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Some thoughts about multicast. This works great on a single subnet, but it's likely to just be treated as broadcast (in the absence of IGMP snooping on the swit

Some thoughts about multicast. This works great on a single subnet, but it's likely to just be treated as broadcast (in the absence of IGMP snooping on the switches). Making multicast work outside a single subnet (even off of the same router) requires multicast routing to be enabled (and it's usually not, because doing it without breaking things is complicated). Now for some comments that apply more generally to multicast (and not for this low volume chat application). Setting TTL=1 for multicast traffic is often bad, because it causes the first hop router to have to generate an ICMP time exceeded message. Having to do this lots and lots and lots of times per second can crush some legacy router platforms. Speaking as someone who builds networks just for multicast, we instruct our application developers to always use a large TTL and tell them we will ensure it doesn't go further than it should. It's just a practical course of action. But good job!

dgregd 14y ago
> Speaking as someone who builds networks just for multicast I heard about IP Multicast more than 10 years ago. But it never took off. Or at least I thought so. I am really curious what are practical applications of the networks you build. Do your clients use them for broadcasting video streams or something else?
- insaneirish 14y ago
  Nearly all financial market data is disseminated from market venues via multicast. It's the most efficient way to do it. However, you are right about multicast not being generally used. The consequences of this fact are that there is a limited subset of engineers who understand it well and that vendor hardware support for it is often substandard. We constantly have to provide feedback to Cisco, etc. on what sort of features should be in their hardware and what sorts of instrumentation can help operate and troubleshoot multicast networks.
- zb 14y ago
  IP multicast is used quite extensively for streaming video. Mostly in combined voice/video/data networks for IPTV (think hotel rooms, but also network service providers), as well for stuff like e.g. security cameras. On the public Internet it's still virtually unheard-of afaik, and I expect it to remain that way indefinitely. (I used to work on IP multicast support for network switches.)
  - fulafel 14y ago
    It used to be pretty popular on the public Internet back when a lot of the use was by academic institutions. I remember seeing some NASA stuff (?) on there before the local university management decided to put up a firewall and break all the interesting IP apps. ref. https://en.wikipedia.org/wiki/Mbone https://en.wikipedia.org/wiki/Mbone

dgregd 14y ago

> Speaking as someone who builds networks just for multicast I heard about IP Multicast more than 10 years ago. But it never took off. Or at least I thought so. I am really curious what are practical applications of the networks you build. Do your clients use them for broadcasting video streams or something else?

insaneirish 14y ago
Nearly all financial market data is disseminated from market venues via multicast. It's the most efficient way to do it. However, you are right about multicast not being generally used. The consequences of this fact are that there is a limited subset of engineers who understand it well and that vendor hardware support for it is often substandard. We constantly have to provide feedback to Cisco, etc. on what sort of features should be in their hardware and what sorts of instrumentation can help operate and troubleshoot multicast networks.
zb 14y ago
IP multicast is used quite extensively for streaming video. Mostly in combined voice/video/data networks for IPTV (think hotel rooms, but also network service providers), as well for stuff like e.g. security cameras. On the public Internet it's still virtually unheard-of afaik, and I expect it to remain that way indefinitely. (I used to work on IP multicast support for network switches.)
- fulafel 14y ago
  It used to be pretty popular on the public Internet back when a lot of the use was by academic institutions. I remember seeing some NASA stuff (?) on there before the local university management decided to put up a firewall and break all the interesting IP apps. ref. https://en.wikipedia.org/wiki/Mbone https://en.wikipedia.org/wiki/Mbone