Recalling Multicast so far

Last day, we started talking about IP Multicast.
We talked about how the basic IP Multicast model is many-to-many and is sometimes called Any Source Multicast (ASM).
There is also a one-to-many variant called Source-Specific-Multicast (SSM).
We said the common protocol for managing multicast groups is IGMP for IPv4 and is MDL for IPv6.
Finally, we described how the significant 28 bits of a IP multi-cast address are mapped to the significant 23 bits of an Ethernet multicast address.

Multicast Forwarding and Routing

A router's unicast forwarding table indicates for any packet destined to an IP address which link to use to forward the packet.
In the multicast setting one might have multiple entries in the forwarding table for a given IP address and the router needs to send the packet to each of them.
Thus, if we look at the set of nodes that a packet might follow in each setting, then in the unicast setting they will form a path in the network graph and in the multicast setting they will form a forest of trees.
This forest is called the multicast distribution trees.
We next look at some different routing algorithms for building multicast forwarding tables.

This is a so-called flood and prune protocol.
When a multicast packet comes into a router with source S, the router checks if it arrived on the port that its fowarding tables says is the shortest path to S. If and only if the answer is yes, the packet is flooded out on each of the other ports.
If two or more routers are connected to the same network, each keeps a parent bit for every forwarding entry. For a given entry, if the parent bit is not set then it doesn't do the forwarding in the above.
Initially this bit is set. When table information is exchanged between routers, the router determines if anyone either has a shorter route or the same length route but smaller address. If so, the bit is turned off.
This completes the description of the flood part of the protocol.
The above mechanism essentially floods the packet to the whole internetwork. Next a pruning mechanism need to be used to reduce this to just the next networks that have members of the multi-cast group.
If the parent router on a network is the only router on that network then it is a leaf.
Periodically each host in the group is supposed to send a packet announcing that it is in the group.
When a parent router of a leaf, exchanges (Destination, Cost) pairs with it neighbors, it augments this with the set of groups for which it has hosts.
This information is then propagated from router to router, so that a given router knows for what groups it should forward multicast packets.

PIM mean the multicast protocol is independent of the unicast routing of the network in question.
The original PIM-DM (dense mode) protocol used a flood and prune algorithm much like DVMRP.
In PIM-SM (sparse mode) has become the dominant multicast routing protocol.
Unlike DVRMP, routers in PIM-SM must explicity join the multicast distribution tree using Join messages.
PIM-SM assigns each group a special router called a rendezvous point (RP).
If a router is configured to be an RP it will periodically announce this candidacy to a bootstrap router.
Groups are assigned to RP's by applying a known hash function to the Group IP address to get an RP number.
The multicast forwarding tree is built as a result of sending Join messages using unicast to the RP.
In order to get to an RP, a host looks up the RP IP address in the BSR that correspond to the RP number it got in applying the hash function to the Group IP.
PIM-SM can operate in two modes shared (anyone can send to group) and source specific.

Once the tree is built, to send to the multicast group a sender sends on its local network to a router known as the desginated router (DR), this then opens a tunnel to the RP using a PIM Register message and the packet is sent there, finally the RP then sends the packet to the group.
To make the scheme more efficient the RP send join messages back along the path to the DR, so the DR can send as native multicast rather than tunneled multicast
The need to make it easier to create so-called source specific multicast led to a variation on PIM-SM called PIM-SSM (source specific multicast), which uses a notion of a channel, which a combination of the source and group address.
To send multi-cast across domains, an extension PIM-SM called Multicast Source Discovery Protocol was devised, that makes use of MSDP peers in each domain.

To finish off our discussion of networking we are going to look at one more important protocol: Multiprotocol Label Switching (MPLS).
The protocol combines some of the properties of virtual circuits with the flexibility and robustness of datagrams.
MPLS is mainly used today for:
- To enable IP capabilities on devices that do not have the capability to forward datagrams in the usual manner.
- To forward IP packets along "explicit routes" -- explicit routes that don't necessarily match those that normal IP would select.
- To support certain types of virtual private networks.

The idea of MPLS is to add a label to an IP packet.
When MPLS is enabled on a router, the router allocates a label for each prefix in its routing table.
Basically, it just uses the index of that entry in the table. So if 18.1.1/24 was the 3rd row in the table, the label might be 3.
It advertises this label and network prefix to its neighbors according to the Label Distribution Protocol.
When a packet arrives at the first router in an MPLS network, a label edge router, a complete IP lookup on the packet is done, and the label of the router that the packet should be sent to is applied to that packet.
The packet is then sent to the next router that understand MPLS. This router only needs to look at the label (not the IP address) to know which port to go out on and which label to replace the current label with.
One advantage to doing this is that ATM networks can now be used to forward IP packets using MPLS rather that need to do an special tricks to try to carry the IP packets across the network.
Labels are attached between the layer 2 and layer 3 headers.