Difference between TCP and UDP?

Question

What is the difference between TCP and UDP?

I know that TCP is used in the case of non-time critical applications, and UDP is used for games or applications that require fast transmission of data. I know that TCP is used for HTTP, HTTPs, FTP, SMTP, and Telnet. I know that UDP is used for DNS and DHCP.

But why? What characteristics of TCP and UDP make it useful for their respective use cases?

Answer 1

TCP establishes a connection before the actual data transmission takes place, UDP does not. In this way, UDP can provide faster delivery. Applications like DNS, time server access, therefore, use UDP.

Unlike UDP, TCP uses congestion control. It responses to the network load. Unlike UDP, it slows down when network congestion is imminent. So, applications like multimedia preferring constant throughput might go for UDP.

Besides, UDP is unreliable, it doesn't react on packet losses. So loss sensitive applications like multimedia transmission prefer UDP. However, TCP is a reliable protocol, so, applications that require reliability such as web transfer, email, file download prefer TCP.

Besides, in today's internet UDP is not as welcoming as TCP due to middle boxes. Some applications like skype fall down to TCP when UDP connection is assumed to be blocked.

Answer 2

Simple Explanation by Analogy

TCP is like this.

Imagine you have a pen-pal on Mars (we communicated with written letters back in the good ol' days before the internet).

You need to send your pen pal the seven habits of highly effective people. So you decide to send it in seven separate letters:

1. Letter 1 - Be proactive
2. Letter 2 - Begin with the end in mind...

etc.

etc..Letter 7 - Sharpen the Saw

Requirements:

You want to make sure that your pen pal receives all your letters - in order and that they arrive perfectly. If your pen pay receives letter 7 before letter 1 - that's no good. if your pen pal receives all letters except letter 3 - that also is no good.

Here's how we ensure that our requirements are met:

• Confirmation Letter: So your pen pal sends a confirmation letter to say "I have received letter 1". That way you know that your pen pal has received it. If a letter does not arrive, or arrives out of order, then you have to stop, and go back and re-send that letter, and all subsequent letters.
• Flow Control: Around the time of Xmas you know that your pen pal will be receiving a lot of mail, so you slow down because you don't want to overwhelm your pen pal. (Your pen pal sends you constant updates about the number of unread messages there are in penpal's mailbox - if your pen pal says that the inbox is about to explode because it is so full, then you slow down sending your letters - because your pen pal won't be able to read them.
• Perfect arrival. Sometimes while you send your letter in the mail, it can get torn, or a snail can eat half of it. How do you know that all your letter has arrived in perfect condition? Well your pen pal will give you a mechanism by which you can check whether they've got the full letter and that it was the exactly the letter that you sent. (e.g. via a word count etc. ). a basic analogy.

Answer 3

One of the differences is in short

UDP : Send message and dont look back if it reached destination, Connectionless protocol
TCP : Send message and guarantee to reach destination, Connection-oriented protocol

Answer 4

Reasons UDP is used for DNS and DHCP:

DNS - TCP requires more resources from the server (which listens for connections) than it does from the client. In particular, when the TCP connection is closed, the server is required to remember the connection's details (holding them in memory) for two minutes, during a state known as TIME_WAIT_2. This is a feature which defends against erroneously repeated packets from a preceding connection being interpreted as part of a current connection. Maintaining TIME_WAIT_2 uses up kernel memory on the server. DNS requests are small and arrive frequently from many different clients. This usage pattern exacerbates the load on the server compared with the clients. It was believed that using UDP, which has no connections and no state to maintain on either client or server, would ameliorate this problem.

DHCP - DHCP is an extension of BOOTP. BOOTP is a protocol which client computers use to get configuration information from a server, while the client is booting. In order to locate the server, a broadcast is sent asking for BOOTP (or DHCP) servers. Broadcasts can only be sent via a connectionless protocol, such as UDP. Therefore, BOOTP required at least one UDP packet, for the server-locating broadcast. Furthermore, because BOOTP is running while the client... boots, and this is a time period when the client may not have its entire TCP/IP stack loaded and running, UDP may be the only protocol the client is ready to handle at that time. Finally, some DHCP/BOOTP clients have only UDP on board. For example, some IP thermostats only implement UDP. The reason is that they are built with such tiny processors and little memory that the are unable to perform TCP -- yet they still need to get an IP address when they boot.

As others have mentioned, UDP is also useful for streaming media, especially audio. Conversations sound better under network lag if you simply drop the delayed packets. You can do that with UDP, but with TCP all you get during lag is a pause, followed by audio that will always be delayed by as much as it has already paused. For two-way phone-style conversations, this is unacceptable.

Answer 5

From the Skullbox article:

TCP (Transmission Control Protocol) is the most commonly used protocol on the Internet. The reason for this is because TCP offers error correction. When the TCP protocol is used there is a "guaranteed delivery." This is due largely in part to a method called "flow control." Flow control determines when data needs to be re-sent, and stops the flow of data until previous packets are successfully transferred. This works because if a packet of data is sent, a collision may occur. When this happens, the client re-requests the packet from the server until the whole packet is complete and is identical to its original.

UDP (User Datagram Protocol) is anther commonly used protocol on the Internet. However, UDP is never used to send important data such as webpages, database information, etc; UDP is commonly used for streaming audio and video. Streaming media such as Windows Media audio files (.WMA) , Real Player (.RM), and others use UDP because it offers speed! The reason UDP is faster than TCP is because there is no form of flow control or error correction. The data sent over the Internet is affected by collisions, and errors will be present. Remember that UDP is only concerned with speed. This is the main reason why streaming media is not high quality.

1) TCP is connection oriented and reliable where as UDP is connection less and unreliable.

2) TCP needs more processing at network interface level where as in UDP it’s not.

3) TCP uses, 3 way handshake, congestion control, flow control and other mechanism to make sure the reliable transmission.

4) UDP is mostly used in cases where the packet delay is more serious than packet loss.

Answer 6

Short and simple differences between Tcp and Udp protocol:

1) Tcp - Transmission control protocol and Udp - User datagram protocol.

2) Tcp is reliable protocol, Where as Udp is a unreliable protocol.

3) Tcp is a stream oriented, where as Udp is a message oriented protocol.

4) Tcp is a slower than Udp.