Multilink point-to-point protocol is defined in the IETF RFC 1990, The PPP Multilink Protocol (MP), and provides a way to distribute data across multiple links within an MLPPP bundle to achieve high bandwidth. MLPPP allows for a single frame to be fragmented and transmitted across multiple links. This allows for lower latency and also allows for a higher maximum receive unit (MRU).
MP is negotiated during the initial LCP option negotiations of a standard PPP session. A router indicates to its peer that it is willing to perform MLPPP by sending the MP option as part of the initial LCP option negotiation. This negotiation indicates the following:
The system offering the option is capable of combining multiple physical links into one logical link.
The system is capable of receiving upper layer protocol data units (PDU) fragmented using the MP header and reassembling the fragments back into the original PDU for processing.
The system is capable of receiving PDUs of size N octets where N is specified as part of the option even if N is larger than the maximum receive unit (MRU) for a single physical link.
After MLPPP has been successfully negotiated, the sending system is free to send PDUs encapsulated and, or fragmented with the MP header.
MP introduces a new protocol type with a protocol ID (PID) of Ox003d. Figure 1 and Figure 2 show the MLPPP fragment frame structure. Framing to indicate the beginning and end of the encapsulation is the same as that used by PPP, and described in PPP in HDLC-like framing [RFC 1662]. MP frames use the same HDLC address and control pair value as PPP, namely: Address - OxFF and Control - Ox03. The two octet protocol field is also structured the same as in PPP encapsulation.
The required and default format for MP is the 24-bit format. During the LCP state the 12-bit format can be negotiated. The SR-series routers can support and negotiate the alternate 12-bit frame format.