Subject: [BLISS] Comments on draft-anil-sipping-bla-04

From: "Elwell, John" <[email-masked]>
Date: Fri, 16 Mar 2007 11:56:07 +0000

If this draft is worked on I think terminology will prove a difficult area.
For example, some people use the term "bridged lined appearance" for the
particular situation where there is bridging (mixing) of media so that users
on different devices can participate in the call simultaneously. We should
avoid the term "bridging" for matters that do not involve this type of
bridging. Also the term "appearance" is misleading, because often people use
the term appearance to refer to a particular registered contact for an AoR,
i.e., a device on which that AoR appears. In the draft, however, it is used
to identify a particular call.

In fact terminology throughout the document seems to be inconsistent. A vast
number of terms are used, often meaning the same thing (I think). Some of my
comments below identify particular terminology issues.

In some places the term "line" is used alone without definition.

"Calls can be joined (also called bridged or conferenced together)". Is this
usual? If two devices A1 and A2 have appearances of the same AoR A, and A1
makes a call to B and A2 makes a call to C, this suggests that you can
bridge together A1, A2, B and C. I have not heard of such a behaviour
(although it could be done by conventional SIP conferencing). I think what
the authors may have in mind is that if A1 makes a call to B, A2 can bridge
into that call. However, this is not reflected by the current text.

Section 3.1:
"Executive/Assistant arrangement: The executive may appear as an
   appearance on the assistant SIP telephony device."
This does not seem to match the description of appearance in section 3. If
an appearance is a call, how can an executive be an appearance?

"taking over a
   call on a particular appearance."
This too does not seem to align with the description of appearance in
section 3.

A BLA call group appears to be just the set of registered contacts for a
given AoR - why do we need a special term? Or can there be registered
contacts that are not members of the BLA call group, in which case how are
such contacts identified?

What are "APPEARANCE group", "BLA group" and "AOR
   group"? How do they relate to "BLA call group"? 

"Virtual BLA: Allows a AOR, not assigned as a primary appearance". What is a
primary appearance? It is not defined and not used elsewhere.

"The Appearance Agent can select the appearance number for an incoming
   call."
What about an outgoing call?

"Some implementations of this feature utilize a URI parameter such as
   "line=3" on the Contact URI."
How can a parameter on the contact URI identify a call? Perhaps it doesn't
matter, because this is not the chosen method. Just rather confusing.

"the proxy could fetch this information by initiating a
   SUBSCRIBE request with Expires: 0 for the AOR to fetch the list of
   lines that are in use."
If an appearance number identifies a call, what is the relevance of the
number of lines in use - shouldn't it be the number of calls (where each
line can have multiple calls potentially)? Of course, this depends exactly
what is meant by "line".

"When calls are placed on hold, the UAs MAY include local session
   description in the dialog package notification.  The element should
   indicate that the call that is the subject of the dialog is being
   held (use of a=inactive or a=sendonly is RECOMMENDED [9])."
Do we really want to get into the parsing of SDP in order to determine the
state of a call? We would start getting into considerations like whether
a=inactive or a=sendonly is at the session level or media level, and if
different media have different values, which is the authoritative value. The
status of the media is of less relevance than whether the call is in a state
where it can be picked up by another device, so we should ensure that
appropriate, easy to interpret indications are given in the dialog event
package document.

"when it is
   attempting to 'seize' a line appearance"
This sounds like an outgoing call attempt, but previously it says that
appearance numbers are assigned only for incoming calls. However, it does
seem reasonable to assign an appearance number to an outgoing call, to allow
it to be picked up, bridged into, etc.

"but this MUST be implemented to resolve glare
   issues."
I think the glare issue talked about is where there is some limit to the
number of calls than can be in progress for a given AoR. This isn't really
discussed much.

"This is needed to resolve conflicts in the use of a particular
   appearance."
What is meant by "appearance" here?

Regards,

John

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From: Alan Johnston <[email-masked]>
Date: Fri, 16 Mar 2007 10:57:12 -0500

John,

Thank you for your comments on the draft.   See my comments below.   
Since this is the BLISS list, I'm hoping to have more discussion about 
the requirements in draft-johnston-bliss-mla-req-00.txt rather then the 
mechanism in this draft.

You are right that terminology is difficult, and despite a major rewrite 
of the document we didn't quite get it right.  Hopefully we will get 
closer in the next version.

Thanks,
Alan

Elwell, John wrote:
> bridging.

We tried to only use the term bridging early in the draft in the 
non-normative sections.  After that, we tried to switch to the 
appearance term.

>  Also the term "appearance" is misleading, because often people use

We are open to other terms, but I thought appearance was pretty good.  
I've never heard of appearance being used to indicated a registered 
contact - is there a document you can point me to here?  We definitely 
want to avoid naming collisions.

> In some places the term "line" is used alone without definition.

Well, there is actually no real definition for a "line" in SIP - it is a 
virtual construct.  Even though I generally dislike long terminology 
sections, we probably should add one to this document.

> bridge together A1, A2, B and C. 

No, this is not the behavior we meant - we'll reword to avoid confusion.

> I have not heard of such a behaviour

Yes, this is what we meant.

> This too does not seem to align with the description of appearance in

You are right - we will fix this wording problem.

>    group"? How do they relate to "BLA call group"? 

I don't believe there are any differences between these.

> What about an outgoing call?

Yes - that is described a few sentences later.

> matter, because this is not the chosen method. Just rather confusing.

Does the description in draft-johnston-bliss-mla-req-00.txt make better 
sense?  The intention was not to discuss the URI contact method in this 
draft but I guess this text got left in by accident.

> line can have multiple calls potentially)? Of course, this depends exactly

We don't use the word line here.  This discussion is about how a proxy 
server, which does not necessarily have any knowledge of which UAs are 
in active sessions, could learn which appearance numbers are in use so 
that an unused one can be assigned.

> appropriate, easy to interpret indications are given in the dialog event

This is a mistake - the next sentence was meant to replace that one:  
"When calls are placed on hold, the "+sip.rendering=no" [RFC 3840 
<http://tools.ietf.org/html/rfc3840>] feature tag MUST be used."  We 
don't want to send SDP in the dialog package notifications.

> it to be picked up, bridged into, etc.

No, it says both earlier, but we'll go over to make sure any confusing 
text is fixed.

> What is meant by "appearance" here?

If two UAs in the group both select appearance number 2 at the same time 
- only one of them may use it, the other must select appearance number 3 
instead.