Dino Farinacci - Control Data - CDCNET TCP/IP Development (dino%CDCCENTR.BITNET@CUNYVM.CUNY.EDU)
Fri, 24 Jun 88 19:17:54 CDT
This is in response to firstname.lastname@example.org's
memo <memo <1919@ssc-vax.UUCP>:
When we (CDC) initially started support of TCP/IP in CDCNET
we made the decision to support all three of the Ethernet
header formats. We were informed with the direction of OSI
standards that 802.2/802.3 was the way to go. Our CDCNET
systems already supported this header format but we needed to
include Ethernet 2 and SNAP headers.
We felt the effort was trivial for incoming frames. Just
check the ether type field in the 802.3 (or version 2
header). If the value was greater than 1518 (maximum length
Ethernet frame), then the header format was Ethernet 2.
Otherwise it was 802.2/802.3. If 802.2 was present than
another check is done to determine if a SNAP header follows.
If so, use the ether type field in the SNAP as one would in
the Ethernet 2 case. If no SNAP, use the 8-bit SAP value in
the 802.2 to multiplex to the correct user.
For outgoing frames, one does not know what header format the
destination system supports. If it was our own system, it
didn't matter (we supported all of them). There are two ways
we allowed this information to be provided. One, staticly
configure the header format with the IP address/ Ethernet
address. Or two, let ARP find out. Obviously it was less of
a burden to the Network Administrator if ARP did it. The
disadvantage was that ARP broadcasts two ARP requests, each
with a differnet header format (Ethernet 2 and SNAP - there
is not an 8-bit 802.2 SAP value assigned for ARP). When the
reply is received, the IP address/Ethernet address and header
format is cached.
We did this over a year and a half ago anticipating that
workstation vendors would go to the SNAP. However,
everything I've heard from the TCP/IP interoperability
conferences indicates no one cares about it and everything
works fine as it is today.
Control Data Corp.
CDCNET TCP/IP Design and Development
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