As the Internet evolves into a universal network for all communication needs, it has to stand up to the high quality standards of traditional networks, such as the telephone network for voice communications. Multimedia applications are particularly sensitive to various impairments introduced by IP networks, such as packet loss, delay and delay jitter. In this paper, we study loss and delay measurements taken over the Internet and we provide a detailed characterization thereof. We focus on wide-area backbone networks, which constitute an important part of long-distance communication. Our study is based on a rich data set that provides valuable insights into the behavior of Internet backbones today, and in particular into how they affect multimedia traffic. We find that most of the problems observed seem more related to reliability, network protocols and router operation rather than to traffic load and traditional quality-of-service issues. Furthermore, the characterization and modeling of packet loss, delay and delay jitter can be used by the research community as input to various problems related to the design and evaluation of network- and application-layer mechanisms.