H3C Technologies H3C S12500-X Series Switches - Manual (Page 110)

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Configuring TCP path MTU discovery
IMPORTANT:
All the 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 that fails to forward the packet because it exceeds the MTU on the outgoing interface
discards the packet and returns an ICMP error message, which 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 to calculate 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 make sure 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 age timer for the path MTU.
• After the age 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 is as large as the MSS negotiated during TCP three-way handshake.
To enable TCP path MTU discovery:
Step Command Remarks
1. Enter system view. system-view N/A
2. Enable TCP path MTU
discovery.
tcp path-mtu-discovery [ aging age-time |
no-aging ] The default setting is
disabled.