Characterizing hop-by-hop and end-to-end performance in a
static wireless multihop environment
Ken Uchida
Ken.Uchida@cobaltech.org
Networking Laboratory
Helsinki University of Technology
P.O. Box 3000, 02015 HUT, Finland
Master’s Thesis: HUT, Networking Laboratory, Espoo, 2006
Supervisor: Prof. Jörg Ott
Presentation: S-38.3310 Research Seminar, 05.09.2006
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 1
Presentation Overview
Agenda
• The goal of this thesis
• Overview of the Internet and TCP/IP
• Transport layer protocols – TCP & UDP
• Emerging networks
• The concept of DTN
• Measurement environment and setup discussion
• Conclusions
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 2
The Goal of this thesis
Main goal: The main goal to study per-hop and hop-by-hop performance
in a static wireless multihop environment.
We have constructed wireless chain network using off-the-shelf products
running embedded Linux systems. This network acts as a platform for
TCP, UDP and DTN measurements.
In the thesis:
• Construct a static wireless multihop network running custom firmware in
wireless routers and cross-compile/compile measurements tools
• Determine link characteristics based on measures
• Perform TCP, UDP and DTN measurements on per-hop and hop-by-hop basis
• Address some practical issues faced during measurements
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 3
The Internet and TCP/IP
•
The Internet – the network of the networks
•
Three layers of protocols
– At the lowest level Internet Protocol (IP)
• Current version IPv4, shifting towards IPv6 ”in progress”
– Transport layer Protocols – TCP & UDP
• TCP provides reliable connection-oriented transfer
• UDP provides connections, best-effort service
– Applications protocol
•
The usability of the Internet depends on some important assumptions:
–
–
–
–
Continuous, bidirectional end-to-end-paths
Short round-trips
Symmetric data rates
Low Error rates
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 4
Transport layer protocols – TCP
• One of the core protocols of the Internet Protocol Suite
• Support many popular applications
– Web, E-mail, Secure shell, etc.
• Plays a dominant role in the Internet
– Accounts for about 90% of the bytes carried by the Internet [1]
•
Provides for upper layer applications:
– Reliability
– End-to-end connectivity
– In-sequence data delivery
[1]: Ad Hoc Networks – Technologies and Protocols, P. Mohapatra, S. V. Krishnamurthy, Springer Verlag, 2005
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 5
Transport layer protocols – UDP
• One of the core protocols of the Internet Protocol Suite
• Minimalistic message-oriented protocol
– No out-of-order detection
– No mechanism for error recovery
– No flow control
• Common network applications that use UDP
– Domain name system (DNS), streaming media applications, Voice over IP,
Trivial file transfer protocol (TFTP), some p2p clients, etc.
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 6
Emerging Networks
•
•
While the Internet plays a dominant role in networks, there are still evolving
networks outside the Internet
Examples of these are:
– Terrestrial civilian networks connecting mobile users
– Wireless military battlefield networks
– Outer-space networks like InterPlaNetary project IPN
•
Problems relating to them are:
– Hazardous, challenged environments
– High bit error rates
• Wireless networks - TCP protocol performance issues
– Intermittent connectivity – no end-to-end guarantee all the time
• Sparse ad hoc networks
• Sensor-nets
• Low Earth orbit satellites
– Longer delays
• Satellite connections
• Inter-planetary communications
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 7
The Motivation for DTN
Many evolving and potential networks do not conform to the Internet’s underlying
assumption. These networks are characterized by:
• Long or variable delay
• Intermittent connectivity, no end-to-end assumption
• Asymmetric data rates
• High error rates
DTN address to these issues by:
• No end-to-end assumption
• Better recovery on device failures
• Long delays
• Minimum end-to-end message exchanges
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 8
DTN Architectural Issues
•
An overlay network
– DTN implementation has ”bundle” layer above transport layer to inter-network
between different heterogeneous networks
•
•
•
End-to-end message delivery
– Virtual message-switching as opposed to packet-switching
– Based on store-and-forward architecture
Voicemail and e-mail style store-and-forward architecture
– The use of persistent storage - Messages are saved in persistent storage in case of
network failures, device reboots, etc.
– Whole messages or pieces of messages are forwarded from a storage place on one
node to a storage place on another node forming a path to destination
A non-conversational protocol
– Designed to use simple sessions with minimal or no round-trips to fight against
long delays in conversational protocols
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 9
Measurement environment & setup
Measurement environment and setup discussion
•
•
•
•
•
•
•
Hardware architecture
Why OpenWRT?
Network configuration
Network placement
Channel interference & operation modes
Determining link characteristics
Measurement software
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 10
Architecture of WRT54GS
Linksys WRT54GSv4 router – Technical specs:
• 802.11b/g + Speedbooster access point
• 5-port 100Mbps Ethernet switch
• BroadCom 200 MHz MIPS processor
• RAM 16MB / NVRAM 4MB for software
• Off-the-shelf product – price about 70€
• Has lots of 3rd party firmwares (DD-WRT, HyperWRT,
OpenWRT, Sveasoft, etc.)
• Firmware source code under GNU GPL licence
 Allows compiling custom firmwares with Buildroot
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 11
OpenWRT – Custom Linux Firmware
Why OpenWRT was selected?
•
•
•
•
•
By default uses Busybox command line tool
 Fully-fledged bash-shell
Not dependant of GUI
Minimal memory use, more memory for applications
Uses Debian style packet manager (ipkg)
 Easy to install/maintain packages
Has huge variety of add-on packages
 Allows customizing to specific needs
Brings Linksys WRT54GS closer to enterprise level products:
(network auditing/monitoring/penetration tools, Asterisk PBX, NFS support,
etc.) [1]
[1] http://linuxdevices.com/news/NS9515501295.html
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 12
Network Configuration
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 13
Measurement Placement
HUT’s Networking laboratory layout – Routers placement
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 14
Channel interference and operating modes
• Channel interference issues
– 802.11b and 802.11g operating modes divide the spectrum into 14
channels (All continents)
– The common concept is to use channels 1, 6 and 11 to maximize
separation of channels (Europe)
– According to channel environmental scan, channels 1 and 11 were most
populated
• We chose to use channel 6
• Wireless LAN operating modes
– Wireless LAN were set to operate at 802.11g mode by setting
correspondent variables in OpenWRT
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 15
Determining Link Characteristics
• Throughput
– Throughput based on both TCP and UDP measurements
• Bandwidth asymmetry
– Determined by running measurements in both directions on both TCP
and UDP measurements – from the client to the server and vice versa
• Packet loss rate
– Determined in the UDP measurements
• End-to-end delay
– Determined in the DTN measurements
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 16
Measurement Software – TCPX/TCPB, RTP
 TCP measurements:
• tcpx tool were run at the both ends of the connection
 Operation based on the traffic specification – defines segment size and wait
time
• tcpb bridge tool were run in wireless routers
 Option to delay incoming traffic before each transmission
 Allows bundling incoming traffic rather forwarding immediately
 Cross-compiled with x86 OpenWRT buildroot to MIPS-platform
 UDP measurements:
• rtpsend – the source application to generate UDP traffic
 Interval, the delay between each frame
 Frame size
• rtpspy – the sink for UDP traffic
 provides statistics of incoming traffic
 The source codes for these programs were provided by Prof. Jörg Ott.
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 17
Measurement Software – DTN2
• DTN2 – and experimental platform and a reference implementation of
the DTN protocols
• Has been written primarily in C++ and ported to platforms like Linux,
Solaris, Win32 Cygwin, FreeBSD, Mac OS X, etc.
• Implementation features include tcl-interpreter and flexible storage
interface like BerkeleyDB, MySQL, PostGres or external filesystem.
• The DTN2 implementation on constant development, the current
version used was 2.2.0.1
• The reference implementation ported to Linksys MIPS-architecture by
Laurent Franck, Felipe Gil Castineita and Simon Paillard
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 18
Conclusions
 Throughput
 At most 25 Mbit/s in any circumstances
 The use of multihop drops significantly throughput performance
 Bandwidth asymmetry
 In TCP and UDP measurements bandwidth asymmetry detected in every case
 Packet loss in UDP
 Heavily dependent of sending rate delay after 10 ms
 100 ms threshold value after packet losses disappear
 End-to-end delay

No constant end-to-end delay guarantees provided for applications
 Deviations in averaged end-to-end delay are large
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 19
Conclusions – practical issues
 Operating in open environment
• Expect to face interference from the wireless users especially
operating at the same channel
• Cross-interference was detected in our measurement – the battle of
single-radio networks
 The limitations in wireless routers
• While using Linux broadens possibilities of use in Linksys routers, the
computational power is still limiting factor
• Internal permanent memory very limited  occasional crashes in DTM
measurements
 Predicting wireless behavior non-practical issue in open environment
• Would mean to predict wireless users behavior
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 20
Future work
 Implementation of a SD-card or USB-port modification for external
persistent storage
 Would solve persistent storage limitation in Linksys routers
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 21
Thank you for
your attention!
Questions?
Nicklas Beijar - Distribution of Numbering Information in Interconnected Circuit and Packet Switched Networks
Slide 22