ToMaTo - Link Emulation

ToMaTo, Topology Management Tool

ToMaTo is topology-oriented, i.e. users build topologies for their experiments.

Devices

produce and consume data
can run software

Connectors

forward and manipulate data
connect devices

Three kinds of devices

KVM devices
OpenVZ devices
Programmable devices

Two kinds of connectors

VPN networks
External networks

Link emulation

Definition

Link emulation is a term that defines methods to apply artificial characteristics or network links and path to a communication link.

Emulatable characteristics

ToMaTo is able to emulate the following link characteristics for all VPN links in both directions (seperately):

Delay
Jitter
Statistical delay distribution
Bandwidth limitation
Packet loss
Packet duplication
Packet corruption

Delay and jitter

Delay

Delay can be added to a link by buffering each packet for a certain time before processing it. Delay can be configured in milliseconds but also fractions can be specified. The resolution depends on the timer resolution of the kernel but is very accurate in general.

Jitter

Jitter is a deviation applied to the per packet delay. Like the delay, jitter can also be specified in milliseconds but also fractions will work. The meaning of the jitter value depends on the selected statistical delay distribution but the jitter is applied in an average-neutral way.

Packet reordering

Packet reordering can be a result of high jitter values. If a packet gets a high delay value and the next packet a low value due to jitter, the later packet could leave the buffer first.

Statistical delay distribution

Uniform distribution

Default distribution
Values evenly distributed within ±jitter

Normal distribution

Higher chance for values near delay
68% of values within ±jitter
95% of values within ±2*jitter
99% of values within ±3*jitter

Pareto distribution

Power law
High chance for small delays, small chance for high delays
Average is configured delay, median is below

Paretonormal distribution

Combination of normal and pareto distribution
Good emulation of most existing links

Other characteristics

Bandwidth limitation

limit the maximal data rate
configured in kilobits per second
default bandwidth limit is 10 MBits/sec

Packet loss

chance for each packet to be lost (dropped)
configured in percent (5 means 5%)
optionally correlated

Packet duplication

chance for each packet to be duplicated
only one duplicate
duplicate counts a traffic
delay and jitter is applied seperately

Packet corruption

chance for each packet to be corrupted
one random bit is flipped

Characteristics configuration

The link emulation properties can be changed on the link properties page for each link. The link characteristics can even be changed for running connections.

Characteristics are additive

The configured link emulation settings are applied additionally to the existing characteristics of the underlying link.

Delay is just added to the underlaying delay
Jitter values and distributions are combined to more complex distributions
Resulting bandwidth limit is the minimum of the existing and the configured limit
Chances for loss, duplication and corruption are inversely multiplied, i.e. p'=1-(1-p₁)*(1-p₂)

Continous measurements

To help users estimate the resulting link characteristics the links between all sites are continously measured and long-time statistics are provided.

Complex link emulation

Sometimes a more complicated link emulation or traffic shaping is needed on links. This can be done by using a link with a device in the middle.

OpenVZ

filtering using the Linux kernel firewall

KVM

can run any custom filtering and link emulation software

Programmable devices

easy link emulation as a program
easy filtering as a program

Obtaining and contributing

How to get ToMaTo

ToMaTo is Open-Source! It can be simply downloaded from the Github page. There is also a step-by-step tutorial on how to setup ToMaTo in a testbed. ToMaTo includes some nice features that make it pretty easy to install it in an experimental facility:

All components packaged for debian (updates come automatically)
Multiple authentication plugins: LDAP, htaccess, SQL-Database, Planet-Lab, ...
Automatic checks and problem reports for the ToMaTo hosts

How to contribute to ToMaTo

As an Open-Source project ToMaTo is open for hints and contributions.

Github offers an easy way to fork the project and offer contributions as pull requests
The wiki is publically editable so everyone can help by adding to the documentation
The issue tracking system can be used for bug reports and feature requests