1. AVPF vs AVP RTCP Intevals

2. 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

3. AVPF vs AVP Problems 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)
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

4. 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)
Td << Trr-int
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 s range
As can be seen the AVPF distribution are mostly uniform in the range *Trr-int, with a little bit of tail
However, still longer tail than AVP

5. 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)
Td>Trr-Int
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 s range, i.e *Trr-int.
That suppression do show up as bit of long tail above 12.5 seconds

6. 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?

7. 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/
Worst case Td = Trr-int: 2.73*Trr-int
Td should preferably be less than 1/4th of Trr-Int
That gives us max tail length of:
1.5*Trr-int + 1.5*Trr-int/(4* ) ≈ 1.81 Trr-int
AVP’s tail ends at 1.5*Tmin/ = 1.231*Tmin
Given that Td is less than Tmin
Otherwise replace Tmin with Td

8. 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 < 1/4th 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.

9. 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<Tmin

10. 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)

11. Number of Reporting Intervals given Trr-int

12. 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

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