Dirk Kutscher

Mobile network designs have to meet several, at first sight contradicting, requirements: maximize resource utilization, provide optimal performannce (user-perceived quality of experience), enable operator-defined "fair usage" policies, maintain user privacy and minimize management complexity.

For 5G networks, virtual network slicing is often mentioned as one the desirable properties, i.e., the ability to run virtual networks for different application classes (service slicing) or different customer groups (MVNOs etc.) over the same physical infrastructure. Virtualizing networks over a larger set of shared resources (radio networks, backhaul, data centers) requires effective and efficient means for capacity sharing.

Capacity sharing can be done in different ways: traditionally, telco network capacity sharing has been inspired by telephony network architectures with an emphasis on control plane-based monitoring, resource allocation and configuration. Such approaches often involve traffic management systems that monitor performance, load etc. of network elements, analyze traffic properties (for example, DPI-based traffic inspection) and configure network elements such as base station and gateways to implement certain rate limits based on operator policies.

Three trends make this difficult in present and future networks:

1. with virtualization, slicing etc., the effort of analyzing every single tenant's flows can be increasingly prohibitive;
2. encryption-by-default with HTTP2 and other protocols that employ connection-based encryption renders DPI-based approaches costly (at best -- if not impossible); and
3. Internet protocols and applications such TCP (transport layer) and DASH-based video streaming over HTTP (application layer) are themselves adaptive to congestion, delay and overall observed performance. New protocols with specific requirements are invented all the time (think IoT, Virtual Reality). Interfering with their control loops through network traffic management may yield bad performance, suboptimal user preference and higher cost overall.

The idea of enabling an effective capacity sharing with a productive cooperation of operator policy decision making and dynamic application/user resource utilization has driven the work in the IETF ConEx working group. Based on earlier work by Bob Briscoe on Re-Feedback, the ConEx WG has defined concepts and (experimental) mechanisms for congestion exposure, enabling a form of capacity sharing that incentivizes senders to respond to congestion signals, while still enabling operators with hooks for auditing and enforcing correct behavior.

RFC 7778 describes how the ConEx mechanisms can be applied to current LTE (EPS) networks, considering their specifics regarding QoS and network architecture. For example, RFC 7778 described how ConEx can

• enable or enhance flow policy-based traffic management;
• reduce the need for complex DPI by allowing for a bulk packet traffic management system that does not have to consider either the application classes flows belong to or the individual sessions; and how it can
• be used to more effectively trigger the offload of selected traffic to a non-3GPP network.

More experiments with ConEx and related capacity sharing mechanisms are needed, but the questions behind ConEx remain important for 5G (and beyond...): How to achieve an effective collaboration of networks and their users (senders and receivers) considering increased need for capacity sharing, increased demand for user privacy (connection encryption) and the permissionless innovation feature of the Internet, i.e., not expecting the network to know all possible application classes and their traffic management requirements.