Detailed answers:
* At a dialup modem pool in AT&T WorldNet, 58% of the
TCP connections advertise ``SACK permitted" on the SYN packet, and
5% of the TCP connections advertise ``SACK permitted" on the SYN ACK.
Thus, only 5% of the TCP connections actually use SACK.
(From Anja Feldmann, trace from 12/99)
Web clients and servers:
* The TBIT tool
reports on the fraction of web servers supporting SACK.
* At a single SACK+RFC1323 capable HTTP client, 15% of the TCP connections
to over 115,000 IP addresses supported SACK.
(From Richard Wendland at Netcraft, data from 3/00)
* 22.7% of the 70,000 web servers measured by the
webstats project
used SACK. (From Wes Eddy, trace from 8/00).
* From December 1998 to February 2000, the
fraction of hosts sampled with SACK-capable TCP has increased
from 8% to 40%. (From Mark Allman).
Implementations:
* The following stacks support SACK: Win98/2000, Linux, Solaris,
IRIX,
OpenBSD, AIX, and possibly BSDI, but SACK is not a given
for high volume servers. (From Zachary Amsden, Richard Wendland,
Venkat Venkatsubra)
* The first SACK implementation to ship in a commercial product was
FTP Software OnNet 2.0 for Win95.
* For Solaris, SACK is used starting with Solaris 7. (From Kacheong Poon at SUN)