D.3.3.4 SCP/SCU Role Selection Negotiation
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#### D.3.3.4 SCP/SCU Role Selection Negotiation

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-requester, 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-requester is SCU only

- Association-requester is SCP only

- Association-requester is both SCU and SCP

If the SCP/SCU Role Selection item is absent the default role of the Association-requester 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-requester 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-requester has not proposed the role, i.e., it has sent a value (0). The Association-requester 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-requester 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

1. The choices made for the default roles are based on clarification made to previous versions of the Standard. Association-requesters 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.

2. If an Association-requestor offers an SCP/SCU Role Selection item for an Abstract Syntax Name but the Association-acceptor does not return a 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-requester is SCU and Association-acceptor is SCP).

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

### Note

1. 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".

2. Both DICOM AEs may be SCU and SCP for the Storage-CT SOP.

3. 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-requester 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 PS3.5.
xxx | SCU-role | This byte field shall contain the SCU-role as defined for the Association-requester 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-requester 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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