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The main purpose of this TCS was to familiarize the students with the PPP encapsulation type. They learned how it functioned, verification protocols, and how to implement PPP onto a router. There were also other learning objectives which had to be completed.

The main learning objectives of the TCS were the following:

1. Describe what layer of the OSI the Point to Point Protocol operates at and what is its function at those layers.

PPP uses a layered architecture. With its lower-level functions, PPP can use:

• Synchronous physical media like those that connect ISDN.

• Asynchronous physical media like those that use basic telephone service for modem dialup connections.

PPP offers a rich set of services that control setting up a data link. These services are options in LCP and are primarily negotiation and checking frames to implement the point-to-point controls an administrator specifies for the call.

With its higher-level functions, PPP carries packets from several network-layer protocols in NCPs. These are functional fields containing standardized codes to indicate the network-layer protocol type that PPP encapsulates.

2. Describe what major types of WAN links are supported by PPP.

PPP runs on the following types of WAN physical interfaces:

• Asynchronous Serial

• ISDN

• Synchronous Serial

PPP uses another of its major components, the Link Control Protocol (LCP), to negotiate and set up control options on the WAN data link. PPP uses its Network Control Programs (NCPs) component to encapsulate multiple protocols.

PPP datagram transmission employs three key components to provide effective data transmission:

• Encapsulation - PPP supports the High-Level Data Link Control (HDLC) protocol to provide encapsulation.

• Link Control Protocol (LCP) - An extensible LCP is used to establish, configure, and test the data link connection.

• Network Control Protocols (NCPs) - A family of NCPs are used to establish and configure different network layer protocols.

3. Describe the functions of PAP and CHAP in a PPP implementation.

CHAP is used at the startup of a link, and periodically, to verify the identity of the remote node using a three-way handshake. CHAP is done upon initial link establishment and can be repeated any time after the link has been established.

After the PPP link establishment phase is complete, the local router sends a "challenge" message to the remote node. The remote node responds with a value calculated using a one-way hash function (typically MD5). The local router checks the response against its own calculation of the expected hash value. If the values match, the authentication is acknowledged. Otherwise, the connection is terminated immediately.

CHAP provides protection against playback attack through the use of a variable challenge value that is unique and unpredictable. The use of repeated challenges is intended to limit the time of exposure to any single attack. The local router (or a third-party authentication server such as TACACS)is in control of the frequency and timing of the challenges.

4. How to implement PPP on a router.

Prompt		Command
router-a>		enable
Password:		cisco
router-a#		config t
router-a(config)#		config int s0
router-a(config-if)#		encapsulation ppp
router-a(config-if)#		exit
router-a(config)#		exit
router-a#		copy run start

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