[djoldman]

This was an open secret. You can also send the start of a datagram on every incoming packet and then abort if you don't want to add/modify/delete an order by splitting your datagram into multiple packets. This worked on some exchanges because they ordered add/modify/deletes by when the beginning of the packet was received as opposed to the end.

If you do too much of this you will get angry exchange network people yelling at you as they may consider it spam/ddos. Some exchanges explicitly limit this.

It's also worth considering the possible gain. If you're in the fpga (0-150ns) or cpu(300ns-3us) space, the math can come out differently.