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RFC1661 - часть 5


This document frequently uses the following terms:

datagram The unit of transmission in the network layer (such as IP).

A datagram may be encapsulated in one or more packets

passed to the data link layer.

frame The unit of transmission at the data link layer. A frame

may include a header and/or a trailer, along with some

number of units of data.

packet The basic unit of encapsulation, which is passed across the

interface between the network layer and the data link

layer. A packet is usually mapped to a frame; the

exceptions are when data link layer fragmentation is being

performed, or when multiple packets are incorporated into a

single frame.

peer The other end of the point-to-point link.

silently discard

The implementation discards the packet without further

processing. The implementation SHOULD provide the

capability of logging the error, including the contents of

the silently discarded packet, and SHOULD record the event

in a statistics counter.

Simpson [Page 3]

RFC 1661 Point-to-Point Protocol July 1994

2. PPP Encapsulation

The PPP encapsulation is used to disambiguate multiprotocol

datagrams. This encapsulation requires framing to indicate the

beginning and end of the encapsulation. Methods of providing framing

are specified in companion documents.

A summary of the PPP encapsulation is shown below. The fields are

transmitted from left to right.

+----------+-------------+---------+

| Protocol | Information | Padding |

| 8/16 bits| * | * |

+----------+-------------+---------+

Protocol Field

The Protocol field is one or two octets, and its value identifies

the datagram encapsulated in the Information field of the packet.

The field is transmitted and received most significant octet

first.

The structure of this field is consistent with the ISO 3309

extension mechanism for address fields. All Protocols MUST be

odd; the least significant bit of the least significant octet MUST

equal "1". Also, all Protocols MUST be assigned such that the




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