Re: broadcast address of a network interface
Re: broadcast address of a network interface
- Subject: Re: broadcast address of a network interface
- From: Jens Alfke <email@hidden>
- Date: Mon, 9 Feb 2009 08:47:28 -0800
On Feb 9, 2009, at 8:37 AM, Joel Reymont wrote:
I would like to broadcast audio from the iPhone to other iPhones or
Macs on a subnet.
This is often done using multiple TCP sockets instead of multicast.
The advantage is that, since TCP is reliable, you don't have to worry
about dealing with dropped packets (which are rare on Ethernet, but
common in busy wireless networks). It also works transparently outside
the local subnet.
It may seem like a waste of bandwidth, but audio doesn't use that much
bandwidth anyway, and it turns out that multicast has bad effects on
802.11 (IIRC, the base station has to drop the network down to low
speed temporarily while it sends a multicast packet). I've had a few
conversations with Bonjour experts about this in the past, and at
least one of them recommended just going to TCP for streaming.
If I go with multicast, how do I programmatically find a proper
multicast address?
There's a limited name- [number-]space of them, so they're assigned
manually by the IETF. If you search around, there's an RFC that's a
registry of registered multicast addresses; you look for an unused
link-local one and request for it to be assigned to you, using the
email address given in the RFC. (Or if you don't think your app will
be widely used, you can just pick a free one and use it as a squatter;
but I don't think the registration process is too onerous.)
—Jens
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