Configuring TCP MSS Adjustment

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

  • Layer 3 GRE tunnel


Note

Subinterfaces do not support TCP MSS Adjust.

Configuring 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 the ip tcp adjust-mss 1452 command.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. interface type number
  4. ip tcp adjust-mss max-segment-size
  5. end

DETAILED STEPS

  Command or Action Purpose

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 1/0/0

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 range is from 500 to 1460.

Step 5

end

Example:

Device(config-if)#end

Exits to global configuration mode.

Configuring the MSS Value for IPv6 Traffic

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. interface type number
  4. ipv6 tcp adjust-mss max-segment-size
  5. end

DETAILED STEPS

  Command or Action Purpose

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 1/0/0

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 value of TCP DF packets going through a device.

The max-segment-size argument is the maximum segment size, in bytes. The range is from 40 to 1440.

Step 5

end

Example:

Device(config-if)#end

Exits interface configuration mode and returns to privileged EXEC mode.

Example: Configuring the TCP MSS Adjustment

Device(config)#vpdn enable 
Device(config)#no vpdn logging
Device(config)#vpdn-group 1
Device(config-vpdn)#request-dialin 
Device(config-vpdn-req-in)#protocol pppoe 
Device(config-vpdn-req-in)#exit
Device(config-vpdn)#exit
Device(config)#interface GigabitEthernet 0/0/0
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: Configuring the TCP MSS Adjustment for IPv6 traffic

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

Feature History and Information for TCP MSS Adjustment

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Release

Modification

Cisco IOS XE Fuji 16.8.1a

This feature was introduced.