Configuring Tcp Path Mtu Discovery - HPE FlexNetwork 10500 Series Configuration Manual

of a TCP segment is smaller than the MSS of the receiver, TCP sends the TCP segment without
fragmentation. If not, it fragments the segment according to the receiver's MSS.
If you set the TCP MSS on an interface, the size of each TCP segment received or sent on the
interface cannot exceed the MSS value.
This configuration takes effect only for TCP connections established after the configuration rather
than the TCP connections that already exist.
This configuration is effective only for IP packets. If MPLS is enabled on the interface, do not set the
TCP MSS on the interface.
To set the TCP MSS for the interface:
Step
1. Enter system view.
2. Enter interface view.
3. Set the TCP MSS for the
interface.

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.
Command
system-view
interface interface-type
interface-number
tcp mss value
141
Remarks
N/A
N/A
By default, the TCP MSS is not set.