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End to End Internet Packet Dynamics. Vern Paxson University of California, Berkeley. Presented by Kiran Komaravolu. End to End characteristics of Internet paths. Network Pathologies Out of order delivery Packet replication Packet corruption Bottleneck Bandwidth Packet Loss Loss rates
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End to End Internet Packet Dynamics Vern Paxson University of California, Berkeley Presented by Kiran Komaravolu
End to End characteristics of Internet paths. • 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 • Out of order delivery is fairly prevalent • Re-ordering is asymmetric • Route-Fluttering • Site-dependant • Impact of re-ordering is not very significant.
Packet Replication • Not very common. • Link level retransmissions are one main reason. • Packet Corruption. • 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 • 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 • 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 • Loaded pkts are those which had to wait for a pending transmission to complete. • 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.
Packet loss • Losses occur in bursts. • TCP retransmissions • Transmitting TCP may retransmit unnecessarily • Because all acks were lost. • Coarse feedback, sequence “holes” not known to sender. • Timed out too early. • Significant portion of retransmissions are “redundant”.
Packet Delays • Timing Compression • 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) • 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 • 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 • 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.
