TCP Friendliness

CMPT771

Spring 2008

Michael Jia

Outline

Background

Classification

Achievements

Challenges

TCP Fairness

R

R

equal bandwidth share

Connection 1 throughputConnect

ion 2

th

roughput

Fair: 1. Equal share

2. Full utilizationif K TCP sessions share same bottleneck link of bandwidth R, each should have average rate of R/K

Why UDP?

UDP Preferred Applications• Video Streaming• VoIP

UDP Advantages• Simplicity • Lower overhead (light weight)• No re-transmission required

Problem with UDP: Unresponsive Flows

• No congestion control• No response to packet drops• TCP competing with unresponsive UDP

– TCP flows reduce sending rates in response to congestion

– Uncooperative UDP flows capture the available bandwidth

– Unfair to TCP, or even starve TCP

Objective: TCP-friendly

TCP TCP

Internetnon-TCP

non-TCP

“ long-term throughput does not exceed the throughput of a conformant TCP connection under the same conditions”

Outline

Background

Classification

Achievements

Challenges

Classification

• Window-Based vs. Rate-Based– window-based:

• Window size controls rate• Sender or receiver(s)• Similar to TCP

– rate-based:• TCP throughput models• More smoother rate• Good for media streams

Classification

• Unicast vs. Multicast– Multicast: more difficult– RTT is required for Rate-based schemes– Window-based approach is more suitable

• Single-rate vs. Multi-rate– Unicast = Single-rate– Multicast: multi-rate protocols are preferred

• More flexible allocation of bandwidth• Layered multicast• Group management

Outline

Background

Classification

Achievements

Challenges

TCP Throughput Equation 1R -- Bandwidth of TCP connection (Long term throughput)T -- Round-trip delay T (RTT)L --Packet size Lp -- Loss event rate p

T. Ott, J.H.B. Kemperman, M. Mathis, 1996

The Stationary Behavior of Ideal TCP Congestion Avoidance

TCP Throughput Equation 2

22 3(3 ) (1 32 )

3 8RTO

LR

q qT T q q

=+ +

Padhye, J., Firoiu, V., Towsley, D., and Kurose, J., Modeling TCP Throughput: a Simple Model and its Empirical Validation, UMASS CMPSCI Tech Report TR98-008,

Feb. 1998.

R -- Bandwidth of TCP connection

T -- Round-trip delay T (RTT)

L --Packet size L

q -- Loss event rate q

TRTO -- Retransmission timeout (~ 4T)

TCP Throughput Equation

M. Mathis, J. Semke, J. Mahdavi, and T. Ott. The macroscopic behavior of the TCP congestion avoidance algorithm.

Computer Communication Review, 27(3), July 1997

• Verify through simulation & live Internet measurements

• Assumption– Steady State (Ignore slow start phase & No timeouts)

– Constant packet size

Achievements

Achievements - TFRC

• TCP-Friendly Rate Control Protocol (2000)• Unicast, rate-based• Based on TCP equation 2

• Using more sophisticated methods to gather parameters– Average-Loss-Interval loss rate estimation

• Stable sending rate• Sufficient responsiveness

Achievements - TEAR

• TCP Emulation At Receivers (2000)• Multicast, single-rate• Rate-based + Window-based• Receiver maintains a congestion window• Receiver calculates average rate

– then send back to the sender– avoid saw-tooth-like behavior

• Scalable in multicast case– use the minimum rate

Achievements – Rainbow

• Rainbow (2000)• Multicast, multi-rate,

window-based• Digital-Fountain• Receivers individually

request each data packet• Routers process requests• Receiver controls

congestion• Limitation – router

supporting

Outline

Background

Classification

Achievements

Challenges

Challenges

• Lack of standard methods for comparison

• Fairness definitions for multicast

• Improvement of the models for TCP traffics

• How to treat short-lived flows

• Much more…

References• Robert Denda Joerg Widmer and Martin Mauve, 2001, A

survey on tcp-friendly congestion control

• T. Ott, J.H.B. Kemperman, M. Mathis, 1996, The Stationary Behavior of Ideal TCP Congestion Avoidance

• Padhye, J., Firoiu, V., Towsley, D., and Kurose, J., 1998, Modeling TCP Throughput: a Simple Model and its Empirical Validation

• M. Mathis, J. Semke, J. Mahdavi, and T. Ott., 1997, The macroscopic behavior of the TCP congestion avoidance algorithm

• Jitendra Padhye Sally Floyd, Mark Handley and Joerg Widmer, 2000, Equation-based congestion control for unicast applications

• Volkan Ozdemir Injong Rhee and Yung Yi., 2000, Tear: Tcp emulation at receivers – flow control for multimedia streaming

Questions?

Thank You