Security function outsourcing has witnessed both research and deployment in the recent years. While most existing services take a straight-forward approach of cloud hosting, on-path transit networks (such as ISPs) are increasingly more interested in offering outsourced security services to end users. Recent proposals (such as SafeBricks and mbTLS) have made it possible to outsource sensitive security applications to untrusted, arbitrary networks, rendering on-path security function outsourcing more promising than ever. However, to provide on-path security function outsourcing, there is one crucial component that is still missing -- a practical end-to-end network protocol. Thus, the discovery and orchestration of multiple capable and willing transit networks for user-requested security functions have only been assumed in many studies without any practical solutions. In this work, we propose Opsec, an end-to-end security-outsourcing protocol that fills this gap and brings us closer to the vision of on-path security function outsourcing. Opsec automatically discovers one or more transit ISPs between a client and a server, and requests user-specified security functions efficiently. When designing Opsec, we prioritize the practicality and applicability of this new end-to-end protocol in the current Internet. Our proof-of-concept implementation of Opsec for web sessions shows that an end user can easily start a new web session with a few clicks of a browser plug-in, to specify a series of security functions of her choice. We show that it is possible to implement such a new end-to-end service model in the current Internet for the majority of the web services without any major changes to the standard protocols (e.g., TCP, TLS, HTTP) and the existing network infrastructure (e.g., ISP's routing primitives).