UDP multicast on iOS with WWAN turned on

[Dries Langsweirdt]

Hello,

I'm developing an iOS gstreamer application which essentially receives
and visualizes an UDP multicast h264 stream. Everything works as
expected, until Mobile network > Mobile data is turned on in the
settings app on an iOS device with a SIM card. As soon as WWAN traffic
is possible, no UDP packages are received by the gstreamer pipeline any
longer (and as such no video displayed).

I have a separate application (no gstreamer involved), which creates an
UDP socket listening on the same multicast address and port as the
pipeline. In this application, turning the mobile data on or off has no
influence on receiving UDP packages over WIFI. As such, I know this is
not a limitation of the hardware or operating system. This has been
confirmed to me by an Apple engineer. Furthermore, the Bonjour protocol
is using UDP multicast as well independent of the mobile data status.

This is the pipeline I'm using:
udpsrc multicast-iface=en0 address=224.1.1.1 port=5004
     ! application/x-rtp, media=(string)video, clock-rate=(int)90000, ....
     ! rtpjitterbuffer latency=400
     ! rtph264depay ! h264parse ! vtdec ! videoconvert
     ! autovideosink

and I'm currently on gstreamer 1.10.2, iOS 9 and 10.

I have looked at the code in gstreamer (gstudpsrc.c > gst_udpsrc_open)
and glib (gsocket.c > g_socket_multicast_group_operation) where the
socket related operations are preformed, but don't see anything
obviously odd at first sight.

I would like to know if I'm looking at an application specific problem
or a general gstreamer problem, so if someone could confirm or
de-confirm this behavior that would be great. Any suggestions as to how
to proceed are greatly appreciated.

Thanks a lot.
--

Kind Regards,
Dries Langsweirdt

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[Sebastian Dröge-3]

On Wed, 2016-12-07 at 01:17 +0100, Dries Langsweirdt wrote:
>
> I would like to know if I'm looking at an application specific problem 
> or a general gstreamer problem, so if someone could confirm or 
> de-confirm this behavior that would be great. Any suggestions as to how 
> to proceed are greatly appreciated.

This would seem like a GStreamer or GLib problem. Can you compare what
that other application does with the socket, and what udpsrc does
different?

Also is switching WWAN on causing a new interface to appear, or what
effects does it have?

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[Dries Langsweirdt]

Hello,

I got this problem solved. I'm leaving this here for future visitors.

Socket code that is working correctly (receiving multicast UDP when mobile data is turned on) is as follows:

----------

fd = socket(AF_INET, SOCK_DGRAM, 0);
    
memset(&addr, 0, sizeof(addr));
addr.sin_len = sizeof(addr);
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = INADDR_ANY;
addr.sin_port = ntohs(5004);
 
bind(fd, (const struct sockaddr *) &addr, sizeof(addr)) == 0;
 
mreq.gr_interface = if_nametoindex(@"en0");
memset(&mreq.gr_group, 0, sizeof(mreq.gr_group));
a = (struct sockaddr_in *) &mreq.gr_group;
a->sin_len = sizeof(addr);
a->sin_family = AF_INET;
a->sin_addr.s_addr = inet_addr("224.1.1.1");
a->sin_port = 0;
    
setsockopt(fd, IPPROTO_IP, MCAST_JOIN_GROUP, &mreq, sizeof(mreq)) == 0;

----------

When I compare this with the socket creation code in Glib, I see a few differences:

- https://github.com/GNOME/glib/blob/master/gio/gsocket.c#L2280 => GlIb is using the group join operation IP_ADD_SOURCE_MEMBERSHIP, while the above code uses MCAST_JOIN_GROUP.
- https://github.com/GNOME/glib/blob/master/gio/gsocket.c#L2236 => GLib is only setting if_nametoindex (iface), but not mc_req.imr_interface.s_addr = g_htonl (INADDR_ANY), while in the above code: addr.sin_addr.s_addr = INADDR_ANY; and mreq.gr_interface = if_nametoindex (iface);

The way I have solved this is by constructing the socket myself, similar to the example code provided, and to set the socket manually on the udpsrc element(s):

----------

gsocket_video = g_socket_new_from_fd(fd, &error);
...
pipeline = gst_parse_launch(pipeline_desc, &error);
...
GstElement* srcvideo = gst_bin_get_by_name((GstBin*)pipeline, "srcvideo");
g_object_set (srcvideo, "socket", gsocket_video, NULL);

----------

To answer this question: "Also is switching WWAN on causing a new interface to appear, or what effects does it have?"
=> There are no new interfaces, the list of real and virtual interfaces on iOS seem fixed. However, in my situation the WIFI is a LAN (so there is no internet access). I assume that when mobile data is turned on the default gateway of the system is changed in order to prefer traffic over WWAN.



On 07/12/16 11:25, Sebastian Dröge wrote:

On Wed, 2016-12-07 at 01:17 +0100, Dries Langsweirdt wrote:

I would like to know if I'm looking at an application specific problem 
or a general gstreamer problem, so if someone could confirm or 
de-confirm this behavior that would be great. Any suggestions as to how 
to proceed are greatly appreciated.

This would seem like a GStreamer or GLib problem. Can you compare what
that other application does with the socket, and what udpsrc does
different?

Also is switching WWAN on causing a new interface to appear, or what
effects does it have?

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[Sebastian Dröge-3]

On Thu, 2016-12-08 at 12:54 +0100, Dries Langsweirdt wrote:

>
> When I compare this with the socket creation code in Glib, I see a
> few differences:
>
> - https://github.com/GNOME/glib/blob/master/gio/gsocket.c#L2280 =>
> GlIb is using the group join operation IP_ADD_SOURCE_MEMBERSHIP,
> while the above code uses MCAST_JOIN_GROUP.
> - https://github.com/GNOME/glib/blob/master/gio/gsocket.c#L2236 =>
> GLib is only setting if_nametoindex (iface), but not
> mc_req.imr_interface.s_addr = g_htonl (INADDR_ANY), while in the
> above code: addr.sin_addr.s_addr = INADDR_ANY; and mreq.gr_interface
> = if_nametoindex (iface);

That would seem like a bug in iOS's network stack then.
IP_ADD_SOURCE_MEMBERSHIP and MCAST_JOIN_GROUP should have the same
effect. But we should probably change GLib to switch to the other one
in that case.

You said, you talked to an Apple engineer. Maybe ask him about the
differences between the two sockopts and if they can fix their code so
they behave the same in the future.


Please also file a bug about this against GLib:
https://bugzilla.gnome.org/enter_bug.cgi?product=glib

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[Sebastian Dröge-3]

On Fri, 2016-12-09 at 07:52 +0200, Sebastian Dröge wrote:

> On Thu, 2016-12-08 at 12:54 +0100, Dries Langsweirdt wrote:
> >
> > When I compare this with the socket creation code in Glib, I see a
> > few differences:
> >
> > > > - https://github.com/GNOME/glib/blob/master/gio/gsocket.c#L2280 =>
> > GlIb is using the group join operation IP_ADD_SOURCE_MEMBERSHIP,
> > while the above code uses MCAST_JOIN_GROUP.
> > > > - https://github.com/GNOME/glib/blob/master/gio/gsocket.c#L2236 =>
> > GLib is only setting if_nametoindex (iface), but not
> > mc_req.imr_interface.s_addr = g_htonl (INADDR_ANY), while in the
> > above code: addr.sin_addr.s_addr = INADDR_ANY; and mreq.gr_interface
> > = if_nametoindex (iface);
>
> That would seem like a bug in iOS's network stack then.
> IP_ADD_SOURCE_MEMBERSHIP and MCAST_JOIN_GROUP should have the same
> effect. But we should probably change GLib to switch to the other one
> in that case.
>
> You said, you talked to an Apple engineer. Maybe ask him about the
> differences between the two sockopts and if they can fix their code so
> they behave the same in the future.
>
>
> Please also file a bug about this against GLib:
> https://bugzilla.gnome.org/enter_bug.cgi?product=glib

I did that now, you can find it here:
https://bugzilla.gnome.org/show_bug.cgi?id=775873

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Sebastian Dröge, Centricular Ltd · http://www.centricular.com
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