Hello all!

This question is the result of two other questions I've asked in the last few days.
I'm creating a new question because I think it's related to the "next step" in my understanding of how to control the flow of my send/receive, something I didn't get a full answer to yet.
The other related questions are:
http://stackoverflow.com/questions/3028376/an-iocp-documentation-interpretation-question-buffer-ownership-ambiguity
http://stackoverflow.com/questions/3028998/non-blocking-tcp-buffer-issues

In summary, I'm using Windows I/O Completion Ports.
I have several threads that process notifications from the completion port.
I believe the question is platform-independent and would have the same answer as if to do the same thing on a *nix, *BSD, Solaris system.

So, I need to have my own flow control system. Fine.
So I send send and send, a lot. How do I know when to start queueing the sends, as the receiver side is limited to X amount?

Let's take an example (closest thing to my question): FTP protocol.
I have two servers; One is on a 100Mb link and the other is on a 10Mb link.
I order the 100Mb one to send to the other one (the 10Mb linked one) a 1GB file. It finishes with an average transfer rate of 1.25MB/s.
How did the sender (the 100Mb linked one) knew when to hold the sending, so the slower one wouldn't be flooded?

Another way to ask this:
Can I get a "hold-your-sendings" notification from the remote side? Is it built-in in TCP or the so called "reliable network protocol" needs me to do so?

Again, I have a loop with many sends to a remote server, and at some point, within that loop I'll have to determine if I should queue that send or I can pass it on to the transport layer (TCP).
How do I do that? What would you do? Of course that when I get a completion notification from IOCP that the send was done I'll issue other pending sends, that's clear.

Another design question related to this:
Since I am to use a custom buffers with a send queue, and these buffers are being freed to be reused (thus not using the "delete" keyword) when a "send-done" notification has been arrived, I'll have to use a mutual exlusion on that buffer pool.
Using a mutex slows things down, so I've been thinking; Why not have each thread have its own buffers pool, thus accessing it , at least when getting the required buffers for a send operation, will require no mutex, because it belongs to that thread only.
The buffers pool is located at the thread local storage (TLS) level.
No mutual pool implies no lock needed, implies faster operations BUT also implies more memory used by the app, because even if one thread already allocated 1000 buffers, the other one that is sending right now and need 1000 buffers to send something will need to allocated these to its own.

This is a long question and I hope none got hurt (:

Thank you all!