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PPP Multilink Protocol (MP) Frame Format
(Page 3 of 4)
PPP Multilink Protocol Fragment Frame Format
The Information field of each
fragment uses a substructure that contains a four-field MP header
along with one fragment of the original PPP frame, as shown in Table 41.
Figure 41: PPP Multilink Protocol Long Fragment Frame Format
The long PPP Multilink Protocol frame format uses a full byte for flags and a 24-bit Sequence Number.
Figure 42: PPP Multilink Protocol Short Fragment Frame Format
The short version of the PPP Multilink Protocol format uses 4 bits for flags and a 12-bit Sequence Number.
As you can see, the MP
frame format comes in two versions: the long format uses a 4-byte header,
while the short format requires only 2 bytes. The default MP header
format uses a 24-bit Sequence Number and has 6 reserved bits,
as shown in Figure 41.
It is possible when MP is set up, for devices to negotiate the Multilink
Short Sequence Number Header Format configuration option. If this
is done successfully, shorter 12-bit Sequence Numbers are used
instead. Four of the reserved bits are also truncated, to save 2 bytes
on each frame, as illustrated in Figure 42.
(Considering that 12 bits still allows for over 4,000 fragments per
PPP frame, this is usually more than enough!)
The Fragment Data field contains
the actual fragment to be sent. Since the original PPP header (including
the Protocol field) is at the start of the original PPP frame,
this will appear at the start of the first fragment. The remaining fragments
will have just portions of the Information field of the original
PPP frame. The last fragment will end with the last bytes of the original
The receiving device will collect
all the fragments for each PPP frame, extract the fragment data and
MP headers from each. It will use the Sequence Numbers to reassemble
the fragments and then process the resulting PPP frame.
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Version 3.0 - Version Date: September 20, 2005
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