User Datagram Protocol

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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? - PowerPoint PPT Presentation

Transcript of User Datagram Protocol

  • User Datagram ProtocolUDP

  • UDPRemember, UDP isNot reliable; data may be dropped No guarantee of in-order delivery Duplicate data is possible No built-in rate limiting

  • UDPWhy 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 receivedIf we are sending continuous updates, this means all data stops until the dropped data is resent and receivedUDP is usually lower latency than TCP

  • UDPBut 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. Whats more, well get the new faster than if we go through a timeout-resend cycleWhat about duplicates and out-of-order? How can you handle that?

  • Sequence NumbersYou can easily handle this by including a monotonically increasing sequence number in your dataIf you get a sequence number equal to or before your last-received sequence number, throw the packet away

  • Stream vs. PacketsTCP 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 ContentsIn 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 binaryIn 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 ContentsBinary float and double value formats vary from CPU to CPU and operating system to operating systemThe good news is that what a float or double is is sort of standardizedThe bad news is that byte order is notDifferent CPUs may arrange the four bytes in a float in different ways

  • Byte OrderFrom 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 computing

  • Byte OrderLuckily, in Java these issues are taken care of for you so long as you stick to the standard input and output streamsIf you work with C or other languages, this is not necessarily taken care of for you, and you need to be alert to the implicationsBut as long as its java on both ends, you will be OK, and maybe even if its not Java on each end

  • ProtocolsHow should we arrange the data? With text, we could simply put it in some easily parsed text format that we could also readWith binary, we need prior knowledge of how the data is laid out, so we can read it correctly. If the data isnt read in the same format as it was written, youll get garbage

  • Java Binary DataByteArrayOutputStream baos = new ByteArrayOutputStream();DataOutputStream dos = new DataOutputStream(baos);dos.writeFloat(17.0f);dos.writeFloat(23.0f);byte[] buffer = Baos.toByteArray();

  • Java Binary DataThe data is now in an array of bytes. This is cross-platform, as long as the receiving side knows the byte orderFloating Point Value 1Floating Point Value 2

  • Binary DataReading is straightforward

    ByteArrayInputStream bais = new ByteArrayInputStream(buffer);DataInputStream dis = new DataInputStream(bais);float x = dis.readFloat();

  • Reading DataWhat 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 dont get garbage outIt is a very good idea to have a protocol version number in the first slot of the binary dataUsually a good idea to have a sequence number as well

  • UDP SocketsYou create UDP (or datagram) sockets in one line. These are unconnected. With TCP, we had a stream connection to another host. With UDP its like an open mailbox that anyone can drop messages intoDatagramSocket socket = new DatagramSocket(4545);Note no other IP, just the UDP port number (which is distinct from the TCP port number)

  • Datagram PacketsThe 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 databyte[] buffer; // fill out contents hereDatagramPacket packet = new DatagramPacket(buffer, buffer.length, ipAddress, port)socket.send(packet);

  • Vs TCPUDP is packet-oriented, TCP is stream-orientedUDP 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 LimitingTCP automatically throttles back the sender if it is overwhelming the receiver. Not so for UDPDue 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 dont denial of service your own software

  • Rate LimitingPackets Sent/secRec/secPackets received at high rates fall off a cliff

  • Data LossHow 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

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