# Establishing PPP connection step-1 and LCP's establishes / negotiates
###### tags: `elearning` `Multimedia Wireless Network`
###### Note Page : [:link:](https://hackmd.io/@YTL0307/HJEb6jWCo)

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## **Establishing PPP connection: Step 1**
- When the two devices want to communicate using PPP, they must first go through a formal connection establishment process using special PPP specific protocols.
- This three-step process is sometimes called a "handshake."
- Before devices can send data using PPP, both ends of the link must send and receive LCP packets to configure and test the link.

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## **How LCP Establishes and Negotiates a Connection**
- To establish a connection using LCP, a device first determines if there is a device at the other end of the link using LCP packets.
- During this first phase, if the devices have different connection maximum receive units or MRUS, or authentication protocols, they must resolve these differences.
- They will also decide if link quality monitoring will occur, which is something built into the protocol to address unreliable links.
- The process of resolving these issues is called negotiation.
- In this example, a device, call it device A wants to establish a PPP connection with another device, device B.
- Device A starts the link establishment and configuration process by sending an LCP configuration request to device B.
- Device A proposes how it will send data to device B, specifying the parameters it wants to use.
- Device A begins the negotiation process by sending a Configure-Request message with three options: X, Y (which it wishes to set to 200), and Z.
- An actual PPP Configure Request might contain the MRU (Maximum Receive Unit), an authentication protocol, and whether link quality monitoring will occur.
- If device B does not support an option or the option is not negotiable, it sends back an LCP Configure-Reject.
- Device B does not understand or support option Z so it sends an LCP Configure-Reject message containing option Z.

- Device A then sends a new LCP Configure-Request message with new parameters, and the process is repeated.
- Device A sends a new LCP Configure Request, which includes option X, option Y set to 200, but does not include option Z.
- If instead Device B receives an LCP Configure-Request, which it understands but which contains an option with unacceptable values, it sends an LCP Configure-Nak, where Nak is short for Negative Acknowledgement.
- Device B prefers that option Y be set to 100, so it sends a Configure Nak indicating its preferred value.

- Device A sends a new Configure-Request which includes option X and option Y set to 100, to which Device B responds with a Configure-ack message, where ack is short for acknowledgement.
- The two devices have now agreed upon how device A should send data to device B.
- With PPP, device A and device B do not have to use the same set of PPP options.
- A separate LCP negotiation must be completed so that Device B can be configured to send data to device A.
- In actual practice, the LCP packets for each negotiation process are typically intermixed during the connection process.

- Device B sends an LCP configure request message with options X, and Y set to 100, and option D, an option not requested by Device A.
- Device A is able to support option D and simply sends an LCP Configure-Ack message.
- Once Device A and B come to an agreement, the status of the link is considered "LCP open" and the devices proceed to the optional authentication phase.
- If the devices cannot agree the physical link is terminated.