[PATCH v6 03/26] docs: Add user documentation for tcp_authopt

[Leonard Crestez <cdleonard@xxxxxxxxx>]

The .rst documentation contains a brief description of the user
interface and includes kernel-doc generated from uapi header.

Signed-off-by: Leonard Crestez <cdleonard@xxxxxxxxx>
---
 Documentation/networking/index.rst       |  1 +
 Documentation/networking/tcp_authopt.rst | 51 ++++++++++++++++++++++++
 2 files changed, 52 insertions(+)
 create mode 100644 Documentation/networking/tcp_authopt.rst

diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst
index 03b215bddde8..294b87137cd2 100644
--- a/Documentation/networking/index.rst
+++ b/Documentation/networking/index.rst
@@ -103,10 +103,11 @@ Contents:
    strparser
    switchdev
    sysfs-tagging
    tc-actions-env-rules
    tcp-thin
+   tcp_authopt
    team
    timestamping
    tipc
    tproxy
    tuntap
diff --git a/Documentation/networking/tcp_authopt.rst b/Documentation/networking/tcp_authopt.rst
new file mode 100644
index 000000000000..72adb7a891ce
--- /dev/null
+++ b/Documentation/networking/tcp_authopt.rst
@@ -0,0 +1,51 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=========================
+TCP Authentication Option
+=========================
+
+The TCP Authentication option specified by RFC5925 replaces the TCP MD5
+Signature option. It similar in goals but not compatible in either wire formats
+or ABI.
+
+Interface
+=========
+
+Individual keys can be added to or removed through an TCP socket by using
+TCP_AUTHOPT_KEY setsockopt and a struct tcp_authopt_key. There is no
+support for reading back keys and updates always replace the old key. These
+structures represent "Master Key Tuples (MKTs)" as described by the RFC.
+
+Per-socket options can set or read using the TCP_AUTHOPT sockopt and a struct
+tcp_authopt. This is optional: doing setsockopt TCP_AUTHOPT_KEY is sufficient to
+enable the feature.
+
+Configuration associated with TCP Authentication is global for each network
+namespace, this means that all sockets for which TCP_AUTHOPT is enabled will
+be affected by the same set of keys.
+
+Manipulating keys requires ``CAP_NET_ADMIN``.
+
+Key binding
+-----------
+
+Keys can be bound to remote addresses in a way that is somewhat similar to
+``TCP_MD5SIG``. By default a key matches all connections but matching criteria can
+be specified as fields inside struct tcp_authopt_key together with matching
+flags in tcp_authopt_key.flags. The sort of these "matching criteria" can
+expand over time by increasing the size of `struct tcp_authopt_key` and adding
+new flags.
+
+ * Address binding is optional, by default keys match all addresses
+ * Local address is ignored, matching is done by remote address
+ * Ports are ignored
+
+RFC5925 requires that key ids do not overlap when tcp identifiers (addr/port)
+overlap. This is not enforced by linux, configuring ambiguous keys will result
+in packet drops and lost connections.
+
+ABI Reference
+=============
+
+.. kernel-doc:: include/uapi/linux/tcp.h
+   :identifiers: tcp_authopt tcp_authopt_flag tcp_authopt_key tcp_authopt_key_flag tcp_authopt_alg
-- 
2.25.1