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,
+403,	0,	stream,
+404,	0,	seqpacket,
+405,	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,	Percent-encoded path to a Unix domain socket
+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,

protocols/unix.md

@@ -1,6 +1,6 @@
 # `unix`
 
-This protocol encodes a Unix domain socket path to a resource. In the string
+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).
 
@@ -12,7 +12,7 @@ representation to avoid ambiguities over which characters were escaped.
 
 ## Examples
 
-The following is a table of examples converting some common Unix paths to their
+The following is a table of examples converting some common UNIX paths to their
 Multiaddr string form.
 
 | Unix Path                   | Multiaddr string form                   |
@@ -33,5 +33,29 @@ Multiaddr string form.
 appear anywhere, for example in the case where we route through some sort of
 proxy server or SSH tunnel.
 
-The absence of a `/` character at the start of the decoded address indicates a
-relative path, otherwise the path is absolute.
+# `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
+for display only.
+
+| 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.