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,

Hitbox, Ohio Network Emulator, Dummynet, NIST Netl Detailed Modelling of Virtual Networks§ VINT/nse, ModelNet, Netbed, IP-TNE, Virtual Routers, PlanetLab

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

the virtual networkl Examples: § VINT/nse, ModelNet, Netbed, IP-TNE, Virtual Routers,

PlanetLab

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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

protocol messages, e.g. ICMP responder, ARP, TCP NATl Application: protocol conformance testing

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VINT/nse Protocol Mode

Send/receive packets from application

ns

TCP Agent

Network

Host A

TCPApplication

Host B

Send/receive packets from

ns

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ModelNet

§ [Vahdat02]§ Edge nodes

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).

l Emulator needs to write custom IP headers.

§ IP-TNE approachl Reading: Packet filtering / capturing (PCAP)l Writing: Raw sockets

§ Alternativesl Proxy sockets§ Problems: restricted to applications with proxy protocols

l Capturing at real hosts§ Problem: Hosts are part of emulation environment and need

special software.

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Virtual Routers§ Virtual Routers (Linux user space processes) [Baumgartner02/03]

l IP packet processing in real-time § Softlink Device

l (sol) forwarding of IP packets to virtual routers via file I/O (cf. TUN/TAP)

VR1 VR2 VR3 VR4IPCUDP

Client

sol0eth0

Server

User Space

OS kernel

VR4

sol0eth0

VR2

eth0

VR3

Socket Socket Socket Socket SocketSocket

Ethernet

VR1

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Implementation of Virtual Routers§ Dynamically loadable components, e.g.

l Active router extension (Python)l Command line interfacel Traffic monitoringl Diagnosis tools (traceroute, ping)l New protocols

§ Configurable interface components, e.g. l Queuing, scheduler, traffic shaperl Network Address Translation

§ API for remote configurationl Request / Response protocol with commands to create, modify, delete

interfaces, routing tables and objects

UDP

Forwarder

Program-mableFilter

Dynamically Loadable Components

Inter-connectHandler

Inter-connectHandler

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Virtual Router Performancertt in unloaded network (chain of routers)

rtt in loaded network (UDP flow using 50 % of link bandwidth)

0

1

2

3

4

5

10 12 14 16

RTTs nondistributedvariance of RTTs nondistributed

RTTs distributedvariance of RTTs distributed

hops

20 4 6 8

RTTs with 16 VRsvariance of RTTs with 16 VRs

RTTs with 32 VRsvariance of RTTs with 32 VRs

RTTs with 64 VRsvariance of RTTs with 64 VRs

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70hops

0

1

2

3

4

5

10 12 14 16

RTTs nondistributedvariance of RTTs nondistributed

RTTs distributedvariance of RTTs distributed

hops

0 2 4 6 8

RTTs with 16 VRsvariance of RTTs with 16 VRs

RTTs with 32 VRsvariance of RTTs with 32 VRs

RTTs with 64 VRsvariance of RTTs with 64 VRs

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70hops

hops

hops

hops

hops

rtt (m

s)rtt

(ms)

rtt (m

s)rtt

(ms)

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Virtual Routers for QoS ResearchDifferentiated Services Experiment§ Queue 1: TCP traffic

(absolute priority, 2 Mbps token bucket rate)§ Queue 2: aggressive UDP traffic§ Total interface bandwidth: 4 Mbps

Q2 Q1

C

S

T

4

3

2

1

00 5 10 15 20 25 30 [sec]

TCP flowUDP flowM

bps

s

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Web Server for Virtual Router Control

§ Download of Java applets embedded into dynamic web pages (PHP) and switching between applets

§ Administrator (Java program) stores shared data§ TCP connection for command exchange

l Storage of datal Allocation of VR resourcesl Retrieval of VR topologiesl ...

Browser

Applet

Web Server

Webpages

Administrator Virtual Routers

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Virtual Router Interactions

Applet Administrator Virtual Router

ifconfig if<number> delete ifconfig if<number> disconnect

ack

ifconfig if<number> delete

ack

if<number> deleted

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Virtual Routers for Educationwww.vitels.ch

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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

(non-overlapping ICMP echo / echo reply messages)§ Queuing delay (back-to-back ICMP echo / echo reply messages)§ Loss (counting of echo reply messages)

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

§ Brian Noble, M. Satyanarayanan, Giao Nguyen, Randy Katz: Trace-Based Mobile Network Emulation, ACM SIGCOMM, Cannes, August 1997

§ Yongguang Zhang, Wei Li: An Integrated Environment for testing Mobile Ad-HocNetworks, ACM MobiHoc 2003, June 9-11, Lausanne, Switzerland

§ Florian Baumgartner, Torsten Braun, Bharat Bhargava: Virtual Routers: A Tool for Emulating IP Routers, 27th IEEE Conference on Local Computer Networks (LCN 2002), Tampa, USA, November 6-8, 2002

§ Florian Baumgartner, Torsten Braun, Eveline Kurt, Attila Weyland: Virtual Routers: A Tool for Networking Research and Education, Computer Communications Review Vol. 33 No. 3, pp. 127-135, July 2003.