The internet has a foundational design flaw that we've papered over for forty years: an IP address means two completely different things at once. It is both where you are (a routing locator) and who you are (an endpoint identifier). TCP connections are bound to IP addresses, so when you change networks — laptop closes, phone hops from Wi-Fi to LTE — your "identity" changes, and every connection breaks. RFC 5201 introduces the Host Identity Protocol (HIP), a serious attempt to surgically separate these two roles at the protocol layer.

HIP inserts a new namespace between the network layer and the transport layer: the Host Identity (HI), which is a public key. From the HI, you derive a fixed-length Host Identity Tag (HIT), a 128-bit value formatted to look exactly like an IPv6 address (it lives in the 2001:20::/28 ORCHID prefix). Transport sockets bind to HITs, not IP addresses. The kernel maintains a mapping from HIT to the host's current set of IP locators, and that mapping can change without TCP ever noticing.

The handshake is where the cryptographic elegance lives. HIP uses a four-packet base exchange — I1, R1, I2, R2 — that performs a Diffie-Hellman key agreement and authenticates both peers via their HIs. But it has a clever wrinkle: the responder embeds a cryptographic puzzle in R1 that the initiator must solve before R2 will be accepted. The responder keeps no state until the puzzle solution arrives in I2. This makes HIP resistant to DoS state-exhaustion attacks in a way that TLS only later approximated with HelloRetryRequest cookies in TLS 1.3.

Key design decisions worth appreciating:

• Identifiers are self-certifying. The HIT is a hash of the public key, so there is no PKI. If you know a peer's HIT, you can verify any signature claiming to come from it. This is the same trick later popularized by Tor onion addresses and IPFS CIDs.
• Mobility is free. Because transport binds to HIT, host mobility (RFC 5206) and multihoming fall out of the architecture rather than being bolted on like Mobile IP's home-agent triangulation.
• IPsec is the data plane. After the base exchange, payload traffic flows over an ESP BEET-mode SA (Bound End-to-End Tunnel) that rewrites the inner HIT addresses to outer routable IPs. You get encryption and integrity by default.
• The control plane is layer 3.5. HIP packets ride directly on IP (protocol 139), not on UDP or TCP, which keeps the architecture clean but makes NAT traversal painful — later RFCs added UDP encapsulation as penance.

Why didn't HIP take over the world? The honest answer is deployment economics. Every host needs a HIP stack, applications need to learn that 2001:20::/28 "addresses" aren't routable, and DNS needs HIP RR records (RFC 5205) to bootstrap discovery. QUIC eventually solved the most painful symptom — connection survival across address changes — using connection IDs at the transport layer, requiring no kernel or network changes. HIP solved the deeper problem; QUIC solved the urgent one.

But HIP did not disappear. It runs in industrial networks, in Boeing's avionics, and inside some 3GPP mobility architectures. More importantly, its identifier/locator split idea is the conceptual ancestor of LISP, ILNP, WireGuard's static-key peers, and the cryptographic identity model in modern overlay networks like Tailscale and Nebula. Anyone designing a zero-trust mesh today is, knowingly or not, reinventing pieces of HIP.