tcp adjust-mss inbound (Tunnel interface view)

Function

The tcp adjust-mss inbound command sets the maximum segment size (MSS) value of inbound TCP SYN/SYN+ACK packets of the GRE tunnel.

The undo tcp adjust-mss inbound command deletes the configuration.

By default, no MSS value is set for inbound TCP SYN/SYN+ACK packets to prevent those packets from being fragmented when they are transmitted along the GRE tunnel.

Format

tcp adjust-mss mss-value inbound

undo tcp adjust-mss mss-value inbound

Parameters

Parameter	Description	Value
mss-value	Specifies the MSS value of inbound TCP SYN/SYN+ACK packets to prevent those packets from being fragmented when they are transmitted along the GRE tunnel.	The value is an integer ranging from 32 to 9500, in bytes.

Parameter

Description

Value

mss-value

Specifies the MSS value of inbound TCP SYN/SYN+ACK packets to prevent those packets from being fragmented when they are transmitted along the GRE tunnel.

The value is an integer ranging from 32 to 9500, in bytes.

Views

Tunnel interface view

Default Level

2: Configuration level

Task Name and Operations

Task Name	Operations
gre	write

Usage Guidelines

Usage Scenario

Duringthe three-way TCP handshakes, a device negotiates the MSS value of subsequent TCP packets. After a TCP connection is established, if the device needs to direct packets to a GRE tunnel, the device encapsulates the GRE header into the packets. In this case, the total packet length may exceed the negotiated MSS value. As a result, the packets are fragmented, lowering transmission efficiency.

To prevent packet fragmentation, run the tcp adjust-mss inbound command on the tunnel interfaces of both ends of the GRE tunnel to set an MSS value for TCP SYN/SYN+ACK packets that can be accepted without being fragmented on a GRE tunnel interface. You can set the MSS value less than the sum of the configured interface MTU and GRE header size to prevent fragmentation for packets transmitted over the GRE tunnel.

Precautions

If the tcp adjust-mss inbound command is run repeatedly, the latest configuration overrides the previous one. The following restrictions apply:

-This command takes effect only on IPv4 GRE tunnels.

-This command takes effect only in IPv4 unicast and L3VPN scenarios.

Example

# Set the MSS value to 1450 bytes for inbound TCP SYN/SYN+ACK packets to prevent those packets from being fragmented when they are transmitted along the GRE tunnel on the GRE tunnel interface named Tunnel10.

<HUAWEI> system-view
[~HUAWEI] interface Tunnel 10
[*HUAWEI-Tunnel10] tunnel-protocol gre
[*HUAWEI-Tunnel10] tcp adjust-mss 1450 inbound