Fred N. van Kempen <waltje_at_pdp11.nl> wrote:
> I actually wrote a driver that did IP-over-DDMCP on the VAX end,
Such a driver has existed in standard BSD since 4.2, except that it's
for the DMC-11/DMR-11/DMV-11 (which do all DDCMP processing on board and
can't be made to talk any other protocol to my knowledge) rather than
for the DMF32 that this thread is presumably about.
The DMF32 sync port supports DDCMP too, but it leaves more work to
the host and supports HDLC as well.
> with a modified IOS running at the other end, on a Cisco 2501.
If you needed to connect the DMF32 sync port to a Cisco router, why didn't
you implement Cisco HDLC on the VAX side instead? (As I've done in 2000
in 4.3BSD-Quasijarus0b on a DSV11.) I'm sure you know the frame format:
<flag> <addr> 00 <Ethertype> packet <FCS-16> <flag>
addr is 0F for unicast or 8F for multicast/broadcast*.
It's the simplest and IMO the best framing format for IP over a sync line,
I've used it on T1s (well, fractional T1 since DSV11 is limited to 256 kbps),
and it comes from Cisco.
*The only thing that I never understood is what the hell did Cisco mean
by multicast or broadcast on a point-to-point link. I guess it dates from
the days before people realised that a point-to-point link is not a network
(no ARP, etc.) and does not need to burn up a net/subnet number, and people
would map a net number (hopefully a /30 subnet) to each point-to-point link.
Then .1 and .2 addrs would be assigned to the ends and I guess the 8F addr
byte was generated on packets addressed to .3... My driver always puts
0F in the address field of transmitted packets and treats 0F and 8F as
equivalent on reception; and I always configure the net interface as
unnumbered (for my end use the same address as assigned to one of my real
Ethernets, for the other end put in whatever addr the ISP gave me for
their router; my driver does not require them to be numerically related
as they would be with a /30 subnet).
The approach of using a /30 subnet for a point-to-point link now has to
be used only when you are forced to use an Ethernet as a point-to-point
link without PPPoE, which is stupid but unfortunately used by some DSL
providers.
> This is why I usually grab all the sync modems I can lay my hands on;
USR Courier V.Everything (external of course) supports sync mode on its
DB25 port. The problem is with the other end of the call... I'm using
such modem for my PPP connection, and while I would love to run sync PPP,
I doubt that the ISP's POP will support it.
MS
Received on Mon Dec 06 2004 - 19:03:17 GMT