User Datagram Protocol

UDP

UDP

Remember, UDP is• Not reliable; data may be dropped• No guarantee of in-order delivery• Duplicate data is possible• No built-in rate limiting

UDP

Why accept these limitations?What happens if the network drops some data

being sent in TCP?The receipt of data on the other application stops

until a time-out expires and the data is resent and received

If we are sending continuous updates, this means all data stops until the dropped data is resent and received

UDP is usually lower latency than TCP

UDP

But what if we are sending position updates every 1/30th of a second (frame rate?)

If one update gets dropped, another one is on the way. What’s more, we’ll get the new faster than if we go through a timeout-resend cycle

What about duplicates and out-of-order? How can you handle that?

Sequence Numbers

You can easily handle this by including a monotonically increasing sequence number in your data

If you get a sequence number equal to or before your last-received sequence number, throw the packet away

Stream vs. Packets

TCP sockets are stream-oriented. You read or sent a stream of bytes, like reading or writing from a file. (InputStream, OutputStream)

UDP is packet-oriented. You compose discrete packets or messages, then send them. (array of bytes, usually)

UDP packets can be big (up to 64K), but you should keep them below 1500 bytes for performance reasons. This is usually easy to do.

Message Contents

In TCP it was best to use text, because it was universal and easy to debug. But with numeric data we often have to translate from text to binary

In UDP we are often dealing with data that is low latency and numeric. Position updates, streaming audio, streaming video, etc.

It usually makes sense to use binary data for this, but this choice introduces all sorts of problems

Message Contents

Binary float and double value formats vary from CPU to CPU and operating system to operating system

The good news is that what a “float” or “double” is is sort of standardized

The bad news is that byte order is notDifferent CPUs may arrange the four

bytes in a float in different ways

Byte Order

From least to most significant bit (integers)

1 2 3 4The same number may be represnted as4 3 2 1On a different CPUThis is called “endian”, as in big endian or

little endian, a hangover from the elder days of computingµ

Byte Order

Luckily, in Java these issues are taken care of for you so long as you stick to the standard input and output streams

If you work with C or other languages, this is not necessarily taken care of for you, and you need to be alert to the implications

But as long as it’s java on both ends, you will be OK, and maybe even if it’s not Java on each end

Protocols

How should we arrange the data? With text, we could simply put it in some easily parsed text format that we could also read

With binary, we need prior knowledge of how the data is laid out, so we can read it correctly. If the data isn’t read in the same format as it was written, you’ll get garbage

Java Binary Data

ByteArrayOutputStream baos = new ByteArrayOutputStream();

DataOutputStream dos = new DataOutputStream(baos);

dos.writeFloat(17.0f);

dos.writeFloat(23.0f);

byte[] buffer = Baos.toByteArray();

Java Binary Data

The data is now in an array of bytes. This is cross-platform, as long as the receiving side knows the byte order

Floating Point Value 1 Floating Point Value 2

Binary Data

Reading is straightforward

ByteArrayInputStream bais = new ByteArrayInputStream(buffer);

DataInputStream dis = new DataInputStream(bais);

float x = dis.readFloat();

Reading Data

What happens if you read an integer from the data input stream instead of a float? A double instead of a float?

This means that you need prior knowledge of what format the data is in so you don’t get garbage out

It is a very good idea to have a protocol version number in the first slot of the binary data

Usually a good idea to have a sequence number as well

UDP Sockets

You create UDP (or “datagram”) sockets in one line. These are unconnected. With TCP, we had a stream connection to another host. With UDP it’s like an open mailbox that anyone can drop messages into

DatagramSocket socket = new DatagramSocket(4545);

Note no other IP, just the UDP port number (which is distinct from the TCP port number)

Datagram Packets

The discrete messages being created are DatagramPackets. These contain an address (the port and IP of the machine they are being sent to) and a payload. The payload is our binary data

byte[] buffer; …// fill out contents here

DatagramPacket packet = new DatagramPacket(buffer, buffer.length, ipAddress, port)

socket.send(packet);

Vs TCP

UDP is packet-oriented, TCP is stream-oriented

UDP is unreliableUDP is not TCP! A common rookie mistake

is to say “I want to use UDP, but I also want it to be reliable”. You then wind up creating a complex protocol that re-invents TCP. Embrace the limitations of UDP

Rate Limiting

TCP automatically throttles back the sender if it is overwhelming the receiver. Not so for UDP

Due to the nature of the software, even if the sender and receiver have the same CPU speed, the sender will at full rate overwhelm the receiver. This means you need to be careful that you don’t denial of service your own software

Rate Limiting

Packets Sent/sec

Rec/sec

Packets received at high rates fall off a cliff

Data Loss

How do you handle losses of position updates?

How about streaming audio data?• One solution is to include redundant

data or hamming codes in multiple packets. If one packet gets lost you can recover the data from other packets

Assignment

Write an application that sends, in binary format, an entity identifier and position