[Fwd: Re: 3 connections as one]

Matthew Dillon dillon at apollo.backplane.com
Thu Jun 26 16:53:41 UTC 2008

    You can do it for outgoing connections fairly easily using the NAT
    trick (with PF), but you can't really load balance multiple links
    without support from some outside entity.  If one of the tunnels goes
    down you can fail-over but any pre-existing connections will die and
    have to be re-established on the remaining link(s).  That generally
    works ok for TCP but is total hell for UDP (because the source address
    will suddenly 'change' on an existing 'connection' and often trigger
    security blocks or simply break the program in question when it does).

    I've got a DSL connection and a Cable internet connection at home now,
    having replaced the T1 I had had for many years.

    I tried using the NAT trick using PF for outgoing but was never happy
    with the results under max load (and my links are typically running
    at 100% 24x7).  I wasn't able to get fail-over to work properly with
    PF at all... the network was actually less reliable instead of more
    reliable and using NAT meant I had very little control over port
    selection or reverse-IP.

    I eventually gave up and now just use my DSL line for all my normal
    traffic, and my cable link for my off-site backup traffic.


    I'm planning out a new solution, one that a friend of mine implemented
    with a portable class C he owns at a colo with a single link which I
    want to extend to multiple links.  The idea is to chop off a subnet from
    the colo-routed class C and run it to the home box over multiple tunnels
    (one over COMCAST, one over DSL).

    I am going to run all the tunnels through a single user program on my
    router box and backhaul it into a TUN interface (using PF on the TUN
    interface for QOS), and have the user program do all the load balancing
    and fail-over.  Since the whole mess is routing a single subnet, no
    NAT tricks are needed and packets can be routed 100% dynamically.
    There would be no disconnections or UDP IP address changes.

    The only caveat is that the colo adds another 10ms to the round-trip
    time verses a direct connection.  But on the plus side the home network
    can operate uninterrupted over however many discrete internet links I
    have access to, including modem dial backup or a directional WIFI link
    between friend's houses.


    I still gotta find time to write that program but there's nothing 
    fancy about the concept.  Maintain multiple links, route packets over
    the links that are up... simple stuff really.  DragonFly has a number
    of utilities that make the job easy which FreeBSD folks might want to
    look into:


	(vknetd is a packet switch, complete with a MAC cache & forwarding).

	+ SOCK_SEQPACKET support in the kernel for unix domain sockets.
	  (it wouldn't be too hard for FreeBSD to implement SOCK_SEQPACKET
	  and stream connection support via unix domain sockets, it took
	  less then a day to get it into DFly).

    Having a packetized stream socket connection to a user program (vknetd)
    which implements a packet switch takes all the effort out of messing
    around with network routing, literally.


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