H3C Technologies H3C S10500 Series Switches - Manual (Page 102)

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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).
NOTE:
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 an 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 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.
Follow these steps to enable TCP path MTU discovery:
To do… Use the command… Remarks
Enter system view system-view —
Enable TCP path MTU
discovery tcp path-mtu-discovery [ aging
minutes | no-aging ] Optional
Disabled by default.
CAUTION:
All the devices on the TCP path must be enabled to send ICMP error messages by using the ip
unreachables enable command.
Configuring the TCP send/receive buffer size
Follow these steps to configure the TCP send/receive buffer size:
To do… Use the command… Remarks
Enter system view system-view —
Configure the size of TCP
send/receive buffer
tcp window window-size
Optional
8 KB by default.