TCP/IP header compression on AX.25

[Anders.Klemets@CS.CMU.EDU]

I have made available a couple of files which allows one to use the Van
Jacobsen TCP/IP header compression when running IP on top of AX.25.
A 40 byte TCP/IP header is usually shrunk to only 6 bytes. This should
give a speed up on a 1200 Baud modem that is comparable to half or two
thirds of the TX delay.

The code is mainly for experimental purposes since there is no way to
dynamically decide when to use compression and when not to use it.
Once the compression is enabled on one interface, all TCP/IP headers
that are sent to that interface will be compressed, regardless of if the
peer station understands it. The compression is enabled by adding the
character  'v' as a last argument to the normal 'attach' command.

A way to use compression only when communicating with certain stations
on a channel would be to attach some sort of pseudo interface. This
pseudo interface would send AX.25 frames to the same TNC as its parent
interface (a normal asy interface) but it would use the compression,
while the parent interface would not. By using simple 'route add'
statements to direct routes to a particular interface one could then
decide exactly for which destinations compression should be used.

I would recommend to use the header compression with connected mode
AX.25 rather then with unnumbered AX.25 frames, unless the channel is
exceptionally good.

The source files for this are available with ftp from sics.se. The
filename is archive/packet/ka9q/nos/axcomp.arc.

-- Anders  W3/SM0RGV
New email address:  klemets@cs.cmu.edu