9.1 Traditional Applications

We begin our discussion of applications by focusing on two of the most popular—the World Wide Web and email. Broadly speaking, both of these applications use the request/reply paradigm—users send requests to servers, which then respond accordingly. We refer to these as "traditional" applications because they typify the sort of applications that have existed since the early days of computer networks (although the Web is a lot newer than email but has its roots in file transfers that predated it). By contrast, later sections will look at a class of applications that have become popular more recently: streaming applications (e.g., multimedia applications like video and audio) and various overlay-based applications. (Note that there is a bit of a blurring between these classes, as you can of course get access to streaming multimedia data over the Web, but for now we'll focus on the general usage of the Web to request pages, images, etc.)

Before taking a close look at each of these applications, there are three general points that we need to make. The first is that it is important to distinguish between application programs and application protocols. For example, the HyperText Transport Protocol (HTTP) is an application protocol that is used to retrieve Web pages from remote servers. Many different application programs—that is, web clients like Internet Explorer, Chrome, Firefox, and Safari—provide users with a different look and feel, but all of them use the same HTTP protocol to communicate with web servers over the Internet. Indeed, it is the fact that the protocol is published and standardized that enables application programs developed by many different companies and individuals to interoperate. That is how so many browsers are able to interoperate with all the web servers (of which there are also many varieties).

This section looks at two very widely used, standardized application protocols:

  • Simple Mail Transfer Protocol (SMTP) is used to exchange electronic mail.

  • HyperText Transport Protocol (HTTP) is used to communicate between web browsers and web servers.

Second, we observe that many application layer protocols, including HTTP and SMTP, have a companion protocol that specifies the format of the data that can be exchanged. This is one reason WHY these protocols are relatively simple: Much of the complexity is managed in this companion standard. For example, SMTP is a protocol for exchanging electronic mail messages, but RFC 822 and Multipurpose Internet Mail Extensions (MIME) define the format of email messages. Similarly, HTTP is a protocol for fetching Web pages, but HyperText Markup Language (HTML) is a companion specification that defines the basic form of those pages.

Finally, since the application protocols described in this section follow the same request/reply communication pattern, you might expect that they would be built on top of a Remote Procedure Call (RPC) transport protocol. This is not the case, however, as they are instead implemented on top of TCP. In effect, each protocol reinvents a simple RPC-like mechanism on top of a reliable transport protocol (TCP). We say "simple" because each protocol is not designed to support arbitrary remote procedure calls of the sort discussed in an earlier chapter, but is instead designed to send and respond to a specific set of request messages. Interestingly, the approach taken by HTTP has proven quite powerful, which has led to it being adopted widely by the Web Services architecture. More on this topic at the end of this section.

Electronic Mail (SMTP, MIME, IMAP)

Email is one of the oldest network applications. After all, what could be more natural than wanting to send a message to the user at the other end of a cross-country link you just managed to get running? It's the 20th century's version of "Mr. Watson, come here... I want to see you." Surprisingly, the pioneers of the ARPANET had not really envisioned email as a key application when the network was created—remote access to computing resources was the main design goal—but it turned out to be the Internet's original killer app.

As noted above, it is important (1) to distinguish the user interface (i.e., your mail reader) from the underlying message transfer protocols (such as SMTP or IMAP), and (2) to distinguish between this transfer protocol and a companion standard (RFC 822 and MIME) that defines the format of the messages being exchanged. We start by looking at the message format.

Message Format

RFC 822 defines messages to have two parts: a header and a body. Both parts are represented in ASCII text. Originally, the body was assumed to be simple text. This is still the case, although RFC 822 has been augmented by MIME to allow the message body to carry all sorts of data. This data is still represented as ASCII text, but because it may be an encoded version of, say, a JPEG image, it's not necessarily readable by human users. More on MIME in a moment.

The message header is a series of <CRLF>-terminated lines. (<CRLF> stands for carriage-return plus line-feed, which are a pair of ASCII control characters often used to indicate the end of a line of text.) The header is separated from the message body by a blank line. Each header line contains a type and value separated by a colon. Many of these header lines are familiar to users, since they are asked to fill them out when they compose an email message; for example, the header identifies the message recipient, and the header says something about the purpose of the message. Other headers are filled in by the underlying mail delivery system. Examples include (when the message was transmitted), (what user sent the message), and (each mail server that handled this message). There are, of course, many other header lines; the interested reader is referred to RFC 822.

RFC 822 was extended in 1993 (and updated quite a few times since then) to allow email messages to carry many different types of data: audio, video, images, PDF documents, and so on. MIME consists of three basic pieces. The first piece is a collection of header lines that augment the original set defined by RFC 822. These header lines describe, in various ways, the data being carried in the message body. They include (the version of MIME being used), (a human-readable description of what's in the message, analogous to the line), (the type of data contained in the message), and (how the data in the message body is encoded).

The second piece is definitions for a set of content types (and subtypes). For example, MIME defines two different still image types, denoted and , each with the obvious meaning. As another example, refers to simple text you might find in a vanilla 822-style message, while denotes a message that contains "marked up" text (text using special fonts, italics, etc.). As a third example, MIME defines an application type, where the subtypes correspond to the output of different application programs (e.g., application/postscript and application/msword).

MIME also defines a multipart type that says how a message carrying more than one data type is structured. This is like a programming language that defines both base types (e.g., integers and floats) and compound types (e.g., structures and arrays). One possible multipart subtype is mixed, which says that the message contains a set of independent data pieces in a specified order. Each piece then has its own header line that describes the type of that piece.

The third piece is a way to encode the various data types so they can be shipped in an ASCII email message. The problem is that, for some data types (a JPEG image, for example), any given 8-bit byte in the image might contain one of 256 different values. Only a subset of these values are valid ASCII characters. It is important that email messages contain only ASCII, because they might pass through a number of intermediate systems (gateways, as described below) that assume all email is ASCII and would corrupt the message if it contained non-ASCII characters. To address this issue, MIME uses a straightforward encoding of binary data into the ASCII character set. The encoding is called base64. The idea is to map every three bytes of the original binary data into four ASCII characters. This is done by grouping the binary data into 24-bit units and breaking each such unit into four 6-bit pieces. Each 6-bit piece maps onto one of 64 valid ASCII characters; for example, 0 maps onto A, 1 maps onto B, and so on. If you look at a message that has been encoded using the base64 encoding scheme, you'll notice only the 52 upper- and lowercase letters, the 10 digits 0 through 9, and the special characters + and /. These are the first 64 values in the ASCII character set.

As one aside, so as to make reading mail as painless as possible for those who still insist on using text-only mail readers, a MIME message that consists of regular text only can be encoded using 7-bit ASCII. There's also a readable encoding for mostly ASCII data.

Putting this all together, a message that contains some plain text, a JPEG image, and a PostScript file would look something like this:

MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="-------417CA6E2DE4ABCAFBC5"
From: Alice Smith <Alice@cisco.com>
To: Bob@cs.Princeton.edu
Subject: promised material
Date: Mon, 07 Sep 1998 19:45:19 -0400

---------417CA6E2DE4ABCAFBC5
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit

Bob,

Here's the jpeg image and draft report I promised.

--Alice

---------417CA6E2DE4ABCAFBC5
Content-Type: image/jpeg
Content-Transfer-Encoding: base64
... unreadable encoding of a jpeg figure
---------417CA6E2DE4ABCAFBC5
Content-Type: application/postscript; name="draft.ps"
Content-Transfer-Encoding: 7bit
... readable encoding of a PostScript document

In this example, the line in the message header says that this message contains various pieces, each denoted by a character string that does not appear in the data itself. Each piece then has its own and lines.

Message Transfer

For many years, the majority of email was moved from host to host using only SMTP. While SMTP continues to play a central role, it is now just one email protocol of several, Internet Message Access Protocol (IMAP) and Post Office Protocol (POP) being two other important protocols for retrieving mail messages. We'll begin our discussion by looking at SMTP, and move on to IMAP below.

To place SMTP in the right context, we need to identify the key players. First, users interact with a mail reader when they compose, file, search, and read their email. Countless mail readers are available, just like there are many web browsers to choose from. In the early days of the Internet, users typically logged into the machine on which their mailbox resided, and the mail reader they invoked was a local application program that extracted messages from the file system. Today, of course, users remotely access their mailbox from their laptop or smartphone; they do not first log into the host that stores their mail (a mail server). A second mail transfer protocol, such as POP or IMAP, is used to remotely download email from a mail server to the user's device.

Second, there is a mail daemon (or process) running on each host that holds a mailbox. You can think of this process, also called a message transfer agent (MTA), as playing the role of a post office: Users (or their mail readers) give the daemon messages they want to send to other users, the daemon uses SMTP running over TCP to transmit the message to a daemon running on another machine, and the daemon puts incoming messages into the user's mailbox (where that user's mail reader can later find them). Since SMTP is a protocol that anyone could implement, in theory there could be many different implementations of the mail daemon. It turns out, though, that there are only a few popular implementations, with the old sendmail program from Berkeley Unix and postfix being the most widespread.

Sequence of mail gateways store and forward email messages.

While it is certainly possible that the MTA on a sender's machine establishes an SMTP/TCP connection to the MTA on the recipient's mail server, in many cases the mail traverses one or more mail gateways on its route from the sender's host to the receiver's host. Like the end hosts, these gateways also run a message transfer agent process. It's not an accident that these intermediate nodes are called gateways since their job is to store and forward email messages, much like an "IP gateway" (which we have referred to as a router) stores and forwards IP datagrams. The only difference is that a mail gateway typically buffers messages on disk and is willing to try retransmitting them to the next machine for several days, while an IP router buffers datagrams in memory and is only willing to retry transmitting them for a fraction of a second. Figure 1 illustrates a two-hop path from the sender to the receiver.

Why, you might ask, are mail gateways necessary? Why can't the sender's host send the message to the receiver's host? One reason is that the recipient does not want to include the specific host on which he or she reads email in his or her address. Another is scale: In large organizations, it's often the case that a number of different machines hold the mailboxes for the organization. For example, mail delivered to bob@cs.princeton.edu is first sent to a mail gateway in the CS Department at Princeton (that is, to the host named cs.princeton.edu), and then forwarded—involving a second connection—to the specific machine on which Bob has a mailbox. The forwarding gateway maintains a database that maps users into the machine on which their mailbox resides; the sender need not be aware of this specific name. (The list of header lines in the message will help you trace the mail gateways that a given message traversed.) Yet another reason, particularly true in the early days of email, is that the machine that hosts any given user's mailbox may not always be up or reachable, in which case the mail gateway holds the message until it can be delivered.

Independent of how many mail gateways are in the path, an independent SMTP connection is used between each host to move the message closer to the recipient. Each SMTP session involves a dialog between the two mail daemons, with one acting as the client and the other acting as the server. Multiple messages might be transferred between the two hosts during a single session. Since RFC 822 defines messages using ASCII as the base representation, it should come as no surprise to learn that SMTP is also ASCII based. This means it is possible for a human at a keyboard to pretend to be an SMTP client program.

SMTP is best understood by a simple example. The following is an exchange between sending host cs.princeton.edu and receiving host cisco.com . In this case, user Bob at Princeton is trying to send mail to users Alice and Tom at Cisco. Extra blank lines have been added to make the dialog more readable.

HELO cs.princeton.edu
250 Hello daemon@mail.cs.princeton.edu [128.12.169.24]

MAIL FROM:<Bob@cs.princeton.edu>
250 OK

RCPT TO:<Alice@cisco.com>
250 OK

RCPT TO:<Tom@cisco.com>
550 No such user here

DATA
354 Start mail input; end with <CRLF>.<CRLF>
Blah blah blah...
...etc. etc. etc.
<CRLF>.<CRLF>
250 OK

QUIT
221 Closing connection

As you can see, SMTP involves a sequence of exchanges between the client and the server. In each exchange, the client posts a command (e.g., QUIT) and the server responds with a code (e.g., 221). The server also returns a human-readable explanation for the code (e.g., ). In this particular example, the client first identifies itself to the server with the HELO command. It gives its domain name as an argument. The server verifies that this name corresponds to the IP address being used by the TCP connection; you'll notice the server states this IP address back to the client. The client then asks the server if it is willing to accept mail for two different users; the server responds by saying "yes" to one and "no" to the other. Then the client sends the message, which is terminated by a line with a single period (".") on it. Finally, the client terminates the connection.

There are, of course, many other commands and return codes. For example, the server can respond to a client's RCPT command with a 251 code, which indicates that the user does not have a mailbox on this host, but that the server promises to forward the message onto another mail daemon. In other words, the host is functioning as a mail gateway. As another example, the client can issue a VRFY operation to verify a user's email address, but without actually sending a message to the user.

The only other point of interest is the arguments to the MAIL and RCPT operations; for example, FROM:<Bob@cs.princeton.edu> and TO:<Alice@cisco.com>, respectively. These look a lot like 822 header fields, and in some sense they are. What actually happens is that the mail daemon parses the message to extract the information it needs to run SMTP. The information it extracts is said to form an envelope for the message. The SMTP client uses this envelope to parameterize its exchange with the SMTP server. One historical note: The reason sendmail became so popular is that no one wanted to reimplement this message parsing function. While today's email addresses look pretty tame (e.g., Bob@cs.princeton.edu), this was not always the case. In the days before everyone was connected to the Internet, it was not uncommon to see email addresses of the form user%host@site!neighbor.

Mail Reader

The final step is for the user to actually retrieve his or her messages from the mailbox, read them, reply to them, and possibly save a copy for future reference. The user performs all these actions by interacting with a mail reader. As pointed out earlier, this reader was originally just a program running on the same machine as the user's mailbox, in which case it could simply read and write the file that implements the mailbox. This was the common case in the pre-laptop era. Today, most often the user accesses his or her mailbox from a remote machine using yet another protocol, such as POP or IMAP. It is beyond the scope of this book to discuss the user interface aspects of the mail reader, but it is definitely within our scope to talk about the access protocol. We consider IMAP, in particular.

IMAP is similar to SMTP in many ways. It is a client/server protocol running over TCP, where the client (running on the user's desktop machine) issues commands in the form of <CRLF>-terminated ASCII text lines and the mail server (running on the machine that maintains the user's mailbox) responds in kind. The exchange begins with the client authenticating him- or herself and identifying the mailbox he or she wants to access. This can be represented by the simple state transition diagram shown in Figure 2. In this diagram, LOGIN and LOGOUT are example commands that the client can issue, while OK is one possible server response. Other common commands include and EXPUNGE, with the obvious meanings. Additional server responses include NO (client does not have permission to perform that operation) and BAD (command is ill formed).

IMAP state transition diagram.

When the user asks to FETCH a message, the server returns it in MIME format and the mail reader decodes it. In addition to the message itself, IMAP also defines a set of message attributes that are exchanged as part of other commands, independent of transferring the message itself. Message attributes include information like the size of the message and, more interestingly, various flags associated with the message (e.g., and Recent). These flags are used to keep the client and server synchronized; that is, when the user deletes a message in the mail reader, the client needs to report this fact to the mail server. Later, should the user decide to expunge all deleted messages, the client issues an EXPUNGE command to the server, which knows to actually remove all earlier deleted messages from the mailbox.

Finally, note that when the user replies to a message, or sends a new message, the mail reader does not forward the message from the client to the mail server using IMAP, but it instead uses SMTP. This means that the user's mail server is effectively the first mail gateway traversed along the path from the desktop to the recipient's mailbox.

World Wide Web (HTTP)

The World Wide Web has been so successful and has made the Internet accessible to so many people that sometimes it seems to be synonymous with the Internet. In fact, the design of the system that became the Web started around 1989, long after the Internet had become a widely deployed system. The original goal of the Web was to find a way to organize and retrieve information, drawing on ideas about hypertext—interlinked documents—that had been around since at least the 1960s. The core idea of hypertext is that one document can link to another document, and the protocol (HTTP) and document language (HTML) were designed to meet that goal.

A short history of the Web provided by the World Wide Web consortium traces its roots to a 1945 article describing links between microfiche documents.

One helpful way to think of the Web is as a set of cooperating clients and servers, all of whom speak the same language: HTTP. Most people are exposed to the Web through a graphical client program or web browser like Safari, Chrome, Firefox, or Internet Explorer. Figure 3 shows the Firefox browser in use, displaying a page of information from Princeton University.

The Firefox web browser.

Clearly, if you want to organize information into a system of linked documents or objects, you need to be able to retrieve one document to get started. Hence, any web browser has a function that allows the user to obtain an object by opening a URL. Uniform Resource Locators (URLs) are so familiar to most of us by now that it's easy to forget that they haven't been around forever. They provide information that allows objects on the Web to be located, and they look like the following:

http://www.cs.princeton.edu/index.html

If you opened that particular URL, your web browser would open a TCP connection to the web server at a machine called www.cs.princeton.edu and immediately retrieve and display the file called index.html. Most files on the Web contain images and text, and many have other objects such as audio and video clips, pieces of code, etc. They also frequently include URLs that point to other files that may be located on other machines, which is the core of the "hypertext" part of HTTP and HTML. A web browser has some way in which you can recognize URLs (often by highlighting or underlining some text) and then you can ask the browser to open them. These embedded URLs are called hypertext links. When you ask your web browser to open one of these embedded URLs (e.g., by pointing and clicking on it with a mouse), it will open a new connection and retrieve and display a new file. This is called following a link. It thus becomes very easy to hop from one machine to another around the network, following links to all sorts of information. Once you have a means to embed a link in a document and allow a user to follow that link to get another document, you have the basis of a hypertext system.

When you ask your browser to view a page, your browser (the client) fetches the page from the server using HTTP running over TCP. Like SMTP, HTTP is a text-oriented protocol. At its core, HTTP is a request/response protocol, where every message has the general form

START_LINE <CRLF>
MESSAGE_HEADER <CRLF>
<CRLF>
MESSAGE_BODY <CRLF>

where, as before, <CRLF> stands for carriage-return+line-feed. The first line (START_LINE) indicates whether this is a request message or a response message. In effect, it identifies the "remote procedure" to be executed (in the case of a request message), or the status of the request (in the case of a response message). The next set of lines specifies a collection of options and parameters that qualify the request or response. There are zero or more of these MESSAGE_HEADER lines—the set is terminated by a blank line—each of which looks like a header line in an email message. HTTP defines many possible header types, some of which pertain to request messages, some to response messages, and some to the data carried in the message body. Instead of giving the full set of possible header types, though, we just give a handful of representative examples. Finally, after the blank line comes the contents of the requested message (MESSAGE_BODY); this part of the message is where a server would place the requested page when responding to a request, and it is typically empty for request messages.

Why does HTTP run over TCP? The designers didn't have to do it that way, but TCP does provide a pretty good match to what HTTP needs, particularly by providing reliable delivery (who wants a Web page with missing data?), flow control, and congestion control. However, as we'll see below, there are a few issues that can arise from building a request/response protocol on top of TCP, especially if you ignore the subtleties of the interactions between the application and transport layer protocols.

Request Messages

The first line of an HTTP request message specifies three things: the operation to be performed, the Web page the operation should be performed on, and the version of HTTP being used. Although HTTP defines a wide assortment of possible request operations—including write operations that allow a Web page to be posted on a server—the two most common operations are GET (fetch the specified Web page) and HEAD (fetch status information about the specified Web page). The former is obviously used when your browser wants to retrieve and display a Web page. The latter is used to test the validity of a hypertext link or to see if a particular page has been modified since the browser last fetched it. The full set of operations is summarized in Table 1. As innocent as it sounds, the POST command enables much mischief (including spam) on the Internet.

Operation Description
OPTIONS Request information about available options
GET Retrieve document identified in URL
HEAD Retrieve metainformation about document identified in URL
POST Give information (e.g., annotation) to server
PUT Store document under specified URL
DELETE Delete specified URL
TRACE Loopback request message
CONNECT For use by proxies

Table 1. HTTP Request Operations.

For example, the START_LINE

GET http://www.cs.princeton.edu/index.html
HTTP/1.1

says that the client wants the server on host to return the page named . This particular example uses an absolute URL. It is also possible to use a relative identifier and specify the host name in one of the MESSAGE_HEADER lines; for example,

GET index.html HTTP/1.1
Host: www.cs.princeton.edu

Here, Host is one of the possible MESSAGE_HEADER fields. One of the more interesting of these is , which gives the client a way to conditionally request a Web page—the server returns the page only if it has been modified since the time specified in that header line.

Response Messages

Like request messages, response messages begin with a single START_LINE. In this case, the line specifies the version of HTTP being used, a three-digit code indicating whether or not the request was successful, and a text string giving the reason for the response. For example, the START_LINE

HTTP/1.1 202 Accepted

indicates that the server was able to satisfy the request, while

HTTP/1.1 404 Not Found

indicates that it was not able to satisfy the request because the page was not found. There are five general types of response codes, with the first digit of the code indicating its type. Table 2 summarizes the five types of codes.

Code Type Example Reasons
1xx Informational request received, continuing process
2xx Success action successfully received, understood, and accepted
3xx Redirection further action must be taken to complete the request
4xx Client Error request contains bad syntax or cannot be fulfilled
5xx Server Error server failed to fulfill an apparently valid request

Table 2. Five Types of HTTP Result Codes.

As with the unexpected consequences of the POST request message, it is sometimes surprising how various response messages are used in practice. For example, request redirection (specifically code 302) turns out to be a powerful mechanism that plays a big role in Content Distribution Networks (CDNs) by redirecting requests to a nearby cache.

Also similar to request messages, response messages can contain one or more MESSAGE_HEADER lines. These lines relay additional information back to the client. For example, the Location header line specifies that the requested URL is available at another location. Thus, if the Princeton CS Department Web page had moved from to , for example, then the server at the original address might respond with

HTTP/1.1 301 Moved Permanently
Location: http://www.princeton.edu/cs/index.html

In the common case, the response message will also carry the requested page. This page is an HTML document, but since it may carry nontextual data (e.g., a GIF image), it is encoded using MIME (see the previous section). Certain of the MESSAGE_HEADER lines give attributes of the page contents, including (number of bytes in the contents), Expires (time at which the contents are considered stale), and (time at which the contents were last modified at the server).

Uniform Resource Identifiers

The URLs that HTTP uses as addresses are one type of Uniform Resource Identifier (URI). A URI is a character string that identifies a resource, where a resource can be anything that has identity, such as a document, an image, or a service.

The format of URIs allows various more specialized kinds of resource identifiers to be incorporated into the URI space of identifiers. The first part of a URI is a scheme that names a particular way of identifying a certain kind of resource, such as mailto for email addresses or file for file names. The second part of a URI, separated from the first part by a colon, is the scheme-specific part. It is a resource identifier consistent with the scheme in the first part, as in the URIs mailto:santa@northpole.org and file:///C:/foo.html.

A resource doesn't have to be retrievable or accessible. We saw an example of this in an earlier chapter—extensible markup language (XML) namespaces are identified by URIs that look an awful lot like URLs, but strictly speaking they are not locators because they don't tell you how to locate something; they just provide a globally unique identifier for the namespace. There is no requirement that you can retrieve anything at the URI given as the target namespace of an XML document. We'll see another example of a URI that is not a URL in a later section.

TCP Connections

The original version of HTTP (1.0) established a separate TCP connection for each data item retrieved from the server. It's not too hard to see how this was a very inefficient mechanism: connection setup and teardown messages had to be exchanged between the client and server even if all the client wanted to do was verify that it had the most recent copy of a page. Thus, retrieving a page that included some text and a dozen icons or other small graphics would result in 13 separate TCP connections being established and closed. Figure 4 shows the sequence of events for fetching a page that has just a single embedded object. Colored lines indicate TCP messages, while black lines indicate the HTTP requests and responses. (Some of the TCP ACKs are not shown to avoid cluttering the picture.) You can see two round trip times are spent setting up TCP connections while another two (at least) are spent getting the page and image. As well as the latency impact, there is also processing cost on the server to handle the extra TCP connection establishment and termination.

HTTP 1.0 behavior.

To overcome this situation, HTTP version 1.1 introduced persistent connections—the client and server can exchange multiple request/ response messages over the same TCP connection. Persistent connections have many advantages. First, they obviously eliminate the connection setup overhead, thereby reducing the load on the server, the load on the network caused by the additional TCP packets, and the delay perceived by the user. Second, because a client can send multiple request messages down a single TCP connection, TCP's congestion window mechanism is able to operate more efficiently. This is because it's not necessary to go through the slow start phase for each page. Figure 5 shows the transaction from Figure 4 using a persistent connection in the case where the connection is already open (presumably due to some prior access of the same server).

HTTP 1.1 behavior with persistent connections.

Persistent connections do not come without a price, however. The problem is that neither the client nor server necessarily knows how long to keep a particular TCP connection open. This is especially critical on the server, which might be asked to keep connections opened on behalf of thousands of clients. The solution is that the server must time out and close a connection if it has received no requests on the connection for a period of time. Also, both the client and server must watch to see if the other side has elected to close the connection, and they must use that information as a signal that they should close their side of the connection as well. (Recall that both sides must close a TCP connection before it is fully terminated.) Concerns about this added complexity may be one reason why persistent connections were not used from the outset, but today it is widely accepted that the benefits of persistent connections more than offset the drawbacks.

Caching

An important implementation strategy that makes the web more usable is to cache Web pages. Caching has many benefits. From the client's perspective, a page that can be retrieved from a nearby cache can be displayed much more quickly than if it has to be fetched from across the world. From the server's perspective, having a cache intercept and satisfy a request reduces the load on the server.

Caching can be implemented in many different places. For example, a user's browser can cache recently accessed pages and simply display the cached copy if the user visits the same page again. As another example, a site can support a single site-wide cache. This allows users to take advantage of pages previously downloaded by other users. Closer to the middle of the Internet, Internet Service Providers (ISPs) can cache pages. Note that, in the second case, the users within the site most likely know what machine is caching pages on behalf of the site, and they configure their browsers to connect directly to the caching host. This node is sometimes called a proxy. In contrast, the sites that connect to the ISP are probably not aware that the ISP is caching pages. It simply happens to be the case that HTTP requests coming out of the various sites pass through a common ISP router. This router can peek inside the request message and look at the URL for the requested page. If it has the page in its cache, it returns it. If not, it forwards the request to the server and watches for the response to fly by in the other direction. When it does, the router saves a copy in the hope that it can use it to satisfy a future request.

There are quite a few issues with this sort of caching, ranging from the technical to the regulatory. One example of a technical challenge is the effect of asymmetric paths, when the request to the server and the response to the client do not follow the same sequence of router hops.

No matter where pages are cached, the ability to cache Web pages is important enough that HTTP has been designed to make the job easier. The trick is that the cache needs to make sure it is not responding with an out-of-date version of the page. For example, the server assigns an expiration date (the Expires header field) to each page it sends back to the client (or to a cache between the server and client). The cache remembers this date and knows that it need not reverify the page each time it is requested until after that expiration date has passed. After that time (or if that header field is not set) the cache can use the HEAD or conditional GET operation (GET with header line) to verify that it has the most recent copy of the page. More generally, there are a set of cache directives that must be obeyed by all caching mechanisms along the request/response chain. These directives specify whether or not a document can be cached, how long it can be cached, how fresh a document must be, and so on. We'll look at the related issue of CDNs—which are effectively distributed caches—in a later section.

Web Services

So far we have focused on interactions between a human and a web server. For example, a human uses a web browser to interact with a server, and the interaction proceeds in response to input from the user (e.g., by clicking on links). However, there is increasing demand for direct computer-to-computer interaction. And, just as the applications described above need protocols, so too do the applications that communicate directly with each other. We conclude this section by looking at the challenges of building large numbers of application-to-application protocols and some of the proposed solutions.

Much of the motivation for enabling direct application-to-application communication comes from the business world. Historically, interactions between enterprises—businesses or other organizations—have involved some manual steps such as filling out an order form or making a phone call to determine whether some product is in stock. Even within a single enterprise it is common to have manual steps between software systems that cannot interact directly because they were developed independently. Increasingly, such manual interactions are being replaced with direct application-to-application interaction. An ordering application at enterprise A would send a message to an order fulfillment application at enterprise B, which would respond immediately indicating whether the order can be filled. Perhaps, if the order cannot be filled by B, the application at A would immediately order from another supplier or solicit bids from a collection of suppliers.

Here is a simple example of what we are talking about. Suppose you buy a book at an online retailer like Amazon.com. Once your book has been shipped, Amazon could send you the tracking number in an email, and then you could head over to the website for the shipping company—http://www.fedex.com, perhaps—and track the package. However, you can also track your package directly from the Amazon.com website. In order to make this happen, Amazon has to be able to send a query to FedEx, in a format that FedEx understands, interpret the result, and display it in a Web page that perhaps contains other information about your order. Underlying the user experience of getting all the information about the order served up at once on the Amazon.com Web page is the fact that Amazon and FedEx had to have a protocol for exchanging the information needed to track packages—call it the Package Tracking Protocol. It should be clear that there are so many potential protocols of this type that we'd better have some tools to simplify the task of specifying them and building them.

Network applications, even those that cross organization boundaries, are not new—email and web browsing cross such boundaries. What is new about this problem is the scale. Not scale in the size of the network, but scale in the number of different kinds of network applications. Both the protocol specifications and the implementations of those protocols for traditional applications like electronic mail and file transfer have typically been developed by a small group of networking experts. To enable the vast number of potential network applications to be developed quickly, it was necessary to come up with some technologies that simplify and automate the task of application protocol design and implementation.

Two architectures have been advocated as solutions to this problem. Both architectures are called Web Services, taking their name from the term for the individual applications that offer a remotely accessible service to client applications to form network applications. The terms used as informal shorthand to distinguish the two Web Services architectures are SOAP and REST. We will discuss the technical meanings of those terms shortly.

The name Web Services is unfortunately so generic sounding that many mistakenly assume that it includes any sort of service associated with the Web.

The SOAP architecture's approach to the problem is to make it feasible, at least in theory, to generate protocols that are customized to each network application. The key elements of the approach are a framework for protocol specification, software toolkits for automatically generating protocol implementations from the specifications, and modular partial specifications that can be reused across protocols.

The REST architecture's approach to the problem is to regard individual Web Services as World Wide Web resources—identified by URIs and accessed via HTTP. Essentially, the REST architecture is just the Web architecture. The Web architecture's strengths include stability and a demonstrated scalability (in the network-size sense). It could be considered a weakness that HTTP is not well suited to the usual procedural or operation-oriented style of invoking a remote service. REST advocates argue, however, that rich services can nonetheless be exposed using a more data-oriented or document-passing style for which HTTP is well suited.

Custom Application Protocols (WSDL, SOAP)

The architecture informally referred to as SOAP is based on Web Services Description Language (WSDL) and SOAP. Both of these standards are issued by the World Wide Web Consortium (W3C). This is the architecture that people usually mean when they use the term Web Services without any preceding qualifier. As these standards are still evolving, our discussion here is effectively a snapshot.

Although the name SOAP originated as an acronym, it officially no longer stands for anything.

WSDL and SOAP are frameworks for specifying and implementing application protocols and transport protocols, respectively. They are generally used together, although that is not strictly required. WSDL is used to specify application-specific details such as what operations are supported, the formats of the application data to invoke or respond to those operations, and whether an operation involves a response. SOAP's role is to make it easy to define a transport protocol with exactly the desired semantics regarding protocol features such as reliability and security.

Both WSDL and SOAP consist primarily of a protocol specification language. Both languages are based on XML with an eye toward making specifications accessible to software tools such as stub compilers and directory services. In a world of many custom protocols, support for automating generation of implementations is crucial to avoid the effort of manually implementing each protocol. Support software generally takes the form of toolkits and application servers developed by third-party vendors, which allows developers of individual Web Services to focus more on the business problem they need to solve (such as tracking the package purchased by a customer).

Defining Application Protocols

WSDL has chosen a procedural operation model of application protocols. An abstract Web Service interface consists of a set of named operations, each representing a simple interaction between a client and the Web Service. An operation is analogous to a remotely callable procedure in an RPC system. An example from W3C's WSDL Primer is a hotel reservation Web Service with two operations, CheckAvailability and MakeReservation.

Each operation specifies a Message Exchange Pattern (MEP) that gives the sequence in which the messages are to be transmitted, including the fault messages to be sent when an error disrupts the message flow. Several MEPs are predefined, and new custom MEPs can be defined, but it appears that in practice only two MEPs are being used: In-Only (a single message from client to service) and In-Out (a request from client and a corresponding reply from service). These patterns should be very familiar, and suggest that the costs of supporting MEP flexibility perhaps outweigh the benefits.

MEPs are templates that have placeholders instead of specific message types or formats, so part of the definition of an operation involves specifying which message formats to map into the placeholders in the pattern. Message formats are not defined at the bit level that is typical of protocols we have discussed. They are instead defined as an abstract data model using XML. XML Schema provides a set of primitive data types and ways to define compound data types. Data that conforms to an XML Schema-defined format—its abstract data model—can be concretely represented using XML, or it can use another representation, such as the "binary" representation Fast Infoset.

WSDL nicely separates the parts of a protocol that can be specified abstractly—operations, MEPs, abstract message formats—from the parts that must be concrete. WSDL's concrete part specifies an underlying protocol, how MEPs are mapped onto it, and what bit-level representation is used for messages on the wire. This part of a specification is known as a binding, although it is better described as an implementation, or a mapping onto an implementation. WSDL has predefined bindings for HTTP and SOAP-based protocols, with parameters that allow the protocol designer to fine-tune the mapping onto those protocols. There is a framework for defining new bindings, but SOAP protocols dominate.

A crucial aspect of how WSDL mitigates the problem of specifying large numbers of protocols is through reuse of what are essentially specification modules. The WSDL specification of a Web Service may be composed of multiple WSDL documents, and individual WSDL documents may also be used in other Web Service specifications. This modularity makes it easier to develop a specification and easier to ensure that, if two specifications are supposed to have some elements that are identical (for example, so that they can be supported by the same tool), then those elements are indeed identical. This modularity, together with WSDL's defaulting rules, also helps keep specifications from becoming overwhelmingly verbose for human protocol designers.

WSDL modularity should be familiar to anyone who has developed moderately large pieces of software. A WSDL document need not be a complete specification; it could, for example, define a single message format. The partial specifications are uniquely identified using XML Namespaces; each WSDL document specifies the URI of a target namespace, and any new definitions in the document are named in the context of that namespace. One WSDL document can incorporate components of another by including the second document if both share the same target namespace or importing it if the target namespaces differ.

Defining Transport Protocols

Although SOAP is sometimes called a protocol, it is better thought of as a framework for defining protocols. As the SOAP 1.2 specification explains, "SOAP provides a simple messaging framework whose core functionality is concerned with providing extensibility." SOAP uses many of the same strategies as WSDL, including message formats defined using XML Schema, bindings to underlying protocols, Message Exchange Patterns, and reusable specification elements identified using XML namespaces.

SOAP is used to define transport protocols with exactly the features needed to support a particular application protocol. SOAP aims to make it feasible to define many such protocols by using reusable components. Each component captures the header information and logic that go into implementing a particular feature. To define a protocol with a certain set of features, just compose the corresponding components. Let's look more closely at this aspect of SOAP.

SOAP 1.2 introduced a feature abstraction, which the specification describes thus: A SOAP feature is an extension of the SOAP messaging framework. Although SOAP poses no constraints on the potential scope of such features, example features may include "reliability," "security," "correlation," "routing," and message exchange patterns (MEPs) such as request/response, one-way, and peer-to-peer conversations. A SOAP feature specification must include:

  • A URI that identifies the feature

  • The state information and processing, abstractly described, that is required at each SOAP node to implement the feature

  • The information to be relayed to the next node

  • (If the feature is a MEP) the life cycle and temporal/causal relationships of the messages exchanged—for example, responses follow requests and are sent to the originator of the request

Note that this formalization of the concept of a protocol feature is rather low level; it is almost a design.

Given a set of features, there are two strategies for defining a SOAP protocol that will implement them. One is by layering: binding SOAP to an underlying protocol in such a way as to derive the features. For example, we could obtain a request/response protocol by binding SOAP to HTTP, with a SOAP request in an HTTP request and a SOAP reply in an HTTP response. Because this is such a common example, it happens that SOAP has a predefined binding to HTTP; new bindings may be defined using the SOAP Protocol Binding Framework.

The second and more flexible way to implement features involves header blocks. A SOAP message consists of an Envelope, which contains a Header that contains header blocks, and a Body, which contains the payload destined for the ultimate receiver. This message structure is illustrated in Figure 6.

SOAP message structure.

It should be a familiar notion by now that certain header information corresponds to particular features. A digital signature is used to implement authentication, a sequence number is used for reliability, and a checksum is used to detect message corruption. A SOAP header block is intended to encapsulate the header information that corresponds to a particular feature. The correspondence is not always one-to-one since multiple header blocks could be involved in a single feature, or a single header block could be used in multiple features. A SOAP module is a specification of the syntax and the semantics of one or more header blocks. Each module is intended to provide one or more features and must declare the features it implements.

The goal behind SOAP modules is to be able to compose a protocol with a set of features by simply including each of the corresponding module specifications. If your protocol is required to have at-most-once semantics and authentication, include the corresponding modules in your specification. This represents a novel approach to modularizing protocol services, an alternative to the protocol layering we have seen throughout this book. It is bit like flattening a series of protocol layers into a single protocol, but in a structured way. It remains to be seen how well SOAP features and modules, introduced in version 1.2 of SOAP, will work in practice. The main weakness of this scheme is that modules may well interfere with each other. A module specification is required to specify any known interactions with other SOAP modules, but clearly that doesn't do much to alleviate the problem. On the other hand, a core set of features and modules that provides the most important properties may be small enough to be well known and well understood.

Standardizing Web Services Protocols

As we've said, WSDL and SOAP aren't protocols; they are standards for specifying protocols. For different enterprises to implement Web Services that interoperate with each other, it is not enough to agree to use WSDL and SOAP to define their protocols; they must agree on—standardize—specific protocols. For example, you could imagine that online retailers and shipping companies might like to standardize a protocol by which they exchange information, along the lines of the simple package tracking example at the start of this section. This standardization is crucial for tool support as well as interoperability. And, yet, different network applications in this architecture must necessarily differ in at least the message formats and operations they use.

This tension between standardization and customization is tackled by establishing partial standards called profiles. A profile is a set of guidelines that narrow or constrain choices available in WSDL, SOAP, and other standards that may be referenced in defining a protocol. They may at the same time resolve ambiguities or gaps in those standards. In practice, a profile often formalizes an emerging de facto standard.

The broadest and most widely adopted profile is known as the WS-I Basic Profile. It was proposed by the Web Services Interoperability Organization (WS-I), an industry consortium, while WSDL and SOAP are specified by the World Wide Web Consortium (W3C). The Basic Profile resolves some of the most basic choices faced in defining a Web Service. Most notably it requires that WSDL be bound exclusively to SOAP and SOAP be bound exclusively to HTTP and use the HTTP POST method. It also specifies which versions of WSDL and SOAP must be used.

The WS-I Basic Security Profile adds security constraints to the Basic Profile by specifying how the SSL/TLS layer is to be used and requiring conformance to WS-Security (Web Services Security). WS-Security specifies how to use various existing techniques such as X.509 public key certificatesand Kerberos to provide security features in SOAP protocols.

WS-Security is just the first of a growing suite of SOAP-level standards established by the industry consortium OASIS (Organization for the Advancement of Structured Information Standards). The standards known collectively as WS-* include WS-Reliability, WS-ReliableMessaging, WS-Coordination, and WS-AtomicTransaction.

A Generic Application Protocol (REST)

The WSDL/SOAP Web Services architecture is based on the assumption that the best way to integrate applications across networks is via protocols that are customized to each application. That architecture is designed to make it practical to specify and implement all those protocols. In contrast, the REST Web Services architecture is based on the assumption that the best way to integrate applications across networks is by re-applying the model underlying the World Wide Web architecture. This model, articulated by Web architect Roy Fielding, is known as REpresentational State Transfer (REST). There is no need for a new REST architecture for Web Services—the existing Web architecture is sufficient, although a few extensions are probably necessary. In the Web architecture, individual Web Services are regarded as resources identified by URIs and accessed via HTTP—a single generic application protocol with a single generic addressing scheme.

Where WSDL has user-defined operations, REST uses the small set of available HTTP methods, such as GET and POST (see Table 1). So how can these simple methods provide an interface to a rich Web Service? By employing the REST model, in which the complexity is shifted from the protocol to the payload. The payload is a representation of the abstract state of a resource. For example, a GET could return a representation of the current state of the resource, and a POST could send a representation of a desired state of the resource.

The representation of a resource state is abstract; it need not resemble how the resource is actually implemented by a particular Web Service instance. It is not necessary to transmit a complete resource state in each message. The size of messages can be reduced by transmitting just the parts of a state that are of interest (e.g., just the parts that are being modified). And, because Web Services share a single protocol and address space with other web resources, parts of states can be passed by reference—by URI—even when they are other Web Services.

This approach is best summarized as a data-oriented or document-passing style, as opposed to a procedural style. Defining an application protocol in this architecture consists of defining the document structure (i.e., the state representation). XML and the lighter-weight JavaScript Object Notation (JSON) are the most frequently used presentation languages for this state. Interoperability depends on agreement, between a Web Service and its client, on the state representation. Of course, the same is true in the SOAP architecture; a Web Service and its client have to be in agreement on payload format. The difference is that in the SOAP architecture interoperability additionally depends on agreement on the protocol; in the REST architecture, the protocol is always HTTP, so that source of interoperability problems is eliminated.

One of the selling features of REST is that it leverages the infrastructure that has been deployed to support the Web. For example, Web proxies can enforce security or cache information. Existing content distribution networks (CDNs) can be used to support RESTful applications.

In contrast with WSDL/SOAP, the Web has had time for standards to stabilize and to demonstrate that it scales very well. It also comes with some security in the form of Secure Socket Layer (SSL)/Transport Layer Security (TLS). The Web and REST may also have an advantage in evolvability. Although the WSDL and SOAP frameworks are highly flexible with regard to what new features and bindings can go into the definition of a protocol, that flexibility is irrelevant once the protocol is defined. Standardized protocols such as HTTP are designed with a provision for being extended in a backward-compatible way. HTTP's own extensibility takes the form of headers, new methods, and new content types. Protocol designers using WSDL/SOAP need to design such extensibility into each of their custom protocols. Of course, the designers of state representations in a REST architecture also have to design for evolvability.

An area where WSDL/SOAP may have an advantage is in adapting or wrapping previously written, "legacy" applications to conform to Web Services. This is an important point since most Web Services will be based on legacy applications for the near future at least. These applications usually have a procedural interface that maps more easily into WSDL's operations than REST states. The REST versus WSDL/SOAP competition may very well hinge on how easy or difficult it turns out to be to devise REST-style interfaces for individual Web Services. We may find that some Web Services are better served by WSDL/SOAP and others by REST.

The online retailer Amazon.com, as it happens, was an early adopter (2002) of Web Services. Interestingly, Amazon made its systems publicly accessible via both of the Web Services architectures, and according to some reports a substantial majority of developers use the REST interface. Of course, this is just one data point and may well reflect factors specific to Amazon.

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