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RFC1661 - часть 3


These links provide full-duplex

simultaneous bi-directional operation, and are assumed to deliver

packets in order. It is intended that PPP provide a common solution

for easy connection of a wide variety of hosts, bridges and routers

[1].

Encapsulation

The PPP encapsulation provides for multiplexing of different

network-layer protocols simultaneously over the same link. The

PPP encapsulation has been carefully designed to retain

compatibility with most commonly used supporting hardware.

Only 8 additional octets are necessary to form the encapsulation

when used within the default HDLC-like framing. In environments

where bandwidth is at a premium, the encapsulation and framing may

be shortened to 2 or 4 octets.

To support high speed implementations, the default encapsulation

uses only simple fields, only one of which needs to be examined

for demultiplexing. The default header and information fields

fall on 32-bit boundaries, and the trailer may be padded to an

arbitrary boundary.

Link Control Protocol

In order to be sufficiently versatile to be portable to a wide

variety of environments, PPP provides a Link Control Protocol

(LCP). The LCP is used to automatically agree upon the

encapsulation format options, handle varying limits on sizes of

packets, detect a looped-back link and other common

misconfiguration errors, and terminate the link. Other optional

facilities provided are authentication of the identity of its peer

on the link, and determination when a link is functioning properly

and when it is failing.

Network Control Protocols

Point-to-Point links tend to exacerbate many problems with the

current family of network protocols. For instance, assignment and

management of IP addresses, which is a problem even in LAN

environments, is especially difficult over circuit-switched

point-to-point links (such as dial-up modem servers). These

problems are handled by a family of Network Control Protocols

(NCPs), which each manage the specific needs required by their




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