CS455 Computer Communications and Networking
1. [10 pt] Suppose that the WFQ scheduling policy is applied to a buffer that supports
three classes, and suppose the weights are 0.5, 0.3, and 0.2 for the three classes.
a. Suppose that each class has a large number of packets in the buffer. In
what sequence might the three classes be served in order to achieve the
WFQ weights? (For round-robin scheduling, a natural sequence is 123123123...).
b. Suppose that classes 1 and 2 have a large number of packets in the buffer,
and there are no class 3 packets in the buffer. In what sequence might the
three classes be served to achieve the WFQ weights?
2. [20 pt] Consider a router that interconnects three subnets: Subnet 1, Subnet 2, and Subnet 3. Suppose all of the interfaces in each of these three subnets are required to have the prefix 223.1.17/24. Also, suppose that Subnet 1 is required to support at least 40 interfaces, Subnet 2 is to support at least 80 interfaces, and Subnet 3 is to support at least 20 interfaces. Provide three network addresses (of the form a.b.c.d/x) that satisfy these constraints.
3. [30 pt] Consider the topology shown in Figure 4.20 in the textbook. Denote the three subnets with hosts (starting clockwise at 12:00) as Networks A, B, and C. Denote the subnets without hosts as Networks D, E, and F.
a. Assign network addresses to each of these six subnets, with the following constraints: All addresses must be allocated from 214.97.254/23; Subnet A should have enough addresses to support 250 interfaces; Subnet B should have enough addresses to support 120 interfaces; and Subnet C should have enough addresses to support 120 interfaces. Of course, subnets D, E, and F should each be able to support two interfaces. For each subnet, the assignment should take the form a.b.c.d/x or a.b.c.d/x – e.f.g.h/y.
b. Using your answer to part (a), provide the forwarding tables (using longest prefix matching) for each of the three routers.
4. [30 pt] Consider the SDN OpenFlow network shown in Figure 4.30 in the textbook.Suppose that the desired forwarding behavior for datagrams arriving at s2 is as follows:
- any datagrams arriving on input port 1 from hosts h5 or h6 that are destinedto hosts h1 or h2 should be forwarded over output port 2;
- any datagrams arriving on input port 2 from hosts h1 or h2 that are destinedto hosts h5 or h6 should be forwarded over output port 1;
- any arriving datagrams on input ports 1 or 2 and destined to hosts h3 or h4 should be delivered to the host specified;
- hosts h3 and h4 should be able to send datagrams to each other.Specify the flow table entries in s2 that implement this forwarding behavior.
5. [10 pt] Consider again the SDN OpenFlow network shown in Figure 4.30 in the textbook. Suppose that the desired forwarding behavior for datagrams arriving from hosts h3 or h4 at s2 is as follows:
a. any datagrams arriving from host h3 and destined for h1, h2, h5 or h6 should be forwarded in a clockwise direction in the network;
b. any datagrams arriving from host h4 and destined for h1, h2, h5 or h6 should be forwarded in a counter-clockwise direction in the network Specify the flow table entries in s2 that implement this forwarding behavior.
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