R V S Network Emulation Torsten Braun Computer Networks and Distributed Systems Institute of Computer Science and Applied Mathematics University of Bern, Switzerland www.iam.unibe.ch/~rvs Ecole d’été Internet Nouvelle Génération, June 14-18, 2004, Obernai
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RVS
Network Emulation
Torsten Braun
Computer Networks and Distributed SystemsInstitute of Computer Science and Applied Mathematics
University of Bern, Switzerlandwww.iam.unibe.ch/~rvs
Ecole d’été Internet Nouvelle Génération, June 14-18, 2004, Obernai
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Overview§ Motivation§ Implementation and Live Tests§ Simulation§ Emulation
l Types of Emulationl Detail of Emulationl Centralized vs. Distributed Emulationl Layers of Emulationl Requirements on Emulation
§ Examplesl Simple Delay Line Modelling§ Network Emulator for Adaptive Applications,
l Emulation in Mobile Networks§ Trace-Based Mobile Network Emulation, MobiEmu
§ Conclusions§ References
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Motivation§ Networks do not only get faster but
become more diverse and carry diverse traffic. § Networks vary
l in bandwidth, l latency, l error and loss rates
and may be asymmetric. § Application demands vary
l Real-time requirementsl Reliability
→ Adaptive applicationsl Behaviour is often not deterministic. l New adaptive applications and protocols need to be evaluated
in non-simplistic network environments that are reproducible. § Evaluation
l Implementation and Live Testsl Simulationl Emulation
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Implementation and Live Tests§ Real environment for running code§ Advantages
l Most realistic environment
§ Problemsl Difficult and expensive to set upl Limited in size and complexityl Interference with production networks l Reproducibility l Restricted to existing technologies
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Simulation§ Synthetic environment for running
representations of code§ Advantages
l Full control over target platforml Investigation of complex network topologies and
conditionsl Not limited by speed of simulation hardwarel Low costl Flexibility
§ Problemsl Modelling of traffic (→ traffic traces)l Independent specification of network code.
Simulator might fail to mimic subtleties of real code. l Missing system interactions
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Emulation
§ Combination of simulation and implementation§ Semi-synthetic environment for running code
l Real network implementation and supplementary means for introducing synthetic delays and faults
l Provides a virtual network to networked devices and applications
§ Applicationsl Debuggingl System designl Development of new protocolsl Performance evaluation
Emulation
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Emulation§ Advantages
l Applications can run on unmodified real devices/systems.l Movement of code between emulated and real networkl Deployment of unmodified software prototypes
in a configurable Internet-like environment§ Important for scalable and reliable Internet services such as
peer-to-peer and overlay networks
l Configurable, controlled, reproducible environmentl Generation of real traffic
§ Problemsl Simulated time = real-time → real-time requirementsl Speed of emulation is limited by underlying simulation hardware.l Mutual interactions between emulation processesl Complexity of network topologies is limited.
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Types of Emulation[Fall99]
Network Emulation§ Simulated components communicate with
protocol implementation in the real world.
Environment Emulation§ Extension of network emulation§ Implementation environment, in which
a real protocol implementation may be executed directly within the simulator
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Detail of Emulation§ Modelling of networks as simple delay lines
l Requires little hardware supportl Examples: § Network Emulator for Adaptive Applications,
Hitbox, ONE, Dummynet, NIST Net
§ Real-time network simulation and detailed modelling of virtual networksl Real network traffic can pass through emulator. l Interaction with synthetically generated traffic within
Centralized vs. Distributed EmulationCentralized Implementation§ Problem
l System bottleneck
§ Advantagesl Dynamic scenariosl Adjustment of network characteristics
(e.g., packet collisions)l Preserves packet order
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Layers of Emulation§ Transport Layer Emulation
l Reproduction of process communication channel characteristics such as TCP channels
l Can be used to measure performance impacts on applications
§ Network Layer Emulationl Mimics end-to-end behaviour of a network
connecting hosts such as packet delays and lossl Can be used for evaluation of applications and
transport protocols
§ Link Layer Emulationl Emulation on single network links such as
bandwidth, frame delays etc. l Allows network layer protocol evaluation
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Requirements on Emulation§ Mimic behaviour of networks and links
as closely as possible§ Emulated network should be transparent to
real devices and applications.§ Network model has to consider
dynamic changes. § Feeding of network traffic may create
some side effects (overhead, delay), which should be minimized.§ Keeping pace with real world events § Scalability
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Network Emulator for Adaptive Applications§ Emulator intercepts UDP packets and introduces a
delay as if the packet would have been transmitted over a slower network [Davies95]l Emulator calls function new_network_name_send (senderNodeID, data)
l Behaviour can be changed dynamically via configuration files. (requires control packet exchange)
§ Packets are routed via a single central point. l Advantage: can easily adapt to network load
§ Emulator requires special data / control packet format to indicate emulator processing.→ modified sendto / recvfrom socket calls to be used by
applications (→ dynamic linking)
§ Implementation on Sparc1 / SunOS 4.1l strong dependency of performance (accuracy)
on packet length and link bandwidth
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Hitbox
§ Hitbox [Ahn95] is added to operating system as pseudo device, which gives programs a handle for parameter (delay, bandwidth, buffer size, drop rate) changes.
§ Installation of hitbox by changing function pointers in output routines
§ Emulation of bidirectional connections requires a pair of hitboxes in two hosts.
§ Hitbox computes delay to model queuing, transmission, propagation delays, assigns send time to the packet, and puts it into a queue.
§ Modification of BSD scheduling resolution (10 → 1 ms)
IP ARP ICMP
Hitbox
Network interface
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Ohio Network Emulator
§ ONE [Allman97] models network by delaying packets arriving on one network interface before forwarding to the other. § Delay components
l Transmission delayl Queuing delayl Propagation delay
§ Experimentsl Difference between expected and measured
transmission / propagation / queuing delay < 10 ms
Emulator
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Dummynet§ Dummynet [Rizzo97/98]
l intercepts packets of the protocol layer under test (e.g. ip_output, tcp_input at the interface between TCP and IP) and
l simulates effects of finite queues (routers), bandwidth limitations, communication delays, lossy links (network links).
§ Packet Processingl Packets are put to rq (limited queue size k)l Packets are moved from rq to pq at maximum rate Bl Packets remain at pq for tp seconds
§ Limitationsl timer granularityl Periodic tasks might run late → real-time OS
§ experience with FreeBSD: rare events§ Extensions: Filtering rules (ipfw) and multiple addresses of a single
interface allow simulation of complex topologies on a single computer.
rq_out
pq_out rq_in
pq_in
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NIST Net§ „network in a box“ [Carson03]§ Specialized router, which emulates statistically
an entire network in a single hop§ NIST Net applies network effects to passing traffic
based on user-supplied settings.
WAN / Internet
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NIST Net Emulator Entries§ Table of matching specifications for packets
(IP and higher layer fields)l Implementation as two-level hash table (default size: 256)
§ Set of effects to be applied to matching packetsl Delay (e.g., heavy tailed, multi-fractal wavelet model)l Lossl Jitterl Reorderingl Duplicationl Bandwidth limitations
§ Statistics about packets matched this entry
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NIST Net Architecture
Main parts1. (Re)Loadable kernel module
l hooks into normal Linux networking and real-time clock codel implements run-time emulatorl exports set of control APIs
2. User interfacesl use APIs to configure and control operation of kernel emulator
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VINT/nse§ Virtual Internet Testbed [Breslau00, Fall99]§ ns (network simulator): widely used discrete event simulator§ nse: emulation with ns simulation engine§ ns includes protocols, buffer management, scheduling etc.
→ network simulation (no environment simulation)§ Challenges
l Real-time synchronization§ Dispatching of pending events at appropriate real-time
by introducing real-time delaysl Packet capturing
§ Architecture
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VINT/nse: Emulation Objects § Interface between ns and
network traffic§ Special Objects for
tunneling live packets inside nsl Network Objects§ Access to live network
via BPF (Berkeley Packet Filter) interface, UDP and raw sockets § Support also access to
trace files
l Tap Agents§ Conversion between
ns and network packet formats
Capture: BPF
ns
Inject: Raw socket
network to ns
ns to network
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VINT/nse and IP Packets§ Ns utilizes own network
address format.→ mapping required
§ Network objects receive IP packets and hand it over to the tap agent.
§ Tap agent creates simulator packet.
§ External payload pointer refers to IP packet.
§ Simulator packets received by tap agentare de-encapsulated using the external payload pointer and written to network object.
Network Packet
SourceDestination
Size
Data
ns Packet Header
SourceDestination
Size
Extern
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VINT/nse: Modes of OperationPacket conversion leads to two modes of operation§ Opaque Mode
l Network packet fields are not interpretedl Live data packets may be dropped, delayed, re-ordered,
duplicated etc. l Application: end-to-end application testing
§ Protocol Model Simulator can interpret and/or generate live network traffic. l Protocol implementations in simulator are able to interact with
peer real-world implementations. l Requires agents within simulator to decode and interpret
l running specific operating systems and application software route packets through ModelNet core.
l Multiplexing of virtual edge nodes on machines of a server cluster
§ Core l Modified FreeBSD kernelsl Responsible for network emulation on a link-by-link basisl Routes traffic through a network of pipes (queues, queuing disciplines)
EdgeNodes
RouterCore
100 MbpsSwitch
1 GbpsSwitch
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ModelNet Core§ Pre-computation of shortest path between
VN pairs§ (Source, destination)
→ set of pipes to be traversedl Hop-by-hop emulation: allows emulation of
congestion and queuing effectsl End-to-end emulation
(collapsing of paths into single pipe): § more efficient, but does not model contention
§ Packet scheduling based on heap of pipes ordered by packet deadlines.
§ Pipes with deadlines > current time are processed and their deadlines are updated.
§ ModelNet priority > interrupt processing priority → processing of packets in the core with higher priority than processing of packets entering the core
§ Multi-core configuration: pipes might be on different core nodes → buffering of packet content at entry core node, data forwarding to exit core node
§ Buffering according to bandwidth delay product (10 Gbps, 200 ms rtt → 250 MB)
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ModelNet Phases§ Create
l generates network topologyl Result: graph based on GML (graph modelling language)
§ Distillationl Transforms GML graph to pipe topology
§ Assignmentl Maps pieces of pipe topology to ModelNet core nodes
(currently: Greedy approach, optimal assignment is NP hard)§ Binding
l Assigns virtual edge nodes (VN) to edge nodesl Configures virtual edge nodes for executing applicationsl Binds physical edge node to a single core nodel Generates configuration scripts for core nodes
(installing pipes and routing tables, IP address configuration)§ Run
l Executes target applications on edge nodes. l Scripts automate execution of multiple instances. l Correct binding of IP addresses at different virtual edge nodes !l Dynamic library to interpose wrappers around socket calls.
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ModelNet Performance§ Emulation accuracy with hardware timer granularity (100 µs) § Hardware: 1.4 GHz Pentium III
l Forwarding rates§ 1 hop per flow: 120’000 packets/s§ 8 hops per flow: 90’000 packets/s
l Overhead per hop: 0.5 µs, overhead per packet: 8.3 µsl Forwarding rate of hardware (without emulation): 250’000 packets/s
§ Multi-core configurationsl 1120 VNs on 20 edge nodes, 4 core nodes, 2 hops per pathl 155 – 460 kpackets/s (100 % - 0 % cross-traffic)l Cross-traffic: Flows must cross from one core node to another one.
§ VN multiplexing effects
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Netbed§ Descendant of Emulab [White02]§ Integrates simulation, emulation, live experimentation § Goal: configuration support for users§ Experimenter can define experiment (virtual topology,
node characteristics) via web interface. l Virtual topology description by ns script (extended language) or
Java GUI. Topology generators can be used. l Virtual nodes may be instantiated from § Local nodes
l Local nodes run 1 virtual nodel 168 PCs at U Utahl Dummynet between physical nodesl VLANs
§ Distributed nodesl Limited number (40) of remote testbed nodesl Sharing of nodes based on FreeBSD Jail isolation (+ raw sockets)
§ Ns simulation (nse)
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Netbed Features§ Global resource allocation
l Over reservation of link bandwidth and monitoring
§ Node self-configuration l Configuration is driven by nodes but entirely
controlled by node state centrally stored in data base. l Swapping of experiments
§ Experiment Controll Event system based on publish/subscribe systeml Dynamic manipulation of link characteristics
§ Pre-emption and Schedulingl Idle detection systeml Manual idle confirmations before swapping out an
experimentl Batch processing
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Netbed Architecture• central point of control• file server
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Netbed Performance§ Up to 150 ns flows (2 Mbps CBR) and 300
nodes on a 850 MHz PC without impact on live TCP connection across nse link§ UDP round trip traffic between two nodes
with / without interposed emulator nodel Saturation of 100 Mbps link with 1500 byte packets
without any effectsl 64 byte packets: 55’000 / 37’000 packets/s
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Netbed: TCP New Reno One Drop TestTest: dropping a single packet§ ns and FreeBSD 4.5 detect 3 duplicate
acknowledgements and retransmit.§ FreeBSD 4.3 did not retransmit until triggered
by timer expiration. l Reason: uninitialized variable
ns FreeBSD 4.3FreeBSD 4.5
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IP-TNEInternet Protocol Traffic and Network Emulator [Simmonds02]§ Simulation based network emulator
l Parallel Distributed Event Simulation kernel l Scalability by parallel simulation
§ Architecture Overviewl Real-Time Endpoints (EPs) represent each real host involved in experiment.
§ Specific routing entries need to be added at real hostsl System being modelled is mapped to logical processes (LPs)
§ Synthetic traffic generators within virtual network§ Operation
l Packet reader captures packets and inserts them into virtual network modelled by simulation engine.
l Emulator dispatches packets to their final destination. l Packets are time-stamped when read and are released when wall clock time
reaches packet's calculated output time.
LAN
Emulator
EPs
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IP-TNE Architecture
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IP-TNE Real-Time Interface§ Simple approach (used in nse)
l Execution of an event is delayed until wall clock reaches timestamp of event.
l Events are not executed ahead of wall clock time in order to avoid causality errors.
l Problems:§ Frequent access to wall clock time (expensive system call)§ The more execution is delayed during low workload periods
the higher is the chance of missing real-time deadlines during high workload periods.
§ IP-TNE approachl EPs need to be ready to receive information from
real-time devices at any point of time.l EPs control data flow to real-time device:
communication only when wall-clock time reaches timestamp of event triggering action.
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IP-TNE Packet Reading and Writing§ Problem
l Standard sockets do not deliver IP packet header information to applications (emulator).
PlanetLab§ Creation of slices (slice = network of virtual machines)
[Chun03]§ Virtual machine runs on a node and
consumes some of the node’s resources. § Multiple virtual machines run on each PlanetLab node. § Implementation based on VServers
l Provides illusion of multiple, independently managed virtual servers on a single Linux machine
l Virtualization above Linux kernell Each VServer has own security context and super user. l Weaker guarantees and protection
§ Protected Raw Socketsl UDP/TCP sockets are bound to specific ports.§ Packet delivery to service that created a socket
l ICMP sockets are bound to specific ICMP identifier. § Resource Limits
l Bandwidth limitation controlled by Linux traffic controll Fairness between processes by Linux CPU scheduler
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PlanetLab: Dynamic Slice Creation§ Resource monitor running on each node
reports resource availability to centralized agent. § Agent issues tickets to service brokers describing
available resources and time frame. Ticket can be redeemed in order to lease resources.
§ Service broker specifies slice specification based on resource requirements and tickets.
§ Node manager runs on each node, receives tickets, performs admission control, reserves resources, creates virtual machines and returns a lease (needed for launching programs).
Node
Node
AgentMonitoringServiceBrokerRequest/
Ticket
Ticket/Lease
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Emulation in Mobile Networks
§ Changing network conditions due to mobility→ Trace-Based Mobile Network Emulation
§ Changing network connectivity→ MobiEmu
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Trace-Based Mobile Network Emulation§ Creation of a synthetic network environment and
execution of real workload [Noble97]. § Experiments using real systems without physical
movements. § Phases of methodology
l Collection§ Performance of a real-wireless network is captured through trace
collection. § Logging of packets, workload: ICMP echo / echo reply§ Wireless LAN signal reports
l DistillationProduction of parameters for a simple network performance model§ End-to-end delay
l Modulation§ Network performance is reproduced based on these parameters
by emulation: Kernel reads replay trace and drops/delays packets.
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MobiEmu§ n Linux machines emulate mobile ad-hoc network with n nodes
[Zhang02]§ Scenario-driven emulation with node locations and movements as
input§ MobiEmu mimics real-world situation by
dynamically installing and removing packet filters. § Master / slave interactions over control channel§ Slave controllers enforce topology
l Options: § Linux Netfilter / iptables§ Packet filtering in user space (library)§ User Mode Linux (UML): dropping of inter-UML packets
Testbed network
Master Controller
Control Channel
Testbed host
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Conclusions
§ Network emulators have evolved from simple delay line models towards complex systems including real-time simulation facilities and detailed network modelling. § Variety of tools exist. Users can select
dependent on their requirements.
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References I§ Mark Carson, Darrin Santay: NIST Net: a Linux-based Network Emulation Tool,
ACM SIGCOMM Computer Communication Review, Volume 33, Issue 3, July 2003, pp. 111 - 126
§ Mark Allman, Adam Caldwell, Shawn Ostermann: ONE: The Ohio Network Emulator, Technical Report TR-19972, Ohio University Computer Science, August 1997.
§ Nigel Davies, Gordon Blair, Keith Cheverst, Adrian Friday: A Network Emulator to Support the Development of Adaptive Applications, Proceedings of the 2nd UsenixSymposium on Mobile and Location Independent Computing, Ann Arbor, USA, April 10-11, 1995, pp. 47-55.
§ Amin Vahdat, Ken Yocum, Kevin Walsh, Priya Mahadevan, Dejan Kostic, Jeff Chase, David Becker: Scalability and Accuracy in a Large-Scale Network Emulator, Proceedings of 5th Symposium on Operating Systems Design and Implementation (OSDI), December 2002.
§ Rob Simmonds, Brian W. Unger: Towards Scalable Network Emulation, Computer Communications, Volume 26, Issue 3, February 2003, pp. 264-277
§ Daniel Herrscher, Kurt Rothermel: A Dynamic Network Scenario Emulation Tool, 11th International Conference on Computer Communications and Networks, Miami, October 2002, pp. 262-267
§ Kevin Fall: Network Emulation in the VINT/ns Simulator, 4th IEEE Symposium on Computers and Communications, July 6-8, 1999 Red Sea, Egypt
§ Lee Breslau, Deborah Estrin, Kevin Fall, Sally Floyd, John Heidemann, Ahmed Helmy, Polly Huang, Steven McCanne, Kannan Varadhan, Ya Xu, Haobo Yu: Advances in Network Simulation, IEEE Computer, Vol. 33, No. 5, pp. 59-67, May, 2000
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References II§ Jong Suk Ahn, Peter B. Danzig, Zhen Liu, Limin Yan: Evaluation of TCP Vegas:
emulation and experiment, ACM SIGCOMM Computer Communication Review, Volume 25, Issue 4, October 1995, pp. 185 - 195
§ Luigi Rizzo: Dummynet: A Simple Approach to the Evaluation of Network Protocols, ACM SIGCOMM Computer Communication Review, Volume 27, Issue 1, January1997, pp. 31 - 41
§ Luigi Rizzo: Dummynet and Forward Error Correction, USENIX 1998 Annual Technical Conference, June 15-19, 1998, New Orleans, USA
§ Brian White, Jay Lepreau, Leigh Stoller, Robert Ricci, Shashi Guruprasad, Mac Newbold, Mike Hibler, Chad Barb, Abhijeet Joglekar: An Integrated Experimental Environment for Distributed Systems and Networks, 5th Symposium on Operating Systems Design & Implementation, pp. 255-270, December 2002
§ Brent Chun, David Culler, Timothy Roscoe, Andy Bavier, Larry Peterson, Mike Wawrzoniak, Mic Bowman: PlanetLab: An Overlay Testbed for Broad-CoverageServices, ACM SIGCOMM Computer Communication Review", Vol. 33, No. 3, pp. 3-12, July 2003
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