4_Routing_spring10 - Chapter 4 Static and Dynamic Routing...

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TCP/IP Essentials A Lab-Based Approach Chapter 4 Chapter 4 Static and Dynamic Static and Dynamic Routing Routing Spring 2010
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Polytechnic Institute of NYU Slide 2 zjzhao Routing Routing is to transfer packets from a source to a destination using network layer protocol information. Two activities: • Determine optimal routing paths • Transport packets through an internetwork Routing table • Records optimal routes. • Gets consulted when a forwarding decision is to be made. • Can be set manually, updated by some ICMP messages, or by using dynamic routing protocols.
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Polytechnic Institute of NYU Slide 3 zjzhao Next-Hop Routing •Direct delivery: send datagram directly through Layer 2 (Ethernet, …) when the source and the destination are on the same (sub)network. •Indirect delivery: when the source and the destination are NOT on the same network Need to send datagram through a router. Consult the routing table to determine the next hop router. Only ONE hop on the path is listed in the routing table.
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Polytechnic Institute of NYU Slide 4 zjzhao Routing Table Entries •Destination address – a specific host or network IP address •Next hop address - the IP address of the next-hop router, or of a directly connected network. •Flags: U: route is up G: route is to a gateway (router); if this flag is not set, destination is directly connected H: route is to a host, i.e., destination address is the complete host address; if this flag is not set, route is to a network and destination address is a netID or subnetID D: route created by redirect M: route modified by redirect •Interface - the name of the interface for the next hop
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Polytechnic Institute of NYU Slide 5 zjzhao Routing Table Lookup To route each IP packet, the destination IP address is first extracted and then • The network prefix gets calculated to determine whether the network prefix matches any directly connected network address so the packet can be delivered directly. • If not direct delivery, a routing table lookup takes place in the following order named as the Longest-prefix-matching rule • Find matching host address • Find matching network address • Find default entry • To keep the routing table small, network-specific entries and default router are often used eth0 0 0 40 UG 0.0.0.0 128.238.4.4 0.0.0.0 lo 0 0 40 U 255.0.0.0 0.0.0.0 127.0.0.0 eth0 0 0 40 U 255.255.255.0 0.0.0.0 128.238.4.0 Iface irtt Window MSS Flags Genmask Gateway Destination
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Polytechnic Institute of NYU Slide 6 zjzhao Statically Setting IP Routing Tables • Static Routing: set IP routing table without a routing protocol • Use static routing when The network is small Only a single connection point to other networks •Ways to set IP routing tables with static routing By default when the interface is configured during bootstrap > e.g., using the Dynamic Host Configuration Protocol (DHCP) Use route command from the system bootstrap file Via ICMP redirect messages Via ICMP router advertisement/router discovery messages
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Polytechnic Institute of NYU Slide 7
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4_Routing_spring10 - Chapter 4 Static and Dynamic Routing...

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