Point-to-Point Protocol (PPP)

1. Introduction

The Point-to-Point Protocol (PPP) directly connects two network devices at the data link layer. It is typically used for internet connections and connecting remote networks via a Wide Area Network (WAN) link. PPP is adaptable, working with different physical layers protocols like serial lines, ISDN, and DSL.

In this tutorial, we’ll explore the PPP’s architecture, features, and operation while including a simple case study.

2. PPP Architecture

PPP is a layered protocol that operates at the data link layer of the OSI (Open Systems Interconnection) model. The PPP protocol consists of a layered protocol with three primary components:

• The Link Control Protocol (LCP): responsible for establishing, configuring, and testing the link between the two devices. It negotiates link parameters like the maximum frame size and compression type while monitoring the link for errors and drops
• The Authentication Protocol (AP): responsible for verifying the identities of the two devices using a range of authentication methods, including passwords, digital certificates, and biometrics
• The Network Control Protocol (NCP): responsible for negotiating the network layer protocol used to transmit data over the connection, supporting a variety of network layer protocols such as IP, IPX, and AppleTalk

The following image depicts the PPP protocol components in the context of the OSI model:

3. PPP Features

PPP provides several features that make it an attractive protocol for establishing network connections:

• Flexibility: PPP is versatile and adaptable to various physical layer and network layer protocols, making it useful in a variety of network configurations
• Error Detection and Correction: PPP includes mechanisms for detecting and correcting errors in transmitted data using checksums and cyclic redundancy checks (CRC), ensuring data transmission accuracy and reliability
• Compression: PPP supports several compression algorithms like Stac LZS and MPPC (Microsoft Point-to-Point Compression), reducing the amount of transmitted data to improve performance and reduce costs
• Encryption: PPP supports several encryption algorithms, including PAP (Password Authentication Protocol), CHAP (Challenge Handshake Authentication Protocol), and EAP (Extensible Authentication Protocol), ensuring the security and privacy of transmitted data

4. PPP Operation

Generally speaking, the PPP operates by encapsulating network layer protocol packets within PPP frames, transmitting them over the physical layer protocol. Thus, the PPP frames consist of a header and a trailer, providing information about the encapsulated packet and ensuring its reliable transmission.

Moreover, the PPP header contains fields for the protocol type, address, control, and protocol identifier. In such a way, the protocol type field specifies the physical layer protocol used, like HDLC (High-level Data Link Control) or SDLC (Synchronous Data Link Control).

Furthermore, the address and control fields specify the source and destination of the PPP frame. The protocol identifier field, in turn, defines the network layer protocol being encapsulated, like IP or IPX.

Additionally, the PPP trailer contains a Frame Check Sequence (FCS) field that detects errors in transmitted data. So, the FCS field includes a checksum or a CRC calculated based on the data in the PPP frame.

5. Case Study

Let’s check a PPP use case study. This case study has two actors: Bob (the user) and an ISP (with whom Bob communicates through a server). Thus, let’s suppose Bob wants to connect his computer to the internet using a modem, and his ISP employs PPP to establish connections:

The following steps illustrate such a process:

• Bob initiates a connection by dialing his ISP’s phone number
• Bob’s modem establishes a physical connection with the ISP’s modem over a phone line
• The two modems exchange LCP packets for negotiating the connection parameters. Parameters are, for instance, the maximum frame size and compression type
• They agree to a maximum frame size of 1500 bytes with no compression
• After the LCP negotiation, Bob’s modem and the ISP’s modem authenticate each other’s identities
• The ISP uses the Challenge-Handshake Authentication Protocol (CHAP) to verify Bob’s identity. Bob, however,  employs the Password Authentication Protocol (PAP) to verify the ISP’s identity
• Once authentication is complete, the two modems exchange NCP packets. So, they determine which network layer protocol will be used to transmit data over the connection
• They agree to use the TCP/IP protocol suite
• With the NCP negotiation complete, we can transmit the data between Bob’s computer and the internet using the TCP/IP protocol suite through encapsulated frames transmitted over the PPP connection
• Finally, when Bob finishes using the internet, he can terminate the PPP connection by disconnecting the modem or logging off the ISP’s server

6. Conclusion

In summary, PPP is a versatile and mature protocol with several features that make it an attractive option for establishing secure and reliable connections between network devices.

The PPP features include flexibility, error detection/correction, compression, and encryption. In this way, through PPP, two entities can successfully establish a secure and reliable connection and transmit data over the internet using the TCP/IP protocol suite, as we saw in the presented case study.