1200 baud links


John Romkey (romkey@BORAX.LCS.MIT.EDU)
Mon, 20 Jan 86 22:10:34 est


The first net interface we supported in PC/IP was the PC's serial line
using (of course) our own packet framing protocol over the serial
line. The protocol is rather stupid in some ways (it's very wasteful
of bytes), but it has worked fine for several years. We also
implemented it for the C Gateway so that the PC's could talk to the
rest of the world. We haven't done much with it since Ethernet and
proNET cards became available for the PC; our interest was mostly in
doing a TCP/IP implementation for the PC, not in investigating serial
line dialups to the Internet, but that gateway is still there. It's
two major uses now are some leased 9600 baud lines going to faculty
members houses and a 1200 baud dialup modem that's on it that some
people used to use to do file transfers. Getting the TFTP timeout
algorithm to work well with line speeds varying from 1200 baud to
56kbps to 10Mbps (well...) was a bit of a challenge...

The gateway supported 8 serial lines; we used a very simple addressing
scheme. We assigned a whole subnet number to the serial lines and then
gave them host numbers 1 through 8. The gateway had a host number on
that subnet as well. Since we tend not to name PC's around here, there
was never a problem with "Jerry Saltzer's home PC" changing its
internet address (actually, the 9600 baud ports are all dedicated,
anyway, so the addresses wouldn't change).
                                        - John Romkey



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