Transcript Lecture3.pptx

CSE 30264
Computer Networks
Prof. Aaron Striegel
Department of Computer Science & Engineering
University of Notre Dame
Lecture 3 – January 14, 2010
Implementing Network Software
Outline
Sockets
Example
Homework 2
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Communication Paradigms
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Stream Paradigm (TCP)
• Stream
– Sequence of bytes flows from one
application program to another
– Think of it like a tube of data
• Bi-directional
– Streams in both directions
– Client to server, server to client
firefo
x
Port: 7583
129.74.200.5
• No boundaries
IIS
Port: 80
129.74.120.75
– Streaming protocol does not
understand boundaries
– Stream of bits
• No size limits*
– One byte -> large blocks of data
– * From the application perspective
firefo
x
129.74.200.5, 7583
IIS
129.74.120.75, 80
• Protocol selects delivery size
– Combine smaller blocks
– Divide large blocks
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Message Paradigm (UDP)
• Message paradigm
– Network accepts and delivers messages
– Datagram service
Shoutcast Port: 8000
Port: 42083
firefox
• Messages are self-contained
– Protocol simply forwards
– No re-assembly, highly aware of packetization
– N bytes in a message, N bytes
received on the other side
– Size limit on messages
62.74.89.71
129.74.200.10
• May be uni-directional or bi-directional
• Spray and pray – no guarantees
– May be lost
– Delivered out-of-order
– May be duplicated
Shoutcast
Destination
129.74.200.10,
42083
• More burden on programmer
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Sockets
Operating
System
• Socket programming
– Adapter that gets an app
onto the network
– API – Application Programming
Interface
– Give me a TCP socket,
hold the aggregation
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Socket
Network
Stack
Device
Drivers
Physical
Adapter
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Socket Operations
• Dependent on type of socket
– Different ops for TCP vs. UDP
• Supported operations
– Specify local/remote communication endpoints
• Who / where -> IP address / port
– Initiate a connection
• Ring the telephone
– Wait for a connection
• Server
– Send/receive data
– Handle incoming data
– Terminate connection gracefully
– Handle connection termination from remote site
– Abort communication
– Handle error conditions or connection abort
– Allocate and release local resources
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Not to be confused
with Sprockets
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Socket Communication
• Very similar to file I/O
– File I/O
– Open, close, read, write, seek, fcntl, ...
• Network communication
– File descriptor for network comms
• Socket is just another file handle
– Is a network connection really a file?
• Initiate, respond to aborted connection
• Socket API was born
– Nominally the same between C and C++
– Similar beyond C, C++
• Java, Perl, PHP, C#
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Upcoming Programming
• Homework 2 - Implement a UDP client
– Connect to a server
– Send a message
– Receive a message
• Project 1 – Implement Client / Server
– Small scale TCP
• Google is your friend
– Just document where you get help from
– C UDP Socket Tutorial
http://www.cs.ucsb.edu/~almeroth/classes/W01.176B/hw2/examples/udp-client.c
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Refresh -> Client-Server Model
•
•
•
•
Server listens for requests from clients
Server: passive open
Client: active open
request
Example:
response
–
–
–
–
–
–
file server
web server
print server
mail server
name server
X window server
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SERVER
CLIENTS
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Overview - TCP
SERVER
socket()
bind()
CLIENT
listen()
socket()
accept()
blocks until connection from client
connection establishment
read()
data request
connect()
write()
process request
write()
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data reply
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read()
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Overview - UDP
SERVER
socket()
CLIENT
socket()
recvfrom()
data
sendto()
sendto()
data
recvfrom()
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Socket Creation
int socket(int family,
int type,
int protocol);
• Return value:
– Socket descriptor
– Use for all subsequent operations
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Socket Creation
#include <sys/types.h>
#include <sys/socket.h>
int s;
s = socket (PF_INET, SOCK_STREAM, 0);
Default
Protocol Family –
Internet
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Socket Type –
Streaming Socket
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Socket Creation
• Family
– PF_INET, PF_UNIX, ...
– Also may be listed as AF_INET (Address Family)
• Socket Types
– Datagram
UDP SOCK_DGRAM
– Reliable stream TCP SOCK_STREAM
– Raw
IP
SOCK_RAW
• Must be root to use
• Protocol
– Typically 0 = default protocol for type
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Endpoint Addresses
• Different protocols may have different representations.
• TCP/IP uses combination of IP address and port.
• Sockets offer different ‘address families’
– TCP/IP uses a single address representation (AF_INET).
• PF_INET, AF_INET often confused
– Use same numeric value.
– Most code
#define PF_INET AF_INET (Linux)
Bind for the local one
Two addresses
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Connect to the remote one
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Socket Addresses
(1)
struct sockaddr {
sa_family_t sa_family;
char sa_data[14];
}
/* sys/socket.h */
/* address family */
/* addressing information */
(2)
struct sockaddr_in {
short sin_family;
u_short sin_port;
struct sin_addr sin_addr;
}
/* AF_INET */
/* network byte order */
/* network address */
Two options
(1) is most common, (2) is actually the recommended approach
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Refresher - C
•
•
•
•
•
•
•
Types – char, short, long
Pointers
size
struct
typecasting
Memset
Endian-ness
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See board in class /
wiki for more examples
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Binding the Local Address
int bind(int s,
struct sockaddr *addr,
int addresslen);
• Socket has no notion of endpoint addresses when
created
– Neither local nor remote
• Necessary on the server side
– Need to specify the local endpoint
• Not always necessary for the client
– May just need to send data towards a destination
– Let OS auto-assign
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Binding the Local Address
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int s;
struct sockaddr_in sin;
s = socket(PF_INET, SOCK_DGRAM, 0);
memset((char *)&sin, 0, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_port = htons (6000);
/* 0 -> let system choose */
sin.sin_addr.s_addr = htonl(INADDR_ANY); /* allow any interface */
bind (s, (struct sockaddr *)&sin, sizeof(sin));
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Breaking the Code Down
memset((char *)&sin, 0, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_port = htons (6000);
/* 0 -> let system choose */
sin.sin_addr.s_addr = htonl(INADDR_ANY); /* allow any interface */
Take the struct and initialize it to zeroes
memset((char *)&sin, 0, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_port = htons (6000);
/* 0 -> let system choose */
sin.sin_addr.s_addr = htonl(INADDR_ANY); /* allow any interface */
Set the address family to Internet
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Breaking the Code Down
memset((char *)&sin, 0, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_port = htons (6000);
sin.sin_addr.s_addr = htonl(INADDR_ANY);
htons – Host to
network short
The port should be set to 6000
memset((char *)&sin, 0, sizeof(sin));
sin.sin_family = AF_INET;
sin.sin_port = htons (6000);
sin.sin_addr.s_addr = htonl(INADDR_ANY);
htonl – Host to
network long
Just use a local address, I don’t care which one
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Converting a String
• What if we need to specify an address?
– inet_addr(char *)
– Takes a string with the dot notation, converts to a
four byte IPv4 value
inet_addr(“129.74.56.5”)
inet_addr(argv[1])
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Breaking the Code Down
bind (s, (struct sockaddr *)&sin, sizeof(sin));
Bind our socket s to the information in the socket
address structure sin and oh by the way, here is the
size of that struct too
Typecast – oh yeah!
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netstat
• HW 1
– netstat –ra
• Routing table
• Shows socket
usage
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setsockopt()/getsockopt()
• May need to tweak the socket
– How long to wait to purge a socket?
• Default is two MSL (Maximum Segment Lengths)
• If not properly terminated, bind() will return EADDRINUSE.
setsockopt(int s, int level, int optname,
const void *optval, int optlen);
getsockopt(int s, int level, int optname,
void *optval, int *optlen);
• Before bind:
setsockopt(s, SOL_SOCKET, SO_REUSEADDR,
(char *)&opt, sizeof(opt));
• Allows re-use of port in use.
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Why reuse ports?
• What if your code
crashes?
• Otherwise, you wait …
wait … wait … wait
• When we write servers,
you will get a range of
ports to use
• Rogue programs
– ps –A –f
kill XXXX (XXXX is PID)
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What do we have so far?
• socket()
– Creates the local “handle”
– Identifier for all socket operations
• bind()
– Allows us to pick the endpoint address
• Port number
• Potentially address on a multi-interface machine
– Must be done on the server
• setsocketopt, getsocketopt
– Change socket options
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Contrast – TCP, UDP
SERVER
TCP Only
socket()
TCP, UDP
bind()
CLIENT
listen()
socket()
accept()
blocks until connection from client
connection establishment
read()
data request
connect()
write(), sendto()
process request
write()
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data reply
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read(), recvfrom()
Technically,
it is possible
to use connect
with UDP
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Overview - UDP
SERVER
socket()
CLIENT
socket()
recvfrom()
data
sendto()
sendto()
data
recvfrom()
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connect()
int connect(int s,
struct sockaddr *addr,
int namelen);
• Client issues connect() to
– Establish remote address and port
– Establish connection
• Fails if host not listening to port
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Class Exercise
• Sketch the code for a connect call to the following
location:
IP Address: 192.168.4.6
Port: 89
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UDP – Send / Receive Data
• UDP -> Spray and Pray
– sendto -> Send a message
– recvfrom -> Listen on the port for a message
• Key parameters
–
–
–
–
–
Socket
Buffer pointer
Buffer size (bytes)
Socket address
Size of socket address struct
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recvfrom
ssize_t recvfrom(int socket,
void *buffer,
size_t length,
int flags,
struct sockaddr *address,
socklen_t *address_len);
Socket
Pointer to a pre-allocated buffer
Size of the buffer (to avoid overflow)
Flags (usually 0)
Address struct – where the packet came from
Length of the address
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May be NULL
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sendto
ssize_t sendto(int socket,
const void *message,
size_t length,
int flags,
const struct sockaddr *dest_addr,
socklen_t dest_len);
Socket
Pointer to a pre-allocated buffer
Size of the buffer (to avoid overflow)
Flags (usually 0)
Address struct – where the packet came from
Length of the address
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In-Class Derivation
• Write a small snippet of code to send a packet via
UDP
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Where next?
• UDP – protocol is up to programmer
– How many messages?
– Who talks when?
– What type of message?
Homework 2: Write a simple UDP client
Read a small text file
Send to the server
Read / display the response
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Revisiting TCP – Server Side
SERVER
TCP Only
socket()
TCP, UDP
bind()
CLIENT
listen()
socket()
accept()
blocks until connection from client
connection establishment
read()
data request
connect()
write(), sendto()
process request
write()
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data reply
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read(), recvfrom()
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listen()
int listen(int s, int backlog);
• Only for stream sockets.
• Socket being listened to can’t be used for client.
• listen() does not wait for connections, but needed
to receive connection attempts.
• Completes 3-way handshake.
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listen()
• Allows backlog pending connection requests
(SYN, completed 3WH) while waiting for
accept().
• Queue full: SYN requests are silently dropped.
• “SYN-flood”: send fake TCP SYN requests to fill
queue.
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TCP Preview
client
port=64444
server
port=9
active open
connect()
SYN j
(64444, 9)
listen()
SYN k
ACK j+1
(9, 64444)
ACK k+1
active close
close()
(64444, 9)
FIN
ACK
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accept()
int accept(int s,
struct sockaddr *addr,
int *addrlen);
• By connection-oriented server, after listen().
• Can’t preview connection: accept and close.
• Returns a new socket.
• Returns address of client.
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Sending/Receiving
• TCP
– read/write: send/receive, no explicit address
– send/recv: send/receive, flags
• UDP
–
–
–
–
sendto: specify destination explicitly
recvfrom: also receive address of peer
sendmsg: msghdr data structure, scatter/gather
recvmsg: msghdr data structure, scatter/gather
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Example: TCP Client
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
int main(int argc, char *argv[]) {
int s, n;
struct sockaddr_in sin;
char msg[80] = “Hello, World!”;
if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
perror(“socket”); return(-1);
}
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Example: TCP Client
sin.sin_family = AF_INET;
sin.sin_port = htons(atoi(argv[2]));
sin_addr.s_addr = inet_addr(argv[1]);
if (connect(s, (struct sockaddr *) &sin, sizeof(sin)) < 0) {
perror(“connect”); return -1;
}
if (write(s, msg, strlen(msg)+1) < 0) {
perror(“write”); return -1;
}
if ((n = read(s, msg, sizeof(msg))) < 0) {
perror(“read”); return -1;
}
printf(“%d bytes: %s\n”, n, msg);
if (close(s) < 0) { perror(“close”); return -1; }
return 0;
}
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Single-Threaded Server
int server;
server = initialize();
while (1) {
wait_for_request(server);
read_request;
send_answer;
}
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Example: TCP Server
int main(int argc, char *argv[]) {
int s, t, n;
struct sockaddr_in sin;
char *r;
char buf[100];
int sinlen;
if ((s = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
perror(“socket); return -1;
}
sin.sin_family = AF_INET;
sin.sin_port = htons(13333);
sin.sin_addr.s_addr = INADDR_ANY;
if (bind (s, (struct sockaddr *) &sin, sizeof(sin)) < 0) {
perror(“bind”); return -1;
}
if (listen(s, 5) < 0) { perror(“listen”); return -1;}
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Example: TCP Server
for (;;) {
sinlen = sizeof(sin);
if ((t = accept(s, (struct sockaddr *) &sin, &sinlen)) < 0) {
perror(“accept”); return -1;
}
if (read(t, &buf, 100) < 0) { perror (“read”); return -1;}
r = gettime();
if (write(t, r, strlen(r)) < 0) { perror (“write”); return -1; }
if (close(t) < 0) { perror(“close”); return -1; }
}
if (close(s) < 0) { perror(“close”); return -1; }
}
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Other Socket Functions
• The socket API contains a variety of support
functions
–
–
–
–
–
–
getpeername
gethostname
setsockopt
getsockopt
gethostbyname
gethostbyaddr
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What About Threaded Servers?
• The socket API works well with concurrent servers
• Implementations of the socket API adhere to the following inheritance
principle:
– each new thread that is created inherits a copy of all open sockets from the
thread that created it
– the socket implementation uses a reference count mechanism to control
each socket
– when a socket is first created
• the system sets the socket’s reference count to 1
• and the socket exists as long as the reference count remains positive.
– when a program creates an additional thread
• the thread inherits a pointer to each open socket the program owns
• and the system increments the reference count of each socket by 1
– when a thread calls close
• the system decrements the reference count for the socket
• if the reference count has reached zero, the socket is removed
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