Problem Statement

1. Problem Statement How does TCP perform for delay-sensitive applications with CBR workloads such as Voice or Video? TCP delay prediction tool • Novel stochastic model for TCP delay • A function of • Network loss rate • Round-trip time • Workload rate and size • Novel • Characterizes delay rather than throughput • Does not assume saturated sender Window and backlog evolutionin congestion avoidance phase Understanding TCP Behavior for Real-time CBR WorkloadsSalman A. Baset, Eli Brosh, Vishal Misra, Dan Rubenstein, and Henning Schulzrinne Computer Science Department, Columbia University Notation: • wi window size at round i • bi backlog size at round i • r packet arrival rate in RTT • s thepacket size in bytes Why is it important? • TCP is used by lots of applications • TCP bypasses NAT and Firewall connectivity restrictions • Lots of TCP variants out there • Who got it right for delay-sensitive applications? Experimental Setup Delay overhead results • Obtained via delay prediction tool • Verified experimentally Experimental results TCP Delay and CWND evolution Reaction to a triple-dup loss event 95% percentile TCP overhead Performance Metrics • TCP Delay: sender + network + receiver • TCP Overhead: sender + receiver (a) ‘Small’ packet size of 200 bytes (b) Full packet size of 1448 bytes Rate 50 p/s, packet size 100 bytes, MSS 1000 bytes The delay overhead of 'small' packet sized flows (MSS/10) is lower then 200ms with probability 0.97 for RTTs< 100ms and loss rates<1% A Flow with ‘small’ packet size is less effected by TCP's window variations than a flow with ‘large’ packet size. Our Contributions • Novel delay prediction tool • Heuristics for reducing delays Conclusions and Observations • TCP can satisfy the delay constraints of a delay-sensitive application over a wide-range network settings • The use of smaller than MSS-sized packets exploits TCP's ACK counting mechanism and thereby limits the sender-side buffering due to congestion window variations Ongoing work Delay reduction HeuristicsIdea: reduce sender-side backlog • Reduce packet size and increase packet rate • Proactive sender-side packet drop • Compare CBR over TCP delays to those of UDP in the Internet using Planet-Lab