What do we currently know about the Hydra tail protocol?

Question

The Hydra version that has already been published in the Hydra and Interhead Hydra papers requires all participants to be online at all times. According to this IOHK blog post and this presentation the so-called tail protocol is supposed to make Hydra usable for low-availability clients such as mobile phones.

Beyond a few mentions I couldn't find any explanation on how the Hydra tail protocol will actually work. Do we have an approximate idea how this protocol is going to work or any similar protocols from other blockchains?

Answer 1

There is some information about the Hydra tail protocol on the GitHub repo Wiki

Tail

In a head, all participants must be online - a symmetric construction. In contrast to that, tails are asymmetric: there is a server, which is always online and responsive, with many clients, which are typically offline (e.g. apps on smartphones). The challenge of the tail is, that servers could cheat on the clients (especially when they are offline). The Hydra Tail protocol tries to solve this using two mechanisms: (1) the server needs to put a collatoral on the mainchain, which he could lose when he cheats and (2) a Challenge-Response-Protocol on the mainchain with which clients can dispute claims by a server.

Head-Tail communication

When multiple Tail servers form a Hydra Head together, we can enable clients of one Tail to communicate with clients of another Tail.

Answer 2

We will have a much better sense of the Hydra Tail protocol once the research paper(s) come out. For now, we can infer its functionality based on some of the design patterns of projects currently building on the Hydra family. For example, SingularityNET's HyperCycle sidechain whitepaper discusses CSL, Plutus, and Hydra integrations, including some talk of Hydra Tail.

In short, Hydra Head is a protocol for validator rings - synchronous, permissioned, high-speed execution environments with CSL snapshots. This is a good way for certain sidechains to interface with Cardano, though others like Hypercycle require that some consensus agents may be partially or completely offline (semi-synchronous or asynchronous). Additional protocols - Hydra Tail - are necessary to account for such agents.

Do note however: the authors of the Hypercycle whitepaper mention that the Hydra Tail protocol is "not yet publicly well specified", and further state the intention "to create a custom version of the Hydra tail protocol that suits Hypercycle’s needs, incorporating mechanisms and ideas beyond those that have previous occurred in the Cardano sphere."

TBE when Hydra Tail paper comes out