Point-to-Point Protocol (PPP) is used to transfer data over serial point-to-point links. It accomplishes
this by using a layer 2 serial encapsulation called High-Level Data Link Control (HDLC).
HDLC is used for frame encapsulation on synchronous serial lines. It uses a link control protocol
(LCP) to manage the serial connection. Network control protocols (NCPs) are used to allow PPP
to use other protocols from layer 3, thus enabling PPP to assign IP addresses dynamically.
PPP uses the same frame structure as HDLC. Figure 36.9 gives you a picture of what the
frame looks like. As always, we move from right to left.
FIGURE 3 6 . 9 PPP packet structure
First, we have the Flag field, which uses one byte to specify the beginning or ending of a
frame. Then there is another byte that is used in the Address field to hold a broadcast address
of 11111111.
The Address field is followed by the one-byte Control field, which requests a transmission
of user data. The two-byte Protocol field follows the Control field. This field indicates the
encapsulated data’s protocol.
The Data field contains the information that will be handed to the upper layer protocols. It is a
variable-length field. After that is the FCS. Like the other protocols, it is used for CRC calculation.