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Section 2.4.4

Design and Feasibility Reports

Design and feasibility reports describe one or more design solutions to a specific problem and determine if the proposed solution is practical andfeasible. Preferably, more than one solution is offered, in which case the report compares thevarious designs and determines which option is best.

Design and feasibility reports are essentially the same type of document, differing only in theamount of emphasis placed on practical and economic viability of the design in comparison withother possible solutions. A design report, often very similar to an internal proposal, focuses on describing one specific implementation. Afeasibility study, on the other hand, also emphasizes the investigation and comparison ofalternative solutions.

Design reports and feasibility reports are crucial for decision making and product development inalmost any technical organization. They document an engineer's thinking through a solution to aproblem, a description of the solution, and the reasons why that solution should be implemented. Managers need accurate and comprehensive feasibility anddesign reports to decide where to commit scarce resources. In addition, an accurate and comprehensive design report helps in developingother documents, such as formal proposals, specifications, and work plans.

Format of Design and Feasibility Reports

Most design and feasibility reports contain the following elements:

  1. An abstract that concludes with a short summary of the recommended design.
  2. An introduction that presents the context of the situation and then gives a clear and concise statement of the problem to solved.
  3. A list of design criteria, in order of importance with the most important first. Clear and concise design criteria are crucial to any design process. They establish the standards by which it possible to determine whether a specific design is successful and to decide intelligently among competing designs.
  4. Descriptions of possible implementations. A design report will often describe only one possible implementation. A feasibility report will often present several possible designs.
  5. A recommendation with a comparison of alternatives. If a design report does not present any alternative designs, it should still explain the reasons for specific design choices with respect to the design criteria. Feasibility reports usually present one (or sometimes two) recommendations and argue for the recommended solution by showing how it best meets the stated criteria. Graphic devices, such as a table listing how each implementation meets each design criterion, are very effective in summarizing the reasons for the specific design recommendation.
  6. Elaboration of design. Design reports and feasibility reports often then give a more detailed description of the recommended design.
  7. Conclusion with recommendations for further actions and a listing of issues that must be resolved before the design can be implemented.

An example of a design and feasibility report follows.

A HyperText Filing Proxy

Debajit Ghosh
Professor Agarwal
REC TR 2pm
March 21, 1996

An Improved HyperText Filing Proxy


This paper discusses a proposed design for a large-scale caching system for World WideWeb (WWW) pages. After examining the criteria the caching system should achieve, this paperreveals some of the issues and possibilities inherent in such a caching system. It then presentsthe decisions made in the new improved HyperText Filing Proxy (HTFP) WWW caching systemand compares HTFP to other web caches used today.

1 Introduction

If the dramatic growth of the number of web sites in the world (from 130 to 90,000 sites betweenJune 1993 and January 1996 [1]) is any indication, web traffic clearlyaccounts for more and more of the traffic on the Internet. Since much of that consists ofrepeated requests for the same pages from different clients, a caching system could reduce theamount of traffic on the net. This reduction not only helps to relieve load on network serversbut also becomes a necessity when traffic is costly.

At the time if this writing, NEARNET is considering charging the MIT community per bytetransmitted across the link to the Internet it provides to MIT. As one might imagine, this couldget very expensive for a community that transfers 40,895,505 kilobytes a day [2] from the Internet, so reducing the amount of this traffic becomes a key issue. As mentioned, caching webpages can reduce the amount of traffic on the net; this can save the MIT community aconsiderable amount of money. Obviously, relying on an individual browser's caching system fora single user would not suffice; these caches are quite volatile and are typically much smallerthan necessary to make a significant reduction in the amount of web traffic across a network. Thus, we developed an improved Hyper Text Filing Proxy (HTFP) to provide such a large-scalecaching system. Basically, HTFP is a proxy server; all web requests are sent to the specifiedproxy server, which either returns a cached version of the requested page or retrieves a fresh onefrom the source. This paper outlines many of the issues considered in the design process,highlights and describes the features of HTFP, and compares HTFP to several other existingWWW caching systems.

2 Background

Before describing how the HTFP system caches web pages, let me give you some background onthe workings of the World Wide Web. This section will explain how the web works and describean important part of many web servers known as a proxy server. I assume that the reader has atleast used a web browser and done a minimal amount of exploration on the World Wide Web.

When a user clicks on a URL (Uniform Resource Locator, an address of a page on the web) inhis or her browser, the browser opens a TCP connection to the URL's target web server andsends a request for the page.

. . .

Many networks now exist behind "firewalls," security systems that block incoming and/or outgoingtraffic. For this reason, individual users' machines cannot send web packets to the rest of thenetwork and browse web pages on the WWW. These networks usually provide a machine knownas a proxy server that can transmit and receive information through the firewall. This serveraccepts web requests from clients within the network, retrieves the requested web pages, andreturns them to the clients. Proxy servers can also be useful in environments not restricted by afirewall, as this paper will explain.

3 Design Criteria

An ideal web caching system would retrieve a web page from an external server only once, placeit in a cache, serve the cached copy transparently to clients requesting that page, andautomatically update the cached copy whenever the original has been modified. Of course, noweb caching system can flawlessly realize all of these features. Accordingly, HTFP prioritizes thefeatures it wishes to achieve from among the ones mentioned above.

Specifically, the most important criterion is that HTFP reduce the amount of web traffic betweenMIT and NEARNET. The next important criterion is that the system be user-transparent; usersshould notice a difference between retrieving pages from their source or retrieving them from thecache. In addition, users should not get stale (outdated) copies of web pages from the cache. Finally, the caching system should be robust, efficient, and as simple as possible while stillmeeting the other criteria.

4 Design Issues and Considerations

HTFP represents but one of many possible implementations of a caching system that attempts tomeet the above criteria. In fact, it represents a mixture of many ideas and possibleimplementations of such a system. In order to understand the thought process behind the designof HTFP, we must first review some of the other possibilities and compare them to themechanisms present and implemented in HTFP.

4.1 Proxy vs. Transparent

Most global web caching systems (as opposed to the personal cache that each browser maintainsfor a specific user) implement the caching system system through a proxy server within theclient's network. Because of the popularity of firewalls in network, proxy servers are alreadypopular, and their very nature makes them an obvious place to implement a cache; the cachesystem could simply intercept web requests and return cached results rather than actuallyforwarding the request to the content provider. After much consideration we decided toimplement our web caching system on a proxy server as well. We did, however, consider thealternatives and ramifications of our decision, as explained below.

. . .

5 Implementation details

This section will outline the exact process by which a web request is handled and discuss otherdetails and features of the HTFP system. First, the client, who is configured to use the masterproxy as the HTTP proxy server for web servers outside the domain, sends a request tothe master proxy server. Based on whether the destination is a .com, a .edu, or another type ofsite (including straight IP address, the master proxy server forwards requests to a sub-pool ofproxies (probably about three servers in each pool). One proxy server is chosen randomly fromthis sub-pool and receives this request.

The chosen proxy then determines whether or not the request is cachable as described above. Ifit is not, the proxy opens a connection to the external server, retrieves the data, and immediatelyreturns the response to the client. Otherwise, the proxy uses an arbitrary hashing function tohash the URL and checks the top-level index of its cache for the page. This index contains theURL, title (if the page is an HTML document), two-character hash code, and actual file on theserver for each page. The index is organized by hash code; the proxy can save time by initiallychecking only the appropriate section based on the hash code of the requested URL. If thisfails, the proxy then hashes the referring URL. For component data, such as images, the refererheader [14], used by the dominant browsers such as Netscape, contains the URL of thecomponent's associated document which it checks against the corresponding section of the index. If this check fails, the proxy then scans its top-level index; it also sends RPC messages to thepeer proxies in its subpools, asking each to check their own caches for the URL and return afind. . . . 

6 Comparison with other caching systems

HTFP borrows and shares many ideas and features with other web caching systems. Implementing the cache in a proxy server is one of the easiest, most network­friendly methods, interms of slowing down the network. Given the structure and information provided by the HTTPprotocol, there are a limited number of ways to address the concern about the expiration andvalidity of cached data. . . . 

Some of the other popular or well-designed web caching systems in use today include W3C httpd,the Harvest cache, and DEC's web relay. Of these, the former two are the most popular andwidely used today, with the Harvest accelerator gaining more and more popularity daily.* W3C httpd and DEC's web relay are rather similar; the caching system is implemented in a single proxy server and distribution of the load complicates the system if maintaining a consistent cache among the servers is desired. Harvest explores and implements some of thehierarchicalcaching system ideas considered and discussed in this paper, such as havingcache-misses on an individual proxy propagate to higher-level servers. HTFP probably mostclosely resembles the Harvest cache; however, even these two systems contain significantdifferences. Table 1 provides a comparison between the key distinguishing features of the aboveproxy-based caching systems. Following the table is an explanation of the presented criteria.

Table 1: Comparison of HTFP with other popular web caching systems

Transparent reduce traffic easily scalable load distribution grouping of documents visibility of cache
W3C httpd yes yes
DEC relay yes yes
Harvest yes yes yes yes
HTFP yes yes yes yes yes yes

. . .

7. Conclusions

HTFP represents a robust, scalable caching system that achieves many of the targeted designcriteria. It brings new ideas and features into a continually explored and developing field. Indesigning this system, I considered many issues and alternatives. Often, as with any system,HTFP had to sacrifice some elegance, simplicity, or transparency in order to maintain efficiencyand usability. With some of these sacrifices, and with the newness of some of HTFP¹s features,this caching system has some tradeoffs and possible negatives. It offers opportunities for furtherdevelopment. Specifically, HTFP could benefit from examining the following hard problems:


* Based on scanning server logs for proxy-type specificationon both the author's Web servers and other Web servers.

** The DEC system accomplished this by modifying themessages the web browser sends to the proxy; obviously, this is not user transparent or practicalfor a large user base or for commercial, unmodifiable browsers.


[1] Measuring the Growth of the Web,

[2] MIT Backbone Traffic Statistics,

[3] Hypertext Transfer Protocol, HTTP/1.0,

[4] The Harvest Cache and Httpd-Accelerator,

[5] Host Distribution by Top-Level Domain Name,

[6] Hypertext Transfer Protocol/1.0.

. . .

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## Design and Feasibility Reports ##
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