Term
| Explain the basic concept of how multicast traffic flows through a network. |
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Definition
| Multicast traffic flows from the source to the multicast group over a distribution tree that connects all of the sources to all the receivers in the group. This tree may be shared by all sources (a shared tree) or a separate distribution tree can be built for each source (a source tree). |
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Term
| Explain the notation of an entry in the multicast routing table. |
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Definition
| The notions used in multicast routing tables include (S, G), which refers to a unicast source for the multicast group G, then the multicast group G itself, and (*, G), which refers to any source for the multicast group G, then again, the multicast group G itself. |
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Term
| What is the simplest for of a multicast distribution tree? |
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Definition
| The simplest form of a multicast distribution tree is a source tree, which has its roots at the source host and has branches forming a spanning tree through the network to the receivers. Because this tree uses the shortest path through the network, it is also referred to as a shortest path tree (SPT). |
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Term
| What is an advantage of using source trees with multicasting? |
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Definition
| Source trees have the advantage of creating the optimal path between the source and the receivers, and this advantage guarantees the minimum amount of network latency for forwarding multicast tree. This optimization does come at a cost, where routers must maintain path information for each source, and in large networks, this can become a resource issue on the routers. |
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Term
| What is an advantage of using shared trees with multicasting? |
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Definition
| Shared trees have the advantage of requiring the minimum amount of state in each router, where this lowers the overall memory requirements for a network that only allowed shared trees. The disadvantage of shared trees is that under certain circumstances the paths between the source and receivers might not be the optimal path, which might introduce some latency in packet delivery. |
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Term
| How is a router able to guarantee that a distribution tree is loop-free? |
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Definition
| A router utilizes Reverse Path Forwarding (RPF), which is the forwarding of multicast traffic away from the router, rather than to the receiver, to help guarantee that the distribution tree is loop-free. When a multicast packet arrives at a router, the router performs an RPF check on the packet, and if the check succeeds, the packet is forwarded. Otherwise, it is dropped. |
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Term
| What does PIM use to determine the correct upstream and downstream neighbors of a distribution tree? |
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Definition
| PIM makes use of the existing unicast routing table to determine the upstream and downstream neighbors. A router will forward a multicast packet only if it is received on the upstream interface. |
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Term
| How does a router check that a multicast packet has arrived through the correct ingress port? |
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Definition
| For traffic flowing down a source tree, the router first looks up the source address in the unicast routing table to determine if the packet has arrived on the interface that is on the reverse path back to the source. If the packet haws arrived on the interface leading back to the source, the RPF check succeeds and the packet is forwarded out the interfaces present in the outgoing interface list of a multicast routing entry. If the RPF check fails, the packet is dropped. |
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Term
| What can a host do to reduce the wait time for joining a multicast group? |
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Definition
| If a host knows of a group it wants to join, it can send a join message directly to its router without waiting for a query. Upon receiving the join request, the router immediately forwards the multicast traffic onto the subnet. |
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Term
| What does a router do it it has no group members on any of its attached subnets? |
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Definition
| If a router has no group members on any of its attached subnets, and the subnets are not transit networks for multicast traffic to other routers, the router itself can request that upstream neighbors not forward multicast traffic to it. If the router then receives a join request on one of its attached subnets, it can send a request upstream to begin receiving the relevant data stream. |
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Term
| Define the issue of join latency. |
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Definition
| Join Latency is the period between the time a host sends a join request and the time the host actually begins receiving group traffic. |
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Term
| Define the issue of leave latency. |
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Definition
| Leave latency is the period of time the last group member on a subnet leaves the group and the time the router stops forwarding group traffic on the subnet. |
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Term
| What can a host do to reduce the leave latency on a network? |
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Definition
| Allowing a host to explicitly notify its local router when it leaves a group can increase efficiency, so that rather than having to wait for no hosts to respond to a router's queries before it implicitly concludes that there are no group members on a subnet, the router can actively determine whether there are remaining members. |
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Term
| What does a router do if it receivers a leave notification from a host? |
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Definition
| When a router receives a leave notification from a host, it immediately sends a query only the subnet, asking qhether there are any remaining members. If no one responds, the router concludes that there are no more members and can cease forwarding packets for the group onto that subnet. If a router knows that it no longer has any group members on any of its subnets, it can "prune" itself from the multicast tree. |
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