Basic understanding of PPP protocol

To master a protocol, we need to understand it based on it. Here we will analyze the content of the PPP protocol. First, let's take a look at the overview of the PPP protocol. Currently, most analog dial-up connections use PPP as the data link protocol, which has the following features:

Error Detection

Automatically negotiate network layer addresses

CHAP or PAP Authentication

Data Compression

Meets ISO standards

PPP and the serial line Internet Protocol SLIP) are often confusing. SLIP only supports the IP protocol, and PPP supports multiple protocols such as IP, IPX, and AppleTalk.

PPP sets the data link layer as three sub-layers:

NCP establishes and negotiates network layer protocols and corresponding addresses

LCP establishes links, authenticates users, and detects link quality

HDLC encapsulates data packets on the Link

Table 1 Overview of the PPP protocol and its Child Layer

OSI Layer common protocols

Layer-3 IP, IPX, and AppleTalk

Layer-3 NCP, LCP, and HDLC

Layer 3 EIA/TIA-232, X.24, V.23, V.35, ISDN, etc.

RFC related to PPP:

Rfc1144tcp/ip packet Compression

RFC1220PPP expansion on the bridge

RFC1334PPP Authentication Protocol

RFC1378PPP AppleTalk control protocol ATCP)

RFC1552PPP Internet Packet Exchange Control Protocol IPXCP)

RFC1570PPP LCP Protocol Expansion

RFC1661PPP PPP)

HDLC encapsulation in RFC1662PPP

RFC1990PPP multi-Link Protocol MP)

Note: At http://www.isi.edu/in-notes/rfcxxxx.txt, xxxxrepresents the rfc's orchestration.

Most of the extended functions of PPP protocols, such as data correction and support for multiple network protocols, are controlled by LCP and NCP. LCP is used to configure and test data links. It works as follows:

Step 1: establish the link-first open the connection, and then determine the communication parameters including MTU, compress type, and link authentication type. After the link is set, confirm the frame, and then the optional link quality validation phase. LCP determines the link quality.

Step 2 (optional) authentication stage-two authentication methods: Question response Handshake Authentication Protocol CHAP) and Password Authentication Protocol PAP ). PPP itself does not require authentication. cisco router asynchronous lines require authentication. We recommend that you use CHAP authentication.

Step 2: Network Layer Protocol stage-LCP guides NCP to activate and configure network layer protocols. Data packets can be transmitted after the end of this phase.

Step 3: link termination-LCP guides NCP to disable layer 3.

LCP uses three types of LCP data frames to complete the preceding steps:

Link establishment frame Link establishment frames)-Establish Link

Link terminateon frames) -- Disable Link

Link maintenance frames) -- maintain links