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Unformatted text preview: t. These smaller headers greatly
improve the interactive response time.
Most SLIP implementations today support CSLIP. Both SLIP links on the author's subnet (see inside front cover)
are CSLIP links. 2.6 PPP: Point-to-Point
PPP, the Point-to-Point Protocol, corrects all the deficiencies in SLIP. PPP consists of three
1. A way to encapsulate IP datagrams on a serial link. 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.
2. A link control protocol (LCP) to establish, configure, and test the data-link connection.
This allows each end to negotiate various options.
3. A family of network control protocols (NCPs) specific to different network layer protocols.
RFCs currently exist for IP, the OSI network layer, DECnet, and AppleTalk. The IP NCP,
for example, allows each end to specify if it can perform header compression, similar to
CSLIP. (The acronym NCP was also used for the predecessor to TCP.)
RFC 1548 [Simpson 1993] specifies the encapsulation method and the link control protocol. RFC
1332 [McGregor 1992] specifies the network control protocol for IP.
The format of the PPP frames was chosen to look like the ISO HDLC standard (high-level data
link control). Figure 2.3 shows the format of PPP frames. file:///D|/Documents%20and%20Settings/bigini/Docum.../homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm (5 of 11) [12/09/2001 14.46.33] file:///D|/Documents%20and%20Settings/bigini/Documenti/homenet2run/tcpip/tcp-ip-illustrated/link_lay.htm Figure 2.3 Format of PPP frames.
Each frame begins and ends with a flag byte whose value is 0x7e. This is followed by an address
byte whose value is always 0xff, and then a control byte, with a value of 0x03.
Next comes the protocol field, similar in function to the Ethernet type field. 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.
The CRC field (or FCS, for frame check sequence) is a cyclic redundancy...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12