Re: Layer 3.5 is actually layer 4

[Masataka Ohta <mohta@necom830.hpcl.titech.ac.jp>]

Christian;

> Thinking of UDP as a transport protocol is misleading. UDP is not really
> a transport protocol, it is a pass-through. It only performs one of the
> classic TP functions: multiplexing.

We can be fine to say UDP is a transport layer protocol to
pass-through.

OTOH, if you implement TCP in a way that kernel does multiplexing
only and all the other TCP functionality (connection management,
congention avoidance, retransmission and so on) is performed in
user space, it is no different from UDP doing similar things in
user space.

So, we can also be fine to say TCP is a combination of a transport
layer protocol to pass-through and an application layer protocol
to do other things.

Note that there are OSes where kernel and user spaces are not
separated.

That is, debate on what funcitonality is of transport and what
else is of application is not meaningful, because the difference
is internal implementation one not visible from the network.

There can be a difference when the network support port number
based QoS assurance, as is the case with networks with RSVP,
where we can definitely say that the multiplexing functionality
does belong to the transport layer.

Anyway, that

> In a multi-homed
> environment, the application will also handle multi-homing.

is enough to deny 3.5 fallacy and UDP is not the only special
case.

						Masataka Ohta