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Re: broadcast address of a network interface
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Re: broadcast address of a network interface


  • Subject: Re: broadcast address of a network interface
  • From: Marshall Eubanks <email@hidden>
  • Date: Mon, 9 Feb 2009 14:15:22 -0500

Well, I may be sending too much information.

On Feb 9, 2009, at 12:20 PM, Joel Reymont wrote:

Let me take a step back then. I would like to "broadcast" audio packets from my iPhone to other Macs (including iPhones), without connecting to every iPhone individually.

What is the best option?

Is BiDir PIM available to me on the iPhone? Should I go with UDP LAN broadcast?


From the standpoint of the phone, the appliance knows little or nothing about multicast routing, so
this would require cooperation from the network operator.


I know PIM is available to me on my home LAN because I have a Cisco 877W but I'd like my app to have a bit more sales potential :-). I also noticed that the Cisco 877W firmware that allows Bonjour, does not come with multicast and vise versa. I would hate to enable multicast and loose my printers and file sharing!

Last but not least, I can't seem to be able to get hold of a broadcast address by enumerating the interfaces, whether on my Mac or iPhone. ifa_broadaddr is null and so is ifa_netmask!


I would not count on a "broadcast" getting everywhere. I believe, for example, that switches may block broadcasts from propagating from one LAN to another.


What is not clear to me is your target environment. Whenever you start talking multicast, you
need to make that clear. It matters a lot.


I am writing this on a laptop on a wireless LAN. This LAN is connected to other wireless LANs and also to a wired LAN here in my office, and to wireless LAN repeaters on other floors. I regard all of this as one "LAN" but from a network point of view it really isn't - I wouldn't, for example, count on getting a broadcast packet from a wireless segment here, through the wireline LAN, and onto the wireless LAN on the floor below.

Can I source a multicast from the 239/8 space or even from IPv6 and have it get through ? Once clue provided on Apple networks by Zeroconf - mDNS (multicast DNS) is used for resource discovery in Zeroconf (AKA Bonjour). If you can "see" your printers on the wireless LAN you should be able to get multicasts to them. (I cannot speak to Cisco firmware; what you reported may be an IPv6 vs IPv4 thing. I have made inquiries to some Cisco engineers about the 877W and will report back if I find anything.)

But, at least in my network with my equipment, multicasts up to 1 Mbps are possible without any configuration. YMMV.

As soon as you leave the local LAN, all bets are off and you cannot assume the
existence of multicast. In a corporate environment, if you are on a routed LAN, not a switched LAN, all bets are also off, and you will need assistance from the network admin to be sure multicast is present and functional.


If I were you, I would use multicast, but with a relay option as a backup. You might consider the use of
mDNS for discovery (both of other devices, and whether or not you can do multicasts at all). In a wireless environment with small, low power devices, you might also consider setting up a P2P relay.


Regards
Marshall


	Thanks, Joel

On Feb 9, 2009, at 5:12 PM, Marshall Eubanks wrote:

Because of this, some wireless AP vendors either don't send multicasts at all, or don't by default. Even though multicasts sources on the wireless LAN are less of a problem, they are sometimes blocked as well. So, you will need to address these issues somehow.

The particular "push to talk" problem you mention is frequently done using BiDir PIM now-a-days, at least on wireline networks. See

--- http://tinyco.de --- Mac & iPhone





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  • Follow-Ups:
    • Re: broadcast address of a network interface
      • From: Joel Reymont <email@hidden>
References: 
 >broadcast address of a network interface (From: Joel Reymont <email@hidden>)
 >Re: broadcast address of a network interface (From: Jens Alfke <email@hidden>)
 >Re: broadcast address of a network interface (From: Joel Reymont <email@hidden>)
 >Re: broadcast address of a network interface (From: Jens Alfke <email@hidden>)
 >Re: broadcast address of a network interface (From: Joel Reymont <email@hidden>)
 >Re: broadcast address of a network interface (From: Marshall Eubanks <email@hidden>)
 >Re: broadcast address of a network interface (From: Joel Reymont <email@hidden>)

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