Справочник по сетевым протоколам

RFC1661 - часть 41

the same value (consider the two binary representations for 3,

00000011 and 00000000 00000011).

When using low speed links, it is desirable to conserve bandwidth

by sending as little redundant data as possible. The Protocol-

Field-Compression Configuration Option allows a trade-off between

implementation simplicity and bandwidth efficiency. If

successfully negotiated, the ISO 3309 extension mechanism may be

used to compress the Protocol field to one octet instead of two.

The large majority of packets are compressible since data

protocols are typically assigned with Protocol field values less

than 256.

Compressed Protocol fields MUST NOT be transmitted unless this

Configuration Option has been negotiated. When negotiated, PPP

implementations MUST accept PPP packets with either double-octet

or single-octet Protocol fields, and MUST NOT distinguish between


The Protocol field is never compressed when sending any LCP

packet. This rule guarantees unambiguous recognition of LCP


When a Protocol field is compressed, the Data Link Layer FCS field

is calculated on the compressed frame, not the original

Simpson [Page 48]

RFC 1661 Point-to-Point Protocol July 1994

uncompressed frame.

A summary of the Protocol-Field-Compression Configuration Option

format is shown below. The fields are transmitted from left to


0 1

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5


| Type | Length |






Simpson [Page 49]

RFC 1661 Point-to-Point Protocol July 1994

6.6. Address-and-Control-Field-Compression (ACFC)


This Configuration Option provides a method to negotiate the

compression of the Data Link Layer Address and Control fields. By

default, all implementations MUST transmit frames with Address and

Control fields appropriate to the link framing.

Since these fields usually have constant values for point-to-point

links, they are easily compressed. This Configuration Option is

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