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On Tue, 26 Oct 1999, Jacob Joseph wrote:
> "Hey, with the ppp_compress modules, what do they actually do? Do I get
> software compression out of them or what? If so, how do I know the
> compression average?
This is Van Jacobsen Compression, same as CSLIP.
If you use the TCP protocol, you should thank Van Jacobsen, since he
pretty much invented it. I hope he's still around. He was awarded
the USENIX lifetime acheivement award in 1994 for "Making TCP
Industrial Strength."
It compresses the TCP Header, not the data.
You might be interested in rfc1144 and rfc1332 for details.
Basically, the problem v-j compression solves is this.
On an interactive sessuibm with a user typing on a terminal,
for instance, the TCP and IP headers for a single byte add
up to 40 bytes. So you are sending 41 bytes for 1 character
(worst case, but not uncommon!). 4000% overhead...
V-J can reduce the overhead for the packet header down to maybe
300%. This was absolutely essential for 2400 baud, and still
a very good thing for any serial link, even now.
I'm not certain whether the net effect of V-J is still a gain
if all one does is bulk data transfer, since it was designed to
bring down the keystroke-echo latency on serial links.
- --
James McGill VOX: (972) 481-5735 (Dallas)
MindSpring Enterprises, Inc. PCS: (214) 641-4458
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