Iptables – Limit bandwidth per connection in Linux

iptables in combination with tc should be able to do this if OP is on Linux. Iptables has a module called connbytes that can match on the number of bytes that has passed the stream so far. Use this to "mark" packets in streams that have sent too many bytes. For example, you may have one rule that marks all packets in streams between 1 MByte and 10 MBytes with mark "1" and another one that marks all packets in streams longer than 10MBytes with mark "2".

Then you set up traffic shaping classes for default (== below 1 Mbyte), for mark "1" and mark "2".

The advantage of this solution is that you need not penalize users unless they collectively consume too much bandwidth. The disadvantage is that these are somewhat complex tools that take som reading to wrap your head around.

iptables and tc is included in most distros. You may also want to look at tcng which makes it radically simpler to formulate tc rule sets.

It's not a TCP requirement. As far as TCP is concerned, only the combination of source IP, source port, destination IP, and destination port needs to be unique. However, in practice most TCP APIs don't provide any way to create more than one connection with the same source port, unless they have different source IP addresses.