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Chapter 6 Delivery & Forwarding of IP Packets

Lecture #4

• Items you should understand by now – before routing

• Physical Addressing – with in the local network

• Network Addressing and subnetting – across interconnected networks

• What is being routed across interconnected networks – IP Datagram – Frame purpose ?

• Access Methods versus Routing versus Switching ??

• Go into Routing now• Routing conceptually

• How routers work

• Routing Protocols versus Routing Algorithms

• Unicast Routing and Multicast Routing

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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)

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Next-hop routingNext-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

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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

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Host-specific RoutingHost-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

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.

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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)

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Simplified forwarding module in classful address without subnetting

For the Classful case, per router, a table was needed for each class – this made the searching simple

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Configuration for routing for R1, Classful Case

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Simplified forwarding module in classful address with subnetting

Recall for the Classful case, subnetting is done within the organization

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Configuration for the Classful and Subnetting Case

Doesn’t know what network is connected to router here

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Simplified forwarding module in classless address

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Routing Table for R1 in the Illustrated Configuration – Classless Case

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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

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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

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Router’s Table LogisticsWhen 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

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Routing Table

FlagsU - 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)

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

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

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A routing example

Router R1 receives 500 packets for destination 192.16.7.14 - how does Router R1 uses it’s routing table ???

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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

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

U case

UGH case

UG case

the router applies the masks to the destination address until a match with the second column

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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

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

U case

UGH case

UG case

Router stops when match is made

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Example 2Make the routing table for router R1 in the Figure

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

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STRUCTURE OF A ROUTER

We represent a router as a black box that accepts incoming We represent a router as a black box that accepts incoming packets from one of the input ports (next hop), uses a routing packets from one of the input ports (next hop), uses a routing table to find the departing output port, and sends the packet from table to find the departing output port, and sends the packet from this output port (interface).this output port (interface).

The topics discussed in this section include:The topics discussed in this section include:

Components Components

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Router components

Performs layer 1 and 2 functions: signal to bits, packet decapsulated from frame, error control performed on bits, buffers packets before going to the switching fabric

Performs layer 1 and 2 functions: bits to signal, packet encapsulated into frame, error control overhead added

This is where delay is incurred

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Crossbar Switching Fabric

Cross Point

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A banyan switch

Uses a binary string to route across the switch

Example

Given a packet came in on port 1 and needed to go out of port 6, the binary string of 110 will be used – explain this