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Transport Layer of TCP/IP Protocol

The Transport layer transports data to and from the correct application. This process is known as end-to-end communication. In TCP/IP the Transport layer  provides a transport service for application data.

The Transport layer header includes a destination port number that identifies the destination application program on the remote machine and a source port number that identifies the application on the originating machine.

In addition, the Transport layer handles error detection, can handle recovery problems, and regulates the flow of information. The way in which the Transport layer handles error detection, the sequence of data,
and flow regulation depends on which protocol is used.

There are two main protocols that operate at the Transport layer, TCP and  UDP. The TCP/IP protocol stack features two Transport layer protocols, TCP and UDP:

• Certified mail -- TCP
• Uncertified mail -- UDP

Both are implemented as a part of the Solaris kernel. There three important binary properties that serve as the base of classification of transport layer protocols:

1. connection-oriented vs. connectionless
2. stateful vs. stateless
3. reliable vs. unreliable

The first important distinction between transport layer protocols is connection-oriented vs. connectionless:

• Connection-Oriented Protocols With connection-oriented protocols, you must establish a logical
  connection with the communication partner before exchanging data. This method of connection:
  • Is highly reliable (because of acknowledgements )
  • Requires more computational processing
  • Has more overhead because of connection establishment and termination
• Connectionless Protocols.  With connectionless protocols, establishing a connection before sending
  data is not necessary. It uses self-contained messages that:
  • Include the full message
  • Do not require any response
  • Has no built-in reliability features, and therefore is best suited for use in highly reliable networks or when application itself can retransmit packets based on some other criteria  (timeout, etc).
  • Has lower overhead because it has no connection and no setup requirements.

The second important distinction is "stateful vs. stateless" protocols:

• Stateful Protocols A stateful protocol is a protocol in which part of the data that is exchanged between the client and the server systems includes state information. Both systems track of the state of the communication session.
• A stateless protocol is a protocol in which neither the client nor the server system has an obligation to keep track of the state of the communication session. A stateless protocol does not support most reliability features;
  and data that were sent can be lost or delivered out-of-sequence. The main advantages is lower overhead and a degree of isolation between the client and the server. Connectionless protocols are typically stateless.

The third important disctinction is "reliable vs. unreliable":

• Reliable Protocols A reliable protocol requires that each transmission is acknowledged by the receiving host. The sender retransmits, if necessary.
• Unreliable Protocols. An unreliable protocol does not require that each transmission is acknowledged by the receiving host.

Based on this classification UDP and TCP looks like complete opposites:

• UDP is connectionless, stateless and unreliable
• TCP is connection-oriented, stateful and reliable

Basic UDP information

UDP is a connectionless, stateless, and unreliable protocol.  It is faster and more efficient for many lightweight or time-sensitive purposes. Also, its stateless nature is useful for servers that answer small queries from huge numbers of clients. UDP is required for broadcast (send to all on local network) and multicast (send to all subscribers).

With UDP, the application is responsible for handling message loss, duplication, sequence (delivery out of order), and loss of connection. UDP receives incoming data from the application and encapsulates the data into UDP datagrams. UDP datagrams have a leading header section hat contains the source and destination port numbers, followed by the data section.

16	32 bits
Source port	Destination port
Length	Checksum
Data

Basic TCP Information

TCP is a connection-oriented, stateful, and reliable protocol. TCP is suited for situations where large volumes of data must travel between systems, particularly across multiple routers and gateways. TCP
has four main features:

• Virtual circuit connection
• Full-duplex connection
• Unstructured stream orientation
• Buffered transfer

The TCP segment header has more fields then UDP header. The TCP header structure is as follows:

16								32 bits
Source port								Destination port
Sequence number
Acknowledgement number
Offset	Resrvd	U	A	P	R	S	F	Window
Checksum								Urgent pointer
Option + Padding
Data
TCP header structure

Recommended Links

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Transport layer - Wikipedia, the free encyclopedia

Examples

From Wikipedia