Have you ever used the term "layer violation", or heard it used by
someone else?
Occasionally :)
Having the application know the network layer address is just slightly
worse than having it know what it's Ethernet address is or what port
it is attached to.
The flip side to this is that an immense amount of additional complexity
is required to truly make applications agnostic of lower layers.
"Classic" TCP/IPv4 was a lot simpler and more deployable than it would
have been with a genuine ID/LOC split. By making the assumption that an
address was "good enough" as an endpoint identifier, made implementation
on modest 1980s era hardware, not to mention slow networks, a lot more
practical. And in the days before laptops and before the net got big
enough to make prefix aggregation necessary, this was a reasonable
assumption.
It seems that a lot of successful protocols were originally optimized
for deployability rather than optimized for longevity. I suspect that
those protocols tend to be more successful, even in the long term, than
those that are initially optimized for longevity over deployability :)
IPv4 is one example, DNS is another, HTTP is a third, I suspect BGP is a
fourth, and I'm sure there are many more.
There are a few cases in which it has a real need to know. In all
except those few, believe me, every whine I hear about renumbering is
in my ears a whine about the layer violation. Make it go away, and the
renumbering problem will be *much* easier to solve.
Well, every time I hear a whine in IETF about how people "should" or
"should not" do things a certain way, I ask myself whether this is
really a reflection of the network architecture's failure to solve some
important problem. After all, if there were a better way to solve the
problem within the bounds of the architecture, people would probably
find it sooner or later. Various practices that some of us might label
as dubious, including NATs, using IP addresses as authentication tokens,
using a single DNS name as a way to name a service that's actually
implemented by a large pool of hosts, or layer violations in general,
are all attributed to failures of the architecture or failures to
implement key features in our core protocols in a timely fashion.
So the reason people are writing applications that look at IP addresses
is because the network doesn't give them the tools that they need to
write good apps without looking at IP addresses. (And they need to do
that even more in a NATted environment, or in an environment where hosts
have multiple source and/or destination addresses with different
characteristics, so it's not like the architecture is moving in a
direction to discourage layer violations). It's a bit unrealistic to
expect those applications to change in order to solve the renumbering
problem, without providing those tools. The incentives all favor
solving your own problem (or making your own customers happy) even at
the risk of creating a problem for someone else, not to solve someone
else's problem even though it makes your own product less functional,
efficient, or reliable.
Keith
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