
mode <iface> [vc | datagram]

     Control the default transmission mode on the specified AX.25
interface. In datagram mode, IP packets are encapsulated in AX.25 UI
frames and transmitted without any other link level mechanisms, such
as connections or acknowledgments.

     In vc (virtual circuit) mode, IP packets are encapsulated in
AX.25 I frames and are acknowledged at the link level according to the
AX.25 protocol.  Link level (i.e. AX.25) connections are opened as
necessary.

     In both modes, ARP is used to map IP to AX.25 addresses.

     (Currently not implemented in NOS: the defaults can be overridden
with the type-of-service (TOS) bits in the IP header.  Turning on the
"reliability" bit causes I frames to be used, while turning on  the
"low delay"  bit uses UI frames.  The effect of turning on both bits
is undefined and subject to change.)

     In both modes, IP-level fragmentation is done if the datagram is
larger  than the interface  MTU.  In Virtual Circuit mode, however,
the resulting datagram (or fragments) is further fragmented at the
AX.25 layer if it (or  they) is still larger  than the AX.25 <paclen>
parameter.  In AX.25 fragmentation, datagrams are broken into several
I frames and reassembled at the  receiving end before being passed to
IP.  This is preferable to IP fragmentation whenever possible because
of decreased overhead (the IP header isn't repeated  in each
fragment) and increased  robustness  (a  lost fragment is immediately
retransmitted by the link layer).

     

