Vyatta is an organization founded by Allan Leinwand in 2005 . Vyatta provides the open source network routing solution such as Open Flexible Router (OFR). OFR is open source router software and provide the routing mechanism for Local Area Network (LAN) and Wide Area Network (WAN). Open Flexible Router (OFR) can run on machine that supported the x86 platforms. Vyatta OFR can install on machine using Hypervisor based virtualization technology.  Open Flexible Router (OFR) got first prise in open source software area. Open Flexible Router (OFR) is covered the security, availability and predefined routing procedures for router e.g. Protocols. The Open Flexible Router (OFR) is manageable for small or medium scale organization. 
Generic router is divided into virtual routers with virtualization. These virtual routers can manage through “virtualization layer”. Each virtual router has assigned a service such as protocol. These days “Virtualization layer” has different name of technology that covers the routers e.g. Vyatta OFR's full “virtualization / para virtualization” or “Hypervisor-based virtualization / OS-based virtualization” technology. The packets routing is core job of router either router is software level or hardware level. Vyatta OFR is software router that route the packets from source to destination on software level. The Network Interface Cards (NIC) is connected via Peripheral Component Interconnect (PCI) buses with Random Access Memory (RAM). Control process of virtual route has APIs that are used to forwarding functionality while switches are used in hardware level router. Number of routing software exist such as XORP  and Quagga . Routing functionality of Vyatta OFR is based on eXtensible Open Router Platform (XORP). [3, 4, 6]
Overview of eXtensible Open Router Platform (XORP)
The eXtensible Open Router Platform (XORP) is open source router software that can route the data packets and forwarding them on software level. According to M. Handley  there are two cores sub-part of XORP namely “higher level” and “lower level” as show in figure 1.
Higher Level-XORP: The higher level is also known as “user level” and in generic router this is called control plane . The control plane in actual router can runs the protocols such as OSPF and RSVP  and in XORP's higher level sub-part holds the routing protocols such as OSPF, BGP and RIP [7,9] as well as routing information. The routing protocols are known as processes and routing information based on it. They designed multi process architecture with configuration, coordination and management process. The coordination between processes is called “XORP Resource Locators (XRLs)”. XRL is human readable and it similarly as Universal Resource Locator (URL). [7, 9]
Lower Level-XORP: In actual router packets are forwarding through Data plane . In Lower Level of XORP is responsible for managing and forwarding packets, and communicates with Higher Level of XORP using APIs. Lower Level of XORP is holding the “Click Modular Router ” that run within kernel of Operating System (OS) and perform the packets processing e.g. forwarding path. [7, 9]
There are four main process of XORP namely “router manager”, “IPC finder” and “routing information base (RIB)” and “Forwarding Engine Abstraction (FEA)” .
Router manager: it is a process that manages the interface configuration, routing protocols information e.g. start, manage and restart in case of failure. In simple word router manager can manage the whole router. [7,9]
IPC finder: The processes of XORP can communicate through IPC-finder without utilize robust mechanism. This process can also manage the information about mapping of interfaces. For example how many interfaces are exist and how many available on router, and finder answer to application on the exacting calls of IPC. 
Routing Information Base (RIB): the routing procedure of XORP send the routing information to RIB. The RIB process can make a decision on receiving information that either transfer these routes into forwarding path or send to routing protocols.RIB can also bring up to date the forwarding table on bases of routing information. [7, 9]
The Forwarding Engine Abstraction (FEA)  is a layer of XORP which divided into two layers routing and forwarding plane (i.e. click element). FEA can manage the forwarding paths and has three main parts: “Forwarding Information Base (FIB) manager”, “Interfaces managed” and “Input output manager (I/O manager)” .
* Forwarding Information Base (FIB) manager: RIB process send forwarding table to FEA process where FIB manager received it and accommodate changes in forwarding table if required. Data plane  mechanism XORP can use “click software”  through plugin to forwarding packet. [7,9]
* Interface manager: When administrator (using command line interface (CLI)) or router manager can configure an interface on router then they issue a command which received by FEA's interface manager. FEA's interface manager activates and resolves this request and sends acknowledge to routing process. 
* Input output manager (I/O manager): every routeing protocols can use an interface (provides I/O manager) for routing information exchange with other neighbour routers instances.
The concrete feature of Vyatta OFR and XORP describes in report . Vyatta OFR can operate and configure through GUI and CLI interfaces which is great feature than XORP. It can also configure the whole network protocol with a single file. Vyatta OFR is superb and second competent router application in open source software. Vyatta OFR can support extensibility through plugin and “iso” file is available for installation on website. 
 L.J. Staff, “New products,” Linux J., vol. 2006, 2006, p. 16.
 L.J. Staff, “New products,” Linux J., vol. 2007, 2007, p. 21.
 E. Hunguana, S. Kafouros, A. Razzaq, X. Ren, and H. Shiode, “Xclouer Distributed Router Field Trial,” TSLab - a creative laboratory | Telecommunication Systems Laboratory (TSLab). http://www.tslab.ssvl.kth.se/csd/projects/0723/files/Final_Report.pdf
 “IBM SystemsVirtualization,” IBM - United States. http://publib.boulder.ibm.com/infocenter/eserver/v1r2/topic/eicay/eicay.pdf
 “Vyatta Open Networking ,” Vyatta Open Networking - The Open-Source Alternative to Cisco. http://www.vyatta.com/index.php
 E. Keller, R.B. Lee, and J. Rexford, “Accountability in hosted virtual networks,” Proceedings of the 1st ACM workshop on Virtualized infrastructure systems and architectures, Barcelona, Spain: ACM, 2009, pp. 29-36.
 M. Handley, O. Hodson, and E. Kohler, “XORP: an open platform for network research,” SIGCOMM Comput. Commun. Rev., vol. 33, 2003, pp. 53-57.
 “Quagga Software Routing Suite,” Quagga Software Routing Suite. http://www.quagga.net
 Y. Li, J. Liu, and R. Rangaswami, “Toward Scalable Routing Experiments with Real-Time Network Simulation,” Proceedings of the 22nd Workshop on Principles of Advanced and Distributed Simulation, IEEE Computer Society, 2008, pp. 23-30.
 T. Spalink, S. Karlin, L. Peterson, and Y. Gottlieb, “Building a robust software-based router using network processors,” Proceedings of the eighteenth ACM symposium on Operating systems principles, Banff, Alberta, Canada: ACM, 2001, pp. 216-229.
 E. Kohler, R. Morris, B. Chen, J. Jannotti, and M.F. Kaashoek, “The click modular router,” ACM Trans. Comput. Syst., vol. 18, 2000, pp. 263-297.
Network Management and Monitoring Tools
The basic Network Management Architecture (NMA) and set of module relationship are same for every Network Management System (NMS) . Network Management Systems (NMSs) are used different approaches such as rule-base and machine learning [5, 6].
Networking monitoring tool can make sure the accessibility of the network using features e.g. FCAPS . On End Station (ES) on the network are manageable devices e.g. computers and network devices. When a problem is occurred on ES then inform to Management Entity (ME) with the help of specific software that run on ES. Then Management Entity (ME) has responsibility to take an action and manage it e.g. generate notification, event, logging off devices or sending polls. Polls are automatically generated or user initiated sent to ES from ME checking for the value of event. The software agent that runs on ME has responsibility to check the value of event and responses to all polls. The main functionality of software agent is to compile the value of event stored in database and inform to ME within NMS with the help of Network Management Protocol (NMP)  e.g. SNMP, CMIP. [1, 2, 9]
According to  OpenNMS and Nagios are used the basic NMA. OpenNMS and Nagios are two Network Management (NM) and Monitoring tools that partially fulfils of above mentioned features [1, 2, 3, 4].
OpenNMS is used a rule-based approach  and used a centralized network management model . It was envisioned in 1999 and now it is1st largest enterprise-grade Network Management System. In 2000 OpenNMS was registered on open “source software development” environment Sourceforge.
OpenNMS was developed by a group of NM professionals and software developers in Raleigh  using the Open Source Model (OSM) . OpenNMS source code is written in JAVA language within 140,745 code lines.
OpenNMS facilitates to the users and administrators of network management e.g.
To required and manage the network by the administrators and have no solutions. The user point of view , they should be replaced the expensive commercial NMS with OpenNMS due to low-cost, use for other platforms and wants to modify the OpenNMS for specific requirements .
OpenNMS is an alternative solution of commercial NMS such as Cisco and IBM .It is web-based NMS and provide full-fledged environment for small and large scale network [1, 2]. OpenNMS runs on Window platform as well as Linux environment . OpenNMS is also incorporate with different open source network monitoring tools i.e. due to producing graph of networks, monitoring capability etc e.g. JRobin  and DRAMA (MANET) .
OpenNMS addressed to how to configure [2, 9]
* Topology of network,
* Services ( based on the status of server, router and switches), and
* Webmail, web server and database of themselves.
All information of configuration is available in extensible markup language (XML) file .
OpenNMS is single elected management station for whole network that has responsibility to monitor different services [2, 7, 9]. OpenNMS supported different network services [1, 2, 7, 9, 10]
* Discovery interfaces, ICMP, Polling, SNMP, Events, Router TCP, Notification, threshold and WebUI, FTP, HTTP, MySQL, Oracle and Postgres databases , DHCP, IMAP, DNS and POP3
Conceptual Architecture of OpenNMS
Figure 1  is describe the normal execute services components and the internal layout of OpenNMS for the reader. Three core areas are covered by the OpenNMS namely Data Collection, Services Polling and Event and Notification Management [2, 14]. To collect the data, store, generates the threshold and report the network information in Data Collection , availability of services are determined and reporting in services Polling, and different events (internal and external) are received and generated a notification in this phase. Conceptual architecture described the relationship among components and their division. This architecture is described and analyzed the OpenNMS system and information of related product. Figure 1  is also envision of the several modules of OpenNMS system.
Administrator Graphical Interface
OpenNMS has a Graphical User Command Interface (GUCI) where administrator can manage and monitor the network services  e.g. generate reports, monitoring and controlling the ES.
Web Server Interface
OpenNMS is easy to handling of web functionality, user functionality [2, 9] through Web Server Interfaces. Web functionalities are dealing with management and manipulation of web node, interface and service information. User functionalities are management and manipulation of web users information, agreement among group servelt, web-graph, web-authentication, and web-polices .
Event Control Unit
Event Control unit is core part of the Network Management Systemused for event handling and event management [2, 9]. The different events are coming from different networks or from administrator also managed by this unit. When the events are occurred on network (servicing, monitoring or configuration) the event-handling listen it and inform (through event) to SNMP  or network administrator when its need. Event Control unit consist different commponents described in below
a. Event Handling
The event handling is central part of management component. This component can be managed the unlike events arriving from the network or the administrators. This core part is listening event from network (e.g. Discovery, monitoring, configuration, and servicing) then processes on event and forwards to Master Station when it needs. Incoming events can be listen through event handler and broadcaster then load and added events into database with the help of “event.conf” and Java Message Services (JMS) .
b. Event Correlation
The auto execution services are placed on event correlation. When the service is receiving an event (either in form of notification, auto action or trouble ticket) then processes it and generated a specific execution command.
c. Event schedule & archive
To store and remove event in database through events schedule & archive. The devices on network are stored in database, for these devices the OutageManger collects the particular event and maintains a historical archive of each outage.
d. Event Notification
When a new node is discovered on network then notifies and polls for this node through event notification. The discover node's data is stored and managed the data into database with the help of event handling component. Notification execution service has sub-component called “Notifd”. When service is received events then execute the suitable commands against the notification.
e. SNMP or JoeSNMP
SNMP protocol is written completely in java language and freely available . Java SNMP known as “JoeSNMP” is used for communication between manager and remote agent with SNMP session. SNMP session is encapsulating different parameters e.g. packet encoding or community strings .
OpenNMS is generated a node report in different ways. The method category-wise-reports in view.xml [2, 9] file are used for different category reports with the help of filter. IP interfaces are getting with the help of filter and node report created by referencing tables stored in database
OpenNMS is stored the inforamtion about network interaces in storage unit known as PostgreSQL [2,9]. Serveral functios are usred for collect data e.g. retrive and sotre. These functions are used to calculate the downtime for all managed services, number of services, percentage availability of services on interfaces between two date .
OpenNMS is used centralized approach to monitor and manage the node to node or node to agent. A management station should be use management protocols e.g. Simple Network Management Protocol (SNMP) [7, 9, 10]. For example the communication session is managed between MS and remote agent. Session is consist different parameters like community string, encoding and version number. When a session is created MS can communicate with remote-agent [2, 9].
The SNMP is not only protocol, but it also used as the same meaning of the Internet Standard Management Framework (ISMT). SNMP has different components such as SNMP engine, manager, protocol (SNMP) and Management Information Base (MIB). SNMP engine is referred to an “agent” that may be software or hardware. The network nodes are remotely managed by the agent via SNMP. The agent can be managed with the application, referred to SNMP manager. Protocol or Simple Network Management Protocol (SNMP) is used for information swap between agent and manager, and Management Information Base (MIB) is well organized information structure. Agent and manager can understand to each other with the help of MIB. MIB structured is same for all other protocols. [11,12]
Management Information Base (MIB)
OpenNMS auto execution is performed on the bases of IP interfaces for specific services or application that run on network [7, 9]. When service is received any event in form of notification, auto action or trouble ticket then appropriate command is generated. Different service has different functionalities e.g. Notification for discover node, services (DHCP request and response), extensible markup language (XML) services for configuration, TCP, filtering and Protocol Data Units (PDU) services .
Several interfaces are defined on network to monitor the services through OpenNMS . Today many different types of networks exist on a single network including IPX, IP version 4 and 6. OpenNMS provides the facility to accommodate and monitor this environment with providing the interface information .
Simple Network Management Protocol (SNMP) is centralized unit that activates the commands for this component. A network can be configure and manage through certain management commands execution. For example application level (i.e. DSL, access register and cable configuration), networking level (i.e. IP and Gateways) and routing level (i.e. switch router and hubs configuration) configurations are performed with the help of configuration management commands [2, 9].
Network Device Interfaces
OpenNMS are monitoring the network interfaces with different network services and protocols. The services and protocol are Simple Mail Transfer Protocol(SMTP), Internet Control Message Protocol (ICMP), File Transfer Protocol(FTP), Hyper Text Transfer Protocol(HTTP), Domain Name Server(DNS), Transfer Communication Protocol (TCP/IP) and Dynamic Host Configuration Protocol (DHCP) [2, 9].
Network monitoring tool can monitors the network services and resources with the intention of failure detection. Nagios is very popular Open Source Framework (OSF) and widely used in academic research area and commercial network monitoring. The network bandwidth and server room temperature is also monitor. In 2000 next step of NetSaint is a Nagios which is stable monitoring tool like Openview. Nagios can monitor the hardware, network devices, services and applications like computers, router, and switches, DNS, DHCP, FTP and Postgresql. Nagios can provide overview and monitor system such as system's status, notification when wrong event occurred, predefined solution to resolve the problem and historical reports (down time). Nagios is similar with other monitoring tools (Big Brother and Openview), and it is available free of cost while Openview is expensive tool. Mostly organization is using to monitor the network services with Nagios platform e.g. JP Morgan Chase, yahoo and European Space Astronomy Center (ESAC).[3,12, 14, 16, 18, 19, 20]
Nagios runs only on LINUX or UNIX platform. An external program can use for window environment known as “plugin”. Nagios can monitors the hosts and services using plugin e.g. Nagios Remote Plugin Executor (NRPE). [3, 12] Nagios can be implemented as centralized or distributed approach depends on network infrastructure. In centralized approach, Nagios server is placed in center and monitors the whole network resources. In distributed approach each resources has a Nagios instance that send the information to the central Nagios or server e.g. Nagios Services Check Acceptor (NCSA). NCSA has ability to transfer the information from remote instances to server. [3, 11, 13, 14, 19]
Nagios is an enterprise level network and monitoring system for small scale network. Each host and its services are manually managed so it is better for small scale network rather than large scale due to time consuming. To configure the hosts (router, switches and services) and services on network with simple set the parameters in configuration files. [3, 11, 12, 13]
Conceptual Architecture of Nagios
Nagios sever is a platform that monitoring the network with the help of plugin. Plugin is an external program directly runs on Nagios server. There are two type of plugin that runs on Nagios server namely “Active check” and “Passive check”. [11, 12]
In Active check plugin the Nagios server itself take the initiated and trigger the test. There are four kinds of plugin namely “check_xyz”, ”check_by_ssh”, ”check_nrpe” and ”check_snmp”. 
Check_xyz plugin can use to monitor the web mail or web server under the Simple Network Management Protocol (SNMP) and Hypertext Transfer Protocol (HTTP). 
Check_by_ssh plugin can use to monitor the local resources such as memory, disk, and CPU usage on remote side via Secure Shell (SSH) or Remote shell (RSH). 
Nagios Remote Plugin Executor (NRPE) is an alternative way of secure shell to install the plugin on local side. Check_nrpe is used to monitor the services with daemon that configure on local resources.SSH is mostly installed on UNIX platform. When SSH is used for monitoring to local resources then it must be configured as enable mode on Nagios server. SSH is connecting with local shell required password to enabling any command to be run on local resource. NRPE is confidential handling and configuring with commands. 
Local resources are also monitored via Simple Network Management Protocol (SNMP). Mostly NMS has assembled SNMP while Nagios are using check_snmp plugin to handle SNMP. The SNMP managed the resource information in MIB and the basic structure of MIB is same as discussed in OpenNMS part. [11,12]
In figure “Conceptual Architecture of Nagios” all phases are active check except external data. [11, 18]
In Passive check plugin the Nagios server are waiting incoming information from client side. The client sends this information with the help of “send_nsca” program to the Nagios server. Nagios Service Check Acceptor (NSCA) is a daemon run on Nagios server and accepts the information from client and forward to the Nagios interface with the help of External commands.
Conceptual Architecture of Nagios 
It contains the Nagios monitoring logic and coordinates (downtime scheduling and escalation). The main configuration file “nagios.cfg” is used to configure Central Daemon. When the Central Daemon is running then Web Interface is activated or access. [11, 18]
Check command is parameter that set to perform the different functionality such as availability of host (check_ping), test the services functionality (check_nrpe) and check the status of Central Daemon (nagios_check_command).
The other commands are used to monitor the external application e.g. NSClient/NC Net or Computing Center Management System framework (CCMS). [11,18]
External applications are run at expected interval to monitor the individual item such as CPU and memory usage etc. 
Web Server or Common Gateway Interface (CGI)
Common Gateway Interface (CGI) is a program using commands to see the working functionality of Nagios. CGI can provide the information about host and service status to the user via web browser, scheduling and alert on respond. Web server is help full to access the web browser e.g. Apache. Web server is must know about the Web and CGI directories. Nagios or Central Daemon can monitor the Web server through plugin check_http and check_ftp with the help of check_squid.pl plugin. [11,18]
Hosts are physically entities such as server, router, switches and station. Services are tangible and intangible items that are associated with hosts. The examples of tangible item are disk space memory usage, and intangible items are Domain Name System (DNS), Hypertext Transfer Protocol (HTTP), and Lightweight Directory Access Protocol (LDAP). Dependencies can be defined among host and services. Dependency is also defined in term of relationship between services such as, parent and child services. Nagios cannot be notified to child services when a parent service is not working. Hosts are monitored when services change their state. [11, 13, 18]
In Nagios hosts and services are monitoring through plugin The return state or value of plugin could be one out of four in case of service e.g. 0 for Ok, 1 for WARNING, 2 for CRITICAL and 3 for UNKNOWN.
* 0 for Ok: system is working in order.
* 1 for WARNING: limit has been exceed but not reached at critical limit.
* 2 for CRITICAL: In case of WARNING when limit is exceed at critical point then it will stop working.
* 3 for UNKNOWN: When wrong parameter is used in plugin.
For host case the possible return values are UP and DOWN
* Up: when the host is reachable it shows UP state.
* Down: when host is out of reachable then show DOWN state, and Nagios cannot test host. [11,18].
Data is stored in hard disk. Nagios stored data are scheduling management (downtime), hosts-services status and configuration (static-dynamic). Nagios can assemble the network using different configuration files e.g. [3, 11, 13, 19]
* hostextinfo.cfg, and
Event handling mechanism is not provided in distributed Nagios. When uncertainly events are occurred then Nagios attempt to resolve in specific number of time. There are two possible states during that time called “Soft state” or “Hard state”. When Nagios is first time check failure event on network then the state of event is known as Soft state and if event does not change the state after final execution of check then it submit to Hard state. Nagios normally send a notification to administrator in hard state. For example if failure service event is occurred then Nagios attempt to restart it without sending any notification to administrator with the help of event handler. If the service does not change the state then it goes to Hard state then send a notification to contact person. This procedure is known as “Escalation management”. Nagios is differentiated between host and services at the level of escalation. [3, 11, 13]
In user machine a web client supports the web-browser. It enhance and support the capability of web-browser when handle the device on remote side. It also refers as plugin or helper of web-browser. When the problem is notified to staff or technician then they use to find the solution with Nagios's web client. In Nagios web-client provide the facility to View of network structured and frequently point out where the problem occurs and what effect of that problem. [3, 21]
In external data the Passive check plugin are used. Nagios server is waiting for incoming information from client side. The client sends this information with the help of “send_nsca” program to the Nagios server. Nagios Service Check Acceptor (NSCA) is a daemon run on Nagios server and accepts the information from client and forward to the Nagios interface with the help of External commands. 
 J. Anaya, D. Conn, A. Floyd, T. Jackson, L. Jones, C. McGuire, S. McPherson, C. Rush, C. Santori, A. Shimkus, K. Smitley, M. Sparavalo, A. Srbinoski, E. Valdivia, M. Wozniewski, M. York, and S. Zaronias, “Network Architecture & Design; Virtualized Network Design for Computer Information Technology Department,” Purdue University Calumet | Computer Information Technology and Graphics, available at http://cit-dept.calumet.purdue.edu/liless/web/virtual_network_design_document.pdf
 “OpenNMS,” OpenNMS. available on internet http://www.opennms.org/ , Last visited
 “Nagios - The Industry Standard in IT Infrastructure Monitoring,” Nagios - The Industry Standard in IT Infrastrure Monitoring. available on internet http://www.nagios.org/
 T. Balog and D. Gregor, “OpenNMS® Installation Guide,” Home Page - The University of Texas at Dallas. available on internet http://www.utdallas.edu/~alston/opennms/install.html
 P. McLachlan, T. Munzner, E. Koutsofios, and S. North, “LiveRAC: interactive visual exploration of system management time-series data,” Proceeding of the twenty-sixth annual SIGCHI conference on Human factors in computing systems, Florence, Italy: ACM, 2008, pp. 1483-1492.
 F. Pouget and M. Dacier, “publidownload.en.htm (application/pdf Object),” EURECOM - Communications Systems - EURECOM .
 C. Chiang, G. Levin, Shihwei Li, C. Serban, M. Wolberg, R. Chadha, G. Hadynski, and L. LaBarre, “Enabling Distributed Management for Dynamic Airborne Networks,” Policies for Distributed Systems and Networks, 2009. POLICY 2009. IEEE International Symposium on, 2009, pp. 102-105.
 “JRobin :: RRDTool Choice for the Java World :: Welcome to JRobin,” JRobin :: RRDTool Choice for the Java World :: Welcome to JRobin. Available on internet http://oldwww.jrobin.org/
 B. SHIHADA, “finalproj.pdf (application/pdf Object).” Available on internet http://shihada.com/finalproj.pdf
 “Internetworking Technology Handbook - Simple Network Management Protocol (SNMP) - Cisco Systems,” Simple Network Management Protocol (SNMP).
 W. BARTH, NAGIOS SYSTEM AND NETWORK MONITORING, Germany: Dr. Markus Wirtz, 2005.
 J. Ellison, “SNMP monitoring with Nagios,” Linux J., vol. 2009, 2009, p. 4.
 E. Imamagic and D. Dobrenic, “Grid infrastructure monitoring system based on Nagios,” Proceedings of the 2007 workshop on Grid monitoring, Monterey, California, USA: ACM, 2007, pp. 23-28.
 A. Marcello, “MarcelloFinalPaper.pdf (application/pdf Object).”
 “HP Software,” Hp Software.
 R.C. Harlan, “Network management with Nagios,” Linux J., vol. 2003, 2003, p. 3.
 “nagios-ltm-dg.pdf(Appliation/pdf Object)”, Deploying the BIG-IP LTM with the Nagios
Open Source Network Monitoring System, available at http://www.f5.com/pdf/deployment-guides/nagios-ltm-dg.pdf
 “Pre graduate projects at the European Space Astronomy Centre (ESAC) in 2006,” Internacionales Ingles. Available at http://www2.uah.es/spas/pdffiles/trainee_2006_PR2.pdf
 C. Lahti, S. Lanza, and R. Peterson, Sarbanes-Oxley IT Compliance Using COBIT and Open Source Tools, Syngress,2005.
 M. Schubert, D. Bennett, J. Gines, A. Hay, and J. Strand, Nagios 3 Enterprise Network Monitoring: Including Plug-Ins and Hardware Devices, Syngress Publishing, 2008.
 T. Andresen, M. Tallaksen, U. Prøitz, and A. Harsjøen, “INF5210 - autumn 2004
Information Infrastructures,” Universitetet i Oslo, available at http://www.uio.no/studier/emner/matnat/ifi/INF5210/h04/prosjekt/oblig4/obl4grpA.pdf