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Published byToby Bink Modified about 1 year ago

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AVPF vs AVP RTCP Intevals

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AVP and AVPF Interop Whe AVP and AVPF is interoperating one must choose certain parameters correctly. In trying to determine the most suitable based on the timing rules a AVPF timing issue was found that is problematic for interop. Based on simulations of the algorithm – There might be bugs in the simulation

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AVPF vs AVP Problems Simulated distributions of RTCP transmission interval Tn is AVPF sending rules without suppression or reconsideration, i.e. Td*rand(0.5,1.5) AVPF(TN no supp) is with reconsideration AVP is RFC 3550 including reconsideration When Td calculation becomes close to, but below Trr-int AVPF suppression pushes transmissions interval distribution upwards and uneven in distribution avg_rtcp_size: 200 bytes RR: 500 bps RS: 1000 bps Senders: 1 Members: 2 We_sent: FALSE Trr-int:5 Tmin:5 (AVP only)

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Td << Trr-int avg_rtcp_size: 200 bytes RR: 10000 bps RS: 1000 bps Senders: 1 Members: 2 We_sent: FALSE Trr-int:5 Tmin:5 (AVP only) Here the RTCP bandwidth portion the calculation uses has been increased from 500 to 10k bps, i.e. 20 times Reduces Td Therefore both Tn and AVPF without suppression are all ending up in the 0.25-0.5 s range As can be seen the AVPF distribution are mostly uniform in the range 0.5- 1.5*Trr-int, with a little bit of tail However, still longer tail than AVP

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Td>Trr-Int avg_rtcp_size: 200 bytes RR: 500 bps RS: 1000 bps Senders: 1 Members: 4 We_sent: FALSE Trr-int:5 Tmin:5 (AVP only) Same RR as the problem slide 500 bps, but now with 4 members, i.e. 3 receivers sharing RR As can be seen here only a little bit of suppression happens at the 4-7.5 s range, i.e. 0.5-1.5*Trr-int. That suppression do show up as bit of long tail above 12.5 seconds

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Question: What gives best interop? So how does an AVPF user set his parameters to best interoperate from a perspective of avoiding accidental timeout: – The Regular RTCP transmission distribution is one factor which is depending on: Trr-int that sets suppression The Td deciding values: – Members – Senders – RR and RS – Average RTCP packet size – The other is when AVPF times out AVP participants 5*Trr-Int given that Trr-int != 0 Thus we must combine Trr-int large enough so that an AVPF particpant do not time out AVP participants At the same time AVPF regular RTCP transmission interval should not be so long that AVP times out How many consecutive packet losses are needed before timeout happens?

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Td parameterization When using AVPF one should try to avoid setting RR and RS so that Td in the given session are close to Trr-int. – The tail in the AVPF RTCP interval distribution is at its extreme at 1.5*Trr-int + 1.5*Td/1.21828 Worst case Td = Trr-int: 2.73*Trr-int – Td should preferably be less than 1/4 th of Trr-Int That gives us max tail length of: – 1.5*Trr-int + 1.5*Trr-int/(4*1.21828) ≈ 1.81 Trr-int – AVP’s tail ends at 1.5*Tmin/1.21828 = 1.231*Tmin Given that Td is less than Tmin Otherwise replace Tmin with Td

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Equalizing the tails If we would like to have equally values for the extreme randomization intervals given that Td is less than Tmin and Trr-int: – Trr-int = 1.231/1.81*Tmin 1.81 (following the Td < 1/4 th Trr-int recommendation) – Tmin = 5 s => Trr-int = 3.40 s This ensures that no AVPF session participant is more likely to time out than an AVP one. However, the reverse is not true.

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Timing out AVP participants An AVPF participant will timeout an participant at 5*Trr-int, – Equivalent of AVP participants using a Trr-int as a factor for number of intervals before timing out. – This assumes Tmin=5s and Td

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Finding the optimal both ways So what is the point of least decrease in timeout robustness performance: Given that Tmin = 5 s and Td < Tmin – AVPF intervals before AVP timeout(target): 5*Tmin/(Trr-int) – AVPF intervals before AVP timeout(1/4 * Trr-int): 5*Tmin/(Trr-int*1.81) – AVPF Intervals before AVP timeout (1/10 * Trr-int): 5*Tmin/(Trr-int*1.623) – AVP intervals before AVPF timeout (target): 5*Trr-int/5 – AVP intervals before AVPF timeout (worst case): 5*Trr-int/(1.231*Tmin)

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Number of Reporting Intervals given Trr-int

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Recommendations Based on these findings it appears that the following recommendations for AVPF vs AVP interop should be given: – RR and RS sufficiently large that Td becomes less than ¼ of Trr-int – Trr-int should be chosen as 4.0

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Future Work? Do the Suppression algorithms bad behavior needs to be addressed?

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