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Network Working Group                                       C. PartridgeRequest for Comments: 1257         Swedish Institute of Computer Science                                                          September 1991Isochronous Applications Do Not Require Jitter-Controlled NetworksStatus 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 theirPartridge                                                       [Page 1]

RFC 1257                 Isochronous and Jitter           September 1991   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 supportPartridge                                                       [Page 2]

RFC 1257                 Isochronous and Jitter           September 1991   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 jitterPartridge                                                       [Page 3]

RFC 1257                 Isochronous and Jitter           September 1991   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 alsoRFC 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.Partridge                                                       [Page 4]

RFC 1257                 Isochronous and Jitter           September 1991Security 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.SEPartridge                                                       [Page 5]