[MS-SMB2] Clarification regarding the relationship between MaxTransactSize and MaxReadSize

Tal Aloni 0

Hello, According to [MS-SMB2] 2.2.4 NEGOTIATE Response: "MaxTransactSize (4 bytes): The maximum size, in bytes, of the buffer that can be used for QUERY_INFO, QUERY_DIRECTORY, SET_INFO and CHANGE_NOTIFY operations. MaxReadSize - The maximum read size, in bytes, that the server will accept in an SMB2 READ Request on this connection" According to 3.2.5.1 Receiving Any Message: "If the message size received exceeds Connection.MaxTransactSize, the client MUST disconnect the connection" So if I'm reading this right, MaxReadSize must be less than or equal to MaxTransactSize, or otherwise the client may request read size greater than MaxTransactSize, causing the server to disconnect the connection. However, I have been informed that Windows Server 2019 does (in some cases) return MaxReadSize value that is greater than MaxTransactSize. A clarification would be appreciated. Thank you!

Obaid Farooqi MSFT 511 Reputation points Microsoft Employee

2024-02-02T07:39:30.4333333+00:00

Hi @Tal Aloni I have alerted open specifications team regarding your inquiry. A member of the team will be in touch soon. Regards, Obaid Farooqi - MSFT
Sreekanth Nadendla 321 Reputation points Microsoft Employee

2024-02-14T17:24:31.94+00:00

Hello Tal, Windows clients do not enforce the MaxTransactSize value (See behavior note 26). If your implementation of server chooses MaxTransactSize that is smaller than MaxReadSize, windows client will work. However, you would choose upper limit values for these so that such situation will not arise while maintaining compliance with the specification. You can see that from a sample Negotiate Response that Windows does this by choosing same value for MaxTransactSize, MaxReadSize and MaxWriteSize.

Regards, Sreekanth Nadendla Microsoft Windows Open Specifications
Tal Aloni 0 Reputation points

2024-02-25T18:04:25.3933333+00:00

Thanks Sreekanth, I find it hard to understand the answer, Are you saying that having the server return MaxTransactSize that is smaller than MaxReadSize is compliant with the specifications? How can you say that knowing that a client that is also compliant with the specifications will be forced to disconnect the connections in some cases?
Sreekanth Nadendla 321 Reputation points Microsoft Employee

2024-02-26T15:21:37.95+00:00

Hello Tal, having the server return MaxTransactSize that is smaller than MaxReadSize is NOT compliant with the specification. However, as documented in the behavior note #26, windows clients do not enforce the MaxTransactSize value. The next updated spec is expected to read as following (change of MUST to SHOULD) 3.2.5.1 Receiving Any Message: "If the message size received exceeds Connection.MaxTransactSize, the client SHOULD disconnect the connection" Our investigation did not show that Windows server using MaxReadSize smaller than MaxTransactSize. Regards, Sreekanth Nadendla Microsoft Windows Open Specifications

1 answer

Tal Aloni 0 Reputation points

2024-03-01T08:36:01.01+00:00

Thanks Sreekanth,

I have initially reported that this behavior was observed with "Windows Server 2019", however after verifying this it turned out that the server in question is not a Microsoft server.

Thanks for the update to the specifications which will clear up the expected client behavior in such case.
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