Протокол PPP: принципы работы и применение в компьютерных сетях.

Data link layer communication protocolIn 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, 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.

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used as a data link layer protocol for connection over synchronous and asynchronous circuits, where it has largely superseded the older Serial Line Internet Protocol (SLIP) and telephone company mandated standards (such as Link Access Protocol, Balanced (LAPB) in the X.25 protocol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, DECnet and AppleTalk. Like SLIP, this is a full Internet connection over telephone lines via modem. It is more reliable than SLIP because it double checks to make sure that Internet packets arrive intact. 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.

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.

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.

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, using a agreed on algorithm such as BSD compress or Deflate. The protocol decompresses the frame at its destination. See 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).

Link Dead This phase occurs when the link fails, or one side has been told to disconnect (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 a connection.

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 technology. Cisco IOS Release 11.1 and later supports Multilink 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

PPTP (Point to Point Tunneling Protocol) is a form of PPP between two hosts via GRE using encryption (MPPE) and compression (MPPC).

Many protocols can be used to tunnel data over IP networks. Some of them, like SSL, SSH, or L2TP create virtual network interfaces and give the impression of direct physical connections between the tunnel endpoints. On a Linux host for example, these interfaces would be called tun0 or ppp0. 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.

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, work with PPP; they can be found on the Datatracker IETF website.