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Network Working Group                                        W. Chimiak
Request for Comments: 1453                                         BGSM
                                                             April 1993

         A Comment on Packet Video Remote Conferencing and the
                        Transport/Network Layers

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard.  Distribution of this memo is


   The new generation of multimedia applications demands new features
   and new mechanisms for proper performance.  ATM technology has moved
   from concept to reality, delivering very high bandwidths and new
   capabilities to the data link layer user.  In an effort to anticipate
   the high bandwidth-delay data link layer, Delta-t [Delta-t], NETBLT
   [RFC 988], and VMTP [RFC 1045] were developed.  The excellent
   insights and mechanisms pioneered by the creators of these
   experimental Internet protocols were used in the design of Xpress
   Transfer Protocol (XTP) [XTP92] with the goal of eventually
   delivering ATM bandwidths to a user process.  This RFC is a vehicle
   to inform the Internet community about XTP as it benefits from past
   Internet activity and targets general-purpose applications and
   multimedia applications with the emerging ATM networks in mind.

1.  Introduction

   Networking is no longer synonymous with analog telephony.  High-
   performance lower-layer networks have made possible exciting new
   applications: collaboratory environments, distributed client/server
   computing, remote conferencing, teleclassrooms, and distributed
   life-sciences imaging.  These applications normally demand a great
   deal of bandwidth and often create operating system bottlenecks.
   Enabling these new multimedia applications entails delivering
   bandwidth to the applications, not just having bandwidth available on
   the network.  This statement may appear obvious, but often solutions
   at the transport layer are satisfied by having bandwidth at that
   layer without sufficient sensitivity to higher-layer access to the
   bandwidth.  The unavailability of bandwidth at upper layers is
   becoming the real issue as the networks are becoming a high-
   performance virtual backplane without concomitant high-performance
   control schemes.  It appears that new services are needed that
   require communication with all layers.  The ATM architecture calls

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RFC 1453             Comments on Video Conferencing           April 1993

   for such an integrated control scheme.

2.  Remote Conferencing

   The challenges of remote conferencing is an application whose
   challenges may be met at the data link layer by the emerging
   broadband networks.  If so, important medical applications such as
   medical imaging for diagnosis and treatment planning would be
   possible [CHIM92].  Remote conferencing would permit imaging
   applications for life sciences through the use of national resource
   centers.  Collaboratory conferences in molecular modeling, design
   efforts, and visualization of data in numerous disciplines could
   become possible.

   At the Second Packet Video Workshop, held December, 1992, at MCNC in
   the Research Triangle Park, North Carolina, a recurrent theme was the
   use of multimedia in remote conferencing.  Its applications included
   the use of interactive, synchronized voice and video transmission,
   multicast transmission, data transfer, graphics transmission,
   noninteractive video and audio transmission, and data base query
   within a virtually shared workspace.  A few participants doubted the
   ability of current computer networks to handle these multimedia
   applications and preferred only connection-oriented, circuit-switched
   services.  Most participants, however, looked forward to using an
   integrated network approach.

2.1.  Remote Conferencing Functions and Requirements

   Remote conferencing as seen at the workshop requires a set of
   functions.  It must provide session scheduling that deals with
   initiating a session, joining in-progress sessions, leaving a session
   without tearing it down if there are multiple participants, and
   terminating a session.

   The remote-conferencing session needs a control subsystem that is
   either tightly controlled for an n-to-n connection for two to 15
   participants, or loosely controlled for a 1-to-n connection for any
   number of participants.  The Multipeer-Multicast Consortium is
   working on defining the control requirements and the mechanisms for
   control.  At the Packet Video Workshop, one participant presented a
   conference control protocol (CCP) shown in Figure 1 [CCP92].  In this
   architecture the CCP controls the Network Voice Protocol (NVP)
   [RFC741] and the Packet Video Protocol (PVP) [PVP81] over the
   experimental Internet Stream Protocol, Version 2 (ST-II) [RFC1190]
   rather than IP.

   Latency and intramedia synchronization and intermedia synchronization
   (lip-sync) are critical for the interactive voice and video streams

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RFC 1453             Comments on Video Conferencing           April 1993

   of remote conferencing.  Client/server applications including data
   base operations are equally important.  The ability to display
   noninteractive video and high-resolution graphics is necessary.

   As with most applications, security will eventually be an issue.  At
   the very least, there must be a mechanism to determine who can find
   out what about conference and who can join a conference.  This
   determination will be part of an upper-layer protocol.

      +--------------+ +--------+ +-----+ +------------+
      |Teleconference| |  File  | |Email| |   Domain   |
      |   (CCP)      | |Transfer| |     | |Name Service|
      +----+-------+-+ +-----+--+ +-+---+ +-----+------+
           |       |         |__  __|           |
           |       |            ||              |
     +-----+--+ +--+-----+    +-++-+       +----+---+
     |Network | | Packet |    | T  |       |    U   |
     | Voice  | | Video  |    | C  |       |    D   |
     |Protocol| |Protocol|    | P  |       |    P   |
     +---+----+ +--+-----+    +-+--+       +--+-----+
         |__     __|            |__         __|
            |   |                  |       |
          +-+---+--+             +-+-------+-+
          | Stream |             |     I     |
          |Protocol|             |     P     |
          +---+----+             +---+-------+
              |                      |

          Figure 1: The Connection Control Protocol Architecture

   The solutions must range in geography from single machines through
   LAN, CAN, MAN, WAN conferences, as well as in data content from PCs
   to high-end workstations.  Implicit in the scaling is the notion of
   product and application interoperability.

   Remote conferencing applications should take advantage of future
   network enhancements, as well as the functions provided by ATM, FDDI,
   and SMDS.  Doing so should provide function versus resource trade-
   offs such as cost versus error control and cost versus rate control.
   As a result, the transport layer should be able to provide the
   services offered by the data link layer.

   Most of the presentation on remote conferencing indicated a need for
   some degree of multicast functionality, ranging from the 1-to-n, with
   conference membership completely known, to conferences for which

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RFC 1453             Comments on Video Conferencing           April 1993

   existence of a group is known, but exact membership is not.

   In remote conferencing, it is preferable to use one network for all
   information traffic.  This network should have an offered quality of
   service (QOS) criteria to accommodate tradeoff metrics, which would
   include guaranteed throughput, connection reliability, high call-
   completion rate, few dropped calls, tolerable error rate, varying
   degrees of compression on the video and audio streams, and tolerable
   motion artifacts, flow control, and latency.  The QOS should be an
   input to the transport layer from the application or transport
   service user.

   The compression/coding function should provide time-stamping and
   packetizing information, as well as real-time echo cancellation.
   These functions are usually at the presentation and session layer of
   the Open System Interconnection (OSI) model or the at the application
   in some Internet models, but not the transport layer.

3.  Potential Solutions

   RFC 1193 deals with the requirements of real-time communications,
   which include some of the same capabilities [RFC1193].  But the
   requirements articulated create the necessity for new
   transport/network protocols.  The new protocols under development by
   the Audio Visual Transport [SCHU92] (RTC, RTCP), and other protocols
   in this area (ST-II) suggest an architecture like that shown in
   Figure 2.

   These approaches may work.  However, they encourage a discipline that
   creates a protocol for each new class of application.  Another
   approach might be to unify the protocols.  It is felt that this is
   one of the main goals of XTP (see Figure 3).

   Other design considerations of XTP include the following:

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RFC 1453             Comments on Video Conferencing           April 1993

             |          media       |
             |       application    |
             |        |RTCP| |      |
             |        +----+ |   T  |
             |         RTP   |   C  |
             +-----+-----+   |   P  |
             |ST-II| UDP |   |      |
             +     +-----+---+------|
             |     |       IP       |
             |    Data Link Layer   |

              Figure 2: One emerging multimedia architecture

     +--------------+  +--------+ +-----+ +------------++-----------+
     |Teleconference|  |  File  | |Email| |   Domain   ||   media   |
     |              |  |Transfer| |     | |Name Service||application|
     +------+-------+  +----+---+ +--+--+ +-----+------++-----+-----+
            |               |        |          |             |
                             |Unified Protocol|
                             |Data Link Layer |

           Figure 3: Another integrated multimedia architecture

   (1)  Developing a protocol based on the work and experience of
        the protocol groups such as IETF, which produced NETBLT,
        VMTP, TCP, IP, and UDP.

   (2)  Incorporating lessons from Delta-t.

   (3)  Observing the design paradigm shift of ATM, SONET, and  VMTP
        in the header, trailer, and information segment construction.

   (4)  Targeting the real-time requirements articulated by the
        Department of Defense SAFENET committee and the French
        Ministry of Defense military real-time specification [GAM-T-103].

   Mechanisms in XTP allow an application to approach the performance
   desired.  A session-scheduling mechanism for joining and leaving a

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RFC 1453             Comments on Video Conferencing           April 1993

   multicast conference exists.  The XTP mechanism for multicast
   satisfies the loosely controlled session requirements.  The tightly
   controlled session would require the use of multiple XTP

   The priority mechanism that uses the 32-bit SORT field permits an
   application to prioritize data.  Because XTP is a transport layer,
   this priority mechanism follows through every node tranversed.  There
   is also an out-of-band delivery mechanism.  However, XTP does not
   offer latency control by itself; it just provides a priority

   The selective acknowledgement, fast negative acknowledgement, and
   selective retransmission permit an application to choose an
   appropriate level of error control.  The combination of the priority
   mechanism and these error-control mechanisms is likely to approach
   the latency and synchronization requirements of remote conferencing.

   Noninteractive audio and video, as well as graphics presentation, can
   be accommodated in many ways by the application.  It is important
   that the transport layer provides adequate mechanisms to deliver the
   appropriate data streams in a manner compatible with the
   applications.  These applications can probably be accomplished by
   means of extant protocols, as well as XTP.

   The scalability of the solution will be a function of the standards
   used.  At the Packet Video Workshop, some of the applications
   sacrificed computer network standards in favor of desired
   performance.  This approach usually impedes scalability.  From the
   explanation of the applications taking this approach, it appeared
   that using XTP would have provided an adequate transport service for
   the applications.

   XTP was designed to provide mechanisms to accommodate application
   requirements, that is, the ability to respond to QOS requests.  For
   example, guaranteed throughput may be accomplished by using XTP's
   rate and burst control together with flow control or no flow control.
   Tolerable error rate can be accomplished with partially error
   controlled connections (PECC), a service which can be placed in the
   application or just above the transport layer [PECC92].  Motion
   artifacts and varying degrees of compression should be done above the
   transport layer in coordination with the transport layer or possibly
   in a network management function.

3.1.  Synthesize the Hardware Fabrication Process into the Design

   To produce an affordable solution, the hardware fabrication process
   should be a design consideration.  Technologies are evolving too

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RFC 1453             Comments on Video Conferencing           April 1993

   rapidly to assume that a generic protocol design will anticipate all
   fabrication advances, but this fact should not impede use of the
   features of advanced hardware fabrication processes.

   System interface problems and VLSI techniques should be considered in
   the specification of the protocol.  An examination of the ATM and
   SONET standards appears to support this philosophy.  Similarly,
   NETBLT and VMTP design efforts seem to support this approach.  XTP
   does use it.

   It is very helpful to break down the protocol into parallel-state
   machines for execution on more inexpensive hardware.  This procedure
   reduces the context switching and interrupt handling requirements of
   the hardware, thereby decreasing production costs while producing a
   scalable protocol machine.

4.  Multimedia Applications over XTP

   In parallel with the IETF efforts to enable multimedia applications
   such as remote conferencing, the XTP forum members have been
   experimenting with major elements of these applications.

   (1)  At the University of Virginia, more than 100 simulated voice
        channels were run on an FDDI network [UVAVOICE92].  The
        purpose of this experiment was to test the limits of FDDI
        and a software version of XTP in a simulated interactive
        voice environment.  Multicasted, noninteractive video has
        been supported there for several years.

        UVa also has a video-mail demo over XTP/FDDI that uses
        Fluent multimedia interface and standard JPEG compression.
        This PC-based demo delivers full frame, full color, 30
        frames/sec video from any network disk to a remote VGA
        screen.  It is important that users could not discern any
        difference  in  playback  between  a local disk and a remote
        disk.  An Xpress File System (XFS) is used on server and
        client systems.

   (2)  The Technical University of Berlin, Germany, reports that
        the coordination, implementation, and operation of
        multimedia services (CIO) of the R&D in Advanced
        Communications Technologies in Europe (RACE) is using XTP as
        a starting point for design [XTPRACE].

   (3)  At the Naval Command, Control, and Ocean Surveillance Center
        Research, Development, Test and Evaluation Division NRaD
        (formerly the Naval Ocean Systems Command (NOSC)), voice is

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RFC 1453             Comments on Video Conferencing           April 1993

        multicasted over XTP/FDDI.  A simple multicast is
        distributed to a group with a latency of around 25 ms, where
        the latency represents delay from the voice signal from the
        microphone to the audio signal to the speaker.  This group
        is currently doing research on an n-party multicast of voice
        (telephone conference-call paradigm [n x n multicast]).

   (4)  Commercially, Starlight Networks Inc., migrated a subset of
        XTP into the transport layer of its video application
        server.  By using XTP rate control, full-motion, full-screen
        compressed video is delivered at a constant 1.2 Mbps, over
        switched-hub Ethernet to viewstations.  This network
        delivers at least 10 simultaneous video streams.

   Therefore, XTP has been used in applications that were previously
   placed over IP or even a data link layer.

5.  Policy versus Mechanism

   Separating protocol policies and mechanisms [XTPbk] permits adoption
   of new policies without altering offered mechanisms.  An excellent
   example is UVa's Partially Error Controlled Connections (PECC).  This
   control system maximizes error control in such a way that receiving
   FIFOs are never starved i.e., the application, driver, or operating
   system buffer control, and not the transport layer becomes the

   Because XTP is mechanism-rich and policy-tolerant, this very dynamic
   error control policy mechanism is possible.  Separating policy and
   mechanism permits an error-control or flow-control policy to adapt to
   the data link layer conditions without shutting down a connection and
   rebuilding (or multiplexing) a new one on a different protocol stack.
   This may also provide an easier way for a network management
   subsystem to maintain a desired QOS.

6.  Summary

   Remote conferencing presents new opportunities for research,
   business, and administration.  Although some are proposing that only
   classical circuit-switched mechanisms be used, most network engineers
   are searching for ways to use the new features of FDDI, SMDS, and ATM
   in multimedia applications such as remote conferencing.  Some new
   applications have been placed directly on a data link layer.  New
   Transport/Network layer combinations have been proposed and are being
   tested.  It is believed that consideration should be given to XTP as
   a possible solution because its forum membership includes commercial,
   government, and research institutions, some of which have implemented
   various applications that contribute to an overall remote-

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RFC 1453             Comments on Video Conferencing           April 1993

   conferencing application.

7.  References

   [CCP92]     Schooler, E., "An Architecture for Multimedia Connection
               Management", in Proceedings of the 4th IEEE ComSoc
               International Workshop on Multimedia Communications,
               Monterey, CA, April 1992.

   [CHIM92]    Chimiak, W., "The Digital Radiology Environment", IEEE
               JSAC, Vol. 10, No. 7, pp. 1133-1144, September 1992.

   [Delta-t]   Watson, R. W., "Delta-t Protocols Specification",
               Lawrence Livermore Laboratory, April 15, 1983.

   [GAM-T-103] French Ministry of Defense, "GAM-T-103 Military
               Real-Time Local Area Network Reference Model
               (Transfer Layer)", February 7, 1987.

   [PECC92]    Dempsey, B., Strayer, T.  and Weaver A., "Adaptive Error
               Control for Multimedia Data Transfer", in Proceedings
               of the IWACA 92, Munich, Germany, pp. 279-288, March

   [PVP81]     Cole, R., "PVP - A Packet Video Protocol", W-Note 28,
               Information Sciences institute, University of
               California, Los Angeles, CA, August 1981.

   [RFC1045]   Cheriton, D., "VMTP: Versatile Message Transaction
               Protocol Specification", RFC 1045, Stanford
               University, February 1988.

   [RFC998]    Clark, D., Lambert, M., and L. Zhang, "NETBLT: A Bulk
               Data Transfer Protocol", RFC 998, MIT, March 1987.

   [RFC1193]   Ferrari, D., "Client Requirements For Real-Time
               Communication Services", RFC 1193, UC Berkeley,
               November 1990.

   [RFC1190]   Topolcic, C., Editor, "Experimental Internet Stream
               Protocol: Version 2 (ST-II)", RFC 1190, CIP Working
               Group, October 1990.

   [SCHU92]    Schulzrinne, H., "A Transport Protocol for Audio and
               Video Conferences and other Multiparticipant
               Real-Time Applications", Internet Engineering Task
               Force, Internet-Draft, October 1992.

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RFC 1453             Comments on Video Conferencing           April 1993

   [UVAVOICE92] Weaver, A. C. and McNabb, J.F., "Digitized Voice
                Distribution Using XTP and FDDI", Transfer, Vol. 5,
                No.  6, pp. 2-7 (November/December 1992).

   [XTP92]     Xpress Transfer Protocol, version 3.6, XTP Forum,
               1900 State Street, Suite D, Santa Barbara, California
               93101 USA, January 11, 1992.

   [XTPbk]     Strayer, W.T., Dempsey, B.J., and Weaver, A.C., "XTP:
               The Xpress Transfer Protocol", Addison-Wesley
               Publishing Company, Inc., 1992.

   [XTPRACE]   Rebensburg, K. and Miloucheva, I., "The Use of XTP in a
               Large European Communication Project", XTP Forum
               Research Affiliate Annual Report, Document 92-183,
               pp. 105-112, 1992.

Security Considerations

   Security issues are discussed in section 2.1.

Author's Address

   William J. Chimiak
   Department of Radiology
   Bowman Gray School of Medicine
   Medical Center Boulevard
   Winston-Salem, NC 27157-1022

   Phone: 919-716-2815
   EMail: chim@relito.medeng.wfu.edu

Chimiak                                                        [Page 10]