One document matched: draft-aboba-rtpscale-00.txt
INTERNET-DRAFT Bernard Aboba
<draft-aboba-rtpscale-00.txt> Microsoft Corporation
Alternatives for Enhancing RTP Scalability
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working docu-
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The distribution of this memo is unlimited. It is filed as <draft-
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ments to the authors.
2. Abstract
This document discusses current issues with RTP scalability as well as
describing the advantages and disadvantages of possible solutions.
3. Requirements
In evaluating a solution to the RTP scaling problem, it is important
to have an understanding of the requirements that a proposed solution
must meet. This document proposes three requirements:
Decrease in convergence time
Decrease in effective RTCP sending population
Improvement in receiver reporting information
3.1. Decrease in convergence time
Currently RTCP relies on multicasting of receiver reports to establish
a receiver-population estimate. This shared-state is then used by
receivers to establish the receiver reporting interval. This mechanism
appears to function satisfactorily for conferences with several thou-
sand members. However, for larger conferences the result is very slow
convergence of receiver population estimates, resulting in long
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receiver reporting intervals, and RTCP bandwidth usage well in excess
of the 5 percent recommendation. This is particularly problematic for
dialup clients, which can experience severe RTCP-induced congestion.
For example, in a large conference the algorithm described in [1] ini-
tially results in a flood of reports from receivers joining the con-
ference, even though some random delay is imposed. This is because the
delay interval will initially be set low since receivers will set the
initial membership estimate to one. For dialup users, the result is
severe RTCP-induced congestion. However, even if all available band-
width is devoted to listening to RTCP receiver reports, a user con-
nected via a 14.4 Kbps modem can only receive a maximum of 1440
receiver reports a minute (assuming 60 octet receiver reports). In
reality, the number of actual reports that can be received will be
much less, since it is likely that the user will want to receiver
other traffic, such as audio/video from the conference they are
attempting to subscribe to. This is likely to reflect itself in having
RTCP packets set at lower priority than RTP packets, resulting in
dropping of RTCP packets during periods of congestion. Therefore in
reality a more realistic maximum convergence rate might be closer to
200 receiver reports a minute. This implies very long convergence
times. In a 20,000 user conference, the algorithm described in [1]
could take well over an hour and a half to converge. By the time we
have achieved convergence, the average reporting interval will be 1.9
hours.
3.2. Decrease in effective RTCP sending population
Today the MBONE uses DVMRP as its multicast routing protocol. DVMRP
generates forwarding cache entries on demand for each (source network,
destination group) pair, and supports source-specific prune messages.
The typical forwarding cache expiration time is 200 seconds.
Since the current RTCP mechanism requires each RTP receiver to become
a sender on the RTCP group, for large conferences, the result is cre-
ation of groups with a large number of senders. For example, an RTP
session with a single sender and 20,000 listeners would result in a
companion RTCP group with 20,001 senders and receivers.
Even given the slow convergence of receiver population estimates, it
is likely that the reporting interval will exceed the forwarding cache
expiration time within the first few minutes after conference initia-
tion. As a result, only a portion of all (source network, destination
group) pairs would be cached by a DVMRP implementation at a given
time. Nevertheless, the router will expend considerable resources in
adding and flushing forwarding cache entries, as well as in responding
to source-specific prune requests.
While future versions of DVMRP may prove more scalable, it is unlikely
that these versions will be widely deployed within the next 18 months.
As a result, it would desirable for an RTP scaling solution to provide
for a decrease in the number senders on the RTCP group.
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3.3. Improvement in receiver reporting information
The RTCP receiver report mechanism provides important information on
listenership, reception quality, and bandwidth availability. This
information is important useful both for engineering and business pur-
poses. Engineers use reception quality information to diagnose prob-
lems with the network. Sending systems can use reception quality and
bandwidth availability information to modify transmission parameters
such as compression ratio and sending rate. Business people use infor-
mation on listenership to track the size of the audience, and recep-
tion quality information to measure the quality of the user experi-
ence.
Since in large conferences the algorithm described in [1] leads to
underestimates of the receiver population and infrequent receiver
reporting, it can be argued that it does not meet the requirements for
accurate and timely receiver reporting. Therefore any scaling
"improvement" should not hamper the ability to collect this informa-
tion, and probably should be expected to result in improved reporting.
4. Scaling alternatives
Several alternatives have been proposed for improving the scalability
of RTCP. These include:
Turning off RTCP receiver reports
Improved convergence algorithms
Use of separate multicast groups for receiver and sender reports
Unicasting of receiver reports
Selective reporting
Use of TTL scoping and summary messages
Use of RTP translators
These alternatives are discussed in turn.
4.1. Turning off RTCP receiver reports
In some applications (such as satellite transmission) it may not be
possible to offer an upstream channel for transmission of RTCP
receiver reports. As a result, it may be desirable to allow receiver
reporting to be turned off. Since there is no receiver reporting,
there would be no way to estimate the receiver population.
Influence on RTCP receivers could be exercised via the SDP announce-
ment mechanism. For example, Mark Handley has proposed that the ses-
sion profile specified in SDP via the "m=" line be used to define the
desired RTCP behavior. This would allow for turning off of RTCP
receiver reports, or another desired mechanism.
While this proposal would eliminate the congestion problem for
receivers, it would deprive interested parties of information on lis-
tenership and reception quality. This will prevent senders from making
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adjustments to their transmission parameters, and would also prevent
RTP monitors from providing listenership estimates needed to justify
advertising rates.
4.2. Improved convergence algorithms
Just as non-linear equation solvers can benefit from improved algo-
rithms, it may be possible to improve RTP receiver population esti-
mates via improvements in the algorithm. These may include improving
the initial population estimate, as well as improve the algorithm by
which receivers estimate the overall population.
Currently the receiver population estimate has an initial value of 1,
but it is possible for an initial population estimate to be supplied
in the session announcement. Successive population estimates could
then be derived via an averaging of the initial estimate and the
receiver's own estimate, so that the effect of the initial estimate
would die out over time.
It is also possible to improve the speed of convergence by allowing
the receiver to use information on the rate at which receiver reports
are being sent, and incorporate this into the population estimate and
receiver reporting interval. For example, due to bandwidth limita-
tions, receivers on higher bandwidth connections have greater knowl-
edge of the overall receiver population. Thus if a receiver were to
note a receiver report from a system advertising high bandwidth avail-
ability, that report could be weighed more heavily in determining the
overall population estimate.
It is also possible to incorporate the receiver report rate into the
reporting interval calculation. For example, if my current population
estimate results in a receiver reporting interval of 3 minutes, and I
am receiving 200 receiver reports/minute, it may be desirable to
incorporate the rate of receiver reports into my calculations,
increasing the reporting interval accordingly.
However, the utility of such methods is limited in the case of dialup
clients, since they will only be able to receive a modest number of
receiver reports/minute, and thus the rate at which receiver reports
are flowing in may not be reflective of the overall population. Thus,
while an improved initial population estimate would result in improved
convergence times, were the initial estimate to be way off, converging
to a better estimate could still take a long time. This leads to the
conclusion that we need to be able to make better use of those
receiver reports that can be received.
Thus while this proposal could lessen the congestion problem for
higher-bandwidth receivers, it would not necessarily improve things
markedly for dialup clients, and would not result in a lower number of
senders on the RTCP receiver report group.
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4.3. Use of separate multicast groups for receiver and sender reports
Currently RTCP sender and receiver reports are sent to the same multi-
cast group, and both RTP senders and receivers join this group. Were
sender and receiver reports to be multicast on different groups, RTP
receivers could be allowed to only join the sender report group, thus
allowing them to avoid listening to receiver report traffic. RTP
senders would still join both groups in order to receive feedback on
listenership and transmission quality, and would need to provide
information on the estimated receiver population within the sender
reports.
While this proposal would lessen the congestion problem for receivers,
it would not improve things for senders who could still be deluged. It
also would only result in improved convergence of receiver population
estimates to the extent that senders can be assumed to have higher
bandwidth connections than receivers. Finally, it would not result in
a lower number of senders on the RTCP receiver report group.
4.4. Unicasting of receiver reports
Instead of multicasting receiver reports, it is possible to unicast
them back to the senders. This would allow RTP listeners to avoid
receiver report traffic, while RTP senders would still receive feed-
back on listenership and transmission quality. In order to control the
receiver report transmission rate, senders would need to provide
information on the estimated receiver population within sender
reports.
While this proposal would lessen the congestion problem for receivers,
it would not necessarily improve things for senders who could still be
deluged. It also would only result in improved convergence of receiver
population estimates to the extent that senders can be assumed to have
higher bandwidth connections than receivers. However, it would elimi-
nate multicasting of RTCP receiver reports, which will be of benefit
to overtaxed routers.
4.5. Selective reporting
RTCP receiver reports serve multiple purposes. One of these is to pro-
vide information on listenership; another is to provide information on
reception quality and bandwidth availability. Given that receiver
reporting intervals will tend to be very long for large conferences,
it can be argued that absent an emergency, it makes sense to provide
information on listenership, reception quality and bandwidth avail-
ability within the RTCP Bye message. Thus on leaving the conference,
the receiver would send a message providing information on the time
period in which they joined the conference, the overall reception
quality and other information. Conventional receiver reports would
then either not be sent at all, or would be sent with a TTL of 1. How-
ever, in order to supply engineers with timely information on network-
related problems, it is necessary to add a mechanism by which
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receivers could report packet losses exceeding some threshold.
While this proposal would lessen the congestion problem at the begin-
ning of a session, it could result in a deluge of receiver reports
toward the end of the session. Given that no receiver population esti-
mate would be available, it appears that this approach could actually
worsen convergence times, unless it were combined with some kind of
summarization mechanism. It would also not reduce the number of RTCP
senders.
4.6. Use of TTL scoping and summary messages
In this approach, RTCP receiver reports would be sent with a TTL of 1,
and a designated summarizer would be elected to provide summary
reports with a larger TTL. This approach has the advantage of increas-
ing the leverage of those RTCP receiver reports that are sent, and is
likely to be particularly effective for conferences in which member-
ship is densely distributed. However, in sparsely distributed confer-
ences, the effect of summarization will be small unless multiple lev-
els of summarization are used. The designated summarizer would not
necesarily be a router; it could also be another receiver, although
this brings up the possibility of how a new summarizer could be
elected if the current summarizer leaves the conference.
Since in this scheme receiver reports are not forwarded, non-
summarizing RTP receivers should use the population estimate gleaned
from local scope reports to calculate their reporting interval. Summa-
rizers and RTP senders will then use global estimates gleaned from
global scope summary reports to calculate their reporting interval. A
recommended bandwidth allocation for reporting is 25 percent for
sender reports, 25 percent for summary reports, and 50 percent for
receiver reports.
Since this proposal decreases the scope of RTCP announcements, it
would substantially reduce the congestion problem. It would also
reduce the number of RTCP senders visible to the multicast backbone,
and would decrease convergence times, since those messages that were
sent would include more information on the receiver population. How-
ever, it would also require substantial modifications in RTP client
behavior.
4.6.1. Summary reports
Via the use of summary reports, privacy can be provided while simulta-
neously providing senders with improved listenership and receiver
quality reporting. This is possible because in the existing implemen-
tation much of the information gained from receiver reports is redun-
dant. For example, if congestion results in packets being dropped on
a particular link, then all receivers downstream from that link will
report the same problem. This overabundance of redundant information
obscures the information of greatest interest, which is information on
global listenership and reception quality. Via introduction of summary
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reports, it is possible to provide more accurate and timely reporting
on listenership and reception quality.
Information useful in summary reports would include:
Total number of systems being summarized
Packets received and lost
Histogram of reception quality (fraction lost)
Histogram of receiver bandwidth
Information on users registering
The total systems summarized number is used in order to provide for
faster convergence times. Information on packets received and lost
will typically be used by network engineers looking to diagnose prob-
lems with the MBONE. The histograms of reception quality and receiver
bandwidth are propagated in order to allow senders to deduce informa-
tion relating to the global user experience. In order to allow for
location of individuals participating in a conference, the summarizer
may wish to relay information on the users in the conference. Alterna-
tively, it may register users in a directory service via use of the
LDAP extensions defined in [4].
4.7. Use of RTP translators
Through the use of RTP translators, it is possible to divide RTP
receivers into areas in much the same way as is accomplished by OSPF.
Through the use of summary receiver reports, information on listener-
ship and receiver report quality can be propagated between areas while
reducing RTCP bandwidth usage and the RTCP sending population on the
MBONE.
In order to insulate receivers within an area from external receiver
reports, the RTP translator must filter external receiver reports,
while allowing external summary reports and sender reports to pass
through. Similarly, the RTP translator will listen to receiver
reports generated from within its area, but will not pass them on to
external areas. Instead, it would issue to external areas two types of
summary reports. The first will be based on the packets it receives
and will be identical to a conventional receiver report, except for
the use of the summary report type; the second will be a true summary
report based on area receiver reports. It is useful to allow receiver
reports from RTP translators to pass through so as to allow diagnosis
of internal distribution problems. The RTP translator will allow
internal sender reports to pass through unmodified.
The introduction of RTP translators has several advantages:
Improved convergence of the receiver population estimate
Decreased RTCP bandwidth
Improved privacy
Listenership and reception quality information available to senders
While most of the above advantages are also available in the receiver
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summary approach, one advantage of the translator approach is that it
provides for greater control by the network administrator. For exam-
ple, since summary reports would be sent by RTP translators rather
than by client summarizers, it would be possible for administrators to
control the propagation of user information on a site-by-site basis,
rather than on a per-session basis. For example, rather than having
this sent in the summary report, it could be stored as a temporary
attribute in the area directory server. This may be perceived as a
substantial advantage by corporations looking to control access to
directory information. For those cases where it is desirable to allow
external access to area registration information, the IP address of
the area directory server may be advertised in the summary report.
This allows senders with appropriate privileges to retrieve conference
registration data from the area directory server via LDAP.
The RTP translator approach has several major disadvantages. These
include:
Requires modifications to routers
Increases loading on routers that now must function as RTP translators
5. Acknowledgements
Thanks to Steve Casner of Precept, Mark Handley of ISI, Bob Hinden of
Ipsilon and Thomas Pfenning and Stefan Solomon of Microsoft for useful
discussions of this problem space.
6. References
[1] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson. "RTP: A
Transport Protocol for Real-Time Applications." RFC 1889, GMD Fokus,
January 1996.
[2] H. Schulzrinne. "RTP Profile for Audio and Video Conferences
with Minimal Control." RFC 1890, GMD Fokus, January 1996.
[3] M. F. Speer, S. McCanne. "RTP Usage with Layered Multimedia
Streams." draft-speer-avt-layered-video-01.txt, Sun Microsystems,
LBNL, June 1996.
[4] Y. Yaacovi, M. Wahl, K. Settle, T. Genovese. "Lightweight Direc-
tory Access Protocol: Extensions for Dynamic Directory Services."
draft-ietf-asid-ldapv3ext-02.txt, Microsoft, Critical Angle, October,
1996.
7. Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
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Phone: 206-936-6605
EMail: bernarda@microsoft.com
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