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Configuring TCP MSS for Session Negotiation

During session connection establishment, two peers agree in negotiations to determine the IP segment size of packets that they will exchange during their communication. The TCP MSS (maximum segment size) value in TCP SYN packets specifies the maximum number of bytes that a TCP packet’s data field, or segment, can contain. An MSS value that is set too high could result in an IP datagram that is too large to send and that must be fragmented. Fragmentation can incur additional overhead cost and packet loss.

To diminish the likelihood of fragmentation and to protect against packet loss, you can use the tcp-mss statement to specify a lower TCP MSS value. The tcp-mss statement applies to all IPv4 TCP SYN packets traversing all the router’s ingress interfaces whose MSS value is higher than the one you specify. You cannot exempt particular ports from its effects.

Note: The tcp-mss statement is only applicable to J-series Services Routers.

To specify a TCP MSS value to be used, include the following statement at the [edit system internet-options] hierarchy level:



[edit system internet-options]

tcp-mss {

mss-value;
}

The value of mss-value is a number in the range from 64 through 65535.

To remove the TCP MSS specification from the configuration, use the following command:



delete system internet-options tcp-mss

For more information about the tcp-mss statement and session negotiation, see the J-series Services Router Basic LAN and WAN Access Configuration Guide.

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