T/TCP useless on FreeBSD 4.7?

misha misha at research.att.com
Fri Aug 1 13:12:21 PDT 2003

Bruce, Michael:

Thanks a lot for the sysctl values tips.
While 4.7 release does not seem to have drop_synfin
option,  syncookies indeed used to be 1 (but now we changed
it to 0) on both machines:

> sysctl net.inet.tcp | grep -E 'rfc1644|drop_synfin|cook'
net.inet.tcp.rfc1644: 1
net.inet.tcp.syncookies: 0

setting it to 0 did not help: the tcp dump looks exactly the same.
In addition, I tried the following changes  on the server
(although it was clear this would not make any diff,  but
just to be diligent)

(a) replace "send" call (as in T/TCP book)
with "sendto" call according to the ttcp manpage:

  sendto(sock, buf, len, MSG_EOF, (struct sockaddr *)0, 0)

 (b) not send any reply at all, just read from the socket and
close it.

(c)  set "TCP_NOPUSH" option on the server's socket  before writing
response to it (with sendto call above).

In all cases, the tcp dump shows a timeout + retransmission by the client...
Also, what do you think about that source ode snippet I included into
my original mail?  It would certainly explain the behavior, except it would
also mean that T/TCP is no longer usable.  Unless that piece of code
is somehow bypassed with proper options...

Thanks again,

Bruce M Simpson wrote:

> On Fri, Aug 01, 2003 at 11:14:12AM -0400, michael rabinovich wrote:
> > Does anyone know the status of T/TCP support on FreeBSD 4.7?
> ...
> > Am I missing something (after all, FreeBSD is supposed to be a ref
> > implementation of T/TCP!) and if not is there is a simple way around
> > this problem, short of going back to earlier FreeBSD releases?
> I don't use T/TCP on my production 4.8-RELEASE system, and the following
> sysctl values look fairly default:-
> net.inet.tcp.rfc1644: 0
> net.inet.tcp.syncookies: 1
> net.inet.tcp.drop_synfin: 0
> SYN cookies and T/TCP can't co-exist. Please do check the above sysctl
> values; I know RFC 1644 has to be enabled, and syncookies have to be disabled,
> as well as drop_synfin.
> HTH,
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