1. Internet and Intranet Protocols and Applications
Lecture 3: Network Programming
Feb 8, 2000
Arthur P. Goldberg
Computer Science Department
New York University

2. Client/Server Paradigm
Basis for most application layer communications
Client (typically)
Initiates communication
Sends requests and receives responses
Interacts with just a few servers at a time
Server (typically)
Waits for incoming requests
Receives requests and sends responses
Interacts with many clients concurrently
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3. How do applications use the transport layer?
Conceptual operations (Comer 4.3)
Allocate local resources for communications
Specify local and remote communications endpoints
Initiate a connection (client)
Wait for an incoming connection (server)
Send or receive data
Terminate a connection
Handle connection termination from the remote site
Abort communications
Handle error conditions or a connection abort
Release local resources when communication finishes
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4. Design CHOICES for Networking API
Assume networking is in the OS
Which system calls?
Define a new set of networking calls
Use existing I/O calls
As in Unix and Win32
Which protocols?
Specific
Generic
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5. Unix system calls, review
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Comer, Fig 4.2

6. Network Client System Calls
socket_descriptor = socket ( family, type, protocol );
rc = connect ( sd, (& sockaddr_in) addr, addr_len );
rc = write ( sd, (& char) buf, buf_len );
rc = read ( sd, (& char) buf, buf_len );
rc = close ( sd );
rc = shutdown ( sd, direction );
unspecified types are int.
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7. Addressing IP Port (& hostent) gethostbyname ((& char) name );
rc = gethostname ((& char) name, namelen );
Port
(& servent) getservbyname ((& char) name, (& char) name );
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8. Minimal TCP Client in C
/* BUGS: NO ERROR CHECKING; DESIGN FLAW: NO MODULARITY */
#include "minimal_tcp.h"
/* usage: TCP_client domain_name port_num */
void main( int argc, char *argv[] ) {
struct hostent *phe; struct protoent *ppe;
struct sockaddr_in sin;
char c; int j, socket_desc; char hi[] = "hello world\n";
phe = gethostbyname( argv[1] );
memcpy ( &sin.sin_addr, phe->h_addr, phe->h_length );
sin.sin_port = atoi(argv[2]); sin.sin_family = AF_INET;
ppe = getprotobyname("tcp");
socket_desc = socket( PF_INET, SOCK_STREAM, ppe->p_proto );
(void) connect( socket_desc, &sin, sizeof(sin) );
(void) write( socket_desc, hi, strlen( hi ) );
for( j = 0; j < strlen( hi ); j++ ) {
(void) read( socket_desc, &c, 1 );
putc ( c , stdout ); }
(void) close( socket_desc );
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9. Minimal TCP Client, include file
/* Comer procedure 7.7 */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
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10. IBQ Assume packets arrive reliably
What are these programs’ minimum duration, in round trip delays?
What are these programs’ maximum duration?
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11. Client design issues Parameterize, with respect to host and server
Connection-based or connectionless
Applications should use TCP unless it
Does not need reliability
Require broadcast or multi-cast
Cannot tolerate TCP’s overhead
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12. Reasons for New Networking System Calls in the API
Greater control over protocol
Eg., send data with SYN message
Performance
Eg., pre-compute checksum for TCP messages
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