1. Networking Programming

2. Outline
Networks
Suggested Reading:
12.2

3. Hardware organization of a network host
main
memory
I/O
bridge MI
ALU
register file
system bus
memory bus
disk
controller
graphics
adapter
USB
mouse
keyboard
monitor
I/O bus
Expansion slots
network
CPU chip

4. Computer networks
A network is a hierarchical system of boxes and wires organized by geographical proximity
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.

5. An internetwork (internet) is an interconnected set of networks.
Computer networks
An internetwork (internet) is an interconnected set of networks.
The IP Internet is the most famous example of an internetwork.
Let’s see how we would build an internetwork from the ground up.

6. 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.
host
hub
100 Mb/s
ports

7. Lowest level: Ethernet segment
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.

8. 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

9. Next level: Bridged Ethernet segment
host
hub
bridge
100 Mb/s
1 Gb/s A B C X Y

10. Conceptual view of LANs
For simplicity, hubs, bridges, and wires are often shown as a collection of hosts attached to a single wire:
host
...

11. Next level: internets
Multiple incompatible LANs can be physically connected by specialized computers called routers.
The connected networks are called an internet.

12. Next level: internets
host
LAN 1
...
LAN 2
router
WAN
LAN 1 and LAN 2 might be completely different, totally incompatible LANs

13. Client-Server Programming Model Global IP Internet Suggested Reading:
Outline
Client-Server Programming Model
Global IP Internet
Suggested Reading:
12.1、12.3

14. A client-server transaction
Every network application is 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.

15. A client-server transaction
process
server
1. client sends request
2. server
handles
request
3. server sends response
4. client
response
resource
Note: clients and servers are processes running on hosts
(can be the same or different hosts).

16. 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.

17. The notion of an internet protocol
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.

18. What does an internet protocol do?
Naming scheme
The 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.

19. What does an internet protocol do?
Delivery mechanism
The 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.

20. Transferring data over an internet
protocol
software
client
LAN1
adapter
Host A
data PH
FH1
FH2
LAN2
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
internet packet
LAN2 frame
Router
LAN1 frame
server
Host B

21. We are glossing over a number of important questions:
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?

22. Other issues
These questions form the heart of the area of computer systems known as networking.

23. Most famous example of an internet.
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.

24. Based on the TCP/IP protocol family.
Global IP Internet
Based on the TCP/IP protocol family.
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 (like files) from process-to-process.
Accessed via a mix of Unix file I/O and functions from the Berkeley sockets interface.

25. Hardware and software organization of an Internet application
Internet client host
Internet server host
client
server
user code
sockets interface
(system calls)
TCP/IP
TCP/IP
kernel code
hardware interface
(interrupts)
network
adapter
network
adapter
hardware
Global IP Internet

26. Programmer’s view of the Internet
Hosts are mapped to a set of 32-bit IP addresses.
The set of IP addresses is mapped to a set of identifiers called Internet domain names.
is mapped to
A process on one host communicates with a process on another host over a connection.

27. Dotted decimal notation
By convention, each by in a 32-bit IP address is represented by its decimal value and separated by a period
IP address 0x8002C2F2 =

28. Dotted decimal notation
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.

29. 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)
/* Internet address structure */
struct in_addr {
unsigned int s_addr; /* network byte order (big-endian) */ };

30. Handy network byte-order functions:
IP Addresses
Handy network byte-order functions:
htonl: convert long int from host to network byte order.
htons: convert short int from host to network byte order.
ntohl: convert long int from network to host byte order.
ntohs: convert short int from network to host byte order.

31. Internet Domain Names mil edu gov com cmu berkeley mit cs ece
kittyhawk
cmcl
unnamed root
pdl
imperial
amazon
www
first-level domain names
second-level domain names
third-level domain names

32. Domain Naming System (DNS)
The Internet maintains a mapping between IP addresses and domain names in a distributed database called DNS.
Conceptually, we can think of the DNS database as being millions of host entry structures:

33. Domain Naming System (DNS)
/* DNS host entry structure */
struct hostent {
/* official domain name of host */
char *h_name;
/* null-terminated array of domain names */
char **h_aliases;
/* host address type (AF_INET) */
int h_addrtype;
/* length of an address, in bytes */
int h_length;
/* null-terminated array of in_addr structs*/
char **h_addr_list; };

34. 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

35. Properties of DNS host entries
Different kinds of mappings are possible:
Simple case: 1-1 mapping between domain name and IP address:
kittyhawk.cmcl.cs.cmu.edu maps to
Multiple domain names mapped to the same IP address:
eecs.mit.edu and cs.mit.edu both map to

36. Properties of DNS host entries
Different kinds of mappings are possible:
Multiple domain names mapped to multiple IP addresses:
aol.com and map to three different IP addresses
Some valid domain name don’t map to any IP address:
for example: cmcl.cs.cmu.edu

37. Domain Naming System (DNS)
Functions for retrieving host entries from DNS:
gethostbyname: query key is a DNS domain name
gethostbyaddr: query key is a an IP address

38. hostname: 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)); }

39. A socket is an endpoint of a connection
Internet connections
Clients and servers communicate by sending streams of bytes of connections:
point-to-point, full-duplex, and reliable.
A socket is an endpoint of a connection
Socket address is an IPaddress:port pair

40. A port is a 16-bit integer that identifies a process:
Internet connections
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)

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

42. Putting it all together: Anatomy of an Internet connection
connection socket pair
( :51213, :80)
server
(port 80)
client
client socket address
:51213
server socket address
:80
client host address
server host address

43. Examples of client programs How does the client find the server?
Clients
Examples of client programs
Web browsers, ftp, telnet, ssh
How does the client find the server?
The address of the server process has two parts: IPaddress:port
The IP address is a unique 32-bit positive integer that identifies the host (adapter).
dotted decimal form: 0x8002C2F2 =

44. How does the client find the server?
Clients
How does the client find the server?
The address of the server process has two parts: IPaddress:port
The port is positive integer associated with a service (and thus a server process) on that machine.
port 7: echo server
port 23: telnet server
port 25: mail server
port 80: web server

45. Using ports to identify services
server machine
client machine
service request for
:80
(i.e., the Web server)
Web server
(port 80)
client
kernel
Echo server
(port 7)
Web server
(port 80)
service request for :7
(i.e., the echo server)
client
kernel
Echo server
(port 7)

46. 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.
A machine that runs a server process is also often referred to as a “server”.

47. Berkeley 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

48. What is a socket?
A socket is a descriptor that lets an application read/write from/to the network.
Key idea: Unix uses the same abstraction for both file I/O and network I/O

49. What is a socket?
Clients and servers communicate with each other by reading from and writing to socket descriptors
Using regular Unix read and write I/O functions
The main difference between file I/O and socket I/O is how the application “opens” the socket descriptors

50. Internet-style sockets are characterized by
Key data structures
Internet-style sockets are characterized by
a 32-bit IP address and a port
Defined in /usr/include/netinet/in.h
/* Internet address */
struct in_addr {
unsigned int s_addr;
/* 32-bit IP address */ };

51. Key data structures /* Internet style socket address */
struct sockaddr_in {
unsigned short int sin_family;
/*Address family (AF_INET) */
unsigned short int sin_port;
/*Port number*/
struct in_addr sin_addr;
/*IP address*/
unsigned char sin_zero[8];
/*Pad to sizeof “struct sockaddr”*/ };

52. Key data structures
/* Generic socket address structure
(for connect, bind, and accept)*/
struct sockaddr {
unsigned short int sin_family;
/*Protocol family */
char sa_data[14]; /*address data*/ };

53. Key data structures Defined in /usr/include/netdb.h
/* Domain Name Service (DNS) host entry */
struct hostent {
char *h_name; /* official name of host */
char **h_aliases; /* alias list */
int h_addrtype; /* host address type */
int h_length; /* length of address */
char **h_addr_list;/*list of addresses */ }

54. Overview of the Sockets Interface
Client
Server
socket
bind
listen
accept
readline
writen
close
connect
connection
request
EOF
Await connection
request from
next client
open_listenfd
open_clientfd

55. accept() illustrated listenfd(3)
1. Server blocks in accept, waiting for connection request on listening descriptor listenfd.
client
server
clientfd
connection
request
listenfd(3)
client
server
2. Client makes connection request by
calling and blocking in connect.
clientfd
listenfd(3)
3. Server returns connfd from accept.
Client returns from connect. Connection is now established between clientfd and connfd.
client
server
clientfd
connfd(4)

56. Echo client int main(int argc, char **argv) { int clientfd, port;
char *host, buf[MAXLINE];
rio_t rio;
if (argc != 3) {
fprintf(stderr,"usage:%s <host> <port>\n",argv[0]);
exit(0); }
host = argv[1];
port = atoi(argv[2]);
clientfd = open_clientfd(host, port);

57. Echo client while (Fgets(buf, MAXLINE, stdin) != NULL) {
Rio_readinitb(&rio, clientfd) ;
while (Fgets(buf, MAXLINE, stdin) != NULL) {
Rio_writen(clientfd, buf, strlen(buf));
Rio_readline(&rio, buf, MAXLINE);
Fputs(buf, stdout); }
Close(clientfd);

58. Echo client: open_clientfd()
int open_clientfd(char *hostname, int port) {
int clientfd;
struct hostent *hp;
struct sockaddr_in serveraddr;
clientfd = Socket(AF_INET, SOCK_STREAM, 0);
/* fill in the server's IP address and port */
hp = Gethostbyname(hostname);
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 */
Connect(clientfd, (SA *) &serveraddr, sizeof(serveraddr));
return clientfd; }

59. Echo client: open_clientfd() (socket)
The client creates a socket that will serve as the endpoint of an Internet (AF_INET) connection (SOCK_STREAM).
socket() returns an integer socket descriptor.
int clientfd; /* socket descriptor */
clientfd = Socket(AF_INET, SOCK_STREAM, 0);

60. Echo client: open_clientfd()
The client builds the server’s Internet address.
struct hostent *hp; /* DNS host entry */
struct sockaddr_in serveraddr; /* server’s IP address */
/* fill in the server's IP address and port */
hp = Gethostbyname(hostname);
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);

61. Echo client: open_clientfd()(connect)
int clientfd; /* socket descriptor */
struct sockaddr_in serveraddr; /* server address */
/* establish a connection with the server */
Connect(clientfd, (SA *) &serveraddr, sizeof(serveraddr));

62. Echo client: open_clientfd()(connect)
Then the client creates a connection with the server
The client process suspends (blocks) until the connection is created with the server.
At this point the client is ready to begin exchanging messages with the server via Unix I/O calls on the descriptor clientfd.

63. Echo server 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 a port passed
on the command line */
listenfd = open_listenfd(port);

64. Echo server clientlen = sizeof(clientaddr);
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); }

65. Echo server: open_listenfd()
int open_listenfd(int port) {
int listenfd;
int optval;
struct sockaddr_in serveraddr;
/* create a socket descriptor */
listenfd = Socket(AF_INET, SOCK_STREAM, 0);
/* eliminates "Address already in use" error from bind. */
optval = 1;
Setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR,
(const void *)&optval , sizeof(int));

66. 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);
Bind(listenfd, (SA *)&serveraddr, sizeof(serveraddr));
/* make it a listening socket ready to accept connection requests */
Listen(listenfd, LISTENQ);
return listenfd; }

67. Echo server: open_listenfd() (setsockopt)
The socket can be given some attributes.
/* eliminates "Address already in use" error from bind. */
optval = 1;
Setsockopt(listenfd,SOL_SOCKET, SO_REUSEADDR,
(const void *)&optval , sizeof(int));

68. Echo server: open_listenfd()(setsockopt)
Handy trick that allows us to rerun the server immediately after we kill it.
Otherwise we would have to wait about 15 secs.
Eliminates “Address already in use” error from bind().
Strongly suggest you do this for all your servers to simplify debugging.

69. Echo server: open_listenfd()
Next, we initialize the socket with the server’s Internet address (IP address and port)
struct sockaddr_in serveraddr; /* server's socket addr */
/* listenfd will be an endpoint for all requests to port
on any IP address for this host */
bzero((char *) &serveraddr, sizeof(serveraddr));
serveraddr.sin_family = AF_INET;
serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
serveraddr.sin_port = htons((unsigned short)port);

70. Echo server: open_listenfd() (initialize socket address)
IP addr 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.

71. Echo server: 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 */
Bind(listenfd, (SA *)&serveraddr, sizeof(serveraddr));

72. Echo server: 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 listenf it a server-side listening socket ready to
accept connection requests from clients */
Listen(listenfd, LISTENQ);

73. 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 line from client */
/* Close(): close the connection */ }

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

75. accept()also fills in client’s address.
Echo server: accept()
accept() returns a connected socket descriptor (connfd) with the same properties as the listening descriptor (listenfd)
Returns when connection between client and server is complete.
All I/O with the client will be done via the connected socket.
accept()also fills in client’s address.

76. 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);

77. EOF notification caused by client calling close(clientfd).
Echo server: echo()
The server uses Unix I/O to read and echo text lines until EOF (end-of-file) is encountered.
EOF notification caused by client calling close(clientfd).
NOTE: EOF is a condition, not a data byte.

78. Echo server: echo() 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); }