I hope you now also understand why IP MTU is 1500 bytes, when the media is Ethernet. But what if we have other encapsulations – i.e., PPP, MPLS, IPSec, etc.?
Example 1: PPPoE
PPP is a layer 2 protocol for WAN connectivity. Though in many cases, it is transported in Ethernet frames and further encapsulated again over ATM. By doing so, PPP built-in features (encryption, authentication, compression) could be used on the payload.
Conceptually, this is how the frame would look like:
If you ever had an ADSL connection using a USB modem connected straight to your computer, then you can safely assume your modem was doing PPPoE encapsulation.
Do you remember you had to install a PPPoE software? It is this piece of software that would:
- encapsulate the IP packets from your NIC within a PPP packet (IPoPPP)
- encapsulate again into a Ethernet packet (IPoPPPoEthernet); send these to the modem, over the Ethernet wire
- the modem encapsulates the ethernet frame further as an ATM cell and sends it over through the ISP’s DSL core (IPoPPPoEoA)
For conveniency, we just call it PPPoE encapsulation.
Example 2: PPPoA
The process describe above is similar with PPPoA only in this case, there won’t be an Ethernet header. Even though the router will (in most cases) receive an Ethernet frame, as it switches the frame to the outgoing ATM interface, it will not add a new Ethernet header; it will instead add the PPP following by the ATM headers.