## 1 Introduction  
The backbone of the Internet is composed of electrical core routers. These routers perform data switching and forwarding digitally, e.g., using synchronized digital hierarchy (SDH) technology, and only make use of fiber optics as the transport medium between adjacent routers. To do so, switches are combined with wavelength-multiplexing systems to increase raw bandwidth at the optical layer, while the IP-layer core electrical routers provide the intelligence needed to route packets between any source and destination on the Internet. The networks forming the Internet backbone consist of multiple service networks on top of a common transport network, which relies on fiber. In this context, the traditional dominant service is voice, but there are also data-oriented services, such as Internet service providers using IP networks, or virtual private networks (VPN) using multi-protocol label switching (MPLS). The complementary electrical and optical technologies of the Internet backbone are converging into IP/optical network architectures. This means that network technologies are evolving from voice- to data-optimized, where IP is the convergence layer. In the first step of this convergence, IP/optical networks are circuit-switched (or wavelength-routed) networks composed of nodes interconnected with fiber optics that act as optical-layer switches and IP routers simultaneously. These IP/optical routers are capable of transporting data at the optical layer by creating optical paths (lightpaths) that bypass the IP routing infrastructure of the network, _i.e._, by offloading data to the optical layer. This is known as wavelength routing and results in low-delay, quality-enabled data delivery at the wavelength level. Typically, IP/optical networks are enabled with control and management infrastructures to establish, modify and release lightpaths, e.g., by using the generalized MPLS (GMPLS) technology. This way, data flows can traverse IP/optical networks through either an optical path or a chain of routing decisions at the IP layer, where IP routers are interconnected with fiber links [1]. This is especially interesting for large flows for two reasons. First, offloading them to the optical layer results in these flows experiencing faster and more reliable transmission with optical switching than with traditional IP routing. Second, remaining flows at the IP level also experience higher quality of service (QoS) because the IP layer is less congested after offloading the large flows to the optical layer. Nowadays, several high-broadband research and education networks are migrating towards this scheme of IP/optical networking. Examples of this migration are Europe's GEANT (www.geant.net), US' Internet2 (www.internet2.edu), The Netherland's SURFnet (www.surfnet.nl) or Canada's CANARIE (www.canarie.ca) networks. These networks transport data of their users and data of applications demanding very high bandwidth and stringent quality levels, such as grid applications, distributed radio telescopes or high-definition television broadcast. Typically, human operators (network managers) are in charge of selecting flows to be placed in the optical layer, in general by establishing lightpaths in advance, either manually or by using automated technologies such as GMPLS. Knowing the heavy-hitter behavior of high-bandwidth demanding applications helps network managers establish lightpaths in advance, because heavy-hitter flows are easily identified as flows with a very large number of packets, as defined by [PERSON] _et al._ in [2]. However, there are other \"big\" IP flows that do not follow heavy-hitter behavior and could also benefit from being moved to lightpaths. These flows are called \"elephants\" and have the following characteristics: they are persistent in time and few in number but account for most of the Internet traffic volume [3].