ch5 Computer network

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Services Provided to the Transport Layer – Connectionless
• • • The network is inherently unreliable The hosts should do error control and flow control themselves Provide SEND PACKET and RECEIVE PACKET primitives Each packet carries full destination address The receiving hosts do packet ordering
• • • • • • • • • • The Optimality Principle Shortest Path Routing Flooding Distance Vector Routing Link State Routing Hierarchical Routing Broadcast Routing Multicast Routing Routing for Mobile Hosts Routing in Ad Hoc Networks
Implementation of Connectionless Service
• The transport-layer code adds a transport header to a message from a process on a host and hands it to the network layer It may be broken into packets – also called datagrams, each individually routed without advance set-up Every router has an internal table telling it where to send packets for each possible destination The routing table may change to take into account the prevailing network conditions – equipment failure, network congestion, etc.
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Shortest Path Routing
The first 5 steps used in computing the shortest path from A to D. The arrows indicate the working node.
Dijkstra's algorithm
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Flooding
• • An incoming packet is sent out on all other outgoing lines A packet may carry a hop counter, initialized to a reasonably large value On decrementing the hop count to zero, a packet is simply discarded Alternatively, a packet may be given a sequence number by the source router; duplicates are ignored Selective flooding – forward a packet on those lines going approximately in the right direction
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Flooding
• • • Robust Concurrent updates to distributed databases Wireless networks where a station can receive all messages within radio range Find the shortest path for benchmarking
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The Optimality Principle
(a) A subnet. (b) A sink tree for router B.
Shortest Path Routing
• • Routers and communication lines are represented as nodes and arcs of a graph The length of an arc could be computed as a function of the distance, bandwidth, average traffic, communication cost, mean queue length, measured delay and other factors For computing the shortest path between two nodes, Dijkstra’s algorithm may be used
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Routing Algorithms
Conflict between fairness and optimality.
The Optimality Principle
• If router J is on the optimal path from router I to router K, then the optimal path from J to K also falls along the same route Otherwise, a better path from J to K can be concatenated to the optimal path from I to J, yielding a better path from I to K A sink tree is the set of optimal routes from all sources to a given destination forming a tree rooted at the destination All routing algorithms try to discover and use the sink trees for all routers
Chapter 5
The Network Layer
Network Layer Design Issues
• • • • • Store-and-Forward Packet Switching Services Provided to the Transport Layer Implementation of Connectionless Service Implementation of Connection-Oriented Service Comparison of Virtual-Circuit and Datagram Subnets
Routing Algorithms
• • • Correctness Simplicity Robustness – able to cope with changes in the topology and traffic Stability – a stable algorithm reaches equilibrium and stays there Fairness and optimality Non-adaptive algorithms – static routing Adaptive algorithms – routing decisions adapt to changes in the topology and traffic
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Services Provided to the Transport Layer – Connection-oriented
• • Quality of service is important It is difficult to achieve quality of service without connections in the network
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Implementation of Connectionless Service
Routing within a diagram subnet.
Implementation of Connection-oriented Service
• At connection set-up, a route from source machine to destination machine is chosen and stored in tables inside the routers Each packet carries an identifier for the virtual circuit it belongs to
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Implementation of Connection-Oriented Service
Routing within a virtual-circuit subnet.
Comparison of Virtual-Circuit and Datagram Subnets
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Routing Algorithms
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Store-and-Forward Packet Switching
fig 5-1
The environment of the network layer protocols.
Services Provided to the Transport Layer
• • • The services should be independent of router technology The transport layer should be shielded from the number, type and topology of the routers present The network addresses made available to the transport layer should use a uniform numbering plan Should the layer provide connection-oriented service or connectionless service?