Networking Alan L. Cox [email protected] T. S. Eugene Ng [email protected] Some slides adapted from CMU 15.213 slides

Networking

Alan L. [email protected]. 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

3

The 2004 A. M. Turing Award Goes to...

Cox & Ng Networking

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The 2004 A. M. Turing Award Goes to...

"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

Bob Kahn Vint Cerf

Cox & Ng Networking

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But at the Same Time...

Source: Akamai Technologies, Inc.

Daily, e.g. 110 attacks from China; 26 attacks from USA

2/24/2008 YouTube traffic mis-routed to PakistanQ2/2008 1000s of Netherlands

DSL customers lost service due to network configuration error11/10/2008 CTBC (Brazil) black-holed

all Internet traffic in some parts of Brazil2/16/2009 Supro (Czech) routing messages

triggered a Cisco router bug world-wide5/2009 AfNOG (Africa) routing messages triggered buffer overflow in open-source

routing software Quagga world-wide

Cox & Ng Networking

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Telephony

Interactive telecommunication between peopleAnalog voice

Transmitter/receiver continuously in contact with electronic circuit Electric current varies with acoustic pressure

Over electrical circuits

Analog/Continuous Signal

Cox & Ng Networking

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Telephony Milestones

1876: Alexander Bell invented telephone1878: 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 SwitchCox & Ng Networking

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1878: First telephone directory; White House line1881: Insulated, balanced twisted pair as local loop1885: AT&T formed1892: First automatic commercial telephone switch1903: 3 million telephones in U.S.1915: First transcontinental telephone line1927: First commercial transatlantic commercial service

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1937: Multiplexing introduced for inter-city calls One link carries multiple conversations

Without Multiplexing With Multiplexing

Cox & Ng Networking

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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

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

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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)

MIT TX-2SDC Q32

dial-up

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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

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• 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

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Internet Hourglass Architecture

Ethernet, WiFi,3G, bluetooth,...

IP

TCP, UDP, …

Email, Web, ssh,...

Cox & Ng Networking


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

Clientprocess

Serverprocess

1. Client sends request

2. Server handlesrequest

3. Server sends response4. Client handles

response

Resource

Note: clients and servers are processes running on hosts (can be the same or different hosts)


Network Hardware

mainmemory

I/O bridge

Bus Interface

ALU

register file

CPU chip

system bus memory bus

disk controller

graphicsadapter

USBcontroller

mousekeyboard monitor

disk

I/O bus

Expansion slots

networkadapter

network


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”)


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

hub100 Mb/s100 Mb/s

ports


Next Level: Bridged Ethernet Segment

Spans room, building, or campusBridges cleverly learn which hosts are reachable from which ports and then selectively copy frames from port to port

host host host host host

hub hubbridge100 Mb/s 100 Mb/s

host

host 1 Gb/s 1 Gb/s

1-10 Gb/s

host host host

bridge

hosthost

bridge

1 Gb/s


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

LAN 1

... host host host

LAN 2

...

router router routerWAN WAN

LAN 1 and LAN 2 might be completely different, totally incompatible LANs (e.g., Ethernet and WiFi, 802.11*, T1-links, DSL, …)


The Internet Circa 1986

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)

In 1986, the Internet consisted of one backbone (NSFNET) that connected 13 sites via 45 Mbps T3 links

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


NSFNET Internet Backbone

source: www.eef.org


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


Current Internet Architecture


Level 3 Backbone


AT&T Backbone


Submarine Cabling


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


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


Transferring Data Over an internet

protocolsoftware

LAN1adapter

Host A

data

data PH FH1

data PH

data PH FH2

LAN1 LAN2

data

data PH

FH1

data PH FH2

(1)

(2)

(3)

(4) (5)

(6)

(7)

(8)

internet packet

LAN2 frame

protocolsoftware

LAN1adapter

LAN2adapter

Router

FH1

LAN1 frame

data PH FH2

protocolsoftware

LAN2adapter

Host B

client server


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)


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


Organization of an Internet Application

TCP/IP

Client

Networkadapter

Global IP Internet

TCP/IP

Server

Networkadapter

Internet client Internet server

Sockets interface(system calls)

Hardware interface(interrupts)

User code

Kernel code

Hardwareand firmware


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


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 orderhtons: convert short int from host to network byte orderntohl: convert long int from network to host byte orderntohs: convert short int 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: 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


IP Address Structure

IP (V4) Address space divided into classes:

Special Addresses for routers and gateways (all 0/1’s)

Loop-back address: 127.0.0.1Unrouted (private) IP addresses:

10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16

Dynamic IP addresses (DHCP)

Class A

Class B

Class C

Class D

Class E

0 1 2 3 8 16 24 310 Net ID Host ID

1 0

1 1 0

Host ID

Host IDNet ID

Net ID

1 1 0

1 11 1

1 Multicast address

Reserved for experiments


Internet Domain Names

.net .edu .gov .com

rice berkeleymit

clear cs

bell128.42.151.14

unnamed root

crystal128.42.151.13

amazon

www72.21.210.11

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:

Functions for retrieving host entries from DNS: gethostbyname: query key is a DNS domain name gethostbyaddr: query key is an IP address

/* 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 */ };


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


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)); }}


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 64.233.187.9964.233.167.99unix> dig +short -x 64.233.167.99 py-in-f99.google.com.


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)


Putting it all Together: Anatomy of an Internet Connection

Connection socket pair(128.2.194.242:51213, 208.216.181.15:80)

Server(port 80)

Client

Client socket address128.2.194.242:51213

Server socket address208.216.181.15:80

Client host address128.2.194.242

Server host address208.216.181.15


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 known ports• Port 7: Echo server• Port 23: Telnet server• Port 25: Mail server• Port 80: Web server


Using Ports to Identify Services

Web server(port 80)

Client host

Server host 128.2.194.242

Echo server(port 7)

Service request for128.2.194.242:80

(i.e., the Web server)

Web server(port 80)

Echo server(port 7)

Service request for128.2.194.242:7

(i.e., the echo server)

Kernel

Kernel

Client

Client


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

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

See /etc/services for a comprehensive list of the services available on a UNIX machine


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


Overview of the Sockets Interface

Client Server

socket socket

bind

listen

accept

rio_readlineb

rio_readlineb

rio_writen

close

rio_readlineb

connect

rio_writen

close

Connectionrequest

EOF

Await connectionrequest fromnext client

open_listenfd

open_clientfd


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


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 (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) */ };


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); }


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


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)


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);


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;

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); }}


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; }


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;


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;


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);


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;


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; }


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 */ }}


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);


Echo Server: accept Illustrated

listenfd(3)

Client

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

Server

listenfd(3)

Client

clientfd

Server2. Client makes connection request by calling and blocking in connect

Connectionrequest

listenfd(3)

Client

clientfd

Server

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

connfd(4)


Connected vs. Listening Descriptors

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

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

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

Why the distinction? Allows for concurrent servers that can communicate

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

handle the request


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);


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); } }


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

bass> echoserver 5000server established connection with KITTYHAWK.CMCL (128.2.194.242)server received 5 bytes: 123server established connection with KITTYHAWK.CMCL (128.2.194.242)server received 8 bytes: 456789

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


Running the Echo Client and Server

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

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

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


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


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


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)


Internet Hosts

ISC Domain Survey Host Count

0

100,000,000

200,000,000

300,000,000

400,000,000

500,000,000

600,000,000Ja

n-9

8

Jan

-99

Jan

-00

Jan

-01

Jan

-02

Jan

-03

Jan

-04

Jan

-05

Jan

-06

Jan

-07

Jan

-08

Source: www.isc.org


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

Webserver

HTTP request

HTTP response(content)

Webclient

(browser)


Web Content

Web servers return content to clients content: a sequence of bytes with an associated

MIME (Multipurpose Internet Mail Extensions) type

Example MIME types text/html HTML document text/plain Unformatted text application/postscript Postcript document image/gif Binary image (GIF format) image/jpeg Binary image (JPEG format)


Static and Dynamic Content

The content returned in HTTP responses can be either static or dynamic

Static content: content stored in files and retrieved in response to an HTTP request• Examples: HTML files, images, audio clips

Dynamic content: content produced on-the-fly in response to an HTTP request• Example: content produced by a program executed by

the server on behalf of the client (i.e., search results)

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


URLs

Each file managed by a server has a unique name called a URL (Universal Resource Locator)

URLs for static content: http://www.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


How Clients and Servers Use URLs

Example URL: http://www.aol.com:80/index.htmlClients use prefix (http://www.aol.com:80) to infer:

What kind of server to contact (http (Web) server) Where the server is (www.aol.com) 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)


Anatomy of an HTTP Transactionunix> telnet www.google.com 80 Client: open connection to serverTrying 209.85.164.104... Telnet prints 3 lines to the terminalConnected to www.l.google.com.Escape character is '^]'.GET / HTTP/1.1 Client: request linehost: www.google.com Client: required HTTP/1.1 HOST header Client: empty line terminates headersHTTP/1.1 200 OK Server: response lineCache-Control: private Server: followed by six response headersContent-Type: text/html; charset=ISO-8859-1Set-Cookie: PREF=ID=<..snip..>Server: gwsTransfer-Encoding: chunkedDate: Tue, 13 Nov 2007 17:25:04 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 bodyConnection closed by foreign host. Server: closes connectionunix> Client: closes connection and terminates


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)


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


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: www.yahoo.com

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 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


GET Request to Apache ServerFrom Firefox Browser

GET /~comp221/proxytest.html HTTP/1.1 Host: www.owlnet.rice.eduUser-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.8) Gecko/20071022 Ubuntu/7.10 (gutsy) Firefox/2.0.0.8Accept: text/xml,application/xml,application/xhtml+xml,text/html; q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5Accept-Language: en-us,en;q=0.5Accept-Encoding: gzip,deflateAccept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7Connection: keep-aliveKeep-Alive: 300CRLF (\r\n)

http://www.owlnet.rice.edu/


GET Response From Apache Server

HTTP/1.1 200 OKDate: Tue, 13 Nov 2007 18:09:01 GMTServer: Apache/1.3.26 (Unix) mod_ssl/2.8.9 OpenSSL/0.9.6eLast-Modified: Tue, 13 Nov 2007 18:08:44 GMTETag: “ac330311-df-4739e82c"Accept-Ranges: bytesContent-Length: 212Connection: closeContent-Type: text/htmlCRLF<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>


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

Client Server

GET /cgi-bin/env.pl HTTP/1.1


Serving Dynamic Content (cont)

The server creates a child process and runs the program identified by the URI in that process Client Server

env.pl

fork/exec


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

env.pl

Content

Content


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 Server

Content

Content

Request

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


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


The add.com Experience

input URL

Output page

host port CGI program args


Serving Dynamic Content With GETQuestion: How does the client pass arguments to the server?

Answer: The arguments are appended to the URI

Can be encoded directly in a URL typed to a browser or a URL in an HTML link

http://add.com/cgi-bin/adder?1&2 adder is the CGI program on the server that will do

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

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


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.



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);



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


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)


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 “_”)


Serving Dynamic Content With GETQuestion: 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);


Serving Dynamic Content With GET bass> ./tiny 8000GET /cgi-bin/adder?1&2 HTTP/1.1Host: bass.cmcl.cs.cmu.edu:8000<CRLF>

kittyhawk> telnet bass 8000Trying 128.2.222.85...Connected to BASS.CMCL.CS.CMU.EDU.Escape character is '^]'.GET /cgi-bin/adder?1&2 HTTP/1.1Host: bass.cmcl.cs.cmu.edu:8000<CRLF>HTTP/1.1 200 OKServer: Tiny Web ServerContent-length: 102Content-type: text/html<CRLF>Welcome to add.com: THE Internet addition portal.<p>The answer is: 1 + 2 = 3<p>Thanks for visiting!Connection closed by foreign host.kittyhawk>

HTTP request received byTiny Web server

HTTP request sent by client

HTTP response generated bythe serverHTTP response generated bythe CGI program


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

Client ProxyOriginServer

HTTP request HTTP request

HTTP responseHTTP response


Why Proxies?

Can perform useful functions as requests and responses pass through

Examples: Caching, logging, anonymization

ClientA

Proxycache

OriginServer

Request foo.html

Request foo.html

foo.html

foo.html

ClientB

Request foo.html

foo.html

Fast inexpensive local network

Slower more expensive

global network


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


Next Time

Concurrency

Documents

Networking Alan L. Cox [email protected] T. S. Eugene Ng [email protected] Some slides adapted from CMU 15.213 slides