Transcript lesson5

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:
request
time
server
application
runs on
station A
response
client
application
runs on
station B
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 );
server
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 );
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 );
client
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 );
‘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
Internet Protocol
Frame Header
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
Header
version length
Type of
Service
Identification
Time-to-Live
Total Length
(in bytes)
Fragment offset
D M
Protocol
ID-number
Header Checksum
Source IP-address
Destination IP-address
Options
The Frame header
14 bytes
Destination MAC-address
(6 bytes)
The unique hardware-address
for the network interface which
should receive this packet
Used for ‘filtering’ packets
that are not intended for
a particular host interface
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
is transmitting this packet
Needed when sending back replies to
requests, and for error-notifications
Algorithm
# Rough idea for a simplified ‘traceroute’ algorithm
int
do
ttl = 1;
{
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
int
ttl = 5;
// maximum of five ‘hops’
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’