Server-Client communication without connection

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.

2
Datagram manipulation 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)

3
UDP Communication with Datagrams
DATAGRAMA an independent self-containing message
sent through the internet whose arrival, delay
and content are not guaranteed
Once a server program is hearing for incoming
datagrams, the client can send one.
www.waseda2.jp
www.waseda1.jp
A SERVER
A CLIENT
?
4444
www.waseda1.jp
4444
message
4
Sending Datagrams
The client must open a UDP socket to send the
datagram which should contain the address and
port of destination. The routing algorithm will
forward it to the destination
www.waseda2.jp
www.waseda1.jp
A SERVER
A CLIENT
?
3333
4444
5
Remtent address
Before the datagram leaves the computer, the
TCP/IP layer puts the address and port of origin
www.waseda2.jp
www.waseda1.jp
A SERVER
A CLIENT
!
3333
4444
6
Receiving Datagrams
The client can now start waiting for an answer of
the server
www.waseda2.jp
www.waseda1.jp
A SERVER
A CLIENT
?
3333
4444
7
Sending datagramas
The server can extract the information about the
address and port from the datagram and prepare
the answer
www.waseda1.jp
www.waseda2.jp
A SERVER
A CLIENT
?
3333
4444
answer
8
Sending datagramas
Finally the answer can be sent sent.
www.waseda2.jp
www.waseda1.jp
A SERVER
A CLIENT
?
3333
4444
9
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 byte256
• InetAddress address InetAddress.getByName(www.c
  tc.cl)
• 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 !!!

10
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 byte256
• 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 use the data variable which points to the
  byte-array

11
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

Pinging.java
12
What about lost packages ?

• UDP Datagrams can get lost like internet frames
• We can check this by numbering the packages and
  controlling the sequence
• Make the following experiments
• package loss between Chile and Germany
• package loss between Japan and Chile
• package loss between Chile and Japan
• Make also the same inside a LAN
• Varying the number of clients

Tiro/ReciboPaquetesNoEnd
13
Bemerkungen über UDP

• Do not try to develop your own fully reliable UDP
  based communication mechanism
• It is only helpful if you want to add some
  reliability
• A server listening for datagrams will receive all
  of them !
• This means, if you want to establish a dialogue
  with a client in a separate thread, do it through
  another available port, like in
  tiro/recibopaquetes

14
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

15
The Multicast paradigm
PROG2
PROG1
PROG2
PROG2
16
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

17
Multicast

• Multicast in Java is similar to UDP except that
  sending-receiving should be implemented on an IP
  number in the range between (224.0.0.0 -
  239.255.255.255)
• 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 !
  

18
What happens when a package is thrown

• 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

19
Multicast support in Java

• MulticastSocket Extension of the
  DatagramSocket
• MulticastSocket( ) binds to any available port
• MulticastSocket(int port) uses a specific port
• Many multicast sockets may be bound the the same
  port at the same time! (contrary to TCP or UDP)
• Inherited methods (send, receive) 3 new
• joinGroup(InetAddress group)
• leaveGroup(InetAddress group)
• setTimeToLive(int ttl)

20
Example of Multicast in Java
import java.io. import java.net. public class
MulticastClient public static void
main(String args) throws IOException
MulticastSocket socket new MulticastSocket(4446)
InetAddress address
InetAddress.getByName("224.2.2
.3") socket.joinGroup(address) byte buf
new byte256 DatagramPacket packet while(tru
e) packet new DatagramPacket(buf,
buf.length) socket.receive(packet)
String received new String(packet.getData())
System.out.println("Received " received)
try Thread.currentThread().sleep(0)
catch (InterruptedException e)
import java.io. import java.net. import
java.util. public class MulticastServer
static public void main(String args)
DatagramSocket socket null BufferedReader
in null boolean moreQuotes true
try socket new DatagramSocket()
while (true) InetAddress grupo
InetAddress.getByName("224.2.2.3")
for (int i1 ilt 1000 i)
String dString i"--"(InetAddress.getLocalHost(
)) byte buf
dString.getBytes() DatagramPacket packet
new DatagramPacket(buf,
buf.length, grupo, 4446)
socket.send(packet) try
Thread.currentThread().sleep(200) catch
(InterruptedException e)
catch (IOException e)

21
Sending video by Multicast
MulticastMovieServer
MulticastMovieClient
22
Group communication with Multicast

• Provides
• Fault tolerance based of replicated services
  there are many servers providing the same
  service. The client sends a request to all by a
  multicasting message
• Discovery of services client and services use
  multicast messages to register their services and
  interfaces. Clients use it to localize them and
  do the lookup of the interfaces.
• Better performance for replicated data when
  clients keep a copy of a certain data locally
  (cache) the server sends the updated data by
  multicast
• Propagation of notification events when clients
  wait for a certain event to react (jini)

23
Possible faults with Multicast

• Because Multicast is based on UDP
• Fault tolerant replicated services If all
  servers start from the same initial state and
  they coordinate themselves by events, if one
  client does not receive the update or receives it
  in a bad order its sate may be inconsistent
• Finding discovery services The servers should
  send in regularly messages telling about their
  presence
• Better performance in replicated data If the
  message of changing of a certain data does not
  arrive to a client it may have an inconsistency
  with the rest
• Propagation of notification events Send
  notifications regularly

24
A peer-to-peer chat based on Multicast

• There is no server.
• Every participant runs the same program joining
  the multicast group defined by addressport
• Messages are distributed to the net as datagrams
  to the multicast group, so every interested
  application will receive
• There is no guarantee that the messages will
  arrive or not, nor if they will arrive in the
  correct order

MulticastChat
25
Spontaneous Networking

• Multicasting is the right way to program systems
  when the participants in the session may come and
  go very frequently
• This is the case of spontaneous networking with
  mobile devices in a room
• Someone announce her presence 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

26
An Awareness Example

• The MulticastRegister program will show all
  people participating in the multicast group
• It implements a thread that will send every
  second a packet with the client's identification
• It starts 3 other threads
• ReceiveThread will listen to packets sent by
  other members of the group. When it receives one,
  it will register it in a vector containing a pair
  (participant-id, time) with the time the last
  packet received from a participant was received
• ChckThread check the vector every 2 seconds the
  vector and deletes the entries from the
  participants whose last package was received more
  than 10 seconds ago
• RefreshThread it simply refreshes the list
  showing the active participants according to the
  vectors content

27
Multicast Model for groups
Multicast has certain characteristics that makes
it efficient to trnasmit messages to all members
of a group Model message(g,m) operation of
sending a message to all members of the
group g deliver(m) operation of
processing the message m sender(m)
identification of the sender of a message
group(m) target group of message m
open/closed group tells if the group is
open or closed for other members to send
messages to the group
28
Reliable Multicast
In order to have reliable multicast, 3
conditions must be fullfilled Integrity
The message that has been sent is the same that
the message that is prodecced. Moreover, no
message is processed two times. A process p
performs the operation deliver(m) only once and
p ? group(m) Validity if a process p
sends a multicast message to the group and p ?
group(m), p will eventually make deliver(m)
Agreement if a process p makes deliver(m) all
the rest will eventually do this also
29
Reliable Multicast with IP !

• Every process maintains a sequence number S(p,g)
  for every multicast group it belongs.
• It also maintains a register R(q,g) which is the
  sequence number of the last processed message
  sent by q to the group g
• When q sends a message to g it includes the
  number S(p,g) and pairs ltq,R(q,g)gt, and then it
  increments S(p,g).
• A process of the group processes a message sent
  by p only if S R(p,g) 1
• if S lt R(p,g) then it is a message already
  processed and it is discarded
• Si S gt R(p,g) 1 means the process has not
  received one of the messages of p and sends a
  negative ack message in order to have p send it
  again
• Integrity is achieved by detection of duplicated
  messages and checksums made by the IP layer over
  the datagrams. Validity is achieved by multicast
  properties.
• In order to reach agreement, every process has to
  keep a copy of the messages sent in order to send
  them again if necessary.

30
Ordering the Multicast messages

• A queue is used to store the messages before
  processing them. For each message, a sequence
  number will be assigned which must be accorded by
  all participants. Every process q in a group g
  maintains a number A(q,g), which is the biggest
  for a sequence previously accorded which has been
  seen for a group g and P(q,g) is the greatest of
  the process own sequence. When p wants to send a
  message
• sends in a secure way ltm,igt being m the message
  and i an unique identifier for m
• Every process q answers to p with a proposal of
  the sequetial number for this message P(q,g)
  Max(A(q,g), P(q,g))1. Every process stores in
  its queue the message with the number it
  provisionally proposed ordering the sequence from
  the smallest to the biggest.
• p collects all numbers and selects the biggest,
  wich will be used to transmit the message in a
  secure way lti,agt
• every process then orders the message queue
  before processing them according to the number
  sequence

31
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 !

32
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

33
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 192.1.2.0 -gt 192.1.2.255
• Para una subred de 16 hosts 197.84.66.192 -gt
  197.84.66.207
• A port number should be agreed

34
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

35
Homework Merge multicastig chat with
multicasting awareness

• Rewrite MulticastChat in order to include
• A window interface like the one seen on the
  TCP/IP chat with central server
• The list of participating people should be
  implemnted according to the MulticastRegister
  schema