Download Sockets

application
transport

Structural view of networks
◦
◦
◦
◦
◦

Physical layer: wire, radio, etc.
Data link layer: protocol principles, etc
Network layer: routing, internetworking
Transport layer: protocols (tcp), etc
Application layer: email, web, class project
network
link
physical
We will use one common method at the transport/network
level (TCP/IP via sockets)
Mobile network


Internetwork: a network of
networks
Uses packet switching to send
data between machines on this
“internet”
Global ISP
Home network
Regional ISP
Institutional network


Each machine has a unique address on the
network (IP address)
Internet Protocol (IP): format of packets used to
create these envelops of data
◦ Like mailing a multipage letter by putting each page
into a separately mailed envelop
◦ May not arrive in correct order

1. Hosts (e.g., computers and other devices) are
mapped to a set of 32-bit* IP addresses.
◦ 147.129.181.14

2. The set of IP addresses is mapped to a set of
identifiers called Internet domain names.
◦ 147.129.56.50 is mapped to www.ithaca.edu

3. A process on one Internet host can communicate
with a process on another Internet host over a
connection.
* this is IPv4. There is a new IP standard iPv6 that uses
64-bit addresses. See http://en.wikipedia.org/wiki/IPv6

Windows NT/2000/XP/Win 7
◦ Click on start, run, type cmd and at the prompt type ipconfig
◦ Type exit to close the command prompt window.

You’ll see something like:
Windows IP Configuration
Ethernet adapter Local Area Connection:
Connection-specific
IP Address. . . . .
Subnet Mask . . . .
Default Gateway . .
DNS
. .
. .
. .
Suffix . :
. . . . . : 192.168.1.101
. . . . : 255.255.255.0
. . . : 192.168.1.1

From the Finder, under Applications -> Utilities launch
Network Utility.
◦ in OS X 9.x look in /System/Library/CoreServices/Applications
◦ Select the correct ethernet interface (normally Ethernet 1 (en0)).
◦ Information and utilities are available from the tabs above.

From the Finder, choose apple->System Preferences.
◦ Then click on “Network” and in the resulting window choose
“ethernet”

Or, start a terminal window.
◦ Type “ifconfig –a”
◦ Look under “en0”

For Linux or Unix users, to view their IP address or network information,
users must have administrator or root privileges.
1.
2.
Open the Linux or Unix shell if you are utilizing a GUI interface for your Linux or
Unix machine.
From the prompt, type ifconfig eth0 and press enter. You’ll see:
eth0 Link encap:Ethernet HWaddr 00:A0:24:72:EB:0A
inet addr:10.10.10.2 Bcast:10.0.0.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:5569799 errors:32 dropped:32 overruns:0 frame:6
TX packets:3548292 errors:0 dropped:0 overruns:0 carrier:3
Collisions:14
Interrupt:18 Base address:0xda00

As seen from the above example, users will commonly see the network
settings for all their network devices when running the "ifconfig"
command. First in the above example we have the network settings for
the "lo" or "local loopback", next is the actual network settings of your
network adapter.

ping
◦ “a computer network administration utility used to test the
reachability of a host on an Internet Protocol (IP) network
and to measure the round-trip time for messages sent from
the originating host to a destination computer”

Open a terminal window (Mac) or do run->cmd
(Windows)
A228536:~ barr$ ping www.ithaca.edu
PING www.ithaca.edu (147.129.56.50): 56
64 bytes from 147.129.56.50: icmp_seq=0
64 bytes from 147.129.56.50: icmp_seq=1
64 bytes from 147.129.56.50: icmp_seq=2
data bytes
ttl=62 time=1.236 ms
ttl=62 time=1.269 ms
ttl=62 time=1.309 ms

The whois utility looks up records in the
databases maintained by several Network
Information Centers (NICs).
whois -h hostname domainName
e.g.
whois ithaca.edu
Or, if you know of a whois server like whois.educause.net (stores edu
names):
whois -h whois.educause.net ithaca.edu
Barr$
whois ithaca.edu
Domain Name: ITHACA.EDU
Registrant:
Ithaca College
Network & Communication Services
110 Phillips Hall
Ithaca, NY 14850-7028
UNITED STATES
Administrative Contact:
Bernie Rhoades
Director, Network & Communication Services
Ithaca College
Information Technology Services
110 Phillips Hall
Ithaca, NY 14850-7028
UNITED STATES
(607) 274-1694
[email protected]
Technical Contact:
William W. Weeks
Network Services Manager
Ithaca College
Information Technology Services
110 Phillips Hall
Ithaca, NY 14850-7028
UNITED STATES
(607) 274-1311
[email protected]
Name Servers:
RESOLVER1.ITHACA.EDU
RESOLVER2.ITHACA.EDU
RESOLVER3.ITHACA.EDU
147.129.1.2
147.129.94.4
208.81.240.2
Domain record activated: 18-Jun-1990
Domain record last updated: 24-Sep-2007
Domain expires:
31-Jul-2009

Transmission Control Protocol (TCP):
◦ Used to create reliable byte-streaming
communication
◦ Ensures that messages are passed error-free
from one machine to another on an internet
◦ Divides data into discrete messages before
passing them on to the IP layer
◦ At the receiving end, protocol used to
“reconstruct” the packets correctly


Applications utilize TCP/IP communication
by having the sender and the receiver
create end points called Sockets
Sockets are created and maintained by the
Operating System (not the network stack)

Created in the early 80’s as part of the original
Berkeley distribution of Unix that contained an
early version of the Internet protocols.

Provides a user-level interface to the network.

Underlying basis for all Internet applications.

Based on client/server programming model.

What is a socket?
◦ To the kernel, a socket is an endpoint of communication.
◦ To an application, a socket is a file descriptor that lets the
application read/write from/to the network.
 In Unix all I/O devices, including networks, are modeled as files.


Clients and servers communicate with each other
by reading from and writing to socket descriptors.
A socket is not a physical device!
◦ your computer may have one physical Ethernet connection
◦ and many sockets

Each socket has a socket number (address)
◦ IP address of the host computer
◦ Port number local to that host to which the socket is
attached
 Port numbers below 1024 are reserved (e.g. for email and
FTP)
 Some addresses are “well known”,
 Port 7: Echo server
 Port 23: Telnet server
 Port 25: Mail server
 Port 80: Web server
 You must choose a port not already in use



To use TCP service a connection must be explicitly
established between a socket on the sending
machine and a socket on the receiving machine
A socket may be used for multiple connections at
the same time.
A computer (and/or application) may have multiple
open socket connections
Computer
IP: 147.129.30.2
OS
application
Port
5678
Physical connection (e.g.
ethernet, RS232)
socket
Port
80
Client
Server
socket
socket
bind
open_listenfd
open_clientfd
listen
connect
Client /
Server
Session
Connection
request
rio_writen
rio_readlineb
rio_readlineb
close
accept
rio_writen
EOF
rio_readlineb
close
Await connection
request from
next client
On Windows:
use netstat.exe program (ships with windows, mac, and linux).

Start ”Command Prompt" using run-> cmd on Windows

type netstat -na and then press Enter,

use command "more" to stop when your listing contains to much lines (netstat -na | more).
Each line of netstat's listing contains these information:
* Protocol type - it can be TCP or UDP. Two sockets can use the same port number when they use different
protocols.
* Local Address - local IP address which is used. In most cases it is 0.0.0.0 (any = not bound to the specific IP
address. It can accept connection from any PC's interface). When IP address is specified then only connection
from interface with the same IP address are accepted.
Local Address field contains port number which is used to accept connection or receive UDP data.
* Foreign Address - address of PC on remote side. It will be displayed when connection is established.
* State - state of socket. You can see this states:
LISTENING - socket waits for connection (or data in case of UDP socket)
ESTABLISHED - connection to the remote side is established
CLOSE_WAIT, TIME_WAIT - socket waits for connection close or other type of negotiation.
To determine whether desired port number is already in use you must check Local Address field on all lines.
Example line:
TCP 0.0.0.0:25 0.0.0.0:0 LISTENING
Some application which uses port 25 (SMTP server port) is running.
On Linux:


Read the /etc/services file.
If you want the list of open ports, try netstat -nlp
◦ Arguments:
n = show network addresses as numbers,
l = show listening sockets,
p = show pid and name of program
On Macs:


Read the /etc/services file.
If you want the list of open ports, try netstat -n
◦ The port follows the IP address



Networked applications may be structured any
number of ways
One popular structure is client-server
Server:
◦ Usually unique
◦ Provides a service
◦ Runs continuously

Clients
◦ Usually many
◦ Send request for service, receive result from server
◦ Connect and disconnect as needed
1. Client sends request
Client
process
4. Client
handles
response
Server
process
3. Server sends response
Resource
2. Server
handles
request
Server host 128.2.194.242
Client host
Service request for
128.2.194.242:80
(i.e., the Web server)
Client
Web server
(port 80)
Kernel
Echo server
(port 7)
Client
Service request for
128.2.194.242:7
(i.e., the echo server)
Web server
(port 80)
Kernel
Echo server
(port 7)

Servers are long-running processes (daemons in Unix).
◦ Created at boot-time (typically) by the init process (process 1)
◦ Run continuously until the machine is turned off.

Each server waits for requests to arrive on a well-known port
associated with a particular service.
◦
◦
◦
◦
◦

Port
Port
Port
Port
See
7: echo server
23: telnet server
25: mail server
80: HTTP server
http://www.answersthatwork.com/Download_Area/ATW_Library/Networking/Networ
k__2-List_of_Common_TCPIP_port_numbers.pdf
A machine that runs a server process is also often referred to
as a “server.”

Web server (port 80)
◦ Resource: files/compute cycles (CGI programs)
◦ Service: retrieves files and runs CGI programs on behalf of
the client

FTP server (20, 21)
◦ Resource: files
◦ Service: stores and retrieve files

Telnet server (23)
See /etc/services for a
comprehensive list of the
services available on a
Linux machine.
◦ Resource: terminal
◦ Service: proxies a terminal on the server machine

Mail server (25)
◦ Resource: email “spool” file
◦ Service: stores mail messages in spool file






Your web browser is the client
Computer containing the pages you are trying
to access runs the web server
Your computer requests the page
Server sends the page
Many other people may be requesting the same
page
Note: these are distinct applications. Your
browser (client) never acts as a server

As you develop networked applications:
◦ Is the client-server model a good model?
 Minimizes network communication?
 Minimizes computational redundancy?

If so:
◦ Which program will be on the server side (e.g.
provide a service)?
◦ Which must be on the client side?


python provides built-in support for TCP/IP
network communication
There are methods for connecting a client and
methods for connecting a server

Within a Client application, create a socket
and connect to a specific server:
#Socket client example in python
import socket
#for sockets
#create an AF_INET, STREAM socket (TCP)
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
Address Family : AF_INET (this is IP version 4 or IPv4)
Type :
SOCK_STREAM (this means connection oriented TCP protocol)

Better to do error checking:
#handling errors in python socket programs
import socket
#for sockets
import sys #for exit
try:
#create an AF_INET, STREAM socket (TCP)
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
except (socket.error, msg):
print ('Failed to create socket. Error code: ' +
str(msg[0]) + ' , Error message : ' + msg[1])
sys.exit()

To connect to a remote server need 2 things, IP
address and port number.
◦ need to know the IP address of the remote server
◦ Here we used the ip address of google.com as a sample.
host = 'www.google.com'
try:
method to obtain an IP
address from a host name
remote_ip = socket.gethostbyname( host )
except socket.gaierror:
#could not resolve
print ('Hostname could not be resolved. Exiting’)
sys.exit()
print ('Ip address of ' + host + ' is ' + remote_ip)

Now connect to google on port 80 (the well known
http port)
#Connect to remote server
port = 80
s.connect((remote_ip , port))
print ('Socket Connected to ' + host + ' on ip ' + remote_ip)

Now we can send and receive messages
◦ sendall method sends bytes through the socket
◦ recv method receives a number of bytes
#Send some data to remote server
message = "GET / HTTP/1.1\r\n\r\n"
try :
#Set the whole string
s.sendall(bytes(message, 'UTF-8'))
except socket.error:
#Send failed
must convert
print ('Send failed')
sys.exit()
print ('Message send successfully’)
#Now receive data
reply = s.recv(4096)
print (reply)
string to bytes

Servers do the following actions:
1. Open a socket
2. Bind to a address(and port).
3.
Listen for incoming connections.
4. Accept connections
5.
Read/Send
open a socket for the server
just as we did for the client
try:
#create an AF_INET, STREAM socket (TCP)
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
except (socket.error, msg):
print ('Failed to create socket. Error code: ' + str(msg[0]) + ' , Error: ' +
msg[1])
sys.exit();



Next must bind the socket to an address and a
port
By doing this we ensure that all incoming data
which is directed towards this port number on this
machine is received by this application.
To bind the socket use the following method:
◦ s.bind(hostname, port number)


Ways of specifying arguments to bind.
to find the name of the host that the program is
running on use:
◦ hostname = socket.gethostname()

to create a “server” socket that is only visible within
the same machine use
◦ s.bind(('localhost', 80)) or
◦ s.bind(('127.0.0.1', 80))

To specify that the socket is reachable by any
address the machine happens to have use
◦ s.bind(('', 80))

To bind the socket to an address and a port
import socket
import sys
the server socket will listen
on port 8888 (not a well
known port) on all interfaces
HOST = ''
# Symbolic name meaning all available interfaces
PORT = 8888 # Arbitrary non-privileged port
try:
s.bind((HOST, PORT))
except socket.error , msg:
print 'Bind failed. Error Code : ' + str(msg[0]) + ' Message ' + msg[1]
sys.exit()
print 'Socket bind complete'


The next step is to make the socket listen for
incoming requests
We must put the socket into listening mode:
◦ s.listen(x)

The parameter x is called the backlog.
◦ It controls the number of incoming connections that are
kept "waiting" if the program is already busy.
◦ x =10 means that if 10 connections are already waiting to
be processed, then the 11th connection request shall be
rejected.


The final step is for the socket to accept incoming
requests
When a request to this machine on the correct port
comes in, the socket must accept the request:
◦ (conn, addr) = s.accept()


The result conn is a new socket with which
we can communicate with the client.
The result addr is an array.
◦ first element is the IP of the client
◦ second element is the port the client is using
(conn, addr) = s.accept()
#display client information
print ('Connected with ' + addr[0] + ':' +str(addr[1]))

This displays the client addr

Now can send & receive over the connection:
#now keep talking with the client
data = conn.recv(1024)
print ("Server received: " + data)
conn.sendall("Server says: Hello client!")

When finished, close the connection & socket:
conn.close()
s.close()

See Class web site:
◦ Course Resources -> Program Examples->find_ip.py
◦ Course Resources -> Program Examples->client.py
◦ Course Resources -> Program Examples->server.py

Calculating area:
◦ Server calculates area given radius
◦ Client gets radius from user, requests area
calculation
server
client
radius