Re: TCP_KEEPINVTL and TCP_KEEPIDLE - Socket Keep Alives not working

[Cary Lewis <cary.lewis@gmail.com>]

That's amazing, thanks. We'll have to try to recompile curl under cygwin to
confirm that it keeps up the constants, and then does the right posix calls.

I will grab the files, and try to have this tested and report back to you.

Take care,

Cary Lewis

On Wed, Jul 1, 2020 at 3:51 PM Corinna Vinschen <corinna-cygwin@cygwin.com>
wrote:

> On Jun 30 18:53, Corinna Vinschen wrote:
> > On Jun 30 09:46, Cary Lewis via Cygwin wrote:
> > > Thanks for the reply. The answer to your question is that the 2 hour
> keep
> > > alive was not sufficient for a particular use case I encountered.
> > >
> > > I was trying to use curl under cygwin to access a very slow REST
> endpoint
> > > that was taking up to 8 minutes to generate download before any data
> flowed
> > > back to the client. This caused the server to abort the socket.
> > >
> > > Accessing the endpoint in chrome or firefox revealed that they set a
> > > keepalive to 45 seconds, which kept the server happy.
> > >
> > > Attempting to set --keepalive-time=45 in cygwin's curl didn't work, and
> > > wireshark revealed that no keepalives were being sent.
> > >
> > > I will attempt to patch cygwin, I got the build to work. Can you point
> me
> > > in the right direction, in terms of where the socket calls get mapped
> to
> > > the winsock calls?
> >
> > Actually, while I'm usually happy to take contributions, you don't have
> > to dig into that yourself.  I already have a few local patches in the
> > loop changing some of the affected code.  I have a good idea what's
> > required to add the keep-alive socket options to that code, so just lay
> > back and stay tuned for now.
>
> Ok, so I added support for a couple more IPPROTO_TCP socket options.
> First of all I fixed TCP_MAXSEG which was using the BSD value, rather
> than the WinSock value.  Then I added TCP_FASTOPEN, TCP_KEEPIDLE,
> TCP_KEEPCNT, TCP_KEEPINTVL, TCP_QUICKACK and TCP_USER_TIMEOUT:
>
> - TCP_FASTOPEN is supported since W10 1607, it's just faked on older
>   systems.
>
> - TCP_KEEPIDLE, TCP_KEEPCNT, TCP_KEEPINTVL are using the options
>   of the same name since W10 1709, WSAIoctl(SIO_KEEPALIVE_VALS)
>   on older systems.
>
>   But here's a problem: Older systems didn't allow to change
>   TCP_KEEPCNT.  It is always fixed to 10.  Mulling over that problem in
>   the shower, I came up with the following solution:
>
>   The max keep-alive timeout is TCP_KEEPIDLE + TCP_KEEPCNT * TCP_KEEPINTVL.
>   This should stay the same from a user space perspective.  So the current
>   code tweaks the TCP_KEEPINTVL given to WinSock so that
>
>   TCP_KEEPCNT * user space TCP_KEEPINTVL == 10 * WinSock TCP_KEEPINTVL
>
>   Example: user space TCP_KEEPCNT 4, TCP_KEEPINTVL 5   (4 * 5 == 20)
>   ==>      WinSock    TCP_KEEPCNT 10, TCP_KEEPINTVL 2  (10 * 2 == 20)
>
>   I hope that makes sense.
>
> - TCP_USER_TIMEOUT is supported with msec granularity since W10 1607
>   (called TCP_MAXRTMS), with 1 secs granularity on older systems
>   (called TCP_MAXRT).  Use the latter on older systems under the expected
>   loss of precision.
>
> - TCP_QUICKACK is supposedly supported on Windows as a socket option
>   but it's still not clear if the net got that right so far.  However,
>   there's WSAIoctl(SIO_TCP_SET_ACK_FREQUENCY) doing the same.
>
> I uploaded developer snapshots to https://cygwin.com/snapshots/,
> please test.
>
> For testing, you'll need at least the DLL, plus the changed headers
> cygwin/socket.h and netinet/tcp.h from the complete tar file
> cygwin-20200701.tar.xz. Or, just take the DLL and fetch the headers
> right from the git repo.
>
>
> Thanks,
> Corinna
>
> --
> Corinna Vinschen
> Cygwin Maintainer
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