Datagram network in Software

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Datagram network
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In this example, all four packets (or datagrams) belong to the same message, but may travel different paths to reach their destination This is so because the links may be involved in carrying packets from other sources and do not have the necessary bandwidth available to carry all the packets from A to X This approach can cause the datagrams of a transmission to arrive at their destination out of order with different delays between the packets Packets may also be lost or dropped because of a lack of resources In most protocols, it is the responsibility of an upper-layer protocol to reorder the datagrams or ask for lost datagrams before passing them on to the application The datagram networks are sometimes referred to as connectionless networks The term connectionless here means that the switch (packet switch) does not keep information about the connection state There are no setup or teardown phases Each packet is treated the same by a switch regardless of its source or destination
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SWITCHING
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Routing Table
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If there are no setup or teardown phases, how are the packets routed to their destinations in a datagram network In this type of network, each switch (or packet switch) has a routing table which is based on the destination address The routing tables are dynamic and are updated periodically The destination addresses and the corresponding forwarding output ports are recorded in the tables This is different from the table of a circuitswitched network in which each entry is created when the setup phase is completed and deleted when the teardown phase is over Figure 88 shows the routing table for a switch
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Figure 88 Routing table in a datagram network
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Destination address Output port
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A switch in a datagram network uses a routing table that is based on the destination address
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Destination Address
Every packet in a datagram network carries a header that contains, among other information, the destination address of the packet When the switch receives the packet, this destination address is examined; the routing table is consulted to find the corresponding port through which the packet should be forwarded This address, unlike the address in a virtual-circuit-switched network, remains the same during the entire journey of the packet
The destination address in the header of a packet in a datagram network remains the same during the entire journey of the packet
Efficiency
The efficiency of a datagram network is better than that of a circuit-switched network; resources are allocated only when there are packets to be transferred If a source sends a packet and there is a delay of a few minutes before another packet can be sent, the resources can be reallocated during these minutes for other packets from other sources
SECTION 83
VIRTUAL-CIRCUIT NETWORKS
Delay
There may be greater delay in a datagram network than in a virtual-circuit network Although there are no setup and teardown phases, each packet may experience a wait at a switch before it is forwarded In addition, since not all packets in a message necessarily travel through the same switches, the delay is not uniform for the packets of a message Figure 89 gives an example of delay in a datagram network for one single packet
Figure 89 Delay in a datagram network
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The packet travels through two switches There are three transmission times (3T), three propagation delays (slopes 3't of the lines), and two waiting times (WI + w2)' We ignore the processing time in each switch The total delay is
Total delay = 3T + 3t + WI + W2
Datagram Networks in the Internet
As we will see in future chapters, the Internet has chosen the datagram approach to switching at the network layer It uses the universal addresses defined in the network layer to route packets from the source to the destination Switching in the Internet is done by using the datagram approach to packet switching at the network layer
VIRTUAL-CIRCUIT NETWORKS
A virtual-circuit network is a cross between a circuit-switched network and a datagram network It has some characteristics of both 1 As in a circuit-switched network, there are setup and teardown phases in addition to the data transfer phase
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