1. Satellite TCP
Lecture 19
04/10/02

2. 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?

3. 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!

4. 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 …

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

6. 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

7. 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

8. WTCP Reliability
SACK algorithm
Probe packets

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

10. 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

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

12. 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?

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

14. 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

15. 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

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

17. 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

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

19. 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

20. 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

21. 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?

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