Modifications to SIP (Session Initiation Protocol) are made which significantly extend the functionality of SIP. SIP messages are associated with computer software code such as Java byte code, Java applets or mobile autonomous software agents. SIP clients are arranged such that on receipt of a SIP message that has been associated with computer software code, that code is executed by a processor associated with the SIP client. In the case that Java applets are contained in a SIP message these are executed by a Java Virtual Machine associated with the SIP client. If a Java mobile agent is contained in the SIP message this executes on a Java Mobile Agent Virtual Machine associated with the SIP client. Preferably an indicator is put into the header of a SIP message to indicate that it has been associated with computer software code, and SIP clients are arranged to detect the presence of such indicators.