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SMART Retransmission:
Concepts and Performance
S. Keshav and S.P. Morgan
IEEE INFOCOM ‘97
Purpose
 Study the effect of retransmission strategy on congestion
 Present new retransmission strategy
 Study its performance
 Show how it cooperates with flow control
 Comparison with existing schemes
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Anatomy of congestion
Source
Data
Router
Sink
Acks
 Overload => loss
 Uncontrolled retransmissions => congestion collapse
 Need both
Good retransmission strategy to prevent collapse
Good flow control to prevent overload
3
Go back N
 Error control window
Packets sent but unacknowledged
 1 2 3 4 5 6 7 8 9 10 11 12
 On loss, retransmit entire error control window
Receiver only accepts in-order packets => no buffering needed
 Overload => loss => excessive retransmissions => more overload
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Selective acknowledgement
 1 2 3 4 5 6 7 8 9 10 11 12
 Bitmask
0 1 0 1 0 in ack
 Sender only retransmits lost packet
 Cons
overhead
even with no loss
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SMART
 Build bitmask at sender
 Receiver sends both cumulative ack, and packet that caused ack
 1 2 3 4 5 6 7 8 9 10 11 12
 (6,6)
(6,8)
(6,10)
send 7
send 9
 On detecting gap, send missing packet, unless already sent
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Lost retransmission?
P was lost, so cumulative ack must be P-1
P
P+1
P
If cumulative ack does not increase by this time, retransmission was lost
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How big an error control window?
 Receiver must buffer at least 1 error control window for reordering
 Small error control window restricts flow
 Large error control window requires large receive buffers
 Error control should be large enough to allow recovery
single loss => allow source to send for ~ 2 RTT without pause
if smaller, flow is restricted
 If loss while recovering, need larger error control window
 Simulations show ~2-4 RTT*bandwidth works well
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How well does it work?
 10 ON-OFF closed-loop sources
 Load varied by changing ratio of ON to OFF time
 Static window flow control
 Bandwidth delay product = 100 packets
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Effect of SMART on static-window flow control
Go back N
SMART
10
SMART alone is not enough
 SMART prevents congestion collapse on packet loss
 We need to prevent packet loss in the first place
 How?
ATM End-to-end rate-based flow control
feedback sustainable rate to source
Packet-pair flow control
estimate rate implicitly by measuring spacing between acks for
back-to-back transmissions
 Either way, source prevents or reacts to overload
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How well does this work?
SMART
+ PP
12
Good flow control drives out bad
PP + SMART
Static Window + SMART
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What about random losses?
 Packet losses can be congestive or random
 Congestive losses are due to buffer overflow
simulated earlier
 Random losses are due to bit errors
primarily on wireless link
 How well does PP + SMART work on such links?
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Performance with random losses
 If one-way loss rate on link is r, and bandwidth delay product is W
 TCP throughput is proportional to 1- rW2/4
 SMART + PP throughput is proportional to 1-2r
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Performance with random losses
TCP
SMART + PP
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Summary
 Bad retransmission strategy can lead to congestion collapse
 SMART can prevent collapse
 Efficiency requires good flow control in addition to good
retransmission strategy
 Good flow control drives out bad
 SMART has excellent performance both with congestive and
random losses
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