PPPoE

PPPoE ( English Point-to-point protocol over Ethernet ) - the network protocol of the data link layer (the second level of the OSI network model ) of PPP frame transmission over Ethernet . Mainly used by xDSL services. Provides additional features ( authentication , data compression , encryption ).

The standard MTU of the protocol is lower than on standard Ethernet (Ethernet - 1500 bytes; PPPoE - 1492 bytes), which sometimes causes problems with poorly configured firewalls .

PPPoE is a tunneling protocol that allows you to configure (or encapsulate) IP or other protocols that are configured for PPP through Ethernet connections, but with the software capabilities of PPP connections, and therefore it is used for virtual “calls” to a neighboring Ethernet machine and installs A connection point that is used to transport IP packets, working with PPP capabilities.

This allows you to use traditional PPP-oriented software to configure the connection, which does not use a serial channel, but a packet-oriented network (like Ethernet ) to organize a classic connection with a login, password for Internet connections. Also, an IP address on the other side of the connection is assigned only when the PPPoE connection is open, allowing dynamic reuse of IP addresses.

PPPoE is developed by UUNET , Redback Networks and RouterWare . The protocol is described in RFC 2516 .

It is worth noting that some equipment vendors ( Cisco and Juniper , for example) use the term PPPoEoE ( PPPoE over Ethernet ), meaning PPPoE that works directly over Ethernet or other IEEE 802.3 networks, and PPPoE that works over Ethernet bridged over ATM , in order to distinguish it from PPPoEoA ( PPPoE over ATM ), which runs on an ATM virtual circuit according to the specifications of RFC 2684 and SNAP and encapsulates PPPoE. PPPoEoA is not the same as Point-to-Point Protocol over ATM ( PPPoA ) because it does not use SNAP.

The work of PPPoE is as follows. There is an Ethernet environment, that is, several connected network cards that are addressed by MAC addresses . Ethernet frame headers contain the sender address of the frame, the destination address of the frame, and the type of frame. One of the cards is listening to the PPPoE server . The client sends a broadcast Ethernet frame to which the PPPoE server should respond (the sender address of the frame is its MAC address, the destination address of the frame is FF: FF: FF: FF: FF: FF and the frame type is PPPoE Active Discovery Initiation). The PPPoE server sends a response to the client (the sender address of the frame is its MAC address, the recipient address is the MAC address of the client and the frame type is PPPoE Active Discovery Offer). If there are several PPPoE servers on the network, then they all send a response. The client selects the appropriate server and sends it a connection request. The server sends a confirmation to the client with a unique session identifier, all subsequent frames in the session will have this identifier. Thus, a virtual channel is created between the server and the client, which is identified by the session identifier and the MAC addresses of the client and server. Then a PPP connection is established in this channel, and IP traffic is already packaged into PPP packets.

Content

PPPoE Discovery (PPPoED)

PADI

PADI - PPPoE Active Discovery Initiation. If a user wants to connect to the Internet via DSL , his machine must first detect an access concentrator ( DSL a ccess c oncentrator or DSL-AC ) on the provider side ( point of presence (POP)). Ethernet communication is only possible through MAC addresses. If the computer does not know the MAC address of the DSL-AC, it sends a PADI packet via Ethernet broadcast (MAC: ff: ff: ff: ff: ff: ff) This PADI packet contains the MAC address of the machine that sent it.

Example PADI package:

  Frame 1 (44 bytes on wire, 44 bytes captured)
 Ethernet II, Src: 00: 50: da: 42: d7: df, Dst: ff: ff: ff: ff: ff: ff 
 PPP-over-Ethernet Discovery
   Version: 1 
   Type 1
   Code Active Discovery Initiation (PADI)
   Session ID: 0000
   Payload Length: 24
 PPPoE Tags
   Tag: Service-Name 
   Tag: Host-Uniq
     Binary Data: (16 bytes)

Src. (= source) represents the MAC address of the machine that sent the PADI.
Dst. (= destination) is a broadcast Ethernet address.
A PADI packet can be received by more than one DSL-AC.

PADO

PADO - PPPoE Active Discovery Offer.

Once the user machine has sent the PADI packet, the DSL-AC responds by sending the PADO packet using the MAC addresses that came from the PADI. The PADO packet contains the DSL-AC MAC addresses, their names (for example, LEIX11-erx for the T-Com DSL-AC hub in Leipzig ) and the service name. If more than one DSL-AC point responded with a PADO packet, the user machine selects a specific POP using the received service names or DSL-AC.

An example of a PADO package:

  Frame 2 (60 bytes on wire, 60 bytes captured)
 Ethernet II, Src: 00: 0e: 40: 7b: f3: 8a, Dst: 00: 50: da: 42: d7: df 
 PPP-over-Ethernet Discovery
   Version: 1 
   Type 1
   Code Active Discovery Offer (PADO)
   Session ID: 0000 Payload Length: 36
 PPPoE Tags
   Tag: Service-Name 
   Tag: AC-Name
     String Data: IpzbrOOl 
   Tag: Host-Uniq
     Binary Data: (16 bytes)

AC-Name - String Data represents the string AC name, in this case “Ipzbr001” (Arcor DSL-AC in Leipzig).
Src. represents the MAC address of the DSL-AC.
The DSL-AC MAC address also identifies the manufacturer of the DSL-AC (in this case, Nortel Networks ).

PADR

PADR stands for PPPoE Active Discovery Request.

As mentioned above, the user machine must select a POP (access point) - this is done using the PADR packet, which is sent to the MAC address of the selected DSL-AC.

PADS

PADS - PPPoE Active Discovery Session-confirmation.

The PADR packet is confirmed by the hub by sending the PADS packet, and in it comes the Session ID. The DSL-AC connection for this access point is now fully established.

PADT

PADT - (PPPoE) Active Discovery Termination.

This packet terminates the connection to the POP. It can be sent either by the user or by the DSL-AC.

Schema Benefits

IP headers in the Ethernet environment are ignored. That is, the user can assign an IP address to his network card, but this will not lead to a “collapse” of the network (theoretically, when working with a network hub, a “collapse” should not occur even when the user changes the MAC address to the server address, and when working with network switch it all depends on the design of the switch).
Each connection is separated from the others (works in its own channel).
Settings (IP address, gateway address, DNS server addresses) can be transmitted by the server.
A PPP connection is easily authenticated and bridged (for example, using RADIUS ).
PPP connection can be encrypted. For example, when working with a network hub (when all Ethernet traffic can be visible on each network card), it is very difficult to read someone else's IP traffic.

Notes

Links

RFC 2516 - A Method for Transmitting PPP Over Ethernet (PPPoE)
RFC 3817 - Layer 2 Tunneling Protocol (L2TP) Active Discovery Relay for PPP over Ethernet (PPPoE)
RFC 4638 - Accommodating a Maximum Transit Unit / Maximum Receive Unit (MTU / MRU) Greater Than 1492 in the Point-to-Point Protocol over Ethernet (PPPoE)