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Data link protocols

Most of the data link protocols are derived from a data link protocol used in IBM’s SNA, called SDLC (Synchronous Data Link Control Protocol). After developing as SDLC, IBM submitted it to ANSI and ISO for acceptance as U. S. and international standards respectively.

ANSI modified it to become ADCCP (Advanced Data Communication Control Procedure), and ISO modified it to become HDLC (High-Level Data Link Control). CCITT then adopted and modified HDLC for its LAP (Link Access Procedure) as part of the X.25 network interface standard but later modified it again to LAPB, to make it more compatible with a later version of HDLC.

All of these protocols are based on the same principles. All are bit-oriented and all use bit stuffing for data transparency. All the bit oriented protocols use the frame structure as shown in the figure below.

Despite its widespread use, HDLC is fat from perfect.
For both the router-router leased line connection and the dial-up host-router connection, some point-to-point data link protocol is required on the line for framing, error control and the other data link layer functions two such protocols are widely used in the internet, SLIP and ppp.

SLIP is the older of the two protocols. It was devised by Rick Adams in 1984 to connect sun workstations to the Internet over a dial-up line using a modern. The protocol, which is described in RFC 1055, is very simple. The workstation just sends raw IP packets over the line, with a special flag byte (OXCO) at the end for framing. If the flag byte occurs inside the IP packets, a form of character stuffing is used, and the two byte sequence (OXDB, OXDC) is sent in its place. If OXDC occurs inside the IP packet, it too, is stuffed. Some SLIP do some TCP and IP header compressions.

Although it is widely used SLIP has some services problems. SLIP is not an approved Internet standard, so many different versions exist. This situation does not make Internet working easier.

To improve the situation, the IETF set up a group to devise a data link protocol for point-to-point lines that solved all these problems and that could become an official Internet standard. This work culminated in PPP (Point-to-point Protocol) which is defined in RFC 1661 and further elaborated on in several other RFCs. PPP handles error detection, supports multiple protocols., allow IP addresses to be negotiated at connection time, permits authentication and has many other improvement over SLIP.

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