Ppp frame serial#
Ppp frame driver#
Ppp frame iso#

Ppp frame serial#

In summary, PPP provides the following advantages over SLIP: (1) support for multiple protocols on a single serial line, not just IP datagrams, (2) a cyclic redundancy check on every frame, (3) dynamic negotiation of the IP address for each end (using the IP network control protocol), (4) TCP and IP header compression similar to CSLIP, and (5) a link control protocol for negotiating many data-link options. Additionally, using the IP network control protocol, most implementations then negotiate to use Van Jacobson header compression (identical to CSLIP compression) to reduce the size of the IP and TCP headers. If we then compare the framing overhead in a PPP frame, versus the 2-byte framing overhead in a SLIP frame (Figure 2.2), we see that PPP adds three additional bytes: 1 byte for the protocol field and 2 bytes for the CRC. Using the link control protocol, most implementations negotiate to omit the constant address and control fields and to reduce the size of the protocol field from 2 bytes to 1 byte. Since PPP, like SLIP, is often used across slow serial links, reducing the number of bytes per frame reduces the latency for interactive applications. It is also possible to use the link control protocol to specify which, if any, of these 32 values must be escaped.

Ppp frame driver#

The reason for doing this is to prevent these bytes from appearing as ASCII control characters to the serial driver on either host or to the modems, which sometimes interpret these control characters specially. (In this case, the complement of the sixth bit turns the bit on, whereas in the two previous examples the complement turned the bit off.) For example, the byte 0x01 is transmitted as the 2-byte sequence 0x7d, 0x21. This is the escape of the escape byte.Р”В«y default, a byte with a value less than 0x20 (i.e., an ASCII control character) is also escaped. 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 theР”Р‡rame 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.Р”В®ach frame begins and ends with a flag byte whose value is 0x7e.

Ppp frame 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,Р”ВECnet, and AppleTalk. This allows each end to negotiate various options.Р”Р„ family of network control protocols (NCPs) specific to different network-layer protocols. 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.Р”Р„ link control protocol (LCP) to establish, configure and test the data-link connection. PPP consists of three components.Р”Р„ 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.