In most cases, the programmability of Software Defined Network (SDN) refers to the flexibility existing in northbound interface that enables network managers to control the behaviors of the networks. However, the lack of flexibility in data plane conversely results in wasting potentially usable information for controlling flows, especially from network services and applications point of view. For example, OpenFlow switches only deal with L2-L4 headers and ignore the other parts of packet. We propose Ouroboros as a programmable switch logic to increase the flexibility of SDN southbound interface. Ouroboros switches not only remove the limitation of regular OpenFlow switches using packet headers as the reference for packet switching, but also provides a highly flexible interface for network managers to conduct application-specific flow control according to packet content at any arbitrary offsets. Ouroboros can penetrate deeply into packet (e.g., RTP or SIP) protocol headers, or further into packet payload, to process user-defined switching protocol. Our evaluations of Ouroboros on 10Gbps traffic indicates the effectiveness of proposed method.