Measurement and classification of out-of-sequence packets in a tier-1 IP backbone

  • Authors:
  • Sharad Jaiswal;Gianluca Iannaccone;Christophe Diot;Jim Kurose;Don Towsley

  • Affiliations:
  • Bell Labs Research India, Alcatel-Lucent, Bangalore, India;Intel Research Cambridge, Cambridge, UK;Thomson Technology Paris Laboratory, Boulogne cedex, France;Computer Science Department, University of Massachusetts at Amherst, Amherst, MA;Computer Science Department, University of Massachusetts at Amherst, Amherst, MA

  • Venue:
  • IEEE/ACM Transactions on Networking (TON)
  • Year:
  • 2007

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Abstract

We present a classification methodology and a measurement study for out-of-sequence packets in TCP connections going over the Sprint IP backbone. Out-of-sequence packets can result from many events including loss, looping, reordering, or duplication in the network. It is important to quantify and understand the causes of such out-of-sequence packets since it is an indicator of the performance of a TCP connection, and the quality of its end-end path. Our study is based on passively observed packets from a point inside a large backbone network--as opposed to actively sending and measuring end-end probe traffic at the sender or receiver. A new methodology is thus required to infer the causes of a connection's out-of-sequence packets using only measurements taken in the "middle" of the connection's end-end path. We describe techniques that classify observed out-of-sequence behavior based only on the previously- and subsequently-observed packets within a connection and knowledge of how TCP behaves. We analyze numerous several-hour packet-level traces from a set of OC-12 and OC-48 links for tens of millions connections generated in nearly 7600 unique ASes. We show that using our techniques, it is possible to classify almost all out-of-sequence packets in our traces and that we can quantify the uncertainty in our classification. Our measurements show a relatively consistent rate of out-of-sequence packets of approximately 4%. We observe that a majority of out-of-sequence packets are retransmissions, with a smaller percentage resulting from in-network reordering.