VAX 11/725

From: Fred N. van Kempen <waltje_at_pdp11.nl>
Date: Mon Dec 6 19:38:51 2004

On Tue, 7 Dec 2004, Michael Sokolov wrote:

> 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.
Indeed. I run DMV11's here. I know.

> The DMF32 sync port supports DDCMP too, but it leaves more work to
> the host and supports HDLC as well.
Correct. Still, it offloads a LOT of the enframe and deframe work
to the board CPU.

> > 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
Probably because that was a test link; the production system (with the
same driver) connected to a real DDMCP network. The choice was to either
change the WAN to HDLC/PPP, or make that one box talk IP over DDMCP.

> 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*.
After 17 years of networking, I guess I'm aware of the various
frame formats, yes.

> *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
Frame filtering.

> 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
There are very good reasons for assigning IP addresses to both ends of
a WAN connection, especially in multipath environments and/or networks
that use link-state routing protocols. We dont all use "route add
default wan0" you know.

> 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.
Actually, by most xDSL providers in Europe. So, apparently it's not
THAT stupid. Its a cheap 10Mbps (or even 100Mbps, if needed) leased
line technology with baseband encoding to them :)

> 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.
If they use TotalControl or akin racks, they will. I used to run that
over my USR Courier I-Modem (ISDN V.Everything with both digital and
analog protocols.)

--f
Received on Mon Dec 06 2004 - 19:38:51 GMT