Luminous Preamble:
If I'd known you will fall in love, fate is you, at the time I was young, I should grasp my right to occupy you, love you, from now on forever.
text:
PPP: Point to Point Protocol
Point to Point Protocol is a link layer protocol running on top of the point to point link.
PPP operation point link may be a serial dial-up telephone line, or the PPPoE, Ethernet analog PPP ( for access control, billing for each host access ) .
IETF PPP initial proposed design requirements;
. Packet framing.
. Transparency. PPP protocol data can not appear in the network layer packet ( header or data ) to make any restrictions.
. Multiple network layer protocols. PPP protocols must be able to simultaneously support multiple network layer protocols running on the same physical link.
. Many types of links.
. Error detection. PPP receiver must be able to detect bit errors in the received frame.
. Active connection. PPP must be able to detect link failure level, and notifies the failure to the network layer.
. The network layer address negotiation. PPP must communication network layer ( e.g. IP) provides a mechanism to learn or configure each network layer address.
. Simplicity.
PPP frame contains the following fields:
· Flag field. Each PPP frame is a value 01111110 of 1 byte as the start and end flag field.
· Address field. The only possible value of this field is 11111111
· Control field. The only possible value of this field is 00000011 .
·protocol. The protocol field tells PPP recipient encapsulating the received data frame ( i.e., PPP contents of the frame information field ) belongs to the upper layer network protocol.
· Checksum. The checksum field used to detect bit errors in the transmitted frame.
Byte stuffing
PPP use 01111110 flags field as the beginning and end of the frame, if the value of the flag field 01111110 appears in the information field, what will happen? PPP uses a technique known as padding bytes to solve this so-called transparent transmission of the problem .
PPP defines a special control byte escape 01111101 . If the flag sequence 01111110 any place other than the flag fields in one frame, PPP is inserted a control such that byte escape character before.
When the receiver see 01111110 a prior 01111101 , removes escape bytes of padding to the control reconstruct the original data.