Transcript PowerPoint Slides - Department of Computer Science and

Stealth Multicast - A New
Paradigm for Incremental
Multicast Deployment
Dr. Aaron Striegel
Dept. of Computer Science & Engineering
University of Notre Dame
Talk Overview
• Information Dissemination
• Motivation
• Stealth Multicast
– Basic Architecture
– Recent work: Dynamic groups
– Preliminary Results
• Future Research
– Inter-domain Peering
– Network stack enhancement
2
Information Dissemination
• Distribute rich content in a timely fashion to users
– Problem: Internet evolved as point-to-point
• Inefficient but currently manageable via unicasts
• Two main approaches
– Active involvement - Multicast
• Close temporal proximity
• Applications, network can participate
– Community participation -> network efficiency
– Passive involvement - Caching
• Multiply-accessed data over time
• No required participation of apps/network
– Exploit existing characteristics of network
» HTTP Caching
» Packet-level caching
3
Multicast
• Operation
– Reduces packet transmission
to an efficient tree
– Relies on network state for
replication
• Benefits
– Reduced bandwidth
• N receivers << N bandwidth
– Bottleneck relief
• Relief close to source
– Simplifies sender management
• Send to group vs. individuals
4
Caching vs. Multicast
• Caching
– Cannot handle rapidly changing data
• Data w/close temporal proximity
– Easy deployment
• No global participation
• Multicast
– Deployment problems
• Global participation
– Addressed by ALM
» Delay-sensitive traffic, rich user base
• Economic incentive
– Bandwidth glut, ISP benefit
– Can handle close temporal data
• Group-oriented activities - synchrony is an issue
5
Current State
• Caching: Yes
Multicast: ??
– Several recent studies [2000, 2003]
• Lackluster adoption 150 groups (1999) -> 250 (2001)
• Most groups are single source (SSM)
– Why have *, G, CBTs, etc.?
» Harder form of multicast anyway
– Key lesson from caching
• Incremental deployment is key
– Big-bang theory is impossible
– Transition as easy as possible (FUD inertia)
• Immediate benefit
– Large benefit with minimal investment
• Directable economic benefit
– Avoid “If you build it, they will come…”
6
Motivation
• Research premise
– Transparent bandwidth conservation technique
– Change the paradigm of multicast
• Incremental deployment
– Zero dependence on external forces
• Immediate benefit
– Exploit the redundancy in the network
• Economics
– Offer a significant and quantifiable benefit
• Stealth Multicast
– Dynamically convert packets to/from multicast
– Target
• Small to medium group-oriented apps
• Delay-sensitive apps
5-500 users
– On-line gaming, video streaming
– Improve ALM-based apps
7
Stealth Multicast
• Two governing principles
– Externally transparent
• Zero modification - application (server/client)
• Zero modification - external Internet
• Seamless operation
– Negligible QoS impact?
• Should not noticeably impact QoS
• What are noticeable QoS changes?
– Depends upon application
» Large buffer - streaming video
» On-line game - zero buffer
– Informal definition
» Additional delay should not make the application
unusable versus separate unicasts
8
Stealth Multicast Model
Conversion
Other Domains
Servers
ISP Domain
Clients
Company LAN
(Content Provider)
Unicast
Multicast
Unicast
9
Multicast Detection
VGDM - Virtual Group Detection Manager
Virtual
Group
Mgr
Disp
Checksum
Tree
Calculation
Construction
H
Rules
State
Management
Filter
Edge Router
Incoming Traffic
Outgoing Traffic
(Unicast only)
(Unicast+Multicast)
10
Further Examination
• Benefits
– Dynamically convert
– Zero modification
– Multicast transport via
virtual groups
– Exact billing
Benefit
Delay
• Drawbacks
–
–
–
–
Non-zero queuing delay
Aggregation effects
Imperfect virtual groups
Not a universal solution
Virtual Group Delay
Minimum gain
Maximum delay
11
Multicast Transition Options
Approach
Pure
Multicast
Application
Assisted
Customer
Aware
Stealth
Mode
Separate
Unicast
Application
Multicast
Stealth
Unicast
Unicast
Unicast
Internet
Multicast
Unicast
Unicast
Unicast
Unicast
ISP
Multicast
Stealth
Stealth
Stealth
Unicast
None
ISP
Company
ISP Edge
None
Perfect
Perfect
Good
OK
None
Customer,
ISP
Customer,
ISP
Customer,
ISP
ISP
None
App change
Deployment
App change
Deployment
Deployment
Deployment
None
VGDM
Detection
Accuracy
Benefits
Costs
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Talk Overview
• Information Dissemination
• Motivation
• Stealth Multicast
– Basic Architecture
– Recent work: Dynamic groups
– Preliminary Results
• Future Research
– Inter-domain Peering
– Network stack enhancement
13
Dynamic Trees
• Implementation of stealth multicast
– Dynamically grow/shrink physical multicast
groups
• Virtual group - snapshot at current time
• Physical group - superset of potential clients
– Defines key issues of stealth multicast
• Triggers - Virtual group release
• Transport - Dynamic groups
• State management - Where to place state
14
Application State
15
Virtual Group Triggers
• Trigger
– Dilemma: Gain for waiting
– When to release the virtual group
• MHT - Maximum Hold Time
• TSW - Time Search Width
• PSW - Packet Search Width
Target packet
MHT
TSW
PSW
time
16
Triggers - Continued
• Triggers/thresholds
– MinGS - Minimum group size
– MaxGS - Maximum group size
– MVG - Maximum virtual groups
New group
Multicast
Unicast
VG 0
..
MaxGS
MVG
MinGS
VG N
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System Balance
• VGDM Limit
– MVG - Maximum Virtual Groups
• Hard limit - should be avoided
• No multicast benefit - overloaded
– Inputs
• Filter effectiveness
– Eliminate non-candidate traffic
• Triggers - dispatch
– PSW, TSW, MHT
– MaxGS
• Tradeoff
– Capacity, QoS vs. efficiency
18
Scalability & Storage
• Examine worst case constraints
– Worst case delay is MHT
• 10% of an RTT of 50 ms
• 5 ms MHT
• Actual delay is MHT / 2
– Worst case storage
• PSW and TSW yield MHT, no matches
• 1 Gb/s link, 1000 byte group overhead
– 1 Gb/s @ 8 bit/byte * 5 ms = 625 kB
– 625 kB/sec / 64 bytes = 9765 packets
– 625 kB + 9765 * 1000 = ~ 11 MB
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Multicast Transport
• Issue
– Members (clients) not known a priori
– Dynamically construct tree
• Approaches
– Exhaustive tree construction
• All variations, all egress points
– Broadcast/hold
• Costly - queuing at edge
– Encapsulation-based
• Embed tree inside the packet
– Dynamic tree construction
• Grow/shrink tree as necessary
20
Application State
21
Egress Node State
22
Control Messages
23
State Management
• Issue
– Unique portions of packet
• Compress multiple packets for different destinations into a
single packet
– Dest port, dest IP
– Who is responsible for exporting?
• Egress A vs. Egress B vs. Egress C
• Approaches
– Include in packet
• Similar to encapsulation-based approach
– Distributed knowledge
• Egress points share knowledge
24
Application-Assisted Method
• Virtual group detection
– Imprecise nature - best guess
• Application-assist
– Application knows about VGDM
– Application sends 1 packet w/state to VGDM
– VGDM constructs tree
• Benefits
– No change to the client - deployment
– Precise group construction
• Issues
– Billing
– Requires change to server app
25
Other Issues - TCP, IPSec, IPv6
• TCP
– Limited benefit
• Why?
– Extra state
– Retransmit of lost packets
– Potential benefit
• Web serving - initial request
– Assume no cookies
• CNN on 9/11
• IPSec / VPNs
• IPv6
26
Simulation Studies
• Simulation setup
– Ns-2 simulator
• Freely available simulator
– GenMCast module for ns-2, GIPSE- simulation management
• Network setup
– Medium-sized multicast groups
• On-line gaming apps - 8 to 64 clients
• UDP traffic - 40 server apps
– Compare various approaches
• Based on VGDM location + others
– Local, Stealth, None, App-Assist, ALM
• Evaluation metrics
– Bandwidth savings
– End-user QoS
27
Effect of Clients - Link BW
No savings, unicast
Higher up-link cost
28
Effect of clients - Domain BW
Increasing savings
vs. unicast
Trades B/W for
client B/W
29
Effect of Clients - QoS (Delay)
Limited impact
of queuing
30
Other Results
• Other aspect
– Out of order delivery
• VGDM Overload
• Traffic aggregation
– OS/app effect
• Spacing between packets
• Live traffic
– Work in progress on prototype
31
Talk Overview
• Information Dissemination
• Motivation
• Stealth Multicast
– Basic Architecture
– Dynamic Groups
– Preliminary Results
• Future Research
– Inter-domain Peering
– Network stack enhancement
32
Immediate Research Areas
• Practical transport
– Encapsulation
– Dynamic groups
– Compare various approaches
• Fixed grouping w/hierarchy
– How to find the group that maps to the egress points
– Combination of groups
• Broadcast w/hold
– Impact of egress point sparsity
• ALM
– Apply ALM on a domain-wise level
– Fixed vs. dynamic groups
33
Future Research Areas
• Inter-domain peering
– Transparent bandwidth conservation
• Packet caching and stealth multicast
• Edge routers of domains exchange info
– Stealth multicast
• Avoid conversion to/from multicast/unicast
• Construct tree for new domain
– Packet caching
• Share cache in other domains
– Issues
• Billing, QoS
• Resource management
• Protocol / security
34
Future Research Areas
• Network stack modification
– Present: Minimize overhead
• Avoid extra IP/TCP or IP/UDP headers
– Premise
• Can we increase redundancy but increase overall system
performance?
– Enhance network stack
• Add End of Data marker - TCP
• Modify sendmail / Apache to use
– Issues
• Benefit to the network
• Downstream impact -> net system impact
35
Conclusions
• Stealth multicast
– New paradigm for multicast
– Offers several key benefits
• Solves multicast deployment issue
– Zero modification outside of the domain
• Inherent resource management
• Offers directable economic benefit
– Interesting research problems
• Transport, state management
• Inter-domain peering, stack optimization
36
Questions?
[email protected]
http://www.cse.nd.edu/~striegel
GenMCast Package (ns-2)
http://www.cse.nd.edu/~striegel/GenMCast
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