TCP Behavior Inference Tool

Jitendra Padhye, Sally Floyd

Presented by Songjie Wei

CISC856 Fall 2005 2

Overview

Motivation

Sample application: initial congestion window

TBIT architecture

Application 1: time wait duration

Application 2: congestion control algorithm

Summary

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Motivation

TCP carries most of the Internet traffic

TCP is a complex protocol with many parameters and

variations

Understanding TCP behaviors is important for:

OS vendors and customers

Internet-related researchers

Application developers

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Testing TCP Behavior of Web Servers The overall congestion control behavior of the

Internet is heavily influenced by TCP

implementation in web servers

Web servers are easy to test

No special privileges required to request information from

web servers

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Example of TBIT Application

Initial value of the congestion window (ICW) Number of bytes sent in a burst after three-way

handshaking, before receiving any ACKs

RFC2581: at most 2*MSS bytes

RFC2414: min(4*MSS, max(2*MSS, 4380))

Some TCP are found to send 8000+ bytes with

MSS of 512

Large bursts of packets: buffering problems, loss,

delay, etc.

TBIT Architecture

Using TBIT to Determine ICW Web server

SYN

“GET / HTTP/1.0” with ACK

SYN+ACK

TCP-PDU n

TCP-PDU 1

TCP-PDU 1

RST

Timeout

TBIT

TCP-PDU 3TCP-PDU 2

No ack to these TPDUs

What is the ICW size measured ?

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Difficulties in Measuring ICW

Web page too small to fill cwnd Use smaller MSS Use URL of bigger object

TPDUs get lost Repeat test multiple times

Multiple computers answer to the same IP address Repeat test multiple times Non-repeatable results?

Machine to test has no web server no solution

A B

SYN

“GET / HTTP/1.0” + ACK

SYN+ACK

TCP-PDU n

TCP-PDU 1

TCP-PDU 1

RST

Timeout

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Using TBIT to Determine Time-wait Duration What is Time-wait?

2 MSL time-wait can avoid

the port-reuse problem

Server should retain

sufficient state information

about the connection during

the time-wait

Many major web servers

use a smaller value of MSL2 MSL

Time-wait

Client Server

Closed

Closed

FIN

FIN + ACK

ACK

Data Transfer

Using TBIT to Determine Time-wait DurationTBIT Web Server

2 MSL

Closed

FIN

FIN + ACK

ACK

SYN

SYN

SYN

SYN + ACK

RST

Data Transfer

Estimated time-wait duration

Should this constant time be a big or small value ?

How about the sequence number of these SYNs?

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Review of TCP Congestion control Tahoe

slow start, congestion avoidance, fast rtx (dupacks)

Reno Tahoe + fast recovery (new acks)

New Reno Reno + modified fast recovery (partial acks)

Using TBIT to Test TCP Congestion ControlTBIT Web Server

Three-way handshake

rwnd=5 MSS

Initial cwnd=2 MSS

TPDU 1TPDU 2

TPDU 3TPDU 4TPDU 5TPDU 6

TPDU 7TPDU 8TPDU 9TPDU 10TPDU 11

TPDU 12TPDU 13TPDU 14TPDU 15TPDU 16

TPDU 17

ACK 1ACK 2

ACK 3ACK 4ACK 5ACK 6

ACK 7ACK 8ACK 9ACK 10ACK 11

ACK 12

ACK 12ACK 12

ACK 12

What happens now ?

“GET /HTTP/1.0”

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TCP without Fast Retransmit

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Tahoe

No retransmit timeout before the retransmission of packet 13

Unnecessary retransmission of packet 17

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Reno

Fast retransmit for packet 13, a retransmit timeout for packet 16

No unnecessary retransmission of packet 17

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NewReno

Fast retransmit for packet 13, no additional fast retransmits or retransmit timeout

No unnecessary retransmission of packet 17

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Characteristics of TCP Variationsunder TBIT Tests TCP without fast retransmission

No fast retransmission Timeout for packet 13

Tahoe No retransmit timeout before retransmission of packet 13 Necessary retransmission of packet 17

Reno Fast retransmit of packet 13 Retransmit timeout for packet 16 No unnecessary retransmission of packet 17

NewReno Fast retransmit of packet 13 No additional fast retransmit or retransmit timeout No unnecessary retransmission of packet 17

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Sample Testing Result (I)

Aug 31, 2003 Total: 439 Error:27 www.joins.com uses 0 www.cnn.com uses 1 www.stoo.com uses 10

Initial window Count

0 4

1 293

2 87

3 17

4 10

10 1

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Sample Testing Result (II)

Experimental results obtained by testing 84394 web servers (27914 classified): NewReno 76% Tahoe 4% (w/o Fast Retransmit) Reno 15% Other 1% Tahoe 4%

Source: Medina, Allman, and Floyd, “Measuring the Evolution of Transport Protocols in the Internet”, May 2004

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Summary TBIT is presented for characterizing the

TCP behaviors TBIT can be used to check any web server

without special privileges, in a non-disruptive manner

More information are obtained by TBIT about the congestion control mechanisms

Source code and detailed results at: http://www.aciri.org/tbit

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References Jitendra Padhye and Sally Floyd,Identifying the TCP Behavior of

Web Servers. June 2001, In Proceedings of SIGCOMM 2001

Alberto Medina, Mark Allman, and Sally Floyd, Measuring

Interactions Between Transport Protocols and Middleboxes.

Internet Measurement Conference 2004, August 2004

Sourabh Ladha, Paul D. Amer, Armando Caro, Jr., Janardhan R.

Iyengar, On the Prevalence and Evaluation of Recent TCP

Enhancements. Globecom 2004, Dallas TX, November 2004

Alberto Medina, Mark Allman, and Sally Floyd, Measuring the

Evolution of Transport Protocols in the Internet. To appear in

Computer Communications Review, April 2005

CISC856 Fall 2005 22

TBIT Homework (due on Nov 22) Design a TBIT test for both of the following:

Whether a web server supports TCP SACK Value and increase of the persistence timer

Requirements Describe the test step by step Draw time time diagram to show what are expected to happen

during the test Consider any possible difficulties in the test, such as

lost/duplicate/reordered packets, etc. State clearly any assumptions made for the local host, the web

server, or the network You don’t really need to install and run TBIT for this homework