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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
2
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
4
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
5
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
6
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
7
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
8
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
9
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
11
How well does this work?
SMART
+ PP
12
Good flow control drives out bad
PP + SMART
Static Window + SMART
13
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?
14
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
15
Performance with random losses
TCP
SMART + PP
16
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
17