Knowledge checks

What's inside a router?



Match the names of the principal router components (A,B,C,D below) with their function and whether they are in the network-layer data plane or control plane.


Question List:
Answer List:

  1. output ports, operating primarily in the data plane.

  2. the switching fabric, operating primarily in the data plane.

  3. the switching fabric, operating primarily in the control plane.

  4. the routing processor, operating primarily in the data plane.

  5. the routing processor, operating primarily in the control plane.

  6. input ports, operating primarily in the control plane.

  7. output ports, operating primarily in the control plane.

  8. input ports, operating primarily in the data plane.


 

That's Correct!

That's Incorrect. Try Again

1/12

Where does destination address lookup happen?



Where in a router is the destination IP address looked up in a forwarding table to determine the appropriate output port to which the datagram should be directed?



 

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That's Incorrect. Try Again

2/12

Where does "match+action" happen?



Where in a router does "match plus action" happen to determine the appropriate output port to which the arriving datagram should be directed?



 

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3/12

Longest prefix matching.



Consider the following forwarding table below. Indicate the output to link interface to which a datagram with the destination addresses below will be forwarded under longest prefix matching. (Note: The list of addresses is ordered below. If two addresses map to the same output link interface, map the first of these two addresses to the first  instance of that link interface.) [Note: You can find more examples of problems similar to this here.]


Question List:
Answer List:

  1. This is the first destination address in the list that maps to output port 1.

  2. This is the first destination address in the list that maps to output port 0.

  3. This is the second destination address in the list that maps to output port 0.

  4. This is the second destination address in the list that maps to output port 1.


  5. This is the first destination address in the list that maps to output port 3.

  6. This is the second destination address in the list that maps to output port 3.

  7. This is the second destination address in the list that maps to output port 2.

  8. This is the first destination address in the list that maps to output port 2.


 

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That's Incorrect. Try Again

4/12

Packet dropping.



Suppose a datagram is switched through the switching fabric and arrives to its appropriate output to find that there are no free buffers.  In this case:



 

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5/12

HOL blocking.



What is meant by Head of the Line (HOL) blocking?




 

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6/12

Packet scheduling (Scenario 1, FCFS).



Consider the pattern of red and green packet arrivals to a router’s output port queue, shown below. Suppose each packet takes one time slot to be transmitted, and can only begin transmission at the beginning of a time slot after its arrival.  Indicate the sequence of departing packet numbers (at t = 1, 2, 3, 4, 5, 7, 8) under FCFS scheduling. Give your answer as 7 ordered digits (each corresponding to the packet number of a departing packet), with a single space between each digit, and no spaces before the first or after the last digit, e.g., in a form like 7 6 5 4 3 2 1).

[Note: You can find more examples of problems similar to this here.]



 

That's Correct!

Not quite. One of more of your packets are in the incorrect order.

7/12

Packet scheduling (Scenario 1, Priority).



Consider the pattern of red and green packet arrivals to a router’s output port queue, shown below. Suppose each packet takes one time slot to be transmitted, and can only begin transmission at the beginning of a time slot after its arrival.  Indicate the sequence of departing packet numbers (at t = 1, 2, 3, 4, 5, 7, 8) under priority scheduling, where red packets have higher priority.

Give your answer as 7 ordered digits (each corresponding to the packet number of a departing packet), with a single space between each digit, and no spaces before the first or after the last digit, e.g., in a form like 7 6 5 4 3 2 1).

[Note: You can find more examples of problems similar to this here.]



 

That's Correct!

Not quite. One of more of your packets are in the incorrect order.

8/12

Packet scheduling (Scenario 1, RR).



Consider the pattern of red and green packet arrivals to a router’s output port queue, shown below. Suppose each packet takes one time slot to be transmitted, and can only begin transmission at the beginning of a time slot after its arrival.  Indicate the sequence of departing packet numbers (at t = 1, 2, 3, 4, 5, 7, 8) under round robin scheduling, where red starts a round if there are both red and green packets ready to transmit after an empty slot.

Give your answer as 7 ordered digits (each corresponding to the packet number of a departing packet), with a single space between each digit, and no spaces before the first or after the last digit, e.g., in a form like 7 6 5 4 3 2 1).

[Note: You can find more examples of problems similar to this here.]



 

That's Correct!

Not quite. One of more of your packets are in the incorrect order.

9/12

Packet scheduling (Scenario 2, FCFS).



Consider the pattern of red and green packet arrivals to a router’s output port queue, shown below. Suppose each packet takes one time slot to be transmitted, and can only begin transmission at the beginning of a time slot after its arrival.  Indicate the sequence of departing packet numbers (at t = 1, 2, 3, 4, 5, 7, 8) under FCFS scheduling. Give your answer as 7 ordered digits (each corresponding to the packet number of a departing packet), with a single space between each digit, and no spaces before the first or after the last digit, e.g., in a form like 7 6 5 4 3 2 1).

[Note: You can find more examples of problems similar to this here.]



 

That's Correct!

Not quite. One of more of your packets are in the incorrect order.

10/12

Packet scheduling (Scenario 2, Priority).



Consider the pattern of red and green packet arrivals to a router’s output port queue, shown below. Suppose each packet takes one time slot to be transmitted, and can only begin transmission at the beginning of a time slot after its arrival.  Indicate the sequence of departing packet numbers (at t = 1, 2, 3, 4, 5, 7, 8) under priority scheduling, where red packets have higher priority.

Give your answer as 7 ordered digits (each corresponding to the packet number of a departing packet), with a single space between each digit, and no spaces before the first or after the last digit, e.g., in a form like 7 6 5 4 3 2 1).

[Note: You can find more examples of problems similar to this here.]



 

That's Correct!

Not quite. One of more of your packets are in the incorrect order.

11/12

Packet scheduling (Scenario 2, RR).



Consider the pattern of red and green packet arrivals to a router’s output port queue, shown below. Suppose each packet takes one time slot to be transmitted, and can only begin transmission at the beginning of a time slot after its arrival.  Indicate the sequence of departing packet numbers (at t = 1, 2, 3, 4, 5, 7, 8) under round robin scheduling, where red starts a round if there are both red and green packets ready to transmit after an empty slot.

Give your answer as 7 ordered digits (each corresponding to the packet number of a departing packet), with a single space between each digit, and no spaces before the first or after the last digit, e.g., in a form like 7 6 5 4 3 2 1).

[Note: You can find more examples of problems similar to this here.]



 

That's Correct!

Not quite. One of more of your packets are in the incorrect order.

12/12

We gratefully acknowledge the programming and problem design work of John Broderick (UMass '21), which has really helped to substantially improve this site.

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