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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols
           9  TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)

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SLIP and PPP Overview and Role In TCP/IP
(Page 2 of 2)

Comparing SLIP and PPP

Both SLIP and PPP are designed for connections that go between just two devices; thus the name “Point-to-Point” Protocol for PPP. They are used in port topology LAN or WAN connections, the simplest type. Since there are only two devices, A and B, communication is straight-forward: A sends to B and B sends to A. Since they only deal with simple two-device connections, they do not have to worry about complexities like media access control, or collisions, or unique addressing schemes, the way technologies like Ethernet must. As mentioned earlier, the primary focus of these protocols is providing framing services to layer three, as well as extra features as needed.

Why have two protocols? They both get the job done; the difference is in how they do it. SLIP is extremely simple and easy to implement but lacks the features of PPP, like authentication, compression, error detection and more. PPP is full-featured but much more complicated.

To draw an analogy, SLIP is a mostly-sturdy, ten-year old compact sedan, while PPP is a shiny new luxury SUV. Both will get you from here to Grandma's house, but the SUV is going to be safer, more comfortable and better able to deal with problems that might crop up on the road. If they cost the same to buy and operate, you'd probably choose the SUV. Both SLIP and PPP cost about the same, and unlike an SUV, PPP causes no air pollution and doesn't guzzle gas. For this reason, PPP is the choice of most serial line connections today, and has all but replaced SLIP.

Key Concept: SLIP and PPP provide layer two connectivity for TCP/IP implementations that run directly over a physical layer link without a layer two technology. SLIP is simple and was more commonly used years ago, but today PPP is favored due to its many features and capabilities.


Are SLIP and PPP “Really” Part of TCP/IP?

Incidentally, I should mention that there are some people who don't even consider SLIP and PPP to be part of the “true” TCP/IP protocol suite. They argue that TCP/IP really is defined at layers three and up on the OSI model, and IP itself is the basis of TCP/IP at layer three. Thus, SLIP and PPP are just “extra” protocols that can be used under TCP/IP. To support their argument they point to the fact that PPP can be used for protocols other than IP (which is true). For its part, SLIP is so simple it could carry any layer-three protocol, but I don't believe it has been implemented for network layer protocols other than IP.

Frankly, I consider these to be “how many angels fit on the head of a pin” type arguments. As far as I am concerned, SLIP and PPP are part of TCP/IP because they were originally designed for the specific purpose of letting TCP/IP run on layer one links. They were defined using the normal Internet RFC process as well. Regardless, whether they are part of TCP/IP or not, they are used by millions of people every day to enable TCP/IP networks to operate, and thus are deserving of a place in this Guide.

 


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TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)
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Version 3.0 - Version Date: September 20, 2005

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