4 ICS132-network-layer

4 ICS132-network-layer - The Network Layer Network Layer...

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The Network Layer
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Network Layer z Recall: ± The network layer is responsible for the routing of packets ± The network layer is responsible for congestion control
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Contents z Connection-Oriented and Connectionless Service z The IP Protocol z IP-Support Protocols z Routing Algorithms z Congestion Control
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1. Connection-Oriented and Connectionless Service z Network layers can offer two types of service to the transport layer: ± Connection-oriented service – Network layer provides the Transport layer with a reliable service: all packets will be delivered (flow control), in- sequence delivery – Connection setup required before communication begins ± Connectionless service – No guarantee – No prior connection setup required; packets are stored and forwarded one at a time by IMPs
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1.1 Connection-Oriented Service z How to provide connection-oriented service: ± Set up a route (virtual circuit) between source and destination ± That route is used for all traffic flowing over the virtual circuit ± IMP maintains an internal table to tell which outgoing line to forward packet on for each active virtual circuit ± Packets must contain a virtual circuit number so that IMP can figure out how to forward them
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Connection-Oriented Service: Analogy z Public Telephone Network ± Set up a virtual circuit (dial a number) ± Transmit data on the circuit (conversation) ± Close down the virtual circuit (hang up) z Two users are provided with the illusion of a dedicated point-to-point channel z Information is delivered to the receiver in the same order in which it is transmitted by the sender
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1.2 Connectionless Service z How to provide connectionless service: ± Send the packet into the network and allow the network to forward it however it likes ± IMPs maintain routing tables to look up the next IMP for each arriving packet ± Each packet must contain a destination address so the IMPs can make routing decisions
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Connectionless Service: An Analogy z Postal service: ± Each packet (letter) is transported as an individual entity ± Each packet (letter) must carry the complete destination address ± If a packet (letter) is lost, error control is the user’s responsibility ± Packets (letters) do not necessarily arrive in the order sent
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1.3 Comparison between Connectionless and Connection-Oriented Services z Connection Setup Procedure: ± Connection-oriented service – Explicit setup and tear-down required – For short transaction oriented communication, the delay of connection setup may be expensive ± Connectionless service – No setup or tear-down required – For long continuous communication, the overhead of packet headers may be expensive
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Comparison (cont’d) z Header Overhead ± Connection-oriented service – Only the virtual circuit number ± Connectionless service – The full destination address is required
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Comparison (cont’d) z Message Sequence: ± Connection-oriented service – Sequence automatically maintained ± Connectionless service – Destination may have to re-sequence out-of- sequence messages
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4 ICS132-network-layer - The Network Layer Network Layer...

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