Point-to-Point Protocol
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In networking, the Point-to-Point Protocol, or PPP, is a data link protocol commonly used in establishing a direct connection between two networking nodes. It can provide connection authentication, transmission encryption privacy, 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. Most Internet service providers (ISPs) use PPP for customer dial-up access to the Internet. 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 (DSL) Internet service.
PPP is commonly used as a data link layer protocol for connection over synchronous and asynchronous circuits, where it has largely superseded the older, non-standard Serial Line Internet Protocol (SLIP) and telephone company mandated standards (such as Link Access Protocol, Balanced (LAPB) in the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 protocol suite). PPP was designed to work with numerous network layer protocols, including Internet Protocol (IP), Novell's Internetwork Packet Exchange (IPX), NBF and AppleTalk.
PPP is also used over broadband connections. RFC 2516 describes Point-to-Point Protocol over Ethernet (PPPoE), a method for transmitting PPP over Ethernet that is sometimes used with DSL. RFC 2364 describes Point-to-Point Protocol over ATM (PPPoA), a method for transmitting PPP over ATM Adaptation Layer 5 (AAL5), which is also sometimes used with DSL.
PPP is specified in RFC 1661.[1]
Contents |
Basic Features
| Application | FTP | SMTP | HTTP | … | DNS | … |
| Transport | TCP | UDP | ||||
| Internet | IP | IPv6 | ||||
| Network access | PPP | |||||
| PPPoE | PPPoA | |||||
| Ethernet | ATM | |||||
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 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).[2]
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.
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. 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.
- 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
| 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, 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.
Encapsulation
PPP frames are encapsulated in a lower-layer protocol that provides framing and may provide other functions such as a checksum 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 (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.
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 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. 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 for set up a VPN, with optional encryption (MPPE) or compression (MPPC). PPP can also be used in conjunction with L2TP which tunnels data over IP network. This technique is used with IPsec (named L2TP/IPsec) to create VPN. One of the reasons to use PPP over L2TP over IPsec is to have virtual IP addresses for connecting clients. Such a scheme is widely standardized and supported by the industry.
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
- ↑ RFC1661 - The Point-to-Point Protocol (PPP)
- ↑ RFC2284 - PPP Extensible Authentication Protocol (EAP)
- ↑ 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