Point-to-point protocol over Ethernet
- with PPPoE-speaking Internet DSL services where a PPPoE-speaking modem-router connects to the DSL service. Here both ISP and modem-router need to speak PPPoE. (Note that in this case, the PPPoE-over-DSL side of things is occasionally referred to as PPPoEoA, for ‘PPPoE over ATM’.)
- or when a PPPoE-speaking DSL modem is connected to a PPPoE-speaking Ethernet-only router using an Ethernet cable. For such two-box systems, DSL modem plus Ethernet-only router, PPPoE is now the preferred solution.
In fact, the term ‘PPPoE’ is used sometimes confusingly to refer to either or both of two things: (i) a modem-to-router protocol; (ii) on the DSL side of things, a part of a protocol stack (part group of nested protocols) which is PPPoEoA. (See the section on protocol stacks below.); or both (i) and (ii) chained together.
It is possible to find DSL modems, such as the Draytek Vigor 120, where PPPoE is confined to the ethernet link between a DSL modem and a partnering router, and the ISP does not speak PPPoE at all (but rather PPPoA).
Older types of link used for connecting modems to PCs or to routers, RS232 and then USB 1.1, are not fast enough to handle DSL services, so by the mid-1990s a new type of link was urgently needed (this being some years before the arrival of USB 2.0), and PPPoE became the solution for tunneling packets over the DSL connection to the ISP's IP network, and from there to the rest of the Internet.
- 1 Original rationale
- 2 Future
- 3 Protocol overhead
- 4 PPPoE stages
- 5 PPPoE Discovery (PPPoED)
- 6 Quirks
- 7 See also
- 8 References
- 9 External links
In late 1998 the DSL service model had a chicken-and-egg problem. ADSL technology had been proposed a decade earlier. Potential equipment vendors and carriers alike recognized that broadband such as cable modem or DSL would eventually replace dialup service, but the hardware (both customer premises and LEC) faced a significant low-quantity cost barrier. Initial estimates for low-quantity deployment of DSL showed costs in the $300–$500 range for a DSL modem and $300/mo access fee from the telco which was well beyond what a home user would pay. Thus the initial focus was on small & home business customers for whom a T1 line (at the time $800–$1500 per month) was not economical, but who needed more than dialup or ISDN could deliver. If enough of these customers paved the way, quantities would drive the prices down to where the home-use dialup user might be interested: more like $50 for the modem and $50/mo for the access.
Different usage profile
The problem was that small business customers had a different usage profile than a home-use dialup user, including
- connecting an entire LAN to the internet
- providing services on a local LAN accessible from the far side of the connection
- simultaneous access to multiple external data sources, such as a company VPN and a general purpose ISP
- continuous usage throughout the workday, or even around the clock
These requirements didn't lend themselves to the connection establishment lag of a dialup process, nor its one-computer-to-one-ISP model, nor even the many-to-one that NAT + dialup provided. A new model was required.
Time to market: simpler is better
A problem with creating a completely new protocol to fill these needs was time. The equipment was available immediately, as was the service, and a whole new protocol stack (Microsoft at the time was advocating fiber-based atm-cells-to-the-desktop, and L2TP was brewing as well, but was not near completion) would take so long to implement that the window of opportunity might slip by. Several decisions were made to simplify implementation and standardization in an effort to deliver a complete solution quickly.
Reuse existing software stacks
PPPoE hoped to merge the widespread Ethernet infrastructure with the ubiquitous PPP, allowing vendors on both sides of the connection to heavily leverage their existing software and deliver products in the very near term. Essentially all operating systems at the time had a PPP stack, and the design of PPPoE allowed for a simple shim at the line-encoding stage to convert from PPP to PPPoE.
Simplify hardware requirements
Competing WAN technologies (T1, ISDN) required a router on the customer premises. PPPoE used a different Ethernet frame type, which allowed the DSL hardware to function as simply a bridge, passing some frames to the WAN and ignoring the others. Implementation of such a bridge is multiple orders of magnitude simpler than a router.
PPPoE was initially designed to provide a small LAN with individual independent connections to the internet at large, but also such that the protocol itself would be lightweight enough that it wouldn't impinge on the hoped-for home usage market when it finally arrived. While success on the second matter may be debated (some complain that 8 bytes per packet is too much) PPPoE clearly succeeded in bringing sufficient volume to drive the price for service down to what a home user would pay. It remains the dominant DSL connectivity mechanism as of 2011, more than a decade later.
PPPoE faces a difficult future. MPLS/IP backbones and IP-DSLAMs are considered the norm and the circuit switched family technologies (ATM/PDH) are being called legacy technologies. Pseudowire and VoIP can be used to deal with legacy customers and equipment. PPPoE has the highest overhead DSL delivery method.
ATM, because of its overhead, is also being purged and PPPoE goes out with that. For example, Verizon's FIOS product has converted to using DHCP Internet access instead of PPPoE delivery. GPON, an upgrade of BPON, added an alternative to ATM, nonetheless, some retail service providers (such as Internode) of Australia's National Broadband Network are using PPPoE over GPON. PPPoE networks face difficulty in adding multicasting and many levels of QoS and lowest overhead for highest speeds for IPTV in today's convergence and triple play networks.
On ATM/DSL (aka PPPoEoA): The amount of overhead added by PPPoEoA on the DSL side of things depends on the packet size because of (i) the absorbing effect of ATM cell-padding which completely cancels out overheads of PPPoEoA in some cases, (ii) the PPPoE overhead can cause an entire additional 53 byte ATM cell to be required, and (iii) IP fragmentation can be induced (which also invokes the first two effects). However ignoring ATM and IP fragmentation for the moment, the basic additional protocol header overheads for AAL5 payload are typically: 2 bytes (for PPP) + 6 (for PPPoE) + 18 (Ethernet framing, variable) + 10 (RFC 2684, variable) = 36 bytes. For very small packets this overhead is even greater because of Ethernet frame padding. However more realistically, the typical DSL/ATM overhead is either zero or 53 bytes as in case (ii) where the ATM cell payload maximum of 48 bytes is exceeded, so requiring an additional 53 byte cell. The 36 byte figure deduced earlier can be slightly reduced by various means: discarding Ethernet FCS loses another 4 bytes, for example, bringing the total down to 32. Compare this with a vastly more header-efficient protocol, PPPoA, with a fixed 10 byte overhead inside the AAL5 payload (i.e. on the DSL side).
On Ethernet: On the Ethernet side of things, additional overhead for PPPoE over Ethernet is a fixed 2 + 6 = 8 bytes as before, unless IP fragmentation is produced.
When the DSL modem sends and receives PPPoE-containing Ethernet frames across the Ethernet link to the router (or PPPoE-speaking single PC), there is an overhead of 8 bytes (2 for PPP, 6 for PPPoE) added within the payload of the Ethernet frame. This added overhead usually means that a reduced limit (so-called ‘MTU’ or ‘MRU’) of 1492 bytes is imposed on the length of IP packets sent or received, as opposed to the usual 1500 bytes for Ethernet networks. Some devices support RFC 4638, which allows a 1508 byte l Ethernet frame, sometimes called ‘baby jumbo frames’, thus allowing a full 1500 byte payload to be transported over a PPPoE session. This is advantageous because of the many companies who (incorrectly) block all ICMP traffic from exiting their network. This stops Path MTU discovery from working correctly and can cause problems for people accessing these networks with a MRU of less than 1500 bytes.
How PPPoE fits in the DSL Internet access architecture
The transport protocol used on the telephone network is ATM. The DSL modem encapsulates PPP packets inside ATM cells and sends them over the WAN. There are several encapsulation methods.
The diagram depicts one of two possible scenarios involving the use of PPPoE to connect an Ethernet router to a DSL modem (both PPPoE-speaking) in order to connect a LAN to a PPPoE-speaking ISP. Here three devices need to speak PPPoE; router, modem and a Broadband Remote Access Server located at the service provider or ISP's office. On the diagram, the deep protocol stack we see between modem and DSLAM is a picture of what we have termed ‘’PPPoEoA in this article. (Note that other authors, such as some Cisco publications also use the term ‘PPPoEoA’ but not necessarily with exactly the same meaning.)
Example: The DSL modem receives an Internet-bound Ethernet frame from the router which contains a PPP frame inside a PPPoE packet. The modem sends on the PPPoE packet toward the DSLAM by wrapping it in several protocol layers as follows: Typically in this PPPoEoA topology, the DSL modem wraps the PPPoE packet within Ethernet frame-wrapping as described in RFC 2684, second part, ‘bridged protocols’, before sending it on to the DSLAM within an AAL5-encapsulated packet over ATM.
This Ethernet frame wrapping around the PPPoE packets represents a significant and quite unnecessary overhead and can be avoided by using the alternate scheme discussed in the next section.
DSL Internet access architecture Router DSL modem DSLAM Remote access server (ISP) (IP) (IP) Ethernet PPP PPP PPP PPP PPP PPP PPPoE PPPoE PPPoE PPPoE backbone backbone Ethernet Ethernet Ethernet Ethernet IP IP RFC2684 RFC2684 backbone AAL5 AAL5 backbone backbone ATM ATM DSL DSL
- Where the diagram shows ‘(IP)’ this could be various kinds of PPP payload, IPv4 and IPv6 being just examples, but the payload need by no means be in any way IP-related. It can be anything that PPP knows how to encapsulate.
- On the diagram, the area shown as ‘backbone’ could also be ATM on older networks, however its architecture is service-provider dependant. There would be additional columns in this area in a more detailed service-provider specific diagram.
- The diagram does not show the whole of the ISP-side. The PPP connection typically terminates at a server further off the right hand side, not shown. The right hand portion of the diagram shown as backbone would typically be PPP/L2TP/IP.
- Where the diagram says ‘router’, a PPPoE-speaking single host PC could be an alternative to an Ethernet router, acting as a PPPoE client and PPP endpoint.
PPPoE to PPPoA
In this diagram, the scenario is the same as before with two exceptions: (i) A more efficient modem architecture is shown, the modem being a PPPoE-to-PPPoA protocol converter. (ii) The service provider offers a PPPoA service and does not understand PPPoE. There is no PPPoEoA in this protocol chain.
In this alternative topology, PPPoE is merely a means of connecting DSL-modems to an Ethernet-only router (again, or to a single host PC). Here it is not concerned with the mechanism employed by an ISP to offer broadband services.
When transmitting packets bound for the internet, the PPPoE-speaking Ethernet router sends Ethernet frames to the (also PPPoE-speaking) DSL modem. The modem extracts PPP frames from within the received PPPoE frames, and sends the PPP frames onwards to the DSLAM by encapsulating them according to RFC 2364 (PPPoA), thus converting PPPoE into PPPoA.
DSL Internet access architecture Router DSL modem DSLAM Remote access server (ISP) (IP) (IP) Ethernet PPP PPP PPP PPP PPPoE PPPoE PPPoA PPPoA backbone backbone Ethernet Ethernet AAL5 AAL5 backbone backbone IP IP ATM ATM DSL DSL
On the diagram, the area shown as ‘backbone’ could also be ATM on older networks, however its architecture is service provider-dependant. On a more detailed, more service-provider specific diagram there would be additional columns in this area.
The PPPoE has two distinct stages:
Since traditional PPP connections are established between two end points over a serial link or over an ATM virtual circuit that has already been established during dial-up, all PPP frames sent on the wire are sure to reach the other end. But Ethernet networks are multi-access where each node in the network can access every other node. An Ethernet frame contains the hardware address of the destination node (MAC address). This helps the frame reach the intended destination.
Hence before exchanging PPP control packets to establish the connection over Ethernet, the MAC address of the two end points should be known to each other so that they can be encoded in these control packets. The PPPoE Discovery stage does exactly this. In addition it also helps establish a Session ID that can be used for further exchange of packets.
Once the MAC address of the peer is known and a session has been established, the Session stage.
PPPoE Discovery (PPPoED)
Although traditional PPP is a peer-to-peer protocol, PPPoE is inherently a client-server relationship since multiple hosts can connect to a service provider over a single physical connection.
The Discovery process consists of four steps between the host computer which acts as the client and the access concentrator at the internet service provider's end acts as the server. They are outlined below. The fifth and last step is the way to close an existing session.
Client to server: Initiation (PADI)
PADI stands for PPPoE Active Discovery Initiation.
If a user wants to "dial up" to the Internet using DSL, then his computer first must find the DSL access concentrator (DSL-AC) at the user's Internet service provider's point of presence (POP). Communication over Ethernet is only possible via MAC addresses. As the computer does not know the MAC address of the DSL-AC, it sends out a PADI packet via an Ethernet broadcast (MAC: ff:ff:ff:ff:ff:ff). This PADI packet contains the MAC address of the computer sending it.
Example of a PADI-packet:
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) holds the MAC address of the computer sending the PADI.
Dst. (=destination) is the Ethernet broadcast address.
The PADI packet can be received by more than one DSL-AC. Only DSL-AC equipment that can serve the "Service-Name" tag should reply.
Server to client: Offer (PADO)
PADO stands for PPPoE Active Discovery Offer.
Once the user's computer has sent the PADI packet, the DSL-AC replies with a PADO packet, using the MAC address supplied in the PADI. The PADO packet contains the MAC address of the DSL-AC, its name (e.g. LEIX11-erx for the T-Com DSL-AC in Leipzig) and the name of the service. If more than one POP's DSL-AC replies with a PADO packet, the user's computer selects the DSL-AC for a particular POP using the supplied name or service.
Here is an example of a PADO packet:
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: AC-Name String Data: IpzbrOOl Tag: Host-Uniq Binary Data: (16 bytes)
AC-Name -> String data holds the AC name, in this case “Ipzbr001” (the Arcor DSL-AC in Leipzig)
Src. holds the MAC address of the DSL-AC.
The MAC address of the DSL-AC also reveals the manufacturer of the DSL-AC (in this case Nortel Networks).
Client to server: request (PADR)
A PADR packet is sent by the user's computer to the DSL-AC following receipt of an acceptable PADO packet from the DSL-AC. It confirms acceptance of the offer of a PPPoE connection made by the DSL-AC issuing the PADO packet.
Server to client: session-confirmation (PADS)
PADS stands for PPPoE Active Discovery Session-confirmation.
The PADR packet above is confirmed by the DSL-AC with a PADS packet, and a Session ID is given out with it. The connection with the DSL-AC for that POP has now been fully established.
Either end to other end: termination (PADT)
PADT stands for PPPoE Active Discovery Termination. This packet terminates the connection to the POP. It may be sent either from the user's computer or from the DSL-AC.
Since the point to point connection established has a MTU lower than that of standard Ethernet (typically 1492 vs Ethernet's 1500), it can sometimes cause problems when Path MTU Discovery is defeated by poorly configured firewalls. Although higher MTUs are becoming more common in providers' networks, usually the workaround is to use TCP MSS (Maximum Segment Size) "clamping" or "rewrite", whereby the access concentrator rewrites the MSS to ensure TCP peers send smaller datagrams. Although TCP MSS clamping solves the MTU issue for TCP, other protocols such as ICMP and UDP may still be affected.
Some vendors (Cisco and Juniper, for example) refer to PPPoEoE (PPPoE over Ethernet), which is PPPoE running directly over Ethernet or other IEEE 802 networks or over Ethernet bridged over ATM, in order to distinguish it from PPPoEoA (PPPoE over ATM), which is PPPoE running over an ATM virtual circuit using RFC 2684 and SNAP encapsulation of PPPoE. (PPPoEoA is not the same as Point-to-Point Protocol over ATM (PPPoA), which doesn't use SNAP).
- Multiprotocol Encapsulation over ATM
- Point-to-Point Protocol daemon
- Point-to-Point Tunneling Protocol
- Point-to-Point Protocol over ATM (PPPoA)
- Point-to-Point Protocol over X (PPPoX)
- Wind River Systems Acquires RouterWare, Inc.. Findarticles.com (1999-07-05). Retrieved on 2011-09-27.
- Richard D. Gitlin, Sailesh K. Rao, Jean-Jacques Werner, Nicholas Zervos (May 8, 1990). "Method and apparatus for wideband transmission of digital signals between, for example, a telephone central office and customer premises". US Patent 4,924,492.
- "TouchWave Partners With Telogy Networks For VoIP Embedded Communications Software". Business Wire. October 5, 1998. Retrieved 16 December 2008.
- Dirk Van Aken, Sascha Peckelbeen Encapsulation Overhead(s) in ADSL Access Networks, June 2003[dead link]
- Encapsulation Overhead(s) in ADSL Access Networks.
- 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)
- RFC 4938 - PPP Over Ethernet (PPPoE) Extensions for Credit Flow and Link Metrics
- US Patent 6891825 Method and system of providing multi-user access to a packet switched network