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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)                 9  Point-to-Point Protocol (PPP)                      9  PPP Feature Protocols

PPP Encryption Control Protocol (ECP) and Encryption Algorithms 1 2 3 PPP Bandwidth Allocation Protocol (BAP) and Bandwidth Allocation Control Protocol (BACP)

Most of the time, there is only a single physical layer link between two devices. There are some situations, however, when there may actually be two layer one connections between the same pair of devices. This may seem strange; why would there be more than one link between any pair of machines?

There are in fact a number of situations in which this can occur. One common one is when two links are intentionally placed between a pair of devices. This is often done to increase performance by “widening the pipe” between two devices, without going to a newer, more expensive technology. For example, if two machines are connected to each other using a regular analog modem and it is too slow, a relatively simple solution is to just use two analog modem pairs connecting the machines to double bandwidth.

A slightly different situation occurs when multiplexing creates the equivalent of several physical layer “channels” between two devices even if they only have one hardware link between them. Consider ISDN for example. The most common form of ISDN service (ISDN basic rate interface or BRI) creates two 64,000 bps B channels between a pair of devices. These B channels are time division multiplexed and carried along with a D channel on a single pair of copper wire, but to the devices they appear as if there were two physical layer links between devices, each of which carries 64 kbps of data. And the ISDN primary rate interface (PRI) actually creates 23 or even more channels, all between the same pair of hardware devices.

In a situation where we have multiple links, we could of course just establish PPP over each connection independently. However, this is far from an ideal solution, because we would then have to manually distribute our traffic over the two (or more) channels or links that connect them. If you wanted to connect to the Internet, you'd have to make separate connections and then choose which to use for each action. Not exactly a recipe for fun, and what's worse is that you could never use all the bandwidth for a single purpose, such as downloading the latest 100 megabyte Microsoft security patch.

What we really want is a solution that will let us combine multiple links and use them as if they were one high-performance link. Some hardware devices actually allow this to be done at the hardware level itself; in ISDN this technology is sometimes called bonding when done at layer one. For those hardware units that don't provide this capability, PPP makes it available in the form of the PPP Multilink Protocol (MP). This protocol was originally described in RFC 1717, and was updated in RFC 1990.

PPP Encryption Control Protocol (ECP) and Encryption Algorithms 1 2 3 PPP Bandwidth Allocation Protocol (BAP) and Bandwidth Allocation Control Protocol (BACP)