Some slides adapted from CMU slides

Some slides adapted from CMU 15.213 slides
Networking Alan L. Cox Some slides adapted from CMU slides

Objectives Be exposed to the basic underpinnings of the Internet
Be able to use network socket interfaces effectively Be exposed to the basic underpinnings of the World Wide Web Cox Networking 2

The 2004 A. M. Turing Award Goes to...
Cox Networking 3

The 2004 A. M. Turing Award Goes to...
Bob Kahn Vint Cerf "For pioneering work on internetworking, including the design and implementation of the Internet's basic communications protocols, TCP/IP, and for inspired leadership in networking." The first Turing Award given to recognize work in computer networking Cox Networking 4

But at the Same Time... Daily, e.g. 110 attacks from China; 26 attacks from USA 2/24/2008 YouTube traffic mis-routed to Pakistan Q2/ s of Netherlands DSL customers lost service due to network configuration error 11/10/2008 CTBC (Brazil) black-holed all Internet traffic in some parts of Brazil 2/16/2009 Supro (Czech) routing messages triggered a Cisco router bug world-wide 5/2009 AfNOG (Africa) routing messages triggered buffer overflow in open-source routing software Quagga world-wide Source: Akamai Technologies, Inc. Cox Networking 5

Analog/Continuous Signal
Telephony Interactive telecommunication between people Analog voice Transmitter/receiver continuously in contact with electronic circuit Electric current varies with acoustic pressure Over electrical circuits Analog/Continuous Signal Cox Networking 6

Telephony Milestones People can talk without being on the same wire!
1876: Alexander Bell invented telephone 1878: Public switches installed at New Haven and San Francisco, public switched telephone network is born People can talk without being on the same wire! Without Switch With Switch Cox Networking 7

Telephony Milestones 1878: First telephone directory; White House line
1881: Insulated, balanced twisted pair as local loop 1885: AT&T formed 1892: First automatic commercial telephone switch 1903: 3 million telephones in U.S. 1915: First transcontinental telephone line 1927: First commercial transatlantic commercial service Cox Networking 8

Telephony Milestones With Multiplexing Without Multiplexing
1937: Multiplexing introduced for inter-city calls One link carries multiple conversations With Multiplexing Without Multiplexing Cox Networking 9

Digital/Discrete Signal
Data or Computer Networks Networks designed for computers to computers or devices vs. communication between human beings Digital information vs. analog voice Not a continuous stream of bits, rather, discrete “packets” with lots of silence in between Dedicated circuit hugely inefficient Packet switching invented Digital/Discrete Signal Cox Networking 10

Major Internet Milestones
Basic concept of “packet switching” was independently developed by Baran (RAND), Kleinrock (MIT) AT&T insisted that packet switching would never work! 1965 First time two computers talked to each other using packets (Roberts, MIT; Marill, SDC) MIT TX-2 SDC Q32 dial-up Cox Networking 11

1968 BBN group proposed to use Honeywell 516 mini-computers for the Interface Message Processors (i.e. packet switches) 1969 The first ARPANET message transmitted between UCLA (Kleinrock) and SRI (Engelbart) We sent an “L”, did you get the “L”? Yep! We sent an “O”, did you get the “O”? Yep! We sent a “G”, did you get the “G”? Crash! Cox Networking 12

1970 First packet radio network ALOHANET (Abramson, U Hawaii) 1973 Ethernet invented (Metcalfe, Xerox PARC) Why is it called the “Inter-net”? 1974 “A protocol for Packet Network Interconnection” published by Cerf and Kahn First internetworking protocol TCP This paper was cited for their Turing Award 1977 First TCP operation over ARPANET, Packet Radio Net, and SATNET 1985 NSF commissions NSFNET backbone 1991 NSF opens Internet to commercial use Cox Networking 13

Internet Hourglass Architecture
, Web, ssh,... TCP, UDP, … IP Ethernet, WiFi, 3G, bluetooth,... Cox Networking 14

A Client-Server Transaction
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 1. Client sends request Client process Server process Resource 4. Client handles response 3. Server sends response 2. Server handles request Note: clients and servers are processes running on hosts (can be the same or different hosts) Cox Networking 15

Network Hardware register file ALU CPU chip system bus memory bus main
Bus Interface I/O bridge Expansion slots I/O bus USB controller graphics adapter disk controller network adapter mouse keyboard monitor disk network Cox Networking 16

Computer Networks A network is a hierarchical system of boxes and wires organized by geographical proximity Cluster network spans cluster or machine room Switched Ethernet, Infiniband, Myrinet, … LAN (local area network) spans a building or campus Ethernet is most prominent example WAN (wide-area network) spans very long distance A high-speed point-to-point link Leased line or SONET/SDH circuit, or MPLS/ATM circuit 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”) Cox Networking 17

Lowest Level: Ethernet Segment
Ethernet segment consists of a collection of hosts connected by wires (twisted pairs) to a hub Operation Each Ethernet adapter has a unique 48-bit address 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 largely obsolete Bridges (switches, routers) became cheap enough to replace them (don’t broadcast all traffic) host host host 100 Mb/s 100 Mb/s hub ports Cox Networking 18

Next Level: Bridged Ethernet Segment
Spans room, building, or campus Bridges cleverly learn which hosts are reachable from which ports and then selectively copy frames from port to port host host host host host hub bridge hub 100 Mb/s 100 Mb/s 1-10 Gb/s host host 1 Gb/s 1 Gb/s host bridge bridge host 1 Gb/s host host host Cox Networking 19

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 Cox Networking 20

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 host LAN 1 LAN 2 router router router WAN WAN LAN 1 and LAN 2 might be completely different, totally incompatible LANs (e.g., Ethernet and WiFi, *, T1-links, DSL, …) Cox Networking 21

The Internet Circa 1986 In 1986, the Internet consisted of one backbone (NSFNET) that connected 13 sites via 45 Mbps T3 links Merit (Univ of Mich) NCSA (Illinois) Cornell Theory Center Pittsburgh Supercomputing Center San Diego Supercomputing Center John von Neumann Center (Princeton) BARRNet (Palo Alto) MidNet (Lincoln, NE) WestNet (Salt Lake City) NorthwestNet (Seattle) SESQUINET (Rice) SURANET (Georgia Tech) Connecting to the Internet involved connecting one of your routers to a router at a backbone site, or to a regional network that was already connected to the backbone Cox Networking 22

NSFNET Internet Backbone
source: Cox Networking 23

After NSFNET Early 90s Commercial enterprises began building their own high-speed backbones Backbone would connect to NSFNET, sell access to companies, ISPs, and individuals 1995 NSFNET decommissioned NSF fostered the creation of network access points (NAPs) to interconnect the commercial backbones Cox Networking 24

Current Internet Architecture
Cox Networking 25

Level 3 Backbone Cox Networking 26

AT&T Backbone Cox Networking 27

Submarine Cabling Cox Networking 28

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 smoothes out the differences between the different networks Implements an internet protocol (i.e., set of rules) that 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 Cox Networking 29

What Does an internet Protocol Do?
1. 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 2. 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 Cox Networking 30

Transferring Data Over an internet
Host A Host B client server (1) (8) data data protocol software protocol software internet packet (2) (7) data PH FH1 data PH FH2 LAN1 frame LAN1 adapter LAN2 adapter Router (3) (6) data PH FH1 data PH FH2 LAN1 adapter LAN2 adapter LAN1 LAN2 frame LAN2 (4) data PH FH1 (5) data PH FH2 protocol software Cox Networking 31

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? We’ll leave the discussion of these question to computer networking classes (COMP 429) Cox Networking 32

Global IP 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 Cox Networking 33

Organization of an Internet Application
Internet client Internet server Client Server User code Sockets interface (system calls) TCP/IP TCP/IP Kernel code Hardware interface (interrupts) Network adapter Hardware and firmware Network adapter Global IP Internet Cox Networking 34

A Programmer’s View of the Internet
Hosts are mapped to a set of 32-bit IP addresses (4 * 8 bits) A set of identifiers called Internet domain names are mapped to the set of IP addresses for convenience is mapped to A process on one Internet host can communicate with a process on another Internet host over a connection Cox Networking 35

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) */ }; Handy network byte-order conversion functions: htonl: convert long int from host to network byte order htons: convert short int from host to network byte order ntohl: convert long int from network to host byte order ntohs: convert short int from network to host byte order Cox Networking 36

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 = Functions for converting between binary IP addresses and dotted decimal strings: inet_pton: converts a dotted decimal string to an IP address in network byte order inet_ntop: converts an IP address in network by order to its corresponding dotted decimal string “n” denotes network representation, “p” denotes presentation representation Cox Networking 37

IP Address Structure IP (V4) Address space divided into classes:
Special Addresses for routers and gateways (all 0/1’s) Loop-back address: Unrouted (private) IP addresses: /8, /12, /16 Dynamic IP addresses (DHCP) Class A Class B Class C Class D Class E Net ID Host ID 1 Multicast address Reserved for experiments Cox Networking 38

Internet Domain Names unnamed root .net .edu .gov .com
First-level domain names mit rice berkeley amazon Second-level domain names clear cs www Third-level domain names bell crystal Cox Networking 39

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 addrinfo structures: Functions for retrieving host entries from DNS: getaddrinfo: query DNS using domain name or IP getnameinfo: query DNS using sockaddr struct struct addrinfo { int ai_flags; /* flags for getaddrinfo */ int ai_family; /* address type (AF_INET or AF_INET6) */ int ai_socktype; /* the socket type */ int ai_protocol; /* the type of protocol */ size_t ai_addrlen; /* length of ai_addr */ struct sockaddr *ai_addr; /* pointer to a sockaddr struct */ char *ai_canonname;/* the canonical name */ struct addrinfo *ai_next; /* pointer to the next addrinfo struct */ }; Cox Networking 40

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 Different kinds of mappings are possible: Simple case: 1 domain name maps to one IP address: forest.owlnet.rice.edu maps to Multiple domain names mapped to the same IP address: ececs.rice.edu, and bianca.cs.rice.edu all map to Multiple domain names mapped to multiple IP addresses: aol.com and map to multiple IP addresses Some valid domain names don’t map to any IP address: for example: clear.rice.edu Cox Networking 41

A Program That Queries DNS
int main(int argc, char **argv) { /* argv[1] is a domain name */ char address[INET_ADDRSTRLEN]; /* or dotted decimal IP addr */ struct sockaddr_in *saddr; struct addrinfo *addrp; struct addrinfo *pp; struct addrinfo hints; hints.ai_flags = AI_CANONNAME; hints.ai_family = AF_INET; hints.ai_socktype = 0; hints.ai_protocol = 0; getaddrinfo(argv[1], NULL, &hints, &addrp); printf("official hostname: %s\n", addrp->ai_canonname); for (pp = addrp; pp != NULL; pp = pp->ai_next) { saddr = (struct sockaddr_in *) pp->ai_addr; inet_ntop(AF_INET, &saddr->sin_addr, address, INET_ADDRSTRLEN); printf("address: %s\n", address); } freeaddrinfo(addrp); return (0); Cox Networking 42

Querying DNS Domain Information Groper (dig) provides a scriptable command line interface to DNS Available on CLEAR Lots of web interfaces (google “domain information groper”) unix> dig +short bianca.cs.rice.edu unix> dig +short -x bianca.cs.rice.edu. unix> dig +short google.com unix> dig +short -x py-in-f99.google.com. Cox Networking 43

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 IP address, 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) Cox Networking 44

Connection socket pair
Putting it all Together: Anatomy of an Internet Connection Client socket address :51213 Server socket address :80 Client Server (port 80) Connection socket pair ( :51213, :80) Client host address Server host address Cox Networking 45

Clients Examples of client programs How does a client find the server?
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 known ports Port 7: Echo server Port 23: Telnet server Port 25: Mail server Port 80: Web server Cox Networking 46

Using Ports to Identify Services
Server host Client host Service request for :80 (i.e., the Web server) Web server (port 80) Client Kernel Echo server (port 7) Web server (port 80) Service request for :7 (i.e., the echo server) Client Kernel Echo server (port 7) Cox Networking 47

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” Cox Networking 48

Server Examples Web server (port 80) FTP server (20, 21)
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 Telnet server (23) Resource: terminal Service: proxies a terminal on the server machine Mail server (25) Resource: “spool” file Service: stores mail messages in spool file See /etc/services for a comprehensive list of the services available on a UNIX machine Cox Networking 49

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 Cox Networking 50

Overview of the Sockets Interface
Client Server socket socket bind open_listenfd open_clientfd listen Connection request connect accept rio_writen rio_readlineb Await connection request from next client rio_readlineb rio_writen EOF close rio_readlineb close Cox Networking 51

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 The main distinction between regular file I/O and socket I/O is how the application “opens” the socket descriptors Cox Networking 52

Socket Address Structures
Generic socket address: Address for connect, bind, and accept C did not have generic (void *) pointers when the sockets interface was designed Internet-specific socket address: Must cast (sockaddr_in *) to (sockaddr *) struct sockaddr { unsigned short sa_family; /* protocol family */ char sa_data[14]; /* address data. */ }; struct sockaddr_in { unsigned short sin_family; /* address family */ unsigned short sin_port; /* port num in network byte order */ struct in_addr sin_addr; /* IP addr in network byte order */ unsigned char sin_zero[8]; /* pad to sizeof(struct sockaddr) */ }; Cox Networking 53

Echo Client Main Routine
int main(int argc, char **argv) /* usage: ./echoclient host port */ { 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); exit(0); Cox Networking 54

Echo Client: open_clientfd
int open_clientfd(char *hostname, int port) { int clientfd; struct hostent *hp; struct sockaddr_in serveraddr; if ((clientfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) return -1; /* check errno for cause of error */ /* 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, (char *)&serveraddr.sin_addr.s_addr, hp->h_length); serveraddr.sin_port = htons(port); /* Establish a connection with the server */ if (connect(clientfd, (SA *) &serveraddr, sizeof(serveraddr)) < 0) return -1; return clientfd; } This function opens a connection from the client to the server at hostname:port Cox Networking 55

open_clientfd (socket)
socket creates a socket descriptor on the client AF_INET: indicates that the socket is associated with Internet protocols SOCK_STREAM: selects a reliable byte stream connection int clientfd; /* socket descriptor */ if ((clientfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) return -1; /* check errno for cause of error */ ... (more) Cox Networking 56

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; bcopy((char *)hp->h_addr, (char *)&serveraddr.sin_addr.s_addr, hp->h_length); serveraddr.sin_port = htons(port); Cox Networking 57

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; Cox Networking 58

Echo Server: Main Routine
int main(int argc, char **argv) { int listenfd, connfd, port; socklen_t clientlen; struct sockaddr_in clientaddr; char host_name[NI_MAXHOST]; char haddrp[INET_ADDRSTRLEN]; port = atoi(argv[1]); listenfd = open_listen(port); while (1) { clientlen = sizeof(clientaddr); connfd = Accept(listenfd, (SA *)&clientaddr, &clientlen); /* determine the domain name and IP address of the client */ getnameinfo((struct sockaddr *)&clientaddr, sizeof(clientaddr), host_name, sizeof(host_name), NULL, 0, 0); inet_ntop(AF_INET, &clientaddr.sin_addr, haddrp, INET_ADDRSTRLEN); printf("server connected to %s (%s)\n", host_name, haddrp); echo(connfd); Close(connfd); } Cox Networking 59

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) ... (more) Cox Networking 60

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 listenfd; } Cox Networking 61

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 int listenfd; /* listening socket descriptor */ /* Create a socket descriptor */ if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) return -1; Cox Networking 62

open_listenfd (setsockopt)
The socket can be given some attributes Handy trick that allows us to rerun the server immediately after we kill it Otherwise we would have to wait about 30 secs Eliminates “Address already in use” error from bind() Strongly suggest you do this for all your servers to simplify debugging ... /* Eliminates "Address already in use" error from bind(). */ if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, (const void *)&optval , sizeof(int)) < 0) return -1; Cox Networking 63

open_listenfd (init socket address)
Next, we initialize the socket with the server’s Internet address (IP address and port) IP address and port stored in network (big-endian) byte order htonl() converts longs from host byte order to network byte order htons() convers shorts from host byte order to network byte order 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_addr.s_addr = htonl(INADDR_ANY); serveraddr.sin_port = htons((unsigned short)port); Cox Networking 64

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; Cox Networking 65

open_listenfd (listen)
listen indicates that this socket will accept connection (connect) requests from clients We’re finally ready to enter the main server loop that accepts and processes client connection requests int listenfd; /* listening socket */ ... /* Make it a listening socket ready to accept connection requests */ if (listen(listenfd, LISTENQ) < 0) return -1; return listenfd; } Cox Networking 66

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 */ } Cox Networking 67

Echo Server: accept accept() blocks waiting for a connection request
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 int listenfd; /* listening descriptor */ int connfd; /* connected descriptor */ struct sockaddr_in clientaddr; int clientlen; clientlen = sizeof(clientaddr); connfd = Accept(listenfd, (SA *)&clientaddr, &clientlen); Cox Networking 68

Echo Server: accept Illustrated
listenfd(3) 1. Server blocks in accept, waiting for connection request on listening descriptor listenfd Client Server clientfd Connection request listenfd(3) Client Server 2. Client makes connection request by calling and blocking in connect clientfd listenfd(3) 3. Server returns connfd from accept. Client returns from connect. Connection is now established between clientfd and connfd Client Server clientfd connfd(4) Cox Networking 69

Connected vs. Listening Descriptors
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 Cox Networking 70

Echo Server: Identifying the Client
The server can determine the domain name and IP address of the client char host_name[NI_MAXHOST]; char haddrp[INET_ADDRSTRLEN]; getnameinfo((struct sockaddr *)&clientaddr, sizeof(clientaddr), host_name, sizeof(host_name), NULL, 0, 0); inet_ntop(AF_INET, &clientaddr.sin_addr, haddrp, INET_ADDRSTRLEN); printf("server connected to %s (%s)\n", host_name, haddrp); Cox Networking 71

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) IMPORTANT: EOF is a condition, not a particular data byte 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) { printf("server received %d bytes\n", n); Rio_writen(connfd, buf, n); } Cox Networking 72

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> Cox Networking 73

Testing the Echo Server With telnet
bass> echoserver 5000 server established connection with KITTYHAWK.CMCL ( ) server received 5 bytes: 123 server received 8 bytes: kittyhawk> telnet bass 5000 Trying Connected to BASS.CMCL.CS.CMU.EDU. Escape character is '^]'. 123 Connection closed by foreign host. 456789 kittyhawk> Cox Networking 74

Running the Echo Client and Server
bass> echoserver 5000 server established connection with KITTYHAWK.CMCL ( ) server received 4 bytes: 123 server received 7 bytes: ... kittyhawk> echoclient bass 5000 Please enter msg: 123 Echo from server: 123 Please enter msg: Echo from server: kittyhawk> Cox Networking 75

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 Complete versions of the echo client and server are developed in the text Available on the course web site You should compile and run them for yourselves to see how they work Feel free to borrow any of this code Cox Networking 76

Web History 1945: Vannevar Bush, “As we may think”, Atlantic Monthly, July, 1945 Describes the idea of a distributed hypertext system A “memex” that mimics the “web of trails” in our minds 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 Berners-Lee writes a graphical browser for Next machines Cox Networking 77

Web History (cont) 1992 1993 1994 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" (became Netscape, then part of AOL) Cox Networking 78

Internet Hosts Source: Cox Networking 79

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 (may) close connection Current version is HTTP/1.1 RFC 2616, June, 1999 HTTP request Web client (browser) Web server HTTP response (content) Cox Networking 80

Web Content Web servers return content to clients Example MIME types
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 (GIF format) image/jpeg Binary image (JPEG format) Cox Networking 81

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 (i.e., search results) Bottom line: All Web content is associated with a file that is managed by the server Cox Networking 82

URLs Each file managed by a server has a unique name called a URL (Universal Resource Locator) URLs for static content: Identifies a file called index.html, managed by a Web server at that is listening on port 80 URLs for dynamic content: Identifies an executable file called adder, managed by a Web server at that is listening on port 8000, that should be called with two argument strings: and 213 Cox Networking 83

How Clients and Servers Use URLs
Example URL: Clients use prefix ( to infer: What kind of server to contact (http (Web) server) Where the server is ( What port the server 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 servers expand to some default home page (e.g., index.html) Cox Networking 84

Anatomy of an HTTP Transaction
unix> telnet Client: open connection to server Trying Telnet prints 3 lines to the terminal Connected to Escape character is '^]'. GET / HTTP/ Client: request line host: Client: required HTTP/1.1 HOST header Client: empty line terminates headers HTTP/ OK Server: response line Cache-Control: private Server: followed by six response headers Content-Type: text/html; charset=ISO Set-Cookie: PREF=ID=<..snip..> Server: gws Transfer-Encoding: chunked Date: Tue, 13 Nov :25:04 GMT Server: empty line (“\r\n”) terminates hdrs <html> Server: first HTML line in response body <..snip..> Server: HTML content not shown. </html> Server: last HTML line in response body Connection closed by foreign host. Server: closes connection unix> Client: closes connection and terminates Cox Networking 85

HTTP Requests HTTP request is a request line, followed by zero or more request headers Request line: <method> <uri> <version> <method> is either GET, POST, OPTIONS, HEAD, PUT, DELETE, or TRACE <uri> is typically URL for proxies, URL suffix for servers <version> is HTTP version of request (HTTP/1.0 or HTTP/1.1) Cox Networking 86

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 Cox Networking 87

HTTP Requests (cont) Request headers: <header name>: <header data> Provide additional information to the server 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: HTTP/1.1 adds additional support for caching Cox Networking 88

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 OK Request was handled without error 403 Forbidden Server lacks permission to access file 404 Not found 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 Cox Networking 89

GET Request to Apache Server From Firefox Browser
GET /~comp221/proxytest.html HTTP/1.1 Host: User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: ) Gecko/ Ubuntu/7.10 (gutsy) Firefox/ Accept: text/xml,application/xml,application/xhtml+xml,text/html; q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5 Accept-Language: en-us,en;q=0.5 Accept-Encoding: gzip,deflate Accept-Charset: ISO ,utf-8;q=0.7,*;q=0.7 Connection: keep-alive Keep-Alive: 300 CRLF (\r\n) Cox Networking 90

GET Response From Apache Server
HTTP/ OK Date: Tue, 13 Nov :09:01 GMT Server: Apache/ (Unix) mod_ssl/2.8.9 OpenSSL/0.9.6e Last-Modified: Tue, 13 Nov :08:44 GMT ETag: “ac df-4739e82c" Accept-Ranges: bytes Content-Length: 212 Connection: close Content-Type: text/html CRLF <html> <head><title>COMP221 Web Proxy Test Page</title></head> <body><h1>COMP221 Web Proxy Test Page</h1> This page is a simple text-only HTML file that can be used to test your web proxy software. </body></html> Cox Networking 91

GET /cgi-bin/env.pl HTTP/1.1
Serving Dynamic Content Client sends request to server Server parses request URI to determine if request is for static or dynamic content In the “old” days, this was as simple as the string “/cgi-bin” starting the URL Web servers are far more flexible and configurable now GET /cgi-bin/env.pl HTTP/1.1 Client Server Cox Networking 92

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 Cox Networking 93

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 Server Content Content env.pl Cox Networking 94

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 Server Content Content Create env.pl Cox Networking 95

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 Cox Networking 96

add.com: THE Internet addition portal!
Ever need to add two numbers together and you just can’t find your calculator? Try the addition service at “add.com: THE Internet addition portal!” Takes as input the two numbers you want to add together Returns their sum in a tasteful personalized message Cox Networking 97

The add.com Experience input URL host port CGI program args
Output page Cox Networking 98

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 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=" Cox Networking 99

URL: Result displayed on browser: Welcome to add.com: THE Internet addition portal. The answer is: = 3 Thanks for visiting! Tell your friends. Cox Networking 100

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.com: QUERY_STRING = “1&2” /* child code that accesses the argument list */ if ((buf = getenv("QUERY_STRING")) == NULL) { exit(1); } /* extract arg1 and arg2 from buf and convert */ ... n1 = atoi(arg1); n2 = atoi(arg2); Cox Networking 101

Question: How does the server pass other info relevant to the request to the child? Answer: In a collection of environment variables defined by the CGI spec Cox Networking 102

Some 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 arguments) 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) Cox Networking 103

Some 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: HTTP_ACCEPT HTTP_HOST HTTP_USER_AGENT (any “-” is changed to “_”) Cox Networking 104

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, so the child (not the server) must generate the corresponding headers /* child generates the result string */ sprintf(content, "Welcome to add.com: THE Internet addition portal\ <p>The answer is: %d + %d = %d\ <p>Thanks for visiting!\r\n", n1, n2, n1+n2); /* child generates the headers and dynamic content */ printf("Content-length: %d\r\n", strlen(content)); printf("Content-type: text/html\r\n"); printf("\r\n"); printf("%s", content); Cox Networking 105

bass> ./tiny 8000 GET /cgi-bin/adder?1&2 HTTP/1.1 Host: bass.cmcl.cs.cmu.edu:8000 <CRLF> kittyhawk> telnet bass 8000 Trying Connected to BASS.CMCL.CS.CMU.EDU. Escape character is '^]'. HTTP/ OK Server: Tiny Web Server Content-length: 102 Content-type: text/html Welcome to add.com: THE Internet addition portal. <p>The answer is: = 3 <p>Thanks for visiting! Connection closed by foreign host. kittyhawk> HTTP request received by Tiny Web server HTTP request sent by client HTTP response generated by the server HTTP response generated by the CGI program Cox Networking 106

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 HTTP request HTTP request Client Proxy Origin Server HTTP response HTTP response Cox Networking 107

Fast inexpensive local network
Why Proxies? Can perform useful functions as requests and responses pass through Examples: Caching, logging, anonymization Client A Request foo.html foo.html Request foo.html Origin Server Proxy cache foo.html Request foo.html foo.html Slower more expensive global network Client B Fast inexpensive local network Cox Networking 108

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 Cox Networking 109

Next Time Concurrency Cox Networking 110

Some slides adapted from CMU slides

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