Previous Lecture. Link Layer & Network Layer. Link Layer. This Lecture. Framing. Sending bits. Chapter 7.C and 7.D

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1 hapter 7. and 7. Previous Lecture Layer & Network Layer The network is organized into layers Prof. ina Katabi Some slides are from lectures by Nick Mckeown, Ion Stoica, Frans Kaashoek, Hari alakrishnan, and Sam Madden Layer Layer Hierarchical ddressing and Routing Problem: eliver data from one end of the link to the other Need to address: its nalog its Framing rrors Medium ccess ontrol (The thernet Paper) 4 its nalog its Sending bits Receiver needs to detect the value of the bits Manchester ncoding: each bit is a transition Having a transition in each bit allows the receiver to synchronize to the sender s clock 0 0 Time Framing Receiver needs to detect the beginning and the end of a frame Use special bit-pattern to separate frames.g., pattern could be (7 ones) it stuffing is used to ensure that a special pattern does not occur in the data If pattern is Whenever the sender sees a sequence of 6 ones in the data, it inserts a zero (reverse this operation at receiver) 5 6

2 etection: rror Handling Use error detection codes, which add some redundancy to allow detecting errors When errors are detected orrection: Some codes allow for correction Retransmition: an have the link layer retransmit the frame (rare) iscard: Most link layers just discard the frame and rely on higher layers to retransmit Layer Hierarchical ddressing and Routing 7 8 The Internet Protocol (IP) The IP Header Protocol Stack vers HLen TOS tal Length pp Hop count I Flags FRG Offset Transport TP / UP ata Hdr TP packet TTL Protocol checksum Network IP ata Hdr IP packet SR IP ddress ST IP ddress (OPTIONS) (P) 9 0 Network Layer: finds a path to the destination and forwards packets along that path ach router has a forwarding table tables are created by a routing protocol R R R R ifference between routing and forwarding is finding the path is the action of sending the packet to the nexthop toward its destination table at R st. ddr

3 In a Router dst is via 4 Inside a router R dst 4 dst, ingress, ingress, ingress 4, ingress, egress, egress, egress 4, egress, ingress, ingress, ingress 4, ingress Table ecision, egress, egress, egress 4, egress 4 an IP Packet Lookup packet s ST in forwarding table If known, find the corresponding outgoing link If unknown, drop packet ecrement TTL and drop packet if TTL is zero; update header hecksum Forward packet to outgoing port Transmit packet onto link Layer Hierarchical ddressing & Routing 5 6 The Routing Problem: Generate forwarding tables Vector Routing Protocol Initialization ach node knows the path to itself For example, initializes its paths ST nd layer null 7 8

4 Vector Step : dvertisement ach node tells its neighbors its path to each node in the graph Vector Step : Update Route Info ach node use the advertisements to update its paths received: From : From : From : null null null For example, receives: From : From : null null From : null 9 updates its paths: ST nd layer null ST nd layer null <> <> <> Note: t the end of first round, each node has learned all one-hop paths 0 Vector Periodically repeat Steps & In round, receives: From : updates its paths: ST nd layer null <> <> <> From : null null <> <> <> <> ST From : null <> <> nd layer null <> <> <> <, > Questions bout Vector How do we ensure no loops? What happens when a node hears multiple paths to the same destination? What happens if the graph changes? Note: t the end of round, each node has learned all two-hop paths Questions bout Vector How do we ensure no loops? When a node updates its paths, it never accepts a path that has itself What happens when a node hears multiple paths to the same destination? It picks the better path (e.g., the shorter number of hops) What happens if the graph changes? lgorithm deals well with new links deal with links that go down, each router should discard any path that a neighbor stops advertising domain- Hierarchical Routing domain- domain- Interior router order router Internet: collection of domains/networks Inside a domain: Route over a graph of routers etween domains: Route over a graph of domains ddress: concatenation of omain Id, Node Id 4 4

5 Hierarchical Routing dvantage scalable Smaller tables Smaller messages elegation ach domain can run its own routing protocol isadvantage Mobility is difficult ddress depends on geographic location Sup-optimal paths.g., in the figure, the shortest path between the two machines should traverse the yellow domain. ut hierarchical routing goes directly between the green and blue domains, then finds the local destination path traverses more routers. 5 5