Session Initiation Protocol (SIP) is one building block for IP Multimedia Subsystem (IMS) and Next Generation Network (NGN), and is becoming ubiquitous in Internet telephony services. Therefore, the performance issue of SIP servers is becoming crucial In this paper, SIP stack processing is identified to be CPU-bound in nature and dominates the system overhead for common SIP server scenarios. These facts provide motivations for the idea of SIP offload. We also find that current protocol offload models have not considered program parallelism and can not analyze throughput and latency in together, which makes them difficult to apply to SIP offload. Therefore, a new model is proposed based on 7 ratios capturing fundamental properties of the protocol stack and offload scheme. The model can be applied in prediction of performance gains, selection of parameters for adaptive offloading schemes, and estimation of software or hardware characteristics using observed performance results.