1. Server-Client communication without connection
When the communication consists of sending and/or receiving datagram packets instead of a data stream it is called a connectionless communication
This means there is no “virtual link” created between both end of a communication.
This is very near to how the packages are actually delivered over the over the internet.
This is why the arriving, order or uniqueness of packages cannot be guaranteed.
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2. Datagram management with JAVA
Communication is based on assembling UDP packages and sending them to the interent. An UDP package consists of:
Data: a bytes array
Destination Port : int
Destination Address: InetAddress
A server start by listening at a certain port for packages.
The client assembles a packages and send it to the net.
The server receives the package (routed by the net to its final destination) and extracts the data.
If the server needs to answer, it extracts the sender address and port (the client must be listening for packages)
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3. Classes for Datagrams in Java: Send
Create a socket for sending a Datagram to the internet
DatagramSocket ds = new DatagramSocket();
Create and assemble the Datagram
byte[] data = new byte[256];
InetAddress address = InetAddress.getByName(“
DatagramPacket pack = new DatagramPacket(data, data.length,address,4444);
Send
ds.send(pack);
Wait for an answer
socket.receive(pack); //make sure it is clean before, perhaps by using a new one !!!
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EchoUDPClient DateUDPClient

4. Classes for Datagrams in Java: Receive
Start listening for Datagrams on a certain socket
socket = new DatagramSocket(4444);
Preparing a Datagram for receiving data
byte[] data = new byte[256];
DatagramPacket pack = new DatagramPacket(data,data.length);
Start listening for a package
socket .receive(pack);
Obtaining the data and address and port of sender
int port = pack.getPort();
InetAddress address = pack getAddress();
String content = new String(pack.getData());
Or just by using the data variable which points to the byte-array
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DateUDPServer EchoUDPServer

5. An UDP Ping Client We will use the echo server by default
In a unix machine there is normally an echo server listening at port 7 for UDP and for TCP requests
It is not the same server, but it is possible to open 2 sever sockets for the same port but for different protocols
The Ping client will send a package to the server with time of issue, which will be returned by the server
By comparing time in the Datagram and current time we can have the round-trip delay
The program will also calculate max/min and avg
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Ping.java

6. Multicasting & Broadcasting
If a server must distribute information to many clients it may overload its resources
This is especially true in situations like videoconferencing (multipoint), where several frames per seconds should be transmitted to may be several clients: this is not possible in the practice!
In multicasting and broadcasting the server transmits this information only once.
This requires the hardware (network) to support this
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7. Multicast & Broadcast
Multicast & Broadcast are protocols which allow an application to put a single package on the net which will be received by many other applications,
Broadcast works only inside a local network. A “broadcasted” package will be received by all.
It requires support from the local network.
Multicast will arrive only to “interested” clients which have registered before
Multicast requires host and routers to support the IGMP routing algorithm
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8. Multicast
Multicast in Java is similar to UDP except that sending-receiving should be implemented on an IP number in the range between ( )
In order to receive the Multicast packs the client must express interest in joining a certain multicast group at a certain multicast address and port. The network, (the routers) will deliver the packs to the interested hosts
Any application can transmit packs to the group !
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9. Multicast
The packet will be picked up my any machines on the local network that are interested in that group.
In addition it will be picked up by routers that will forward it as appropriate to adjacent networks that are interested
The significant complexity of multicast is how routers will know what adjacent networks are interested
This requires the storage of additional information in the routing table
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10. Time to Live
Multicast packets include a TTL field that limits the propagation across the internet
In general, every time a packet is relayed by a router or a tunnel the value of TTL will be decreased
When it reaches cero the package is discarded
This concept is also present on every internet packet !
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11. Java Support for Multicast
MulticastSocket: extension of DatagramSocket
MulticastSocket( ) bounds it to any available port
MulticastSocket(int port) bounds it to a specific port
Many multicast socekts can be bound to the same port! (contrary to TCP or UDP sockets)
Methods are inherited (send, receive) + 3 new
joinGroup(InetAddress group)
leaveGroup(InetAddress group)
setTimeToLive(int ttl)
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12. A Multicast Based Chat There is no server.
Each participant runs the exactly same program, joining a common Multicast group
The messages are “multicasted” over the net, thus everyone joining the group will receive them
There is no guarantee about the arriving, arriving time, or duplication of messages
NOTAS
MulticastChat

13. Spontaneous Networking
Multicasting is the right way to program systems when the participants in the session may come and go very frecuently
This is the case of spontaneous networking with mobile devices in a room
Someone “announce” her precence to the other members by sending message to all at regular intervals
The fact that someone has left is recorded by the others when there have been no messages from her since a certain period of time
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MulticastRegister

14. MBone
Multicast is currently not widely deployed on the Internet so it is not possible to implement it across different networks. This is mainly because of the routers not supporting the IGMP
There is a subnet called MBone which communicate multicast-enabled islands, allowing the transport of multicast packets through tunnels.
A tunnel communicates the routers of two networks which are not physically adjacent.
Packages will be forwarded from router to the other as if there were actually neighboring networks
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15. Broadcast Broadcast is similar a Multicast but in a local network
Every Broadcast based network (like ethernet) has a broadcast IP address. Any message sent to this address will be received by all computers on the local network
Usually this is the last IP address of the subnet:
Clase C: >
Para una subred de 16 hosts >
A port number should be agreed
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16. ¿ Broadcast or Multicast ?
If you can chose it is better to use Multicast because it does not disturb other machines
Sometimes is necessary to have privileges to write on a broadcast address.
Multicast allows many multicast groups in the same network
The generated traffic is the same: one package which is received by all members
Broadcast works in Java like common UDP. Only the IP address is special
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