User Datagram Protocol

User Datagram Protocol An initial look at using the UDP transport protocol for sending and receiving network packets

The Client/Server Paradigm • A great many network applications employ this asymmetrical program-design idea: client application runs on station B server application runs on station A request time response

The sockets API for server struct sockaddr_in saddr = {0}; int salen = sizeof( saddr ); saddr.sin_family = AF_INET; saddr.sin_port = htons( port ); saddr.sin_addr = htonl( INADDR_ANY ); sock = socket( AF_INET, SOCK_DGRAM, IPPROTO_UDP ); bind( sock, (sockaddr*)&saddr, salen ); struct sockaddr_in peer = {0}; int plen = sizeof( peer ); char buf[ BUFSIZ ] = {0}; recvfrom( sock, buf, BUFSIZ, 0, (sockaddr*)&peer, &plen ); sendto( sock, buf, BUFSIZ, 0, (sockaddr*)&peer, plen ); close( sock ); server

The sockets API for client struct hostent *pp = gethostbyname( peername, NLEN ); struct sockaddr_in peer = {0}; int plen = sizeof( peer ); peer.sin_family = AF_INET; peer.sin_port = htons( port ); peer.sin_addr.s_addr = *(uint32_t*)pp->h_addr; sock = socket( AF_INET, SOCK_DGRAM, IPPROTO_UDP ); char msg[ MSGSIZ ] = “Hello, world! \n”; sendto( sock, msg, MSGSIZ, 0, (sockaddr*)&peer, plen ); char buf[ BUFSIZ ] = {0}; recvfrom( sock, buf, BUFSIZ, 0, (sockaddr*)&peer, &plen ); close( sock ); client

‘udpserver.cpp’ and ‘udpclient.cpp’ • This pair of network application-programs provides us with a simple illustration of the basic paradigm: • First, launch ‘udpserver’ on station A • Then launch ‘udpclient’ on station B • We can watch the ethernet frames being sent and received using our ‘nicwatch’

The packet format Ethernet Frame Header Internet Protocol Header UDP Header application’s message This message is written to the socket by the application This header is added by the transport layer This header is added by the network layer This header is added by the link layer

The UDP header 32 bits Source port Destination port UDP Length UDP Checksum The UDP Length field is the total number of bytes of data, plus the 8 bytes that comprise this UDP Header structure The UDP Checksum field is computed using an algorithm based upon ones-complement addition of the 16-bit words in the entire UDP segment (its data and its header), along with an extra structure known as the UDP Pseudo-Header

The IP header 32 bits IP version Header length Type of Service Total Length (in bytes) Identification D M Fragment offset Time-to-Live Protocol ID-number Header Checksum Source IP-address Destination IP-address Options

The Frame header 14 bytes Destination MAC-address (6 bytes) Source MAC-address (6 bytes) Type/Length (2 bytes) An integer which describes the type of this packet, or its length in bytes The unique hardware-address for the network interface which should receive this packet The unique hardware-address for the network interface which is transmitting this packet Used for ‘filtering’ packets that are not intended for a particular host interface Needed when sending back replies to requests, and for error-notifications

Algorithm # Rough idea for a simplified ‘traceroute’ algorithm int ttl = 1; do { send UDP message toward host using ttl; receive response from router or from host; if ‘Resource temporarily unavailable’, break; if ‘No route to Host’, then show who sent it; } while ( ++ttl < 30 ); Implementing this basic ‘traceroute’ algorithm would require us to modify the value of the ‘Time-to-Live’ field in an outgoing packet’s IP-header, but doing that directly is prohibited by our lack of access to kernel data

Using ‘setsockopt()’ • There is a socket-option at the IP-Level which allows an application program to adjust the ‘Time-to-Live’ value assigned to any outgoing UDP packet’s IP header unsigned char ttl = 5; // maximum of five ‘hops’ int tlen = sizeof( ttl ); // length of the option data if ( setsockopt( sock, SOL_IP, IP_TTL, &ttl, tlen ) < 0 ) { perror( “setsockopt TTL” ); exit(1); }

Demo: ‘tweakttl.cpp’ • This program allows a user to specify the destination hostname, the port-number, and the desired ‘Time-to-Live’ value • For example: $./tweakttl stargate 54321 5 • You can watch the outgoing packet, and any ICPM reply-message, with ‘nicwatch’

User Datagram Protocol