Whatmechanismsareavailablefora
programmerwhowritesnetworkapplications?
SocketProgramminginC
CNT4007C,CISEdept.
UniversityofFlorida
(SlidesAdaptedon
JrnAltmannsSlides)
QuestionsthatwillbeAddressed
Howtowriteanetworkapplicationthatsends
packetsbetweenhosts(clientandserver)
acrossanIPnetwork?
Answer:socketAPI
Client
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IPNetwork
Server
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SocketProgramming
TableofContents
NetworkApplicationProgrammingInterface:
SocketsandInternetSockets
NetworkProgrammingTips
ClientServerArchitecture
Example:ClientProgramming
Example:ServerProgramming
NetworkProgrammersMistakes
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LayersoftheIPProtocolSuite
ApplicationLayer
TransportLayer
NetworkLayer
LinkLayer
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e.g.ftp
e.g.TCP,UDP
e.g.IP
Ethernet
ApplicationLayer
TransportLayer
NetworkLayer
LinkLayer
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ProtocolSuiteLocation
InternetProtocolLayer
ApplicationLayer
TransportLayer(TCP,UDP)
Location
Applications
(e.g.browser,game,ftp)
ApplicationProgramming
Interface(API)
(e.g.networkAPI)
OperatingSystem
(e.g.Unix)
NetworkLayer(IP)
InterfacetotheNetworkCard
LinkLayer
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NetworkCard&
DeviceDriver
(e.g.Ethernetcard)
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NetworkAPI
OperatingsystemprovidesApplicationProgramming
Interface(API)fornetworkapplication
APIisdefinedbyasetoffunctiontypes,data
structures,andconstants
Desirablecharacteristicsofthenetworkinterface
Simpletouse
Flexible
independentfromanyapplication
allowsprogramtouseallfunctionalityofthenetwork
Standardized
allowsprogrammertolearnonce,writeanywhere
ApplicationProgrammingInterfacefornetworksis
calledsocket
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Sockets
Socketsprovidemechanismstocommunicate
betweencomputersacrossanetwork
Therearedifferentkindofsockets
DARPAInternetaddresses(InternetSockets)
Unixinterprocesscommunication(UnixSockets)
CCITTX.25addresses
andmanyothers
BerkeleysocketsisthemostpopularInternetSocket
runsonLinux,FreeBSD,OSX,Windows
fedbythepopularityofTCP/IP
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InternetSockets
Supportstreamanddatagrampackets(e.g.TCP,UDP,
IP)
IsSimilartoUNIXfileI/OAPI(providesafiledescriptor)
BasedonC,singlethreadmodel
doesnotrequiremultiplethreads
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TypesofInternetSockets
Differenttypesofsocketsimplementdifferent
communicationtypes(streamvs.datagram)
Typeofsocket:streamsocket
connectionoriented
twowaycommunication
reliable(errorfree),inorderdelivery
canusetheTransmissionControlProtocol(TCP)
e.g.telnet,ssh,http
Typeofsocket:datagramsocket
connectionless,doesnotmaintainanopen
connection,eachpacketisindependent
canusetheUserDatagramProtocol(UDP)
e.g.IPtelephony
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Othertypesexist:similartotheoneabove
NetworkProgrammingTips
ByteOrdering
Naming
Addressing
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ByteOrderingofIntegers
DifferentCPUarchitectureshavedifferentbyte
ordering
memory
address A +1
memory
address A
Storedatlittleendiancomputer
high-order byte
Integerrepresentation(2byte)
D3
F2
low-order byte
high-order byte
Storedatbigendiancomputer
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low-order byte
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ByteOrderingProblem
Question:Whatwouldhappeniftwocomputers
withdifferentintegerbyteorderingcommunicate?
Answer:
Nothingiftheydonotexchangeintegers!
But:Iftheyexchangeintegers,theywouldgetthe
wrongorderofbytes,therefore,thewrongvalue!
Example:
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Messageis:[Hello,512]
Messageissent
acrossNetwork
MessageinMemoryof
ofbigendianComputer
Processing
MessageinMemoryof
littleendianComputer
Processing
Messageis:[Hello,1]
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ByteOrderingSolution
Therearetwosolutionsifcomputerswithdifferentbyte
orderingsystemwanttocommunicate
Theymustknowthekindofarchitectureofthesending
computer
(badsolution,ithasnotbeenimplemented)
Introductionofanetworkbyteorder.Thefunctionsare:
uint16_t
uint32_t
uint16_t
uint32_t
htons(uint16_t
htonl(uint32_t
ntohs(uint16_t
ntohs(uint32_t
host16bitvalue)
host32bitvalue)
net16bitvalue)
net32bitvalue)
Note:useforallintegers(shortandlong),which
aresentacrossthenetwork
IncludingportnumbersandIPaddresses
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NetworkProgrammingTips
ByteOrdering
Naming
Addressing
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NamingandAddressing
Hostname
identifiesasinglehost(seeDomainNameSystem
slides)
variablelengthstring(e.g.www.berkeley.edu)
ismappedtooneormoreIPaddresses
IPAddress
writtenasdottedoctets(e.g.10.0.0.1)
32bits.Notanumber!Butoftenneedstobe
convertedtoa32bittouse.
Portnumber
identifiesaprocessonahost
16bitnumber
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ClientServerArchitecture
response
Client
Server
request
Clientrequestsservicefromserver
Serverrespondswithsendingserviceor
errormessagetoclient
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SimpleClientServerExample
Client
socket()
connect()
send()
recv()
close()
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response
request
Connection
establishment
Server
socket()
bind()
listen()
accept()
Datarequest
recv()
Dataresponse
send()
Endoffilenotification
recv()
close()
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Example:ClientProgramming
Createstreamsocket(socket())
Connecttoserver(connect())
Whilestillconnected:
sendmessagetoserver(send())
receive(recv())datafromserverand
processit
CloseTCPconnectionandSocket
(close())
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socket():InitializingSocket
Gettingthefiledescriptor
int chat_sock;
if ((chat_sock = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
perror("socket");
printf("Failed to create socket\n");
abort ();
}
1.parameterspecifiesprotocol/addressfamily
2.parameterspecifiesthesockettype
Otherpossibilities:SOCK_DGRAM
3.parameterspecifiestheprotocol.
0meansprotocolischosenbytheOS.
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IPAddressDataStructure
struct sockaddr_in {
short int
unsigned short int
struct in_addr
unsigned char
};
sin_family; //Ad d re s s fa m ily
sin_port;
//P o rtnum b e r
sin_addr;
//Inte rn e ta d d re s s
sin_zero[8];
struct in_addr {
unsigned long
};
s_addr;
//4 b yte s
Paddingofsin_zeros:struct sockaddr_in
hassamesizeasstruct sockaddr
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connect():MakingTCP
ConnectiontoServer
struct sockaddr_in sin;
struct hostent *host = gethostbyname (argv[1]);
unsigned int server_address = *(unsigned long *) host->h_addr_list[0];
unsigned short server_port = atoi (argv[2]);
memset (&sin, 0, sizeof (sin));
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = server_address;
sin.sin_port = htons (server_port);
if (connect(chat_sock, (struct sockaddr *) &sin, sizeof (sin) ) < 0) {
perror("connect");
printf("Cannot connect to server\n");
abort();
}
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send():SendingPackets
int send_packets(char *buffer, int buffer_len) {
sent_bytes = send(chat_sock, buffer, buffer_len, 0);
if (send_bytes < 0) {
perror (send");
}
return 0;
}
Needssocketdescriptor,
Buffercontainingthemessage,and
Lengthofthemessage
Canalsousewrite()
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ReceivingPackets:
SeparatingDatainaStream
Fixed length
record
A
0
receive
buffer
B
1
2
C
3
4
5
D
6
7
8
9
slidethrough
length
record
Fixed Userecords(datastructures)to
partitionthedatastream
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ReceivingPackets
int receive_packets(char *buffer, int buffer_len, int *bytes_read)
{
int left = buffer_len - *bytes_read;
received = recv(chat_sock, buffer + *bytes_read, left, 0);
if (received < 0) {
buffer_len
buffer
perror (recv");
}
if (received <= 0) {
return close_connection();
}
*bytes_read
*bytes_read += received;
while (*bytes_read > RECORD_LEN) {
process_packet(buffer, RECORD_LEN);
*bytes_read -= RECORD_LEN;
memmove(buffer, buffer + RECORD_LEN, *bytes_read);
}
return 0;
}
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Canalsouseread()
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ServerProgramming:Simple
Createstreamsocket(socket())
Bindporttosocket(bind())
Listenfornewclient(listen())
While
acceptuserconnectionandcreateanew
socket(accept())
dataarrivesfromclient(recv())
datahastobesendtoclient(send())
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bind():AssignIPandPort
struct sockaddr_in sin;
struct hostent *host = gethostbyname (argv[1]);
unsigned int server_address = *(unsigned long *) host->h_addr_list[0];
unsigned short server_port = atoi (argv[2]);
memset (&sin, 0, sizeof (sin));
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = server_address;
sin.sin_port = htons (server_port);
if (bind(chat_sock, (struct sockaddr *) &sin, sizeof (sin) ) < 0) {
perror("bind");
printf("Cannot bind server application to network\n");
abort();
}
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bind():
bind()tellstheOStoassignalocalIPaddress
andlocalportnumbertothesocket.
ManyapplicationslettheOSchooseanIP
address.
UsewildcardINADDR_ANYaslocaladdressinthiscase.
Atserver,userprocessmustcallbind()to
assignaport
Atclient,bind()isnotrequiredsinceOSmay
assignavailableportandIPaddress
Theserverwillgettheportnumberoftheclientthroughthe
UDP/TCPpacketheader
Note:Eachapplicationisrepresentedbya
Cserverportnumber
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listen():WaitforConnections
int listen(int sockfd, int backlog);
Putssocketinalisteningstate,willingto
handleincomingTCPconnectionrequest.
Backlog:numberofTCPconnectionsthatcan
bequeuedatthesocket.
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ServerExample
#include<stdio.h>
#include<string.h>
#include<unistd.h>
#include<pthread.h>
#include<arpa/inet.h>
#include<sys/types.h>
#include<sys/socket.h>
#define MYPORT 3490 // the port users will be connecting to
#define BACKLOG 10 // how many pending connections queue will hold
int main(void) {
int sockfd, new_fd; // listen on sockfd, new connection on new_fd
struct sockaddr_in my_addr; // my address information
struct sockaddr_in their_addr; // connector's address information
int sin_size;
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
exit(1);
}
perror("socket");
my_addr.sin_family = AF_INET;
// host byte order
my_addr.sin_port = htons(MYPORT);
// short, network byte order
my_addr.sin_addr.s_addr = INADDR_ANY; // auto. filled with local IP
memset(&(my_addr.sin_zero), '\0', 8);
// zero the rest of the struct
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if (bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct sockaddr)) == -1) {
perror("bind");
exit(1);
}
if (listen(sockfd, BACKLOG) == -1) {
perror("listen");
exit(1);
}
while(1) {
// main accept() loop
sin_size = sizeof(struct sockaddr_in);
if ((new_fd = accept(sockfd, (struct sockaddr *)&their_addr, &sin_size)) == -1) {
perror("accept");
continue;
}
printf("server: got connection from %s\n", inet_ntoa(their_addr.sin_addr));
if (send(new_fd, "Hello, world!\n", 14, 0) == -1)
perror("send");
close(new_fd);
}
return 0;
}
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ClientExample
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h>
#include<arpa/inet.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<netdb.h>
#define PORT 3490
#define MAXDATASIZE 100
int main(int argc, char *argv[]) {
int
sockfd, numbytes;
char
buf[MAXDATASIZE];
struct hostent *he;
struct sockaddr_in their_addr;
// the port client will be connecting to
// max number of bytes we can get
// at once
// server's address information
if (argc != 2) {
fprintf(stderr,"usage: client hostname\n");
exit(1);
}
if ((he=gethostbyname(argv[1])) == NULL) { // get the host info
perror("gethostbyname");
exit(1);
}
if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
perror("socket");
exit(1);
}
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their_addr.sin_family = AF_INET;
// host byte order
their_addr.sin_port = htons(PORT);
// short, network byte order
their_addr.sin_addr = *(struct in_addr *)*he->h_addr_list; // already network byte order
memset(&(their_addr.sin_zero), '\0', 8); // zero the rest of the struct
if (connect(sockfd, (struct sockaddr *)&their_addr, sizeof(struct sockaddr)) == -1){
perror("connect");
exit(1);
}
if ((numbytes=recv(sockfd, buf, MAXDATASIZE-1, 0)) == -1) {
perror("recv");
exit(1);
}
buf[numbytes] = '\0';
printf("Received: %s",buf);
close(sockfd);
return 0;
}
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NetworkProgrammers
Mistakes
byteordering
separatingrecordsinstreams
useofselect()
misinterpretingtheprojectspecification
notknowingallavailablesystemcalls
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TherearemoreSystemCalls
Dependsoncommunicationtype
Datagramsocketsuserecvfrom()andsendto()
forreceivingandsendingdata
Closingconnection:close(),shutdown()
Convenientfunctions(onUNIX)
inet_aton,inet_ntoa
inet_pton,inet_ntop
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Compile&Run
Compile
Server
$sand>gccoserverserver.clnsllsocketlpthread
Client
$rain>gccoclientclient.clnsllsocket
Run
Server
$sand>./server
Client
$rain>./clientsand.cise.ufl.edu
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Literature
short tutorial: Beej's Guide to Network
Programming
http://www.ecst.csuchico.edu/~beej/guide/net/
http://beej.us/guide/bgnet/
UnixNetworkProgramming,volumes1and
2byW.RichardStevens.Publishedby
PrenticeHall;ISBNsforvolumes1and2:
013490012X,0130810819.
AdvancedProgrammingintheUnix
EnvironmentbyW.RichardStevens.
PublishedbyAddisonWesley.ISBN
0201563177.
manpagesonaUnixcomputer
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JavaReferences
JavaSocketProgrammingTutorials
Textbook
p159~p177
http://perleybrook.umfk.maine.edu/slides/spring
%202004/cos420/Socket_Programming.ppt
http://www.cs.rpi.edu/academics/courses/netprog/
c09.html
http://www.cs.odu.edu/~cs476/SocketProgrammin
g/java/java.htm
http://zerioh.tripod.com/ressources/sockets.html
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