Packets and IP

Why a 5 MB photo crosses the internet as thousands of tiny pieces

The internet never sends a file in one go. It cuts it into packets: chunks of at most about 1,500 bytes, each with an address label called the IP header (IP, the Internet Protocol, is the rulebook all connected devices follow).

A 5 MB photo becomes about 3,400 packets. Each header holds the source and destination IP addresses and a TTL (time to live): a counter cut by one at every router. At zero the packet is dropped, so it can't loop forever.

IP is best effort. Packets can be dropped, duplicated or arrive out of order, and IP itself doesn't fix that. It only promises to try to deliver each packet to its address.

Packets and IP

Run traceroute and watch your packets hop from router to router

Type traceroute google.com on a Mac or Linux machine, or tracert google.com on Windows, and you get a list of routers between you and Google: your home router, your provider's routers, then Google's network.

It uses the TTL: it sends packets with TTL 1, 2, 3…, each dies one router further on, and that router sends back an error revealing its address. Run it twice and the list can change: packets have no fixed path.

Packets and IP

Routers only know the next hop, so a cut cable gets routed around

No router knows the whole route. Each reads the packet's destination, looks it up in its forwarding table (address ranges mapped to next routers) and passes it one hop on. Every packet is judged alone.

If a cable between two cities breaks, routers update their tables and later packets go another way. With no line reserved for you, one cable carries packets from thousands of people at once, unlike an old phone circuit.

The cost is that your 3,400 photo packets may take different paths and arrive shuffled or with gaps. Putting them back in order and resending missing ones is a job for a layer above IP.