarillo         Phone :  [phone]
> > > Oy L M Ericsson Ab        Mobile:  [phone]
> > > Telecom R&D               Fax   :  [phone]
> > > FIN-02420 Jorvas          Email :  [e-mail]
> > > Finland                   http://www.hut.fi/~gonzalo
> > > 
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