X4: Mechanism Adaptivity in Heterogeneous Multi-Stream Transport Protocols

Within in the Internet protocol stack, the transport protocol arguably has the highest level of variety in terms of ser-vice models and expected QoS targets. The de-facto standard transport protocols UDP and TCP provide unreliable datagram service and reliable bytestream service, respectively. Application demands range from reliable or unrelia-ble message passing and elastic reliable in-order delivery to low-latency best-effort media streaming. While most applications with a single type of traffic and a small number of parallel communication relations can be served satis-factorily within this framework, it comes to its limits if multiple (logical) communications or a mix of data traffic service classes between the same end points are active at the same time.

A protocol design that tackles the questions posed above for such diverse application requirements demands a solu-tion that selects, configures and adapts mechanisms of the transport layer (including reliability, scheduling, segmen-tation, flow control, congestion control, path MTU discovery, and more) according to the current traffic demand and the current set of streams. In the subproject, we will consider a single pair of end points between which a (potentially dynamic) set of streams with not necessarily homogeneous service and QoS requirements is established. Our goal is to dynamically select appropriate mechanisms to realize these services based on a best-effort packet service. Co-operative transitions between the end points will allow to dynamically adjust the protocol behavior to changing re-quirements and varying network conditions. The selected and configured mechanisms can operate per-stream or they can be shared among streams. We do not aim for a complete and exhaustive solution in the sense of a one-size-fits-all transport protocol, but we intend to explore the solution space and the interdependencies between ser-vices, mechanisms, mechanism selection and mechanism adaptation, and a prototype transport protocol design that demonstrates the viability of supporting heterogeneous streams.