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The PPP Multilink Protocol (MP) described in the previous topic allows multiple links between a pair of devices, whether physical or in the form of virtual channels, to be combined into a “fat pipe” (high-capacity channel). This offers tremendous advantages to many PPP users, as it lets them make optimal use of all their bandwidth, especially for applications such as Internet connectivity. It's no surprise, then, that MP has become one of the most popular features of PPP.
The original standard defining MP basically assumed that multiple links would be combined into a single bundle. For example, if you had two modem links they would both be connected and then combined, or two B channels in an ISDN link would be combined. After MP was set up the bundle would be available for either device to use in its entirety.
There's one drawback to this system: the “fat pipe” is always enabled, and in many cases, it is expensive to have this set up all the time. It often costs more to connect two or more layer one links than a single one, and it's not always needed. For example, some ISDN services charge per minute for calls on either of the B channels. In the case of modem dialup there are per-minute charges in some parts of the world. Even where regular phone calls are “free”, there is a cost in the form of tying up a phone line. Consider that in many applications, the amount of bandwidth needed varies over time.
It would be better if we could set up MP so that it could dynamically add links to the bundle when needed, such as when we decided to download some large files, and then automatically drop them when no longer required. This enhancement to the basic MP package was provided in the form of a pair of new protocols described in RFC 2125:
Key Concept: BAP and BACP are used to provide dynamic control over how the PPP Multilink Protocol functions.