R211x-HP Flexfabric 11900 Layer 3 - IP Services Configuration Guide

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Configuring TCP MSS for an interface
The maximum segment size (MSS) option informs the receiver of the largest segment that the sender can
accept. Each end announces its MSS during TCP connection establishment. If the size 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 configure a 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 configure the
TCP MSS on the interface.
To configure a TCP MSS of the interface:
Step Command Remarks
1. Enter system view.
system-view N/A
2. Enter interface view.
interface interface-type
interface-number
N/A
3. Configure a TCP MSS for the
interface.
tcp mss value By default, no TCP MSS is configured.
Configuring TCP path MTU discovery
IMPORTANT:
A
ll the devices on a TCP connection must be enabled to send ICMP error messa
g
es by usin
g
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,