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SMB and TCP packets - TCP retransmissions

asked 2021-03-31 11:35:36 +0000

bb25 gravatar image

updated 2021-03-31 11:49:38 +0000

grahamb gravatar image


I'm looking for some help on analysis some packets.

The pcap i have consists of TCP and SMB packets.

From the IO graph, I have shown TCP errors (which are showing as retransmissions) and all SMB packets.

I have noticed form the graph that when there are any TCP errors, the SMB packet numbers lower drastically:

^this is an image of my graph.

If anyone could help to explain why this is or why the retransmissions are occuring id appreciate it.

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Could you please share us a trace?

Christian_R gravatar imageChristian_R ( 2021-04-01 00:18:12 +0000 )edit

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answered 2021-04-03 16:18:49 +0000

Eddi gravatar image

The screenshot suggests that the systems use SMB (not SMB2) to exchange data.

Most applications make blocking calls when they read or write files. In other words: Send data, wait for an acknowledgement, then send the next block etc. In case of an error, the client has to wait until the TCP retransmission has been resolved.

One of the improvements with SMB2 was the introduction of pipelining, where clients could fire away a bunch of requests at once. A sequence like "Create File, Query Information, Close" takes only one TCP turn.

I wonder, how your application reacts, if SMB2 is used. Of course, the calls have to be non-blocking ...

Good luck


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Asked: 2021-03-31 11:35:36 +0000

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Last updated: Apr 03