  Trust, security, and privacy are three of the major pillars to assemble the
fifth generation network and beyond. Despite such pillars are principally
interconnected, they arise a multitude of challenges to be addressed
separately. 5G ought to offer flexible and pervasive computing capabilities
across multiple domains according to user demands and assuring trustworthy
network providers. Distributed marketplaces expect to boost the trading of
heterogeneous resources so as to enable the establishment of pervasive service
chains between cross-domains. Nevertheless, the need for reliable parties as
``marketplace operators'' plays a pivotal role to achieving a trustworthy
ecosystem. One of the principal blockages in managing foreseeable networks is
the need of adapting previous trust models to accomplish the new network and
business requirements. In this regard, this article is centered on trust
management of 5G multi-party networks. The design of a reputation-based trust
framework is proposed as a Trust-as-a-Service (TaaS) solution for any
distributed multi-stakeholder environment where zero trust and zero-touch
principles should be met. Besides, a literature review is also conducted to
recognize the network and business requirements currently envisaged. Finally,
the validation of the proposed trust framework is performed in a real research
environment, the 5GBarcelona testbed, leveraging 12% of a 2.1GHz CPU with 20
cores and 2% of the 30GiB memory. In this regard, these outcomes reveal the
feasibility of the TaaS solution in the context of determining reliable network
operators.
