Measuring the Congestion Responsiveness of Internet

Traffic

Ravi Prasad&

Constantine Dovrolis

Networking and Telecommunications GroupCollege of Computing

Georgia Tech

Outline Motivation Session arrival models

Closed-loop Open-loop

Congestion Responsiveness Metric Closed-loop Traffic Ratio (CTR)

CTR Measurements Methodology Results

Summary

Congestion Responsiveness Congestion Responsive Traffic: Reduces the offered

load in the event of congestion Conventional wisdom: the Internet traffic is congestion

responsive due to TCP TCP carries more than 90% of Internet traffic TCP reduces offered load (send window) upon sign of

congestion Negative-feedback loop, stabilizing queuing system Key modeling unit: persistent flows (they last

forever!) Most Internet flows are non-persistent Is an aggregate of non-persistent TCP flows

congestion responsive?

Flows are generated by users/applications, not by the transport layer!

Examples: user clicks web page, p2p transfers, machine-generated periodic FS synchronization

Session: Set of finite (i.e., non-persistent) flows, generated by single user action

Key issue: session arrival process

Does the session arrival rate reduce during congestion?

Receiver Sender

Transport

Application

ResponseRequest

Network

Two session arrival models Closed-loop model

Fixed number of users, each user can generate one session at a time

New session arrival: depends on completion of previous session

E.g., ingress traffic in campus network

Open-loop model Sessions arrive in network

independently of congestion

Theoretically, infinite population of users

E.g., egress traffic at popular Web server

1

2

3

N

Closed-loop model

N users: cycles of transfer and idle periods S : Average session size

TT : Average transfer

duration

TI : Average idle time

Na: Number of active sessions

Congestion responsive Congestion increases TT :

reduces offered load Roffered

1][

1,][

1,1

][

a

Ia

a

N

C

T

SE

S

CTNNE

NE

TIoffered TT

NSR

ICT

NS

Open-loop model

Poisson session arrivals S : Average session size : Session arrival rate Stable only if <1

Congestion unresponsive Offered load Roffered

independent of congestion

C

S

1),1(][ CT

SE

SRoffered

Mixed Traffic Internet traffic: mix of open-loop and

closed-loop traffic Mixed traffic can be characterized by

Closed-loop Traffic Ratio (CTR)

load trafficTotal

model loop closed from load TrafficCTR

Measuring Congestion Responsiveness

Direct congestion responsiveness measurements difficult Require highly intrusive experiments to cause

congestion Require access at bottleneck link

Alternative: Measure CTR (Closed-loop Traffic Ratio) Indirect metric for congestion responsiveness High CTR: more congestion responsive Low CTR: less congestion responsive

CTR estimation (overview) Start with packet trace from Internet link

Per-packet: arrival time, src/dst address & ports, size Focus only on TCP traffic: HTTP and well-known ports

Identify users: Downloads: user is associated with unique DST

address Uploads: user is associated with unique SRC address

For each user, identify sessions: Session: one or more connections (“transfers”)

associated with same user action E.g., Web page download: multiple HTTP

connections Classify sessions as open-loop or closed-loop:

Successive sessions from same user: closed-loop Session from a new user, or session arriving from

known user after a long idle period: open-loop

From Connections to Transfers An HTTP 1.1 connection

can stay alive across multiple sessions

Transfer : Segment of TCP connection that belongs to a single session

Intra-transfer packet interarrivals: TCP and network-dependent (short)

Inter-transfer packet interarrivals: caused by user actions (long)

Classify interarrivals based on Silence Threshold (STH)

1105126179.423931 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.478309 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.478438 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.478554 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.488433 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.488666 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.488918 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.539748 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.539870 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.539993 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.549085 163.157.239.61 127.207.1.255 80 2290 154 T 114

1105126179.549399 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.611572 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.611702 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.612235 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.612507 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.612752 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.613121 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.672432 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

Inter transfer gap Intra transfer gap

Silence Threshold (STH) estimation

Inter tranfer gap Intra transfer gap

STH=40sec

Group transfers from same user in sessions Intuition: transfers

from same session will have short interarrivals (machine-generated)

Minimum Session Interarrival (MSI) threshold

MSI aims to distinguish machine-generated from user-initiated events MSI = 1-5 seconds

1105126179.423931 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.478309 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.478438 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.478554 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.488433 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.488666 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.488918 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.539748 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.539870 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.539993 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.549085 163.157.239.61 127.207.1.255 80 2290 154 T 114

1105126179.549399 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.611572 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.611702 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.612235 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.612507 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.612752 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.613121 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.672432 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

Inter transfer gap Intra transfer gap

<MSI >MSI

session 1 session 2 session 3

Classify sessions as open/closed-loop

First session from a user is always open-loop

Session from a returning user is also open-loop, if it starts Before last session finish,

or Long time after

completion of last session

Long time = MTT: Maximum Think Time

1105126179.423931 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.478309 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.478438 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.478554 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.488433 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.488666 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.488918 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.539748 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.539870 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.539993 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.549085 163.157.239.61 127.207.1.255 80 2290 154 T 114

1105126179.549399 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.611572 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.611702 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.612235 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.612507 163.157.239.61 127.207.1.255 80 2289 1420 T 1380

1105126179.612752 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.613121 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

1105126179.672432 163.157.239.61 127.207.1.255 80 2290 1420 T 1380

Inter transfer gap Intra transfer gap

<MSI >MSI

session 1Open

session 2Open

session 3Close

> MTT < MTT

Robustness to MSI & MTT thresholds

Examined CTR variation in the following ranges: Minimum Session Interarrival (MSI): 0.5sec-2sec Maximum Think Time (MTT) : 5min-25min

CTR variation < 0.1 Linear regression:

CTR/MSI = 0.0232/sec CTR/MTT = 0.0020/min

We use: MSI=1sec. MTT=15min.

Sample CTR measurementsLink location

Year Direction Duration TCP Well-known ports

GB(%) Bytes(%) CTR

Georgia Tech.

05 In 2Hr. 129(97) 63.5 0.71

Out 2Hr. 208(99) 47.9 0.57

Los Nettos 04 Core 1Hr. 59(95) 65.6 0.77

UNC, Chapel Hill

03 In 1Hr. 41(87) 26.6 0.76

Out 1Hr. 153(97) 35.8 0.61

Abilene, Indianapolis

02 Core 1Hr. 172(96) 41.9 0.70

Core 1Hr. 178(85) 47.3 0.64

Univ. of Auckland, NZ

01 In 6Hr. 0.6(95) 73.0 0.73

Out 6Hr. 1.4(98) 78.0 0.67

Summary TCP or TCP-like protocols are necessary but

not sufficient for a congestion responsive aggregate

Show importance of arrival process for non-persistent transfers Focus on open-loop and closed-loop models Closed-loop Traffic Ratio (CTR) used to

characterize traffic in a given link Measurements show CTR values of 60-80%

for most Internet links we examined Session level feedback could be making internet

traffic congestion responsive

Thank you!