What disadvantages are there of the layered approach to network
protocols (eg. the OSI model)


NOTE: This question will be cancelled on 17th May, as it is irrelevant
after this date.

The main disadvantages of layered systems consist primarily of
overhead both in computation and in message headers caused by the
abstraction barriers between layers. Because a message often has to
pass through many (10 or more) protocol layers the overhead of these
boundaries is often more than the actual computation being done.

One other disadvantage is that the higher level layers can't see
what's in the lower layers, implying that an application can't debug
where in a connection a problem is, or exactly what the problem is.

Another disadvantage is the higher level layers can't control all
aspects of the lower layers, so they can't modify the transfer system
if beneficial (like controlling windowing, header compression,
CRC/parity checking, et cetera), nor specify routing, and must rely on
the lower protocols working, and can't specify alternatives when
there's problems.

Cheers for that pcjunkie-ga, thats what I was looking for.

One further point to note (although I'm not sure on its validity): The
benefits of layering are modularity and managability; however, these
can only be achieved up to a certain number of layers. Once the number
of layers goes beyond this point the complexity will increase.