HPE FlexNetwork 5130 EI Series Layer 3-Ip Services Configuration Guide

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system-view
2. Enter interface view.
interface interface-type interface-number
3. Set the TCP MSS for the interface.
tcp mss value
By default, the TCP MSS is not set.
Configuring TCP path MTU discovery
About TCP path MTU discovery
TCP path MTU discovery (in RFC 1191) discovers the path MTU between the source and destination
ends of a TCP connection. 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.
TCP path MTU discovery 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.
The aging mechanism of the path MTU is as follows:
• 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.
Prerequisites
Make sure all devices on a TCP connection are enabled to send ICMP error messages by using the
ip unreachables enable command.
Procedure
1. Enter system view.
system-view
2. Enable TCP path MTU discovery.
tcp path-mtu-discovery [ aging age-time | no-aging ]
By default, TCP path MTU discovery is disabled.

HPE FlexNetwork 5130 EI Series Specifications

General

Brand	HPE
Model	FlexNetwork 5130 EI Series
Category	Switch
Language	English