The migration of network functions (NFs) to datacenters, as promoted by Network Function Virtualization (NFV), raises the need for service chaining (i.e., steering traffic through a sequence of NFs). Service chaining is typically performed by installing forwarding entries in switches within datacenters (DCs). However, as the number of service chains in DCs grows, switches will be required to maintain a large amount of forwarding state. This will raise a dataplane scalability issue, due to the relatively small flow table size of switches. To mitigate this problem, we present a software-defined network (SDN) based source routing architecture for scalable service chaining, at which the NF-path is encoded into the packet header obviating the need for any forwarding state and lookup in the switches. We assess the feasibility and efficiency of our architecture using a prototype implementation.