CSMA/CD Vs. CSMA/CA And IP Datagram Error Reporting In ICMP Messages

Comparison of CSMA/CD and CSMA/CA

1. a) Carrier Sense Multiple Access (CSMA) in general is an access control protocol that is used by the MAC to coordinate the transfer and flow of data in mediums. This protocol ensures data packets are never lost or affected thus maintaining their integrity. Now, the CSMA/CD (Collision Detection) is one of the variations seen with the protocol where its operation is executed by it detecting the presence of collisions in mediums. In effect, after detecting a collision, transmissions are usually terminated to avoid wastage of time by the transmitter. On the other hand, CSMA/CA (collision avoidance) as the name suggests does not concern itself with the recovery procedures, instead, it checks the state of the medium to coordinate transmission (Snoeren 2013). CSMA/CA, therefore, allows transmission only when the medium is idle which minimises the occurrence of collisions. Based on these operations states/models CSMA/CD is usually used in wired connections while CSMA/CA is used in wireless connections.

In summary, CSMA/CD takes action after the occurrence of collisions while the CSMA/CA operates before collisions. Secondly, CSMA/CD reduces the occurrence of collisions while CSMA/CD just reduces the recovery times. Finally, CSMA/CD is used in wired connections while CSMA/CA is used in wireless connections.

1. First, connectionless services do not need time to establish connections between the sender and receiver. In fact, transmission starts from the get go with data being transmitted in one direction which saves time as the time for establishing the connection (session) is avoided.
2. Secondly, when network devices fail, it’s a good method for transmitting data. Therefore, when faced with the possibility of network failures this method provides the necessary resilience and robustness to execute operations.
3. Furthermore, unlike connected-oriented services, connectionless services can be used with subnets that do not have host virtual circuits.
4. Moreover, when faced with a rugged environment i.e. extreme difficulties in transmission, it can be used as it facilitates the re-transmission of data over the affected region or medium.
5. Finally, it is a good transmission method for connectionless transmission protocols as it has minimal overhead requirements (Veeraraghavan & Karol 2015).

1. Connectionless services are highly unreliable as compared to the connection-oriented services as they fail to assess the connections before transmitting information.
2.  Furthermore, the absence of medium assessment or evaluations makes congestion control impossible as there are no systems or procedures to coordinate transmission. Simply put, transmission just happens.
3. Finally, even though they have minimal overheads, they require longer header fields (Tiwari 2017).

1. Large packet sizes favour the transmission of large data particularly when the transmission is done over large distances.
2. Secondly, large packet units have adequate room for data transmission which minimises the loss of datagrams which lack reliable connections in their transfer process.
3. Large MTU does not require the fragmentation of data while it transmits information which minimises the transmission time. This outcome also facilitates a faster delivery time which is also combined with a higher reception time because the process lacks a unique time for assembling data.
4. Finally, large MTUs have minimal overheads as fewer packets are used to transmit files which make them more efficient in data transmission (Murray, Koziniec, Lee & Dixon 2011).

1. Small packet sizes have minimal delays, therefore, are advantageous when transferring time-sensitive content such as audio and visual files.
2. Secondly, small MTUs are efficient when transferring multiple signals in single channels i.e. multiplexing, a common occurrence in communication.

Several reasons can be attributed to the presence of the IP header and the first 8 bytes of the datagram data on the ICMP error messages. For one, these values help to match the transferred packets to their place of originality. Consider an ICMP errors message that signifies an unreachable device. Now, the IP address of the source can be used to determine the router interface on which the message (ICMP) was sent from thus facilitating the troubleshooting process. Therefore, these data facilitates the identification of interfaces that are unreachable based on the errors message received. Moreover, in case the ports are identified to have a problem, the same data is used to highlight the exact ports that are affected. Consider the protocol unreachable message, this message is accompanied by the datagrams identified above, whose first byte identifies the TCP ports i.e. source and destination. Again, this information is useful as it helps identify the application ports that the communication was trying to connect to.

Furthermore, the same content is useful in TTL expiration message as well as source quench messages as it helps identify the problems that cause routers to perform their operations in loops resulting in errors (Vlajic 2016).

3. a) All ICMP messages contain the IP datagram which is usually used to describe the section of the IP data that is affected by the error. In essence, the IP datagram will contain several fields arranged in a sequential manner i.e. the type of service, TTL, protocol, checksum, source and destination address among others. Since, all ICMP messages contain these fields, the troubleshooting process is easier to conduct because the source of the errors are outlined by the available information (i.e. IP datagram even without seeking extra data elsewhere) (Vlajic 2016). 

ICMPv6	ICMPv4
Destination unreachable error message is of Type 1 and is usually designated with seven different codes.	Destination unreachable is highlighted as type 3 error message having sixteen different possible codes. However, only four are mostly used i.e. host (1), port (3), fragmentation required (9) and communication admin prohibition (13).
ICMPv6 replaces the fragmentation required (9) of ICMPv4 with a new message known as Packet Too Big (PTG), which corresponds to a totally new Type message i.e. Type 2.	Fragmentation required (code 9) is part of the destination unreachable error message.
Host unreachable is generated when a given host is unresponsive more so during the neighbour discovery process.	Host unreachable message is produced when hosts e.g. routers are unable to send IP datagrams to certain destinations. This outcome can be experienced if a router’s last hop is unable to receive its ARP request since the requested router is missing
Time exceeded (ICMPv6 Type 3) are generated when the hop limit is too low. Hop limit dictates the time a datagram is allowed to remain active in a given network.	Time exceeded (ICMPv4 Type 11) is generated when the Time to Live (TTL) is too low. The original field that specified the active duration for a datagram.
ICMPv6 error messages are never sent for: Other ICMPv6 error messages redirect messages, exempted messages, and a packet without unique source addresses.	ICMPv4 error messages are never sent for; other ICMPv4 error messages, datagram lacking unique source address, IPv4 broadcast and multicast addresses (Shichao 2017).

1. Classless IP address; 160.70.60.12/28

Advantages and disadvantages of connectionless services

Address is 160.70.60.12 with network mask of 255.255.255.240

To obtain the subnet mask, network, and broadcast address convert the IP address and network mask above into binary.

160.70.60.12 = 10100000 01000110 00111100 00001100

255.255.255.240 = 11111111 11111111 11111111 11110000

Then compare the two to get the network address (ANDing)

IP address: 10100000 01000110 00111100 00001100

Mask:         11111111 11111111 11111111 11110000

Network:     10100000 01000110 00111100 00000000

Network address in decimal: 160.70.60.0

Broadcast address/ compare network address and network mask then convert all host addresses to network addresses i.e. all 0’s in host to 1’s

Net address: 10100000 01000110 00111100 00000000

Net mask:  11111111 11111111 11111111 11110000

Broadcast:  10100000 01000110 00111100 11111111

Thus is; 160.70.60.255

Host address 2⁴ – 2 = 14 address

Subnets will be 2⁴ = 16 subnets

Thus the host address are 160.70.60.0 – 160.70.60.255, i.e. 160.70.60.1 to 160.70.60.254

2. Router’s table has 20 entries

1. Periodic timer to handle the table: one periodic time is needed to handle the data in this table, this because RIP handles 25 routes per time thus the 20 are accommodated.
2. Expiration time will be 20 because each entry requires a single time.
3. Finally, if 5 routers are identified as invalid, then 5 garbage timers will be needed as each router will require 1 timer for each invalid entry.

3. Given value for a TCP header dump (values in hexadecimal):

05320017 00000001 00000000 500207FF 00000000₁₆

Therefore based on the TCP header size and structure we have the following:

a. Source port number is the first 4 digits of the first octet

Which is 0532 in decimal 0x0532 = 1330

b. Destination port number the last 4 in the first octet

Which is 0017 and in decimal 0x0017 = 23

c. The next octet in a TCP header represents the sequence number

00000001 thus in decimal will be 0x00000001 = 1

d. Third octet is the acknowledgement number

00000000 convert to decimal 0x00000000 = 0

e. Length of the header

5, 0x5 = 5

f. The type of segment

002 which amounts to 0x002 = 2 and this highlights a Synchronise (SYN) packet.

g. The window size is given by the rest of the octet

i.e. 07FF, 0x07FF = 2047

Part C

1. Given values: IANA has assigned the following classless network, 160.50.1.1/23

The number of customers served is 16 each having a requirement of 18-30 addresses.

Subnet mask and addresses needed to satisfy these customers.

Therefore; hosts needed are a multiple of the customer by the maximum addresses needed, which is 16 x 30 = 480 hosts.

In simple terms each subnet should have 30 usable addresses which can be easily met by 5 bits since 2⁵ = 32 but on subtracting the network and broadcast address we get 30 usable addresses. Thus we can have:

Subnetting 160.50.1.1 255.255.254.0 (23) based on this requirement

Subnet in binary: 11111111 11111111 11111110 00000000 (highlighted bits become ours hosts while the rest is left for the network addresses).

11111111 11111111 11111111 11100000 = 255.255.255.224 (Green highlight is the increment)

The increment is: 128 64 32 16 8 4 2 1

Therefore, the subnets are (add 32 to 160.50.1.1 in the last octet to align with the subnets given):

160.50.1.1 – 160.50.1.33; usable addresses .2-.31 (30 hosts)

160.50.1.34 – 160.50.1.66; usable addresses .35-.65 (30 hosts)

160.50.1.67 – 160.50.1.99; usable address .68-.98 (30 hosts)

160.50.1.100 – 160.50.1.132; usable address .101-131 (30 hosts)

And so on till the 16^th subnet.

Proof that there are 16 subnets? Well, from 11111111 11111111 11111111 11100000 (255.255.255.224). We have 4 bits for network which translates to 2⁴ = 16 subnets. While the hosts are 2⁵ = 32 -2 = 30 usable hosts.

2. After directing the employee to use ipconfig on the command prompt the values given are identified i.e. 169.254.14.11 and having a subnet mask of 255.255.0.0

From this figure, one can deduce that the IP address was automatically assigned to the employee’s PC by the APIPA (automatic private IP addressing) which is a common feature in most computer’s operating systems. Therefore, the TCP/IP configurations on the machines are done automatically based on how it was originally configured. Furthermore, the address identified illustrates that the PC did not reach a DHCP server and not only that but also, no other alternative procedures are put in place to access another connection. In this case, the specified configuration lack a default gateway for the access interface.

References

Murray. D, Koziniec. T, Lee. K & Dixon. M, 2011, Large MTUs and Internet Performance. School of Information Technology. Available at: https://pdfs.semanticscholar.org/23b6/90b7188d2075a400264636a092c21f689d1e.pdf [Accessed 10 June 2017]

Shichao, 2017, Chapter 8. ICMPv4 and ICMPv6: Internet Control Message Protocol. Online. Available at: https://notes.shichao.io/tcpv1/ch8/ [Accessed 10 June 2017]

Snoeren. A, 2013, Lecture 8: Carrier Sense Multiple Access. CSE 123: Computer Networks. Online. Available at: https://cseweb.ucsd.edu/classes/fa13/cse123-a/lectures/123-fa13-l8.pdf [Accessed 10 June 2017]

Tiwari. R, 2017, List some advantages and disadvantages of connection-oriented internetworking and connectionless internetworking. Online. Available at: https://mpstudy.com/list-some-advantages-and-disadvantages-of-connection-oriented-internetworking-and-connectionless-internetworking/ [Accessed 10 June 2017]

Veeraraghavan. M & Karol. M, 2015, exploiting the advantages of connectionless and connection-oriented network. Online. Available at: https://pdfs.semanticscholar.org/1db4/3a25d866af6e18e9050b75665c2c2adcbe0e.pdf [Accessed 10 June 2017]

Vlajic. N, 2016, Network layer (6): ICMP. Online. Available at: https://www.eecs.yorku.ca/course_archive/2015-16/W/3214/CSE3214_12_ICMP_2016_posted_part1.pdf [Accessed 10 June 2017]