Test: TCP/UDP - Computer Science Engineering (CSE) MCQ

Data-Link Layer: Bridges operate at the Data-Link layer of the OSI Model. They can distinguish between local and remote data, so data traveling from one workstation to another in the same segment doesn't have to cross the bridge.

TCP/IP Model has Four-Layer Architecture
Application Layer

• The application layer interacts with an application program, which is the highest level of the OSI model. The application layer is the OSI layer, which is closest to the end-user. It means the OSI application layer allows users to interact with other software applications.

Internet Layer

• An internet layer is a second layer of TCP/IP layers of the TCP/IP model. It is also known as a network layer. The main work of this layer is to send the packets from any network, and any computer still they reach the destination irrespective of the route they take.

Transport Layer

• The transport layer builds on the network layer in order to provide data transport from a process on a source system machine to a process on a destination system. It is hosted using single or multiple networks, and also maintains the quality of service functions.

The Network Interface Layer

• Network Interface Layer is this layer of the four-layer TCP/IP model. This layer is also called a network access layer. It helps you to defines details of how data should be sent using the network.

Difference Between OSI and TCP/IP

Concept:

• TCP uses the persistence timer to deal with a zero-window-size.
• Zero-window-size occurs when the TCP sender receives an acknowledgement with a window of size zero. And this is when the persistence timer starts.
• When this timer goes off, the TCP sender sends a special segment called probe.
• This probe causes the receiving TCP to resend the acknowledgment which was lost.
   

Explanation:
Consider the following situation

• Sender receives an acknowledgment from the receiver with zero window size. This indicates the sender to wait.
• Later, receiver updates the window size and sends the segment with the update to the sender. This segment gets lost.
• Now, both sender and receiver keep waiting for each other to do something. To deal with such a situation, TCP uses a persistent timer.
• Sender starts the persistent timer on receiving an ACK from the receiver with a zero-window size. When persistent timer goes off, sender sends a special segment to the receiver. This special segment is called as probe segment and contains only 1 byte of new data.
• Response sent by the receiver to the probe segment gives the updated window size. If the updated window size is non-zero, it means data can be sent now. If the updated window size is still zero, the persistent timer is set again and the cycle repeats.

Key Points
TCP/IP, the protocol stack that is used in communication over the Internet and most other computer networks, has a five-layer architecture. The TCP/IP model is based on a five-layer model for networking. From bottom to top, there are,

• Physical layer
• Data link layer
• Net- work layer
• Transport layer
• Application layers

 Not all layers are completely defined by the model, so these layers are “filled in” by external standards and protocols.

Additional Information

• TCP/  IP model includes 4 layers also.
• The four-layer TCP/IP model has the layers Application Layer, Transport Layer, Internet Layer, and Network Access Layer.
• The Internet layer in the above 4 layer model is the same thing as the network layer defined in the 5 layer model.
• Whereas
• The network Access layer in the 4 layer model is the data link layer and the physical layer of the 5 layer model is combined into a single layer in the 4 layer model.

 Hence the correct answer C.

In the TCP header checksum calculation includes the header, data, and pseudo-header.
All these values are added and stored in one’s complement form.

Important Point:
Checksum is present in IPv4 header format

• Elastic traffic has the ability of making adjustment of wide-ranging changes in delay and throughput across the internet and still meets the needs of the applications.
• It adjusts its throughput between end hosts in response to network condition. Network load or congestion may cause packet loss.
• TCP elastic traffic is generated by the traditional “data” applications in the Internet, such as web browsing, peer-to-peer file sharing, ftp, e-mail and other.
• These applications are built on top of TCP, which provides reliable transfers and adjusts the sending rate to the network conditions to achieve the maximum possible throughput, a feature that makes TCP flows to be called “elastic”.

 (Hence option 2 is correct)

• From the point of view of the network, TCP elastic traffic requires the maximum possible throughput above a minimum value, a network service that we call the Minimum Throughput Service (MTS).
   

Number of sequence number bits in TCP header = 32
Therefore, 2³² sequence number are possible  
For given Bandwidth, 1 sec → 8 GB
1 sec = 2³³ sequence numbers (TCP is Byte stream, 1 Byte = 1 Sequence number)
For a lifetime of TCP segment
4 sec → 2³³ × 4 sequence numbers
2³⁵ sequence numbers
32 bits are available in TCP header.
So, 3 bits are required from options.

At network layer a connectionless service may mean different paths for different datagrams belonging to the same message.
At transport layer, connectionless service means independency between packets. (Connection orient) means dependency.

TCP (Transmission Control Protocol):

• TCP (Transmission control protocol) is a connection-oriented reliable transport protocol. It provides a process to process communications using port numbers.
•  It provides services like flow control, error control, congestion control. A packet that is sent is known as a segment in TCP.
• In TCP, transmission requires three phases: connection establishment, data transfer, and connection termination. It uses three-way handshaking mechanism for connection establishment and termination.
   

UDP (User datagram protocol):

• UDP (User datagram protocol) is called a connectionless, unreliable transport protocol.
• It does not add anything to the services of IP except providing a process to process communication.
• There is no flow control, error control and congestion control mechanism in UDP. To send a message from one process to another,
• UDP protocol encapsulates and decapsulates messages in an IP datagram.

Key Points

• ​Source port: this is a 16 bit field that specifies the port number of the sender.
• Destination port: this is a 16 bit field that specifies the port number of the receiver.
• Sequence number: the sequence number is a 32 bit field that indicates how much data is sent during the TCP session.
• Acknowledgment number: this 32 bit field is used by the receiver to request the next TCP segment.
• Header Length: this is the 4 bit data offset field, also known as the header length.
• Reserved bits: these are 3 bits for the reserved field.
• Flags: there are 9 bits for flags, we also call them control bits.
• Window: the 16 bit window field specifies how many bytes the receiver is willing to receive. 
• Checksum: 16 bits are used for a checksum to check if the TCP header is OK or not.
• Urgent pointer: these 16 bits are used when the URG bit has been set, the urgent pointer is used to indicate where the urgent data ends.
• Options: this field is optional and can be anywhere between 0 and 320 bits.
  
  Hence the correct answer is 16 bits.

RFC 1122

Application Layer of TCP/IP corresponds to the Application layer, Session layer, the Presentation layer of OSI.
Therefore, the TCP/IP model does not have Session and  Presentation layers but the Application layer includes the required functions of these layers.

Transmission Control Protocol (TCP):

• TCP is a reliable connection-oriented protocol that can be used in any application where reliability is important. 
• TCP explicitly defines to provide a connection establishment, data transfer, and connection teardown phases to provide a connection-oriented service.
• TCP connections are full-duplex
• TCP uses GBN and SR protocols to attain reliability. Therefore it has selective acknowledgment. 
• It provides end-to-end communication due to the port number of source and destination. 
• TCP connections are byte streams. 
   

Hence statement I is correct

TCP stands for Transmission Control Protocol a communications standard that enables application programs and computing devices to exchange messages over a network. It is designed to send packets across the internet and ensure the successful delivery of data and messages over networks.
Hence the correct answer is Transmission Control Protocol.

X = 301
Since packet with sequence (301–400) is lost
After 3 duplicate acknowledgements (mild congestion) – (301–400) will be sent again.
Y = 301
Z = 400
X + Y – Z = 301 + 301 – 400 = 602 – 400 = 202

Size of the data = 3880-20 = 3860
Since, 3860 > MTU of the link, the intermediate router will perform fragmentation.
MTU = 520 Bytes
The size of data for each fragment possible is = 520 – 20 = 500 bytes but it should be divisible by 8, hence we take data part to be 496B (we take the max possible)
Number of fragments = ceil [3860/496] = 8

fragment offset = Number of data bytes ahead of a fragment / 8

1. False: The fragment offset of the 3rd fragment is 124
2. True: The fragment offset of the 4th fragment is 186
3. False: The fragment size of the intermediate fragment is (496 bytes + 20 bytes) = 516 bytes.
4. False: The fragment size of the last fragment is (388 + 20) bytes = 408 bytes and more fragment bit = 0 as there is no more fragment following