TCP/IP Sockets in Java: Practical Guide for Programmers Kenneth L. Calvert Michael J. Donahoo

Computer Chat How do we make computers talk? How are they interconnected? Internet Protocol (IP)

Internet Protocol (IP) Datagram (packet) protocol Best-effort service Loss Reordering Duplication Delay Host-to-host delivery Host-to-host delivery means that IP takes packets from one host to another host. Datagram service analogy to the post office.

IP Address 32-bit identifier Dotted-quad: 192.118.56.25 www.mkp.com -> 167.208.101.28 Identifies a host interface (not a host) We need some way to identify computers on an internetwork The “dotted-quad” representation is just a more readable version of a real 32-bit number 192.18.22.13 209.134.16.123

Transport Protocols Best-effort not sufficient! Add services on top of IP User Datagram Protocol (UDP) Data checksum Best-effort Transmission Control Protocol (TCP) Reliable byte-stream delivery Flow and congestion control

Ports Identifying the ultimate destination IP addresses identify hosts Host has many applications Ports (16-bit identifier) Ports are analogous to phone extensions from main switchboard Application WWW E-mail Telnet Port 80 25 23 192.18.22.13

Sockets Identified by protocol and local/remote address/port Applications may refer to many sockets

Clients and Servers Client: Initiates the connection Server: Passively waits to respond Server: Jane Client: Bob “Hi, Bob. I’m Jane” “Hi. I’m Bob.” “Nice to meet you, Jane.”

TCP Client/Server Interaction Server starts by getting ready to receive client connections… Server Create a TCP socket Repeatedly: Accept new connection Communicate Close the connection Client Create a TCP socket Communicate Close the connection Students should follow along with source code in the book

TCP Client/Server Interaction ServerSocket servSock = new ServerSocket(servPort); Server Create a TCP socket Repeatedly: Accept new connection Communicate Close the connection Client Create a TCP socket Communicate Close the connection Students should follow along with source code in the book Server specified “well-known” port

TCP Client/Server Interaction for (;;) { Socket clntSock = servSock.accept(); Server Create a TCP socket Repeatedly: Accept new connection Communicate Close the connection Client Create a TCP socket Communicate Close the connection Students should follow along with source code in the book

TCP Client/Server Interaction Server is now blocked waiting for connection from a client Server Create a TCP socket Repeatedly: Accept new connection Communicate Close the connection Client Create a TCP socket Communicate Close the connection Students should follow along with source code in the book

TCP Client/Server Interaction Later, a client decides to talk to the server… Server Create a TCP socket Repeatedly: Accept new connection Communicate Close the connection Client Create a TCP socket Communicate Close the connection Students should follow along with source code in the book

TCP Client/Server Interaction Socket socket = new Socket(server, servPort); Server Create a TCP socket Repeatedly: Accept new connection Communicate Close the connection Client Create a TCP socket Communicate Close the connection Students should follow along with source code in the book Client must specify server address (name or IP) and port

TCP Client/Server Interaction OutputStream out = socket.getOutputStream(); out.write(byteBuffer); Server Create a TCP socket Repeatedly: Accept new connection Communicate Close the connection Client Create a TCP socket Communicate Close the connection Students should follow along with source code in the book Socket has OutputStream for writing

TCP Client/Server Interaction Socket clntSock = servSock.accept(); Server Create a TCP socket Repeatedly: Accept new connection Communicate Close the connection Client Create a TCP socket Communicate Close the connection Students should follow along with source code in the book accept( ) returns a new, connected Socket instance

TCP Client/Server Interaction InputStream in = clntSock.getInputStream(); recvMsgSize = in.read(byteBuffer); Server Create a TCP socket Repeatedly: Accept new connection Communicate Close the connection Client Create a TCP socket Communicate Close the connection Students should follow along with source code in the book Socket has InputStream for reading

TCP Client/Server Interaction close(sock); close(clntSocket) Server Create a TCP socket Bind socket to a port Set socket to listen Repeatedly: Accept new connection Communicate Close the connection Client Create a TCP socket Establish connection Communicate Close the connection Students should follow along with source code in the book Note that server closes connection socket, not listening socket

TCP Tidbits Client Server Client knows server address and port No correlation between send() and recv() Client out.write(“Hello Bob”) in.read() -> “Hi Jane” Server in.read() -> “Hello ” in.read() -> “Bob” out.write(“Hi ”) out.write(“Jane”)

Closing a Connection Client Server close() used to delimit communication Analogous to EOF Client out.write(string) while (not received entire string) in.read(buffer) out.write(buffer) close(socket) Server in.read(buffer) while(client has not closed connection) out.write(buffer) close(client socket) In echo application, how does server know when client is done considering it doesn’t know echo string length? Role of closer can (and is in HTTP) be reversed

Constructing Messages …beyond simple strings

TCP/IP Byte Transport TCP/IP protocols transports bytes Application protocol provides semantics Application Application byte stream byte stream Here are some bytes. I don’t know what they mean. I’ll pass these to the app. It knows what to do. TCP/IP TCP/IP TCP does not examine or modify the bytes

Application Protocol Encode information in bytes Sender and receiver must agree on semantics Data encoding Primitive types: strings, integers, and etc. Composed types: message with fields

Primitive Types String Character encoding: ASCII, Unicode, UTF Delimit: length vs. termination character 0 77 0 111 0 109 0 10 M o m \n 3 77 111 109

Primitive Types Integer Strings of character encoded decimal digits Advantage: 1. Human readable 2. Arbitrary size Disadvantage: 1. Inefficient 2. Arithmetic manipulation 49 55 57 57 56 55 48 10 ‘1’ ‘7’ ‘9’ ‘9’ ‘8’ ‘7’ ‘0’ \n Inefficient: Need only 4 bits to represent values 0-9 Arithmetic manipulation: Cannot add and subtract strings. Must convert to primitive type. If you must convert then you lose arbitrary size advantage.

Primitive Types Integer Native representation Network byte order (Big-Endian) Use for multi-byte, binary data exchange htonl(), htons(), ntohl(), ntohs() Little-Endian 0 0 92 246 4-byte two’s-complement integer 23,798 Big-Endian If the byte order is already big-endian, then the [hn]to[ns][sl]( ) functions are no-ops. 246 92 0 0

Message Composition Message composed of fields Fixed-length fields Variable-length fields integer short M i k e 1 2 \n