In networking A computer network, often simply referred to as a network, is a collection of computers and devices connected by communications channels that facilitates communications among users and allows users to share resources with other users. Networks may be classified according to a wide variety of characteristics. This article provides a general, the Point-to-Point Protocol, or PPP, is a data link The Data Link Layer is Layer 2 of the seven-layer OSI model of computer networking. It corresponds to or is part of the link layer of the TCP/IP reference model protocol In computing and telecommunications, a protocol or communications protocol is a formal description of message formats and the rules for exchanging those messages. Protocols may include signaling, authentication and error detection and correction capabilities. In its simplest form, a protocol can be defined as the rules governing the syntax, commonly used to establish a direct connection between two networking nodes In communication networks, a node is a connection point, either a redistribution point or a communication endpoint (some terminal equipment). The definition of a node depends on the network and protocol layer referred to. A physical network node is an active electronic device that is attached to a network, and is capable of sending, receiving, or. It can provide connection authentication Authentication is the act of establishing or confirming something (or someone) as authentic, that is, that claims made by or about the subject are true ("authentification" is a French language variant of this word). This might involve confirming the identity of a person, tracing the origins of an artifact, ensuring that a product is what, transmission encryption In cryptography, encryption is the process of transforming information using an algorithm (called cipher) to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information (in cryptography, referred to as ciphertext). In many contexts, the word encryption privacy, and compression In computer science and information theory, data compression or source coding is the process of encoding information using fewer bits than an unencoded representation would use, through use of specific encoding schemes.

PPP is used over many types of physical networks including serial cable, phone line A telephone line or telephone circuit is a single-user circuit on a telephone communications system. Typically this refers to the physical wire or other signaling medium connecting the user's telephone apparatus to the telecommunications network, and usually also implies a single telephone number for billing purposes reserved for that user, trunk line In modern communications, trunking is a concept by which a communications system can provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. This is analogous to the structure of a tree with one trunk and many branches. Examples of this include telephone systems and the VHF radios, cellular telephone A mobile phone or mobile is an electronic device used for mobile telecommunications (mobile telephone, text messaging or data transmission) over a cellular network of specialized base stations known as cell sites. Mobile phones differ from cordless telephones, which only offer telephone service within limited range, e.g. within a home or an office,, specialized radio links, and fiber optic links such as SONET Synchronous Optical Networking or Synchronous Digital Hierarchy (SDH) are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or light-emitting diodes (LEDs). Lower rates can also be transferred via an electrical interface. The method was developed to replace the Plesiochronous Digital. Most Internet service providers An Internet service provider , also sometimes referred to as an Internet access provider (IAP), is a company that offers its customers access to the Internet[citation needed]. The ISP connects to its customers using a data transmission technology appropriate for delivering Internet Protocol Paradigm, such as dial-up, DSL, cable modem, wireless or (ISPs) use PPP for customer dial-up access Dial-up Internet access is a form of Internet access that uses telephone lines. The user's computer or router uses an attached modem connected to a telephone line to dial into an Internet service provider's node to establish a modem-to-modem link, which is then used to route Internet Protocol packets between the user's equipment and hosts to the Internet The Internet is a global system of interconnected computer networks that use the standard Internet Protocol Suite to serve billions of users worldwide. It is a network of networks that consists of millions of private, public, academic, business, and government networks of local to global scope that are linked by a broad array of electronic and. Two encapsulated forms of PPP, Point-to-Point Protocol over Ethernet (PPPoE) and Point-to-Point Protocol over ATM (PPPoA), are used by Internet Service Providers (ISPs) to connect Digital Subscriber Line Digital Subscriber Line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term Digital Subscriber Line is widely understood to mean Asymmetric Digital Subscriber Line (ADSL), the most commonly (DSL) Internet service.

PPP is commonly used as a data link layer The Data Link Layer is Layer 2 of the seven-layer OSI model of computer networking. It corresponds to or is part of the link layer of the TCP/IP reference model protocol for connection over synchronous Synchronization or synchronisation is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time. Systems operating with all their parts in synchrony are said to be synchronous or in sync. Some systems may be only approximately synchronized, or and asynchronous In specific terms of digital logic and physical layer of communication, an asynchronous process does not require a clock signal. Contrast with plesiochronous systems circuits, where it has largely superseded the older, non-standard Serial Line Internet Protocol (SLIP) and telephone company mandated standards (such as Link Access Protocol, Balanced (LAPB) in the X.25 X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange (PSE) nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links. X.25 is a family of protocols that was used especially during the 198 protocol suite). PPP was designed to work with numerous network layer The Network Layer is responsible for end-to-end packet delivery including routing through intermediate hosts, whereas the Data Link Layer is responsible for node-to-node (hop-to-hop) frame delivery on the same link protocols, including Internet Protocol The Internet Protocol is a protocol used for communicating data across a packet-switched internetwork using the Internet Protocol Suite, also referred to as TCP/IP (IP), Novell's Internetwork Packet Exchange (IPX), NBF NetBIOS Frames or NBF protocol is a non-routable network- and transport-level data protocol most commonly used as one of the layers of Microsoft Windows networking in the 1990s. NBF protocol or NetBIOS over IEEE 802.2 LLC is used by a number of network operating systems released in the 1990s, such as LAN Manager, LAN Server, Windows for Workgroups, and AppleTalk AppleTalk is a proprietary suite of protocols developed by Apple Inc. for networking computers. It was included in the original Macintosh released in 1984, and is now unsupported with the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking. AppleTalk's Datagram Delivery Protocol corresponds closely to the Network layer of the Open.

PPP is also used over broadband The term broadband refers to a telecommunications signal of greater bandwidth, in some sense, than another standard or usual signal. Different criteria for "broad" have been applied in different contexts and at different times connections. RFC 2516 describes Point-to-Point Protocol over Ethernet (PPPoE), a method for transmitting PPP over Ethernet Ethernet is a family of frame-based computer networking technologies for local area networks . The name came from the physical concept of the ether. It defines a number of wiring and signaling standards for the Physical Layer of the OSI networking model as well as a common addressing format and Media Access Control at the Data Link Layer that is sometimes used with DSL Digital Subscriber Line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term Digital Subscriber Line is widely understood to mean Asymmetric Digital Subscriber Line (ADSL), the most commonly. RFC 2364 describes Point-to-Point Protocol over ATM (PPPoA), a method for transmitting PPP over ATM Asynchronous Transfer Mode is a standardized digital data transmission technology. ATM is implemented as a network protocol and was first developed in the mid 1980s. The goal was to design a single networking strategy that could transport real-time video conference and audio as well as image files, text and email. The International Adaptation Layer 5 (AAL5), which is also sometimes used with DSL.

PPP is specified in RFC In computer network engineering, a Request for Comments is a memorandum published by the Internet Engineering Task Force (IETF) describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems 1661.^[1]

Basic Features

PPP was designed somewhat after the original HDLC specifications. The designers of PPP included many additional features that had been seen only in proprietary data-link protocols up to that time.

Automatic self configuration

Link Control Protocol (LCP) 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 A datagram is a basic transfer unit associated with a packet-switched network in which the delivery, arrival time and order are not guaranteed. A datagram consists of header and data areas, where the header contains information sufficient for routing from the originating equipment to the destination without relying on prior exchanges between the 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 In computing, the Challenge-Handshake Authentication Protocol authenticates a user or network host to an authenticating entity. That entity may be, for example, an Internet access provider (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 Extensible Authentication Protocol, or EAP, is a universal authentication framework frequently used in wireless networks and Point-to-Point connections. It is defined in RFC 3748, which has been updated by RFC 5247. Although the EAP protocol is not limited to wireless LANs and can be used for wired LAN authentication, it is most often used in (EAP).^[2]

After the link has been established, additional network (layer 3 The Network Layer is responsible for end-to-end packet delivery including routing through intermediate hosts, whereas the Data Link Layer is responsible for node-to-node (hop-to-hop) frame delivery on the same link) 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 very popular.^{[citation needed]} Internet Protocol Version 6 Control Protocol (IPv6CP) will see extended use in the future, when IPv6 Internet Protocol version 6 is an Internet Protocol version which is designed to succeed IPv4, the first implementation which is still in dominant use currently[update]. It is an Internet Layer protocol for packet-switched internetworks. The main driving force for the redesign of Internet Protocol is the foreseeable IPv4 address exhaustion. IPv6 replaces IPv4 Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet. IPv4 is still by far the most widely deployed Internet Layer protocol. As of 2010['s position 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.

For example, Internet Protocol (IP) uses the IP Control Protocol (IPCP), and Internetwork Packet Exchange (IPX) uses the Novell IPX Control Protocol (IPXCP). NCPs include fields containing standardized codes to indicate the network layer protocol type that the PPP connection encapsulates.

Looped link detection

PPP detects looped links using a feature involving magic numbers This type of magic number was initially found in early Seventh Edition source code of the Unix operating system and, although it has lost its original meaning, the term magic number has become part of computer industry lexicon. 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.

Most important features

• Link Control Protocol initiates and terminates connections gracefully, allowing hosts to negotiate connection options. It also supports both byte- and bit-oriented encodings.^{[citation needed]}
• Network Control Protocol is used for negotiating network-layer information, e.g. network address or compression options, after the connection has been established.

PPP Configuration Options

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

Authentication - Peer routers exchange authentication messages. Two authentication choices are Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP). Authentication is explained in the next section.

Compression - Increases the effective throughput on PPP connections by reducing the amount of data in the frame that must travel across the link. The protocol decompresses the frame at its destination. Two compression protocols available in Cisco routers are Stacker and Predictor.

Error detection - Identifies fault conditions. The Quality and Magic Number options help ensure a reliable, loop-free data link. The Magic Number field helps in detecting links that are in a looped-back condition. Until the Magic-Number Configuration Option has been successfully negotiated, the Magic-Number must be transmitted as zero. Magic numbers are generated randomly at each end of the connection.

Multilink - Cisco IOS Release 11.1 and later supports multilink PPP. This alternative provides load balancing over the router interfaces that PPP uses. Multilink PPP (also referred to as MLPPP, MP, MPPP, MLP, or Multilink) provides a method for spreading traffic across multiple distinct PPP connections.

PPP frame

The Protocol field indicates the type of payload packet (e.g. LCP, NCP, IP, IPX, AppleTalk, etc.).

The Information field contains the PPP payload; it has a variable length with a negotiated maximum called the Maximum Transmission Unit. By default, the maximum is 1500 octets Octet refers to an entity having exactly eight bits. As such, it is often used where the term byte might be ambiguous. For that reason, computer networking standards almost exclusively use octet. It is prominently used in Requests for Comments published by the Internet Engineering Task Force. The earliest example is RFC 635 from 1974. In France,. It might be padded on transmission; if the information for a particular protocol can be padded, that protocol must allow information to be distinguished from padding.

Encapsulation

PPP frames are encapsulated in a lower-layer protocol that provides framing and may provide other functions such as a checksum A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and comparing it with the stored one. If the checksums to detect transmission errors. PPP on serial links is usually encapsulated in a framing similar to HDLC, described by IETF RFC 1662.

The Flag field is present when PPP with HDLC-like framing is used.

The Address and Control fields always have the value hex FF (for "all stations") and hex 03 (for "unnumbered information"), and can be omitted whenever PPP LCP Address-and-Control-Field-Compression (ACFC) is negotiated.

The Frame Check Sequence (FCS) field is used to determine whether an individual frame has an error. It contains a checksum A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and comparing it with the stored one. If the checksums computed over the frame to provide basic protection against errors in transmission. This is a CRC A cyclic redundancy check or polynomial code checksum is a non-secure hash function designed to detect accidental changes to raw computer data, and is commonly used in digital networks and storage devices such as hard disk drives. A CRC-enabled device calculates a short, fixed-length binary sequence, known as the CRC code or just CRC, for each code similar to the one used for other layer two protocol error protection schemes such as the one used in Ethernet. According to RFC 1662, it can be either 16 bits (2bytes) or 32 bits (4 bytes) in size (default is 16 bits - Polynomial x¹⁶ + x¹² + x⁵ + 1).

The FCS is calculated over the Address, Control, Protocol, Information and Padding fields after the message have been escaped.

PPP line activation and phases

The phases of the Point to Point Protocol according to RFC 1661 are listed below:

• Link Dead. This phase occurs when the link fails, or one side has been told not to connect (e.g. a user has finished his or her dialup connection.)
• Link Establishment Phase. This phase is where Link Control Protocol negotiation is attempted. If successful, control goes either to the authentication phase or the Network-Layer Protocol phase, depending on whether authentication is desired.
• Authentication Phase. This phase is optional. It allows the sides to authenticate each other before a connection is established. If successful, control goes to the network-layer protocol phase.
• Network-Layer Protocol Phase. This phase is where each desired protocols' Network Control Protocols are invoked. For example, IPCP is used to establish IP service over the line. Data transport for all protocols which are successfully started with their network control protocols also occurs in this phase. Closing down of network protocols also occur in this phase.
• Link Termination Phase. This phase closes down this connection. This can happen if there is an authentication failure, if there are so many checksum errors that the two parties to the link decide to tear down the link automatically, if the link suddenly fails, or if the user decides to hang up his connection. This phase tries to close everything down as gracefully as possible depending on the circumstances.

Multiclass PPP

MP's monotonically increasing sequence numbering (contiguous numbers are needed for all fragments of a packet) does not allow suspension of the sending of a sequence of fragments of one packet in order to send another packet. The obvious approach to providing more than one level of suspension with PPP Multilink is to run Multilink multiple times over one link. Multilink as it is defined provides no way for more than one instance to be active. Each class runs a separate copy of the mechanism defined i.e. uses a separate sequence number space and reassembly buffer. See RFC 2686.

Other features

Numerous documents on PPP have been published through the RFC process since July 1990, including various authentication, encryption, and compression methods, and the use of PPP in conjunction with other network protocols.

RFC 2615 is also used in Packet over SONET/SDH (PoS) transmissions.

PPTP is a form of PPP between two hosts via GRE. It is often used to set up a VPN A virtual private network is a network that uses a public telecommunication infrastructure, such as the Internet, to provide remote offices or individual users with secure access to their organization's network. It aims to avoid an expensive system of owned or leased lines that can only be used by one organization. The goal of a VPN is to provide, with optional encryption (MPPE) or compression (MPPC).

RFCs

PPP is defined in RFC 1661 (The Point-to-Point Protocol, July 1994). RFC 1547 (Requirements for an Internet Standard Point-to-Point Protocol, December 1993) provides historical information about the need for PPP and its development. A series of related RFCs have been written to define how a variety of network control protocols-including TCP/IP, DECnet, AppleTalk, IPX, and others-work with PPP.^[3]

• RFC 1661, Standard 51, The Point-to-Point Protocol (PPP)
• RFC 1662, Standard 51, PPP in HDLC-like Framing
• RFC 1994, PPP Challenge Handshake Authentication Protocol (CHAP)
• RFC 2153, Informational, PPP Vendor Extensions
• RFC 2687, Proposed Standard, PPP in a Real-time Oriented HDLC-like Framing
• RFC 5072, IP Version 6 over PPP
• RFC 5172, Negotiation for IPv6 Datagram Compression Using IPv6 Control Protocol

References

1. ^ http://www.ietf.org/rfc/rfc1661.txt
2. ^ RFC2284 - PPP Extensible Authentication Protocol (EAP)
3. ^ PPP (Point-to-Point Protocol) (Linktionary term)

See also

• Extensible Authentication Protocol
• Hayes command set
• Link Access Procedure for Modems (LAPM)
• RADIUS
• Diameter
• PPPoA
• PPPoE
• Point-to-Point Protocol daemon (PPPD)
• PPPoX
• Multiprotocol Encapsulation (MPE) for MPEG transport stream
• Unidirectional Lightweight Encapsulation (ULE) for MPEG transport stream

Categories: Link protocols | Logical Link Control | Internet standards | Modems

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