The Point-to Point Protocol and the Serial Line Internet Protocol


Communication on the Internet is based on the TCP/IP suite. TCP/IP is covered in detail in Chapter 25, "Overview of the TCP/IP Protocol Suite"; other services, applications, and tools designed to work with TCP/IP are discussed in Chapters 26, "Basic TCP/IP Services and Applications," 28, "Troubleshooting Tools for TCP/IP Networks," and 29, "BOOTP and Dynamic Host Configuration Protocol (DHCP)."

Although TCP/IP is a great LAN/WAN networking solution, it does not provide for dial-up connections. A dial-up connection is a point-to-point link using a phone line. Because of this, a router or server on the remote network will be your connection point to that network using a modem. The remote access server at the Internet service provider (ISP, or maybe your corporate dial-in modem bank) creates point-to-point connections with dial-in clients .

This connection needs a method for sending IP or other protocols across this point-to-point connection (where addressing doesn't matter because the conversation has only two parties) transparently to the actual network transport protocol. The Serial Line Internet Protocol (SLIP) was the first widely adopted protocol, and was initially found mainly on Unix systems. Many operating systems (including Unix, Linux, and Windows) still support SLIP today. However, far fewer use SLIP as compared to the Point-to-Point Protocol (PPP). PPP is more robust and has generally replaced SLIP in all but the most unique cases during the past few years .

PPP makes up for many of the shortcomings of SLIP. For example:

  • PPP offers synchronous as well as asynchronous communications, whereas SLIP offers only synchronous.

  • PPP includes error correction. SLIP relies on either the error-correction capabilities of the hardware used to make the connection or the error-correction capabilities of TCP/IP.

  • PPP provides for automatic, dynamic address assignment. SLIP must be manually configured prior to dialing or during the initial session setup.

  • PPP provides for compression. SLIP ”for the most part ”does not. There are variations of SLIP, such as Compressed SLIP, or CSLIP, that enable compression, but these are uncommon and often proprietary implementations .

  • Multiple protocols can use the same PPP link (such as IP and IPX). With SLIP, you are limited to IP.

Both of these protocols are nonroutable, due to their point-to-point connection. There are only two parties to the connection, so there is no need for routing. Of course, this applies only to PPP or SLIP. Both can encapsulate other protocols that are then passed to routers or other devices. At the receiving end of a connection, the PPP or SLIP information is stripped off and the protocol (such as IP) that was sent across the serial link (such as a modem) is then transmitted through a network as if it were coming from a computer or another device attached to the network.

Note

Encapsulate is a networking term that means a particular protocol is carrying, in its payload section, another packet generated by a different protocol. SLIP is one of the simplest of the protocols developed for this purpose, using only two characters to successfully transmit other protocols across a point-to-point connection. PPP is more complicated, adding its own protocol header information to the packet (and stripping it off at the other endpoint), to make communications a little more flexible and reliable.



Upgrading and Repairing Networks
Upgrading and Repairing Networks (5th Edition)
ISBN: 078973530X
EAN: 2147483647
Year: 2003
Pages: 434

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net