Why can't you raise net.max_halfopen

I've read several times on this forum that you shouldn't raise the net.max_halfopen and the recommended setting is 8.

Why is that? Doesn't the higher amount of half open ports = faster downloads? I've ran the Windows patch to modify the tcpipsys file on Vista and set it to 100. Is that necessary to raise my speeds?

Explanations are appreciated. Thanks!

No it does not. Speeds are dependent upon the peers upload. You will get the same speeds from those peers whether your half-open is at 0 or 100.

Half open count is a common cause for malfunctions and internet disconnection. It is therefore not recommended, and ESPECIALLY for those having trouble while torrenting.

What does increasing half-open ports do?

Why does BitComet recommend its users to increase it?

The half-open connection (technically embryonic connection, but practically no one calls it that, so oh well) limit controls how many connections µTorrent will attempt to establish simultaneously at any given time. Half-open connections are just like phone calls that haven't yet been picked up by the other end of the line. The half-open connection limit controls the number of such "calls" you can attempt to make at any given time, but does not limit how many fully-established connections (phone calls that are successfully picked up from the other end) you can make in total.

More importantly, the half-open connection limit does not affect the overall speed at which the you can download/upload. At best, increasing the half-open limit may decrease the amount of time it takes for µTorrent to attain decent speeds, but that only lasts for the first few minutes at worst. After several minutes have passed, the connection limit will have already been reached if there are enough peers anyhow, so at that point, there is no difference between a low half-open connection limit and a high limit.

High half-open limits, on the other hand, cause connection problems for many users because Microsoft limits the number of half-open connections able to be made globally in Windows from XP SP2 and newer. It can be patched, but for little gain, as I already explained earlier (and Microsoft resets the limit every so often with some Windows Update anyhow). Even if your half-open connection count isn't limited by TCPIP.sys (either through patching, or you're using an OS that doesn't impose such a limit), an increase in the half-open limit would fall into the exact same limitations in efficacy as described above.

Why anyone would recommend that users increase their half-open limits as a general speed tweak that everyone should apply is beyond me. It doesn't actually help with speeds beyond the first few minutes at most, and comes with the detrimental effect of potentially killing connections for many users. The only potential exception to the "you don't need to increase your half-open connection limit" rule of thumb is if you have a connection with a very fast upload rate (think "several megabits per second"), and have many torrent jobs started simultaneously. In that situation, it may be beneficial to increase the half-open connection limit in order for µTorrent to be able to communicate (in a reasonable amount of time) with the large pool of peers and trackers associated with starting many torrent jobs simultaneously.

In instances where raising the half-open connection limit is unavoidable or necessary, the user should keep in mind that µTorrent's net.max_halfopen limit should never be set to a value greater than ~80% of the TCPIP.sys half-open connection limit. So if the TCPIP.sys limit is 100, the highest anyone should set µTorrent's net.max_halfopen should be 80. That doesn't mean it needs to be 80% either -- it can be less. That's just a safe limit guideline to maintain quality of connection.

Just as a "Real Life™" example, I never patch my TCPIP.sys, and run with net.max_halfopen set to 2, and I'm able to utilize my connection quite well (10mbit/s down, 384kbit/s up).

Half open connection limit also has NO effect on how fast you can get incoming connections.

(...Though you usually have to be unfirewalled to get incoming connections.)