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.
PPP Link Control Protocol (LCP)
(Page 2 of 3)
LCP Link Configuration
Link configuration is arguably the
most important job that LCP does in PPP. During the Link Establishment
phase, LCP frames are exchanged that enable the two physically-connected
devices to negotiate the conditions under which the link will operate.
shows the entire procedure, which we will now examine in detail.
The process starts with the initiating
device (let's call it device A, yeah, isn't that original) creating
a Configure-Request frame that contains a variable number of
configuration options that it wants to see set up on the link. This
is basically device A's wish list for how it wants
the link created.
Figure 27: PPP LCP Link Configuration Process
This flowchart shows in more detail the negotiation process undertaken to configure the link by LCP. This process begins when the PPP link enters the Link Establishment phase. After successful configuration, the connection transitions to the Authentication phase.
The main PPP document (RFC
1661) defines a number of different configuration options that the initiator
can specify in this request. Any one of these can be included and if
so, filled in with the value corresponding to what device A wants
for that option. If absent, this means device A is neither requesting
nor specifying that option. The six options are:
- Maximum-Receive-Unit (MRU): Lets
device A specify the maximum size datagram it wants the link
to be able to carry.
- Authentication-Protocol: Device
A can indicate the type of authentication protocol it wishes
to use (if any).
- Quality-Protocol: If device A
wants to enable quality monitoring on the link, what protocol to use
(though there is only one currently defined: LQR).
- Magic-Number: Used to detect looped
back links or other anomalies in the connection.
- Protocol-Field-Compression: Allows
device A to specify that it wants to use compressed
(8 bit) Protocol fields in PPP data frames instead of the normal
16 bit Protocol field. This provides a small but free savings
(one byte) on each PPP frame. Note that this has nothing to do with
the compression feature offered by CCP. See the PPP general frame format
topic for more on this feature.
- Address-and-Control-Field-Compression (ACFC):
The same as the option just above but used to compress the Address
and Control fields, again for small bandwidth savings. Again,
PPP general frame format topic for more.
Other options may also be added to
this list by optional feature protocols. For example, Multilink
PPP adds several options that must be
negotiated during link setup.
The other device (let's call it say
device B J)
receives the Configure-Request and processes it. It then has
three choices of how to respond:
- If every option in it is acceptable
in every way, device B sends back a Configure-Ack (acknowledge).
The negotiation is complete.
- If all the options that device A
sent are valid ones that device B recognizes and is capable of
negotiating, but it doesn't accept the values device A sent,
then device B returns a Configure-Nak (negative
acknowledge) frame. This message includes a copy of each configuration
option that B found unacceptable.
- If any of the options that A
sent were either unrecognized by B, or represent ways of using
the link that B considers not only unacceptable but not even
subject to negotiation, it returns a Configure-Reject containing
each of the objectionable options.
The difference between a Configure-Nak
and a Configure-Reject is that the former is like device B
saying I don't accept your terms, but I'm willing to haggle,
while the latter is device B basically saying No way Jose.
For example, if device A tries to request PAP as the authentication
protocol but device B wants to use CHAP, it will send a Configure-Nak.
If device B doesn't support authentication at all, it will send
Note: Even after receiving a reject, device A can retry the negotiation with a new Configure-Request.
|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!|
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.