June 30, 2006 Chapter 7: Second Level Commands
SG1-UM-8500-03 7-39
MAXIMUM SEGMENT SIZE (MSS) CHANGINGUsing the ip tcp adjust-mss command
The system, when configured so that ip tcp adjust-mss is set to on, should adjust the TCP MSS option value on
SYN packets to 1436 (for MSS option larger than 1436) in both directions for each connected user. (Note: When the
mtu is configured to 1544, there is no need to adjust the mss.)
• The system should extract 64 from the MTU=1500 because of the TCP header (20 bytes).
• This command configures the system to work in adjust-mss mode.
• The system default adjust-mss value is on.
• When the system adjust-mss is configured to off it should not appear in the write terminal command.
Usage
ip tcp adjust-mss <on | off>
Example(s)
Host(config)# ip tcp adjust-mss on
Term Descriptions
•MTU: The maximum transmission unit is a link layer restriction on the maximum number of bytes of data in a
single transmission.
•Path MTU: The smallest MTU of any link on the current path between two hosts. This may change over time
since the route between two hosts, especially on the Internet, may change over time. It is not necessarily sym-
metric and can even vary for different types of traffic from the same host.
•MSS: The MSS is the maximum segment size. It can be announced during the establishment of a TCP con-
nection to indicate to the other end the largest amount of data in one packet that should be sent by the remote
system. Normally the packet generated will be 40 bytes larger than this: 20 bytes for the IP header and 20 for
the TCP header. Most systems announce an MSS determined from the MTU on the interface that traffic to the
remote system passes out through.
Host(config)# ip tcp adjust-mss