Multicast Routing Algorithms n Multicast routing n Flooding and Spanning Tree n Forward Shortest Path algorithm n Reversed Path Forwarding (RPF) algorithms n Steiner Tree (ST) algorithm n Shared tree algorithms

Multicast Routing

Construction of Distribution Tree in Case of Many Sources  For forwarding multicast packets across internet, we should use multicast protocols  Several algorithms may be used by the protocols  In case of many sources the multicast routing algorithms can handle the distribution of the packets in three ways: n Primitive methods –Flooding –Spanning Tree n Source Based Tree –Forward Shortest Path –(Reverse Path Forwarding (RPF) algorithms – not detailed) –Steiner Trees (ST) n Shared Tree –Center Based Trees (CBT)

Primitive Methods n These do not take into account: –the multicast traffic refers to different multicast groups –which group member is sender and which is receiver n Such are the Flooding and the Spanning Tree algorithms n These are regarded as basic procedures –They are used as building blocks of the more sophisticated algorithms regarding construction of distribution tree

Flooding n When the router receives a multicast packet first it checks whether it has already met with this n If this is the first occasion, the router forwards it to its all interfaces, except that from which the packet arrived n If it has already met with that packet, the router will drop it out n In such a way all router will get the packet once at least n Its disadvantages: –It generates high number multiplied packets –It wastes network bandwidth –Every router must maintain table of the lately arrived packets, which requires a huge memory

Flooding n When a router receives a packet, the router will determine whether it’s the first time it receives the packet. If so, the packet will be delivered to all interfaces except the one on which it arrived, otherwise the packet will be discarded. n No routing tables needed n Inefficient use of bandwidth

The Operation of the Flooding

Spanning Tree  Only one active path connects any two of routers  The multicast packets will not loop and reach all routers  May not provide the most efficient path between the source subnetwork and group members

Spanning Tree

The Operation of the Spanning Tree Algorithm

n Distribution trees – Source tree n Uses more memory O(N S x N G ) but you get optimal paths from source to all receivers, minimizes delay – Shared tree n Uses less memory O(N G ) but you may get suboptimal paths from source to all receivers, may introduce extra delay Multicast Routing Algorithms—Characteristics

Source Based Tree n Each source creates its own distribution tree n The advantages of such a kind of algorithms are –the higher end-to-end power »E.g., smaller delay –sharing the traffic load coming from each source on the network n Disadvantage is that each router must generate large routing table, since one routing item is needed for each source and each group

Forward Shortest Path n In case of link-state unicast routing algorithms every router know the whole network topology –Thus they can calculate the shortest path form the source to the receivers n This tree is different from the reverse shortest path (later) if the links are non- symmetrical

Reverse Path Forwarding n What is RPF? –A router forwards a multicast datagram if received on the interface used to send unicast datagrams to the source B C AF SourceReceiver Unicast Multicast DE

Reverse Path Forwarding n If the RPF check succeeds, the datagram is forwarded n If the RPF check fails, the datagram is typically silently discarded n When a datagram is forwarded, it is sent out each interface in the outgoing interface list n Packet is never forwarded back out the RPF interface!

Shortest Path or Source Distribution Tree Receiver 1 E BADF Source Notation: (S, G) S = Source G = Group C Receiver 2

The Reverse Path Forwarding (RPF) Algorithm-Family n These forward packet on their output interfaces if and only if the packet arrived on the interface, which is on the shortest path to the source n The “reverse path” naming is originated from the fact that after the multicast tree was built, the routers have to listen to the reverse path to the multicast source –If a packet arrive in an other interface, which is not in the shortest path to the source, the packet will be dropped out by the router

Finding the Shortest Path n If a packet comes and another packet has already arrived on a shorter way from the sender, then the new packet will be dropped out n But if the new packet arrived in the shorter path, from this point its path is the current shortest path n It is important, since the packet must carry the IP addresses of the routers along the path, and in case of longer path the size of the packet increases n This algorithm does not need any additional method to prevent against the packet multiplication, e.g., automatic hop count limiting mechanism, since the algorithm automatically limits the path of the packets to the shortest path

The RPF Tree Rooted in the Router A

The RPF Tree Rooted in the Router C

The Different Reverse Path Forwarding (RPF) Algorithms n Reverse Path Forwarding/Flooding/Broadcast (RPF) n (Sophisticated/Extended) Reverse Path Broadcast (RPB) n Truncated Reverse Path Broadcast (TRPB) n Reverse Path Multicast (RPM)

Reverse Path Forwarding/Flooding/Broadcast (RPF)

RPF A local router only accepts packets from the ‘nearest’ source (parent), otherwise the packets are discarded. Accepted packets will be forwarded to all interfaces except the source It does not take into account multicast group membership when building the distribution tree, so packets may be forwarded to non-membership subnetwork

(Sophisticated/Extended) Reversed Path Broadcast (RPB)

The RPB Tree

Truncated Reverse Path Broadcast (TRPB)

Truncated Reverse Path Broadcasting (TRPB) G1 G2 G3 Truncated Source, G1 It’s an enhancement of RPB With the help of IGMP, multicast routers determine the group membership on each leaf subnetwork, thus avoiding forwarding packets to non- members Eliminates unnecessary traffic on leaf subnetworks, but does not consider group memberships when building the branches of the distribution tree

TRPB Based Multicast Delivery

Reverse Path Multicasting (RPM)  RPM creates a delivery tree that spans only Subnetworks with group members, andSubnetworks with group members, and Routers along the shortest path to subnetworks with group membersRouters along the shortest path to subnetworks with group members  First packet will be sent to all interfaces except the source. If none of the subnetworks connected to the leaf router have group members, the leaf router may transmit a "prune" message on its parent link informing the upstream router not to forward packets in this group to the leaf  In RPM, ‘first packet’ still needs to be forwarded throughout the network.  Each router has to maintain state information for all groups. It will be impossible as the number of groups and sources increases.

Reversed Path Multicast (RPM)

Building The Distribution Tree using RPM

Steiner Trees (ST)

Shared Tree n The all sources sends the packet to the same tree n The same tree distributes the all packets n This algorithm concentrates the traffic to the small set of the network connections n This algorithm is not optimal if all sources are active in the same time –E.g., distributed interactive simulation application n This algorithm works well, when the source operates alternatively –E.g., audio-conference n Its advantage is that it needs one item per group, only n Such algorithm is e.g., Center Based Trees

Center Based Trees (CBT)  If a host wants to join one group, it has to send a unicast “join request” message to the core.  Compared to previous algorithms, CBT can use bandwidth and router resources more efficiently  CBT may result in traffic concentration and bottlenecks near core routers since traffic from all sources traverses the same set of links as it approaches the core  A single shared tree may create trees not as optimal as source-trees

Operation of the Center Based Trees (CBT) Algorithm

Shared Distribution Tree Receiver 1 B E A D (Shared Root) F Source 1Notation: (*, G) * = All Sources G = Group C Receiver 2 Source 2

Center Based Multicast Distribution Tree

Comparison of the properties related to the construction of the distribution tree

Comparison of the properties related to the traffic on the tree