The Model Context Protocol (MCP) facilitates structured communication between AI models and external tools or data sources. Understanding the internal workings of an MCP server involves exploring its architecture, transport mechanisms, and communication protocols.
An MCP server operates within a client-server architecture, where:
- Host: Manages multiple client instances, enforcing security policies and coordinating AI integrations.
- Client: Establishes individual sessions with servers, handling protocol negotiations and message routing.
- Server: Provides specialized contexts, tools, or data, operating independently to fulfill client requests.
This structure ensures modularity and clear separation of responsibilities, enhancing security and scalability.
MCP supports multiple transport mechanisms for communication between clients and servers:
- STDIO Transport:
- Communication: Utilizes standard input (stdin) and standard output (stdout) streams for message exchange.
- Operation:
- The client launches the MCP server as a subprocess.
- The server reads
JSON-RPCmessages from stdin and writes responses to stdout. - Messages are newline-delimited and must not contain embedded newlines.
- Use Cases: Ideal for command-line tools, local process communication, and development scenarios.
- Limitations: Supports only single client connections and lacks network accessibility and authentication mechanisms.
- HTTP with Server-Sent Events (SSE):
- Communication: Employs HTTP POST for client-to-server messages and SSE for server-to-client messages.
- Operation:
- The server provides two endpoints:
- SSE Endpoint: Clients establish a connection to receive messages from the server.
- HTTP POST Endpoint: Clients send messages to the server.
- Upon connection, the server sends an endpoint event containing a URI for client messages.
- The server provides two endpoints:
- Use Cases: Suitable for web applications and distributed systems requiring network-based communication and support for multiple clients.
- Features: Supports authentication mechanisms such as
JWTandAPI keys.
MCP relies on JSON-RPC 2.0 for message formatting, ensuring a standardized structure for requests, responses, and notifications. This protocol-agnostic approach allows MCP to operate over various transport mechanisms, maintaining consistency across different communication channels.
For HTTP-based transports, MCP servers listen on configurable ports to handle client connections. The specific port numbers depend on the server's configuration and the network environment in which it operates.
For instance, in a Mirantis Cloud Platform (MCP) environment, services like OpenStack Dashboard and Keystone are exposed through endpoints terminated on a reverse proxy server, typically using ports 443 and 5000, respectively.
MCP servers implement security measures appropriate to the chosen transport mechanism:
- STDIO Transport: Inherently secure due to process isolation, suitable for local integrations without network exposure.
- HTTP with SSE: Requires authentication mechanisms (e.g., JWT, API keys) and transport security (e.g., TLS) to protect data over network communications.
By adhering to these structured protocols and transport mechanisms, MCP servers ensure reliable and secure interactions between AI models and external resources.
To illustrate the internal workings of an MCP (Model Context Protocol) server, let's use Mermaid diagrams to visualize its architecture and communication flows.
This diagram showcases the core components of the MCP architecture, highlighting the relationships between hosts, clients, and servers.
graph TD
subgraph Host Application
A[Host]
B[Client]
end
subgraph External Resources
E[Tool]
F[Resource]
end
A --> B
B --> C[Server]
C --> E
C --> F
In this architecture:
- Host: Manages the overall system and initiates connections.
- Client: Maintains a one-to-one connection with the server.
- Server: Provides access to tools and resources.
- Tool: Represents executable functions or services.
- Resource: Denotes data sources or files.
This structure ensures modularity and clear separation of responsibilities, enhancing security and scalability.
The STDIO transport mechanism utilizes standard input and output streams for communication between the client and server.
sequenceDiagram
participant Client
participant Server
Client->>Server: Launches Server as Subprocess
Client->>Server: Writes JSON-RPC Messages to stdin
Server->>Client: Writes Responses to stdout
This mechanism is ideal for local process communication and development scenarios.
This transport mechanism employs HTTP POST for client-to-server messages and SSE for server-to-client messages.
sequenceDiagram
participant Client
participant Server
Client->>Server: HTTP POST Request
Server->>Client: SSE Connection Established
Server->>Client: Sends Messages via SSE
Client->>Server: Sends Messages via HTTP POST
This approach is suitable for web applications and distributed systems requiring network-based communication.
MCP relies on JSON-RPC 2.0 for message formatting, ensuring a standardized structure for requests, responses, and notifications.
sequenceDiagram
participant Client
participant Server
Client->>Server: JSON-RPC Request
Server->>Client: JSON-RPC Response
This protocol-agnostic approach allows MCP to operate over various transport mechanisms, maintaining consistency across different communication channels.
These diagrams provide a detailed visualization of the internal workings of an MCP server, illustrating its architecture, transport mechanisms, and communication protocols.
sequenceDiagram
participant Host
participant Client
participant Server
participant Tool
participant Resource
Note over Host,Client: Host initializes and manages the Client
Host->>Client: Initialize Client
Note over Client,Server: Client establishes a connection with the Server
Client->>Server: Establish Connection
Note over Server: Server advertises available Tools and Resources
Server->>Client: Advertise Capabilities
Note over Client,Server: Client requests to use a specific Tool
Client->>Server: Request Tool Execution
Note over Server,Tool: Server invokes the Tool
Server->>Tool: Invoke Tool
Note over Tool,Server: Tool performs the task and returns the result
Tool->>Server: Return Result
Note over Server,Client: Server sends the result back to the Client
Server->>Client: Send Result
Note over Client,Host: Client provides the result to the Host
Client->>Host: Provide Result
Note over Client,Server: Client requests access to a Resource
Client->>Server: Request Resource Access
Note over Server,Resource: Server retrieves the Resource
Server->>Resource: Retrieve Resource
Note over Resource,Server: Resource provides data to the Server
Resource->>Server: Provide Data
Note over Server,Client: Server sends the Resource data to the Client
Server->>Client: Send Resource Data
Note over Client,Host: Client provides the Resource data to the Host
Client->>Host: Provide Resource Data
This diagram depicts the interactions among the core components of the MCP architecture:
- Host: Manages the overall system and initiates the Client.
- Client: Establishes connections with Servers to access Tools and Resources.
- Server: Provides access to Tools and Resources, advertising its capabilities to Clients.
- Tool: Represents executable functions or services that perform specific tasks.
- Resource: Denotes data sources or files that can be accessed.
The sequence illustrates:
- The Host initializes the Client.
- The Client establishes a connection with the Server.
- The Server advertises its available Tools and Resources.
- The Client requests to execute a Tool or access a Resource.
- The Server processes the request by invoking the Tool or retrieving the Resource.
- The result or data is sent back through the Server to the Client, and finally to the Host.
This structured interaction ensures modularity, clear separation of responsibilities, and efficient communication within the MCP framework.