What is PPP Point To Point Protocol?
A fundamental data link layer (Layer 2) communication protocol called the Point-to-Point Protocol (PPP) is used to create and maintain a direct connection between two networking nodes, such as routers. PPP, created by the Internet Engineering Task Force (IETF) and outlined in RFC 1661, is widely acknowledged as an industry-standard protocol.
With features like error detection and support for different protocols, PPP was created in the late 1980s to overcome the shortcomings of its predecessor, the Serial Line Internet Protocol (SLIP). Serial cables, phone lines, ISDN, and fiber optic links are among the duplex circuits over which it is intended to function.
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Core Functions and Features
PPP provides several essential services and features for reliable and flexible point-to-point data transmission:
- Connection Establishment and Link Management: PPP is in charge of negotiating and creating a connection between two devices, taking care of everything from setup to disconnecting the connection.
- Encapsulation: Network-layer protocols, like IP packets, are encapsulated and sent over a point-to-point link by being wrapped in PPP frames. This procedure enables the simultaneous support of many network protocols, such as IP, AppleTalk, and IPX.
- Authentication: Before a connection is fully established, authentication confirms each device’s identification to provide safe access.
- Error Detection: To guarantee data integrity, PPP has error detection features that usually use a Frame Check Sequence (FCS) checksum in each frame.
- Compression and Multilink: It can negotiate data compression to boost effective throughput and supports Multilink PPP (MLPPP), which raises bandwidth by combining several physical links into a single logical link.
- Loop Detection: To identify and stop looped links, PPP takes advantage of a feature in LCP messages called “magic numbers.”
PPP Components (The Protocol Suite)
PPP is a suite of protocols, mainly comprising three components that handle the various functions of establishing and managing a connection:
| Component | Function |
|---|---|
| Link Control Protocol (LCP) | Responsible for establishing, configuring, maintaining, and terminating the physical data link connection. LCP negotiates link configuration parameters, such as the maximum frame size, authentication method, and compression settings. |
| Network Control Protocols (NCPs) | A family of protocols used to configure and enable specific network-layer protocols (Layer 3) to transmit data over the PPP link. For example, the Internet Protocol Control Protocol (IPCP) is used to configure IP settings, such as negotiating IP address assignment. |
| Authentication Protocols (APs) | Optional protocols are used to verify the identity of the communicating nodes. Common methods include the less secure Password Authentication Protocol (PAP), which sends credentials in plain text, and the more secure Challenge Handshake Authentication Protocol (CHAP), which uses a challenge-response mechanism. The Extensible Authentication Protocol (EAP) is also supported. |
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PPP Connection Phases
A PPP session goes through distinct phases to establish and terminate a connection:
- Link Dead: The physical layer is still unfinished.
- Link Establishment Phase: To negotiate link setup choices (such as frame size and error detection), LCP packets are sent.
- Authentication Phase (Optional): If agreed upon, the peer identification is confirmed via PAP, CHAP, or EAP protocols. Termination of the link results from failure in this step.
- Network-Layer Protocol Phase: To set up the network layer protocols for data transit, NCP packets like IPCP are traded.
- Link Open/Data Transfer Phase: PPP frames containing data are transmitted.
- Link Termination Phase: A timeout or link failure causes termination, or an endpoint sends an LCP message to cease the connection.
PPP Frame Structure
PPP data is sent in frames that share a basic structure similar to High-Level Data Link Control (HDLC) frames:
- Flag: Marks the beginning and end of the frame (1 byte, value
01111110binary). - Address: Standard broadcast address (1 byte, value
11111111binary). - Control: Usually set to
00000011binary for connection-less data link (1 byte). - Protocol: Identifies the network protocol of the datagram contained in the data field (1 or 2 bytes).
- Information/Payload: Contains the upper layer datagram, typically a network layer packet. Its maximum length is generally 1500 bytes. Byte stuffing is used if the flag sequence appears in the payload.
- FCS (Frame Check Sequence): A checksum used for error detection (2 or 4 bytes).
Applications
Even though PPP and its variations were once well-known for dial-up Internet access using modems, they are still in use today:
- Broadband Access: By utilizing PPP’s strong authentication and configuration capabilities included within Ethernet frames, ISPs frequently employ PPPoE (PPP over Ethernet) to offer DSL and fiber broadband services.
- VPNs: PPP serves as the foundation for a number of tunneling protocols, including PPTP (Point-to-Point Tunneling Protocol), which offers data encapsulation and authentication for virtual private networks.
- Dedicated Serial Links: Dedicated or leased lines are utilized for dependable communication between routers.
Analogy: The Point-to-Point Protocol (PPP) can be compared to a normal shipping procedure for establishing a courier route between two particular distribution centers. The paperwork phase, or LCP, involves deciding on the shipment’s terms, such as the maximum box size, if insurance (error detection) is included, and who is permitted to send packages (authentication). The NCPs are particular labels affixed to the packages after those terms are decided upon, guaranteeing that the receiving center handles and routes various kinds of cargo (IP data, AppleTalk data, etc.) appropriately. The standardized, tamper-evident transport box is called the PPP Frame.
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