L3 Fragmentation

Name

L3FRAG -- NetRom Layer 3 Fragmentation.

Description

Long ago there was a proposed scheme for fragmenting NetRom L3 datagrams that were larger than the PACLEN of the underlying AX25 L2 link. This involved breaking large datagrams into PACLEN (or smaller) fragments, and manipulating bits in the L2 Protocol Identifier (PID), to indicate whether the L2 frame contained the first, intermediate or last fragment of a NetRom datagram.

The PID values were as follows:

	Hex   Dec   Meaning
	---------------------------------------------
	CF    207   First & last (i.e. only) fragment
	8F    128   First fragment    
	4F     64   Last fragment
	0F     15   Intermediate fragment.

The scheme was implemented in XRouter, but was not understood by BPQ, which was a popular node software of the time. Therefore the code was disabled. It is not known which, if any, modern software implements this scheme.

The fragmentation scheme described above is believed to have been superceded by a general-purpose L2 fragmentation scheme as described in the MAN page entitled L2FRAG.

See also

L2FRAG(9) -- AX25 Layer 2 Fragmentation.
PACLEN(2) -- AX25 Max Packet Length.