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INFORMATIONAL
Network Working Group C. Partridge
Request for Comments: 1257 Swedish Institute of Computer Science
September 1991
Isochronous Applications Do Not Require Jitter-Controlled Networks
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
This memo argues that jitter control is not required for networks to
support isochronous applications. A network providing bandwidth and
bounds delay is sufficient. The implications for gigabit
internetworking protocols are briefly considered.
Introduction
An oft-stated goal of many of the ongoing gigabit networking research
projects is to make it possible to support high bandwidth isochronous
applications. An isochronous application is an application which
must generate or process regular amounts of data at fixed intervals.
Examples of such applications include telephones, which send and
receive voice samples at regular intervals, and fixed rate video-
codecs, which generate data at regular intervals and which must
receive data at regular intervals.
One of the properties of isochronous applications like voice and
video data streams is that their users may be sensitive to the
variation in interarrival times between data delivered to the final
output device. This interarrival time is called "jitter" for very
small variances (less than 10 Hz) and "wander" if it is somewhat
larger (less than one day). For convenience, this memo will use the
term jitter for both jitter and wander.
A couple of examples help illustrate the sensitivity of applications
to jitter. Consider a user watching a video at her workstation. If
the screen is not updated regularly every 30th of a second or faster,
the user will notice a flickering in the image. Similarly, if voice
samples are not delivered at regular intervals, voice output may
sound distorted. Thus the user is sensitive to the interarrival time
of data at the output device.
Observe that if two users are conferring with each other from their
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workstations, then beyond sensitivity to interarrival times, the
users will also be sensitive to end-to-end delay. Consider the
difference between conferencing over a satellite link and a
terrestrial link. Furthermore, for the data to be able to arrive in
time, there must be sufficient bandwidth. Bandwidth requirements are
particularly important for video: HDTV, even after compression,
currently requires bandwidth in excess of 100 Mbits/second.
Because multimedia applications are sensitive to jitter, bandwidth
and delay, it has been suggested that the networks that carry
multimedia traffic must be able to allocate and control jitter,
bandwidth and delay [1,2].
This memo argues that a network which simply controls bandwidth and
delay is sufficient to support networked multimedia applications.
Jitter control is not required.
Isochrony without Jitter Control
The key argument of this memo is that an isochronous service can be
provided by simply bounding the maximum delay through the network.
To prove this argument, consider the following scenario.
The network is able to bound the maximum transit delay on a channel
between sender and receiver and at least the receiver knows what the
bound is. (These assumptions come directly from our assertion that
the network can bound delay). The term "channel" is used to mean
some amount of bandwidth delivered over some path between sender and
receiver.
Now imagine an operating system in which applications can be
scheduled to be active at regular intervals. Further assume that the
receiving application has buffer space equal to the channel bandwidth
times the maximum interarrival variance. (Observe that the maximum
interarrival variance is always known - in the worst case, the
receiver can assume the maximum variance equals the maximum delay).
Now consider a situation in which the sender of the isochronous data
timestamps each piece of data when it is generated, using a universal
time source, and then sends the data to the receiver. The receiver
reads a piece data in as soon as it is received and and places the
timestamped data into its buffer space. The receiver processes each
piece of data only at the time equal to the data's timestamp plus the
maximum transit delay.
I argue that the receiver is processing data isochronously and thus
we have shown that a network need not be isochronous to support
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isochronous applications.
A few issues have to be resolved to really make this proof stick.
The first issue is whether the operating system can be expected to
schedule applications to be active at regular intervals. I will
argue that whether or not the network is isochronous, the operating
system must be able to schedule applications at regular intervals
Consider an isochronous network which delivers data with a tight
bound on jitter. If the application on the receiving system does not
wake up when new data arrives, but waits until its next turn in the
processor, then the isochrony of the network service would be lost
due to the vagaries of operating system scheduling. Thus, we may
reasonably expect that the operating system provides some mechanism
for waking up the application in response to a network interrupt for
a particular packet. But if the operating system can wake up an
application in response to an interrupt, it can just as easily wake
the application in response to a clock interrupt at a particular
time. Waking up to a clock interrupt provides the regular scheduling
service we wanted.
Observe that the last paragraph suggests an application of the End-
To-End Principle [3]. Given that the operating system must provide a
mechanism sufficient for restoring isochrony, regardless of whether
the network is isochronous, it seems unreasonable to require the
network to redundantly provide the same service.
Another issue is the question of whether all receiving systems will
have memory for buffering. For example, the telephone network is
required to deliver its data isochronously because many telephones do
not have memory. However, most receiving devices do have memory, and
those devices, like telephones, that do not currently have memory
seem likely to have memory in the future. Many telephones have a
modest amount of memory now. Furthermore, even if the end nodes
require isochronous traffic it is possible that last switch before
delivery to the end node could provide the necessary buffer space to
restore isochrony to the data flow.
Readers may wonder if the assumption of a universal time source is
reasonable. The Network Time Protocol (NTP) has been widely tested
on the Internet and is capable of distributing time accurately to the
millisecond [4]. Its designer is currently contemplating the
possibility of distributing time accurate to the microsecond.
Some Implications
The most important observation that can be made is that jitter
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control is not required for networks to be able to support
isochronous applications. A corollary observation is that if we are
to design an internetworking protocol for isochronous applications,
that internetworking protocol should probably only offer control over
delay and bandwidth. (There may exist networks that simply manage
delay and bandwidth. We know that's sufficient for multimedia
networking so our multimedia internetworking protocol should be
capable of running over those networks. But if the multimedia
internetworking protocol requires control over jitter too, then
jitter control must be implemented on those subnetworks that don't
have it. Implementing jitter control is clearly feasible - the
method for restoring jitter in the last section could be used on a
single network. But if we know jitter control isn't needed, why
require networks to implement it?)
Note that the argument simply says that jitter control is not
required to support isochronous applications. It may be the case
that jitter control is useful for other reasons. For example, work
at Berkeley suggests that jitter control makes it possible to reduce
the amount of buffering required in intermediate network nodes [Y].
Thus, even if applications express their requirements only in terms
of bandwidth and delay, a network may find it useful to try to limit
jitter and thereby reduce the amount of memory required in each node.
Acknowledgements
Thanks to the members of the End-To-End Interest mailing list who
provided a number of invaluable comments on this memo.
References
[1] Leiner, B., Editor, "Critical Issues in High Bandwidth
Networking", Report to DARPA, August 1988.
[2] Ferrari, D., "Client Requirements for Real-Time Communication
Services", IEEE Communications Magazine, November 1990. See also
RFC 1193, November, 1990.
[3] Saltzer, J., Reed D., and D. Clark, "End-To-End Arguments in
System Design", ACM Transactions on Computer Systems, Vol. 2, No.
4, November 1984.
[4] Mills, D., "Measured Performance of the Network Time Protocol in
the Internet System", RFC 1128, UDEL, October 1989.
[5] Verma, D., Zhang H., and D. Ferrari. "Guaranteeing Delay Jitter
Bounds in Packet Switching Networks", Proceedings of TriComm '91,
Chapel Hill, North Carolina, April 1991.
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Security Considertaions
Security issues are not discussed in this memo.
Author's Address
Craig Partridge
Swedish Institute of Computer Science
Box 1263
164 28 Kista
SWEDEN
Phone: +46 8 752 1524
EMail: craig@SICS.SE
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