Application layer multicast routing

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Application layer multicast routing
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What Is Multicast?
Key Unicast transfer Broadcast transfer
Multicast transfer

• Unicast
• One-to-one
• Destination unique receiver host address
• Broadcast
• One-to-all
• Destination address of network
• Multicast
• One-to-many
• Multicast group must be identified
• Destination address of group

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Multicast application examples

• Financial services
• Delivery of news, stock quotes, financial
  indices, etc
• Remote conferencing/e-learning
• Streaming audio and video to many participants
  (clients, students)
• Interactive communication between participants
• Data distribution

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IP Multicast
Gatech
Stanford

CMU
Berkeley

• Highly efficient bandwidth usage
• Key Architectural Decision Add support for
  multicast in IP layer

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So what is the big issue

• 15 years since proposal, but no wide area IP
  multicast deployment
• Scalability (with number of groups)
• -- Routers maintain per-group state
• -- Require every group to dynamically obtain a
  globally unique address from the multicast
  address space
• IP Multicast best-effort multi-point delivery
  service
• -- Providing higher level features such as
  reliability, congestion control, flow control,
  and security has shown to be more difficult than
  in the unicast case
• Other issues
• -- Change in infrastructure
• -- DOS attacks
• -- Network management, billing etc.
• -- works across space, not across time
• Can we achieve efficient multi-point delivery
  without IP-layer support?

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Application layer multicast
Stan1
Gatech
Stanford
Stan2

CMU
Berk1

Berkeley
Berk2
Overlay Tree
Stan1
Gatech

Stan2
CMU
Berk1
Berk2
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Pros and Cons

• Scalability
• Routers do not maintain per-group state
• End systems do, but they participate in very few
  groups
• No need for globally consistent naming, allows
  application specific naming
• Easier to deploy
• No infrastructural support
• Potentially simplifies support for higher level
  functionality
• Leverage computation and storage of end systems
• Leverage solutions for unicast congestion, error
  and flow control
• Efficiency concerns
• redundant traffic on physical links
• increase in latency due to end-systems

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Open questions!

• What are the performance implications of using an
  overlay?
• How do end systems with limited topological
  information
• cooperate to construct good overlay structures?
• What metric should the optimization be based on?
• End-to-end overlay vs. proxy-based overlay?
• Does high bandwidth data dissemination require
  special attention?
• A lot of work has been done to tackle these
  questions, for eg.
• Narada, Yoid, Scattercast, ALMI, Overcast, ROMA,
  NICE, Bullet,
• Selectcast, Scribe .

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Overcast Motivation
Key Line thickness indicates desired
bandwidth usage by sending entity

• Offering bandwidth-intensive content on demand
• - primarily video content
• - necessary to maintain full fidelity
• Long-running content availability for multiple
  clients
• - data distribution system for businesses
• Bandwidth bottlenecks develop as multiple
  requests are made

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What is Overcast?

• An application level multicasting system
• Provides scalable and reliable single-source
  multicast
• Goals
• Overlay structured to maximize bandwidth
• Utilize network topology efficiently
• - limit repeated usage of physical links
• No change to existing routers
• - easy deployment
• Draws upon work in content distribution, caching,
  and server replication

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Key insight and contributions

• Add storage to the network fabric for reliability
  and scalability
• - use disk-space to time shift multicast
  distribution
• - trade-off between disk-space and bandwidth
• Contributions
• a simple protocol for forming efficient and
  scalable distribution trees that dynamically
  adapt to changes
• a protocol for maintaining global status at the
  root of the changing distribution tree

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System structure

• The overlay comprises of
• A central source (may be replicated for fault
  tolerance)
• A no of overcast nodes (standard PCs with lots
  of storage)
• - organized into a distribution tree rooted
  at the source
• - bandwidth efficient trees
• Final Consumers members of the multicast group
  
• - allows unmodified HTTP clients to join
• The business model involves a content provider
  who installs these
• nodes (proxies) in the fabric and the overlay
  acts as a data
• distribution system for businesses

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Bandwidth Efficient Overlay Trees
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100 Mb/s
10 Mb/s
100 Mb/s
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three ways of organizing the root and the nodes into a distribution tree. three ways of organizing the root and the nodes into a distribution tree. three ways of organizing the root and the nodes into a distribution tree.

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How Does Overcast Build Bandwidth Efficient Trees?

• Goal maximize bandwidth to root for all nodes
• Places a new node as far away from root as
  possible without sacrificing bandwidth
• Additional details
• Bandwidth measured by timing a 10KB download
• Hystersis nodes with bandwidth within 10 of
  each other considered equal
• in case of a tie, choose the closest parent as
  determined by traceroute
• Wont the results of this algorithm change over
  time?

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The node addition algorithm
5
10
10
3
8
1
7
5
2
Physical network substrate
Overcast distribution tree
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But .

• What is the 10 Kb download measuring?
• TCP begins with a slow start
• Download is over by the time it switches to AIMD
• Does not give a measure of long-term TCP
  throughput
• Where is the bottleneck bandwidth?
• Nodes on the edges -- access links (leads to
  linear trees)
• Nodes in the core
• - fat pipes (a 10 Kbps download does not
  give the bandwidth)
• - bottleneck due to congestion, which can
  vary on a short time scale
• May have been more relevant at the time of the
  work .
• The impact of the child nodes on the bandwidth .

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Dynamic Topology

• A node periodically reevaluates its position in
  the tree by measuring the bandwidth been itself
  and
• its parent (baseline)
• its grandparent
• all its siblings
• Node can relocate to become
• child of a sibling
• sibling of a parent
• Inherently tolerant of non-root failures
• if the parent dies, a node moves up the ancestry
  tree

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Interactions Between Node Adding And Dynamic
Topology
10
20
1
15
2
Physical network substrate
Overcast distribution tree Round 1
Overcast distribution tree Round 2
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But .

• What happens when bandwidth keeps fluctuating?
• 10 hysterisis gap is ineffective given the way
  the bandwidth is measured
• Smart choice of bandwidths for the network edges
  in the generated graph.
• No evaluation in case of bandwidth flux
• Even if the system does converge, the convergence
  time is limited by the probes to siblings and
  ancestors

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State tracking the Up/Down protocol

• An efficient mechanism is needed to exchange
  information between nodes
• must scale sublinearly in terms of network usage
• may scale linearly in terms of storage
• Assumes information either
• changes slowly (E.g., Health status of nodes)
• can be combined efficiently from multiple
  children into a single description (E.g., Totals
  from subtrees)
• Each node maintains state about all nodes in its
  subtree

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Management of information with the Up/Down
protocol

• Each node periodically contacts its parent
• Parents assume a child (and all descendants) has
  died if the child fails to contact within some
  interval
• During contact, a node reports to its parent
• Death certificates
• Birth certificates
• Extra information
• Information propagated from children
• Sequence numbers used to prevent race conditions

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Scaling sublinearly in terms of network
usage
1

• A node (and descendants) relocates under a
  sibling
• The sibling must learn about all the nodes
  descendants
• Birth certificates
• The sibling passes this information to the
  (original nodes) parent
• The parent recognizes no changes and halts
  further propagation

No change observed. Propagation halted.
1.1
1.2
1.3
Birth certificates for 1.2.2, 1.2.2.1
1.2.1
1.2.3
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The client side how to join a multicast group

• Clients join a multicast group through a typical
  HTTP GET request
• Root determines where to connect the client to
  the multicast tree using
• Pathname of URL (multicast group being joined)
• Status of Overcast nodes
• Location of client
• Root selects best server and redirects the
  client to that server

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Client joins
Key Content query (multicast join) Query
redirect Content delivery
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Evaluation

• Based on simulations with GT-ITM
• Five 600-node graphs
• 3 transit domains (backbone)
• 8 stub networks per domain
• 25 nodes per stub
• Assigned Bandwidth
• 45Mbps, 1.5Mbps, 100Mbps
• T3, T1, Fast Ethernet
• One node supports 20 clients
• (MPEG-1 video)

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Bandwidth utilization

• Backbone
• Adds transit nodes
• first
• Random
• All nodes chosen
• randomly
• Fraction Overcast bandwidth/Optimal bandwidth
• At full participation distribution trees are
  different

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Discussion (cont.)

• Is a tree structure effective for high bandwidth
  data dissemination?
• Kostic et. al. differ!
• Bullet achieves this by distributing data in a
  disjoint manner to strategic points - the nodes
  organize as a mesh
• For good performance, the content provider will
  have to manually choose strategically placed
  nodes in the core which are anyway connected by
  big fat pipes so what is the relevance of the
  provided automation?
• What are the reliability semantics?
• - what if a node is down while a transmission
  completes how does the log help, does the node
  receive the file, does the root know of this?
• - when is data removed from the storage?
• Not suitable for live streams
• - although the authors were never aiming for
  this!

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Discussion (cont.)

• What about NATs
• - how can any overcast node be inside a NAT?
• In case of a tie in bandwidth measure, why not
  use latency instead of hop count leads to a
  shortest widest kind of selection
• Where does the root get the location of client
  from, when doing server selection?
• Simply coupled TCP connections not the best way
  to realize the bandwidth potential of the
  topology
• - May be a problem in case of heterogeneous
  receivers
• - ROMA uses loosely coupled connections
  with fast forward error correction
• Seems more like a contribution to smarter content
  distribution systems than to application level
  multicasting
• Bit Torrent does the same thing!

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Enabling Conferencing Applications on the
Internet using an Overlay Multicast Architecture
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Past Work

• Self-organizing protocols
• Yoid (ACIRI), Narada (CMU), Scattercast
  (Berkeley), Overcast (CISCO), Bayeux (Berkeley),
  
• Construct overlay trees in distributed fashion
• Self-improve with more network information
• Performance results showed promise, but
• Evaluation conducted in simulation
• Did not consider impact of network dynamics on
  overlay performance

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Focus of This Paper

• Can End System Multicast support real-world
  applications on the Internet?
• Study in context of conferencing applications
• Show performance acceptable even in a dynamic and
  heterogeneous Internet environment
• First detailed Internet evaluation to show the
  feasibility of End System Multicast

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Enhancements of Overlay Design

• Two new issues addressed
• Dynamically adapt to changes in network
  conditions
• Optimize overlays for multiple metrics
• Latency and bandwidth
• Study in the context of the Narada protocol
  (Sigmetrics 2000)
• Techniques presented apply to all self-organizing
  protocols

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Optimize Overlays for Dual Metrics
Source rate 2 Mbps
60ms, 2Mbps
Receiver X
Source
30ms, 1Mbps

• Prioritize bandwidth over latency
• Break tie with shorter latency

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Adapt to Dynamic Metrics

• Adapt overlay trees to changes in network
  condition
• Monitor bandwidth and latency of overlay links
• Link measurements can be noisy
• Aggressive adaptation may cause overlay
  instability

transient do not react
persistentreact

• Capture the long term performance of a link
• Exponential smoothing, Metric discretization

raw estimate
bandwidth
time
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Evaluation overview
Compare performance of their scheme with --
Benchmark (IP Multicast) -- Other overlay
schemes that consider fewer network metrics
Overlay Scheme Choice of Metrics Choice of Metrics
Overlay Scheme Bandwidth Latency
Bandwidth-Latency
Bandwidth-Only
Latency-Only
Random
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Three Scenarios Considered
Primary Set 1.2 Mbps
Primary Set 1.2 Mbps
Primary Set 2.4 Mbps
Extended Set 2.4 Mbps

• Does ESM work in different scenarios?
• How do different schemes perform under various
  scenarios?

(lower) ? stress to overlay schemes ? (higher)
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BW, Primary Set, 1.2 Mbps
Naïve scheme performs poorly even in a less
stressful scenario
RTT results show similar trend
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Scenarios Considered
Primary Set 1.2 Mbps
Primary Set 2.4 Mbps
Extended Set 2.4 Mbps
(lower) ? stress to overlay schemes ? (higher)

• Does an overlay approach continue to work under a
  more stressful scenario?
• Is it sufficient to consider just a single
  metric?
• Bandwidth-Only, Latency-Only

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BW, Extended Set, 2.4 Mbps
Optimizing only for latency has poor bandwidth
performance
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RTT, Extended Set, 2.4Mbps
Optimizing only for bandwidth has poor latency
performance
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Discussion

• Does a source rate of 2.4 Mbps represent a
  realistic setting!
• Issues not addressed
• Scalability
• - how to lower network costs for large sized
  groups
• - how to make the tree building algorithm
  more scalable
• Extremely dynamic environments
• - how to achieve shorter time scale
  adaptation
• - trade-off between overlay stability and
  the detection time
• Optimizing for 2 dynamic metrics
• Optimizing for both bandwidth and latency may be
  tricky the discretization and hysterisis
  combined with the probe latency may affect the
  granularity of adjustment

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Discussion (cont)

• Is end-system multicast suitable for real time
  applications?
• Does provide distribution across time
• But for applications with timing constraints, it
  may not be such a good idea!
• These are some of the most well connected
  machined on the Internet!
• Is the internet a good place for such
  measurements and comparisons?
• The authors do a decent job of making the
  analysis fair and objective

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On a general note .

• What is the impact of such overlays on the other
  traffic?
• The usefulness of the overlay concept
• Will they be a success iff nobody uses them?
• Interaction between multiple independent
  overlays
• What is the motivation for their deployment?
• Users come in and go out !
• Cheating by end hosts
• All the ALMs provide a best effort service!
• are they comparable?
• which one provides the best best-effort service