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

We present a measurement study and classification methodology for out-of-sequence packets in TCP connections observed within the Sprint IP backbone. Such out-of-sequence packets can result from many causes 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 they are one indication of the "health" of an end-end TCP connection. Our first contribution is methodological. Because we measure out-of-sequence packets at a single point in the backbone (rather than by sending and measuring end-end probe traffic at the sender or receiver), a new methodology is required to infer the causes of a connection\´s out-of-sequence packets based only on measurements taken in the "middle" of the connection. We thus describe techniques that classify the causes of observed out-of-sequence behavior based only on the previously- and subsequently-observed packets within a connection and knowledge of how TCP behaves. We show that using these simple 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 second contribution is the characterization of the out-of-sequence behavior itself. We analyze numerous several-hour packet-level traces from a set of OC-3 and OC-12 links for several million connections generated in nearly 4,300 unique ASs. Our measurements show a relatively consistent amount of out-of-sequence packets of approximately 5%. We find that few out-of-sequence packets result from pathological problems such as routing loops or in network duplication/reordering.