[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 RFC 4586

Internet Draft                                           C. Burmeister
draft-burmeister-avt-rtcp-feedback-sim-06.txt             R. Hakenberg
Expires: October 2004                                      A. Miyazaki
                                                            Matsushita

                                                                J. Ott
                                              University of Bremen TZI

                                                               N. Sato
                                                           S. Fukunaga
                                                                   Oki

                                                            April 2004



              Extended RTP Profile for RTCP-based Feedback
               - Results of the Timing Rule Simulations -


Status of this Memo

   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC 2026.


   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other
   documents at any time.  It is inappropriate to use Internet-Drafts
   as reference material or to cite them other than as "work in
   progress."

   The list of current Internet-Drafts can be accessed at
        http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at
        http://www.ietf.org/shadow.html.

Copyright Notice

      Copyright (C) The Internet Society (2004).  All Rights
   Reserved.


Abstract

   This document describes the results achieved when simulating the
   timing rules of the Extended RTP Profile for RTCP-based Feedback,


Burmeister et al.        Expires October 2004                        1
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   denoted AVPF.  Unicast and multicast topologies are considered as
   well as several protocol and environment configurations.  The
   results show that the timing rules result in better performance
   regarding feedback delay and still preserve the well accepted RTP
   rules regarding allowed bit rates for control traffic.


Table of Contents

   1 Introduction
   2 Timing rules of the extended RTP profile for RTCP-based feedback
   3 Simulation Environment
   4 RTCP Bit Rate Measurements
   5 Feedback Measurements
   6 Investigations on "l"
   7 Applications Using AVPF
   8 Summary
   9 Security Considerations
   10 Informative References
   11 IPR Notices
   12 Authors' Address
   13 Full Copyright Statement

1 Introduction

   The Real-time Transport Protocol (RTP) is widely used for the
   transmission of real-time or near real-time media data over the
   Internet.  While it was originally designed to work well for
   multicast groups in very large scales, its scope is not limited to
   that.  More and more applications use RTP for small multicast
   groups (e.g. video conferences) or even unicast (e.g. IP telephony
   and media streaming applications).

   RTP comes together with its companion protocol Real-time Transport
   Control Protocol (RTCP), which is used to monitor the transmission
   of the media data and provide feedback of the reception quality.
   Furthermore, it can be used for loose session control.  Having the
   scope of large multicast groups in mind, the rules when to send
   feedback were carefully restricted to avoid feedback explosion or
   feedback related congestion in the network.  RTP and RTCP have
   proven to work well in the Internet, especially in large multicast
   groups, which is shown by its widespread usage today.

   However the applications that transmit the media data only to
   small multicast groups or unicast may benefit from more frequent
   feedback. The source of the packets may be able to react to
   changes in the reception quality, which may be due to varying
   network utilization (e.g. congestion) or other changes.  Possible
   reactions include transmission rate adaptation according to a


Burmeister et al.        Expires October 2004                        2
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   congestion control algorithm or the invocation of error resilience
   features for the media stream (e.g. retransmissions, reference
   picture selection, NEWPRED, etc.).

   As mentioned before, more frequent feedback may be desirable to
   increase the reception quality, but RTP restricts the use of RTCP
   feedback.  Hence it was decided to create a new extended RTP
   profile, which redefines some of the RTCP timing rules, but keeps
   most of the algorithms for RTP and RTCP, which have proven to work
   well.  The new rules should scale from unicast to multicast, where
   unicast or small multicast applications have the most gain from
   it.  A detailed description of the new profile and its timing
   rules can be found in [1].

   This document investigates the new algorithms by the means of
   simulations.  We show that the new timing rules scale well and
   behave in a network-friendly manner.  Firstly, the key features of
   the new RTP profile that are important for our simulations are
   roughly described in Section 3.  After that, we describe the
   environment that is used to conduct the simulations in Section 4.
   Section 5 describes simulation results that show the backwards
   compatibility to RTP and that the new profile is network-friendly
   in terms of used bandwidth for RTCP traffic.  In Section 6, we
   show the benefit that applications could get from implementing the
   new profile.  In Section 7 we investigated the effect of the
   parameter "l" (used to calculate the T_dither_max value) upon the
   algorithm performance and finally in Section 8 we show the
   performance gain we could get for a special application, namely
   NEWPRED in [6] and [7].


2 Timing rules of the extended RTP profile for RTCP-based feedback

   As said above, RTP restricts the usage of RTCP feedback.  The main
   restrictions on RTCP are as follows:

   - RTCP messages are sent in compound packets, i.e. every RTCP
   packet
     contains at least one sender report (SR) or receiver report (RR)
     message and a source description (SDES) message.
   - The RTCP compound packets are sent in time intervals (T_rr),
   which
     are computed as a function of the average packet size, the
   number
     of senders and receivers in the group and the session bandwidth
     (5% of the session bandwidth is used for RTCP messages; this
     bandwidth is shared between all session members, where the
   senders
     may get a larger share than the receivers.)
   - The average minimum interval between two RTCP packets from the
   same source


Burmeister et al.        Expires October 2004                        3
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



     is 5 seconds.

   We see that these rules prevent feedback explosion and scale well
   to large multicast groups.  However, they not allow timely
   feedback at all.  While the second rule scales also to small
   groups or unicast (in this cases the interval might be as small as
   a few milliseconds), the third rule may prevent the receivers from
   sending feedback timely.

   The timing rules to send RTCP feedback from the new RTP profile
   [1] consist of two key components.  First the minimum interval of
   5 seconds is abolished.  Second, receivers get once during their
   (now quite small) RTCP interval the chance to send an RTCP packet
   "early", i.e. not according to the calculated interval, but
   virtually immediately.  It is important to note that the RTCP
   interval calculation is still inherited from the original RTP
   specification.

   The specification and all the details of the extended timing rules
   can be found in [1].  We shall describe the algorithms here, but
   rather reference these from the original specification where
   needed.  Therefore we use also the same variable names and
   abbreviations as in [1].


3 Simulation Environment

   This section describes the simulation testbed that was used for
   the investigations and its key features.  The extensions to the
   simulator that were necessary are roughly described in the
   following sections.


3.1 Network Simulator Version 2

   The simulations were conducted using the network simulator version
   2 (ns2).  ns2 is an open source project, written in a combination
   of Tool Command Language (TCL) and C++.  The scenarios are set-up
   using TCL.  Using the scripts it is possible to specify the
   topologies (nodes and links, bandwidths, queue sizes or error
   rates for links) and the parameters of the "agents", i.e. protocol
   configurations.  The protocols themselves are implemented in C++
   in the agents, which are connected to the nodes.  The
   documentation for ns2 and the newest version can be found in [4].


3.2 RTP Agent

   We implemented a new agent, based on RTP/RTCP.  RTP packets are
   sent at a constant packet rate with the correct header sizes.
   RTCP packets are sent according to the timing rules of [2] and


Burmeister et al.        Expires October 2004                        4
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   also its algorithms for group membership maintenance are
   implemented.  Sender and receiver reports are sent.

   Further, we extended the agent to support the extended profile
   [1].  The use of the new timing rules can be turned on and off via
   parameter settings in TCL.


3.3 Scenarios

   The scenarios that are simulated are defined in TCL scripts.  We
   set-up several different topologies, ranging from unicast with two
   session members to multicast with up to 25 session members.
   Depending on the sending rates used and the corresponding link
   bandwidths, congestion losses may occur.  In some scenarios, bit
   errors are inserted on certain links.  We simulated groups with
   RTP/AVP agents, RTP/AVPF agents and mixed groups.

   The feedback messages are generally NACK messages as defined in
   [1] and are triggered by packet loss.


3.4 Topologies

   Mainly four different topologies are simulated to show the key
   features of the extended profile.  However, for some specific
   simulations we used different topologies.  This is then indicated
   in the description of the simulation results.  The main four
   topologies are named after the number of participating RTP agents,
   i.e. T-2, T-4, T-8 and T-16, where T-2 is a unicast scenario, T-4
   contains four agents, etc.  The figures below illustrate the main
   topologies.





















Burmeister et al.        Expires October 2004                        5
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



                                                   A5
                                     A5            |   A6
                                    /              |  /
                                   /               | /--A7
                                  /                |/
                    A2          A2-----A6          A2--A8
                   /           /                  /        A9
                  /           /                  /        /
                 /           /                  /        /---A10
   A1-----A2   A1-----A3   A1-----A3-----A7   A1------A3<
                 \           \                  \        \---A11
                  \           \                  \        \
                   \           \                  \        A12
                    A4          A4-----A8          A4--A13
                                                   |\
                                                   | \--A14
                                                   |  \
                                                   |  A15
                                                  A16

       T-2         T-4            T-8               T-16

   Figure 1: Simulated Topologies.


4 RTCP Bit Rate Measurements

   The new timing rules allow more frequent RTCP feedback for small
   multicast groups.  In large groups the algorithm behaves similarly
   to the normal RTCP timing rules.  While it is generally good to
   have more frequent feedback it cannot be allowed at all to
   increase the bit rate used for RTCP above a fixed limit, i.e. 5%
   of the total RTP bandwidth according to RTP.  This section shows
   that the new timing rules keep RTCP bandwidth usage under the 5%
   limit for all investigated scenarios, topologies and group sizes.
   Furthermore, we show that mixed groups, i.e. some members using
   AVP some AVPF, can be allowed and that each session member behaves
   fairly according to its corresponding specification.  Note that
   other values for the RTCP bandwidth limit may be specified using
   the RTCP bandwidth modifiers as in [10].


4.1 Unicast

   First we measured the RTCP bandwidth share in the unicast topology
   T-2.  Even for a fixed topology and group size, there are several
   protocol parameters which are varied to simulate a large range of
   different scenarios.  We varied the configurations of the agents
   in the sense that the agents may use the AVP or AVPF.  Thereby it
   is possible that one agent uses AVP and the other AVPF in one RTP



Burmeister et al.        Expires October 2004                        6
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   session.  This is done to test the backwards compatibility of the
   AVPF profile.

   First we consider scenarios where no losses occur.  In this case
   both RTP session members transmit the RTCP compound packets at
   regular intervals, calculated as T_rr, if they use the AVPF, and
   use a minimum interval of 5s (in average) if they implement the
   AVP.  No early packets are sent, because the need to send early
   feedback is not given.  Still it is important to see that not more
   than 5% of the session bandwidth is used for RTCP and that AVP and
   AVPF members can co-exist without interference.  The results can
   be found in table 1.

   |         |      |      |      |      | Used RTCP Bit Rate |
   | Session | Send | Rec. | AVP  | AVPF | (% of session bw)  |
   |Bandwidth|Agents|Agents|Agents|Agents|  A1  |  A2  | sum  |
   +---------+------+------+------+------+------+------+------+
   |  2 Mbps |  1   |  2   |  -   | 1,2  | 2.42 | 2.56 | 4.98 |
   |  2 Mbps | 1,2  |  -   |  -   | 1,2  | 2.49 | 2.49 | 4.98 |
   |  2 Mbps |  1   |  2   |  1   |  2   | 0.01 | 2.49 | 2.50 |
   |  2 Mbps | 1,2  |  -   |  1   |  2   | 0.01 | 2.48 | 2.49 |
   |  2 Mbps |  1   |  2   | 1,2  |  -   | 0.01 | 0.01 | 0.02 |
   |  2 Mbps | 1,2  |  -   | 1,2  |  -   | 0.01 | 0.01 | 0.02 |
   |200 kbps |  1   |  2   |  -   | 1,2  | 2.42 | 2.56 | 4.98 |
   |200 kbps | 1,2  |  -   |  -   | 1,2  | 2.49 | 2.49 | 4.98 |
   |200 kbps |  1   |  2   |  1   |  2   | 0.06 | 2.49 | 2.55 |
   |200 kbps | 1,2  |  -   |  1   |  2   | 0.08 | 2.50 | 2.58 |
   |200 kbps |  1   |  2   | 1,2  |  -   | 0.06 | 0.06 | 0.12 |
   |200 kbps | 1,2  |  -   | 1,2  |  -   | 0.08 | 0.08 | 0.16 |
   | 20 kbps |  1   |  2   |  -   | 1,2  | 2.44 | 2.54 | 4.98 |
   | 20 kbps | 1,2  |  -   |  -   | 1,2  | 2.50 | 2.51 | 5.01 |
   | 20 kbps |  1   |  2   |  1   |  2   | 0.58 | 2.48 | 3.06 |
   | 20 kbps | 1,2  |  -   |  1   |  2   | 0.77 | 2.51 | 3.28 |
   | 20 kbps |  1   |  2   | 1,2  |  -   | 0.58 | 0.61 | 1.19 |
   | 20 kbps | 1,2  |  -   | 1,2  |  -   | 0.77 | 0.79 | 1.58 |

   Table 1: Unicast simulations without packet loss.

   We can see that in configurations where both agents use the new
   timing rules each of them uses, at most, about 2.5% of the session
   bandwidth for RTP, which sums up to 5% of the session bandwidth
   for both.  This is achieved regardless of the agent being a sender
   or a receiver.  In the cases where agent A1 uses AVP and agent A2
   AVPF, the total RTCP session bandwidth is decreased.  This is due
   to the fact that agent A1 can send RTCP packets only with an
   average minimum interval of 5 seconds.  Thus only a small fraction
   of the session bandwidth is used for its RTCP packets.  For a high
   bit rate session (session bandwidth = 2 Mbps) the fraction of the
   RTCP packets from agent A1 is as small as 0.01%.  For smaller
   session bandwidths the fraction increases, because the same amount
   of RTCP data is sent.  The bandwidth share that is used by RTCP


Burmeister et al.        Expires October 2004                        7
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   packets from agent A2 is not different from what was used, when
   both agents implemented the AVPF.  Thus the interaction of AVP and
   AVPF agents is not problematic in these scenarios at all.

   In our second unicast experiment, we show that the allowed RTCP
   bandwidth share is not exceeded, even if packet loss occurs.  We
   simulated a constant byte error rate (BYER) on the link.  The byte
   errors are inserted randomly according to an uniform distribution.
   Packets with byte errors are discarded on the link; hence the
   receiving agents will not see the loss immediately.  The agents
   detect packet loss by a gap in the sequence number.

   When an AVPF agent detects a packet loss the early feedback
   procedure is started.  As described in AVPF [1], in unicast
   T_dither_max is always zero, hence an early packet can be sent
   immediately if allow_early is true.  If the last packet was
   already an early one (i.e. allow_early = false), the feedback
   might be appended to the next regularly scheduled receiver report.
   The max_feedback_delay parameter (which we set to 1 second in our
   simulations) determines if that is allowed.

   The results are shown in table 2, where we can see that there is
   no difference in the RTCP bandwidth share, whether losses occur or
   not.  This is what we expected, because even though the RTCP
   packet size grows and early packets are sent, the interval between
   the packets increases and thus the RTCP bandwidth stays the same.
   Only the RTCP bandwidth of the agents that use the AVP increases
   slightly.  This is because the interval between the packets is
   still 5 seconds (in average), but the packet size increased
   because of the feedback that is appended.


   |         |      |      |      |      | Used RTCP Bit Rate |
   | Session | Send | Rec. | AVP  | AVPF | (% of session bw)  |
   |Bandwidth|Agents|Agents|Agents|Agents|  A1  |  A2  | sum  |
   +---------+------+------+------+------+------+------+------+
   |  2 Mbps |  1   |  2   |  -   | 1,2  | 2.42 | 2.56 | 4.98 |
   |  2 Mbps | 1,2  |  -   |  -   | 1,2  | 2.49 | 2.49 | 4.98 |
   |  2 Mbps |  1   |  2   |  1   |  2   | 0.01 | 2.49 | 2.50 |
   |  2 Mbps | 1,2  |  -   |  1   |  2   | 0.01 | 2.48 | 2.49 |
   |  2 Mbps |  1   |  2   | 1,2  |  -   | 0.01 | 0.02 | 0.03 |
   |  2 Mbps | 1,2  |  -   | 1,2  |  -   | 0.01 | 0.01 | 0.02 |
   |200 kbps |  1   |  2   |  -   | 1,2  | 2.42 | 2.56 | 4.98 |
   |200 kbps | 1,2  |  -   |  -   | 1,2  | 2.50 | 2.49 | 4.99 |
   |200 kbps |  1   |  2   |  1   |  2   | 0.06 | 2.50 | 2.56 |
   |200 kbps | 1,2  |  -   |  1   |  2   | 0.08 | 2.49 | 2.57 |
   |200 kbps |  1   |  2   | 1,2  |  -   | 0.06 | 0.07 | 0.13 |
   |200 kbps | 1,2  |  -   | 1,2  |  -   | 0.09 | 0.08 | 0.17 |
   | 20 kbps |  1   |  2   |  -   | 1,2  | 2.42 | 2.57 | 4.99 |
   | 20 kbps | 1,2  |  -   |  -   | 1,2  | 2.52 | 2.51 | 5.03 |
   | 20 kbps |  1   |  2   |  1   |  2   | 0.58 | 2.54 | 3.12 |


Burmeister et al.        Expires October 2004                        8
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   | 20 kbps | 1,2  |  -   |  1   |  2   | 0.83 | 2.43 | 3.26 |
   | 20 kbps |  1   |  2   | 1,2  |  -   | 0.58 | 0.73 | 1.31 |
   | 20 kbps | 1,2  |  -   | 1,2  |  -   | 0.86 | 0.84 | 1.70 |

   Table 2: Unicast simulations with packet loss.


4.2 Multicast

   Next, we investigated the RTCP bandwidth share in multicast
   scenarios, i.e. we simulated the topologies T-4, T-8 and T-16 and
   measured the fraction of the session bandwidth that was used for
   RTCP packets.  Again we considered different situations and
   protocol configurations (e.g. with or without bit errors, groups
   with AVP and/or AVPF agents, etc.).  For reasons of readability,
   we present only selected results.  For a documentation of all
   results, see [5].

   The simulations of the different topologies in scenarios where no
   losses occur (neither through bit errors nor through congestion)
   show a similar behavior as in the unicast case.  For all group
   sizes the maximum RTCP bit rate share used is 5.06% of the session
   bandwidth in a simulation of 16 session members in a low bit rate
   scenario (session bandwidth = 20kbps) with several senders.  In
   all other scenarios without losses the RTCP bit rate share used is
   below that.  Thus, the requirement that not more than 5% of the
   session bit rate should be used for RTCP is fulfilled with
   reasonable accuracy.

   Simulations where bit errors are randomly inserted in RTP and RTCP
   packets and the corrupted packets are discarded give the same
   results.  The 5% rule is kept (at maximum 5.07% of the session
   bandwidth is used for RTCP).

   Finally we conducted simulations where we reduced the link
   bandwidth and thereby caused congestion related losses.  These
   simulations are different from the previous bit error simulations,
   in that the losses occur more in bursts and are more correlated,
   also between different agents.  The correlation and burstiness of
   the packet loss is due to the queuing discipline in the routers we
   simulated; we used simple FIFO queues with a drop-tail strategy to
   handle congestion.  Random Early Detection (RED) queues may
   enhance the performance, because the burstiness of the packet loss
   might be reduced, however this is not the subject of our
   investigations, but is left for future research.  The delay
   between the agents, which also influences RTP and RTCP packets, is
   much more variable because of the added queuing delay.  Still the
   RTCP bit rate share used does not increase beyond 5.09% of the
   session bandwidth.  Thus also for these special cases the
   requirement is fulfilled.



Burmeister et al.        Expires October 2004                        9
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004




4.3 Summary of the RTCP bit rate measurements

   We have shown that for unicast and reasonable multicast scenarios,
   feedback implosion does not happen.  The requirement that at
   maximum 5% of the session bandwidth is used for RTCP is fulfilled
   for all investigated scenarios.


5 Feedback Measurements

   In this chapter we describe the results of feedback delay
   measurements, which we conducted in the simulations.  Therefore we
   use two metrics for measuring the performance of the algorithms,
   these are the "mean waiting time" (MWT) and the number of feedback
   packets that are sent, suppressed or not allowed.  The waiting
   time is the time, measured at a certain agent, between the
   detection of a packet loss event and the time when the
   corresponding feedback is sent.  Assuming that the value of the
   feedback decreases with its delay, we think that the mean waiting
   time is a good metric to measure the performance gain we could get
   by using AVPF instead of AVP.

   The feedback an RTP/AVPF agent wants to send can be either sent or
   not sent.  If it was not sent, this could be due to the feedback
   suppression, i.e. another receiver already sent the same feedback
   or because the feedback was not allowed, i.e. the
   max_feedback_delay was exceeded.  We traced for every detected
   loss, if the agent sent the corresponding feedback or not and if
   not, why.  The more feedback was not allowed, the worse the
   performance of the algorithm.  Together with the waiting times,
   this gives us a good hint of the overall performance of the
   scheme.


5.1 Unicast

   In the unicast case, the maximum dithering interval T_dither_max
   is fixed and set to zero.  This is due to the fact that it does
   not make sense for a unicast receiver to wait for other receivers
   if they have the same feedback to send.  But still feedback can be
   delayed or might not be permitted to be sent at all.  The
   regularly scheduled packets are spaced according to T_rr, which
   depends in the unicast case mainly on the session bandwidth.

   Table 3 shows the mean waiting times (MWT) measured in seconds for
   some configurations of the unicast topology T-2.  The number of
   feedback packets that are sent or discarded is listed also
   (feedback sent (sent) or feedback discarded (disc)).  We do not
   list suppressed packets, because for the unicast case feedback



Burmeister et al.        Expires October 2004                       10
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   suppression does not apply.  In the simulations, agent A1 was a
   sender and agent A2 a pure receiver.

   |         |       |          Feedback Statistics          |
   | Session |       |       AVP         |       AVPF        |
   |Bandwidth|  PLR  | sent |disc| MWT   | sent |disc| MWT   |
   +---------+-------+------+----+-------+------+----+-------+
   |  2 Mbps | 0.001 |  781 |  0 | 2.604 |  756 |  0 | 0.015 |
   |  2 Mbps | 0.01  | 7480 |  0 | 2.591 | 7548 |  2 | 0.006 |
   |  2 Mbps | cong. |   25 |  0 | 2.557 | 1741 |  0 | 0.001 |
   | 20 kbps | 0.001 |   79 |  0 | 2.472 |   74 |  2 | 0.034 |
   | 20 kbps | 0.01  |  780 |  0 | 2.605 |  709 | 64 | 0.163 |
   | 20 kbps | cong. |  780 |  0 | 2.590 |  687 | 70 | 0.162 |


   Table 3: Feedback Statistics for the unicast simulations.

   From the table above we see that the mean waiting time can be
   decreased dramatically by using AVPF instead of AVP.  While the
   waiting times for agents using AVP is always around 2.5 seconds
   (half the minimum interval average) it can be decreased to a few
   ms for most of the AVPF configurations.

   In the configurations with high session bandwidth, normally all
   triggered feedback is sent.  This is because more RTCP bandwidth
   is available.  There are only very few exceptions, which are
   probably due to more than one packet loss within one RTCP
   interval, where the first loss was by chance sent quite early.  In
   this case it might be possible that the second feedback is
   triggered after the early packet was sent, but possibly too early
   to append it to the next regularly scheduled report, because of
   the limitation of the max_feedback_delay.  This is different for
   the cases with a small session bandwidth, where the RTCP bandwidth
   share is quite low and T_rr thus larger.  After an early packet
   was sent the time to the next regularly scheduled packet can be
   very high.  We saw that in some cases the time was larger than the
   max_feedback_delay and in these cases the feedback is not allowed
   to be sent at all.

   With a different setting of max_feedback_delay it is possible to
   have either more feedback that is not allowed and a decreased mean
   waiting time or more feedback that is sent but an increased
   waiting time.  Thus the parameter should be set with care
   according to the application's needs.


5.2 Multicast

   In this section we describe some measurements of feedback
   statistics in the multicast simulations.  We picked out certain
   characteristic and representative results.  We considered the


Burmeister et al.        Expires October 2004                       11
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   topology T-16.  Different scenarios and applications are simulated
   for this topology.  The parameters of the different links are set
   as follows.  The agents A2, A3 and A4 are connected to the middle
   node of the multicast tree, i.e. agent A1, via high bandwidth and
   low delay links.  The other agents are connected to the nodes 2, 3
   and 4 via different link characteristics.  The agents connected to
   node 2 represent mobile users.  They suffer in certain
   configurations from a certain byte error rate on their access
   links and the delays are high.  The agents that are connected to
   node 3 have low bandwidth access links, but do not suffer from bit
   errors.  The last agents, that are connected to node 4 have high
   bandwidth and low delay.

5.2.1 Shared Losses vs. Distributed Losses

   In our first investigation, we wanted to see the effect of the
   loss characteristic on the algorithm's performance.  We
   investigate the cases where packet loss occurs for several users
   simultaneously (shared losses) or totally independently
   (distributed losses).  We first define agent A1 to be the sender.
   In the case of shared losses, we inserted a constant byte error
   rate on one of the middle links, i.e. the link between A1 and A2.
   In the case of distributed losses, we inserted the same byte error
   rate on all links downstream of A2.

   These scenarios are especially interesting because of the feedback
   suppression algorithm.  When all receivers share the same loss, it
   is only necessary for one of them to send the loss report.  Hence
   if a member receives feedback with the same content that it has
   scheduled to be sent, it suppresses the scheduled feedback.  Of
   course, this suppressed feedback does not contribute to the mean
   waiting times.  So we expect reduced waiting times for shared
   losses, because the probability is high that one of the receivers
   can send the feedback more or less immediately.  The results are
   shown in the following table.

   |     |                Feedback Statistics                |
   |     |  Shared Losses          |  Distributed Losses     |
   |Agent|sent|fbsp|disc|sum | MWT |sent|fbsp|disc|sum | MWT |
   +-----+----+----+----+----+-----+----+----+----+----+-----+
   |  A2 | 274| 351|  25| 650|0.267|   -|   -|   -|   -|    -|
   |  A5 | 231| 408|  11| 650|0.243| 619|   2|  32| 653|0.663|
   |  A6 | 234| 407|   9| 650|0.235| 587|   2|  32| 621|0.701|
   |  A7 | 223| 414|  13| 650|0.253| 594|   6|  41| 641|0.658|
   |  A8 | 188| 443|  19| 650|0.235| 596|   1|  32| 629|0.677|

   Table 4: Feedback statistics for multicast simulations.

   Table 4 shows the feedback statistics for the simulation of a
   large group size.  All 16 agents of topology T-16  joined the RTP
   session.  However only agent A1 acts as an RTP sender, the other


Burmeister et al.        Expires October 2004                       12
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   agents are pure receivers.  Only 4 or 5 agents suffer from packet
   loss, i.e. A2, A5, A6, A7 and A8 for the case of shared losses and
   A5, A6, A7 and A8 in the case of distributed losses.  Since the
   number of session members is the same for both cases, T_rr is also
   the same on the average.  Still the mean waiting times are reduced
   by more than 50% in the case of shared losses.  This proves our
   assumption that shared losses enhance the performance of the
   algorithm, regardless of the loss characteristic.

   The feedback suppression mechanism seems to be working quite well.
   Even though some feedback is sent from different receivers (i.e.
   1150 loss reports are sent in total and only 650 packets were
   lost, resulting in loss reports being received on the average 1.8
   times) most of the redundant feedback was suppressed.  That is,
   2023 loss reports were suppressed from 3250 individual detected
   losses, which means that more than 60% of the feedback was
   actually suppressed.


6 Investigations on "l"

   In this section we want to investigate the effect of the parameter
   "l" on the T_dither_max calculation in RTP/AVPF agents.  We
   investigate the feedback suppression performance as well as the
   report delay for three sample scenarios.

   For all receivers the T_dither_max value is calculated as
   T_dither_max = l * T_rr, with l = 0.5.  The rationale for this is
   that, in general, if the receiver has no RTT estimation, it does
   not know how long it should wait for other receivers to send
   feedback.  The feedback suppression algorithm would certainly fail
   if the time selected is too short.  However, the waiting time is
   increased unnecessarily (and thus the value of the feedback is
   decreased) in case the chosen value is too large.  Ideally, the
   optimum time value could be found for each case but this is not
   always feasible.  On the other hand, it is not dangerous if the
   optimum time is not used.  A decreased feedback value and a
   failure of the feedback suppression mechanism do not hurt the
   network stability.  We have shown for the cases of distributed
   losses that the overall bandwidth constraints are kept in any case
   and thus we could only lose some performance by choosing the wrong
   time value.  On the other hand, a good measure for T_dither_max
   however is the RTCP interval T_rr.  This value increases with the
   number of session members.  Also, we know that we can send
   feedback at least every T_rr.  Thus increasing T_dither max beyond
   T_rr would certainly make no sense.  So by choosing T_rr/2 we
   guarantee that at least sometimes (i.e. when a loss is detected in
   the first half of the interval between two regularly scheduled
   RTCP packets) we are allowed to send early packets.  Because of
   the randomness of T_dither we still have a good chance to send the
   early packet in time.


Burmeister et al.        Expires October 2004                       13
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004




   The AVPF profile specifies that the calculation of T_dither_max,
   as given above, is common to session members having an RTT
   estimation and to those not having it.  If this were not so,
   participants using different calculations for T_dither_max might
   also have very different mean waiting times before sending
   feedback, which translates into different reporting priorities.
   For example, in an scenario where T_rr = 1s and the RTT = 100 ms,
   receivers using the RTT estimation would, on average, send more
   feedback than those not using it.  This might partially cancel out
   the feedback suppression mechanism and even cause feedback
   implosion.  Also note that, in a general case where the losses are
   shared, the feedback suppression mechanism works if the feedback
   packets from each receiver have enough time to reach each of the
   other ones before the calculated T_dither_max seconds.  Therefore,
   in scenarios of very high bandwidth (small T_rr) the calculated
   T_dither_max could be much smaller than the propagation delay
   between receivers, which would translate into a failure of the
   feedback suppression mechanism.  In these cases, one solution
   could be to limit the bandwidth available to receivers (see [10])
   such that this does not happen.  Another solution could be to
   develop a mechanism for feedback suppression based on the RTT
   estimation between senders.  This will not be discussed here and
   may be object of another document.  Note, however, that a really
   high bandwidth media stream is not that likely to rely on this
   kind of error repair in the first place.

   In the following, we define three representative sample scenarios.
   We use the topology from the previous section, T-16.  Most of the
   agents contribute only little to the simulations, because we
   introduced an error rate only on the link between the sender A1
   and the agent A2.

   The first scenario represents those cases, where losses are shared
   between two agents.  One agent is located upstream on the path
   between the other agent and the sender.  Therefore, agent A2 and
   agent A5 see the same losses that are introduced on the link
   between the sender and agent A2.  Agents A6, A7 and A8 do not join
   the RTP session.  From the other agents only agents A3 and A9
   join.  All agents are pure receivers, except A1 which is the
   sender.

   The second scenario represents also cases, where losses are shared
   between two agents, but this time the agents are located on
   different branches of the multicast tree.  The delays to the
   sender are roughly of the same magnitude.  Agents A5 and A6 share
   the same losses.  Agents A3 and A9 join the RTP session, but are
   pure receivers and do not see any losses.

   Finally, in the third scenario, the losses are shared between two
   agents, A5 and A6.  The same agents as in the second scenario are


Burmeister et al.        Expires October 2004                       14
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   active.  However, the delays of the links are different.  The
   delay of the link between agent A2 and A5 is reduced to 20ms and
   between A2 and A6 to 40ms.

   All agents beside agent A1 are pure RTP receivers.  Thus these
   agents do not have an RTT estimation to the source.  T_dither_max
   is calculated with the above given formula, depending only on T_rr
   and l, which means that all agents should calculate roughly the
   same T_dither_max.


6.1 Feedback Suppression Performance

   The feedback suppression rate for an agent is defined as the ratio
   of the total number of feedback packets not sent out of the total
   number of feedback packets the agent intended to send (i.e. the
   sum of sent and not sent).  The reasons for not sending a packet
   include: the receiver already saw the same loss reported in a
   receiver report coming from another session member or the
   max_feedback_delay (application-specific) was surpassed.

   The results for the feedback suppression rate of the agent Af that
   is further away from the sender, are depicted in Table 10.  In
   general it can be seen that the feedback suppression rate
   increases with an increasing l.  However there is a threshold,
   depending on the environment, from which the additional gain is
   not significant anymore.

   |      |  Feedback Suppression Rate  |
   |  l   | Scen. 1 | Scen. 2 | Scen. 3 |
   +------+---------+---------+---------+
   | 0.10 |  0.671  |  0.051  |  0.089  |
   | 0.25 |  0.582  |  0.060  |  0.210  |
   | 0.50 |  0.524  |  0.114  |  0.361  |
   | 0.75 |  0.523  |  0.180  |  0.370  |
   | 1.00 |  0.523  |  0.204  |  0.369  |
   | 1.25 |  0.506  |  0.187  |  0.372  |
   | 1.50 |  0.536  |  0.213  |  0.414  |
   | 1.75 |  0.526  |  0.215  |  0.424  |
   | 2.00 |  0.535  |  0.216  |  0.400  |
   | 3.00 |  0.522  |  0.220  |  0.405  |
   | 4.00 |  0.522  |  0.220  |  0.405  |

   Table 10: Fraction of feedback that was suppressed at agent Af of
   the total number of feedback messages the agent wanted to send

   Similar results can be seen for the agent that is nearer to the
   sender in Table 11.

   |      |  Feedback Suppression Rate  |
   |  l   | Scen. 1 | Scen. 2 | Scen. 3 |


Burmeister et al.        Expires October 2004                       15
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   +------+---------+---------+---------+
   | 0.10 |  0.056  |  0.056  |  0.090  |
   | 0.25 |  0.063  |  0.055  |  0.166  |
   | 0.50 |  0.116  |  0.099  |  0.255  |
   | 0.75 |  0.141  |  0.141  |  0.312  |
   | 1.00 |  0.179  |  0.175  |  0.352  |
   | 1.25 |  0.206  |  0.176  |  0.361  |
   | 1.50 |  0.193  |  0.193  |  0.337  |
   | 1.75 |  0.197  |  0.204  |  0.341  |
   | 2.00 |  0.207  |  0.207  |  0.368  |
   | 3.00 |  0.196  |  0.203  |  0.359  |
   | 4.00 |  0.196  |  0.203  |  0.359  |

   Table 11: Fraction of feedback that was suppressed at agent An of
   the total number of feedback messages the agent wanted to send

   The rate of feedback suppression failure is depicted in Table 12.
   The trend of additional performance increase is not significant
   beyond a certain threshold. Dependence on the scenario is
   noticeable here as well.

   |      |Feedback Suppr. Failure Rate |
   |  l   | Scen. 1 | Scen. 2 | Scen. 3 |
   +------+---------+---------+---------+
   | 0.10 |  0.273  |  0.893  |  0.822  |
   | 0.25 |  0.355  |  0.885  |  0.624  |
   | 0.50 |  0.364  |  0.787  |  0.385  |
   | 0.75 |  0.334  |  0.679  |  0.318  |
   | 1.00 |  0.298  |  0.621  |  0.279  |
   | 1.25 |  0.289  |  0.637  |  0.267  |
   | 1.50 |  0.274  |  0.595  |  0.249  |
   | 1.75 |  0.274  |  0.580  |  0.235  |
   | 2.00 |  0.258  |  0.577  |  0.233  |
   | 3.00 |  0.282  |  0.577  |  0.236  |
   | 4.00 |  0.282  |  0.577  |  0.236  |

   Table 12: The ratio of feedback suppression failures.

   Summarizing the feedback suppression results, it can be said that
   in general the feedback suppression performance increases with an
   increasing l.  However, beyond a certain threshold, depending on
   environment parameters such as propagation delays or session
   bandwidth, the additional increase is not significant anymore.
   This threshold is not uniform across all scenarios; a value of
   l=0.5 seems to produce reasonable results with acceptable (though
   not optimal) overhead.


6.2 Loss Report Delay




Burmeister et al.        Expires October 2004                       16
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   In this section we show the results for the measured report delay
   during the simulations of the three sample scenarios.  This
   measurement is a metric of the performance of the algorithms,
   because the value of the feedback for the sender typically
   decreases with the delay of its reception.  The loss report delay
   is measured as the time at the sender between sending a packet and
   receiving the first corresponding loss report.

   |      |   Mean Loss Report Delay    |
   |  l   | Scen. 1 | Scen. 2 | Scen. 3 |
   +------+---------+---------+---------+
   | 0.10 |  0.124  |  0.282  |  0.210  |
   | 0.25 |  0.168  |  0.266  |  0.234  |
   | 0.50 |  0.243  |  0.264  |  0.284  |
   | 0.75 |  0.285  |  0.286  |  0.325  |
   | 1.00 |  0.329  |  0.305  |  0.350  |
   | 1.25 |  0.351  |  0.329  |  0.370  |
   | 1.50 |  0.361  |  0.363  |  0.388  |
   | 1.75 |  0.360  |  0.387  |  0.392  |
   | 2.00 |  0.367  |  0.412  |  0.400  |
   | 3.00 |  0.368  |  0.507  |  0.398  |
   | 4.00 |  0.368  |  0.568  |  0.398  |

   Table 13: The mean loss report delay, measured at the sender.

   As can be seen from Table 13 the delay increases in general with
   an increasing value of l. Also, a similar effect as for the
   feedback suppression performance is present: beyond a certain
   threshold, the additional increase in delay is not significant
   anymore.  The threshold is environment dependent and seems to be
   related to the threshold, where the feedback suppression gain
   would not increase anymore.


6.3 Summary of "l" investigations

   We have shown experimentally that the performance of the feedback
   suppression mechanisms increases with an increasing value of l.
   The same applies for the report delay, which increases also with
   an increasing l.  This leads to a threshold where both the
   performance and the delay does not increase any further.  The
   threshold is dependent upon the environment.

   So finding an optimum value of l is not possible because it is
   always a trade-off between delay and feedback suppression
   performance.  With l=0.5 we think that a tradeoff was found that
   is acceptable for typical applications and environments.


7 Applications Using AVPF



Burmeister et al.        Expires October 2004                       17
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   NEWPRED is one of the error resilience tools, which is defined in
   both ISO/IEC MPEG-4 visual part and ITU-T H.263.  NEWPRED achieves
   fast error recovery using feedback messages.  We simulated the
   behavior of NEWPRED in the network simulator environment as
   described above and measured the waiting time statistics, in order
   to verify that the extended RTP profile for RTCP-based feedback
   (AVPF)[1] is appropriate for the NEWPRED feedback messages.
   Simulation results, which are presented in the following sections,
   show that the waiting time is small enough to get the expected
   performance of NEWPRED.


7.1 NEWPRED Implementation in NS2

   The agent that performs the NEWPRED functionality, called NEWPRED
   agent, is different from the RTP agent we described above.  Some
   of the added features and functionalities are described in the
   following points:

   Application Feedback
     The "Application Layer Feedback Messages" format is used to
     transmit the NEWPRED feedback messages.  Thereby the NEWPRED
     functionality is added to the RTP agent.  The NEWPRED agent
     creates one NACK message for each lost segment of a video frame,
     and then assembles multiple NACK messages corresponding
     to the segments in the same video frame into one Application
     Layer Feedback Message.  Although there are two modes, namely
     NACK mode and ACK mode, in NEWPRED [6][7], only NACK mode is
   used
     in these simulations.

     The parameters of NEWPRED agent are as follows:
           f: Frame Rate(frames/sec)
         seg: Number of segments in one video frame
          bw: RTP session bandwidth(kbps)

   Generation of NEWPRED's NACK Messages
     The NEWPRED agent generates NACK messages when segments are
   lost.
     a. The NEWPRED agent generates multiple NACK messages per
        one video frame when multiple segments are lost.  These
        are assembled into one FCI message per video frame.  If there
        is no lost segment, no message is generated and sent.
     b. The length of one NACK message is 4 bytes.  Let num be the
        number of NACK messages in one video frame (1 <= num <= seg).
        Thus, 12+4*num bytes is the size of the low delay RTCP
   feedback
        message.

   Measurements
     We defined two values to be measured:


Burmeister et al.        Expires October 2004                       18
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



     - Recovery time
       The recovery time is measured as the time between the
   detection
       of a lost segment and reception of a recovered segment.  We
       measured this "recovery time" for each lost segment.
     - Waiting time
       The waiting time is the additional delay due to the feedback
       limitation of RTP.

     Fig.1 depicts the behavior of a NEWPRED agent when a loss
   occurs.
     The recovery time is approximated as follows:
       (Recovery time) = (Waiting time) +
                         (Transmission time for feedback message) +
                         (Transmission time for media data)

     Therefore, the waiting time is derived as follows:

       (Waiting time) = (Recovery time) - (Round-trip delay), where

       (Round-trip delay ) = (Transmission time for feedback message)
   +
                             (Transmission time for media data)





        Picture Reference                            |: Picture
   Segment
                 ____________________                %: Lost Segment
                /_    _    _    _    \
               v/ \  / \  / \  / \    \
               v   \v   \v   \v   \    \
   Sender   ---|----|----|----|----|----|---|------------->
                    \    \                 ^ \
                     \    \               /   \
                      \    \             /     \
                       \    v           /       \
                        \    x         /         \
                         \   Lost     /           \
                          \    x     /             \
   _____
                           v    x   / NACK          v
   Receiver ---------------|----%===-%----%----%----|----->
                                |-a-|               |
                                |-------  b  -------|

                          a: Waiting time
                          b: Recover time (%: Video segments are
   lost)


Burmeister et al.        Expires October 2004                       19
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004




   Fig.1: Relation between the measured values at the NEWPRED agent


7.2 Simulation

   We conducted two simulations (Simulation A and Simulation B).  In
   Simulation A, the packets are dropped with a fixed packet loss
   rate on a link between two NEWPRED agents.  In Simulation B,
   packet loss occurs due to congestion from other traffic sources,
   i.e. ftp sessions.

7.2.1. Simulation A - Constant Packet Loss Rate

   The network topology, used for this simulation is shown in Fig.2.




                  Link 1         Link 2        Link 3
        +--------+      +------+       +------+      +--------+
        | Sender |------|Router|-------|Router|------|Receiver|
        +--------+      +------+       +------+      +--------+
                 10(msec)       x(msec)       10(msec)


   Fig2. Network topology that is used for Simulation A

   Link1 and link3 are error free, and each link delay is 10 msec.
   Packets may get dropped on link2.  The packet loss rates (Plr) and
   link delay (D) are as follows:

      D [ms] = {10, 50, 100, 200, 500}
      Plr    = {0.005, 0.01, 0.02, 0.03, 0.05, 0.1, 0.2}
      Session band width, frame rate and the number of segments are
      shown in Table 14

   +------------+----------+-------------+-----+
   |Parameter ID| bw(kbps) |f (frame/sec)| seg |
   +------------+----------+-------------+-----+
   | 32k-4-3    |     32   |      4      |  3  |
   | 32k-5-3    |     32   |      5      |  3  |
   | 64k-5-3    |     64   |      5      |  3  |
   | 64k-10-3   |     64   |     10      |  3  |
   | 128k-10-6  |    128   |     10      |  6  |
   | 128k-15-6  |    128   |     15      |  6  |
   | 384k-15-6  |    384   |     15      |  6  |
   | 384k-30-6  |    384   |     30      |  6  |
   | 512k-30-6  |    512   |     30      |  6  |
   | 1000k-30-9 |   1000   |     30      |  9  |
   | 2000k-30-9 |   2000   |     30      |  9  |


Burmeister et al.        Expires October 2004                       20
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   +------------+----------+-------------+-----+

   Table 14: Parameter sets of the NEWPRED agents

   Figure3 shows the packet loss rate vs. mean of waiting time.  A
   plotted line represents a parameter ID ( "[session bandwidth] -
   [frame rate] - [the number of segments] - [link2 delay]" ).  E.g.
   384k-15-9-100 means the session of 384kbps session bandwidth, 15
   frames per second, 9 segments per frame and 100msec link delay.

   When the packet loss rate is 5% and the session bandwidth is
   32kbps, the waiting time is around 400msec, which is just
   allowable for reasonable NEWPRED performance.

   When the packet loss rate is less than 1%, the waiting time is
   less than 200msec. In such a case, the NEWPRED allows as much as
   200msec additional link delay.

   When the packet loss rate is less than 5% and the session
   bandwidth is 64kbps, the waiting time is also less than 200msec.

   In 128kbps cases, the result shows that when the packet loss rate
   is 20%, the waiting time is around 200msec.  In cases with more
   than 512kbps session bandwidth, there is no significant delay.
   This means that the waiting time due to the feedback limitation of
   RTCP is neglectable for the NEWPRED performance.

   +------------------------------------------------------------+
   |           | Packet Loss Rate =                             |
   | Bandwidth | 0.005| 0.01 | 0.02 | 0.03 | 0.05 |0.10  |0.20  |
   |-----------+------+------+------+------+------+------+------|
   |       32k |130-  |200-  |230-  |280-  |350-  |470-  |560-  |
   |           |   180|   250|   320|   390|   430|   610|   780|
   |       64k | 80-  |100-  |120-  |150-  |180-  |210-  |290-  |
   |           |   130|   150|   180|   190|   210|   300|   400|
   |      128k | 60-  | 70-  | 90-  |110-  |130-  |170-  |190-  |
   |           |    70|    80|   100|   120|   140|   190|   240|
   |      384k | 30-  | 30-  | 30-  | 40-  | 50-  | 50-  | 50-  |
   |           |    50|    50|    50|    50|    60|    70|    90|
   |      512k | < 50 | < 50 | < 50 | < 50 | < 50 | < 50 | < 60 |
   |           |      |      |      |      |      |      |      |
   |     1000k | < 50 | < 50 | < 50 | < 50 | < 50 | < 50 | < 55 |
   |           |      |      |      |      |      |      |      |
   |     2000k | < 30 | < 30 | < 30 | < 30 | < 30 | < 35 | < 35 |
   +------------------+------+------+------+------+------+------+

   Fig. 3 The result of simulation A


7.2.2. Simulation B - Packet Loss due to Congestion



Burmeister et al.        Expires October 2004                       21
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   The configuration of link1, link2, and link3 are the same as in
   simulation A except that link2 is also error-free, regarding bit
   errors.  However in addition, some FTP agents are deployed to
   overload link2.  See Figure 4 for the simulation topology.






                      Link1         Link2          Link3
           +--------+      +------+       +------+      +--------+
           | Sender |------|Router|-------|Router|------|Receiver|
           +--------+    /|+------+       +------+|\    +--------+
                   +---+/ |                       | \+---+
                 +-|FTP|+---+                   +---+|FTP|-+
                 | +---+|FTP| ...               |FTP|+---+ | ...
                 +---+  +---+                   +---+  +---+

                  FTP Agents                      FTP Agents


                  Fig4. Network Topology of Simulation B



   The parameters are defined as for Simulation A with the following
   values assigned:

      D[ms] ={10, 50, 100, 200, 500}
      32 FTP agents are deployed at each edge, for a total of 64 FTP
      agents active.
      The sets of session bandwidth, frame rate, the number of
   segments
      are the same as in Simulation A (Table 14)

   We provide the results for the cases with 64 FTP agents, because
   these are the cases where packet losses could be detected to be
   stable.  The results are similar to the Simulation A except for a
   constant additional offset of 50..100ms.  This is due to the delay
   incurred by the routers' buffers.

7.3 Summary of Application Simulations

   We have shown that the limitations of RTP AVPF profile do not
   generate such high delay in the feedback messages that the
   performance of NEWPRED is degraded for sessions from 32kbps to
   2Mbps.  We could see that the waiting time increases with a
   decreasing session bandwidth and/or an increasing packet loss
   rate.  The cause of the packet loss is not significant; congestion
   and constant packet loss rates behave similarly.  Still we see


Burmeister et al.        Expires October 2004                       22
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   that for reasonable conditions and parameters the AVPF is well
   suited to support the feedback needed for NEWPRED.


8 Summary

   The new RTP profile AVPF was investigated regarding performance
   and potential risks to the network stability.  Simulations were
   conducted using the network simulator, simulating unicast and
   several differently sized multicast topologies.  The results were
   shown in this document.

   Regarding the network stability, it was important to show that the
   new profile does not lead to any feedback implosion, or use more
   bandwidth as it is allowed.  Thus we measured the bandwidth that
   was used for RTCP in relation to the RTP session bandwidth.  We
   have shown that, more or less exactly, 5% of the session bandwidth
   is used for RTCP, in all considered scenarios.  Other RTCP
   bandwidth values could be set using the RTCP bandwidth modifiers
   [10].  The scenarios included unicast with and without errors,
   different sized multicast groups, with and without errors or
   congestion on the links.  Thus we can say that the new profile
   behaves network-friendly in the sense that it uses only the
   allowed RTCP bandwidth, as defined by RTP.

   Secondly, we have shown that receivers using the new profile
   experience a performance gain.  This was measured by capturing the
   delay that the sender sees for the received feedback.  Using the
   new profile this delay can be decreased by orders of magnitude.

   In the third place, we investigated the effect of the parameter
   "l" on the new algorithms.  We have shown that there does not
   exist an optimum value for it but only a trade-off can be
   achieved.  The influence of this parameter is highly environment-
   specific and a trade-off between performance of the feedback
   suppression algorithm and the experienced delay has to be met.
   The recommended value of l= 0.5 given in the draft seems to be
   reasonable for most applications and environments.


9 Security Considerations

   This document describes the simulation work carried out to verify
   the correct working of the RTCP timing rules specified in the AVPF
   profile [1].  Consequently, security considerations concerning
   these timing rules are described in that document.


10 Informative References




Burmeister et al.        Expires October 2004                       23
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   1 J. Ott, S. Wenger, N. Sato, C. Burmeister, and J. Rey, "Extended
     RTP Profile for RTCP-based Feedback", Internet Draft, draft-
     ietf-avt-rtcp-feedback-07.txt, Work in Progress, June 2003.

   2 H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, " RTP
     - A Transport Protocol for Real-time Applications, RFC 3550,
     July 2003.

   3 H. Schulzrinne, S. Casner, "RTP Profile for Audio and Video
     Conferences with Minimal Control", RFC 3551, July 2003.

   4 Network Simulator Version 2 - ns-2, available from
     http://www.isi.edu/nsnam/ns.

   5 C. Burmeister, T. Klinner, "Low Delay Feedback RTCP - Timing
     Rules Simulation Results".  Technical Report of the Panasonic
     European Laboratories, September 2001, available from:
     http://www.informatik.uni-bremen.de/~jo/misc/SimulationResults-
     A.pdf.

   6 ISO/IEC 14496-2:1999/Amd.1:2000, "Information technology -
     Coding of audio-visual objects - Part2: Visual", July 2000.

   7 ITU-T Recommendation, H.263. Video encoding for low bitrate
     communication. 1998.

   8 S. Fukunaga, T. Nakai, and H. Inoue, "Error Resilient Video
     Coding by Dynamic Replacing of Reference Pictures," IEEE Global
     Telecommunications Conference (GLOBECOM), pp.1503-1508, 1996.

   9 H. Kimata, Y. Tomita, H. Yamaguchi, S. Ichinose, T. Ichikawa,
     "Receiver-Oriented Real-Time Error Resilient Video Communication
     System: Adaptive Recovery from Error Propagation in Accordance
     with Memory Size at Receiver," Electronics and Communications in
     Japan, Part 1, vol.84, no.2, pp.8-17, 2001.

   10 S. Casner, "Session Description Protocol (SDP) Bandwidth
     Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556,
     July 2003.


11 IPR Notices

   The IETF takes no position regarding the validity or scope of any
   intellectual property or other rights that might be claimed to
   pertain to the implementation or use of the technology described
   in this document or the extent to which any license under such
   rights might or might not be available; neither does it represent
   that it has made any effort to identify any such rights.
   Information on the IETF's procedures with respect to rights in
   standards-track and standards-related documentation can be found


Burmeister et al.        Expires October 2004                       24
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   in BCP 11 [13].  Copies of claims of rights made available for
   publication and any assurances of licenses to be made available,
   or the result of an attempt made to obtain a general license or
   permission for the use of such proprietary rights by implementers
   or users of this specification can be obtained from the IETF
   Secretariat.

   The IETF invites any interested party to bring to its attention
   any copyrights, patents or patent applications, or other
   proprietary rights which may cover technology that may be required
   to practice this standard.  Please address the information to the
   IETF Executive Director.


12 Authors' Address

   Carsten Burmeister
   Panasonic European Laboratories GmbH
   Monzastr. 4c, 63225 Langen, Germany
   mailto: burmeister@panasonic.de

   Rolf Hakenberg
   Panasonic European Laboratories GmbH
   Monzastr. 4c, 63225 Langen, Germany
   mailto: hakenberg@panasonic.de

   Akihiro Miyazaki
   Matsushita Electric Industrial Co., Ltd
   1006, Kadoma, Kadoma City, Osaka, Japan
   mailto: akihiro@isl.mei.co.jp

   Joerg Ott
   Universitaet Bremen TZI
   MZH 5180, Bibliothekstr. 1, 28359 Bremen, Germany
   {sip,mailto}: jo@tzi.uni-bremen.de

   Noriyuki Sato
   Oki Electric Industry Co., Ltd.
   1-16-8 Chuo, Warabi, Saitama 335-8510 Japan
   mailto: sato652@oki.com

   Shigeru Fukunaga
   Oki Electric Industry Co., Ltd.
   2-5-7 Honmachi, Chuo-ku, Osaka 541-0053 Japan
   mailto: fukunaga444@oki.com


13 Full Copyright Statement

   "Copyright (C) The Internet Society (2004).  All Rights Reserved.



Burmeister et al.        Expires October 2004                       25
RTP/AVPF Profile     Timing Rules Simulation Results      April 2004



   This document and translations of it may be copied and furnished
   to others, and derivative works that comment on or otherwise
   explain it or assist in its implementation may be prepared,
   copied, published and distributed, in whole or in part, without
   restriction of any kind, provided that the above copyright notice
   and this paragraph are included on all such copies and derivative
   works.  However, this document itself may not be modified in any
   way, such as by removing the copyright notice or references to the
   Internet Society or other Internet organizations, except as needed
   for the  purpose of developing Internet standards in which case
   the procedures for copyrights defined in the Internet Standards
   process must be followed, or as required to translate it into
   languages other than English.

   The limited permissions granted above are perpetual and will
   not be revoked by the Internet Society or its successors or
   assigns.

   This document and the information contained herein is provided on
   an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR
   PURPOSE."




























Burmeister et al.        Expires October 2004                       26


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/