Infinity, Speedtest and TCP

I am on BT Infinity FTTC 40/10, using a Draytek 2750 router, connected via Ethernet.

I have been trying to understand why Speedtest.net gives me very differing upload speeds between my standard BT router/modem, and my 2750. With the 2750, I get round about 7Mbps, but with the BT setup, I get over 8Mbps. The download speeds are about the same - about 37.4Mbps on both. Using TBB's speedtester, the speeds on both routers are very similar - 36Mbps down and 6Mbps up.

I have captured the Speedtest packets using Microsoft's Network Monitor 3.4.

With the 2750, the payload length on each TCP packet sent by my PC is normally 536, and occasionally 1072. However, with the BT modem/router, the payload length is often 2824, with a frame length of 2878. Occasionally, the payload length can reach 5648, with a frame length of 5702.

This could easily explain the difference in upload results between the two routers, because of the extra overheads involved with the shorter payloads.

However, when I used to work with TCP/IP and Ethernet, many years ago, I thought that Ethernet frames above about 1500 bytes were illegal. Has this changed, now that Ethernet is not usually a shared media any more?

And the other, more important, question is :- how can I make my Draytek connection use the larger payload sizes, thus improving the upload Speedtest results?

I think you might need jumbo frames http://www.codinghorror.com/blog/2009/03/the-promise...

Thank you. That explains the large (jumbo) packets.

And, having read your link, I can now see the difference between the BT router and the Draytek. In both cases, when starting the upload test, they start with very large frames. The BT router handles these, and the transfer continues. But the Draytek drops them, and the upload restarts with smaller frames.

The problem is in the PPPoE configuration of the 2750, which has a maximum MTU of 1492.

It means that I can't do anything about it, but at least I understand what is going on.

I think that I will raise it with Draytek, and see what happens.

Jumbo Frames currently are only relevant to LAN data transfer, they are not used on the Internet as a whole. In fact the router would have to fragment those frames before transmitting them to your ISP, so potentially it could actually make performance worse.

They were primarily introduced for Gigabit ethernet as the normal frame size is perfectly sufficient for maxing out 100Mbit ethernet. In fact, if you look at the linked article above again you will notice it shows the speed BEFORE Jumbo Frames was 46MB/s. That is about four times the speed Infinity/VDSL2 could do if BT had no speed limits on it, around ten times the speed of Infinity today. So clearly, Jumbo Frames are not the answer.

On the other hand there are a lot of TCP/IP parameters that can affect broadband speed, but if you are using a modern operating system on your computer that is highly unlikely to be the problem as they automatically optimise themselves for faster speeds.

Its perfectly normal for speed tests to not show your full speed. On a 40/10 connection like Infinity the average speed is more like 37/8 but can vary. So it sounds to me like your connection is working perfectly normally.

Edited by alexatkin (Wed 11-Apr-12 03:18:36)