[RFCv3 02/15] docs: Add user documentation for tcp_authopt

[Leonard Crestez <cdleonard@gmail.com>]

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@gmail.com>
---
 Documentation/networking/index.rst       |  1 +
 Documentation/networking/tcp_authopt.rst | 44 ++++++++++++++++++++++++
 2 files changed, 45 insertions(+)
 create mode 100644 Documentation/networking/tcp_authopt.rst

diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst
index 58bc8cd367c6..f5c324a060d8 100644
--- a/Documentation/networking/index.rst
+++ b/Documentation/networking/index.rst
@@ -100,10 +100,11 @@ 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..484f66f41ad5
--- /dev/null
+++ b/Documentation/networking/tcp_authopt.rst
@@ -0,0 +1,44 @@
+.. 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 from a 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 indepedently attached to
+each TCP socket. After listen and accept the newly returned socket gets an
+independent copy of relevant settings from the listen socket.
+
+Key binding
+-----------
+
+Keys can be bound to remote addresses in a way that is similar to TCP_MD5.
+
+ * The full address must match (/32 or /128)
+ * Ports are ignored
+ * Address binding is optional, by default keys match all addresses
+
+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