Neon-komputadór

Modem Protocols (SLIP, PPP, LCP)

With the sort of point-to-point connection between two computers that one achieves through a modem, the complications of a networking protocol like TCP/IP is not necessary. After all, competition between data packets, flow control and addressing is pretty self-evident. However, with the arrival of local networks and the Internet the use of dial-up connections as a means of providing network access became an issue. Originally, the computer being attached to the network assumed the responsibility for the material being transmitted to the network. The remote computer basically acted like a terminal that directed the host computer to perform network tasks. This model placed undue strain on the host computer and underused the processing ability of the remote computer.

Dial-up protocols started to work directly with TCP/IP and have become a integral part of that protocol stack. The protocols examined here are Serial Line Internet Protocol (SLIP), Point-to Point Protocol (PPP) and the role of the Link Control Protocol (LCP) within PPP. In general, the PPP dial-up protocol is replacing SLIP.

SLIP was an early attempt to direct integrate modem protocols with TCP/IP. It was later implemented on Berkeley UNIX systems and spread widely from there. Whilst considered somewhat obsolete, there are some situations where it is beneficial. SLIP operates by transmitting IP datagrams across a modem line. It provides no physical address or error control and depends on other protocols to fulfill these functions. SLIP simply sends the data with a special signal marking the end of the data transmission.

Some of the key characteristics of SLIP are as follows: a) both computers need to know each other's IP address. SLIP does not support dynamic IP addresses and is therefore impractical for ISPs which lease IP addresses b) SLIP has no Type field, which means that SLIP is incapable of supporting multiple protocols simultaneously, it cannot multiplex/demultiplex c) SLIP does not provide error correction d) SLIP does not support header compression.

When the computer industry developed PPP they had a much better idea of what features would be useful. Thus PPP includes the following a) capacity for multiprotocol datagrams b) A Link Control Protocol (LCP) that establishes, configures and tests the connection and c) a whole suite of Network Control Protocols (NCPs). The data format of a PPP frame consists of a one or two-byte field providing the identification number for the protocol of the enclosed packed, based on a standard list maintained by the Internet Assigned Numbers Authority (IANA), the enclosed data, and optional adding of variable length which depends on the particular protocol being used.

Much of the flexibility of PPP comes form the inclusion of the LCP that establishes, manages and terminates connections. There are three types of LCP packets, one for configuration, one for maintenance, and one for termination. Assuming that the LCP negotiates a connection, PPP uses NCP packets to specify protocol-specific configuration for each supported protocol suite. It then transits datagrams received from the upper-layer protocols. Assuming this is successful, PPP then closes the connection through an exchange of LCP termination packets.