6 Appendix A: Product Behavior

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The information in this specification is applicable to the following Microsoft products or supplemental software. References to product versions include updates to those products.

  • Windows Vista operating system

  • Windows Server 2008 operating system

  • Windows 7 operating system

  • Windows Server 2008 R2 operating system

  • Windows 8 operating system

  • Windows Server 2012 operating system

  • Windows 8.1 operating system

  • Windows Server 2012 R2 operating system

  • Windows 10 operating system

  • Windows Server 2016 operating system

  • Windows Server operating system

  • Windows Server 2019 operating system

  • Windows Server 2022 operating system

  • Windows 11 operating system

  • Windows Server 2025 operating system

Exceptions, if any, are noted in this section. If an update version, service pack or Knowledge Base (KB) number appears with a product name, the behavior changed in that update. The new behavior also applies to subsequent updates unless otherwise specified. If a product edition appears with the product version, behavior is different in that product edition.

Unless otherwise specified, any statement of optional behavior in this specification that is prescribed using the terms "SHOULD" or "SHOULD NOT" implies product behavior in accordance with the SHOULD or SHOULD NOT prescription. Unless otherwise specified, the term "MAY" implies that the product does not follow the prescription.

<1> Section 3.1.5.1: In Windows Vista, Windows 7, Windows 8, Windows 8.1, and Windows 10, the client initializes the Version field with RDP_PRECONNECTION_PDU_V1 because the Id field is sufficient to identify the wanted RDP source. The Id is initialized to a value that identifies the server. This value is obtained by the client through an external mechanism (such as fax, email, or telephone) that is outside this protocol. The external mechanism that is used by the Remote Assistance Initiation Protocol (see [MS-RAI]) exchanges the Id as described in [MS-RAI] section 2.2.2. (The SID XML attribute in the Remote Assistance Connection String 2 contains the Id value.)

<2> Section 3.1.5.2: In the Windows Server 2008 operating system, Windows Server 2008 R2, Windows Server 2012, Windows Server 2012 R2, Windows Server 2016, Windows Server operating system, and Windows Server 2019 virtual machine implementation, the client initializes the Version field to RDP_PRECONNECTION_PDU_V2 and sets the Id field to zero. The string is used to identify the name of the wanted virtual machine. If the string is not null-terminated, the server will overwrite the last Unicode character with NULL prior to further processing.

The Windows Server 2008, Windows Server 2008 R2, Windows Server 2012, Windows Server 2012 R2, Windows Server 2016, Windows Server operating system, and Windows Server 2019 virtual machine sends a GUID instead of a virtual machine name.

GUID formatting is as follows: 00000000-0000-0000-0000-000000000000. The hexadecimal numbers are separated by '-' characters; however, without any braces {} around the GUID. The byte ordering is standard GUID byte ordering (identical for all GUIDs that are represented as strings) and is case-insensitive for the hexadecimal digits. No other form is allowed.

<3> Section 3.2.2: In Windows implementations, the server associates a time stamp for each new connection. The time stamp is used to verify whether all the preconnection PDU data for a connection was received in the specified 10-second interval. (See section 3.2.2.) If the data was not received, the client is then disconnected. However, the checks are only performed when a new connection is created or when data is received on one of the pending connections. Otherwise, the server does not check the time stamp, and the client is not disconnected.

<4> Section 3.2.5.1: In Windows implementations, the server associates a time stamp for each new connection. The time stamp is used to verify whether all the preconnection PDU data for a connection is received in the specified 10-second interval. (See section 3.2.2.) If the data is not received, the client is disconnected. However, these checks are only performed when a new connection is created or when data is received on a pending connection. Otherwise, the server does not check the time stamp, and the client is not disconnected.

<5> Section 3.2.5.1: On a Windows Server 2008, Windows Server 2008 R2, Windows Server 2012, Windows Server 2012 R2, Windows Server 2016, Windows Server operating system, or Windows Server 2019 virtual machine, RDP_PRECONNECTION_PDU_V2 is used. The Id is ignored by the server. The server always expects a wszPCB string. The string represents the GUID of the virtual machine to which the client tries to connect. The server expects a null-terminated string. The server does not verify that the last character is a null terminator; instead, the server overwrites the last character with the null-terminated character. The server will use the GUID to identify the virtual machine the connection is intended for. If a matching virtual machine is not found on the server the connection will be terminated.

On Windows Server 2012 R2, Windows Server 2016, Windows Server operating system, and Windows Server 2019 the wszPCB string contains virtual machine GUID and connection-specific information. The following syntax is used for that pre-connection string:

GUID;key1=value1;key2=value2;key3=value3...

"GUID" is followed by a semi-colon, which is then followed by connection-specific key-value pairs, with each pair separated by a semi-colon delimiter. The key and value in the key-value pair are separated by an equal sign ("="). There are no spaces in the string.

The only supported key in Windows Server 2012 R2, Windows Server 2016, Windows Server operating system, and Windows Server 2019 is "EnhancedMode". Its value is set to 1 for connections that expect to use RDP features.

<6> Section 3.2.5.1: In Windows implementations, the server will disconnect the client if the cbSize value is 17 bytes or less than 16 bytes.

<7> Section 3.2.5.1: In Windows implementations, the server disconnects the client if the cbSize value is greater than 16 bytes and the Version field indicates that it is RDP_PRECONNECTION_PDU_V1.

<8> Section 3.2.5.1: Windows virtual machine clients are disconnected if the cbSize field verification fails. If the cbSize value indicates that the payload is bigger than the length of the wszPCB field the bytes after the wszPCB field up to cbSize will be ignored.

<9> Section 3.2.5.1: In Windows Vista, Windows 7, Windows 8, Windows 8.1, and Windows 10 Remote Assistance, the type of the PDU is always expected to be RDP_PRECONNECTION_PDU_V1, and if the server receives a RDP_PRECONNECTION_PDU_V2, it disconnects the client. In Windows Server 2008 Hyper-V, Windows Server 2008 R2 Hyper-V, Windows Server 2012 Hyper-V, Windows Server 2012 R2 Hyper-V, Windows Server 2016 Hyper-V, Windows Server operating system Hyper-V, and Windows Server 2019 Hyper-V, the type of the PDU is always expected to be RDP_PRECONNECTION_PDU_V2, and if a RDP_PRECONNECTION_PDU_V1 is received in this case, the server will disconnect the client.

<10> Section 3.2.6: In Windows implementations, the server associates a time stamp for each new connection. The time stamp is used to verify whether all the preconnection PDU data for a connection is received in the specified 10-second interval. (See section 3.2.2.) If the data is not received, the client is disconnected. However, these checks are only performed when a new connection is created or when data is received on a pending connection. Otherwise, the server does not check the time stamp, and the client is not disconnected.