• Open Menu Close Menu
  • Apple
  • Shopping Bag
  • Apple
  • Mac
  • iPad
  • iPhone
  • Watch
  • TV
  • Music
  • Support
  • Search apple.com
  • Shopping Bag

Lists

Open Menu Close Menu
  • Terms and Conditions
  • Lists hosted on this site
  • Email the Postmaster
  • Tips for posting to public mailing lists
Re: broadcast address of a network interface
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: broadcast address of a network interface


  • Subject: Re: broadcast address of a network interface
  • From: Josh Graessley <email@hidden>
  • Date: Mon, 09 Feb 2009 09:58:30 -0800


Your best bet really is to use something like Bonjour to discover the service running on all the other devices, open individual TCP connections to all of the clients and send data to over each TCP connection. Just because the feature is "broadcast" doesn't mean the implementation needs to do that under the covers.


Multicast and broadcast over WiFi is extremely expensive. As someone pointed out, the access point doesn't know which wireless clients want the multicast/broadcast. Most unicast packets over the WiFi link are actually acked and retransmitted if they didn't make. Multicast/ broadcats are not. To increase reliability, they're transmitted at a very slow rate (often 2megabit) instead of 54 or whatever. The multicast packet takes about 25 times as long to transmit. In other words, in the time it takes to send a single multicast packet, the network could have sent 25 unicast packets. In the real world you don't really get 54 megabit and there's some overhead at the 802.11 layer with the acknowledgements and whatnot but even if you drop that optimistic 25 to 1 number down to 10 to 1, you're probably still going to see a performance win using unicast.

Bonjour uses a trick where it multicasts a question and sets a special bit to request that the clients respond using unicast. Normally, a multicast DNS answer to a query is sent using multicast to make duplicate detection quicker. When a computer joins a WiFi network for the first time or upon waking from sleep, it often has a number of open browse requests. Browsing for all of those services at once tends trigger a large number of answers. mDNSResponder will set a special bit requesting that answers be unicast. After the first query that collects lots of unicast answers, mDNSResponder sends another query allowing multicast answers for clients that don't understand the unicast answer bit. The query includes a list of known answers received via unicast so all those responders that see their answer already in the known answers section don't ever need to send a multicast. There is an exception where a responder will send a multicast if it hasn't sent a multicast in a while to help with duplicate detection still occurs.

-josh

On Feb 9, 2009, at 9:20 AM, 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?

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!

	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




_______________________________________________ Do not post admin requests to the list. They will be ignored. Macnetworkprog mailing list (email@hidden) Help/Unsubscribe/Update your Subscription: This email sent to email@hidden

_______________________________________________ Do not post admin requests to the list. They will be ignored. Macnetworkprog mailing list (email@hidden) Help/Unsubscribe/Update your Subscription: This email sent to email@hidden
  • 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>)

  • Prev by Date: Re: broadcast address of a network interface
  • Next by Date: Re: broadcast address of a network interface
  • Previous by thread: Re: broadcast address of a network interface
  • Next by thread: Re: broadcast address of a network interface
  • Index(es):
    • Date
    • Thread