[Dovecot] Dovecot IDLE bandwidth usage

[Fredrik Tolf]

Hi list,

I just recently discovered the IMAP IDLE command and tried it out with
my Dovecot server. It seems to work fine and all, but it seems to be
using quite a deal of bandwidth, because of the "* OK Still here"
messages sent by the server at two minute intervals. I would argue that
is a bit of a problem when using the IDLE command for "Push mail" on a
mobile phone.

I don't know exactly what parts of the packets that mobile carriers
collect payment for, but the Ethernet bandwidth consumed by one such
message is 149 bytes (83 bytes for the message itself, and 66 bytes for
the TCP ACK from the client). At two minute intervals, that becomes over
100 KB per day, and over 3 MB per month. At GRPS rates typical here in
Sweden, that becomes about 5-7 USD per month.

That might not seem tremendously much, but seeing how it is what takes
up the vast majority of the IMAP bandwidth, it is still quite annoying.

Are those messages really necessary? I mean, Dovecot still times out
after half an hour if the client doesn't "finish" the IDLE command and
re-issues it, right? (Coincidentally, I would like to argue that it
would be better not to time out at all during an IDLE command and just
turn on TCP keepalives instead, but that is the minor issue here)

I'll apologize in advance if this is already a tunable option, but I
haven't found anything about it in the configuration file.

Cheers,
Fredrik Tolf