New Asus RT-N16 Router

[ecaddict]

At least you're making some tests now.

Now not to make your theory somewhat incorrect we have these possible causes why Linux is saying no:

• The broadcom drivers are bad programmed - totally agree, still not sure if that causes this

Wrong. The ethernet driver deals with frames and it does no care if the IP packet carries UDP or TCP.
And the same ethernet driver can produce with NFS over UDP over 12 MB/s "download".

• The bus the router uses to communicate doesn't like reading - quite unlikely

I don't know what kind of bus you mean. If you mean the connection between the SoC (BCM4718) and ethernet chip (BCM53115) that probably also does not care if UDP or TCP is used or what direction frames are sent.

• The ethernet ports have wrong settings concerning packet size - bigger packet size is always good in a network with few clients and little mixed traffic, still it usually only increases performance with 5% or so

Wrong. Jumbo frames cannot be used as discussed earlier and otherwise as it can be seen from the capture maximum size ethernet frames (1500 bytes) are used (exception is the last one of the block, this is normal).

• Linux is wrong about the cpu time - well, could be in theory: maybe a kernel bug

I thought that you posted calendar time. Anyhow that's not the cause but the phenomenon.

• Samba is a non optimized program - also somewhat unlikely considering the popularity

Wrong. Not just Samba "upload" is faster than download. FTP or NFS over TCP has also this. Exception is NFS over UDP. It's faster in the "download" direction (and the fastest file transfer among any other protocols in any other direction).

So my suspect number one is still TCP.

I just don't have enough time to check this issue in depth in a decent book like TCP/IP Architecture, Design and Implementation in Linux (from Sameer Seth and M. Ajaykumar Venkatesulu).