Internet Info 1997 December/Internet_Info_CD-ROM_Walnut_Creek_December_1997.iso/ietf/mmusic/mmusic-minutes-93nov.txt

Text File | 1994-02-23 | 11KB | 246 lines

CURRENT_MEETING_REPORT_

Reported by Abel Weinrib/Bellcore

Minutes of the Multiparty Multimedia Session Control Working Group
(MMUSIC)

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.

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 a presentation of an algorithm and
framework for managing shared session state.  The second meeting focused
on preliminary ideas as to what might comprise shared session state for
a couple of different session types, and three short presentations on
related work.


Overview and Framework

Abel Weinrib 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:


   o 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.

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

   o 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.

   o 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 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 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:


   o It was observed 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 than for
     human members who can more easily detect the corruption.

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

   o 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's talk was devoted to the interpretation and usage of the
agreement protocol for teleconference session control.  Discussion
attempted 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:


   o Issues of media typing and the addressing of media agents are
     related to problems that need to be solved for WWW as well as
     XMosiac naming and MIME mailcap media descriptions.

   o 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 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
via email for a copy of his paper.

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 discussed the relationship of RTCP, the ``real time control
protocol'' defined by the Audio/Video Transport Working Group (AVT), 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
teleconferencing (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 presented a list of 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

Lou Berger               [e-mail]
David Borman             [e-mail]
Stephen Casner           [e-mail]
Ping Chen                [e-mail]
George Clapp             [e-mail]
Steve DeJarnett          [e-mail]
David Dubois             [e-mail]
Ed Ellesson              [e-mail]
Julio Escobar            [e-mail]
William Fenner           [e-mail]
James Fielding           [e-mail]
Ron Frederick            [e-mail]
Atanu Ghosh              [e-mail]
Fengmin Gong             [e-mail]
John Hanratty            [e-mail]
Ken Hayward              [e-mail]
Van Jacobson             [e-mail]
Yasuhiro Katsube         [e-mail]
Charley Kline            [e-mail]
Jim Knowles              [e-mail]
Ted Kuo                  [e-mail]
Paul Lambert             [e-mail]
Mark Laubach             [e-mail]
Jim Martin               [e-mail]
Thomas Maslen            [e-mail]
Donald Merritt           [e-mail]
Karen O'Donoghue         [e-mail]
Laura Pate               [e-mail]
J. Mark Pullen           [e-mail]
Bala Rajagopalan         [e-mail]
Steven Richardson        [e-mail]
Eve Schooler             [e-mail]
Henning Schulzrinne      [e-mail]
Scott Shenker            [e-mail]
Michael Speer            [e-mail]
John Stewart             [e-mail]
Daniel Swinehart         [e-mail]
Matsuaki Terada          [e-mail]
Claudio Topolcic         [e-mail]
Abel Weinrib             [e-mail]
Taehwan Weon             [e-mail]
John Wroclawski          [e-mail]
Shinichi Yoshida         [e-mail]
Lixia Zhang              [e-mail]
Weiping Zhao             [e-mail]