I'm a little confused about terminology in the Host Identity Protocol (HIP)

The two distinct terms Host Identity and Host Identifier are described in https://tools.ietf.org/html/rfc4423#section-3 (section 3.2) as

Host Identity | An abstract concept assigned to a 'computing platform'


Host Identifier | A public key used as a name for a Host Identity.

It is also described earlier

Each host will have at least one Host Identity, but it will typically have more than one. Each Host Identity uniquely identifies a single host; i.e., no two hosts have the same Host Identity. The Host Identity, and the corresponding Host Identifier, can be either public (e.g., published in the DNS) or unpublished. Client systems will tend to have both public and unpublished Identities. There is a subtle but important difference between Host Identities and Host Identifiers. An Identity refers to the abstract entity that is identified. An Identifier, on the other hand, refers to the concrete bit pattern that is used in the identification process.

It is stated "a Host Identity is an abstract concept" which is rather vague. What actually is it? It is also described that an "Host Identity uniquely identifies a single host", but not how. On the other hand a Host Identifier is very concret and easily understandable: It's a public key used as a name (identifier) for a host.

What also confuses me is that two books about HIP I started reading already got in wrong on the first pages. E.g. "Towards the secure mobile Internet": "The transport protocols use a cryptographic host identity instead of IP addresses." (whereas the Host Identifier should be used here, as it is cryptographic)

Also "Host Identity Protocol (HIP): Towards the Secure Mobile Internet Andrei Gurtov 2008 John Wiley & Sons, Ltd. ISBN: 978-0-470-99790-1" states on Chapter 1 Overview Page 1:

"HIP also includes security as an inherent part of its design, because its “host identities” are cryptographic keys that can be used with many established security algorithms"

Am I right in saying the books got in wrong? What actually is a Host Identity?

  • An identifier is what is used to refer to an identity. I think what the IETF means is that what exactly is being identified doesn't have to be known in the protocol, only the identifier itself is manipulated. This being said, in many cases the two words would be interchangeable. It's arguable whether Gurtov is wrong since he is part of the team that invented HIP. – Steve Dodier-Lazaro May 20 '15 at 16:14
  • Yeah my comment ('they got it wrong') was maybe a little offensive there... Of course most likely I am wrong. In "Towards the secure mobile Internet" on chapter 3.2 page 46 is is said: "a pair of self-generated public and private keys provides the Host Identity". So an identifier is the public part and the Identity is the concept of having an cryptographic identifier and validation of it..? – jannikb May 22 '15 at 15:04
  • I don't understand the IETF's stressing of how the two concepts are distinct either, though I'm happy to see that HIP is being standardised because it's a pretty cool concept :-) – Steve Dodier-Lazaro May 22 '15 at 15:05
  • Hipv2 recently got the "proposed standard" status in IETF terminology. So maybe we will see more of that in the future – jannikb May 22 '15 at 15:37

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