How 'equality' can squeeze polite users to dial-up speeds

The principle of equal flow rates for each data stream in a communications channel seems irreproachable until you start looking at the detail. BT's Bob Briscoe points out that some users get far more than their share by streaming continuously and grabbing multiple data streams.

It is not the streaming itself he objects to. It is the fact that current protocols needlessly slow down light users.

To see what he means, consider the two diagrams below. Both depict 100 users with access rated at 2Mbits/sec sharing a 10Mbit line.

The blue portions show the times at which they are using the line ­ - that is, contending for a share of its capacity. If all users were engaged in traditional ‘bursty’ web tasks, each would be using the link for only five per cent of the time and would get something close to their full 2Mbits/sec.

Figure 1 shows what happens if 20 of them, each using just a single sTransport Control Protocol (TCP) stream begin continuous downloads. Each is still entitled to the same share at any point as a light user even though they are using the channel 20 times as much (100 per cent compared with five per cent).

Peer-to-peer (P2P, and other applications generating heavy traffic, make things worse by use multiple TCP streams to speed their data flows. If the 20 heavy users from Figure 1 each grabs 25 streams (Figure 2), each is using the channel 500 times more often (20x25) than a light user politely taking a single stream once in a while.

This could drag a light user’s speed down to 20Kbits/sec - ­ slower than dial-up unless the service provider takes counter-measures.