CS 457, Fall 2004

Computer Networks

Practice questions - Instead of Homework 8

1. A server using HTTP 1.0 is downloading a web page with 3 embedded objects. How many TCP connections are used?
2. Repeat question 1 if the server was using HTTP 1.1.
3. Discuss the advantages and disadvantages of HTTP 1.0 and HTTP 1.1.
4. A TCP connection timed out when the congestion window was 16. After the timeout, the application still had 56 packets to send. How many roudtrips will it take to send them?
5. Discuss the advantages and disadvantages of LRU and LFU caching policies
6. A page is requested at times 0, 10, 15, and 20. The delay between the requesting client and a cache is 1. The delay between the cache and the server is 2. The cache uses a TTL-based algorithm with TTL=6. Draw a diagram showing which requests are served from the cache without contacting the server, which requests are forwarded to the server, and which requests result in if-modified-since requests to the server, if any.
7. If the page in question 6 is modified at the server at time 4 and 16, do any of the aforementioned requests get served a stale (i.e., not up to date) copy of the page? If so, which one(s)?
8. Repeat questions 6 and 7 if the cache is using update contracts with contract time = 6.

9. Which types of consistency management on the left-hand-side below are most appropriate for the content types on the right-hand-side? (Note: the higher the tolerance to staleness, the less important it is to serve the most up to date content.)


TTL-schemes                                 Frequently updated but rarely
                                            requested pages with low tolerance
                                            to staleness

Update contracts		            Content with high tolerance to
					    staleness


Invalidate contracts			    Very popular pages that are rarely
					    updated and that have a low tolerance
					    to staleness.

10. What is the "listen queue" of a server?