Why PPP procotol is know as Point to point ?

[Ravi VK]

Hi ,

I am a newbie working on PPP/PPPoE . I have read the RFC's related to PPPoE/PPP.

I understand that PPPoE/PPP runs between CPE and BRAS/BNG edge router.

But want to know PPPoE is called as point to point ?

[Masoud Pourshabanian]

Hello,

The better definition for PPPoE is:

PPPoE is a protocol that enables you to establish a point-to-point communications channel through an essentially non point-to-point network which is ethernet.

PPPoE allows multiple hosts connect to a service provider over a single physical connection.

PPPoE enjoys advantages of two protocols. Ethernet which is everywhere and PPP which is a desirable protocol for ISPs.

PPP provides several features for ISPs. Authentication and authorization and assigning temperary IPs and sessions and so on.

Now to your question.

For answering your question about PPP, we need to take a look at PPP header. PPP header has only an "address" field which is always 1111111. there is no destination and source address so whatever you put on the line, it will receive by other side. Because of this nature, it is called point to point protocol. Please check PPP header in the link below.

http://www.tcpipguide.com/free/t_PPPGeneralFrameFormat.htm

Hope it helps,

Masoud

[Ravi VK]

Thanks for you answer.

I have one more doubt :

CPE <--------------> DSLAM -------------> BRAS

Once PPPoE/PPP LCP and NCP negotiations are done , CPE and BRAS know each others MAC addresses. When a data packet encapsulated in PPP/PPPoE is sent from CPE , it will have Source MAC (say SrcMAC1) and destination MAC of BRAS (say DstMAC1) . Packet is sent from CPE to DSLAM . Will DSLAM put its source MAC here ? As per my knowledge DSLAM will not add its source mac .

Please clarify , how packet is forwarded from CPE to BRAS and vice a versa.

[Masoud Pourshabanian]

Hello,

Client craetes an IP packet. IP packet will be encapsulated in a PPP frame. PPP frame will be encapsulated in an ethernet frame. Ethernet frame arrives to the modem. Modem adds ATM header. DSLAM receives ATM frames. It removes ATM header and sends the ethernet frame to BRAS.

DSLAM does not replace any MAC address.

IP over Ethernet is routed to the ISP via the BRAS. The identity of the customer is maintained at Layer 2 by a unique customer source MAC address all the way to the BRAS.

Please check also the link below

"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."

https://en.wikipedia.org/wiki/Point-to-point_protocol_over_Ethernet

Hope it helps,

Masoud