THE WORLD WIDE WEB - WWW PART- 4




HTTP—The HyperText Transfer Protocol

Now that we have an understanding of Web content and applications, it is time to look at the protocol that is used to transport all this information between Web servers and clients. It is HTTP (HyperText Transfer Protocol), as specified in RFC 2616.

HTTP is a simple request-response protocol that normally runs over TCP. It specifies what messages clients may send to servers and what responses they get back in return. The request and response headers are given in ASCII, just like in SMTP. The contents are given in a MIME-like format, also like in SMTP. This simple model was partly responsible for the early success of the Web because it made development and deployment straightforward.

In this section, we will look at the more important properties of HTTP as it is used nowadays. However, before getting into the details we will note that the way it is used in the Internet is evolving. HTTP is an application layer protocol because it runs on top of TCP and is closely associated with the Web. That is why we are covering it in this chapter. However, in another sense HTTP is becoming more like a transport protocol that provides a way for processes to communicate content across the boundaries of different networks. These processes do not have to be a Web browser and Web server. A media player could use HTTP to talk to a server and request album information. Antivirus software could use HTTP to download the latest updates. Developers could use HTTP to fetch project files. Consumer electronics products like digital photo frames often use an embedded HTTP server as an interface to the outside world. Machine-to-machine communication increasingly runs over HTTP. For example, an airline server might use SOAP (an XML RPC over HTTP) to contact a car rental server and make a car reservation, all as part of a vacation package. These trends are likely to continue, along with the expanding use of HTTP.

Connections

The usual way for a browser to contact a server is to establish a TCP connection to port 80 on the server’s machine, although this procedure is not formally required. The value of using TCP is that neither browsers nor servers have to worry about how to handle long messages, reliability, or congestion control. All of these matters are handled by the TCP implementation.

Early in the Web, with HTTP 1.0, after the connection was established a single request was sent over and a single response was sent back. Then the TCP connection was released. In a world in which the typical Web page consisted entirely of HTML text, this method was adequate. Quickly, the average Web page grew to contain large numbers of embedded links for content such as icons and other eye candy. Establishing a separate TCP connection to transport each single icon became a very expensive way to operate.

This observation led to HTTP 1.1, which supports persistent connections. With them, it is possible to establish a TCP connection, send a request and get a response, and then send additional requests and get additional responses. This strategy is also called connection reuse. By amortizing the TCP setup, startup, and release costs over multiple requests, the relative overhead due to TCP is reduced per request. It is also possible to pipeline requests, that is, send request 2 before the response to request 1 has arrived.

The performance difference between these three cases is shown in Fig. 7-36. Part (a) shows three requests, one after the other and each in a separate connection. Let us suppose that this represents a Web page with two embedded images on the same server. The URLs of the images are determined as the main page is fetched, so they are fetched after the main page. Nowadays, a typical page has around 40 other objects that must be fetched to present it, but that would make our figure far too big so we will use only two embedded objects.

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In Fig. 7-36(b), the page is fetched with a persistent connection. That is, the TCP connection is opened at the beginning, then the same three requests are sent, one after the other as before, and only then is the connection closed. Observe that the fetch completes more quickly. There are two reasons for the speedup. First, time is not wasted setting up additional connections. Each TCP connection requires at least one round-trip time to establish. Second, the transfer of the same images proceeds more quickly. Why is this? It is because of TCP congestion control. At the start of a connection, TCP uses the slow-start procedure to increase the throughput until it learns the behavior of the network path. The consequence of this warmup period is that multiple short TCP connections take disproportionately longer to transfer information than one longer TCP connection.

Finally, in Fig. 7-36(c), there is one persistent connection and the requests are pipelined. Specifically, the second and third requests are sent in rapid succession as soon as enough of the main page has been retrieved to identify that the images must be fetched. The responses for these requests follow eventually. This method cuts down the time that the server is idle, so it further improves performance.

Persistent connections do not come for free, however. A new issue that they raise is when to close the connection. A connection to a server should stay open while the page loads. What then? There is a good chance that the user will click on a link that requests another page from the server. If the connection remains open, the next request can be sent immediately. However, there is no guarantee that the client will make another request of the server any time soon. In practice, Persistent connections do not come for free, however. A new issue that they raise is when to close the connection. A connection to a server should stay open while the page loads. What then? There is a good chance that the user will click on a link that requests another page from the server. If the connection remains open, the next request can be sent immediately. However, there is no guarantee that the client will make another request of the server any time soon. In practice,clients and servers usually keep persistent connections open until they have been idle for a short time (e.g., 60 seconds) or they have a large number of open connections and need to close some.

The observant reader may have noticed that there is one combination that we have left out so far. It is also possible to send one request per TCP connection, but run multiple TCP connections in parallel. This parallel connection method was widely used by browsers before persistent connections. It has the same disadvantage as sequential connections—extra overhead—but much better performance. This is because setting up and ramping up the connections in parallel hides some of the latency. In our example, connections for both of the embedded images could be set up at the same time. However, running many TCP connections to the same server is discouraged. The reason is that TCP performs congestion control for each connection independently. As a consequence, the connections compete against each other, causing added packet loss, and in aggregate are more aggressive users of the network than an individual connection. Persistent connections are superior and used in preference to parallel connections because they avoid overhead and do not suffer from congestion problems.

Methods

Although HTTP was designed for use in the Web, it was intentionally made more general than necessary with an eye to future object-oriented uses. For this reason, operations, called methods, other than just requesting a Web page are supported. This generality is what permitted SOAP to come into existence.

Each request consists of one or more lines of ASCII text, with the first word on the first line being the name of the method requested. The built-in methods are listed in Fig. 7-37. The names are case sensitive, so GET is allowed but not get.

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 .The GET method requests the server to send the page. (When we say ‘‘page’’ we mean ‘‘object’’ in the most general case, but thinking of a page as the contents of a file is sufficient to understand the concepts.) The page is suitably encoded in MIME. The vast majority of requests to Web servers are GETs. The usual form of GET is GET filename HTTP/1.1

where filename names the page to be fetched and 1.1 is the protocol version. The HEAD method just asks for the message header, without the actual page. This method can be used to collect information for indexing purposes, or just to test a URL for validity.

The POST method is used when forms are submitted. Both it and GET are also used for SOAP Web services. Like GET, it bears a URL, but instead of simply retrieving a page it uploads data to the server (i.e., the contents of the form or RPC parameters). The server then does something with the data that depends on the URL, conceptually appending the data to the object. The effect might be to purchase an item, for example, or to call a procedure. Finally, the method returns a page indicating the result.

The remaining methods are not used much for browsing the Web. The PUT method is the reverse of GET: instead of reading the page, it writes the page. This method makes it possible to build a collection of Web pages on a remote server. The body of the request contains the page. It may be encoded using MIME, in which case the lines following the PUT might include authentication headers, to prove that the caller indeed has permission to perform the requested operation.

DELETE does what you might expect: it removes the page, or at least it indicates that the Web server has agreed to remove the page. As with PUT, authentication and permission play a major role here.

The TRACE method is for debugging. It instructs the server to send back the request. This method is useful when requests are not being processed correctly and the client wants to know what request the server actually got.

The CONNECT method lets a user make a connection to a Web server through an intermediate device, such as a Web cache.

The OPTIONS method provides a way for the client to query the server for a page and obtain the methods and headers that can be used with that page.

Every request gets a response consisting of a status line, and possibly additional information (e.g., all or part of a Web page). The status line contains a three-digit status code telling whether the request was satisfied and, if not, why not. The first digit is used to divide the responses into five major groups, as shown in Fig. 7-38. The 1xx codes are rarely used in practice. The 2xx codes mean that the request was handled successfully and the content (if any) is being returned. The 3xx codes tell the client to look elsewhere, either using a different URL or in its own cache (discussed later). The 4xx codes mean the request failed due to a client error such an invalid request or a nonexistent page. Finally, the 5xx errors mean the server itself has an internal problem, either due to an error in its code or to a temporary overload.

THE WORLD WIDE WEB - WWW PART- 4

Message Headers

The request line (e.g., the line with the GET method) may be followed by additional lines with more information. They are called request headers. This information can be compared to the parameters of a procedure call. Responses may also have response headers. Some headers can be used in either direction. A selection of the more important ones is given in Fig. 7-39. This list is not short, so as you might imagine there is often a variety of headers on each request and response.

The User-Agent header allows the client to inform the server about its browser implementation (e.g., Mozilla/5.0 and Chrome/5.0.375.125). This information is useful to let servers tailor their responses to the browser, since different browsers can have widely varying capabilities and behaviors.

The four Accept headers tell the server what the client is willing to accept in the event that it has a limited repertoire of what is acceptable. The first header specifies the MIME types that are welcome (e.g., text/html). The second gives the character set (e.g., ISO-8859-5 or Unicode-1-1). The third deals with compression methods (e.g., gzip). The fourth indicates a natural language (e.g., Spanish). If the server has a choice of pages, it can use this information to supply the one the client is looking for. If it is unable to satisfy the request, an error code is returned and the request fails.

The If-Modified-Since and If-None-Match headers are used with caching. They let the client ask for a page to be sent only if the cached copy is no longer valid. We will describe caching shortly. 

The Host header names the server. It is taken from the URL. This header is mandatory. It is used because some IP addresses may serve multiple DNS names and the server needs some way to tell which host to hand the request to.

The Authorization header is needed for pages that are protected. In this case, the client may have to prove it has a right to see the page requested. This header is used for that case.

The client uses the misspelled Referer header to give the URL that referred to the URL that is now requested. Most often this is the URL of the previous page.

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This header is particularly useful for tracking Web browsing, as it tells servers how a client arrived at the page.

Although cookies are dealt with in RFC 2109 rather than RFC 2616, they also have headers. The Set-Cookie header is how servers send cookies to clients. The client is expected to save the cookie and return it on subsequent requests to the server by using the Cookie header. (Note that there is a more recent specification for cookies with newer headers, RFC 2965, but this has largely been rejected by industry and is not widely implemented.)

Many other headers are used in responses. The Server header allows the server to identify its software build if it wishes. The next five headers, all starting with Content-, allow the server to describe properties of the page it is sending.

The Last-Modified header tells when the page was last modified, and the Expires header tells for how long the page will remain valid. Both of these headers play an important role in page caching.

The Location header is used by the server to inform the client that it should try a different URL. This can be used if the page has moved or to allow multiple URLs to refer to the same page (possibly on different servers). It is also used for companies that have a main Web page in the com domain but redirect clients to a national or regional page based on their IP addresses or preferred language.

If a page is very large, a small client may not want it all at once. Some servers will accept requests for byte ranges, so the page can be fetched in multiple small units. The Accept-Ranges header announces the server’s willingness to handle this type of partial page request.

Now we come to headers that can be used in both directions. The Date header can be used in both directions and contains the time and date the message was sent, while the Range header tells the byte range of the page that is provided by the response.

The ETag header gives a short tag that serves as a name for the content of the page. It is used for caching. The Cache-Control header gives other explicit instructions about how to cache (or, more usually, how not to cache) pages.

Finally, the Upgrade header is used for switching to a new communication protocol, such as a future HTTP protocol or a secure transport. It allows the client to announce what it can support and the server to assert what it is using.

Caching

People often return to Web pages that they have viewed before, and related Web pages often have the same embedded resources. Some examples are the images that are used for navigation across the site, as well as common style sheets and scripts. It would be very wasteful to fetch all of these resources for these pages each time they are displayed because the browser already has a copy.

Squirreling away pages that are fetched for subsequent use is called caching. The advantage is that when a cached page can be reused, it is not necessary to repeat the transfer. HTTP has built-in support to help clients identify when they can safely reuse pages. This support improves performance by reducing both network traffic and latency. The trade-off is that the browser must now store pages, but this is nearly always a worthwhile trade-off because local storage is inexpensive. The pages are usually kept on disk so that they can be used when the browser is run at a later date.

The difficult issue with HTTP caching is how to determine that a previously cached copy of a page is the same as the page would be if it was fetched again. This determination cannot be made solely from the URL. For example, the URL may give a page that displays the latest news item. The contents of this page will be updated frequently even though the URL stays the same. Alternatively, the contents of the page may be a list of the gods from Greek and Roman mythology. This page should change somewhat less rapidly.

HTTP uses two strategies to tackle this problem. They are shown in Fig. 7-40 as forms of processing between the request (step 1) and the response (step 5). The first strategy is page validation (step 2). The cache is consulted, and if it has a copy of a page for the requested URL that is known to be fresh (i.e., still valid), there is no need to fetch it anew from the server. Instead, the cached page can be returned directly. The Expires header returned when the cached page was originally fetched and the current date and time can be used to make this determination.

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However, not all pages come with a convenient Expires header that tells when the page must be fetched again. After all, making predictions is hard—especially about the future. In this case, the browser may use heuristics. For example, if the page has not been modified in the past year (as told by the Last-Modified header) it is a fairly safe bet that it will not change in the next hour. There is no guarantee, however, and this may be a bad bet. For example, the stock market might have closed for the day so that the page will not change for hours, but it will change rapidly once the next trading session starts. Thus, the cacheability of a page may vary wildly over time. For this reason, heuristics should be used with care, though they often work well in practice.

Finding pages that have not expired is the most beneficial use of caching because it means that the server does not need to be contacted at all. Unfortunately, it does not always work. Servers must use the Expires header conservatively, since they may be unsure when a page will be updated. Thus, the cached copies may still be fresh, but the client does not know.

The second strategy is used in this case. It is to ask the server if the cached copy is still valid. This request is a conditional GET, and it is shown in Fig. 7-40 as step 3. If the server knows that the cached copy is still valid, it can send a short reply to say so (step 4a). Otherwise, it must send the full response (step 4b).

More header fields are used to let the server check whether a cached copy is still valid. The client has the time a cached page was last updated from the LastModified header. It can send this time to the server using the If-Modified-Since header to ask for the page only if it has been changed in the meantime.

Alternatively, the server may return an ETag header with a page. This header gives a tag that is a short name for the content of the page, like a checksum but better. (It can be a cryptographic hash, which we will describe in Chap. 8.) The client can validate cached copies by sending the server an If-None-Match header listing the tags of the cached copies. If any of the tags match the content that the server would respond with, the corresponding cached copy may be used. This method can be used when it is not convenient or useful to determine freshness. For example, a server may return different content for the same URL depending on what languages and MIME types are preferred. In this case, the modification date alone will not help the server to determine if the cached page is fresh.

Finally, note that both of these caching strategies are overridden by the directives carried in the Cache-Control header. These directives can be used to restrict caching (e.g., no-cache) when it is not appropriate. An example is a dynamic page that will be different the next time it is fetched. Pages that require authorization are also not cached.

There is much more to caching, but we only have the space to make two important points. First, caching can be performed at other places besides in the browser. In the general case, HTTP requests can be routed through a series of caches. The use of a cache external to the browser is called proxy caching. Each added level of caching can help to reduce requests further up the chain. It is common for organizations such as ISPs and companies to run proxy caches to gain the benefits of caching pages across different users. We will discuss proxy caching with the broader topic of content distribution in Sec. 7.5 at the end of this chapter.

Second, caches provide an important boost to performance, but not as much as one might hope. The reason is that, while there are certainly popular documents on the Web, there are also a great many unpopular documents that people fetch, many of which are also very long (e.g., videos). The ‘‘long tail’’ of unpopular documents take up space in caches, and the number of requests that can be handled from the cache grows only slowly with the size of the cache. Web caches are always likely to be able to handle less than half of the requests. See Breslau et al. (1999) for more information.

Experimenting with HTTP

Because HTTP is an ASCII protocol, it is quite easy for a person at a terminal (as opposed to a browser) to directly talk to Web servers. All that is needed is a TCP connection to port 80 on the server. Readers are encouraged to experiment with the following command sequence. It will work in most UNIX shells and the command window on Windows (once the telnet program is enabled).

telnet www.ietf.org 80

GET /rfc.html HTTP/1.1

Host: www.ietf.org

This sequence of commands starts up a telnet (i.e., TCP) connection to port 80 on IETF’s Web server, www.ietf.org. Then comes the GET command naming the path of the URL and the protocol. Try servers and URLs of your choosing. The next line is the mandatory Host header. A blank line following the last header is mandatory. It tells the server that there are no more request headers. The server will then send the response. Depending on the server and the URL, many different kinds of headers and pages can be observed.



Frequently Asked Questions

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Ans: The static page model we have used so far treats pages as multimedia documents that are conveniently linked together. It was a fitting model in the early days of the Web, as vast amounts of information were put online. view more..
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Ans: The basis of the Web is transferring Web pages from server to client. In the simplest form, Web pages are static. That is, they are just files sitting on some server that present themselves in the same way each time they are fetched and viewed. view more..
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Ans: The Web, as the World Wide Web is popularly known, is an architectural framework for accessing linked content spread out over millions of machines all over the Internet. view more..
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Ans: Now that we have an understanding of Web content and applications, it is time to look at the protocol that is used to transport all this information between Web servers and clients. It is HTTP (HyperText Transfer Protocol), as specified in RFC 2616. view more..
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Ans: The Web is used from most every type of computer, and that includes mobile phones. Browsing the Web over a wireless network while mobile can be very useful. It also presents technical problems because much Web content was designed for flashy presentations on desktop computers with broadband connectivity. view more..
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Ans: Web applications and the mobile Web are not the only exciting developments in the use of networks. For many people, audio and video are the holy grail of networking. When the word ‘‘multimedia’’ is mentioned, both the propellerheads and the suits begin salivating as if on cue. view more..
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Ans: Now that we know all about the ear, it is time to move on to the eye. (No, this section is not followed by one on the nose.) The human eye has the property that when an image appears on the retina, the image is retained for some number of milliseconds before decaying. If a sequence of images is drawn at 50 images/sec, the eye does not notice that it is looking at discrete images. All video systems exploit this principle to produce moving pictures. view more..
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Ans: Let us now move on to network applications. Our first case is streaming media that is already stored in files. The most common example of this is watching videos over the Internet view more..
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Ans: It is not only recorded videos that are tremendously popular on the Web. Live media streaming is very popular too. Once it became possible to stream audio and video over the Internet, commercial radio and TV stations got the idea of broadcasting their content over the Internet as well as over the air. view more..
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Ans: Once upon a time, voice calls were carried over the public switched telephone network, and network traffic was primarily voice traffic, with a little bit of data traffic here and there. Then came the Internet, and the Web. view more..
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Ans: he Internet used to be all about communication, like the telephone network. Early on, academics would communicate with remote machines, logging in over the network to perform tasks. People have used email to communicate with each other for a long time, and now use video and voice over IP as well. view more..
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Ans: The Web designs that we have seen so far have a single server machine talking to multiple client machines. To build large Web sites that perform well, we can speed up processing on either the server side or the client side. On the server side, more powerful Web servers can be built with a server farm, in which a cluster of computers acts as a single server. view more..
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Ans: Server farms and Web proxies help to build large sites and to improve Web performance, but they are not sufficient for truly popular Web sites that must serve content on a global scale. For these sites, a different approach is needed. view more..
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Ans: Not everyone can set up a 1000-node CDN at locations around the world to distribute their content. (Actually, it is not hard to rent 1000 virtual machines around the globe because of the well-developed and competitive hosting industry. view more..
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Ans: For the first few decades of their existence, computer networks were primarily used by university researchers for sending email and by corporate employees for sharing printers. Under these conditions, security did not get a lot of attention. view more..
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Ans: In a substitution cipher, each letter or group of letters is replaced by another letter or group of letters to disguise it. One of the oldest known ciphers is the Caesar cipher, attributed to Julius Caesar. view more..
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Ans: Modern cryptography uses the same basic ideas as traditional cryptography (transposition and substitution), but its emphasis is different. Traditionally, cryptographers have used simple algorithms. view more..
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Ans: Despite all this complexity, AES (or DES, or any block cipher for that matter) is basically a monoalphabetic substitution cipher using big characters (128-bit characters for AES and 64-bit characters for DES). Whenever the same plaintext block goes in the front end, the same ciphertext block comes out the back end. view more..




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