DHT-based Multicast

Motivation 1.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 2.Assumption that DHT is a globally pervasive service –What if this is not the case? –Medium sized groups (1000  5000, e.g.)

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

Scribe New member Underlying Pastry DHT

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

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

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

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

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

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

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 didn’t think through this because this case would be rare!

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 