> For the complete documentation index, see [llms.txt](https://developer.convergegate.com/legacy/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://developer.convergegate.com/legacy/card-server-to-server/overview.md).

# Overview

API Commands are send from merchant’s system to the Gateway with HTTP protocol. Request need to be sent to URL provided by Converge Gate. In both request and response POST HTTP headers API command, the "Content-Type" header is employed to indicate the XML format (Content-Type: application/xml) and convey presentation details about the transmitted XML data.

## Authentication

{% content-ref url="/pages/Muq149Jd67EO1Mr4zUJ0" %}
[Authentication](/legacy/card-server-to-server/overview/authentication.md)
{% endcontent-ref %}

## Process Flow

{% content-ref url="/pages/tMM5IcfASm3reTDRuKIu" %}
[Process Flow](/legacy/card-server-to-server/overview/process-flow.md)
{% endcontent-ref %}

## API Endpoint

{% content-ref url="/pages/QdS2Oz3gVMwtzkaSZpJZ" %}
[API Endpoints](/legacy/card-server-to-server/overview/api-endpoints.md)
{% endcontent-ref %}

## Testing

{% content-ref url="/pages/f7JXj0N4nWUjopcgUI4e" %}
[Testing](/legacy/card-server-to-server/overview/testing.md)
{% endcontent-ref %}

## API Commands

{% content-ref url="/pages/5i4E8tmzZSWnE4u2dKgt" %}
[API Commands](/legacy/card-server-to-server/api-commands.md)
{% endcontent-ref %}

## Notifications

{% content-ref url="/pages/Utw9YDcUavQaEGuWwCKE" %}
[Notifications](/legacy/card-server-to-server/notifications.md)
{% endcontent-ref %}

## Transaction Status Codes

{% content-ref url="/pages/rW0BcFVNtDGJCw2Fal5v" %}
[Transaction Status Codes](/legacy/card-server-to-server/transaction-status-codes.md)
{% endcontent-ref %}

## 3D Secure Integration

{% content-ref url="/pages/RRNeqpD5rODuyTEvSmbf" %}
[3D Secure Integration](/legacy/card-server-to-server/3d-secure-integration.md)
{% endcontent-ref %}

{% hint style="info" %}
**Good to know:** Using the 'Page Link' block lets you link directly to a page. If this page's name, URL or parent location changes, the reference will be kept up to date. You can also mention a page – like [Authentication](/legacy/card-server-to-server/overview/authentication.md) – if you don't want a block-level link.
{% endhint %}