Installing and Administering PPP

76 Chapter 4
Common pppd Options
Compression
Van Jacobson TCP Header Compression
Each layer a TCP/IP datagram passes through adds a header to the user
data. For example, many streams can potentially pass between two
hosts. Therefore, in addition to its source and destination addresses,
each packet contains a tag to identify which stream it belongs to. These
headers are very large, and a comparison between successive packets
reveals strong similarities.
RFC 1144 ’VJ’ TCP header compression reduces the packet header size
by transmitting only the header segments that change from one packet
to the next. TCP and IP header overhead is reduced from over 40 octets
to as few as 4. TCP header compression has a dramatic effect on
interactive responsiveness over low-speed links, because it reduces a
typical single-character Telnet or rlogin packet from over 40 octets to 5 or
6 octets. It has a much smaller effect on batch data throughput, like X
bitmap displays, or FTP or rcp file transfers. That sort of data flows in
much larger packets. Reducing frame size from 1500 to 1460 octets
produces a much smaller percentage improvement than a reduction from
45 to 5 octets.
The option vjslots followed by a numerical value between 3 and 256 sets
the number of compression slots for Van Jacobson compression. The
default is 16 slots. See the pppd man pages for other options for VJ
compression.
TCP header compression is enabled by default on asynchronous PPP and
SLIP links, and disabled by default on synchronous PPP links.
PPP Link Compression
Like in-modem data compression, PPP link compression reduces the
amount of data that must flow across a low-bandwidth telephone line,
thus increasing its effective bandwidth. Since PPP link compression in
pppd is performed on the UNIX system, less data flows across the serial
interface. This is advantageous in the following situations:
• The host's serial interfaces are incapable of high asynchronous data
rates.
• The host's inefficient serial I/O subsystem causes an onerous
interrupt load on the host processor.
• Two computers are directly connected and there are no modems to
compress the data.