Download TCP/IP Socket Programming

Communication Networks
Recitation 1
Netcomm 2010 - Recitation 1: Sockets
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Administrative
• Hillel Avni
– Email: [email protected]
– website:
http://www.cs.tau.ac.il/~hillelav/courses/co
mnet10.html
– Grader: Eran Cohen
– [email protected]
Netcomm 2010 - Recitation 1: Sockets
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TCP/IP Socket Programming
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What is a socket?
• An interface between application and
the network
• The application can send/receive data
to/from the network -- communicate
Application
Network API
Protocol A
Protocol B
Protocol C
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A Socket-eye view of the Internet
medellin.cs.columbia.edu
(128.59.21.14)
newworld.cs.umass.edu
(128.119.245.93)
cluster.cs.columbia.edu
(128.59.21.14, 128.59.16.7,
128.59.16.5, 128.59.16.4)
• Each host machine has an IP address
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Ports
• Each host has
65,536 ports
• Some ports are
reserved for specific
apps
Port 0
Port 1
Port 65535
– 20,21: FTP
– 23: Telnet
– 80: HTTP
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Address Pair
• An address is an IP+port
• A socket provides an interface to an
IP:port pair
Local IP: 111.22.3.4
Local Port: 2249
Remote IP: 123.45.6.78
Remote Port: 3726
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Functions needed:
• Specify local and remote
communication endpoints
• Initiate a connection
• Send and receive data
• Terminate a connection
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Socket Creation in C: socket()
• int s = socket(domain, type, protocol);
– s: socket descriptor (an integer, like a file-handle)
– domain: integer, communication domain
• e.g., PF_INET (IPv4 protocol) – typically used
– type: communication type
• SOCK_STREAM: reliable, 2-way, connection-based
service
• SOCK_DGRAM: unreliable, connectionless,
– protocol: specifies protocol (see file /etc/protocols
for a list of options) - usually set to 0
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Two essential types of sockets
• SOCK_STREAM
– a.k.a. TCP
– reliable delivery
– in-order guaranteed
– connection-oriented
– bidirectional
App
3 2
1
• SOCK_DGRAM
– a.k.a. UDP
– unreliable delivery
– no order guarantees
– no notion of “connection”
– can send or receive
D1
App
socket
Dest.
3 2
1
D2
socket
D3
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The bind() function
• associates and (can exclusively) reserves a
port for use by the socket
• Done by the server
• int status = bind(sockid, &addrport, size);
– status: error status, = -1 if bind failed
– sockid: integer, socket descriptor
– addrport: struct sockaddr, the (IP) address and
port of the machine. (address usually set to
INADDR_ANY – chooses a local address)
– size: the size (in bytes) of the addrport structure
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The struct sockaddr
• The Internet-specific:
• The generic:
struct sockaddr {
u_short sa_family;
char sa_data[14];
};
– sa_family
• specifies which
address family is
being used
• determines how
the remaining 14
bytes are used
struct sockaddr_in {
short sin_family;
u_short sin_port;
struct in_addr sin_addr;
char sin_zero[8];
};
– sin_family = AF_INET
– sin_port: port # (065535)
– sin_addr: IP-address
– sin_zero: unused
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Address and port byte-ordering
 Problem:
different machines / OS’s use different word
orderings
• little-endian: lower bytes first
• big-endian: higher bytes first
 these machines may communicate with one another
over the network

128.119.40.12
128
Big-Endian
machine
119
40
12
Little-Endian
machine
128
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12.40.119.128
40
12
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Network Byte Order
• All values stored in a sockaddr_in
must be in network byte order.
• Whenever the source of the address
isn’t the network, use htonl and htons
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Example
int sock;
sock = socket(PF_INET, SOCK_STREAM, 0);
if (sock<0) { /* ERROR */ }
struct sockaddr_in myaddr;
myaddr.sin_family = AF_INET;
myaddr.sin_port = htons( 80 );
myaddr.sin_addr = htonl( INADDR_ANY );
bind(sock, &myaddr, sizeof(myaddr));
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Connection Setup (SOCK_STREAM)
• A connection occurs between two kinds of
participants
– passive: waits for an active participant to request
connection
– active: initiates connection request to passive side
• Once connection is established, passive and
active participants are “similar”
– both can send & receive data
– either can terminate the connection
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Connection setup cont’d
• Passive participant
– step 1: listen (for
incoming requests)
– step 3: accept (a
request)
– step 4: data transfer
• The accepted
connection is on a
new socket
• The old socket
continues to listen
• Active participant
– step 2: request &
establish connection
– step 4: data transfer
Passive Participant
a-sock-1
l-sock
a-sock-2
socket
socket
Active 1
Active 2
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Connection est.: listen() & accept()
• Called by passive participant
• int status = listen(sock, queuelen);
– status: 0 if listening, -1 if error
– sock: integer, socket descriptor
– queuelen: integer, # of active participants that can
“wait” for a connection
• int s = accept(sock, &name, &namelen);
– s: integer, the new socket (used for data-transfer)
– sock: integer, the orig. socket (being listened on)
– name: struct sockaddr, address of the active
participant
– namelen: sizeof(name): value/result parameter
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Connection est.: connect()
• Called by active participant
• int status = connect(sock, &name, namelen);
– status: 0 if successful connect, -1 otherwise
– sock: integer, socket to be used in connection
– name: struct sockaddr: address of passive
participant
– namelen: integer, sizeof(name)
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Server example
int sock;
sock = socket(PF_INET, SOCK_STREAM, 0);
if (sock<0) { /* ERROR */ }
struct sockaddr_in myaddr;
myaddr.sin_family = AF_INET;
myaddr.sin_port = htons( 80 );
myaddr.sin_addr = htonl( INADDR_ANY );
bind(sock, &myaddr, sizeof(myaddr));
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Server example
listen(sock, 5);
int new_sock;
struct sockaddr_in their_addr;
sin_size = sizeof(struct sockaddr_in);
new_sock = accept(sock, (struct sockaddr
*)&their_addr, &sin_size);
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Client example
int sock;
sock = socket(PF_INET, SOCK_STREAM, 0);
if (sock<0) { /* ERROR */ }
struct sockaddr_in dest_addr;
dest_addr.sin_family = AF_INET;
dest_addr.sin_port = htons( 80 );
dest_addr.sin_addr = inet_addr(“128.2.5.10”);
connect(sock, (struct sockaddr *)&dest_addr,
sizeof(struct sockaddr));
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