1. Point-to-Point Protocol
Semester 4, Chapter 4
Allan Johnson

2. Table of Contents Introduction to PPP PPP Session Establishment
Go There!
Introduction to PPP
Go There!
PPP Session Establishment
Go There!
PPP Authentications

3. Introduction to PPP
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4. PPP Replaced SLIP Control of data link setup
PPP was created specifically to address the limitations of SLIP (Serial Line Internet Protocol) and to allow...
PPP is a versatile, widely used WAN protocol with the following features:
Control of data link setup
dynamic assignment of IP addresses
Network protocol multiplexing
Link configuration and quality testing
Error detection
Negotiation options

5. PPP and Data Links
PPP operates at the Data Link layer. Components of PPP include:
A method for encapsulating packets (datagrams) over serial links
The Link Control Protocol (LCP) to estab., maintain, test and terminate the data-link connection
The Network Control Protocol (NCP) to allow simultaneous encapsulation of multiple network layer protocols across the same data-link
At the physical layer, PPP can be used across synchronous (e.g., ISDN) and asynchronous (e.g., modem dialup) data links.

6. PPP Layer Functions

7. PPP Layer Functions Network Control Protocol (NCP) Data Link
(specific to ea. Network layer protocol)
Link Control Protocol (LCP) (Authentication, other options)
High-Level Data Link Control (HDLC)
Physical
Synchronous or asynchronous physical media

8. PPP Frame Formats Flag - Indicates the beginning or end of a frame1 2
Variable
2 or 4
Flag
Address
Control
Protocol
Data
FCS
Flag - Indicates the beginning or end of a frame
Address - broadcast address; PPP does not assign individual station addresses.
Control - 1 byte calling for a connection-less data link
Protocol - identifies the network protocol
Data - contains the upper layer datagram
FCS - characters added to a frame for error control purposes.

9. PPP Session Establishment
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10. Phases of PPP Establishment
PPP goes through four distinct phases to provide communications over point-to-point links
Link establishment & configuration negotiation
Link quality determination
Network-layer protocol configuration negotiation
Link termination

11. 1. Link Estab. & Config. Negotiation
Link establishment occurs at the data link layer with each PPP device sending LCP packets.
The Link Control Protocol packets contain a configuration field for options such as...
MTU
compression
link authentication
LCP must establish the link before any network layer protocols can be exchanged
This phase is complete when a configuration acknowledgement has been sent and received.

12. 2. Link-Quality Determination
PPP provides optional testing to determine whether the link is good enough to bring up network layer protocols.
In addition, if authentication is required it occurs during this phase:
PAP (Password Authentication Protocol)
CHAP (Challenge Handshake Authentication Protocol)
Authentication occurs before the network layer configuration phase begins.

13. 3. Network-Layer Negotiation
Once LCP finishes the link-quality phase, network layer protocols can be configured by the appropriate NCP
NCPs are sent for each protocol (e.g., IP, IPX, AppleTalk)
If LCP terminates the link, it informs NCP so it can take the appropriate action
To view the status of LCP and NCP, use the show interfaces command

14. 4. Link Termination Requested by user (closing internet connection)
LCP can terminate the link at any time. Reasons include:
Requested by user (closing internet connection)
Loss of carrier at the physical layer

15. PPP Authentications
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16. Authentication PAP (Password Authentication Protocol)
If chosen, occurs during the link-quality determination phase.
Requires that the calling side of the link enter authentication information.
Ensures that the users has net. admin.’s permission to make the call
The two authentication options supported by PPP are:
PAP (Password Authentication Protocol)
CHAP (Challenge Handshake Authentication Protocol)

17. PAP Simple authentication requiring a two-way handshake.
A username/password pair is repeatedly sent to the called router until authentication is acknowledge or the link is terminated.
Passwords are sent as clear text (not secure)
The remote router is control of the frequency and timing of the login attempts and send password repeatedly

18. Configuring PAP
On each router, define the user name and password to except from the remote router.
Enable PPP and PAP on the interface.
Lab-A(config)#username Lab-B password class
Lab-A(config-if)#encap ppp
Lab-A(config-if)#ppp authentication pap
Lab-A(config-if)#ppp pap sent-username Lab-A password cisco
Lab-B(config)#username Lab-A password cisco
Lab-B(config-if)#encap ppp
Lab-B(config-if)#ppp authentication pap
Lab-B(config-if)#ppp pap sent-username Lab-B password class

19. CHAP
CHAP use a three-way handshake to periodically verify the identity of the calling router.
This is done during link establishment and (unlike PAP) can be repeated any time during link maintenance.
Does not allow caller to attempt authentication without a challenge (a variable value that is unique and unpredictable)
The local router is in charge of timing and frequency of authentication.
Passwords are encrypted.

20. Configuring CHAP
You can use the same hostname on multiple routers when you want remote users to think they are logging into the same router.
Lab-A(config)#username Lab-B password cisco
Lab-A(config-if)#encap ppp
Lab-A(config-if)#ppp authentication chap
Lab-B(config)#username Lab-A password cisco
Lab-B(config-if)#encap ppp
Lab-B(config-if)#ppp authentication chap

21. Verifying Authentication
To verify that you have PAP or CHAP configured correctly, use the debug features of Cisco’s IOS.
Close all telnet sessions first to return to the original consoled router.
In Privileged Exec. Mode, enter the command…
Lab-A#debug ppp authentication
Go to the ppp interface. Shut it down and then bring it back up. You should see PAP or CHAP info come across the link as it comes back up and the routers authenticate each other.

22. Table of Contents
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