Question

(a) Describe briefly the packetizing services provided by network-layer of the TCP/IP (3 marks) (b) Describe briefly the multiplexing services provided by the network-layer of the protocol suite. TCP/IP protocol suite. How can the multiplexed TCP and UDP packets be (4 marks) (c) Describe briefly the impact of congestion on packet delay and throughput. Describe (6 marks) (d) Fig. Q1 shows a network adopting the virtual-circuit approach to transmit demultiplexed at the destination? the strategy used in the Internet to reduce its impact. datagrams. Suppose that a virtual circuit has been setup between host A and host B via R1, R3 and R4 i. Describe how the forwarding table was setup at R1, R3 and R4 (8 marks) ii. Describe the forwarding process when host A sends datagram to host B (4 marks) A to B Incoming Outgoing Pon Labe Por Label 1 14 RI 2 R2 R5 R3 R4 Incoming Oulgoing Port Label Por Label 1 22 4 77 A to B

Answer 1

a) Every network has a limit on the packet size it can forward. It is known as Maximum Transmission Unit (MTU). Every packet that arrieves at the network layer is checked wether it can be forwarded or not by looking at the size of the packet and MTU of network. If the size of the packet is more than the size of the network, it is broken down into several pieces to meet the requirement of the network. This process is known as fragmentation.

b) At the sender's side a large numer of processes are present that wishes to send the data to the network. There is many to one relationship as many processes want to send data to one network. So, multiplexing is done at the recievers's end by the help of port number. Every process on a machine is assigned a port number and data it want to send is stamped with this port number as well as the port number of process it want to send the data to.

At the reciever's end, data coming from the network has to be distributed to different processes on the machine. So, demultiplexing is required. Again it is performed with the help of destination port number stamped by the sender while multiplexing.

c) There are mainly four type of delay;

Processing delay: Time taken by the routers to process the data packet header.

Queuing delay: Time taken by the data packet in routing queues.

Transmission delay: Time taken to load a data packet onto the transmission channel

Propagation delay - Time taken by the data packet to reach from source to destination

Due to congestion, typw of delay that is affected is queuing delay. Due to the congestion in the network, a packet spends lots of time in queue waiting for its turn to get forwarded.Hence congestion affects the delays in network.

Throughput refers to the amount of data that is passed successfully through the network in a given amount of time. Due to congestion, there is delay which means packet is forwarded late. So, amount of data sent in a unit time decreases and throughput of the network also decreases.

We can implement the congestion congestion control either in the network layer or in the transport layer.

In transport layer, it is implemented with the help of congestion window. The window that can be used by the sender is decided by  the current sending window and the last received receive window. TCP congestion control increments the congestion window by MSS bytes every round-trip-time. In the TCP literature, this phase is often called the congestion avoidance phase. The Multiplicative Decrease part of the TCP congestion control divides the current value of the congestion window once congestion has been detected.

d) (i) When th A send the data to the network, it reaches the router R1, R1 on recieving data know by looking ata the address that this packet is from A. so, it adds an entry to its table that A is at port 1. But it do not knows the address of B. So, it forward the packet to all the ports except port1. This packet is recieved by the R2, R3 and R5. All of these three routers also use the same approach and froward the packet to whole network. Similarly, every router gets the entry for the B also when the B forward the packet.

Note that there can be several path but only best paths are choosen.forwarding table of R3 will look like:

Port	Label	Port	Label
1	28(a number)	3	65(a number)

ii) when the packet reaches A, it checks the entry in its datagram forwarding table from A to B and sends the data to the port 3.

Data then reaches R3, which then forward the data as per its table to the port 3.

Data reaches the R4 from where it is forwarded to the destination B.