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Fundamentals of Computer Networks ECE 478/578 Lecture #13: Packet Switching (2) Instructor: Loukas Lazos Dept of Electrical and Computer Engineering University of Arizona

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Source Routing Idea: Include the entire route to be followed in the header Nodes must know the network topology in advance 2

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Possible Header Implementations Header has variable length Used in both datagram and VC switching (connection setup) May be “loose” or “strict” routing 3

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Bridges Bridges are special switches that interconnect Ethernet networks Act as simple nodes on each Ethernet Early bridges forwarded all packets between the two networks 4

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Bridges Vs. Repeaters Repeaters are analog amplifiers of the signal Monitor the existence of analog signal on the line Amplify and repeat the signal Physical layer function, no message parsing Bridges “repeat” frames rather than signals They forward received frames among different LANs Message parsing is performed to decide whether to fwd the packet or not 5

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Limitations of Repeaters All hosts connected via repeaters belong to the same collision domain Every bit is sent everywhere So, aggregate throughput is limited E.g., three departments each get 10 Mbps independently … and then connect via a hub and must share 10 Mbps Multiple LAN technologies not compatible No message parsing – analog process No interoperability between different rates and formats E.g., 10 Mbps Ethernet and 100 Mbps Ethernet Limited geographical distances and # of nodes 6

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Advantages of Bridges Facilitate breaking of network into isolated collision domains Message parsing may avoid unnecessary packet forwarding 7 Collision domain 1 Collision domain 2

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Bridge Forwarding Table Not all packets need to be forwarded. E.g., packets from A to B need not be forwarded on port 2 Bridges maintain a forwarding table 8 HostPort A1 B1 C1 X2 Y2 Z2

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Forwarding Algorithm 9 Look up Destination Address Forward designated port Flood all ports but incoming one Destination not found broadcast Discard Same as incoming port Destination found Different port

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Problem with Flooding 10

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Some Preliminaries on Graphs Graph G(V, E) : Collection of vertices and edges V: Set of vertices E: Set of edges, e(x, y) = 1, if x, y connected, 0 otherwise Path: sequence of vertices with an edge between subsequent vertices (can be defined as sequence of edges as well) 11 V1V1 V3V3 V2V2 V4V4 V5V5 V6V6 Path: {V 1, V 3, V 4 } Vertex Edge

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Connected Graph A graph is said to be connected, if there is a path from any node to any other node 12 V1V1 V3V3 V2V2 V4V4 V5V5 V6V6 V1V1 V3V3 V2V2 V4V4 V5V5 V6V6

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k-Connected Graph Vertex degree: # of edges adjacent to the vertex A graph is k-connected if the degree of each vertex is at least k. Alternatively, if removal of any k-1 edges does not leave the graph disconnected A graph of N vertices is fully connected if each node has a degree (N – 1) (directly connected network) 13 V1V1 V3V3 V4V4 V5V5 V6V6 Number of edges: N(N-1)/2

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(A)Cyclic Graphs Cycle: a path with the same first and last vertex Cyclic graph: A graph that contains at least one cycle Acyclic graph: A graph with no cycles Tree: An acyclic connected graph (spanning tree, spans all vertices) Forest: A disconnected acyclic graph (a graph with many trees) 14 V1V1 V3V3 V4V4 V5V5 V6V6 V1V1 V3V3 V4V4 V5V5 V6V6 Cyclic graph Tree

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Mapping Extended LAN to a Graph Bridges and LANs become vertices, ports become edges Goal: Make a tree that spans only LANs (red vertices) 15 B3B3 A B5B5 B7B7 B K F B1B1 B2B2 B6B6 B4B4 J H I G E C D

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Spanning Tree Algorithm(1) Each bridge has a unique identifier Pick bridge with smallest ID, make it the root of the tree 16 F B1B1 H G E D

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Spanning Tree Algorithm(2) Compute shortest path (in terms of hops) from each bridge to root 17 B3B3 A B5B5 B7B7 B K F B1B1 B2B2 B6B6 B4B4 J H I G E C D

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Spanning Tree Algorithm(3) Each LAN remains connected to the bridge with shortest path to the root. In case of a tie use smallest ID 18 B3B3 A B5B5 B7B7 B K F B1B1 B2B2 B6B6 B4B4 J H I G E C D

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Message Exchange Initially, all bridges consider themselves as roots Broadcast on all ports ID, root bridge, and distance to root If bridge hears a message from another bridge with smaller ID it adjusts root/distance, and forwards Ex., Let (Y, d, X) denote (root, distance, bridge ID) B3 receives (B2, 0, B2) B3 accepts B2 as root B3 adds one on its distance to B2, and fwds to B5 (B2, 1, B3) B2 accepts B1 as root and updates B3 with (B1, 1, B2) B5 accepts B1 as root and updates B3 with (B1, 1, B3) B3 accepts B1 as root, and note that both B2, B5 are closer to root in both ports. Hence, B3 blocks all ports 19

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