Configuring TCP path MTU discovery

	IMPORTANT: All devices on a TCP connection must be enabled to send ICMP error messages by using the ip unreachables enable command.

TCP path MTU discovery (in RFC 1191) discovers the path MTU between the source and destination ends of a TCP connection. It works as follows:

1. A TCP source device sends a packet with the Don't Fragment (DF) bit set.

2. A router discards the packet that exceeds the MTU of the outgoing interface and returns an ICMP error message. The error message contains the MTU of the outgoing interface.

3. Upon receiving the ICMP message, the TCP source device calculates the current path MTU of the TCP connection.

4. The TCP source device sends subsequent TCP segments that each are smaller than the MSS (MSS = path MTU – IP header length – TCP header length).

If the TCP source device still receives ICMP error messages when the MSS is smaller than 32 bytes, the TCP source device will fragment packets.

An ICMP error message received from a router that does not support RFC 1191 has the MTU of the outgoing interface set to 0. Upon receiving the ICMP message, the TCP source device selects the path MTU smaller than the current path MTU from the MTU table as described in RFC 1191. Based on the selected path MTU, the TCP source device calculates the TCP MSS. The MTU table contains MTUs of 68, 296, 508, 1006, 1280, 1492, 2002, 4352, 8166, 17914, 32000, and 65535 bytes. Because the minimum TCP MSS specified by the system is 32 bytes, the actual minimum MTU is 72 bytes.

After you enable TCP path MTU discovery, all new TCP connections will detect the path MTU. The device uses the path MTU to calculate the MSS to avoid IP fragmentation.

The path MTU uses the following aging mechanism to ensure that the source device can increase the path MTU when the minimum link MTU on the path increases:

• When the TCP source device receives an ICMP error message, it reduces the path MTU and starts an aging timer for the path MTU.

• After the aging timer expires, the source device uses a larger MSS in the MTU table, as described in RFC 1191.

• If no ICMP error message is received within two minutes, the source device increases the MSS again until the MSS negotiated during TCP three-way handshake is reached.

To enable TCP path MTU discovery: