L2FRAG(9)               XR32 REFERENCE MANUAL                 7/2/2013

NAME
        L2FRAG -- AX25 Layer 2 Fragmentation.

DESCRIPTION
 Data originated by XR32 is always packaged into the correct
 sized frames for the underlying link.  For AX25, this size is
 specified by PACLEN, which defines the largest payload of an
 information-bearing frame.

 However XR32 has no control over the size of NetRom datagrams
 originated at other systems, which may be larger than the
 PACLEN of the AX25 L2 link onto which they are to be routed.

 In order to cope with this situation, XR32 includes a layer 2
 fragmentation scheme, which (if enabled) breaks the datagram
 into two or more PACLEN (or smaller) chunks.  This scheme is
 only used for connected-mode AX25 (LAPB).  It supersedes an
 earlier NetRom-specific fragmentation scheme which appears to
 be no longer used.

 A fragmented datagram is indicated by the presence of a
 2-byte PID (Protocol Identifier) field in place of the usual
 single-byte field.  The first byte has the decimal value 8,
 indicating that a second PID byte follows.  The second byte
 indicates how many segments (up to 127) remain to be sent.
 The first segment is indicated by the value 128, and the last
 segment by 0, i.e. no fragments left.

 Whether or not XR32 uses fragmentation is controlled by bit 4
 of CFLAGS (decimal 16).  If this bit is set, XR32 will
 fragment outgoing datagrams if necessary.  If the flag is not
 set, datagrams will be sent unfragmented.  This flag should
 only be set if the peer is known to be capable (xNOS systems
 usually are).

 If a fragmented datagram is received from a peer, that peer
 is assumed to be capable of fragmentation, and XR32 will
 enable fragmentation (and re-assembly of received fragments)
 regardless of the setting of bit 4 of CFLAGS.

        Compatible Systems
        ~~~~~~~~~~~~~~~~~~
        XRouter, XR32 and xNOS are known to be compatible with this
        scheme. The status of others such as BPQ32, PE1CHL Net etc.
 are unknown at this time. Reports would be appreciated.

SEE ALSO
        CFLAGS(7) -- Connection Control Flags
        L3FRAG(9) -- NetRom Layer 3 Fragmentation.
        PACLEN(1) -- AX25 Max Packet Length.

L2FRAG(9)             END OF DOCUMENT