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Introduction to Computer Systems15-213/18-243, spring 200922nd Lecture, Apr. 9th

Instructors: Gregory Kesden and Markus Püschel

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Exam 2 (This Section)

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Last Time: Open Files in Unix Two descriptors referencing two distinct open disk files.

Descriptor 1 (stdout) points to terminal, and descriptor 4 points to open disk file

fd 0fd 1fd 2fd 3fd 4

Descriptor table[one table per process]

Open file table [shared by all processes]

v-node table[shared by all processes]

File posrefcnt=1

...

File posrefcnt=1

...

stderrstdoutstdin File access

...

File sizeFile type

File access

...

File sizeFile type

File A (terminal)

File B (disk)

Info in stat struct

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Last Time: Unix I/O

file fd

memory

pos pos (after read or write)

n bytes

buf

read write

ssize_t read(int fd, void *buf, size_t n)ssize_t write(int fd, void *buf, size_t n)

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Last Time: Standard I/O (Buffering)

memory

fread fwrite

file fd

buffer

data transferred in chunks

abstracted as stream:

FILE

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Last Time: Robust I/O rio_readn/* * rio_readn - robustly read n bytes (unbuffered) */ssize_t rio_readn(int fd, void *usrbuf, size_t n) { size_t nleft = n; ssize_t nread; char *bufp = usrbuf;

while (nleft > 0) {if ((nread = read(fd, bufp, nleft)) < 0) { if (errno == EINTR) /* interrupted by sig handler return */

nread = 0; /* and call read() again */ else

return -1; /* errno set by read() */ } else if (nread == 0) break; /* EOF */nleft -= nread;bufp += nread;

} return (n - nleft); /* return >= 0 */}

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Today System level I/O

Unix I/O Standard I/O RIO (robust I/O) package Conclusions and examples

Internetworking Networks Global IP Internet

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Buffered I/O: Motivation I/O Applications Read/Write One Character at a Time

getc, putc, ungetc gets

Read line of text, stopping at newline Implementing as Calls to Unix I/O Expensive

Read & Write involve require Unix kernel calls > 10,000 clock cycles

Buffered Read Use Unix read() to grab block of bytes User input functions take one byte at a time from buffer

Refill buffer when empty

unreadalready readBuffer

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unread

Buffered I/O: Implementation For reading from file File has associated buffer to hold bytes that have been read

from file but not yet read by user code

Layered on Unix File

already readBuffer

rio_bufrio_bufptr

rio_cnt

unreadalready readnot in buffer unseen

Current File Position

Buffered Portion

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Buffered I/O: Declaration All information contained in struct

typedef struct { int rio_fd; /* descriptor for this internal buf */ int rio_cnt; /* unread bytes in internal buf */ char *rio_bufptr; /* next unread byte in internal buf */ char rio_buf[RIO_BUFSIZE]; /* internal buffer */} rio_t;

unreadalready readBuffer

rio_bufrio_bufptr

rio_cnt

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RIO: Robustness and Buffering

Robust

Buffered

read

rio_read

rio_readn

rio_readnb

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Buffered RIO Input Functions Efficiently read text lines and binary data from a file partially

cached in an internal memory buffer

rio_readlineb reads a text line of up to maxlen bytes from file fd and stores the line in usrbuf

Especially useful for reading text lines from network sockets Stopping conditions

maxlen bytes read EOF encountered Newline (‘\n’) encountered

#include "csapp.h"

void rio_readinitb(rio_t *rp, int fd);

ssize_t rio_readlineb(rio_t *rp, void *usrbuf, size_t maxlen);

Return: num. bytes read if OK, 0 on EOF, -1 on error

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Buffered RIO Input Functions (cont)

rio_readnb reads up to n bytes from file fd Stopping conditions

maxlen bytes read EOF encountered

Calls to rio_readlineb and rio_readnb can be interleaved arbitrarily on the same descriptor

Warning: Don’t interleave with calls to rio_readn

#include "csapp.h"

void rio_readinitb(rio_t *rp, int fd);

ssize_t rio_readlineb(rio_t *rp, void *usrbuf, size_t maxlen);ssize_t rio_readnb(rio_t *rp, void *usrbuf, size_t n);

Return: num. bytes read if OK, 0 on EOF, -1 on error

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RIO Example Copying the lines of a text file from standard input to

standard output

#include "csapp.h"

int main(int argc, char **argv) { int n; rio_t rio; char buf[MAXLINE];

Rio_readinitb(&rio, STDIN_FILENO); while((n = Rio_readlineb(&rio, buf, MAXLINE)) != 0)

Rio_writen(STDOUT_FILENO, buf, n); exit(0);}

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Today System level I/O

Unix I/O Standard I/O RIO (robust I/O) package Conclusions and examples

Internetworking Networks Global IP Internet

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Choosing I/O Functions General rule: use the highest-level I/O functions you can

Many C programmers are able to do all of their work using the standard I/O functions

When to use standard I/O When working with disk or terminal files

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

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

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For Further Information The Unix bible:

W. Richard Stevens & Stephen A. Rago, Advanced Programming in the Unix Environment, 2nd Edition, Addison Wesley, 2005

Updated from Stevens’ 1993 book

Stevens is arguably the best technical writer ever. Produced authoritative works in:

Unix programming TCP/IP (the protocol that makes the Internet work) Unix network programming Unix IPC programming

Tragically, Stevens died Sept. 1, 1999 But others have taken up his legacy

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Fun with File Descriptors (1)

What would this program print for file containing “abcde”?

#include "csapp.h"int main(int argc, char *argv[]){ int fd1, fd2, fd3; char c1, c2, c3; char *fname = argv[1]; fd1 = Open(fname, O_RDONLY, 0); fd2 = Open(fname, O_RDONLY, 0); fd3 = Open(fname, O_RDONLY, 0); Dup2(fd2, fd3); Read(fd1, &c1, 1); Read(fd2, &c2, 1); Read(fd3, &c3, 1); printf("c1 = %c, c2 = %c, c3 = %c\n", c1, c2, c3); return 0;}

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Fun with File Descriptors (2)

What would this program print for file containing “abcde”?

#include "csapp.h"int main(int argc, char *argv[]){ int fd1; int s = getpid() & 0x1; char c1, c2; char *fname = argv[1]; fd1 = Open(fname, O_RDONLY, 0); Read(fd1, &c1, 1); if (fork()) { /* Parent */ sleep(s); Read(fd1, &c2, 1); printf("Parent: c1 = %c, c2 = %c\n", c1, c2); } else { /* Child */ sleep(1-s); Read(fd1, &c2, 1); printf("Child: c1 = %c, c2 = %c\n", c1, c2); } return 0;}

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Fun with File Descriptors (3)

What would be the contents of the resulting file?

#include "csapp.h"int main(int argc, char *argv[]){ int fd1, fd2, fd3; char *fname = argv[1]; fd1 = Open(fname, O_CREAT|O_TRUNC|O_RDWR, S_IRUSR|S_IWUSR); Write(fd1, "pqrs", 4); fd3 = Open(fname, O_APPEND|O_WRONLY, 0); Write(fd3, "jklmn", 5); fd2 = dup(fd1); /* Allocates descriptor */ Write(fd2, "wxyz", 4); Write(fd3, "ef", 2); return 0;}

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Accessing Directories Only recommended operation on a directory: read its entries

dirent structure contains information about a directory entry DIR structure contains information about directory while stepping

through its entries

#include <sys/types.h>#include <dirent.h>

{ DIR *directory; struct dirent *de; ... if (!(directory = opendir(dir_name))) error("Failed to open directory"); ... while (0 != (de = readdir(directory))) { printf("Found file: %s\n", de->d_name); } ... closedir(directory);}

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Unix I/O vs. Standard I/O vs. RIO Standard I/O and RIO are implemented using low-level

Unix I/O

Which ones should you use in your programs?

Unix I/O functions (accessed via system calls)

Standard I/O functions

C application program

fopen fdopenfread fwrite fscanf fprintf sscanf sprintf fgets fputs fflush fseekfclose

open readwrite lseekstat close

rio_readnrio_writenrio_readinitbrio_readlinebrio_readnb

RIOfunctions

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Pros and Cons of Unix I/O Pros

Unix I/O is the most general and lowest overhead form of I/O All other I/O packages are implemented using Unix I/O

functions Unix I/O provides functions for accessing file metadata

Cons Dealing with short counts is tricky and error prone Efficient reading of text lines requires some form of buffering, also

tricky and error prone Both of these issues are addressed by the standard I/O and RIO

packages

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Pros and Cons of Standard I/O Pros

Buffering increases efficiency by decreasing the number of read and write system calls

Short counts are handled automatically

Cons Provides no function for accessing file metadata Standard I/O is not appropriate for input and output on network

sockets There are poorly documented restrictions on streams that interact

badly with restrictions on sockets

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Today System level I/O

Unix I/O Standard I/O RIO (robust I/O) package Conclusions and examples

Internetworking Networks Global IP Internet

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A Client-Server Transaction

Clientprocess

Serverprocess

1. Client sends request

2. Server handlesrequest

3. Server sends response4. Client handles

response

Resource

Most network applications are based on the client-server model: A server process and one or more client processes Server manages some resource Server provides service by manipulating resource for clients Server activated by request from client (vending machine analogy)

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

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Hardware Organization of a Network Host

mainmemory

I/O bridgeMI

ALU

register fileCPU chip

system bus memory bus

disk controller

graphicsadapter

USBcontroller

mouse keyboard monitordisk

I/O bus

Expansion slots

networkadapter

network

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Computer Networks A network is a hierarchical system of boxes and wires

organized by geographical proximity SAN (System Area Network) spans cluster or machine room

Switched Ethernet, Quadrics QSW, … LAN (Local Area Network) spans a building or campus

Ethernet is most prominent example WAN (Wide Area Network) spans country or world

Typically high-speed point-to-point phone lines

An internetwork (internet) is an interconnected set of networks The Global IP Internet (uppercase “I”) is the most famous example

of an internet (lowercase “i”)

Let’s see how an internet is built from the ground up

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Lowest Level: Ethernet Segment

Ethernet segment consists of a collection of hosts connected by wires (twisted pairs) to a hub

Spans room or floor in a building Operation

Each Ethernet adapter has a unique 48-bit address (MAC address) Hosts send bits to any other host in chunks called frames Hub slavishly copies each bit from each port to every other port

Every host sees every bit Note: Hubs are on their way out. Bridges (switches, routers) became cheap enough

to replace them (means no more broadcasting)

host host host

hub100 Mb/s100 Mb/s

port

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Next Level: Bridged Ethernet Segment

Spans 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

hub

bridge

100 Mb/s 100 Mb/s

host host

hub

100 Mb/s 100 Mb/s

1 Gb/s

host host host

bridge

hosthost

hub

A B

C

X

Y

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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...

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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...

WAN WAN

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

router router routerLAN LAN

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Logical Structure of an internet

Ad hoc interconnection of networks No particular topology Vastly different router & link capacities

Send packets from source to destination by hopping through networks Router forms bridge from one network to another Different packets may take different routes

router

router

routerrouter

router

router

hosthost

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The Notion of an internet Protocol How is it possible to send bits across incompatible LANs

and WANs?

Solution: protocol software running on each host and router smooths out the differences between the different networks

Implements an internet protocol (i.e., set of rules) governs how hosts and routers should cooperate when they

transfer data from network to network TCP/IP is the protocol for the global IP Internet

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What Does an internet Protocol Do? Provides a naming scheme

An internet protocol defines a uniform format for host addresses Each host (and router) is assigned at least one of these internet

addresses that uniquely identifies it

Provides a delivery mechanism An internet protocol defines a standard transfer unit (packet) Packet consists of header and payload

Header: contains info such as packet size, source and destination addresses

Payload: contains data bits sent from source host

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LAN2

Transferring Data Over an internet

protocolsoftware

client

LAN1adapter

Host ALAN1

data(1)

data PH FH1(4)

data PH FH2(6)

data(8)

data PH FH2 (5)

LAN2 frame

protocolsoftware

LAN1adapter

LAN2adapter

Routerdata PH(3) FH1

data PH FH1(2)

internet packet

LAN1 frame

(7) data PH FH2

protocolsoftware

server

LAN2adapter

Host B

PH: Internet packet headerFH: LAN frame header

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Other Issues We are glossing over a number of important questions:

What if different networks have different maximum frame sizes? (segmentation)

How do routers know where to forward frames? How are routers informed when the network topology changes? What if packets get lost?

These (and other) questions are addressed by the area of systems known as computer networking

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Today System level I/O

Unix I/O Standard I/O RIO (robust I/O) package Conclusions and examples

Internetworking Networks Global IP Internet

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Global IP Internet Most famous example of an internet

Based on the TCP/IP protocol family IP (Internet protocol) :

Provides basic naming scheme and unreliable delivery capability of packets (datagrams) from host-to-host

UDP (Unreliable Datagram Protocol) Uses IP to provide unreliable datagram delivery from

process-to-process TCP (Transmission Control Protocol)

Uses IP to provide reliable byte streams from process-to-process over connections

Accessed via a mix of Unix file I/O and functions from the sockets interface

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Hardware and Software Organization of an Internet Application

TCP/IP

Client

Networkadapter

Global IP Internet

TCP/IP

Server

Networkadapter

Internet client host Internet server host

Sockets interface(system calls)

Hardware interface(interrupts)

User code

Kernel code

Hardwareand firmware

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Basic Internet Components Internet backbone:

collection of routers (nationwide or worldwide) connected by high-speed point-to-point networks

Network Access Point (NAP): router that connects multiple backbones (often referred to as peers)

Regional networks: smaller backbones that cover smaller geographical areas

(e.g., cities or states) Point of presence (POP):

machine that is connected to the Internet Internet Service Providers (ISPs):

provide dial-up or direct access to POPs

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NAP-Based Internet Architecture NAPs link together commercial backbones provided by

companies such as AT&T and Worldcom

Currently in the US there are about 50 commercial backbones connected by ~12 NAPs (peering points)

Similar architecture worldwide connects national networks to the Internet

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Internet Connection HierarchyNAP NAP

Backbone BackboneBackboneBackbone

NAP

POP POP POP

Regional net

POPPOP POP

POPPOP

Small Business

Big BusinessISP

POP POP POP POP

Pgh employee

Cablemodem

DC employee

POP

T3

T1

ISP (for individuals)

POP

DSLT1

Colocationsites

Private“peering”

agreementsbetween

two backbonecompanies

often bypassNAP

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Network Access Points (NAPs)

Source: Boardwatch.com

Note: Peers in this context are commercial backbones (droh)

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Source: http://personalpages.manchester.ac.uk/staff/m.dodge/cybergeography/atlas/

MCI/WorldCom/UUNET Global Backbone

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Naming and Communicating on the Internet Original Idea

Every node on Internet would have unique IP address Everyone would be able to talk directly to everyone

No secrecy or authentication Messages visible to routers and hosts on same LAN Possible to forge source field in packet header

Shortcomings There aren't enough IP addresses available Don't want everyone to have access or knowledge of all other hosts Security issues mandate secrecy & authentication

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Evolution of Internet: Naming Dynamic address assignment

Most hosts don't need to have known address Only those functioning as servers

DHCP (Dynamic Host Configuration Protocol) Local ISP assigns address for temporary use

Example: My laptop at CMU

IP address 128.2.220.249 (bryant-tp3.cs.cmu.edu) Assigned statically

My laptop at home IP address 205.201.7.7 (dhcp-7-7.dsl.telerama.com) Assigned dynamically by my ISP for my DSL service

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Evolution of Internet: Firewalls

Firewalls Hides organizations nodes from rest of Internet Use local IP addresses within organization For external service, provides proxy service

1. Client request: src=10.2.2.2, dest=216.99.99.992. Firewall forwards: src=176.3.3.3, dest=216.99.99.993. Server responds: src=216.99.99.99, dest=176.3.3.34. Firewall forwards response: src=216.99.99.99, dest=10.2.2.2

Corporation X

Firewall

Internet

10.2.2.214 2

3

176.3.3.3

216.99.99.99

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Virtual Private Networks

Supporting road warrior Employee working remotely with assigned IP address 198.3.3.3 Wants to appear to rest of corporation as if working internally

From address 10.6.6.6 Gives access to internal services (e.g., ability to send mail)

Virtual Private Network (VPN) Overlays private network on top of regular Internet

Corporation X

Internet

10.x.x.x 198.3.3.3Firewall 10.6.6.6

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A Programmer’s View of the Internet Hosts are mapped to a set of 32-bit IP addresses

128.2.203.179

The set of IP addresses is mapped to a set of identifiers called Internet domain names 128.2.203.179 is mapped to www.cs.cmu.edu

A process on one Internet host can communicate with a process on another Internet host over a connection

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IP Addresses 32-bit IP addresses are stored in an IP address struct

IP addresses are always stored in memory in network byte order (big-endian byte order)

True in general for any integer transferred in a packet header from one machine to another.

E.g., the port number used to identify an Internet connection.

/* Internet address structure */struct in_addr { unsigned int s_addr; /* network byte order (big-endian) */};

Useful network byte-order conversion functions:

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

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Dotted Decimal Notation By convention, each byte in a 32-bit IP address is represented

by its decimal value and separated by a period IP address: 0x8002C2F2 = 128.2.194.242

Functions for converting between binary IP addresses and dotted decimal strings: inet_aton: dotted decimal string → IP address in network byte order inet_ntoa: IP address in network byte order → dotted decimal string

“n” denotes network representation “a” denotes application representation

Blackboard?

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Dotted Decimal Notation By convention, each byte in a 32-bit IP address is represented

by its decimal value and separated by a period IP address: 0x8002C2F2 = 128.2.194.242

Functions for converting between binary IP addresses and dotted decimal strings: inet_aton: dotted decimal string → IP address in network byte order inet_ntoa: IP address in network byte order → dotted decimal string

“n” denotes network representation “a” denotes application representation

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IP Address Structure IP (V4) Address space divided into classes:

Network ID Written in form w.x.y.z/n n = number of bits in host address E.g., CMU written as 128.2.0.0/16

Class B address Unrouted (private) IP addresses:

10.0.0.0/8 172.16.0.0/12 192.168.0.0/16

Class A

Class B

Class C

Class D

Class E

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

Host ID

Host IDNet ID

Net ID

Multicast address

Reserved for experiments

1 01 01

1 1 01

1 1 11

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Internet Domain Names

.net .edu .gov .com

cmu berkeleymit

cs ece

kittyhawk128.2.194.242

cmcl

unnamed root

pdl

imperial128.2.189.40

amazon

www208.216.181.15

First-level domain names

Second-level domain names

Third-level domain names

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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 */ };

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Properties of DNS Host Entries Each host entry is an equivalence class of domain names and

IP addresses Each host has a locally defined domain name localhost

which always maps to the loopback address 127.0.0.1 Different kinds of mappings are possible:

Simple case: one-to-one mapping between domain name and IP address: kittyhawk.cmcl.cs.cmu.edu maps to 128.2.194.242

Multiple domain names mapped to the same IP address: eecs.mit.edu and cs.mit.edu both map to 18.62.1.6

Multiple domain names mapped to multiple IP addresses: aol.com and www.aol.com map to multiple IP addresses

Some valid domain names don’t map to any IP address: for example: cmcl.cs.cmu.edu

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A Program That Queries DNSint main(int argc, char **argv) { /* argv[1] is a domain name */ char **pp; /* or dotted decimal IP addr */ struct in_addr addr; struct hostent *hostp;

if (inet_aton(argv[1], &addr) != 0) hostp = Gethostbyaddr((const char *)&addr, sizeof(addr), AF_INET); else hostp = Gethostbyname(argv[1]); printf("official hostname: %s\n", hostp->h_name); for (pp = hostp->h_aliases; *pp != NULL; pp++) printf("alias: %s\n", *pp);

for (pp = hostp->h_addr_list; *pp != NULL; pp++) { addr.s_addr = ((struct in_addr *)*pp)->s_addr; printf("address: %s\n", inet_ntoa(addr)); }}

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Querying DNS from the Command Line Domain Information Groper (dig) provides a scriptable

command line interface to DNS

linux> dig +short kittyhawk.cmcl.cs.cmu.edu 128.2.194.242 linux> dig +short -x 128.2.194.242 KITTYHAWK.CMCL.CS.CMU.EDU. linux> dig +short aol.com 205.188.145.215 205.188.160.121 64.12.149.24 64.12.187.25 linux> dig +short -x 64.12.187.25 aol-v5.websys.aol.com.

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Internet Connections Clients and servers communicate by sending streams of bytes

over connections: Point-to-point, full-duplex (2-way communication), and reliable.

A socket is an endpoint of a connection Socket address is an IPaddress:port pair

A port is a 16-bit integer that identifies a process: Ephemeral port: Assigned automatically on client when client makes a

connection request Well-known port: Associated with some service provided by a server

(e.g., port 80 is associated with Web servers)

A connection is uniquely identified by the socket addresses of its endpoints (socket pair) (cliaddr:cliport, servaddr:servport)

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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