Multimedia Streaming using Multiple TCP Connections

Transcription

1 Multimedia Streaming using Multiple TCP Connections

2 INTRODUCTION In recent years, there have been an explosive growth of multimedia applications over the Internet Several companies offer video on demand to broadband subscribers Quality of videos being streamed are often of low quality (Due to insufficient bandwidth, packet loss, and delay) To view a DVD quality video download either the entire video a large portion of the video before playback time (To avoid pauses caused by insufficient bandwidth during a streaming session)

3 Straightforward approach Transmit standard-based multimedia via existing IP protocols TCP UDP Why do we need multiple TCP connections?

4 TCP Single TCP connection is not suitable for multimedia transmission because of its congestion control UDP Ability to control the sending rate is essential to interactive and live streaming applications So majority of multimedia streaming systems use UDP Not congestion aware protocol Lack reliable transmission

5 MULTI TCP A receiver-driven, TCP-based system Provides resilience against short-term insufficient bandwidth by using multiple TCP connections Enables the application to achieve and control the sending rate during congested period Implemented at the application layer No kernel modification to TCP is necessary

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7 SINGLE TCP Controls the sending rate based on a window-based congestion control Window-based Congestion Control Sender keeps track of a window of maximum number of unacknowledged packets Sender increases the window size W by 1/W upon successfully receiving an acknowledged packet Upon encountering a loss, the window size is reduced by half

8 The throughput reduction of TCP is attributed to the combination of Reduction of the sending rate by half upon detection of a loss event The slow increase of sending rate afterward

9 Alleviating Throughput Reduction In a single TCP: When there is no congestion, the receiver can control the streaming rate quite accurately by setting the maximum receiver s window size W max Throughput during this period is approximately equal to T = W max MTU/RTT -RTT denotes the round trip time -MTU denotes the TCP maximum transfer unit

10 Single TCP with single loss: Area of the inverted triangular region in fig(a) ) indicates the amount of data that would have been transmitted if there was no loss event. Thus, the amount of data reduction D equals to D = (1/2)(W max MTURTT/2)(W max /2RTT) =W 2 max max MTU/8

11 One TCP connection with single loss (a) Two TCP connections with single loss (b) Two TCP connections with double losses (c)

12 If two TCP connections are used Keep the same total streaming rate W max /RTT Set W max = W max /2 for each of the two connections in fig(b) Assuming that only a single loss event happens in one of the connection, the total throughput reduction would be equal to D = (W max MTU)/8 =(W 2 max =D/4 max MTU)/32

13 Even when there are simultaneously losses on both connections as indicated in fig(c), the throughput reduction is half of that of the single TCP In general, the amount of throughput reduction equals to D N = nw 2 max MTU/N 2 -N N denote the number of TCP connections -n n denote the number of TCP connections that suffer simultaneous losses during short congestion period

14 Control Streaming Rate in a Congested Network In a lightly loaded network condition, one can set the desired throughput T d by simply setting the receiver window W max = T d RTT/MTU In a moderately or heavily congested network, the throughput of a TCP does not depend on W max It is determined by the degree of congestion For achieving a higher throughput than the available TCP throughput multiple TCP connections can be used The algorithm used maintains a relatively stable number of TCP connections while varies the size of the receiver windows to achieve the desired throughput

15 Algorithm Input : Desired user s throughput,t d and Number of TCP connections, N Initializing steps: 1. Set N, the number of TCP connections to the user input 2. Set the receiver window size w i = T d RTT/(MTU)N for connection i. Running steps: The actual throughput T m is measured at every δ second and the algorithm dynamically changes the window size based on the measured T m as follows. 3. If both of the following conditions (a) T m < T d, and (b) W s = Σ i w i ft d RTT/MTU where f > 2 are true, run AdjustWindow(T d, T m ). 4. If T m > T d + λ,, run AdjustWindow(T d, T m ). 5. Else, keep the receiver window size the same.

16 Suppose there are 5 connections denoted by TCP1 to TCP5 If none of TCP connections is blocked Packet1 would be sent by TCP1 Packet 2 by TCP2, and so on If TCP1 is blocked TCP2 would send packet1 TCP3 would send packet 2 and so on When it is TCP1 s turn again and if TCP1 is not blocked, it would send packet 5