ARC TCP Workshop, ENS, Paris, November 5-7, 2003

1. Equation-Based Rate Control: Is it TCP-friendly ?Milan VojnovicJoint work with Jean-Yves Le Boudec ARC TCP Workshop, ENS, Paris, November 5-7, 2003

2. The Axiom: TCP-friendliness Requires adaptive sources to obey to TCP in the following sense:TCP-friendliness (late 1990’s) “A flow that is not TCP-friendly is one whose long-term arrival rate exceeds that of any conformant TCP in the same circumstances.” Floyd and Fall, 1999

3. Equation-Based Rate Control: Basic Control Estimator of 1/p: Send rate: Example Protocol: TFRC (RFC 3448, IETF proposed standard, Jan 2003)

4. Is Equation-Based Rate Control a TCP Friend ? We deduce: the Engineering Intuition p -> f(p) is TCP loss-throughput formula So, it must be that if I adjust the send rate at loss-events to f(), evaluated at the on-line estimated loss-event rate, my new protocol will be TCP-friendly Problem: When the Intuition is True and when Not ?

5. Outline 1. Breakdown the TCP-friendliness into sub-conditions, study the sub-conditions separately • Why the common evaluation practice to verify TCP-friendliness is not good ? 2. TCP-friendliness is difficult to verify • Counterexamples to TCP-friendliness 3. Conservativeness is easier • Sufficient conditions for conservativeness • Or bounded non-conservativeness

6. measured throughputs x x’ TCP Test: TCP-friendly iff x <= x’ 1. Common Evaluation Practice Common Practice: Non-TCP Why the common evaluation practice is NOT GOOD ?- hides a cause of the observed throughput deviation- may lead a protocol designer to an improper adjustment

7. Breakdown the TCP-Friendliness Condition (I) Does the source verify x <= f(p,r) ? (II) Does the source attain the same loss-event rate as TCP ? (III) Does the source see the same average round-trip time as TCP ? (IV) Does TCP verify its throughput formula ? Important to BREAKDOWN the TCP-friendliness conditioninto sub-conditions, and study them separately !

8. (x, p, r) (x’, p’, r’) TCP Equation-Based Rate Control throughput loss-event rate average RTT Breakdown the TCP-Friendliness Condition (Cont’d) (I) Conservativenessx<= f(p, r) (II) Loss-Event Rates p>= p’ (III) Round-Trip Times r>= r’ (IV) Obedience of TCP to the Formula x’ >= f(p’, r’) If (I), (II), (III), and (IV) hold, that implies TCP-friendliness.

9. Ass. EBRC uses f(p) in (1) r r AIMD (a,b) EBRC (1) TCP-like (b=1/2) p’/p=16/9 (approx. 1.7778) 2. Counterexample to TCP-Friendliness:AIMD experiences larger loss rate than EBRC Example 1: Either One AIMD or One EBRC over a Link Ob: p’ > p <=> non-TCP-friendliness

10. Convergence for One EBRC over a Link slope K2/2

11. Convergence for One EBRC over a Link (Cont’d) Can be seen as Jacobi iterative solving of: The equilibrium point: If stable: Remarks • both AIMD and EBRC are rate-based • both AIMD and EBRC are fluid, no packetization effects => the deviation of the loss-event rates is intrinsic to the very nature of the dynamics of the two controls

12. Validation by ns-2 Simulation x/x’ TFRC b pakets b TCP b pakets x/f(p,r) p’/p r’/r x’/f(p’,r’) Breakdown:

13. AIMD sees larger loss rate than EBRC (Cont’d) Example 2: One AIMD and One EBRC Competing for a Link • time t is a loss-event iff at t-the sum of the send rates of the two sources = r • a loss-event is assigned to either AIMD or EBRC • Zn = 1 iff the nth loss-event is assigned to EBRC, else Zn=0 g : R+L+1 -> R+ is a non-linear function; the system is non-linear

14. Example 2: Numerical Simulations

15. Example 2: Validation by ns-2 Simulation x/x’ TCP TFRC b pakets b x/f(p,r) p’/p r’/r x’/f(p’,r’) Breakdown:

16. Internet Measurements EPFL Long-lived transmissions with TFRC and TCP Estimated: loss-event rates, average round-trip times, throughputs INRIA, KTH, UMASS,UMELB

17. Breakdown into Sub-Conditions: x/f(p,r) p’/p r’/r x’/f(p’,r’) EPFL to UMASS TFRC/TCP throughput x/x’

18. assume: the send rate is a stationary ergodic process • The send rate control: 3. Conservativeness Convergence: • The estimator is updated at special points in time Q. Is x <= f(p) ?

19. Conditions for Conservativeness In practice: • the conditions are true, or almost • the result explains overly conservativeness

20. Is Negative or Slightly Positive ? InternetLAN to LANEPFL sender InternetLAN to cable-modem at EPFL Lab

21. Cause: convexity of 1/f(1/x) PFTK-simplified 16 8 PFTK 4 L=2 SQRT Throughput-Drop Puzzle Empirical indications: TFRC looses throughput for large loss-event rates E.g. Bansal et al (ACM SIGCOMM 2001): “ … in return to for smoother transmission rates, slowly-responsive algorithms lose throughput to faster ones (like TCP) under dynamic network conditions.” Why ?

22. What Causes Excessive Conservativeness ? Palm inversion: Throughput: May make the control conservative ? !

23. What Causes Excessive Conservativeness ? (Cont’d) • 1/f(1/x) is assumed to be convex, thus, it is above its tangents • take the tangent at 1/p • the “overshoot” bounded by a function of p and

24. Conclusion 1. Breakdown the TCP-friendliness into sub-conditions, study the sub-conditions separately 2. TCP-friendliness is difficult to verify 3. Conservativeness is easier