Auto Tuning - TCP/IP receive level

Information

The TCP receive window size (RWIN) is the amount of data that a TCP receiver (your computer) allows a TCP sender (other internet or local network computer) to send before having to wait for an acknowledgement. After the connection is established, the receive window size is advertised in each TCP segment. Advertising that maximum amount of data that the sender can send is a receiver-side flow control mechanism that prevents the sender from sending data that the receiver cannot handle. A sending host can only send at a maximum the amount of data advertised by the receiver before waiting for an acknowledgment and a receive window size update. Auto Tuning adjusts the receive windows size continually based upon the changing network conditions. This usually happens if you have a network device (EX: Router, modem, etc.) that doesn’t support RFC 1323 “TCP Extensions for High Performance”.

NOTE

The TCP window size field controls the flow of data and is limited to between 2 and 65,535 bytes, and cannot be expanded anymore. Thus, a scaling factor is used to get a larger TCP receive window size to achieve more efficient use of high bandwidth networks. The TCP window scale option is used to increase the maximum window size from 65,535 bytes to 1 Gigabyte. Scaling up to larger TCP congestion window sizes is a part of what is necessary for TCP Tuning. When dealing with slow network performance in Vista, the problem may be due to the above mentioned TCP Window Scaling option, where many older routers and and packet firewalls do not properly implement TCP Window Scaling (doesn’t support RFC 1323), and rewrite the window scaling factor during a transmission, and causing sending and receiving sides to assume different TCP window sizes. The situation causes malfunction Internet connection, non-stable traffic that is very slow, or network connection that is only available intermittently. Beside, Windows Vista enhances further TCP window scaling with auto tuning feature, where TCP stack in Vista system will auto tune and adjust the RWIN size to increase the percentage of full-sized TCP segments that are used during bulk data transmission based on the network scenario it encountered. And thus it’s no longer possible to specify a custom size for the TCP/IP Receive Window (RWIN) size with the inclusion of new auto tuning feature, unless the application it self uses SO_RCVBUF. However, Vista TCP auto tuning feature may get things wrong sometimes. Instead of optimal true receive window size, incompatible and out of range RWIN size may be used. By default, Vista in normal auto tuning level will use RWIN size of 256 bytes with a scale factor of 8. This value is not suitable for all routers and servers which does not support TCP scale factor, and continue to communicate with 65536 bytes.

Tip

Possible TCP/IP Auto Tuning Related Problems: Indefinite delay (hang) when opening the Certificate Services snap-in. Slow (sometimes no) group policy application. Trying to select a domain user in order to add that principal to a local security group (the object picker) would hang indefinitely. Instant Messaging is not working well. Sometimes not at all by not being able to sign in. Access to local file servers (EX: networked drives, etc.) are extremely slow and sometimes did not succeed at all (appears to hang). Internet connection problems. Extremely slow file transfers. Other strange unexplained network connecting issues. Wireless network connection problems Website browsing and downloads are extremely slow.

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1. Open the Start Menu.
2. In the white line (Start Search) area, type cmd.
3. Right click cmd (at top) and click Run as administrator.
4. Click Continue button for UAC prompt.
5. In command prompt, type: netsh interface tcp show global
6. Press Enter. (See screenshot below)



7. Close command prompt when done.
8. Run through the steps below to see if you have a TCP/IP Auto Tuning issue

1. Open the Start Menu.
2. In the white line (Start Search) area, type cmd.
3. Right click cmd (at top) and click Run as administrator. (See screenshot below)



4. Click Continue button for UAC prompt.

5. To Restrict Auto Tuning -
NOTE: Allows for the receive window to grow beyond the default value of 65,535 bytes, but limits such growth in some scenarios.

A) In command prompt, type netsh interface tcp set global autotuninglevel=restricted and press Enter. (See screenshot below)
B) You will get a OK response if successful. If not, repeat step 5A.
C) Go to step 7.

6. To Highly Restrict Auto Tuning -
NOTE: Allows for the receive window to grow beyond the default value of 65,535 bytes, but doing so very conservatively. In this mode, Vista will by default use a RWIN of 16,384 bytes with a scale factor of 2.

A) In command prompt, type netsh interface tcp set global autotuninglevel=high and press Enter. (See screenshot below)
B) You will get a OK response if successful. If not, repeat step 6A.

7. Close command prompt window and restart the computer to apply the changes.
8. If it fixed your problem stop here. If not, continue to Step Two below.

1. Open the Start Menu.
2. In the white line (Start Search) area, type cmd.
3. Right click cmd (at top) and click Run as administrator.
4. Click Continue button for UAC prompt.
5. In command prompt, type netsh interface tcp set global autotuninglevel=disabled
6. Press Enter. (See screenshot below)
7. You will get a OK response. If not, repeat step 4.



8. Close command prompt and restart computer to apply changes.
9. If it fixed your problem stop here. If not, you may not have a issue with Auto Tuning and will need to turn it back on. Continue to Step 3 below.

1. Open the Start Menu.
2. In the white line (Start Search) area, type cmd.
3. Right click on cmd and click Run as administrator.
4. Click Continue button for UAC prompt.
5. In command prompt, type:
netsh interface tcp set global autotuninglevel=normal
6. Press Enter. (See screenshot below)
7. You will get a OK response if successful. If not, repeat step 4.



8. Close command prompt.
9. Restart the computer to apply changes.