Everyone:
I have recently been building FreeBSD VPN servers which can accept
50 to 100 PPTP connections. PPTP is, essentially, PPP over GRE
(with a TCP control connection), so we have large numbers of
packets passing in and out using GRE. Unfortunately, GRE on FreeBSD
doesn't currently have a multiplexing function as does TCP. If
userland PPP and pptpd are used to handle the PPTP sessions, each
GRE packet is passed to the first pptpd process. If the call ID
doesn't match, it's passed to the next, and then the next, and so
on. What's more, each test requires a "bounce" into and out of the
kernel. mpd, which uses netgraph, does more of the work within the
kernel, but the testing still takes place in linear time -- and the
potential delay increases with the number of PPTP sessions that
have been established. The packet is bounced from one netgraph node
to another until one of them accepts it or the packet falls off the
end of the chain.
It seems to me that it might be worth it to implement a
multiplexing function that dispatches the packet directly to the
right process or netgraph node rather than passing it from hand to
hand. Thoughts?
--Brett Glass
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