1. Boots Cassel Villanova University
Computer Networks
Boots Cassel
Villanova University

2. Client - Server Computing
Direction of initiation determines which side is which
Client initiates
Server responds
Same system may be client and server

3. Parameterization of clients
Parameter specifies machine where server resides
Optional parameter specifies the port where the server listens
Include the port number parameter in defining clients
Especially useful for testing new versions

4. Addressing Specify the machine Specify the port number
name gets converted to IP address
Specify the port number
Which running process to connect to.

5. How do you know Which machine Which port
you have to know the name or the IP address of the partner
Names get converted by a Domain Name Service
Which port
Common services have assigned port numbers
telnet = 23, http = 80, etc.

6. Domain Name Service Structure of Internet Names
gadget.netlab.csc.villanova.edu
Lookup in order from right to left
netlab and csc are both entries in the villanova DNS, so that lookup can go directly from villanova to netlab without using the csc DNS
That is always an option in configuring the servers

7. DNS - Christie’s table ; Addresses for the canonical names ;
localhost.netlab.csc.villanova.edu. IN A
christie.netlab.csc.villanova.edu IN A
doyle.netlab.csc.villanova.edu IN A
gadget.netlab.csc.villanova.edu IN A
missmarple.netlab.csc.villanova.edu. IN A
lordpeter.netlab.csc.villanova.edu. IN A
poirot.netlab.csc.villanova.edu IN A
koko.netlab.csc.villanova.edu IN A
sherlock.netlab.csc.villanova.edu IN A
cadfael.netlab.csc.villanova.edu IN A
columbo.netlab.csc.villanova.edu IN A
weber.netlab.csc.villanova.edu IN A
matlock.netlab.csc.villanova.edu IN A
samspade.netlab.csc.villanova.edu IN A
poe.netlab.csc.villanova.edu IN A

8. Port number usage
Some implementations do not allow the user to specify a port number
uses the protocol’s reserved number only
Allowing a port number to be specified makes it easier to test the implementation
allow the “real” version to run while testing a different version

9. Server Usually runs in privileged condition Responsibilities
needs to access protected resources
be careful about what access it gives to clients
Responsibilities
Authenticate client identity
Enforce authorization restrictions
Data protection: security and privacy
Protection: system resources from abuse

10. Connectionless vs Connection-oriented
Directly related to the choice of transport protocol
UDP: connectionless
TCP: connection-oriented
Can build connection-oriented client/server application over a connectionless transport layer
Silly to do the converse

11. Server state Stateless server Stateful server
no memory of past interaction
each request must be fully qualified
Stateful server
records state information
reduces needed content in subsequent messages
depends on reliable network communication
vulnerable to system failures, reboots

12. Idempotent operations
An operation that always yields the same result, regardless of repetition or missing prior steps
READ or WRITE, without automatic increment of pointer
not idempotent - increment, add amount to total, etc.

13. Concurrent processes Multiprogramming Multiprocessing
Apparent simultaneous execution
Multiprocessing
Real simultaneous execution

14. Server Concurrency Need to serve requests from multiple clients
Process
Fundamental unit of computation
Address space plus at least one thread of execution
instruction pointer tells where in the process the thread is currently executing

15. Threads Similar to a process except
a thread is owned by a process
a thread may share common resources belonging to a process
A thread has a private copy of local variables

16. Concurrent processes In a uniprocessor system
Each process gets a slice of the CPU time in turn
All appear to run at the same time
If each process has only one thread of execution, that process shares the CPU with other processes on a time sharing plan
If a process has more than one thread, the CPU time is divided among the threads.
OS varies: divide time among all threads or divide among processes and multiple threads share the time allotted to their parent process
In a multiprocessor system, each process or thread may be allocated a separate CPU

17. Threads in servers
A process may respond to client requests by spawning a separate thread for each client
Clients appear to be served simultaneously but actually receive intermingled slices of the CPU time.

18. Unix concurrency
Different operating systems handle concurrency in idfferent ways
unix: fork() creates an exact copy of the calling process and continues execution from exactly the same place in the code.
Copy includes the stack showing function calls, local and global variables
Original and forked process continue from the same point in execution
One distinction: fork returns a value: new process returns 0, original returns non zero value

19. Unix concurrency example
Sum = 0;
pid = fork();
if (pid != 0 ) { /* original */
printf (“The original process\n”);
}else { /* the new process */
printf (“The new process\n”);

20. Concurrency in Windows
_beginthread
parameter specifies what is to be executed
Not necessarily executing the same code as the calling process

21. Sockets A de facto standard interface to TCP/IP protocols
Defined in BSD unix
Adapted by other operating systems

22. Initiating concurrent subprocesses
Windows:
_beginthread
parameter specifies what to execute as thread
unix
fork
new process is a copy of the initiating process
pid identifies each