Edit: the following can be used to check your current setting, look for 'Receive Window Auto-Tuning Level:' netsh interface tcp show global it seems like the setting is already 'normal' for a lot of people, but if yours isn't, it could be hampering your speed without your knowledge. Receive Window Auto-Tuning Level is set to normal. To disable Receive Window Auto-Tuning Level, click Start, and in the Search box type 'command.' In the results, right-click Command Prompt, and then click Run as administrator. At the command prompt, run the following command: netsh int tcp set global autotuning=disabled For more information, see. By default, Windows 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 (64kB). 64kB is the fixed default TCP receive buffer, and through autotuning it can scale up to 16 MB. Aug 05, 2016 Locate the 'Receive Window Auto-Tuning Level' value under TCP Global Parameters. If it is not set to disabled, it is being used by Windows to optimize TCP connections. You may want to disable the feature to run connection tests and see if it is the cause for the limited Internet speed that you are getting out of the connection.
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TCP tuning techniques adjust the network congestion avoidance parameters of Transmission Control Protocol (TCP) connections over high-bandwidth, high-latency networks. Well-tuned networks can perform up to 10 times faster in some cases.[1] However, blindly following instructions without understanding their real consequences can hurt performance as well.
Bandwidth-delay product (BDP) is a term primarily used in conjunction with TCP to refer to the number of bytes necessary to fill a TCP 'path', i.e. it is equal to the maximum number of simultaneous bits in transit between the transmitter and the receiver.
High performance networks have very large BDPs. To give a practical example, two nodes communicating over a geostationary satellite link with a round-trip delay time (or round-trip time, RTT) of 0.5 seconds and a bandwidth of 10 Gbit/s can have up to 0.5×1010bits, i.e., 5 Gbit = 625 MB of unacknowledged data in flight. Despite having much lower latencies than satellite links, even terrestrial fiber links can have very high BDPs because their link capacity is so large. Operating systems and protocols designed as recently as a few years ago when networks were slower were tuned for BDPs of orders of magnitude smaller, with implications for limited achievable performance.
The original TCP configurations supported TCP receive window sizebuffers of up to 65,535 (64 KiB - 1) bytes, which was adequate for slow links or links with small RTTs. Larger buffers are required by the high performance options described below.
Buffering is used throughout high performance network systems to handle delays in the system. In general, buffer size will need to be scaled proportionally to the amount of data 'in flight' at any time. For very high performance applications that are not sensitive to network delays, it is possible to interpose large end to end buffering delays by putting in intermediate data storage points in an end to end system, and then to use automated and scheduled non-real-time data transfers to get the data to their final endpoints.
Maximum achievable throughput for a single TCP connection is determined by different factors. One trivial limitation is the maximum bandwidth of the slowest link in the path. But there are also other, less obvious limits for TCP throughput. Bit errors can create a limitation for the connection as well as RTT.
In computer networking, RWIN (TCP Receive Window) is the amount of data that a computer can accept without acknowledging the sender. If the sender has not received acknowledgement for the first packet it sent, it will stop and wait and if this wait exceeds a certain limit, it may even retransmit. This is how TCP achieves reliable data transmission.
Even if there is no packet loss in the network, windowing can limit throughput. Because TCP transmits data up to the window size before waiting for the acknowledgements, the full bandwidth of the network may not always get used. The limitation caused by window size can be calculated as follows:
where RWIN is the TCP Receive Window and RTT is the round-trip time for the path.
At any given time, the window advertised by the receive side of TCP corresponds to the amount of free receive memory it has allocated for this connection. Otherwise it would risk dropping received packets due to lack of space.
The sending side should also allocate the same amount of memory as the receive side for good performance. That is because, even after data has been sent on the network, the sending side must hold it in memory until it has been acknowledged as successfully received, just in case it would have to be retransmitted. If the receiver is far away, acknowledgments will take a long time to arrive. If the send memory is small, it can saturate and block emission. A simple computation gives the same optimal send memory size as for the receive memory size given above.
When packet loss occurs in the network, an additional limit is imposed on the connection.[2] In the case of light to moderate packet loss when the TCP rate is limited by the congestion avoidance algorithm, the limit can be calculated according to the formula (Mathis, et al.):
where MSS is the maximum segment size and Ploss is the probability of packet loss. If packet loss is so rare that the TCP window becomes regularly fully extended, this formula doesn't apply.
A number of extensions have been made to TCP over the years to increase its performance over fast high-RTT links ('long fat networks' or LFNs).
TCP timestamps (RFC 1323) play a double role: they avoid ambiguities due to the 32-bit sequence number field wrapping around, and they allow more precise RTT estimation in the presence of multiple losses per RTT. With those improvements, it becomes reasonable to increase the TCP window beyond 64 kB, which can be done using the window scaling option (RFC 1323).
The TCP selective acknowledgment option (SACK, RFC 2018) allows a TCP receiver to precisely inform the TCP sender about which segments have been lost. This increases performance on high-RTT links, when multiple losses per window are possible.
Path MTU Discovery avoids the need for in-network fragmentation, increasing the performance in the presence of packet loss.
Hi, my name is Katarzyna and I am the Program Manager within the Internet Protocols team. I have been asked a few times about the Receive Window Auto-Tuning feature on Vista and some associated issues people are having.
One of the many cool new features on Windows Vista, Receive Window Auto-Tuning enables the networking stack to receive data more efficiently than on XP. Auto-Tuning allows the operating system to continually monitor the routing conditions (bandwidth, network delay, application delay) and configure connections (scale the TCP Receiving Window) so as to maximize the network performance.In some high bandwidth, high latency links, we have seen SMB performance improvement up to 20 times!
In every TCP packet there is a 'window' field, which informs the receiver how much data the sender can accept back. This window controls the flow by setting a threshold on data kept 'in flight' and prevents overwhelming the receiver with data that it cannot accept.
The TCP window field is 16 bits wide, allowing for a maximum window size of 64KB, which used to meet requirements of many older networks. Nowadays, however, network interfaces can handle larger packets and keep more of them in flight at any given time. Thus, a larger TCP window has become necessary; especially on high-speed, high latency networks. To fill such a long, fat pipe and make use of the available bandwidth, the sending system can often require very large windows for good performance.
The solution to this demand is called 'window scaling”, described back in 1992 in RFC 1323. It introduces an eight-bit scale factor, which serves as a multiplication factor for the window width. After the factor has been negotiated, window values used by that system on a given connection will be shifted to the left by that scale factor; a window scale of zero, thus, implies no scaling at all, while a scale factor of six implies that window sizes should be shifted six bits, thus multiplied by 2^6 = 64. Now a window greater than 64KB can be easily expressed (e.g., 128KB) by setting the scale factor (e.g., 6) and keeping the window field under the original 16 bits (here, 2048).
The window size included in all packets is modified by the scale factor, which is negotiated once at the very beginning of a TCP connection. The connection requestor suggests window scaling factor in its original SYN packet and if the SYN+ACK packet sent in response contains the option, then this particular value will be used on this connection. The scale factor cannot be changed after the initial setup handshake; remaining data transfers on this connection will implicitly use the negotiated value.
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Older routers and firewalls however do not handle window scaling correctly leaving the option in the original SYN packet but setting the connection’s scale factor to zero. Seeing the option on, the receiver responds with its own window scale factor. Believing that its scale factor has been accepted, the initiator scales the window appropriately while the receiver thinks that a scale factor of zero is applied and thus a small window of data should follow. As a result, the communication is slow at best. Sometimes, small window packets are dropped by the routers, essentially breaking the connection.
The resulting slow data transfers or loss of connectivity, users may experience as slow or hung networking applications. Remote Desktop Connection and network file copy are two scenarios particularly hurt by misbehaving routers.
If your connection from a Vista machine appears slow or hung, here are some steps to isolate the cause:
Please refer to the following KB articles for more information:
-- Katarzyna
Updated: Broken link to KB 932170
Update 2: Changed the guidance to do restricted before disabled.
Update 3: tunning doesn't have two 'n's. :)
Update 4: no really, tuning doesn't have two 'n's.