TDS 协议字段详情描述

晓天望 30

最近在研究tds协议标准规范，对于tds协议中，sql批处理消息与rpc消息中有很多字段仅仅使用了在tds 7.x 这种描述，在官网中我并没有找到关于tds 7.x这种文档，并且很多字段直接用的 L_VARBYTE 这种字段描述，对于理解tds消息有着较大的难度，不知道是我没有找到tds相关文档还是对文档理解不够深刻，希望能拿到更详细一点的协议说明，如果可以的话。感谢

Tom Jebo 1,996 Reputation points Microsoft Employee

2023-08-24T14:52:55.9833333+00:00

Hi @晓天望,

Thanks for your question about MS-TDS. One of the Open Specifications team members will respond to discuss and assist you with your questions.

Best regards,
Tom Jebo
Microsoft Open Specifications Support
晓天望 30 Reputation points

2023-08-27T04:41:46.15+00:00

I am very sorry for not seeing this reply recently. In the subsequent research, I have already known many definition positions in the tds protocol, but I still found some problems that I could not understand. For example, UDT messages are delivered to the server in rpc messages. However, I have not found a clear definition of how many bytes the length field in the value of the udt message takes, can you help me
Sreekanth Nadendla 411 Reputation points Microsoft Employee

2023-08-30T17:39:29.0666667+00:00

Hello @晓天望,

UDT describes a data type which can be anything from a simple string to a complex structure type with additional members inside. The following section from MS-TDS provides that information. The internal structure details of it are described in the [MS-SSCLRT] specification. Please let us know if you have additional questions.

2.2.5.5.2 Common Language Runtime (CLR) Instances

The following data type definition stream is used for UDT_INFO in TYPE_INFO. This data type was introduced in TDS 7.2. DB_NAME = B_VARCHAR ; database name of the UDT SCHEMA_NAME = B_VARCHAR ; schema name of the UDT TYPE_NAME = B_VARCHAR ; type name of the UDT MAX_BYTE_SIZE = USHORT ; max length in bytes ASSEMBLY_QUALIFIED_NAME = US_VARCHAR ; name of the CLR assembly UDT_METADATA = ASSEMBLY_QUALIFIED_NAME UDT_INFO_IN_COLMETADATA = MAX_BYTE_SIZE DB_NAME SCHEMA_NAME TYPE_NAME UDT_METADATA UDT_INFO_IN_RPC = DB_NAME ; database name of the UDT SCHEMA_NAME ; schema name of the UDT TYPE_NAME ; type name of the UDT UDT_INFO = UDT_INFO_IN_COLMETADATA ;when sent as part of COLMETADATA / UDT_INFO_IN_RPC ;when sent as part of RPC call

MAX_BYTE_SIZE is only sent from the server to the client in COLMETADATA (section 2.2.7.4) and is an unsigned short with a value within the range 1 to 8000 or 0xFFFF. The value 0xFFFF signifies the maximum LOB size indicating a UDT with a maximum size greater than 8000 bytes (also referred to as a Large UDT; introduced in TDS 7.3). MAX_BYTE_SIZE is not sent to the server as part of RPC calls.

Note UserType in the COLMETADATA stream is either 0x0000 or 0x00000000 for UDTs, depending on the TDS version that is used. The actual data value format associated with a UDT data type definition stream is specified in [MS-SSCLRT]

Regards,

Sreekanth Nadendla

Microsoft Windows Open Specifications

Accepted answer

MojiTMJ 690 Reputation points

2023-08-23T18:03:30.2833333+00:00

Hello 晓天望,

Thank you for reaching out in Microsoft Q&A.

Note sure if i get it right, take a look at this link: https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-tds/b46a581a-39de-4745-b076-ec4dbb7d13ec

https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-drm/fbb909e3-96bf-4fc0-843e-b56736711d6c

Note: If you found this response helpful, please acknowledge it to help others facing similar challenges.

Best of luck!
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MojiTMJ 690 Reputation points

2023-08-27T09:43:57.4733333+00:00

For the L_VARBYTE field description, The length of the is variable. It is specified by the previous byte array length field. For example, if the previous byte array length field is 2, then the length of the L_VARBYTE field is 2 bytes.

The L_VARBYTE field is used to represent a variable-length byte array. The length of the byte array is stored in the previous byte array length field. The value of the byte array is stored in the L_VARBYTE field.

Therefore, the length field in the value of the UDT message is 2 bytes long.

Github TdsLib

For example, let's say the previous byte array length field is 0x03. This means that the previous byte array is 3 bytes long. The L_VARBYTE field will be 3 bytes long plus the length of the value of the L_VARBYTE field.

If the value of the L_VARBYTE field is 10 bytes long, then the total length of the L_VARBYTE field will be 13 bytes.

Here is a diagram that illustrates the relationship between the previous byte array length field and the L_VARBYTE field:

Byte offset Field

0 Previous byte array length (2 bytes)

2 L_VARBYTE (3 + length of value) bytes

I hope this helps!

晓天望 30 Reputation points

2023-08-27T10:08:48.4866667+00:00

Could you please tell me in which chapter of the document this standard is defined? Thank you very much

MojiTMJ 690 Reputation points

2023-08-28T09:22:56.02+00:00

The VARBYTE is defined in section 2.2.5.2.2, "Variable-Length Data Streams", of the MS-TDS documentation. The document can be found HERE.
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TDS 协议字段详情描述

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Byte offset	Field
0	Previous byte array length (2 bytes)
2	L_VARBYTE (3 + length of value) bytes