Scalable source routing is like a map that tells computer messages where to go. Imagine you are sending a letter to a friend. You know their address, so you write it on the envelope, put some stamps on it, and drop it in the mailbox. The post office then takes your letter and uses a map to figure out which trucks and planes to put it on to get it to your friend's house.
Computers work the same way, but instead of addresses, they use numbers called IP addresses. These numbers help computers find each other on the internet. When one computer wants to send a message to another computer, it uses a map called a routing table to figure out where the message needs to go.
Scalable source routing is a way to make this map even better. It lets the computer sending the message decide exactly which path the message should take to its destination. This is like if you could tell the post office which trucks and planes to put your letter on instead of letting them decide.
The advantage of this is that it can make messages travel faster and more efficiently. Just like how sometimes there is more than one way to get to your friend's house, there may be more than one way for a message to travel on the internet. With scalable source routing, the computer sending the message can pick the best path based on things like how crowded the roads are or how fast the connection is.
Overall, scalable source routing is a really useful tool for making sure messages get delivered quickly and efficiently on the internet.