Multilink PPP (MP) allows for the aggregations of multiple links into one logical channel. This may be needed in situations for which only low throughput technologies (like dial-up modems) are available to provide the point-to-point connection.
Multilink PPP is specified in RFC 1717.
Other IETF standards, Bandwidth Allocation Protocol (BAP) and Bandwidth Allocation Control Protocol (BACP) are defined in RFC 2125. BAP and BACP allow for automatic bandwidth management (such as allocating traffic on a per-link basis).
References:
http://www.cisco.com/en/US/tech/tk713/tk507/technologies_tech_note09186a0080112d3b.shtml