1. EECS 122: Introduction to Computer Networks Transport Layer - Advanced Variations 
Computer Science Division
Department of Electrical Engineering and Computer Sciences
University of California, Berkeley
Berkeley, CA

2. Quick Review Slow-Start: cwnd++ upon every new ACK
Congestion avoidance: AIMD if cwnd > ssthresh
ACK: cwnd = cwnd + 1/cwnd
Drop: ssthresh =cwnd/2 and cwnd=1
Fast Recovery:
duplicate ACKS: cwnd=cwnd/2
Timeout: cwnd=1

3. TCP Flavors TCP-Tahoe TCP-Reno TCP-newReno TCP-Vegas, TCP-SACK
cwnd =1 whenever drop is detected
TCP-Reno
cwnd =1 on timeout
cwnd = cwnd/2 on dupack
TCP-newReno
TCP-Reno + improved fast recovery
TCP-Vegas, TCP-SACK

4. TCP Vegas Improved timeout mechanism
Decrease cwnd only for losses sent at current rate
avoids reducing rate twice
Congestion avoidance phase:
compare Actual rate (A) to Expected rate (E)
if E-A > , decrease cwnd linearly
if E-A < , increase cwnd linearly
rate measurements ~ delay measurements
see textbook for details!

5. TCP-SACK SACK = Selective Acknowledgements
ACK packets identify exactly which packets have arrived
Makes recovery from multiple losses much easier

6. Standards? How can all these algorithms coexist?
Don’t we need a single, uniform standard?
What happens if I’m using Reno and you are using Tahoe, and we try to communicate?

7. Equation-Based CC Simple scenario Observations:
assume a drop every k’th RTT (for some large k)
w, w+1, w+2, ...w+k-1 DROP (w+k-1)/2, (w+k-1)/2+1,...
Observations:
In steady state: w= (w+k-1)/2 so w=k-1
Average window: 1.5(k-1)
Total packets between drops: 1.5k(k-1)
Drop probability: p = 1/[1.5k(k-1)]
Throughput: T ~ (1/RTT)*sqrt(3/2p)

8. Equation-Based CC Idea: Approach:
Forget complicated increase/decrease algorithms
Use this equation T(p) directly!
Approach:
measure drop rate (don’t need ACKs for this)
send drop rate p to source
source sends at rate T(p)
Good for streaming audio/video that can’t tolerate the high variability of TCP’s sending rate

9. Question!
Why use the TCP equation?
Why not use any equation for T(p)?

10. Cheating Three main ways to cheat:
increasing cwnd faster than 1 per RTT
using large initial cwnd
Opening many connections

11. Increasing cwnd FasterB x D E y y C
x increases by 2 per RTT
y increases by 1 per RTT
Limit rates:
x = 2y x

12. Increasing cwnd FasterB x D E y

13. Larger Initial cwnd A B D E x y x starts SS with cwnd = 4
y starts SS with cwnd = 1

14. Open Many Connections A B D E x y Assume A starts 10 connections to B
D starts 1 connection to E
Each connection gets about the same throughput
Then A gets 10 times more throughput than D

15. Cheating and Game TheoryB x D E y
D  Increases by 1 Increases by 5 A
Increases by 1
Increases by 5
(x, y)
22, 22
10, 35
35, 10
15, 15
Too aggressive
Losses
Throughput falls
Individual incentives: cheating pays
Social incentives: better off without cheating
Classic PD: resolution depends on accountability

16. Lossy Links TCP assumes that all losses are due to congestion
What happens when the link is lossy?
Recall that Tput ~ 1/sqrt(p) where p is loss prob.
This applies even for non-congestion losses

17. Example
p = 0
p = 1%
p = 10%