Subject: [Sipping] Some thoughts and comments regarding draft-sinnreich-sipdev-req-00

From: [email-masked]
Date: Tue, 22 Apr 2003 22:56:03 EDT

Henry,

Sorry for the late comments (I only recently got to your draft).

As someone who has been closely involved with application of SIP in the 
mobile phone environment (3GPP) I have some concern with the initial version 
of draft-sinnreich-sipdev-req.

Although the Abstract indicates that the scope of the draft includes mobile 
phones the contents do not seem to take into account the needs of mobile 
phones and the requirements in draft-garcia-sipping-3gpp-reqs-03.

Specifically (but not necessarily a complete list):

1. The Link Layer Requirements only address Ethernet and wireless LAN and do 
not include 3GPP GSM/GPRS and UMTS or indeed other cellular access systems 
such as CDMA 2000 systems. Mobile phones should not be required to support 
Ethernet or WLAN access or though multiple mode devices may. Bluetooth and 
other access technologies may also be supported.

2. The IP Requirements does not include IP address aquisition using GPRS/UMTS 
mechanisms as utilised by 3GPP.

3. SIP User Agent Services:
SIP-13 client-based conferencing: SIP devices MUST be able to 
  support handset based conferencing.  A SIP client MUST be able to 
  initiate and mix the audio streams of at least 2 separate calls (i.e. 
  3 way conference calling).  
Making this mandatory in the mobile environment is problematic because this 
type of conferencing is not really suitable in the cellular environment where 
bandwith is scarce.

4. SIP-14 and the requirement that mandates G.711 is not acceptable in the 
cellular environment where bandwidth is scarce. If a single mandator codec is 
felt to be essential then it must be a cellular codec such as AMR (although a 
single cellular codec is not likely to be acceptable to all the other mobile 
standardisation organisations out there). In practice the use of transcoders 
in the network although undesirable is likely to be necessary as agreeing on 
a single mandatory codec is like mandating that everyone in the world must 
speak English.

A general comment is that the are a lot of MAYs in the draft. In my opinion 
any option in the SIP or other IP protocols MAY be supported by any SIP 
client and therefore are unnecessary to be specified here. 

Another general comment is that a lot of the mandatory requirements seem to 
me to be optional features not mandatory requirements (for instance Message 
Waiting indicator). Some of these may be mandatory for certain types of 
devices but not others. 

I understand that specifying a minimum set of requirements will aid 
interoperability and also assist in product comparision and procurement. 
However I think that the one shoe fits all approach is not practical. 
Therefore I would propose that classes of devices are defined and then 
minimum requirements for each class are defined.

For instance a SIP mobile phone would be a different class device from a PC 
based SIP UA as would be a dedicated SIP phone. This I think would make the 
draft more structured/readable and useful. The current draft already has 
caveats that state that this requirement only applies to certain types of 
device which I think reinforces that certain requirements only apply to 
certain device classes.

So I think that either the scope needs to be restricted to avoid applying to 
mobile phones or much more work needs to be done to include the requirements 
for the mobile phone class (which should be gathered from the 3GPP and other 
relevent specifcation body work or specified by reference to those wireless 
specifcations).

Some of the relevent specifcations from 3GPP are:

TS 22.228, TS 23.228, TS 24.229 located at <A HREF="http://www.3gpp.org/ftp/specs/latest/Rel-5/">
http://www.3gpp.org/ftp/specs/latest/Rel-5/</A>

regards
Andrew