IP-Multicast (outline)

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IP-Multicast (outline)

• Motivation and Background
• Multicast vs. unicast
• Multicast Applications
• Delivery of Multicast
• Local delivery and multicast addressing
• WAN delivery and its model
• Group Membership Protocol (IGMP)
• Multicast Algorithms and Concepts
• Flooding, Spanning Tree, Reverse Path
  Broadcasting (RPB), Truncated RPB, Reverse Path
  Multicasting, Center-Based Trees

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Outline (Contd.)

• Multicast Routing Protocols
• Dense vs. Sparse Multicast
• DVMRP
• MOSPF
• PIM (PIM-DM, PIM-SM)
• Multicast and the Internet
• The MBONE
• Recent deployment

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Unicast vs. Multicast

• Multicast provides multipoint-to-multipoint
  communication
• Today majority of Internet applications rely on
  point-to-point transmission (e.g., TCP).
• IP-Multicast conserves bandwidth by replicating
  packets in the network only when necessary

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Unicast vs. Multicast
S
S
R1
R1
R2
R2
R3
R3
R4
R4
Multiple unicasts
Multicast
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Example Multicast Applications

• One-to-Many
• Scheduled audio/video distribution lectures,
  presentations
• Push media news headlines, weather updates
• Caching web site content other file-based
  updates sent to distributed replication/caching
  sites
• Announcements network time, configuration
  updates
• Monitoring stock prices, sensor equipment

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IP Multicast Applications (contd.)

• Many-to-One
• Resource discovery
• Data collection and sensing
• Auctions
• Polling

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IP Multicast Applications (contd.)

• Many-to-Many
• Multimedia Teleconferencing (audio, video, shared
  whiteboard, text editor)
• Collaboration
• Multi-Player Games
• Concurrent Processing
• Chat Groups
• Distributed Interactive Simulation

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• Currently The Multicast Backbone (MBONE) carries
  audio and video multicasts of IETF meetings, NASA
  space shuttle missions,.. etc.

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• How do hosts know about new groups?
• The Session Directory (SD) tool lists active
  multicast sessions on MBONE and allows to join a
  conference using MBONE tools
• vat (visual audio tool), rat (robust audio tool)
• vic (video tool)
• wb (shared white board)
• nte (network text editor), .. etc.

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More Applications ...

•  Resource Discovery
• Multicast may be used (instead of broadcast) to
  transmit to group members on the same LAN.
• Multicast may be used for resource discovery
  within a specific scope using the TTL field in
  the IP header.

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Multicast Scope ControlTTL Expanding-Ring Search

• to reach or find a nearby subset of a group

s
1
2
3
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Multicast Scope Control Administrative TTL
Boundaries

• to keep multicast traffic within an
  administrative domain, e.g., for privacy reasons

the rest of the Internet
TTL threshold set oninterfaces to these
links,greater than the diameterof the admin.
domain
an administrative domain
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Multicast Scope Control Administratively-Scoped
Addresses

• RFC 1112
• uses address range 239.0.0.0 239.255.255.255

the rest of the Internet
address boundary set oninterfaces to these links
an administrative domain
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Transmission and Delivery of Multicast Datagrams

• Over the same (LAN)
• The source addresses the IP packet to the
  multicast group
• The network interface card maps the Class D
  address to the corresponding IEEE-802 multicast
  address
• Receivers notify their IP layer to receive
  datagrams addressed to the group.
• Key issue is addressing filtration

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• Over different subnets
• Routers implement a multicast routing protocol
  that constructs the multicast delivery trees and
  supports multicast data packet forwarding.
• Routers implement a group membership protocol to
  learn about the existence of group members on
  directly attached subnets.
• Hosts implement the group membership protocol
  that provides the IP-multicast host model

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Addressing

• Types of IP addresses
• Unicast used to transmit packets to one
  destination.
• Broadcast used to send datagrams to entire
  subnet.
• Multicast used to deliver datagrams to a set of
  hosts (members of a multicast group) in various
  scattered subnets.

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• IP-Multicast is a best-effort service.
• Reliable/ordered delivery are not guaranteed.
• Reliability may be provided by upper-layer
  protocols (e.g., reliable multicast protocols).
• IP-Multicast packets include a "group address"
  (Class D) in the Destination field of the IP
  header.

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Multicast Addressing

•  An IP multicast group is identified by a Class D
  address.
• Multicast group addresses range from (224.0.0.0)
  to (239.255.255.255).

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• The Internet Assigned Numbers Authority (IANA)
  registers IP multicast groups.
• The block of multicast addresses ranging from
  (224.0.0.1) to (224.0.0.255) is reserved for
  local LAN multicast
• used by routing protocols and other low-level
  topology discovery or maintenance protocols
• E.g., "all-hosts" group (224.0.0.1),
  "all-routers group (224.0.0.2), "all DVMRP
  routers", etc.
• The range (239.0.0.0) to (239.255.255.255) are
  used for site-local "administratively scoped"
  applications.

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Mapping Class D to Ethernet Address

• All multicast addresses in IANA's reserved block
  begin with 01-00-5E (hex)
•  Mapping between a Class D and an Ethernet
  multicast address is obtained by
• placing the low-order 23 bits of the Class D
  address into the low-order 23 bits of IANA's
  reserved address block.

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• How multicast group address 224.10.8.5
  (E0-0A-08-05) is mapped into an Ethernet
  (IEEE-802) multicast address.
• The mapping may place up to 32 different IP
  groups into the same Ethernet address because the
  upper five bits of the IP multicast group ID are
  ignored.

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 The Multicast Host Model

• Hosts can join or leave a group at any time
• A host may be a member of multiple groups
• Senders need not be members of the group
• Participants do not know about each other
• The two components of IP-multicast
• the group membership protocol
• the multicast routing protocol

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Group Membership Protocol

• Routers need to learn about the presence of group
  members on directly attached subnets
• When a host joins a group
• it transmits a group membership message for the
  group(s) that it wishes to receive
• sets its IP process and network interface card to
  receive packets sent to those groups.

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• This receiver-initiated join process scales well
• as the group size increases, it becomes more
  likely for a new member to locate a nearby branch
  of the multicast distribution tree.

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• Multicast Routing Protocols
• Run on routers and establish the multicast
  distribution tree to forward packets from
  sender(s) to group members.
• Based on unicast routing concepts
• DVMRP is a distance-vector routing protocol,
• MOSPF is an extension to the OSPF link-state
  unicast routing protocol.
• Center-based trees (e.g., CBT PIM-SM) introduce
  the notion of the tree core.

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Internet Group Management Protocol (IGMP)

• IGMP runs between hosts and their immediately
  neighboring multicast routers.
• The protocol allows a host to inform its
  first-hop router that it wishes to receive
  packets destined to a specific group.

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Router Operation in IGMP

• Routers periodically query the LAN to determine
  if group members are still active.
• One router per LAN is elected as "querier" to
  query for group members.
• Through IGMP a router determines which multicast
  traffic needs to be forwarded to each of its
  "leaf" subnets.

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IGMP Version 1

• RFC-1112
• To determine local group membership
• Multicast routers periodically transmit Host
  Membership Query messages
• Queries are ddressed to the all-hosts group
  (224.0.0.1) with TTL 1 (i.e., not forwarded by
  any other multicast router).

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Hosts Joining Groups

•  Upon receiving a Query, a host responds with a
  Host Membership Report for each group that it
  wishes to Join
• Observation The router only needs to know of at
  least one group member on the leaf subnet

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Report Suppression Mechanism

• To avoid Report implosion
• Each host starts a random delay timer for its
  Reports.
• If during the delay period another Report is
  heard for the same group, the host resets its
  timer
• Otherwise, the host transmits a Report causing
  other group members to reset their timers
• Thus, Reports are spread out over time and Report
  traffic is minimized

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• IGMP-Query Message

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Updating Local Membership

• The querier periodically transmits Queries to
  update local membership
•  If no Report is received for a group after a
  number of Queries, the router assumes that
  members are no longer present on that LAN
• the group is removed from the membership list of
  that interface/subnet

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Reducing Join Latency

• When a host first joins a group, it immediately
  transmits a Report for the group rather than
  waiting for a router Query.

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IGMP Version 2 (IGMP V2)

•  IGMP V2 was part of IP-mcast (V3.3-3.8)
• spec ltdraft-ietf-idmr-igmp-v2-01.txtgt
• IGMP V2 enhances IGMP V1
• IGMP V2 elects one querier for each LAN, the
  router with the lowest IP address.
• In V1, the querier election was done by the
  multicast routing protocol (different multicast
  routing protocol used different methods).

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•  IGMP V2 defines a new Query message, the
  Group-Specific Query, to Query a specific group
  rather than all groups

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Reducing Leave Latency

• To reduce leave latency V2 defines a Leave
  Group message
• When a host leaves a group, it sends a Leave
  Group to the all-routers group (224.0.0.2) with
  the group field set to the group to be left.
• Upon receiving a Leave from a LAN, the querier
  sends Group-Specific Query on that LAN.
• If there are no Reports in response to the
  Group-Specific Query, the group is removed from
  the membership list of that subnet.

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• Observation With IGMP V1 and V2, if a host wants
  to receive any sources from a group, the traffic
  from all sources for the group has to be
  forwarded onto the subnet.

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IGMP Version 3 (IGMP v3)

• Spec ltdraft-ietf-idmr-igmp-v3-03.txtgt
• IGMP V3 supports Group-Source Reports
• A host can elect to receive traffic from specific
  sources of a multicast group.
• An inclusion Group-Source Report specifies the
  sources a host wants to receive.
• An exclusion Group-Source Report identifies the
  sources a host does not want to receive.

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• IGMP v3 enhances support for Leave Group messages
  to support Group-Source Leave messages
• A host can leave an entire group or specific
  (source, group) pair(s).

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Multicast Forwarding Algorithms

• A multicast routing protocol is responsible for
  the establishment of the multicast distribution
  tree and for performing packet forwarding.

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• Several algorithms may be employed by multicast
  routing protocols
• Flooding
• Spanning Trees
• Reverse Path Broadcasting (RPB)
• Truncated Reverse Path Broadcasting (TRPB)
• Reverse Path Multicasting (RPM)
• Core-Based Trees

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• These algorithms are implemented in the most
  prevalent multicast routing protocols in the
  Internet today.
• Distance Vector Multicast Routing Protocol
  (DVMRP)
• Multicast OSPF (MOSPF)
• Protocol-Independent Multicast (PIM) PIM-DM and
  PIM-SM

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Flooding

• The simplest technique for multicast delivery.
• When a router receives a multicast packet it
  determines whether or not this is the first time
  it has seen this packet.
• On first reception, a packet is forwarded on all
  interfaces except the one on which it arrived.
• If the router has seen the packet before, it is
  discarded.

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•  A router does not maintain a routing table, but
  needs to keep track of recently seen packets.
• Flooding does not scale for Internet-wide
  application
• Generates a large number of duplicate packets and
  uses all available paths across the internetwork.
  
• Routers maintain a distinct table entry for each
  recently seen packet (consumes memory).

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 Spanning Tree

• More effective than flooding
• Defines a tree structure where one active path
  connects any two routers on the Internet.
• Spanning Tree rooted at R

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•  A router forwards each multicast packet to
  interfaces that are part of the spanning tree
  except the receiving interface.
• A spanning tree avoids looping of multicast
  packets and reaches all routers in the network.

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• A spanning tree algorithm is easy to implement
• However, a spanning tree solution
• may centralize traffic on small number of links
• may not provide the most efficient path between
  the source and the group members.

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Reverse Path Broadcasting (RPB)

• More efficient than building a single spanning
  tree for the entire Internet.
• Establishes source-rooted distribution trees for
  every source subnet.
• A different spanning tree is constructed for each
  active (source, group) pair.

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RPB Algorithm

• For each (source, group) pair
• if a packet arrives on a link that the router
  considers to be the shortest path back to the
  source of the packet
• then the router forwards the packet on all
  interfaces except the incoming interface.
• Otherwise, the packet is discarded.

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• The interface over which a router accepts
  multicast packets from a particular source is
  called the "parent" link.
• The outbound links over which a router forwards
  the multicast packets are called the "child"
  links.

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• Reverse Path Broadcasting (RPB) Forwarding

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•  Enhancement to reduce packet duplication
• A router determines if a neighboring router
  considers it to be on the shortest path back to
  the source.
• If Yes, the packet is forwarded to the neighbor.
• Otherwise, the packet is not forwarded on that
  potential child link.

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•  To derive the parent-child information
• link-state routing protocol already has it (since
  each router maintains a topological database for
  the entire routing domain).
• distance-vector routing protocol uses poison
  reverse
• a neighbor can either advertise its previous hop
  for the source subnet as part of its routing
  update messages or "poison reverse" the route.

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•  Example of Reverse Path Broadcasting

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Benefits

• Reasonably efficient and easy to implement.
• Does not require keeping track of previous
  packets, as flooding does.
• Multicast packets follow the "shortest" path from
  the source to the group members.
• Avoids concentration over single spanning tree

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Limitations

• Does not take into account group membership when
  building the distribution tree.
• As a result, packets may be unnecessarily
  forwarded to subnets with no group members.

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Truncated Reverse Path Broadcasting (TRPB)

• Using IGMP, routers discover group members and
  avoid forwarding packets onto leaf subnets with
  no members.
• The spanning delivery tree is "truncated" if a
  leaf subnet has no group members.

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•  Truncated Reverse Path Broadcasting (TRPB)

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• TRPB eliminates unnecessary traffic on leaf
  subnets
• But it does not consider group membership when
  building the branches of the distribution tree.

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Reverse Path Multicasting (RPM)

• RPM enhances TRPB.
• RPM creates a delivery tree that spans only
  -Subnets with group members-Routers and subnets
  along the shortest path to group members
• In RPM, non-member branches are pruned
• Packets are forwarded only along branches leading
  to group members.

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RPM Operation

• The first multicast packet is forwarded (using
  TRPB) to all routers in the network.
• Routers at edges of the network with no
  downstream routers are called leafrouters.
• A leaf router with no downstream members sends a
  "prune" message on its parent link to stop packet
  flow down that branch.

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• Prune messages are sent hop-by-hop back toward
  the source.
• A router receiving a prune message stores the
  prune state in memory.
• A router with no local members that receives
  prunes on all child interfaces sends a prune one
  hop back toward the source.
• This succession of prune messages creates a
  multicast forwarding tree that contains only
  branches that lead to group members.

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• Reverse Path Multicasting (RPM)

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•  To adapt to membership/network dynamics, the
  prune state is timed out periodically, and
  packets are broadcast throughout the network.
• This may result in a burst of prune messages.

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Limitations

• Despite improvements over RPM, there are scaling
  issues and limitations
• Multicast packets are periodically forwarded to
  every router in the network.
• Routers maintain prune state off-tree for all
  (source,group) pairs.
• These limitations are amplified with increase in
  number of sources and groups.

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Center/Core-Based Trees (CBT)

• Earlier algorithms build source-based trees
• CBT builds a single delivery tree (rooted at the
  core) that is shared by all group members.
• Multicast traffic for each group is sent and
  received over the shared tree, regardless of the
  source.

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CBT Operation

• A core-based tree involves one or more cores in
  the CBT domain.
• Each leaf-router of a group sends a hop-by-hop
  "join" message toward the "core tree" of that
  group.
• Routers need to know the group core to send the
  join request.

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• Intermediate routers process the join request
• The interface on which the join was received is
  added to the delivery tree.
• Intermediate routers forward join requests toward
  the core until the join reaches a core or a
  router on the distribution tree.
• Senders unicast their packets toward the core.
• When the unicast packet reaches a member of the
  delivery tree, the packet is multicast to all
  outgoing interfaces that are part of the tree.

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Benefits

• Advantages over RPM, in terms of scalability
• A router maintains state information for each
  group, not for each (source, group) pair.
• Multicast packets only flow down branches leading
  to members (not periodically broadcast).
• Only join state is kept on-tree

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Limitations

• CBT may result in traffic concentration near the
  core since traffic from all sources traverses the
  same set of links as it approaches the core.
• A single shared delivery tree may create
  sub-optimal routes resulting in increased delay.
• Core management issues
• dynamic core selection
• core placement strategies

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Multicast Routing Protocols

• In general, there are two classes of multicast
  routing protocols
• Dense-mode protocols (broadcast-and-prune)
• DVMRP, PIM-DM, (MOSPF!)
• Sparse-mode protocols (explicit-join)
• PIM-SM, CBT, BGMP

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Dense vs. Sparse Mode Multicast
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R1
R2
R3
R4
Dense-Mode Multicast
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Dense vs. Sparse Mode Multicast
S
R1
Root
R2
R3
R4
Sparse-Mode Multicast
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Distance Vector Multicast Routing Protocol (DVMRP)

• DVMRP constructs source-rooted trees using
  variants of RPM.
• Many MBONE routers run DVMRP
• DVMRP was first defined in RFC-1075.
• The original spec was derived from the Routing
  Information Protocol (RIP) and used TRPB.
• Mrouted version 3.8 uses RPM.

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DVMRP Basic Operation

• DVMRP implements RPM.
• The first packet for any (source, group) pair is
  broadcast to the entire network, providing the
  packet's TTL permits.
• Leaf routers with no local members send prune
  messages back toward the source.

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• Prune messages cause the removal of branches that
  do not lead to group members
• The result is source-specific shortest path tree
  with all leaves having group members.
• After a period of time, the pruned branches grow
  back and the packets are broadcast throughout the
  network.
•  A graft mechanism helps to quickly
  re-establish previously pruned branches.

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• A new member joining the group causes the
  first-hop router to send a graft message to the
  group's previous-hop router.
• When an upstream router receives a graft message,
  it removes the prune state.
• Graft messages may cascade back toward the source
  allowing previously pruned branches to be
  restored.

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Example DVMRP Scenario
g
g
s
g
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Initial Broadcast using Truncated Broadcast
g
g
s
g
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Prune non-member branches
g
g
prune (s,g)
prune (s,g)
s
g
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Graft new members
g
g
g
report (g)
graft (s,g)
graft (s,g)
s
g
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DVMRP Distribution Tree
g
g
g
s
g
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Avoiding duplicates on LANs

• To avoid duplicates, one router per LAN is
  elected the Dominant Router

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• The router with lowest metric to the source
  subnet (with the lowest IP address as tie
  breaker) becomes the Dominant router
• A dominant router is the forwarder for the LAN
  for traffic from the source subnet

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DVMRP Forwarding Table

• Entries in a typical DVMRP forwarding table
•  Source Subnet
•  Multicast Group
•  InPort - The parent port for the (S, G) pair. A
  "Pr" indicates that a prune was sent to upstream.
  
•  OutPorts - The child ports over which packets
  for the (S, G) pair are forwarded. A p
  indicates prune message received on that
  interface.

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DVMRP Forwarding Table
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Multicast Extensions to OSPF (MOSPF)

• OSPF V2 is defined in RFC-1583.
• OSPF is a unicast routing protocol that
  distributes topology information and calculates
  routes for a single domain.
• MOSPF is defined in RFC-1584.
• MOSPF routers maintain a current image of the
  network topology through the unicast OSPF
  link-state routing protocol.

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• MOSPF does not support tunnels
• Basic MOSPF runs in a single OSPF domain
• MOSPF uses IGMP to discover members on directly
  attached subnets.
• The Designated Router (DR) is responsible for
  sending membership information to all routers in
  the OSPF domain.
• The DR floods Group-Membership Link State
  Advertisements (LSAs) throughout the OSPF domain.

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Building the Shortest Path Tree

• The shortest path tree for (S, G) pair is built
  "on demand" when a router receives the first
  packet for (S,G).
• When the initial packet arrives, the source
  subnet is located in MOSPF link state database.
• MOSPF LS-DB OSPF LS-DB Group-Membership LSAs

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• Source-rooted shortest-path tree is constructed
  using Dijkstra's algorithm.
• To forward packets to downstream members, each
  router determines its position in the shortest
  path tree
• After the tree is built, Group-Membership LSAs
  are used to prune those branches that do not lead
  to group members.

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• All routers within an OSPF domain calculate the
  same shortest path trees.
• MOSPF LS-DB allow a router to perform RPM
  computation "in memory".
• No need for broadcast and prune.

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Forwarding Cache

• Forwarding cache entry contains the (source,
  group) pair, the upstream node, and the
  downstream interfaces.
• MOSPF Forwarding Cache

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•  The forwarding cache contains
•  Destination The group address
•  Source The packets source subnet.
•  Upstream The interface from which (S,G) packets
  are received.
•  Downstream The interfaces to which (S,G)
  packets are forwarded
•  TTL min. number of hops a packet needs to reach
  the group members. This allows the router to
  discard packets with no chance of reaching the
  members.

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• The forwarding cache is not aged. The forwarding
  cache will change when
• The topology of the OSPF network changes, forcing
  all of the datagram shortest-path trees to be
  recalculated.
• There is a change in the Group-Membership LSAs
  indicating that the distribution of individual
  group members has changed.

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Limitations

• Limited to OSPF domains
• Flooding membership information does not scale
  well for Internet-wide multicsat

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Protocol-Independent Multicast (PIM)

• Design Rationale
• Broadcast and prune keeps state off-tree and is
  suitable when members are densely distributed
• Explicit join/center-based approach keeps state
  on-tree and is suitable when members are sparsely
  distributed
• PIM attempts to combine the best of both worlds

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Design Choices

• Shared trees or shortest path trees?
• Both use shared trees to Rendezvous then
  switch to shortest path to deliver
• DV or LS for routing?
• Use routing tables regardless of which protocol
  created them (hence the name Protocol
  Independent)

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PIM Operation Modes

• PIM provides both dense-mode (DM) and sparse-mode
  (SM) protocols
• PIM-DM similar to DVMRP but does not build its
  own routing table
• PIM-SM similar to CBT but provides switching to
  SPT and bootstrap mechanism for electing the tree
  center dynamically

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How PIM-DM works

• Packets initially flow on broadcast tree
• Forwarded away from source using the RPF
  algorithm
• A router forwards a multicast datagram if
  received on the interface used to send unicast
  datagrams to the source
• Then, Prunes are sent up the tree to remove
  branches with no members

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How PIM-DM works
Source
Receiver 1
Receiver 2
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How PIM-DM works
Source
Prune
Receiver 2
Receiver 1
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How PIM-DM works
Source
Asserts
Receiver 2
Receiver 1
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How PIM-DM works
Source
Receiver 2
Receiver 1
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How PIM-DM works
Source
Prune
Join Override
Prune
Receiver 2
Receiver 1
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How PIM-DM works
Source
Receiver 1
Receiver 2
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How PIM-DM works
Source
Graft
Receiver 2
Receiver 1
Receiver 3
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How PIM-DM works
Source
Receiver 1
Receiver 2
Receiver 3
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How PIM-SM works

• A Rendezvous Point (RP) is chosen as tree center
  per group to enable members and senders to meet
• Members send their explicit joins toward the RP
• Senders send their packets to the RP
• Packets flow only where there is join state
• (,G) any-source,group state is kept in routers
  between receivers and the RP

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How PIM-SM works

• When should we use shared-trees versus
  source-trees?
• Source-trees tradeoff low-delay from source with
  more router state
• Shared-trees tradeoff higher-delay from source
  with less router state
• Switch to the source-tree if the data rate is
  above a certain threshold

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How PIM-SM works
Source
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How PIM-SM works
Source
B
A
D
RP
E
C
(, G) Join
Receiver 2
Receiver 1
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How PIM-SM works
Source
B
A
D
RP
E
C
Receiver 2
Receiver 1
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How PIM-SM works
Source
Register
B
A
D
RP
E
C
Receiver 2
Receiver 1
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How PIM-SM works
Source
(S, G) Join
(S, G) Join
B
A
D
RP
E
C
Receiver 2
Receiver 1
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How PIM-SM works
Source
Register-Stop
B
A
D
RP
E
C
Receiver 2
Receiver 1
121
How PIM-SM works
Source
B
A
D
RP
(S, G) Join
E
C
Receiver 2
Receiver 1
122
How PIM-SM works
Source
(S, G) Prune
B
A
D
RP
E
C
(S, G) RP Bit Prune
Receiver 2
Receiver 1
123
How PIM-SM works
Source
B
A
D
RP
E
C
(, G) Join
Receiver 2
Receiver 1
124
How PIM-SM works
Source
B
A
D
RP
E
C
Receiver 2
Receiver 1