GRE Mux

[Brett Glass]

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