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PPP Multilink Protocol (MP) Frame Format
(Page 2 of 4)
PPP Multilink Protocol Frame Fragmentation Process
To accomplish this fragmentation
process, a three step process is followed:
- Original PPP Frame Creation:
The data or other information to be sent is first formatted as a whole
PPP frame, but in a modified form, as described below.
- Fragmentation: The full-sized
PPP frame is chopped into fragments by MP.
- Encapsulation: Each fragment
is encapsulated in the Information field of a new PPP MP fragment
frame, along with control information to allow the fragments to be reassembled
by the recipient.
Several of the fields that normally
appear in a whole PPP frame arent needed if that frame
is going to then be divided and placed into other PPP Multilink frames,
so when fragmentation is to occur, they are omitted when the original
PPP frame is constructed for efficiencys sake. Specifically:
- The Flag fields at the start and end are
used only for framing for transmission and arent needed in the
logical frame being fragmented.
- The FCS field is not needed, because each
fragment has its own FCS field.
- The special compression options that
are possible for any PPP frame are used when creating this original
and Control Field Compression (APCP) and Protocol Field Compression
(PFC). This means that there are no Address
or Control fields in the frame, and the Protocol field
is only one byte in size. Note that this inherently restricts fragments
to carrying only certain types of information.
These changes save a full eight bytes
on each PPP frame to be fragmented. As a result, the original PPP frame
has a very small header, consisting of only a one-byte Protocol
field. The Protocol value of each fragment is set to 0x003D to
indicate a MP fragment, while the Protocol field of the original
frame becomes the first byte of data in the first fragment.
Key Concept: The PPP Multilink Protocol normally divides data amongst physical links by creating an original PPP frame with unnecessary headers removed, and then dividing it into fragment frames. Each fragment includes special headers to allow reassembly of the original frame by the recipient device.
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Version 3.0 - Version Date: September 20, 2005
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