Boots Cassel Villanova University Computer Networks Boots Cassel Villanova University

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

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

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

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.

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

DNS - Christie’s table ; Addresses for the canonical names ; localhost.netlab.csc.villanova.edu. IN A 127.0.0.1 christie.netlab.csc.villanova.edu. IN A 153.104.203.200 doyle.netlab.csc.villanova.edu. IN A 153.104.203.210 gadget.netlab.csc.villanova.edu. IN A 153.104.203.42 missmarple.netlab.csc.villanova.edu. IN A 153.104.203.7 lordpeter.netlab.csc.villanova.edu. IN A 153.104.203.41 poirot.netlab.csc.villanova.edu. IN A 153.104.203.148 koko.netlab.csc.villanova.edu. IN A 153.104.203.45 sherlock.netlab.csc.villanova.edu. IN A 153.104.203.44 cadfael.netlab.csc.villanova.edu. IN A 153.104.203.43 columbo.netlab.csc.villanova.edu. IN A 153.104.203.46 weber.netlab.csc.villanova.edu. IN A 153.104.203.205 matlock.netlab.csc.villanova.edu. IN A 153.104.203.203 samspade.netlab.csc.villanova.edu. IN A 153.104.203.204 poe.netlab.csc.villanova.edu. IN A 153.104.203.50

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

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

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

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

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.

Concurrent processes Multiprogramming Multiprocessing Apparent simultaneous execution Multiprocessing Real simultaneous execution

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

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

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

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.

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

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”);

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

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

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