What Is LPC and How Does It Work?

Computer networking has tons of somewhat nebulous acronyms, like LPC. However, these often have an explicit meaning and use. LPC, just as an example, is something used exclusively by Microsoft computers as a method of communication.

This guide will take a closer look at LPC, how it works, and how that applies to modern computing and networking protocols. Even if you never touch a PowerShell instance, you’ll have a greater understanding of one of the most interesting internal mechanisms to be found in the Microsoft Windows ecosystem.

Defining LPC

Before we get started, it is important to note there is more than one name for LPC. You’ll find it referred to as a Local Procedure Call, Lightweight Procedure Call, and even Local Inter-Process Communication. While all these names are different, they all refer to the same exact method of communication.

LPC is a messaging protocol similar to the open-platform RPC but intended solely for Microsoft Windows operating systems. It was introduced as a core function of the NT kernel but has since seen future revisions still very much present in Windows 10 and 11 today. Unlike RPC, there isn’t readily available documentation to access LPC as a standalone function.

How Is LPC Used?

So, now that the definition is out of the way, how is LPC actually used? There are a few different ways to call upon protocol to send communications. It is important to note that the actual usage of LPC is not a native function, but rather one implemented directly in the Windows Native API. Documentation for the API is intended for internal Microsoft use.

That said, you can send messages of your own accord if you know what you’re looking at. While there are multiple undocumented functions in Microsoft Windows for internal development, LPC serves an important function as a whole.

Communication Between Processes

The biggest use of this messaging protocol is in providing communication between Windows processes. While the RPC messaging protocol is used to communicate between client and server architecture, the full name denotes usage ranging between local and remote usage. LPC is intended solely for internal networks.

You’ll find LPC is often used for inter-server communication on a local area network. It is also often used with core Windows functions like LSASS, or for simple communication between processing threads.

The messaging protocol serves as a vital backbone for inter-component communication in the Windows operating system. It also provides key communications between server processes on those supported operating systems. Home users are leveraging LPC, even if they don’t fully realize it themselves.

API Calls in Windows

Developers are no doubt familiar with leveraging API calls in their own projects. Application programming interfaces are a common part of software development and allow for certain functions, libraries, and communication to occur between different pieces of software. The Native API in Windows isn’t readily available to developers outside of Microsoft.

However, there are ways to coax LPC communications to different processes as needed. The messaging protocol is unique in that it allows both smaller packs of data and far larger amounts to be sent as simple transmissions between processes.

Developers can call on LPC messages fairly easily by calling on native Windows functions, accessing drivers, and even sending communications to processor threads. When calling upon the Native API, something called a port object is created. This port object prepares the intended recipient to receive a native message using the lightweight process call protocol.

How Does LPC Work?

So, you’ve got the use and definition of this messaging protocol, but how does LPC work? For those without a fair amount of computer development under their belt, the actual particulars might seem a bit vague.

The messaging protocol itself isn’t terribly complex, at least when compared to some standards. When a message call is made to the Native API, a function is called upon to open a port for communication. From there, the port mentioned above is routed to receive a communication.

After opening the port, the server constructs the message, with a header to mark its destination along with a form of identification to address the CPU thread and exact process for the recipient. Following this, if communication is needed, the connection will be completed and the LPC message will be sent.

The process itself can happen nearly instantly depending on the length and the size of the message. As mentioned, there are different types of data you can send with the LPC protocol. The overall process remains the same, however, with a local handshake between both client and server.

The entire process can also occur locally and does so when you do something as simple as powering your PC on. Processes like LSASS rely on LPC messaging to authenticate that you’re the intended user for the profile, just as a simple example.

Is LPC Still In Use?

As you can imagine, if it isn’t broken, there’s zero need to fix something. As such, LPC is still in use by modern Windows operating systems. However, the name has changed, and it has some added features.

Modern Revisions and Implementations

LPC was effectively phased out as a whole around the release of Windows 7. Prior to that, you’d find LPCs most commonly in NT-based Windows operating systems. NT systems were usually intended for business and server usage. Windows Vista introduced ALPC, or the Advanced Local Procedure Call, which has a number of noted improvements.

Before we depart, let’s discuss the changes made in the latest version of the messaging protocol and how they affect modern Windows usage.

Accessing LPC

Sending LPC communications largely hasn’t changed in terms of methodology. You’re still accessing the same Native API functions to provide messaging services for low-level to high-level processes on local client/server relationships. Instead, what ALPC brings to the fore is scalability.

You’ll still be making the same API calls, as the overall construction of the message hasn’t changed. They still contain a header, any pertinent information needed to be communicated, and a destination.

In fact, the method of message exchange is largely unchanged. The local server still requests an open port on the client machine. The client machine then can either accept or deny. More seasoned developers are likely familiar with the process. However, if you’re new to the field the documentation is largely unchanged.

Performance Enhancements

It is not a secret that organizations and businesses around the world have grown more complex and scaled to exponential extremes. The old days of computer networking may have been a nightmare in terms of the amount of legwork needed for anything, but needs have changed.

Most operating systems, especially for server use, are meant to scale these days. As such, an antiquated messaging protocol like LPC isn’t entirely built with scalability in mind. Microsoft’s revision of LPC to ALPC definitely brought the ability to scale, as you’ll see in most modern business networks.

ALPC also brought the ability to communicate with the then-new User-Mode Driver Framework. That requires a reliable method of communication between the device and the host operating system.

Is LPC Important for Modern Computing?

So, is LPC a crucial part of modern computing? If you’re running Windows, it certainly is. The entire backbone of the operating system seems to rely on it. The messaging service allows near bare-metal and higher-level communications between all the parts. It also acts as an effective method of communication between machines on a LAN.

Simply put, LPC and ALPC are crucial parts of the Windows operating system. Given the general prevalence of Windows both in the workplace and at home. LPC serves a key part in allowing modern computing and networking to exist in its current state.

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