Well, the ping of 1496 confirms that you have an MUT issue. What to note is that when pinging, you sent a packet of 1496 bytes and also recieved 1496. So it does not really matter where you are pinging from, the MTU issue can be in eihter direction.
When downloading a file, the request and acknowledgement packets (upstream) are small, much more lower than 1500 bytes. But if the file is larger than 1500 bytes, it is broken down into chuncks of 1500bytes and sent like that.
I'm not sure of why the download goes one, (it could be TCP MSS setting on the server or routers). Please also note that MTU applies to the transmission, and that the trasmitting interface from A to B, would be a recieving interface from B to A.
Do you have vlans? I would watch out for there because of your 4 bytes overhead. It could also be a label impostion.
Check the following link to help you troubleshoot. While it does not mention MPLS, the technique for resolution are similar.
I think, this problem occurs because server message block (SMB) write operations to a Windows 2000 may experience a delay of up to 200 milliseconds.You can edit the TcpDelAckTicks registry value to adjust the TCP delayed ACK timer. If you change the TCP delayed ACK timer to a lower value, the server sends an ACK packet more frequently but at shorter intervals.Apply the latest service for Windows 2000 so that you can modify the delayed ACK timer value.From MS:
1. Start Registry Editor (Regedt32.exe).
2. Locate and click the following key in the registry, where Adapter GUID is the globally unique identifier (GUID) for the network adapter that connects to the clients:
3. On the Edit menu, click Add Value, and then add the following registry value:
Value name: TcpDelAckTicks
Data type: REG_DWORD
Value data: You can set this value to a range from 0 to 6. The default setting is 2 (200 milliseconds).
4. Quit Registry Editor.
5. Restart Windows for this change to take effect.
This registry value specifies the number of 100-millisecond intervals to use for the delayed ACK timer on a per-interface basis. By default, the delayed ACK timer value is 200 milliseconds. If you set the TcpDelAckTicks value to 0, delayed acknowledgments are disabled. This setting causes the computer to immediately send an ACK packet for every packet it receives.
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