|Friday, December 13, 2002
18:07 - What makes IE so fast?
Internet Explorer on Windows always seems either to run impossibly fast
(page requests are fulfilled almost before the mouse button has returned to
its original unclicked position), or ridiculously slow (as with the weird
stalling-on-connect problem that many people, including myself, have
One possible explanation is something that my team and I noticed a couple of years ago, in analyzing packet traces of IE's connection setup procedure. Microsoft might have fixed this since then; I'm not sure. But it's a possible culprit.
First of all, for those rusty on their TCP/IP-- here's how a normal HTTP request over TCP should work:
This is how the client and server synchronize their sequence numbers, which is how a connection gets established. The client sends a synchronization request, the server acknowledges it and sends a synchronization request of its own, and the client acknowledges that. Only then can the HTTP request proceed reliably.
The server's SYN (synchronize) and ACK (acknowledgement) packets are combined for speed; there's no reason to send two separate packets, when you're trying to get a connection established as quickly as possible. Another speed enhancement that Mac OS 9's stack uses, by the way, is to combine the client's ACK and the HTTP request into a single packet; this is legal, but not frequently done. The idea is that within the structure of TCP/IP, you want to minimize the number of transactions that need to take place in setting up the two-way handshake necessary before you can send the HTTP request.
When tearing down a connection, it looks like this:
This generally takes four steps, and the FIN/ACK packets are usually not consolidated because connection teardown is nowhere near as speed-sensitive as startup is. (The FIN sequence can be initiated either by the client or the server.)
Many very stupid companies have tried to come up with overly clever ways to speed up TCP/IP. TCP, by its nature, is a stateful and bidirectional protocol that requires all data packets to be acknowledged; this makes the data flow reliable, by providing a mechanism for dropped packets to be retransmitted; but this also makes for a more strictly regimented flow structure involving more packets transmitted over the wire than in simpler, non-reliable protocols like UDP-- and therefore it's slower. One company that thought itself a lot smarter than it really was, called RunTCP, came up with the idea of "pre-acking" TCP packets; it would send out the acknowledgments for a whole pile of data packets in advance, thus freeing them from the onerous necessity of double-checking that each packet actually got there properly. And it worked great, speeding up TCP flows by a significant margin-- in the lab, under ideal test conditions. The minute you put RunTCP's products out onto the real Internet, everything stopped working. Which stands to reason-- their "solution" was to tear out all the infrastructure that made TCP work reliably, under competing load and in adverse conditions, in the first place. Dumbasses.
So then there's this thing we discovered in the lab. We noticed that when you entered a URL in Internet Explorer 5, its sequence of startup packets didn't look like the one shown above. Instead, it looked like this:
In other words, instead of sending a SYN packet like every other TCP/IP application in the world, IE would send out the request packet first of all. Just to check. Just in case the HTTP server was, oh, say, a Microsoft IIS server. Because IIS' HTTP teardown sequence looked like this:
...And that's it. The client doesn't FIN, and the server doesn't ACK. In other words, the connection is kept "half-open" on the server end. The reason for this? Why, to make subsequent connections from IE clients faster. If the connection isn't torn down all the way, all IE has to do is send an HTTP request, with no preamble-- and the server will immediately respond. Ingenious!
They probably called it "Microsoft Active Web AccelerationX™©®" or something.
(I may be remembering this incorrectly; it might be that the client does FIN, and the server simply keeps the connection around after it ACKs it. Instead of shutting down the connection entirely, it just waits to see if that client will come back, so it can open the connection back up immediately instead of having to go through that whole onerous SYN-SYN/ACK procedure. Damn rules!)
Now, what does this mean for non-IIS servers? It means that if you use IE to connect to them, it first tries to send that initial request packet, without any SYNs-- and then it only proceeds with the standard TCP connection setup procedure if the request packet gets a RST or no response (either of which is a valid way for a legal stack to deal with an unsynchronized packet). But IIS, playing by its own rules, would respond to that packet with an HTTP response right away, without bothering to complete the handshake. So IE to IIS servers will be nice and snappy, especially on subsequent connections after the first one. But IE to non-IIS servers waste a packet at the beginning of each request-- and depending on how the server handles that illegal request, it might immediately RST it, or it might just time out... which would make the browser seem infuriatingly slow to connect to new websites.
This is only marginally less stupid than RunTCP's "solution"-- and I say "marginally" only because in the grand scheme of things, this probably makes sense to Microsoft's network engineers. After all, eventually all clients will be Windows platforms running IE, and all servers will be Windows platforms running IIS. And then we can break all kinds of rules! Rules are only there to hold us back and force us to play nice with other vendors. Well, once the other vendors are all gone, who cares about some stupid RFC?
I have to admire their arrogance and their confidence. But it'll be some time before I can bring myself to admire their technical integrity.
UPDATE: Since this post got Slashdotted, I've been getting a pretty fair amount of e-mail, suggesting that the behavior we observed here might be anything from T/TCP to HTTP/1.1 pipelining to delirium tremens. Well, I should point out that this phenomenon was something we observed in 1997, before HTTP/1.1 was in wide use; both the client and server were using vanilla HTTP/1.0. As it turned out, it was actually the NT stack that was causing this to happen-- it didn't matter what client or server software you used. It even happened with our home-grown network test tools.