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

["Price, Richard" <[e-mail]>]

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/current/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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