Multicast Member Priority

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Hi,

I have come across a problem whose specifics are as follows:

I have a multicast group in which i have observed that, a udp packet sent by a member in a multicast group is received by other membes based on the sequence in which they have joined the group.

for eg: If server1,2,3,4 join groups in respective sequence then if a packet is received in the group it is received first by 1 then 2 and so on.

I have checked this behaviour via some test applications aswell.

I am using Winsock2 ,Please let me know if their is sequencing at kernel level and to priortize the group members if possible.

Posted 22-Oct-12 18:58pm

ganeshkawade2003

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michaelmel · Answer 1 · 2012-10-24T20:43:00

This type of prioritization doesn't make sense and it cannot be a general rule.

Consider a multicast group, and assume all your computers are on the same Ethernet segment for the moment to illustrate the point.

When a UDP packet is broadcast, all other members of the multicast group will receive it at the same point in time (excluding propagation delay) as everything happens, effectively, on the same piece of wire.

The processing will take different time depending on the state of the receiving computer. There is no way that the receiving computers have any way of determining which one should react to the packet first and so on.

The same would apply to a situation where computers are on more than just one subnet.

So what you are observing is specific to your setup. May be those computers that have joined the multicast group later have done so because they are located further (e.g. separated by more switches/routers). If this is indeed the case, you will observe the behaviour that you have described. However, the root cause is NOT the order of joining the multicast group, but the topology of the network.