Routing — Hop by Hop

A packet from a Delhi player's laptop to the Mumbai match server may pass through a dozen machines on the way. Not one of them knows the whole route. Each makes one small decision and hands the packet on.

The rule

Each router reads only the destination and picks the next hop.

First, how the packet is built. The game's bytes get the game header and a UDP header, and then IP wraps the lot in a header of its own:

The IPv4 header's most important fields:

• Source and destination address — where the packet came from and where it is going.
• Protocol — what is inside: 6 means TCP, 17 means UDP, 1 means ICMP, the protocol routers use to report problems.
• TTL (time to live) — a counter, often starting at 64, that every router lowers by one. When it reaches 0, the router drops the packet and usually sends an ICMP "time exceeded" message back to the sender. This stops packets from looping forever, and it is how traceroute maps a path: it sends packets with a TTL of 1, 2, 3 and so on, and lists which router replies each time.
• Total length, a header checksum and fragment fields — the next two lessons cover these.

IPv6 uses a fixed 40-byte header with the same ideas under new names: next header instead of protocol, and hop limit instead of TTL.

Each router holds a routing table of address prefixes. For every packet it finds all the entries that match the destination and uses the most specific one, the longest prefix match. 0.0.0.0/0 matches every address, so it serves as the default route.

Routers keep no memory of earlier packets. If routes change, two packets from the same match can travel different paths and wait in different queues, which is one reason packets arrive out of order and delays wobble.

The shortcut

For a lookup, list the entries that match, then take the one with the longest prefix. A /24 fixes the first 24 bits (the first three numbers); a /28 fixes 28 bits, which leaves a block of 16 addresses.

Drills

Answer fast — trust the rule.

Remember

• Each router reads only the destination and picks the next hop.
• Longest prefix match: list the matching entries, then take the most specific.
• Trap: expecting one fixed path. Every packet is routed on its own.