Transcript TDC561 - Network Programming

TDC561
Network Programming
Week 2 – part II:
Socket Application Programming Interface – UDP Socket;
Camelia Zlatea, PhD
Email: [email protected]
UDP Clients and Servers
 Connectionless clients and servers create a socket using
SOCK_DGRAM instead of SOCK_STREAM
 Connectionless servers do not call listen() or accept(), and
usually do not call connect()
 Since connectionless communications lack a sustained
connection, several methods are available that allow you to
specify a destination address with every call:
– sendto(sock, buffer, buflen, flags, to_addr,
tolen);
– recvfrom(sock, buffer, buflen, flags, from_addr,
fromlen)
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Datagram Socket Transaction (UDP Connection)
Server
Client
socket()
socket()
bind()
sendto()
data
recvfrom()
data
sendto()
recvfrom()
close()
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UDP Client and Server Client Flows
 Client Flow
– Create UDP socket.
– Create sockaddr with address of server.
– Call sendto(), sending request to the server.
– If a reply is needed, call recvfrom()
 Server Flow
– Create UDP socket and bind to well known address.
– Call recvfrom() to get a request, notifying the address of the client.
– Process request and send reply back with sendto().
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Creating a UDP socket
int socket(int family,int type,int proto);
int sock;
sock = socket(AF_INET,
SOCK_DGRAM,
0);
if (sock<0) { /* ERROR */ }
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Binding to well known address
int mysock;
struct sockaddr_in myaddr;
mysock = socket(AF_INET,SOCK_DGRAM,0);
myaddr.sin_family = AF_INET;
myaddr.sin_port = htons( 6990 );
myaddr.sin_addr = htonl( INADDR_ANY );
bind(mysock, &myaddr, sizeof(myaddr));
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Sending UDP Datagrams
ssize_t sendto( int sockfd,
void *buff,
size_t nbytes,
int flags,
const struct sockaddr* to,
socklen_t addrlen);
sockfd is a UDP socket
buff is the address of the data (nbytes long)
to is the address of a sockaddr containing the destination address.
Return value is the number of bytes sent, or -1 on error.
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Sending UDP Datagrams
 Return Value sendto()
– indicates how much data was accepted by the Kernel (OS) for
sending as a datagram
– does NOT say how much data actually reached the destination.
 Errors from sendto()
EBADF, ENOTSOCK: bad socket descriptor
EFAULT: bad buffer address
EMSGSIZE: message too large
ENOBUFS: system buffers are full
 No error condition to indicate that the destination did not get receive
the datagram
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Receiving UDP Datagrams
ssize_t recvfrom( int sockfd,
void *buff,
size_t nbytes,
int flags,
struct sockaddr* from,
socklen_t *fromaddrlen);
sockfd is a UDP socket
buff is the address of a buffer (nbytes long)
from is the address of a sockaddr.
Return value is the number of bytes received and put into buff, or -1 on error.
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Receiving UDP Datagrams




If buff is not large enough, any extra data is lost
The sockaddr at from is filled in with the address of the sender.
The fromaddrlen should be set before invoking.
If from and fromaddrlen are NULL there is no way to identify who
sent the data.
 Same errors as sendto, but also:
– EINTR: System call interrupted by signal.
 Blocking operation ( by default )
– recvfrom doesn’t return until there is a datagram available
– waiting/blocking time can be indefinite
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UDP Echo Server
int mysock;
struct sockaddr_in myaddr, cliaddr;
char msgbuf[MAXLEN];
socklen_t clilen;
int msglen;
mysock = socket(AF_INET,SOCK_DGRAM,0);
myaddr.sin_family = AF_INET;
myaddr.sin_port = htons( S_PORT );
myaddr.sin_addr = htonl( INADDR_ANY );
bind(mysock, &myaddr, sizeof(myaddr));
while (1) {
len=sizeof(cliaddr);
msglen=recvfrom(mysock,msgbuf,MAXLEN,0,cliaddr,&clilen);
sendto(mysock,msgbuf,msglen,0,cliaddr,clilen);
}
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UDP Daytime Client and Server
 Server waits for requests on a known port.
 Client sends a UDP request to server.
 Server responds with daytime info.
 No connection establishment!
 No reliability!
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UDP Server
#include
#include
"unp.h"
<time.h>
int main(int argc, char **argv)
{
int
sockfd, clilen;
struct sockaddr_in servaddr, cliaddr;
char buff[MAXLINE], req[REQ_LEN];
time_t ticks;
/* Create a socket */
sockfd = Socket(AF_INET, SOCK_DGRAM, 0);
/* Initialize server’s address and well-known port */
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family
= AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port
= htons(13);
/* daytime server */
/* Bind server’s address and port to the socket */
Bind(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr));
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UDP Server
for ( ; ; ) {
/* Wait for client request */
len = sizeof(cliaddr);
n = Recvfrom( sockfd, req, REQ_LEN, 0, &cliaddr, &clilen);
/* Retrieve the system time */
ticks = time(NULL);
snprintf(buff, sizeof(buff), "%.24s\r\n", ctime(&ticks));
/* Send to client*/
Sendto(sockfd, buff, strlen(buff), 0, &cliaddr, clilen);
}
}
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UDP Client
#include
"unp.h"
int main(int argc, char **argv)
{
int sockfd, n, servlen;
char req[10], recvline[MAXLINE + 1];
struct sockaddr_in servaddr;
if( argc != 2 )err_quit(“usage : gettime <IP address>”);
/* Create a UDP socket */
sockfd = Socket(AF_INET, SOCK_DGRAM, 0);
/* Specify server’s IP address and port */
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(13); /* daytime server port */
Inet_pton(AF_INET, argv[1], &servaddr.sin_addr);
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UDP Client
/* Send message to server requesting date/time */
strcpy(req, “GET_TIME”);
Sendto(sockfd, req, strlen(req), 0, (struct sockaddr *)&servaddr,
sizeof(servaddr));
/* Read date/time from the socket */
servlen = sizeof(servaddr);
n= Recvfrom(sockfd, recvline, MAXLINE, 0, (struct sockaddr
*)&servaddr, &servlen);
recvlen[n] = 0;
printf(“%s”, recvlen);
close(sockfd);
}
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Background Slides
UDP Protocol - Terms and Concepts
TCP/IP Summary
 IP: network layer protocol
– unreliable datagram delivery between hosts.
 UDP: transport layer protocol - provides fast / unreliable datagram
service. Pros: Less overhead; fast and efficient
– minimal datagram delivery service between processes.
– unreliable, since there is no acknowledgement of receipt, there is no way to
know to resend a lost packet
– no built-in order of delivery, random delivery
– connectionless; a connection exists only long enough to deliver a single
packet
– checksum to guarantee integrity of packet data
 TCP: transport layer protocol . Cons: Lots of overhead
– connection-oriented, full-duplex, reliable, byte-stream delivery service
between processes.
– guaranteed delivery of packets in order of transmission by offering
acknowledgement and retransmission:
– sequenced delivery to the application layer, by adding a sequence number to
every packet.
– checksum to guarantee integrity of packet data
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End-to-End (Transport) Protocols
 Underlying best-effort network
– drops messages
– re-orders messages
– delivers duplicate copies of a given message
– limits messages to some finite size
– delivers messages after an arbitrarily long delay
 Common end-to-end services
– guarantee message delivery
– deliver messages in the same order they are sent
– deliver at most one copy of each message
– support arbitrarily large messages
– support synchronization
– allow the receiver to apply flow control to the sender
– support multiple application processes on each host
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UDP
Application
process
Application
process
Application
process
Ports
Queues
Packets
demultiplexed
UDP
Packets arrive
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UDP
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Simple Demultiplexor
Unreliable and unordered datagram service
Adds multiplexing
No flow control
Endpoints identified by ports
– servers have well-known ports
– see /etc/services on Unix

Optional checksum
– pseudo header + udp header + data
UDP Packet Format
Header

0
16
31
Src Port Address
Dst Port Address
Checksum
Length of DATA
DATA
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Datagram Sockets
 Connectionless sockets, i.e., C/S addresses are passed along
with each message sent from one process to another
 Peer-to-Peer Communication
 Provides an interface to the UDP datagram services
 Handles network transmission as independent packets
 Provides no guarantees, although it does include a checksum
 Does not detect duplicates
 Does not determine sequence
– ie information can be lost, wrong order or duplicated
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