CHARLES STURT UNIVERSITYINTERNETWORKING WITH TCP/IP

Student ID: 11452282Student Name: Claudia CardenasLecturer Name: Chandana Penatiyana Withanage

Sydney AustraliaApril – 2010

ASSIGNMENT 1

Question 1:

Discuss Why packet switched networks are used for data communication instead of circuit switched.

In order to note what are the advantages that packet switched offers to data communication and circuit switched not, it is need to know what circuit and packet switched are; also it is need to know how they works.

In circuit switched the connection between the sender and receiver is set up in advance as well as the path (Uruth, N.D.). This path is decided by taking into account different elements such as algorithms which are used for optimizing the use of resources. One of these resources could be bandwidth (Kozierok.2005). There could be more than one possible path; however, only one route will be used from the beginning to the end of the connection.

Figure 1 Circuit Switching from Kozierok 2005

In contrast packet switched networks works a bit different. First it is need to understand the meaning of a packet. It is also called as a datagram or segment (Uruth, N.D.). Each of these units of data contains information about its destination, header, and sequence numbers. In Packed Switching unlike circuit switching there is not a specific route or path for the entire connection. In addition the original data is fragmented in one or more units (packets); then each packet is routed by different paths in order to achieve its destination node. Once the packets have arrived to the destination nodes, the reassembly process takes place (Kozierok.2005).

Figure 2 Packet Switching from Kozierok 2005

Packet Switching Circuit Switching

More modern Old and expensive

Less reliability More reliability

Multiple paths One path

The connection is set up before the begginning of the communication

After reading the above paragraphs it is possible to conclude that packet switching is more suitable for data communications because it is less expensive; it is not need to have dedicated circuits for each connection; therefore the channels can be shared for more than one connection. Even though circuit switching is more reliable; however, packet switching uses the functionalities of different protocols such as TCP to makes it more reliable. Packet switching is less expensive and simple.

Question 2:

Given an IP address of 17.212.89.111 and that each subnet has been designed to accommodate 700 hosts.

a) What is the subnet mask in dotted decimal notation and slash notation?

The default subnet mask is: 255.0.0.0 because it is class A.

However we need to accommodate at least 700 hosts for each subnet; therefore we need to borrow 7 bits.

So the custom subnet mask is:

Dotted decimal Notation: 255.255.252.0

Slash Notation; 17.212.89.111/22

b) How many usable hosts are there per subnet?

The total number of host addresses per subnet is 1024; it is because 28=1024, however, there are two addresses that are designated for the network address and broadcast address; so the number of usable hosts is 1022 per subnet.

c) What is the network and the broadcast address of the subnet the specific host belongs to?

0 0 0 1 0 0 0 1 . 1 1 0 1 0 1 0 0 . 0 1 0 1 1 0 0 1 . 0 1 1 0 1 1 1 1

1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 1 1 . 1 1 1 1 1 1 0 0 . 0 0 0 0 0 0 0 0

0 0 0 1 0 0 0 1 . 1 1 0 1 0 1 0 0 . 0 1 0 1 1 0 0 0 . 0 0 0 0 0 0 0 0

The IP address 17.212.89.111 belongs to the network address 17.212.88.0, and its broadcast address is 17.212.91.254.

Question 3:

Consider the diagram below. Which of the two central routers (R1) and (R2) is more critical for network operations and why?

Based in the diagram, we should consider both routers as critical to perform routing operations such as task to determinate the best path to send an incoming packet based on a routing table that contains the mapping from the network and host address that is made using routing algorithms (Fairhust. 2001)(Martin. 2002)). The reason is because through R1 and R2 any hot that is located inside the network is able to reach other host in the networks N1, N2, N3, N4, N5 and N6. However, through R1 any packet is able to access more networks such as N1 can be reached by using router A, N2 can be reached by using router B, N3 can be reached by using router C, N4 can be reached by using router D and N5 can be reached by using routers C and E; however through R2 N3 can be reached by using routers E and C, N4 can be reached by using router D, N5 can be reached by using router E and N6 can be reached by using router F. So, in terms of accessibility the router R1 is more critical than R2.

Question 4:

Why is reassembly done at the ultimate destination instead of inside the network?

As it was explained in the question one the packets are fragmented in order to make the packets more manageable. This process could happen more than once depends on the routers and their capacities. Once all the packets are arrived to the destination, they are reassembled (Hampton, Donahue, Castanho. 2001). This is one of the reasons because the reassembly is done at the destination.

The other reason is related to packet switching. It is because in packet switching not all the packets use the same route; therefore it will be impossible to carry out the reassembly process inside the network. In addition it is need to highlight that the packets do not arrive in the correct order, neither would they arrive at the same time (NetworkWorld. 2007).

What are the problems with reassembling at the destination?

According to Uruth the packets could choose different routes or paths in order to get the destination node in the most efficient way. However this means that the packets may arrive to the destination node in different order than they suppose to be assembled. This problem affects more to voice or video than other kinds of data such as e-mails (NetworkWorld, 2007). Other problem could be the loss of certain data. However a protocol such as TCP/IP takes care of this concern.

Question 5:

Compare and Contrast DHCP and the ICMP messages that were previously used for startup configuration. What are the advantages of DHCP that caused to be the popular solution?

DHCP as well called as Dynamic Host Configuration Protocol is based on the Bootstrap protocol BOOTP with an additional capability such as dynamic allocation of network addresses that can be reusable (Droms. 1997). Unlike ICM messages which are used to report different kind of errors in the process of the datagram; this report is carried out between the router/host source (sender) and

router/host destination (receiver). This communication is possible by using the IP support (Postel. 1981).

It is possible to note that the main purpose of these two protocols are totally different however they need each other, because ICMP could not be able to send any error message to any host if it does not have a valid IP address. In the same way DHCP needs the services of ICMP messages in order to know if the assigned IP addresses are working correctly or not.

The figure 3 and 4 show that ICMP message is much simple and has less number of fields than DHCP message.

Figure 3DHCP Message Format from Corner

Figure 4 ICMP Error Message from Postel

There are different types of error messages. These errors will be show in the following figure

Figure 5 ICMP Error Messages from Streve and Perkins

It is need to note that ICMP also has vulnerabilities. These vulnerabilities are:

The ICMP message could be used to produce a Denial of Service attack and Man in the Middle attack. This is because to the lack of authentication field in the message format (Streve and Perkins. 2010).

A fake error message reply could cause problems in the network by not providing the services to the “unreachable address” (Streve and Perkins. 2010 ).

On the other hand DHCP as well hast its own advantages such as

The management of Internet Protocol addresses is done automatically. This could help to prevent issues related to IP addresses duplication.

Easy transition from BOOTP networks to DHCP.

DHCP servers are able to decide which workstations can use dynamic addresses according with their MAC addresses.

DHCP provides more functionalities and options configurations than BOOTP.

Reduce wasted addresses.

The pools of available range of addresses can be defined by the administrator

A complete list of leasing addresses and available addresses is maintained in a central location.

However DHCP also has few disadvantages such as:

Increase the network traffic.

Increase the start up time for establishing the connection.

REFERENCES

Corner, D. 2010. One Byte at a Time: Bootstrapping with BOOTP and DHCP. Cisco Systems. Retrieved March 26, 2010 from: www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_5-2/dhcp.html

Droms, R. 1997. Dynamic Host Configuration Protocol. Request for Comments 2131. Network Working Group. Bucknell University. Retrieved March 26, 2010 from: http://www.ietf.org/rfc/rfc2131.txt

Faihust G. 2001. Operation of a Router. University of Aberdeen. Retrieved March 26, 2010. From www.erg.abdn.ac.uk/users/gorry/course/inet-pages/router-opn.html

Hampton, M. Donahue, S. Castanho, R. 2001. Segmentation and Reassembly of Packets. Retrieved March 25, 2010 from: http://www.arl.wustl.edu/~lockwood/class/cs535/project/sar/SAR.PDF

Kozierok, C. 2005. Circuit Switching and Packet Switching Networks. The TCP/IP Guide. Retrieved March23, 2010. From: http://www.tcpipguide.com/free/t_CircuitSwitchingandPacketSwitchingNetworks.htm

Martin R. 2002. Internet Technology Routing. Rutgers University. Retrieved March 28, 2010. From http://remus.rutgers.edu/cs352/F02/lectures/routing.pdf

NetworkWorld. 2007 Packet Switching Retrieved March 25, 2010 from: http://www.networkworld.com/community/node/20096

Postel, J. 1981. Internet Control Message Protocol. Request for Comments 792. Network Working Group. ISI. Retrieved March 25, 2010 from: http://www.rfc-editor.org/rfc/rfc792.txt

Streve, M. Perkins,C. 2010. TCP/IP from a Security Viewpoint. Microsoft Corporation. Retrieved March 26, 2010 from: http://technet.microsoft.com/en-us/library/cc750854.aspx

Uruth, N. N.D. Circuit Switching Vs. Packet Switching. About.com:Voice Over IP. Retrieved March 24, 2010 from: http://voip.about.com/od/voipbasics/a/switchingtypes.htm