Once the PPPoE Session phase begins, PPP data is sent as in any other PPP encapsulation. That is to say that the LCP negotiation takes place and NCPs are opened as needed. All Ethernet frames are unicast between the aggregation router and PPPoE client at this point.
RFC 2516 specifies a Maximum Receivable Unit (MRU) for PPPoE negotiated payload size at 1492 bytes. The PPPoE header is 6 bytes in length with a Protocol-ID field of 2 bytes. This keeps PPPoE in line with Ethernet's 1500-byte maximum payload.
The ETHER_TYPE field, in the Ethernet header, is set to 0x8864. The PPPoE CODE field must be set to 0x00. The SESSION_ID field must not change for that PPPoE Session and must be the value assigned in the Discovery stage. The PPPoE payload contains a PPP frame. The frame begins with the PPP Protocol-ID (PID).
Once the Session stage is complete, the PPP LCP options can engage. As mentioned previously, the Session is stateless until the PPP connection is negotiated, including authentication, and any additionally or optionally configured LCP options.
The needs of the subscriber community served by a particular service provider are nearly as diverse as the population itself. With that in mind, flexibility is a key benefit in the marketplace. It is crucial that a balance be struck in the offered options and the ease of support. Allowing too much hardware and configuration diversity will affect the provider's ability to support the solution when need arises. Typically, three options are made available in some form or fashion to the subscriber:
■ Placing a DSL-capable router at the subscriber home—This router will have an integrated DSL modem and built-in PPPoE client capabilities, allowing the router to be configured in an always-on service offering. No additional software is needed on the subscriber computer. It also remedies the need to have a subscriber install PPPoE client software on all machines that wish to be connected to the network. This router will provide DHCP, NAT/PAT, and other relevant services to the subscriber home network.
■ Placing a non-DSL-capable router at the subscriber home—This requires the additional placement of an external DSL modem at the subscriber premises to terminate the DSL connection. The router should have PPPoE client capabilities in order to provide the always-on service. This router, too, will provide DHCP, NAT, and PAT services.
■ Placing an external DSL modem at the subscriber home to terminate the DSL
connection—PPPoE client software is installed on the subscriber hosts wishing to connect to the network.
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