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Cisco IOS Software Releases 12.2 T

TCP MSS Adjustment

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Table Of Contents

TCP MSS Adjustment

Contents

How to Configure TCP MSS Adjustment

Setting the MTU on the Local Hosts

Verifying TCP MSS Adjustments

Configuration Examples for TCP MSS Adjustment

TCP MSS Adjustment Configuration: Example

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Command Reference

ip tcp adjust-mss

Feature Information for TCP MSS Adjustment


TCP MSS Adjustment

First Published: October 15, 2001
Last Updated: June 19, 2006

The TCP MSS Adjustment feature enables the configuration of the maximum segment size (MSS) for transient packets that traverse a router, specifically TCP segments in the SYN bit set, when PPP over Ethernet (PPPoE) is being used in the network. PPPoE truncates the Ethernet maximum transmission unit (MTU) 1492, and if the effective MTU on the hosts (PCs) is not changed, the router in between the host and the server can terminate the TCP sessions. The ip tcp adjust-mss command specifies the MSS value on the intermediate router of the SYN packets to avoid truncation.

Finding Feature Information in This Module

Your Cisco IOS software release may not support all of the features documented in this module. To reach links to specific feature documentation in this module and to see a list of the releases in which each feature is supported, use the "Feature Information for TCP MSS Adjustment" section.

Finding Support Information for Platforms and Cisco IOS and Catalyst OS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

•How to Configure TCP MSS Adjustment

•Configuration Examples for TCP MSS Adjustment

•Additional References

•Command Reference

•Feature Information for TCP MSS Adjustment

How to Configure TCP MSS Adjustment

This section contains the following procedures:

•Setting the MTU on the Local Hosts (required)

•Verifying TCP MSS Adjustments (optional)

Setting the MTU on the Local Hosts

Perform this task to set the MTU on the local hosts. Network Address Translation (NAT) does not have to be configured to specify the MTU.

SUMMARY STEPS

1. enable

2. configure terminal

3. interface type number

4. ip tcp adjust-mss max-segment-size

5. ip mtu bytes

6. end

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

interface type number

Example:

Router(config)# interface ethernet0/1

Configures an interface type and enters interface configuration mode.

Step 4 

ip tcp adjust-mss max-segment-size

Example:

Router(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 

ip mtu bytes

Example:

Router(config-if) ip mtu 1491

Sets the MTU size of IP packets, in bytes, sent on an interface.

Step 6 

end

Example:

Router(config-if) end

Exits to global configuration mode.

Verifying TCP MSS Adjustments

In the following examples, there are outputs that assist in verifying the TCP MSS adjustments.

Command Configured on Single Interface

Step 1 Configure the interface adjustment value. 

interface ethernet1/1

 ip tcp adjust 500

Step 2 Telnet from router A to router C, with B having the MSS adjustment configured. 

telnet

00:09:15:%SYS-5-CONFIG_I: Configured from console by consolenet 10.0.1.3

Trying 10.0.1.3... Open

Step 3 Observe the debug output from router C. 

*Mar 10 14:49:45.045: tcp0: I LISTEN 172.16.0.1:11001 10.0.1.3:23 seq 2012812244

        OPTS 4 SYN WIN 4128

*Mar 10 14:49:45.045: TCP0: state was LISTEN -> SYNRCVD [23 -> 172.16.0.1(11001)]

*Mar 10 14:49:45.045: TCP0: Connection to 172.16.0.1:11001, received MSS 500, 

MSS is 500

*Mar 10 14:49:45.045: TCP: sending SYN, seq 1091096877, ack 2012812245

The MSS gets adjusted to 500 as configured.

Command Configured on Two Interfaces

Step 1 Configure the command on both the interfaces using different values. 

interface Ethernet1/1

 ip address 172.17.0.2 255.0.0.0

 ip tcp adjust-mss 505

 duplex half


interface Ethernet1/3

 ip address 172.18.1.1 255.255.255.0

 ip tcp adjust-mss 500

 duplex half

Step 2 Telnet from router A to router C. 

telnet 10.0.1.3

     Trying 10.0.1.3... Open

Step 3 Observe the debug output from router A. 

06:06:49: TCP: sending SYN, seq 979045471, ack 0

     06:06:49: TCP0: Connection to 10.0.1.3:23, advertising MSS 536

Step 4 Observe the debug output from router C. 

I LISTEN 172.16.0.1:11003 10.0.1.3:23 seq 979045471

       OPTS 4 SYN WIN 4128

       TCP0: state was LISTEN -> SYNRCVD [23 -> 172.16.0.1(11003)]

       TCP0: Connection to 172.16.0.1:11003, received MSS 500, MSS is 500

The TCP MSS is successfully adjusted to 500.

Command Used with Process Switching

Step 1 Verify the configuration of the TCP MSS adjustment. 

Router# show running-config interface ethernet1/1


Building configuration...


Current configuration: 95 bytes

!

interface ethernet1/1

 ip address 10.0.0.2 255.0.0.0

 ip tcp adjust-mss 505

 duplex half

Step 2 Telnet from router A to router C. 

telnet 10.0.1.3

 Trying 10.0.1.3... 

    TCP: sending SYN, seq 886170752, ack 0

    TCP0: Connection to 10.0.1.3:23, advertising MSS 536

tcp0: O CLOSED 10.0.1.3:23 172.16.0.1:11008 seq 886170752

          OPTS 4 SYN WIN 4128

Step 3 Observe the debug output on router C. 

tcp0: I LISTEN 172.16.0.1:11008 10.0.1.3:23 seq 886170752

           OPTS 4 SYN WIN 4128

    TCP0: state was LISTEN -> SYNRCVD [23 -> 172.16.0.1(11008)]

    TCP0: Connection to 172.16.0.1:11008, received MSS 505, MSS is 505

The TCP MSS value gets adjusted to the configured value of 505.

Configuration Examples for TCP MSS Adjustment

This section provides the following example:

•TCP MSS Adjustment Configuration: Example

TCP MSS Adjustment Configuration: Example

The following example shows how to configure one interface with CEF switching turned on: 

interface ethernet1/1

 ip tcp adjust 500

The following example shows how to configure two interfaces: 

interface ethernet1/1

 ip address 10.0.0.2 255.0.0.0

 ip tcp adjust-mss 505

 duplex half


interface ethernet1/3

 ip address 10.0.1.1 255.255.255.0

 ip tcp adjust-mss 500

 duplex half

Additional References

The following sections provide references related to TCP MSS Adjustment.

Related Documents

Related Topic
Document Title

WAN commands: complete command syntax, command mode, command history, defaults, usage guidelines, and examples

Cisco IOS Wide-Area Networking Command Reference, Release 12.2SR

TCp configuration information: Configuring TCP Performance Parameters, TCP Window Scaling, TCP Congestion Avoidance, TCP Explicit Congestion Notification

TCP section of the Cisco IOS IP Application Services Configuration Guide, Release 12.4.


Standards

Standards
Title

None

—


MIBs

MIBs
MIBs Link

None

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs

RFCs
Title

None

—


Technical Assistance

Description
Link

The Cisco Technical Support & Documentation website contains thousands of pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.

http://www.cisco.com/techsupport


Command Reference

This section documents one modified command only.

•ip tcp adjust-mss

ip tcp adjust-mss

To adjust the maximum segment size (MSS) value of TCP SYN packets going through a router, use the ip tcp adjust-mss command in interface configuration mode. To return the MSS value to the default setting, use the no form of this command.

ip tcp adjust-mss max-segment-size

no ip tcp adjust-mss max-segment-size

Syntax Description

max-segment-size

Maximum segment size, in bytes. The range is from 500 to 1460.


Command Default

The MSS is determined by the originating host.

Command Modes

Interface configuration

Command History

Release
Modification

12.2(4)T

This command was introduced.

12.2(8)T

This command was changed from ip adjust-mss to ip tcp adjust-mss.

12.2(28)SB

This command was integrated into Cisco IOS Release 12.2(28)SB.

12.2(33)SRA

This command was integrated into Cisco IOS Release 12.2(33)SRA.


Usage Guidelines

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 maximum transmission unit (MTU) configuration on the host. The default MSS value for a PC is 1500 bytes.

The PPP over Ethernet (PPPoE) standard supports a 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 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.

If you are configuring the ip mtu command on the same interface as the ip tcp adjust-mss command, it is recommended that you use the following commands and values:

•ip tcp adjust-mss 1452

•ip mtu 1492

Examples

The following example shows the configuration of a PPPoE client with the MSS value set to 1452: 

vpdn enable

no vpdn logging

!

vpdn-group 1

request-dialin

protocol pppoe

!

interface Ethernet0

 ip address 192.168.100.1.255.255.255.0

 ip tcp adjust-mss 1452

 ip nat inside

!

interface ATM0

 no ip address

 no atm ilmi-keepalive

 pvc 8/35

 pppoe client dial-pool-number 1

!

dsl equipment-type CPE

dsl operating-mode GSHDSL symmetric annex B

dsl linerate AUTO

!

interface Dialer1

 ip address negotiated

 ip mtu 1492

 ip nat outside

 encapsulation ppp

 dialer pool 1

 dialer-group 1

 ppp authentication pap callin

 ppp pap sent-username sohodyn password 7 141B1309000528

!

ip nat inside source list 101 Dialer1 overload

ip route 0.0.0.0.0.0.0.0 Dialer1

access-list permit ip 192.168.100.0.0.0.0.255 any

Related Commands

Command
Description

ip mtu

Sets the MTU size of IP packets sent on an interface.


Feature Information for TCP MSS Adjustment

Table 1 lists the release history for this feature.

Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 1 lists only the Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release train. Unless noted otherwise, subsequent releases of that Cisco IOS software release train also support that feature.

Table 1 Feature Information for TCP MSS Adjustment 

Feature Name
Releases
Feature Information

TCP MSS Adjustment

12.2(4)T
12.2(8)T
12.2(28)SB
12.2(33)SRA

The TCP MSS Adjustment feature enables the configuration of the maximum segment size (MSS) for transient packets that traverse a router, specifically TCP segments in the SYN bit set, when PPP over Ethernet (PPPoE) is being used in the network.

In 12.2(4)T, this feature was introduced.

In 12.2(8)T, the command that was introduced by this feature was changed from ip adjust-mss to ip tcp adjust-mss.

In 12.2(28)SB and 12.2(33)SRA, this feature was enhanced to be configurable on subinterfaces.



Any Internet Protocol (IP) addresses used in this document are not intended to be actual addresses. Any examples, command display output, and figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses in illustrative content is unintentional and coincidental.

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