Chapter 6 Delivery & Forwarding of IP Packets Dr. Clincy

ROUTING METHODS There are various routing methods: Next-Hop Routing – table only holds the address of the next hop (instead info regarding the entire route) – routing table for each host Network-Specific Routing – instead of an entry for each host (on the same network), only one entry for the network is defined Host-Specific Routing – for a specific destination host, you might want to control the exact route – in this case, the actual Rx is listed in the routing table and the desired next hop is listed Default Routing – instead of listing all of the various networks in the Internet, Tx host would use one entry called the Default (network address 0.0.0.0) Dr. Clincy

Next-hop routing Next-Hop Routing – table only holds the address of the next hop (instead info regarding the entire route) Show more routers in better illustrating the routing table Dr. Clincy

Network-specific routing Network-Specific Routing – instead of an entry for each host on the same physical network, only one entry for the network is defined Dr. Clincy

Host-specific Routing Host-Specific Routing – for a specific destination host, you might want to control the exact route – in this case, the actual Rx is listed in the routing table and the desired next hop is listed R1 In this case, you want every packet traveling to Host B to traverse through R3. For the other hosts on N2 and N3, the Network-specific routing approach is used. Dr. Clincy

Default Routing Default Routing – instead of listing all of the various networks in the Internet, Tx host would use one entry called the Default (network address 0.0.0.0) In this case, R1 sends to a specific network however, R2 sends to the remainder of the Internet (default) Dr. Clincy

Simplified forwarding module in classful address without subnetting Using the next-hop address and interface number, ARP searches for the physical address in facilitating the actual hop For the Classful case, per router, a table was needed for each class – this made the searching simple Dr. Clincy

Configuration for routing for R1, Classful Case Dr. Clincy

Simplified forwarding module in classful address with subnetting Recall for the Classful case, subnetting is done within the organization Dr. Clincy

Configuration for the Classful and Subnetting Case Doesn’t know what network is connected to router here Dr. Clincy

Simplified forwarding module in classless address Dr. Clincy

Routing Table for R1 in the Illustrated Configuration – Classless Case Dr. Clincy

Address aggregation With the classless approach, routing tables increased – in reducing the size of some tables, use a router to represent multiple blocks – address aggregation Dr. Clincy

STATIC VERSUS DYNAMIC ROUTING Host or router uses a routing table Table can be either static or dynamic in nature A static routing table contains information entered manually. A dynamic routing table is updated periodically using one of the dynamic routing protocols such as RIP, OSPF, or BGP Regarding dynamic routing table: if fiber cut or router failure, the tables are updated Dr. Clincy

Router’s Table Logistics When the router is looking for the route, it: First check for direct delivery Then host-specific delivery, The network-specific delivery, and Finally, default delivery This order can be organized with in the routing table Dr. Clincy

Routing Table Mask: used to extract the net id of the Rx. For Host-Specific Routing - the mask is 255.255.255.255 and for Default Routing – the mask is 0.0.0.0. Destination Address: either the destination host address or destination network address Next-hop Address: next hop router address Flags U - The router is up and running. If router is down, the packet discarded G - The destination is in another network. If G flag present, indirect delivery (if not, direct delivery) H – If H flag present, destination field contains Host-specific address (if not present, network address) D – If D flag present, routing info added to host routing table via ICMP’s redirection (cover later) M - If M flag present, routing info was modified via ICMP’s redirection (cover later) Reference count: # of users using this route at any moment Use: # of packets transmitted through this router for the corresponding Rx Interface: name of the interface Dr. Clincy

A routing example Router R1 receives 500 packets for destination 192.16.7.14 - how does Router R1 uses it’s routing table ??? Dr. Clincy

Mask Dest. Next Hop I. 255.0.0.0 111.0.0.0 -- m0 255.255.255.224 193.14.5.160 - m2 255.255.255.224 193.14.5.192 - m1 ---------------------------------------------------------------------------255.255.255.255 194.17.21.16 111.20.18.14 m0 ---------------------------------------------------------------------------- 255.255.255.0 192.16.7.0 111.15.17.32 m0 255.255.255.0 194.17.21.0 111.20.18.14 m0 0.0.0.0 0.0.0.0 111.30.31.18 m0 U case UGH case UG case the router applies the masks to the destination address until a match with the second column Direct delivery 192.16.7.14 & 255.0.0.0  192.0.0.0 no match 192.16.7.14 & 255.255.255.224  192.16.7.0 no match 192.16.7.14 & 255.255.255.224  192.16.7.0 no match Dr. Clincy

Mask Dest. Next Hop I. 255.0.0.0 111.0.0.0 -- m0 255.255.255.224 193.14.5.160 - m2 255.255.255.224 193.14.5.192 - m1 ---------------------------------------------------------------------------255.255.255.255 194.17.21.16 111.20.18.14 m0 ---------------------------------------------------------------------------- 255.255.255.0 192.16.7.0 111.15.17.32 m0 255.255.255.0 194.17.21.0 111.20.18.14 m0 0.0.0.0 0.0.0.0 111.30.31.18 m0 U case UGH case UG case Host-specific 192.16.7.14 & 255.255.255.255 192.16.7.14 no match Network-specific 192.16.7.14 & 255.255.255.0 192.16.7.0 match Router stops when match is made Dr. Clincy

Example 2 Make the routing table for router R1 in the Figure U UG Mask Destination Next Hop I. 255.255.0.0 134.18.0.0 -- m0 255.255.0.0 129.8.0.0 222.13.16.40 m1 255.255.255.0 220.3.6.0 222.13.16.40 m1 0.0.0.0 0.0.0.0 134.18.5.2 m0 U UG Dr. Clincy