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How to Configure, Verify, and Troubleshoot TCP Window Scaling on a NetScaler Appliance

  • CTX113656
  • Created onMar 26, 2014
  • Updated onApr 17, 2014
Article Topic Networking

Objective

This article describes how to configure, verify, and troubleshoot TCP window Scaling on a NetScaler appliance.

A TCP window is an amount of outstanding data, the data not acknowledged by the recipient, which sender can send on a connection before receiving an acknowledgment from the receiver about the receipt of some of the data. The primary reason for the window is to manage traffic congestion.

For example, if a pair of hosts communicates over a TCP connection that has a TCP window size of 64 kilobytes (KB), the sender can only send 64 KB of data. After sending 64 KB of data, the sender must stop and wait for an acknowledgment from the receiver about the receipt of some or entire data. If the receiver acknowledges that the entire data is received, the sender is free to send another 64 KB of data. If the sender receives an acknowledgment from the receiver that only first 32 KB of data is received, then the sender can only send another 32 KB of data because the sender cannot have unacknowledged outstanding data of more than 64 KB.

The receiver might not have received the remaining 32 KB of data because any of the following reasons:

  • The remaining 32 KB of data is still in transit.

  • The remaining 32 KB of data is lost.

The TCP window size is controlled by the window size parameter in the TCP header, which is 16 bits long. This size limits the ability of the sender to advertise the window size to 65536, which is 2^16. The following table demonstrates the TCP header format that contains the window size as well as other TCP protocol specifications, as defined in the RFC 793:

Source Port
(16 bits)
Destination Port
(16 bits)
Sequence Number
(32 bits)
Acknowledgment Number (32 bits)
Len
(4 bits)
Unused
(6 bits)
Flags
(6 bits)
Window Size
(16 bits)
Checksum
(16 bits)
Urgent Pointer
(16 bits)
Type
(8 bits)
Length
(8 bits)
Data
(16 bits)

You can use a tcpdump network trace to determine the window size. The following is an example of a typical network trace displayed by expanding the TCP stack in the Wireshark application.

Ethernet II, Src: HewlettP_33:2e:a1 (00:11:0a:33:2e:a1), Dst: Iwill_09:d4:86 (00:d0:68:09:d4:86)
Internet Protocol, Src: 10.3.250.11 (10.3.250.11), Dst: 10.3.250.16 (10.3.250.16)
Transmission Control Protocol, Src Port: 3215 (3215), Dst Port: http (80), Seq: 280837115, Len: 0
    Source port: 3215 (3215)
    Destination port: http (80)
    Sequence number: 280837115
    Header length: 32 bytes
    Flags: 0x02 (SYN)
    Window size: 64512
    Checksum: 0x115e [correct]
    Options: (12 bytes)
Note: In the preceding example, the window size advertised by the sender is in bold face for your reference.

The TCP window Scale extension expands the definition of the TCP window to 32 bits by using a scale factor to carry this 32 bit value in the 16 bit window field of the TCP header, SEG.WND as defined in the RFC 793. The scale factor is carried in a new TCP option, window Scale. This option is only sent in a SYN segment, a segment with the SYN bit on, and the SYN-ACK segment, a segment with the SYN and ACK bits on. Therefore, the window scale is fixed in each direction when a connection is opened. This extension is defined in the RFC 1323.

You can verify the window scale by using a tcpdump network trace. The following is an example of a typical network trace displayed by expanding the TCP stack for the SYN and SYN-ACK packets in the Wireshark application.

Ethernet II, Src: Iwill_0c:a2:23 (00:d0:68:0c:a2:23), Dst: VRID_ff (00:00:5e:00:01:ff)
Internet Protocol, Src: 10.128.116.22 (10.128.116.22), Dst: 10.119.159.152 (10.119.159.152)
Transmission Control Protocol, Src Port: 22157 (22157), Dst Port: http (80), Seq: 2105340, Len: 0
Source port: 22157 (22157)
Destination port: http (80)
Sequence number: 2106340
Header length: 32 bytes
Flags: 0x02 (SYN)
Window size: 62645
Checksum: 0x0000 [Checksum Offloaded]
Options: (12 bytes)
Maximum segment size: 1460 bytes
NOP
Window scale: 3 (multiply by 8)
NOP
NOP
SACK permitted

You can use the following formula to calculate the actual Window size used by the sender:

Actual window size = (<Window_Size>) * (2 ^ <Window_Scale>)
For example, the following is the calculation for the actual window size in the preceding sample:
Actual window size = (62645) * (2 ^ 3)
= 501106

Note: The window size sent, even if a Scale factor is set, is never greater than 65535. The window scale calculation is actually done at each host.

Instructions

To configure, verify, and troubleshoot the TCP window scaling on a NetScaler appliance, refer to the appropriate section from the following sections:

Configuring the TCP Window Scaling on a NetScaler Appliance

To configure the TCP Window scaling on a NetScaler appliance, run the following command from the command line interface of the appliance: 

nsroot> set ns tcpparam -WS ENABLED -WSVal 4

Done
-WS is used to ENABLE/DISABLE the feature.
-WSVal is used to set the scale value.

Note: The preceding command changes the TCP settings at a global level, you can change the TCP setting at the virtual server level using TCPprofiles and binding it to appropriate virtual server.

add ns tcpProfile <name> [-WS ( ENABLED | DISABLED )] [-WSVal <positive_integer>]

Verifying the TCP Window Scaling on a NetScaler Appliance

To verify the settings of the TCP Window scaling and other TCP parameters on a NetScaler appliance, run the following command from the command line interface of the appliance:

nsroot> show ns tcpparam

TCP Parameters
Window Scaling status : ENABLED
Window Scaling factor : 4
SACK status : DISABLED
MaxBurst setting : 6 MSS
Initial cwnd setting : 4 MSS
TCP Receive Buffer : 8190 bytes

The following table lists the scale factors you can use and the respective Window scale size:

Scale Factor

Scale Value

Initial Window

Window Scaled

0

1

65535 or less

65535 or less

1

2

65535

131,070

2

4

65535

262,140

3

8

65535

524,280

4

16

65535

1,048,560

5

32

65535

2,097,120

6

64

65535

4,194,240

7

128

65535

8,388,480

8

256

65535

16,776,960

9

512

65535

33,553,920

10

1024

65535

67,107,840

11

2048

65535

134,215,680

12

4096

65535

268,431,360

13

8192

65535

536,862,720

14

16384

65535

1,073,725,440

Note: Setting a high value for the Scale Factor, -WSVal, might have an adverse impact on the appliance and the network.

Troubleshooting the TCP Window Scaling on a NetScaler Appliance

Consider the following points when troubleshooting TCP Window Scaling on a NetScaler appliance:

  • Verify if the TCP parameters are set correctly on the appliance by running the show ns tcpparam command.
  • Verify if the TCP parameters are set correctly on the host computers.
  • Verify if the TCP parameters are set correctly by recording the packet traces of the actual traffic and checking the TCP headers. It is a best practice to record the network packet traces on the appliance to record the packet traces of both sides of the communication.
  • When analyzing the network packet trace files in the Wireshark application, the applications sometimes display different Window sizes for seemingly the same session. These applications only calculate the scaled Window sizes if the initial handshake is within the time when packet traces for the traffic are recorded. If the TCP handshake is not included in the time span of recording the packet traces, then the TCP header displays the 16 bits value for the Window size.
Note: For a Windows Scale to be accomplished, both hosts in a TCP session must send a Window Scale factor. If only one side of a connection is set, Windows scaling is set. Each connection of the same session, such as TCP session between Client and NetScaler appliance, and TCP session between NetScaler appliance and Server with the same request/response, are the independent Window Scaling sessions. It is possible to have Window scaling between the client and NetScaler appliance, and not the between the NetScaler appliance and server.

Additional Resources

Refer to the NetScaler product documentation suite for more information about window scaling on the NetScaler appliance.

Note: The Citrix NetScaler product documentation can be accessed from the Citrix Knowledge Center by NetScaler customers who are logged on and have valid NetScaler maintenance agreements.

For more information about the TCP window scaling, refer to the Wikipedia Web site.

Applicable Products

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