Why do we need two programs for connecting to the Internet?
Normally, if you google for a HowTo that helps you to connect a personal computer with the Internet over ADSL, you are proposed to use two ethernet networking adapters if you wish to act as an gateway for a small home network. But our SEGA Dreamcast only has one single extension slot. So, where should we take this second adapter?
But, RFC 2516 [RFC2516] helps us. That Request for Comments describes how to build PPP sessions and encapsulate PPP packets over Ethernet. That memo describes the theoretical backgrounds necessary for implementing such a protocol. And even that is the key for us. PPPoE is "only" a protocol. Thus, nothing contradicts the use of only one adapter due to the use of multiple protocols assigned to that adapter. We just load multiple protocols for this adapter: The TCP/IP and the PPPoE protocol.
It's up to the protocol and the Linux kernel to distinguish "simple" TCP/IP - packets from PPPoE packets (see figure 5.1). We cause a lot of traffic on that adapter and in the LAN, but ADSL has only a limited downstream of 768 kbps (about 90 kBytes/s), so, those carry no weight.
