Network Layer 4.2
SCHEDULING Consider the pattern of red and green packet arrivals to a router's output port queue. 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).
1 2 3 4 5 6 7
LONGEST PREFIX MATCHING [exact image] Consider the following forwarding table. Indicate the output to link interface to which a datagram with the destination addresses below will be forwarded under longest prefix matching.
11001000 00010111 00010010 10101101 - first destination address in the list that maps to output port 0 11001000 00010111 00011000 00001101 - first dest addr in the list that maps to output port 1 11001000 00010111 00011001 11001101 - first dest addr in the list that maps to output port 2 10001000 11100000 00011000 00001101 - first dest addr in the list that maps to output port 3 11001000 00010111 00011000 11001111 - second dest addr in the list that maps to output port 1 11001000 00010111 00010001 01010101 - second dest addr in the list that maps to output port 0 11001000 00010111 00011101 01101101 - the second dest addr in the list that maps to output port 2
HOL BLOCKING What is meant by Head of the Line (HOL) blocking?
A queued datagram waiting for service at the front of a queue prevents other datagrams in queue from moving forward in the queue book: A queued packet in an input queue must wait for transfer through the fabric, even though its output port is free) because it is blocked by another packet at the head of the line
Interactive Problems: Scheduling Scheduling Discipline: FIFO
At t=1, which packet is sent out? 1 At t=2, which packet is sent out? n/a At t=3, which packet is sent out? 2 At t=4, which packet is sent out? 3 At t=5, which packet is sent out? 4 At t=6, which packet is sent out? 5 At t=7, which packet is sent out? 6 At t=8, which packet is sent out? 7
Interactive Problems: Scheduling Scheduling Discipline: RoundRobin
At t=1, which packet is sent out? 1 At t=2, which packet is sent out? n/a At t=3, which packet is sent out? 2 At t=4, which packet is sent out? 3 At t=5, which packet is sent out? 6 At t=6, which packet is sent out? 4 At t=7, which packet is sent out? 7 At t=8, which packet is sent out? 5
Interactive Problems: Scheduling Scheduling Discipline: Priority
At t=1, which packet is sent out? 1 At t=2, which packet is sent out? n/a At t=3, which packet is sent out? 2 At t=4, which packet is sent out? 3 At t=5, which packet is sent out? 6 At t=6, which packet is sent out? 7 At t=7, which packet is sent out? 8 At t=8, which packet is sent out? 4
Interactive Problems: Scheduling Scheduling Discipline: WFQ
At t=1, which packet is sent out? 1 At t=2, which packet is sent out? n/a At t=3, which packet is sent out? 2 At t=4, which packet is sent out? 4 At t=5, which packet is sent out? 5 At t=6, which packet is sent out? 7 At t=7, which packet is sent out? 8 At t=8, which packet is sent out? 3
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)?
At the input port where a packet arrives
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?
At the input port where a packet arrives.
Match names of principal router components (A, B, C, D) with their function and whether they are in the network-layer data plane or control plane D A B C
D = the routing processor, operating primarily in the control plane A = input ports, operating primarily in the data plane B = the switching fabric, operating primarily in the data plane C = output ports, operating primarily in the data plane
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:
The packet will either be dropped or another packet will be removed (lost) from the buffer to make room for this packet, depending on the policy. But the packet will definitely not be sent back to the input port.
