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SMB and TCP packets - TCP retransmissions

Hi,

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.

bb25's avatar
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bb25
asked 2021-03-31 11:35:36 +0000
grahamb's avatar
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grahamb
updated 2021-03-31 11:49:38 +0000
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Could you please share us a trace? https://blog.packet-foo.com/2016/11/t...

Christian_R's avatar Christian_R (2021-04-01 00:18:12 +0000) edit
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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

Eddi

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