What Is a Point-to-Point Protocol?

Point-to-point protocol (PPP) is mainly used to establish a connection and send data between two network nodes through dial-up or dedicated line. PPP is a solution for simple connections between various types of hosts, bridges and routers. The PPP protocol is described in detail in RFC1661. The PPP protocol is one of the most widely used protocols on the WAN at present. Its advantages are simplicity, user authentication, and IP address resolution.

Chinese name: Point-to-point protocol
Foreign name: Point to Point Protocol

Abbreviation: PPP
stand by: Bit-oriented synchronization link

Introduction to Point-to-Point Protocol

The acronym for Point to Point Protocol (PPP) is a member of the TCP / IP network protocol package. PPP is an extension of TCP / IP, which adds two additional functional groups:

(1) It can transmit TCP / IP packets through a serial interface.

(2) It can log in securely.

When using a serial interface as part of a public telephone system, care must be taken to ensure the authenticity of all communications. This terminal PPP combines user name and password security. Therefore, when a router or server receives a request through PPP, if the source of the request is not secure, this requires authorization. This authorization is part of the PPP. Because of its ability to route TCP / IP packets through a serial interface and its authorization capabilities, ISPs (Internet Service Providers) usually use PPP to allow dial-up users to connect to the Internet.

Point-to-point protocol protocol overview

As a data link layer layer 2 protocol, PPP supports both synchronous link connections and asynchronous link connections. The PPP protocol is designed to work with multiple network layer protocols (such as IP, IPX, and AppleTalk) and is designed to replace the non-standard protocol SLIP at the data link layer.
The PPP protocol was designed after the original HDLC specification. So the designers of the PPP protocol included many extra features that were not considered in the WAN data link layer protocol until then. ^[1]

PPP Point-to-point protocol PPP component

PPP is a layered protocol, which consists of three parts:

1) Establish, configure and test the Link Control Protocol (LCP) of the data link. It allows the communicating parties to negotiate to determine different options.

2) Network control protocol (NCP: Network Control Protocol) system for different network layer protocols. NCP negotiates optional configuration parameters for the network layer.

3) Authentication protocol: The most commonly used are password authentication protocol PAP and challenge handshake authentication protocol CHAP. PAP and CHAP are commonly used to provide security authentication over PPP-encapsulated serial lines.

PPP Frame format of point-to-point protocol PPP

Flag field (1B)

Address field (1B)

Control field (1B)

Agreement (2B)

Information section

FCS (2B)

Flag field (1B)

The first three fields (Flag, Address, Control) of the PPP frame format are fixed as: 0x7E, 0xFF, and 0x03.
Protocol field: indicates the type of protocol package encapsulated by Information (such as LCP, NCP, IP, etc.).
Data field: Contains the payload of PPP. The length of the data is determined through negotiation and is variable length.
The FCS field is a cyclic redundancy check code for the entire frame and is used to detect possible data errors during transmission. ^[2]

Three major achievements of PPP:

1. Clearly divide the framing of the tail of one frame and the head of the next frame. This frame format also handles error detection.

2. When the lines are no longer needed, pick them out, test them, negotiate options, and carefully release the link control protocol again. This protocol is called link control protocol (LCP).

3 Negotiate which network layer options to use in a way that is independent of the network layer protocol used. For each supported network layer, the method chosen has a different network control protocol (NCP).

PPP frames can be sent not only through dial-up telephone lines, but also over SONET or truly bit-oriented HDLC lines (that is, routers connected to routers).

PPP Point-to-point protocol PPP features

Compared with other protocols, PPP has the following characteristics:

PPP has the ability to dynamically assign IP addresses, allowing IP addresses to be negotiated at connection time.
PPP supports multiple network protocols, such as TCP / IP, NetBEUI, NWLINK, etc.
PPP has error detection and correction capabilities and supports data compression.
PPP has an identity verification function, and two interconnected devices can verify each other's identity, enhancing security.
PPP can be used on many types of physical media, including serial lines, telephone lines, mobile phones, and optical fibers (such as SDH). PPP is also used for Internet access.

Point-to-point protocol enhanced error checking

The PPP protocol uses the frame check serial FCS (Frame Check Sequence) to check whether each individual frame has an error. PPP can also monitor which frames always receive errors when received, and can be configured to reduce this occurrence. Wrong interface.

Point-to-point protocol link loopback check

The link control protocol LCP (as an integral part of the PPP protocol and PPP is defined in the same RFC) uses special numbers identifying itself as features to find loops. When using the PPP protocol, the endpoint sends out an LCP message with a special digital identity that is different from the other endpoints. If there is a loop on the line, the endpoint that sent this message will receive the information containing its own identity and not the identity of others .

The PPP protocol provides hooks for each end user to automatically configure network interfaces (setting IP addresses, default gateways, etc.) and identity authentication.

PPP Point-to-point protocol PPP multi-connection protocol

The PPP multi-connection protocol can provide multiple connections between two systems to increase additional bandwidth. When performing remote resource fetches, the PPP multi-connection protocol allows combining two bandwidths into one or combining physical communication lines such as analog modems, ISDN and other analog or digital links to improve overall throughput. IETF RFC 1717 describes the PPP multi-connection protocol.

PPP Point-to-point protocol PPP application scenarios

PPP has now become the most widely used data link layer protocol for Internet access. PPP can be combined with ADSL, Cable Modem, LAN and other technologies to complete various types of broadband access. The most widely used broadband access method in our home is PPPoE (PPP over Ethernet). This is a PPP technology that uses Ethernet resources to run PPP on Ethernet to authenticate users' access. PPP is responsible for establishing a communication link between the client and the access server of the operator.

Similarly, the method of running the PPP protocol to manage user authentication on an ATM (Asynchronous Transfer Mode) network is called PPPoA (PPP over ATM). It has the same principle and function as PPPoE, but complies with ATM network standards.

PPP Point-to-point protocol PPP troubleshooting commands

debug ppp negotiation -Determines whether the client can negotiate through PPP; this is when you check the address negotiation.

debug ppp authentication -Determines whether the client can pass authentication. If you are using a version before Cisco IOS Software Release 11.2, issue the debug ppp chap command.

debug ppp error- displays protocol errors and statistics errors related to PPP connection negotiation operations.

debug aaa authentication -To determine which method is being used for authentication (should be RADIUS unless the RADIUS server fails), and whether the user passed the authentication.

debug aaa authorization -To determine which method is being used for authentication and whether the user is authenticated.

debug aaa accounting -View the records sent.

debug radius -View attributes exchanged by user and server.

PPP Point-to-Point Protocol PPP Frequently Asked Questions

1LCP Point-to-point protocol 1) What is LCP

Link Control Protocol (LCP) LCP establishes a point-to-point link, which is a practical part of PPP. The LCP sits above the physical layer and is responsible for establishing, configuring, and testing data link connections. The LCP is also responsible for negotiating and setting control options on the WAN data link, which are handled by the NCP.

2NCP Point-to-point protocol 2) What is NCP

PPP allows multiple network protocols to share a link. The Network Control Protocol (NCP) is responsible for connecting PPP (Layer 2) and Network Protocol (Layer 3). For each network layer protocol used, PPP uses a separate NCP to connect. For example, IP uses the IP Control Protocol (IPCP) and IPX uses the Novell IPX Control Protocol (IPXCP).