CS558 Sylabus

Sending and receiving with UDP sockets

Sending Packets using a UDP port

System call to send data with (bound) UDP socket:

int sendto(int sock, void *data, int len, // Transmitted data int flags, // Transmit options struct sockaddr *to, int to_len); // Destination network address

Meaining of the parameters:

sock = socket variable (should be bounded) data = pointer to data buffer location len = number bytes in data buffer to be sent flag = flags (indicate send options) E.g., Set MSG_OOB bit for "out-of-band" (high priority) data to = Internet address (typed) variable contains: network address of the destination to_len = length of the "to" data structure

Effect:

Transmits a UDP message containing the data to the specify destination (IP address + UDP port#)
The UDP mesasge will contain the network address (IP addr + UDP port#) of the socket s

Graphically:

Return value:

Error: returns -1 Otherwise: returns len (≥ 0)

General error processing:

If a system call (such as sendto returns an error ), you can use:
to print the error message to the terminal

Example: send the string "Hello"

char data[] = "Hello"; // String has 6 characters: H e l l o \0 !!! struct sockaddr_in dest_addr; // Internet address type /* ----------------------------------------- Set up destination: (IPaddress, Port#) ----------------------------------------- */ dest_addr.sin_family = AF_INET; dest_addr.sin_addr.s_addr = htonl(DestIPaddr); dest_addr.sin_port = htons(DestPosrtNumber); if ( sendto(s, data, 6, 0 /* no flags */, (struct sockaddr *)&dest_addr, sizeof(dest_addr)) == -1 ) { perrer("sendto()"); }

Example Program: (Demo above code)

Prog file: click here

How to run the program:

Right click on link(s) and save in a scratch directory
To compile: gcc -o udp4-s1 udp4-s1.c
To run: udp4-s1 Dest-IP-addr Dest-Port#
Example: udp4-s1 2827975501 (=170.140.150.1) 9000

Note:

use host NameOfComputer and bc to compute the destination IP address (in binary)
Warning: you won't see anything....

Remarks about the UDP send program

Important observation:

When you run the above demo program, you should notice that:
This can be done because:

Analogy:

sendto( ) is like mailing a letter
The sender does not know the result of the transmission

Sending with an unbounded UDP socket

Hacker's note:

It is possible to send UDP messages using an unbounded UDP socket:

int IPAddress; (a 32 bit IP address) short PortNumber; (a 16 bit port number) int s; struct sockaddr_in in_addr; s = socket(AF_INET, SOCK_DGRAM, 0); // Create socket // s is unbounded /* -------------------------------- OMIT the bind step -------------------------------- */ in_addr.sin_family = AF_INET; /* Protocol domain */ in_addr.sin_addr.s_addr = htonl(IPAddress); /* IP address of socket */ in_addr.sin_port = htons(PortNumber); /* UDP port # of socket */ /* -------------------------------- Bind socket to socket address -------------------------------- */ if ( bind(s, (struct sockaddr *)&in_addr, sizeof(in_addr)) != 0 ) perror("Bind error: "); /* ----------------------------------------- Set up destination: (IPaddress, Port#) ----------------------------------------- */ dest_addr.sin_family = AF_INET; dest_addr.sin_addr.s_addr = htonl(DestIPaddr); dest_addr.sin_port = htons(DestPosrtNumber); if ( sendto(s, data, 6, 0 /* no flags */, (struct sockaddr *)&dest_addr, sizeof(dest_addr)) == -1 ) { perrer("sendto()"); }

(An unbounded socket is a socket variable s where you has not called bind(s, ....)

Effect of sending with an unbounded socket:

UDP Messages sent with an unbounded UDP socket is:

Possible results:

Some system will bind the socket (to an available UDP port) first
Other systems will send UDP messages with:
(I.e.: anonymous IP messages !!!)