Subject: MMusic minutes from Houston IETF

From: [email-masked] (Abel Weinrib)
Date: Fri, 19 Nov 93 09:59:37 EST

These notes were prepared by Abel Weinrib, [email-masked].  An
on-line copy of the minutes and the accompanying slides may be
found in the directory venera.isi.edu:confctrl/minutes as files
ietf.11.93 and slides.[a-d].11.93.ps.

      Multiparty Multimedia Session Control WG (MMusic)
                Minutes from the 28th IETF
                       Houston, TX
                   November 1 and 2, 1993

               Eve Schooler, [email-masked]
               Abel Weinrib, [email-masked]

The MMusic working group met for two sessions at the Houston IETF
meeting. The first day was dedicated to a short overview of the goals
and context for the working group and then to a presentation of an
algorithm and framework for managing shared session state.  The second
meeting focused on preliminary ideas on what might comprise shared
session state for a couple of different session types, and then three
short presentations on related work.  Overall, about 45 people attended
the two meetings; a list of the attendees is attached at the end of the
minutes.

Overview and Framework (Abel Weinrib, slides.a.11.93.ps)

Abel presented an overview of the goals of the MMusic working group
and discussed the framework for the work.  This presentation was
basically a review of the work of previous working group meetings;
refer to the minutes of those meetings available from the confctrl
archives for more detail.

In setting the context for the next two talks, a distinction was
made between the "agreement algorithm" and the "session control
protocol."  The agreement algorithm supports generic control of
group membership and enforces correctness and other policies on
state shared among the members.  This agreement layer "understands"
membership and policies, but views the rest of the domain-specific
session state as opaque.  The session control protocol understands
the domain-specific session state, using the services of the
agreement protocol to manage the state shared among the members. 
The session protocol may also use other services in addition to the
agreement services, such as services that support soft state sharing 
and recovery. 

Issues that were raised during discussion included:

Where should a "session manager" that terminates a session control
protocol reside?  Various alternatives are on a workstation
(as shown in the framework slide) for one or multiple users, or one
per domain that could act as a demultiplexing agent by passing on
session control messages for users in that domain to the
appropriate place.  The second alternative may provide hooks for
supporting user mobility and may deal well with security firewalls.

Should floor control be done through the session control protocol
or through some other mechanism?

Should policies be chosen from a predefined set or should they be
defined in all of their generality by each application?  This has
implications on interoperability and the complexity of the
applications.

In the framework, resource reservation is separate from session
management.  The session control protocol is used to propagate a
shared view of the state, which includes descriptions of the media
streams required by a conference.

An Algorithm for Managing Shared Teleconferencing State
	(Scott Shenker, slides.b.11.93.ps)

Scott described some preliminary ideas being developed for
expressing policies about how session state can be changed and the
degree to which members agree on their views of the state.   Policy
can be expressed along three dimensions:  voting policies,
consistency policies, and initiator policies.  Voting policy
defines which members must agree for a state change to take place. 
Consistency policies describe how the state seen by different
members may differ.  Initiator policies set which members may
initiate changes to the state.   The policy framework provides the
vocabulary for concretely describing various session styles.

He then presented an algorithm that supports operations on the
shared state while enforcing the policies associated with the
session.  These operations might be adding a member, changing the
policies themselves, or modifying some other domain specific state
variable such as an encryption key.  The basic mechanism is a group
agreement algorithm based on a two-phase commit procedure when
or correctness.

For additional information on this work there is a rough draft
document in the confctrl archives in docs/agree.ps.  Notice of the
availability of more complete drafts of the document will be sent
to the confctrl mailing list.

Some points raised in the discussion during and following the talk
included:

The observation that some members of a session may be programs
running on computers.  The fall-back position of always allowing
members to leave a corrupted session may be less useful for them
than for human members who can more easily detect the corruption.

Critical and non-critical membership allows there to be a core of
members that control the conference and a potentially much larger
set of members that can more easily enter and leave.

This talk is about agreement, not negotiation.  The distinction is
that there is no support for multiple rounds of proposals and
counter-proposals.  This could be future work, or could be done at
the application level building on top of the basic agreement
service.

Session Control Above the Agreement Protocol
	(Eve Schooler, slides.c.11.93.ps)

Eve's talk was devoted to the interpretation and usage of
the agreement protocol for teleconference session control.
Discussion aimed to place the agreement protocol in the context of
a traditional protocol stack and to hint at implementation concerns.
Examples were given for generic and domain-specific session operations,
as well as for the array of potentially interesting state attributes
(session-wide, membership-related, or media- and policy-specific).
To illustrate the range of sessions that can be constructed
from different sets of policies, two example paradigms were presented;
one for an open hailing-channel session with little coordination among
members, and another for a minimal invitation-only session.

The second half of the presentation focused on several open issues:
Tradeoffs between different end-system organizations, addressing
issues related to the use of unicast and multicast and to the
interaction of media agents and session agents, and alternate
techniques for user rendezvous that resemble what is currently in
place on the MBone for session directories. 

For additional information on this work there is a very rough draft
document in the confctrl archives in docs/usage.txt.  Notice of the
availability of more complete drafts of the document will be sent
to the confctrl mailing list.

Some points raised in the discussion during and following the talk
included:

Issues of media typing and addressing of media agents are related
to problems that need to be solved for www and xmosiac naming and
MIME mailcap media descriptions.

It would be nice if session control did not assume that the media
used by the conference is necessarily carried over an IP network.

Consensus and Control in Wide-Area Communication (Bala Rajagopalan)

Bala briefly presented his work on agreement and control of group
membership in wide area communications.  He also handed out a 
paper that presents his model and algorithm in more detail; 
contact him for a copy of his paper at [email-masked].

The model allows a group to (eventually) come to consensus on its
membership in the presence of unreliable message delivery.  His
algorithm uses wide area multicast and a coordinator for each
partition's "view" of the membership state.  Operations on groups
include join, leave, delete, reform, merge.  One underlying assumption
of this work that led to some heated discussion during the meeting is
that connectivity is transitive, meaning that if A is connected to B
and B is connected to C, then A is connected to C; this assumption may
break down during certain failure scenarios in the Internet.

This work appears to be relevant to the concerns of the MMusic
working group.  More effort is required to understand how and where it 
might fit into the MMusic charter.

RTCP Implications for MMusic (Steve Casner, slides.d.11.93.ps)

Steve discussed the relationship of RTCP, the "real time control
protocol" defined by the A/V Transport working group, to the MMusic
working group effort.  RTCP is separate from the RTP protocol
(which supports transport of time-critical media streams) and may
in the future be replaced by a higher level control protocol, such
as the MMusic session control protocol.  In particular, he
described the functions that RTCP currently provides, and discussed
other functions that would be useful in supporting an application
such as multimedia telconferencing (see the slides).  He concluded
that it may make sense to use some part of the RTCP in conjunction
with a higher level control protocol.

Session Control Work at BBN (Julio Escobar)

Julio briefly listed relevant work at BBN that is addressing
similar issues to the MMusic working group.  He mentioned Chip
Elliott's work on the "sticky" protocol (Chip had actually
presented this work at an earlier MMusic/confctrl BOF), Lou Berger's
simulation exercise management tool, and Walter Milliken's work on
resource coordination objects.  Julio promised to send additional
information on this work to the confctrl mailing list (which he has
done).

Attendees:

Abel Weinrib		[email-masked]
Eve Schooler		[email-masked]
Scott Shenker		[email-masked]		
Dan Swinehart		[email-masked]
Jim Martin		[email-masked]
Bala Rajagopalan	[email-masked]
Fengim Gong		[email-masked]
Ted Kuo			[email-masked]		
Lixia Zhang		[email-masked]		
John Stewart		[email-masked]
Ed Ellesson		[email-masked]
Shin Yoshida		[email-masked]
Ping Chen		[email-masked]			
			[email-masked]
Matsuaki Terada		[email-masked]		
Mark Pullen		[email-masked]		
Michael F. Speer	[email-masked]			
Steve Richardson	[email-masked]			
Bill Fenner		[email-masked]		
Ron Frederick		[email-masked]	
Claudio Topolcic	[email-masked]	
Donald Merritt		[email-masked]		
James Fielding		[email-masked]		
Karen O'Konoghue	[email-masked]	
Ken Hayward		[email-masked]		
David Borman		[email-masked]			
Steve Casner		[email-masked]
Taehwan Weon		[email-masked]		
Yasahiro Katsube	[email-masked]	
Weiping Zhao		[email-masked] 
Henning	Schulzrinne	[email-masked]
Julio Escobar		[email-masked]
John Wroclawski		[email-masked]
Laura Pate		[email-masked]
Steve DeJarnett		[email-masked]
Lou Berger		[email-masked]
Atanu Ghosh		[email-masked]
John Hanratty		[email-masked]
Paul lambert		[email-masked]	
Charly Kline		[email-masked]
Gerge Clapp		[email-masked]
Mark Laubach		[email-masked]
David Dubois		[email-masked]		
Thomas Maslen		[email-masked]
Van Jacobson		[email-masked]
Jim Knowles		[email-masked]

---

From: [email-masked]
Date: Tue, 23 Nov 93 09:37 EST

>From Abel Weinrib, Re: Houston IETF minutes

>contact him for a copy of his paper at [email-masked].

Until it is added to the confctrl archives, the postscript
version of this paper can be obtained through anonymous ftp
from ds.internic.net. The file is "consensus.ps" and it is
in the directory, "/pub/current-ietf-docs/tsv/mmusic".

Bala Rajagopalan
[email-masked]