kern/82470: FreeBSD advertises wrong window scale in some situations

[Pieter de Boer]

>Number:         82470
>Category:       kern
>Synopsis:       FreeBSD advertises wrong window scale in some situations
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Tue Jun 21 08:30:18 GMT 2005
>Closed-Date:
>Last-Modified:
>Originator:     Pieter de Boer
>Release:        5.4-RELEASE
>Organization:
SNB at UvA
>Environment:
FreeBSD client.wind.surfnet.nl 5.4-RELEASE FreeBSD 5.4-RELEASE #0: Thu Jun 16 17:09:33 CEST 2005     root@:/usr/obj/usr/src/sys/SMP  i386

>Description:
      Setting a recv buffer of (65535*2)+1 lets FreeBSD advertise a window of 65535<<2 (window scale 2, 256KB) in stead of 65535<<1 (window scale 1, 128KB). So FreeBSD is advertising a window bigger than the actual available receive space. When setting the buffer to 65535*2 exactly, the window scale option is set correctly. 
>How-To-Repeat:
      sysctl net.inet.tcp.recvspace=131071

Then set up a connection from the FreeBSD system to something else. FreeBSD will advertise a windowsize of 256KB, although there's only 1 byte more than 128KB available.
>Fix:
      Haven't had time to look into it yet, sorry.
>Release-Note:
>Audit-Trail:
>Unformatted: