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TCP Sockets ( In Java)

TCP Sockets ( In Java). Don McGregor Research Associate MOVES Institute. mcgredo@nps.edu. TCP. As mentioned before, TCP sits on top of other layers (IP, hardware) and implements • Reliability • In-order delivery • No duplicates • Rate-limiting

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TCP Sockets ( In Java)

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  1. TCP Sockets(In Java) Don McGregor Research Associate MOVES Institute mcgredo@nps.edu

  2. TCP As mentioned before, TCP sits on top of other layers (IP, hardware) and implements • Reliability • In-order delivery • No duplicates • Rate-limiting It allows you to write to another host on the network as if it were a file stream TCP only passes bytes; it is completely agnostic about what they mean. Making sense of the bytes is up to you, the programmer (or the protocol receiving the bytes)

  3. Review To connect to a host, we need to specify: • What host we will be talking to, either directly via IP number, or indirectly via DNS name • What port on the host we will be talking to; a number in the range 0-64K

  4. Socket A “socket” describes a connection between two hosts. To set up a stream between two hosts you need a “socket pair” that describes the connection between the client and the server Client Server Port TCP 4567 Port TCP 4485 172.20.40.88 172.20.40.12

  5. Socket The socket pair can be described as ((172.20.40.12, 4567), (172.20.40.88, 4485)) The client IP, a client port, the server IP, and the well-known port we connect to on the server If you’re picky, you also need to specify whether this is TCP or UDP. This is often omitted, though, because it’s usually clear from the context which is being used This “socket pair” uniquely describes a connection on the network

  6. Server Socket The standard technique is: client initiates connection to server Before we can establish a connection, we have to have something waiting for the connection on the server side This is a “server socket”, a piece of software that waits for clients to connect.

  7. Server Socket Waiting for a connection at 172.20.40.88, 4485 Server Client Port TCP 4567 Port TCP 4485 172.20.40.88 172.20.40.12

  8. Multiple Connections What if two people want to connect to a web server at the same time? Notice that the socket connection has a unique ID based on the socket pair. If two hosts connect, or even if two programs from the same host connect You can see this with “netstat -an”

  9. Netstat hodad-5:MV3500 mcgredo$ netstat -an Active Internet connections (including servers) Proto Recv-Q Send-Q Local Address Foreign Address (state) tcp4 0 411 192.168.1.6.57583 74.125.53.95.80 ESTABLISHED tcp4 0 406 192.168.1.6.57582 74.125.53.95.80 ESTABLISHED tcp4 0 0 192.168.1.6.57581 64.236.68.229.80 ESTABLISHED

  10. Two Connections from Same Host What are the socket pairs here? Client Port TCP 4568 Port TCP 4485 Server Port TCP 4567 Port TCP 4485 172.20.40.88 172.20.40.12

  11. Connection Process (Java) The server starts a server socket on a port; this will listen on, for example 172.20.81.12 on port 1234 The client initiates a connection from its IP to the server. The client picks an unused port on the client machine, and this port is used in the socket pair. The client port is sometimes called an “ephemeral port” At this point we have a socket pair. The client code returns a socket object, and the server code returns a similar socket object Note that a Socket object is not a ServerSocket object!

  12. Socket InputStream OutputStream Socket OutputStream InputStream The inputStream on one side of the Socket is connected to the outputStream On the other side, and vice versa

  13. Input & Output Streams Input & Output streams are standard Java objects that know how to read & write bytes only. They don’t know about higher level concepts like Unicode characters, integers, floating point numbers, etc But: are you convinced that with enough work you could write a class that figured out what four bytes represented as a floating point number? Luckily for you, there are classes in the standard Java library that do this for you

  14. Input & Output Streams: Text Unicode representation of data PrintStream OutputStream Buffered Reader InputStream Reader InputStream

  15. Other Streams There are also Java library streams for reading & writing binary values, and streams for reading and writing Java objects (!) Remember, bytes are bytes; sockets are agnostic about what they mean. We have to come to some sort of a priori agreement on what we’ll be sending so we’ll know what to expect on the receiving side The string value “17.4” is not the same as a binary floating point number representing 17.4

  16. Framing OutputStream (Sender) InputStream (Receiver) Command 3 Command 2 Command 1 Notice that the TCP socket is sending a stream of bytes. How do the programs on either side know where a particular message starts and ends? Remember, the TCP socket treats this as one long array of bytes

  17. Framing Command1\n Command2\n Command3\n The standard way to do this when the contents of the TCP stream is text is to use newline characters. When the reader comes across a newline it realizes that the command has ended and it can process it Be careful to define an agreed-upon newline character—not all operating systems use the same character(s)

  18. Example Code See TcpServer.java, TcpClient.java This simply establishes a connection and sends a simple message and response

  19. Message Format? So what type message should you send? Since you get to make it up, it’s good for you if you pick something simple and robust For TCP sockets sending state updates or commands, this is usually ASCII text Why? • Binary is different from host to host • You can easily debug ASCII • it is usually fast enough for what TCP does You can use binary formats, but your default first attempt should be ASCII Binary content on TCP sockets are good for bulk transfers of data

  20. Telnet • Telnet is a very old utility that has a handy side effect: it can be used to establish a client TCP connection to a port server, and send commands • On windows, install with • pkgmgr /iu:”TelnetClient” • Should already be present on OS X • Install with yum install telnet on RHEL • Install with apt-get install telnet on Ubuntu

  21. Examples of TCP Message Formats You can establish your own interactive TCP connection to a server with “telnet <hostname> <port>” (usually typing blind on windows machines) HTTP (web servers): • mcgredo$ telnet nps.edu 80 Trying 205.155.65.20… Connected to nps.edu. Escape character is '^]’. GET /index.html HTTP/1.0

  22. HTTP HTTP/1.1 200 OK Cache-Control: no-cache Content-Type: text/html Last-Modified: Tue, 16 Sep 2014 21:01:17 GMT Accept-Ranges: bytes ETag: "a34be15bf1d1cf1:0" Server: Microsoft-IIS/7.5 X-Powered-By: ASP.NET Access-Control-Allow-Origin: * Date: Tue, 16 Sep 2014 22:58:25 GMT Connection: close Content-Length: 11200 telnet www.nps.edu80 GET /index.html HTTP/1.0 (two returns)

  23. SFTP sftpmcgredo@savage.nps.edu Connected to savage.nps.edu. sftp> ls xj3d_code xj3d_code.dump xj3d_website xj3d_website.dump cd ppp-2.4.5 ls Changes-2.3 FAQ Secure File Transfer Protocol (SFTP) allows you to transfer files and list directory contents on a remote host sftpmcgredo@savage.nps.edu

  24. Stateless vs. Stateful Protocols Note a subtle difference between the HTTP and SFTP protocols HTTP is stateless--you send one text command, the server processes that, and then the socket connection can be torn down and the server completely forgets that it ever talked to you before A stateful protocol depends on prior commands sent from the client. The ls and cd commands depend on the prior commands sent. Stateless protocols are good. They scale well, and are simple. Stateful protocols are inherently more complex, because you need to keep track of program state

  25. Sequential vs Parallel What happens to our simple ping-pong example if the server takes a long time to process a command? How can we fix this? Use a thread per client connection--then we can go back and do another accept() wile the first command is still processing This is a parallel server

  26. TCP Protocols • You want to use text for the protocol commands if you possibly can • Keep it simple. Simple can be debugged and may actually work. Complex will not. If you possibly can, start off with a sequential, stateless server

  27. Matrix Sequential Parallel Stateless X Stateful Try for a stateless, sequential protocol if you can, on the grounds that it’s simple. Going to a parallel implementation is not bad, but a stateful protocol can be much more complex

  28. Assignment Write a client and server that accepts a simple position command for an entity. Include an entity identifier and position (x,y,z) Write a simple HTTP client program that sends a request for a URL to a server and gets a response back

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