PPP Multilink Protocol

The PPP Multilink Protocol (MP) protocol is defined in RFC 1990 and used to aggregate multiple physical links into a single logical link. A good example is the aggregation of both B-channels of an ISDN Basic Rate Interface (BRI) connection. MP fragments, sequences, and re-orders alternating packets sent across multiple physical connections so that the end result is a single logical link with the combined bandwidth of all of the aggregated physical links. MP is the recommended method of combining multiple B-channels of a BRI connection because the support for bonding — the combining of ISDN B-channels through hardware support — can be specific to the ISDN adapter. MP can be done for any ISDN adapter. MP must be supported on both sides of the connection.

Figure 7.16 illustrates the structure of an MP frame. The payload of a Multilink PPP packet is either a fragment of a PPP frame or the entire PPP frame. Multilink PPP fragmentation need not occur if the Multilink PPP packet fits within the MRU of the link. To prevent improper ordering of the datagrams or fragments across multiple links, additional fields are used between the PPP Protocol field and the IP datagram. Multilink PPP uses the PPP Protocol ID of 0x00-3D.

Figure 7.16 Multilink PPP

RFC 1717 defines two different packet formats for short sequence numbers and long sequence numbers. When using either short or long sequence numbers, the sequence number is used to prevent misordering of frames that are sent across multiple links, not to sequence fragments.

For short sequence numbers, a two byte Delimitation/Sequence # field consists of four bits used for delimitation and 12 bits used for the sequence number. Within the delimitation field are two flags. The first bit (the Beginning bit) is an indicator that this fragment begins a sequence of fragments corresponding to a packet. The second bit (the Ending bit) is an indicator that this fragment ends a sequence of fragments corresponding to a packet. The other bits in the first four bits of the short sequence number header are set to 0.

For a PPP frame that is sent without fragmentation, both the Beginning and Ending bits are set. For a PPP frame that is larger than the MRU of the physical link, the PPP frame is fragmented, and each fragment is sent as a separate PPP packet. MP performs a data-link layer fragmentation that is not related to IP fragmentation.

For long sequence numbers, a four byte Delimitation/Sequence # field consists of eight bits (one byte) used for delimitation, and 24 bits (3 bytes) used for the sequence number. Within the delimitation field, the same bits as the short sequence number header define the Beginning bit and the Ending bit. The other bits in the first byte of the long sequence number header are set to 0. The long sequence number header is used by default, unless the short sequence number is chosen during LCP negotiation.

Table 7.16 lists Multilink LCP options negotiated by Microsoft PPP peers. For information about other Multilink options, see RFC 1990.

Table   7.16 Multilink LCP Options