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Client designs Connection-oriented clients

Connectionless clients

Client examples TCP DAYTIME

UDP TIME

TCP and UDP ECHO

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hostentry

ip address(32b)

ip addressdot.decimal

gethostbyname

gethostbyaddr

inet_aton

serviceentry

port #(16b)

getservbyname

protocolentry

portocol #getprotobyname

getsockname

getsockopt

setsockopt

host nt

addr type

name

alias

length

addr list

icmp 1igmp 2tcp 6udp 17rsvp 46sctp 132raw 255

s rv nt

port

name

alias list

protocol

proto nt

protocol #

name

alias list

http 80dns 53smtp 25telnet 23daytime 13ftp 21

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Domain Name System (DNS) System configuration files, e.g.

/etc/hosts, /etc/services, /etc/resolv.conf

Convert between names and numeric values gethostbyname() and gethostbyaddr()

Convert between hostnames and IPv4 addresses

getservbyname() and getservbyport()

Convert between service names and port numbers getaddrinfo() and getnameinfo()

Two protocol-independent functions

Convert between hostnames and IP addresses andbetween service names and port numbers

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Typical arrangement of clients, resolvers andname servers

/etc/resolv.conf(IP addresses oflocal name servers)

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#include

struct hostent *gethostbyname(const char *hostname);

struct hostent {

char *h_name; /* official (canonical) name of host */

char **h_aliases; /* ptr to array of ptrs to alias names */

int h_addrtype; /* host address type: AF_INET */

int h_length; /* length of address: 4 */

char **h_addr_list; /* ptr to array of ptrs with IPv4 addrs */

}

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#include

struct servent *getservbyname(const char *servname,

const char *protoname);

struct servent *getservbyport(int port,

const char *protoname);

struct servent {

char *s_name; /* official service name */

char **s_aliases; /* aliases list */

int s_port; /* port number, network byte order */int s_proto; /* protocol to use */

}

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struct servent *sptr;

sptr = getservbyname(domain, udp); // DNS using UDP

sptr = getservbyname(ftp, tcp); // FTP using tCP

sptr = getservbyname(ftp, udp); // This call will fail

sptr = getservbyport(htons(21), tcp); // FTP using TCP

sptr = getservbyport(htons(21), NULL); // FTP using TCP

sptr = getservbyport(htons(21), udp); // This call will fail

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1. Find the IP address and protocol portnumber of the server

2. Allocate a socket

3.

Specify the local port number or allow TCPto choose one

4. Connect the socket to the server

5. Communicate with the server

6. Close the connection

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1a) Find servers IP address Converting from dotted decimal to 32-bit binary

notation#include

int inet_aton(const char *strp, struct in_addr *inp);in_addr_t inet_addr(const char *strp); // deprecated

Converting from symbolic name to 32-bit binarynotation

#include

struct hostent *gethostbyname(const char *hostname);

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1b) Find servers port number by service#include

struct servent *getservbyname(const char *name,

const char *protoname);

namedesired service name, e.g. smtp protonametransport protocol being used, e.g. tcp

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2. Allocate a socket#include

#include

int sd; /* socket descriptor */

sd = socket(PF_INET, SOCK_STREAM, 0);

// valid only if sd > 0

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3. Specify local port # or allow TCP to choose one Which port number is right for client?

Users may choose the specific port Dont have to use a known port number

Possible, cause an error if the port is in use

struct sockaddr_in sockinfo;

sockinfo.sin_port = htons(8000);

Or let the system choose one (recommended)

sockinfo.sin_port = htons(0);

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4. Connect the socket to the server#include

int connect(int sockfd,

const struct sockaddr *servaddr,

socklen_t addrlen);

What does it do? Ensure socket is valid and unconnected

Enter remote endpoint into socket descriptor table Select and enter local endpoint information

Complete 3-way handshake and establish TCPconnection

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Why there is no source address specified inthe interface? The system chooses the right IP address for you

Otherwise, users should figure out it if there are

multiple network interfaces available Not sure which IP address should be used in the local

address

Wrong address could result in inefficient routingcausing significant delays

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5. Communicate with the server Reading from the connection

recv(socket, msg, msglen, flags) or

read(socket, msg, msglen)

Writing into the TCP connection send(socket, msg, msglen, flags) or

write(socket, msg, msglen)

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TCP is a stream oriented protocol TCP packets could be fragmented during transmission

Need to loop until all the data are receivedn = recv(sd, buf, buflen, 0);

while (n != SOCKET_ERROR && n != 0) {

buf += n;

buflen -= n;

n = recv(sd, buf, buflen, 0);

}

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6. Close the connection What if there are outstanding data to

send/receive when closing the connection?

Better coordination: Partial close#include

int shutdown(int sockfd, int howto);

SHUT_RD no further reception allowed

SHUT_WR no further transmission allowed

SHUT_RDWR no further receptions & transmission allowed

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close() Only closes the socket ID for the process

Leaves the connection open if there is anotherprocess using the same socket

shutdown() Breaks the connection for all the processes using

the socket

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sd= connect();if (fork() == 0) { /* Child: send input */

while (gets(buf) > 0) {

write(sd, buf, strlen(buf));

}

close(sd);

exit(0);} else { /* Parent: read from server */

while ((datalen = read(sd, buf, buflen)) > 0) {

do_something(buf, datalen);

}

wait(0);exit(0);

}

sdis used by both read and write processes.

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1. Find the IP address and protocol portnumber of the server

Similar to connection-oriented clients

2. Allocate a socket

socket(PF_INET, SOCK_DGRAM, 0);

3. Specify the local port number or allow UDPto choose one

Like connection-oriented clients

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4. Specify the server to which the message issent

5. Communicate with the server Unconnected UDP sockets

Connected UDP sockets

6. Close the socket or partial close the socket

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Socket is NOT tied to a specific server Should specify remote address every time called

Can interact with multiple servers

sendto(socket, msg, msglen, flags, to, tolen)

recvfrom(socket, buf, buflen, flags,

from, fromlen)

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The socket is tied to a specific server afterconnect call Remote endpoint info is stored in socket descriptor

table

Dont have to specify remote address every time called recv() and send() can be used

Interact with only one server

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What should be different? What should becareful in UDP programming? What if the packet is dropped?

What if the packet is delivered in out of order?

For reliability Packet sequencing

Acknowledgements

Timeouts

Retransmission

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DAYTIME service TCP client for DAYTIME

TIME service UDP client for the TIME service

ECHO service TCP and UDP clients for the ECHO service

Use common procedures

connectTCP(host, service) connectUDP(host, service) connectsock(host, service, transport) errexit(format, )

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Available as TCP and UDP services at port 13 How to support both TCP and UDP services?

E.g. select(), multi-threaded, or multi-process

select()

Poll multiple file/socket descriptors Allow to handle whichever comes available first

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Server sends daytime as a character string Example:

Wednesday, September 10, 2014 18:30:40-PDT

TCP version Server sends daytime message whenever it accepts

a connection

UDP version Server sends daytime message whenever it receives

a datagram

Example code TCPdaytime.c in the textbook

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Available as TCP and UDP services at port 37 Server responds with time in Universal

Coordinated Time (UCT) 32-bit integer as the number of seconds since

January 1, 1900.

Requesting the time TCP: connection triggers a response

UDP: client datagram triggers a response

Example code: UDPtime.c

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Available as TCP and UDP services at port 7 Test connectivity

debug protocols

find routing problems

Server sends back to client all the data itreceives from the client

Example code: TCPecho.c and UDPecho.c