The%20Impact%20of%20Multicast%20Layering%20on%20Network%20Fairness

1
The Impact of Multicast Layering on Network
Fairness

• Dan Rubenstein
• Jim Kurose
• Don Towsley

2
Motivation

• How should multicast flows share bandwidth
  fairly within a network?

3
Layered Multicast

• Layering permits multi-rate sessions
• receivers in same session receive at differing
  rates

• Used for
• multicast video MJV96/97
• (reliable) data via FEC RV98, BLMR98

Q How can multi-rate sessions affect fairness
within a network?
4

• Formally extend max-min fair (MMF) definition to
  cover multi-rate (i.e., layered) sessions
• Demonstrate that desirable fairness properties
  hold in multi-rate max-min fair rate allocations
• Quantify a practical coordination problem within
  multi-rate sessions redundancy
• Examine how redundancy impacts fairness of a
  practical congestion control protocol

5
Unicast MMF H 81, , BG 92

• When possible, take from the rich, give to the
  poor...
• rates are max-min fair when for all rates B,
  either
• B uses all link bandwidth on some link
• increasing B causes a decrease in some other
  rate A, where initially A B

r1
r1
r2
r2
S1
S1
S2
S2
S3
S3
r3
r3
Not MMF!
6
Unicast MMF H 81, , BG 92

• When possible, take from the rich, give to the
  poor...
• rates are max-min fair when for all rates B,
  either
• B uses all link bandwidth on some link
• increasing B causes a decrease in some other
  rate A, where initially A B

r1
r1
r2
r2
S1
S1
S2
S2
S3
S3
r3
r3
MMF!
Not MMF!

• Hayden proves For any unicast network, there is
  a unique max-min fair allocation.

7
Single-rate Multicast MMF TS97

• all receivers in session must receive at same
  rate
• fairness applies to session rates
• a sessions link BW is identical on all utilized
  links

r2,2
S1
r1,1
S2
r2,1

• TS97 proves for any single-rate multicast
  network
• there is a unique max-min fair
  allocation

8
Multi-rate MMF

• receivers in a session can receive at differing
  rates
• make receiving rates fair
• sessions link BW is the maximum used on
  downstream links (like layered protocols)

• We prove for any multi-rate multicast network,
• there is a unique max-min fair
  allocation
• (Proofs extend to networks w/ mix of single-rate
  multi-rate sessions)

9
Why is multi-rate MMF desirable?

• We identify desirable fairness properties
• Derived from desirable properties of unicast
  max-min fair allocations
• e.g., Same-path-receiver-fairness (SPRF) 2
  rcvrs with same paths from sources should receive
  at identical rates.

SPRF!
Multi-rate MMF
10
The other fairness properties...

• Deal with competing sessions rates link
  utilizations (give from richer to poorer)...
• Fully-Utilized-Receiver-Fairness Each receiving
  rate should be no poorer than other rates over
  some competing link
• Per-Session-Link-Fairness Each session should be
  no poorer than other sessions on some link over
  some branch of the sessions multicast tree.
• Per-Receiver-Link-Fairness Each session should
  be no poorer than other sessions on some link
  over every branch of the sessions multicast
  tree.

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Multi-rate MMF is desirable

• We prove these desirable properties hold within
  the multi-rate MMF allocation in any network.
• We show these properties need not hold within the
  single-rate MMF allocation
• (e.g., Same-path-receiver-fairness)
• We measure desirability of the MMF allocation
  as individual session types (single- or
  multi-rate) vary
• construct allocation ordering relation
• show desirability increases as sessions switch
  to multi-rate.

12
Practicalities...

• In practice finite of layers small set of
  available rates
• Problem fixed set of layers might not yield fair
  rate
• Solution join and leave layers to achieve
  desired (fair) average rate
• Leads to another problem...

Layer 2
MMF rate

Layer 1
time
13
Redundancy

• Lack of intra-session join/leave coordination
  increases shared link usage

Link usage
r1,1
coord
uncoord
14
Redundancy in Practice

• Simple example
• one layer
• Unsynched (random) joins and leaves
• redundancy highest when all receivers touch a
  layer

• Also find using multiple layers reduces
  redundancy

15
Redundancy vs. Fair rates

• E.g., single bottleneck link redundancy lowers
  fair rates
• Less impact on rates when fraction, f, of
  sessions with redundancy is small.
• f likely to be small

16
Redundancy in Practice

• What is the redundancy of a practical, layered,
  congestion control protocol?
• Protocol we consider (simple model based on the
  work of VCR98)
• Lose a packet, leave a layer
• Join layer, 2 versions
• uncoordinated points in time
• coordinated by sender

Independent-loss links

• Markov models and simulation on mod-star topology

Shared-loss link
17
A practical CC protocol redundancy

• Results w/ 100 rcvrs
• allocations are close to multi-rate max-min
  fair
• sender-coordinated joins keeps redundancy smaller
  than 3
• Redundancy lt 3 means fair rate within .9 of
  optimal!

18
Conclusion

• Multi-rate sessions change the rules for
  fairness
• can achieve desirable fairness properties
• Keep redundancy low
• simple techniques likely to do quite well in
  practice

19
Open Issues

• Extensions to other fairness definitions
• TCP fairness
• Proportional fairness
• Eliminating redundancy
• Network support (router filters)
• Prioritized layers
• Effects of join/leave latencies
• Stability
• How fast should/can protocols join/leave layers