HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 3 - IP Services Conf - Page 80

Configuring the TCP send/receive buffer size, Configuring TCP timers

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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 an 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 1. Enter system view. 2. Enable TCP path MTU discovery. Command system-view tcp path-mtu-discovery [ aging minutes | no-aging ] Remarks N/A Optional. Disabled by default. Configuring the TCP send/receive buffer size Step 1. Enter system view. 2. Configure the size of TCP send/receive buffer. Command system-view tcp window window-size Remarks N/A Optional. 8 KB by default. Configuring TCP timers You can configure the following TCP timers: • synwait timer-When sending a SYN packet, TCP starts the synwait timer. If no response packet is received within the synwait timer interval, the TCP connection cannot be created. • finwait timer-When a TCP connection is changed into FIN_WAIT_2 state, the finwait timer is started. If no FIN packet is received within the timer interval, the TCP connection is terminated. If a FIN packet is received, the TCP connection state changes to TIME_WAIT. If a non-FIN packet is 72

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72
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 an 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
minutes
|
no-aging
]
Optional.
Disabled by default.
Configuring the TCP send/receive buffer size
Step
Command
Remarks
1.
Enter system view.
system-view
N/A
2.
Configure the size of TCP
send/receive buffer.
tcp window
window-size
Optional.
8 KB by default.
Configuring TCP timers
You can configure the following TCP timers:
•
synwait timer
—When sending a SYN packet, TCP starts the synwait timer. If no response packet is
received within the synwait timer interval, the TCP connection cannot be created.
•
finwait timer
—When a TCP connection is changed into FIN_WAIT_2 state, the finwait timer is
started. If no FIN packet is received within the timer interval, the TCP connection is terminated. If a
FIN packet is received, the TCP connection state changes to TIME_WAIT. If a non-FIN packet is