Ethernet – Why does PPP need an underlying protocol

ethernetlayer2osippppppoe

The Point-to-Point Protocol is (at least according to Wikipedia) a Datalink Layer Protocol.

I know that different use cases exist for it but on examining the image below from Wikipedia one can see that it always seems to need a wrapping protocol like PPPoE and PPPoA.

Thus, my questions are: Why does PPP need a wrapping protocol? Which concrete functionalities from these protocols are needed? Why can't I just use PPP over Ethernet instead of PPPoE? (for the last one I would guess routing)

Best Answer

( As others have already said, PPPoE is literally PPP over Ethernet. And similarly PPPoA is PPP over ATM. )

Ethernet and ATM are oddities in the networking world as they define both a layer-1 and layer-2 component. In the case of ethernet, it's layer-1 has always used it's layer-2; no one ever built it any other way. (Ethernet's layer-2 protocol, however, has been used over many different layer-1's. For example, 802.11 WiFi, DOCSIS.) ATM went the other direction; it's layer-1 is virtually unheard of, but it's layer-2 can be found lots of places... T1, T3, SONET (OC-X), sync-serial (V.35, HSSI), ADSL, etc.

PPP was invented in the era of async-serial (RS-232) and dialup modems, and runs "naked" on those physical layers. As technology advanced, network operators wanted to continue using the same "dialup era" infrastructure and processes (i.e. account management), thus methods of carrying the familiar PPP were needed. An ethertype was registered for transporting PPP frames over ethernet. (and similar processes for ATM, and SONET. In the case of SONET, a bit of HDLC was needed to make the framing work with the SONET NRZ encoding.)

Related Solutions

Cisco – PPPoA ADSL Configuration with external modem on Cisco 891

Technically, you're doing PPPoEoA.

The VC Mux part will have to be set on the modem, as that's the only thing that has access to the ATM layer. If you had a dsl interface in the router, you'd do:

blue-gw#show run int atm0.1
Building configuration...

Current configuration : 131 bytes
!
interface ATM0.1 point-to-point
 bandwidth 6000
 pvc dsl 8/35 
  encapsulation aal5mux ppp dialer
  dialer pool-member 1
 !
end

Other devices will vary.

Ip – PPP, VPN, Tunneling and OSI model

Your question is really too broad, and home networking and consumer-grade devices are explicitly off-topic here, but I will try to give you some general answers.

PPP is a layer-2 protocol, the same way that ethernet, HDLC, frame relay, ATM, etc. are. PPP is specifically a point-to-point protocol with only two endpoints. Other protocols, e.g. ethernet, are designed to work with multiple possible endpoints. Because ethernet, and other IEEE LAN protocols can have multiple connection, they use MAC addresses, but PPP doesn't.
I don't really understand your second question. PPP is a layer-2 protocol, but IP is a layer-3 protocol. In general, layer-2 protocols don't care about layer-3, and they can carry any number of layer-3 protocols.
Ethernet is both a layer-1 and a layer-2 protocol. PPPoE uses PPP for the layer-2 protocol on an ethernet layer-1. The PPP frames are encapsulated in ethernet frames. As far as your traffic is concerned, it is passing through an interface that uses PPP as the layer-2 protocol. PPP can be used on multiple different layer-1 interfaces, e.g. serial, ATM, etc. PPP adds some feature that allow you to negotiate layer-3 addressing, data compression, authentication, etc. that do not exist in other layer-2 protocols.
Your fourth question doesn't really make much sense. You seem to be comparing apples to oranges. PPP is a layer-2 protocol, but PPTP basically allows you to extend a LAN on top of other protocols. As you wrote, it is above layer-4, and the layer-2 protocol used when using PPTP could be PPP. Unfortunately protocols above layer-4 are explicitly off-topic here.
PPP frames are not wrapped in IP packets. It is the other way around; IP packets can be wrapped in PPP (or other layer-2 protocol) frames. Neither layer-2 nor layer-3 know anything about ports, which are layer-4 addresses for some layer-4 protocols. Routers strip off layer-2 frames from the layer-3 packets, so the router on the other end of the PPP link will strip off the PPP frame. A router will then route the layer-3 packet, and it will create a new frame for the new interface for the packet. By the time the packet gets to a server, the PPP frame will be long gone. The server will have a network stack through which the frame it receives passes. When it gets to TCP at layer-4, then it will have the port used, and it will know to which process in the server it should send the TCP segment.

Best Answer

Related Solutions

Cisco – PPPoA ADSL Configuration with external modem on Cisco 891

Ip – PPP, VPN, Tunneling and OSI model

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