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Keywords: XTP
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
unlimited.
Abstract
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
Chimiak [Page 1]
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
Chimiak [Page 2]
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 |
+---+----+ +---+-------+
| |
+-----+----------------------+----+
|IEEE_802.X,FDDI,DARTnet,ATOMIC...|
+---------------------------------+
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
Chimiak [Page 3]
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:
Chimiak [Page 4]
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
Chimiak [Page 5]
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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
associations.
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
mechanism.
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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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
Chimiak [Page 7]
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
bottleneck.
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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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
1992.
[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.
Chimiak [Page 9]
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]