computer science

Point-to-Point Protocol

Point-to-Point Protocol is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Point-to-Point Protocol rather than just read about it. In short: In computer networking, Point-to-Point Protocol (PPP) is a data link layer (layer 2) communication protocol between two routers directly without any host or any other networking in between. It can provide loop detection, authentication, transmission encryption, and data compression.

Key takeaways

Point-to-Point Protocol belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Point-to-Point Protocol to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Point-to-Point Protocol from memory before moving on to harder problems.

Reference excerpt

In computer networking, Point-to-Point Protocol (PPP) is a data link layer (layer 2) communication protocol between two routers directly without any host or any other networking in between. It can provide loop detection, authentication, transmission encryption, and data compression. PPP is used over many types of physical networks, including serial cable (PPPoS), phone line, trunk line, cellular telephone, specialized radio links, ISDN, and fiber optic links such as SONET. Since IP packets cannot be transmitted over a modem line on their own without some data link protocol that can identify where the transmitted frame starts and where it ends, Internet service providers (ISPs) have used PPP for customer dial-up access to the Internet. PPP is used on former dial-up networking lines. Two derivatives of PPP, Point-to-Point Protocol over Ethernet (PPPoE) and Point-to-Point Protocol over ATM (PPPoA), are used most commonly by ISPs to establish a digital subscriber line (DSL) Internet service LP connection with customers.

Description PPP is very commonly used as a data-link-layer protocol for connection over synchronous and asynchronous circuits, where it has largely superseded the older Serial Line Internet Protocol (SLIP) and telephone company mandated standards (such as Link Access Protocol, Balanced (LAPB) in the X.25 protocol suite). The only requirement for PPP is that the circuit provided be duplex. PPP was made to work with numerous network-layer protocols, including Internet Protocol (IP), TRILL, Novell's Internetwork Packet Exchange (IPX), NBF, DEC net and AppleTalk. Like SLIP, PPP enables a full Internet connection over telephone lines via modem. It is more reliable than SLIP because it double checks to ensure Internet packets arrive intact, resending any damaged packets. PPP was designed somewhat after the original HDLC specifications. The people who had designed PPP included many additional features that had been seen only in proprietary data-link protocols up to that time. PPP is specified in RFC 1661. RFC 2516 describes Point-to-Point Protocol over Ethernet (PPPoE) as a method for transmitting PPP over Ethernet that is sometimes used with DSL. RFC 2364 describes Point-to-Point Protocol over ATM (PPPoA) as a method for transmitting PPP over ATM Adaptation Layer 5 (AAL5), which is also a common alternative to PPPoE used with DSL. PPP, PPPoE and PPPoA are widely used in WAN lines. PPP is a layered protocol that has three components:

An encapsulation component that is used to transmit datagrams over the specified physical layer. A Link Control Protocol (LCP) to establish, configure, and test the link as well as negotiate settings, options and the use of features. One or more Network Control Protocols (NCP) used to negotiate optional configuration parameters and facilities for the network layer. There is one NCP for each higher-layer protocol supported by PPP.

Automatic self configuration LCP initiates and terminates connections gracefully, allowing hosts to negotiate connection options. It is an integral part of PPP, and is defined in the same standard specification. LCP provides automatic configuration of the interfaces at each end (such as setting datagram size, escaped characters, and magic numbers) and for selecting optional authentication. The LCP protocol runs on top of PPP (with PPP protocol number 0xC021) and therefore a basic PPP connection has to be established before LCP is able to configure it. RFC 1994 describes Challenge-Handshake Authentication Protocol (CHAP), which is preferred for establishing dial-up connections with ISPs. Although deprecated, Password Authentication Protocol (PAP) is still sometimes used. Another option for authentication over PPP is Extensible Authentication Protocol (EAP) described in RFC 2284. After the link has been established, additional network (layer 3) configuration may take place. Most commonly, the Internet Protocol Control Protocol (IPCP) is used, although Internetwork Packet Exchange Control Protocol (IPXCP) and AppleTalk Control Protocol (ATCP) were once popular. Internet Protocol Version 6 Control Protocol (IPv6CP) will see extended use in the future, when IPv6 replaces IPv4 as the dominant layer-3 protocol.

Multiple network layer protocols

PPP permits multiple network layer protocols to operate on the same communication link. For every network layer protocol used, a separate Network Control Protocol (NCP) is provided in order to encapsulate and negotiate options for the multiple network layer protocols. It negotiates network-layer information, e.g. network address or compression options, after the connection has been established. For example, IP uses IPCP, and Internetwork Packet Exchange (IPX) uses the Novell IPX Control Protocol (IPX/SPX). NCPs include fields containing standardized codes to indicate the network layer protocol type that the PPP connection encapsulates. The following NCPs may be used with PPP:

IPCP for IP, protocol code number 0x8021, RFC 1332 the OSI Network Layer Control Protocol (OSINLCP) for the various OSI network layer protocols, protocol code number 0x8023, RFC 1377 the AppleTalk Control Protocol (ATCP) for AppleTalk, protocol code number 0x8029, RFC 1378 the Internetwork Packet Exchange Control Protocol (IPXCP) for the Internet Packet Exchange, protocol code number 0x802B, RFC 1552 the DEC net Phase IV Control Protocol (DNCP) for DNA Phase IV Routing protocol (DEC net Phase IV), protocol code number 0x8027, RFC 1762 the NetBIOS Frames Control Protocol (NBFCP) for the NetBIOS Frames protocol (or NetBEUI as it was called before that), protocol code number 0x803F, RFC 2097 the IPv6 Control Protocol (IPV6CP) for IPv6, protocol code number 0x8057, RFC 5072

Looped link detection PPP detects looped links using a feature involving magic numbers. When the node sends PPP LCP messages, these messages may include a magic number. If a line is looped, the node receives an LCP message with its own magic number, instead of getting a message with the peer's magic number.

Configuration options The previous section introduced the use of LCP options to meet specific WAN connection requirements. PPP may include the following LCP options:

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Point-to-Point Protocol

Start with the simplest possible case. Write down what Point-to-Point Protocol claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Point-to-Point Protocol before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Point-to-Point Protocol ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Point-to-Point Protocol

In research: Point-to-Point Protocol appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Point-to-Point Protocol in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Point-to-Point Protocol is common in secondary-school and first-year university syllabi. It links to neighbouring topics Internet Standards, Link protocols, Logical link control, so understanding it makes those chapters shorter.
In everyday life: Look for Point-to-Point Protocol outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Point-to-Point Protocol in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Point-to-Point Protocol means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Point-to-Point Protocol out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Point-to-Point Protocol in simple terms?

Why does Point-to-Point Protocol matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Point-to-Point Protocol?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Point-to-Point Protocol.