PPPoE Technology White Paper Keywords: PPP, Ethernet, PPPoE Abstract: Point-to-Point Protocol over Ethernet (PPPoE) provides access to the Internet for hosts on an Ethernet through a remote access device and implements access control and accounting on a perhost basis. This document introduces the background, technical benefits, working mechanism, and application scenarios of PPPoE. Acronyms: Acronym Full spelling ADSL DSLAM PPP PPPoE PADI PADO PADR PADS PADT Asymmetric Digital Subscriber Line Digital Subscriber Line Access Multiplexer Point to Point Protocol PPP Over Ethernet PPPoE Active Discovery Initiation PPPoE Active Discovery Offer PPPoE Active Discovery Request PPPoE Active Discovery Session-confirmation PPPoE Active Discovery Terminate Hangzhou H3C Technologies Co., Ltd. 1/11
Table of Contents Overview 3 Background 3 Benefits 3 PPPoE Implementation 3 PPPoE Network Structure 3 PPPoE Packet Format 5 How PPPoE Works 6 Discovery Stage 6 Session Stage 6 Terminate Stage 7 Application Scenarios 7 Connecting a LAN to the Internet Using an ADSL Modem 7 Network Requirements 7 Wireless PPPoE Access 8 Network Requirements 8 References 9 Appendix 9 Packet Structure 9 Hangzhou H3C Technologies Co., Ltd. 2/11
Overview Background In some cases, it is desirable to connect multiple hosts at a remote site through the same customer premise access device that can provide access control and accounting functions, similar to those provided in dial-up services using PPP. Since the most economical way to connect multiple hosts to an access device is Ethernet, and PPP can provide access control and accounting functions, Point-to- Point Protocol over Ethernet (PPPoE) was developed to transmit PPP packets over Ethernet, which makes full use of the economy of Ethernet and the scalability and management functions of PPP. In particular, because PPPoE supports accounting on a per-host basis, it gains considerable popularity among broadband access service providers. Benefits For end users, the benefits of PPPoE are as follows: Adopting the traditional dial-up access mode, which allows end users to use the familiar hardware and similar software to access the Internet. Compatible with all the existing xdsl modems. No complicated configurations are required on the clients xdsl modems. Using Ethernet adapters to connect PCs and xdsl modems, which allows PCs to share xdsl lines and thus saves investment. For service providers, the benefits of PPPoE are as follows: Allowing service providers to provide broadband access services for a large number of end users through digital subscriber lines, cable modems, or wireless access devices. Allowing service providers to quickly deploy high-speed Internet services by using reliable, mature technologies, thus minimizing impact on the current networks. Allowing service providers to authenticate end users with the access control function, perform accounting with the accounting function, and monitor user behavior so as to ensure network security. Allowing end users to access network services provided by different service providers at the same time. This dynamic subscription mode enables the service providers to easily develop and provide new services. PPPoE Implementation PPPoE Network Structure PPPoE adopts the client/server model. The PPPoE client sends a connection request to the PPPoE server. After the PPP negotiation between the two succeeds, the PPPoE server provides functions such as access control and authentication for the PPPoE client. There are two networking modes for PPPoE: Hangzhou H3C Technologies Co., Ltd. 3/11
In the first mode, a PPP session is established between two network devices, all hosts connected to the PPPoE client transmit data using this PPP session, and the PPPoE client dial-up software is not required on the hosts. This mode is used typically when an enterprise uses a single account for Internet access. Figure 1 is a network diagram for this PPPoE networking mode. In the diagram, the PPPoE client is on the enterprise's Intranet, and the PPPoE server is a device on the service provider s network. Figure 1 Network diagram for PPPoE networking mode 1 Service provider device DSLAM PPPoE Server Internet Router B Client device Modem Router A PPPoE Client Host A Host B Host C In the second mode, a PPPoE session is established between each host and a router (PPPoE server), as shown in Figure 2. Each host is a PPPoE client and has an Internet access account, which makes it easy for the service provider to perform access control and accounting on a perhost basis. The PPPoE client dial-up software must be installed on each host. Figure 2 Network diagram for PPPoE networking mode 2 Hangzhou H3C Technologies Co., Ltd. 4/11
PPPoE Packet Format Essentially, a PPPoE packet is a PPP packet encapsulated in an Ethernet frame, as shown in Figure 3. Figure 3 PPPoE packet format The field descriptions are as follows: Destination_address: A unicast Ethernet destination address or Ethernet broadcast address (0xffffffff). In a discovery packet, this field is a unicast or broadcast address. The PPPoE client uses a broadcast address to discover the PPPoE server. After the PPPoE server is determined, unicast addresses are used for communication. During the Session stage, this field must be the unicast address of the peer determined during the Discovery stage. Source_address: Ethernet MAC address of the source device. Ether_type: 0x8863 during the Discovery or Terminate stage, or 0x8864 during the Session stage. Ver: PPPoE version number, 4 bits in length, with the value 0x1. Type: PPPoE type, 4 bits in length, with the value 0x1. Code: PPPoE packet type, 8 bits in length. Code value 0x00 indicates session data; 0x09 indicates PADI packets; 0x07 indicates PADO or PADT packets; 0x19 indicates PADR packets; 0x65 indicates PADS packets. For detailed description about these packets, see Appendix. Session_ID: Unique identifier of a PPP session, 16 bits in length. It is fixed for a given PPP session and in fact, defines a PPP session together with the Source_address and Destination_address fields. The value 0xffff is reserved for future use and must not be used. Length: The Length field is 16 bits long and represents the length of the PPPoE Payload. It does not indicate the length of the Ethernet or PPPoE headers. Hangzhou H3C Technologies Co., Ltd. 5/11
How PPPoE Works Figure 4 shows the PPPoE negotiation process. Figure 4 PPPoE negotiation process PPPoE Client PPPoE Server PADI Discovery PADO PADR PADS PPP negotiation Session Teminate Data transmission PADT PPPoE undergoes three stages, namely, Discovery, Session, and Terminate. Discovery Stage The Discovery stage includes the following four steps. When this stage is over, both peers know the PPPoE session ID and the MAC address of the peer, which together determine a unique PPPoE session. 1) The PPPoE client broadcasts a PADI packet that contains information about the service type it requests. 2) After receiving a PADI packet that it can serve, a PPPoE server replies with a PADO packet. The destination address of the PADO packet is the unicast packet of the host that sent the PADI. 3) Depending on the network topology, since the PADI was broadcast, the PPPoE client may receive PADO packets sent by multiple PPPoE servers. Among these PPPoE servers, the PPPoE client selects the one whose PADO packet arrived the earliest and unicasts a PADR packet to the PPPoE server. 4) The PPPoE server generates a unique session ID for the session and sends the session ID to the PPPoE client through a PADS packet. If no error occurs, the session will thus be established and PPPoE moves on to the Session stage. Session Stage The Session stage can be divided into two steps, namely, PPP negotiation step and PPP packet transmission step. The PPP negotiation for a PPPoE session can be further divided into three stages, namely, LCP, authentication, and NCP. Hangzhou H3C Technologies Co., Ltd. 6/11
1) LCP is a stage where parameters used to establish, configure, and test the data-link connection are negotiated. 2) After the LCP negotiation succeeds, the authentication is performed with the authentication protocol (CHAP or PAP) agreed upon in LCP negotiation. 3) After the authentication succeeds, PPP enters the NCP stage where different network layer protocols are configured. The PPPoE server and client typically use the IP Control Protocol (IPCP) to configure IP addresses and DNS server information for end users. After PPP negotiation succeeds, the PPPoE session can carry PPP data packets. In the Session stage, all Ethernet data packets are sent in unicast mode. Terminate Stage The PPPoE server and client should use the PPP protocol itself (for example, PPP termination packets) to terminate the PPPoE session between them. When PPP cannot be used, the PADT packets may be used. In PPPoE Session stage, the PPPoE server and client both can terminate the PPPoE connection by sending a PADT packet in unicast mode at any time. After sending or receiving a PADT packet, PPP traffic, even normal PPP termination packets, is not allowed to be sent using the session. Application Scenarios Connecting a LAN to the Internet Using an ADSL Modem Network Requirements As shown in Figure 5, the enterprise wishes that all its employees access the Internet using a common account: Router A serves as the PPPoE client and accesses the Internet with an account, which is the common account used by all the employees to access the Internet. Router B serves as the PPPoE server. It connects to a DSLAM on one end, providing functions such as RADIUS authentication and accounting, and to the Internet on the other end. Hangzhou H3C Technologies Co., Ltd. 7/11
Figure 5 Network diagram for connecting a LAN to the Internet using an ADSL modem PPPoE Technology White Paper Service provider device DSLAM PPPoE Server Internet Router B Client device Modem Router A PPPoE Client Host A Host B Host C Wireless PPPoE Access Network Requirements As shown in Figure 6, the APs serve as PPPoE clients and the broadband remote access server (BRAS) serves as the PPPoE server. End users access the Internet through wireless networks. Figure 6 Network diagram for wireless PPPoE access PPPoE Server BRAS Internet Distribution switch Distribution switch PPPoE Client APs Client A Client B Client C Client D Hangzhou H3C Technologies Co., Ltd. 8/11
References RFC1661: The Point-to-Point Protocol (PPP) RFC2516: A Method for Transmitting PPP Over Ethernet (PPPoE) Appendix Packet Structure PPPoE Active Discovery Initiation (PADI) The Code value is 0x09. The SESSION_ID value is 0x0000. The TAG_TYPE value is 0x0101 (Service-Name), indicating that a service name follows. There is only one tag with the TAG_TYPE value being Service-Name. The other tags are optional. Figure 7 PADI packet structure PPPoE Active Discovery Offer (PADO) The Code value is 0x07. The SESSION_ID value is 0x0000. The TAG_TYPE value being 0x0101 (Service-Name) indicates that a service name follows; being 0x0102 (AC-Name) indicates that the following string uniquely identifies a particular access concentrator. There is only one tag with the TAG_TYPE value being AC-Name, and at least one tag with the TAG_TYPE value being Service-Name. Figure 8 PADO packet structure Hangzhou H3C Technologies Co., Ltd. 9/11
PPPoE Active Discovery Request (PADR) The Code value is 0x19. The SESSION_ID value is 0x0000. The TAG_TYPE value is 0x0101 (Service-Name), indicating that a service name follows. There is only one tag with the TAG_TYPE value being Service-Name. The other tags are optional. Figure 9 PADR packet structure PPPoE Active Discovery Session-confirmation (PADS) The Code value is 0x65. The SESSION_ID value is the one determined during the Discovery stage. The TAG field is optional. Figure 10 PADS packet structure PPPoE Active Discovery Terminate (PADT) The Code value is 0xa7. The SESSION_ID value is the one determined during the Discovery stage. The TAG field is not available. Hangzhou H3C Technologies Co., Ltd. 10/11
Figure 11 PADT packet structure Copyright 2009 Hangzhou H3C Technologies Co., Ltd. All rights reserved. No part of this manual may be reproduced or transmitted in any form or by any means without prior written consent of Hangzhou H3C Technologies Co., Ltd. The information in this document is subject to change without notice. Hangzhou H3C Technologies Co., Ltd. 11/11