What is the difference between TCP and UDP?

TCP (transmission control protocol) and UDP (user datagram protocol) are two very important protocols. These two protocols are transportation protocol.  These two protocols are counted in the core protocols of the IP suite. 

These two protocols operate at the 4^th layer i.e., the transport layer of the TCP/ IP model but the usage of the both the protocols is used very differently.

1. Reliability: 

- UDP is a connection-less protocol whereas TCP is a protocol that is connection  oriented. 

- Whenever a message is sent, it will not get delivered if there is a connection failure. 

- If during the delivery of the message the connection gets lost, the server will send a request to get the lost part. 

- During the transfer of a message, there is no corruption. 

- The reliability of the UDP is less and if a message is sent, there is no guarantee that it will get delivered, it may get lost on the way. 

- The message might get corrupted during transfer.

2. Ordered: 

- If at the same time two messages are sent along the same connection, one after the other, it is sure that the message which is the first in the line will get delivered there first. 

- The data is therefore delivered always in the same order. 

- You do not have to worry about the order of the arriving data. 

- In the case of UDP, the order of the arrival of data is not sure. 

- The second one can arrive there first before the first one. 

3. Heavyweight: 

- When the order of arrival of the low level parts of the transmission stream is wrong, the requests have to be sent again and again. 

- All the lost parts of the message have to be put together in a proper sequence. 

- So it takes some time for putting back the parts together. 

- On the other hand, UDP is lightweight.

- After sending the message, the user cannot think about tracking connections or ordering of the messages etc. 

- This indeed makes it lot quicker and therefore there is very less work for the network or the OS card for translating the data obtained from the data packets.

4. Streaming: 

- In TCP, the data is read in form of a stream.

- There is nothing that distinguishes one data from another. 

- Per read call there can be a number of packets. 

- In UDP each of the data packets is sent individually and if they arrive, they do so in whole form. 

- Here per read call, only one packet is sent.

5. Some examples of the TCP are FTP or file transfer protocol, World Wide Web (such as Apache TCP port 80), secure shell such as open SSH port 22 and so on. Examples of UDP are TFTP (trivial file transfer protocol), VoIP (voice over IP), IPTV, online multiplayer games, domain name system (such as DNS UDP port 53) etc.

6. Error-checking: 

- The TCP protocol offers extensive error checking mechanisms for the acknowledgement of the data, flow control and so on. 

In TCP, a connection is a must to be established in order to transfer data. Datagram mode is the mode in which the user data gram protocol operates. You can choose between the two protocols depending up on the requirements. If the guaranteed delivery of data is required, then the transmission control protocol must be chosen. The user data gram protocol comes only with the basic error checking mechanism. It checks the data by the means of the check sums. 

UDP - User Datagram Protocol

The User Datagram Protocol (UDP) is a transport layer protocol defined for use with the IP network layer protocol.UDP is often used in videoconferencing applications or computer games specially tuned for real-time performance.
- UDP network traffic is organized in the form of data grams.A data-gram comprises one message unit. The first eight bytes of a data-gram contain header information and the remaining bytes contain message data.
- UDP can be used in networks where TCP is traditionally implemented.
- It does not guarantee reliability or the correct sequencing of data.
- UDP makes use of a simple communication model without implicit transmission checks for guaranteeing reliability, sequencing, or data-gram integrity.
- UDP considers that error checks and corrections should be carried out in the communicating application, and not at the network layer.
- UDP makes the protocol that much faster and more efficient because it does not have the overhead of checking whether the data has reached the destination every time it is sent.
- UDP is a stateless protocol. UDP is used for packet broadcast or multi-casting whereby the data is sent to all the clients in the network.

The UDP header consists of four fields each of 2 bytes in length :

- Source Port : Source port recognizes the sending port and should be understood to be the port to respond to if required. If not used, then its value should be zero.

- Destination Port : UDP packets from a client use this as a service access point (SAP) to indicate the service required from the remote server.

- UDP length : The number of bytes comprising the combined UDP header information and payload data.

- UDP Checksum : A checksum to verify that the end to end data has not been corrupted by routers or bridges in the network or by the processing in an end system.

The Transport Layer - Layer 4 of OSI model

The Transport Layer of the OSI model is responsible for delivering messages between networked hosts. The Transport Layer should be responsible for fragmentation and reassembly.

- This layer converts the data received from the upper layers into segments and prepares them for transport.
- The Transport layer is responsible for end-to-end (source-to-destination) delivery of entire messages.
- It allows data to be transferred reliably and uses sequencing to make sure that the order of packets is maintained.
- It also provides services such as error checking and flow control.
- In case IP, lost packets arriving out of order must be reordered.
- The size and complexity of a transport protocol depends on the type of service it can get from the network layer.
- The transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer.
- Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer.
- TCP establishes connections between two hosts on the network through 'sockets' which are determined by the IP address and port number. It keeps a track of the packet delivery order and the packets that must be resent.
- UDP provides a low overhead transmission service, but with less error checking.
- The Transport layer protocols are either connectionless or connection-oriented.
- Connection-oriented means that a connection (a virtual link) must be established before any actual data can be exchanged. e.g. TCP.
- In Connectionless, the sender does not establish a connection before it sends data, it just sends it without guaranteeing delivery. e.g. UDP.

Data Segmentation

Data segmentation is the process by which the transport layer uniquely handles all data passed to and from different upper-level applications. For example, if a user is browsing the web and checking email at the same time, each program would be passing data and waiting for a reply on a unique port number. The Transport layer ensures that data is passed to the correct application.