Process-to-process Communication

• The transport layer is responsible for process-to-process communication.

• Port Number : the port number defines one of the processes on this particular host

lANA Ranges

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

Socket Addresses

• The combination of an IP address and a port number is called a socket
  • The client socket address defines the client process uniquely just as the server socket address defines the server process uniquely address.

Connectionless Versus Connection-Oriented Service:-

• A transport layer protocol can either be connectionless or connection-oriented.

1. Connectionless Service

• In a connectionless service, the packets are sent from one party to another with no need for connection establishment or connection
  • The packets are not numbered;
  • They may be delayed or lost or may arrive out of sequence.
  • There is no acknowledgment either.
  • UDP, is connectionless.
  release.

2. Connection Oriented Service

• a connection is first established between the sender and the
  • Data are transferred. At the end, the connection is released.
  • TCP and SCTP are connection-oriented protocols.
  receiver.

Reliable Versus Unreliable

• The transport layer service can be reliable or unreliable

Reliable :

• by implementing flow and error control at the transport layer.
• This means a slower and more complex service.

Unreliable

• uses its own flow and error control mechanism or it needs fast service or the nature of the service.
• does not demand flow and error control

Two Protocols

1. UDP
2. TCP
   1. USER DATAGRAM PROTOCOL (UDP)
      • 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.
      • Also, it performs very limited error checking.

Well-Known Ports for UDP

User Datagram

• UDP packets, called user datagrams, have a fixed-size header of 8 bytes.

Source port number :

• 16 bits long range from 0 to
  • If the source host is the client – an ephemeral port number requested by the process and chosen by the UDP software running on the source host.
  • If the source host is the server the port number, in most cases, is a well-known port number.65,535.

Destination port number :

• 16 bits long.
  • If the destination host is the server, the port number, is a well-known port number.
  • If the destination host is the client the port number, is an ephemeral port number.

Length :

• 16-bit field – defines the total length of the user datagram, header plus data.
• The 16 bits can define a total length of 0 to 65,535 bytes

UDP length = IP length – IP header’s length

Checksum :

• This field is used to detect errors over the entire user datagram (header plus data).

UDP Operation

1. Connectionless Services

• Each user datagram sent by UDP is an independent datagram
• The user datagrams are not numbered.
• Also, there is no connection establishment and no connection termination

2.Flow and Error Control

• There is no error control mechanism in UDP except for the checksum

3.Encapsulation and Decapsulation

• UDP protocol encapsulates and decapsulates messages in an IP datagram.

4.Queuing

• In UDP, queues are associated with ports

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 FrP 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)

TCP

• TCP, like UDP, uses port
• Unlike UDP, TCP is a connection oriented protocol; it creates a virtual connection between two TCPs to send data.
• In addition, TCP uses flow and error control mechanisms at the transport level.
• TCP is called a connection-oriented, reliable transport protocol.
• It adds connection-oriented and reliability features to the services of IP.numbers.

TCP Services

• Like UDP, TCP provides process-to-process communication using port numbers.

Stream Delivery Service

• TCP, unlike UDP, is a stream-oriented protocol.
• TCP creates an environment in which the two processes seem to be connected by an imaginary “tube” that carries their data across the Internet.

• TCP needs buffers for storage

Full-Duplex Communication

• Each TCP has a sending and receiving buffer, and segments move in both directions.

Connection-Oriented Service

1. The two TCPs establish a connection between them.
2. Data are exchanged in both directions.
3. The connection is terminated.

Reliable Service

• TCP is a reliable transport protocol. It uses an acknowledgment mechanism to check the safe and sound arrival of data.

TCP Features

Numbering System

• The bytes of data being transferred in each connection are numbered by TCP.
• The numbering starts with a randomly generated number.

Flow Control

• The numbering system allows TCP to use a byte-oriented flow control

Error Control

• error control is byte-oriented

Congestion Control

• TCP, unlike UDP, takes into account congestion in the network.

Segment

• A packet in TCP is called a segment.

Format

• The format of a segment is shown 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.

Sequence number :

• 32-bit field & the number assigned to the first byte of data contained in this segment. As we said before, TCP is a stream transport protocol.

Acknowledgment number :

• 32-bit field & the byte number that the receiver of the segment is expecting to receive from the other party.
• If the receiver of the segment has successfully received byte number x from the other party, it defines x + I as the acknowledgment number. Acknowledgment and data can be piggybacked together.

Header length :

• 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.

Reserved :

• 6-bit field reserved for future use.

Control :

• defines 6 different control bits or flags as shown in Figure below.
• One or more of these bits can be set at a time.

Window size :

• defines the size of the window, in bytes, 16 bits, i.e 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.

Checksum :

1. bit field contains the checksum.

Urgent pointer :

• l6-bit field, valid only if the urgent flag is set, is used when the segment contains urgent data.
• It defines the number that must be added to the sequence number to obtain the number of the last urgent byte in the data section of the segment.

Options :

• There can be up to 40 bytes of optional information in the TCP header.
• We will not discuss these options here; please refer to the reference list for more information.

A TCP Connection –

• TCP is connection-oriented.
• A connection-oriented transport protocol establishes a virtual 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.
• 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.
• In TCP, connection-oriented transmission requires three phases:
  1. Connection establishment,
  2. Data transfer, and
  3. Connection termination.