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

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

3. 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.

4. IP Address 32-bit identifier Dotted-quad: 192.118.56.25
->
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

5. 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

6. 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 Telnet
Port

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

8. 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.”

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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”)

20. 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

21. Constructing Messages
…beyond simple strings

22. 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

23. 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

24. Primitive Types String Character encoding: ASCII, Unicode, UTF
Delimit: length vs. termination character
M o m \n

25. Primitive Types Integer Strings of character encoded decimal digits
Advantage: 1. Human readable
2. Arbitrary size
Disadvantage: 1. Inefficient
2. Arithmetic manipulation
‘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.

26. Primitive Types Integer Native representation
Network byte order (Big-Endian)
Use for multi-byte, binary data exchange
htonl(), htons(), ntohl(), ntohs()
Little-Endian
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.

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