Satellite TCP Lecture 19 04/10/02

Announcements … Project (preliminary report) Generalization of model, DSR changes required Individual extensions to be tested and evaluated first Extra-credit questions posted on web Any other questions?

Puzzle 14 Consider a professor telling his class: “There will be a surprise exam during class next week” 5 days in a week, class meets every day Can the professor give a surprise exam? Surprise: Student does not know at the beginning of class that the exam is during that class SEP Paradox!

WTCP – Recap For wide-area wireless networks Rate-based transmissions Inter-packet delay as congestion indicator Packet-pair based start-up Non-congestion loss detection No RTOs Reliability …

Reliability Selective acknowledgements SACKs very useful in TCP when random losses occur Better utilization of the network resources (no unnecessary retransmissions)

Reliability (contd.) No retransmit timeouts Since RTOs cannot be relied upon, WTCP does not use RTOs Use probe packets to recover from suffix losses ACK frequency tuned by sender based on factors such as (a) observed loss rate on reverse path, (b) round-trip time, (c ) half-duplex or full-duplex nature of channel

WTCP Algorithm Congestion control 3 phases: increase, decrease, and maintain Chosen based on long-term and short term averages in inter-packet separation RTT computation (for probing) Using loss profile Increase, decrease, maintain phases

WTCP Reliability SACK algorithm Probe packets

Satellite Networks LEOs (~1500Km), MEOs (20000Km), GEOs (35000Km) Inter-satellite Network

Satellite Network Characteristics High latencies GEOs: 550ms MEOs: 250ms LEOs: 50ms Asymmetric paths Downlink: up to a few Mbps Uplink: up to a few tens of kilobits Transmission errors

High Latencies More time in slow-start Large bandwidth delay products

Slow-start and Delays Time required by slow-start to reach a target bit rate B: T = rtt * (1 + log2B*rtt/l) Assuming 1Mbps, 1KB LEO: 180ms MEO: 1.5 seconds GEO: 3.91 seconds Connections might entirely be in the slow-start phase before they terminate Why is this bad?

Approaches to handle large delays Increasing initial window size Split connection approaches Fast-start Use values from previous connections Use low-priority packets Cons?

TCP-Peach Sudden start Send 1 real packet, and N dummy packets (N = rwnd-1) After 1 round-trip time, depending upon the number of ACKs received, set the congestion window Assume dummy packets can have lower priority

TCP Peach – Sudden start 0 < t < RTT Sender transmits a data-segment first and (rwnd-1) dummy segments, in such a way and the data and dummy segment transmissions are uniformly distributed in an interval equal to RTT RTT < t < 2*RTT ACKs arrive at sender (for each ACK, increase congestion window by 1) t > 2*RTT Congestion avoidance

High Bandwidth-delay products Default TCP advertised window can scale up to 64KB GEOs: 10 Mbps and 0.5 seconds (5 Mb)! Lower utilization

Large BDPs Window scaling 32 bit windows (instead of 16 bits) with a “scale factor” agreed upon through the exchange of an option during start-up Multiple TCP connections for the same application

Transmission Errors TCP infers random wireless losses as congestion related losses Consequently, it reduces congestion window drastically

TCP Peach – Rapid Recovery Similar to sudden start approach Perform fast-retransmit just as in TCP Reno However, during fast recovery, while limiting the congestion window to cwnd/2, also transmit N dummy packets (set to cwnd) After receiving cwnd/2 dummy ACKs, increase congestion window by 1 for every ACK thereafter

Path Asymmetry Remove the necessity for per-packet ACKs STP (Satellite Transport Protocol) Periodic NACKs or status update (STAT) messages from the receiver to the sender Sender (based on the periodicity of the NACKs or STATs) computes the number of packets it can transmit and spaces the transmissions accordingly

Puzzle 15 A candy store has two types of candy: C1 and C2. C1 costs one dollar while C2 costs seventy five cents. The costs are put up on a black board visible to anyone who enters. Person A enters a candy store, gives one dollar to the person at the counter, and asks for a piece of candy. The counter-clerk not surprisingly, asks A which of the two A wants Person B enters the store, gives one dollar to the clerk. The clerk gives B candy C1 (both C1 and C2 are in stock). Why?

Recap Satellite Networks 3 problems Large BDPs Large start-up delays Transmission errors Path asymmetry