Your software release might not support all the features documented in this module. For the latest caveats and feature information, see the Bug Search Tool at https://tools.cisco.com/bugsearch/ and the release notes for your software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "New and Changed Information"chapter or the Feature History table in this chapter.
Feature History for IP TCP MSS
Table 1 Feature History for IP TCP MSS
IP TCP MSS
The IP TCP Maximum Segment Size (MSS) feature enables a switch to set a maximum segment size for all the TCP connections that originate from or terminate at a Cisco Nexus 7000 Series Switch.
Information About IP TCP MSS
The IP TCP Maximum Segment Size (MSS) feature enables a switch to set a maximum segment size for all TCP connections that originate or terminate at a Cisco Nexus 7000 Series switch. The MSS in a TCP header field is the maximum data size or payload that a host can send or receive in a single segment. By default, a Cisco Nexus 7000 Series switch sets the MSS value to 536 bytes for IPv4 TCP connections and 1240 bytes for IPv6 TCP connections. This default value is set by the switch during the initial TCP connection establishment.
The following figure shows how the MSS value is set between two switches for IPv4 TCP connections:
The ip tcp mss<2000> command is used on Host A to set the MSS to 2000 bytes. The difference between the route interface MTU (1500 bytes) and the protocol header (40 bytes) is 1460 bytes.
For IPv6 TCP connections, the difference between the route interface MTU (1500 bytes) and the protocol header (60 bytes) is 1440 bytes.
The switch from which the TCP connection originates will always set the MSS to the user-configured MSS or the difference between the route interface MTU and the protocol header, whichever is lower. Thus, Host A sends a SYN packet with the proposed MSS of 1460 bytes to Host B. After receiving the SYN packet with the proposed MSS, Host B sends a SYN-ACK packet to Host A, accepting the proposed MSS value for the TCP connection. Host A sends an ACK packet to Host B, setting the MSS value to 1460 for the TCP connection.
Licensing Requirements for IP TCP MSS
IP TCP MSS requires no license. Any feature that is not included in a license package is bundled with the Cisco NX-OS system images and is provided at no extra charge to you. For a complete explanation of the Cisco NX-OS licensing scheme, see the Cisco NX-OS Licensing Guide.
Default Settings for IP TCP MSS
IP TCP MSS
536 bytes for IPv4 TCP connections
1240 bytes for IPv6 TCP connections
Guidelines and Limitations for IP TCP MSS
If the MSS has to be set to a value that is more than 1460 bytes for IPv4 TCP connections, the corresponding MTU value should be set to the required MSS value plus 40 bytes. If the MSS has to be set to a value that is more than 1440 bytes for IPv6 TCP connections, the corresponding MTU value should be set to the required MSS value plus 60 bytes.