A Measurement Study of Internet Delay Asymmetry
RTT has been widely used as a metric for peer/server selection. However, many applications involving closest peer/server selection such as streaming, tree-based multicast services and other UDP and TCP based services would benefit more from knowing one-wa
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Abstract. RTT has been widely used as a metric for peer/server selection. However, many applications involving closest peer/server selection such as streaming, tree-based multicast services and other UDP and TCP based services would benefit more from knowing one-way delay (OWD) rather than RTT. In fact, RTT is frequently used as as an approximate solution to infer forward and reverse delays by many protocols and applications which assume forward and reverse delay to be equal to half of RTT. In this paper, we compare and contrast one-way delays and corresponding RTTs using a wide selection of routes in the Internet. We first measure the extent and severeness of asymmetry in forward and reverse OWD in the Internet. We then attempt to isolate the causes of OWD asymmetry by correlating OWD asymmetry with the route asymmetry. Finally, we investigate the dynamics of delay asymmetry. We find there exists a weak correlation between the fluctuation of RTT and OWD but a strong correlation between OWD change and the corresponding route change.
1 Introduction Today’s Internet is rife with several wide-area network applications: real-time applications such as voice over IP [1] and multicast streaming applications [2,3,4,5], data transfer applications that perform locality-aware redirection and server selection [6], and services such as proximity-aware DHTs [7,8] and positioning systems [9,10,11]. A common thread in all these applications is the requirement to perform proximity measurements. For example, in multicast applications, proximity is used to choose a suitable parent/child in the tree; in positioning systems, proximity to landmarks is used for localization. For some applications, the proximity of interests can be measured using the roundtrip time (RTT) between two end hosts (say A and B), defined as the sum of forward delay from A to B and the reverse delay from B to A. For example, if the interaction between remote hosts typically involves only one or a few request and reply messages, for example, a DNS lookup, or a small HTTP document download, then RTT is a good indication of the completion time of the interaction. For other applications, however, the proximity of direct relevance is the one-way delay (OWD) from the client to the servers/peers or along the other direction, rather than RTT. An asymmetry in OWD could hurt such applications. For example, in multicast streaming applications, since M. Claypool and S. Uhlig (Eds.): PAM 2008, LNCS 4979, pp. 182–191, 2008. c Springer-Verlag Berlin Heidelberg 2008
A Measurement Study of Internet Delay Asymmetry
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the data always flows from a parent node to a child node in the overlay multicast tree, optimizing the OWD from the parent to the child is more beneficial. As another example, ACK/NAK data systems such as the Transmission Control Protocol (TCP) estimate the available bandwidth of the unidirectional route from the sender to the receiver using the round-trip time as an approximation. If the reverse path taken by ACK packets has a much larger delay than the for
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