Process-to-process Communication

Process-to-process Communication

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.

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.

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.


  • 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
      • 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.


  • 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, 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.


  • A packet in TCP is called a segment.


  • 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.

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