Finding Feature Information, page 1 Feature History for IP TCP MSS, page 2 Information About IP TCP MSS, page 2 Licensing Requirements for IP TCP MSS, page 3 Default Settings for IP TCP MSS, page 3 Guidelines and Limitations for IP TCP MSS, page 3, page 3 Configuration Examples for IP TCP MSS, page 5 Verifying IP TCP MSS, page 5 Additional References for IP TCP MSS, page 5 Finding Feature Information 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. 1
Feature History for IP TCP MSS Feature History for IP TCP MSS Table 1: Feature History for IP TCP MSS Feature Name IP TCP MSS Release 8.2(1) Feature Information 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. 2
Licensing Requirements for IP TCP MSS Note 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 Parameter IP TCP MSS Default Setting 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. Setting the MSS for TCP Connections Procedure Step 1 Step 2 Enter global configuration mode: switch# Set a maximum segment size: switch(config)# ip tcp mss <bytes> 3
Removing a Set IP TCP MSS Step 3 Note The IP TCP MSS range is from 68 to 10000 bytes. (Optional) Display the configured IP TCP MSS: Example: Running Configuration This example shows a running configuration, followed by a verification command that displays the configured IP TCP MSS: ip tcp mss 5000 Setting TCP MSS to 5000 bytes TCP MSS value 5000 bytes Removing a Set IP TCP MSS Procedure Step 1 Step 2 Step 3 Enter global configuration mode: switch# Remove the configured IP TCP MSS and set the IP TCP MSS to default values: switch(config)# no ip tcp mss (Optional) Display the configured IP TCP MSS: Example: Running Configuration This example shows a running configuration, followed by a verification command that displays the configured IP TCP MSS: no ip tcp mss 5000 Setting default MSS value is 536 bytes TCP MSS value 536 bytes 4
Configuration Examples for IP TCP MSS Configuration Examples for IP TCP MSS Example: Setting the MSS for TCP Connections ip tcp mss 2000 Example: Removing a Set IP TCP MSS no ip tcp mss Verifying IP TCP MSS Command show ip tcp mss Purpose Displays the set IP TCP MSS. Additional References for IP TCP MSS This section describes additional information related to configuring IP TCP MSS. Related Documents Related Topic Cisco NX-OS licensing guide Command reference Document Title Cisco NX-OS Licensing Guide Cisco Nexus 7000 Series NX-OS Interfaces Command Reference 5
Additional References for IP TCP MSS 6