Cisco Networking Assessment - Essay Example

CISCO NETWORK ASSIGNMENT Question 5 marks) Error detection occurs at both the Network and Transport layers of the OSI reference model. Explain why this occurs and explain what action is taken at each layer when an error occurs when using the TCP/IP protocol architecture. The reason why error detection occurs is explained briefly by the article provided by Wikipedia (Wikipedia, 2008). It states: Error detection is the ability to detect the presence of errors caused by noise or other impairments during transmission from the transmitter to the receiver. Error correction is the additional ability to reconstruct the original, error-free data." The reason error checking occurs is to ensure good Quality Of Service (QOS), which governs how network quality is to be achieved (The TCP/IP Guide, 2005). There are vaious types of error detection. One example of this the Cyclic Redundancy Check (CRC) code. Data is transmitted as a packet of information. A packet of information is the format in which a message is transmitted over a network. There are a few parts to a typical message which is transmitted over a network. Error correction also includes the additional ability to reconstruct the original message, with error-free data. Two basic methods have been designed to handle the error correcting system. These are called the Automatic repeat-request (ARQ), and the Forward Error Correction (FEC). The ARQ sends that data with an error detection code, which the receiver user to check for errors. In the FEC method the transmitter includes an error-correcting code (ECC) and sends the coded message. The receiver never sends any data to the transmitter, it just decodes what it thinks is the most "likely" data. At the network later and the transport layer the actions taken to correct errors are follows: Network Layer: The Network Layer deals with errors via a variety of means (Wikipedea, 2008). These are to ensure the QOS is maintained. These can be items such as the Parity check, which means that if any bits are corrupted in the message, they are replaced to the expected format. A checksum, which involves an algorithm to check the validity of the data, Hash functions, which perform a similar function and repetition schemes. The full list of methods is stated here: Repetition schemes Parity schemes Checksum Cyclic redundancy checks Hamming distance based checks Hash function Cryptographic message digest Horizontal and vertical redundancy check Polarity schemes Transport Layer: The Transport Layer uses 2 two way error correction protocol, which involves a discussion between the sender and the receiver. This is where the receiver informs the transmitter to resend a message when an error occurs. The two basic types for each packet transferred are called a NACK (Not Acknowledged), when a failure occurs and an ACK (Acknowledged) when a packet has been successfully received. The way it does this is to check the header/footers of packets to see if it is in the format expected. This is usually achieved by CRC (Cyclic Redundancy Check) checksums which are calculated based on the data transmitted. Summary: Basically, the Network Layer function is to check the validity of the data at a lower level than the Transport Layer, and communicates the problem up the hierarchy. The Transport Layer in turn, informs the transmitter to resend the erroneous data. The Transport layer has other functions, such as allowing a mechanism where the receiver informs the transmitter what to send next and what is expected. Question 2 (5 marks) What are the different types of collisions that can occur on an Ethernet network Briefly explain each. There are the types of collision where two packets are sent over the network at the same time, and this is handled by the CSMA/CD (Carrier Sense Multiple Access / Collision Detect) protocol and a 'Collision Domain'. A Collision Domain happens when two devices on the network transmit at the same time (The TCP/IP Guide, 2005). An Ethernet network can be composed of a single or a multiple segment. This assists with prevention of collisions on a large network. Question 3 (5 marks) Using at least three (3) different references, describe the differences between Link State and Distance Vector routing protocols. Wikipedea (Wikipedea 2008), states the following about each protocol: "A link-state routing protocol is one of the two main classes of routing protocols used in packet-switched networks for computer communications. Examples of link-state routing protocols include OSPF and IS-IS." "This contrasts with distance-vector routing protocols, which work by having each node share its routing table with its neighbours. In a link-state protocol, the only information passed between the nodes is information used to construct the connectivity maps." CCNP Routing Exam Cram, Chapter 3, Routing Protocol Overview, (Eric McMasters, Brian Morgan, 2000) states the following about each protocol: "Link-state protocols were developed specifically to overcome the limitations of distance-vector protocols. Whereas distance-vector protocols simply broadcast updates out of each active interface when the update timer expires, link-state protocols send an update only when there is an update to send, and even then only the precise change goes out. Link-state protocols actually enter into a conversational state with their directly connected neighbours. This ongoing conversation consists of routing updates as well as paranoid updates, which occur usually every thirty minutes if there are no other routing changes. A paranoid update is simply one router asking its neighbours whether they are still there." Distance-vector protocols were the first in the family of dynamic routing protocols. They function on the premise of metric addition. This algorithm is known as the Bellman-Ford algorithm. Consider a ticket line in front of a theater. You are third in the line. What if you could not see the ticket window due to an obstruction such as a wall Now, you know that you are third in the line only because the person ahead of you told you he is second in line. How does he know he is second in line He knows only because the person ahead of him claims to be first. What is the ultimate destination The destination is the ticket window. You can only know for certain that there is a person ahead of you and a person behind you. That's the principle behind distance-vector technology. A router knows its position in the network only because its neighbours told it their positions in the network. The function of this type of protocol is commonly referred to as routing by rumour. Distance-vector protocols generally use periodic updates. A periodic update is one that's on a timed interval. Once the interval timer reaches zero, a routing update is broadcast out of every active interface, whether there have been changes or not. Because distance-vector protocols use these periodic timers, they tend to be slow to react to topological changes in the internetwork. To that end, convergence tends to be somewhat slow. In some cases, several of these protocols have implemented triggered updates to compensate for a loss of signal on the wire. The following is a list of various types of distance-vector routing protocols" Network Routing Basics: Understanding IP Routing in Cisco Systems, by James Macfarlane (James Macfarlane, 2006) states for both protocols: "Link-state-based routing protocols (also known as shortest path first protocols) are a step up the routing protocol evolutionary ladder. Link-state protocols are based on the Dijkstra algorithm. Rather than relying on routing information learned through hearsay, link-state routers receive first-hand routing information from each router in the area. The information is transmitted in the form of Link-State Advertisements, or LSAs, which are passed from router to router and include information about the state of each router's directly connected links (IP address, speed of link, and so on). Unlike distance vector protocols which only compile lists of networks and the interface to forward packets through in order to reach those networks, link-state protocols allow each router to know how many routers are out there and what networks are connected to each router. Every router ends up with a topology map of the system. Because the routers are only passing information about their own links, rather than the entire routing table, initial convergence is improved. Furthermore, LSAs are usually only sent when the status of a link changes. This important feature also improves convergence.[5.]" "Distance vector protocols are based on what is colloquially known as "routing by rumor." Routers running a distance vector protocol don't receive routing information from each router in the system. Rather, a router running a distance vector protocol is told about networks known to other routers by its immediate neighbours. Each router shares routing information with its neighbours in the form of routing update packets. The neighbouring routers receive these updates, add the information about the newly learned networks to their own route tables, and then send the combined information to their neighbours. In this manner, each router in the system eventually learns the path to all networks, but much of the information is second-hand-thus the "routing by rumor" moniker Figure 4-1. illustrates in basic form how a distance vector protocol communicates." Figure 4.1 To summarise these quotations, we can see that the routing protocol determines the path to networks. This in turn determines the make up of the route table on each router. Therefore Routing protocols either use distanced vector or link-state methods. Question 4 (5 marks) Cisco extended access lists can use an optional key word 'established'. What is the purpose of the 'established' key word in access control lists, how does it work, and discuss an example of when it may be used The purpose of the keyword 'established' (Managing Cisco Network Security, 2002) is used to permit a return of traffic if it is a response to something that was originated inside the original network. It is an additional security measure, put in place to prevent Hackers from gaining access to the network. It works by allowing a range of workstations access to say, the internet without having an entry for all the machines on the network. For example, we may want to allow all workstations access to the Internet, but obviously, we cannot create individual lines in an access list to permit traffic back from every Web server on the entire Internet. To solve this problem, the established keyword is added to the extended access list. If the established keyword is used on a line of the access list, it will only allow a packet through if it matches the line of the list, and has either the ACK or RST bit set in the TCP header. A typical access list, including the keyword 'Established' is as follows: access-list 111 permit tcp any host 172.17.0.0 0.0.25.255 established access-list 111 permit tcp any host 172.17.11.19 eq 25 access-list 111 permit tcp 12.0.0.0 0.255.255.255 172.22.114.0 0.0.0.255 eq 23 References: Books The TCP/IP Guide by Charles M. Kozierok (2005) CCNP Routing Exam Cram, Chapter 3, Routing Protocol Overview, Eric McMasters, Brian Morgan, (2000) Network Routing Basics: Understanding IP Routing in Cisco Systems, by James Macfarlane (2006) Managing Cisco Network Security, Second Edition by Eric Knipp et al (2002). Websites Wikipeda (2008) http://en.wikipedia.org/wiki/Error_detection http://en.wikipedia.org/wiki/Link_state http://en.wikipedia.org/wiki/Network_layer Read More