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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)

TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP) 1 2 Serial Line Internet Protocol (SLIP)

The TCP/IP protocol suite was generally designed to provide implementation of the networking stack from the network layer (layer three) and above. The core protocols of TCP/IP operate at layers three and four of the OSI model, corresponding to the Internet layer and Host-to-Host Transport layer of the TCP/IP architectural model. Other support protocols are defined at these two layers, and many application protocols run there too, as well as at the upper layers of the protocol stack.

However, the TCP/IP architectural model also defines the Network Interface layer, which corresponds to the data link layer (layer two) in the OSI scheme. In most classical networks, TCP/IP doesn't define any protocols operating at this layer. TCP/IP assumes that layer two functionality is provided by a WAN or LAN technology like Ethernet, Token Ring, or IEEE 802.11. These technologies are responsible for the classical layer two functions: physical layer addressing, media access control, and especially, layer two framing of datagrams received from layer three.

There's a problem with the assumption that IP can run on top of an existing layer two protocol: sometimes there isn't one. There are certain technologies that establish only a basic, low-level connection at the physical layer. The most common example of this is a simple serial connection established between two devices. Years ago, it was fairly common for two computers to just be connected using serial ports instead of a full-fledged LAN protocol. Today, we see this much less, but there's another type of serial connection that is very popular: serial dial-up networking. When you use a modem to connect to a modem at your Internet Service Provider, the modems negotiate a connection that architecturally exists only at the physical layer.

Since the Internet Protocol assumes certain services will be provided at layer two, there is no way to make it operate directly over a serial line or other physical layer connection. The most important layer two function that is required at a minimum is some mechanism for framing the IP datagram for transmission—that is, providing the necessary data packaging to let datagrams be transmitted over the physical layer network. Without this, IP datagrams cannot be sent over the link.

We need something to “fill the gap” between IP at layer three and the physical connection at layer one. To this end, a pair of special protocols have been defined that operate at layer two and provide the services that IP requires to function. These are:

• Serial Line Internet Protocol (SLIP): A very simple layer two protocol that provides only basic framing for IP.
  
  
• Point-to-Point Protocol (PPP): A more complex, full-featured data link layer protocol that provides framing as well as many additional features that improve security and performance.

TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP) 1 2 Serial Line Internet Protocol (SLIP)