Usuario:Amitie 10g/Abandonados/PPPoE
| Application | FTP | SMTP | HTTP | … | DNS | … |
| Transport | TCP | UDP | ||||
| Internet | IP | IPv6 | ||||
| Network access | PPP | |||||
| PPPoE | ||||||
| Ethernet | ||||||
The Point-to-Point Protocol over Ethernet (PPPoE) is a network protocol for encapsulating Point-to-Point Protocol (PPP) frames inside Ethernet frames. It is used mainly with DSL services where individual users connect to the DSL modem over Ethernet and in plain Metro Ethernet networks. It was developed by UUNET, Redback Networks and RouterWare (now Wind River Systems) [1] and is available as an informational RFC 2516.
Ethernet networks are packet-based and have no concept of a connection or circuit and also lack basic security features to protect against IP and MAC 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 systems and fits perfectly into the ATM backbones. The protocol also permits very easy unbundling of digital subscriber line access multiplexers (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) are being called legacy technologies[cita requerida]. Pseudowire and VoIP can be used to deal with legacy customers and equipment. PPPoE has the highest overhead DSL delivery method.[2] The amount of overhead added by PPPoE depends on the packet size because PPPoE adds 8 bytes to each packet.[3] 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.[4] 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. 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 networks.
How PPPoE fits in the DSL Internet access architecture[editar]
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.
PPPoEoA[editar]
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 IP PPP PPP PPPoE PPPoE Ethernet Ethernet Ethernet Ethernet ATM ATM DSL DSL backbone backbone
PPPoE to PPPoA[editar]
The PPP frames are extracted from the PPPoE packets and encapsulated in ATM, thus converting PPPoE into PPPoA (RFC 2364).
DSL Internet access architecture Host PC DSL modem DSLAM Remote access server IP IP PPP PPP PPPoE PPPoE PPPoA PPPoA Ethernet Ethernet ATM ATM DSL DSL backbone backbone
PPPoE stages[editar]
The PPPoE has two distinct stages:
PPPoE discovery[editar]
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.
PPP session[editar]
Once the MAC address of the peer is known and a session has been established, the Session stage.
PPPoE Discovery (PPPoED)[editar]
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)[editar]
PADI stands for PPPoE Active Discovery Initiation.[5]
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)[editar]
PADO stands for PPPoE Active Discovery Offer.[6]
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)[editar]
PADR stands for PPPoE active discovery request.[7]
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)[editar]
PADS stands for PPPoE active discovery session-confirmation.[8]
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)[editar]
PADT stands for PPPoE active discovery termination.[9]
This packet terminates the connection to the POP. It may be sent either from the user's computer or from the DSL-AC.
Quirks[editar]
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 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.
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).
See also[editar]
- Multiprotocol Encapsulation over ATM
- Point-to-Point Protocol daemon
- Point-to-Point Tunneling Protocol
- Point-to-Point Protocol over ATM (PPPoA)
References[editar]
- ↑ Wind River Systems Acquires RouterWare, Inc.. Findarticles.com (1999-07-05). Retrieved on 2011-09-27.
- ↑ Dirk Van Aken, Sascha Peckelbeen Encapsulation Overhead(s) in ADSL Access Networks, June 2003
- ↑ An Explanation of DSL Overhead. Pflog.net (2006-02-26). Retrieved on 2010-10-27.
- ↑ centurytel.net - Support - Online Helpdesk. Centuryinter.net. Retrieved on 2010-10-27.
- ↑ http://tools.ietf.org/html/rfc2516#section-5.1
- ↑ http://tools.ietf.org/html/rfc2516#section-5.2
- ↑ http://tools.ietf.org/html/rfc2516#section-5.3
- ↑ http://tools.ietf.org/html/rfc2516#section-5.4
- ↑ http://tools.ietf.org/html/rfc2516#section-5.5
External links[editar]
- 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)