TCP uses a sliding window mechanism to provide flow control. The destination advertises how much space it has available in its buffers, and the source restricts its transmissions so that the receiver is not overloaded. A small flow control window, however, can impact the throughput of the TCP connection. In the extreme, a small flow control window can cause TCP to act like a stop-and-wait protocol. A very large flow control window would allow the TCP source to transmit continuously.
Objective
To examine the throughput of a TCP connection as the flow control window size is varied.
1. Duplicate your scenario and name it WFQ (for Weighted Fair Queuing). Select both routers and choose the Protocols tab => IP => QoS => Configure QoS... Set the QoS Scheme to WFQ. Note that the QoS profile is ToS Based, which means that the router will use the ToS field that we defined for the application to determine which packets get priority.
Click on the radio button next to Interfaces on selected router(s).
Click on OK to close the window.
Rerun the simulation and examine the relative packet drop rate, delay, delay variation, and buffer occupancy. Explain your results.
2. Duplicate your scenario and name it FIFO. Select the two routers and choose the Protocols tab => IP => QoS => Configure QoS... Set the QoS Scheme to FIFO. FIFO routers treat all packets the same. No priorities are implemented.
Click on the radio button next to Interfaces on selected router(s).
Click on OK to close the window.
Rerun the simulation and examine the relative packet drop rate, delay, delay variation, and buffer occupancy. Explain your results.
3. Compare the Weighted Fair Queuing, Priority Queuing, and FIFO schemes in terms of drop rate, delay, and delay variation. What are the advantages and disadvantages of each?
4. Edit the attributes of the QoS Parameters object. Look in the WFQ Profiles attribute at the ToS based profile. What weights are associated with the various ToS values in the WFQ scheme? How do these weights affect the operation of the router?
5. Edit the attributes of the QoS Parameters object. Look in the Priority Queuing Profiles at the ToS based profile. How many priority levels are defined? What ToS values are associated with each priority level?
6. Edit the attributes of the QoS Parameters object. Look in both the Priority Queuing Profiles and the WFQ Profiles. Other than ToS, how else may packets be classified for priority service?