Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

Enjoy The TCP/IP Guide? Get the complete PDF!
The TCP/IP Guide

Custom Search







Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Network Interface Layer (OSI Data Link Layer) Protocols
           9  TCP/IP Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP)
                9  Point-to-Point Protocol (PPP)
                     9  PPP Protocol Frame Formats

Previous Topic/Section
PPP General Control Protocol Frame Format and Option Format
Previous Page
Pages in Current Topic/Section
1
Next Page
PPP Authentication Protocol (PAP, CHAP) Frame Formats
Next Topic/Section

PPP Link Control Protocol (LCP) Frame Formats

The previous topic described the general format used by the various protocols in PPP that exchange control messages. Of the many control protocols in PPP, the Link Control Protocol (LCP) is the most important, as it is responsible for basic PPP link setup and operation. It is also the protocol used as a “template” for many of the other control protocols.

Since it is so central to PPP, and since many of the other protocols use a similar messaging system, I want to make the general frame format seen in the previous page more “concrete” by showing the specific frame formats used for each of the LCP control frames. There are thirteen different ones, however, and since they have many fields in common, I didn't want to put here thirteen tables all of which look almost the same.

To save space and show how the messages differ, I have put them into a single large summary table instead of making thirteen smaller ones. Table 38 shows the contents and meaning for each of the fields in the thirteen LCP frame types:


Table 38: PPP Link Control Protocol (LCP) Frame Types and Fields

Frame Type

Code Field

Identifier Field

Length Field

Data Field

Configure-Request

1

New value generated for each frame.

4 + Length of all included configuration options.

Configuration options to be negotiated by the two peers on a link. The previous topic describes the general format of configuration options.

Configure-Ack

2

Copied from the Identifier field of the Configure-Request frame for which this Configure-Ack is a reply.

4 + Length of all included configuration options.

Configuration options being positively acknowledged (accepted during negotiation of the link).

Configure-Nak

3

Copied from the Identifier field of the Configure-Request frame for which this Configure-Nak is a reply.

4 + Length of all included configuration options.

Configuration options being negatively acknowledged (renegotiation requested).

Configure-Reject

4

Copied from the Identifier field of the Configure-Request frame for which this Configure-Reject is a reply.

4 + Length of all included configuration options.

Configuration options being rejected (since the device cannot negotiate them).

Terminate-Request

5

New value generated for each frame.

4 (or more if extra data is included).

Not required. See note below.

Terminate-Ack

6

Copied from the Identifier field of the matching Terminate-Request.

4 (or more if extra data is included).

Not required. See note below.

Code-Reject

7

New value generated for each frame.

4 + Length of rejected frame.

A copy of the LCP frame that was rejected. This is not the complete PPP frame, just the LCP control portion from its Information field.

Protocol-Reject

8

New value generated for each frame.

6 + Length of rejected frame.

The first two bytes contain the Protocol value of the frame rejected. The rest contains a copy of the Information field from the frame rejected.

Echo-Request

9

New value generated for each frame.

8 (or more if extra data is included).

Contains a 4-byte “magic number” used to detect looped-back links, if the appropriate configuration option has been negotiated; otherwise, set to zero.

May also contain additional “uninterpreted data”; see note below.

Echo-Reply

10

Copied from the Identifier field of the matching Echo-Request.

8 (or more if extra data is included).

Contains a 4-byte “magic number” used to detect looped-back links, if the appropriate configuration option has been negotiated; otherwise, set to zero.

May also contain additional “uninterpreted data”; see note below.

Discard-Request

11

New value generated for each frame.

8 (or more if extra data is included).

Contains a 4-byte “magic number” used to detect looped-back links, if the appropriate configuration option has been negotiated; otherwise, set to zero.

May also contain additional “uninterpreted data”; see note below.

Identification

12

New value generated for each frame.

8 (or more if extra data is included).

Contains a 4-byte “magic number” used to detect looped-back links, if the appropriate configuration option has been negotiated; otherwise, set to zero.

May also contain additional “uninterpreted data”; see note below.

Time-Remaining

13

New value generated for each frame.

12 (or more if extra data is included).

Contains a 4-byte “magic number” used to detect looped-back links, if the appropriate configuration option has been negotiated; otherwise, set to zero.

Also contains a 4-byte value indicating the number of seconds remaining in the current session. A value of all ones in this field is interpreted as “forever”, meaning the session will not expire.

May also contain additional “uninterpreted data”; see note below.


Note: LCP frame types 5, 6, 9, 10, 11, 12 and 13 allow an additional amount of data to be included in the Data field whose use is not strictly described by the protocol. The PPP standard says that this may be zero or more octets that “contains uninterpreted data for use by the sender” and “may consist of any binary value”. The inclusion of this “uninterpreted data” is left as an implementation-dependent option.


All LCP control frames are encapsulated into a PPP frame by placing the frame structure into its Information field as discussed in the previous topic. The Protocol field is set to 0xC021 for LCP. For an explanation of how the frames are used, see the operational description of LCP.


Previous Topic/Section
PPP General Control Protocol Frame Format and Option Format
Previous Page
Pages in Current Topic/Section
1
Next Page
PPP Authentication Protocol (PAP, CHAP) Frame Formats
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.