SplitStream: High- Bandwidth Multicast in Cooperative Environments Monica Tudora

Contents ► Introduction ► SplitStream approach ► Background ► SplitStream design ► Experimental evaluation ► Conclusions

Introduction ► in tree-based multicast systems, a relatively small number of interior nodes carry the load of forwarding multicast messages. ► SplitStream:  enables efficient cooperative distribution of high- bandwidth content in a peer-to-peer system.  splits content into stripes and multicasts each stripe in a separate tree.  peers join as many trees as there are stripes and specify an upper bound  improved robustness to node failure and sudden node departures

SplitStream approach ► a participating node: leaf or interior node. ► splitting the multicast stream into multiple stripes ► use of separate multicast trees to distribute each stripe ► participating peers can control their inbound or outbound bandwidth requirements.

SplitStream approach

► Applications need to encode the content such that:  each stripe requires approximately the same bandwidth  the content can be reconstructed from any subset of the stripes of sufficient size (ex: media stream encoded using MDC, multicasting of file data with erasure coding)  Control when to create and tear down a splitstream forest

SplitStream approach ► Notations:  N – number of nodes, k – numbers of stripes  I i – desired indegree, C i – forwarding capacity  S – source nodes ► Conditions:  1 - the sum of the desired indegrees cannot exceed the sum of the forwarding capacities: ∑I i <= ∑C i  2 - Ci > Ii ⇒ Ii + Ti = k.

Background ► implementation using Pastry and Scribe. ► Pastry:  scalable, self-organizing structured peer-to-peer overlay network  nodes and objects are asigned random identifiers -> nodeIds and keys.  routes the message to the node with the nodeId that is numerically closest to the key.  each node maintains a routing table and a leaf set.

Background ► ► Scribe:   application-level group communication system built upon Pastry   groupId chosen for each multicast tree   the multicast tree is formed by the union of the Pastry routes from each group member to root.

SplitStream design ► ► Building interior-node-disjoint trees:   separate Scribe multicast tree for each stripe   nodeIds of all interior nodes share some number of digits with the tree’s groupId   condition is to choose groupIds for the trees that all differ in the most significant digit

SplitStream design

► ► Limiting node degree:   inbound bandwidth is proportional to the desired indegree   Scribe has a built-in mechanism (called “push- down”) to limit a node’s outdegree ► ► Locating parents:   the node adopts the prospective child regardless of the outdegree limit   evaluates the new set of children and chooses one to reject

SplitStream design   Cryterias: ► ► reject children in stripes whose stripeIds do not share a prefix with the node’s nodeID ► ► the child whose nodeId has the shortest prefix match with that stripeId

SplitStream design ► ► Spare capacity group:   the orphan node sends an anycast message to the group   members – nodes that have less children in stripe trees than their forwarding capacity   conditions to take an orphan as a child: ► ► it is not an ancestor ► ► you receive the stripe needed by the child

SplitStream design   Failure scenarios: ► ► the spare capacity group is empty ► ► if attaching the orphan to receive the stripe from any of the members causes a cycle ► ► no member of the spare capacity group provides any of the desired stripes.

EXPERIMENTAL EVALUATION ► ► Experimental setup:   Network simulation: ► ► network simulator ► ► 3 network topologies: GATech, Mercator and CorpNet.   SplitStream configuration: ► ► leaf set size with l = 16, number of stripes k = 16 ► ► six different configurations of node degree constraints: 16x16, 16x18, 16x32, 16xNB.

EXPERIMENTAL EVALUATION   SplitStream implementation: 3 optimisations ► ► Forest construction overhead:   2 metrics: node stress and link stress.

EXPERIMENTAL EVALUATION

  Link stress

EXPERIMENTAL EVALUATION ► ► Forest multicast performance   comparison between SplitStream, IP multicast and Scribe

EXPERIMENTAL EVALUATION

► ► Resilience to node failures:   Catastrophic failures: nodes, 2500 fail

Conclusions ► ► balance forwarding load ► ► tolerate faults ► ► respect individual node bandwidth constraints ► ► small overhead of forest construction and maintenanace ► ► multicasts using forest do not load nodes beyond their bandwidth constraints