# Introduction to Model Context Protocol (MCP)
The Model Context Protocol (MCP) is designed with a flexible and extensible architecture to facilitate seamless communication between LLM applications and various integrations.
## Overview of MCP Architecture
MCP operates on a client-server architecture, defining specific roles for different components:
* **Hosts**: These are LLM applications, such as Claude Desktop or Integrated Development Environments (IDEs), responsible for initiating connections.
* **Clients**: These components maintain one-to-one connections with servers, operating within the host application.
* **Servers**: These entities provide essential context, tools, and prompts to the clients.
```mermaid
flowchart LR
subgraph "Host"
client1[MCP Client]
client2[MCP Client]
end
subgraph "Server Process"
server1[MCP Server]
end
subgraph "Server Process"
server2[MCP Server]
end
client1 <-->|Transport Layer| server1
client2 <-->|Transport Layer| server2
```
## Core Components of MCP
MCP's architecture is built upon two primary layers: the Protocol layer and the Transport layer.
### Protocol Layer
The protocol layer is responsible for managing high-level communication patterns, including message framing and the linking of requests to their corresponding responses. Key classes within this layer include `Protocol`, `Client`, and `Server`.
### Transport Layer
The transport layer handles the actual communication mechanics between MCP clients and servers. MCP supports multiple transport mechanisms, all of which utilize [JSON-RPC 2.0](https://www.jsonrpc.org/) for message exchange. The transport layer is responsible for converting MCP protocol messages to JSON-RPC format for transmission and vice-versa for reception.
## Message Types in MCP
MCP defines four main types of messages for communication:
1. **Requests**: These messages are sent with the expectation of a response from the receiving party.
```typescript
interface Request {
method: string;
params?: { ... };
}
```
2. **Results**: These represent successful responses to a previously sent request.
```typescript
interface Result {
[key: string]: unknown;
}
```
3. **Errors**: These messages indicate that a request has failed.
```typescript
interface Error {
code: number;
message: string;
data?: unknown;
}
```
4. **Notifications**: These are one-way messages that do not require or expect a response.
```typescript
interface Notification {
method: string;
params?: { ... };
}
```
## Connection Lifecycle
An MCP connection follows a defined lifecycle, from its establishment to its termination.
### 1. Initialization
The connection initialization phase involves a handshake between the client and server:
```mermaid
sequenceDiagram
participant Client
participant Server
Client->>Server: initialize request
Server->>Client: initialize response
Client->>Server: initialized notification
Note over Client,Server: Connection ready for use
```
1. The Client sends an `initialize` request, providing its protocol version and capabilities.
2. The Server responds with its own protocol version and capabilities.
3. The Client sends an `initialized` notification to acknowledge the setup.
4. Upon completion of these steps, normal message exchange can begin.
### 2. Message Exchange
After successful initialization, clients and servers can exchange messages using two primary patterns:
* **Request-Response**: Either the client or the server can send requests and await responses.
* **Notifications**: Both parties can send one-way messages that do not expect a response.
### 3. Termination
A connection can be terminated by either party through a clean shutdown using `close()`, a transport disconnection, or due to error conditions.
## Error Handling
MCP defines standard error codes, which are based on JSON-RPC error codes:
```typescript
enum ErrorCode {
// Standard JSON-RPC error codes
ParseError = -32700,
InvalidRequest = -32600,
MethodNotFound = -32601,
InvalidParams = -32602,
InternalError = -32603
}
```
SDKs and applications have the flexibility to define their own custom error codes, provided they are above -32000. Errors are propagated through error responses to requests, error events on transports, and protocol-level error handlers.
## Transports in Detail
Transports are fundamental to MCP, handling the underlying mechanics of message transmission and reception.
### Message Format
As mentioned, MCP uses JSON-RPC 2.0 as its wire format. The transport layer is responsible for converting MCP protocol messages to and from this format. The three types of JSON-RPC messages used are:
* **Requests**:
```typescript
{
jsonrpc: "2.0",
id: number | string,
method: string,
params?: object
}
```
* **Responses**:
```typescript
{
jsonrpc: "2.0",
id: number | string,
result?: object,
error?: {
code: number,
message: string,
data?: unknown
}
}
```
* **Notifications**:
```typescript
{
jsonrpc: "2.0",
method: string,
params?: object
}
```
### Built-in Transport Types
MCP includes two standard transport implementations:
1. **Standard Input/Output (stdio) Transport**
The stdio transport facilitates communication via standard input and output streams. It is particularly well-suited for local integrations and command-line tools.
**Use cases for stdio transport:**
* Building command-line tools.
* Implementing local integrations.
* Requiring simple process communication.
* Working with shell scripts.
2. **HTTP with Server-Sent Events (SSE) Transport**
The SSE transport enables server-to-client streaming, while client-to-server communication is handled via HTTP POST requests.
**Use cases for SSE transport:**
* When only server-to-client streaming is required.
* Operating in restricted network environments.
* Implementing simple updates.
### Custom Transports
MCP allows for the implementation of custom transports to meet specific communication needs. Any custom transport must conform to the `Transport` interface. This flexibility enables integration with custom network protocols, specialized communication channels, existing systems, or for performance optimization.
### Transport Error Handling
Transport implementations should be robust in handling various error scenarios, including connection errors, message parsing errors, protocol errors, network timeouts, and ensuring proper resource cleanup.
### Best Practices for Transports
When implementing or using MCP transports, it is recommended to:
* Handle the connection lifecycle properly.
* Implement robust error handling.
* Clean up resources upon connection closure.
* Utilize appropriate timeouts.
* Validate messages before sending them.
* Log transport events for debugging purposes.
* Implement reconnection logic where suitable.
* Manage backpressure in message queues.
* Monitor connection health.
* Implement proper security measures.
### Security Considerations for Transports
Security is paramount when implementing transports:
* **Authentication and Authorization**: Implement proper authentication mechanisms, validate client credentials, use secure token handling, and implement authorization checks.
* **Data Security**: Employ TLS for network transport, encrypt sensitive data, validate message integrity, implement message size limits, and sanitize input data.
* **Network Security**: Implement rate limiting, use appropriate timeouts, handle denial of service scenarios, monitor for unusual patterns, and implement proper firewall rules.
## General Best Practices
### Transport Selection
* For **local communication** between processes on the same machine, the stdio transport is efficient and simple.
* For scenarios requiring **HTTP compatibility** or remote communication, SSE is a suitable choice, with due consideration for security implications like authentication and authorization.
### Message Handling
* **Request Processing**: Thoroughly validate inputs, use type-safe schemas, handle errors gracefully, and implement timeouts.
* **Progress Reporting**: For long-running operations, use progress tokens to report progress incrementally, including total progress when known.
* **Error Management**: Use appropriate error codes, include helpful error messages, and ensure resources are cleaned up on errors.
## General Security Considerations
* **Transport Security**: Always use TLS for remote connections, validate connection origins, and implement authentication when necessary.
* **Message Validation**: Validate all incoming messages, sanitize inputs, check message size limits, and verify the JSON-RPC format.
* **Resource Protection**: Implement access controls, validate resource paths, monitor resource usage, and rate limit requests.
* **Error Handling**: Avoid leaking sensitive information in error messages, log security-relevant errors, implement proper cleanup, and handle Denial of Service (DoS) scenarios.
## Debugging and Monitoring
* **Logging**: Log protocol events, track message flow, monitor performance, and record errors.
* **Diagnostics**: Implement health checks, monitor connection state, track resource usage, and profile performance.
* **Testing**: Test different transports, verify error handling, check edge cases, and load test servers.
