IP TCP adjust-mss

ip tcp adjust-mss

To adjust the maximum segment size (MSS) value of TCP synchronize/start (SYN) packets that go 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 Modes

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

Usage Guidelines

When a host (usually a PC) initiates a TCP session with a server, the host 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 size is 1460 bytes, when the default MTU of the containing IP datagram 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 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 Internet Control Message Protocol (ICMP) error messages that must be relayed from the host 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 that pass through the router.

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

If you are configuring the ip mtu command on the same interface as the ip tcp adjust-mss command, we recommend 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 interface Dialer1 overload

ip route 0.0.0.0 0.0.0.0 Dialer1

access-list 101 permit ip 192.168.100.0 0.0.0.255 any

Related Commands

ip mtu

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