Universal Connection Establishment
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All-In-One-Mediator enabled findbugs and checkstyle in UCE. Made some cosmetic changes in… Apr 20, 2013
connectivitymanager.demo.complete optimization Jun 22, 2013
connectivitymanager.demo fixed checkstyle errors. Jun 10, 2013
connectivitymanager Added missing hashCode(). Jul 4, 2013
core fixed checkstyle errors. Jun 10, 2013
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holepunching.mediator got rid of more warnings, added sbo's socket switch impl, added comme… Jan 17, 2013
holepunching.message removed more warnings Jan 18, 2013
holepunching Transformed cast to long into multiplication with long. Jun 27, 2013
master.server Added missing hashCode(). Jul 4, 2013
mediator failure handling in pligin loading Sep 19, 2014
parent work on checkstyle Jul 14, 2013
plugininterface Fixed bunch of checkstyle errors. Jun 27, 2013
relaying.mediator got rid of more warnings, added sbo's socket switch impl, added comme… Jan 17, 2013
relaying.message Fixed bunch of checkstyle errors. Jun 27, 2013
relaying.server Fixed bunch of checkstyle errors. Jun 27, 2013
relaying Transformed cast to long into multiplication with long. Jun 27, 2013
reversal.mediator got rid of more warnings, added sbo's socket switch impl, added comme… Jan 17, 2013
reversal Transformed cast to long into multiplication with long. Jun 27, 2013
stun.server Stun and Relay server are build as executable jars with their depende… Apr 15, 2013
stun Fixed bunch of checkstyle errors. Jun 27, 2013
uce added master server to UCE Apr 26, 2013
.travis.yml Updated README. Added flags to maven output Jun 24, 2013
LICENSE readme, parent May 24, 2012
README.md updated readme with maintain info Nov 13, 2015


Copyright (c) 2012 HTWG Konstanz, 

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.

UCE - Universal Connection Establishment


Universal Connection Establishment (UCE) is a framework for firewall and NAT traversal techniques that is designed to provide universal IP connectivity with minimal administrative and configuration overhead.

Current build status:

Build Status

NAT introduction

Network Address (and Port) Translation (NAT / NAPT) allows address sharing of public IP addresses and hiding private networks from the public. The downside is that NAT breaks the end-to-end principle and prevents incoming pakets that are not related to outgoing traffic from traversing the NAT device. As a result, hosts behind NAT are not reachable from the public internet. For an explanation of NAT and it's terminology see RFC2663. This is especially problematic for P2P applications that require direct connections between participating peers.

One can distinguish between NAT mapping, that is how a NAT maps public endpoints (IP, Port) to private endpoints, and NAT filtering, that is how the NAT deals with incoming pakets and connection requests. Four strategies are common (see RFC5128 for more explanation):

  • Endpoint-Independent (EI)
  • Address-Dependent (AD)
  • Adress- and Port-Dependent (APD)
  • Connection-Dependent (CD)

The mapping and filtering behavior on a NAT device is usually called the NAT behavior. There can be multiple NAT devices between two peers, the sum of all NAT behaviors on the path is called NAT situation. The NAT situation is decisive for the success of NAT traversal strategies.

NAT Traversal Components

NAT traversal describes the process of traversing NAT devices from the outside world with the intend of enabling incoming pakets that are unrelated to outoing traffic.

The following NAT traversal techniques have been proposed in literature. See P2PNat, RFC5128.

  • Direct connect

    If the target peer is publicly available or has a persistent mapping on the NAT or a user defined mapping the source can directly connect to the public address.

  • Relaying

    Instead of directly communicating with each other, the peers connect to a public relay server. This always works but puts pressure on the relays and might increase delay and latency which is undesirable in certain applications.

  • Connection Reversal

    If only the target (serving) peer is behind NAT, or the source (requesting) peer's NAT is configured for persistent port forwarding or otherwise has a persistent non-filtering mapping, the connection can be reversed. In this case the target peer connects to the source peer. This requires the target to maintain a connection to a rendezvous or mediator that signals the connection request and tells the target to reverse the connection.

  • Hole Punching

    Both peers open connections to the outside, thus creating a mapping in the NAT, effectively punching a hole. This mapping can then be used to connect to. Requires some sort of rendezvous server to exchange public and private endpoints.

  • UPnP

    If the NAT box supports UPnP it can be directed to automatically create persistent mappings. The peer behind this NAT box is then effectively visible from the outside.

There are other solutions like NAT-PMP or SOCKS which are not discussed here. A good overview can be found on Wikipedia.

So what's UCE and what's different

UCE tries to unify a multitude of NAT traversal techniques in one library (and application) that is able to guarantee connectivity in basically any network environment. It tries to collect information about the NAT situation and then uses some logic to find the best suitable NAT traversal method.

In that regard it is quite similar to ICE or ANTS. ICE, however, is only defined for UDP-based media streams whereas UCE focuses entirely on TCP. ANTS promises similar behavior for TCP. Unfortunately, there is no implementation publicly available. You can test their NAT-Analyzer (you can also download the code there if you look closely)

An essential part of UCE is exposure to Java RMI. That allows building RMI applications over NAT and firewall boundaries.


All modules and bundles are located in a flat hierarchy parallel to parent. Modules are components of UCE. Bundles are different from modules in the way that they extend and bundle modules to new releases. E.g. 'UCE Messages' is a module that implements all the core messages, while 'UCE Core' is a bundle that bundles all UCE core functionality to one release.

  • README.md

    This readme file


    The license file. GPLv3.

  • parent/

    The parent project with all global definitions and dependencies.

  • uce/

    The UCE build project. Defines and builds all modules and bundles. Here run mvn compile or mvn package

  • core/

    Core functionality that is used by multiple other modules.

  • plugininterface/

    UCE is built to be extendible using plugins and the java Service Loader. The plugininterface includes all common interfaces for NAT behavior, situation traversal techniques, method handlers and so on. All NAT traversal techniques for instance must implement the interface NATTraversalTechnique.

  • stun/

    Another core technology in UCE this is an implementation of the RFC5389 STUN message standard. Only the messages are implemented here. Custom message handlers as well as custom NAT traversal STUN messages are implemented alongside specific NAT traversal techniques.

  • stun.server/

    An implementation of a STUN server. This is not compliant to RFC 5389 since it uses TCP exclusivly and also opens TCP connections to the STUN client in order to investigate NAT filtering behavior.

  • connectivitymanager/

    The connectivity manager implements all the logic of collecting NAT information, identifying the NAT situation and then choosing a suitable NAT traversal techniques. For that it always has a control connection open to a publicly available mediator (see below). The connectivity manager requires the implementation of at least one NAT traversal techniques as plugin in it's classpath or plugin path. For easy use in your application, use the connectivitymanager, to manage you connections transparently.

  • connectivitymanager.demo/

    A demo chat application that uses the connectivity manager to establish a peer-to-peer connection behind NATs and provides a simple CLI chat between the peers. You need to manually copy the NAT traversal jar files into the plugin directory for testing.

  • connectivitymanger.demo.complete/

    This is a bundle of the connectivitymanager demo and all related plugins for the demo.

  • mediator/

    The mediator is a publicly available rendezvous server that handles peers, peer requests, and exchanges endpoints. As the connectivity manager it requires the presense of NAT traversal plugins for the messages it should handle.

  • All-In-One-Mediator/

    The All-In-One-Mediator contains a mediator and all provided plugins from UCE.

  • master.server/

    The master server is a bundle of the stun, relay and mediator server with all plugins for the mediator.

  • directconnection/

    Implementation of the direct connection NAT traversal (not really a traversal method though).

  • directconnection.mediator/

    Direct connect request handler for the mediator.

  • directconnection.message/

    STUN messages for direct connection.

  • holepunching/

    Implementation of parallel TCP holepunching NAT traversal.

  • holepunching.mediator/

    Holepunching request handler

  • holepunching.message/

    Holepunching STUN messages.

  • relaying/

    Implementation of a TURN like relaying NAT traversal.

  • relaying.mediator/

    Relaying request handler.

  • relaying.message/

    Relaying STUN messages.

  • relaying.server/

    Implementation of a TURN like Relay server. Not compliant to the standard.

  • reversal/

    Implementation of connection reversal NAT traversal.

  • reversal.mediator/

    Connection reversal request handler.

  • reversal.message/

    Connection reversal STUN messages.

  • socketswitch/

    A proof-of-concept implementation of TCP socket switching. Is able to switch connections from one socket to another. This should enable to change sockets on-the-fly while maintaining an active connection from an application point of view. Not very well tested, code is a mess right now and it is not yet integrated into the UCE framework.

State of UCE

UCE originates from a BMBF funded research project. Most components are in a proof-of-concept or prototype state. See the project page for detailed information, references, publications and presentations.

Currently implemented are the NAT traversal techniques mentioned above as well as the connectivity manager, a modular mediator, STUN and relay server. There are no plans from our side to extend this for support of further techniques.

Clone and build UCE

  • Clone the repo

    git clone git@github.com:htwg/UCE.git
  • Compile and install

    cd UCE/uce
    mvn install

Test the connectivity manager demo

  • Copy the connectivitymanager.demo.complete-1.0-bin.[tar.gz, zip] archive from connectivitymanager.demo.complete/target to a target and source machine. Both or one of them can be behind a NAT device.

  • Unpack both.

  • Cd into connectivitymanager.demo.complete-1.0

  • On the target machine run:

    java -cp connectivitymanager.demo-1.0-jar-with-dependencies.jar de.fhkn.in.uce.connectivitymanager.demo.chat.ChatTarget <targetID>

    Where targetID is any arbitrary string you want as identifier for the target

  • On the source machine run:

    java -cp connectivitymanager.demo-1.0-jar-with-dependencies.jar de.fhkn.in.uce.connectivitymanager.demo.chat.ChatSource <targetID>

    Where targetID is the same identifier you used before.

You should now have a running CLI-based chat between the (NATed) peers.

BEWARE The demo application uses a built-in mediator and STUN server IP. These servers are hosted by us and might not always be available.

To see how to integrate UCE into your own application see the connectivitymanager.demo implementation. It boils down to do sth like the following:

    import de.fhkn.in.uce.connectivitymanager.connection.UCESocket;
    import de.fhkn.in.uce.connectivitymanager.connection.UCEUnsecureSocketFactory;

    UCESocket socketTpPartner = UCEUnsecureSocketFactory.getInstance().createTargetSocket(targetId);

Build your own bundle

  • Copy the connectivitymanager.demo.complete directory and change the folder name

  • Cd into this directory, head to the pom.xml and change the following:

    <artifactId>: Id of your bundle
    <name>: Name of your bundle
  • Edit the source path in the <file>-tag in config/assembly-bin.xml.

  • Add your new module in the module section of uce/pom.xml.

  • Build UCE as above.

Run your own mediator

  • Build UCE as above.

  • cd All-In-One-Mediator/target

  • Unpack the UCE-All-In-One-Mediator-1.0-bin.[tar.gz, zip] archive in your destination folder.

  • cd to UCE-All-In-One-Mediator-1.0 and execute

    java -jar mediator-1.0.jar <port> <user clean interval> <max lifetime>


    java -jar mediator-1.0.jar 10140 300 600

The mediator is now working on your own machine. To use it from the connectivity manager, you hava to change the mediator.properties. Unfortunately it does not currently work to change the system properties from the command line. To change the mediator.properties goto the directory:


Edit the file mediator.properties like this:

mediator.ip=<your mediator ip>
mediator.port=<your mediator port>

Afterwards you have to rebuild the connectivity manager and the demo.

Similarly, you can also change the plugin directory location of the connectivity manager and the mediator. Just look into the appropriate resources directories, find the registry / techniqueregistry dirs and change the file nattraversalregistry.properties.

Run the master server

  • Build UCE as above.

  • Unpack the master.server-1.0-bin.[tar.gz, zip] archive in to your destination folder.

  • Cd into master.server-1.0.

  • Now you have four posibilities to run the master server:

    1. Add your values into the provided properties file (config/master.server.properties).

    2. Set the values with system properties.



    1. Provide the values as command line arguments:



    -StunFirstIP=  or  StunFirstIP=
    1. A mixture of these three.
  • Example of a correct start of the master server using command line arguments:

    java -jar master.server-1.0.jar StunFirstIP= StunSecondIP= MediatorPort=14001 RelayPort=14000 MediatorLifeTime=1 MediatorIteration=1


The properties will be read and overwritten in the following ascending order:

properties file < system properties < command line arguments

To start the master server correctly the following arguments need to be set:

RelayPort           (optional)

Plugin development

If you want to use your own plugins, please provide them in the corresponding plugins/ folder in the mediator and connectivitymanager.demo. For the mediator it should look like this:


And for the demo it should look like this:


How it works

A target behind NAT that wants to be publicly available registers with a publicly available mediator. In addition a public UCE STUN server can be used to investigate the NAT behavior. A client that wants to connect needs to know the mediator and the registered target ID. The cnnectivity manager on the source side decides in which sequence NAT traversal techniques should be tried and using the mediator issues connection requests and starts connecting. As a last resort, relaying is tried.

What's left to do and how to contribute

Although UCE work has proven to work quite well in our tests there are some open issues.

  • UPnP support would be nice, although most UPnP implementations have severe security issues and should not really be activated. It is important to note here that all NAT traversal techniques implement the equals method correctly.

  • Although UCE is intended for TCP, it would be nice to have a UDP HP implementation and be able to use UDP tunnels for TCP.

  • UCE is both targetted towards RMI and P2P. But both is not yet really well supported. Although RMI has in all development stages been present and well tested (it works), the refactored implementation lacks the required RMI Factory classes.

  • P2P support would require a mechanism to decentralize the mediators. This in turn would require a better user management as now (none at all). Sth like uuid@MediatorIP:MediatorPort would uniquely identify a user on a specific mediator (like SIP or Diaspora does). Then of course mediator-to-mediator communication must be implemented, that is, forwarding requests and responses between mediators. Same would be nice for relay servers too. Stun servers, however, do not require such a behavior.

If you like to contribute and think that you could deal with one of the issues above, or you have your own ideas / improvements, just fork the repo, do your stuff and send us a pull request. Also feel free to contact the project maintainer at any time.


  • Maintainer:

    none, project concluded in 2013

  • Lead:

    Thomas Zink, tzink {at-sign} htwg-konstanz {a dot} de

  • Contributors:

    • Daniel Maier: former lead and all initial code (see UCE_deprecated)
    • Stefan Lohr: connection reversal, web hole punching, some demos
    • Alexander Diener: connectivity manager, STUN, refactoring to plugins
    • Ellen Wieland / Steven Boeckle: socket switching
    • Robert Danczak: Configuration of bundles, CI with Travis and Jenkins, Master Server