用户接入Internet，在传送数据时都需要有数据链路层协议，其中最为广泛的是串行 线路网际协议（SLIP）和点对点协议（PPP）。由于SLIP具有仅支持IP等缺点，主要用于低速（不超过19.2kbit/s）的交互性业务，它并未成为Internet的标准协议。为了改进SLIP，人们制订了点对点PPP（Point-to-Point Protocol）。RFC1661、RFC1662、RFC1663。(When users access Internet, they need data link layer protocol to transmit data, among which SLIP and PPP are the most widely used. Because SLIP has the disadvantage of supporting IP only, it is mainly used in interactive services with low speed (no more than 19.2kbit/s), and it has not become the standard protocol of Internet. In order to improve SLIP, point to point PPP (Point-to-Point Protocol) has been developed. RFC1661, RFC1662, RFC1663.)