Point-to-Point Protocol or PPP for short is a layer 2 protocol for sending layer 3 packets over synchronous (ISDN) and asynchronous (dial-up) WAN links.
Just like for every topic on the CCNA Exam there are many new terms that you will need to learn and understand about PPP. So, lets begin!
Link Control Protocol
The Link Control Protocol (LCP) is used by PPP to manage connections and uses the following four steps:
(1) Establish – Before any layer 3 packets can be sent a configuration acknowledgment needs to be sent out to the receiving router and returned.
(2) Testing – Is optional, but is used to determine whither the link quality is good enough for sending Layer 3 data
(3) Maintenance – This is performed by the Network Control Protocol.
(4) Termination - The LCP is in charge of ending the connection. This can be done manually by an administrator or automatically if the link goes down or some other error.
Authentication
PPP supports two different ways to authenticate connections Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP). PAP is not very secure because it only uses a two-way handshake and it sends its passwords in plain text. CHAP uses a three-way handshake that is encrypted. The three-way handshake works like this:
CHAP’s Three-Way Handshake
(1) RouterA sends a challenge message to RouterB.
(2) RouterB must then calculate the correct value from a one way hash function which is usually MD5 encrypted and send that to RouterA.
(3) RouterA then acknowledges back to RouterB if it was correct and establishes the secure connection. If RouterB did not calculate the correct value then the link is terminated.
Another benefit to CHAP is that it can prevent playback attacks. A playback attack which is also known as replay attacks occurs when somebody (AttackerC) listening in on the authentication process between RouterA and RouterB intercepts the correct hash calculated by RouterB when it responds back to RouterA. Then when AttackerC disguises itself as RouterB to establish a connection with RouterA it can use the same authentication it stole from RouterB. To prevent this CHAP uses challenge value that changes every time it starts a new authentication session. So even if AttackerC did intercept the correct calculated has value from RouterB it wouldn’t matter because the next time AttackerC tried to authenticate with RouterA the the challenge value would be different. This challenge value is unique every time and is unpredictable preventing AttackerC from ever being able to authenticate with RouterA using the calculated hash value from RouterB.
Compression
PPP has two different types of compression techniques stacker and predictor. Compressing your data before you send it off can really help increase sending speeds especially for low bandwidth links. When applying any type of compression on Cisco routers you need to keep in mind the available bandwidth and memory on the router. Stacker requires more CPU power than memory, while predictor requires more memory than CPU power. Stacker supports the HDLC, PPP, LAPB, and X.25 protocols while predictor only supports PPP and LAPB.
Default Encapsulation
Keep in mind that the default encapsulation type cisco uses for serial connections is HDLC. Some other encapsulation types include PPP, SDLC, LAPB, Frame Relay, X.25, and ATM.
Network Control Protocol
One advantage to using PPP is that it allows support for several different Layer 3 Network Protocols at the same time. It supports IP, IPX, and AppleTalk. The Network Control Protocol (NCP) takes care of all the Layer 3 information.
Recap
Here is a list of all the terms that you should have learned about PPP which you will be required to know and understand for the CCNA Exam and ICND2 Exam:
PPP, LCP, NCP, Asynchronous Serial, Synchronous Serial, PAP, CHAP, Stacker, Predictor, HDLC.
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