HDLC and PPP
Serial links are often used in WANs to provide long-distance data transmission. High-Level Data Link Control (HDLC) and Point to Point Protocol (PPP) are two typical serial port encapsulation protocols.
HDLC: Rarely used.
PPP: The main protocol used.
Serial link data transfer method
Serial links are commonly used in wide area networks. Two data transmission methods are defined in serial links: asynchronous and synchronous.
- Asynchronous transfer
Asynchronous transfers transfer data in units of bytes and require additional start and stop bits to mark the beginning and end of each byte. The start bit is binary value 0 and the stop bit is binary value 1. In this transmission mode, the start and stop bits occupy a considerable proportion of the transmitted data, and each byte of transmission requires additional overhead.
Asynchronous: There are start bits and stop bits, if not specified, conflicts may occur. The amount of data carried each time is also small.
- Synchronous transmission
Synchronous transmission is to transmit data in units of frames, and a clock needs to be used to synchronize the communication between the local and peer devices during communication. DCE is data communication equipment, which provides a clock signal for synchronizing data transmission between DCE equipment and DTE equipment. DTE stands for data terminal equipment, which usually uses a clock signal generated by DCE.
Synchronization: DCE timing, DTE learning time.
- HDLC protocol application
High-level Data Link Control, HDLC for short, is a bit-oriented link layer protocol (full duplex).
HDLC is a bit-oriented link layer protocol applied to synchronous transmission. It inherits the advantages of synchronous transmission. It transmits data frames and does not depend on any character set. It also has a check mechanism to encode the transmitted information to prevent Missed and so on.
- HDLC structure
HDLC has three types of frames: information frames (I), supervisory frames (S), and unnumbered frames (U).
- PPP protocol application
PPP protocol is a point-to-point link layer protocol, which is mainly used for point-to-point data transmission on full-duplex synchronous and asynchronous links.
- PPP link establishment process
The description of the PPP link establishment process is as follows:
The Dead phase is also known as the physical layer unavailable phase. When both ends of the communication parties detect that the physical line is activated, they will migrate from the Dead phase to the Establish phase, that is, the link establishment phase.
In the Establish phase, the PPP link negotiates LCP parameters. The negotiated content includes options such as the maximum receiving unit (MRU), authentication method, and magic number (Magic Number). After the LCP parameter negotiation is successful, it will enter the Opened state, indicating that the underlying link has been established.
In most cases, the devices at both ends of the link need to go through the authentication phase (Authenticate) before they can enter the network layer protocol phase. PPP links do not require authentication by default. If authentication is required, the authentication protocol must be specified during the link establishment phase. The authentication method is negotiated by both parties during the link establishment phase. If the Configure-Request message is received again at this stage, it will return to the link establishment stage.
In the Network phase, the PPP link performs NCP negotiation. Select and configure a network layer protocol and negotiate network layer parameters through NCP negotiation. Only after the negotiation of the corresponding network layer protocol is successful, the network layer protocol can send packets through this PPP link. If the Configure-Request message is received at this stage, it will also return to the link establishment stage.
After the NCP negotiation is successful, the PPP link will remain in the communication state. During the PPP operation, the connection can be interrupted at any time. For example, the physical link is disconnected, the authentication fails, the timeout timer expires, and the administrator closes the connection through configuration, which may cause the link to enter the Terminate phase.
In the Terminate phase, if all the resources are released, the two communicating parties will return to the Dead phase until the communicating parties re-establish the PPP connection.
- LCP link parameter negotiation
If the RTA does not receive the Configure-Ack message, it will retransmit the Configure-Request message every 3 seconds. The peer end is unavailable and stops sending Configure-Request packets.
Completing the above process only indicates that the RTB considers the link parameter configuration on the RTA to be acceptable. The RTB also needs to send a Configure-Request message to the RTA, so that the RTA can detect whether the link parameters on the RTB are acceptable.
After RTB receives the Configure-Request message sent by RTA, if RTB can identify all link layer parameters carried in this message, but considers that some or all of the parameter values are unacceptable, that is, the parameter value negotiation is unsuccessful. , the RTB needs to respond with a Configure-Nak message to the RTA. (If it is not recognized, it will reply to the Configure-Reject message)
In this Configure-Nak message, only unacceptable link layer parameters are included, and the link layer parameters included in this message are all modified to acceptable values (or value ranges) on RTB.
After receiving the Configure-Nak message, the RTA needs to reselect other locally configured parameters according to the link layer parameters in this message, and resend a Configure-Request.
- PPP authentication-PAP mode
- PPP authentication-CHAP mode
IPCP static address negotiation
In the PPP protocol, the addresses of both ends do not need to be on the same network segment to be able to communicate.
Running the PPP protocol on the Ethernet is called PPPOE.
PPPoE packets are encapsulated in Ethernet format.
PPPoE Discovery Phase
The client discovers the access server by broadcasting PADI packets.
After receiving the PADI message, all PPPoE servers compare the service requested by the client with the service that they can provide, and if they can provide the service, they will unicast and reply the PADO message.
The PPPoE client selects the PPPoE server corresponding to the first received PADO packet, and unicasts a PADR packet.
The PPPoE server generates a unique PPPoE Session ID and sends a PADS packet to the client, and the session is established successfully.
PPPoE session phase
The PPP negotiation on the PPPoE session is the same as the common PPP negotiation method, which is divided into three stages: LCP, authentication, and NCP.
After the PPP negotiation of the PPPoE session is successful, the PPP data can be transmitted.
PPPoE session termination
When the PPPOE client or server wants to close the connection, it can send a PADT message to the opposite end to close the connection.
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