Using Loss Pairs to Discover
Network Properties
Jun Liu, Mark Crovella
Computer Science Dept.
Boston University
Discovering Network Properties
What is the state of the network
when packets are being dropped?
E.g., What is the buffer occupancy when
packets are being dropped?
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For a DropTail queue, indicates buffer size;
For an AQM queue, indicates dropping policy.
Benefits and Difficulties
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Why is this useful?
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Characterizing existing networks;
Using information gained to adapt applications
to network status;
Debugging network elements.
Why is this hard?
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Dropped packets carry no information to the
endpoint;
Noisy measurements require robust estimation
methods.
The Basic Idea
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A packet traveling close to a dropped
packet sees similar network state as
the dropped one.
We propose a method called Loss
Pairs to characterize the dropping
function of a network element under
some assumptions.
Loss Pairs
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A non-dropped packet that is close to a dropped
packet can inform the endpoint about network
state seen by the dropped packet.
We define a loss pair as two packets p1 and p2
such that
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p2 initially follows p1 with time  between the trailing
edge of p1 and the leading edge of p2;
Exactly one of p1 and p2 happens to be dropped in the
network;
p1 and p2 traverse the same sequence of links and
routers up to the point one is dropped.
Generally, we consider  to be 0.
Assumptions For Using Loss
Pairs in Practice
1.
2.
3.
4.
Most of the packet losses and delays
happen at the bottleneck.
The round-trip path and the bottleneck
stay stable during measurement.
To estimate queue state, packet
scheduling must be FCFS.
To convert queue occupancy to bytes,
bottleneck bandwidth must be known.
Network Setting For Evaluation
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We consider a 3-hop sample network setting.
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Capable of varying cross traffic upstream, downstream
and at the bottleneck.
Workload : heavy tailed ON/OFF TCP sources.
Goal : Characterize the queue between B and C by
passive measurements taken at the sender.
Upstream
Cross Traffic
Visable
Traffic
A
Bottleneck
Cross Traffic
Downstream
Cross Traffic
Bottleneck
Link
B
C
D
Varying Buffer Size Of A
DropTail Router
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The queue state seen by a dropped packet is a
linear function of buffer size.
Estimating Buffer Size of
DropTail Routers
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Tq = the most common RTT of Loss
Pairs.
Tp = minimum RTT of all non-dropped
packets.
C = link bandwidth of the bottleneck
link
Buffer size = C * (Tq – Tp)
Filtering Ability of Loss Pairs
Normal RTT
Corresponds
to empty
buffer
RTT of Loss Pairs
Corresponds
to full buffer
Estimation Accuracy Under
Light Cross Traffic
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Each crossing path
has 1/10th the
sources of the main
path.
The estimation
results are quite
good -- all
assumptions are
met.
Estimation Accuracy Under
Moderate Cross Traffic
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Each crossing path has
50% of the sources of
the main path.
Estimation results are
still acceptable -assumptions are
partially met.
Queue delays in nonbottleneck queues are
prominent for small
buffer sizes.
Estimation Accuracy Under
Heavy Cross Traffic
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Each crossing path has
as many sources as the
main path.
Estimation results are
poor for small buffers
-- assumptions are
violated.
However, the results
are acceptable on
large buffers.
Effect of Cross Traffic
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Heavy cross traffic upstream ,
downstream or on return path
affects accuracy.
Cross traffic at the bottleneck
doesn’t affect accuracy -- only
affects the number of samples on
Loss Pairs.
Characterizing Dropping
Curves of AQM Routers
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New assumption : packet drops are
independent.
Estimation Method:
# loss pairs with RTT = x
Dropping ratio = ------------------------------# trial pairs with RTT = x
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A trial pair with RTT = x is a pair of
packets with at least one packet not
having been dropped.
Characterizing the Dropping
Curve of a RED Router
Parameters: min_threshold=9KB, max_threshold=18KB, Mp=0.1
Measured By Loss Pairs
Actual Behavior
Characterizing the Dropping
Curve of a BLUE Router
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Parameters: B=500 pkts, Incr=Decr=0.0025, Holding time = 0.01 sec.
Related Work
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Measurement of bandwidth by Packet Pairs
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Measurement of bandwidth, delay, mean queue
occupancy, etc. by individual packet
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Keshav (1991), Bolot (1993), Paxson (1995), Carter &
Crovella (1996, Bprobe)
Jacobson (1997, pathchar), Downey (1999, pchar), Mah
(1999, clink), Lai and Baker (2000), Harfoush et al. (2001)
Measurement of loss rate
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Duffield et. al. (1999, MINC)
Harfoush et. al. (2000, MINT)
Conclusion
We propose a new technique for characterizing
the packet dropping behavior of network
elements.
By simulation, we have shown that this method is
effective in characterizing the dropping patterns
of routers.
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This method can be used to characterize the queue
management scheme being used at the bottleneck link.
For DropTail queues, this method can be used to
determine router buffer size.