Digital Imaging and Communications in Medicine (DICOM)

9.8 Protocol Data Unit 199
9.8.1
A-Associate-RQ
As you can see on Figs. 61 and 62, A-Associate-RQ consists of several familiar
structures, such as Application Contexts and Presentation Contexts. The
A-Associate-RQ is the information that any calling AE hands out to its called
AEs, proposing to establish a working DICOM association. Most importantly,
the proposition includes one or more Presentation Context items (Abstract and
Transfer Syntaxes) that reflect the DICOM functionality of the calling AE requesting
the association.
Both calling and called AE titles must not be blank (undefined) and should
correspond to the real, existing AEs on the given DICOM network. Because
the AE VR uses 16 characters (see 5.3, Table 2), the A-Associate-RQ provides
16-character fields to hold calling and called AE titles. Shorter AE titles are
padded with trailing blanks to ensure that all 16 bytes are used. Many PACS
workstations and archives, acting as called AEs, first verify whether the calling
AE title is present in their “white list” (list of permitted callers usually configured
by the PACS administrator). If the calling AE is not on the white list, the
A-Associate-RQ will likely be rejected as a stranger, with the A-Associate-RJ
message. The messages often contains the infamous “no reason given” reason
for rejection, which we will see more of in the following sections. This archaic,
primitive, and easy-to-bypass, security check nonetheless remains one of the
most currently used in PACS networks: sometimes serving its purpose, and
sometimes creating implementation problems.
The only new item in the A-Associate-RQ is the 2-byte “protocol version”.
This is a constant with the first (zero) bit set to 0 to indicate version 1
of the DICOM UL protocol. Because the values of the other bits are not used,
Fig. 61 A-Associate-RQ message structure, from PS3.8