University of Amsterdam Computer Systems – Iterative server Arnoud Visser 1 Computer Systems Iterative server

University of Amsterdam

Computer Systems – Iterative serverArnoud Visser 1

Computer Systems

Iterative server



Web Servers

• Clients and servers communicate using the HyperText Transfer Protocol (HTTP)– Client and server establish TCP connection– Client requests content– Server responds with requested content– Client and server close connection (usually)

Webserver

HTTP request

HTTP response(content)

Webclient

(browser)



Web Content• Web servers return content to clients

– content: a sequence of bytes with an associated MIME (Multipurpose Internet Mail Extensions) type

• Example MIME types– text/html HTML document– text/plain Unformatted text– application/postscript Postcript document– image/gif Binary image encoded in GIF format– image/jpeg Binary image encoded in JPEG format



Static and Dynamic Content• The content returned in HTTP responses can be

either static or dynamic.– Static content: content stored in files and retrieved in

response to an HTTP request• Examples: HTML files, images, audio clips.

– Dynamic content: content produced on-the-fly in response to an HTTP request

• Example: content produced by a program executed by the server on behalf of the client.

• Bottom line: All Web content is associated with a file that is managed by the server.



URLs• Each file managed by a server has a unique name called

a URL (Universal Resource Locator)• URLs for static content:

– http://www.cs.cmu.edu:80/index.html– http://www.cs.cmu.edu/index.html– http://www.cs.cmu.edu

• Identifies a file called index.html, managed by a Web server at www.cs.cmu.edu that is listening on port 80.

• URLs for dynamic content:– http://brooks.science.uva.nl:8008/cgi-bin/adder?33&9

• Identifies an executable file called adder, managed by a Web server runing on brooks that is listening on port 8008, that should be called with two argument strings: 33 and 9.



How Clients and Servers Use URLs• Example URL: http://www.aol.com:80/index.html• Clients use prefix (http://www.aol.com:80) to

infer:– What kind of server to contact (Web server)– Where the server is (www.aol.com)– What port it is listening on (80)

• Servers use suffix (/index.html) to:– Determine if request is for static or dynamic content.

• No hard and fast rules for this.• Convention: executables reside in cgi-bin directory

– Find file on file system.• Initial “/” in suffix denotes home directory for requested content.• Minimal suffix is “/”, which all servers expand to some default

home page (e.g., index.html).



Clients• Examples of client programs

– Web browsers, ftp, telnet, ssh

• How does a client find the server?– The IP address in the server socket address identifies the

host (more precisely, an adapter on the host)– The (well-known) port in the server socket address

identifies the service, and thus implicitly identifies the server process that performs that service.

– Examples of well know ports• Port 7: Echo server• Port 23: Telnet server• Port 25: Mail server• Port 80: Web server



Using Ports to Identify Services

Web server(port 80)

Client host

Server host 128.2.194.242

Echo server(port 7)

Service request for128.2.194.242:80

(i.e., the Web server)

Web server(port 80)

Echo server(port 7)

Service request for128.2.194.242:7

(i.e., the echo server)

Kernel

Kernel

Client

Client



Sockets Interface

• Created in the early 80’s as part of the original Berkeley distribution of Unix that contained an early version of the Internet protocols.

• Provides a user-level interface to the network.

• Underlying basis for all Internet applications.

• Based on client/server programming model.



Sockets• What is a socket?

– To the kernel, a socket is an endpoint of IP communication.

TCP/IP

Client

Networkadapter

Global IP Internet

TCP/IP

Server

Networkadapter

Internet client host Internet server host

Sockets interface(system calls)

Hardware interface(interrupts)

User code

Kernel code

Hardware



Sockets• What is a socket?

– To an application, a socket is a file descriptor that lets the application read/write from/to the network.

• Remember: All Unix I/O devices, including networks, are modeled as files.

• Clients and servers communicate with each by reading from and writing to socket descriptors.

• The main distinction between regular file I/O and socket I/O is how the application “opens” the socket descriptors.



A server listens to a portlistenfd(3)

Client

1. Server blocks in accept, waiting for connection request on listening descriptor listenfd.

clientfd

Server

listenfd(3)

Client

clientfd

Server 2. Client makes connection request bycalling and blocking in connect.

listenfd(3)

Client

clientfd

Server

3. Server returns connfd from accept.Client returns from connect. Connection is now established between clientfd and connfd.

connectionrequest

connfd(4)



System calls of the Sockets InterfaceClient Server

socket socket

bind

listen

accept

rio_readlineb

rio_readlineb

rio_writen

close

rio_readlineb

connect

rio_writen

close

Connectionrequest

EOF

Await connectionrequest fromnext client

open_listenfdopen_clientfd



Echo Client Main Routine#include "csapp.h"

/* usage: ./echoclient host port */int main(int argc, char **argv){ int clientfd, port; char *host, buf[MAXLINE]; rio_t rio; host = argv[1]; port = atoi(argv[2]); clientfd = Open_clientfd(host, port); Rio_readinitb(&rio, clientfd); while (Fgets(buf, MAXLINE, stdin) != NULL) { Rio_writen(clientfd, buf, strlen(buf)); Rio_readlineb(&rio, buf, MAXLINE); Fputs(buf, stdout); } Close(clientfd); }



Echo Server: Main Routineint main(int argc, char **argv) { int listenfd, connfd, port, clientlen; struct sockaddr_in clientaddr; struct hostent *hp; char *haddrp;

port = atoi(argv[1]); /* the server listens on a port passed on the command line */ listenfd = open_listenfd(port);

while (1) { clientlen = sizeof(clientaddr); connfd = Accept(listenfd, (SA *)&clientaddr, &clientlen); hp = Gethostbyaddr((const char *)&clientaddr.sin_addr.s_addr, sizeof(clientaddr.sin_addr.s_addr), AF_INET); haddrp = inet_ntoa(clientaddr.sin_addr); printf("server connected to %s (%s)\n", hp->h_name, haddrp); echo(connfd); Close(connfd); }}



Echo Server: echovoid echo(int connfd) { size_t n; char buf[MAXLINE]; rio_t rio; Rio_readinitb(&rio, connfd); while((n = Rio_readlineb(&rio, buf, MAXLINE)) != 0) { printf("server received %d bytes\n", n); Rio_writen(connfd, buf, n); } }

• The server uses RIO to read and echo text lines until EOF (end-of-file) is encountered.– EOF notification caused by client calling close(clientfd).

– IMPORTANT: EOF is a condition, not a particular data byte.



Unix I/O vs. Standard I/O vs. RIO• Standard I/O and RIO are implemented using

low-level Unix I/O.

• Which ones should you use in your programs?

Unix I/O functions (accessed via system calls)

Standard I/O functions

C application program

fopen fdopenfread fwrite fscanf fprintf sscanf sprintf fgets fputs fflush fseekfcloseopen readwrite lseekstat close

rio_readnrio_writenrio_readinitbrio_readlinebrio_readnb

RIOfunctions



Pros and Cons of Standard I/O

• Pros:– Buffering increases efficiency by decreasing the

number of read and write system calls.– Short counts are handled automatically.

• Cons:– Provides no function for accessing file metadata– Standard I/O is not appropriate for input and output

on network sockets– There are poorly documented restrictions on streams

that interact badly with restrictions on sockets



Pros and Cons of Standard I/O

• Restrictions on (full-duplex) streams:– Restriction 1: input function cannot follow output function

without intervening call to fflush, fseek, fsetpos, or rewind.

• Latter three functions all use lseek to change file position.– Restriction 2: output function cannot follow an input function

with intervening call to fseek, fsetpos, or rewind.

• Restriction on sockets:– You are not allowed to change the file position of a socket.



Choosing I/O Functions• General rule: Use the highest-level I/O functions you

can.– Many C programmers are able to do all of their work using the

standard I/O functions.

• When to use standard I/O?– When working with disk or terminal files.

• When to use raw Unix I/O – When you need to fetch file metadata.– In rare cases when you need absolute highest performance.

• When to use RIO?– When you are reading and writing network sockets or pipes.– Never use standard I/O or raw Unix I/O on sockets or pipes.



Testing the Echo Server With telnetbass> echoserver 5000server established connection with KITTYHAWK.CMCL (128.2.194.242)server received 5 bytes: 123server established connection with KITTYHAWK.CMCL (128.2.194.242)server received 8 bytes: 456789

kittyhawk> telnet bass 5000Trying 128.2.222.85...Connected to BASS.CMCL.CS.CMU.EDU.Escape character is '^]'.123123Connection closed by foreign host.kittyhawk> telnet bass 5000Trying 128.2.222.85...Connected to BASS.CMCL.CS.CMU.EDU.Escape character is '^]'.456789456789Connection closed by foreign host.kittyhawk>



Running the Echo Client and Server

bass> echoserver 5000server established connection with KITTYHAWK.CMCL (128.2.194.242)server received 4 bytes: 123server established connection with KITTYHAWK.CMCL (128.2.194.242)server received 7 bytes: 456789...

kittyhawk> echoclient bass 5000Please enter msg: 123Echo from server: 123

kittyhawk> echoclient bass 5000Please enter msg: 456789Echo from server: 456789kittyhawk>



A server listens to a portlistenfd(3)

Client

1. Server blocks in accept, waiting for connection request on listening descriptor listenfd.

clientfd

Server

listenfd(3)

Client

clientfd

Server 2. Client makes connection request bycalling and blocking in connect.

listenfd(3)

Client

clientfd

Server

3. Server returns connfd from accept.Client returns from connect. Connection is now established between clientfd and connfd.

connectionrequest

connfd(4)



Connected vs. Listening Descriptors

• Listening descriptor– End point for client connection requests.– Created once and exists for lifetime of the server.

• Connected descriptor– End point of the connection between client and server.– A new descriptor is created each time the server accepts a

connection request from a client.– Exists only as long as it takes to service client.

• Why the distinction?– Allows for concurrent servers that can communicate over

many client connections simultaneously.• E.g., Each time we receive a new request, we fork a child to handle

the request.



Anatomy of an HTTP Transaction

unix> telnet www.aol.com 80 Client: open connection to serverTrying 205.188.146.23... Telnet prints 3 lines to the terminalConnected to aol.com.Escape character is '^]'.GET / HTTP/1.1 Client: request linehost: www.aol.com Client: required HTTP/1.1 HOST header Client: empty line terminates headers.HTTP/1.0 200 OK Server: response lineMIME-Version: 1.0 Server: followed by five response headersDate: Mon, 08 Jan 2001 04:59:42 GMTServer: NaviServer/2.0 AOLserver/2.3.3Content-Type: text/html Server: expect HTML in the response bodyContent-Length: 42092 Server: expect 42,092 bytes in the resp body Server: empty line (“\r\n”) terminates hdrs<html> Server: first HTML line in response body... Server: 766 lines of HTML not shown.</html> Server: last HTML line in response bodyConnection closed by foreign host. Server: closes connectionunix> Client: closes connection and terminates



The add.com Experienceinput URL

Output page

host port CGI program args



Serving Dynamic Content With GET• Question: How does the client pass arguments to

the server?• Answer: The arguments are appended to the URI• Can be encoded directly in a URL typed to a

browser or a URL in an HTML link – http://add.com/cgi-bin/adder?1&2– adder is the CGI program on the server that will do the

addition.– argument list starts with “?”– arguments separated by “&” – spaces represented by “+” or “%20”

• Can also be generated by an HTML form<form method=get action="http://add.com/cgi-bin/postadder">



Assignment

• Adder.com: Make and Start your own tiny webserver

http://staff.science.uva.nl/~arnoud/onderwijs/CS/conc/tiny.tar.gz

• Problem 11.10Access the adder-code with a form

See http://staff.science.uva.nl/~arnoud/onderwijs/CS/Evaluation.html

<form method=get action="http://add.com/cgi-bin/postadder">

Documents

University of Amsterdam Computer Systems – Iterative server Arnoud Visser 1 Computer Systems Iterative server