According to RFC 793 (the original TCP specification), the client may
(even should) wait at least one second before retransmitting any
However, RFC 2001 describes Fast Retransmission, where the third
acknowledgment for the same segment should be interpreted as an
indication of packet loss, and cause an immediate retransmission
(without waiting for the timeout of at least one second).
I'd have expected Mac OS X to both implement this and enable it by
default, but maybe I'm wrong.
There's a sysctl net.inet.tcp.newreno which defaults to 0, and which
seems to affect things. If you google, you find patches like
which doesn't just contain SACK code, but possibly fixes fast
retransmissions when newreno is not set. I'm not sure if disabling the
newreno sysctl should disable fast retransmissions, or whether that's a
So, the Mac OS X client is not wrong in honouring RFC 793 alone. It just
suffers badly from any packet loss.
For you, the more relevant question is why there's 1.2% packet loss
between the Mac OS X client and FreeBSD server, even when connected
directly with crossover.