Kunchan Lan

1
Network Measurements, Modeling and Simulations

• Kun-chan Lan
• Department of Computer Science and Information
  Engineering
• klan_at_csie.ncku.edu.tw

2
Some Admin stuff

• Paper review list due next week
• The references for homework is post on the course
  webpage
• We will have a guest speaker on 3/18 to talk
  about how to do game measurements (related to
  your homework 2) No class lecture

3
A quick survey

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

4

• 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!)

5
Outline

• Model and simulate Internet traffic
• Its hard to model and simulate Internet
• Use measurement to improve the realism of your
  model
• We advocate trace-driven simulation
• Internet and wireless measurements

6
The challenges in modeling and simulating
Internet traffic
7
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.
• ..

8
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

9
What simulation doest do

• Realism
• Details of simulation matters!
• Its 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

10
Its hard to simulate Internet

• Network heterogeneity
• Rapid and unpredictable change

11
Network heterogeneity

• Topology
• Link properties
• Protocol
• traffic
• All the above matter when you do the simulation

12
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

13
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

14
Difficulty in modeling protocol

• Differences in implementations
• 400 different TCP implementations
• Different applications and different traffic mix

15
Difficulty in modeling traffic

• Traffic is different everywhere
• Effect of background traffic
• Queuing, congestion
• Some application are adaptive to network
  conditions

16
Rapid and unpredictable changes

• Change in TCP Reno -gt NewReno/SACK
• Change in devices PC-gthandheld
• Change in web caching -gt CDN
• Change in killer applicaton
• web-gtp2p-gtVoIP?
• Change in physical layer wired -gt wireless

17
Coping strategy

• OK, so its hard to simulate Internet, but can we
  do something about it?
• Yes
• Systematically explore important parameters
• Searching for invariants

18
Network behavior as a function

• Explore network behavior as a function of
  changing parameters
• ltobserved trafficgt 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.)

19
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!

20
Question?
21
Outline

• Model and simulate Internet traffic
• Its hard to model and simulate Internet
• Internet and wireless measurements
• Case study modeling heavy-hitter traffic

22
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)

23
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!
24
TCP over GPRS network
How fair is TCP over GPRS?
25
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?

26
Measure the Internet

• What can you measure
• Traffic
• Routing
• Topology
• Performance
• Multicast
• Wireless/Mobility

27
Tool for measuring traffic

• Tcpdump/etherreal (libpcap)
• Netflow
• NetTrMet/RTG (SNMP)

28
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.

29
Usage of tcpdump

• tcpdump -adeflnNOpqStvx -c count
• -F file   -i interface -r file -s
  snaplen
• -T type -w file expression
• Must run as root or have sudo permission

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ltoptiongt

• -i Listen on interface. If unspecified, tcpdump
  searches the system interface list for the lowest
  numbered, configured up interface (excluding
  loopback)
• -n Don't convert addresses (i.e., host
  addresses, port numbers, etc.) to names

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ltoptiongt

• -p Don'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
  -''.

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ltoptiongt

• -w Write 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 -''.
• -r Read packets from file (which was created
  with the -w option). Standard input is used if
  file is -''.
• -S Print absolute, rather than relative, TCP
  sequence numbers

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ltoptiongt

• -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.

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ltoptiongt

• -t Don't print a timestamp on each dump line.
• -tt Print 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.
• -vv Even more verbose output. For example,
  additional fields are printed from NFS reply
  packets.
• -x Print each packet in hex.

35
ltexpressiongt

• 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.
• lttypegt ltdirgt ltprotogt

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ltqualifiergt

• lttypegt
• what kind of thing the id name or number refers
  to
• Possible types are host, net and port
• E.g., host csie.ncku.edu.tw', net 146.132',
  port 20'
• If there is no type qualifier, host is assumed.

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ltqualifiergt

• ltdirgt
• specify a particular transfer direction to and/or
  from id.
• Possible directions are src, dst, src or dst and
  src and dst.
• E.g., src csie.ncku.edu.tw', dst net 146.132',
  src or dst port ftp-data'.
• If there is no dir qualifier, src or dst is
  assumed

38
ltqualifiergt

• ltprotogt
• 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 server1.ncku.edu.tw', arp net
  128.3', tcp port 21'.
• If there is no proto qualifier, all protocols
  consistent with the type are assumed. E.g., src
  mail.ncku.edu.tw' means (ip or arp or rarp) src
  mail.ncku.edu.tw'

39
Complex expression

• complex filter expressions are built up by using
  the words and, or and not to combine primitives.
• E.g., host csie.ncku.edu.tw 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'.

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Allowable primitives

• dst host host
• src host host
• host host
• ether dst ehost
• ether src ehost
• ether host ehost
• gateway host

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Allowable primitives

• dst net net
• src net net
• net net
• net net mask 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

42
Allowable primitives

• less length
• True if the packet has a length less than or
  equal to length. This is equivalent to len lt
  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

43
Allowable primitives

• ether multicast
• ip multicast
• ip, arp, rarp, decnet
• short for ether proto p where p is one of the
  above protocols.
• tcp, udp, icmp
• short for ip proto p

44
Relation operator

• expr relop expr
• relop is one of gt, lt, gt, lt, , !
• 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. tcp0 means the first byte of the
  TCP header
• For example, ether0 1 ! 0' catches all
  multicast traffic. The expression ip0 0xf !
  5' catches all IP packets with options.

45
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 ace is short for not host
  vs and host ace, which should not be confused
  with not ( host vs or ace )

46
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.

47
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 gt t
• Long-lived flows (duration gt 30min)
• TCP connections with FIN or RST
• when cache becomes full (applying some heuristics
  to age flows)

48
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

49
NetTrMet

• 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

50
RTG

• 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

51
Tool for measuring routing

• Traceroute
• tracert command for Windows
• RouteView

52
traceroute

• 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.

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(No Transcript)
54
RouteView

• 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

55
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

56
Path vector protocol
AS100 180.10.0.0/16 100
AS200 180.10.0.0/16 200 100
time
AS300 180.10.0.0/16 300 200 100
57
Tool for measuring topology

• traceroute-based
• Skitter
• Rocketfuel

58
Skitter

• 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
• collected

59
Rocketfuel

• Input
• traceroute (utilizing public available tracroute
  servers)
• BGP
• DNS
• Output (per ISP)
• Backbone
• POP
• Peer links

60
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

61
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

62
Use BGP to choose traceroute
destination
AS path
closer to destination ?
1.2.3.0/24 8 11 4 2 5

6 2 5

• traceroutes that are likely to traverse AS 2
• from servers in AS 8, 11, 4, 6 to prefix
  1.2.3.0/24
• If ALL paths to 1.2.3.0/24 includes AS 2
• from anywhere to 1.2.3.0/24
• from 1.2.3.0/24 to anywhere

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Path reduction

• Skip repeated traces of the same path
• Same destination, same ingress point
• Same ingress point, same egress point

64
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

65
Alias resolution

• Alias traceroute reports the IP address of the
  interface on the router (not the router!)
• The router might have multiple interfaces
• Routers interfaces may be numbered from entirely
  different IP prefixes
• Need to know interface 1 and
• 2 are on the same router

66
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

67
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 lt y lt z, and z x is small

68
Identify router location

• Utilize DNS names
• ISP typically use certain naming convention to
  name their routers
• s1-bb11-nyc-3-0.sprintlink.net
• A Sprint backbone router (bb11) in New York city
  (nyc)
• p4-0-0-0.r01.miamfl01.us.bb.verio.net
• A Verio backbone router (bb) in Miami, Florida
• s1-neighborname.sprintlink.net
• A neighboring router of Sprint

69
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

POP
70
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

71
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

72
iperf

• Need to setup a client and a server
• Iperf -s -c lthostnamegt

73
Bottleneck Link Bandwidth Estimation

• RTT variation
• Dispersion of packet pairs/trains

74
RTT variation
s data packet size ste ICMP packet size bi
available bandwidth c light speed fi process
packet
75
Pathchar

• 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)

76
Packet Pair (ideal)

• send a sequence of TCP probe packets
• packets are queued before entering the bottleneck
• a gap PrPb is created by the bottleneck link
• bottleneck link bandwidth packet size / As

77
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

78
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)

79
bprobe
80
Tool for measuring multicast

• Mtrace (IGMP)
• mHealth (RTCP Mtrace)
• Mlisten (RTP/RTCP)
• RTPmon/RTPtools
• Mantra

81
Mtrace

• 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

82
IGMP
83
Reverse Path Lookup

• Multicast IGMP Query packet on ALL-ROUTERS
  multicast address (224.0.0.2)
• 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

84
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

85
Mhealth

• 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

86
Mlisten

• 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

87
RTPMon

• 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)

88
RTPtools

• 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

89
Mantra..

• 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

90
..Mantra

• 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

91
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

92
netstumbler

• 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

93
Wireless extension

• API that allows a driver to access to the
  configuration and statistics of WLAN
• Components
• User interface and tool
• Driver interface

94
User interface and tool

• cat /proc/net/wireless
• Iwconfig
• Iwspy
• For mobile IP test
• Allow driver to add
• new addresses

95
Driver interface

• Defined in /usr/include/linux/wireless.h
• Example
• get_wireless_stat
• ioctl calls SIOCSIWFREQ

96
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

97
triangulating
(A10, B1) is a different location from (A1,
B5)
10
10
A
B
10
10
5
5
1
1
98
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/etherea
  l/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.

99
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

100
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

101
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

102
Ad-hoc mode

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

103
Infrastructure mode

• Infrastructure Mode
• A client setting providing connectivity to APs
• As oppose to AdHoc Mode

104
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
105
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

106
Non-overlapping channels
107
DSSS Channel
108
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

109
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

110
Snort format

• ltactiongt wifi ltmacgt ltdirectiongt ltmacgt (ltrule
  optionsgt)
• Use source and destination MAC address instead of
  IP address

111
ltactiongt

• 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

112
ltmacgt

• Format
• Single MAC Address00DEADBEEF00
• MAC Address List 00DEADBEEF00,
  00DEADC0DE00, ....

113
ltdirectiongt

• -gt
• From source to destination
• ltgt
• Both directions

114
What info you can get from wireless packets

• timestamp
• Signal strength
• SSID
• Sender/receiver
• Retransmission
• Mobility (association/disassociation)

115
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).

116
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.

117
What is signal strength?

• Four common units for measuring RF signal
  strength
• mW
• dBm
• RSSI
• percentage

118
mW lt-gt 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
• Its cumbersome to talk about 96 dBm as
  0.0000000002511 mW

119
RSSI

• 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

120
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

121
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

dBm
5mW
122
RSSI lt-gt dBm

• Most vendors use a table to map RSSI to dBm
• Atheros
• dBm RSSI 95
• Cisco

123
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 RSSI0
• Impossible to measure RSSI0
• Cant decode a packet
• The higher data rate, the high receive
  sensitivity required

124
Percentage metrics

• RSSI RSSI_MAX percentage
• E.g. for Atheros card, 50 60 50 RSSI 30
• Good for site survey

125
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 XORed 11-bit-long PN code
  (Barkers sequence, 101100111000)

126
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

127
Question?
128
Things to know when making measurements

• Its 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

129
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

130
precision

• Consider a tcpdump record
• 1092727442.276251 IP 192.168.0.12022 gt
  192.168.0.1379320
• How precise is it?
• Answer at most 1 us, but perhaps much less

131
How precise is the packet captured by tcpdump?

• Snapshot length limits the total data
• filtering

132
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

133
Accuracy

• 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

134
Not measuring what you think youre 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?

135
Calibration

• Detect problems of precision/accuracy/misconceptio
  n
• 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?

136
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

137
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

138
Compare multiple measurements

• Compare packets at both ends
• Compare packet headers with payload
• Compare measurements collected at different times

139
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

140
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

141
But

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

142
Strategies

• 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

143
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

144
Measurement infrastructure

• Administrative issues
• Its not easy to get fresh data by yourself
• Places where you can get some existing data
• NLANR
• ITA
• MAWI
• NIMI
• CAIDA
• Internet 2

145
NLANR

• Passive Measurement Analysis (PMA)
• Active Measurement Project (AMP)

146
PMA

• 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

147
AMP

• 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

148
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.
  

149
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

150
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

151
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 (www.antc.uoregon.edu/route-views)
  and Merit's IPMA (www.merit.edu/ipma/)
• active measurement from skitter

152
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

153
Wireless traces

• Crawdad (Dartmouth)
• Mobilib (UFL)
• Reality Mining (MIT)
• DiselNet (UMass)