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There are 3 key components to SNMP: Managed Devices, Agents, and Network Management Systems (NMSs). These are shown in Figure 1 below.
The Managed Devices contain the SNMP Agent and can consist of routers, switches, hubs, pcs, printers, and items such as these. They are responsible for collecting information and making it available to the NMSs.
The Agents contain software that have knowledge of management information and translates this information into a form compatible with SNMP. They are located on a managed device.
The NMSs execute applications that monitor and control the managed devices. Processing and memory resources that are needed for network management are provided by the NMSs. A minimum of one NMS must exist on any managed network. SNMP can act solely as a NMS or an agent, or can perform the duties of both. There are four basic commands used by SNMP NMS to monitor and control the managed devices: read, write, trap, and traversal operations. The read command examines the variables that are kept by the managed devices. The write command changes the values of the variables stored by the managed devices. Traversal operations look to find out what variables a managed devices supports and gathers information from the supported variable tables.The trap command is used by the managed devices to report the occurrence of certain events to the NMS.
SNMP uses four protocol operations in order to operate: Get, GetNext, Set, and Trap. The Get command is used when the NMS issues a request for information to managed devices. The SNMPv1 message (request) that is sent consists of a message header and a Protocol Data Unit (PDU). The PDU of the message contains the information that is needed to successfully complete a request that will either retrieve information from the agent or set a value within the agent. The managed devices use the SNMP agents located on them to retrieve the needed information, and then respond to the NMS with an answer to the request. If the agent does not have any information in regards to the request, it does not return anything. The GetNext command will then retrieve the value of the next object instance. It is also possible for the NMS to send a request (Set operation) that sets the values of items within the agents. When an agent needs to inform the NMS of an event, it will use the Trap operation.
As discussed, SNMP is an Application Layer protocol that uses passive sensors to help administrators monitor network traffic and performance. Although SNMP can be a helpful tool to Network Administrators it does create a vulnerability to security threats because it lacks any authentication capabilities. It is unlike Remote Monitoring (RMON) that is discussed in the following section in that RMON monitors at the Network Layer and below, rather than at the Application Layer.
While there are 3 key components to the SNMP monitoring environment there are only 2 in the RMON environment. They are shown in Figure 2 below.
The 2 components of RMON are the probe also known as the agent or monitor, and the client also know as the management station. Not unlike SNMP the RMON probe or agent gathers and stores the network information. The probe is embedded software on the network hardware, such as routers and switches. The probe can also run on a pc. The probes must be put on each different LAN or WAN segment as they only are able to see traffic that flows through only their link, and are unaware of outside links. The Client is usually a management station that communicates with the probe using SNMP to obtain and correlate the RMON Data.
RMON [RMON] uses 9 different monitoring groups to obtain information about the network.
As stated above RMON, builds upon the SNMP protocol. Although traffic monitoring can be performed with these techniques, analysis of the information provided by SNMP and RMON takes a little extra work. Netflow, which is discussed in the next section, works well with many analysis software packages to help make the job of administrators a little easier.
The flow caching analyzes [NetFlow06]and collects the IP data flows that enter an interface and prepares the data for exportation.
The following information can be obtained from Netflow packets: [NetflowAbout06]
The first packet of a flow through the standard switching path is processed to create the cache. Packets with similar flow characteristics are used to create a flow record which is kept in the cache for all active flows. The flow record tracks the packets and bytes per flow. The cache information is then periodically exported to the Flow Collector.
The Flow Collector [NetFlow06] is responsible for the data collection, filtering, and storage. It contains a history of the flow information that was switched within the interface. Data volume reduction is also done by the Flow Collector through selective filtering and aggregation.
Data Analyzer [NetFlow06] is then responsible for presentation of the data. As shown in the figure the data collected can be used for various purposes other than network monitoring such as network planning and accounting and billing.
The advantage of Netflow over other monitoring methods such as SNMP and RMON is that there are numerous traffic analysis software packages (data analyzers) that exist to pull the data from Netflow packets and present it in a more user friendly way.
By using a tool such as Netflow Analyzer [NetflowWhitePaper05] (just one tool that is available for analyzing Netflow packets) the information above can be pulled out of the Netflow packets to create charts and usage graphs that an Administrator can study to maintain an understanding of their network. The biggest benefit of using Netflow in combination with one of the available Analysis packages is that numerous different graphs detailing network activity can be created on the spur of the moment.
Commonly used tools such as ping, which measures delay and loss of packets, and traceroute which helps determine topology of the network, are examples of basic active measurement tools. They both send ICMP packets (probes) to a designated host and wait for the host to respond back to the sender. Figure 4 is an example of the ping command that uses active measurements by sending an Echo Request from the source host through the network to a specified destination. The destination then sends an Echo Response back to the source it received the request from.
Not only can a person collect the metrics above from active measurements, one can also determine the network topology. Another common example of an active measurement tool is iperf. Iperf is a tool that measures TCP and UDP bandwidth performance. It reports bandwidth, delay jitter, and loss.
The problem that exists with active monitoring is that introducing probes into the network can be an interference to the normal traffic on the network. [UnivPenn02] Often times the active probes are treated differently than normal traffic as well, which causes the validity of the information provided from these probes to be questioned.
As a result of the information detailed above, active monitoring is very rarely implemented as a stand-alone method of monitoring as a good deal of overhead is introduced. On the other hand passive monitoring does not introduce much if any overhead into the network.
Passive measurements deal with information such as: Traffic and protocol mixes Accurate bit or packet rates Packet timing and inter-arrival timing
Passive monitoring can be achieved with the assistance of any packet sniffing program.
Although passive monitoring does not have the overhead that active monitoring has, it has its own set of downfalls. [UnivPenn02] With passive monitoring, measurements can only be analyzed off-line and not as they are collected. This creates another problem with processing the huge data sets that are collected.
As one can see passive monitoring my be better than active monitoring in that overhead data is not added into the network but post-processing time can take a large amount of time. This is why a combination of the two monitoring methods seems to be the route to go.
The kernel level packet trace facility is responsible for capturing the information associated with incoming and outgoing packet. Figure 6 lists the information that is gathered for each packet. A buffer was added to the Web100 kernel to collect these characteristics. Access to the buffer is through 2 system calls. One call starts the trace and provides the information needed to conduct it while another call retrieves the trace from the kernel.
The packet trace facility is able to coordinate measurements between the different machines. One machine will trigger the other machine by setting a flag in the header of outgoing packets to start tracing the same range of packets that it is tracing. The other machine will in turn trace all packets that it sees with the same header flag set. This coordination ensures that the information about the same packets is stored at each end of the connection regardless of what happens in between.
The user level trace analyzer is the other level in the WREN environment. It is the component that begins any packet traces and collects and processes the data returned from the kernel level trace facility. By design the user-level components are not required to read the information from the packet trace facility at all times. It can be analyzed immediately after the trace is completed to make runtime decisions or stored for future analysis.
When traffic is low, WREN will actively introduce traffic into the network in order to maintain a continuous flow of measurements. After numerous studies, it was found that WREN produced the same measurements in congested and un-congested environments.
In the current implementation of WREN users are not constrained to capturing only the traces that were initiated by them. Although any user is able to trace another users application traffic they are restricted to the information that can be obtained from another users trace. They are only able to get the sequence and acknowledgement numbers but not the actual data segments of the packets.
In summary, WREN is a very useful tool that utilizes the benefits of both active and passive monitoring. Although it is in its early stages WREN can provide Administrators with a valuable resource in the monitoring and analyzing their network. Self Configuring Network Monitor (SCNM) is another tool that uses both active and passive monitoring techniques.
The hardware is installed at critical points in the network. It is responsible for passively collecting the packet headers. The software runs on the endpoints of the network. Figure 7 below shows the software components of the SCNM environment.
The software is responsible for creating and sending the activation packets that are used to start the monitoring of the network. A user will send an activation packet out into the network containing the details about the packets they want to monitor and gather. The user does not need to know the location of the SCNM hosts due to the fact that all hosts listen for packets. Based on the information that is within the activation packet a filter is set up within a data collection daemon that is also running on an endpoint. The network and transport layer headers of packets that correspond to the filter are collected. The filter will automatically time out after a specified amount of time unless it receives another application packet. The packet capture daemon which runs on the SCNM host uses a tcpdump like packet capture program in order to receive requests and to record the traffic that corresponds to the requests.
When a problem is detected by the passive monitoring tools, traffic can be generated using the active tools, allowing one to collect additional data to further study the problem. By having these monitors deployed at every router along the path, we can study only the section of network that seems to be having the problem. [Tierney04].
SCNM [Agarwal03] is intended to be installed and used mainly by network administrators; however average users can use a subset of its functionality. Although average users are capable of using part of the SCNM monitoring environment they are only allowed to monitor their own data.
In conclusion, SCNM is another combinational monitoring tool that utilizes both active and passive monitoring to help administrators monitor and analyze their networks.
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Being able to monitor and analyze networks is vital in the job of Network Administrators. They must strive to keep the networks they oversee in good health as to not disrupt productivity within a company and to not disrupt any essential public services. As summarized throughout this paper several router based and non-router based techniques are available to assist Network Administrators in the day to day monitoring and analysis of their networks. SNMP, RMON, and Cisco's NetFlow are a few of the router based techniques that are briefly reviewed. The non-router based techniques that were discussed were Active, Passive, and Combinational monitoring tools.
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|ICMP||Internet Control Message Protocol|
|LAN||Local Area Network|
|MIB||Management Information Database|
|NMS||Network Management System|
|PDU||Protocol Data Unit|
|SCNM||Self Configuring Network Monitor|
|SNMP||Simple Network Management Protocol|
|ToS||Type of Service|
|WAN||Wide Area Network|
|WREN||Watching Resources from the Edge of the Network|
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