[e2e] Lost Layer?
touch at isi.edu
Tue Feb 11 06:29:20 PST 2014
On 2/11/2014 1:09 AM, Fred Baker (fred) wrote:
> In real operational networks, in 2014, we have at least three common
> sub-layers within the network layer. One is what we call the
> Internetwork Layer and should be called, perhaps, the Inter-network
> sub-layer. It provides the end to end datagram service that TCP and
> other transports ride atop. ...
The layer that TCP rides over that spans heterogeneous network
technologies is IP, the Internet(working) layer.
> ...Another might, by analogy, be called the
> Intra-network sublayer. It connects systems that are not necessarily
> directly connected, but use the same technology and are operated by a
> common administration.Switched Ethernets, 802.11 networks, MPLS,
> ATM, Frame Relay, and X.25 are all examples of Intra-network
In ISO that's the link layer (the Internet originally called these
subnetworks), the homogeneous network technology span.
> And then there is what one might call the virtualization
> sublayer, which is when, whatever we call it, we use an IP tunnel
> between the Internetwork and Intranetwork layers. Static IP/IP and
> GRE/IP tunnels, LISP, Mobile IP, L2TP,
Tunnels don't map to any static definition of layers, exactly because
they can always be placed between any pair of layers - including other
tunnel layers. A tunnel is link, not a network. A set of tunnels can
represent a network to the layer above it.
This last reality is why it's so easy to get tunneling wrong. E.g., a
tunnel can transit a "largest" packet - which defines the tunnel MTU;
that MTU is *not* the 'largest unfragmented message' that can transit
the tunnel. We don't have a concept of 'natural chunksize', and
confusion between that and the largest message transmissible is the
reason for a lot of really badly crafted tunnel specs.
So all these layers end up being defined by the layer above and the
layer below, e.g., if IP runs over them, they had better act a lot like
what RFC3819 expects, or things will (and do) work badly. (that relative
relationship is what RNA is all about).
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