Point-to-Point Protocol (PPP) Frame Format
Point-to-Point Protocol (PPP), which is frequently used at the data link layer and is typically the default RAS protocol in Windows, is needed to encapsulate higher network-layer protocols in order to pass them over synchronous and asynchronous communication lines. Link Control Protocol controls and manages link establishment primarily in PPP (LCP). Additionally, a modem connection from the home computer to the ISP server is necessary. ISPs also started using it to merely offer dial-up Internet Access.
Services Provided By PPP
The primary functions of the Point-to-Point Protocol are
- Defining the data frame format for transmission.
- Describing the steps involved in creating a link between two points and data exchange.
- Stating the method of encapsulation of network layer data in the frame.
- Stating authentication rules of the communicating devices.
- Providing an address for network communication.
- Providing connections over multiple links.
- Supporting a variety of network layer protocols by providing a range of services.
Services Not Provided By The PPP protocol
- It does not support flow control mechanism.
- It has a very simple error control mechanism.
- As PPP only supports point-to-point communication, it is unable to handle frames in multipoint configurations.
Components Of PPP
Point – to – Point Protocol is a layered protocol having three components
Encapsulation Component
It encapsulates the datagram so that it can be transmitted over the specified physical layer.
Link Control Protocol (LCP)
It is in charge of creating, setting up, testing, keeping up with and terminating links for transmission. Additionally, it teaches the two endpoints of the link how to negotiate the setting up of options and the use of features.
Authentication Protocols (AP)
For service use, these protocols authenticate endpoints. There are two PPP authentication protocols:
- Password Authentication Protocol (PAP)
- Challenge Handshake Authentication Protocol (CHAP)
Network Control Protocols (NCPs)
These protocols are used to negotiate the network layer’s parameters and facilities. One NCP exists for each higher-layer protocol that PPP supports. Several of the PPP’s NCPs include:
- Internet Protocol Control Protocol (IPCP)
- OSI Network Layer Control Protocol (OSINLCP)
- Internetwork Packet Exchange Control Protocol (IPXCP)
- DECnet Phase IV Control Protocol (DNCP)
- NetBIOS Frames Control Protocol (NBFCP)
- IPv6 Control Protocol (IPV6CP)
Various Fields Of Frame
Flag field
PPP frames always start and end with the conventional HDLC flag, just like HDLC frames do. It always has a value of 1 byte or the binary value 01111110.
Address field
The broadcast address is essentially the address field. All 1’s in this case simply mean that every station is prepared to accept a frame. Its value is 1 byte or the binary value 11111111. PPP, on the other hand, neither assigns nor provides specific station addresses.
Control field
This field primarily uses HDLC’s U-frame or Unnumbered frame, format. In HDLC, the control field is necessary for a number of reasons, but in PPP, this field is set to 1 byte or the binary value 00000011. For a connectionless data link, this 1 byte is used.
Protocol field
This field essentially identifies the datagram’s network protocol. The type of packet in the data field or what is specifically being carried in the data field is typically identified. This field, which is 1 or 2 bytes long, aids in identifying the PDU (Protocol Data Unit) that the PPP frame is encasing.
Data field
Typically, it includes the datagram from the top layer. For regular PPP data frames, network layer datagrams are specifically encapsulated in this field. The length of this field fluctuates rather than being constant.
FCS field
Typically, the checksum in this field serves only to identify errors. Its size options are 16 bits or 32 bits. Additionally, it is calculated over the fields of address, control, protocol and even information. To control and handle errors, additional characters are added to the frame.
Notes
- It has the ability to negotiate IP Addresses dynamically.
- It contains Link Control Protocol (LCP) simply to develop link options.
- It contains error checking for each PPP frame.
- It also has the ability to transport several protocols on a single serial connection.