kern/43150: TCP/ECN behaves different to CE/CWR than ns2 reference

[rs%netapp.com@localhost]

>Number:         43150
>Category:       kern
>Synopsis:       TCP/ECN behaves different to CE/CWR than ns2 reference
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Apr 11 23:20:00 +0000 2010
>Originator:     Richard Scheffenegger
>Release:        ---
>Organization:
NetApp
>Environment:
---
code inspection, day-1 problem
>Description:
Used TBIT (www.icir.org/tbit/) to verify the RFC3168 compliance of the FreeBSD 
ECN implementiation.

Unfortunately, RFC3168 has slightly ambigous wording, but the reference 
implementation of ECN in ns2, done by the authors of the RFC, can be used as a 
best practise implementation.

The Bug prevents the deployment of further enhancements into TCP CC algorithms 
which make smart use of CE marked frames.

This issue occurs with CWR/CW frames (both TCP CWR flag, and IP CE codepoint 
are set). In the reference implementation, this frame results in a ECE/ACK, 
while in most BSD stacks, ECE will NOT be set.

>How-To-Repeat:

Establish a ECN connection;
Mark one data frame towards the NetBSD host with both CE and CWR.
Expected result: A ECE frame
Current result: A normal ACK 
>Fix:
See http://www.freebsd.org/cgi/query-pr.cgi?pr=145600

relevant codebase is identical, therefore even the patch should work.