PPP, VPN, Tunneling and OSI model

[NetworkDummy]

I am really confused about tunneling protocol family and all related protocols. I read a bunch of articles, rfc but still cannot get the idea.

I will put my questions between my explanation.

A little bit about my network configuration.
1. I have home router (wifi), it is connected to my ISP via PPPoE.
2. All packets were captured while I was connected to the remote VPN server over PPTP protocol.

In computer networking, Point-to-Point Protocol (PPP) is a data link (layer 2) protocol used to establish a direct connection between two nodes.
Q-1: What does point to point in general mean ? How can I connect not point to point ? I have only one cable, in case of my network, my router is connected to the switch with the cable ?

I have captured packets, here is wireshark information about the PPP packet. http://imgur.com/VPYk9a3

Q-2: Point-to-Point Protocol (PPP) is a data link Hmm. Why then this packet is encapsulated inside IP -> GRE ? It is not second layer

Q-3: How PPP (PPPoe) differs from simple Data Link ethernet frame (used by switches) ?

Also VPN connection is called PPTP, so as I could guess PPTP is used just for session establishment, nothing else. Because I can see PPTP packets only serveral times while communication.

Captured packet http://imgur.com/a/w9qTk

And this goes even further above Layer 4 (TCP).

So I am really confused about all these stuff, so I try to explain how I understand a process is general.

First of all PPP is just standard for special packet format on the data link layer, it is used because this protocol (PPP) is implemented above different physical environments like Ethernet, ATM...

Why is it used ? Because it provides auth on the second layer of OSI using LCP, NCP protocols. So basically when I am connecting to my provider I use credentials to establish the connection.

Sometimes encapsulation is mentioned as an advantage of this protocol

Q-4: Why this is an advantage ? OSI stack assumes encapsulation out of the box, each higher layers is just encapsulated inside payload field of the the lower one !!

So basically PPP allows authentication and compression.

When I am using VPN (PPTP) as far as PPTP server is specific application that uses port (1723) in order to listen inbound connections.Thats why it is located above TCP. And this protocol is just used to establish communication with the server.

Q-5: How does further communication work if PPP packets are just wrapped in IP and no port is specified ? How does VPN server machine know that this packet is tunnel and it should be redirected to the 1723 port application in order to proceed ?

Please help me to understand this topic, please answer my questions. I would be grateful for any help

[deanwebb]

OK, lots of questions...

But, basically, PPP is just what carries a payload packet from one end of a WAN connection to another. It's the wrapper for IP, not the other way around, so the last question is not valid. PPP handles the layer 2 work. Other protocols take care of layers 3 and 4.

PPPoE and PPTP are basically connections to get PPP to work well with layers 1 and 3+, respectively.

[icecream-guy]

This networking stuff is not always straight forward. there are lots of little eccentricities involved to make this work, like MPLS being not at layer 2 or 3, but shimmed in at layer 2 1/2. Books teach the base knowledge, life provides the experience.

[LynK]

Q-1: What does point to point in general mean ? How can I connect not point to point ? I have only one cable, in case of my network, my router is connected to the switch with the cable ?

Traditional Point to point is defined as a direct connection between two devices. Layer 2 frames, and L3 packets go directly from one endpoint to the other.



Q-5: How does further communication work if PPP packets are just wrapped in IP and no port is specified ? How does VPN server machine know that this packet is tunnel and it should be redirected to the 1723 port application in order to proceed ?

You are not understanding correctly how PPP works. Essentially PPP is a layer 2 encapsulation process, because of this it needs a layer 3 transport. It uses GRE as the means to the end. This allows non directly connected links to ask as a directly connected link.

The way the end tunnel device knows how to get back to the originating host initiating the PPP is by utilizing pre-configured parameters to establish two way communication.