Every DICOM Association starts with a process of negotiation whereby the Application_Entities involved exchnage sufficient information about themselves to allow proper communication. The information passed in the initial request includes:

• The AETs of the two applications
• The list of Presentation_Contexts request, each containing a list of Transfer_Syntaxes
• Any Reverse_Role_Negotiation requests
• Any Extended_Negotiation requests
• Miscellaneous "User Information" including
  • The name and UID of the software
  • The maximum data packet size that may be received

The acceptance packet contains mostly the same information but each Presentation_Context is either accepted or rejected, and if accepted, only a single Transfer Syntax" per Presentation_Context is specified.