  Network Function Virtualization (NFV), network slicing, and Software-Defined
Networking (SDN) are the key enablers of the fifth generation of mobile
networks (5G). Service Function Chaining (SFC) plays a critical role in
delivering sophisticated service per slice and enables traffic traversal
through a set of ordered Service Functions (SFs). In fully symmetric SFCs, the
uplink and downlink traffic traverse the same SFs, while in asymmetric SFC, the
reverse-path may not necessarily cross the same SFs in the reverse order.
Proposed approaches in the literature support either full symmetry or no
symmetry. In this paper, we discuss the partial symmetry concept, that enforces
the reverse path to traverse the SFs only when needed. Our contribution is
threefold. First, we propose a novel SFC framework with an abstraction layer
that can dynamically create partial or full symmetric SFCs across multiple
administrative and technological cloud/edge domains. According to the Key
Performance Indicators (KPIs) and desired objectives specified at the network
slice intent request, the abstraction layer would automatise different SFC
operations, but specifically generating partial or full symmetric SFCs. Second,
we propose an algorithm to dynamically calculate the reverse path for an SFC by
including only SFs requiring symmetry. Third, we implement a prototype
application to test the performance of the partial symmetry algorithm. The
obtained results show the advantages of partial symmetry in reducing both the
SFC delivery time and the load on VNFs.
