TCP_KEEPINVTL and TCP_KEEPIDLE - Socket Keep Alives not working

[Cary Lewis]

Corinna, thank you.

I have tested and confirmed that the patched cywin1.dll, and include
files /usr/include/netinet/tcp.h and /usr/include/cygwin/socket.h,  work as
expected after I re-compile and link curl (specifically lib/connect.c).

I look forward to seeing this patch included in a future release, and
hopefully that will include rebuilt versions of curl, and other network
utilities, such as wget, nc, etc.

It's always a pleasure working with you.




On Thu, Jul 2, 2020 at 10:52 AM Cary Lewis <cary.lewis@gmail.com> wrote:

> 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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>