Multicast for Video Streaming

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Multicast for Video Streaming

• EE290T Spring 2002
• Puneet Mehra
• pmehra_at_eecs.berkeley.edu

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IP Multicast Overview

• Semantics
• 1 -gt Many or Many -gt Many
• Approach
• Build tree connecting source and receivers on
• Current Infrastructure in Net 1
• Group Addressing provides flexibility
• Receivers/senders unaware of each other
• Packets delivered throughout tree.
• Dynamic changes to tree
• New Receiver -gt graft path onto tree
• Receiver leaving -gt pruning path from tree
• Uses UDP so no reliability
• Challenges
• Efficient routing of data to receivers

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Video Multicast Over Net1

• Issues in Multicast over Best Effort
• Fixed Frame Rate regardless of delay/jitter
• Losses degradation, possibly ungraceful
• Heterogeneity of receivers
• Approaches to Multicast
• QoS resource reservation for Multicast
• Adaptive Rate Control
• Techniques for Rate Adaptation
• Single Stream Video Multicast
• Replicated Stream Video Multicast
• Layered Video Multicast

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Single Stream Video Multicast

• Only send 1 stream to all receivers.
• Pros
• Easy To Implement
• Cons
• Ignores Receiver Heterogeneity
• Feedback Implosion
• INRIA Video Conferencing System
• Feedback Problem handled through probabilistic
  receiver response
• Tradeoff granularity of control vs B/W efficiency

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Efficiency Tradeoff in Single Stream Approach
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Replicated-Stream Video Multicast

• Destination Set Group (DSG)
• Small of video streams of varying quality sent
  to different multicast groups
• Intra-stream Rate control to adjust stream rate
  by receivers
• Inter-stream protocol used by receivers to switch
  streams
• Pros
• deals with heterogeneity more fair
• Scalable since receiver-driven
• Cons Network carries redundant info

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Layered Video Multicast

• Receiver-Driven Layered Multicast (RLM)
• Send different layers to multicast groups, and
  receiver subscribes as needed -gt scalable
  solution
• Congestion -gt layer dropping
• Spare B/W -gt layer adding
• Receivers conduct group join experiments and
  share info with others.

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Layered Video Multicast Cont.

• Layered Video Multicast with Retrans. (LVMR)
• Improve reception w/in a layer by retransmission
• Deal w/ congestion using Hierarchical Rate
  Control
• Hierarchical Rate Control (HRC)
• Congestion info distributed at both
  sender/receivers
• Intelligent partitioning of info -gt concurrent
  experiments w/ less overhead
• Use hierarchy to only inform those who need to
  know about an experiment affected regions
• Collaborative layer drop better approach to
  congestion

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Error Control in Video Multicast

• Pure FEC
• ARQ From LVMR
• Local Recovery - designated receivers at each
  level in tree help w/ rtx. of pkts -gt lower
  latency
• Dont rtx packets past deadline
• Receivers can trade reliability/latency by
  picking parent with desired attributes

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Multicast Routing 2,3

• Routing construct efficient tree from source to
  receivers
• Theoretical Results 3
• Steiner Tree minimize total cost of a multicast
  tree. NP-Complete. So use heuristics to provide a
  good approx. to Steiner Tree.
• Constrained Steiner Tree impose b/w delay
  constraints on links to receivers. Also
  NP-Complete. So must use heuristics
• All practical algorithms based on shortest path
  tree minimize sum of weights on links along
  each path from source to receiver

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Intra-Domain Routing

• Source-based Routing
• Tree rooted at source
• Dense-mode routing works best when topology
  densely populated with receivers
• Core-based Approach
• Select a Rendezvous Point (RP) to root the tree
• Sparse Mode Routing More efficient than dense
  mode when few, wide-spread receivers

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Dense Mode Protocols

• Distance Vector Multicast Routing Protocol
• Uses broadcast prune technique to build reverse
  shortest path trees (RSP)
• Steps
• Src bcasts pkt on Lan. Local router fwds pkt on
  all ifaces
• If pkt received on RPF iface, then it is
  forwarded.
• Leaf routers send prune toward src if no attached
  receivers
• Prune message forwarded to source, and send own
  prune if receive prune message on all ifaces.
• A lot of state info kept in ALL routers in net.
• Multicast extensions to OSPF
• Uses IGMP locally, then floods info along with
  link state to net.
• PIM-DM
• Less complex than DVMRP since no RPF check is
  done. More inefficient as a result

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Tree Construction in DVMRP3

• S Source. Black Circles Receivers
• Periodically flood net w/ datagrams
• Leaf routers send prune toward source if there
  are no group members on leaf subnet
• Final Tree is shown in (d).

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Core-Based Routing

• General Approach
• A core, or rendezvous point (RP) is configured
  for a multicast group
• Info about the RP mapping from group to RP is
  discovered by routers using bootstrap protocol
  (also finds alternate RP in case of failure)
• Receivers explicitly join tree -gt contact RP
• Src sends data to RP which sends down tree
• More efficient since state only kept in routers
  on path from src/receivers to RP.
• Examples
• CBT Core-Based Trees
• PIM-SM Protocol Independent Multicast/Sparse
  Mode

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Tree construction in CBT

• The Join Process for a new node
• Receiver Contacts Local Router
• Router sends JOIN_REQUEST to the core router
• When msg reaches on-tree router, a JOIN_ACK is
  sent back
• every router receiving JOIN_ACK updates state
  information

• Periodically send echo-request to parent router.
  If echo not received in time, then router sends
  quit-notification upstream and deletes state
  information.

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Inter-Domain Routing

• Probs w/ multicast described
• Large flat topology -gt complexity and instability
  since no BGP-like protocol
• No mechanism to build hierarchical mcast routing
• Solution Immediate Future
• Introduce Hierarchy multi-protocol extensions
  to BGP (MBGP)
• Each router only knows topology of its own domain
  how to reach other domains
• Used to determine next hop for a host

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Inter-Domain Routing Cont.

• What if you have a src in one domain receivers
  in others?
• Multicast Source Discovery Protocol
• When src registers w/ RP -gt a source active (SA)
  msg is sent to MSDP peers
• Prevent loops w/ per-RPF flooding (ie if msg
  received on correct iface -gt flood)
• If MSDP is aware of local group members (use
  IGMP), then it will send a join to the src

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Long-Term Inter-Domain Proposals

• Border Gateway Multicast Protocol
• Bidirectional shared trees between domains with
  single root. Need strict allocation of addresses
  among domains.
• Address Allocation Protocols
• Multi Address Set Claim Helps allocate
  addresses dynamically across domains
• GLOP a glop of addresses statically allocated
  among domains

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Problems Deploying IP Multicast 4

• Complexity
• Cant put it in core routers
• Hardware more difficult to manage (probs w/
  firewalls)
• Makes old routers useless
• disrupts ISP router migration model (routers
  generally migrate from core to edge)
• Domain Independence
• ISPs dont want to rely on remote RPs
• Dont want to be RP for non-customers
• Security anyone can send/listen
• Address Allocation anyone can pick a class D
  addr.

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References

• 1 Video Multicast over the Internet. Xue Li
  et al. IEEE Network. 1999.
• 2 The Evolution of Multicast From the MBone
  to Interdomain Multicast to Internet2
  Deployment. Kevin Almeroth. IEEE Network. 2000.
• 3 Multicast Routing and Its QoS Extension
  Problems, Algorithms, and Protocols. Bin Wang
  and Jennifer C. Hou. IEEE Network. 2000.
• 4 Deployment Issues for the IP Multicast
  Service and Architecture. Christophe Diot et Al.
  IEEE Network. 2000.