1. Chapter 6 Delivery & Forwarding of IP Packets
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2. 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 )
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3. 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
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4. 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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5. 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.
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6. 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 )
In this case, R1 sends to a specific network however, R2 sends to the remainder of the Internet (default)
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7. 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
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8. Configuration for routing for R1, Classful Case
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9. Simplified forwarding module in classful address with subnetting
Recall for the Classful case, subnetting is done within the organization
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10. Configuration for the Classful and Subnetting Case
Doesn’t know what network is connected to router here
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11. Simplified forwarding module in classless address
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12. Routing Table for R1 in the Illustrated Configuration – Classless Case
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13. 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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14. 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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15. 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
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16. Routing Table
Mask: used to extract the net id of the Rx. For Host-Specific Routing - the mask is and for Default Routing – the mask is
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
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17. A routing example
Router R1 receives 500 packets for destination how does Router R1 uses it’s routing table ???
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18. Mask Dest. Next Hop I. m0 m2 m1 m0 m0 m0 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
&  no match
&  no match
&  no match
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19. Mask Dest. Next Hop I. m0 m2 m1 m0 m0 m0 m0
U case
UGH case
UG case
Host-specific
&  no match
Network-specific
&  match
Router stops when match is made
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20. Example 2 Make the routing table for router R1 in the Figure U UG
Mask Destination Next Hop I. m0 m1 m1 m0 U UG
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21. STRUCTURE OF A ROUTER
We represent a router as a black box that accepts incoming packets from one of the input ports (interfaces), uses a routing table to find the departing output port, and sends the packet from this output port.
The topics discussed in this section include:
Components
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22. 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
This is where delay is incurred
Performs layer 1 and 2 functions: bits to signal, packet encapsulated into frame, error control overhead added
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23. Crossbar Switching Fabric
Cross Point
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24. 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
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25. (How the routers’ tables are filled in)
Chapter 11
Unicast Routing
Protocols
(RIP, OSPF, BGP)
(How the routers’ tables are filled in)
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26. Explain how a router uses a routing table when a packet arrives ?
Before Starting
Explain how a router uses a routing table when a packet arrives ?
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27. Routing Protocols
At this stage, we understand how a router uses a routing table in making a next hop decision
However, what dictates HOW the routing tables are filled in ?
Tables are concerned about the next hop only
What’s responsible for looking across the entire path or route – what makes the decision of the best route ?
Routing Protocols and Algorithms are used
Routing protocols allow routers to share info with one another dynamically - as the Internet makes changes, the routing protocols allow routers to inform other routers
Routers communicate to their neighboring routers - gossip
Routing protocols implement the procedures for combining info received from other routers
Routing Algorithms – decision making analysis – the “brains” – using the info provided
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28. Autonomous systems
Because the Internet is so large, one protocol cannot handle all of the updating of tables – create groups and networks and routers called Autonomous Systems
Routing within the autonomous system is called “interior routing”
Routing between the autonomous systems is called “exterior routing”
NOTE: different interior routing protocols can be used for each autonomous systems HOWEVER, only one exterior routing protocol is used
R1, R2, R3 and R4 use an interior and exterior routing protocol – all other routers only use an interior routing protocol
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29. How does it work ? Tx Rx
A certain “cost” or “metric” is assigned each network
In figuring out the best route from Tx to Rx, the set of networks with the smallest sum is chosen
More generically, the set of networks best meeting the “metric’s” objective is chosen
If #hops was the metric, we would want to traverse the least number of networks in going from Tx to Rx
If max throughput was the metric, a fiber optic network would have a better metric than a coaxial network.
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30. Explain routing using your street/highway analogy
Autonomous systems
Default routing
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31. Popular Unicast Routing protocols
RIP – Routing Information Protocol – treats each network the same (assigns the same cost for each network)
OSPF – Open Shortest Path First protocol – assigns a cost for passing through a network based on the type of service required – routes through the network can have different cost – each router would have several tables
BGP – Border Gateway Protocol – is an exterior routing protocol that uses a policy that defines what paths should be chosen
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32. RIP Algorithm Recall: each router sends message to it’s neighbor
Distance Vector Algorithm built from Bellman-Ford Algorithm
Recall: each router sends message to it’s neighbor
For the router receiving a RIP response
1st – add one to hop count for each destination advertised
2nd – repeat the following steps for each advertised destination
1. If destination is not in table
add destination to table
2. Else if destination is in table
1. If next-hop field is the same
replace entry in table with advertised one
2. Else next-hop different
replace entry if advertised hop count is less
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33. Explain RIP in Simple English
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34. Example RIP Algorithm Router receives RIP message for some router C
The RIP message list destination networks, corresponding hop count and next hop (not listed in diagram)
1st step: increment hop count
Net1: no news, don’t change
Net2: same next hop, so replace 2 with 5
Net 3: new router, so add
Net 6: different next hop, new hop count less, so replace
Net 8: different next hop, new hop count the same, don’t change
Net 9: different next hop, new hop count larger, do not change
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35. Initial routing tables in a small
autonomous system
Initial tables are created from config file (and hop counts are set to 1) – next hop fields are empty initially because all networks are directly connected
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36. Final routing tables for the previous figure
For example, suppose packet hitting Router A first had a destination of Net 66 ?
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