Multicast congestion control on manytomany videoconferencing

1
Multicast congestion control on many-to-many
videoconferencing

• Xuan Zhang
• zhangx_at_cernet.edu.cn
• Network Research Center
• Tsinghua University, China

2
Outline

• Introduction
• Packets transmission model in multicast based
  many-to-many videoconference
• End-to-end packet loss rate measurement and
  aggregation
• Locate congestion link via tomography method
• Congestion control via adjusting senders bit
  rates
• Simulation and experiment
• Conclusions

3
1 Introduction

• Motivation
• Application background
• Many-to-many videoconferencing is N-to-N
  application which consumes large bandwidth
  Multicast is an efficient technology for
  many-to-many videoconferencing for it could save
  bandwidth.( Fig.1 )
• Problems?
• However, network congestion would cause serious
  end-to-end packets loss and delay which would
  debase the quality of videoconferencing.
• Goals ?
• Propose a valid and efficient multicast
  congestion control scheme for many-to-many
  videoconference.

4
Fig.1 The multicast-based many-to-many
videoconferencing on CERNET
5
Introduction (2)

• Overview of the scheme
• Judge the congestion
• Judge the congestion via end-to-end packet loss
  rate (PLR) measurement
• Locate the congestion links
• Locate the congestion links (bottleneck links) on
  shared multicast tree via tomography method based
  on packets transmission model and end-to-end PLR.
• Congestion control via adjusting senders bit
  rate.
• Adjust the sending bit rate of senders which
  effect the congestion links to achieve
  sender-fairness.

6
2 Packets transmission model in multicast based
many-to-many videoconference

• 2.1 Packets transmission model on one path
• In one path with r links, Suppose link i drops
  packets with probability p(i), then the
  successful probability of packets transmission in
  the path could be presented with link packet loss
  rate p(i) as following
• 
  (1)
• Linear expression with logarithm
• 
  (2)

7
2.1 Packets transmission model on one path
(example)

• The packets transmission model on one path (A,C)
• Its linear expression

8
2.2 Packet transmission model on N-to-N multicast
based videoconferencing

• The shared multicast tree and shared links.
• In many-to-many videoconferencing, all users
  share one session, the router would process their
  packets by using same strategy and same priority.
  That is said, all the media data packets from the
  senders in one multicast session have the same
  packet loss rate p(i) at link i .
• The transmission model
• In N-to-N videoconferencing, there are r
  N(N-1) paths, we could get r linear equation of
  formula (2).
• Let p(path,k) be the end-to-end PLR of jth path,
  vector b be a column vector with elements b(k)
  log(1-p(path,k)).
• The r equations of form (2) could be expressed as
  linear equations with matrix G ( rs ), which is
  the transmission model
• G x b (3)

9
2.2 Packet transmission model on N-to-N multicast
based videoconferencing (2)

• Gx b (packets transmission model for N-to-N
  mulitcast
• G
• Row vector of matrix G corresponding path
  vector
• G(k,i)1 means link i on path k (Figure)
• x
• x(i)log(1-p(i)) for link i
• b(k)
• End-to-end PLR (packets loss rate) of one path
• b(k) log(1- p(path,k) )

10
3 End-to-end packet loss rate measurement and
aggregation -- b(k)

• One End-to-end PLR of path(k) measurement
• Aggregate all N(N-1) PLR to get b
• PLR(x,t,ssrc) is one PLR from sender ssrc to
  receiver x. In a multicast group with N users,
  each receiver could get N-1 PLR
• We build a performance server to collect and
  aggregate all r N(N-1) PLR from N users to
  get b
• b(k) log(1- p(path,k) ) , k 1 ,2 ,3 , r

11
4 Locate congestion link via tomography method (1)

• Judge congestion and locate bottleneck links via
  tomography method
• Congestion judging
• If end-to-end PLR of path(k) b(k) is larger
  than one threshold , we could judge congestion
  occurred on path(k).
• Locate the bottleneck link via tomography method
• We could judge if link i is congested according
  to x(i) values.
• x(i) could be gained via solving Gx b (packets
  transmission model) in which G and b have been
  known or measured beforehand.
• We call these method as tomography method.
• Two method to get x(i) while solve Gxb
• Solve the linear equations of Gx b directly
  when rank(G) r ,
• Adopt Good path method to simplify G when G
  is rank deficient (rank (G) lt s )

12
4 Locate congestion link via tomography method (2)

• Good path method
• In practice, we find most paths more than 50
  paths in Internet have no loss or have only very
  low loss rate, we got it from our experiments.
• We know if the path loss rate is zero, all the
  link and logical links in the path are zero loss.
• We call this path with zero loss rate as zero
  loss path or good path, and we could remove
  zero loss links from matrix G to get simplified
  sub-matrix G, and links number s lt s until
  rank(G) s. So we could locate the bottleneck
  link by solving Gx b via good path
  methodology.

13
4 Locate congestion link via tomography method (3)

• Good path method example
• If PLR(C,D) and PLR(B,A) is zero, then link
  8,9,5,2 is good link with zero loss rate and
  column 8,9,5,2 could be removed from G matrix
• If path(C,A) is congested with PLR(C,A) is
  larger than threshold link 7 could be
  identified as the bottleneck link

14
5 Congestion control via adjusting senders bit
rates

• Determine the senders whose rate should be
  adjusted
• We could determine the congestion paths and
  corresponding senders related to the bottleneck
  link from matrix G.
• when link i is a bottleneck link, all paths whose
  row vectors at ith elements equal to 1 belongs to
  the congestion paths related to link i.
• We should adjust sending rate of related senders
  to reduce the traffic load on link j to achieve
  sender-fairness.
• Determine acceptable sending rate and adjust the
  senders bit rates fairly
• Calculation the adjusting ratio of sending bit
  rate.
• ratio Throughput (e, path))/ Rate(O,sender)
  
• ratio means the sending rate should be adjusted
  from origin rate to acceptable rate with ratio
  percentage of origin rate
• All the senders who affect the same bottleneck
  link should be adjusted with the same adjusting
  ratio to achieve the sender-fairness

15
6 Simulation and experiments

• Simulation setup
• RED queues are used at the routers for
  intra-protocol fairness.
• PIM-DM is deployed for multicast routing.
• UDP data and CBR traffic type is adopted to
  simulate videoconferencing
• Simulation process without congestion control
• At beginning user A , D and E join the group and
  send traffic with primal rate,
• After 5 seconds, user B joins the multicast group
  and start traffic sending.
• After 10 seconds user C joins and start traffic
  sending.
• After 25 seconds, all users leave the group and
  stop sending, simulation ends

16
6 Simulation and experiments (2)

• Left Prim congestion with end-to-end packet
  loss
• Right Packet loss rate drop to zero via
  congestion control method
• At 15th seconds, A and B which affect bottleneck
  link 6 are adjusted from 1.2Mbps and 0.8Mbps to
  0.9Mbps and 0.6Mbps with the same ratio 0.75.
• At 18th second, the sending rates of A, B are
  adjusted to 0.72Mbps and 0.48Mbps, and C, E are
  both adjusted to 0.375Mbps .

17
6 Simulation and experiment--Experiment on
CERNET-MVC
18
7 Conclusion

• Congestion control is important for the quality
  many-to-many videoconferencing based on
  multicast.
• The congestion link could be located via
  tomography method, which based on multicast
  packets transmission model and end-to-end PLR
  measurement
• By adjusting the senders sending bit rate who
  affect on the bottleneck links, the multicast
  congestion control on many-to-many
  videoconferencing could be realized with
  sender-fairness.
• Simulation and experiments shows the congestion
  control scheme is valid .

19

• Thanks!