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Advanced Transport Protocol Design for Efficient Multimedia Communication

This technical document outlines the design of a reliable transport protocol aimed at achieving high throughput and fairness in multimedia communications. It covers the TCP/IP overview, congestion control models, and innovations in loss discrimination and modified AIMD techniques. The system model incorporates a hybrid wired/wireless architecture, focusing on improved congestion detection and response strategies. Future work is suggested in the realm of forward error correction and congestion avoidance approaches to enhance overall network efficiency and performance.

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Advanced Transport Protocol Design for Efficient Multimedia Communication

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  1. Advanced Transport Protocol Design Nguyen Nguyen Multimedia Communications Laboratory March 23, 2005

  2. Outline • Introduction • Overview of TCP/IP • System model • Queueing model for congestion • Loss discrimination • Modified AIMD • Future work

  3. Introduction • Transport protocol • End-to-end data transmission • Sequencing, flow control, congestion control etc. • Transport protocol measures of performance • Throughput (bytes/second) • Fairness • Latency • TCP-friendliness

  4. Introduction (2) • Goal: design a reliable transport protocol that achieves high throughput and fairness • Key: congestion control • Congestion control design • Congestion detection (loss discrimination) • Response to congestion

  5. Overview of TCP/IP • Layered network architecture

  6. Overview of TCP/IP (2) • Internet Protocol (IP) • End-to-end data transmission • Routing • Best-effort • User Datagram Protocol (UDP) • Basically raw IP • Fast, but unreliable

  7. Overview of TCP/IP (3) • Transmission Control Protocol (TCP) • Connection-oriented • Not as fast as UDP, but reliable • Sliding window transmission policy

  8. Overview of TCP/IP (4) • Reliability through retransmission • Retransmit lost packets (triple duplicate ACK or timeout) • Flow control • Buffer advertisements from the receiver • Congestion control • Congestion indicator = packet loss

  9. Overview of TCP/IP (5) • Additive increase, multiplicative decrease (AIMD) Additive increase Timeout Window Triple-duplicate ACK Slow-start Time (RTT)

  10. Overview of TCP/IP (6) • Problem 1. Inaccurate congestion indicator • Packet loss in wireless networks is mainly due to random transmission error (i.e. fading) • Problem 2. Response to congestion • TCP is too conservative because it does not have an up to date notion of the available bandwidth

  11. System model • Network model Internet destination1 source1 MH BS destination2 source2 MH

  12. System model (2) • Adjust the rate of the sender subject to the following constraints • Hybrid wired/wireless network topology • No help from intermediate routers • Unsynchronized clocks • Online

  13. Queueing model • Single-server queueing system • Customer: packet from primary source plus preceding cross-traffic Primary flow Cross-traffic

  14. Queueing model (2) • {X2, X3, …} - sequence of interarrival times • {S1, S2, …} - sequence of service times • {Q(t) : t ≥ 0} - number of customers in queue • Traffic intensity: ratio of average service time to average interarrival time

  15. Queueing model (3) • D/G/1 queueing system • Case 1: Independent, identically distributed (IID) service times • {S1, S2, …} is a sequence of IID r.v.’s • Theorem 1. Let Dn be the departure time of the nth customer. Then {Q(Dn) : n ≥ 1} is a Markov chain.

  16. Queueing model (4) • Proof. Let Un be the number of customers arriving during the service time Sn+1 of the (n+1)th customer. But Un = T-1Sn+1 and service times are independent.

  17. Queueing model (5) • Case 2: Dependent service times • {S1, S2, …} is a stationary, ergodic process • {Q(Dn) : n ≥ 1} is not a Markov chain • Theorem 2 [Grimmett]. The waiting time distribution, P(W ≤ w), is non-defective if • (a) ρ < 1, or • (b) ρ = 1 and Var(S – X) = 0.

  18. Queueing model (6) • Long-term properties • Average number of customers in the system • Average number of customers in queue, average delay through the system, average waiting time can also be derived

  19. Loss discrimination • Improved congestion detection • Packet loss • Delay • Theorem 2: Long-term stability achieved if average service rate > average arrival rate • The long-term does not exist in our problem

  20. Loss discrimination (2) • Sample traffic intensity • Step 1. Calculate the “short-term” average over a time interval

  21. Loss discrimination (3) • Condition 1. If > 1 and increasing trend of traffic intensity is observed, congestion ifthen congestion_loss endif

  22. Loss discrimination (4) • Condition 2. If a large, sudden spike in traffic intensity is observed, congestion ifthen congestion_loss endif

  23. Loss discrimination (5) • Step 2. Communicate cause of loss to the sender via a feedback message. • Step 3. Retransmit. If cause of loss was congestion, sender adjusts its rate

  24. Modified AIMD • Maintain up to date estimate of bandwidth • Sample bandwidth • Step 1. Calculate smoothed average

  25. Modified AIMD (2) • Step 2. Communicate bandwidth estimate to sender via feedback message. • Step 3. Set sending window accordingly • Step 4. Additive-increase.

  26. Future work • Reliability through forward error correction (FEC) instead of retransmission • LDPC code • Interleaver • Congestion avoidance instead of AIMD

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