1. Transport Layer
Transport Services
Addressing
Internet Transport Protocols TCP (Transmission Control Protocol) and UDP (User Datagram Protocol)
Introduction to UDP (Operation of UDP, Characteristics of UDP, Application of UDP)
Introduction to TCP (Operation of TCP, Characteristics of TCP, TCP three-way handshake process, Application of TCP)
Relationship between TCP & IP
Standard TCP / IP services
Port numbers and socket address
Overview of BSD Socket API
By: Er. Bednidhi Rijal

2. Transport Service The transport service are of two types:
Connection Oriented Service
Connectionless Service
Provides reliable service
Acknowledge facility is incorporated
TCP (Transmission control protocol) is used for this purpose
Connection-oriented communication includes the following steps
Connection Establishment: The source and destination machine establishes a connection before any data is transferred. Connection is established by having both sides initialize variables and counters
Data Transfer: In this phase, the frames are actually transmitted
Connection Release: In this phase, the connection is released. The buffers and other variable to maintain connection are freed up.
By: Er. Bednidhi Rijal

3. Contd..
Connectionless
Packets are sent from one party to another with no need for connection establishment or connection release
Packets are not numbered; they may be delayed or lost or may arrive out of sequence. (I.e. Provides unreliable service)
No acknowledge facility is incorporated
UDP (User datagram protocol) is used for this purpose
By: Er. Bednidhi Rijal

4. Quality of Service
The transport layer provides certain Quality of service (QOS)
It denotes a set of parameters which collectively describe the quality of the transport service requested by the user which are explained below:
Connection Establishment Delay
The time difference between the instant at which the transport connection is requested and the instant at which it is confirmed is called connection Establishment Delay
Shorter the delay, better is the service
Connection Establishment Failure Probability
This is the probability that connection is not established even after maximum delay
This may be due to network congestion
Throughput
It measures the number of bytes a user transfers per second, over a time interval
By: Er. Bednidhi Rijal

5. Contd.. Transit Delay Residual error ratio Protection
It is the time delay between a message sent by the transport layer source machine and the message being received by the destination machine's transport layer
Residual error ratio
It measures the number of lost or garbled message as a function of the total message sent
Ideally the ratio should be zero, and practically it should be as small as possible.
Protection
This parameter protects the transmitted data from being read or modified by unauthorized parties
By: Er. Bednidhi Rijal

6. Contd.. Priority Resilience
This parameter provides a way for the users to show that some its connection is more important (higher priority) than the others.
This is important while handling data on congestion
Higher priority connection gets service before low priority connection
Resilience
Due to internal problem or congestion, the transport layer spontaneously terminates connection
The resilience parameter gives the probability of such termination
By: Er. Bednidhi Rijal

7. Addressing
At the transport layer, Port number is used as a transport layer address to choose among multiple processes running on the destination host.
The destination port number is needed for delivery; the source port number is needed for the reply.
In the Internet model, the port numbers are 16-bit integers between 0 and 65,535.
The client program defines itself with a port number, chosen randomly by the transport layer software running on the client host. This is the ephemeral port number or temporary.
By: Er. Bednidhi Rijal

8. Contd..
The server process must also define itself with a port number but it cannot be chosen randomly, instead it uses universal port numbers called well-known port numbers (permanent).
Every client process must knows the well-known port number of the corresponding server process to communicate
Fig: port number
By: Er. Bednidhi Rijal

9. IP Address VS Port Address
The destination IP address defines the host among the different hosts in the world.
After the host has been selected, the port number defines one of the processes on this particular host
By: Er. Bednidhi Rijal

10. IP vs Port number
By: Er. Bednidhi Rijal

11. IANA Ranges
The IANA (Internet Assigned Number Authority) has divided the port numbers into three ranges:
well known,
registered, and
dynamic (or private)
Well-known ports
The ports ranging from 0 to 1023 are assigned and controlled by IANA. These are the well-known ports.
Registered ports
The ports ranging from 1024 to 49,151 are not assigned or controlled by IANA.
They can only be registered with IANA to prevent duplication.
By: Er. Bednidhi Rijal

12. Contd.. Dynamic ports Fig: IANA Ranges
The ports ranging from 49,152 to 65,535 are neither controlled nor registered. They can be used by any process.
These are the ephemeral ports.
Fig: IANA Ranges
By: Er. Bednidhi Rijal

13. Socket Addresses
Process-to-process delivery needs two identifiers, IP address and the port number, at each end to make a connection.
The combination of an IP address and a port number is called a socket address
The client socket address defines the client process uniquely and the server socket address defines the server process uniquely
A transport layer protocol needs a pair of socket addresses: the client socket address and the server socket address.
By: Er. Bednidhi Rijal

14. Contd..
These four pieces of information are part of the IP header and the transport layer protocol header.
The IP header contains the IP addresses; the UDP or TCP header contains the port numbers.
By: Er. Bednidhi Rijal

15. Transmission Control Protocol (TCP)
TCP is called a connection-oriented; it creates a virtual connection between two TCPs to send data
TCP uses flow and error control mechanisms at the transport level.
It adds connection-oriented and reliability features to the services of IP.
The major purpose of the TCP is to give the reliable and secure logical connection, service or circuit between the pairs of the processes
By: Er. Bednidhi Rijal

16. Common TCP and UDP application and server port assignment
By: Er. Bednidhi Rijal

17. Characteristics of TCP
The major characteristics are:
Reliable:
Reliable protocol ensures that data sent from one machine to another will eventually be communicated correctly
It does not guarantee that this data will be transmitted correctly within any particular amount of time-just that given amount time, it will arrive
Connection-oriented:
It involves a connection or session between the endpoints. In other words, each host is aware of the other and can maintain information about the state of communication between them
The connection needs to be initialized and destroyed
Full-duplex:
By full-duplex we mean a mode of communication such that both sides can send and receive concurrently
By: Er. Bednidhi Rijal

18. TCP Header The format of a segment is shown below:
By: Er. Bednidhi Rijal

19. Contd..
The segment consists of a 20 to 60 byte header, followed by data from the application program. The header is 20 bytes if there are no options and up to 60 bytes if it contains options.
TCP headers fields are described below:
Source port address. (16 bit field)
Defines the port number of the application program in the host that is sending the segment.
Destination port address. (16 bit field)
Defines the port number of the application program in the host that is receiving the segment.
By: Er. Bednidhi Rijal

20. Contd.. Sequence number. (32 bit field)
Defines the number assigned to the first byte of data contained in this segment.
To ensure connectivity, each byte to be transmitted is numbered.
The sequence number tells the destination which byte in this sequence comprises the first byte in the segment.
Acknowledgment number. (32 bit field)
This is the sequence number of the last byte being acknowledged. This is a piggy-backed acknowledgement.
By: Er. Bednidhi Rijal

21. Contd.. Window size. Header length. Reserved.
This 4-bit field indicates the number of 4-byte words in the TCP header.
The length of the header can be between 20 and 60 bytes.
Therefore, the value of this field can be between 5 (5 x 4 =20) and 15 (15 x 4 =60).
Reserved.
This is a 6-bit field reserved for future use.
Window size.
This field defines the size of the window, in bytes, that the other party must maintain.
Note that the length of this field is 16 bits, which means that the maximum size of the window is 65,535 bytes.
This value is normally referred to as the receiving window (rwnd) and is determined by the receiver.
The sender must obey the dictation of the receiver in this case.
By: Er. Bednidhi Rijal

22. Contd.. Checksum. Urgent pointer (16 bit field) Options. Padding: Data
This 16-bit field contains the checksum.
Checksum is used to detect the unusual types of errors.
Urgent pointer (16 bit field)
Provision for TCP for some urgent for some urgent data messages to be sent bypassing the normal sequence number system.
This field is used to indicate where such data is stored in the packets.
Options.
There can be up to 40 bytes of optional information in the TCP header.
Padding:
It is added to make the header a multiple of 32 bits long and is only used when options are used.
Data
The data field is passed intact to the program which is receiving packets addressed to this port
By: Er. Bednidhi Rijal

23. Control field '
By: Er. Bednidhi Rijal

24. Relation between TCP and IP
TCP is connection oriented protocol but IP is connectionless protocol.
TCP uses the services of IP to deliver individual segments to the receiver, but it controls the connection itself. If a segment is lost or corrupted, it is retransmitted. Unlike TCP, IP is unaware of this retransmission.
If a segment arrives out of order, TCP holds it until the missing segments arrive; IP is unaware of this reordering.
By: Er. Bednidhi Rijal

25. A TCP Connection TCP is connection-oriented.
A connection-oriented transport protocol establishes a virtual path (not physical path) between the source and destination.
All the segments belonging to a message are then sent over this virtual path.
Using a single virtual pathway for the entire message facilitates the acknowledgment process as well as retransmission of damaged or lost frames.
TCP operates at a higher level.
In TCP, connection-oriented transmission requires three phases:
connection establishment,
data transfer, and
connection termination.
By: Er. Bednidhi Rijal

26. Connection Establishment
TCP transmits data in full-duplex mode. For this each party must initialize communication and get approval from the other party before any data are transferred.
TCP uses a three-way handshake to establish a connection
Guarantees both sides ready to transfer data
Allows both sides to agree on initial sequence numbers
Initial sequence number (ISN) must be chosen so that each incarnation of a specific TCP connection between two end-points has a different ISN.
The connection establishment process involves:
By: Er. Bednidhi Rijal

27. Contd..
Step 1
Client sends first segment with SYN flag set
Includes source and destination port numbers
Includes initialization sequence number for numbering client’s segments
Step 2
Server sends a segment with both SYN and ACK flags set
The ACK flag acknowledges receipt of the client’s SYN segment
The ACK number is the client’s ISN + 1 in this case because client’s segment had no payload data
The SYN number is the server’s ISN
Step 3
Client sends segment with ACK flag set to acknowledge server’s segment
The ACK number is the server’s ISN + 1 because there was no payload data
By: Er. Bednidhi Rijal

28. Connection establishment using three-way handshaking
Normally, client initiates the connection
By: Er. Bednidhi Rijal

29. TCP Connection termination
Termination is a 4-step process
Step 1
Client sends a segment with the FIN flag set
Step 2
Server sends an ACK back
Step 3
Server finishes sending data if it has more to send
When all server data has been sent, server sends a FIN segment
Step 4
Client sends and ACK back to server
By: Er. Bednidhi Rijal

30. Contd..
Normally, client performs active close and server performs passive close
By: Er. Bednidhi Rijal

31. User Datagram Protocol (UDP)
The User Datagram Protocol (UDP) is called a connectionless, unreliable transport protocol.
It does not add anything to the services of IP except to provide process-to process communication instead of host-to-host communication.
It only performs very limited error checking.
UDP is a very simple protocol using a minimum of overhead and can be used to send message where the reliability issue is not concerned. For example: Real time communication like voice, video etc.
By: Er. Bednidhi Rijal

32. UDP Operation Connectionless services Flow and Error control
Each user datagram sent by UDP is an independent datagram
The user datagrams are not numbered
No connection establishment and no connection termination (each user datagram can travel on a different path)
Flow and Error control
Simple, unreliable transport protocol
No flow control and hence no window mechanism
Receiver may overwhelm with incoming message
No error control mechanism except for the checksum
Encapsulation and De-capsulation
To send a message from one process to another, the UDP protocol encapsulates and de-capsulates messages in an IP datagram
By: Er. Bednidhi Rijal

33. UDP Format
By: Er. Bednidhi Rijal

34. Contd.. Source port number (16 bit long)
This is the port number used by the process running on the source host.
If the source host is the client (a client sending a request), the port number is an ephemeral port and If the source host is the server the port number is a well-known port number.
Destination port number (16 bit long)
This is the port number used by the process running on the destination host.
If the destination host is the server the port number is a well-known port number and If the destination host is the client the port number is an ephemeral port number
By: Er. Bednidhi Rijal

35. Contd.. Length (16 bit long) Checksum Data
Contains the total length of the user datagram, header plus data.
Checksum
This field is used to detect errors over the entire user datagram (header + data)
Data
This field is passed to the relevant program
By: Er. Bednidhi Rijal

36. Use of UDP The following lists some uses of the UDP protocol:
UDP is suitable for a process that requires simple request-response communication with little concern for flow and error control. It is not usually used for a process such as FTP that needs to send bulk data
UDP is suitable for a process with internal flow and error control mechanisms. For example, the Trivial File Transfer Protocol (TFTP) process includes flow and error control. It can easily use UDP.
UDP is a suitable transport protocol for multicasting. Multicasting capability is embedded in the UDP software but not in the TCP software.
UDP is used for management processes such as SNMP.
UDP is used for some route updating protocols such as Routing Information Protocol (RIP)
By: Er. Bednidhi Rijal

37. Uses of TCP
TCP: Almost anything where you have to get all transmitted data
Web
SSH, FTP, telnet
SMTP, sending mail
IMAP/POP, receiving mail
By: Er. Bednidhi Rijal

38. Overview of BSD Socket API (BERKELEY)
The Microsoft Windows platform supplies a windows socket API called Winsock that implements the BSD socket API
Comprises a library for developing applications in C language that performs inter-process communication in computer network
Allows application processes to open the communication “sockets” that can then be assigned to a particular integer port number on the host
Once a connection is established between two stream sockets, either side can send and receive guaranteed data to the other.
Note: API:- Application Programming Interface
By: Er. Bednidhi Rijal

39. Contd..
Sockets created to use the TCP stream protocol can either be set to “listen()” for connections from remote hosts, or can be set to “connect()” to a remote host that is currently listening for incoming connections
The socket API contains other utility routines for performing operations such as host name resolution and socket options configuration, such as:
Socket() – create new socket
Accept() – accepting connection
Send(), write()
Read(), sendto()
Close()
By: Er. Bednidhi Rijal

40. Contd..
Sockets can also be created in datagram mode, in which case they will use the underlying UDP protocol for transmission of datagram packets
Since the datagram socket mode is connectionless, the destination IP address and port must be specified with each data packet sent.
By: Er. Bednidhi Rijal