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PPP Multilink Protocol (MP) Frame Format
(Page 1 of 4)
Some devices are connected not by a single physical layer link but by two or more. These may be either multiple physical connections such as two connected pairs of modems, or multiplexed virtual layer one connections like ISDN B channels. In either case, the PPP Multilink Protocol (MP, which also goes by several aliases such as MLPPP and MLP) can be used to aggregate the bandwidth of these physical links to create a single, high-speed bundle. I describe how this is done in the operational topic on MP.
Field Name |
Size (bytes) |
Description |
B |
1/8 (1 bit) |
Beginning Fragment Flag: When set to 1, flags this fragment as the first of the split-up PPP frame. It is set to 0 for other fragments. |
E |
1/8 (1 bit) |
Ending Fragment Flag: When set to 1, flags this fragment as the last of the split-up PPP frame. It is set to 0 for other fragments. |
Reserved |
2/8 (2 bits) |
Reserved: Not used, set to zero. |
Sequence Number |
1
1/2 (12 bits) |
Sequence Number: When a frame is split up, the fragments are given consecutive sequence numbers so the receiving device can properly reassemble them. |
Fragment Data |
Variable |
Fragment Data: The actual fragment from the original PPP frame. |
After MP is configured and starts working, it operates by employing a strategy for dividing up regular PPP frames amongst the many individual physical links that comprise the MP bundle. This is usually accomplished by chopping up the PPP frames into pieces called fragments and spreading these fragments across the physical links. This allows the traffic on the physical links to be easily balanced.
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