TCP protocol parameters for Windows

Registry Editor: Regedit

Table Entries: Hkey_local_machinesystemcurentcontrolsetservicestcpipparameters

Window enlargement factor & timestamp options

Tcp1323Opts REG_DWORD 0x00000001 (1)

The values of the tcp1323opts are as follows:

0-disable RFC 1323 options. Disables the window enlargement factor and timestamp option, which is the default value.

1-window scaling enabled only. Enable the window enlargement factor option only.

2-timestamp enabled only. Enable timestamp option only.

3-both options enabled. Enable both the window enlargement factor and the timestamp option.

Sack Options

SackOpts REG_DWORD 0x00000001 (1)

SackOpts value range is 0 | 1, the default value is 1.

Describe:

Enables and disables the selective Acknowledgement (SACK) feature of Windows Server 2003 TCP/IP.

SACK is specified in RFC 2018.

Tcpmaxdupacks

Tcpmaxdupacks REG_DWORD 0x00000002 (2)

The value range for Tcpmaxdupacks is 1-3, which defaults to 2. The default value for Linux is 3.

Describe:

Specifies how many duplicate ACks (ACks for the same sequence numbers) constitute a signal to

Retransmit a segment.

When data arrives with a sequence number which is greater than expected, the receiver assumes that data

With the expected number is dropped, and it immediately sends an ACK with the ACK number set to the

Expected sequence number. The receiver sends ACKs set to the same missing number each time it receives

A TCP segment that has a sequence number greater than expected.

The sender recognizes the duplicate ACKs and sends the missing segment.

Congestion Control algorithm

Microsoft's latest TCP Congestion control algorithm is compound TCP (CTCP), and Windows 7 is not enabled by default.

(1) Windows VISTA/2008/7

CTCP is enabled by default in computers running beta versions of Windows Server and disabled

By default in computers running Windows Vista and 7.

CTCP can is enabled with the command:

> netsh interface tcp set global CONGESTIONPROVIDER=CTCP

or disabled with the command:

> netsh interface tcp set global Congestionprovider=none

To display the current setting for CTCP use:

> netsh interface tcp show global

Parameter "Add-on congestion Control Provider" would either has a value of "none" if CTCP is disabled

or "ctcp" if it is enabled.

(2) Windows 8

CTCP is now-enabled using PowerShell as netsh is deprecated:

> Set-nettcpsetting-settingname custom-congestionprovider CTCP

(3) Windows 2003 & XP x64

A hotfix is available This adds CTCP support to a bit Windows XP and Windows Server 2003.

The following registry key can is set to 1 and enable, or 0 to disable:

> Hkey_local_machinesystemcurrentcontrolsetservicestcpipparameterstcpcongestioncontrol

Maximum Receive Window

TcpWindowSize REG_DWORD Number of bytes

Value range: 0-0x3fffffff.

Describe:

In practice the TCP/IP stack would round the number set to the nearest multiple of maximum segment size

(MSS). Values greater than 64KB can be achieved if connecting to other systems

RFC 1323 Window Scaling.

Initial value

The smaller of the following values:

0xFFFF (65536)

Globalmaxtcpwindowsize

The larger of four times the MSS (5840)

16384 rounded up to an even multiple of the MSS

The stack also tunes itself based on the media speed:

Below 1 mbps:8kb

1 Mbps-100 mbps:17kb

Greater than MBPS:64KB

This parameter determines the maximum TCP receive window size offered.

Global Maximum Receive Cache

Globalmaxtcpwindowsize REG_DWORD Number of bytes

Value range: 0-0x3fffffff.

Default:this parameter does not exist by default.

Describe:

Determines the largest TCP receive window that the system offers.

The TcpWindowSize parameter can be used to set the Receive window on a per-interface basis.

This parameter can is used to set a global limit for the TCP window size on a system-wide basis.