EEC-484/584 Computer Networks Discussion Session for HTTP and DNS Wenbing Zhao [email protected]

EEC-484/584EEC-484/584Computer NetworksComputer Networks

Discussion Session for Discussion Session for HTTP and DNSHTTP and DNS

Wenbing ZhaoWenbing Zhao

[email protected]@ieee.org

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Fall Semester 2008Fall Semester 2008 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao

OutlineOutline

• Web Caching

• DNS Resource Records

• Exercises

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Web CachingWeb Caching

• user sets browser: Web accesses via proxy server

• browser sends all HTTP requests to proxy server– object in cache: returns

cached object

– else cache requests object from origin server, then returns object to client

Goal: satisfy client request without involving origin server

client

Proxyserver

client

HTTP request

HTTP request

HTTP response

HTTP response

HTTP request

HTTP response

origin server

origin server

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More about Web CachingMore about Web Caching

• Proxy server acts as both client and server

• Typically proxy server is installed by ISP (university, company, residential ISP)

Why Web caching?• Reduce response time for

client request• Reduce traffic on an

institution’s access link• Internet dense with

caches: enables “poor” content providers to effectively deliver content

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Non-Caching Example Non-Caching Example

Assumptions• Average object size =

100,000 bits• Avg. request rate from

institution’s browsers to origin servers = 15/sec

• Delay from institutional router to any origin server and back to router = 2 sec

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

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Non-Caching Example Non-Caching Example

Consequences• Utilization on LAN = 15%• Utilization on access link =

100%• Total delay = Internet

delay + access delay + LAN delay

= 2 sec + minutes + milliseconds

originservers

public Internet



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Non-Caching ExampleNon-Caching Example

Possible solution• Increase bandwidth of

access link to, say, 10 Mbps

Consequences• Utilization on LAN = 15%• Utilization on access link = 15%• Total delay = Internet delay +

access delay + LAN delay

= 2 sec + msecs + msecs• Often a costly upgrade

originservers

public Internet


10 Mbps access link

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Caching ExampleCaching Example

Install proxy server• Suppose hit rate is 0.4

Consequence• 40% requests will be satisfied almost

immediately• 60% requests satisfied by origin

server• Utilization of access link

reduced to 60%, resulting in negligible delays (say 10 msec)

• Total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + .4*milliseconds < 1.4 secs

originservers

public Internet



InstitutionalProxy server

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URLURL

• Q1. Imagine that someone in the CS Department at Stanford has just written a new program that he wants to distribute by FTP. He puts the program in the FTP directory ftp/pub/freebies/newprog.c. What is the URL for this program likely to be?

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HTTP and CachingHTTP and Caching• Q2. The If-Modified-Since header can be used to check

whether a cached page is still valid. Requests can be made for pages containing images, sound, video, and so on, as well as HTML. Do you think the effectiveness of this technique is better or worse for JPEG images ascompared to HTML?

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Web and CachingWeb and Caching• Q3. Consider an institutional network connected to the

internet. Suppose that the average object size is 900,000 bits and that the average request rate from the institution’s browsers to the origin servers is 1.5 requests per second. The bandwidth of the access link is 1.5 Mbps. Also suppose that the amount of time it takes from when the router on the Internet side of the access link forwards an HTTP request until it receives the response in two seconds on average. Model the total average response time as the sum of the average access delay (that is, the delay from Internet router to institution router) and the average Internet delay. (continued on next slide)

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Web and CachingWeb and Caching• Q3 (cont’d) For the average access delay, use /(1-),

where is the average time required to send an object over the access link and is the arrival rate of objects to the access link.– Find the total average response time.– Now suppose a cache is installed in the institutional LAN.

Suppose the high rate is 0.4. Find the total response time.

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DNSDNS

• Q4. DNS typically uses UDP instead of TCP. If a DNS packet is lost, there is no automatic recovery. Does this cause a problem, and if so, how is it solved?

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DNSDNS

• Q5. Although it was not mentioned in the text, an alternative form for a URL is to use the IP address instead of its DNS name. An example of using an IP address is http://192.31.231.66/index.html. How does the browser know whether the name following the scheme is a DNS name or an IP address.

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DNS, Web, HTTPDNS, Web, HTTP• Q6. Suppose within your Web browser you click on a link to

obtain a Web page. The IP address for the associated URL is not cached in your local host, so a DNS look-up is necessary to obtain the IP address. Suppose that n DNS servers are visited before your host receives the IP address from DNS; the successive visits incur an RTT of RTT1, …, RTTn. Further suppose that the Web page associated with the link contains exactly one object, consisting of a small amount of HTML text. Let RTT0 denote the RTT between the local host and the server containing the object. Assuming 0 transmission time of the object, how much time elapses from when the client clicks on the link until the client receives the object?

Documents

EEC-484/584 Computer Networks Discussion Session for HTTP and DNS Wenbing Zhao [email protected]