Configuration Guide Configuring TCP
Run the ip tcp mss max-segment-size command in global configuration mode to set an MSS. It ranges from 68 to 1000
bytes. By default, the MSS is calculated based on MTU. If an MSS is configured, the effective MSS is the smaller one
between the calculated MSS and the configured MSS.
An excessively small MSS reduces transmission performance. You can promote TCP transmission by increasing the
MSS. Choose an MSS value by referring to the interface MTU. If the former is bigger, TCP packets will be fragmented
and transmission performance will be reduced.
8.3.5 Configuring MSS Value for SYN Packet
Working Principle
When a client initiates a TCP connection, it negotiates with the server on the total amount of data contained in a TCP
segment through the MSS field in TCP SYN packets. The MSS value in the SYN packets indicates the largest amount of data
that the server sends in a single and unfragmented piece.
For example, in the following figure, the MSS negotiated between a PC and a HTTP server is 1460, but TCP packets carrying
1460-byte data should be fragmented as they cannot directly pass R1 and R2 connected by a tunnel with an MTU of less
than 1500. Modify the MSS value in SYN packets on interfaces (1) and (2) of R2 to enable TCP packets to pass R1 and R2.
Figure 8-3
Related Configuration
Configuring MSS Value for TCPv4 SYN Packets
By default, the MSS value in TCPv4 SYN packets is not modified.
Run the ip tcp adjust-mss max-segment-size command in interface configuration mode to set an MSS, which ranges
from 500 to 1460 bytes.
To avoid packet fragmentation in the case of a small path MTU, you may configure an MSS for TCPv4 SYN packets.
The MSS in TCPv4 SYN packets will change to the configured value once the device receives the packets. You may
configure an MSS value with reference to the interface MTU.
This configuration applies to a new connection but does not take effect for an existing TCP connection.
Configuring MSS Value for TCPv6 SYN Packets
By default, the MSS value in TCPv6 SYN packets is not modified.
Run the ipv6 tcp adjust-mss max-segment-size command in interface configuration mode to set an MSS for TCPv6
SYN packets, which ranges from 1220 to 1440 bytes.