> On Feb 20, 2019, at 11:31 PM, Grant Taylor via cctalk <cctalk@classiccmp.org>
> wrote:
>
> On 2/20/19 12:23 PM, Paul Koning via cctalk wrote:
>> Please note that among LANs, there is Token Ring (802.5) and there is
>> everything else.
>
> I think it really depends on how you look at them.
>
> From a frame formatting point of view, Ethernet is the odd ball when looking
> at how TCP/IP is carried.
>
> Everything other than Ethernet (802.3) uses 802.2 or a medium specific
> varient of 802.2. Then there's Ethernet which predominantly uses either
> Ethernet II for TCP/IP or 802.3 (a.k.a. "Raw") Ethernet frames for IPX.
"raw 802.3" is a bug, caused by a programmer not understanding how the specs
work.
The mapping from Ethernet to 802.2 SNAP is trivial, but yes, you do need that
mapping.
>> FDDI is like Ethernet and like 802.4. Token Ring is the oddball because (a)
>> it doesn't have proper multicast addresses, and (b) for some reason IBM
>> invented source-routed bridging and tied that to Token Ring.
>
> Does it actually need a broadcast address like Ethernet when the ring passes
> through all the stations? Or is that functionally comparable to a multicast?
Broadcast is just a special case of multicast. My point was that 802.5, at
least as IBM thinks of it, doesn't have proper multicast addresses (low bit set
plus 47 bits to say what the address is). Instead, it has "functional
addresses" which have a fixed beginning plus one of 32 bits that is set. So
instead of 2^47 possible values, you have 32. I don't know why this was done.
Perhaps their chip designers thought hash or CAM address matching was too hard?
So if you have a protocol that uses multicast, like DECnet, you have to
translate the real multicast address to the corresponding "functional" address
in an Ethernet-802.5 bridge. And you have to make up a functional address,
since the address space of 32 values is too small to have globally assigned
multicast addresses as you do with other LANs.
The ring passes through all stations of a LAN segment, just as the Ethernet bus
(in the original version) passes through all stations of the segment. The
point of multicast is that it recognized by multiple stations, not just one.
But multicast matters to bridges because they have to forward it differently
than individual addresses: individual is forwarded according to the address
database entry if there is one, while multicast is not learned and always
flooded along the spanning tree.
>> FDDI is in no way at all like Token Ring. The only thing the two have in
>> common is "token" and "ring". The MAC protocol is utterly different; the
>> closest relative is 802.4 Token Bus.
One example of this is the behavior under high load. At one time, token ring
marketeers claimed it was better because it wouldn't "collapse" under load
"like Ethernet". That is actually false, but in any event, 802.5 worst case
latency is incredibly large for large rings. 802.4 and FDDI with their "timed
token protocol" have far lower worst case latency.
paul
