Know Networking Series – Point-to-Point Protocol

Ever wondered what happens when you connect your laptop/PC with the modem and how it connects to the Internet?! Before we go about learning how PPP works, we should have an idea of where we lie in terms of the entire network.

One of these multiple PCs is your home system. You connect to the modem and it in turn connects to the D-SLAM.

D-SLAM expands to Digital Subscriber Line Access Multiplexer. It acts like a multiplexer that takes multiple inputs and gives one single output line. This output is  ideally directed to your service provider’s router from where they connect to internet.

Now that we know where devices sit in the network, we shall go ahead and see how these devices communicate with each other. Yes, the obvious answer is they are physically connected. But over this physical connection we need a protocol to tell these devices what to do when they receive packets.  That is where PPP comes into picture.

PPP denotes Point-to-Point Protocol. The specialty of this fellow is that, he can connect to almost anything like serial cable, telephone lines, fiber optic cables, radio and cellular networks,etc., This makes PPP the most popular protocol. PPP ideally provides connectivity between two nodes.

Looking closer into the OSI model:

The physical layer functions are taken care by the cables and PPP sits on the Data-Link Layer. Now what does that mean anyway?!  Quite clearly, Data-link layer provides connectivity. So the major factors it has to take care of are:

• Authentication – Validate the connection by authentication mechanisms
• Encryption – Encrypt what you send with the connection
• Compression – Compress so that it fits the capacity of the physical connection.

PPP meets all these three requirements and hence the obvious choice for the Internet Service Providers (ISPs). Apart from these, the other reasons why PPP is preferred is:

• Seamless integration with the other AAA [Authentication Accounting and Authorization] protocols
• PPP Multi-link feature
• Dynamic address allocation

The IP packets can be carried over an ethernet connection or ATM connection. PPP also provides variations based on it. We have PPPoE i.e., PPP over Ethernet and PPPoA i.e., PPP over ATM.

Now lets ponder the details of how PPPoE works. There are 2 stages of operation:

1. Discovery Stage
2. Session Stage

PPP Discovery Stage:

PPP works in the client server model. The discovery has following stages:

Step 1: PADI – PPP Active Discovery Initiation – Client to Server

So the first logical step is when you want to call a person through phone is that, you have to reach the other end, for which you will need to know their number. Same case applies here. In order to reach the ISP’s router you need to know their Point of Presence (POP). Computers communicate via MAC address before they get an IP assigned.  To know the MAC address of the DSL Access Concentrator [DSL -AC], PADI packet is sent.

Step 2: PADO – PPP Active Discovery Offer – Server to Client

The PADI packets are generally broadcast. So every DSL-AC that receives it will reply back with a PADO. The reply/offer consists of the MAC address of the client to which it offers and the name or service offered by the DSL-AC. This name or service field is used by the client to choose from the multiple PADOs it received.

Step 3: PADR – PPP Active Discovery Request – Client to Server

The PADR packets are unicast to the accepted PADO DSL-AC alone. This is just to acknowledge the DSL that the client has accepted the PADO from him.

Step 4: PADS – PPP Active Discovery Session – Server to Client

PADS is the reply to PADR from the DSL, which now enables a connection between DSL and the client. Session ID is assigned along the reply.

Now I am all set to send data via PPP  connection. Once I am done with the connection and data transfer, I initiate termination of the session with the PADT – PPP Active Discovery Terminate. This can either be client to server or vice versa. This is the high level overview of the protocol. In further posts, the intricate details will be covered.