Download Internet Programming - Seneca

Windows
Programming Using
C#
Internet Programming
Contents
Basic Internet classes
 Internet protocols
 Client TCP/IP sockets
 Server TCP/IP sockets
 UDP sockets

2
DNS Class
The DNS class provides a simple interface
to the domain name system
 This allows you to translate names to IP
addresses and vice versa
 Remember that a computer with one name
might have several IP addresses if it has
several network cards

3
DNS Class

Methods
 IPHostEntry
GetHostEntry(string host)
Returns an IPHostEntry for the host
 The host string can be a host name or a dotted
address
 If an empty string is passed as the host,
information is returned about the local machine

4
IPHostEntry Class


This holds all the information on a particular host
Properties
 IPAddress[] AddressList


Returns an array of IP addresses for the host
If the host cannot be resolved, this list has zero length
 string[] Aliases

Returns a list of aliases for the host
 string

HostName
Returns the primary name for the host
5
IPAddress Class
This represents an IP address with
support for IPV6
 Methods

 byte[]

Returns an array of bytes in the address
 string

GetAddressBytes()
ToString()
Returns the address in the usual dotted notation
6
IPAddress Class

Many useful, pre-defined fields are
provided
 Any
Matches any IP address
 Used for listening to accept any host

 Broadcast

An address which sends to all hosts on the local
area network
7
IPAddress Class
 Loopback

The loopback address for the local host
 None

An address which does not match any real
address
8
Communication Modes

An internet connection is one of two types
 Connection-oriented
Similar to a phone connection
 A virtual path is set up between two hosts and the
communication parameters are negotiated
 Guarantees that the packets are delivered and in
the correct order

9
Communication Modes
 Connectionless
This is like sending a letter
 There is no permanent connection between sender
and recipient
 This saves

Setup time
 Additional information in the packets to ensure they are
received and in order


Connectionless protocols do not guarantee
delivery or delivery in the right order
10
Protocols

TCP
 Transmission control protocol
 Connection-oriented protocol with
guaranteed
delivery
 Used for most communication with servers

UDP
 User datagram protocol
 A connectionless protocol
 More efficient than TCP
 Used
without guaranteed delivery
for streaming media
11
Sockets


The software connection to the internet is called a socket
A socket combines





The IP address of a machine
A port number
Each program which opens a socket to communicate on
the internet uses one of 65,535 sockets on the machine
Data from the network is delivered to a particular socket
and then to a particular program listening on that socket
This allows one computer to have many network
connections active at once.
12
Sockets

A socket can be of different types
 TCP/IP


Acts as a data stream between two computers which can be
read from and written to
The socket is set up to establish a virtual circuit to a single
host
 UDP



Send discrete messages between computers
Each message can be sent to a different computer
There is no permanent connection between computers
13
IPEndPoint Class
Represents one end of a socket
connection
 Includes

 The
IP address of a computer
 A socket number
14
AddressFamily
An enumeration of the types of addressing
that can be used by a socket
 Common values include

 AppleTalk
 InterNetwork
 InterNetworkV6
// IPV4
// IPV6
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SocketType


An enumeration of the types of sockets that can
be created
Common values
 Dgram
 Connectionless UDP
 Raw
 Access to underlying protocol
 Used for ICMP
 Stream
 Connection-oriented TCP/IP
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ProtocolFamily

The protocol used by a socket
 Raw

Raw protocol
 Tcp

Transmission control protocol
 Udp

User datagram protocol
17
Socket Class
This creates a real socket
 The namespace is System.Net.Sockets
 To create a socket

 Socket
s = new Socket(
AddressFamily.InterNetwork,
SocketType.Stream,
ProtocolType.Tcp);
18
Socket Class

To connect a socket
IPHostEntry hostEntry = Dns.GetHostEntry(hostName);
IPAddress[] addresses = hostEntry.AddressList;
IPEndPoint endPoint = new IPEndPoint(addresses[0],
port);
s.Connect(endPoint);
19
Socket Class

To check to see if the socket is connected
 If(s.Connected)

To write data to the socket
 s.Send(byte[],

length, offset);
To read from a socket
 nread

{…}
= s.Receive(byte[], length, offset);
*see my_curl
20
Servers

A server
 Creates
a TcpListener
 This listens on a particular port for connections
 When a connection is received, a socket for the
connection is returned
 Data is exchanged over the socket
 The socket is closed
 The listener listens for the next connection
21
TcpListener

To create a server
int port = 2001;
IPAddress localAddr = IPAddress.Parse("127.0.0.1");
TcpListener listener = new TcpListener(localAddr, port);
listener.Start();
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TcpListener

To wait for a connection
 Socket

sock = listener.AcceptSocket();
To communicate on a socket
 Use
the Send and Receive methods
 Close the socket when finished
23
To Connect to a Server

There are two ways
 Use

a TcpClient
Create a client and connect


Get a stream to read and write to the server


TcpClient client = new TcpClient(server, port);
NetworkStream stream = client.GetStream();
Read/write the stream
stream.Write(data, 0, data.Length);
 Int32 bytes = stream.Read(data, 0, data.Length);

24
To Connect to a Server
 Use
a Socket
Create a normal Socket and connect to the host
and port
 Use Send and Receive to communicate
 Close the socket when done

 *see
hello_client and hello_server
25
UDP
This sends discrete messages rather than
a stream of bytes
 Each message can be sent to a different
computer
 UDP is much more efficient than TCP/IP
 UDP is used for sending audio and video

26
UdpClient


While you can use a socket for UDP
communication, it is easier to use a UdpClient
To create a client and listen on a port
 UdpClient

client = new UdpClient(port);
To send bytes to a computer
 byte[]
sendBytes = Encoding.ASCII.GetBytes(msg);
 client.Send(sendBytes, sendBytes.Length, host,
remotePort);
27
UdpClient

To receive bytes from any computer
IPEndPoint remoteIpEndPoint = new IPEndPoint(
IPAddress.Any, 0);
Byte[] receiveBytes = client.Receive(ref remoteIpEndPoint);
string returnData = Encoding.ASCII.GetString(receiveBytes);


The IPEndPoint passed to Receive is filled in with the
address of the sender of the message
To find the message sender


remoteIpEndPoint.Address
remoteIpEndPoint.Port
28
Socket Timeouts
The Receive call blocks until it receives a
message
 This can be a problem if nobody wants to
talk to you
 You can only wait for the phone so long…
 You need to set a timeout which will
interrupt the receive operation

29
Socket Timeouts

The time is expressed in milliseconds
 client.Client.SetSocketOption(
SocketOptionLevel.Socket,
SocketOptionName.ReceiveTimeout, 5000);

When the timeout fires, an exception is
thrown, which must be caught

* see UdpChat
30
Broadcast
Ever feel like just calling anybody?
 Then broadcast it!
 A broadcast message will be picked up by
any listener on the local area network
 Broadcast messages will not pass through
routers

31
Broadcast

To send a broadcast message use
IPAddress.Broadcast
IPEndPoint remoteIpEnd =
new IPEndPoint(
IPAddress.Broadcast, remotePort);
Send(sendBytes, sendBytes.Length, remoteIpEnd);
32
Finding Servers





Normally, servers operate on well-known ports
These are ports which are less that 1024 and
are assigned for specific purposes
Eg. Web servers are on port 80
This tells us what port the server is on
What if we don’t know which machine is hosting
the server?
33
Finding Servers

In this case, we can broadcast for a server
 Send
out a broadcast message to all machines on the
LAN
 Wait for a reply
 Use that machine as a server


You can broadcast with UDP on one port to
locate the servers
You can then stay with UDP to talk to the servers
on another port or switch to TCP/IP on another
port
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