Please consider backporting server-side-only ECN support to RHEL5:
Discounting the updates to Documentation/networking/ip-sysctl.txt, this is literally a two-line patch, and it doesn't alter the kernel ABI.
RHEL6 already has server-side-only ECN support.
Thank you for submitting this issue for consideration. Red Hat Enterprise Linux 5 has reached the end of Production 1 Phase of its Life Cycle. Red Hat does not plan to incorporate the suggested capability in a future Red Hat Enterprise Linux 5 minor release. If you would like Red Hat to re-consider this feature request and the requested functionality is not currently in Red Hat Enterprise Linux 6, please re-open the request via appropriate support channels and provide additional supporting details about the importance of this issue.