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TCP Configurations

January 4, 2021
Contributed by: 
C

TCP configurations for a Citrix ADC appliance can be specified in an entity called a TCP profile, which is a collection of TCP settings. The TCP profile can then be associated with services or virtual servers that want to use these TCP configurations.

A default TCP profile can be configured to set the TCP configurations that will be applied by default, globally to all services and virtual servers.

Note

When a TCP parameter has different values for service, virtual server, and globally, the value of the most-specific entity (the service) is given the highest precedence.  The Citrix ADC appliance also provides other approaches for configuring TCP. Read on for more information.

Supported TCP configuration

The Citrix ADC appliance supports the following TCP capabilities:

Defending TCP against spoofing attacks

The Citrix ADC implementation of window attenuation is RFC 4953 compliant.

Explicit Congestion Notification (ECN)

The appliance sends notification of the network congestion status to the sender of the data and takes corrective measures for data congestion or data corruption. The Citrix ADC implementation of ECN is RFC 3168 compliant.

Round trip time measurement (RTTM) using the timestamp option

For the TimeStamp option to work, at least one side of the connection (client or server) must support it. The Citrix ADC implementation of TimeStamp option is RFC 1323 compliant.

Detection of spurious retransmissions

This can be done using TCP duplicate selective acknowledgment (D-SACK) and forward RTO-Recovery (F-RTO). If there is spurious retransmissions, the congestion control configurations are reverted to their original state. The Citrix ADC implementation of D-SACK is RFC 2883 compliant, and F-RTO is RFC 5682 compliant.

Congestion control

This functionality use New-Reno, BIC, CUBIC, Nile, and TCP Westwood algorithms.

Window scaling

 This increases the TCP receive window size beyond its maximum value of 65,535 bytes.

Points to consider before you configure window scaling

  • You do not set a high value for the scale factor, because this can have adverse effects on the appliance and the network.
  • You do not configure window scaling unless you clearly know why you want to change the window size.
  • Both hosts in the TCP connection send a window scale option during connection establishment. If only one side of a connection sets this option, window scaling is not used for the connection.
  • Each connection for same session is an independent window scaling session. For example, when a client’s request and the server’s response flow through the appliance, it is possible to have window scaling between the client and the appliance without window scaling between the appliance and the server.

TCP maximum congestion window

The window size is a user configurable one. The default value is 8190 bytes.

Selective acknowledgment (SACK)

This uses the data receiver (either a Citrix ADC appliance or a client) notifies the sender about all the segments that have been received successfully.

Forward acknowledgment (FACK)

This functionality avoids TCP congestion by explicitly measuring the total number of data bytes outstanding in the network, and helping the sender (either a Citrix ADC or a client) control the amount of data injected into the network during retransmission timeouts.

TCP connection multiplexing

This functionality enables reuse of existing TCP connections. The Citrix ADC appliance stores established TCP connections to the reuse pool. Whenever a client request is received, appliance checks for an available connection in the reuse pool and serves the new client if the connection is available. If it is unavailable, the appliance creates a connection for the client request and stores the connection to the reuse pool. Citrix ADC supports connection multiplexing for HTTP, SSL, and DataStream connection types.

Dynamic receive buffering

This allows the receive buffer to be adjusted dynamically based on memory and network conditions.

MPTCP Connection

MPTCP connections between client and Citrix ADC. MPTCP connections are not supported between Citrix ADC and the back-end server. The Citrix ADC implementation of MPTCP is RFC 6824 compliant.

You can view MPTCP statistics such as active MPTCP connections and active subflow connections by using the command line interface.

At the command prompt, type one of the following commands to display a summary or detailed summary of MPTCP statistics, or to clear the statistics display:

  1. Stat MPTCP

  2. Stat mptcp –detail

  3. Clearstats basic

Note:

To establish an MPTCP connection, both the client and the Citrix ADC appliance must support the same MPTCP version. If you use the Citrix ADC appliance as an MPTCP gateway for your servers, the servers do not have to support MPTCP. When the client starts a new MPTCP connection, the appliance identifies the client’s MPTPC version from the MP_CAPABALE option in the SYN packet. If the client’s version is higher than the one supported on the appliance, the appliance indicates its highest version in the MP_CAPABALE option of the SYN-ACK packet. The client then falls back to a lower version and sends the version number in the MP_CAPABALE option of the ACK packet. If that version is supportable, the appliance continues the MPTCP connection. Otherwise, the appliance falls back to a regular TCP. The Citrix ADC appliance does not initiate subflows (MP_JOIN’s). The appliance expects the client to initiate subflows.

Extracting the TCP/IP path overlay option and inserting client-IP HTTP header

Extracting TCP/IP path overlay and inserting client-IP HTTP header. Data transport through overlay networks often uses connection termination or Network Address Translation (NAT), in which the IP address of the source client is lost. To avoid this, the Citrix ADC appliance extracts the TCP/IP path overlay option and inserts the source client’s IP address into the HTTP header. With the IP address in the header, the web server can identify the source client that made the connection. The extracted data is valid for lifetime of the TCP connection and therefore, this prevents the next hop host from having to interpret the option again. This option is applicable only for web services that have the client-IP insertion option enabled. For more information, see Insert the IP address of the client in the request header topic.

TCP segmentation offload

Offloads TCP segmentation to the NIC. If you set the option as “AUTOMATIC”, TCP segmentation is offloaded to the NIC, if NIC is supported.

This is used for resisting SYN flood attacks. You can enable or disable the SYNCOOKIE mechanism for TCP handshake with clients. Disabling SYNCOOKIE prevents SYN attack protection on the Citrix ADC appliance.

Learning MSS to enable MSS learning for all the virtual servers configured on the appliance

Supportable TCP Parameters

The following table provides a list of TCP parameters and its default value configured on a Citrix ADC appliance.

Parameter Default Value Description
Window Management    
TCP Delayed-ACK Timer 100 millisec Timeout for TCP delayed ACK, in milliseconds.
TCP minimum Retransmission Timeout(RTO) in millisec 1000 millisec Minimum retransmission timeout, in milliseconds, specified in 10-millisecond increments (value must yield a whole number if divided by 10)
Connection idle time before starting keep-alive probes 900 seconds Silently drop TCP established connections on idle timeoutcp established connections on idle timeout
TCP Timestamp Option ENABLED The timestamp option allows for accurate RTT measurement. Enable or Disable TCP Timestamp option.
Multipath TCP session timeout 0 seconds MPTCP session timeout in seconds. If this value is not set, idle. MPTCP sessions are flushed after virtual server’s client idle timeout.
Silently Drop HalfClosed connections on idle timeout 0 seconds Silently drop TCP half closed connections on idle timeout.
Silently Drop Established connections on idle timeout DISABLED Silently drop TCP established connections on idle timeout
Memory Management    
TCP Buffer Size 131072 bytes TCP buffer size is the receive buffer size on Citrix ADC. This buffer size is advertised to clients and servers from Citrix ADC and it controls their ability to send data to Citrix ADC. The default buffer size is 8K and usually it is safe to increment this when talking to internal server farms. The buffer size is also impact by the actual application layer in Citrix ADC like for SSL endpoint cases it is set to 40 K and for Compression it is set to 96 K.
Note: The buffer size argument must be set for dynamic adjustments to take place.    
TCP Send Buffer Size 8190 bytes TCP Send Buffer Size
TCP Dynamic Receive Buffering DISABLED Enable or disable dynamic receive buffering. When enabled, it allows the receive buffer to be adjusted dynamically based on memory and network conditions. Note: The buffer size argument must be set for dynamic adjustments to take place
TCP Max congestion window(CWND) 524288 bytes TCP Maximum Congestion Window
Window Scaling status ENALBED Enable or disable window scaling.
Window Scaling factor 8 Factor used to calculate the new window size. This argument is needed only when window scaling is enabled.
Connection Setup    
Keep-alive probes DISABLED Send periodic TCP keep-alive (KA) probes to check if peer is still up.
Connection idle time before starting keep-alive probes 900 seconds Duration, in seconds, for the connection to be idle, before sending a keep-alive (KA) probe.
Keep-alive probe interval 75 seconds Time interval, in seconds, before the next keep-alive (KA) probe, if the peer does not respond.
Maximum keep-alive probes to be missed before dropping connection. 3 Number of keep-alive (KA) probes to be sent when not acknowledged, before assuming the peer to be down.
RST window attenuation (spoof protection). DISABLED Enable or disable RST window attenuation to protect against spoofing. When enabled, will reply with corrective ACK when a sequence number is invalid.
Accept RST with last acknowledged sequence number. ENABLED  
Data transfer    
Immediate ACK on PUSH packet ENABLED Send immediate positive acknowledgment (ACK) on receipt of TCP packets with PUSH flag.
Maximum packets per MSS 0 Maximum number of octets to allow in a TCP data segment
Nagle’s Algorithm DISABLED Nagle’s Algorithm fights with the problem of small packets in TCP transmission. Applications like Telnet and other real time engines which require every key stroke to be passed to the other side often create small packets. With Nagle’s algorithm Citrix ADC can buffer such small packets and sends them together to increase on the connection efficiency. This algorithm needs to work along with other TCP optimization techniques in Citrix ADC.
Maximum TCP segments allowed in a burst 10 MSS Maximum number of TCP segments allowed in a burst
Maximum out-of-order packets to queue 300 Maximum size of out-of-order packets queue. A value of 0 means no limit
Congestion Control    
TCP Flavor CUBIC  
Initial congestion window(cwnd) setting 4 MSS Initial maximum upper limit on the number of TCP packets that can be outstanding on the TCP link to the server
TCP Explicit Congestion Notification(ECN) DISABLED Explicit Congestion Notification (ECN) provides end to end notification of network congestion without dropping packets.
TCP Max congestion window(CWND) 524288 bytes TCP maintains a congestion window (CWND), limiting the total number of unacknowledged packets that may be in transit end-to-end. In TCP, the congestion window is one of the factors that determines the number of bytes that can be outstanding at any time. The congestion window is a means of stopping a link between the sender and the receiver from becoming overloaded with too much traffic. It is calculated by estimating how much congestion there is on the link.
TCP Hybrid Start (HYSTART) 8 bytes  
TCP minimum Retransmission Timeout(RTO) in millisec 1000 Minimum retransmission timeout, in milliseconds, specified in 10-millisecond increments (value must yield a whole number if divided by 10).
TCP dupack threshold DISABLED  
Burst Rate Control 3 TCP Burst Rate Control DISABLED/FIXED/DYNAMIC. FIXED requires a TCP rate to be set
TCP Rate DISABLED TCP connection payload send rate in Kb/s
TCP Rate Maximum Queue 0 Maximum connection queue size in bytes, when BurstRateControl is used.
MPTCP    
Multipath TCP DISABLED Multipath TCP (MPTCP) is a set of extensions to regular TCP to provide a Multipath TCP service, which enables a transport connection to operate across multiple paths simultaneously.
Multipath TCP drop data on pre-established subflow DISABLED Enable or disable silently dropping the data on Pre-Established subflow. When enabled, DSS data packets are dropped silently instead of dropping the connection when data is received on pre established subflow.
Multipath TCP fastopen DISABLED Enable or disable Multipath TCP fastopen. When enabled, DSS data packets are accepted before receiving the third ack of SYN handshake.
Multipath TCP session timeout 0 seconds MPTCP session timeout in seconds. If this value is not set, idle MPTCP sessions are flushed after virtual server’s client idle timeout.
Security    
SYN spoof protection DISABLED Enable or disable drop of invalid SYN packets to protect against spoofing. When disabled, established connections are reset when a SYN packet is received.
TCP Syncookie DISABLED This is used for resisting SYN flood attacks. Enable or disable the SYNCOOKIE mechanism for TCP handshake with clients. Disabling SYNCOOKIE prevents SYN attack protection on the Citrix ADC appliance.
Loss Detection and Recovery    
Duplicate Selective Acknowledgement (DSACK) ENABLED A Citrix ADC appliance uses Duplicate Selective Acknowledgment (DSACK) to determine if a retransmission was sent in error.
Forward RTO recovery (FRTO) ENABLED Detects spurious TCP retransmission timeouts. After retransmitting the first unacknowledged segment triggered by a timeout, the algorithm of the TCP sender monitors the incoming acknowledgments to determine whether the timeout was spurious. It then decides whether to send new segments or retransmit unacknowledged segments. The algorithm effectively helps to avoid additional unnecessary retransmissions and thereby improves TCP performance in the case of a spurious timeout.
TCP Forward Acknowledgment (FACK) ENABLED Enable or disable FACK (Forward ACK).
Selective Acknowledgement(SACK) status ENABLED TCP SACK addresses the problem of multiple packet loss which reduces the overall throughput capacity. With selective acknowledgment the receiver can inform the sender about all the segments which are received successfully, enabling sender to only retransmit the segments which were lost. This technique helps Citrix ADC improve overall throughput and reduce the connection latency.
Maximum packets per retransmission 1 Allows Citrix ADC to control how many packets to be retransmitted in one attempt. When Citrix ADC receives a partial ACK and it has to do retransmission then this setting is considered. This does not impact the RTO based retransmissions.
TCP Delayed-ACK Timer 100 millisec Timeout for TCP delayed ACK, in milliseconds
TCO Optimization    
TCP Optimization mode TRANSPARENT TCP Optimization modes TRANSPARENT/ENDPOINT
Apply adaptive TCP optimizations DISABLED Apply Adaptive TCP optimizations
TCP Segmentation Offload AUTOMATIC Offload TCP segmentation to the NIC. If set to AUTOMATIC, TCP segmentation is offloaded to the NIC, if the NIC supports it.
ACK Aggregation DISABLED Enable or disable ACK Aggregation

Setting Global TCP Parameters

The Citrix ADC appliance allows you to specify values for TCP parameters that are applicable to all Citrix ADC services and virtual servers. This can be done using:

  • Default TCP profile
  • Global TCP command
  • TCP buffering feature

Note:

The recvBuffSize parameter of the set ns tcpParam command is deprecated from release 9.2 onwards. In later releases, set the buffer size by using the bufferSize parameter of the set ns tcpProfile command. If you upgrade to a release where the recvBuffSize parameter is deprecated, the bufferSize parameter is set to its default value.

Default TCP profile

A TCP profile, named as nstcp_default_profile, is used to specify TCP configurations that will used if no TCP configurations are provided at the service or virtual server level.

Note:

Not all TCP parameters can be configured through the default TCP profile. Some settings have to be performed by using the global TCP command (see section below). The default profile does not have to be explicitly bound to a service or virtual server.

To configure the default TCP profile

  • Using the command line interface, at the command prompt enter:

    set ns tcpProfile nstcp_default_profile …

  • On the GUI, navigate to System > Profiles, click TCP Profiles and update nstcp_default_profile.

Global TCP command

Another approach you can use to configure global TCP parameters is the global TCP command. In addition to some unique parameters, this command duplicates some parameters that can be set by using a TCP profile. Any update made to these duplicate parameters is reflected in the corresponding parameter in the default TCP profile.

For example, if the SACK parameter is updated using this approach, the value is reflected in the SACK parameter of the default TCP profile (nstcp_default_profile).

Note:

Citrix recommends that you use this approach only for TCP parameters that are not available in the default TCP profile.

To configure the global TCP command

  • Using the command line interface, at the command prompt enter:

    set ns tcpParam …

  • On the GUI, navigate to System > Settings, click Change TCP parameters and update the required TCP parameters.

TCP buffering feature

Citrix ADC provides a feature called TCP buffering that you can use to specify the TCP buffer size. The feature can be enabled globally or at service level.

Note. The buffer size can also be configured in the default TCP profile. If the buffer size has different values in the TCP buffering feature and the default TCP profile, the greater value is applied.

To configure the TCP buffering feature globally

  • At the command prompt enter:

    enable ns mode TCPB

    set ns tcpbufParam -size <positiveInteger> -memLimit <positiveInteger>

  • On the GUI, navigate to System > Settings, click Configure Modes and select TCP Buffering.

    And, navigate to System > Settings, click Change TCP parameters and specify the values for Buffer size and Memory usage limit.

Setting Service or Virtual Server Specific TCP Parameters

Using TCP profiles, you can specify TCP parameters for services and virtual servers. You must define a TCP profile (or use a built-in TCP profile) and associate the profile with the appropriate service and virtual server.

Note:

You can also modify the TCP parameters of default profiles as per your requirements. You can specify the TCP buffer size at service level using the parameters specified by the TCP buffering feature.

To specify service or virtual server level TCP configurations by using the command line interface

At the command prompt, perform the following:

  1. Configure the TCP profile.

    set ns tcpProfile <profile-name>...

  2. Bind the TCP profile to the service or virtual server.

set service <name> ....

**Example**:

`> set service service1 -tcpProfileName profile1`

To bind the TCP profile to the virtual server:

`set lb vserver <name> ....`

**Example**:

`> set lb vserver lbvserver1 -tcpProfileName profile1`

To specify service or virtual server level TCP configurations by using the GUI

At the GUI, perform the following:

  1. Configure the TCP profile.

    Navigate to System > Profiles > TCP Profiles, and create the TCP profile.

  2. Bind the TCP profile to the service or virtual server.

    Navigate to Traffic Management > Load Balancing > Services/Virtual Servers, and create the TCP profile, which should be bound to the service or virtual server.

Built-in TCP Profiles

For convenience of configuration, the Citrix ADC provides some built-in TCP profiles. Review the built-in profiles listed below and select a profile and use it as it is or modify it to meet your requirements. You can bind these profiles to your required services or virtual servers.

Built-in profile Description
nstcp_default_profile Represents the default global TCP settings on the appliance.
nstcp_default_tcp_lan Useful for back-end server connections, where these servers reside on the same LAN as the appliance.
nstcp_default_WAN useful for WAN deployments.
nstcp_default_tcp_lan_thin_stream Similar to the nstcp_default_tcp_lan profile. However, the settings are tuned too small size packet flows.
nstcp_default_tcp_interactive_stream Similar to the nstcp_default_tcp_lan profile. However, it has a reduced delayed ACK timer and ACK on PUSH packet settings.
nstcp_default_tcp_lfp Useful for long fat pipe networks (WAN) on the client side. Long fat pipe networks have long delay, high bandwidth lines with minimal packet drops.
nstcp_default_tcp_lfp_thin_stream Similar to the nstcp_default_tcp_lfp profile. However, the settings are tuned for small size packet flows.
nstcp_default_tcp_lnp Useful for long narrow pipe networks (WAN) on the client side. Long narrow pipe networks have considerable packet loss occasionally.
nstcp_default_tcp_lnp_thin_stream Similar to the nstcp_default_tcp_lnp profile. However, the settings are tuned for small size packet flows.
nstcp_internal_apps Useful for internal applications on the appliance (for example, GSLB site syncing). This contains tuned window scaling and SACK options for the desired applications. This profile should not be bound to applications other than internal applications.
nstcp_default_Mobile_profile Useful for mobile devices.
nstcp_default_XA_XD_profile Useful for a XenApp or XenDesktop deployment.

Sample TCP Configurations

Sample command line interface examples for configuring the following:

Defending TCP against spoofing attacks

Enable the Citrix ADC to defend TCP against spoof attacks. By default the “rstWindowAttenuation” parameter is disabled. This parameter is enabled to protect the appliance against spoofing. If you enable, it replies with corrective acknowledgment (ACK) for an invalid sequence number. Possible values are Enabled, Disabled.

Where RST window attenuate parameter protects the appliance against spoofing. When enabled, will reply with corrective ACK when a sequence number is invalid.

```
set ns tcpProfile profile1 -rstWindowAttenuate ENABLED -spoofSynDrop ENABLED
Done
set lb vserver lbvserver1 -tcpProfileName profile1
Done
```

Explicit Congestion Notification (ECN)

Enable ECN on the required TCP profile

```
set ns tcpProfile profile1 -ECN ENABLED
Done
set lb vserver lbvserver1 -tcpProfileName profile1
Done
```

Selective ACKnowledgment (SACK)

Enable SACK on the required TCP profile.

```
> set ns tcpProfile profile1 -SACK ENABLED
Done
> set lb vserver lbvserver1 -tcpProfileName profile1
Done
```

Forward ACKnowledgment (FACK)

Enable FACK on the required TCP profile.

```
set ns tcpProfile profile1 -FACK ENABLED
Done
set lb vserver lbvserver1 -tcpProfileName profile1
Done
```

Window Scaling (WS)

Enable window scaling and set the window scaling factor on the required TCP profile.

```
set ns tcpProfile profile1 –WS ENABLED –WSVal 9
Done
set lb vserver lbvserver1 -tcpProfileName profile1
Done
```

Maximum Segment Size (MSS)

Update the MSS related configurations.

```
set ns tcpProfile profile1 –mss 1460 - maxPktPerMss 512
Done
set lb vserver lbvserver1 -tcpProfileName profile1
Done
```

Citrix ADC to learn the MSS of a virtual server

Enable the Citrix ADC to learn the VSS and update other related configurations.

```
set ns tcpParam -learnVsvrMSS ENABLED –mssLearnInterval 180 -mssLearnDelay 3600
Done
```

TCP keep-alive

Enable TCP keep-alive and update other related configurations.

```
> set ns tcpProfile profile1 –KA ENABLED –KaprobeUpdateLastactivity ENABLED -KAconnIdleTime 900 -KAmaxProbes 3 -KaprobeInterval 75
Done
> set lb vserver lbvserver1 -tcpProfileName profile1
Done
```

Buffer size - using TCP profile

Specify the buffer size.

```
> set ns tcpProfile profile1 –bufferSize 8190
Done
> set lb vserver lbvserver1 -tcpProfileName profile1
Done
```

Buffer size - using TCP buffering feature

Enable the TCP buffering feature (globally or for a service) and then specify the buffer size and the memory limit.

```
> enable ns feature TCPB
Done
> set ns tcpbufParam -size 64 -memLimit 64
Done
```

MPTCP

Enable MPTCP and then set the optional MPTCP configurations.

```
> set ns tcpProfile profile1 -mptcp ENABLED
Done
> set ns tcpProfile profile1 -mptcpDropDataOnPreEstSF ENABLED -mptcpFastOpen ENABLED -mptcpSessionTimeout 7200
Done
> set ns tcpparam -mptcpConCloseOnPassiveSF ENABLED -mptcpChecksum ENABLED -mptcpSFtimeout 0 -mptcpSFReplaceTimeout 10
-mptcpMaxSF 4 -mptcpMaxPendingSF 4 -mptcpPendingJoinThreshold 0 -mptcpRTOsToSwitchSF 2 -mptcpUseBackupOnDSS ENABLED
Done
```

Congestion control

Set the required TCP congestion control algorithm.

```
> set ns tcpProfile profile1 -flavor Westwood
Done
> set lb vserver lbvserver1 -tcpProfileName profile1
Done
```

Dynamic receive buffering

Enable dynamic receive buffering on the required TCP profile.

```
> set ns tcpProfile profile1 -dynamicReceiveBuffering ENABLED
Done
> set lb vserver lbvserver1 -tcpProfileName profile1
Done
```

Support for TCP Fast Open (TFO) in Multipath TCP (MPTCP)

A Citrix ADC appliance now supports TCP Fast Open (TFO) mechanism for establishing Multipath TCP (MPTCP) connections and speed up data transfers. The mechanism allows subflow data to be carried during the initial MPTCP connection handshake in SYN and SYN-ACK packets and also enables data to be consumed by the receiving node during the MPTCP connection establishment.

For more information, see TCP Fast Open topic.

A Citrix ADC appliance now enables you to configure a variable length TCP Fast Open (TFO) cookie of a minimum size of 4 bytes and a maximum size of 16 bytes in a TCP profile. By doing this, the appliance can respond with the configured TFO cookie size in the SYN-ACK packet to the client.

To configure TCP Fast Open (TFO) cookie in a TCP profile by using the command line interface

At the command prompt, type:

set tcpProfile nstcp_default_profile -tcpFastOpenCookieSize <positive_integer>

Example

set tcpProfile nstcp_default_profile -tcpFastOpenCookieSize 8

To configure TCP Fast Open (TFO) cookie in a TCP profile by using GUI

  1. Navigate to Configuration > System > Profiles.
  2. In the details pane, go to TCP Profiles tab and select a TCP profile.
  3. In the Configure TCP Profile page, set the TCP Fast Open cookie size.
  4. Click OK and Done.

In a Citrix ADC appliance, by default, the SYN cookie parameter on the TCP profile is enabled to resist SYN attacks. If you prefer to detect an attack for a virtual server and check the SYN-ACK retransmission rate, the SYN cookie value is toggled from Enabled to Disabled state. However, the toggling effect of the cookie from Enabled to Disabled state and vice versa causes a configuration inconsistency between nodes in a cluster deployment. To overcome this, you can use the second SYN cookie parameter maintained on each virtual server of the appliance. When a SYN attack is detected, the SYN cookie on the TCP profile is disabled and the second SYN cookie value on the server is toggled from Disabled to Enabled state. The cookie remains in Enabled state for an autosyncookietimeout interval that you can specify. During the timeout interval, the virtual server fights the SYN attack in SYN Cookie mode. After the interval times out, the SYN Cookie value is toggled back to Disabled state if the virtual server’s SYN-ACK retransmission rate is below the threshold. Otherwise, if the rate is still above the threshold, it implies it is a SYN attack and the cookie is again toggled to Enabled state for the specified timeout period.

To configure the maximum SYN ACK retransmission threshold by using the command line interface

At the command prompt, type:

```
set ns tcpparam [-maxSynAckRetx <positive_integer>]
Set ns tcpparam [-maxSynAckRetx 150]
```

To configure auto SYN cookie timeout interval by using the command line interface

At the command prompt, type:

set ns tcpparam [-autosyncookietimeout <positive_integer>]

Set ns tcpparam [-autosyncookietimeout 90]

TCP Configurations
January 4, 2021
Contributed by: 
C
January 4, 2021
Contributed by: 
C

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