Bell Labs/Alcatel-Lucent AT&T Labs Columbia University Sonus Networks Overload occurs in Session Initiation Protocol (SIP) networks when SIP servers have insufficient resources to handle all SIP messages they receive. Even though the SIP protocol provides a limited overload control mechanism through its 503 (Service Unavailable) response code, SIP servers are still vulnerable to overload. This document discusses models and design considerations for a SIP overload control mechanism. This document is not an Internet Standards Track specification; it is published for informational purposes.