TCP MODELING

Submitted by:Varsha Anandani (13MIT0062)

TCP = Transmission Control Protocol Connection-oriented protocol Provides a reliable unicast end-to-end byte

stream over an unreliable internetwork

TCP

1) The sheer scale of systems in which we find

TCP operating is tremendous.

2) There still remain many unknowns in the environment in which TCP is operating.

3) TCP was not designed using optimization.

Motivation for mathematical modeling of TCP

TCP operating scale is very large

Models are required to gain deeper understanding of TCP dynamics

Uncertainties can be modeled as stochastic processes

Drive the design of TCP-friendly algorithms for multimedia applications

Optimize TCP performance

Motivation for TCP Modeling

Link speed the number of bits per second that can be transmitted. Propagation delay, the time it takes the actual electronic, optical, or other

signals to travel from one end of the connection to the other. Window size, the amount of unacknowledged data that can be

outstanding on a TCP connection. Link reliability. Network and intermediate device congestion. Path maximum transmission unit (PMTU).

FACTORS THAT AFFECT THROUGHPUT

TCP Modeling Essentials

Mainly Reno flavors are modeled Two main features are modeled

Window dynamics Packet loss process

Window Dynamics

Linear increase and multiplicative decrease is modelled.

The standard assumption X(t) = W(t)/RTT

Packet Loss Process

Packet loss triggers window decrease Packet loss is uncertain This uncertainty is typically modeled as a

stochastic process E.g. probability p of losing a packet

Gallery of TCP Models

Periodic model Detailed packet loss model Finite state machine Fluid flow model And others…

Periodic Model

Simplest model for TCP No specific

version assumed

Assumes a periodic pattern of congestion window

X(p) = (1/RTT)*Root(3/2p)

P is the packet loss probability

TCP Congestion Control: window algorithm

Window: can send W packets

increase window by one per RTT if no loss, W <- W+1 each RTT

decrease window by half on detection of loss W <- W/2

sender

receiver

W

1 RTT

TCP throughput/loss relationship

Idealized model: W is maximum supportable

window size (then loss occurs)

TCP window starts at w/2 grows to W, then halves, then grows to W, then halves…

one window worth of packets each rtt

to find: throughput as function of loss, RTT

TCPwindow

size

time (rtt)

W/2

W

loss occurs

period

# packets sent per “period”

period

222

8

3

8

1

4

1WWW

Example 1 - Question

If a TCP connection has an average RTT of 200ms, and packet loss probability 0.05, what is the average throughput?