Point to Point Protocol in Computer Network

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Transition states, PPP layers, Link Control Protocol (LCP), Authentication, Network Control Protocol.

Point To Point protocol

 Point To Point Protocol in computer network

The first protocol devised for this direction was serial line internet protocol (SLIP). However SLIP has some deficiencies it doesn’t support protocols aside from internet protocol (IP), it had been not allowed the IP address to be assigned dynamically, and it does not support authentication of the user. The point-to-point protocol (PPP) is a protocol designed to respond to these deficiencies.


 Point To Point Protocol in computer network

The difference phases through which a Point to point protocol in computer network connection goes can be described using a transition state.

  • IDLE STATE- the idle state means that the link isn’t being used. There is no active carrier and therefore the line is quiet.
  • ESTABLISHING STATE- when one of the endpoints starts the communication, the connection goes into the established state. In the state, options are negotiated between the 2 parties. If the negotiation is successful, then the system goes to the authenticating state or directly to the networking state. The LCP packets, discussed shortly, are used for this direction.
  • AUTHENTICATING STATE- the authenticating state is optional the two endpoints may decide, during the established state, not to go through this state. If they decide to proceed with authenticating, they send several authenticating packets, discussed in a later section. If the result’s successful, the connection goes to the networking state.
  • NETWORKING STATE- the networking state is that the heart of the transition states. When a connection reaches this state, the exchange of user control and data packets are often started. The connection remains in this state until one of the endpoints wants to conclude the connection.
  • TERMINATING STATE- when the connection is in the terminating state, several packets are exchanged between the two ends for house cleaning and closing the link.


Point to point protocol in computer network has only physical and data link layers. This means that a protocol that wants to use the services of PPP should have other layers of networks, transport, and so on.

Point to Point Protocol in Computer Network
  • PHYSICAL LAYER- no specific protocol for the physical layer in Point to point protocol in computer network. Instead, it’s left to the implementer to use whatever is out there PPP supports any of the protocol recognized by ANSI.
  • DATA LINK LAYER- at the info data link layer, PPP employs a version of HDLC. The format of the Point to point protocol in the computer network frame.


The descriptions of the fields are as follows:

Point to Point Protocol in Computer Network
  • FLAG FIELD –  the flag field, like one in HDLC, identifies the boundaries of a PPP frame. Its value is 0111110.
  • ADDRESS FIELD – because the PPP frame is employed for a point-to-point connection, it uses the broadcast address of HDLC, 11111111, to avoid a data link address in the protocol.
  • CONTROL FIELD – the control field uses the format of the U-frame in HDLC. The value is 11000000 to show that the frame doesn’t contain any sequence numbers and there’s no flow and error control.
  • PROTOCOL FIELD – the protocol field defines what is being carried in the data field user data or other information. We will discuss this field in detail shortly.
  • FCS – the frame check sequence field, as in HDLC, is simply a two-byte or four-byte CRC.


The link control protocol (LCP) is liable for established, maintaining, configuring, and terminating links.

It also provides negotiation mechanisms to set options between the two endpoints. Both endpoints of the link must reach an agreement about the choices before the link can be established.

All LCP packets are carried within the payload field of the Point to point protocol in computer network frame.

When the defines the frame as one carrying an LCP packet is the value of the protocol field, which should be set to C02116.

Point to Point Protocol in Computer Network

The descriptions of the fields are as follows:

  • CODE – this field defines the sort of LCP packet.
  • ID – this field holds a value used to match a request with the reply. One endpoint inserts a worth during this field, which can be copied within the reply packet
  • LENGTH – This field defines the length of the whole LCP packet.
  • INFORMATION – this field contains extra information needed for some LCP packet.


0116Configure-requestContains the list of proposed options and their values
0216Configure-ackAccepts all options proposed
0316Configure-nakAnnounces that some options are not acceptable
0416Configure-rejectAnnounces that some options are not recognized
0516Terminate-requestRequest to shut the line down
0616Terminate-ackAccepts the shut-down request
0716Code-rejectAnnounces an unknown code
0816Protocol-rejectAnnounces an unknown protocol
0916Echo-requestA type of hello message to check if the other end is alive
0A16Echo-replyThe response to the echo-request message
0B16Discard-requestA request to discard the packet
LCP packets and their codes

Configuration packets configuration packets are wont to negotiate the options between two ends. 4 different packets are used for this purpose: configure-request, configure-ack, configure-nak, and configure-reject.

  • CONFIGURE-REQUEST – the endpoint that wishes to start a connection sends a configure-request message with an inventory of zero or more options to the other endpoint.
  • CONFIGURE-ACK – if all of the options listed within the configure-request packet are accepted by the receiving end, it will send a configure-ack, which repeats all of the options requested.
  • CONFIGURE-NAK – if the receiver of the configure-request packet packets recognizes all of the options but finds that some should be omitted or revised, it sends a configure-nak packet to the sender. The sender should then omit or revise the options and a totally new configure-request packet.
  • CONFIGURE-REJECT – if some of the options are not recognized by the receiving party, it responds with a configure-reject packet, marking those options that aren’t recognized. The sender of the request should revise the configure-request message and send a completely new one.

LINK TERMINATION PACKETS the termination packets are used to disconnect the link between two end points.

  • TERMINATE-REQUEST – either party can terminate the link by sending a terminate-request packet.
  • TERMINATE-ACK – the party that receives the terminate-request packet should answer with a terminate-ack packet.

LINK MONITORING AND DEBUGGING PACKETS – these packets are used to monitoring and debugging the link.

  • CODE-REJECT – if the endpoint receives a packet with an unrecognized code in the packet, it sends a code-reject packet.
  • PROTOCOL-REJECT – if the endpoint receives a packet with an unrecognized protocol within the frame, it sends a protocol-reject packet.
  • ECHO-REQUEST – this packet is sent to monitor the link. Its purpose is to ascertain if the link is functioning. The sender expects to receive an echo-reply packet from the opposite side as proof.
  • ECHO-REPLY – this packet is shipped in response to an echo-request. The information field within the echo-request packet is exactly duplicated and sent back to the sender of the echo-request packet.
  • DISCARD-REQUEST – this is often an of the loopback test packet. It is used by the sender to check its own loopback condition. The receiver of the packet just discards it.  


Authentication plays a very important role in Point to point protocol in computer network because Point to point protocol in computer network is designed for use over dial-up links where verification of user identity is necessary.

Authentication means validating the identity of a user who needs to access a set of resources.

Point to point protocol in computer network has created two protocols for authentication: password authentication protocol (PAP) and challenge handshake authentication protocol (CHAP).

PAP The password authentication protocol (PAP) may be a simple authentication procedure with a two-step process.

Point to Point Protocol in Computer Network
  • The user who wants to access a system sends an authentication identification (usually the user name) and a password.
  • The system checks the validation of the identification and password and either accepts or denies the connection.

For those systems that require more security. PAP isn’t enough a 3rd party with access to the link easily pick up the password and accesses the system resources.

PAP packets – PAP packets are encapsulated during a PPP frame. What distinguishes a PAP packet from other packets is that the value of the protocol field, C02316.

There are three PAP packets: Authenticate-request, authenticate-cat, and authenticate-nak. The first packet is employed by the user to send the user name and password. The second is employed by the system to allow access. The third is employed by the system to deny access.

CHAP The challenge handshake authentication protocol (CHAP) is a three way handshaking authentication protocol that gives more security than PAP. In this method, the password is kept private it is never sent on line.

  • The system sends to the user a challenge packet containing value, usually a few bytes.
  • The user applies a predefined function that takes the challenge value and therefore the users own password and created a result. The user sends the result in the reply packet to the system.
  • The system does the same. It applies the same function to the password of the user and therefore the challenge value to create a result. If the result created is that the same as the result sent in the reply packet, access is granted otherwise it’s denied.
Point to Point Protocol in Computer Network

 CHAP is more protected than PAP, especially if the system continuously changes the challenge value. Even if the intruder learns the challenge value and the result, the password remains secret.

CHAP packets CHAP packets are encapsulated in the PPP frame. What distinguishes a CHAP packet from other packets is that the value of the protocol field, C22316.

There are four CHAP packets: challenge, response, success, and failure. The first packet is applied by the system to send the challenge value.

The second is applied by the user to return the result of the calculation. The third is applied by the system to allow access to the system. The fourth is applied by the system to deny access to the system.


After the link has been established and authentication has been successful, the connection goes to the networking state.

In this state, PPP uses another protocol called network control protocol (NCP). NCP is a set of the control point to point protocol in computer network to allow the encapsulation of data coming from network layer protocol (such as IP, IPX, and apple talk) in the PPP frame.

IPCP The set of packets that established and terminate a network layer connection for IP packets is called the internetwork protocol control protocol (IPCP).

The value of the protocol field, 802116, defines the packets encapsulated within the protocol as an IPCP packet.

Seven packets are defined for the IPCP protocol, distinguished by their code values defined in the table.

Code value for IPCP packets

A party uses the configure- request packet to negotiate options with the opposite party and to set the IP addresses and so on.

After configuration, the link is prepared to carry IP protocol data within the payload field of a PPP frame. Time, the value of the protocol field is 002116 to show that the IP data packet, not the IPCP packet, is being carried across the link. After IP has sent all of its packets the IPCP can take hold and use the terminate-request and terminate-ack packets to finish the network connection.  


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