In this post, I will discuss about The Point-to-Point Protocol, The PPP full frame format for unnumbered mode operation, and Explanation of Method to bring Internet Lines Up and Down in Computer Networks.
The Point-to-Point Protocol:
Point to Point protocol is used for various reasons like router to router traffic, error detection, supports multiple protocols, IP addresses can be negotiated during connection. PPP provides three features:
- A framing method that without separates, one frame to another without complexity and also handles error detection.
- A link control protocol is used for bringing lines up and down when required, negotiating options, testing and is called as LCP (Link Control Protocol). It supports synchronous and asynchronous circuits and bit-oriented and byte-oriented encodings.
- A way to negotiate network-layer options in a way that is independent of the network layer protocol to be used. The method chosen is to have a different NCP (Network Control Protocol) for each network layer supported.
We will see what happens when a home user calling up an Internet service provider to make a home PC a temporary Internet host. PC calls the provider’s router via a modem and when the router’s modem has answered the phone then physical connection is made. PC sends the router a series of LCP packets in the payload field of one or more PPP frames.
After this a series of NCP packets are sent for the purpose to configure the network layer. The PC needs an IP address to run a TCP/IP protocol stack. Usually each Internet provider gets a block of IP and then dynamically assigns one to each newly attached PC for the duration of its login session because there are not much IP. If a provider owns x IP addresses, it can have up to z machines logged in simultaneously, but its total customer base may be many times that. The NCP for IP assigns the IP address.
Now, the PC is now an Internet host and can send and receive IP packets, just as hardwired hosts can. When the user is finished, NCP breaks the network layer connection and IP address is taken back. Then LCP shuts down the data link layer connection. Finally, the computer tells the modem to hang up the phone, releasing the physical layer connection
The PPP frame format was chosen to closely resemble the HDLC frame format. The major difference between PPP and HDLC is that PPP is character oriented instead of bit oriented. In particular, PPP uses byte stuffing on dial-up modem lines. Not only can PPP frames be sent over dial-up telephone lines, but they can also be sent over SONET.
The PPP full frame format for unnumbered mode operation:
BYTES 1 1 1 1 or 2 VARIABLE 2 or 4 1
| FLAG 01111110 | ADDRESS 11111111 | CONTROL 00000011 | PROTOCOL | PAYLOAD | CHECKSUM | FLAG 01111110 |
All frames in PPP frames starts with the standard HDLC flag byte (01111110), if it occurs within the payload field, it is byte stuffed. Address field is followed next, which is always set to the binary value 11111111 to indicate that all stations are to accept the frame. Using this value avoids the issue of having to assign data link addresses.
The Address field is followed by the Control field, the default value of which is 00000011. This value indicates an unnumbered frame. In other words, PPP does not provide reliable transmission using sequence numbers and acknowledgements as the default. Since the Address and Control fields are always constant in the default configuration, LCP provides the necessary mechanism for the two parties to negotiate an option to just omit them altogether and save 2 bytes per frame.
Fourth PPP field is the Protocol field which tells what kind of packet is in the Payload field Protocols starting with a 1 bit are used to negotiate other protocols. These include LCP and a different NCP for each network layer protocol supported. The default size of the Protocol field is 2 bytes, but it can be negotiated down to 1 byte using LCP.
The Payload field is variable length, up to some negotiated maximum.If the length whose default length of 1500 bytes is used. Padding may follow the payload if need be. After the Payload field comes the Checksum field, which is normally 2 bytes, but a 4-byte checksum can be negotiated.
Explanation of Method to bring Internet Lines Up and Down:
Figure shows the phases that a line goes through when it is brought up, used, and taken down again. This sequence applies both to modem connections and to router-router connections.
The DEAD state means that no physical layer carrier is present and no physical layer connection exists. Then the physical connection is ESTABLISHED. At that point LCP option negotiation begins, which, if successful, leads to AUTHENTICATE. Now the two parties can check on each other’s identities if desired. When the NETWORK phase is entered, the appropriate NCP protocol is invoked to configure the network layer. If the configuration is successful, OPEN is reached and data transport can take place. After data transport, the line moves into the TERMINATE phase, and from there, back to DEAD when the carrier is dropped.
During the ESTABLISH phase LCP negotiates data link protocol options. LCP provides a way for the initiating process to make a effort for the responding process to accept or reject it, in whole or in part, also line quality is verified for two process to setup connection if line quality is good.. Finally, the LCP protocol also allows lines to be taken down when they are no longer needed.
LCP have several Frames. These are mentioned in the table below:
| NAME | DIRECTION | DESCRIPTION |
| Configure-request | I→R | List of proposed options and values. |
| Configure-ack | I←R | All options are accepted |
| Configure-nak | I←R | Some options are not accepted. |
| Configure-reject | I←R | Some options are not negotiable. |
| Terminate-request | I→R | Request to shut the line down. |
| Terminate-ack | I←R | OK, line shut down |
| Code-reject | I←R | Unknown request received. |
| Protocol-reject | I←R | Unknown protocol requested. |
| Echo-request | I→R | Please send this frame. |
| Echo-reply | I←R | Here is the frame back. |
| Discard-request | I→R | Just discard this frame (for testing). |