My naive understanding of how the internet works is as follow :
Physical layer (1) Link layer (2) - Inside frames IP layer (3) - Inside IPv4 or IPv6 packets Transport layer (4) - Inside TCP or UDP Application layer
Where does L2TP protocol stands? The name suggests it is a layer 2 protocol, but the wiki article says L2TP packets are inside UDP datagrams. This, to me, suggests it is an application protocol.
Is L2TP a layer 2 protocol? Is it using another "kind of frame" different thant the common one I am used to? Or is it actually an application protocol contained within "normal" frames, IP packets and UDP datagrams?
Tunneling protocols like L2TP, GRE, IPSEC and MPLS don't fit well into the OSI model. L2TP tunnels (encapsulates) L2 frames in UDP so it can be transported over layer 3.
Fixed layer count models and tunneling don't match up very well.
Our stack might look something like.
- Appliction data.
- TCP (inner network)
- IP (inner network)
- Ethernet framing (inner network)
- UDP (outer network)
- IP (outer network)
- Ethernet framing (outer network)
- Ethernet medium access control
- Ethernet physical
What layer we regard L2TP as depends on what perspective we are looking from. From the perspective of the "outer" network it looks like an application protocol. From the perfective of the "inner" network it looks like an Ethernet port.