1. Berkeley Socket Abstraction
Tom Skoczlas

2. History Created at UC Berkeley for use on UNIX
Provides an API for which to “abstract” TCP/IP communication
“adopted” by Microsoft and renamed Winsock

3. Client-Server Model In software, it’s all client-server.
Application Layer Protocol describes the way client and server programs communicate

4. TCP vs UDP sockets TCP Sockets – Connection-oriented.
Idea of a “stream”
UDP Sockets- Connection-less.
One transmittion per send operation

5. Stateful vs Stateless Servers
Stateful – A server that keeps an active record of the communication.
Example - Telnet
Stateless – A server that keeps no record of the communication
Example- Web (in general terms)

6. Client Algorithm (TCP)
Find IP and port of server
Allocate a Socket
Specify an unused port
Communicate with the Server
Close Connection

7. Client Functions (TCP)
socket()
connect()
send()
recv()
closesocket()

8. Server Algorithm (TCP)
Create a socket and bind it to a port.
Place the socket in passive mode.
Accept the incoming connection.
Repeatedly receive a request, formulate and send back a response.
When finished, close the connection and return to “acceptive” state.

9. Server Functions (TCP)
socket()
bind()
listen()
accept()
recv()
send()
closesocket()

10. UDP differences Client – can “skip” the connect() call.
In this case the sendto() function is used, and the endpoint must be specified each time
Server – server is always receiving, since there is no “connection”
No call to listen() or accept()

11. Concurrent vs Iterative Servers
Iterative: Block on I/O operations
Concurrent: Allow multiple I/O operations at once
Possible through:
Multiple Threads
Multiple Processes
Software Polling

12. Winsock differences
Winsock requires the use of the Winsock dynamically-linked library.
Requires use of WSAstartup() and WSAcleanup to initialize and uninitialize the dll

13. Questions?
Good reference – Internetworking with TCP/IP, VOL. 3: Client-Server Programming and Applications
Comer & Stevens