Servers will process many request from clients, but servers have a limit to how many connections can be serviced at one time, an example would be a web server. A load balancer will distribute network data across multiple interfaces or hosts. It is an intelligent appliance that can determine if one interface or server is managing too much of a load and direct network traffic to another interface or server.
When a business cannot keep up with orders they will tend to hire more employees to manage demand. Load balancers in effect function the same way, they notice demand and transfer that demand onto an unused or less utilized resource.
A proxy server has the ability to cache or store information, so when a resource such as a web site is requested again, no unnecessary processing needs to take place. For instance, a user requests the web page www.yahoo.com; the proxy server queries DNS for the location, IP address and other data, stores this data in cache, and provides the user with the www.yahoo.com webpage.
When another user or host request the same webpage, the proxy server already has the required information and can provide the requestor with the web page content. This type of functionality decreases the amount bandwidth required on a network. Imagine 100 users requesting the same information, each request utilizing bandwidth, when a proxy server could have stored the request from one of the 100 users and serviced the other 99 users. Proxy servers eliminate unnecessary redundancy.
A Content Filter will examine data packets and block or filter those data packets whose content is not allowed on the network. For instance some network administrators may not allow Facebook or other social media on the network. Any data packet with the IP address or name of www.facebook.com will be blocked. Websense and Barracuda are examples of content Filter Appliances. For more information please review the following: Content-control software.
A virtual private network (VPN) concentrator is used to manage large network environments that have many users’ that will require VPN connections. VPN concentrators manage VPN clients and the tunnels created between them. VPN is a more secure, encrypted and up– to– date way of providing remote access to internal network resources when compared to Dial-up RADIUS installations.
Performance and Optimization Strategies
Strategies are needed for managing performance and optimization. A highly available system is one that is accessible even when a problem occurs. You have already been introduced to traffic shaping, load balancing, high availability, and caching strategies to increase network performance. Two additional strategies include: Fault Tolerant systems, and Common Address Redundancy Protocol (CARP).
1. A fault tolerant system means that the system can still function if the primary resource becomes unavailable. Provided below is an example of applying the fault tolerant strategy.
A primary web server in Los Angeles, CA is impacted by a city wide power outage, but a backup or “standby” web server located in Seattle, WA picks up the primary role so users that access the website hosted on the web server are not adversely impacted.
Redundant Array of Independent Disks (RAID) is another fault tolerant system. RAID disk provide a mechanism to recover data if a particular disk drive crashes or becomes unavailable.
2. Common Address Redundancy Protocol (CARP) allows a pool, or group of computer, or interfaces to share one IP address. This allows a router with various network interfaces to automatically failover if one of the interfaces fails or goes down.
Quality of Service (QoS)
QoS or Quality of Service places a priority or weight on different types of network traffic. For instance, video and VoIP data require greater bandwidth than an email message traveling through the network. QoS ensures that there is enough network bandwidth to allow the best possible transmission of high priority data traffic such as VoIP, or video. QoS is the measurement of expected vs. actual results. Whether you are placing a phone call on PSTN or VoIP, you expect a clear, crisp, uninterrupted conversation.
QoS uses the following techniques to achieve expected results:
1. Resource Reservation Protocol (RSVP) makes an attempt to reserve network resources before transmitting. This allows for the transmission path between sender and receiver to be adequately appropriated for the signal to arrive without delay. (Imagine if on your way to work, you had a police escort that allowed you to bypass stop lights and maintain high speed rates.) RSVP reservations are transmitted by sender and receiver, and take up a considerable amount of bandwidth. RSVP is scalable to small networks, but large networks require a more efficient QoS solution.
2. Differentiated Service (DiffServ) considers all types of data traffic and assigns priority accordingly. It inserts data within the datagram or data packet which defines how a router should forward the data. There are two forwarding types: Expedited Forwarding (EF) and Assured Forwarding (AF).
- Expedited Forwarding (EF) each data packet is assigned a minimum departure rate from the router. This prevents delays that would normally slow normal data from reaching its destination in sequence. Imagine you are on the freeway following a family member to their house. All the cars are traveling at different speeds some fast, and others very slow. During the drive you become separated from your family member, because they drive a little faster than you do. Other vehicles slide in between you and the person you are following. Now imagine if all the cars on the freeway had to drive the same speed; it is less likely that you and your family member would become separated and all traffic would flow smoothly allowing for consistent estimated time of arrival.
- Assured Forwarding (AF) each data packet is assigned a departure rate from the router based on the type of data being forwarded. Data packets are prioritized, but not guaranteed to arrive in sequence. Imagine the same example used with Expedited Forwarding, although this time you take the (HOV) lane, where you are allowed to travel at a higher rate of speed than your family member on a separate section of the highway. It is probably unlikely that you and your family member will arrive at your destination at the same time or in the correct order.
3. Multiprotocol Label Switching (MPLS) places a label on the data packet. This label tells the router where to forward the packet next. No routing table lookups are required since the label has all the necessary information to get data from source to destination. This technique allows for fast delivery which is an attribute of video and VoIP communications.
Networks often combine multiple QoS techniques to achieve desired performance and optimization. Latency can be described as the time it takes for a data packet to reach its destination. The time the data packet is held by a network device before forwarded contributes to latency.
