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Versions: 00                                                            
Internet Engineering Task Force                 Audio/Video Transport wg
Internet Draft                              J. Rosenberg, H. Schulzrinne
draft-ietf-avt-byerecon-00.txt              Bell Laboratories/Columbia U.
November 13, 1997
Expires: May 1998


                     New Results in RTP Scalability

STATUS OF THIS MEMO

   This document is an Internet-Draft. Internet-Drafts are working docu-
   ments of the Internet Engineering Task Force (IETF), its areas, and
   its working groups.  Note that other groups may also distribute work-
   ing 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
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   Distribution of this document is unlimited.

1 Abstract

   Recently, a number of problems related to RTP scalability to large
   multicast groups have been identified. The main problem, congestion
   of RTCP packets due to near-simultaneous joins, has been resolved in
   previous work. In this document, we present some additional problems
   and describe the solutions. In particular, we discuss the problem of
   BYE floods and premature timeouts. To resolve the BYE flood problem,
   we propose a BYE reconsideration mechanism. To help alleviate prema-
   ture timeouts, we propose a reverse timer reconsideration algorithm.
   Both algorithms are simple, and require minimal state and computa-
   tion.

2 Introduction

   Recently, a number of problems related to RTP [1] scalability to
   large multicast groups have been identified [2]. The most serious of
   these problems is RTCP congestion in simultaneous joins. In this sce-
   nario, a large number of users join a session at nearly the same
   time. This can happen if the join is an automatic response to an
   announced session on the mbone [3], or if RTP is being used for
J. Rosenberg, H. Schulzrinne                                  [Page 1]


   broadcast applications, and a popular show comes on. When this hap-
   pens, each user joins the group believing that they are the only
   user. They then send their initial RTCP packets within a short period



Internet Draft             RTP Scalability 2           November 13, 1997


   of time, causing a flood of RTCP reports. This can result in network
   and/or access link congestion. For modem dial-in users, the slow
   access links can cause congestion even with moderate simultaneous
   joins.

   To combat this problem, an algorithm called reconsideration has been
   proposed [4] [5]. This algorithm causes backoff in the transmission
   of RTCP packets as group sizes increase.

   However, several new problems have recently been discovered relating
   to RTCP scalability. These difficulties are similar to the step-join
   congestion problem, but different in that they require additional
   algorithms to resolve. This document describes the two new difficul-
   ties which have been encountered - BYE floods and premature timeouts.

3 BYE Floods

   The BYE flood problem is very similar to the simultaneous join prob-
   lem. Instead of many users joining at the same time, many users leave
   the group at the same time. Since an RTP client sends an RTCP BYE
   packet when it leaves the group, this causes a flood of BYE packets,
   which congests the network. Users can be expected to leave a group
   simultaneously for much the same reasons they might join simultane-
   ously - an automatic leave as a result of the end of the session (as
   indicated in the SDP [6] announcement for the session), or because
   the show is over, and the users manually exit their applications.

   A number of aspects of the BYE flood problem make it different than
   the simultaneous join problem. These must be taken into consideration
   when designing an algorithm to reduce the flood. We therefore state
   the goals of the BYE flood prevention algorithm as follows:

     oUsers often terminate their applications just after leaving the
      session. The algorithm must be aware of this possibility, and
      define the appropriate behavior if an application decides to ter-
      minate.

     oThe algorithm should behave gracefully; when very few users are
      leaving the group simultaneously, users should generally be
      allowed to send their BYE packets right away. It is only in the
      presence of a large number of BYE packets that the algorithm
      should kick in, and force users to hold back on sending their BYE
      packets.

     oThe algorithm should be simple, requiring minimal computation and
      storage.

We propose an algorithm called BYE reconsideration to accomplish these


J. Rosenberg, H. Schulzrinne                                  [Page 2]


Internet Draft             RTP Scalability 2           November 13, 1997


goals. The algorithm operates much like standard
reconsideration. How- ever, instead of counting other users, and using
the resulting count as a multiplier for the packet transmission
interval, the client counts BYE packets, and uses the number of BYE
packets received thus far as the multiplier for the interval. The
operation of the algorithm is as fol- lows.

At some time tl, the user decides to leave the session. The application
first checks to see if it has ever sent an RTCP packet. If it has not,
the application must not send a BYE packet. Instead, it should leave the
session silently. Without having sent an RTCP packet, the BYE packet
provides no useful information. Next, the application checks to see if
the group size is less than some threshold, Bt (a value of 50 seems rea-
sonable). If it is, the application may send a BYE packet immediately,
and then leave the session. For small groups, BYE packets are of signif-
icant value (for loose session management), and sending them immediately
is quite important. Since the group contains only a small number of par-
ticipants, the flood of packets is limited.

It is possible that a user has a lowball group size estimate if they
leave a session quickly after joining it. If this session is large, and
there are many users coming and going fairly quickly (typical of a chan-
nel surfer), it might appear that this can cause a steady flow of BYE
packets. However, if these clients implement the forward reconsideration
algorithms, they generally will have never sent an RTCP packet. This is
because the new users' RTCP packet transmission will be constantly
reconsidered, as the new user will be receiving RTCP packets at a steady
rate from other users already in the group. This constant reception of
packets will cause the new user to see a steady growth in the group
size, causing its own RTCP packet transmission to be pushed into the
future. Since a user who never sends an RTCP packet cannot send a BYE
packet, this will generally cause these channel surfers to neither send
RTCP SDES or RR information, nor a BYE packet.

If the user has sent an RTCP packet previously, and the group size
exceeds Bt, the application computes a time interval T as:

T = R(1/2) max(T_min, n_l * C)

Where Tmin is 2.5 seconds, C=avgpktsz/(bw*.05), R(1/2) is a random vari-
able uniformly distributed between 1/2 and 3/2, avgpktsz is the average
size of all BYE packets received thus far, and bw is the session band-
width. The average packet size is computed using the same exponential
weighted average filter used to compute the average RTCP packet size in
the current specification. The value is updated through the filter every
time a BYE packet is received or transmitted (not when it is reconsid-
ered).



J. Rosenberg, H. Schulzrinne                                  [Page 3]


Internet Draft             RTP Scalability 2           November 13, 1997


The user then schedules the BYE packet to be sent at time tl+T. Between
tl and this time, the user increments nl for each BYE packet that is
received. In this fashion, nl counts the number of BYE's from other
users since deciding to leave the session.

When this time arrives, the user recomputes T according to the previous
equation. If tl+T is less than the current time, the BYE packet may be
sent. If tl+T is more than the current time, the BYE packet transmission
is rescheduled for time tl+T. At that time, the computation and compari-
son are repeated. All along, nl is incremented for each BYE packet
received.

A BYE packet which is from an SSRC which already sent a BYE (a dupli-
cate) is ignored. Furthermore, the application should not increment nl
if it receives a BYE from a user which has never sent an RTCP packet.
Under normal situations, an application should never send a duplicate
BYE packet, or send a BYE if an RTCP packet was never sent. However, a
malicious user may send many BYE packets. If this check were not made,
these BYE's would cause the variable nl to increase, and effectively
prevent any other user from sending a BYE.

If an application wishes to terminate before it can send a BYE RTCP
packet according to these rules, it must not send a BYE packet. Instead,
it should terminate silently. The BYE reconsideration algorithm will
effectively re-allocate the bandwidth from users who leave without BYE's
to those who wait around to send a BYE.

The effect of this algorithm is to restrict the BYE packet transmission
rate to at most an additional 10% of the session bandwidth (assuming a
very large simultaneous leave). At the same time, if only a few users
are leaving the group (even for a large group), they will get to send
their BYE packets in a timely fashion. This meets the design objectives
described in the beginning of the section.

We ran numerous simulations to verify the performance of the algorithm.
Even with as many as 10,000 users simultaneously leaving the session,
the BYE reconsideration algorithm maintained the BYE transmission rate
at 10%. This is demonstrated in Figure 1, which depicts the cumulative
number of RTCP packets (BYE and others) send to the multicast group over
time. At time 10,000, almost all of the users leave the group. The top
line depicts the performance without BYE reconsideration, where some
10,000 BYE packets are sent all at once. The lower curve shows perfor-
mance for BYE reconsideration. Note how there is only a small increase
in packet transmission rates.

[Figure available in Postscript version only]

Figure 1: BYE Reconsideration Performance


J. Rosenberg, H. Schulzrinne                                  [Page 4]


Internet Draft             RTP Scalability 2           November 13, 1997


4 Premature Timeouts

   We have observed a secondary effect when many users simultaneously
   leave a group. There are many applications where not all of the users
   will leave - some will stick around. An example is a distance learn-
   ing application. There are perhaps several hundred students in the
   class. When the class ends, most of the students leave at about the
   same time. However, some stay behind to talk with the professor.

   We have observed that rapid leaves can cause the remaining users to
   time each other out. It can take a significant amount of time for the
   users to return. This implies that each user will not see the other
   users, which is undesirable for the post-class discussion scenario
   just mentioned.

4.1 Quantifying the Problem

   The difficulty is related to the way timeouts are handled. In the
   current specification [1], a user is timed out if they have not sent
   an RTP or RTCP packet within the last five RTCP intervals. In dynamic
   groups, the interval itself is dynamic. As many users leave a group,
   their BYE packets cause the group size estimate to rapidly decrease.
   This, in turn, decreases the timeout interval. If the number of users
   who leave the group is sufficiently large, the timeout interval may
   decrease so much that the remaining users will time out.

   For example, consider a group of 505 users. If the total RTCP inter-
   val is to be limited to 1 packet per second, each user sends RTCP
   packets once every 505 seconds (on average). Assume user 1 last sent
   an RTCP packet at time 0. The user schedules the next RTCP packet for
   time 505. At time 490, 500 of the 505 members (not including user 1)
   leave the group, and send BYE packets (assume for the moment that
   there is no BYE flood prevention algorithm). Shortly thereafter (say
   time 500) the BYE packets have been received, and the remaining 5
   users perceive the group size to be 5. Based on this, the timeout
   interval is 25 seconds. Any user who has not sent a packet since time
   475 will therefore be timed out. User 1 last sent a packet at time 0,
   so they are timed out. In fact, odds are good that most of the
   remaining 5 users sent their last packet before time 475. Thus, every
   user will time out all of the other users. Furthermore, it may take a
   long time for those users to come back. Consider user 2, who did not
   leave the group, and who was unfortunate enough to have last sent an
   RTCP packet at time 450. They then scheduled their next RTCP packet
   for time 955 (since there were still 505 users at the time). After
   the exodus at time 500, user 2 will remain, but will not send the
   next RTCP packet until time 955 (unless the user sends data, in which
   case they will be known via the RTP packet).



J. Rosenberg, H. Schulzrinne                                  [Page 5]


Internet Draft             RTP Scalability 2           November 13, 1997


   The first question to ask is whether BYE flood prevention helps alle-
   viate this problem. Since the algorithm is designed to reduce the
   flood of BYE packets, the group size cannot drop so rapidly. This
   does help, of course, but not completely. With network delays, there
   still can be spikes in BYE packets. It does not require many BYE
   packets for this phenomenon to surface; it only requires that the
   ratio of users left after the leave to the number before the leave be
   less than around 1/5. This can occur in both small and large groups
   alike.

   Even assuming the rate of BYE packets is restricted to some constant
   factor, the efficacy of the timeout algorithm is reduced. To show
   this, we computed the number of packets sent by a user, on average,
   during the timeout interval, as the group size decreases linearly
   from some initial value Ns to some final value Nf. We denote the
   slope of the decrease by r users per second. We also define C as the
   multiplier to convert from group size to interval (so that if there
   are N group members, each member sends RTCP packets every CN seconds,
   on average), and M as the factor of 5 timeout multiplier. If r times
   C is much smaller than 1, and less than (1/CM)(Ns/Nf - 1)the number of
   packets sent during the timeout window, is on average:

   N_p = (-1)/(ln(1 - rC)) ln(1 + rCM)

   and for rC larger than 1, but less than (1/cM)(Ns/Nf-1):

   N_p <= (M)/(1 + rCM)

   In the limit as r goes to zero (that is, nobody leaving the group),
   the number of packets is M (according to the first equation), as
   expected. However, this quantity decreases rapidly as the slope, r,
   increases relative to 1/C. With the BYE prevention algorithm, the
   flood of packets can be shown to be bounded at an average rate of 2/C
   in the absence of network delays. With network delays, there can be a
   spike of packets, but following this spike, the BYE packet rate will
   also settle to 2/C initially, gradually decreaseing to 1/C. Plugging
   in r=2/C into the second equation above, the number of packets sent
   is:

   N_p <= 5/11

   This means that each sender will send only half a packet during the
   timeout window, on average. In reality, this means that half of the
   users remaining will send 1, and the other half will not. Therefore,
   many users will still timeout. Those users which do manage to send a
   packet may still timeout if the packet is lost.

   Note that both equations above rely on many users leaving the group


J. Rosenberg, H. Schulzrinne                                  [Page 6]


Internet Draft             RTP Scalability 2           November 13, 1997


   at the same time (the constraint that r<(1/MC)(Nf/Ns-1)).If the num-
   ber who leave is small relative to the slope of the leave (r>(1/MC)
   (Nf/Ns-1)),the effects are less severe.


4.2 Reverse Reconsideration

   One of the major factors contributing to the premature timeout effect
   is the delay between when the group size decreases, and when users
   begin to send packets using the resulting smaller interval. In the
   example in the previous section, user 2 sent an RTCP packet at time
   450, and scheduled the next for time 955. After the exodus at time
   490, user 2 knows that the group size has dropped - but does nothing.
   The user instead waits until time 955, sends the packet, and then
   computes the next send time. Since the group size is now 5, user 2
   will schedule the next packet 5 seconds later, on average. There is
   thus a 500 second delay between the exodus and when user 2 gets
   around to scheduling a packet using the new, smaller interval.

   To resolve this problem, we propose an algorithm called reverse
   reconsideration. The idea is simple. If the group membership
   decreases, each user reschedules their next packet immediately. The
   packet is rescheduled for a time earlier than previously. The amount
   earlier is made to depend on how much the group size has decreased.

   More specifically, assume that the last time a user sent an RTCP
   report is tp. The next report is scheduled for time tn, and tc is the
   current time. Before the arrival of a BYE packet at the current time
   tc, there were np users. There are now nc users. Before the BYE, RTCP
   packet transmissions should be uniformly scheduled over time. That
   means that there should have been nc packet transmissions scheduled
   between tc and tc+C*nc. Now, however, the group size has decreased to
   np. This should cause there to be np packet transmissions scheduled
   between tcandC*np. To accomplish this, every user should compress the
   interval between the current time and their next packet transmission
   by nc/np.This implies that the next packet transmission should be
   rescheduled for time tn:

   t_n = t_c + (n_c/n_p)(t_n - t_c)

   This new value for tn has two key properties:

     1.   The new time is always earlier than the previous time.

     2.   The new time can never be before the current time.

The second property is key; it guarantees that there will not be a spike
of packets transmitted due to a sharp decrease in group size.


J. Rosenberg, H. Schulzrinne                                  [Page 7]


Internet Draft             RTP Scalability 2           November 13, 1997


On the surface, it would seem that an alternate algorithm might be a
direct application of regular (or forward) reconsideration. Such an
implementation might work as follows. At tc, when the BYE arrives, the
user recomputes the transmission interval T, based on the new group size
nc. This interval is then added to the previous packet transmission time
tp. If the result is a time before the current time, the packet is sent,
else it is rescheduled for tp+T. This algorithm does not work. Even a
moderate decrease in the group size would cause many users to send their
RTCP packets immediately, causing an additional spike. This is because
this version of the algorithm does not maintain property 2 - the new
transmission time can be before the current time.

There is one additional aspect to the reverse reconsideration algorithm
that must be considered - how it interacts with forward reconsideration.
Consider the following example. There are 100 users in a group. The con-
stant C is equal to 1 packet per second. At time 0, user A sends an RTCP
packet, and schedules the next for time 100. At time 50, 50 users leave
the group. User A executes the reverse reconsideration algorithm, and
reschedules their packet for time 50 + (50/100)(100 - 50) = 75. At time
60, one more user joins the group. At time 75, user A executes the for-
ward reconsideration algorithm (we assume conditional reconsideration).
Since the group size has increased (51 vs. 50), user A recomputes the
interval - which is now 51 seconds on average. This is then added to the
previous transmission time, which is time 0. The result is t=51, signif-
icantly earlier than the current time t=75. This will cause the user
(and in fact, all other users) to send their packets immediately, even
if the group size further increases. The problem is that while we have
adjusted the next packet transmission time, tn, with reverse reconsider-
ation, we have not adjusted the value of the previous packet transmis-
sion time, tp. This quantity is used for forward reconsideration, and
must be adjusted as well in order to maintain consistency.

The adjustment algorithm is simple. The value for tp is updated when tn
is updated by reverse reconsideration. Its value is adjusted to:

t_p = t_c - (n_c/n_p)(t_c - t_p)

The nature of this adjustment is the same as for tn. In the previous
example, it would have caused tp to be adjusted from 0 to (50 -
(50/100)(50 - 0)) = 25. At time 75, when the user is performing the for-
ward reconsideration algorithm, the interval T=51 is added to tp=25,
yielding t=76, slightly ahead of the current time t=75, as expected
(since the group has only increased by one member).

4.3 Performance

   What is the improvement due to reverse reconsideration? We have been
   able to prove that the number of packets sent during the timeout


J. Rosenberg, H. Schulzrinne                                  [Page 8]


Internet Draft             RTP Scalability 2           November 13, 1997


   window has an achievable lower bound of:

   N_p = (1 / rC) * ln(1 + rCM)

   This time, independent of the relative value of rC. Again, this holds
   only when the actual number of users who leave is a significant frac-
   tion of the current group size (Nf/Ns<1/(1+MCr)).Assuming that BYE
   reconsideration is being used, the BYE rate is limited to around
   r=2/C when network delays are small. Plugging this in:

   N_p = 1/2 ln 11 = 1.19

   This means that a user will send 1.19 packets on average, during the
   timeout window. This is to be compared to the situation before
   reverse reconsideration, where Np=5/11. Therefore, reverse reconsid-
   eration affords a factor of three improvement in performance. Now, no
   users will timeout under normal circumstances (on average). However,
   a single packet loss may cause a user to timeout prematurely.

5 Conclusion

   We presented two additional problems with RTP scalability - BYE
   floods and premature timeouts. BYE floods are caused when many users
   simultaneously leave a group. To fix the problem, we presented an
   algorithm called BYE reconsideration, which works much like forward
   reconsideration, but for BYE packets. The second problem, premature
   timeouts, is a secondary effect, but still problematic. To help
   resolve it, we presented an algorithm called reverse reconsideration,
   which shows a threefold factor of improvement. Both algorithms are
   extremely simple, requiring no additional memory beyond forward
   reconsideration, and requiring a single O(1) computation upon recep-
   tion of a BYE packet.

6 Bibliography

   [1] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, RTP: a
   transport protocol for real-time applications, Tech. Rep. RFC 1889,
   Internet Engineering Task Force, Jan. 1996.

   [2] B. Aboba, Alternatives for enhancing RTCP scalability, Internet
   Draft, Internet Engineering Task Force, Jan. 1997.  Work in progress.

   [3] M. Handley, SAP: Session announcement protocol, Internet Draft,
   Internet Engineering Task Force, Nov. 1996.  Work in progress.

   [4] J. Rosenberg and H. Schulzrinne, Timer reconsideration for
   enhanced RTP scalability, Internet Draft, Internet Engineering Task
   Force, July 1997.  Work in progress.

J. Rosenberg, H. Schulzrinne                                  [Page 9]


Internet Draft             RTP Scalability 2           November 13, 1997


   [5] J. Rosenberg and H. Schulzrinne, Timer reconsideration for
   enhanced rtp scalability, in   To appear in Proceedings of IEEE Info-
   com '98 , 1998.

   [6] M. Handley and V. Jacobson, SDP: Session description protocol,
   Internet Draft, Internet Engineering Task Force, Mar. 1997.  Work in
   progress.

7 Authors' Addresses


   Jonathan Rosenberg
   Bell Laboratories, Lucent Technologies
   101 Crawfords Corner Rd.
   Holmdel, NJ 07733
   Rm. 4C-526
   email: jdrosen@bell-labs.com

   Henning Schulzrinne
   Columbia University
   M/S 0401
   1214 Amsterdam Ave.
   New York, NY 10027-7003
   email: schulzrinne@cs.columbia.edu


























J. Rosenberg, H. Schulzrinne                                 [Page 10]