MOSPF PIM Multicast Open Shortest Path First Protocol Independent Multicast

MOSPF PIM Multicast Open Shortest Path First Protocol Independent Multicast

(MOSPF) Multicast Open Shortest Path First Protocol Independent Multicast

Taxonomy of common multicast protocols

Taxonomy of common multicast protocols

  • Multicast link state routing uses the source based tree approach
  • Group membership LSA is flooded throughout the AS
  • The router calculates the shortest path trees on demand (when it receives the first multicast packet)
  • MOSPF is a data-driven protocol; the first time a MOSPF router see a datagram with a given source and group address, the router constructs the Dijkstra shortest path tree
  • The shortest path tree is made all at once instead of gradually (i.e. pre-made, pre-pruned, ready to use)

MOSPF with Areas

Group management

  • Group-membership LSA is flooded in the same area.
  • Inter-area multicast forwarders (area border routers) summarize their attached areas’ group membership to the backbone.

Data routing

  • Introduction of the wild-card multicast receivers (area border routers)
  • In the presence of OSPF areas, during tree pruning care must be taken so that the branches leading to other areas remain since it is unknown whether there are group members in these (remote) areas. For this reason, only those branches having no group members nor wild-card multicast receivers are pruned when producing the datagram shortest-path tree.
  • 1. Source area: building intra-area shortest path tree (forward cost) with leaf nodes including wild-card multicast receivers.
  • 2. Backbone area: each wild-card multicast receiver of the source area calculates the shortest path from the source to the multicast forwarders (with group members) of other areas using the reverse cost.
  • (You must know the reason why using the reverse cost in this case)E.g. In Figure 4 of the sample MOSPF area configuration in the supplementary document, RT3 and RT4 can calculate and compare to determine which one of them should construct the shortest path from the source to RT7, RT10, and RT11 (the multicast forwarders in non-source areas). The result tree is shown in Figure 9 in the supplementary document.
  • 3. Destination areas: The corresponding multicast forwarder (e.g. RT7 in area 2) constructs the shortest path using the reverse cost to each network (with group members) in its own area.


MOSPF Tree in Source Area

MOSPF Tree in Source Area

MOSPF Tree in Backbone Area

MOSPF Tree in Backbone Area

MOSPF Tree in Destination Area

MOSPF Tree in Destination Area

MOSPF PIM Multicast Open Shortest Path First Protocol Independent Multicast

Protocol Independent Multicast (PIM)

  • To develop a scalable protocol independent of any particular unicast protocol
    ANY unicast protocol to provide a routing table
  • A router can switch between DM and SM depending on the group

PIM Dense Mode (MOSPF)

  • Deployed in resource-rich environments
    RPM Algorithm
    Similar to DVMRP
  • Algorithm
    A datagram is forwarded if the arriving interface is the shortest path back to the source
    Datagram forwarded on all outgoing interfaces initially
    Create forwarding cache entry
    Prune and graft messages used to prune the tree
  • Leaf Network Detection
  • The absence of PIM Hello messages
  • No host membership reports
  • Pruning on a Multi-access LAN
  • A prune is sent upstream when the outgoing interface list is empty
    Upstream router schedules the interface for deletion (Delay 3 sec)
    Any other routers on the LAN that depend on the upstream router send a PIM-Join
    Deletion request for interface cancelled
    Randomly delay Join message to reduce traffic
    Prunes are flushed periodically
  • Graft and Graft ACKs
  • Designated Router in Multi-access LANs
  • Highest IP address router as seen in “Hello”
    Assert Messages
  • When duplicate packets arrive on a multi-access LAN
    Send Assert with metric for that source
    Choose router with a lower metric
    Equal metrics, higher IP address prevails
    Modify upstream and downstream neighbours

Sparse – Mode (MOSPF)

    group members are sparsely distributed throughout the network
    BW not widely available
    Receiver-initiated construction of the spanning tree
    Limit multicast traffic and hence improve scalability
    Define a Rendezvous Point (RP) and build the multicast tree around it
  • Algorithm
    The sender sends data to the RP
    Receivers JOIN the RP tree
    Difference from DM
    Receiver-initiated vs Data Driven
    SM routers maintain state info ( Primary RP)
    Conserve network resources
    Decreased amount of info in routers
    The concentration of traffic around RP
    Sub-optimal trees increase Latency

PIM Sparse Mode (MOSPF)

PIM Sparse Mode

  • Independent of particular unicast routing protocol
    Senders send data to the RP
    Receivers JOIN the tree
    Unwanted branches pruned
    Receivers can switch trees
  • Designated Router
    Multiple routers on a LAN: Highest IP address
    IGMP Queries
    JOIN/Prune messages towards RP
    Maintain status of active RP
    Route Entry
    Source address, Group Address
    Incoming interface (towards RP)
    Outgoing interfaces (towards receivers)
    WC-bit: Any source would match (*, G)
    RPT-bit: Join sent up the shared RP tree
  • Receivers: when a new host report received
    Look up primary RP for that group
    Unicast JOIN message to the RP
    Payload: Address G, Join = RP, WC_bit, RPT_bit, PRUNE = NULL
    Create forwarding route entry for (*, G) pair
    Delete cache entry when no more members
    Intermediate routers transmit JOIN to RP
    Create forwarding route
    entry for (*, G)

PIM Sparse

  • Hosts sending to Group
    Its DR looks up the active RP for that group
    Unicast data, encapsulated in a PIM-SM-Register to the RP
    Active RP sends a PIM-Join to the source DR
    Intermediate routers maintain state info (*, G)
    When source gets a Join, it sends further packets without encapsulation
    RP resends all data on the shared tree

PIM Sparse Mode Host Sending to Group

  • If data rate warrants an SPT, an (S, G) state created
    RP sends periodic Join/Prune to the source
    Intermediate routers maintain (S, G) state info
    Sources stop encapsulating data when they receive Register-Stop messages
    RP has no downstream members for that group or source
    RP already receives native data from (S, G) tree
  • Switching from RP-Shared tree to SPT
    Depending on the data rate of a particular source, a switchover may be initiated
    Only by RP or routers with members
    Activate (S,G) route entry
  • JOIN/PRUNE message from the Rx towards the source
    Payload: Address G, Join = S, Prune = NULL
    Prune towards the RP for that (S, G)
  • The steady state of the distribution tree
    Each router periodically sends JOIN/PRUNE for each active route entry
    To the neighbour indicated in the route entry
    Helps capture changes in topology/state/membership

PIM Sparse Mode Steady State of Distribution Tree

  • RP Information
    Bootstrap messages are used to distribute RP information within the domain
    Domains’ Bootstrap Router (BSR) elected from a set of candidates
    A set of RP candidates periodically advertise to the BSR the groups associated with them
    C-RP-Advertisements: Address of C-RP, Group address and mask
    C-RP-Advs distributed in BS messages
    The Advs are used by DRs
    use a Hash function to map a group address to one C-RP whose ad includes the group
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