Installing and Administering PPP

Chapter 4 75
Common pppd Options
Compression
Compression
In addition to in-modem data compression, PPP supports compression at
several different layers of the communications stack.
HDLC Frame Compression
The PPP frame format is based on the established HDLC format.
Synchronous PPP links almost always use the full PPP/HDLC frame
because the link hardware supports it. But lower-speed asynchronous
links typically handle framing in software and several of the fields carry
the same contents in each message. Therefore, it makes sense to amend
the full HDLC frame for asynchronous PPP.
Address and Control Field Compression
A full PPP frame contains the HDLC ALLSTATIONS value (0xFF) in the
address field, and the HDLC Un-numbered information value (0x03) in
the control field. These fields are unnecessary because they always carry
the same information and increase the latency of a low-speed link.
During the Link Control Protocol (LCP) phase, asynchronous PPP links
usually negotiate to drop both fields. When the LCP layer is opened, the
fields are not transmitted in PPP frames.
Protocol Field Compression
The two-octet PPP Protocol field in an asynchronous PPP frame tells the
receiving PPP whether the incoming frame carries an IP network
datagram, a Link Quality Report, a link option (re)negotiation, or any of
several other types of data. Though option negotiations and other
link-level traffic always require two octets, most of an established link's
traffic is network-layer (for example, IP) datagrams. PPP’s
network-layer protocol field values always place a null octet (0x00) before
the octet that distinguishes IP (0x21) from Appletalk (0x29) datagrams.
That octet almost always contains the same value (0x00), and
asynchronous PPP links usually negotiate it away to reduce latency.