IP Datagrams

1
IP Datagrams

• Chapter 16,17

2
Connectionless service

• Extension of packet switching
• Individual packets can be transmitted across the
  internet.
• Each packet travels independently and contains
  information that identifies the recipient.

3
The IP datagram

• Datagram refers to an internet packet.
• The amount of data carried is not fixed but
  varies from 1 octet to 64K octets including the
  header.
• The datagram header contains information to route
  the datagram across the network.

4
Forwarding an IP Datagram

• Each router along the path receives the datagram,
  extracts the destination address and uses it to
  determine the next hop to which the datagram
  should be sent.
• Each IP router keeps information in a routing
  table.

Table found at R2
5
IP addresses Routing table entries
Specifies which bits of address correspond to the
network prefix
In practice routing is more complex
6
The Mask Field

• The process of selecting the next hop is called
  routing.
• The mask field is used to extract the network
  part of an address during lookup.

7
Best-Effort Delivery

• IP does not guarantee that it will handle the
  problems of
• Datagram duplication
• Delayed or out-of-order delivery
• Corruption of data
• Datagram loss
• IP datagrams may be lost, duplicated, delivered
  out of order or delivered with corrupted data.

8
Datagram header format
Each field has a fixed size
9
IP Encapsulation
10
MTU, Datagram size encapsulation

• Each hardware technology specifies the maximum
  amount of data that a frame can carry. This limit
  is the MTU
• MTU restrictions can cause a problem

Splitting datagram into fragments
11
Fragment Re-assembly

• All fragments have the same destination address
  as original datagram. Final fragment identified
  with additional bit set in header.
• Each fragment contains of original datagram
  identification number fragment offset.
• Reassembled at final destination when all
  fragments have arrived.
• IP specifies a maximum time to hold fragments.
• Re-assembly is whole-or-nothing

12
Sending Datagrams in Java

• Datagrams are up encapsulated using to two java
  classes
• The DatagramSocket class
• The DatagramPacket class

13
DatagramSocket Class

• This class encapsulates a socket used for sending
  and receiving datagram packets. Since been
  routing Information is contained in the packets
  themselves, The constructors are not concerned
  with the destination for the data at all. You can
  create a datagram socket object with the default
  constructor
• DatagramSocket socket new DatagramSocket()
• To send a datagram you call the send() method for
  a the DatagramSocket, and pass the DatagramPacket
  object (which encapsulates the datagram ) to be
  sent as an argument.

14
Receiving a Datagram in Java

• A socket used to receive a Datagram needs to have
  a specific fixed port address. Because the
  machine sending the data would have included the
  receiving port number as part of the packet
  header information. To create a receiving
  Datagram socket you can use the following
  constructor which takes an int parameter.
• DatagramSocket receiver new DatagramSocket(portN
  umber)
• To receive a datagram packet you call the
  DatagramSocket receive() method.

15
The DatagramPacket Class

• The DatagramPacket class encapsulates a datagram
  packet. A datagram packet is an uninterpreted
  sequence of bytes therefore a byte array must be
  created containing the information you wish to
  send.
• Two constructors are used
• byte buffer new bytelength
• DatagramPacket packetIn new DatagramPacket(buffe
  r,packetLength)
• this is for packets used to receive data

16
DatagramPacket Class

• DatagramPacket packetOut new datagramPacket(buff
  er, // byte
• packetlength, // int
• destination, //InetAddress
• portNumber) // int
• this is for sending packets of data
• The portNumber is used to specify the process on
  the host.

17
Packet data

• To obtain the data held in a packet call the
  getData() function.
• byte data packet.getData()
• To set the data in a packet call the setData()
  function.
• packet.setData(newData)