Both SLIP and PPP are most common TCPIP modem protocols used for establishing dial-up internet connection. SLIP stands for Serial Line Internet protocol and PPP stands for Point-to-Point Protocol. Initially SLIP was introduced, which has few limitations and hence was later replaced by PPP. Now PPP is the most common protocol used to establish internet connection. This page covers difference between SLIP and PPP protocols. SLIP is described in RFC 1055 and PPP is described in RFC1661.


The function of the SLIP is to transmit IP datagrams across modem line. SLIP data format consists of data followed by special END character(decimal 192) which marks end of the data. To avoid false stop in case data itself contains END character, then SLIP adds ECS character before actual END character so that receiver will be able to receive complete message/data. RFC 1055 specifies maximum packet size as 1006 bytes.

Characteristics and deficiencies of SLIP protocol:

•  Both the computers(sender and receiver) must know each other's IP address in order to communicate. SLIP does not support DHCP which is used to assign dynamic IP. Hence it is not appropriate to be used by most of the ISPs of providing dial-up internet services.
•  It does not have type field and hence there is no way to specify protocol type. Hence SLIP can not be used for multiple protocol functionalities. This is take care by PPP. Also SLIP can not multiplex and demultiplex other protocol systems along with TCPIP, the same is supported by PPP.
•  SLIP does not take care of error correction, this need to be done by upper layers.
•  SLIP does not support header compression and hence is not ideal for slow speed internet connection.


As mentioned PPP is designed to overcome the shortcomings studied with the introduction of SLIP. PPP is divided into 3 basic functionalities as outlined below.
•  Method of encapsulation of multiprotocol datagrams.
•  LCP(Link Control Protocol) used by PPP to establish,manage and to terminate the dial up connections.
• NCPs(Network Control Protocols) to support upper layer protocols.

PPP frame consists of protocol field,followed by data and then padding. The data part carry control packet or upper layer datagrams. protocol field is 1 or 2 bytes long and identifies protocol type of the enclosed packet. Possible types can be LCP packet,NCP packet,IP packet or OSI network layer protocol packet.

Difference between HDLC and PPP

Refer HDLC vs PPP which mentions difference between HDLC and PPP protocols.


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