Stealth Multicast - A New Paradigm for Incremental

Multicast Deployment

Dr. Aaron Striegel

Dept. of Computer Science & Engineering

University of Notre Dame

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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

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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

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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

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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

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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…”

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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 5-500 users• Delay-sensitive apps

– On-line gaming, video streaming

– Improve ALM-based apps

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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

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Stealth Multicast Model

Unicast

Company LAN(Content Provider)

ISP Domain

Other DomainsServers

Clients

Multicast Unicast

Conversion

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Multicast Detection

Edge Router

Filter

VirtualGroupMgr

Disp

ChecksumCalculation

HRules

TreeConstruction

StateManagement

Incoming Traffic (Unicast only)

Outgoing Traffic(Unicast+Multicast)

VGDM - Virtual Group Detection Manager

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Further Examination

• Benefits– Dynamically convert– Zero modification– Multicast transport via

virtual groups– Exact billing

• Drawbacks– Non-zero queuing delay– Aggregation effects– Imperfect virtual groups– Not a universal solution

Virtual Group Delay

Benefit

Delay

Minimum gain

Maximum delay

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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

VGDM None ISP Company ISP Edge None

Detection

Accuracy Perfect Perfect Good OK None

Benefits Customer, ISP

Customer,

ISP

Customer, ISP

ISP None

Costs App change

Deployment

App change

Deployment

Deployment Deployment None

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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

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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

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Application State

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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

time

MHTTSWPSW

Target packet

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Triggers - Continued

• Triggers/thresholds– MinGS - Minimum group size– MaxGS - Maximum group size– MVG - Maximum virtual groups

UnicastMulticastNew group

MaxGS MinGS

VG 0

VG N

.. MVG

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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

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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

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Application State

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Egress Node State

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Control Messages

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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

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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

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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

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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

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Effect of Clients - Link BW

No savings, unicast

Higher up-link cost

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Effect of clients - Domain BW

Trades B/W forclient B/W

Increasing savingsvs. unicast

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Effect of Clients - QoS (Delay)

Limited impactof queuing

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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

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Talk Overview

• Information Dissemination• Motivation• Stealth Multicast

– Basic Architecture– Dynamic Groups– Preliminary Results

• Future Research– Inter-domain Peering– Network stack enhancement

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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

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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

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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

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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

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Questions?

striegel@cse.nd.edu

http://www.cse.nd.edu/~striegel

GenMCast Package (ns-2)http://www.cse.nd.edu/~striegel/GenMCast