DHTbased Multicast

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DHT-based Multicast
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Motivation

• How does it perform in the presence of
  heterogeneity?
• DHTs are good at balancing load
• However, resources are usually NOT equally
  distributed in the real world
• Assumption that DHT is a globally pervasive
  service
• What if this is not the case?
• Medium sized groups (1000? 5000, e.g.)

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Roadmap

• Study concrete protocol Scribe
• Study heterogeneity handling mechanisms
• Pushdown
• Difficulties/Limitations
• Effect on metrics we care
• Depth
• Maintenance overhead
• RDP (?)

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Scribe
Underlying Pastry DHT
New member
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Scribe tree construction
Underlying Pastry DHT
groupID
New member
Rendezvous point
Route towards multicast groupID
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Scribe tree construction
Underlying Pastry DHT
groupID
New member
Route towards multicast groupID
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Pushdown

• When node is full
• Add incoming node anyways
• Kick out one child
• Pick the farthest child from me
• Orphan selects one of his former siblings as
  parent
• Selection depends on total latency to parent

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Problems with pushdown

• What happens if leaves have zero-degree?
• How does search continue?
• Paper punts on this (assumes degree gt 1)
• Creates loops (when failures!)
• Non-DHT links!!
• Additional maintenance traffic
• Increases depth by restricting search?
• If significant number ? what did DHT buy us?

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Goal

• Quantify pushdown effects
• Depth
• Overhead
• RDP
• Other goals
• RDP with group size 1000 ? 5000
• Tree/DHT structure when everybody knows the same
  bootstrap node

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Simulation efforts

• Simulator does not implement pushdown!
• implemented it
• Does not implement loop detection
• Paper says this is rare
• I did not find this rare, with failures
• implemented loop detection, path updates

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Efforts (continued)

• What to do when we detect a loop?
• Paper says randomized join (??) through Pastry
• Miguel
• Some hooky explanation about going via the
  leafset
• Caveat says they didnt think through this
  because this case would be rare!

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Splitstream

• Why splitstream?
• 1 tree ? simulates scribe!
• Uses anycast for looking up resources
• This primitive solves some problems
• Code is much more functional
• Possible problems?
• Trees constructed are not delay-sensitive
• Currently experimenting
• Several visual studio nightmares ?