Did Microsoft make their services run faster on IE6? Did IE6 send information only to Microsoft?
Yes. Internet Explorer used to send out-of-spec TCP packets during the initial handshake that IIS would respond to also in an out of spec way. This had the effect of speeding up subsequent requests, but only in the case of IE talking to IIS.
It is perfectly within spec to send data on a half-closed TCP connection, from the other end. That is exactly what the shutdown(SD_SEND) socket call is for. It signals the stack to send a FIN so that the other side can send data but knows to not expect more.
They were probably implementing a version of HTTP 1.1 (before it became a formalized standard) that did not break existing clients (or servers) but speeded up theirs.
BTW, I find it hard to believe IIS was sending more data after sending a FIN. Let's just say I worked in the vicinity of the Windows networking org in those days.
I know it's not a popular history to believe among the SV crowd, but Microsoft in those days would bend over backwards for ISVs (Independent Software Vendors, or companies that wrote software for Windows) if they just whispered "your products are using undocumented APIs that I don't have access to". Regardless of whether that claim was true or not, they would work overtime to dispel the notion.
The were just getting put under the anti-trust microscope, with a consent decree signed in 1994 and a bigger suit was obviously coming soon.
Of course, when google does something similar with QUIC/SPDY, it's the second coming of Christ. Definitely not a way to boost their browser/core business.
edit:typos
The web is the new OS. Microsoft made use of certain private Windows APIs to win the browser war (by providing features exclusive to IE). I think it’s a pretty apt analogy.
Yes, the pillar of Web 2.0, the AJAX, was implemented in IE5.5 first with XMLHTTPReques.