OSI Transport Layer

Data networks and the Internet support the human network by supplying seamless, reliable communication between people - both locally and around the globe. On a single device, people can use multiple services such as e-mail, the web, and instant messaging to send messages or retrieve information. Applications such as e-mail clients, web browsers, and instant messaging clients allow people to use computers and networks to send messages and find information.

Data from each of these applications is packaged, transported, and delivered to the appropriate server daemon or application on the destination device. The processes described in the OSI Transport layer accept data from the Application layer and prepare it for addressing at the Network layer. The Transport layer is responsible for the overall end-to-end transfer of application data.

Transport Layer – segment the data and manages the separation of data for different application. Multiple applications running on device receive the correct data.
Some protocols at the transport layer provide:
•Connection-oriented conversations
•Reliable delivery
•Ordered data reconstruction
•Flow control



Congestion – the state of a network when there is not sufficient bandwidth to support the amount of network traffic.

UDP – (user datagram protocol) a simple, connectionless protocol, described in RFC 768. It has the advantage of providing for low overhead data delivery. The pieces of communication in UDP are called datagrams.

Applications that use UDP include:
•Domain Name System (DNS)
•Video Streaming
•Voice over IP (VoIP)

TCP – (transmission control protocol) a connection-oriented protocol, described in RFC 793. TCP incurs additional overhead to gain functions. Additional functions specified by TCP are the same order delivery, reliable delivery, and flow control.

Applications that use TCP are:
•Web Browsers
•E-mail
•File Transfers

IANA – (Internet Assigned Numbers Authority) assigned port numbers. A standard body that is responsible for assigning various addressing standards.

There are different types of port numbers:
Well Known Ports (Numbers 0 to 1023) - These numbers are reserved for services and applications. They are commonly used for applications such as HTTP (web server) POP3/SMTP (e-mail server) and Telnet.

Registered Ports (Numbers 1024 to 49151) - These port numbers are assigned to user processes or applications. These processes are primarily individual applications that a user has chosen to install rather than common applications that would receive a Well Known Port.

Dynamic or Private Ports (Numbers 49152 to 65535) - Also known as Ephemeral Ports, these are usually assigned dynamically to client applications when initiating a connection. It is not very common for a client to connect to a service using a Dynamic or Private Port (although some peer-to-peer file sharing programs do).

Netstat - an important network utility that can be used to verify connections. It list the protocol in use, the local address and port number, the foreign address and port number, and the state of the connection.

Three-way handshake – a process that establishes a TCP session between two endpoints. The process is as follows:
1.A client wishes to communicate with a server.
2.In response, the server responds with a SYN-ACK.
3.The client then sends an ACK (usually called SYN-ACK-ACK) back to the other end and the session is established.



Within the TCP segment header, there are six 1-bit fields that contain control information used to manage the TCP processes. Those fields are:
URG - Urgent pointer field significant
ACK - Acknowledgement field significant
PSH - Push function
RST - Reset the connection
SYN - Synchronize sequence numbers
FIN - No more data from sender

These fields are referred to as flags, because the value of one of these fields is only 1 bit and, therefore, has only two values: 1 or 0. When a bit value is set to 1, it indicates what control information is contained in the segment.

Window size – to determine the number of segments sent by the sending device before the receiving device sends a confirmation. It is a field in the TCP header that enables the management of lost data and flow control.



Flow control – assists the reliability of TCP transmission by adjusting the effective rate of data flow between the two services in the session. When the source is informed that the specified amount of data in the segments is received, it can continue sending more data for this session.