When the Microsoft Skype for Business Server 2015 Front End (FE) server communicates together with a peer server through the Transmission Control Protocol (TCP) protocol, the FE service (Rtcsrv.exe) crashes. Additionally, the following event is logged in Application log:
Microsoft strongly recommends that you use Transport Layer Security (TLS) to help provide superior security through encryption and peer server authentication. Support for TCP connections to trusted applications is currently limited (for example, there is no support for IPv6), and the support may be deprecated in future releases of the product.
This issue can occur if the EnableTCP parameter is used together with the New-CsTrustedApplication cmdlet. This parameter tells Lync that the trusted application will use TCP. Use this parameter only if the trusted application is not a Microsoft Unified Communications Managed API (UCMA) application. This is because UCMA applications support only the mutual Transport Layer Security (MTLS) protocol. If you don't specify the Force parameter together with the EnableTCP parameter, you receive a confirmation prompt before the new trusted application is created.