The byte 0x7d is transmitted as the 2-byte sequence 0x7d, 0x5d. The byte 0x7e is transmitted as the 2-byte sequence 0x7d, 0x5e. Whenever this escape character appears in a PPP frame, the next character in theįrame has had it's sixth bit complemented, as follows: On asynchronous links, the special byte 0x7d is used as an escape character. On an asynchronous link, this is done by the hardware using a technique called bit stuffing. Since the byte value, 0x7e is the flag character, PPP needs to escape this byte when it appears in the information field. The CRC field (or FCS, for frame check sequence) is a cyclic redundancy check, to detect errors in the frame. A value of 0x0021 means the information field is an IP datagram, a value of 0xc021 means the information field is link control data and a value of 0x8021 is for network control data. Next comes the protocol field, similar in function to the Ethernet type field. Is always 0xff, and then a control byte, with a value of 0x03. This is followed by an address byte whose value Figure 2.3 shows the format of PPP frames.Įach frame begins and ends with a flag byte whose value is 0x7e.
#A PPP FRAME INCLUDE A FCS WHICH IS A ISO#
The format of the PPP frames was chosen to look like the ISO HDLC standard (high-level data link control). RFC 1332 specifies the network control protocol for IP. RFC 1548 specifies the encapsulation method and the link control protocol. The IP NCP, for example, allows each end to specify if it can perform header compression, similar to CSLIP. RFCs currently exist for IP, the OSI network layer,ĭECnet, and AppleTalk. This allows each end to negotiate various options.Ī family of network control protocols (NCPs) specific to different network-layer protocols.
#A PPP FRAME INCLUDE A FCS WHICH IS A SERIAL#
PPP supports either an asynchronous link with 8 bits of data and no parity (i.e., the ubiquitous serial interface found on most computers) or bit-oriented synchronous links.Ī link control protocol (LCP) to establish, configure and test the data-link connection.
PPP consists of three components.Ī way to encapsulate IP datagrams on a serial link.
PPP, the Point-to-Point Protocol, corrects all the deficiencies in SLIP.
Today, I would like to share with you the PPP protocol knowledge.
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