A Typical HTTP Session

In web communications, Hypertext Transfer Protocol (HTTP) is the cornerstone of data exchange between clients (typically web browsers) and servers. An HTTP session represents the lifecycle of the single request-response cycle between these two entities. Let us explore the steps involved in a typical HTTP session from initiating a connection to receiving the response.

Below are the topics we are going to cover in the given article:

Establishing the Connection

HTTP relies on the TCP (Transmission Control Protocol) to establish a reliable connection between the client (like a web browser) and the server. The process involves:

• The DNS Resolution: The client converts the domain name (like www.example.com) into an IP address by using a Domain Name System (DNS).
• TCP Handshake: The client and the server perform a 3-step process known as the TCP handshake to establish a connection:
  • The Client sends the SYN request to the server.
  • The Server responds with SYN-ACK.
  • The Client sends an ACK to confirm the connection.

Once the handshake is complete, the client can send an HTTP request.

Sending the Request

After the TCP connection is established client sends an HTTP request to the server. An HTTP request consists of several components:

Request Line

The request line is the first line of an HTTP request and specifies the HTTP method (e.g., GET, POST), the target URL, and the HTTP version (e.g., HTTP/1.1). It tells the server what action to perform on the specified resource. This line is crucial as it defines the intent of the request which includes:

• HTTP Method: Defines action to be performed (e.g., GET, POST, PUT, DELETE).
• URL: The resource being requested (e.g., /index.html).
• HTTP Version: Version of the HTTP protocol being used (e.g., HTTP/1.1).

Request Header Fields

Request header fields provide additional information about HTTP requests such as the type of content that the client can accept or details about the client making the request. Common headers include:

• Host: Specifies domain of the server (e.g., Host: www.example.com).
• User-Agent: Identifies client making the request (e.g., browser, application).
• Accept: Lists types of content the client can process (e.g., Accept: text/html).
• Authorization: Contains credentials for accessing the protected resources (e.g., API keys, tokens).

Above headers help server to process request accurately and deliver appropriate response.

Empty Line

In the HTTP request, the empty line serves as crucial separator between request headers and optional message body. After all the request headers are specified, a blank line (just newline character) signals end of header section. This is important because it tells server that no more headers are coming and any data following this line belongs to message body (if present). Without this empty line the server would not be able to correctly parse request and distinguish between headers and body content.

Message Body (Optional)

The message body in HTTP request contains actual data being sent to the server such as form inputs or the JSON payloads. It is typically used with methods like POST or PUT. The message body is optional and is separated from the headers by an empty line.

Example: In given example, Client is making the GET request to fetch the index.html page from the server, specifying that it can handle the HTML content.

GET /index.html HTTP/1.1
Host: www.example.com
User-Agent: Mozilla/5.0
Accept: text/html

Interpreting the Response

After processing request, Interpreting the response involves analyzing the server reply to the HTTP request. The response includes elements which can help client to understand the outcome of request and act accordingly. The elements that include:

Status Line

Status line is the first line of the HTTP response which indicates result of the request for helping the client to understand whether operation was successful or if there were issues and includes:

• HTTP Version: HTTP protocol version used by the server.
• Status Code: The three-digit number indicating result (e.g., 200 for the success, 404 for not found and 500 for internal server error).
• Status Message: The textual explanation of a status code (e.g., OK, Not Found, Internal Server Error).

HTTP Response Headers

Response headers provide additional information about the server and the response. Common headers include:

• Content-Type: Specifies a type of content that being returned (e.g., Content-Type: text/html).
• Content-Length: It Indicates the size of response body in the bytes.
• Set-Cookie: Instructs client to store the cookie for future requests (useful for the session management).

Message Body (Optional)

Message body in the HTTP response contains actual content returned by the server such as an HTML page, JSON data or the file. It follows the headers and is optional depending on the nature of request and response. The client processes this content based on its type indicated by the Content-Type header.

Conclusion

The HTTP session is structured process that ensures reliable communication between clients and the servers. It involves establishing the connection like sending well-formed request and receiving the structured response. The Key components include the request line, headers and an optional message body. Responses include the status line, headers and if it necessary, the message body with requested content. For security, the HTTPS adds encryption using SSL/TLS.

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Article Tags :

• Computer Networks
• HTTP