Maximum simultaneous NSURLConnections

max_nsurlconnections

When connecting to a public HTTP server, a client should not maintain more
than two open connections to that server at one time. (RFC2616 §8.1.4) An
HTTP server is likely to impose its own limit on the number of simultaneous
connections from a single IP address anyway. Furthermore opening a large
number of simultaneous connections to a server may be seen as a denial of
service (DOS) attack upon the server.

Queuing HTTP 1.1 requests on a single, asynchronous connection such as
NSURLConnection is likely to offer the best throughput anyway - since both
the requests and the responses can be "pipelined" (sent back to back without
any pause between them). Furthermore, an asynchronous connection makes the
most effective use of the client's processor cycles; because it allows the
client to perform local processing (such as parsing HTML) more or less
continuously - and be interrupted only when data is available.

Generally, most HTTP requests are I/O bound - so transferring the data from
the server to the client's machine in the shortest amount of time achieves
the best performance. Client "load balancing" can only interfere with that
goal. And in fact a client has no reason for balancing the load of its
requests - because unlike a server - it can (and should) simply limit how
many outstanding requests it has to deal with at any one time.

Greg

http://lists.apple.com/archives/cocoa-dev/2006/Mar/msg01058.html