Keith Sklower <sklower(_at_)vangogh(_dot_)cs(_dot_)berkeley(_dot_)edu> writes:
[This begs the numerous PPP protocol id's
less then 600 hex for the protocols themselves
which you wouldn't want to use in ethernet packets
because they might get taken for 802.3 lengths, and
there are already common other numbers for them,
like 800 for IP packets as an ethertype].
I also asked Drew Perkins why it was that PPP
protocol id's had an even first byte and an odd
second byte. He said that was so they could be used
as an HDLC two-byte address, but in fact this
was something that was a hedge against a future
day where it might be useful, and wasn't currently
an absolute requirement; if there was a really good
reason to give it up, current implementations
of PPP wouldn't choke. I'm not saying there is
any good reason to give it up right now!
(If they were used as an HDLC address, you would
need to follow them with a UI, which is currently
not being done).
Keith, we somehow miscommunicated. I did NOT say that we wanted to
use protocol ids as addresses. What I did try to say is the following:
1. We wanted to have two octet addresses for high-speed links where
processing speed was an important criteria and alignment issues
might exist.
2. We wanted to have one octet addresses for all important
network-layer protocols when used with low-speed links where
latency was an important criteria and alignment issues didn't exist.
3. We realized that extensible protocol numbers would solve our
problem.
4. We realized that CCITT had an existing methodology for implementing
extensible fields, namely using the least-significant bit, and
this was already in use by the HDLC address field (which we were
supporting).
5. Therefore, it made sense to use the same methodology for our
protocol fields as we used for our HDLC address fields.
You also mentioned that changing this now would not break any
implementations. This is definitely NOT true; it cannot now be changed.
Drew Perkins
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