For peer-to-peer traffic, different operating systems have different limitations on the maximum number of peer-to-peer connections; Windows XP Pro has a limit of ten simultaneous peer-to-peer connections, Windows XP Home (for the client) has a limit of five, and an OS X client has a limit of ten [ P2P].

Stanislav = Shalunov
BitTorrent Inc
shalunov@bittorrent.com
personal: http://shlang.com

On Nov 13, 2009, at 11:20 AM, Bob Briscoe = wrote:

Reinaldo,

RFC5290 (individual submission via = RFC Editor) repeats a fallacy widely available from Internet searches = about OS limits to numbers of connections (see S.3.2.2). It describes a = commercial limit to file & printer sharing connections in desktop = licensed OSs but says it's a limit on all TCP connections. We should = also clarify confusions with limits on half-open TCP = connections.

Below is the mail I sent to the authors. But the = fallacy remained in the published RFC (being an individual RFC, there = was no working group to oversee its publication).

Could you add = appropriate text in ledbat-app-practices-recommendations to correct the = misinformation in RFC5290.

[I just said in the LEDBAT meeting = that I had posted the mail below on the tsvwg list - it was actually = only a private mail to the authors. But it seems to have been overlooked = anyway.]

Bob
Date: Tue, 16 Oct 2007 19:08:48 +0100
To: = "FLOYD, Sally" <floyd@acm.org>,= "ALLMAN, Mark" <mallman@icir.org>
From: Bob = Briscoe <rbriscoe@jungle.bt.co.uk><= br> Subject: Correction to draft-floyd-tsvwg-besteffort-01

= Sally, Mark,

Off list.

In section 3.2.1, you have an = inaccuracy. You say
= " &= nbsp; &nb= sp; ...
For =
peer-to-peer traffic, different
   operating systems have different limitations on the maximum
number of
   peer-to-peer connections; Windows XP Pro has a limit of ten
   simultaneous peer-to-peer connections, Windows XP Home (for
the
   client) has a limit of five, and an OS X client has a limit
of ten
  
[
P2P].
..."
I suggest you avoid quoting forums on this subject - = it is an area in which you will get as many opinions as postings (which = is why this is off-list - even tho I've been careful to check my = sources, some of the info below is less solidly cited than others).
= First a picky point. There are no OSs that specifically limit "p2p = connections", given the OS wouldn't be able to tell whether a connection = was p2p. But there's a more serious misunderstanding here...

All = the limits you cite above are solely limits on the number of remote = devices connecting to the built-in services of the respective OSs, like = file and printer sharing and more recently, in the case of Windows, the = built-in Web server called IIS. They are not limits to the number of IP = sockets in listen mode.

These limits were introduced long before = p2p file-sharing became popular (I recall they were around in Windows = NT4.0), in order to protect sales of the server version of each OS for = use as enterprise file-servers. I believe most p2p filesharing software = wouldn't work if these limits applied to IP server sockets = generally.

There is a limit in Windows (XP from service pack 2, = and Vista) on the number of TCP connections in half open state - this = limits the rate the OS can open new TCP connections, whether as a client = or a server, to 10 per RTT.
Clarification: This is a = limit on the number of half-open connections to *different* IP = addresses.

But = there's no limit on the opening rate of non-TCP connections (e.g. UDP). = This was designed to slow worm propagation, but as collateral damage, it = also hits a lot of p2p software that opens a hundred or so connections = at once. Microsoft hard coded this limit into the TCP/IP stack, but two = workrounds have been published (at least one is just a binary edit of = the TCPIP.SYS file).

Murari Sridharan says that this = half-open connections limit only applies to Windows XP-SP2+ & Vista. =

More
= ----
Even the article you quote says there are no limits in Mac OS X = to the number of server connections, other than for Apple's file sharing = service (Apple file protocol - AFP), which is limited to 10. The latter = part is confirmed by apple:
< = http://docs.info.apple.com/article.html?artnum=3D107237>
= Windows 2000, XP & Vista have very similar limits to OS X. The = important text in the Microsoft article below is "The connection limit = refers to the number of redirector-based connections and is enforced for = any file, print, named pipe, or mail slot session. The TCP connection = limit is not enforced, but it may be bound by legal agreement to not = permit more than 10 clients."
< = http://support.microsoft.com/default.aspx?scid=3Dkb;en-us;314882>
The XP Pro license says:
"You may permit a maximum of ten = (10) computers or other electronic devices (each a =93Device=94) to = connect to the Workstation Computer to utilize the services of the = Product solely for File and Print services, Internet Information = Services, and remote access (including connection sharing and telephony = services)."

There is plenty of ambiguity in these terms, but I = think remote access is usually used by Microsoft to mean access to a = remote device through this computer (not access by a remote device to = this computer). The examples in parentheses support this assumption. All = the mentioned services are those Windows actually limits. While general = IP connections are separate and not limited by the OS software. So I = believe the licence is reinforcing what the OS actually limits, rather = than widening the definition of the limits by legal means.

Win = 95/98 had a limit to the TCP/IP stack of 100 concurrent connections, but = that was configurable via the registry <= http://support.microsoft.com/kb/158474/EN-US/>.

I don't = know about Mac OS X limits to general IP server sockets (in listen mode) = but I doubt there are any (if you find anyone complaining of a limit, = let me know, but I can't find any via a brief scan of the results from a = Web search engine).

-----
BTW, the Azureus app (popular = BitTorrent s/w) typically holds more than a hundred connections open to = other peers, of which about half (by symmetry) will be server sockets. = Usually, only about 10% are actively transferring data at any one time, = but all hundred or so are open TCP connections. E.g. This calculator is = a popular way to decide how to config Bit Torrent clients:
< = http://infinite-source.de/az/az-calc.html>
If you put in = 256kbps upload (typical for 2M or 8Mbbps download DSL services), it = advises 134 max connections.

BTW^2, Microsoft advise users how = to increase the limit on HTTP downloads in a couple of places:
= < http://support.microsoft.com/kb/835821/en-gb> = (Find "MaxConnectionsPerServer" within the page)
< = http://support.microsoft.com/kb/282402> (Note the limp caveat at = the bottom of the page)

BTW^3 (nothing to do with no's of = connections), were you aware that Joost, the p2p video on demand service = that is becoming increasingly popular, uses UDP not TCP for its media = transfers. Currently at about a total of 700kbps inward and 220kbps = outward in 1kB packets. According to the Joost data centre network = architect, this is because "congestion control would kill video = streaming":
< = http://www.uknof.org.uk/uknof7/MacCarthaigh-Joost.pdf>
This = article giving an initial analysis of Joost traffic is worth reading = through (this one is based on some actual experiments)...
< = http://blog.ipdev.net/2007/04/joost-analysis-of-bandwidth-hog.html = >

Bob

= __________________________________________________________________________= __
Bob Briscoe, <bob.briscoe@bt.com>  &= nbsp;   Networks Research Centre, BT Research
B54/77 = Adastral Park,Martlesham Heath,Ipswich,IP5 3RE,UK.    +44 = 1473 645196

= ________________________________________________________________
Bob = Briscoe,           =             &n= bsp;        BT Innovate & = Design
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