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Network Measurements, Modeling and Simulations. Kun-chan Lan Department of Computer Science and Information Engineering [email protected] Some Admin stuff. Paper review 2 paper reviews instead one as we originally planned since now the number of enrollment is reduced to 7

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Network measurements modeling and simulations

Network Measurements, Modeling and Simulations

Kun-chan Lan

Department of Computer Science and Information Engineering

[email protected]

Some admin stuff
Some Admin stuff

  • Paper review

    • 2 paper reviews instead one as we originally planned since now the number of enrollment is reduced to 7

    • Please send the titles of the paper you will review to TA by the end of this week

Guess talk next week
Guess talk next week

  • Talk: 如何一邊玩線上遊戲一邊寫論文?

  • 陳昇瑋: 中央研究院

  • The talk will be related to your 2nd homework

  • No class lecture next week

Homework 2
Homework 2

  • Similar to the experiments done in the paper “Online Game QoE Evaluation using Paired Comparisons “

  • Compare gaming performance using different wireless media

    • WiFi vs. 3G vs. WiMax

  • The experiments will be done in LENS lab using the multihomed mobile router

Multi homed mobile router
Multi-homed mobile router

  • A mobile router with multiple interfaces

  • Will be setup in LENS lab

  • The mobile router will periodically change to a difference interface

game client

Game server

Multi homed mobile router1
Multi-homed mobile router

  • You job:

    • Play game

    • Make comparison about your perceived gaming performance

    • Create graph similar to what shown in the paper (you don’t need to write program for that purpose. Some programs will be offered to you to generate the plots)

A quick survey

A quick survey…

Why do you come to this class? What do you want to get out of this course?

  • Learn about ns-2?

  • Learn how to measure traffic?

  • Learn how to use emulator?

  • Somebody suggested you to try it out?

  • None of the above (you have no idea why you came here!)


  • Model and simulate Internet traffic

    • It’s hard to model and simulate Internet

      • Use measurement to improve the realism of your model

      • We advocate trace-driven simulation

    • Internet and wireless measurements

What is a model
What is a model?

  • Abstraction of real world

  • Base of a network simulation

    • Topology model

      • e.g. “a dumbbell topology”

    • Traffic model

      • “80% TCP + 20% UDP”

    • Queuing model

      • e.g. “FIFO”, “Fair queuing”, etc.

    • …..

Role of simulation
Role of simulation

  • Based on some particular models

    • Topology: e.g. dumbell vs. tree

    • Traffic: e.g. TCP vs. UDP

  • Widely used by researcher to study Internet

    • Millions of hosts in different administrative domains

  • Simulation vs. experiment (Why simulation?)

    • Repeatability

    • Configurability

    • Scalability

    • Explore complicated scenarios

    • Study “future” application/prtotocol/network

What simulation does t do
What simulation does’t do

  • Realism

    • Details of simulation matters!

    • It’s your responsibility to know what level of details you need to capture in the simulation

  • Prove correctness of the model

    • Only for validation!

    • The value of simulation relies on a good model

It s hard to simulate internet
It’s hard to simulate Internet

  • Network heterogeneity

  • Rapid and unpredictable change

Network heterogeneity
Network heterogeneity

  • Topology

  • Link properties

  • Protocol

  • traffic

  • All the above matter when you do the simulation

Difficulty in modeling topology
Difficulty in modeling topology

  • Constantly changing

    • Routing change

    • Link/node up and down

  • ISPs typically do not make topological information available

  • There is no “typical” topology

    • Depends on what are you simulating

Difficulty in modeling links
Difficulty in modeling links

  • large diversities

    • Speed: e.g. modem vs. fiber optic link

    • Loss: e.g. cooper wire vs. 802.11

    • Transmission: point-to-point vs. broadcast

    • Latency: DSL vs. satellite links

  • Routing-dependent

  • Asymmetry

Difficulty in modeling protocol
Difficulty in modeling protocol

  • Differences in implementations

    • 400 different TCP implementations

  • Different applications and different traffic mix

Difficulty in modeling traffic
Difficulty in modeling traffic

  • Traffic is different everywhere

  • Effect of background traffic

    • Queuing, congestion

  • Some application are adaptive to network conditions

Rapid and unpredictable changes
Rapid and unpredictable changes

  • Change in TCP: Reno -> NewReno/SACK

  • Change in devices: PC->handheld

  • Change in web: caching -> CDN

  • Change in killer applicaton:

    • web->p2p->VoIP?

  • Change in physical layer: wired -> wireless

Coping strategy
Coping strategy

  • OK, so it’s hard to simulate Internet, but can we do something about it?

  • Yes

    • Systematically explore important parameters

    • Searching for invariants

Network behavior as a function
Network behavior as a function

  • Explore network behavior as a function of changing parameters

    • <observed traffic> = f(x1,x2,x3,…..)

  • Impossible to explore the whole set of parameters

    • Challenge: identify important parameters

    • Example parameters to which a simulation might be sensitive

      • Congestion

      • Topology

      • Router mechanism (routing, scheduling, etc.)

Search for invariants
Search for Invariants

  • Invariant: behavior that holds in a very wide range of environment

  • Examples

    • Diurnal patterns

    • Self-similarity

    • Poisson session arrival

    • Heavy-tailed distribution

    • Geographical topology

  • Extract invariants from real world data

    • Extensive measurements!


  • Model and simulate Internet traffic

    • It’s hard to model and simulate Internet

    • Internet and wireless measurements

    • Case study: modeling heavy-hitter traffic

Why measuring
Why measuring?

  • To tell us what are the invariants, and what are just artifacts of the system

    • A base for realistic modeling and simulation

  • A common practice in other science disciplines (physics, biology, etc)

A measurement plan
A measurement plan

  • What questions you want to answer?

  • Testbed setup

  • How to collect the traces? And for how long?

  • What to collect? (what is your performance metrics)

  • Data analysis

All of these should be in your project report!

Tcp over gprs network
TCP over GPRS network

How fair is TCP over GPRS?

Things i am going to tell you next
Things I am going to tell you next

  • What can you measure?

  • Things that you need to know when you measure

  • Where can you get Internet traffic measurements for free?

Measure the internet
Measure the Internet

  • What can you measure

    • Traffic

    • Routing

    • Topology

    • Performance

    • Multicast

    • Wireless/Mobility

Tool for measuring traffic
Tool for measuring traffic

  • Tcpdump/etherreal (libpcap)

  • Netflow

  • NetTrMet/RTG (SNMP)

Tcpdump ethereal

  • tcpdump

    • Most commonly used packet collector

    • based on libpcap API

    • Output can be easily analyzed using awk/perl scripts

  • Ethereal

    • GUI-based

    • Support various trace formats, including tcpdump, snoop, etc.

    • Support various link-layer headers, including 802.11, ATM, etc.

  • tcpdpriv

    • A commonly used packet anonymizer (to share traces with the others)

    • Libpcap-based

    • Link-level headers are passed through unchanged.

Usage of tcpdump
Usage of tcpdump

tcpdump [ -adeflnNOpqStvx ] [ -ccount ]

[-Ffile ]   [ -iinterface ] [ -rfile ] [ -ssnaplen ]

[-Ttype ] [ -wfile ] [expression ]

Must run as root or have sudo permission


-iListen on interface. If unspecified, tcpdump searches the system interface list for the lowest numbered, configured up interface (excluding loopback)

-nDon't convert addresses (i.e., host addresses, port numbers, etc.) to names


-pDon't put the interface into promiscuous mode.

-q Quick (quiet?) output. Print less protocol information so output lines are shorter.

-r Read packets from file (which was created with the -w option). Standard input is used if file is ``-''.


-wWrite the raw packets to file rather than parsing and printing them out. They can later be printed with the -r option. Standard output is used if file is ``-''.

-rRead packets from file (which was created with the -w option). Standard input is used if file is ``-''.

-SPrint absolute, rather than relative, TCP sequence numbers


-s snarf snaplen bytes of data from each packet rather than the default of 68. 68 bytes is adequate for IP, ICMP, TCP and UDP but may truncate protocol information from name server and NFS packets. Packets truncated because of a limited snapshot are indicated in the output with ``[|proto]'', where proto is the name of the protocol level at which the truncation has occurred.

Taking larger snapshots both increases the amount of time it takes to process packets and, effectively, decreases the amount of packet buffering. This may cause packets to be lost.

- Limit snaplen to the smallest number that will capture the protocol information you're interested in.


-tDon't print a timestamp on each dump line.

-ttPrint an unformatted timestamp on each dump line.

-v(Slightly more) verbose output. For example, the time to live and type of service information in an IP packet is printed.

-vvEven more verbose output. For example, additional fields are printed from NFS reply packets.

-xPrint each packet in hex.


  • selects which packets will be dumped. If no expression is given, all packets will be dumped. Otherwise, only packets for which expression is `true' will be dumped.

  • The expression consists of one or more primitives. Primitives usually consist of an id (name or number) preceded by one or more qualifiers.

  • There are three different kinds of qualifier.

    <type> <dir> <proto>



  • what kind of thing the id name or number refers to

  • Possible types are host, net and port

  • E.g., `host', `net 146.132', `port 20'

  • If there is no type qualifier, host is assumed.



  • specify a particular transfer direction to and/or from id.

  • Possible directions are src, dst, src or dst and src anddst.

  • E.g., `src', `dst net 146.132', `src or dst port ftp-data'.

  • If there is no dir qualifier, src or dst is assumed



  • restrict the match to a particular protocol.

  • Possible protos are: ether, fddi, ip, arp, rarp, decnet, lat, sca, moprc, mopdl, tcp and udp.

  • E.g., `ether src', `arp net 128.3', `tcp port 21'.

  • If there is no proto qualifier, all protocols consistent with the type are assumed. E.g., `src' means `(ip or arp or rarp) src'

Complex expression
Complex expression

  • complex filter expressions are built up by using the words and, or and not to combine primitives.

  • E.g., `host and not port ftp and not port ftp-data'.

  • Iidentical qualifier lists can be omitted.

    E.g., `tcp dst port ftp or ftp-data or domain' == `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.

Allowable primitives
Allowable primitives

  • dst host host

  • src host host

  • host host

  • ether dst ehost

  • ether src ehost

  • ether host ehost

  • gatewayhost

Allowable primitives1
Allowable primitives

  • dst net net

  • src net net

  • net net

  • net netmask mask

  • net net/len

    True if the IP address matches net a netmask len bits wide. May be qualified with src or dst.

  • dst port port

    • src port port

  • port port

Allowable primitives2
Allowable primitives

  • less length

    True if the packet has a length less than or equal to length. This is equivalent to: len <= length.

  • greater length

  • ip proto protocol

    • True if the packet is an ip packet of protocol type protocol. Protocol can be a number or one of the names icmp, igrp, udp, nd, or tcp. Note that the identifiers tcp, udp, and icmp are also keywords and must be escaped via backslash (\)

  • ether broadcast

  • ip broadcast

Allowable primitives3
Allowable primitives

  • ether multicast

  • ip multicast

  • ip, arp, rarp, decnet

    short for: ether proto pwhere p is one of the above protocols.

  • tcp, udp, icmp

    short for: ip proto p

Relation operator
Relation operator

  • expr relop expr

  • relop is one of >, <, >=, <=, =, !=

  • expr is an arithmetic expression composed of integer constants, the normal binary operators [+, -, *, /, &, |], a length operator, and special packet data accessors.

  • To access data inside the packet, use the following syntax: proto [ expr : size ]Proto is one of ether, fddi, ip, arp, rarp, tcp, udp, or icmp. E.g.tcp[0] means the first byte of the TCP header

  • For example, `ether[0] & 1 != 0' catches all multicast traffic. The expression `ip[0] & 0xf != 5' catches all IP packets with options.

Combining primitives
Combining primitives

  • Primitives may be combined using:

    • Negation (`!' or `not').

    • Concatenation (`&&' or `and').

    • Alternation (`||' or `or').

  • Negation has highest precedence. Alternation and concatenation have equal precedence and associate left to right..

  • If an identifier is given without a keyword, the most recent keyword is assumed.

    • E.g.,not host vs and aceis short for not host vs and host ace,which should not be confused with not ( host vs or ace )

  • Netflow

    • Built-in service for most Cisco router/switch that runs Cisco IOS

    • Provide flow-level information

    • First packet in a flow is used to build an entry in the cache

    • Per-interface basis

    • Useful for accounting/billing, traffic monitoring, user profiling, data mining, etc.

    More on netflow
    More on Netflow

    • Typical cache size: 4K-128K (typical DRAM size: 2M-8M)

      • Need to use the cache efficiently

    • When to expire netflow cache entries

      • Idle time > t

      • Long-lived flows (duration > 30min)

      • TCP connections with FIN or RST

      • when cache becomes full (applying some heuristics to age flows)

    Management of netflow
    Management of Netflow

    • Netflow FlowCollector

      • can collect flow info from multiple NetFlow-enabled devices

      • data volume reduction through selective filtering and aggregation

      • store flow information for off-line analysis

    • Netflow FlowAnalyzer

      • data visualization: graphical data display

      • data export to external applications (such as Excel)

    • Netflow Server

      • collect flow statistics from multiple FlowCollector

      • further summarize NetFlow statistics by enabling bi-directional consolidation

      • store NetFlow statistics in a common commercial RDBMS (can be queried via SQL later)

      • encrypt and compress NetFlow statistics


    • Collect flow data via SNMP

    • builds up packet and byte counts for traffic flows

    • Flows are defined by their end-point addresses

      • Address can be ethernet addresses, IP address or the combination of both

    • Can specify a set of rules to filter the flows of interest

    • Run under dos or Unix


    • A SNMP statistics monitoring system

    • Commonly used by ISPs

    • collect time-series SNMP data from a large number of interfaces

    • Run as a daemon

    • All collected data is inserted into a relational database where complex queries and reports may be generated via SQL

    • can poll at sub-one-minute intervals

    • utilities are included to generate traffic reports, 95th percentile reports and graphical data plots

    Tool for measuring routing
    Tool for measuring routing

    • Traceroute

      • tracert command for Windows

    • RouteView


    • Trace the path from a source to a destination

    • Show how many hops a packet required to reach the destination and how long each hop takes.

    • Utilize IP Time-to-Live (TTL) field

    • TTL value specifies how many hops a packet is allowed to travel (decremented by 1 at each hop). An ICMP TIME_EXCEEDED response is returned to the source once TTL reaches 0.

    • Send a series of packets and incrementing the TTL value with each successive packet.


    • A large collection of BGP routing tables from several backbones (from 60 vantage points and 400+ AS)

    • Aim to provide network operators the information about the global routing system from various locations around the Internet

    Bgp basics
    BGP basics

    • BGP: an inter-gateway protocol to route packets between Autonomous System (AS)

      • AS: a group of networks that is controlled by a common network administrator on behalf of a single administrative entity. Each AS is assigned a globally unique number

      • Convey information about AS path topology

      • Run on top of TCP (port 179)

      • A path vector protocol

    Path vector protocol
    Path vector protocol

    AS100: 100

    AS200: 200 100


    AS300: 300 200 100

    Tool for measuring topology
    Tool for measuring topology

    • traceroute-based

      • Skitter

      • Rocketfuel


    • effort of CAIDA

    • ICMP-based: similar to traceroute

    • probing the paths from a source to many destinations IP addresses spread throughout the IPv4 address space

    • RTT and forward paths are



    • Input

      • traceroute (utilizing public available tracroute servers)

      • BGP

      • DNS

    • Output (per ISP)

      • Backbone

      • POP

      • Peer links

    Path discovery
    Path discovery

    • Use 750 public available traceroute sources

    • Merge traceroute paths from multiple sources to multiple destinations to obtain network map

    • Brute-force (all src × all dest) approach does not work

      • Too many addresses to probe (150M!)

      • Too much load for the traceroute server

      • Too much traffic for the network

    • Approach

      • Only probe the paths which are most “relevant”

        • Paths that transit the targeted ISP

      • Omit redundant paths

    • Other challenges

      • Alias: one router might have multiple IP addresses, one for each of its interfaces

      • Geographical location of the router

    Selected measurements
    Selected measurements

    • per-ISP map

      • Only choose traceroutes that are expected to transit the ISP (direct probing)

        • Use BGP routing tables

        • Data: from RouteView

      • Path reduction

        • Some probes might have identical paths inside the ISP

    Use bgp to choose traceroute







    9 5

    Use BGP to choose traceroute


    AS path

    closer to destination  8 11 42 5

    62 5

    • traceroutes that are likely to traverse AS 2

    • from servers in AS 8, 11, 4, 6 to prefix

    • If ALL paths to do not include AS 2

      • from anywhere to

      • from to anywhere

    Path reduction
    Path reduction

    • Skip repeated traces of the same path

      • Same destination, same ingress point

      • Same ingress point, same egress point

    Effectiveness of selected measurements
    effectiveness of selected measurements

    • Brute-force (all servers to all BGP prefix)

      • 150 million traceroutes required

    • Direct probing

      • 15 million traceroutes required

    • Direct probing + path reduction

      • 300 thousand traceroutes required

    Alias resolution
    Alias resolution

    • Alias: traceroute reports the IP address of the interface on the router (not the router!)

    • The router might have multiple interfaces

    • Router’s interfaces may be numbered from entirely different IP prefixes

    • Need to know interface 1 and

      2 are on the same router

    Alias probe
    Alias probe

    • If you send an UDP packet to interface A of a router and address to a non-existing port

      • By default, the router will return a ICMP “port unreachable” response back to you

      • The source address of ICMP packet will be the outgoing interface for the unicast route to you (interface B)

    • if we probe interface X and Y

      and the resulting ICMP packets

      have the same source address Z,

      then we know X and Y are on

      the same router

    Other tricks for resolving alias
    Other tricks for resolving alias

    • Compare TTL

    • Compare IP identifier (ID)

      • Packets sent consecutively will have consecutive IP identifier

      • Send probe packets to two potential aliases

      • Send another packet to the address that responded first

      • Aliases: if x < y < z, and z – x is small

    Identify router location
    Identify router location

    • Utilize DNS names

    • ISP typically use certain naming convention to name their routers


        • A Sprint backbone router (bb11) in New York city (nyc)


        • A Verio backbone router (bb) in Miami, Florida


        • A neighboring router of Sprint

    A typical pop structure
    A typical POP structure

    • POP (Point Of Presence)

      • Consist of a set of backbone and access routers

    • Backbone routers

      • connect to other ISPs

      • typically fully connected

        within the POP

    • Access routers

      • Connect to customers

        • Connect to routers from

          the neighboring domains

        • Connect to two backbone routers

          for redundancy


    Isp peering structure
    ISP peering structure

    • Using BGP table

      • AS level: whether two ASes peer with each other

    • Using Rocketfuel

      • Router level: where and how many places these two ASes exchange traffic

    • Skewed distribution

      • ISP typically peer in a lot of places with a small number of other ISPs, and peer in only a few places with the most of other ISPs

    Tool for measuring performance
    Tool for measuring performance

    • Throughput

      • iperf

    • Bottleneck link Bandwidth

      • Pathchar

      • Packet Pair (Bprobe/Nettimer)

    • Latency

      • Ping

        • One second resolution

        • Hping3 can provide a higher resolution

      • traceroute

    • Loss

      • tcpdump


    • Need to setup a client and a server

      • Iperf -s | -c <hostname>

    Bottleneck link bandwidth estimation
    Bottleneck Link Bandwidth Estimation

    • RTT variation

    • Dispersion of packet pairs/trains

    Rtt variation
    RTT variation

    s: data packet size

    ste: ICMP packet size

    bi: available bandwidth

    c: light speed

    fi: process packet


    Utilize RTT variation

    • increase the packet size and repeat (1) again

    • Estimate the link bandwidth by solving the linear equations obtained from (1)(2)

    • Repeat (1)-(3) for each link on the path

    • Find the minimum of (4)

    Packet pair ideal
    Packet Pair (ideal)

    • send a sequence of TCP probe packets

    • packets are queued before entering the bottleneck

    • a gap Pr=Pb is created by the bottleneck link

    • bottleneck link bandwidth = packet size / As

    Life is not perfect
























    Life is not perfect

    • Lots of noise will affect the estimated bandwidth!

      • Effect of cross traffic

        • Packets are not queued before the bottleneck (case B)

        • Packets are queued again after the bottleneck (case C)

      • Packets arrive out-of-order

        • Packets traverse different path

      • Bottleneck changes over the course of connection

      • Router does not use a FIFO queue

      • Clock resolution

    Filter the noise


    Filter the noise

    • Assumption: correct estimate will appear more frequent than incorrect ones

      • Choose the one has higher density

        • histogram (bprobe)

        • kernel density estimator (nettimer)

    Tool for measuring multicast
    Tool for measuring multicast

    • Mtrace (IGMP)

    • mHealth (RTCP + Mtrace)

    • Mlisten (RTP/RTCP)

    • RTPmon/RTPtools

    • Mantra


    • Multicast version of traceroute

    • Show the route from a receiver to the source

    • Traceroute

      • Based on increasing ICMP TTL

      • Does not work for multicast

        • ICMP TIME_EXCEED is typically disabled by multicast router

    • Use IGMP (Internet Group Management Protocol)

      • Multicast router keeps the state of incoming/outgoing interfaces of (S,G)

      • Reverse path lookup

        • Start at the receiver and trace back toward the source

    • Allow 3rd-party mtrace

    Reverse path lookup
    Reverse Path Lookup

    • Multicast IGMP Querypacket on ALL-ROUTERS multicast address (

    • The last hop router of the receiver begins a mtrace after receiving the Query packet

    • The last hop router appends its info and change the packet type from Query to Request

    • The last hop router forward the packet via unicast to the previous router, the incoming interface of (S,G)

    • Same process is repeated until the source is reached

    • The router that connects to the source appends its info and change packet type from Request to Response

    • Response packet is then sent to the mtrace initiator

    Rtp rtcp

    • RTP (Real Time Protocol)

      • TCP does not work for real time multicast

        • ACK implosion and timing requirements

        • Application Layer Framing (ALF): between Transport and Application

      • Commonly used in Mbone and streaming tools

      • Payload type ID, sequence numbering, timestamping

      • Consist of a data channel and a control channel (RTCP)

    • RTCP

      • A control protocol of RTP

      • Function

        • Deliver quality

        • Canonical name: synchronize data from multiple tools (audio/video)

        • Estimate group size

        • Distribution of group membership info

      • Packet format

        • Sender report

        • Receiver report

        • Source description

          • Canonical name

        • BYE


    • A graphical multicast monitor tool

    • Collect data of a MBone session

      • listen RTCP traffic to obtain group information and deliver quality

      • Use Mtrace to trace the hops from each receiver to the source


    • A tool for collecting info when members join and leave a multicast group

    • Continuously monitor well-known multicast address used to advertise Mbone session

    • For each session, Mlisten join the audio and video groups and collect control and data packets

    • For each packet received, Mlisten record

      • Sender

      • Session name

      • Time received

    • At periodic interval, Mlisten identify any session or group members who has no activity for a threshold of period (session: 2hr, member: 2 min) and record them


    • A tool that display the statistics of a RTP session by passively monitoring the RTCP traffic

      • Startup time

      • Sender

      • Receivers

      • Traffic statistics for each (sender,receiver) pair

        • Data sent

        • Loss

        • jitter

      • Route from the sender to a receiver (via Mtrace)


    • a number of applications that can be used for processing RTP data

    • rtpsend

      • generate RTP packets from a text file, generated by hand or rtpdump

    • rtpdump

      • capture and print RTP packets, generating output files suitable for rtpplay and rtpsend

    • rtpplay

      • play back RTP sessions recorded by rtpdump


    • A tool that collect multicast from multiple multicast-enabled routers

      • FIXW: the largest multicast exchange point in west coast of US

      • STARTAP: a core router between Interenet2 and commodity Internet

      • DANTE: an exchange point between US and European research backbone

      • ORIX

      • Router View


    • Data collection

      • MBGP (Multicast Border Gateway Protocol)

        • A router exchange protocol that propagate topology information between domains

      • DVMRP (Distance Vector Multicast Routing Protocol)

        • Within the same domain

      • MSDP (Multicast Source Discovery Protocol)

        • A protocol that propagates info about active sources

      • Router forwarding tables

    Tools for measuring wireless
    Tools for measuring wireless

    • Prismdump (or newer version of tcpdump)

      • 802.11

    • Ethereal (tcpdump with a GUI)

      • tethreal

    • netstumbler

    • wireless extension

    • Snort-wireless

      • A wireless intrusion detection system


    • A tool for detecting 802.11 WLAN

    • Usage

      • Verify if the WLAN is setup correctly

      • Detect other interfering WLANs in your area

      • Help aim directional antenna for long-haul WAN link

      • WarDriving

    Wireless extension
    Wireless extension

    • API that allows a driver to access to the configuration and statistics of WLAN

    • Components

      • User interface and tool

      • Driver interface

    User interface and tool
    User interface and tool

    • cat /proc/net/wireless

    • Iwconfig

    • Iwspy

      • For mobile IP test

      • Allow driver to add

        new addresses

    Driver interface
    Driver interface

    • Defined in /usr/include/linux/wireless.h

      • Example

        • get_wireless_stat

        • ioctl calls: SIOCSIWFREQ

    Measuring mobility
    Measuring mobility

    • GPS

    • Association/disassociation patterns from base stations/access points

      • Tools: SNMP, Syslog

    • Wireless signal strength

      • infer user location based on analysis of signal strength

        • triangulating


    (A:10, B:1) is a different location from (A:1, B:5)











    What is war driving
    What is War Driving?

    • One popular wireless measurement activity

    • Record the activities of wireless LANs from place to place

    • What do you need for War driving

      • a device capable of receiving an 802.11b signal (notebook w/ wireless card)

      • a device capable of moving around (some transportation)

      • A software that can log data (netstumbler/ethereal/GPS)

    • Then you just sit back and relax

      • You move these devices from place to place

      • Over time, you build up a database comprised of the network name, signal strength, location, and ip/namespace in use.

    What is wireless lan
    What is Wireless LAN?

    • It is a LAN

    • Extension of Wired LAN

    • Use High Frequency Radio Wave (RF)

    • Speed : 2Mbps to 54Mbps

    • Distance 100 feet to 15 miles

    Different version of 802 11
    Different version of 802.11

    • 802.11

      • IEEE family of specifications for WLANs

      • 2.4GHz 2Mbps

    • 802.11a

      • 5GHz, 54Mbps

    • 802.11b

      • Often called Wi-Fi, 2.4GHz, 11Mbps

    • 802.11e

      • QoS & Multimedia support to 802.11b & 802.11a

    • 802.11g

      • 2.4GHz, 54Mbps

    • 802.11i

      • An alternative of WEP

    • 802.11n

      • Antenna diversity

    Access points
    Access points

    • Access Point (AP)

      • A device that serves as a communications "hub" for wireless clients and provides a connection to a wired LAN

    • Beacon

      • Message transmitted at regular intervals by the Aps (100ms by default for many vendors)

      • Used to maintain and optimize communications to automatically connect to the AP

    Ad hoc mode
    Ad-hoc mode

    • Ad Hoc Mode

      • Wireless client-to-client communication, the opposite is Infrastructure Mode

    Infrastructure mode


    Infrastructure mode

    • Infrastructure Mode

      • A client setting providing connectivity to APs

      • As oppose to AdHoc Mode

    Basic service set


    (Basic Service Set Identifier)




    Basic service set

    • SSID or BSSID

      • Basic Service Set Identifier

    • BSS

      • An AP forms an association with one or more wireless clients is referred to as a Basic Service Set

    Extended service set


    (Extended Service Set Identifier)

    Extended service set

    • ESS

      • In order to increase the range and coverage of the wireless network, one needs to add more strategically placed APs to the environment to increase density. This is referred to as an Extended Service Set

    • ESSID

      • Extended Service Set Identifier

    The rfmon mode
    The RFMON mode

    • Like promiscuous mode in wired

      • Listen(Receive) only

      • Also known as “Monitor Mode”

      • You can capture raw 802.11 (such MAC-layer packets in this mod)

    • Many drivers now support RFMOD mode

      • Prism2

      • madwifi

    Snort wireless

    • Extended from Snort (an IDS for Internet) for wireless

    • allow one to specify custom rules for detecting specific 802.11 frames, rogue APs, AdHoc networks, and Netstumbler-like behaviour in the vicinity of the Snort-Wireless sensor

    Snort format
    Snort format

    • <action> wifi <mac> <direction> <mac> (<rule options>)

    • Use source and destination MAC address instead of IP address


    • tells Snort what to do when it finds a packet that matches the rule criteria

      • alert: generate an alert and then log the packet

      • Log: log the packet

      • pass: ignore the packet

      • Activate: alert and then turn on another dynamic rule

      • Dynamic: remain idle until activated by an activate rule , then act as a log rule


    • Format

      • Single MAC Address00:DE:AD:BE:EF:00

      • MAC Address List [00:DE:AD:BE:EF:00, 00:DE:AD:C0:DE:00, ....]


    • ->

      • From source to destination

    • <>

      • Both directions

    What info you can get from wireless packets
    What info you can get from wireless packets

    • timestamp

    • Signal strength

    • SSID

    • Sender/receiver

    • Retransmission

    • Mobility (association/disassociation)

    Received signal strength indication
    Received Signal Strength Indication

    • In arbitrary units (different vendors define it in different ways)

    • RSSI is typically used to determine when the amount of radio energy in the channel is below a certain threshold at which point the network card is clear to send (CTS).

    Noise floor
    Noise floor

    • Typically assumed as a constant

    • the noise power N = kTB

      • where k is Boltzmann's constant, T is the temperature in Kelvin, B is the system bandwidth

      • For a 20Mhz OFDM channel we have -174 + 10log10(20x106), or -101.7dBm thermal noise at the antenna. After including an additional 5dBm noise from the amplifier chain, we have -96dBm

    • RSSI 10: weak, 20: ok, 40: good

    • RSSI changes with time due to interference, channel fading etc.

    What is signal strength
    What is signal strength?

    • Four common units for measuring RF signal strength

      • mW

      • dBm

      • RSSI

      • percentage

    Mw dbm
    mW <-> dBm

    • dBm = log10(mW) x 10

    • Example

      • 100mW = log10(100) x 10 = 20 dBm

      • 50mW = log10(50) x 10 = 16.9 dBm

      • 1mW = log10(1) x 10 = 0 dBm

      • 0.5mW = log10(0.5) x 10 = -3.01 dBm

    • It’s cumbersome to talk about –96 dBm as 0.0000000002511 mW


    • 802.11 standard

      • A mechanism by which RF energy is measured on the circuitry of a wireless NIC

      • An allowable range from 0 to 255

    • In reality

      • No vendor actually measures 256 different signal strength level

      • Use RSSI_Max

        • Cisco: 100

        • Symbol: 30

        • Atheros: 60

    Use rssi
    Use RSSI

    • Chipset uses RSSI to decide if the channel clear

      • Clear channel threshold

      • Roaming threshold

    • RSSI_MAX is different from vendor to vendor

      • Clear channel/roaming threshold is different from vendor to vendor

    Granularity of rssi
    Granularity of RSSI

    • RSSI are discrete integer numbers

      • Can not represent all possible energy levels (mW or dBm)

      • Many vendors map RSSI to dBm because of the logarithmic nature of dBm



    Rssi dbm
    RSSI <-> dBm

    • Most vendors use a table to map RSSI to dBm

    • Atheros

      • dBm = RSSI – 95

    • Cisco

    Receive sensitivity
    Receive sensitivity

    • The minimum level of RF energy for the receiver to extract bit-stream

    • A NIC spec measured in dBm

    • Signal and noise are not distinguishable below receive sensitivity

    • Very close to RSSI=0

      • Impossible to measure RSSI=0

        • Can’t decode a ‘packet’

    • The higher data rate, the high receive sensitivity required

    Percentage metrics
    Percentage metrics

    • RSSI = RSSI_MAX * percentage

      • E.g. for Atheros card, 50% = 60 * 50% = RSSI 30

    • Good for site survey

    What is signal quality
    What is signal quality?

    • In 802.11b standard

      • PN code correlation strength

      • In the context of DSSS modulation

    • Symbol

      • Data bits + PN code (called spreading)

      • E.g. At 1Mbps/2Mbps

        • 1 single bit of data XOR’ed 11-bit-long PN code (Barker’s sequence, 101100111000)

    Symbol correlation
    Symbol correlation

    symbol for ‘1’ 101100111000

    received symbol 101100111001

    symbol for ‘0’ 010011000111

    received symbol 101100111001

    the received symbol is ‘closer’ to 1 than to 0

    signal quality == percentage of ‘correct’ bits

    == reflect the corruption between AP and client

    but not necessarily equal to SNR

    Things to know when making measurements
    Things to know when making measurements

    • It’s not just plugging in a box and then start sniffing traffic

    • Administrative issue

      • Privacy and security

    • Technical issue

      • Error and imperfections

      • Large volume of data

      • Reproducible results

      • Making data publicly available

    Error and imperfections
    Error and imperfections

    • Precision

      • Limited by the measurement devices

      • Clock precision

      • How much details to record

    • Accuracy

      • Packet drops during recording or filtering

      • Duplicate or re-ordering due to packet filter

      • Clocks

        • Un-synchronized clocks

        • Buffered packets at NIC

      • Effect of middle-box

      • Trace edge-effect

    • Representative data


    Consider a tcpdump record

    1092727442.276251 IP >

    How precise is it?

    Answer: at most 1 us, but perhaps much less

    How precise is the packet captured by tcpdump
    How precise is the packet captured by tcpdump?

    • Snapshot length limits the total data

    • filtering

    Maintain meta data
    Maintain meta data

    • E.g. when, where, how the traces are recorded

    • Giving the measurements a context

    • Meta data is important when the measurement is used by other people later for different purposes

    • Existing tools are weak here

      • Can be your potential project topic


    • An even harder problem than precision

    • Examples

      • Clock

        • arbitrarily off from true time

        • Jump forward or backward

        • Fail to move

        • Run arbitrarily fast or slow

      • Packet filter

        • Drop packets

        • Fail to report drops

        • Report drops that did not occur

        • Reorder packets

        • Duplicate packets

        • Record the wrong packets

    Not measuring what you think you re measuring
    Not measuring what you think you’re measuring

    • Examples

      • Measuring TCP packet losses by counting retransmission

        • Packets can be replicated by the network

      • Counting TCP connection size by counting the difference between SYN and FIN

        • What if the remote host was down?


    • Detect problems of precision/accuracy/misconception

    • Goal: Fix these problems post facto

    • Identify and remove faulty measurements

      • Find the outliers

        • E.g. what are the biggest and smallest RTT in the measurements?

    Techniques to detect inaccuracy
    Techniques to detect inaccuracy

    • Examine outliers and spikes

      • Outliers: unusually low or high values

      • Spikes: values that appear a lot

      • E.g. extremely small RTT or extremely large connection

    • Consistency check

      • Compare against normal protocol/traffic behavior

    • Comparing multiple measurements

      • From different time

      • From different places

    • Use synthetic data to verify the correctness of software

    Self consistency check
    Self-consistency check

    • Check against the expected protocol behavior

    • E.g. if a TCP receiver acknowledged data never sent, something must be wrong

      • Filter drops the data

      • Packet took another route

      • Data was sent before you measured

      • The TCP receiver is broken

    Compare multiple measurements
    Compare multiple measurements

    • Compare packets at both ends

    • Compare packet headers with payload

    • Compare measurements collected at different times

    Large volume of data
    Large volume of data

    • Disk space

    • Number of files

    • Process time

    • Memory usage

    • Maximum file size

      • 2G for older version of Linux

    • Software limitation

      • The number of data points can be input

    • Statistical limitation

      • Large datasets do not have statistically exact description

    • Tip: early analysis with a smaller dataset

    Re producible analysis
    Re-producible analysis

    A typical scenario

    • you collected the measurements, did the analysis and submitted the results to a conference

    • Months later, you got a feedback from the reviewer that asks you to re-do the measurements with a tweak

    • What would you do?

      • Introduce the tweak, re-crunch the numbers, update the table and then call it done

      • Or, you first re-run your scripts to understand how you got those numbers in the first place


    • For a good-sized measurement study, you often can not re-produce the exact earlier numbers…

      • You’ve lost the previous mental context of fudge factors, glitch removals, script inconsistency

        • Ad hoc notes

        • Removal of outliers

        • Random fixes

        • Different versions of analysis scripts

        • Rounding the numbers


    • One single master that builds all results from raw data

    • Keep intermediary form of the data

    • Maintain a notebook

      • What have been done and what happened

    • Use version control

    • Need a way to visualize the changes after the re-run

      • Another potential project topic

    Make data publicly available
    Make data publicly available

    • Comment details about how measurements were taken

      • Where and when

      • Link properties (speed, utilization, loss, etc.)

    • Include analysis scripts that were used

    • Anonymization

      • Security, privacy, business sensitivity

    • Data-reduction request

    Measurement infrastructure
    Measurement infrastructure

    • Administrative issues

      • It’s not easy to get fresh data by yourself

    • Places where you can get some existing data

      • NLANR

      • ITA

      • MAWI

      • NIMI

      • CAIDA

      • Internet 2


    • Passive Measurement Analysis (PMA)

    • Active Measurement Project (AMP)


    • Collect passive IP header trace ranging from OC3 to OC192 links

    • Each monitor captures a unique portion of overall network data

    • Capture 8 samples per day

    • 2 minutes per sample

    • 3.2G data per day

    • A number of OC48 long, continuous traces

      • From 1 hour to 45 days


    • 150 sites in US and some in other countries

    • Site to site measurements

    • Two meshes

      • HPC mesh (all in US, ~140 sites)

      • International mesh

    • Data measured

      • round trip time (RTT)

      • packet loss

      • topology

      • throughput

    Internet traffic archive ita
    Internet Traffic Archive (ITA)

    • Founded by Vern Paxson since 1996

    • Mainly are Web traces (and some wide-area TCP and traceroute traces)

    • Most traces are in the format of tcpdump or http log

    • Trace duration ranges from 2 hours to 6 months

    • Related software

      • tcpdpriv

        • Remove private information of tcpdump

      • tcp-reduce

        • a collection of shell scripts for reducing a tcpdump trace file to a summary of the corresponding TCP connections.

      • tracelook

        • a program for graphically viewing tcpdump traces.

    Mawi wide project
    MAWI (WIDE project)

    • Japan research efforts

    • Traffic from several trans-Pacific T1 lines, an US-Japan OC-3 line and 6Bone

      • Daily traces

      • 2 million packets per hour for trans-pacific lines

      • 6Bone traffic is still light (mainly BGP and ICMPv6)

    • Traces are in tcpdump format and anonymized with tcpdpriv

    Nimi national internet measurement infrastructure
    NIMI (National Internet Measurement Infrastructure )

    • A set of measurement servers (probes) running on a set of hosts

      • Function

        • Receive and authenticate request

        • Execute the request at the appropriate time

        • Send the result back the requester

      • Daemon

        • nimid: communicate with outside world

        • scheduled: scheduling, execute measurements and packaging results

    • CPOC (Configuration Point Of Contact)

      • Configure and administer a set of NIMI probes in the same administration domain

    • Measurement client (MC)

      • A tool that allow end-user to send measurement request to NIMI probe

    • Data Analysis Client (DAC)

      • Where the measurement results are returned

      • The address of DAC is included in the request sent by MC

    Caida cooperative association for internet data analysis
    CAIDA (Cooperative Association for Internet Data Analysis )

    • Affiliated with UCSD

    • Provide tools, data and analysis for research community

    • Data sources

      • Exchange points: e.g. San Diego Network Access Point (SD-NAP)

      • Data from FIX-West

      • routing data from University of Oregon's Route Views project ( and Merit's IPMA (

      • active measurement from skitter

    Internet 2
    Internet 2

    • Goal

      • A large-scale edge network for research community

      • Enable revolutionary application

      • Transfer new application/service to commercial Internet

    • Consist of 207 universities connected by 3 networks: Abilene, Quilt, ARENA

    • The participants collaborate with each other on studying and identifying, developing, and testing advanced network services, applications and technologies

    • Focus on end-to-end performance measurements

      • Active: routing, delay, loss

      • Passive: SNMP, Netflow