Question

True or false? a. A user requests a Web page that consists of some text and three images. For this page, the client will send one request message and receive four response messages. b. Two distinct Web pages (for example, www. mit. edu/research. html and www. mit. edu/students. html) can be sent over the same persistent connection. c. With nonpersistent connections between browser and origin server, it is possible for a single TCP segment to carry two distinct HTTP request messages. d. The Date: header in the HTTP response message indicates when the object in the response was last modified. e. HTTP response messages never have an empty message body.

Short answer

a. False, b. True, c. False, d. False, e. False.

Step-by-step solution

Understanding HTTP Request and Response

When a user requests a web page that includes text and images, the browser sends one request for the text, and after receiving the response, sends an additional request for each image. Therefore, the user initially sends one request and receives a separate response for each requested item: one for the text and one for each image.

Multiple Pages on a Persistent Connection

Persistent connections allow the client to maintain a single TCP connection open, over which multiple HTTP requests and responses can be sent. Therefore, two distinct web pages can indeed be sent over the same persistent connection.

Understanding TCP Segments and Nonpersistent Connections

With nonpersistent connections, each HTTP request/response pair uses a separate TCP connection. It is not possible for two distinct HTTP requests to be carried over the same TCP segment because each pair operates in its own TCP connection.

Date Header Clarification

The Date: header in an HTTP response message indicates when the response message was sent by the server, not necessarily when the object was last modified. The Last-Modified header indicates the latter.

HTTP Response Message Body

HTTP response messages can have an empty body, especially in the case of responses to requests like HEAD requests or in responses with status codes indicating absence of message body, such as 204 No Content or 304 Not Modified.

Key concepts

HTTP Persistent Connections

HTTP persistent connections are a key feature that enhances the efficiency of web communications. When a client and a server establish a persistent connection, it means they keep a single TCP connection open. This allows the client to send several HTTP requests without needing to re-establish a new TCP connection for each one.
Persistent connections help to minimize latency and improve the loading speed of web pages. This is because the overhead of setting up and tearing down connections is reduced, saving both time and resources. With persistent connections, two different web pages, such as `www.site.com/page1` and `www.site.com/page2`, can be delivered over the same TCP connection. This was addressed in part b of the original exercise, and it is indeed true that these pages can share the same persistent connection.

HTTP Request and Response

The process of HTTP request and response is the core of web communications. When a user requests a web page, the browser sends an HTTP request message to the server. This message specifies the desired resource, such as a web page or an image.
Upon receiving the request, the server processes it and sends back an HTTP response message. This response contains the requested content and information about the resource, such as its type and length. In the original exercise, it was pointed out that a single web page request often leads to multiple responses if the page contains multiple resources, like images. For each resource (text or image), a separate response is sent back to the browser.

TCP Connections

TCP connections form the backbone of data transmission over the Internet. Every HTTP interaction relies on the underlying TCP (Transmission Control Protocol) to establish a connection between the client and server.
With nonpersistent connections, each request-response pair utilizes its own separate TCP connection. This means that each request causes the setup and breakdown of a new TCP connection. In comparison, persistent connections keep the TCP link open, reducing the need for repeated handshakes. In the exercise, it was highlighted that nonpersistent connections cannot transport two HTTP requests in a single TCP segment. This limitation stems from each request having its own TCP connection.

HTTP Headers

HTTP headers play a crucial role in both requests and responses, providing important information about the message and facilitating communication between the client and server. They can include metadata about the request or response, like content type, length, and caching details.
The `Date:` header, part of the response headers, signals when the server sent the response, not the last modification time of the resource. The modification time is delivered through the `Last-Modified` header instead. Heads up! Some responses may not even have a body, as addressed in the original exercise. For instance, in responses with status codes like 204 or 304, the message body can be empty.

Web Page Loading

The process of web page loading encompasses various steps, from initiating communication with a server to rendering the page in a browser. A single web page often comprises multiple components, such as HTML documents and images.
When a browser requests a web page, it combines multiple HTTP request-response cycles. Each component of the web page requires at least one request and response, influencing how swiftly the page loads. Thanks to HTTP persistent connections, this process is streamlined, allowing different components of a web page to be loaded using the same ongoing connection.
This makes the loading of web pages quicker and more efficient, as fewer resources are consumed and less time is spent waiting for connections to establish.