Computer networks have become a critical infrastructure. Especially in shared environments such as datacenters it is important that a correct, consistent and secure network operation is guaranteed at any time, even during routing policy updates. In particular, at no point in time should it be possible for packets to bypass security critical waypoints~(such as a firewall or IDS) or to be forwarded along loops. This paper studies the problem of how to change routing policies in a transiently consistent manner. Transiently consistent network updates have been proposed as a fast and resource efficient alternative to per-packet consistent updates. Our main result is a negative one: we show that there are settings where the two basic properties waypoint enforcement and loop-freedom cannot be satisfied simultaneously. Even worse, we rigorously prove that deciding whether a waypoint enforcing, loop-free network update schedule exists is NP-hard. These results hold for both kinds of loop-freedom used in the literature: strong and relaxed loop-freedom. This paper also presents optimized, exact mixed integer programs to compute optimal update schedules. We report on extensive simulation results and initiate the discussion of scenarios where multiple waypoints need to be ensured (also known as service chains).

In this work, we propose utilizing the rich connectivity between IXPs and ISPs for inter-domain path stitching, supervised by centralized QoS brokers. In this context, we highlight a novel abstraction of the Internet topology, i.e., the inter-IXP multigraph composed of IXPs and paths crossing the domains of their shared member ISPs. This can potentially serve as a dense Internet-wide substrate for provisioning guaranteed end-to-end (e2e) services with high path diversity and global IPv4 address space reach. We thus map the IXP multigraph, evaluate its potential, and introduce a rich algorithmic framework for path stitching on such graph structures.