Module 5

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Questions and Answers

What is the purpose of the receiver informing the sender of the free buffer space in TCP?

To prevent the sender from overflowing the receiver's buffer (correct)

To ensure the sender sends data in a specific sequence

To establish a handshake between the sender and receiver

To increase the speed of data transmission

What is the main characteristic of UDP that makes it suitable for streaming multimedia applications?

Error correction and retransmission

Guaranteed delivery of data

No need for handshaking between sender and receiver (correct)

Segments are handled in a specific sequence

What is the purpose of the UDP checksum?

To detect errors in data transmission (correct)

To multiplex and demultiplex data

To establish a handshake between sender and receiver

To increase the speed of data transmission

What is the function of the length field in the UDP header?

To specify the length of the application data Signup and view all the answers

What is the term used to describe the process of directing data to the correct application layer socket?

Demultiplexing Signup and view all the answers

What is the main difference between TCP and UDP?

TCP is connection-oriented, while UDP is connectionless Signup and view all the answers

What is the purpose of the source port number and destination port number in the UDP header?

To multiplex and demultiplex data Signup and view all the answers

What is the term used to describe the process of combining multiple data streams into a single stream?

Multiplexing Signup and view all the answers

What type of service is provided by TCP?

Reliable, connection-oriented Signup and view all the answers

What is the purpose of multiplexing at the sender side?

To multiplex data coming from multiple processes Signup and view all the answers

What is used to direct a received UDP segment to the correct socket?

Destination IP address and port number Signup and view all the answers

What is the purpose of the sequence number in a TCP segment header?

To specify the byte-stream number of the first byte in the segment's data Signup and view all the answers

What is the purpose of the acknowledgment number in a TCP segment header?

To specify the sequence number of the next byte expected to be received Signup and view all the answers

What is the purpose of the receive window field in a TCP segment header?

To control the flow of data from the sender Signup and view all the answers

What is the purpose of the three-way handshake in TCP?

To open and close a TCP connection, and to synchronize both ends Signup and view all the answers

What is the purpose of the SYN message in TCP?

To initiate a connection establishment Signup and view all the answers

What is the purpose of the ACK message in TCP?

To acknowledge the received SYN message Signup and view all the answers

What is the purpose of the FIN message in TCP?

To close a TCP connection Signup and view all the answers

Study Notes

Transport Services and Protocols

Transport layer provides logical communication between application processes running on different hosts
Transport layer converts application layer messages into transport layer packets called segments
Transport layer breaks application messages into smaller chunks, adds a transport layer header to create a segment, and passes it to the network layer where the segment is encapsulated into a datagram

Transmission Control Protocol (TCP)

Provides a reliable, connection-oriented service
Provides reliable and in order data transfer, subject to congestion control and flow control
Connection state is maintained at both the sender and receiver sides

User Datagram Protocol (UDP)

Provides an unreliable, connectionless service
A best-effort, no-frills delivery service that does not guarantee that data sent will arrive intact or arrive at all

Multiplexing/Demultiplexing

Multiplexing at sender side: transport layer multiplexes data coming from processes, taking the data, putting it into segments, and adding information into the transport header
Demultiplexing at receiver side: transport layer uses header information to deliver the contents of the received segments to the correct socket

Demultiplexing

Each packet received by the host has a source IP address and destination IP address
Each packet also carries one transport layer segment
Transport layer segment also has a header containing source port number and destination port number
Host uses these IP addresses and port numbers to direct the segment to the correct socket

Connection-Oriented Demultiplexing

Receiver uses a 4-tuple to direct the segment to the appropriate socket:
- Source IP address
- Source port number
- Destination IP address
- Destination port number
A server can have many simultaneous TCP sockets, each identified by its own 4-tuple, and each socket is associated with a different connecting client process

Connectionless Demultiplexing

Receiving host receives UDP segment, checks the destination port number, then directs the UDP segment to the socket associated with that port number
When there are multiple clients sending datagrams to the same UDP port number at a destination, UDP datagrams with the same destination port number, coming from different source IP addresses and source port numbers, are directed to the same socket at the receiving host

Reliable Delivery Protocols – TCP

Operates in a point-to-point manner between a single sender and a single receiver
Provides a full-duplex service where data payload can flow in both directions
Provides cumulative ACKs
Uses pipelining, allowing the sender to have multiple transmitted segments but yet-to-be-acknowledged segments outstanding at any given time
Connection-oriented, with a handshake between the sender and receiver before data begins to flow
Provides a flow-control service to eliminate the possibility of the sender overflowing the receiver's buffer

TCP Segment Structure

TCP segment consists of header fields and a data field
In the header fields, there's a source port number and a destination port number which are used for multiplexing and demultiplexing from/to upper-layer applications
TCP header contains a 32-bit sequence number and a 32-bit acknowledgement number which are used by the TCP sender and receiver in implementing a reliable data transfer service
The 16-bit receive window field is used for flow control
The 4-bit header length field specifies the length of the TCP header
The TCP header can be of variable length due to the options field
The flag field contains 6 bits:
- A bit or ACK is used to indicate that the value carried in the acknowledgment field is valid
- R bit or RST, S bit or SYN and F bit or FIN bits are used for connection management

Connection Establishment and Termination

TCP uses a three-way handshake to open and close a connection and to synchronize both ends of the connection
Client and server both create a TCP socket and enter the LISTEN state
Client connects to the server by sending a SYN message and enters a SYN SENT state
Server waits for connection, receives the SYN message and enters a SYN RECEIVED state
Server sends a SYNACK message back to the client
Client sends an ACK message to the server and enters the ESTABLISHED state at the client side
When server receives the ACK message, it then enters the ESTABLISHED state at the server side

Reliability and Error Recovery

TCP provides a flow-control service to eliminate the possibility of the sender overflowing the receiver's buffer
Receiver informs the sender how much free buffer space there is
Sender will be limited to send no more than the amount of data the receiver is willing to accept
The value will change as the amount of free buffer space fluctuates over time

Best Effort Delivery Protocols – UDP

UDP is no frills, bare bones and simple because of the service that it provides
There's no need for handshaking between the UDP sender and receiver and there's no need for the sender and receiver to have a shared state
Each UDP segment is handled independently of all the arriving segments
UDP makes it useful for a set of applications such as streaming multimedia applications, Domain Name System (DNS), and Simple Network Management Protocol (SNMP)

UDP Segment

The header only has four fields, two of those fields are source port number and destination port number used for multiplexing and demultiplexing
The length field is needed because application data or payload part of the UDP segment can be a variable length
The checksum field is for error detection