Pointto Point Protocol

The International Telecommunications Union (ITU), then known as the Consultative Committee for International Telecommunications Technologies (CCITT), first defined HDLC. Later, the Internet Engineering Task Force (IETF) saw the need for another data-link protocol for use between routers over a point-to-point link. In RFC 1661, the IETF created the Point-to-Point Protocol (PPP).

Comparing the basics, PPP behaves exactly like HDLC. The framing looks identical. There is an address field, but the addressing does not matter. PPP does discard errored frames that do not pass the FCS check. And PPP uses a 2-byte Protocol Type field—although PPP's Protocol Type field is defined by the protocol, as opposed to being a Cisco proprietary feature added later.

PPP was defined much later than the original HDLC specifications. As a result, the creators of PPP included many additional features that had not been seen in WAN data-link protocols up to that time. As a result, PPP has become the most popular and feature-rich of WAN data link layer protocols.

PPP-unique features fall into two main categories:

■ Those needed regardless of the Layer 3 protocol sent across the link

■ Those specific to each Layer 3 protocol

So, the PPP specifications actually include several different protocols. One protocol, the PPP Link Control Protocol (LCP), focuses on the features that apply regardless of the Layer 3 protocol used. LCP performs most of its work when the line comes up, so it has a lot more work to do with dialed links, which come up and down a lot, versus leased lines, which hopefully seldom fail.

PPP also defines several control protocols (CPs), which are used for any special purposes for a particular Layer 3 protocol. For instance, the IP Control Protocol (IPCP) provides for IP address assignment over a PPP link. When a user dials a new connection to an ISP using a modem, PPP typically is used, with IPCP assigning an IP address to the remote PC.

Each link that uses PPP has one LCP per link and one CP for each Layer 3 protocol defined on the link. If a router is configured for IPX, AppleTalk, and IP on a PPP serial link, the router configured for PPP encapsulation automatically tries to bring up the appropriate control protocols for each Layer 3 protocol.

LCP provides a variety of optional features for PPP besides just managing the link. You should at least be aware of the concepts behind these features, as summarized in Table 4-5.

Table 4-5 PPP LCP Features

Function

LCP Feature

Description

Error detection

Link quality monitoring (LQM)

PPP can take down a link based on the percentage of errors on the link using LQM.

Looped link detection

Magic number

The telco might reflect the data that a router sends it back to the router, to test a circuit. PPP uses a feature called magic numbers to detect a looped link and takes down the link.

Multilink support

Multilink PPP

This allows multiple parallel serial links to be connected between the same two routers, balancing traffic across the links.

Authentication

PAP and CHAP

Particularly useful for dial-up links, PPP initiates an authentication process to verify the identity of the device on the other end of the serial link.

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