Network Control Protocol (NCP) – Full Details with Example

1. Introduction to Network Control Protocol (NCP)

Network Control Protocol (NCP) is a set of protocols used in computer networks to establish and configure different network layer protocols in a Point-to-Point Protocol (PPP) environment. NCP is a part of PPP (Point-to-Point Protocol), which is used to establish a direct connection between two nodes.

NCP operates at the Network Layer (Layer 3) of the OSI model and is responsible for managing and negotiating the parameters of network-layer protocols such as:

• Internet Protocol (IP)
• Internetwork Packet Exchange (IPX)
• AppleTalk
• DECnet
• VJ Compression

2. Role of NCP in PPP

PPP consists of three main components:

1. Link Control Protocol (LCP): Establishes, configures, and tests the data link connection.
2. Authentication Protocols (optional): PAP or CHAP for validating user identity.
3. Network Control Protocols (NCPs): Configure and enable network-layer protocols.

NCPs allow multiple network-layer protocols to operate simultaneously over the same PPP link. Each protocol is handled by its own NCP.

3. How NCP Works

Here’s a step-by-step description of how NCP functions in the PPP session:

1. PPP Link Establishment (LCP Phase):
   PPP uses LCP to establish and configure the link (set options like authentication, error detection, etc.).
2. Authentication Phase (optional):
   If authentication is required, PAP or CHAP is used after LCP.
3. Network Protocol Configuration (NCP Phase):
   NCP configures network-layer protocols (e.g., IPCP for IP, IPXCP for IPX).
4. Network Layer Protocols Activated:
   Once NCP negotiation is successful, data transmission for that network-layer protocol can begin.
5. Link Termination:
   When the session ends, NCP and then LCP are used to terminate the connection.

4. Examples of NCPs

5. Example: IPCP (IP Control Protocol)

Scenario: A PPP link is established between a router and a client device. The goal is to assign an IP address to the client using IPCP.

Step-by-step IPCP negotiation example:

1. Client sends IPCP Configure-Request:

   IPCP Configure-Request
   - IP Address: 0.0.0.0 (Requesting address)

2. Server responds with Configure-Nak:

   IPCP Configure-Nak
   - Suggested IP Address: 192.168.1.10

3. Client sends Configure-Request again:

   IPCP Configure-Request
   - IP Address: 192.168.1.10 (Accepting suggestion)

4. Server sends Configure-Ack:

   IPCP Configure-Ack
   - IP Address: 192.168.1.10

After this exchange, the client is assigned the IP 192.168.1.10 and can now send/receive IP packets over the PPP link.

6. NCP Packet Structure

Each NCP packet generally includes:

• Code: Type of message (Configure-Request, Configure-Ack, etc.)
• Identifier: Helps match requests and replies.
• Length: Total length of the packet.
• Data: Protocol-specific configuration options.

7. Importance of NCP

• Flexibility: NCP allows PPP to work with many different network-layer protocols.
• Standardization: Ensures proper configuration and operation of protocols over a PPP link.
• Extensibility: New NCPs can be added for new network protocols.

8. Real-world Usage of NCP

• Dial-up internet: PPP was commonly used for dial-up connections, with IPCP used to assign IP addresses.
• VPNs: Some VPNs use PPP and NCP to configure protocols over virtual links.
• Mobile networks: PPP with NCPs can be used in cellular networks for IP address assignment and negotiation.

9. Summary Table

10. Conclusion

NCP is a crucial component of PPP-based networking that ensures proper communication between devices using different network-layer protocols. It negotiates options, assigns addresses, and prepares the network-layer environment for data transfer, playing a vital role especially in legacy and mobile systems.