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HTTP keeps nothing between requests, so a server that wants to know who is asking must be told on every request. Basic authentication, defined in RFC 7617 , does it the simplest way: every request carries the username and password in a header. The simulator shows each message whole, so you can read the password …

A session moves the password off the wire. The browser sends it once, in a login form; the server then creates a record, "this random id belongs to alice", and hands the id back in a cookie. The browser returns the cookie on every request, and the server looks the id up to learn who is asking. The simulator shows each …

A home or café router gives its clients private addresses and has a single public address of its own, so it rewrites every packet that crosses it: the source on the way out, the destination on the way back. The simulator shows that rewrite on each packet and the NAT table that drives it. The background is in NAT - …

One idea per line, in the order a connection lives through them. A read more link points at the RFC section behind a line; RFC 9293 is today's TCP specification. The TCP Simulator shows most of these lines happening. The basic units: segment, connection, stream Segment: a TCP header plus the data it carries, sent …

TCP numbers every byte it sends, and every ACK names the next byte it expects, so each acknowledgment can be traced back to the segments it covers. The simulator draws that tie between the two directions of a connection: Host A sends from left to right, Host B from right to left, and every segment is one you send and …

When a link fails, the routers at its two ends describe the network afresh and flood the news to everyone in the area; every router then reruns SPF on the new database. The demo cuts the R1B–R1D link in area 1 of the network from the earlier demos, while a host on R1A's LAN sends a steady stream of packets to ABR-1B, …

Inside an area every router holds the whole map, but no router LSA ever leaves its area. Routes between areas are carried by the ABRs as summary LSAs: a prefix and a cost, with nothing of the links behind them. This demo follows one such route from area 2 to area 1 across the network from the Hello , Database Exchange …

Every router in an OSPF area holds the same database, and runs Shortest Path First (SPF) on it to find the cheapest path to everywhere in the area. This demo takes area 1 of the network from the Hello and Database Exchange demos, strips it down to a graph, and runs Dijkstra's algorithm on it one decision at a time. …

Two OSPF neighbors that have agreed to become adjacent still hold different link-state databases. The database exchange makes them identical: each describes what it holds, asks for what it lacks, and receives it. This demo runs that exchange on the same network as the Hello protocol demo , one adjacency at a time. Open …

The Hello protocol is how OSPF routers find each other, agree that they can talk, and elect a Designated Router on a shared segment. This demo runs it on a fixed network of eleven routers; there is nothing to configure, only a scenario to watch. Open full screen ↗ recommended: the drawing is large Next: OSPF Database …