Download From TCP to Net Neutrality and Back

Removing Exponential Backoff
from TCP
Amit Mondal
Aleksandar Kuzmanovic
EECS Department
Northwestern University
http://networks.cs.northwestern.edu
TCP Congestion Control
Sending Rate
packet loss
• Slow-start phase
• Double the sending ...
... rate each round-trip ...
time
• Reach high throughput
...quickly
Time
2
A. Mondal
Removing Exponential Backoff from TCP
TCP Congestion Control
Sending Rate
packet loss
• Additive Increase –
...Multiplicative Decrease
• Fairness among flows
Time
3
A. Mondal
Removing Exponential Backoff from TCP
TCP Congestion Control
Sending Rate
packet loss
• Exponential
•.backoff
• System stability
Time
4
A. Mondal
Removing Exponential Backoff from TCP
Our breakthrough
Sending Rate
packet loss
• Exponential backoff
• fundamentally wrong!
Time
5
A. Mondal
Removing Exponential Backoff from TCP
Contribution
Untangle retransmit timer backoff mechanism
Challenge the need of exponential backoff in
TCP
Demonstrate exponential backoff can be
removed from TCP without causing congestion
collapse
Incrementally deployable two-step task
6
A. Mondal
Removing Exponential Backoff from TCP
Implications
Dramatically improve performance of shortlived and interactive applications
Increase TCP's resiliency against low-rate
(shrew attack) and high-rate (bandwidth
flooding) DoS attack
Other impacts
7
A. Mondal
Removing Exponential Backoff from TCP
Background
Origin on RTO backoff
Adopted from classical Ethernet protocol
– IP gateway similar to 'ether' in shared-medium Ethernet
network
Exponential backoff is essential for Internet
stability
– "an unstable system (a network subject to random load
shocks and prone to congestion collapse) can be stabilized by
adding some exponential damping (exponential timer backoff)
to its primary excitation (senders, traffic sources)“
[Jacobson88]
8
A. Mondal
Removing Exponential Backoff from TCP
Rationale behind revisions
No admission control in the Internet
– No bound on number of active flows
– Stability results in Ethernet protocol not applicable
IP gateway vs classical Ethernet
– Classical Ethernet: Throughput reduces to zero in overloaded
scenarios
– IP gateway: Forwards packets at full capacity even in extreme
congested scenarios
Dynamic network environment
Finite flow sizes and skewed traffic distribution
Increased bottleneck capacities
9
A. Mondal
Removing Exponential Backoff from TCP
Implicit Packet Conservation Principle
RTO > RTT
– Karn-Partridge algorithm and Jacobson's algorithm
ensures this
End-to-end performance cannot suffer if
endpoints uphold the principle
– Formal proof for single bottleneck case in paper
– Extensive evaluation with network testbed
• Single bottleneck
• Multiple bottleneck
• Complex topologies
10
A. Mondal
Removing Exponential Backoff from TCP
Experimental methodology
Testbed
–
–
–
–
–
Emulab
64-bit Intel Xeon machine + FreeBSD 6.1
RTT in 10ms - 200ms
Bottleneck 10Mbps
TCP Sack + RED
Workload
– Trace-II: Synthetic HTTP traffic based on empirical distribution
– Trace-I : Skewed towards shorter file-size
– Trace-III: Skewed towards longer file-size
NS2 simulations
11
A. Mondal
Removing Exponential Backoff from TCP
Evaluation
TCP*(n) : sub exponential
backoff algorithms
– No backoff for first “n”
consecutive timeouts
Impact of RTO backoff
mechanism on response
time
Impact of minRTO and
initRTO on end-to-end
performance
12
A. Mondal
Removing Exponential Backoff from TCP
Sub-exponential backoff algorithms
End-to-end performance does not degrade
after removing exponential backoff from TCP
Trace-I
Trace-II
Trace-III
A. Mondal
Removing Exponential Backoff from TCP
13
Impact of (minRTO, initRTO) parameters
RFC 2988 recommendation
– (1.0s, 3.0s)
Current practice
– (0.2s, 3.0s)
Aggressive version
– (0.2s, 0.2s)
14
A. Mondal
Removing Exponential Backoff from TCP
Impact of minRTO and initRTO
 Poor performance of
(1.0s,3.0s) RTO pair, the
CCDF tail is heaviest
Aggressive minRTO and initRTO parameters do
not hurt
e2e performance as long as endpoints
TCP
uphold implicit packet conservation principle
TCP*(3)
TCP*(∞)
 Improved performance both
for (0.2s, 3.0s) and (0.2s, 02s)
pair
15
A. Mondal
Removing Exponential Backoff from TCP
Role of bottleneck capacity
TCP*(∞) out performs classical TCP independent
of bottleneck capacity
16
A. Mondal
Removing Exponential Backoff from TCP
Dynamic environments
ON-OFF flow arrival period
Inter-burst: 50ms – 10s
17
A. Mondal
Removing Exponential Backoff from TCP
Dynamic environments
ON-OFF flow arrival period
Inter-burst: 1 sec
Time series of active connections
18
A. Mondal
Removing Exponential Backoff from TCP
TCP variants and Queuing disciplines
TCP Tahoe, TCP Reno, TCP Sack
Droptail, RED
The backoff-less TCP stacks outperform regular
stacks irrespective of TCP versions and queuing
disciplines
19
A. Mondal
Removing Exponential Backoff from TCP
Multiple bottlenecks
Dead packets
 Packets that exhaust
In multiple bottleneck scenario
is a chance
network there
resources
that dead packets impact
the performance
upstream,
but are thenof
flows sharing the upstream
bottleneck.
dropped
downstream
Topology
We do modeling and extensive experiment to
explore such scenarios
S1
S2
L1
S0
R1
R2
R3
L2
R4
C0
L0
p1
C1
p2
C2
20
A. Mondal
Removing Exponential Backoff from TCP
Impact on network efficiency
Fraction of dead packet at upstream bottleneck:
< 5% flows experience multiple
bottleneck
α = 0.002475 for (1%, 5%) very small
21
A. Mondal
Removing Exponential Backoff from TCP
Impact on end-to-end performance
What happens if the percent of multiplebottleneck flows increases dramatically?
What is the impact of backoff-less TCP
approach on end-to-end performance in such
scenarios?
Emulab experiment
– Set L0/(L0+L1)= 0.25 >> current situation
22
A. Mondal
Removing Exponential Backoff from TCP
Impact on end-to-end performance
Improves response
times distributions of
both set of flows
Similarimprove
result as their response
Multiple-bottlenecked flows
Trace-II
Trace-II
times without causing catastrophic
effect other
flows
even when their presence is significant
Multiple-bottlenecked flows
Trace-I
improve response times, while
upstream single-bottlenecked
flows only marginally
degrades response times
Trace-III
23
A. Mondal
Removing Exponential Backoff from TCP
Realistic network topologies
 Orbis-scaled HOT topology




10 Gbps core link
100 Mbps server edge link
1 – 10Mbps client side link
10ms link delay
 Workload
Response
times distribution
The improvement is more
significant
 HTTP
improves in absence of p2p
in presence
 HTTP
+ P2P of p2p traffic
traffic
24
A. Mondal
Removing Exponential Backoff from TCP
Incremental deployment
TCP's performance degrades non-negligibly
when present with TCP*(∞)
Two-step Task
– TCP to TCP*(3)
– TCP*(3) to TCP*(∞)
25
A. Mondal
Removing Exponential Backoff from TCP
Summary
Challenged the need of RTO backoff in TCP
End-to-end performance can only improve if
endpoints uphold implicit packet conservation
principle
Extensive testbed evaluation for single
bottleneck and multiple bottleneck scenario,
and with complex topologies
Incrementally deployable two-step task
26
A. Mondal
Removing Exponential Backoff from TCP
Thank you
27
A. Mondal
Removing Exponential Backoff from TCP
Impact of minRTO and initRTO
Aggressive minRTO and initRTO parameters do
not hurt e2e performance as long as endpoints
uphold implicit packet conservation principle
TCP
TCP*(3)
TCP*(∞)
28
A. Mondal
Removing Exponential Backoff from TCP
Impact of minRTO and initRTO
Aggressive minRTO and initRTO parameters do
not hurt e2e performance as long as endpoints
uphold implicit packet conservation principle
TCP
TCP*(3)
TCP*(∞)
29
A. Mondal
Removing Exponential Backoff from TCP