In February 1999, the ATM Forum international consortium approved the first version of its security specifications aiming to protect communications over Asynchronous Transfer Mode (ATM) networks by offering data confidentiality, partners authentication, etc. In order to improve those specifications, a new approach to negotiate some security services has been presented at the ATM Forum and will be integrated into the version 1.1 of the specifications. Contrary to the ATM Forum approaches, this new approach allows security partners to negotiate some new security parameters at any time during a connection in progress. The idea rests on fixed-size ATM management cells used to transfer security information. To cope with cell losses during transfer, a cell-loss recovery protocol is defined. This article describes this new approach, the cell-loss recovery protocol and the validation of this protocol using SDL tools.