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PPP is a network-specific standard protocol with STD number 51. Its status is elective. It is described in RFC 1661 and RFC 1662.
There are a large number of proposed standard protocols which specify the operation of PPP over different kinds of point-to-point link. Each has a status of elective. The reader is advised to consult STD 1 - Internet Official Protocol Standards for a list of PPP-related RFCs which are on the Standards Track.
Point-to-Point circuits in the form of asynchronous and synchronous lines have long been the mainstay for data communications. In the TCP/IP world, the de facto standard SLIP protocol has served admirably in this area, and is still in widespread use for dial-up TCP/IP connections. However, SLIP has a number of drawbacks:
However, many SLIP implementations now use Van Jacobsen Header Compression. This is used to reduce the size of the combined IP and TCP headers from 40 bytes to 8 bytes by recording the states of a set of TCP connections at each end of the link and replacing the full headers with encoded updates for the normal case where many of the fields are unchanged or are incremented by small amounts between successive IP datagrams for a session. This compression is described in RFC 1144.
PPP has three main components:
Before a link is considered to be ready for use by network-layer protocols, a specific sequence of events must happen. The LCP provides a method of establishing, configuring, maintaining and terminating the connection. LCP goes through the following phases:
In this phase, link control packets are exchanged and link configuration options are negotiated. Once options are agreed upon, the link is open, but not necessarily ready for network-layer protocols to be started.
This phase is optional. PPP does not specify the policy for determining quality, but does provide low-level tools, such as echo request and reply.
This phase is optional. Each end of the link authenticates itself with the remote end using authentication methods agreed to during phase 1.
Once LCP has finished the previous phase, network-layer protocols may be separately configured by the appropriate NCP.
LCP may terminate the link at any time. This will usually be done at the request of a human user, but may happen because of a physical event.
The IP Control Protocol (IPCP) is the NCP for IP and is responsible for configuring, enabling and disabling the IP protocol on both ends of the point-to-point link. The IPCP options negotiation sequence is the same as for LCP, thus allowing the possibility of reusing the code.
One important option used with IPCP is Van Jacobsen Header Compression which is used to reduce the size of the combined IP and TCP headers from 40 bytes to approximately 4 by recording the states of a set of TCP connections at each end of the link and replacing the full headers with encoded updates for the normal case where many of the fields are unchanged or are incremented by small amounts between successive IP datagrams for a session. This compression is described in RFC 1144.
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