1. 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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2. 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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3. Crossbar Switching Fabric
Cross Point
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4. 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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5. (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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6. 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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7. 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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8. 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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9. 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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10. Explain routing using your street/highway analogy
Autonomous systems
Default routing
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11. 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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12. 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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13. Explain RIP in Simple English
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14. 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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15. 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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16. 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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17. RIP message format
Command – 8-bit field specifying the type of message: response (2) or request (1)
Version – 8-bit field specifying RIP version
Family – 16-bit specifying protocol family (TCP/IP=2)
Network Address – address of the destination network
Distance – 32-bit field defining the hop count from advertising router to destination network
NOTE: Request can be issued by a newly added router or by a router seeking certain info
NOTE: 2 response types: Solicited – response to request, Unsolicited – periodic updates
Gray fields repeated for each destination network
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18. Example 1
What is the periodic response sent by router R1 in the figure below. Assume R1 knows about the whole autonomous system.
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19. Solution
R1 can advertise three networks , , and The periodic response (update packet) is shown below
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20. RIP timers
Periodic Timer – each router has timer set to secs and when the timer counts down, an update message is sent
Expiration Timer – governs the validity of the next-hop – when router receives next-hop update, timer is set to 180 sec. If there is a problem and the router doesn’t receive it’s 30 sec update, the route info expires (invalid) after the 180 sec count down – then the hop count is set to 16 (infinity)
Garbage Collection Timer – once the route expires, this timer is set to 120 sec and counts downs – allows neighbors time to become aware of invalidity – after count down, info is purged
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21. Problems with RIP
Slow Convergence – the time it takes a change in the Internet to propagate through the rest of the Internet – recall the periodic updates with neighbors. DEPENDING ON THE DATA RATE, millions or billions of bits could be sent in that time – therefore possibly lost
nx15s
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22. RIP Problem - Instability
RA and RB has hop counts 1 and 2, respectively for Net1
Net1 goes down – RA can update fast due to direct connection – sets hop count to 16
RA has to wait 30 sec to update RB (this is the problem)
In meanwhile, RB sends update to RA with hop count 2 for Net1 (incremented to 3)
Now when RA finally send the update to RB, it sends a hop count of 3 (incremented to 4) – RA thinks it’s another route to Net1
This INSTABILITY (back-and-forth) continues until both set hop count to 16
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23. Exam 3 Results & Grading Scale
Average Score = 28
Standard Deviation= 14
A-grade (3 students)
B-grade (3 students)
C-grade (13 students)
D-grade (12 students)
F-grade (0 students)
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Lecture