nat tcp-mss

Function

The nat tcp-mss command changes the maximum segment size (MSS) value in TCP SYN packets after NAT processes the packets.

The undo nat tcp-mss command does not change the MSS value in TCP SYN packets after NAT processes the packets.

By default, the MSS value in TCP SYN packets after NAT processes the packets is not changed.

This command is supported only on the NetEngine 8000 F1A.

Format

nat tcp-mss mss-value

undo nat tcp-mss [ mss-value ]

Parameters

Parameter	Description	Value
tcp-mss mss-value	Sets the MSS value in TCP SYN packets.	The value is an integer ranging from 536 to 1460, in bytes.

Parameter

Description

Value

tcp-mss mss-value

Sets the MSS value in TCP SYN packets.

The value is an integer ranging from 536 to 1460, in bytes.

Views

System view

Default Level

2: Configuration level

Task Name and Operations

Task Name	Operations
nat	write

Usage Guidelines

Usage Scenario

To change the MSS value in TCP SYN packets after NAT processes the packets, run the nat tcp-mss command.

The MSS value defined in TCP specifies the length of a TCP packet. The MSS value is carried in the SYN packet and is negotiated when the TCP connection is being established.

When the link MTU is small, NAT packet fragments may be generated. You can change the MSS value in TCP, so that the packets for NAT are not fragmented. This helps improve NAT efficiency.

Configuration Impact

After the nat tcp-mss command is run in the system view, the NAT device changes the MSS value in all TCP packets for NAT. If the negotiated MSS value is greater than the MSS value configured using the parameter, the negotiated MSS value is changed to the configured value. If the negotiated MSS value is smaller than the MSS value configured using the parameter, the negotiated MSS value remains unchanged.

Example

# Set the MSS value in TCP SYN packets after NAT processes the packets to 1300 bytes.

<HUAWEI> system-view
[~HUAWEI] nat tcp-mss 1300