Multicast - duplicate packets

Answered Question
Sep 6th, 2007
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Hi all


apologies for the long post


The rules regarding multicast over multiple paths are clear. AD, then metric is used and in case of a tie, highest NH IP address router becomes the forwarder. so in no case duplicate packets would be received (constantly) by receivers.

the above applies when ONE source is used.


My issue is that I am planning on using 2 servers to multicast TV content. so, packets sent will be identical but sources different. there is no built in failover mechanism between these servers. so both have to be active and sending.


Using pim dense mode:

Routers in my network will be forwarding both packets as there will be 2 different entries ( S1,G) & (S2,G).


using pim sparse:

mroute will only have (*,G). does this mean rules I listed above will apply and routers will ensure only no packet duplication will occur ?


TIA


Sam

Correct Answer by Harold Ritter about 9 years 6 months ago

Sam,


It doesn't really matter which RP is active. If the sources are active at the same time then the two streams will get to the receivers.


If the receiver is not able to choose between the two sources for the same stream then you need to make sure only one source is active at any given time for a specific multicast group.


Regards,

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Pavel Bykov Thu, 09/06/2007 - 05:14
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When (*,G) is not used by any downstream destination, the PRUNE message is sent to the upstream router.


When a subscriber (e.g. IGMP) wants to use (*,G), the router sends a GRAFT to an upstream.



It sounds a bit off topic - but it's not. What I am trying to point out, is that a subscriber wants to listen to only one (*,G). That (*,G) is reached using RPF up to the location where the Source Tree is not pruned - the timer resets back to 3min and data starts flowing downstream through a GRAFTed tree.


Because PIM is protocol independent, if you have two sources with the same group address, only one source should be activated, because only one path to the source will be used (the closest one).


I think the theory of it is similar with Anycast-RP described in RFC 3446.


I'm assuming you are using two servers for backup purposes, therefore there is no point to discuss Pragmatic General Multicast.



How far apart are your servers, metric/cost wise?



Hope this helps.

Please rate all helpful posts

Harold Ritter Thu, 09/06/2007 - 05:27
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Are the two servers used for redundancy purposes?


In my experience with IPTV deployment, the receiver is usually capable of receiving the same stream from multiple sources and of deciding which one to select based on quality and or availability.


Regards,

cisco_lad2004 Thu, 09/06/2007 - 05:55
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Thank u both for the replies !


Yes, 2 servers will be used for redundancy.


I don't know if Set Top Boxes can pick or drop packets.


The 2 sources are as below.


S1........ S2

|....................|

RP1--------RP2

|....................|

N------------N


Please ignore the dots, I had to use them to plot space between devices.


so, 2 sources hanging off 2 RPs, which act as gateways to a ring of routers (N for network).


I am making the below points, please correct me if I am wrong:


1-when using 2 RPs, RP with higher IP will be the active RP.


2-if only one one RP is active, it will be teh only root to RPT. And then only one RP will forward what is received by source(if there is a listener and assuming sparse mode only is used and static RP).


3-If RP1 is active, when it receives Unicast register message from S2, it will drop them and send a register stop to S2.


if the above 3 points stand, I am back to One RP One source set up and duplicate packets.



TIA


Sam

Correct Answer
Harold Ritter Thu, 09/06/2007 - 07:40
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Sam,


It doesn't really matter which RP is active. If the sources are active at the same time then the two streams will get to the receivers.


If the receiver is not able to choose between the two sources for the same stream then you need to make sure only one source is active at any given time for a specific multicast group.


Regards,

cisco_lad2004 Thu, 09/06/2007 - 07:52
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Hi Harold



"there can be only one", This was also my initial conclusion and fear :-(


I will design a workaround to ensure minimal downtime should one source fail...even if it means a Technician enabling PIM on one link and disabling another.


Thanks again for ur replies.


Sam

Pavel Bykov Thu, 09/06/2007 - 09:50
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Oh, I misread your question. I thought you decided for PIM DM.


Please read RFC 3446 and other documents regarding Anycast-RP


That could greatly improve your multicast network.

Pavel Bykov Thu, 09/06/2007 - 09:55
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In PIM DM case, what if one source will be one hop further away? Will that make furthest server standing by because PIM relies on unicast routing protocol?

cisco_lad2004 Thu, 09/06/2007 - 10:24
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I dont think so.

both sources will stream, which will result on 2 sets of (S,G). so both duplication will occur.


I was investigating that Sparse mode only, will get rid of duplication as (*,G) will be generated by RP. I assumed only active RP will forward and become source of tree...I was mistaken.


Sam

cisco_lad2004 Fri, 09/07/2007 - 04:33
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Hi Mike


Very good paper that covers my needs !


Regards


Sam

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