Point-to-Point Protocol over Ethernet

PPPoE and TCP/IP protocol stack
Protocol stack
The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them....
Application	FTP File Transfer Protocol File Transfer Protocol is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server...	SMTP	HTTP	…	DNS Domain name system The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...	…
Transport	TCP Transmission Control Protocol The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...				UDP User Datagram Protocol The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...
Internet	IP Internet Protocol The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...			IPv6 IPv6 Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
Network access	PPP Point-to-Point Protocol In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
	PPPoE
	Ethernet Ethernet Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

The Point-to-Point Protocol over Ethernet (PPPoE) is a network protocol for encapsulating

Encapsulation (networking)

In computer networking, encapsulation is a method of designing modular communication protocols in which logically separate functions in the network are abstracted from their underlying structures by inclusion or information hiding within higher level objects....

Point-to-Point Protocol

In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...

(PPP) frames inside Ethernet

Ethernet

Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

frames. It is used mainly with DSL services where individual users connect to the DSL modem over Ethernet and in plain Metro Ethernet

Metro Ethernet

A Metro Ethernet is a computer network that covers a metropolitan area and that is based on the Ethernet standard. It is commonly used as a metropolitan access network to connect subscribers and businesses to a larger service network or the Internet...

networks. It was developed by UUNET

UUNET

UUNET founded in 1987, was one of the largest Internet service providers and one of the nine Tier 1 networks. It was based in Northern Virginia and was the first commercial Internet service provider...

, Redback Networks

Redback Networks

Redback Networks was a telecommunications equipment company, specializing in hardware and software used by ISPs to manage broadband services. In December 2006, Ericsson and Redback announced they had signed a definitive agreement under which Ericsson would acquire Redback...

and RouterWare (now Wind River Systems

Wind River Systems

Wind River Systems, Inc. is a company providing embedded systems, development tools for embedded systems, middleware, and other types of software. The company was founded in Berkeley, California in 1981 by Jerry Fiddler and David Wilner. On June 4, 2009, Wind River announced that Intel had bought...

) and is available as an informational RFC 2516.

Ethernet networks are packet-based and have no concept of a connection or circuit

Virtual circuit

In telecommunications and computer networks, a virtual circuit , synonymous with virtual connection and virtual channel, is a connection oriented communication service that is delivered by means of packet mode communication...

and also lack basic security features to protect against IP

IP address

An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...

and MAC

MAC address

A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...

conflicts and rogue DHCP servers. By using PPPoE, users can virtually "dial" from one machine to another over an Ethernet network, establish a point to point connection between them and then securely transport data packets over the connection. It is mainly used by telephone companies, since PPPoE is easily integrated with legacy dial-up AAA

AAA protocol

In computer security, AAA commonly stands for authentication, authorization and accounting.- Authentication :Authentication refers to the process where an entity's identity is authenticated, typically by providing evidence that it holds a specific digital identity such as an identifier and the...

systems and fits perfectly into the ATM

Asynchronous Transfer Mode

Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...

backbones. The protocol also permits very easy unbundling

Unbundled Access

Unbundled access is an often practiced form of regulation during liberalization, where new entrants of the market are offered access to facilities of the incumbent, that are hard to duplicate...

of digital subscriber line access multiplexer

Digital subscriber line access multiplexer

A digital subscriber line access multiplexer is a network device, located in the telephone exchanges of the telecommunications operators. It connects multiple customer digital subscriber line interfaces to a high-speed digital communications channel using multiplexing techniques...

s (DSLAM) where required by regulators, since the user would simply use a different login into PPP, then the ATM circuit would be routed to the user's ISP. Also pre-paid traffic bucket business models can be created with PPPoE more easily than with DHCP or multiplexing multiple users with different speed tiers or QoS through 1 DSL modem or by creating a different login for each static IP purchased by customers.

PPPoE faces a difficult future. MPLS/IP backbones and IP-DSLAMs are considered the norm and the circuit switched family technologies (ATM/PDH

Plesiochronous Digital Hierarchy

The Plesiochronous Digital Hierarchy is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems...

) 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. The amount of overhead added by PPPoE depends on the packet size because PPPoE adds 8 bytes to each packet. If packets are large, say 1492 bytes, the overhead is only 0.54 % ((1500-1492)/1492). If packets are small though, such as the traffic required by certain web applications, VoIP or online games, the overhead can be much greater. As an example, if a specific VoIP connection used a packet size of 60 bytes, a PPPoE connection adds a 15.3 % overhead.
ATM, because of its overhead, is also being purged and PPPoE goes out with that. For example, Verizon's FIOS

Fíos

Fíos is one of 17 parishes in Parres, a municipality within the province and autonomous community of Asturias, in northern Spain....

product has converted to using DHCP internet access instead of PPPoE delivery.
GPON, an upgrade of BPON, added an alternative to ATM

Asynchronous Transfer Mode

. PPPoE networks face difficulty in adding multicasting and many level of QoS and lowest overhead for highest speeds for IPTV in today's convergence and triple play

Triple play (telecommunications)

In telecommunications, triple play service is a marketing term for the provisioning of two bandwidth-intensive services, high-speed Internet access and television, and a less bandwidth-demanding service, telephone, over a single broadband connection. Triple play focuses on a combined business...

networks.

How PPPoE fits in the DSL Internet access architecture

The transport protocol used on the telephone network is ATM

Asynchronous Transfer Mode

. The DSL modem encapsulates PPP packets inside ATM cells and sends them over the WAN. There are several encapsulation methods.

PPPoEoA

Ethernet packets containing the PPPoE packets are bridged over ATM (RFC 2684).

**DSL Internet access architecture**
Host PC	DSL modem		DSLAM		Remote access server
IP Internet Protocol The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...					IP Internet Protocol The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
PPP Point-to-Point Protocol In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...					PPP Point-to-Point Protocol In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
PPPoE					PPPoE
Ethernet Ethernet Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....	Ethernet Ethernet Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....	Ethernet Ethernet Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....			Ethernet Ethernet Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
		ATM Asynchronous Transfer Mode Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...			ATM Asynchronous Transfer Mode Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
		DSL	DSL	backbone	backbone

PPPoE to PPPoA

The PPP frames are extracted from the PPPoE packets and encapsulated in ATM, thus converting PPPoE into PPPoA

Point-to-Point Protocol over ATM

The Point-to-Point Protocol over ATM is a network protocol for encapsulating PPP frames in AAL5. It is used mainly with DOCSIS and DSL carriers....

(RFC 2364).

**DSL Internet access architecture**
Host PC	DSL modem		DSLAM		Remote access server
IP Internet Protocol The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...					IP Internet Protocol The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
PPP Point-to-Point Protocol In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...					PPP Point-to-Point Protocol In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
PPPoE	PPPoE	PPPoA			PPPoA
Ethernet Ethernet Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....	Ethernet Ethernet Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....	ATM Asynchronous Transfer Mode Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...			ATM Asynchronous Transfer Mode Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
		DSL	DSL	backbone	backbone

PPPoE discovery

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

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.

PPP session

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

Internet service provider

An Internet service provider is a company that provides access to the Internet. Access ISPs directly connect customers to the Internet using copper wires, wireless or fiber-optic connections. Hosting ISPs lease server space for smaller businesses and host other people servers...

's point of presence

Point of presence

A point of presence is an artificial demarcation point or interface point between communications entities. It may include a meet-me-room.In the US, this term became important during the court-ordered breakup of the Bell Telephone system...

(POP). Communication over Ethernet is only possible via MAC address

MAC address

es. As the computer does not know the MAC address of the DSL-AC, it sends out a PADI packet via an Ethernet broadcast

Broadcasting (networks)

right|250pxIn telecommunication and information theory, broadcasting refers to a method of transferring a message to all recipients simultaneously...

(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

Leipzig

Leipzig Leipzig has always been a trade city, situated during the time of the Holy Roman Empire at the intersection of the Via Regia and Via Imperii, two important trade routes. At one time, Leipzig was one of the major European centres of learning and culture in fields such as music and publishing...

) 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)

PADR stands for PPPoE active discovery request.

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.

Quirks

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

Path MTU discovery

Path MTU Discovery is a standardized technique in computer networking for determining the maximum transmission unit size on the network path between two Internet Protocol hosts, usually with the goal of avoiding IP fragmentation...

is defeated by poorly configured firewalls. Although higher MTUs are becoming more common in providers' networks, usually the fix 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. In practice, this does not present significant issues for residential users since most content is served over TCP.

RFC 4638 allows PPPoE devices to negotiate an MTU of greater than 1492 if the underlying Ethernet layer is capable of Jumbo Frames

Jumbo Frames

In computer networking, jumbo frames are Ethernet frames with more than 1500 bytes of payload. Conventionally, jumbo frames can carry up to 9000 bytes of payload, but variations exist and some care must be taken when using the term. Many Gigabit Ethernet switches and Gigabit Ethernet network...

.

Some vendors (Cisco

Cisco Systems

Cisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...

and Juniper

Juniper Networks

Juniper Networks is an information technology and computer networking products multinational company, founded in 1996. It is head quartered in Sunnyvale, California, USA. The company designs and sells high-performance Internet Protocol network products and services...

, for example) refer to PPPoEoE (PPPoE over Ethernet), which is PPPoE running directly over Ethernet or other IEEE 802

IEEE 802

IEEE 802 refers to a family of IEEE standards dealing with local area networks and metropolitan area networks.More specifically, the IEEE 802 standards are restricted to networks carrying variable-size packets. IEEE 802 refers to a family of IEEE standards dealing with local area networks and...

networks or over Ethernet bridged

Bridging (networking)

Bridging is a forwarding technique used in packet-switched computer networks. Unlike routing, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on flooding and examination of source addresses in received packet headers to locate unknown...

over ATM

Asynchronous Transfer Mode

, in order to distinguish it from PPPoEoA (PPPoE over ATM), which is PPPoE running over an ATM virtual circuit using RFC 2684 and SNAP

Subnetwork Access Protocol

The Subnetwork Access Protocol is a mechanism for multiplexing, on networks using IEEE 802.2 LLC, more protocols than can be distinguished by the 8-bit 802.2 Service Access Point fields. SNAP supports identifying protocols by Ethernet type field values; it also supports vendor-private protocol...

encapsulation of PPPoE. (PPPoEoA is not the same as Point-to-Point Protocol over ATM

Point-to-Point Protocol over ATM

The Point-to-Point Protocol over ATM is a network protocol for encapsulating PPP frames in AAL5. It is used mainly with DOCSIS and DSL carriers....

(PPPoA), which doesn't use SNAP).

External 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)

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

How PPPoE fits in the DSL Internet access architecture

PPPoEoA

PPPoE to PPPoA

PPPoE discovery

PPP session

PPPoE Discovery (PPPoED)

Client to server: Initiation (PADI)

Server to client: Offer (PADO)

Client to server: request (PADR)

Server to client: session-confirmation (PADS)

Either end to other end: termination (PADT)

Quirks

See also

External links