Configure TCP MSS Adjustment

Feature History for TCP MSS Adjustment

This table provides release and related information for the features explained in this module.

These features are available in all the releases subsequent to the one they were introduced in, unless noted otherwise.

Table 1. Feature history for TCP MSS Adjustment

Release

Feature

Feature information

Cisco IOS XE 26.2.1

Transmission Control Protocol (TCP) Maximum Segment Size (MSS) Adjustment: The TCP MSS Adjustment feature has been introduced.

Cisco C9610 Series Smart Switches

Cisco C9550 Series Smart Switches

Cisco IOS XE 26.1.1

Transmission Control Protocol (TCP) Maximum Segment Size (MSS) Adjustment: The TCP MSS Adjustment feature enables the configuration of the maximum segment size for transient packets that traverse a router, specifically TCP segments with the SYN bitset. This feature helps prevent TCP sessions from being dropped by adjusting the MSS value of the TCP SYN packets.

Cisco C9350 Series Smart Switches

Use the Cisco Feature Navigator to find information about platform and software image support.

Restrictions for TCP MSS Adjustment

  • Subinterfaces do not support TCP MSS Adjust.

  • TCP MSS adjustment configuration works only if applied on an ingress interface. This configuration does not work if applied on an egress interface.

Information about TCP MSS Adjustment

The Transmission Control Protocol (TCP) Maximum Segment Size (MSS) Adjustment feature enables the configuration of the maximum segment size for transient packets that traverse a router, specifically TCP segments with the SYN bit set. Use the ip tcp adjust-mss command in interface configuration mode to specify the MSS value on the intermediate router of the SYN packets to avoid truncation.

When a host (usually a PC) initiates a TCP session with a server, it negotiates the IP segment size by using the MSS option field in the TCP SYN packet. The value of the MSS field is determined by the MTU configuration on the host. The default MSS value for a PC is 1500 bytes.

The PPP over Ethernet (PPPoE) standard supports an MTU of only 1492 bytes. The disparity between the host and PPPoE MTU size can cause the router in between the host and the server to drop 1500-byte packets and terminate TCP sessions over the PPPoE network. Even if the path MTU (which detects the correct MTU across the path) is enabled on the host, sessions may be dropped because system administrators sometimes disable the ICMP error messages that must be relayed from the host in order for path MTU to work.

The ip tcp adjust-mss command helps prevent TCP sessions from being dropped by adjusting the MSS value of the TCP SYN packets.

The ip tcp adjust-mss command is effective only for TCP connections passing through the router.

In most cases, the optimum value for the max-segment-size argument of the ip tcp adjust-mss command is 1452 bytes. This value plus the 20-byte IP header, the 20-byte TCP header, and the 8-byte PPPoE header add up to a 1500-byte packet that matches the MTU size for the Ethernet link.


Note

TCP MSS adjustment-based traffic is always software switched.

Supported Interfaces

TCP MSS Adjust is supported only on the following interfaces:

  • Physical Layer 3 interface

  • SVI

  • Layer 3 port channel

How to Configure TCP MSS Adjustment

The following sections provide configuration information for TCP MSS adjustment.

Configure the MSS Value for Transient TCP SYN Packets

Before you begin

Perform this task to configure the MSS for transient packets that traverse a router, specifically TCP segments with the SYN bit set.

We recommend that you use ip tcp adjust-mss 1452 command.

Procedure

Step 1

enable

Example:

Device>enable

Enables privileged EXEC mode.

Enter your password if prompted

Step 2

configure terminal

Example:

Device#config terminal

Enters the global configuration mode.

Step 3

interface type number

Example:

Device(config)#interface GigabitEthernet 1/0/1

Configures an interface type and enters interface configuration mode.

Step 4

ip tcp adjust-mss max-segment-size

Example:

Device(config-if)#ip tcp adjust-mss 1452

Adjusts the MSS value of TCP SYN packets going through a router.

The max-segment-size argument is the maximum segment size, in bytes.

  • The configurable ranges are:

    • IPv4: 500–9176 bytes

    • IPv6: 1220–9156 bytes

    With the interface MTU set to 1500, typical MSS values are:

    • IPv4: 1460 bytes

    • IPv6: 1440 bytes

Step 5

end

Example:

Device(config-if)#end

Exits to global configuration mode.

Configure the MSS Value for IPv6 Traffic

Procedure

Step 1

enable

Example:

Device>enable

Enables privileged EXEC mode.

Enter your password if prompted

Step 2

configure terminal

Example:

Device#config terminal

Enters the global configuration mode.

Step 3

interface type number

Example:

Device(config)#interface GigabitEthernet 1/0/1

Configures an interface type and enters interface configuration mode.

Step 4

ipv6 tcp adjust-mss max-segment-size

Example:

Device(config-if)#ipv6 tcp adjust-mss 1440

Adjusts the MSS option in IPv6 TCP SYN packets, including SYN-ACK packets, traversing the device. The adjustment is applied on ingress interfaces.

The max-segment-size argument is the maximum segment size, in bytes. The range is from 1220–9156.

Step 5

end

Example:

Device(config-if)#end

Exits interface configuration mode and returns to privileged EXEC mode.

Configuration Examples for TCP MSS Adjustment

The following sections provide configuration examples for TCP MSS adjustment.

Example: Configure the TCP MSS Adjustment

Device(config)#interface GigabitEthernet 1/0/1
Device(config-if)#ip address 192.168.100.1.255.255.255.0
Device(config-if)#ip tcp adjust-mss 1452
Device(config-if)#ip nat inside
Device(config-if)#exit

Example: Configure the TCP MSS Adjustment for IPv6 traffic

Device>enable
Device#configure terminal
Device(config)#interface GigabitEthernet 1/0/1
Device(config)#ipv6 tcp adjust-mss 1440
Device(config)#end