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SMB Transfer Upload fast, Download slow

asked 2019-05-17 10:20:42 +0000

sugar76 gravatar image

updated 2019-05-17 12:29:56 +0000

I have problems within my local network.

I have a Windows Server 2019 with some shares (no Active Directory). Clients are all Windows (mostly Windows 10). No virtualization is being used.

Uploading files to the share is pretty fast, about 60 MByte/sec. But downloading files from the share is extremely slow, between 1-4 MBytes/sec. These results come from a wifi connection but are similar using an ethernet connection. So it is not a wifi problem.

You find the captured results here: Download:!AphLkdj_V9pOpTL2D... Upload:!AphLkdj_V9pOpTHs9...

I noticed quite a lot TCP Dup Ack, Out-Of-Order and Retransmission packets.

Turning off Windows Defender and Firewall on the client did not change anything.

Using iperf, i measured the bandwith between client and server:

  • Download: 20 Mbit/sec
  • Upload: 1 Gbit/sec (ethernet), 300 Mbit/sec (wifi)

So it seems not to be an SMB problem.

Does anybody has an idea what the reason could be or how I could investigate further?

EDIT: Network topology between Client and Server:

  1. Client (wifi)
  2. Router
  3. Switch 1
  4. Switch 2
  5. Server
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Thanks for the capture files, however they would be much more useful if you include the TCP handshake. Can you please redo both and include the TCP handshake? Also, can you share details about the topology of the connection between server and client?

NJL gravatar imageNJL ( 2019-05-17 10:52:52 +0000 )edit

Not sure how to capture TCP handshake. Followed this description: Updated the pcapng files in onedrive.

Regarding network topology please see my edits.

sugar76 gravatar imagesugar76 ( 2019-05-17 12:31:06 +0000 )edit

1 Answer

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answered 2019-05-19 07:19:13 +0000

The main reason is, that the download trace shows continuous packet loss, which causes the slow transfer.

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Thanks. It's weird that packet loss only occurs in one direction (download) whereas upload is pretty fast. Do you have any idea how I can investigate further to find out the problem's cause?

sugar76 gravatar imagesugar76 ( 2019-05-19 12:43:56 +0000 )edit

One-way packet-loss could be caused by a dirty fiber-optics cable, bad ethernet cable or connector or a duplex mismatch on a switchport. Did you check for interface errors on all the involved interfaces?

SYN-bit gravatar imageSYN-bit ( 2019-05-19 12:53:15 +0000 )edit

Damn, it seems the ethernet cable was the cause. After unplugging it and plugging it again upload AND download speed is pretty fast. What seems weird to me that the problem occurred only in one direction.

sugar76 gravatar imagesugar76 ( 2019-05-19 19:28:56 +0000 )edit

After all it seems the CAT6 ethernet cable (which is brand new) was the cause. After unplugging it and plugging again, transfer is pretty fast in BOTH directions.

I wonder why the problem only raised when transferring from server to client.

To be on the safe side, I will change the cable.

sugar76 gravatar imagesugar76 ( 2019-05-19 19:46:05 +0000 )edit

@sugar76 thank you for telling us the solution!

Christian_R gravatar imageChristian_R ( 2019-05-19 21:57:37 +0000 )edit

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Asked: 2019-05-17 10:20:42 +0000

Seen: 2,282 times

Last updated: May 19 '19