[Dovecot] v2.0.alpha1 released
lists at wildgooses.com
Fri Oct 16 14:54:44 EEST 2009
Timo Sirainen wrote:
> On Thu, 2009-10-15 at 23:56 +0100, Ed W wrote:
>> I'm currently having problems with my vpn when using a UK "Three" SIM.
>> I haven't fully investigated, but I *think* it's because the IP changes
>> regularly even while the connection is up and in progress? I think this
>> setup is rare, but exists (on some broadband connections also
>> apparently?). I actually think that in this situation all the tcp
>> connections *should* die...?
> Not necessarily. Laptops don't kill existing WiFi connections when RJ45
> is plugged into them.
Actually you raise an interesting issue there and actually it would be
worth doing a bit of experimentation to see what really happens.
What I think will happen depends on the circumstances:
- Wifi and Wired on the same network, no NAT: On most unix operating
systems, plugging in a new net card simply changes the default gateway,
so the existing connection will try and go out via the new gateway and
assuming no clever firewall on the remote side and no NAT, then the TCP
sequence numbers should match and the connection should continue normally...
- Wifi and Wired on the same network, but with a NAT: Now in this case I
would expect the NAT not to know that the continuing TCP stream from a
new IP address is connected with the old stream? I would imagine all
kinds of things will break, but I wouldn't expect the connection to
- Wifi and Wired on Windows XP and earlier (possibly vista also?) - now
XP does something clever, it appears to have connection tracking in
place and once a connection is started on a given interface then that
connection continues via the same interface even if the default gateway
is changed, ie default gateway only affects new tcp connections and old
connections are automatically routed through their initial net device.
This allows you to do some funky stuff such as remote controlling a
machine over a fast connection whilst getting it to connect to some
dialup connection, you can continue to control the machine even after
the dialup device is brought up, ie the remote control app doesn't
suddenly switch to the new connection. I'm not 100% sure, but I think
if you kill the wifi connection then actually it will drop all TCP
connections on the wifi interface rather than switching them to the
wired interface - so I don't think it's actually possible to achieve the
effect you described?
Curiously you can achieve the same effect on linux at least as the XP
situation, but you need to add some extra firewalling rules to setup
connection tracking. Read the LARTC faq for more...
So actually I think that your scenario is actually quite rare? In
general I think the connection will die (for the mobile user). I think
the main situation where the connection won't die is only where you have
wired/wireless connected to the same subnet and you switch between
them. In general I don't actually mind if my imap connection drops
while switching and it seems low priority to preserve it, but agree it's
desirable if it's possible
> Perhaps mobile phones can also use 3G for some
> connections and when WiFi shows up starts using them for the new
> connections. Why should it kill any existing connections in those
I don't think it will kill the existing connections, but I think either
it will switch them incorrectly (and they will die) or else they will
continue in place over their respective networks (which may not actually
be what the user wishes in the case of imap connections...)
Additionally it's hard to distinguish between one device using two net
connections and two devices each on different/same connections...
>> I think the main cases to optimise for are a) mobile users behind a NAT,
>> b) users possibly leaving one desktop machine on broadband, but checking
>> the same account via a mobile device (same login). I think this coverst
>> the 90% situation?
> I think including only username in the hash works nicely for just about
> everyone. I now committed the code:
Well, I would claim that it's only *important* to *synchronise*
communications with a hash of username+IP (where IP is a proxy for
communication interface in use on a given device). I can't immediately
see the implications of syncing all communications with a given user,
but I think it's possible to be more specific if this is useful?
The logic is that we want the radio card to be as idle as possible for
as long as possible, so we can treat a wifi interface and a 3G interface
as separate problems and keep them both quiet for as long as possible.
The point is once we wakeup say the wifi card we desire ONLY to wakeup
the wifi card and additionally to get as much done as possible before we
stick it back in idle so that we can maximise it's next idle period.
Same also with the 3G card. Of course if we can get away with waking up
only one of the cards and NOT the other card then this is desirable
because it's one less wakeup
Does that make sense? I think it argues in favour of tracking idle per
user, per network card - given this is hard to do in the general case I
think it suggests that tracking username + IP may be beneficial in terms
of syncing communication and batching packets?
Turning it around, a typical use case might be to assume one user, one
mailbox, one username/login, but two email programs in use simultaneously:
- User sits in office using office wifi/wired network to read email on
desktop. a) device uses same subnet and hence same NAT, b) device uses
3G connection. In both cases the desktop machine will be generating
traffic and the device sits idle and we want to minimise wakeups to the
- User leaves office, office machine remains on and IDLEing, but
otherwise unused. Device and desktop now on different networks. User
is a) leaving device in pocket, so also idling or b) actively using
device, ie generating traffic - in both cases the desktop is also
idling, but we desire to minimise the effects that idle causes on the
Next usecase is:
One user, multiple mailboxes, multiple email programs... Eg I have a
desktop machine and mobile device, I also have a personal email address,
main office sales address and main office support email address. At the
moment we use one username for each mailbox and in the case of the
sales/support accounts we have every desktop user login with the same
username for each account (eg everyone logs in as both sales@ and support@)
- In this case I desire to sync IDLE packets for multiple username
combinations, but going back to the same IP address, ie actually I want
to minimise wakeups per device and per device interface, ie if my 3G
card wakes up then I want to get all the data in for all three logins...
How can I address this use-case? Perhaps in this case its better to use
a single login and make the other accounts shared subfolders of that
account? This isn't something I have tried so far though?
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