Transcript Networking - Rice University
Networking
Alan L. Cox [email protected]
T. S. Eugene Ng [email protected]
Some slides adapted from CMU 15.213 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 & Ng Networking 2
The 2004 A. M. Turing Award Goes to...
Cox & Ng 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 only Turing Award given to-date to recognize work in computer networking Cox & Ng Networking 4
But at the Same Time...
Daily, e.g. 110 attacks from China; 26 attacks from USA 2/24/2008 YouTube traffic DSL customers lost service due to 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
Cox & Ng Networking
Source: Akamai Technologies, Inc.
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Telephony
Interactive telecommunication between people Analog voice
Transmitter/receiver continuously in contact with electronic circuit
Electric current varies with acoustic pressure Analog/Continuous Signal Over electrical circuits Cox & Ng Networking 6
Telephony Milestones
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
Cox & Ng Networking
With Switch
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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 & Ng Networking 8
Telephony Milestones
1937: Multiplexing introduced for inter-city calls
One link carries multiple conversations
Without Multiplexing
Cox & Ng Networking
With Multiplexing
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Data or Computer Networks
Networks designed for computers to computers or devices
vs. communication between human beings Digital information
vs. analog voice Digital/Discrete Signal Not a continuous stream of bits, rather, discrete “packets” with lots of silence in between
Dedicated circuit hugely inefficient
Packet switching invented Cox & Ng Networking 10
Major Internet Milestones
1960-1964 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) Cox & Ng
MIT TX-2 dial-up
Networking
SDC Q32
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Major Internet Milestones
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 & Ng Networking 12
Major Internet Milestones
• 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 & Ng Networking 13
Internet Hourglass Architecture
Email, Web, ssh,...
TCP, UDP, … IP Ethernet, WiFi, 3G, bluetooth,...
Cox & Ng 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 4. Client handles response
Client process
3. Server sends response
Server process
2. Server handles request
Resource Cox & Ng
Note: clients and servers are processes running on hosts (can be the same or different hosts)
Networking 15
Network Hardware
register file CPU chip ALU system bus memory bus Bus Interface I/O bridge main memory I/O bus Cox & Ng USB controller mouse keyboard graphics adapter monitor Networking disk controller disk Expansion slots network adapter network 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 & Ng Networking 17
Lowest Level: Ethernet Segment
Ethernet segment consists of a collection of hosts connected by wires (twisted pairs) to a hub host host host 100 Mb/s 100 Mb/s hub Operation
ports
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) Cox & Ng 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 Cox & Ng host host host host hub 100 Mb/s bridge 100 Mb/s hub host 1 Gb/s 1-10 Gb/s bridge host 1 Gb/s bridge host host 1 Gb/s Networking host host host 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 & Ng 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 WAN router WAN router Cox & Ng LAN 1 and LAN 2 might be completely different, totally incompatible LANs (e.g., Ethernet and WiFi, 802.11*, T1 links, DSL, …) 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 & Ng Networking 22
NSFNET Internet Backbone
Cox & Ng Networking source: www.eef.org
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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 & Ng Networking 24
Current Internet Architecture
Cox & Ng Networking 25
Level 3 Backbone
Cox & Ng Networking 26
AT&T Backbone
Cox & Ng Networking 27
Submarine Cabling
Cox & Ng 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 & Ng 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 & Ng Networking 30
Transferring Data Over an internet
Host A Host B client server (1) data (8) data (2)
internet packet
protocol software data PH
LAN1 frame
FH1 LAN1 adapter (3) data PH FH1 LAN1 (4) data protocol software (7) data PH FH1 LAN2 adapter LAN1 Router adapter (6) data LAN2 adapter
LAN2 frame
data PH FH2 (5) protocol software PH FH2 PH FH2 LAN2 Cox & Ng 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 & Ng 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 & Ng Networking 33
Organization of an Internet Application
Sockets interface (system calls)
Internet client Client
User code
TCP/IP
Kernel code Hardware interface (interrupts)
Network adapter
Hardware and firmware
Global IP Internet Internet server Server TCP/IP Network adapter Cox & Ng Networking 34
A Programmer’s View of the Internet
Hosts are mapped to a set of 32-bit IP addresses
128.42.1.125 (4 * 8 bits) A set of identifiers called Internet domain names are mapped to the set of IP addresses for convenience
www.cs.rice.edu is mapped to 128.42.1.125
A process on one Internet host can communicate with a process on another Internet host over a connection Cox & Ng 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 ntohs: convert short int from network to host byte order from network to host byte order Cox & Ng 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 = 128.2.194.242
Functions for converting between binary IP addresses and dotted decimal strings:
inet_aton : converts a dotted decimal string to an IP address in network byte order inet_ntoa : converts an IP address in network by order to its corresponding dotted decimal string
“ n ” denotes network representation, “ a ” denotes application representation Cox & Ng Networking 37
IP Address Structure
IP (V4) Address space divided into classes:
Class A 0 1 2 3 8 16 24 31
0
Net ID Host ID Class B
1 0
Class C
1 1 0
Net ID Net ID Host ID Host ID Class D
1 1 1 0
Multicast address Class E
1 1 1 1
Reserved for experiments
Special Addresses for routers and gateways (all 0/1’s) Loop-back address: 127.0.0.1
Unrouted (private) IP addresses:
10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 Dynamic IP addresses (DHCP) Cox & Ng 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 72.21.210.11
Third-level domain names
bell 128.42.151.14
crystal 128.42.151.13 Cox & Ng 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 host entry structures: /* DNS host entry structure */ struct hostent { char *h_name; /* official domain name of host */ char **h_aliases; /* null-terminated array of domain names */ int h_addrtype; /* host address type (AF_INET) */ int h_length; /* length of an address, in bytes */ char **h_addr_list; /* 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 Cox & Ng 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 127.0.0.1
Different kinds of mappings are possible:
Simple case: 1 domain name maps to one IP address:
• forest.owlnet.rice.edu maps to 10.130.195.71
Multiple domain names mapped to the same IP address:
• www.cs.rice.edu, ececs.rice.edu, and bianca.cs.rice.edu all
map to 128.42.1.125
Multiple domain names mapped to multiple IP addresses:
• aol.com and www.aol.com map to multiple IP addresses
Some valid domain names don’t map to any IP address:
• for example: clear.rice.edu
Cox & Ng Networking 41
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 = *((unsigned int *)*pp); printf("address: %s\n", inet_ntoa(addr)); } } Cox & Ng 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 128.42.1.125 unix> dig +short -x 128.42.1.125 bianca.cs.rice.edu. unix> dig +short google.com 72.14.207.99 64.233.187.99
64.233.167.99
unix> dig +short -x 64.233.167.99 py-in-f99.google.com. Cox & Ng 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 & Ng Networking 44
Putting it all Together: Anatomy of an Internet Connection
Client socket address
128.2.194.242
: 51213
Server socket address
208.216.181.15
: 80 Client Client host address 128.2.194.242
Connection socket pair ( 128.2.194.242
: 51213 , 208.216.181.15
: 80 ) Server (port 80) Server host address 208.216.181.15
Cox & Ng Networking 45
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 known ports
• Port 7: Echo server • Port 23: Telnet server • Port 25: Mail server • Port 80: Web server
Cox & Ng Networking 46
Using Ports to Identify Services
Server host 128.2.194.242
Client host Service request for 128.2.194.242:80 (i.e., the Web server) Web server (port 80) Kernel Client Echo server (port 7) Client Cox & Ng Service request for 128.2.194.242:7 (i.e., the echo server) Networking Kernel Web server (port 80) Echo server (port 7) 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 & Ng Networking 48
Server Examples
Web server (port 80)
Resource: files/compute cycles (CGI programs)
Service: retrieves files and runs CGI programs on behalf of the client See /etc/services for a FTP server (20, 21) comprehensive list of the services
Resource: files available on a UNIX machine Service: stores and retrieve files 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 Cox & Ng 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 & Ng Networking 50
Overview of the Sockets Interface
Client socket Server socket open_clientfd bind listen open_listenfd connect Connection request accept Cox & Ng rio_writen rio_readlineb close rio_readlineb rio_writen EOF Networking rio_readlineb close Await connection request from next client 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 & Ng 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 struct sockaddr { unsigned short sa_family; /* protocol family */ char sa_data[14]; /* address data. */ }; Internet-specific socket address:
Must cast (sockaddr_in *) to (sockaddr *) struct sockaddr_in { unsigned short sin_family; /* address family (always AF_INET) */ 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 & Ng 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 & Ng Networking 54
Echo Client: open_clientfd
int open_clientfd(char *hostname, int port) { int clientfd; struct hostent *hp; struct sockaddr_in serveraddr; This function opens a connection from the client to the server at hostname:port 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; Cox & Ng Networking 55
open_clientfd (socket)
socket client
creates a socket descriptor on the 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 & Ng 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 & Ng 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 & Ng Networking 58
Echo Server: Main Routine
int 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 the given port */ 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); } } Cox & Ng 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) return -1; ... (more) Cox & Ng 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 -1; } return listenfd; Cox & Ng 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 & Ng Networking 62
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 30 secs
Eliminates “Address already in use” error from bind () Strongly suggest you do this for all your servers to simplify debugging Cox & Ng Networking 63
open_listenfd (init socket address)
Next, we initialize the socket with the server’s Internet address (IP address and port) 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); 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 Cox & Ng 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 & Ng Networking 65
open_listenfd (listen)
listen indicates that this socket will accept connection ( connect ) requests from clients 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 Cox & Ng 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 & Ng Networking 67
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 Cox & Ng Networking 68
Echo Server: accept Illustrated
Client clientfd listenfd(3) Server 1. Server blocks in accept , waiting for connection request on listening descriptor listenfd Connection request Client clientfd listenfd(3) Server Client clientfd Cox & Ng 2. Client makes connection request by calling and blocking in connect listenfd(3) Server connfd(4) Networking 3. Server returns connfd from accept . Client returns from connect . Connection is now established between clientfd and connfd 69
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 Cox & Ng Networking 70
Echo Server: Identifying the Client
The server can determine the domain name and IP address of the client struct hostent *hp; /* pointer to DNS host entry */ char *haddrp; /* pointer to dotted decimal string */ 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); Cox & Ng 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; } Cox & Ng Rio_readinitb(&rio, connfd); while((n = Rio_readlineb(&rio, buf, MAXLINE)) != 0) { printf("server received %d bytes\n", n); Rio_writen(connfd, buf, n); } 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
Creates a connection with a server running on
Testing the Echo Server With telnet
bass> echoserver 5000 server established connection with KITTYHAWK.CMCL (128.2.194.242) server received 5 bytes: 123 server established connection with KITTYHAWK.CMCL (128.2.194.242) server received 8 bytes: 456789 kittyhawk> telnet bass 5000 Trying 128.2.222.85...
Connected to BASS.CMCL.CS.CMU.EDU.
Escape character is '^]'.
123 123 Connection closed by foreign host.
kittyhawk> telnet bass 5000 Trying 128.2.222.85...
Connected to BASS.CMCL.CS.CMU.EDU.
Escape character is '^]'.
456789 456789 Connection closed by foreign host.
kittyhawk> Cox & Ng Networking 74
Running the Echo Client and Server
bass> echoserver 5000 server established connection with KITTYHAWK.CMCL (128.2.194.242) server received 4 bytes: 123 server established connection with KITTYHAWK.CMCL (128.2.194.242) server received 7 bytes: 456789 ...
kittyhawk> echoclient bass 5000 Please enter msg: 123 Echo from server: 123 kittyhawk> echoclient bass 5000 Please enter msg: 456789 Echo from server: 456789 kittyhawk> Cox & Ng 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 & Ng 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 & Ng Networking 77
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" (became Netscape, then part of AOL) Cox & Ng Networking 78
Internet Hosts
ISC Domain Survey Host Count 600,000,000 500,000,000 400,000,000 300,000,000 200,000,000 100,000,000 0 Cox & Ng Networking Source: www.isc.org
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) HTTP response (content) Web server Cox & Ng Networking 80
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 HTML document Unformatted text application/postscript Postcript document
image/gif image/jpeg Binary image (GIF format) Binary image (JPEG format) Cox & Ng 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 & Ng Networking 82
URLs
Each file managed by a server has a unique name called a URL (Universal Resource Locator) URLs for static content:
http://www.rice.edu:80/index.html
http://www.rice.edu/index.html
http://www.rice.edu
• Identifies a file called
index.html
, managed by a Web server at www.rice.edu
that is listening on port 80 URLs for dynamic content:
http://www.cs.cmu.edu:8000/cgi-bin/adder?15000&213
• Identifies an executable file called
adder , managed by a Web server at www.cs.cmu.edu
that is listening on port 8000, that should be called with two argument strings: 15000 and 213 Cox & Ng Networking 83
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 ( http Where the server is ( www.aol.com
) (Web) server) 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 & Ng Networking 84
Anatomy of an HTTP Transaction
unix> telnet www.google.com 80
Client: open connection to server
Trying 209.85.164.104...
Telnet prints 3 lines to the terminal
Connected to www.l.google.com.
Escape character is '^]'.
GET / HTTP/1.1
Client: request line
host: www.google.com
Client: required HTTP/1.1 HOST header Client: empty line terminates headers
HTTP/1.1 200 OK
Server: response line
Cache-Control: private
Server: followed by six response headers
Content-Type: text/html; charset=ISO-8859-1 Set-Cookie: PREF=ID=<..snip..> Server: gws Transfer-Encoding: chunked Date: Tue, 13 Nov 2007 17:25:04 GMT
Server: empty line (“ \r\n ”) terminates hdrs
Server: first HTML line in response body <..snip..> Server: HTML content not shown.
Server: last HTML line in response body
Connection closed by foreign host.
Server: closes connection
unix>
Client: closes connection and terminates
Cox & Ng Networking 85
HTTP Requests
HTTP request is a request line, followed by zero or more request headers Request line:
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 & Ng Networking 87
HTTP Requests (cont)
Request headers: :
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: www.yahoo.com
HTTP/1.1 adds additional support for caching Cox & Ng Networking 88
HTTP Responses
HTTP response is a response line followed by zero or more response headers Response line:
• 200 • 403
OK Forbidden Request was handled without error Server lacks permission to access file
• 404
Not found Server couldn’t find the file Response headers:
Provide additional information about response Content-Type : MIME type of content in response body Content-Length : Length of content in response body Cox & Ng Networking 89
GET Request to Apache Server From Firefox Browser
GET /~comp221/proxytest.html HTTP/1.1 Host: www.owlnet.rice.edu
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.8) Gecko/20071022 Ubuntu/7.10 (gutsy) Firefox/2.0.0.8
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-8859-1,utf-8;q=0.7,*;q=0.7
Connection: keep-alive Keep-Alive: 300 CRLF (\r\n) Cox & Ng Networking 90
GET Response From Apache Server
HTTP/1.1 200 OK Date: Tue, 13 Nov 2007 18:09:01 GMT Server: Apache/1.3.26 (Unix) mod_ssl/2.8.9 OpenSSL/0.9.6e
Last-Modified: Tue, 13 Nov 2007 18:08:44 GMT ETag: “ac330311-df-4739e82c" Accept-Ranges: bytes Content-Length: 212 Connection: close Content-Type: text/html CRLF COMP221 Web Proxy Test Page
This page is a simple text-only HTML file that can be used to test your web proxy software.
Cox & Ng Networking 91
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 & Ng 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 & Ng 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 Content Server Content env.pl
Cox & Ng 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 Client Request Content Server Content Create env.pl
Cox & Ng 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 & Ng 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 & Ng Networking 97
The add.com Experience
input URL host port CGI program args Cox & Ng Networking Output page 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
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
Serving Dynamic Content With GET
URL:
http://add.com/cgi-bin/adder?1&2 Result displayed on browser:
Welcome to add.com: THE Internet addition portal.
The answer is: 1 + 2 = 3 Thanks for visiting! Tell your friends.
Cox & Ng Networking 100
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.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 & Ng Networking 101
Serving Dynamic Content With GET
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 & Ng Networking 102
Some CGI Environment Variables
General
SERVER_SOFTWARE
SERVER_NAME GATEWAY_INTERFACE (CGI version) Request-specific
SERVER_PORT REQUEST_METHOD QUERY_STRING ( GET , POST , etc) (contains GET arguments) REMOTE_HOST REMOTE_ADDR (domain name of client) (IP address of client) CONTENT_TYPE (for POST , type of data in message body, e.g., text/html ) CONTENT_LENGTH (length in bytes) Cox & Ng 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 & Ng Networking 104
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 socket to redirect stdout to its connected 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\
The answer is: %d + %d = %d\
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 & Ng Networking 105
Serving Dynamic Content With GET
bass> ./tiny 8000 GET /cgi-bin/adder?1&2 HTTP/1.1
Host: bass.cmcl.cs.cmu.edu:8000
Connected to BASS.CMCL.CS.CMU.EDU.
Escape character is '^]'.
GET /cgi-bin/adder?1&2 HTTP/1.1
Host: bass.cmcl.cs.cmu.edu:8000 The answer is: 1 + 2 = 3 Thanks for visiting!
Connection closed by foreign host.
kittyhawk> HTTP request sent by client HTTP response generated by the server HTTP response generated by the CGI program Welcome to add.com: THE Internet addition portal.
Cox & Ng 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 Client HTTP response Proxy HTTP request HTTP response Origin Server Cox & Ng Networking 107
Why Proxies?
Can perform useful functions as requests and responses pass through
Examples: Caching, logging, anonymization Client A Request foo.html
foo.html
Proxy cache Request foo.html
Client B foo.html
Request foo.html
foo.html
Slower more expensive global network Origin Server Fast inexpensive local network Cox & Ng 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 & Ng Networking 109
Next Time
Concurrency Cox & Ng Networking 110