Evaluating and improving SIP non-INVITE transaction to alleviate the losing race problem

Abstract

SIP (Session Initiation Protocol) is developed by IETF for creating and managing sessions such as Internet calls. The exchange of SIP messages is controlled by two types of transactions: INVITE transaction for session setup; non-INVITE transaction (NIT) for other purposes, e.g. canceling a session. NIT was identified to have a race condition under which it will fail if a final response by server is not received by the client in time. An update to NIT thus has been proposed by IETF to reduce the risk of NIT losing the race. Although the update has been implemented in some SIP products, no report on the effectiveness of the update has been seen. In this paper, firstly the performance of the update is evaluated by simulation using Coloured Petri Nets (CPNs). The results show that the update does not improve the situation in most cases. Therefore we examine the causes of the losing race problem and propose an improvement to enhance the performance of NIT under the race condition. The analysis suggests that the improvement can effectively alleviate the losing race problem. We hope that this research can contribute to the SIP community and demonstrate the usefulness of CPNs in protocol development.