wip: heuristic for browser UI

Author: lidel

docs/heuristic-for-browser-ui.md

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+# Implementation Notes for Browsers and URI parsers
+
+Below is the suggested heuristic for detecting content-addressed resources
+on the web, and presenting additional user interface for interacting with them.
+
+TBD if this document  will live in this repo, https://docs.ipfs.tech or become part of https://github.com/ipfs/specs/
+
+## Detecting IPFS Resources and Hints
+
+```mermaid
+graph TD
+    Start[Top-level navigation event]
+    IsGateway{Is it a Gateway URL?<br/>foo.tld/ip?s/id/..<br/>id.ns.foo.com/..}
+
+    Start -->|new URL in address bar| IsGateway
+    IsGateway -->|YES| IsPathOrSubdomain
+    IsPathOrSubdomain -->|Path Gateway| ExtractFromPath
+    IsPathOrSubdomain -->|Subdomain Gateway| ExtractFromSubdomain
+
+    subgraph GwURLToPath [Convert Gateway URL to a Content Path]
+        IsPathOrSubdomain{Is the Gateway URL 'path' <br/> or 'subdomain' based?}
+        ExtractFromPath[Extract URL.pathname]
+        ExtractFromSubdomain[Read namespace and identifier from <br/> URL.hostname and  prepend to URL.pathname. <br/> E.g., turn id.ns.foo.com/pathname <br/> into /ns/id/pathname]
+    end
+
+    PotentialContentPath[Found a Potential Content Path]
+
+    ExtractFromPath --> PotentialContentPath
+    ExtractFromSubdomain --> PotentialContentPath
+    PotentialContentPath -->|/ipxs/id/sub/path..| IsIpfsOrIpns
+
+    subgraph PathValidation [Is the Content Path valid?]
+        IsIpfsOrIpns{Is it /ipfs/ or /ipns/?}
+        ValidateRootCID{Is 'id' is a valid CID?}
+        ValidateIPNSKey{Is 'id' is a valid CID <br/> with libp2p-key codec?}
+        ValidateDNSLink{Is 'id' a DNSLink name <br/>  with TXT record? <br/> dnslink=/ipfs/<br/>  or =/ipns/}
+
+        IsIpfsOrIpns --->|/ipfs/id/..| ValidateRootCID
+        IsIpfsOrIpns -->|/ipns/id/..| ValidateIPNSKey
+        ValidateIPNSKey -->|NO| ValidateDNSLink
+    end
+
+    subgraph FoundValidIPFSContent [Confirmed we've found a Valid IPFS Resource]
+        ValidIPFSCID[Valid /ipfs/cid/.. Content Path]
+        ValidIPNSKey[Valid /ipns/key/.. Content Path]
+        ValidDNSLink[Valid /ipns/dnslink/.. Content Path]
+
+        ValidateRootCID --->|YES, /ipfs/cid/..| ValidIPFSCID
+        ValidateIPNSKey --->|YES, /ipns/key/..| ValidIPNSKey
+        ValidateDNSLink --->|YES, /ipfs/dnslink/..| ValidDNSLink
+    end
+
+    subgraph NonGwURLToPath [See if non-Gateway URL has a Content Path]
+        IsCachedDNSLink{Does URL.hostname <br/> match a cached <br/> DNSLink domain?}
+        IsHeaderPresent{Is X-Ipfs-Path <br/> header present?}
+        IsError{Did request fail? <br/> HTTP error>500 <br/> or network error}
+        IsDNSLinkAtHostname{Does DNSLink exists <br/> at URL.hostname?<br/>/ipns/example.com}
+        PathFromHeader[Read value from X-Ipfs-Path]
+    end
+
+    IsGateway -->|NO| IsCachedDNSLink
+    IsCachedDNSLink -->|YES| ValidDNSLink
+    IsCachedDNSLink -->|NO| IsHeaderPresent
+    IsHeaderPresent -->|YES| IsDNSLinkAtHostname
+    IsHeaderPresent -->|NO| IsError
+    IsError -->|YES| IsDNSLinkAtHostname
+    IsDNSLinkAtHostname -->|YES| ValidDNSLink
+    IsDNSLinkAtHostname -->|NO| PathFromHeader
+    PathFromHeader --> PotentialContentPath
+```
+
+## What to do with detected Content Paths?
+
+### Immutable `/ipfs/cid/..`
+
+- Display "Open via IPFS" button in UI
+  - Clicking it should open `ipfs://cid/path?query#hash` (preserving any `?query` or `#hash` from the original HTTP URL)
+- If IPFS Gateway Redirect is enabled, and the HTTP URL was a gateway one, redirect automatically to `ipns://dnslink/path?query#hash`
+
+### Mutable `/ipns/key/..`
+- Display "Open via IPFS" button in UI
+  - Clicking it should open `ipns://key/path?query#hash` (preserving any `?query` or `#hash` from the original HTTP URL)
+- If IPFS Gateway Redirect is enabled, and the original HTTP URL was a gateway one, redirect automatically to `ipns://dnslink/path?query#hash`
+
+### Mutable `/ipns/dnslink/..`
+
+- Display "Open via IPFS" button in UI
+  - Clicking it should open `ipns://dnslink/path?query#hash` (preserving `?query` or `#hash` from the original HTTP URL)
+- If DNSLink Website redirect is enabled, redirect automatically to `ipns://dnslink/path?query#hash`
+- It is a good practice to internally cache the fact that domain has a valid DNSLink.
+  - TTL from TXT record can be used as a hint when to expire cache entry.
+  - Performance can be improved further by using cached flag and revalidating it asynchronously, without blocking browser navigation.