What is the value of TCP's MSL variable on SLES

This document (7018997) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 10
SUSE Linux Enterprise Server 11
SUSE Linux Enterprise Server 12

Situation

TCP by definition (RFC 793) has connection states.

They include LISTEN, ESTABLISHED, FIN-WAIT-1, FIN-WAIT-2, CLOSING, TIME-WAIT, CLOSE-WAIT, LAST-ACK, and the virtual state of CLOSED.

When a TCP connection has passed through an Active Close and is now in a TIME-WAIT state, it MUST remain in that state by TCP specification for 2*MSL.

That brings up the question,
What is the value MSL on a SLES server?

Resolution

It turns out that the value is hard coded in the kernel and 2*MSL = 60 seconds.
You can find where it is hard coded here:
$(kernel_code)/include/net/tcp.h: ... #define TCP_TIMEWAIT_LEN (60*HZ) /* how long to wait to destroy TIME-WAIT
* state, about 60 seconds */
...
The sysctl -a | grep -i fin command at the terminal returns net.ipv4.tcp_fin_timeout = 60.
This would appear to be a way to access and alter this functionality.
But
since it is hard coded in the kernel there is no way to change it at the terminal.

Cause


Additional Information

Sometimes the hyphenated states are given with underscores like this:
FIN_WAIT_1, FIN_WAIT_2, TIME_WAIT, CLOSE_WAIT, LAST_ACK

Disclaimer

This Support Knowledgebase provides a valuable tool for SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.

  • Document ID:7018997
  • Creation Date: 16-May-2017
  • Modified Date:03-Mar-2020
    • SUSE Linux Enterprise Server

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