The invention relates to a method and a device for transmitting and receiving attachment data of extra attachments in session initiation messages of a session initiation protocol that is defined to initiate, maintain and terminate sessions with one or more terminals in a network. As the available space is limited for such attachments in a session initiation message the invention proposes to fragment the attachment data in attachment fragments and attach the attachment fragments to successive session initiation messages. Thereby the size of attachment data is increased considerably.