What are Max Parallel HTTP Connections in a Browser?

When you visit a website, your web browser sends a request to the server to retrieve the content of the page. The server then sends back a response containing the content of the page along with other information such as the status of the request and the content type. The process of sending a request and receiving a response is known as an HTTP transaction.

In a typical scenario, a single user might open multiple tabs in their web browser and visit several websites simultaneously. Each tab would represent a separate HTTP transaction. But what happens when a user tries to load multiple pages or resources from the same website at the same time? How does the browser handle these requests and responses?

To understand this, we need to first understand the concept of max parallel HTTP connections in a browser.

What are Max Parallel HTTP Connections in a Browser?

Max parallel HTTP connections in a browser refer to the maximum number of HTTP transactions that a browser can initiate and maintain at the same time with a single web server. In other words, it is the maximum number of parallel requests that a browser can send to a server.

For example, if the max parallel HTTP connections for a browser are set to 4, it means that the browser can initiate and maintain up to 4 HTTP transactions with a server at the same time. If a user tries to load a fifth page or resource from the same server, the browser will have to wait until one of the existing transactions is completed before it can initiate the fifth transaction.

Why are Max Parallel HTTP Connections Important?

Max parallel HTTP connections play a crucial role in determining the performance and speed of a website. If the number of parallel connections is too low, it can lead to a bottleneck and slow down the loading of the website. On the other hand, if the number of parallel connections is too high, it can lead to server overload and degrade the performance of the website.

Therefore, it is important to strike the right balance between the number of parallel connections and the performance of the website.

How are Max Parallel HTTP Connections Controlled?

There are two main factors that control the max parallel HTTP connections in a browser:

The Browser

Each browser has a different default value for the maximum parallel connections. For example, in Google Chrome, the default value is 6 connections per domain. This means that Chrome will only make a maximum of 6 HTTP requests to a single domain at the same time. If the website has more than 6 resources (such as images, CSS files, and JavaScript files) that need to be loaded, the browser will queue the additional requests until one of the existing connections becomes available.

The maximum parallel connections can be adjusted in the browser’s settings. However, changing this value can have unintended consequences, such as increasing the load on the server and potentially causing it to crash. It is generally best to leave this setting at the default value unless there is a specific reason to change it.

Here are the steps to adjust the maximum parallel connections in some popular browsers:

In Google Chrome:

• Open Chrome and click on the three dots in the top-right corner of the window.
• Click on “Settings.”
• Scroll down and click on “Advanced.”
• Scroll down to the “System” section and click on “Open proxy settings.”
• In the Internet Properties window, click on the “Connections” tab.
• Click on “LAN settings.”
• In the “Connections” window, click on the “Advanced” button.
• In the “Advanced Connections Settings” window, scroll down to the “Max connections per server” field and enter the desired value.

In Mozilla Firefox:

• Open Firefox and click on the three horizontal lines in the top-right corner of the window.
• Click on “Options.”
• In the left-hand menu, click on “General.”
• Scroll down to the “Performance” section and click on “Settings.”
• In the “Performance” window, click on the “Network” tab.
• In the “Connections” section, adjust the value in the “Maximum number of connections per server” field.

In Microsoft Edge:

• Open Edge and click on the three dots in the top-right corner of the window.
• Click on “Settings.”
• In the left-hand menu, click on “System.”
• Scroll down and click on “Open proxy settings.”
• In the Internet Properties window, click on the “Connections” tab.
• Click on “LAN settings.”
• In the “Connections” window, click on the “Advanced” button.
• In the “Advanced Connections Settings” window, scroll down to the “Max connections per server” field and enter the desired value.

The Web Server

Web servers control the max parallel HTTP connections in a browser by setting the “Keep-Alive” header in the HTTP response. The “Keep-Alive” header tells the browser to maintain a persistent connection with the server for a specified period of time, during which the browser can initiate multiple requests without establishing a new connection for each request.

The “Keep-Alive” header also specifies the maximum number of requests that can be made over the persistent connection. This value is known as the “max Keep-Alive requests” and it acts as an upper limit on the max parallel HTTP connections that a browser can initiate with the server.

For example, if the “Keep-Alive” header specifies a “max Keep-Alive requests” value of 100, it means that the browser can initiate up to 100 requests over the persistent connection before it has to re-establish a new connection.

By setting the “Keep-Alive” header, web servers can effectively control the number of parallel connections that a browser can initiate and maintain with the server. This can help to optimize the performance and speed of the website by reducing the number of new connections that have to be established and maintaining a persistent connection for multiple requests.

Here is an example of how the maximum parallel connections can affect the loading time of a website:

Imagine that a website has 10 images that need to be loaded. If the browser’s maximum parallel connections is set to 2, it will take 5 requests to load all of the images. If the maximum parallel connections is set to 6, it will only take 2 requests to load all of the images. In this case, increasing the maximum parallel connections from 2 to 6 would significantly reduce the loading time of the website. However, it is important to note that increasing the maximum parallel connections can also increase the load on the server, which may result in slower loading times for other users accessing the website.

It is generally best to leave the maximum parallel connections at the default value unless there is a specific reason to change it. For example, if a website is hosted on a server with a high capacity and is able to handle a large number of requests without becoming overloaded, increasing the maximum parallel connections may improve the loading time of the website for the user. However, it is important to consider the potential impact on the server and other users before making any changes to this setting.

How to Optimize Max Parallel HTTP Connections?

There are several ways to optimize the max parallel HTTP connections in order to improve the performance and speed of a website. Some of the key strategies include:

• Reduce the number of HTTP requests: One of the main causes of slow loading times is the number of HTTP requests that a browser has to make to retrieve all the resources required to render a page. By reducing the number of requests, you can decrease the number of parallel connections required and improve the loading speed of the website. You can do this by optimizing images, using CSS sprites, and combining JavaScript and CSS files.
• Enable HTTP/2: HTTP/2 is the latest version of the HTTP protocol and it offers several improvements over HTTP/1.1, including multiplexing and server push. HTTP/2 allows multiple requests to be sent over a single connection and allows the server to push resources to the client proactively. By enabling HTTP/2, you can improve the performance of your website and reduce the need for parallel connections.
• Use a CDN: A content delivery network (CDN) is a network of servers located in different parts of the world that serve content to users based on their geographical location. By using a CDN, you can reduce the load on your main server and improve the loading speed of your website. CDNs also have a large number of servers, which means that they can handle a higher number of parallel connections.
• Set the “Keep-Alive” header: As mentioned earlier, the “Keep-Alive” header allows the browser to maintain a persistent connection with the server and initiate multiple requests over that connection. By setting the “Keep-Alive” header, you can reduce the number of new connections that the browser has to establish and improve the performance of your website.