9 Difference between TCP and UDP Protocol - Java Network Interview Question

TCP and UDP are two transport layer protocols, which are extensively used on the internet for transmitting data from one host to another. Good knowledge of how TCP and UDP work is essential for any programmer. That's why the difference between TCP and UDP is one of the most popular programming interview questions. I have seen this question many times in various Java interviews, especially for server-side Java developer positions. Since FIX (Financial Information Exchange) protocol is also a TCP based protocol, several investment banks, hedge funds, and exchange solution provider looks for Java developer with good knowledge of TCP and UDP. 

Writing fix engines and server-side components for high-speed electronic trading platforms need capable developers with a solid understanding of fundamentals including data structure, algorithms, and networking.

By the way, the use of TCP and UDP is not limited to one area, it's at the heart of the internet. The protocol which is the core of the internet, HTTP is based on TCP. One more reason, why Java developers should understand these two protocols in detail is that Java is extensively used to write multi-threaded, concurrent, and scalable servers.

Java also provides a rich Socket programming API for both TCP and UDP based communication. In this article, we will learn key differences between TCP and UDP protocol, which is useful to every Java programmer.

To start with, TCP stands for Transmission Control Protocol and UDP stands for User Datagram Protocol, and both are used extensively to build Internet applications.

Difference between TCP vs UDP Protocol

I love to compare two things at different points, this not only makes them easy to compare but also makes it easy to remember differences. When we compare TCP to UDP, we learn the difference in how both TCP and UDP works, we learn which provides reliable and guaranteed delivery and which doesn't. Which protocol is fast and why, and most importantly when to choose TCP over UDP while building your own distributed application.

In this article, we will see the difference between UDP and TCP in 9 points, e.g. connection set-up, reliability, ordering, speed, overhead, header size, congestion control, application, different protocols based on TCP and UDP and how they transfer data.

By learning these differences, you not only able to answer this interview question better but also understand some important details about two of the most important protocols of the internet.

1. Connection-oriented vs Connectionless

The first and foremost difference between them is TCP is a connection-oriented protocol, and UDP is a connectionless protocol. This means a connection is established between client and server before they can send data. The connection establishment process is also known as TCP handshaking where control messages are interchanged between client and server. 

The attached image describes the process of TCP handshake, for example, which controls messages are exchanged between client and server.

The client, which is the initiator of the TCP connection, sends a SYN message to the server, which is listening on a TCP port. The server receives and sends an SYN-ACK message, which is received by the client again and responded to using ACK. Once the server receives this ACK message, a TCP connection is established and ready for data transmission.

On the other hand, UDP is a connectionless protocol, and point to point connection is not established before sending messages. That's the reason, UDP is more suitable for multicast distribution of the message, one to many distributions of data in a single transmission.

2. Reliability

TCP provides the delivery guarantee, which means a message sent using TCP protocol is guaranteed to be delivered to the client. If a message is lost in transit then its recovered using resending, which is handled by TCP protocol itself. On the other hand, UDP is unreliable, it doesn't provide any delivery guarantee. A datagram package may be lost in transits. That's why UDP is not suitable for programs that require guaranteed delivery.

3. Ordering

Apart from the delivery guarantee, TCP also guarantees the order of messages. The message will be delivered to the client in the same order, the server has sent, though it's possible they may reach out of order to the other end of the network. TCP protocol will do all sequencing and order for you. UDP doesn't provide any ordering or sequencing guarantee.

Datagram packets may arrive in any order. That's why TCP is suitable for application which needs delivery in a sequenced manner, though there is UDP based protocol as well which provides ordering and reliability by using the sequence number and redelivery like TIBCO Rendezvous, which is actually a UDP based application.

4. Data Boundary

TCP does not preserve data boundary, UDP does. In Transmission control protocol, data is sent as a byte stream, and no distinguishing indications are transmitted to signal message (segment) boundaries.

On UDP, Packets are sent individually and are checked for integrity only if they arrived. Packets have definite boundaries which are honored upon receipt, meaning a read operation at the receiver socket will yield an entire message as it was originally sent.

Though TCP will also deliver the complete message after assembling all bytes. Messages are stored on TCP buffers before sending them to make optimum use of network bandwidth.

5. Speed

In one word, TCP is slow and UDP is fast. Since TCP does have to create a connection, ensure guaranteed and ordered delivery, it does a lot more than UDP. This cost TCP in terms of speed, that's why UDP is more suitable where speed is a concern, for example, online video streaming, telecast, or online multiplayer games.

If you want to learn more about why UDP is faster than TCP then consider reading The TCP/IP Guide, a Comprehensive, Illustrated Internet Protocols Reference 1st Edition
by Charles M. Kozierok, one of the must-read books to learn TCP/IP protocol.

6. Heavyweight vs Lightweight

Because of the overhead mentioned above, the Transmission control protocol is considered as heavyweight as compared to the lightweight UDP protocol. The simple mantra of UDP to deliver a message without bearing any overhead of creating connection and guaranteeing delivery or order guarantee keeps it lightweight. This is also reflected in their header sizes, which are used to carry metadata.

7. Header size

TCP has a bigger header than UDP. The usual header size of a TCP packet is 20 bytes which is more than double of 8 bytes, header size of a UDP datagram packet. TCP header contains Sequence Number, Ack number, Data offset, Reserved, Control bit, Window, Urgent Pointer, Options, Padding, CheckSum, Source Port, and Destination port. 

While the UDP header only contains Length, Source port, Destination port, and CheckSum. Here is how TCP and UDP header looks like:

TCP Header Format

UDP Header Format 

8. Congestion or Flow control

TCP does Flow Control. TCP requires three packets to set up a socket connection before any user data can be sent. TCP handles reliability and congestion control. On the other hand, UDP does not have an option for flow control. See TCP/IP Illustrated - The Protocol Volume 1  By W. Richard Stevens to learn more about how TCP does flow control

9.Usage and Applications 

Where do TCP and UDP are used on the internet? After knowing the key differences between TCP and UDP, we can easily conclude, which situation suits them. Since TCP provides delivery and sequencing guaranty, it is best suited for applications that require high reliability, and transmission time is relatively less critical.

While UDP is more suitable for applications that need a fast, efficient transmission, such as games. UDP's stateless nature is also useful for servers that answer small queries from huge numbers of clients. In practice, TCP is used in the finance domain like FIX protocol is a TCP based protocol, UDP is used heavily in gaming and entertainment sites.

10. TCP and UDP based Protocols 

One of the best examples of TCP based higher end protocol is HTTP and HTTPS, which is everywhere on the internet. In fact, most of the common protocols you are familiar with like Telnet, FTP, and SMTP all are based on Transmission Control Protocol. 

UDP doesn't have anything as popular as HTTP but UDP is used in a protocol like DHCP (Dynamic Host Configuration Protocol) and DNS (Domain Name System). Some of the other protocol, which is based on the User Datagram protocol is the Simple Network Management Protocol (SNMP), TFTP, BOOTP, and NFS (early versions).

Btw, while working in TCP/UDP based applications on Linux, it's also good to remember basic networking commands like telnet and netstat, they help tremendously to debug or troubleshoot any connection issue.


That's all about the difference between TCP and UDP protocol. Always remember to mention that TCP is connection-oriented, reliable, slow, provides guaranteed delivery and preserves the order of messages, while UDP is connectionless, unreliable, no ordering guarantee, but a fast protocol. TCP overhead is also much higher than UDP, as it transmits more metadata per packet than UDP.

It's worth mentioning that the header size of the Transmission control protocol is 20 bytes, compared to 8 bytes header of the User Datagram protocol. Use TCP, if you can't afford to lose any message, while UDP is better for high-speed data transmission, where the loss of a single packet is acceptable e.g. video streaming or online multiplayer games.