PPP Principle

Components of PPP :

→ Datagram encapsulation method : define the method of encapsulating multi-protocol datagram

→ Link Control Protocol : define method of establishing, configuring, and testing data link conn.

→ Network Control Protocol : define a set of protocol for establishing connection and negotiating parameters for different network-layer protocols.

* | Protocol (2B) | Information | Padding (optional) |

\____________/

Max receive unit (MRU) [default: 1500 bytes]

0x0021 : IP datagram + padding (optional)

0xc021 : LCP + padding (optional)

0x8021 : IP control protocol (IPCP) + padding (optional)

| Flag | Address | Control | PPP frame | FCS | Flag |

01111110 11111111 00000011 * 16bits 01111110

\____HDLC standard___________/ \___follow HDLC standard__/

Messages Used by LCP Negotiation

→ Configure-Request : the beginning of link-layer parameter negotiation of the two ends

→ Configure-Ack : if the values of negotiated parameters are acceptable

→ Configure-Nak : if the values of negotiated parameters are not acceptable

→ Configure-Reject : if the values of negotiated parameters can not be identified

→ Terminate-Request : asks to close connection. 2 * 3 s. if not receive TA, forced close connection

→ Terminate-Ack : acknowledge terminate request from the peer.

→ Echo-Request : checks the status of link [on VRP every 10 s]

→ Echo-Reply : telling to the peer that the link is normal.

[Success] LCP negotiation : RTA sends CR to RTB. RTB sends CA. If CA is not received 10 * 3 s CR.

[Unsuccessful] : RTA sends CR. RTB sends CN. RTA re-sends modified CR. max 5 CR

[Unidentified] : RTA sends CR. RTB can not identify so returns CJ. RTA re-sends deleted CR params.

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Principle of PAP Authentication Mode → 2 way

RTA [Authenticator] –––––––––––––––– RTB [Authenticated]

RTB – Authenticate-Request (user name + password) → Authenticator.

RTA – Authenticate-Ack/Authenticate-Nak → RTB.

Principle of CHAP Authentication Mode → 3 way

RTA [Authenticator] –––––––––––––––– RTB [Authenticated]

→ RTA send CHALLENGE. RTB encrypt MD5 { identifier+password+challenge }=16-byte digest. RTB sends RESPONSE packet (CHAP user name & digest) to authenticator. A Success/Failure?→ B

Network Control Protocol

NCP has same mechanism (CR,CA,CN,CJ) like LCP but it doesn't invoke LCP.

→ NCP static configuration

→ NCP dynamic configuration : CR (use 0.0.0.0 address) → CN with IP → CR → CA → CR₂ → CA