Update2: I'm not sure why this is getting downvotes, but my technical information is very much correct. I have included a script that can impact the TCP Window size scaling factor at the end of this comment. Not sure why correct technical information is being downvoted, but at the possibility of getting more negative votes, I'm including full demonstration code that shows this is accurate at the end of this post. /Update2

Sorry, but that is clearly not what the RFC I linked to said. Quoting here:

The maximum receive window, and therefore the scale factor, is determined by the maximum receive buffer space. In a typical modern implementation, this maximum buffer space is set by default but can be overridden by a user program before a TCP connection is opened. This determines the scale factor, and therefore no user interface is needed for window scaling.

Update: So, breaking this down, the above quote says "the scaling factor is determined by the buffer space, which the application has access do. Thus we do not need to give the userland API additional control over further tuning on top of kernel support." /Update

Notice the one important piece here: userland applications can take advantage of window-scaling (again, provided the administrator and kernel are configured for such support, which I have mentioned several times now,) without needing any additional API.

Perhaps I am mis-understanding the point of contention for you, but you seem to be suggesting that there is no API that can control this. The RFC, and system-documentation, says otherwise. Obviously if the feature is disabled in the kernel or by the admin, one cannot have it. However, reading the RFC, it's possible for an application to avoid window-scaling by setting the buffers small enough that the kernel will not enable that feature.

I never said an application got to set kernel-wide params. I point out last comment and again here that they have an impact on it. It is very possible to change the buffers (which are socket-level options) which do have an impact on the window-scaling. Again, reference the quote above and the tcp manpage, referenced earlier in the thread.

Part of Update2: Code follows that shows you can impact the window-scaling factor from userland follows. If nothing else, this should help others understand how socket code works and what control userland has over the process.

What you are describing is this:

1. If your tcpip stack supports window scaling;
2. And if it is enabled for your machine and your userid/group;
3. And if you set the receive buffer size immediately after connect
4. And if the remote host tcpip stack also supports window scaling;
5. And if it is enabled;
6. And if all the firewalls, gateway devices and bridges between the ends support window scaling; have it enabled and are set to monitor the scaling factors;
7. And if the bandwidth-delay product between them indicates a benefit from increasing the buffer size;
8. And if the application retrieve rate is sufficiently high to merit it;
9. And if the stack-wide maximum scaling factor permits it;

Then setting the receive buffer size may, heuristically allow an application to influence the setting of the window size/scale factor product.

That's an awful lot of ifs & buts, to be claiming "an application can set the window size".

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In the name of avoiding a reply-war, I'll make this (brief) comment my last in the thread. For the record, I never said "an application can set the window size". What I did say is that an application can:

tune window-scaling per-socket

..and..

impact the window scaling

..and..

adjust this in the application level (where "this" is socket buffer sizes)

from the application. Apologies if there was any confusion where someone thought I said you had direct application-level control over the window-scaling. The application can influence this, which is described not only in system manpages, but in RFC-1323, section 2.1. This has been my only point for a while, and it's not worth any more attention, I suspect.