Presentation Title 1 1/31/2014 Lucent Technologies - Proprietary RTP Scalability in Large Multicast Groups Jonathan Rosenberg High Speed Networks Research 11/10/97

Presentation Title 2 1/31/2014 Lucent Technologies - Proprietary Contents RTP and RTCP Primer Large scale applications Problems –Congestion –State Storage –Delay –BYE Storms –Premature Timeouts Reconsideration –Conditional –Unconditional –Reverse –BYE Polling Group Size Estimation Conclusions

Presentation Title 3 1/31/2014 Lucent Technologies - Proprietary RTP and RTCP RTP provides for –Real time transport –Resequencing –Payload type identification –Intra and Inter media synchronization –Encryption –Multicast Per User demultiplexing - SSRC RTP does not –Provide QoS –Require RSVP RTP is a framework –Specific payload formats defined for H.263, MPEG etc. –UDP Port numbers based on application

Presentation Title 4 1/31/2014 Lucent Technologies - Proprietary RTCP Real Time Control Protocol RTP port + 1 Used for –QoS Reporting Sender reports: packets sent, bytes sent Receiver reports (per sender): loss, delay, jitter observed; instantaneous and cumulative –Media Synchronization NTP and RTP Timestamp correlation –Loose Session Control Hello, Bye messages SDES - , username, CNAME, etc.

Presentation Title 5 1/31/2014 Lucent Technologies - Proprietary Large Scale Applications Cable Distribution –one sender, tens of thousands of receivers –QoS reporting critical –Nielsen - group size estimation Distributed real time sensors Van Jacobson RLM - Receiver Driven Layered Multicast MBONE concerts, talks

Presentation Title 6 1/31/2014 Lucent Technologies - Proprietary Achieving Scalability RTP –RTP scales if number of data senders is small or controlled –Generally not a problem RTCP –Both data senders AND receivers send reports –Reports are periodic –Period must scale with group size to avoid implosion –Period is defined to scale linearly Each system listens to group and counts number of other users heard from - L(t) is group size estimate Period is set to CL(t); C depends on application Works fine if group size is static $10M question: How does this work when L(t) is dynamic?

Presentation Title 7 1/31/2014 Lucent Technologies - Proprietary Other Applications Problem is general; applies when –Some unkown number of users wish to share a bandwidth resource equally –The users are on a shared medium, and can hear each other, like multicast –The number of users grows/shrinks over time Examples –CBR traffic on Ethernet –Route updates among BGP peers –Distributed Sensors

Presentation Title 8 1/31/2014 Lucent Technologies - Proprietary RTCP Intervals Require RTCP reporting to scale with multicast group size Total RTCP bw should be no more than 5% of session bw Divide 5% among participants - 75% senders, 25% receivers Algorithm: –At time t, emit packet –Compute interval T –Schedule packet for t+T –Repeat at t+T T MIN is 2.5s for first packet, 5s for all others Add randomization to avoid synchronization

Presentation Title 9 1/31/2014 Lucent Technologies - Proprietary Problems Congestion –Simultaneous Joins (ER at 10pm) –RTCP Interval based on group size estimate –N users, each think there is only ONE group member –Multicast flood over initial 2.5s interval - all N users send –Packet losses -> further congestion State Storage –To count group size, must store state for each other group member –Set top boxes - memory size limitations

Presentation Title 10 1/31/2014 Lucent Technologies - Proprietary Problems Delay –Frequency of reports from any user becomes small –Average loss reports over large intervals useless –Time to converge on group size exceeds membership times –DVMRP cache times -> EVERY RTCP packet is flooded! Premature Timeouts –Must timeout members if you havent heard from them in M transmission intervals –Drop in group membership reduces transmission interval –Sharp drop can cause a user to prematurely timeout other users BYE Storms –Many users simultaneously leave and send a BYE message

Presentation Title 11 1/31/2014 Lucent Technologies - Proprietary Reconsideration Avoid Congestion Last packet send time was t n-1 About to send at time t n Reconsider if number of users changed since t n-1 Compute interval based on new observation - > T, add to t n-1. If result is less than current time, send now, else reschedule for t n-1 + T Conditional: As above Unconditional: Recompute even if number of users unchanged Backwards compatible Effectiveness grows with usage Safe t n-1 tntn 3 new users observed t n-1 + T

Presentation Title 12 1/31/2014 Lucent Technologies - Proprietary Simulation Assumptions 28.8 kbps access links downstream Infinitely fast upstream links Access server buffers = 100kB per user Network adds delay uniform between 0 and 600ms Full connectivity - multicast join latency not considered Step join 10K users at t=0. Each have a homogeneous view of state

Presentation Title 13 1/31/2014 Lucent Technologies - Proprietary Packets Sent

Presentation Title 14 1/31/2014 Lucent Technologies - Proprietary Analytical Results Can derive a differential equation for L(t) given a step join! For small t, solution is linear when there is no network delays Can also show that packet rate is roughly derivative of L(t) Gives an estimate of network bandwidth

Presentation Title 15 1/31/2014 Lucent Technologies - Proprietary Add Delay Real network has delay - significant impact on performance Arrival of packets used to increase group size estimate and back-off -> network delays cause initial bunch of senders not to see packets Result is a spike of packets After packets arrive, causes rapid increase in group size estimates Result is a cessation of packet transmission: plateau effect What is size of spike? Again, analytic solutions exist! Let D be fixed network delays, m is modem rate in packets/s In limit of N large, distribution of delays doesnt matter! Conditional - first a linear growth in packets, then a growth as t 2 - much smaller t Packets Sent NpNp

Presentation Title 16 1/31/2014 Lucent Technologies - Proprietary Unconditional Unconditional works much better - growth in number of packets consists only of small t 2 component Total packets sent grows with square of delay to T MIN quotient Reason - unconditional skews distribution of packet transmissions towards later in time t Packets Sent NpNp

Presentation Title 17 1/31/2014 Lucent Technologies - Proprietary State Storage Problem: –Must maintain group size estimate –To distinguish new users from old, must maintain SSRC list for all users –In a group of thousands, can be problematic Solution: –Dynamic sampling –Given memory size M, store all SSRC until memory full –Discard all SSRC with MSB=1 (roughly 1/2) –Sample probability is 2 -k –Group size estimate is N s 2 k

Presentation Title 18 1/31/2014 Lucent Technologies - Proprietary Delay May not desire RTCP in current distributed form –When you know group size is large apriori Central monitor polls stations for reports Monitor sends SSRC mask, and interval T Stations whose SSRC match mask respond to poller within T seconds Must gradually decrease size of mask to avoid flood (Bolot, Wakeman) Standard sampling theorems allow group size estimation and confidence computations IDEA - Use non-uniform response distribution, and allow poller to cancel poll If poller gets lots of responses early, it can expect tons later, so cancel the poll, decrease probability Like the beach parking lot problem!

Presentation Title 19 1/31/2014 Lucent Technologies - Proprietary Premature Timeouts Subtle problem –Timeout if no packet seen in MCL(t) –Rapid leave causes L(t) to drop; timeout interval drops –Remaining users wont send packets immediately; change in interval size takes effect after next packet transmission Original Timeout Window Timeout Window after Flood RTCP Packets BYE Flood 012 3

Presentation Title 20 1/31/2014 Lucent Technologies - Proprietary Solution Two-fold Reverse Reconsideration –When BYE arrives, each user reschedules their next packet a little earlier –Allows packet transmission rate to closely track group membership Group Size filtering –Dont use L(t) for timeout - use filtered version! –How to filter? –Optimal timeout window size topt is solution to:

Presentation Title 21 1/31/2014 Lucent Technologies - Proprietary BYE Storms Users simultaneously leave also! Usually send a BYE - flood of packets Solution: Apply reconsideration to BYE packets –At leave time, schedule BYE packet –Count number of BYEs from other users since my leave time –Use that as group size estimate in reconsideration equations –Works very well

Presentation Title 22 1/31/2014 Lucent Technologies - Proprietary Conclusions RTP Scalability to large multicast groups has several components –Step joins and leaves –State storage –Premature timeouts Problem is general –Desire equal bandwidth sharing among users –Number of users is known in a distributed way by observing transmission –Number of users is very dynamic