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1. I/O
2. Networks and the Internet
I/O





Unix I/O
RIO (robust I/O) package
Metadata, sharing, and redirection
Standard I/O
Conclusions and examples
Unix Files

A Unix file is a sequence of m bytes:
 B0 , B1 , .... , Bk , .... , Bm-1

All I/O devices are represented as files:
 /dev/sda2 (/usr disk partition)
 /dev/tty2 (terminal)

Even the kernel is represented as a file:
 /dev/kmem
 /proc
(kernel memory image)
(kernel data structures)
Unix File Types

Regular file
 File containing user/app data (binary, text, whatever)
 OS does not know anything about the format


other than “sequence of bytes”, akin to main memory
Directory file
 A file that contains the names and locations of other files

Character special and block special files
 Terminals (character special) and disks (block special)

FIFO (named pipe)
 A file type used for inter-process communication

Socket
 A file type used for network communication between processes
Unix I/O

Key Features
 Elegant mapping of files to devices allows kernel to export simple
interface called Unix I/O
 Important idea: All input and output is handled in a consistent and
uniform way

Basic Unix I/O operations (system calls):
 Opening and closing files
open()and close()
 Reading and writing a file
 read() and write()
 Changing the current file position (seek)
 indicates next offset into file to read or write
 lseek()

B0 B1 • • •
Bk-1 Bk Bk+1 • • •
Current file position = k
Opening Files

Opening a file informs the kernel that you are getting ready to
access that file
int fd;
/* file descriptor */
if ((fd = open("/etc/hosts", O_RDONLY)) < 0) {
perror("open");
exit(1);
}

Returns a small identifying integer file descriptor
 fd == -1 indicates that an error occurred

Each process created by a Unix shell begins life with three open
files associated with a terminal:
 0: standard input
 1: standard output
 2: standard error
Closing Files

Closing a file informs the kernel that you are finished
accessing that file
int fd;
/* file descriptor */
int retval; /* return value */
if ((retval = close(fd)) < 0) {
perror("close");
exit(1);
}


Closing an already closed file is a recipe for disaster in
threaded programs (more on this later)
Moral: Always check return codes, even for seemingly
benign functions such as close()
Reading Files

Reading a file copies bytes from the current file position to
memory, and then updates file position
char buf[512];
int fd;
/* file descriptor */
int nbytes;
/* number of bytes read */
/* Open file fd ... */
/* Then read up to 512 bytes from file fd */
if ((nbytes = read(fd, buf, sizeof(buf))) < 0) {
perror("read");
exit(1);
}

Returns number of bytes read from file fd into buf
 Return type ssize_t is signed integer
 nbytes < 0 indicates that an error occurred
 Short counts (nbytes < sizeof(buf) ) are possible and are not
errors!
Writing Files

Writing a file copies bytes from memory to the current file
position, and then updates current file position
char buf[512];
int fd;
/* file descriptor */
int nbytes;
/* number of bytes read */
/* Open the file fd ... */
/* Then write up to 512 bytes from buf to file fd */
if ((nbytes = write(fd, buf, sizeof(buf)) < 0) {
perror("write");
exit(1);
}

Returns number of bytes written from buf to file fd
 nbytes < 0 indicates that an error occurred
 As with reads, short counts are possible and are not errors!
Simple Unix I/O example

Copying standard in to standard out, one byte at a time
#include "csapp.h"
int main(void)
{
char c;
while(Read(STDIN_FILENO, &c, 1) != 0)
Write(STDOUT_FILENO, &c, 1);
exit(0);
}
cpstdin.c
Note the use of error handling wrappers for read and write
(Appendix A).
Dealing with Short Counts

Short counts can occur in these situations:
 Encountering (end-of-file) EOF on reads
 Reading text lines from a terminal
 Reading and writing network sockets or Unix pipes

Short counts never occur in these situations:
 Reading from disk files (except for EOF)
 Writing to disk files

One way to deal with short counts in your code:
 Use the RIO (Robust I/O) package from your textbook’s csapp.c
file (Appendix B)
I/O
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
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

Unix I/O
RIO (robust I/O) package
Metadata, sharing, and redirection
Standard I/O
Conclusions and examples
The RIO Package

RIO is a set of wrappers that provide efficient and robust I/O
in apps, such as network programs that are subject to short
counts

RIO provides two different kinds of functions
 Unbuffered input and output of binary data
rio_readn and rio_writen
 Buffered input of binary data and text lines
 rio_readlineb and rio_readnb
 Buffered RIO routines are thread-safe and can be interleaved
arbitrarily on the same descriptor

Unbuffered RIO Input and Output


Same interface as Unix read and write
Especially useful for transferring data on network sockets
#include "csapp.h"
ssize_t rio_readn(int fd, void *usrbuf, size_t n);
ssize_t rio_writen(int fd, void *usrbuf, size_t n);
Return: num. bytes transferred if OK, 0 on EOF (rio_readn only), -1 on error
 rio_readn returns short count only if it encounters EOF
Only use it when you know how many bytes to read
 rio_writen never returns a short count
 Calls to rio_readn and rio_writen can be interleaved arbitrarily on
the same descriptor

Implementation of 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 */
}
csapp.c
Buffered I/O: Motivation

Applications often read/write one character at a time
 getc, putc, ungetc
 gets, fgets


Read line of text on character at a time, stopping at newline
Implementing as Unix I/O calls expensive
 read and write require Unix kernel calls


> 10,000 clock cycles
Solution: 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
Buffer already read
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
rio_cnt
Buffer already read
rio_buf

unread
rio_bufptr
Layered on Unix file:
Buffered Portion
not in buffer
already read
unread
Current File Position
unseen
Buffered I/O: Declaration

All information contained in struct
rio_cnt
Buffer already read
rio_buf
unread
rio_bufptr
typedef struct {
int rio_fd;
int rio_cnt;
char *rio_bufptr;
char rio_buf[RIO_BUFSIZE];
} rio_t;
/*
/*
/*
/*
descriptor for this internal buf */
unread bytes in internal buf */
next unread byte in internal buf */
internal buffer */
Buffered RIO Input Functions

Efficiently read text lines and binary data from a file partially
cached in an internal memory buffer
#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
 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
Buffered RIO Input Functions (cont)
#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
 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

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);
}
cpfile.c
Today
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Unix I/O
RIO (robust I/O) package
Metadata, sharing, and redirection
Standard I/O
Conclusions and examples
File Metadata
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
Metadata is data about data, in this case file data
Per-file metadata maintained by kernel
 accessed by users with the stat and fstat functions
/* Metadata returned by the stat and fstat functions */
struct stat {
dev_t
st_dev;
/* device */
ino_t
st_ino;
/* inode */
mode_t
st_mode;
/* protection and file type */
nlink_t
st_nlink;
/* number of hard links */
uid_t
st_uid;
/* user ID of owner */
gid_t
st_gid;
/* group ID of owner */
dev_t
st_rdev;
/* device type (if inode device) */
off_t
st_size;
/* total size, in bytes */
unsigned long st_blksize; /* blocksize for filesystem I/O */
unsigned long st_blocks;
/* number of blocks allocated */
time_t
st_atime;
/* time of last access */
time_t
st_mtime;
/* time of last modification */
time_t
st_ctime;
/* time of last change */
};
Example of Accessing File Metadata
/* statcheck.c - Querying and manipulating a file’s meta data */
#include "csapp.h"
unix> ./statcheck statcheck.c
type: regular, read: yes
int main (int argc, char **argv)
unix> chmod 000 statcheck.c
{
unix> ./statcheck statcheck.c
struct stat stat;
type: regular, read: no
char *type, *readok;
unix> ./statcheck ..
type: directory, read: yes
Stat(argv[1], &stat);
unix> ./statcheck /dev/kmem
if (S_ISREG(stat.st_mode))
type: other, read: yes
type = "regular";
else if (S_ISDIR(stat.st_mode))
type = "directory";
else
type = "other";
if ((stat.st_mode & S_IRUSR)) /* OK to read?*/
readok = "yes";
else
readok = "no";
printf("type: %s, read: %s\n", type, readok);
exit(0);
}
statcheck.c
How the Unix Kernel Represents Open Files

Two descriptors referencing two distinct open disk files.
Descriptor 1 (stdout) points to terminal, and descriptor 4
points to open disk file
Descriptor table
[one table per process]
Open file table
[shared by all processes]
v-node table
[shared by all processes]
File A (terminal)
File access
File pos
File size
refcnt=1
File type
File B (disk)
File pos
refcnt=1
...
...
stdin fd 0
stdout fd 1
stderr fd 2
fd 3
fd 4
File access
File size
File type
Info in
stat
struct
...
...
File Sharing

Two distinct descriptors sharing the same disk file through
two distinct open file table entries
 E.g., Calling open twice with the same filename argument
Descriptor table
[one table per process]
Open file table
[shared by all processes]
v-node table
[shared by all processes]
File A (disk)
File access
File pos
File size
refcnt=1
File type
File B (disk)
File pos
refcnt=1
...
...
stdin fd 0
stdout fd 1
stderr fd 2
fd 3
fd 4
...
How Processes Share Files: Fork()

A child process inherits its parent’s open files
 Note: situation unchanged by exec functions (use fcntl to change)

Before fork() call:
Descriptor table
[one table per process]
Open file table
[shared by all processes]
v-node table
[shared by all processes]
File A (terminal)
File access
File pos
File size
refcnt=1
File type
File B (disk)
File pos
refcnt=1
...
...
stdin fd 0
stdout fd 1
stderr fd 2
fd 3
fd 4
File access
File size
File type
...
...
How Processes Share Files: Fork()


A child process inherits its parent’s open files
After fork():
 Child’s table same as parent’s, and +1 to each refcnt
Descriptor table
[one table per process]
Parent
v-node table
[shared by all processes]
File A (terminal)
File access
File pos
File size
refcnt=2
File type
Child
File B (disk)
File pos
refcnt=2
...
...
fd 0
fd 1
fd 2
fd 3
fd 4
File access
File size
File type
...
...
fd 0
fd 1
fd 2
fd 3
fd 4
Open file table
[shared by all processes]
I/O Redirection

Question: How does a shell implement I/O redirection?
unix> ls > foo.txt

Answer: By calling the dup2(oldfd, newfd) function
 Copies (per-process) descriptor table entry oldfd to entry newfd
Descriptor table
before dup2(4,1)
fd 0
fd 1 a
fd 2
fd 3
fd 4 b
Descriptor table
after dup2(4,1)
fd 0
fd 1 b
fd 2
fd 3
fd 4 b
I/O Redirection Example

Step #1: open file to which stdout should be redirected
 Happens in child executing shell code, before exec
Descriptor table
[one table per process]
Open file table
[shared by all processes]
v-node table
[shared by all processes]
File A
File access
File pos
File size
refcnt=1
File type
File B
File pos
refcnt=1
...
...
stdin fd 0
stdout fd 1
stderr fd 2
fd 3
fd 4
File access
File size
File type
...
...
I/O Redirection Example (cont.)

Step #2: call dup2(4,1)
 cause fd=1 (stdout) to refer to disk file pointed at by fd=4
Descriptor table
[one table per process]
Open file table
[shared by all processes]
v-node table
[shared by all processes]
File A
File access
File pos
File size
refcnt=0
File type
File B
File pos
refcnt=2
...
...
stdin fd 0
stdout fd 1
stderr fd 2
fd 3
fd 4
File access
File size
File type
...
...
Fun with File Descriptors (1)
#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;
}
ffiles1.c

What would this program print for file containing “abcde”?
Fun with File Descriptors (2)
#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;
}
ffiles2.c

What would this program print for file containing “abcde”?
Fun with File Descriptors (3)
#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;
}
ffiles3.c

What would be the contents of the resulting file?
I/O





Unix I/O
RIO (robust I/O) package
Metadata, sharing, and redirection
Standard I/O
Conclusions and examples
Standard I/O Functions

The C standard library (libc.so) contains a collection of
higher-level standard I/O functions
 Documented in Appendix B of K&R.

Examples of standard I/O functions:




Opening and closing files (fopen and fclose)
Reading and writing bytes (fread and fwrite)
Reading and writing text lines (fgets and fputs)
Formatted reading and writing (fscanf and fprintf)
Standard I/O Streams

Standard I/O models open files as streams
 Abstraction for a file descriptor and a buffer in memory.
 Similar to buffered RIO

C programs begin life with three open streams
(defined in stdio.h)
 stdin (standard input)
 stdout (standard output)
 stderr (standard error)
#include <stdio.h>
extern FILE *stdin; /* standard input (descriptor 0) */
extern FILE *stdout; /* standard output (descriptor 1) */
extern FILE *stderr; /* standard error (descriptor 2) */
int main() {
fprintf(stdout, "Hello, world\n");
}
Buffering in Standard I/O

Standard I/O functions use buffered I/O
buf
printf("h");
printf("e");
printf("l");
printf("l");
printf("o");
printf("\n");
h
e
l
l
o
\n
.
.
fflush(stdout);
write(1, buf, 6);

Buffer flushed to output fd on “\n” or fflush() call
Standard I/O Buffering in Action

You can see this buffering in action for yourself, using the
always fascinating Unix strace program:
#include <stdio.h>
int main()
{
printf("h");
printf("e");
printf("l");
printf("l");
printf("o");
printf("\n");
fflush(stdout);
exit(0);
}
linux> strace ./hello
execve("./hello", ["hello"], [/* ... */]).
...
write(1, "hello\n", 6)
= 6
...
exit_group(0)
= ?
I/O





Unix I/O
RIO (robust I/O) package
Metadata, sharing, and redirection
Standard I/O
Conclusions
Unix I/O vs. Standard I/O vs. RIO

Standard I/O and RIO are implemented using low-level
Unix I/O
fopen
fread
fscanf
sscanf
fgets
fflush
fclose
fdopen
fwrite
fprintf
sprintf
fputs
fseek
open
write
stat

C application program
Standard I/O
functions
read
lseek
close
RIO
functions
Unix I/O functions
(accessed via system calls)
Which ones should you use in your programs?
rio_readn
rio_writen
rio_readinitb
rio_readlineb
rio_readnb
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.
 Unix I/O functions are async-signal-safe and can be used safely in
signal handlers.


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.
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 functions are not async-signal-safe, and not
appropriate for signal handlers.
 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 (CS:APP2e, Sec 10.9)
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
 Inside signal handlers, because Unix I/O is async-signal-safe.
 In rare cases when you need absolute highest performance.

When to use RIO
 When you are reading and writing network sockets.
 Avoid using standard I/O on sockets.
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’s 1993 classic text.
Networks and the Internet
A Client-Server Transaction
1. Client sends request
Client
process
4. Client
handles
response
Server
process
3. Server sends response
Resource
2. Server
handles
request
Note: clients and servers are processes running on hosts
(can be the same or different hosts)

Most network applications are based on the client-server
model:




A server process and one or more client processes
Server manages some resource
Server provides service by manipulating resource for clients
Server activated by request from client (vending machine analogy)
Hardware Organization of a Network Host
CPU chip
register file
ALU
system bus
memory bus
main
memory
I/O
bridge
MI
Expansion slots
I/O bus
USB
controller
mouse keyboard
graphics
adapter
disk
controller
network
adapter
disk
network
monitor
Computer Networks

A network is a hierarchical system of boxes and wires
organized by geographical proximity
 SAN (System Area Network) spans cluster or machine room
Switched Ethernet, Quadrics QSW, …
 LAN (Local Area Network) spans a building or campus
 Ethernet is most prominent example
 WAN (Wide Area Network) spans country or world
 Typically high-speed point-to-point phone lines


An internetwork (internet) is an interconnected set of
networks
 The Global IP Internet (uppercase “I”) is the most famous example
of an internet (lowercase “i”)

Let’s see how an internet is built from the ground up
Lowest Level: Ethernet Segment
host
100 Mb/s
host
hub
host
100 Mb/s
port

Ethernet segment consists of a collection of hosts connected
by wires (twisted pairs) to a hub

Spans room or floor in a building

Operation
 Each Ethernet adapter has a unique 48-bit address (MAC address)
E.g., 00:16:ea:e3:54:e6
 Hosts send bits to any other host in chunks called frames
 Hub slavishly copies each bit from each port to every other port

Every host sees every bit
 Note: Hubs are on their way out. Bridges (switches, routers) became cheap enough
to replace them (means no more broadcasting)

Next Level: Bridged Ethernet Segment
A
host
host
hub
B
host
host
X
100 Mb/s bridge
100 Mb/s hub
1 Gb/s
hub
100 Mb/s
bridge
host
100 Mb/s
host
host
hub
Y
host
host
host
host
host
C

Spans building or campus

Bridges cleverly learn which hosts are reachable from which
ports and then selectively copy frames from port to port
Conceptual View of LANs

For simplicity, hubs, bridges, and wires are often shown as a
collection of hosts attached to a single wire:
host
host ... host
Next Level: internets


Multiple incompatible LANs can be physically connected by
specialized computers called routers
The connected networks are called an internet
host
host ...
host
host
host ...
LAN
host
LAN
router
WAN
router
WAN
router
LAN 1 and LAN 2 might be completely different, totally incompatible
(e.g., Ethernet and Wifi, 802.11*, T1-links, DSL, …)
Logical Structure of an internet
host
router
host
router
router
router
router

router
Ad hoc interconnection of networks
 No particular topology
 Vastly different router & link capacities

Send packets from source to destination by hopping through
networks
 Router forms bridge from one network to another
 Different packets may take different routes
The Notion of an internet Protocol

How is it possible to send bits across incompatible LANs
and WANs?

Solution:
 protocol software running on each host and router
 smooths out the differences between the different networks

Implements an internet protocol (i.e., set of rules)
 governs how hosts and routers should cooperate when they
transfer data from network to network
 TCP/IP is the protocol for the global IP Internet
What Does an internet Protocol Do?

Provides a naming scheme
 An internet protocol defines a uniform format for host addresses
 Each host (and router) is assigned at least one of these internet
addresses that uniquely identifies it

Provides a delivery mechanism
 An internet protocol defines a standard transfer unit (packet)
 Packet consists of header and payload


Header: contains info such as packet size, source and destination
addresses
Payload: contains data bits sent from source host
Transferring Data Over an internet
LAN1
(1)
client
server
protocol
software
data
PH
data
PH
LAN2
(8)
data
(7)
data
PH
FH2
(6)
data
PH
FH2
protocol
software
FH1
LAN1 frame
(3)
Host B
data
internet packet
(2)
Host A
LAN1
adapter
LAN2
adapter
Router
FH1
LAN1
adapter
LAN2
adapter
LAN2 frame
(4)
PH: Internet packet header
FH: LAN frame header
data
PH
FH1
data
protocol
software
PH
FH2
(5)
Other Issues

We are glossing over a number of important questions:
 What if different networks have different maximum frame sizes?
(segmentation)
 How do routers know where to forward frames?
 How are routers informed when the network topology changes?
 What if packets get lost?

These (and other) questions are addressed by the area of
systems known as computer networking
Global IP Internet

Most famous example of an internet

Based on the TCP/IP protocol family
 IP (Internet protocol) :
Provides basic naming scheme and unreliable delivery capability
of packets (datagrams) from host-to-host
 UDP (Unreliable Datagram Protocol)
 Uses IP to provide unreliable datagram delivery from
process-to-process
 TCP (Transmission Control Protocol)
 Uses IP to provide reliable byte streams from process-to-process
over connections


Accessed via a mix of Unix file I/O and functions from the
sockets interface
Hardware and Software Organization
of an Internet Application
Internet client host
Internet server host
Client
User code
Server
TCP/IP
Kernel code
TCP/IP
Sockets interface
(system calls)
Hardware interface
(interrupts)
Network
adapter
Hardware
and firmware
Global IP Internet
Network
adapter
A Programmer’s View of the Internet

Hosts are mapped to a set of 32-bit IP addresses
 140.192.36.43

The set of IP addresses is mapped to a set of identifiers
called Internet domain names
 140.192.36.43 is mapped to cdmlinux.cdm.depaul.edu
IP Addresses

32-bit IP addresses are stored in an IP address struct
 IP addresses are always stored in memory in network byte order
(big-endian byte order)
 True in general for any integer transferred in a packet header from one
machine to another.
 E.g., the port number used to identify an Internet connection.
/* Internet address structure */
struct in_addr {
unsigned int s_addr; /* network byte order (big-endian) */
};
Useful network byte-order conversion functions (“l” = 32 bits, “s” = 16 bits)
htonl: convert uint32_t from host to network byte order
htons: convert uint16_t from host to network byte order
ntohl: convert uint32_t from network to host byte order
ntohs: convert uint16_t from network to host byte order
Dotted Decimal Notation

By convention, each byte in a 32-bit IP address is represented
by its decimal value and separated by a period


IP address: 0x8002C2F2 = 128.2.194.242
Functions for converting between binary IP addresses and
dotted decimal strings:
 inet_aton: dotted decimal string → IP address in network byte order
 inet_ntoa: IP address in network byte order → dotted decimal string
 “n” denotes network representation
 “a” denotes application representation
Internet Domain Names
unnamed root
.net
.edu
smith depaul
cti
.gov
berkeley
ece
.com
amazon
www
207.171.166.252
cstsis
reed
140.192.32.110
ctilinux3
140.192.36.43
First-level domain names
Second-level domain names
Third-level domain names
Domain Naming System (DNS)

The Internet maintains a mapping between IP addresses and
domain names in a huge worldwide distributed database called
DNS
 Conceptually, programmers can view the DNS database as a collection of
millions of host entry structures:
/* DNS host entry structure
struct hostent {
char
*h_name;
/*
char
**h_aliases;
/*
int
h_addrtype;
/*
int
h_length;
/*
char
**h_addr_list; /*
};

*/
official domain name of host */
null-terminated array of domain names */
host address type (AF_INET) */
length of an address, in bytes */
null-terminated array of in_addr structs */
Functions for retrieving host entries from DNS:
 gethostbyname: query key is a DNS domain name.
 gethostbyaddr: query key is an IP address.
Properties of DNS Host Entries



Each host entry is an equivalence class of domain names and
IP addresses
Each host has a locally defined domain name localhost
which always maps to the loopback address 127.0.0.1
Different kinds of mappings are possible:
 Simple case: one-to-one mapping between domain name and IP address:

reed.cs.depaul.edu maps to 140.192.32.110
 Multiple domain names mapped to the same IP address:

eecs.mit.edu and cs.mit.edu both map to 18.62.1.6
 Multiple domain names mapped to multiple IP addresses:

google.com maps to multiple IP addresses
A Program That Queries DNS
int main(int argc, char **argv) { /* argv[1] is a domain name */
char **pp;
/* or dotted decimal IP addr */
struct in_addr addr;
struct hostent *hostp;
if (inet_aton(argv[1], &addr) != 0)
hostp = Gethostbyaddr((const char *)&addr, sizeof(addr),
AF_INET);
else
hostp = Gethostbyname(argv[1]);
printf("official hostname: %s\n", hostp->h_name);
for (pp = hostp->h_aliases; *pp != NULL; pp++)
printf("alias: %s\n", *pp);
for (pp = hostp->h_addr_list; *pp != NULL; pp++) {
addr.s_addr = ((struct in_addr *)*pp)->s_addr;
printf("address: %s\n", inet_ntoa(addr));
}
}
Using DNS Program
$ ./hostinfo reed.cs.depaul.edu
official hostname: reed.cti.depaul.edu
alias: reed.cs.depaul.edu
address: 140.192.39.42
$ ./hostinfo 140.192.39.42
official hostname: reed.cti.depaul.edu
address: 140.192.39.42
$ ./hostinfo www.google.com
official hostname: www.google.com
address: 173.194.73.104
address: 173.194.73.105
address: 173.194.73.99
address: 173.194.73.147
address: 173.194.73.103
address: 173.194.73.106
Querying DIG

Domain Information Groper (dig) provides a scriptable
command line interface to DNS
$ dig +short reed.cs.depaul.edu
reed.cti.depaul.edu.
140.192.39.42
$ dig +short -x 140.192.39.42
reed.cti.depaul.edu.
baf346.cstcis.cti.depaul.edu.
$ dig +short www.google.com
173.194.73.99
173.194.73.147
173.194.73.103
173.194.73.106
173.194.73.104
173.194.73.105
Internet Connections

Clients and servers communicate by sending streams of bytes
over connections:
 Point-to-point, full-duplex (2-way communication), and reliable.

A socket is an endpoint of a connection
 Socket address is an IPaddress:port pair

A port is a 16-bit integer that identifies a process:
 Ephemeral port: Assigned automatically on client when client makes a
connection request
 Well-known port: Associated with some service provided by a server
(e.g., port 80 is associated with Web servers)

A connection is uniquely identified by the socket addresses
of its endpoints (socket pair)
 (cliaddr:cliport, servaddr:servport)
Putting it all Together:
Anatomy of an Internet Connection
Client socket address
128.2.194.242:51213
Client
Client host address
128.2.194.242
Server socket address
208.216.181.15:80
Connection socket pair
(128.2.194.242:51213, 208.216.181.15:80)
Server
(port 80)
Server host address
208.216.181.15
Clients

Examples of client programs
 Web browsers, ftp, telnet, ssh

How does a client find the server?
 The IP address in the server socket address identifies the host
(more precisely, an adapter on the host)
 The (well-known) port in the server socket address identifies the
service, and thus implicitly identifies the server process that performs
that service.
 Examples of well know ports
 Port 7: Echo server
 Port 23: Telnet server
 Port 25: Mail server
 Port 80: Web server
Using Ports to Identify Services
Server host 128.2.194.242
Client host
Client
Service request for
128.2.194.242:80
(i.e., the Web server)
Web server
(port 80)
Kernel
Echo server
(port 7)
Client
Service request for
128.2.194.242:7
(i.e., the echo server)
Web server
(port 80)
Kernel
Echo server
(port 7)
Servers

Servers are long-running processes (daemons)
 Created at boot-time (typically) by the init process (process 1)
 Run continuously until the machine is turned off

Each server waits for requests to arrive on a well-known port
associated with a particular service





Port 7: echo server
Port 23: telnet server
Port 25: mail server
Port 80: HTTP server
A machine that runs a server process is also often referred to
as a “server”
Server Examples

Web server (port 80)
 Resource: files/compute cycles (CGI programs)
 Service: retrieves files and runs CGI programs on behalf of the client

FTP server (20, 21)
 Resource: files
 Service: stores and retrieve files

See /etc/services for a
comprehensive list of the port
mappings on a Linux machine
Telnet server (23)
 Resource: terminal
 Service: proxies a terminal on the server machine

Mail server (25)
 Resource: email “spool” file
 Service: stores mail messages in spool file