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When a file is sent over the network the file is chopped up in small packets, which are then sent to the destination. The TCPIP protocol is a secure protocol and demands that each packet is acknowledged, so the receiver has to respond with a matching ACK packet. It is important for the sender not to flood the receiver and for the receiver not to flood the sender.

The Nagle algorithm (by John Nagle) is a method for congestion control (RFC 896), so the sender won't flood the receiver with data. When the sender sends a packet to the receiver, then the sender will wait for an ACK from the receiver before sending the following packets.

The delayed ACK algorithm is also a method for congestion control (RFC 2581), so the receiver won't flood the network with ACK packets. When the receiver has to sent an ACK in response to a packet, then it waits a little (200 ms or until it has 2 outstanding ACKs) to see if more packets should arrive that it can acknowledge with that single ACK.

The delayed ACK algorithm can slow down the network, if the sender doesn't send the next packet before receiving the ACK of the previous packet (200 ms delay).
The delayed ACK algorithm can get even worse when combined with the Nagle algorithm, if the sender sends two packets and waits for ACK, as nagling will hold back the second packet until recieving the delayed ACK for the first packet (400 ms delay).

The solution to such slow downs is to disable or lower the delayed ACK timeout.

To configure the max outstanding ACKs in Windows XP/2003/Vista/2008/7:

[HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \Tcpip \Parameters \Interfaces \{Adapter-id}]
TcpAckFrequency = 2 (Default=2, 1=Disables delayed ACK, 2-n = If n outstanding ACKs before timed interval, sent ACK)

More Info MS KB Q328890
More Info MS KB 815230 (XP/2003 needs hotfix or SP2 for it to work)
More Info MS KB 935458 (Vista/2008 needs hotfix or SP1 for it to work)
More Info MS KB 2020559 (Applies also to Win7/Win2008 R2)

To configure the interval timeout in Windows 2000/XP/2003:

[HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \Tcpip \Parameters \Interfaces \{Adapter-id}]
TcpDelAckTicks = 1 (Default=2, 0=Disables delayed ACK, 1-6 = 100-600 ms)

More Info MS KB Q311833 (Win2000 requires SP3)
More Info MS KB Q321098
More Info MS KB Q321169

To configure the interval timeout in WinNT SP4 (Go to the Services-key and do a search for "TCPIP" to find the different adapters using TCPIP):

[HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \{Adapter-Name} \Parameters \Tcpip]
TcpDelAckTicks = 1 (Default=2, 0=Disables nagling, 1-6 = 100-600 ms)

Note if disabling or shortening delayed ACK on a few machines (Like a file-server or domain-controller), then it will probably result in greater network performance for those machines. If on large corporate network and disabling delayed ACK for all computers, then it will most likely lower the available bandwidth for actual filetransfer as more of the bandwidth is used for sending ACKs.

Note before trying to disable ACK delay (RFC 1122) one should at least consider the following:

• Increased performance will only be seen if requests are sent to your machine, and the requesters doesn't request anything else before your machine replies back(ACK) to the first request.
• Some additions to the above statement:
  • If the application doing socket communication uses the socket option TCP_NODELAY, then it will disable the nagle algorithm but not the delayed ACK.
  • If all of the upload bandwidth is already used (easy if slow connection), then then disabling delayed ACK will lower performance because it will generate even more upload traffic.
  • If on a half duplex connection, then disabling delayed ACK will lower performance because only one party can sent at a time (Receiver will block the sender when sending ACK).
  • If on a ethernet hub with other computers(Instead of a switch), then disabling delayed ACK will lower performance because the increased traffic will increase chance of collision and require retransmissions.

Note Explorer.exe doesn't copy the next file before the previous file was acknowledged (XCOPY doesn't have this behavior). This means it that the receiver will only accept a file at every ACK interval, and as the default ACK interval is 200 ms, which means that the it will copy max 5 files/sec for a single connection (Imagine copying 1000 files of 1 Kbyte). The performance can be improved some if dragging a folder containing the files instead of selecting all the files and dragging.

Note SMB Signing requires that SMB commands are processed synchronously, so a client is only allowed to send the next SMB command when it receives ACK of the previous one (Only one outstanding). This means that a client can max sent 5 SMB Commands/sec, as it has to wait for the Server's 200 ms ACK delay before it is allowed to sent the next SMB Command. This can cause very low performance when copying small files to a Server with SMB signing enabled (Imagine copying 1000 files of 1 Kbyte).

Note if a computer's only job is to receive large files or streaming data, one can increase performance by increasing the number of outstanding ACKs before it sends an ACK (TcpAckFrequency). It will allow acknowledgment of large chunks of data with a single ACK packet instead of sending ACK for every 2 packet. Make sure that the TCPIP RWIN is larger than TcpAckFrequency*MTU, as the sender will stop sending data if it fills the TCPIP RWIN without getting an ACK. Recommended values:

• 1 GigaBit: TcpAckFrequency = 13 (RWIN = 64 KByte)
• 100 MegaBit: TcpAckFrequency = 5 (RWIN = 17 KByte)
• 10 MegaBit: TcpAckFrequency = 2 (RWIN = 8 KByte)

More Info MS KB Q214397
More Info MS KB Q823764