1. Congestion Control for Multipoint Communications in ATM Networks
U.T. Nguyen
Dept. of Computer Science and Engineering
York University
5/20/2019
Department of Computer Science

2. Presentation Contents
Motivations
Outline of research topics and contributions
Explicit rate control for ABR multipoint-to-point
Explicit rate control for ABR multipoint-to-multipoint
Reliable multicast at ATM layer
 motivations, proposed solution, contributions
Conclusion and future work
5/20/2019

3. Congestion Control Is Essential!
For any type of network
Traffic demands are unpredictable
Congestion occurs when rate of arrival exceeds rate of processing/transmitting
Consequences of congestion:
– collapse of network throughput
– long delay experienced by users
– data loss
5/20/2019

4. Even More Essential ... In ATM networks
several service classes with different QoS guarantees
high bit rates
When congestion occurs in ATM networks,
data queue up quickly at switches
users suffer long delay, excessive cell loss
network can’t deliver QoS guarantees
For multipoint communication (and more challenging!)
coordination of multiple senders and receivers
much higher bandwidth consumption compared with unicast connections
5/20/2019

5. Multipoint Communications
Our Research Focus
Congestion Control
for
Multipoint Communications in
ATM Networks
5/20/2019

6. ATM Congestion Control
There are 3 mechanisms:
Cell discarding
When buffer occupancy reaches a threshold
Applies to all service classes: UBR  ABR  VBR/CBR
Explicit rate control for ABR services
Available bandwidth after VBR/CBR usage is divided evenly among ABR connections
RM cells convey the allowed cell rate info to ABR sources
Explicit forward congestion indication
Switches set EFCI bit when experiencing congestion
Actions to control congestion are application-dependent
5/20/2019

7. Contributions of the Research
Explicit rate (ER) control for ABR services
We propose effective and flexible algorithms for
Explicit rate control for ABR MP-P
Explicit rate control for ABR MP-MP
Cell discarding
Existing reliable multicast protocols are at transport layer
One cell dropped  the whole packet retransmitted
Multicast: many copies of a packet are re-sent redundantly
Consequences: increase of congestion level; high recovery latency
We propose: loss detection/retransmission at ATM layer, cell level
 Reliable multicast at the ATM layer
that reduces congestion and minimizes recovery latency
5/20/2019

8. Explicit Rate Control for ABR Multipoint-to-point Services
Motivations
Several bandwidth allocation definitions (BADs)
Source-based
VC/source-based
Weighted-source-based
Flow-based
VC/flow-based
Weighted-flow-based
No consensus on which BAD is best
Existing explicit rate control schemes for ABR MP-P support 1 or 2 BADs, or are not effective:
Ren, Siu and Suzuki:
source-based
incorrect w/ VC merge
Moh and Chen
weighted-source-based
incorrect in some cases
Fahmy, et al.:
source-based only
5/20/2019

9. Feasibility of BADs For simplicity: For fairness of pricing:
source-based  feasible
VC/source-based: MP sources are penalized for sharing resources
For fairness of pricing:
weighted-source-based is the best  most feasible
Extending weighted-source-based definition even further:
every source, unicast or MP, has its own weight
more overheads; more complex to implement
ABR is a best-effort service  not worth the increased cost
BADs based on flows:
sources merging many times may get a smaller share
advantage: simpler to implement than BADs based on sources
5/20/2019

10. Advantages of NK Algorithm
Flexible
supporting both VP merge and VC merge
supporting a large set of BADs
Minimal overhead
overhead for implementing weighted-source-based BAD
accommodating other BADs with no overhead increase
easy setting using only a few software parameters
Scalable and efficient
one FRM is replied by exactly one BRM
running time of NK algorithm: O(B)
Effective allocation, fast convergence, robustness
by design (e.g., using fair share, computing ER in both directions)
confirmed by simulation results
5/20/2019

11. Experimental Results Simulation Measures Performance of NK algorithm
ATM network simulator version 4.1 by NIST
code added to implement MP-P routing, NK and FJ algorithms
Measures
ACRs of sources: effectiveness of allocation, convergence time
queue lengths at switches: cell loss, stability
Performance of NK algorithm
under normal condition, heavy load, with join/leave operations
correct allocation, fast convergence, stable after convergence
minimal rate oscillation during transient periods
adjusting well to heavy traffic
Comparing NK and FJ algorithms: NK algorithm performs better
faster convergence
shorter queue lengths during transient periods  less cell loss
much smaller rate fluctuations
5/20/2019

12. Explicit Rate Control for ABR Multipoint-to-multipoint Services
Motivations
One single VC for the entire MP-MP group
Several bandwidth allocation definitions (BADs)
No consensus on which BAD is best
Existing explicit rate control schemes for ABR MP-MP support only source-based allocation, or are not effective or scalable:
Ren, Siu and Suzuki
source-based allocation
incorrect calculation with VC merge
Pao
source-based allocation only
Cavendish and Gerla
MP-MP group with n members = n P-MP connections  not scalable
5/20/2019

13. Advantages of MPER Algorithm
Flexible
supporting both VP merge and VC merge
implementing a large set of BADs
Minimal overhead
overhead for implementing weighted-source-based BAD
easy setting using only a few software parameters
Scalable and efficient
a MP-MP group is supported by a single VC
one FRM is replied by exactly one BRM
running time of MPER algorithm: O(B)
Feedback consolidation at branch points
minimizing rate fluctuation and cell loss rate
maintaining stability after convergence
Effective allocation, fast convergence, robustness
by design (e.g., using fair share, computing ER in both directions)
confirmed by simulation results
5/20/2019

14. Experimental Results Simulation Measures Performance of MPER algorithm
ATM network simulator version 4.1 by NIST
Code added to implement MP-MP routing and MPER algorithm
Measures
ACRs of sources: effectiveness of allocation, convergence time
Queue lengths at switches: cell loss, stability
Performance of MPER algorithm
under normal condition and heavy load
correct allocation, fast convergence
minimal rate oscillation during transient periods
stable after convergence thanks to feedback consolidation
adjusting well to high traffic demands
5/20/2019

15. Reliable Multicast at the ATM Layer
Motivations
Existing reliable multicast protocols are at transport layer
one cell dropped  the whole packet retransmitted
multicast: many copies of a packet are retransmitted redundantly
Consequences:
increase of congestion level
high recovery latency
Our Objective
A reliable multicast protocol that
minimizes redundant retransmission (exposure)
offers low recovery latency, scalability
We propose:
loss detection/retransmission done at ATM layer
unit of recovery: ATM cells, not packets
5/20/2019

16. REMAT Protocol Adding 2-byte sequence numbers to ATM cells
sufficient to support applications requiring both reliability and real-time delivery
incurring less overhead than AAL3/4 (2 bytes versus 4 bytes)
Request/retransmission algorithm
uses the same ATM routing tree
local recovery performed by retransmission servers:
child-parent request/retransmission
scalable; low recovery latency
no implosion of requests or repair data
exposure, if any, is limited to one level of children
Message stability detection: a feedback control mechanism
for managing buffers
that can be used for rate-based congestion control
5/20/2019

17. Experimental Results Simulation Comparing Measures
ATM network simulator version 4.1 by NIST
code added to implement P-MP routing, REMAT and LSM protocols
Comparing
ATM-layer recovery: REMAT protocol
transport-layer recovery: LSM-like protocol
Measures
average retransmission delay
recovery latency
the faster packets are cleared, the better to alleviate congestion
connection throughput P= S/T
performance gain/loss
Results: ATM-layer recovery offers
much lower average retransmission delay in all cases
better connection throughput under moderate to heavy congestion
5/20/2019

18. Summary Congestion control is crucial to
high speed networks like ATM
multipoint communications
delivering QoS guarantees
We propose flexible and effective algorithms for
Explicite rate control for ABR multipoint-to-point services
Explicite rate control for ABR multipoint-to-multipoint services
fast convergence, minimal rate fluctuation
adapting well to heavy traffic demands
minimal implementation overhead
We propose REMAT protocol that performs loss recovery at the ATM layer
scalable: no implosion, minimal exposure
reducing unnecessary retransmission of transport-layer reliable multicast
much lower recovery latency than that of transport-layer reliable multicast
5/20/2019

19. Future Work
Defining MCRs and PCRs of multipoint connections (per VC or per source?). How should the rate calculation algorithm and connection admission protocol be modified?
Supporting heterogeneous receivers in an MP-MP group using layered data approach
Developing the message stability mechanism of REMAT protocol into a rate control scheme for flow/congestion control
5/20/2019