D.3.3.4 SCP/SCU Role Selection Negotiation

D.3.3.4 SCP/SCU Role Selection Negotiation (Current)
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The SCP/SCU Role Selection Negotiation allows peer AEs to negotiate the roles in which they will serve for each SOP Class or Meta SOP Class supported on the Association. This negotiation is optional.

The Association-requestor, for each SOP Class UID or Meta SOP Class UID, may use one SCP/SCU Role Selection item. The SOP Class or Meta SOP Class shall be identified by its corresponding Abstract Syntax Name followed by one of the three role values:

Association-requestor is SCU only
Association-requestor is SCP only
Association-requestor is both SCU and SCP

If the SCP/SCU Role Selection item is absent the default role of the Association-requestor shall be SCU and the default role of the Association-acceptor shall be SCP.

The Association-acceptor, for each SCP/SCU Role Selection item offered, either accepts the Association-requestor proposal by returning the same value (1) or turns down the proposal by returning the value (0). The Association-acceptor shall not return the value (1) if the Association-requestor has not proposed the role, i.e., it has sent a value (0). The Association-requestor shall ignore the response if it has not proposed the role.

If the SCP/SCU Role Selection item is not returned by the Association-acceptor then the role of the Association-requestor shall be SCU and the role of the Association-acceptor shall be SCP. Figure D.3-7 illustrates the SCP/SCU Role Selection Negotiation.

If the SCP/SCU Role Selection items do not exist in the A-ASSOCIATE indication they shall be omitted in the A-ASSOCIATE response.

Note

The choices made for the default roles are based on clarification made to previous versions of the Standard. Association-requestors that wish to offer Abstract Syntax Names using the SCP role must support this item. Association-acceptors that wish to accept Abstract Syntax Names using the SCU role must support this item.
If an Association-requestor offers an SCP/SCU Role Selection item for an Abstract Syntax Name but the Association-acceptor does not return an SCP/SCU Role Selection item for the same Abstract Syntax Name then the proposed roles have not been accepted and the default roles apply (i.e., Association-requestor is SCU and Association-acceptor is SCP).

Figure D.3-7. SCU/SCP Role Selection Negotiation

Note

DICOM AE "B" accepts DICOM AE "A"'s proposed role as an SCU for the Storage-MR SOP; therefore, DICOM AE "B" will perform in the SCP role. DICOM AE "B" turns down the SCP proposal from DICOM AE "A".
Both DICOM AEs may be SCU and SCP for the Storage-CT SOP.
DICOM AE "B" accepts DICOM AE "A"'s proposed role as an SCU for the Print-SOP; therefore, DICOM AE "B" will perform in the SCP role.

D.3.3.4.1 SCP/SCU Role Selection Sub-Item Structure (A-ASSOCIATE-RQ)

The SCP/SCU Role Selection Sub-Item shall be made of a sequence of mandatory fields. This Sub-Item is optional and if supported, one or more SCP/SCU Role Selection Sub-Items may be present in the User Data Item of the A-ASSOCIATE-RQ. The Association-requestor may only offer one SOP Class SCP/SCU Role Selection Sub-Item for each SOP Class UID or Meta SOP Class that is present in the A-ASSOCIATE request. Table D.3-9 shows the sequence of the mandatory fields.

Table D.3-9. SCP/SCU Role Selection Sub-Item Fields (A-ASSOCIATE-RQ)

Item Bytes	Field Name	Description of Field
1	Item-type	54H
2	Reserved	This reserved field shall be sent with a value 00H but not tested to this value when received.
3-4	Item-length	This Item-length shall be the number of bytes from the first byte of the following field to the last byte of the SCP Role field. It shall be encoded as an unsigned binary number.
5-6	UID-length	This UID-length shall be the number of bytes from the first byte of the following field to the last byte of the SOP-class-uid field. It shall be encoded as an unsigned binary number.
7 -xxx	SOP-class-uid	This variable field shall contain the SOP Class UID or Meta SOP Class UID that may be used to identify the corresponding Abstract Syntax for which this Sub-Item pertains. It shall be encoded as a UID as defined in Section 9 “Unique Identifiers (UIDs)” in PS3.5.
xxx	SCU-role	This byte field shall contain the SCU-role as defined for the Association-requestor in Section D.3.3.4. It shall be encoded as an unsigned binary and shall use one of the following values: 0 - non support of the SCU role 1 - support of the SCU role
xxx	SCP-role	This byte field shall contain the SCP-role as defined for the Association-requestor in Section D.3.3.4. It shall be encoded as an unsigned binary and shall use one of the following values: 0 - non support of the SCP role 1 - support of the SCP role.

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