Transcript CS335 Networking & Network Administration

CS335
Networking &
Network Administration
Tuesday, May 25, 2010
Client-server





Network applications
Protocol software cannot initiate contact with, or
accept contact from a remote computer
Two application programs must participate in any
communication
One application initiates the communication and the
other accepts it
In the client-server paradigm, a server application
waits passively for a client application to initiate
communication
Characteristics of client/server

Client software






Is an arbitrary application program that becomes a client
temporarily when remote access is needed, but also
performs other computation locally
Is invoked directly by the user, and executes for only one
session
Runs locally on a user’s personal computer
Actively initiates contact with the server
Can access multiple services as needed, but actively
contacts one remote server at a time
Does not require special hardware or OS
Server programs







Special-purpose, privileged program dedicated to providing one
service, but can handle multiple remote clients at one time
Is invoked automatically when a system boots, and continues to
execute through many sessions
Runs on a shared computer
Waits passively for contact from arbitrary remote clients
Accepts contact from arbitrary clients, but offers a single service
Requires powerful hardware and a sophisticated OS also known
as a server-class computer
Software technically is the server
Data flow



Information can pass in either or both
directions between a client and server
Can be a series of request with response
form the server
Can be a continuous flow of data from server
as soon as client contacts
Client-server


TCP/IP stack as the transport protocol
For troubleshooting need to know what
transport protocol is being used
Multiple services



Sufficient hardware resources
Network OS that allows multiple concurrent
applications
Practical to run multiple server apps, but…
Multiple services











Groupwise
Follett library software
Winschool
Macschool
File server services
Mealtime
Filemaker pro
Open district – Oracle
Sophos
Antivirus server
Proxy server
Protocol port numbers




TCP uses 16-bit integer values to identify
services
Assigns a unique protocol port number to
each service
Both client and server specify the service
identifier (port number)
Protocol software uses the identifier to direct
each incoming request to the correct server
software
Concurrency



A computer system that permits multiple
application programs to execute at the same
time
Multiple process, task, or thread
Concurrent server offers service to multiple
clients without requiring clients to wait for
other clients to finish
Dynamic server creation



Main server thread creates a new service
thread for each new client connection
Service tread handles one request, then
terminates
Main thread keeps the server alive waiting for
requests
Transport protocols


Transport protocols assign an identifier to
each client as well as to each service
Protocol software on server uses the
combination of client and server identifiers to
choose the correct copy of the concurrent
server
Connection-oriented transport




Establish connection
Send data
Terminate
TCP
Connectionless transport




Application can send a message to any
destination at any time
UDP
Client-server apps can use either approach
Can also offer both with the choice of
transport left to the client
Sockets



Interface between an application program
and the communication protocols in an OS is
known as the Application Program Interface
or API
The socket API is a de facto standard
A protocol standard might suggest an
operation is needed to allow an application to
send data, and the API specifies the exact
name of the function and the type of each
argument
Socket libraries





Socket functions are part of BSD UNIX
Other vendors started adding a socket API to their
systems
Instead of modifying the OS, programmers added a
socket library that provides the socket API
Socket library can provide apps with a socket API on
systems that do not provide native sockets
When an app calls one of the socket procedures,
control passes to a library routine that makes calls to
the underlying OS
Sockets, descriptors, I/O


When an app creates a socket, the app is
given a small integer descriptor used to
reference the socket
If a system uses the same descriptor space
for sockets and other I/O, a single application
can be used for network communication as
well as for local data transfer
Socket procedures










Create
Close
Bind
Listen
Accept
Connect
Send
Receive
Read
Write
Socket API routines




Socket API
The Socket API is the standard API for TCP/IP programming in the UNIX world. In the Windows world, the Winsock API is based on and shares a lot
with the BSD Socket API. This makes conversion between the two fairly easy.
All the routines that application programmers would expect are available, including (but not limited to):
accept()
bind()
bindresvport()
connect()
dn_comp()
dn_expand()
endprotoent()
endservent()
gethostbyaddr()
gethostbyname()
getpeername()
getprotobyname()
getprotobynumber()
getprotoent()
getservbyname()
getservent()
getsockname()
getsockopt()
herror()
hstrerror()
htonl()
htons()
h_errlist()
h_errno()
h_nerr()
inet_addr()
inet_aton()
inet_lnaof()
inet_makeaddr()
inet_netof()
inet_network()
inet_ntoa()
Socket API routines


ioctl()
listen()
ntohl()
ntohs()
recv()
recvfrom()
res_init()
res_mkquery()
res_query()
res_querydomain()
res_search()
res_send()
select()
send()
sendto()
setprotoent()
setservent()
setsockopt()
shutdown()
socket()
The common daemons and utilities from the Internet will easily port or just compile in this
environment. This makes it easy to leverage what already exists for your applications.
Client Server examples

Chapter 30