End to End Internet
Packet Dynamics
Vern Paxson
University of California, Berkeley
Presented by Kiran Komaravolu
End to End characteristics of Internet
paths.
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Network Pathologies
 Out of order delivery
 Packet replication
 Packet corruption
Bottleneck Bandwidth
Packet Loss
 Loss rates
 Data vs. Ack loss
 Loss Bursts
 TCP retransmission
Packet Delays
Out of Order Delivery
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Out of order delivery is fairly prevalent
Re-ordering is asymmetric
Route-Fluttering
Site-dependant
Impact of re-ordering is not very
significant.
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Packet Replication
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Not very common.
Link level retransmissions are one main reason.
Packet Corruption.
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Evidence suggests data packet corruption rate of
0.02%.
Pure acks less prone to packet corruption.
TCP provides a 16-bit checksum. Thus it can lose
1 in 65,536 packets to errors.
On average 65,536 x 5000 = 300 million packets
will cause an bad packet to be accepted. Paxson
argues this is too high.
Bottleneck bandwidth
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Self-interference time constant
 Qb = packet size / bottleneck bandwidth.
Packet pair Algorithm.
 Two packets are sent with interval Δts < Qb.
 When they arrive at receiver Δtr = Qb.
 ICMP echo packets could be used to measure
bandwidth with this idea.
Difficulties
 Out of order delivery
 Clock resolution limitation
 Change in Bottleneck bandwidth
 Multichannel links
Packet Loss
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Loss rates doubled in 1995 (from 2.7% to 5.2
%)
Bigger windows do not make any impact on
Loss rates.
Loaded and Unloaded pkts
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Loaded pkts are those which had to wait for a
pending transmission to complete.
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About 2/3rd of all packets are loaded.
Loaded packets are more prone to be lost.
Acks are more prone to be lost than unloaded pkts.
Packet loss maybe asymmetric.
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Packet loss
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Losses occur in bursts.
TCP retransmissions
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Transmitting TCP may retransmit
unnecessarily
Because all acks were lost.
 Coarse feedback, sequence “holes” not known to
sender.
 Timed out too early.
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Significant portion of retransmissions are
“redundant”.
Packet Delays
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Timing Compression
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Timing Compression occurs when a flight (burst) of
packets sent over a period Ts arrives at the
receiver over an interval Tr and Tr < Ts.
Zhang et al predicted that ack compression could
occur is a flight arrives at a router and faces no
cross traffic. (router is under-utilised)
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E = (Tr + Cr) / (Ts – Cs)
Group is compressed if E < 0.75.
Most connections experience at least one
compression event.
Ack compression occurs more often for dup-acks.
(they are sent with less spacing between them.)
Packet Delays
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Compressed acks advance sender TCP window at
a faster rate. May lead to network stress.
Bandwidth measurement techniques would fail with
ack compression.
Data Packet Timing Compression
 Can occur due to sudden advance in receiver
advertised window.
 Bottleneck will spread out the packets.
 Packets “may” still be compressed after the
bottleneck.
Data pkt timing compression is rarer than ack
compression.
Available Bandwidth
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B = (N/W resources used by the connection/
(resources it used) + (resources competitors use).
B is a figurative expression for available bandwidth.
B = 1 means entire bottleneck was available.
Internet connections have been found to encounter
a broad range of available bandwidth.
Q’s
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