[e2e] Lost Layer?

[Joe Touch]

On 2/11/2014 1:09 AM, Fred Baker (fred) wrote:
>
> On Feb 10, 2014, at 6:31 PM, Joe Touch <touch at isi.edu> wrote:
>
>> There are three layers, but it's TCP that's incomplete. I don't at
>> all understand the difference between a "network layer" and an
>> "internetwork layer".
>
> Well, the deal is that layers can be sub-layered.

If you mean that every layer expects similar things from the layer 
below, I agree (that's RNA). It expects a way to transit a set of nodes 
using a path, and accepts that there will need to be a way to map nodes 
at the upper layer to nodes at the lower layer (e.g., resolution, ala 
Google, BGP, and ARP, depending on what layer you look at).

> Yes, the
> Internetwork layer is perhaps unfortunately named, in that it doesn't
> always interconnect networks.

But you can't tell the difference when it isn't.

> But it comes down to this.
>
> First, consider that each layer answers a fundamental question.

Each answers exactly the same fundamental question - how do I transit 
hops at my layer using what I think are links at my layer, by using what 
look like hops and links that are really a service provided by the next 
layer down.

> The
> physical layer provides the physical interconnect between a system and a
> neighboring system.

The physical layer is just the base case where the signal receives this 
'service' from a real, physical entity.

 > The Link Layer provides the interpretation of
> signals on the physical medium connecting neighboring systems.

That translation of format can happen at every layer in a stack of 
layers. It happens when tunnels encrypt/decrypt. It happens when a 
stream of messages are FEC encoded. It happens when we stripe over 
different channels to emulate a mega-channel.

 > The
> network layer connects a system to another system that it is not
> necessarily directly connected to.

That happens at every layer, because every layer can include forwarding. 
Link layers forward, and transport layers can relay (forward) too.

> The Transport Layer provides needed
> services end to end across that network.

That's what IP provides across different link layers. It's what a link 
layer provides over separate physical connections. Again, all the same.

 > In TCP's case, the service is
> that of a sequential, reliable, octets stream; in the case of UDP...
> SCTP..., and so on.

And SONET provides a stream over frames. Ethernet provides packet relay 
over packet links, as does IP.

Joe