[e2e] Lost Layer?
touch at isi.edu
Tue Feb 11 06:39:36 PST 2014
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
> 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.
> 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.
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