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JUNOSe 11.0.x IP, IPv6, and IGP Configuration Guide
NOTE: The MSS value is equal to the MTU value minus the IPv6 and TCP headers,
so the MSS value is generally 60 bytes less than the MTU.
Configuring Path MTU Discovery
IPv6 hosts transmit large amounts of data to other hosts using a series of IPv6
datagrams. To best use resources, increase performance, and avoid difficult
reassembly, hosts try to send datagrams that are as large as possible without requiring
fragmentation anywhere along the path from the source to the destination. This
datagram size is referred to as the path MTU (PMTU), and it is equal to the smallest
MTU for each hop in the path.
Path MTU discovery is the process of discovering the PMTU value and using that
value when transmitting IP datagrams.
Enabling PMTU Discovery
Use the tcp path-mtu-discovery command to enable PMTU discovery on the active
virtual router.
tcp path-mtu-discovery
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IPv6 TCP Configuration
Use to specify the MSS value for TCP to use.
Use the vrfName variable to specify a VRF to which you want to assign the TCP
MSS value.
Example
host1(config)#tcp mss 1000
Use the no version to remove the MSS value so that the router uses the advertised
MSS derived from the MTU of the output interface.
See tcp mss
Use to enable and configure path MTU discovery on the virtual router.
Issue the command without any keywords to enable path MTU discovery.
Issue the age-timer keyword to set the time (minutes) that TCP waits before
attempting to increase the path MTU after receiving an ICMP Too Big message
or after previously increasing the PMTU successfully (minutes2). The range of
these two timers is 1–30 minutes. The timer defaults to 10 minutes.
Issue the age-timer indefinite keyword to disable PMTU aging functions.
Example 1 Enables path MTU discovery
host1:VR1(config)#tcp path-mtu-discovery
Example 2 Sets path MTU discovery age timers differently
host1:VR1(config)#tcp path-mtu-discovery age-timer 20 15
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