mcast-2 - Protocol Design Concept: Soft State vs. Hard...

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Ahmed Helmy - UF 1 Protocol Design Concept: Soft State vs. Hard State Soft State: A state is refreshed periodically, and if it is not refreshed it times out and is removed. Example: in PIM-SM a state is created by the receipt of a Join message. A Join message is not acknowledged, but is sent periodically (every minute approx.)
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Ahmed Helmy - UF 2 A router maintains an entry timer for every state created. When a router receives a Join it restarts (or refreshes) that timer. When a router does not receive a Join for approx. 3 x Join refresh period (approx 3 min.s) it times out the timer and the entry is removed.
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Ahmed Helmy - UF 3 Hard state A state in a router is created once, and it remains until another message is sent to remove it. Usually uses an acknowledged message, Simple example: DVMRP uses a graft message to create a state.
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Ahmed Helmy - UF 4 The graft message is acknowledged. When a router receives a graft, it creates the state, and the state remains until a prune is sent to remove the forwarding state in that router.
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Ahmed Helmy - UF 5 Advantages and disadvantages For soft state: Since, in general, it is not acknowledged, it may lead to 'join latency'. For example, if a receiver joins the group and the join is lost, it has to wait until the next refresh period to send another join.
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Ahmed Helmy - UF 6 In hard state: Since, in general, it is acknowledged, it incurs less join latency (since the ack timer is probably 3 seconds while the refresh timer is approx 1 min.). The main advantage for soft state (vs. hard state) is its robustness to failures.
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Ahmed Helmy - UF 7 Crash scenarios: If A sends a graft message, it gets acknowledged, and B creates state A crashes and loses state State will remain permanently in B Packets will keep on getting to A unless/until a prune is sent B A Graft Graft Ack
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Ahmed Helmy - UF 8 If router B crashes and comes back up again, there is no way to recreate the state in B (because A already got ack for its graft). So DVMRP uses periodic broadcast to take care of this situation. In PIM-SM, the soft state (periodic refresh) mechanisms take care of the above crash scenarios.
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Ahmed Helmy - UF 9 Host A B C 1. IGMP Host- Membership Query 2. IGMP Host- Membership Report for G 3. Create (*,G) entry: Multicast address=G RP-address=C,WC=1,RP=1 outgoing interface list={1} incoming interface=2 4. Send Join/Prune message to B: Multicast address=G Join={C,WC,RP} Prune=Null 5. Create (*,G) entry: Multicast address=G RP-address=C,WC=1,RP=1 outgoing interface list={1} incoming interface=3 6. Send Join/Prune message to C: Multicast address=G Join={C,WC,RP} Prune=Null D 7. Create (*,G) entry: Multicast address=G RP-address=C,WC=1,RP=1 outgoing interface list={1} incoming interface=Null 1 2 1 2 3 1 ...
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mcast-2 - Protocol Design Concept: Soft State vs. Hard...

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