User Datagram Protocol

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 not • Different CPUs may arrange the four bytes in a float in different ways

Byte Order • From least to most significant bit (integers) • 1 2 3 4 • The same number may be represnted as • 4 3 2 1 • On a different CPU • This 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 unreliable • UDP 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 received at high rates fall off a cliff Rec/sec Packets Sent/sec

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

User Datagram Protocol