8 Oct
2014
8 Oct
'14
4:31 a.m.
On Mon, 6 Oct 2014, Reuben Martin wrote:
On Monday, October 06, 2014 04:10:59 PM you wrote:
Sometimes, it is a good idea to read about the parts of things one thinks
they already know. IGMP is a very basic part of multicasting and while I
understood the idea of multicasting, the Linux multicasting how-to was
quite a good starting point. It is quite old (pre 2000 and 2.0 kernel) has
a few mistakes, but explains the basics pretty good. It talks about IGMP
v2. but I notice this machine (kernel 3.13) has IGMP v3.
AES67, however, requires IGMP v2 and notes:
IGMPv2 support is required because IGMPv3 devices operating on an IGMPv2
network experience a two minute startup delay looking for IGMPv3 services
on the network.
I don't know if the linux IGMP can be set to run v2ish or not (or if it
would even be a good idea).
With regard to the idea of generating a word clock from ptpd system
clock... I think this would be pushing the low latency limits of most
systems. Using this clock to generate a period clock would make more
sense. I do not know that period clock would be useful for generating a
word clock in HW though. I can understand why switches with a PTP clock
are expensive/rare. I can also see why digital audio devices often have
more than one processor (each having multi cores).
--
Len Ovens
www.ovenwerks.net
On Mon, 6 Oct 2014, Reuben Martin wrote:
IGMP is the protocol used to solicit membership to a multicast group. IGMP
snooping is the switch probing layer3 in order to monitor which ports have
connections subscribed to multicast groups, and uses that to determine which
multicast traffic should be sent to which port. Without IGMP snooping, the
multicast traffic is sent to all ports, which makes it no different than
broadcast traffic. And with multicast flooding all ports, you actually eat up
more bandwidth than if you sent duplicate unicast traffic to multiple locations.
IGMP Snooping built into the switch is very convenient. Otherwise the
multicast audio transport will flood your ports.
IGMP is one of those things mentioned in the AES67 spec. that I have to
read still. For some reason they have not bothered to spell out what IGMP
stands for as they have with just about everything else.