Multicast Forwarding and Application State Scalability in the Internet
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Multicast Forwarding and Application State Scalability
in the Internet
Tina Wong
Dissertation SeminarComputer Science Division
University of California, BerkeleyOctober 16, 2000
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Challenge
“… in the long run, the biggest issue facing multicast deployment is likely to be the scalability of multicast forwarding state as the number of multicast groups increases.”
--Thaler and Handley 2000
The memory required to store multicast forwarding entries at a router with 32 interfaces is 1024 TB for IPv6, assuming 50% address space utilization
--Radoslavov, Govindan and Estrin 1999
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Outline
• Introduction, background, motivation• Multicast state scaling trends in Internet • Preference clustering protocol• Application-driven tunable reliability• Conclusions and future work
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IP Multicast
• Efficient point-to-multipoint delivery mechanism
• Packets travel on common parts of the network only once
S
R R R
R
R
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Multicast Routing
S
R R R
Broadcast
DVMRP• Per-source reverse shortest
path tree• Broadcast-and-prune• MBone
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Multicast Routing
S
R R R
Prune
DVMRP• Per-source reverse shortest
path tree• Broadcast-and-prune• MBone
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Multicast Routing
S
R R R
Forward Data
DVMRP• Per-source reverse shortest
path tree• Broadcast-and-prune• MBone
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Multicast Routing
• PIM-Dense Mode / Sparse Mode– Unidirectional shared tree– Explicit joins– Core location a problem
• Core Based Trees (CBT)– Bi-directional shared tree– More optimal data paths– Few routing vendors support
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Multicast Forwarding State
• Router maintains membership state to achieve forwarding
• State scales linearly with number of concurrent groups
• No natural aggregation
• Number of concurrent multicast groups limited by router memory
• Heartbeat messages to maintain state incur processing costs
oif0 oif1 oif2
iif0
s = 0.0.0.0/0G = 224.0.1.2/32iif0, oif1, oif2
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Motivation
Lots of simultaneously active multicast groups on the Internet?
• Many small, group-based applications– Few participants form a single multicast group– E.g. internet video conferencing, games, events
notifications, etc
• Few large-scale applications– Lots of users form many multicast groups– E.g. Content delivery, stock quotes, DIS, etc
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Related Work
• Multicast state reduction– Leaky and non-leaky state aggregation– Tunneling in backbone (MPLS, DCM)– Non-branching state elim (DTM, REUNITE)
• Application-level multicast– End Sytsem Multicast, YOID, Scattercast
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Contributions
• Comprehensive analysis on multicast state– Understand scaling trends in the Internet– Predict future growth– Estimate potentials for reduction– Apply to network provisioning, protocol and
application design
• Mechanisms for network and end-host state scalability in large-scale applications– Interest-based content delivery– Application-driven loss recovery
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Outline
• Introduction, background, motivation• Multicast state scaling trends in Internet• Preference clustering protocol• Application-driven tunable reliability• Conclusions and future work
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Questions: Scaling Trends
Much research and engineering effort into making IP multicast widely deployed...
• How do multiplying peering agreements among parallel backbone networks affect multicast state scalability?
• How do rising subscriptions to individual applications increase multicast state?
• What are the state scaling properties when more and more applications use multicast?
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Questions: State Concentration
An intuition: multicast state scalability is most critical at “core” routers…
• How concentrated is multicast state at “core” routers?
• How much benefit from tunneling?
“Core”
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Questions: State Reduction
An intuition: delivery trees of sparse multicast groups tend to have large number of non-branching routers...
• How prominent are non-branching routers?
• Are these routers stateful?
S
R
R
R R
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Basic Model
Local state• Fraction of concurrent
multicast groups
True local state• Local state with only
multicast forwarding
Independent of address space size and number of concurrent groups
5 concurrent groupsLocal state = 2/5True local state = 1/5
oif0 oif1 oif2
iif0
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Methodology
• Simulations– Extends upon SGB package
• Parameters– Topology – Session density– Membership model
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Topology
• 4 AS graphs from Nov97 to Jan00– Connectivity among Internet autonomous
systems– Study multicast state at inter-domain level– Over 3 year timespan
• Mbone graph from Feb99– Study multicast state at intra-domain level
• Generated graphs– TIERS– Transit-stub
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Session Density
• Graphs have different number of nodes, from 1000 to 6474– Session density instead of absolute size– 0.1% to 0.9%, 1% to 9%, 10% to 90%– E.g., session with 0.1% density in AS-Jan00
with 6500 nodes involves 7 domains– E.g., session with 10% density in Mbone
with 4200 nodes involves 420 routers
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Membership Taxonomy
Topological Correlationwithin one group
Subscription correlationacross multiple groups
NO
NO
YES
YES
1
random distrclusters
2
affinity/disaffinity
3
interest4
layeredinterest
5 6
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Experiments
• For each experiment, fix topology, session density and membership model– (1) Pick a set of nodes with these parameters– (2) Build shortest path tree rooted at a random
node from this set– Repeat (1) & (2) 1000 times– Calculate local state and true local state on each
node in topology
• All combinations of parameters used, yielding 945 experiments and results!
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Answers: Scaling Trends 1
• How do multiplying peering agreements among parallel backbone networks affect multicast state scalability?
– More state at a handful of core routers– Offset by reduced state in majority of
routers
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Topological Properties
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Hypothesis
• In a more connected network– Trees have larger fanouts and shorter
heights– Only a few highly peered routers involved in
most concurrent multicast trees
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Hypothesis
• In a less connected network– Trees have smaller fanouts and taller
heights– Backbone routers share responsibility of
multicast forwarding -- “load balancing”?
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Path Lengths
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Node Degrees
AS-Nov97 MBone
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Past and Future ScalingTrends
• Implication– If Internet continues to evolve as it has been,
multicast memory requirements at most of border routers actually decline, all things remain equal
• Evidence– Peering increases for past 3 years– Maximum domain degree from 605 to 1459, roughly
50% expansion each year– Slight decrease in state for majority of nodes– Slight increase for rest of nodes
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Answers: Scaling Trends 2
• How do rising subscriptions to individual applications affect multicast state?
– Follows power law• fraction of stateful routers grows proportional to
some constant power of multicast group size
– Exponents within each membership for the Internet similar over past 3 years
– Predictive of future state growth
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Answers: State Concentration
• How concentrated is multicast state at the “core” routers?
– State concentration does not follow “10/90” rule even when session density is 0.1%
– Application-driven membership significantly impact state distribution and concentration
– Tunneling useful for multicast applications with very sparse and spread-out membership
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Answers: State Reduction
• How prominent are non-branching routers? Are these routers stateful?
– Very prominent– Up to 2 orders of magnitude reduction is
possible even at top 10% most stateful nodes
– Substantial even at 90% session density– Promising approach
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Outline
• Introduction, background, motivation• Multicast state scaling trends in Internet• Preference clustering protocol• Application-driven tunable reliability• Conclusions and future work
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Large-scale Applications
• Large-scale applications: many receivers, many sources, rich data types, UI
• Multicast uses one data stream to satisfy potentially heterogeneous receivers
• Lead to Preference Heterogeneity– Users differ in interest on application data– E.g. Content delivery, news dissemination,
stock quotes, network games, DIS, etc
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Example: Stock Quotes Service
...
AABC BACU CABL DAGR EACO FACO
www.StockCentral.com
Amy
INTCDELLCSCOMSFT
Bob
AAPLAMZNEWEBMSFTGABCQCOM
Cathy
PWBCSISIYHOO
...
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Example: Network GamesA player's position in virtual environmentdrives its preferences on entity updates
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Preference Heterogeneity
• Assign each logical data stream a unique multicast address ?
+No superfluous data
–Multicast routing state scalability
–Multicast address allocation and scarcity
–End-host connection maintenance
• 100% reliability not necessary– Different levels of reliability desired– Help to reduce NACK implosion
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The Clustering Concept
completeheterogeneity
completesimilarity
UNICAST MULTICAST
CLUSTER
approximately similar sources and receivers into like groups
many smallgroups
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Preference Clustering Protocol
• Clustering algorithm– On-line and adaptive to changes in preferences– Customizable to different application and data
types
• Signaling protocol– Coordinate clustering within an application– Scalable, fault tolerant and reliable through
decentralization, soft state and sampling
• API • Detailed evaluation
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App-Level Tunable Reliability
• Consider application semantics in loss recovery decisions– Meta-data to describe data content– Temporal: statistics on update frequency– Semantic: magnitude or importance of
change– Policy-driven by individual receivers
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Outline
• Introduction, background, motivation• Multicast state scaling trends in Internet• Preference clustering protocol• Application-driven tunable reliability• Conclusions and future work
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Conclusions
• Comprehensive study on multicast state scalability– Scaling trends confirmed with past 3 years– State distribution and concentration– Potentials for reduction
• Mechanisms to accommodate problem for large-scale applications– Customizable and adaptive preference
clustering protocol– Tunable reliable multicast protocol
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Future Directions
• Compare and contrast methodologically IP multicast and application-level multicast– Params: Topology, session density,
membership– Apps: Few-to-few, one-to-many– Metric: Bandwidth, latency, complexity, etc
• Placement of service agents in Internet– Spawning of new agents – Coalescing based on topology, user
population, network measurements