C-GET and SCP/SCU Role Selection

The final wrinkle in presentation context negotiation is SCP/SCU Role Selection, which allows an association requestor to propose it’s role (SCU, SCP, or SCU and SCP) for each proposed abstract syntax. Role selection is used for services such as the Query/Retrieve Service’s C-GET requests, where the association acceptor sends data back to the requestor.

To propose SCP/SCU Role Selection as a requestor you should include SCP_SCU_RoleSelectionNegotiation items in the extended negotiation, either by creating them from scratch or using the build_role() convenience function:

from pynetdicom import AE, build_role
from pynetdicom.pdu_primitives import SCP_SCU_RoleSelectionNegotiation
from pynetdicom.sop_class import CTImageStorage, MRImageStorage

ae = AE()
ae.add_requested_context(CTImageStorage)
ae.add_requested_context(MRImageStorage)

role_a = SCP_SCU_RoleSelectionNegotiation()
role_a.sop_class_uid = CTImageStorage
role_a.scu_role = True
role_a.scp_role = True

role_b = build_role(MRImageStorage, scp_role=True)

assoc = ae.associate("127.0.0.1", 11112, ext_neg=[role_a, role_b])

When acting as the requestor you can set either or both of scu_role and scp_role, with the non-specified role assumed to be False.

To support SCP/SCU Role Selection as an acceptor you can use the scu_role and scp_role keyword parameters in AE.add_supported_context():

from pynetdicom import AE
from pynetdicom.pdu_primitives import SCP_SCU_RoleSelectionNegotiation
from pynetdicom.sop_class import CTImageStorage

ae = AE()
ae.add_supported_context(CTImageStorage, scu_role=True, scp_role=False)
ae.start_server(("127.0.0.1", 11112))

When acting as the acceptor both scu_role and scp_role must be specified. A value of True indicates that the acceptor will accept the proposed role. pynetdicom uses the following table to decide the outcome of role selection negotiation:

Requestor		Acceptor		Outcome		Notes
scu_role	scp_role	scu_role	scp_role	Requestor	Acceptor
N/A	N/A	N/A	N/A	SCU	SCP	Default
True	True	False	False	N/A	N/A	Rejected
			True	SCP	SCU
		True	False	SCU	SCP	Default
			True	SCU/SCP	SCU/SCP
True	False	False	False	N/A	N/A	Rejected
		True		SCU	SCP	Default
False	True	False	False	N/A	N/A	Rejected
			True	SCP	SCU
False	False	False	False	N/A	N/A	Rejected

As can be seen there are four possible outcomes:

• Requestor is SCU, acceptor is SCP (default roles)

• Requestor is SCP, acceptor is SCU

• Requestor and acceptor are both SCU/SCP

• Requestor and acceptor are neither (context rejected)

Warning

Role selection negotiation is not very well defined by the DICOM Standard, so different implementations may not give the same outcomes.