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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!