Specify /unix. Add /unix-abstract. Add /stream, /seqpacket, /dgram

Based on #174

Author: nabijaczleweli

README.md

@@ -152,6 +152,8 @@ TODO: most of these are way underspecified
 - /ipcidr
 - /dns4, /dns6
 - [/dnsaddr](protocols/DNSADDR.md)
+- [/unix](protocols/unix.md)
+- [/unix-abstract](protocols/unix.md)
 - /tcp
 - /udp
 - /utp

protocols.csv

@@ -2,6 +2,9 @@ code,	size,	name,	comment
 4,	32,	ip4,
 6,	16,	tcp,
 273,	16,	udp,
+7,	0,	stream,
+8,	0,	seqpacket,
+274,	0,	dgram,
 33,	16,	dccp,
 41,	128,	ip6,
 42,	V,	ip6zone,	rfc4007 IPv6 zone
@@ -13,7 +16,8 @@ code,	size,	name,	comment
 132,	16,	sctp,
 301,	0,	udt,
 302,	0,	utp,
-400,	V,	unix,
+400,	V,	unix,	Percent-encoded path to a unix-domain socket
+401,	V,	unix-abstract,	Percent-encoded address of a linux abstract unix-domain socket
 421,	V,	p2p,	preferred over /ipfs
 421,	V,	ipfs,	backwards compatibility; equivalent to /p2p
 444,	96,	onion,
@@ -39,4 +43,4 @@ code,	size,	name,	comment
 280,	0,	webrtc-direct, ICE-lite webrtc transport with SDP munging during connection establishment and without use of a STUN server
 281, 	0,	webrtc, webrtc transport where connection establishment is according to w3c spec
 290,	0,	p2p-circuit,
-777,	V, memory, in memory transport for self-dialing and testing; arbitrary 
+777,	V, memory, in memory transport for self-dialing and testing; arbitrary

protocols/unix.md

@@ -0,0 +1,58 @@
+# `unix`
+
+This protocol encodes a UNIX-domain socket path to a resource. In the string
+representation, the path is encoded in a way consistent with a single URI Path
+segment per [RFC 3986 Section 3.3](https://datatracker.ietf.org/doc/html/rfc3986#autoid-23).
+
+Specifically following the grammar of a single `segment-nz`. In the binary
+representation, no encoding is needed as the value is length prefixed.
+
+When comparing multiaddrs, implementations should compare their binary
+representation to avoid ambiguities over which characters were escaped.
+
+## Examples
+
+The following is a table of examples converting some common UNIX paths to their
+Multiaddr string form.
+
+| Unix Path                   | Multiaddr string form                   |
+| --------------------------- | --------------------------------------- |
+| socket                      | `/unix/socket`                          |
+| /run/unbound.ctl            | `/unix/%2Frun%2Funbound.ctl`            |
+| /run/user/1000/gnupg/S.g... | `/unix/%2Frun%2Fuser%2F1000%2Fgnupg...` |
+| real space                  | `/unix/fake%20space`                    |
+| real/slash                  | `/unix/real%2Fslash`                    |
+| fake%20space                | `/unix/fake%2520space`                  |
+| fake%2Fslash                | `/unix/fake%252Fslash`                  |
+
+## Usage
+
+`/unix` would typically be found at the start of a multiaddr, however it may
+appear anywhere, for example in the case where we route through some sort of
+proxy server or SSH tunnel.
+
+# `unix-abstract`
+
+This protocol encodes a Linux UNIX-domain abstract socket address,
+which are distinguished by their first byte being 0.
+It is encoded the same way as `unix`;
+the marker byte is not part of the path.
+
+## Examples
+
+In the following table, the address column follows the userspace convention
+of 0 bytes in the address being rendered as an `@` for abstract addresses.
+
+| Rendered Address                             | multiaddr string form                                              |
+| -------------------------------------------- | ------------------------------------------------------------------ |
+| @f87a1c847a4ecaf3/bus/systemd/bus-api-system | `/unix-abstract/f87a1c847a4ecaf3%2Fbus%2Fsystemd%2Fbus-api-system` |
+| @/run/fsid.sock@@@@@@@@@@@@@@@@@@@@@@@@@@... | `/unix-abstract/%2Frun%2Ffsid.sock%00%00%00%00%00%00%00%00%00...`  |
+
+# `stream`, `seqpacket`, `dgram`
+
+These correspond to the *type* of UNIX-domain socket:
+`SOCK_STREAM`, `SOCK_SEQPACKET`, `SOCK_DGRAM`.
+
+Previous versions of this specification did not contain these types;
+for compatibility, their absence should not indicate an error,
+if a default makes sense when decoding.