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04/01/2019 192 views

I'm trying to solve an issue where a workstation used as a POS in a school lunchroom will suddenly run slow or freeze, and at other times lose connection to the numeric keypads used by students to input lunch number IDs. During operations the POS computer is connected to a vendor database via a secure session (HTTPS) so processing occurs on the vendor servers. Loss of the keypad connection seems to result from the assigned com ports changing. On these machines I have seen  RoCE related errors. Researching the errors I found that under certain circumstances misconfigured RDMA over Converged Ethernet can cause momentary loss of network connections. That in turn might cause Windows to change the com ports. But there have been no consistent errors.  Everything has seemed to be at random so far.

The server admin and the network engineer have said RDMA is not being used by us.  Using Microsoft Message Analyzer I captured traffic between my PC and a lunchroom computer. I retrieved this breakdown of a sample packet:

SMB2_CHANNEL_RDMA_V1_INVALIDATE             (..............................0.) This value is valid only for the SMB 3.02 dialect. One or more SMB_DIRECT_BUFFER_DESCRIPTOR_V1 structures as specified in [MS-SMBD] section 2.2.3.1 are present in the channel information specified by the WriteChannelInfoOffset and WriteChannelInfoLength fields. The server is requested to perform remote invalidation when responding to the request as specified in [MS-SMBD] section 3.1.4.2.                769         1


Name    Value    Bit Offset            Bit Length           Type     

SMB2_CHANNEL_RDMA_V1       (...............................0) One or more SMB_DIRECT_BUFFER_DESCRIPTOR_V1 structures as specified in [MS-SMBD] section 2.2.3.1 are present in the channel information specified by WriteChannelInfoOffset and WriteChannelInfoLength fields.        768         1

Name    Value    Bit Offset            Bit Length           Type     

SMB2_WRITEFLAG_WRITE_THROUGH   (...............................0) The write data should be written to persistent storage before the response is sent regardless of how the file was opened.  This value is only supported for the SMB 2.1 dialect. 864         1                             

SMB2_WRITEFLAG_WRITE_UNBUFFERED             (..............................0.) The server or underlying object store SHOULD NOT cache the write data at intermediate layers and SHOULD allow it to flow through to persistent storage. This bit is only valid for the SMB 3.02 dialect.           865         1                                                                                              

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This appears to me to be a request for an RDMA connection from one computer to another that is sent in SMB 3.02 dialect, and the receiving PC cannot receive that dialect, and refuses the connection.

However I'm learning this as I go and I'm not confident in my interpretation of the messages. Has anyone dealt with this type of message before?

Thanks,

Zayess



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