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1. Networks and the Internet 2. Network programming 3. Web services A Client-Server Transaction 1. Client sends request Client process 4. Client handles response Server process 3. Server sends response Resource 2. Server handles request Note: clients and servers are processes running on hosts (can be the same or different hosts) Most network applications are based on the client-server model: A server process and one or more client processes Server manages some resource Server provides service by manipulating resource for clients Server activated by request from client (vending machine analogy) Hardware Organization of a Network Host CPU chip register file ALU system bus memory bus main memory I/O bridge MI Expansion slots I/O bus USB controller mouse keyboard graphics adapter disk controller network adapter disk network monitor Computer Networks A network is a hierarchical system of boxes and wires organized by geographical proximity SAN (System Area Network) spans cluster or machine room Switched Ethernet, Quadrics QSW, … LAN (Local Area Network) spans a building or campus Ethernet is most prominent example WAN (Wide Area Network) spans country or world Typically high-speed point-to-point phone lines An internetwork (internet) is an interconnected set of networks The Global IP Internet (uppercase “I”) is the most famous example of an internet (lowercase “i”) Let’s see how an internet is built from the ground up Lowest Level: Ethernet Segment host 100 Mb/s host hub host 100 Mb/s port Ethernet segment consists of a collection of hosts connected by wires (twisted pairs) to a hub Spans room or floor in a building Operation Each Ethernet adapter has a unique 48-bit address (MAC address) E.g., 00:16:ea:e3:54:e6 Hosts send bits to any other host in chunks called frames Hub slavishly copies each bit from each port to every other port Every host sees every bit Note: Hubs are on their way out. Bridges (switches, routers) became cheap enough to replace them (means no more broadcasting) Next Level: Bridged Ethernet Segment A host host hub B host host X 100 Mb/s bridge 100 Mb/s hub 1 Gb/s hub 100 Mb/s bridge host 100 Mb/s host host hub Y host host host host host C Spans building or campus Bridges cleverly learn which hosts are reachable from which ports and then selectively copy frames from port to port Conceptual View of LANs For simplicity, hubs, bridges, and wires are often shown as a collection of hosts attached to a single wire: host host ... host Next Level: internets Multiple incompatible LANs can be physically connected by specialized computers called routers The connected networks are called an internet host host ... host host host ... LAN host LAN router WAN router WAN router LAN 1 and LAN 2 might be completely different, totally incompatible (e.g., Ethernet and Wifi, 802.11*, T1-links, DSL, …) Logical Structure of an internet host router host router router router router router Ad hoc interconnection of networks No particular topology Vastly different router & link capacities Send packets from source to destination by hopping through networks Router forms bridge from one network to another Different packets may take different routes The Notion of an internet Protocol How is it possible to send bits across incompatible LANs and WANs? Solution: protocol software running on each host and router smooths out the differences between the different networks Implements an internet protocol (i.e., set of rules) governs how hosts and routers should cooperate when they transfer data from network to network TCP/IP is the protocol for the global IP Internet What Does an internet Protocol Do? Provides a naming scheme An internet protocol defines a uniform format for host addresses Each host (and router) is assigned at least one of these internet addresses that uniquely identifies it Provides a delivery mechanism An internet protocol defines a standard transfer unit (packet) Packet consists of header and payload Header: contains info such as packet size, source and destination addresses Payload: contains data bits sent from source host Transferring Data Over an internet LAN1 (1) client server protocol software data PH data PH LAN2 (8) data (7) data PH FH2 (6) data PH FH2 protocol software FH1 LAN1 frame (3) Host B data internet packet (2) Host A LAN1 adapter LAN2 adapter Router FH1 LAN1 adapter LAN2 adapter LAN2 frame (4) PH: Internet packet header FH: LAN frame header data PH FH1 data protocol software PH FH2 (5) Other Issues We are glossing over a number of important questions: What if different networks have different maximum frame sizes? (segmentation) How do routers know where to forward frames? How are routers informed when the network topology changes? What if packets get lost? These (and other) questions are addressed by the area of systems known as computer networking Global IP Internet Most famous example of an internet Based on the TCP/IP protocol family IP (Internet protocol) : Provides basic naming scheme and unreliable delivery capability of packets (datagrams) from host-to-host UDP (Unreliable Datagram Protocol) Uses IP to provide unreliable datagram delivery from process-to-process TCP (Transmission Control Protocol) Uses IP to provide reliable byte streams from process-to-process over connections Accessed via a mix of Unix file I/O and functions from the sockets interface Hardware and Software Organization of an Internet Application Internet client host Internet server host Client User code Server TCP/IP Kernel code TCP/IP Sockets interface (system calls) Hardware interface (interrupts) Network adapter Hardware and firmware Global IP Internet Network adapter A Programmer’s View of the Internet Hosts are mapped to a set of 32-bit IP addresses 140.192.36.43 The set of IP addresses is mapped to a set of identifiers called Internet domain names 140.192.36.43 is mapped to cdmlinux.cdm.depaul.edu IP Addresses 32-bit IP addresses are stored in an IP address struct IP addresses are always stored in memory in network byte order (big-endian byte order) True in general for any integer transferred in a packet header from one machine to another. E.g., the port number used to identify an Internet connection. /* Internet address structure */ struct in_addr { unsigned int s_addr; /* network byte order (big-endian) */ }; Useful network byte-order conversion functions (“l” = 32 bits, “s” = 16 bits) htonl: convert uint32_t from host to network byte order htons: convert uint16_t from host to network byte order ntohl: convert uint32_t from network to host byte order ntohs: convert uint16_t from network to host byte order Dotted Decimal Notation By convention, each byte in a 32-bit IP address is represented by its decimal value and separated by a period IP address: 0x8002C2F2 = 128.2.194.242 Functions for converting between binary IP addresses and dotted decimal strings: inet_aton: dotted decimal string → IP address in network byte order inet_ntoa: IP address in network byte order → dotted decimal string “n” denotes network representation “a” denotes application representation Internet Domain Names unnamed root .net .edu smith depaul cti .gov berkeley ece .com amazon www 207.171.166.252 cstsis reed 140.192.32.110 ctilinux3 140.192.36.43 First-level domain names Second-level domain names Third-level domain names Domain Naming System (DNS) The Internet maintains a mapping between IP addresses and domain names in a huge worldwide distributed database called DNS Conceptually, programmers can view the DNS database as a collection of millions of host entry structures: /* DNS host entry structure struct hostent { char *h_name; /* char **h_aliases; /* int h_addrtype; /* int h_length; /* char **h_addr_list; /* }; */ official domain name of host */ null-terminated array of domain names */ host address type (AF_INET) */ length of an address, in bytes */ null-terminated array of in_addr structs */ Functions for retrieving host entries from DNS: gethostbyname: query key is a DNS domain name. gethostbyaddr: query key is an IP address. Properties of DNS Host Entries Each host entry is an equivalence class of domain names and IP addresses Each host has a locally defined domain name localhost which always maps to the loopback address 127.0.0.1 Different kinds of mappings are possible: Simple case: one-to-one mapping between domain name and IP address: reed.cs.depaul.edu maps to 140.192.32.110 Multiple domain names mapped to the same IP address: eecs.mit.edu and cs.mit.edu both map to 18.62.1.6 Multiple domain names mapped to multiple IP addresses: google.com maps to multiple IP addresses A Program That Queries DNS int main(int argc, char **argv) { /* argv[1] is a domain name */ char **pp; /* or dotted decimal IP addr */ struct in_addr addr; struct hostent *hostp; if (inet_aton(argv[1], &addr) != 0) hostp = Gethostbyaddr((const char *)&addr, sizeof(addr), AF_INET); else hostp = Gethostbyname(argv[1]); printf("official hostname: %s\n", hostp->h_name); for (pp = hostp->h_aliases; *pp != NULL; pp++) printf("alias: %s\n", *pp); for (pp = hostp->h_addr_list; *pp != NULL; pp++) { addr.s_addr = ((struct in_addr *)*pp)->s_addr; printf("address: %s\n", inet_ntoa(addr)); } } Using DNS Program $ ./hostinfo reed.cs.depaul.edu official hostname: reed.cti.depaul.edu alias: reed.cs.depaul.edu address: 140.192.39.29 $ ./hostinfo 140.192.39.29 official hostname: reed.cti.depaul.edu address: 140.192.39.29 $ ./hostinfo www.google.com official hostname: www.google.com address: 74.125.225.20 address: 74.125.225.17 address: 74.125.225.18 address: 74.125.225.19 address: 74.125.225.16 Querying DIG Domain Information Groper (dig) provides a scriptable command line interface to DNS $ dig +short reed.cs.depaul.edu reed.cti.depaul.edu. 140.192.39.29 $ dig +short -x 140.192.39.29 ipdstdwkr.cstcis.cti.depaul.edu. reed.cti.depaul.edu. $ dig +short www.google.com 74.125.225.19 74.125.225.16 74.125.225.20 74.125.225.17 74.125.225.18 Internet Connections Clients and servers communicate by sending streams of bytes over connections: Point-to-point, full-duplex (2-way communication), and reliable. A socket is an endpoint of a connection Socket address is an IPaddress:port pair A port is a 16-bit integer that identifies a process: Ephemeral port: Assigned automatically on client when client makes a connection request Well-known port: Associated with some service provided by a server (e.g., port 80 is associated with Web servers) A connection is uniquely identified by the socket addresses of its endpoints (socket pair) (cliaddr:cliport, servaddr:servport) Putting it all Together: Anatomy of an Internet Connection Client socket address 128.2.194.242:51213 Client Client host address 128.2.194.242 Server socket address 208.216.181.15:80 Connection socket pair (128.2.194.242:51213, 208.216.181.15:80) Server (port 80) Server host address 208.216.181.15 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 Server host 128.2.194.242 Client host Client Service request for 128.2.194.242:80 (i.e., the Web server) Web server (port 80) Kernel Echo server (port 7) Client Service request for 128.2.194.242:7 (i.e., the echo server) Web server (port 80) Kernel Echo server (port 7) Servers Servers are long-running processes (daemons) Created at boot-time (typically) by the init process (process 1) Run continuously until the machine is turned off Each server waits for requests to arrive on a well-known port associated with a particular service Port 7: echo server Port 23: telnet server Port 25: mail server Port 80: HTTP server A machine that runs a server process is also often referred to as a “server” Server Examples Web server (port 80) Resource: files/compute cycles (CGI programs) Service: retrieves files and runs CGI programs on behalf of the client FTP server (20, 21) Resource: files Service: stores and retrieve files See /etc/services for a comprehensive list of the port mappings on a Linux machine Telnet server (23) Resource: terminal Service: proxies a terminal on the server machine Mail server (25) Resource: email “spool” file Service: stores mail messages in spool file 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 communication 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 other by reading from and writing to socket descriptors Client clientfd Server serverfd The main distinction between regular file I/O and socket I/O is how the application “opens” the socket descriptors Example: Echo Client and Server On Client On Server $ ./echoserveri 28888 $ ./echoclient localhost 28888 server connected to localhost.localdomain (127.0.0.1) type: hello there server received 12 bytes echo: hello there type: ^D Connection closed Overview of the Sockets Interface Client Server socket socket bind open_listenfd open_clientfd listen connect Client / Server Session Connection request accept rio_writen rio_readlineb rio_readlineb rio_writen close EOF rio_readlineb close Await connection request from next client Socket Address Structures Generic socket address: For address arguments to connect, bind, and accept Necessary only because C did not have generic (void *) pointers when the sockets interface was designed struct sockaddr { unsigned short sa_family; char sa_data[14]; }; /* protocol family */ /* address data. */ sa_family Family Specific Socket Address Structures Internet-specific socket address: Must cast (sockaddr_in *) to (sockaddr *) for connect, bind, and accept struct sockaddr_in { unsigned short sin_family; unsigned short sin_port; struct in_addr sin_addr; unsigned char sin_zero[8]; }; sin_port AF_INET /* /* /* /* address family (always AF_INET) */ port num in network byte order */ IP addr in network byte order */ pad to sizeof(struct sockaddr) */ sin_addr 0 0 sa_family sin_family Family Specific 0 0 0 0 0 0 Echo Client Main Routine #include "csapp.h" Send line to server Receive line from server /* 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); printf("type:"); fflush(stdout); while (Fgets(buf, MAXLINE, stdin) != NULL) { Rio_writen(clientfd, buf, strlen(buf)); Rio_readlineb(&rio, buf, MAXLINE); printf("echo:"); Fputs(buf, stdout); printf("type:"); fflush(stdout); } Close(clientfd); exit(0); } Read input line Print server response Overview of the Sockets Interface Client Server socket socket bind open_clientfd listen connect Connection request accept open_listenfd Echo Client: open_clientfd int open_clientfd(char *hostname, int port) { int clientfd; This function opens a connection struct hostent *hp; from the client to the server at struct sockaddr_in serveraddr; hostname:port } if ((clientfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) return -1; /* check errno for cause of error */ Create socket /* Fill in the server's IP address and port */ if ((hp = gethostbyname(hostname)) == NULL) return -2; /* check h_errno for cause of error */ bzero((char *) &serveraddr, sizeof(serveraddr)); serveraddr.sin_family = AF_INET; bcopy((char *)hp->h_addr_list[0], (char *)&serveraddr.sin_addr.s_addr, hp->h_length); serveraddr.sin_port = htons(port); Create address /* Establish a connection with the server */ if (connect(clientfd, (SA *) &serveraddr, sizeof(serveraddr)) < 0) return -1; return clientfd; Establish connection Echo Client: open_clientfd (socket) socket creates a socket descriptor on the client Just allocates & initializes some internal data structures AF_INET: indicates that the socket is associated with Internet protocols SOCK_STREAM: selects a reliable byte stream connection provided by TCP int clientfd; /* socket descriptor */ if ((clientfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) return -1; /* check errno for cause of error */ ... <more> Echo Client: open_clientfd (gethostbyname) The client then builds the server’s Internet address int clientfd; /* socket descriptor */ struct hostent *hp; /* DNS host entry */ struct sockaddr_in serveraddr; /* server’s IP address */ ... /* fill in the server's IP address and port */ if ((hp = gethostbyname(hostname)) == NULL) return -2; /* check h_errno for cause of error */ bzero((char *) &serveraddr, sizeof(serveraddr)); serveraddr.sin_family = AF_INET; serveraddr.sin_port = htons(port); bcopy((char *)hp->h_addr_list[0], (char *)&serveraddr.sin_addr.s_addr, hp->h_length); Echo Client: open_clientfd (connect) Finally the client creates a connection with the server Client process suspends (blocks) until the connection is created After resuming, the client is ready to begin exchanging messages with the server via Unix I/O calls on descriptor clientfd int clientfd; /* socket descriptor */ struct sockaddr_in serveraddr; /* server address */ typedef struct sockaddr SA; /* generic sockaddr */ ... /* Establish a connection with the server */ if (connect(clientfd, (SA *)&serveraddr, sizeof(serveraddr)) < 0) return -1; return clientfd; } Echo Server: Main Routine int main(int argc, char **argv) { int listenfd, connfd, port, clientlen; struct sockaddr_in clientaddr; struct hostent *hp; char *haddrp; unsigned short client_port; 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); client_port = ntohs(clientaddr.sin_port); printf("server connected to %s (%s), port %u\n", hp->h_name, haddrp, client_port); echo(connfd); Close(connfd); } } Overview of the Sockets Interface Client Server socket socket bind open_listenfd open_clientfd listen connect Connection request accept Office Telephone Analogy for Server Socket: Bind: Listen: Accept: Buy a phone Tell the local administrator what number you want to use Plug the phone in Answer the phone when it rings Echo Server: open_listenfd int open_listenfd(int port) { int listenfd, optval=1; struct sockaddr_in serveraddr; /* Create a socket descriptor */ if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) return -1; /* Eliminates "Address already in use" error from bind. */ if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, (const void *)&optval , sizeof(int)) < 0) return -1; ... <more> Echo Server: open_listenfd (cont.) ... /* Listenfd will be an endpoint for all requests to port on any IP address for this host */ bzero((char *) &serveraddr, sizeof(serveraddr)); serveraddr.sin_family = AF_INET; serveraddr.sin_addr.s_addr = htonl(INADDR_ANY); serveraddr.sin_port = htons((unsigned short)port); if (bind(listenfd, (SA *)&serveraddr, sizeof(serveraddr)) < 0) return -1; /* Make it a listening socket ready to accept connection requests */ if (listen(listenfd, LISTENQ) < 0) return -1; return listenfd; } Echo Server: open_listenfd (socket) socket creates a socket descriptor on the server AF_INET: indicates that the socket is associated with Internet protocols SOCK_STREAM: selects a reliable byte stream connection (TCP) int listenfd; /* listening socket descriptor */ /* Create a socket descriptor */ if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) return -1; Echo Server: open_listenfd (setsockopt) The socket can be given some attributes ... /* Eliminates "Address already in use" error from bind(). */ if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, (const void *)&optval , sizeof(int)) < 0) return -1; Handy trick that allows us to rerun the server immediately after we kill it Otherwise we would have to wait about 15 seconds Eliminates “Address already in use” error from bind() Strongly suggest you do this for all your servers to simplify debugging Echo Server: open_listenfd (initialize socket address) Initialize socket with server port number Accept connection from any IP address struct sockaddr_in serveraddr; /* server's socket addr */ ... /* listenfd will be an endpoint for all requests to port on any IP address for this host */ bzero((char *) &serveraddr, sizeof(serveraddr)); serveraddr.sin_family = AF_INET; serveraddr.sin_port = htons((unsigned short)port); serveraddr.sin_addr.s_addr = htonl(INADDR_ANY); IP addr and port stored in network (big-endian) byte order sin_port AF_INET sa_family sin_family sin_addr INADDR_ANY 0 0 0 0 0 0 0 0 Echo Server: open_listenfd (bind) bind associates the socket with the socket address we just created int listenfd; /* listening socket */ struct sockaddr_in serveraddr; /* server’s socket addr */ ... /* listenfd will be an endpoint for all requests to port on any IP address for this host */ if (bind(listenfd, (SA *)&serveraddr, sizeof(serveraddr)) < 0) return -1; Echo Server: open_listenfd (listen) listen indicates that this socket will accept connection (connect) requests from clients LISTENQ is constant indicating how many pending requests allowed int listenfd; /* listening socket */ ... /* Make it a listening socket ready to accept connection requests */ if (listen(listenfd, LISTENQ) < 0) return -1; return listenfd; } We’re finally ready to enter the main server loop that accepts and processes client connection requests. Echo Server: Main Loop The server loops endlessly, waiting for connection requests, then reading input from the client, and echoing the input back to the client. main() { /* create and configure the listening socket */ while(1) { /* Accept(): wait for a connection request */ /* echo(): read and echo input lines from client til EOF */ /* Close(): close the connection */ } } Overview of the Sockets Interface Client Server socket socket bind open_listenfd open_clientfd listen connect Client / Server Session Connection request accept rio_writen rio_readlineb rio_readlineb rio_writen close EOF rio_readlineb close Await connection request from next client Echo Server: accept accept() blocks waiting for a connection request int listenfd; /* listening descriptor */ int connfd; /* connected descriptor */ struct sockaddr_in clientaddr; int clientlen; clientlen = sizeof(clientaddr); connfd = Accept(listenfd, (SA *)&clientaddr, &clientlen); accept returns a connected descriptor (connfd) with the same properties as the listening descriptor (listenfd) Returns when the connection between client and server is created and ready for I/O transfers All I/O with the client will be done via the connected socket accept also fills in client’s IP address Echo Server: accept Illustrated listenfd(3) Client Server clientfd Connection request Client 1. Server blocks in accept, waiting for connection request on listening descriptor listenfd listenfd(3) Server 2. Client makes connection request by calling and blocking in connect clientfd listenfd(3) Client clientfd Server connfd(4) 3. Server returns connfd from accept. Client returns from connect. Connection is now established between clientfd and connfd 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 Echo Server: Identifying the Client The server can determine the domain name, IP address, and port of the client struct hostent *hp; /* pointer to DNS host entry */ char *haddrp; /* pointer to dotted decimal string */ unsigned short client_port; hp = Gethostbyaddr((const char *)&clientaddr.sin_addr.s_addr, sizeof(clientaddr.sin_addr.s_addr), AF_INET); haddrp = inet_ntoa(clientaddr.sin_addr); client_port = ntohs(clientaddr.sin_port); printf("server connected to %s (%s), port %u\n", hp->h_name, haddrp, client_port); Echo Server: echo 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) void echo(int connfd) { size_t n; char buf[MAXLINE]; rio_t rio; Rio_readinitb(&rio, connfd); while((n = Rio_readlineb(&rio, buf, MAXLINE)) != 0) { upper_case(buf); Rio_writen(connfd, buf, n); printf("server received %d bytes\n", n); } } Testing Servers Using telnet The telnet program is invaluable for testing servers that transmit ASCII strings over Internet connections Our simple echo server Web servers Mail servers Usage: unix> telnet <host> <portnumber> Creates a connection with a server running on <host> and listening on port <portnumber> Testing the Echo Server With telnet $ ./echoserveri 28888 Use separate SSH sessions $ telnet localhost 28888 Trying ::1... Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. Hello hello For More Information W. Richard Stevens, “Unix Network Programming: Networking APIs: Sockets and XTI”, Volume 1, Second Edition, Prentice Hall, 1998 THE network programming bible Unix Man Pages Good for detailed information about specific functions Web Services Web History 1989: Tim Berners-Lee (CERN) writes internal proposal to develop a distributed hypertext system. Connects “a web of notes with links.” Intended to help CERN physicists in large projects share and manage information 1990: Tim BL writes a graphical browser for Next machines. Web History (cont) 1992 NCSA server released 26 WWW servers worldwide 1993 Marc Andreessen releases first version of NCSA Mosaic browser Mosaic version released for (Windows, Mac, Unix). Web (port 80) traffic at 1% of NSFNET backbone traffic. Over 200 WWW servers worldwide. 1994 Andreessen and colleagues leave NCSA to form “Mosaic Communications Corp” (predecessor to Netscape). Internet Hosts Web Servers HTTP request 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 (eventually) Current version is HTTP/1.1 RFC 2616, June, 1999. Web client (browser) Web server HTTP response (content) HTTP TCP IP Web content Streams Datagrams http://www.w3.org/Protocols/rfc2616/rfc2616.html 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 text/plain application/postscript image/gif image/jpeg HTML document Unformatted text Postcript document Binary image encoded in GIF format 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. Request identifies content file 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. Request identifies file containing executable code 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://reed.cs.depaul.edu:80/index.html http://reed.cs.depaul.edu/index.html http://reed.cs.depaul.edu Identifies a file called index.html, managed by a Web server at reed.cs.depaul.edu that is listening on port 80. URLs for dynamic content: http://riely373.cdm.depaul.edu:8000/cgi-bin/adder?15000&213 Identifies an executable file called adder, managed by a Web server at riely373.cdm.depaul.edu that is listening on port 8000, that should be called with two argument strings: 15000 and 213. How Clients and Servers Use URLs Example URL: http://www.depaul.edu:80/index.html Clients use prefix (http://www.depaul.edu:80) to infer: What kind of server to contact (Web server) Where the server is (www.depaul.edu) 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). Anatomy of an HTTP Transaction $ telnet reed.cs.depaul.edu 80 Trying 140.192.39.42... Connected to reed.cti.depaul.edu. Escape character is '^]'. GET / HTTP/1.1 host: reed.cs.depaul.edu HTTP/1.1 200 OK Server: Apache-Coyote/1.1 Accept-Ranges: bytes ETag: W/"2285-1357855910000" Last-Modified: Thu, 10 Jan 2013 22:11:50 GMT Content-Type: text/html Content-Length: 2285 Date: Mon, 04 Mar 2013 04:01:00 GMT <html> <head> <META http-equiv="Content-Type" content="text/html; charset=UTF-8” ... HTTP Requests HTTP request is a request line, followed by zero or more request headers Request line: <method> <uri> <version> <version> is HTTP version of request (HTTP/1.0 or HTTP/1.1) <uri> is typically URL for proxies, URL suffix for servers. A URL is a type of URI (Uniform Resource Identifier) See http://www.ietf.org/rfc/rfc2396.txt <method> is either GET, POST, OPTIONS, HEAD, PUT, DELETE, or TRACE. HTTP Requests (cont) HTTP methods: GET: Retrieve static or dynamic content Arguments for dynamic content are in URI Workhorse method (99% of requests) POST: Retrieve dynamic content Arguments for dynamic content are in the request body OPTIONS: Get server or file attributes HEAD: Like GET but no data in response body PUT: Write a file to the server! DELETE: Delete a file on the server! TRACE: Echo request in response body Useful for debugging. Request headers: <header name>: <header data> Provide additional information to the server. HTTP Versions Major differences between HTTP/1.1 and HTTP/1.0 HTTP/1.0 uses a new connection for each transaction. HTTP/1.1 also supports persistent connections multiple transactions over the same connection Connection: Keep-Alive HTTP/1.1 requires HOST header Host: www.depaul.edu Makes it possible to host multiple websites at single Internet host HTTP/1.1 supports chunked encoding (described later) Transfer-Encoding: chunked HTTP/1.1 adds additional support for caching HTTP Responses HTTP response is a response line followed by zero or more response headers. Response line: <version> <status code> <status msg> <version> is HTTP version of the response. <status code> is numeric status. <status msg> is corresponding English text. 200 301 403 404 OK Moved Forbidden Not found Request was handled without error Provide alternate URL Server lacks permission to access file Server couldn’t find the file. Response headers: <header name>: <header data> Provide additional information about response Content-Type: MIME type of content in response body. Content-Length: Length of content in response body. GET Request From Chrome Browser URI is just the suffix, not the entire URL GET / HTTP/1.1\r\n Host: reed.cs.depaul.edu\r\n Connection: keep-alive\r\n Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8\r\n User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.22 (KHTML, like Gecko) Chrome/25.0.1364.97 Safari/537.22\r\n Accept-Encoding: gzip,deflate,sdch\r\n Accept-Language: en-US,en;q=0.8\r\n Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3\r\n Cookie:__utma=114012434.756988690.1360702406.1360702406.1360874291.2; __utmz=114012434.1360874291.2.2.utmcsr=cdm.depaul.edu|utmccn=(referral )|utmcmd=referral|utmcct=/academics/Pages/bs%20computerscience%20stand ard.aspx\r\n \r\n GET Response From Apache Server HTTP/1.1 200 OK Server: Apache-Coyote/1.1\r\n Accept-Ranges: bytes\r\n ETag: W/”2285-1357855910000”\r\n Last-Modified: Thu, 10 Jan 2013 22:11:50 GMT\r\n Content-Type: test/html\r\n Content-Length: 2285\r\n Date: Mon, 04 Mar 2013 04:58:40 GMT\r\n \r\n <html>\n <head>\n ... Tiny Web Server Tiny Web server described in text Tiny is a sequential Web server. Serves static and dynamic content to real browsers. text files, HTML files, GIF and JPEG images. 226 lines of commented C code. Not as complete or robust as a real web server Tiny Operation Read request from client Split into method / uri / version If not GET, then return error If URI contains “cgi-bin” then serve dynamic content Fork process to execute program Otherwise serve static content Copy file to output Tiny Serving Static Content /* Send response headers to client */ From tiny.c get_filetype(filename, filetype); sprintf(buf, "HTTP/1.0 200 OK\r\n"); sprintf(buf, "%sServer: Tiny Web Server\r\n", buf); sprintf(buf, "%sContent-length: %d\r\n", buf, filesize); sprintf(buf, "%sContent-type: %s\r\n\r\n", buf, filetype); Rio_writen(fd, buf, strlen(buf)); /* Send response body to client */ srcfd = Open(filename, O_RDONLY, 0); srcp = Mmap(0, filesize, PROT_READ, MAP_PRIVATE, srcfd, 0); Close(srcfd); Rio_writen(fd, srcp, filesize); Munmap(srcp, filesize); Serve file specified by filename Use file metadata to compose header “Read” file via mmap Write to output Serving Dynamic Content Client sends request to server. If request URI contains the string “/cgi-bin”, then the server assumes that the request is for dynamic content. GET /cgi-bin/env.pl HTTP/1.1 Client Server Serving Dynamic Content (cont) The server creates a child process and runs the program identified by the URI in that process Client Server fork/exec env.pl Serving Dynamic Content (cont) The child runs and generates the dynamic content. The server captures the content of the child and forwards it without modification to the client Client Content Server Content env.pl Issues in Serving Dynamic Content How does the client pass program arguments to the server? How does the server pass these arguments to the child? How does the server pass other info relevant to the request to the child? How does the server capture the content produced by the child? These issues are addressed by the Common Gateway Interface (CGI) specification. Request Client Content Content Server Create env.pl CGI Because the children are written according to the CGI spec, they are often called CGI programs. Because many CGI programs are written in Perl, they are often called CGI scripts. However, CGI really defines a simple standard for transferring information between the client (browser), the server, and the child process. The cdmlinux addition portal input URL host port CGI program args Output page 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://cdmlinux.cdm.depaul.edu/cgi-bin/adder?n1=4&n2=7 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” URI often generated by an HTML form <FORM METHOD=GET ACTION="cgi-bin/adder"> <p>X <INPUT NAME="n1"> <p>Y <INPUT NAME="n2"> <p><INPUT TYPE=submit> </FORM> Serving Dynamic Content With GET URL: cgi-bin/adder?4&7 Result displayed on browser: Welcome to THE Internet addition portal. The answer is: 4+7=11 Thanks for visiting! Serving Dynamic Content With GET Question: How does the server pass these arguments to the child? Answer: In environment variable QUERY_STRING A single string containing everything after the “?” For add: QUERY_STRING = “4&7” Additional CGI Environment Variables General SERVER_SOFTWARE SERVER_NAME GATEWAY_INTERFACE (CGI version) Request-specific SERVER_PORT REQUEST_METHOD (GET, POST, etc) QUERY_STRING (contains GET args) REMOTE_HOST (domain name of client) REMOTE_ADDR (IP address of client) CONTENT_TYPE (for POST, type of data in message body, e.g., text/html) CONTENT_LENGTH (length in bytes) Even More CGI Environment Variables In addition, the value of each header of type type received from the client is placed in environment variable HTTP_type Examples (any “-” is changed to “_”) : HTTP_ACCEPT HTTP_HOST HTTP_USER_AGENT Serving Dynamic Content With GET Question: How does the server capture the content produced by the child? Answer: The child generates its output on stdout. Server uses dup2 to redirect stdout to its connected socket. Notice that only the child knows the type and size of the content. Thus the child (not the server) must generate the corresponding headers. /* Make the response body */ From adder.c sprintf(content, "Welcome to add.com: "); sprintf(content, "%sTHE Internet addition portal.\r\n<p>", content); sprintf(content, "%sThe answer is: %s\r\n<p>", content, msg); sprintf(content, "%sThanks for visiting!\r\n", content); /* Generate the HTTP response */ printf("Content-length: %u\r\n", (unsigned) strlen(content)); printf("Content-type: text/html\r\n\r\n"); printf("%s", content); Serving Dynamic Content With GET $ telnet perko406.cdm.depaul.edu 8000 Trying 140.192.39.11... Connected to perko406.cdm.depaul.edu. Escape character is '^]'. GET /cgi-bin/adder?4&7 HTTP/1.0 HTTP/1.0 200 OK Server: Tiny Web Server Content-length: 97 Content-type: text/html HTTP request sent by client HTTP response generated by the server Welcome to THE Internet addition portal. <p>The answer is: 4 + 7 = 11 <p>Thanks for visiting! HTTP response generated by Connection closed by foreign host. the CGI program $ Tiny Serving Dynamic Content /* Return first part of HTTP response */ sprintf(buf, "HTTP/1.0 200 OK\r\n"); Rio_writen(fd, buf, strlen(buf)); sprintf(buf, "Server: Tiny Web Server\r\n"); Rio_writen(fd, buf, strlen(buf)); From tiny.c if (Fork() == 0) { /* child */ /* Real server would set all CGI vars here */ setenv("QUERY_STRING", cgiargs, 1); Dup2(fd, STDOUT_FILENO); /* Redirect stdout to client */ Execve(filename, emptylist, environ);/* Run CGI prog */ } Wait(NULL); /* Parent waits for and reaps child */ Fork child to execute CGI program Change stdout to be connection to client Execute CGI program with execve Proxies A proxy is an intermediary between a client and an origin server. To the client, the proxy acts like a server. To the server, the proxy acts like a client. 1. Client request Client 2. Proxy request Origin Server Proxy 4. Proxy response 3. Server response Why Proxies? Can perform useful functions as requests and responses pass by Examples: Caching, logging, anonymization, filtering, transcoding Client A Request foo.html foo.html Request foo.html Client B Request foo.html Proxy cache foo.html Fast inexpensive local network foo.html Slower more expensive global network Origin Server For More Information Study the Tiny Web server described in your text Tiny is a sequential Web server. Serves static and dynamic content to real browsers. text files, HTML files, GIF and JPEG images. 220 lines of commented C code. Also comes with an implementation of the CGI script for the add.com addition portal. See the HTTP/1.1 standard: http://www.w3.org/Protocols/rfc2616/rfc2616.html