UDP

Hi it’s the user datagram protocol!

Unit Goals

To understand the role of UDP in a network and appreciate the simplicity of its packet format.

What UDP Is

It’s known as the IP of the transport layer, or “IP between applications.”

Like IP, it is:

Connectionless (any app can send to any other app at any time)
Unreliable
Without guarantees of out-of-order delivery
Without guarantees of duplicates
Best effort

What UDP is Used For

UDP is used for applications that just send messages back and forth, not streams.

There are no sequence numbers. If an application wants to sequence messages, then the application has to put sequence numbers in the packet body and the application has to do the reordering and re-transmission.

Applications that might use UDP:

Audio
Video
Multiplayer interactive games

If you need streams, reliability, and all that, use TCP instead.

So why might UDP be good? Well, it’s really fast as there’s very little overhead.

UDP Packet Format

UDP is a transport layer protocol, so it only cares about port numbers, not about networks and hosts. As a message-based protocol, there is no notion of sequence numbers, fragments, or really anything else. The packet format is just this:

0 0	0 1	0 2	0 3	0 4	0 5	0 6	0 7	0 8	0 9	1 0	1 1	1 2	1 3	1 4	1 5	1 6	1 7	1 8	1 9	2 0	2 1	2 2	2 3	2 4	2 5	2 6	2 7	2 8	2 9	3 0	3 1
Source Port																Destination Port
Message Length																Checksum
Body

Discussion:

So 16 bits for ports....yep, that’s why ports are numbered from 0 to 65535.
The message length is the length of the whole datagram in octets.
The sending and receiving hosts are not shown because that is in the IP packet which encloses the message.
The checksum is actually optional. If the receiver sees a 0 in that field it is an indication not to compute it.
If a checksum is to be used, it is computed not just from the UDP header but from parts of the enclosing IP header.

Exercise: Do some research to find out exactly how the UDP Header Checksum is computed.

Choose message sizes wisely
Although messages can be up to 64K in size, if you make them too big they will get fragmented at the IP layer. Remember that IP fragments in order to fit its packets in the source and destination’s networks’ frames, so generally keep the UDP messages around 1400-1450 bytes (leaving room for the IP header, etc.)

Using UDP in Applications

UDP applications may be client-server or peer-to-peer.

Python

TODO

Java

TODO

JavaScript

TODO

Summary

We’ve covered:

What UDP is
What UDP is for
The UDP packet format