Multicast EECS 122: Lecture 16

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MulticastEECS 122 Lecture 16

• Department of Electrical Engineering and Computer
  Sciences
• University of California
• Berkeley

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Broadcasting to Groups

• Many applications are not one-one
• Broadcast
• Group collaboration
• Proxy/Cache updates
• Resource Discovery
• Packets must reach a Group rather than a single
  destination
• Group membership may be dynamic
• More than one group member might be a source
• Idea After a group is established
• Interested receivers join the group
• The network takes care of group management
• Recall RSVP

Webcasts Radio/TV Push/IE Channels
Chats Video Conferencing Audio Conferencing
Caches and Proxies
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The Multicast service Model

• Membership access control
• open group anyone can join
• closed group restrictions on joining
• Sender access control
• anyone can send to group
• anyone in group can send to group
• only one host can send to group
• Packet delivery is best effort

R0
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Net
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Multicast and Layering

• Multicast can be implemented at different layers
• data link layer
• e.g. Ethernet multicast
• network layer
• e.g. IP multicast
• application layer
• e.g. as an overlay network like Kazaa
• Which layer is best?

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Multicast Implementation Issues

• How are multicast packets addressed?
• How is join implemented?
• How is send implemented?
• How does multicast traffic get routed?
• This is easy at the link layer and hardest at the
  network layer
• How much state is kept and who keeps it?

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Ethernet Multicast

• Reserve some Ethernet MAC addresses for multicast
• To join group G
• network interface card (NIC) normally only
  listens for packets sent to unicast address A and
  broadcast address B
• to join group G, NIC also listens for packets
  sent to multicast address G (NIC limits number of
  groups joined)
• implemented in hardware, so efficient
• To send to group G
• packet is flooded on all LAN segments, like
  broadcast
• can waste bandwidth, but LANs should not be very
  large
• Only host NICs keep state about who has joined ?
  scalable to large number of receivers, groups

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Limitations of Data Link Layer Multicast

• Single LAN
• limited to small number of hosts
• limited to low diameter latency
• essentially all the limitations of LANs compared
  to internetworks
• Broadcast doesnt cut it in larger networks

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IP Multicast Interconnecting LANS

• Interconnected LANs
• LANs support link-level multicast
• Map globally unique multicast address to
  LAN-based multicast address (LAN-specific
  algorithm)
• IP Group addresses are class D addresses
• 1110/28 or 224.0.0.0 to 239.255.255.255

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Internet Group Management ProtocolIGMP

• Operates between Router and local Hosts,
  typically attached via a LAN (e.g., Ethernet)
• Query response architecture
• Router periodically queries the local Hosts for
  group membership information
• Can be specific or general
• Hosts receiving query set a random timer before
  responding
• First host to respond sends membership reports
• All the other hosts observe the query and
  suppress their own repots.
• To Join send a group send an unsolicited Join
• Start a group by joining it
• To leave dont have to do anything
• Soft state

Query to 224.0.0.1
Report
Suppresses Report
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Naïve Routing Option Dont change anything
Point-to point routing
R0
R1
S
Net
. . .
Rn-1
Group abstraction not implemented in the network
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This approach does not scale
Broadcast Center
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Instead build trees
Copy data at routers At most one copy of a data
packet per link
Broadcast Center

• Routers keep track of groups in real-time
• Path computation is Tree computation

• LANs implement layer 2 multicast by broadcasting

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Routing Approaches

• Kinds of Trees
• Shared Tree
• Source Specific Trees
• Tree Computation Methods
• Intradomain Update methods
• Build on unicast Link State MOSPF
• Build on unicast Distance Vector DVMRP
• Protocol Independent PIM
• Interdomain routing BGMP
• This is still evolving

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Source Specific Trees
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Each source is the route of its own tree.
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Source Specific Trees
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Each source is the route of its own tree. One
tree for each source
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Can pick good trees but lots of state at the
routers!
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Shared Tree
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One tree used by all
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Cant pick good trees but minimal state at the
routers
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Tree Computation

• A tree which connects all the group nodes is a
  Steiner Tree
• Finding the min cost Steiner Tree is NP hard

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Tree Computation

• A tree which connects all the group nodes is a
  Steiner Tree
• Finding the min cost Steiner Tree is NP hard

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Tree Computation

• A tree which connects all the group nodes is a
  Steiner Tree
• Finding the min cost Steiner Tree is NP hard
• The tree does not span the network
• Heuristics are known

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Tree Computation

• A tree that connects all of the nodes in the
  graph is a spanning tree
• Finding a minimum spanning tree is much easier

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Tree Computation

• A tree that connects all of the nodes in the
  graph is a spanning tree
• Finding a minimum spanning tree is much easier

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Tree Computation

• A tree that connects all of the nodes in the
  graph is a spanning tree
• Finding a minimum spanning tree is much easier
• Prune back to get a multicast tree

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Tree Computation

• A tree that connects all of the nodes in the
  graph is a spanning tree
• Finding a minimum spanning tree is much easier
• Prune back to get a multicast tree

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Link State Protocols e.g. MOSPF

• Use in conjunction with a link state protocol for
  unicast
• Enhance the LSP updates with group membership
• Compute best tree from source
• Flood Membership in link state advertisements
• Dynamics are a problem

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Distance Vector Multicast Routing

• An elegant extension to DV routing
• Use shortest path DV routes to determine if link
  is on the source-rooted spanning tree

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Distance Vector Multicast

• Extension to DV unicast routing
• Packet forwarding
• iff incoming link is shortest path to source
• out all links except incoming
• Reverse Path Flooding (RPF)
• packets always take shortest path
• assuming delay is symmetric
• Issues
• Every link receives each multicast packet, even
  if no interested hosts Pruning
• Some links (LANs) may receive multiple copies
  Reverse Path Broadcasting

s3
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Example

• Flooding can cause a given packet to be sent
  multiple times over the same link
• Solution Reverse Path Broadcasting

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duplicate packet
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Reverse Path Broadcasting (RPB)

• Extend DV to eliminate duplicate packets
• Choose parent router for each link
• router with shortest path to source
• lowest address breaks ties
• each router can compute independently from
  already known information
• each router keeps a bitmap with one bit for each
  of its links
• Only parent forwards onto link

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Identify Child Links

• Routing updates identify parent
• Since distances are known, each router can easily
  figure out if it's the parent for a given link
• In case of tie, lower address wins

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Reverse Path Broadcasting (RPB)
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child link of x for S
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Dont really want to flood!

• This is still a broadcast algorithm the traffic
  goes everywhere
• Need to Prune the tree when there are subtrees
  with no group members
• Strategy
• Identify leaf networks with no members
• IGMP
• Propagate this information up the subtree

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How much to Prune?

• Truncated Reverse Path Broadcasting Prunes to
  prevent flooding of all packets
• Reverse Path Multicasting More aggressive. Scale
  router state with the number of active groups
• Use on-demand pruning so that router group state
  scales with number of active groups (not all
  groups)

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Pruning Details

• Prune (Source,Group) at leaf if no members
• Send Non-Membership Report (NMR) up tree
• If all children of router R prune (S,G)
• Propagate prune for (S,G) to parent R
• On timeout
• Prune dropped
• Flow is reinstated
• Down stream routers re-prune
• Note again a soft-state approach

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Pruning Details

• How to pick prune timers?
• Too long ? large join time
• Too short ? high control overhead
• What do you do when a member of a group
  (re)joins?
• Issue prune-cancellation message (grafts)

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MBONE

• What to do if most of the routers in the internet
  are not multicast enabled?
• Tunnel between multicast enabled routers
• Creates an overlay network but both operate at
  Level 3
• This is how multicast was first deployed

IP
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RMP Scaling

• State requirements
• O(Sources ? Groups) active state
• How to get better scaling?
• Hierarchical Multicast
• Core-based Trees

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Core Based Trees (CBT)

• Pick a rendevouz point for the group called the
  core.
• Shared tree
• Unicast packet to core and bounce it back to
  multicast group
• Tree construction is receiver-based
• Joins can be tunneled if required
• Only nodes on One tree per group tree involved
• Reduce routing table state from O(S x G) to O(G)

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Example

• Group members M1, M2, M3
• M1 sends data

root
M1
M2
M3
control (join) messages
data
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Disadvantages

• Sub-optimal delay
• Single point of failure
• Core goes out and everything lost until error
  recovery elects a new core
• Small, local groups with non-local core
• Need good core selection
• Optimal choice (computing topological center) is
  NP complete

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PIM

• Popular intradomain method
• UUNET streaming using this
• Recognizes that most groups are very sparse
• Why have all of the routers participate in
  keeping state?
• Two modes
• Dense mode flood and prune
• Sparse mode Core-based shared tree approach with
  a twist

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PIM Sparse Mode

• Routers explicitly issue JOIN and Prune messages
  to the Core
• Recievers typically send a Join message of the
  form (,G)
• As it propagates towards the core it establishes
  a new branch of the shared tree
• To send on the tree, tunnel to the core and then
  traverse the shared tree
• This can lead to bad performance
• To optimize sending from S, the core can send
  Join message of the form (S,G) to S.
• Creates a specific path from S to the core
• Receivers can send (S,G) messages as well to S
  and gradually replace the shared tree with a
  source specific tree

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Problems with Network Layer Multicast

• Scales poorly with number of groups
• A router must maintain state for every group that
  traverses it
• many groups traverse core routers
• Supporting higher level functionality is
  difficult
• NLM best-effort multi-point delivery service
• Reliability and congestion control for NLM
  complicated
• Deployment is difficult and slow
• Difficult to debug problems given the service
  model

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NLM Reliability

• Assume reliability through retransmission
• Sender can not keep state about each receiver
• e.g., what receivers have received
• number of receivers unknown and possibly very
  large
• Sender can not retransmit every lost packet
• even if only one receiver misses packet, sender
  must retransmit, lowering throughput
• N(ACK) implosion
• described next

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(N)ACK Implosion

• (Positive) acknowledgements
• ack every n received packets
• what happens for multicast?
• Negative acknowledgements
• only ack when data is lost
• assume packet 2 is lost

R1
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NACK Implosion

• When a packet is lost all receivers in the
  sub-tree originated at the link where the packet
  is lost send NACKs

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Scalable Reliable Multicast (SRM)

• Randomize NACKs (request repairs)
• All traffic including request repairs and repairs
  are multicast
• A repair can be sent by any node that heard the
  request
• A node suppresses its request repair if another
  node has just sent a request repair for the same
  data item
• A node suppresses a repair if another node has
  just sent the repair

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Avoiding NACK Implosions

• Every node estimates distance (in time) from
  every other node
• Information is carried in session reports (lt 5
  of bandwidth)
• Nodes use randomized function of distance to
  decide when to
• Send a request repair
• Reply to a request repair

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ISPs charge by bandwidth
Broadcast Center
Remember what interdomain protocols optimize for.
They make more money without multicast
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Application Layer Multicast

• Provide multicast functionality above the IP
  unicast
• Gateway nodes could be the hosts or multicast
  gateways in the network
• Advantages
• No multicast dial-tone needed
• Performance can be optimized to application
• Loss, priorities etc.
• More control over the topology of the tree
• Easier to monitor and control groups
• Disadvantages
• Scale
• Performance if just implemented on the hosts (not
  gateways)

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Summary

• Large amount of work on multicast routing
• Major problems
• preventing flooding
• minimizing state in routers
• denial-of-service attacks
• deployment
• Multicast can be implemented at different layers
• lower layers optimize performance
• higher layers provide more functionality
• IP Multicast still not widely deployed
• Ethernet multicast is deployed
• application layer multicast systems are promising