Point-to-Point Protocol
Encyclopedia
In networking
, the Point-to-Point Protocol (PPP) is a data link
protocol commonly used in establishing a direct connection between two networking nodes
. It can provide connection authentication
, transmission encryption
, and compression
.
PPP is used over many types of physical networks including serial cable
, phone line, trunk line
, cellular telephone, specialized radio links, and fiber optic links such as SONET
.
PPP is also used over Internet access
connections (now marketed as "broadband").
Internet service provider
s (ISPs) have used PPP for customer dial-up access
to the Internet
, since IP packets cannot be transmitted over a modem
line on their own, without some data link protocol. Two encapsulated forms of PPP, Point-to-Point Protocol over Ethernet
(PPPoE) and Point-to-Point Protocol over ATM
(PPPoA), are used most commonly by Internet Service Providers (ISPs) to establish a Digital Subscriber Line
(DSL) Internet service connection with customers.
PPP is 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circuits, where it has largely superseded the older Serial Line Internet Protocol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). PPP was designed to work with numerous network layer
protocols, including Internet Protocol
(IP), TRILL
, Novell's Internetwork Packet Exchange (IPX), NBF and AppleTalk
.
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.
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 is specified in RFC
1661.
(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 very popular. Internet Protocol Version 6 Control Protocol (IPv6CP) will see extended use in the future, when IPv6
replaces IPv4
's position as the dominant layer-3 protocol.
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, Internet Protocol (IP) uses the IP Control Protocol (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.
. 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.
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
. 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.
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 for determining whether an individual frame has an error. It contains a checksum
computed over the frame to provide basic protection against errors in transmission. This is a CRC
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 (2 bytes) 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.
On a single PPP line frames cannot arrive out of order, but this is possible when the frames are divided among multiple PPP connections. Therefore Multilink PPP must number the fragments so they can be put in the right order again when they arrive.
Multilink PPP is an example of a link aggregation
technology. Cisco IOS Release 11.1 and later supports Multilink PPP.
That's not possible with Multilink PPP either. Multilink PPP uses contiguous numbers for all the fragments of a packet, and as a consequence it is not possible to suspend the sending of a sequence of fragments of one packet in order to send another packet. This prevents from running Multilink PPP multiple times on the same links.
Multiclass PPP is a kind of Multilink PPP where each "class" of traffic uses a separate sequence number space and reassembly buffer. Multiclass PPP is defined in RFC 2686.
using encryption (MPPE) and compression (MPPC
).
data over IP networks. Some of them, like SSL, SSH
, or L2TP
create virtual network interface
s and give the impression a direct physical connections between the tunnel endpoints. On a Linux
host for example, these interfaces would be called tun0.
As there are only two endpoints on a tunnel, the tunnel is a point-to-point connection and PPP is a natural choice as a data link layer protocol between the virtual network interfaces. PPP can assign IP addresses to these virtual interfaces, and these IP addresses can be used, for example, to route between the networks on both sides of the tunnel.
IPsec
in tunneling mode does not create virtual physical interfaces at the end of the tunnel, since the tunnel is handled directly by the TCP/IP stack. L2TP
can be used to provide these interfaces, this technique is called L2TP/IPsec. In this case too, PPP provides IP addresses to the extremities of the tunnel.
, AppleTalk
, IPX
, and others-work with PPP.
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
, the Point-to-Point Protocol (PPP) is a data link
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 commonly used in establishing a direct connection between two networking nodes
Node (networking)
In communication networks, a node is a connection point, either a redistribution point or a communication endpoint . The definition of a node depends on the network and protocol layer referred to...
. It can provide connection authentication
Authentication
Authentication is the act of confirming the truth of an attribute of a datum or entity...
, transmission encryption
Encryption
In cryptography, encryption is the process of transforming information using an algorithm 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...
, and compression
Data compression
In computer science and information theory, data compression, source coding or bit-rate reduction is the process of encoding information using fewer bits than the original representation would use....
.
PPP is used over many types of physical networks including serial cable
Serial cable
A serial cable is a cable that can be used to transfer information between two devices using serial communication. The form of connectors depends on the particular PHY used...
, phone line, trunk line
Trunking
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...
, cellular telephone, specialized radio links, and fiber optic links such as SONET
Sonet
Sonet may refer to:* Sonet Records, European record label* Synchronous optical networking * Saab Sonett...
.
PPP is also used over Internet access
Internet access
Many technologies and service plans for Internet access allow customers to connect to the Internet.Consumer use first became popular through dial-up connections in the 20th century....
connections (now marketed as "broadband").
Internet service provider
Internet service provider
An Internet service provider is a company that provides access to the Internet. Access ISPs directly connect customers to the Internet using copper wires, wireless or fiber-optic connections. Hosting ISPs lease server space for smaller businesses and host other people servers...
s (ISPs) have used PPP for customer dial-up access
Dial-up access
Dial-up Internet access is a form of Internet access that uses the facilities of the public switched telephone network to establish a dialled connection to an Internet service provider via telephone lines...
to the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
, since IP packets cannot be transmitted over a modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
line on their own, without some data link protocol. Two encapsulated forms of PPP, Point-to-Point Protocol over Ethernet
Point-to-Point Protocol over Ethernet
The Point-to-Point Protocol over Ethernet is a network protocol for encapsulating Point-to-Point Protocol frames inside Ethernet frames. It is used mainly with DSL services where individual users connect to the DSL modem over Ethernet and in plain Metro Ethernet networks...
(PPPoE) and Point-to-Point Protocol over ATM
Point-to-Point Protocol over ATM
The Point-to-Point Protocol over ATM is a network protocol for encapsulating PPP frames in AAL5. It is used mainly with DOCSIS and DSL carriers....
(PPPoA), are used most commonly by Internet Service Providers (ISPs) to establish a Digital Subscriber Line
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 DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
(DSL) Internet service connection with customers.
PPP is commonly used as a data link layer
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
Synchronization 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....
and asynchronous
Asynchrony
Asynchrony, in the general meaning, is the state of not being synchronized.* Asynchronous learning* Collaborative editing systemsIn specific terms of digital logic and physical layer of communication, an asynchronous process does not require a clock signal, in contrast with synchronous and...
circuits, where it has largely superseded the older Serial Line Internet Protocol
Serial Line Internet Protocol
The Serial Line Internet Protocol is an encapsulation of the Internet Protocol designed to work over serial ports and modem connections. It is documented in RFC 1055...
(SLIP) and telephone company mandated standards (such as Link Access Protocol, Balanced
LAPB
Link Access Procedure, Balanced implements the data link layer as defined in the X.25 protocol suite. LAPB is a bit-oriented protocol derived from HDLC that ensures that frames are error free and in the right sequence. LAPB is specified in and ISO/IEC 7776...
(LAPB) in the X.25
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 nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
protocol suite). PPP was designed to work with numerous network layer
Network layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
protocols, including Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP), TRILL
TRILL (computing)
TRILL is a IETF Standard implemented by devices called RBridges or Routing Bridges. TRILL combines the advantages of bridges and routers and is the application of link state routing to the VLAN-aware customer-bridging problem. RBridges are compatible with and can incrementally replace previous...
, Novell's Internetwork Packet Exchange (IPX), NBF and AppleTalk
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, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...
.
Description
| Application | FTP File Transfer Protocol File Transfer Protocol is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server... |
SMTP Simple Mail Transfer Protocol Simple Mail Transfer Protocol is an Internet standard for electronic mail transmission across Internet Protocol networks. SMTP was first defined by RFC 821 , and last updated by RFC 5321 which includes the extended SMTP additions, and is the protocol in widespread use today... |
HTTP Hypertext Transfer Protocol The Hypertext Transfer Protocol is a networking protocol for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web.... |
… | DNS Domain name system The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities... |
… |
| Transport | TCP Transmission Control Protocol The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP... |
UDP User Datagram Protocol The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring... |
||||
| Internet | IP Internet Protocol The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite... |
IPv6 IPv6 Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4... |
||||
| Network access | PPP | |||||
| PPPoE | PPPoA | |||||
| Ethernet Ethernet Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies.... |
ATM Asynchronous Transfer Mode Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that... |
|||||
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.
RFC 2516 describes Point-to-Point Protocol over Ethernet
Point-to-Point Protocol over Ethernet
The Point-to-Point Protocol over Ethernet is a network protocol for encapsulating Point-to-Point Protocol frames inside Ethernet frames. It is used mainly with DSL services where individual users connect to the DSL modem over Ethernet and in plain Metro Ethernet networks...
(PPPoE) as a method for transmitting PPP over Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
that is sometimes used with DSL
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 DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
. RFC 2364 describes Point-to-Point Protocol over ATM
Point-to-Point Protocol over ATM
The Point-to-Point Protocol over ATM is a network protocol for encapsulating PPP frames in AAL5. It is used mainly with DOCSIS and DSL carriers....
(PPPoA) as a method for transmitting PPP over ATM
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
Adaptation Layer 5 (AAL5), which is also a common alternative to PPPoE used with DSL.
PPP is specified in RFC
Request for Comments
In computer network engineering, a Request for Comments is a memorandum published by the Internet Engineering Task Force describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems.Through the Internet Society, engineers and...
1661.
Automatic self configuration
Link Control ProtocolLink Control Protocol
In computing, the Link Control Protocol forms part of the Point-to-Point Protocol. In setting up PPP communications, both the sending and receiving devices send out LCP packets to determine the standards of the ensuing data transmission...
(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
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....
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
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 service provider. CHAP is specified in RFC 1994....
(CHAP), which is preferred for establishing dial-up connections with ISPs.
Although deprecated, Password authentication protocol
Password authentication protocol
A password authentication protocol is an authentication protocol that uses a password.PAP is used by Point to Point Protocol to validate users before allowing them access to server resources. Almost all network operating system remote servers support PAP.PAP transmits unencrypted ASCII passwords...
(PAP) is still sometimes used.
Another option for authentication over PPP is Extensible Authentication Protocol
Extensible Authentication Protocol
Extensible Authentication Protocol, or EAP, is an authentication framework frequently used in wireless networks and Point-to-Point connections. It is defined in RFC 3748, which made RFC 2284 obsolete, and was updated by RFC 5247....
(EAP) described in RFC 2284.
After the link has been established, additional network (layer 3
Network layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
) configuration may take place. Most commonly, the Internet Protocol Control Protocol
Internet Protocol Control Protocol
In computer networking, Internet Protocol Control Protocol is a network control protocol for establishing and configuring Internet Protocol over a Point-to-Point Protocol link. IPCP uses the same packet exchange mechanism as the Link Control Protocol...
(IPCP) is used, although Internetwork Packet Exchange Control Protocol
IPX/SPX
IPX/SPX stands for Internetwork Packet Exchange/Sequenced Packet Exchange. IPX and SPX are networking protocols used primarily on networks using the Novell NetWare operating systems.-Protocol Layers:...
(IPXCP) and AppleTalk Control Protocol
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, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...
(ATCP) were once very popular. Internet Protocol Version 6 Control Protocol (IPv6CP) will see extended use in the future, when IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
replaces IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and 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...
's position as the dominant layer-3 protocol.
Multiple network layer protocols
| IP Internet Protocol The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite... |
|||
| LCP Link Control Protocol In computing, the Link Control Protocol forms part of the Point-to-Point Protocol. In setting up PPP communications, both the sending and receiving devices send out LCP packets to determine the standards of the ensuing data transmission... |
CHAP 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 service provider. CHAP is specified in RFC 1994.... PAP Password authentication protocol A password authentication protocol is an authentication protocol that uses a password.PAP is used by Point to Point Protocol to validate users before allowing them access to server resources. Almost all network operating system remote servers support PAP.PAP transmits unencrypted ASCII passwords... EAP Extensible Authentication Protocol Extensible Authentication Protocol, or EAP, is an authentication framework frequently used in wireless networks and Point-to-Point connections. It is defined in RFC 3748, which made RFC 2284 obsolete, and was updated by RFC 5247.... |
IPCP Internet Protocol Control Protocol In computer networking, Internet Protocol Control Protocol is a network control protocol for establishing and configuring Internet Protocol over a Point-to-Point Protocol link. IPCP uses the same packet exchange mechanism as the Link Control Protocol... |
|
| PPP encapsulation | |||
| HDLC High-Level Data Link Control High-Level Data Link Control is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization... -like Framing |
PPPoE Point-to-Point Protocol over Ethernet The Point-to-Point Protocol over Ethernet is a network protocol for encapsulating Point-to-Point Protocol frames inside Ethernet frames. It is used mainly with DSL services where individual users connect to the DSL modem over Ethernet and in plain Metro Ethernet networks... |
PPPoA Point-to-Point Protocol over ATM The Point-to-Point Protocol over ATM is a network protocol for encapsulating PPP frames in AAL5. It is used mainly with DOCSIS and DSL carriers.... |
|
| RS-232 RS-232 In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports... |
POS Packet over SONET/SDH Packet over SONET/SDH, abbreviated POS, is a communications protocol for transmitting packets in the form of the Point to Point Protocol over SDH or SONET, which are both standard protocols for communicating digital information using lasers or light emitting diodes over optical fibre at high... |
Ethernet Ethernet Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies.... |
ATM Asynchronous Transfer Mode Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that... |
| SONET/SDH Synchronous optical networking Synchronous Optical Networking and Synchronous Digital Hierarchy are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or highly coherent light from light-emitting diodes . At low transmission rates data can also be transferred via an... |
|||
PPP permits multiple network layer protocols to operate on the same communication link. For every network layer protocol used, a separate Network Control Protocol
Network Control Protocol
A Network Control Protocol is a protocol that runs atop the Point-to-Point Protocol and that is used to negotiate options for a network layer protocol running atop PPP...
(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, Internet Protocol (IP) uses the IP Control Protocol (IPCP), and Internetwork Packet Exchange (IPX) uses the Novell IPX Control Protocol (IPX/SPX
IPX/SPX
IPX/SPX stands for Internetwork Packet Exchange/Sequenced Packet Exchange. IPX and SPX are networking protocols used primarily on networks using the Novell NetWare operating systems.-Protocol Layers:...
). 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 numbersMagic number (programming)
In computer programming, the term magic number has multiple meanings. It could refer to one or more of the following:* A constant numerical or text value used to identify a file format or protocol; for files, see List of file signatures...
. 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.
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. See RFC 1962 for more details.
- 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 - Provides load balancing several interfaces used by PPP through Multilink PPP (see below).
Structure of a PPP frame
| Name | Number of bytes | Description |
|---|---|---|
| Protocol | 1 or 2 | setting of protocol in data field |
| Information | variable (0 or more) | datagram |
| Padding | variable (0 or more) | optional padding |
The Protocol field indicates the type of payload packet (e.g. LCP
Link Control Protocol
In computing, the Link Control Protocol forms part of the Point-to-Point Protocol. In setting up PPP communications, both the sending and receiving devices send out LCP packets to determine the standards of the ensuing data transmission...
, NCP
Network Control Protocol
A Network Control Protocol is a protocol that runs atop the Point-to-Point Protocol and that is used to negotiate options for a network layer protocol running atop PPP...
, IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
, IPX
IPX
Internetwork Packet Exchange is the OSI-model Network layer protocol in the IPX/SPX protocol stack.The IPX/SPXM protocol stack is supported by Novell's NetWare network operating system. Because of Netware's popularity through the late 1980s into the mid 1990s, IPX became a popular internetworking...
, AppleTalk
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, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...
, etc.).
The Information field contains the PPP payload; it has a variable length with a negotiated maximum called the Maximum Transmission Unit
Maximum transmission unit
In computer networking, the maximum transmission unit of a communications protocol of a layer is the size of the largest protocol data unit that the layer can pass onwards. MTU parameters usually appear in association with a communications interface...
. By default, the maximum is 1500 octets
Octet (computing)
An octet is a unit of digital information in computing and telecommunications that consists of eight bits. The term is often used when the term byte might be ambiguous, as there is no standard for the size of the byte.-Overview:...
. 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 checksumChecksum
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...
to detect transmission errors. PPP on serial links is usually encapsulated in a framing similar to HDLC, described by IETF RFC 1662.
| Name | Number of bytes | Description |
|---|---|---|
| Flag | 1 | indicates frame's begin or end |
| Address | 1 | broadcast address |
| Control | 1 | control byte |
| Protocol | 1 or 2 | setting of protocol in information field |
| Information | variable (0 or more) | datagram |
| Padding | variable (0 or more) | optional padding |
| FCS | 2 (or 4) | error check |
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
Frame Check Sequence
A frame check sequence refers to the extra checksum characters added to a frame in a communication protocol for error detection and correction. Frames are used to send upper-layer data and ultimately the user application data from a source to a destination. The data package includes the message...
(FCS) field is used for determining whether an individual frame has an error. It contains a checksum
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...
computed over the frame to provide basic protection against errors in transmission. This is a CRC
Cyclic redundancy check
A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
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 (2 bytes) 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 ProtocolNetwork Control ProtocolA Network Control Protocol is a protocol that runs atop the Point-to-Point Protocol and that is used to negotiate options for a network layer protocol running atop PPP...
s are invoked. For example, IPCPInternet Protocol Control ProtocolIn computer networking, Internet Protocol Control Protocol is a network control protocol for establishing and configuring Internet Protocol over a Point-to-Point Protocol link. IPCP uses the same packet exchange mechanism as the Link Control Protocol...
is used in establishing 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 decide to tear down the link automatically, if the link suddenly fails, or if the user decides to hang up his connection.
Multilink PPP
Multilink PPP (also referred to as MLPPP, MP, MPPP, MLP, or Multilink) provides a method for spreading traffic across multiple distinct PPP connections. It is defined in RFC 1990. It can be used, for example, to connect a home computer to an Internet Service Provider using two traditional 56k modems, or to connect a company through two leased lines.On a single PPP line frames cannot arrive out of order, but this is possible when the frames are divided among multiple PPP connections. Therefore Multilink PPP must number the fragments so they can be put in the right order again when they arrive.
Multilink PPP is an example of a link aggregation
Link aggregation
Link aggregation or trunking or link bundling or Ethernet/network/NIC bonding or NIC teaming are computer networking umbrella terms to describe various methods of combining multiple network connections in parallel to increase throughput beyond what a single connection could sustain, and to provide...
technology. Cisco IOS Release 11.1 and later supports Multilink PPP.
Multiclass PPP
With PPP, one cannot establish several simultaneous distinct PPP connections over a single link.That's not possible with Multilink PPP either. Multilink PPP uses contiguous numbers for all the fragments of a packet, and as a consequence it is not possible to suspend the sending of a sequence of fragments of one packet in order to send another packet. This prevents from running Multilink PPP multiple times on the same links.
Multiclass PPP is a kind of Multilink PPP where each "class" of traffic uses a separate sequence number space and reassembly buffer. Multiclass PPP is defined in RFC 2686.
PPP and tunnels
| Application | FTP File Transfer Protocol File Transfer Protocol is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server... |
SMTP Simple Mail Transfer Protocol Simple Mail Transfer Protocol is an Internet standard for electronic mail transmission across Internet Protocol networks. SMTP was first defined by RFC 821 , and last updated by RFC 5321 which includes the extended SMTP additions, and is the protocol in widespread use today... |
HTTP Hypertext Transfer Protocol The Hypertext Transfer Protocol is a networking protocol for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web.... |
… | DNS Domain name system The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities... |
… |
| Transport | TCP Transmission Control Protocol The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP... |
UDP User Datagram Protocol The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring... |
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| Network | IP Internet Protocol The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite... |
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| Data Link | PPP | |||||
| Application | SSH | |||||
| Transport | TCP Transmission Control Protocol The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP... |
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| Network | IP Internet Protocol The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite... |
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| Data Link | Ethernet Ethernet Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies.... |
ATM Asynchronous Transfer Mode Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that... |
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| Physical | Cables, NICs, and so on | |||||
Derived protocols
PPTP is a form of PPP between two hosts via GREGeneric Routing Encapsulation
Generic Routing Encapsulation is a tunneling protocol developed by Cisco Systems that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links over an Internet Protocol internetwork.-Overview:...
using encryption (MPPE) and compression (MPPC
Microsoft Point-to-Point Compression
Microsoft Point-to-Point Compression is a streaming data compression algorithm based on an implementation of Lempel–Ziv using a sliding window buffer...
).
PPP as a layer 2 protocol between both ends of a tunnel
Many protocols can be used to tunnelTunneling protocol
Computer networks use a tunneling protocol when one network protocol encapsulates a different payload protocol...
data over IP networks. Some of them, like SSL, SSH
Secure Shell
Secure Shell is a network protocol for secure data communication, remote shell services or command execution and other secure network services between two networked computers that it connects via a secure channel over an insecure network: a server and a client...
, or L2TP
L2TP
In computer networking, Layer 2 Tunneling Protocol is a tunneling protocol used to support virtual private networks . It does not provide any encryption or confidentiality by itself; it relies on an encryption protocol that it passes within the tunnel to provide privacy.-History:Published in 1999...
create virtual network interface
Network interface
Network interface may refer to:* Network interface controller, the device a computer uses to connect to a computer network* Network interface device, a demarcation point for a telephone network...
s and give the impression a direct physical connections between the tunnel endpoints. On a Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
host for example, these interfaces would be called tun0.
As there are only two endpoints on a tunnel, the tunnel is a point-to-point connection and PPP is a natural choice as a data link layer protocol between the virtual network interfaces. PPP can assign IP addresses to these virtual interfaces, and these IP addresses can be used, for example, to route between the networks on both sides of the tunnel.
IPsec
IPsec
Internet Protocol Security is a protocol suite for securing Internet Protocol communications by authenticating and encrypting each IP packet of a communication session...
in tunneling mode does not create virtual physical interfaces at the end of the tunnel, since the tunnel is handled directly by the TCP/IP stack. L2TP
L2TP
In computer networking, Layer 2 Tunneling Protocol is a tunneling protocol used to support virtual private networks . It does not provide any encryption or confidentiality by itself; it relies on an encryption protocol that it passes within the tunnel to provide privacy.-History:Published in 1999...
can be used to provide these interfaces, this technique is called L2TP/IPsec. In this case too, PPP provides IP addresses to the extremities of the tunnel.
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, DECnetDECnet
DECnet is a suite of network protocols created by Digital Equipment Corporation, originally released in 1975 in order to connect two PDP-11 minicomputers. It evolved into one of the first peer-to-peer network architectures, thus transforming DEC into a networking powerhouse in the 1980s...
, AppleTalk
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, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...
, IPX
IPX
Internetwork Packet Exchange is the OSI-model Network layer protocol in the IPX/SPX protocol stack.The IPX/SPXM protocol stack is supported by Novell's NetWare network operating system. Because of Netware's popularity through the late 1980s into the mid 1990s, IPX became a popular internetworking...
, and others-work with PPP.
- RFC 1661, Standard 51, The Point-to-Point Protocol (PPP)
- RFC 1662, Standard 51, PPP in HDLC-like Framing
- RFC 1717, The PPP Multilink Protocol (MP) (obsoleted by RFC 1990)
- RFC 1990, The PPP Multilink Protocol (MP)
- RFC 1994, PPP Challenge Handshake Authentication Protocol (CHAP)
- RFC 2686, The Multi-Class Extension to Multi-Link PPP
- RFC 2153, Informational, PPP Vendor Extensions
- RFC 2284, PPP Extensible Authentication Protocol (EAP)
- RFC 2615, PPP over SONET/SDH
- 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
See also
- Extensible Authentication ProtocolExtensible Authentication ProtocolExtensible Authentication Protocol, or EAP, is an authentication framework frequently used in wireless networks and Point-to-Point connections. It is defined in RFC 3748, which made RFC 2284 obsolete, and was updated by RFC 5247....
- Hayes command setHayes command setThe Hayes command set is a specific command-language originally developed for the Hayes Smartmodem 300 baud modem in 1981. The command set consists of a series of short text strings which combine together to produce complete commands for operations such as dialing, hanging up, and changing the...
- Link Access Procedure for Modems (LAPM)
- SLIPSlip- In science and technology :* Slip , an aqueous suspension of minerals, and frequently deflocculant.* Slip , a positional displacement in a sequence of transmitted symbols...
an earlier dialup protocol that PPP supplanted - RADIUSRADIUSRemote Authentication Dial In User Service is a networking protocol that provides centralized Authentication, Authorization, and Accounting management for computers to connect and use a network service...
- Diameter
- PPPoA
- PPPoE
- Point-to-Point Protocol daemon (PPPD)
- PPPoXPPPoXPPPoX designates a family of encapsulating communications protocols implementing Point-to-Point Protocol.Specific implementations:* PPPoE* PPPoA* PPTP* PPPoEoVLAN...
- Multiprotocol EncapsulationMultiprotocol EncapsulationMultiprotocol Encapsulation, or MPE for short, is a Data link layer protocol definedby DVB which has been published as part of ETSI EN 301 192...
(MPE) for MPEG transport stream - Unidirectional Lightweight EncapsulationUnidirectional Lightweight EncapsulationThe Unidirectional Lightweight Encapsulation is a Data link layer protocol for the transportation of network layer packets over MPEG transport streams....
(ULE) for MPEG transport stream