ipv6: ioam: Documentation for new IOAM sysctls

Add documentation for new IOAM sysctls:
 - ioam6_id and ioam6_id_wide: two per-namespace sysctls
 - ioam6_enabled, ioam6_id and ioam6_id_wide: three per-interface sysctls

Example of IOAM configuration based on the following simple topology:

 _____              _____              _____
|     | eth0  eth0 |     | eth1  eth0 |     |
|  A  |.----------.|  B  |.----------.|  C  |
|_____|            |_____|            |_____|

1) Node and interface IDs can be configured for IOAM:

  # IOAM ID of A = 1, IOAM ID of A.eth0 = 11
  (A) sysctl -w net.ipv6.ioam6_id=1
  (A) sysctl -w net.ipv6.conf.eth0.ioam6_id=11

  # IOAM ID of B = 2, IOAM ID of B.eth0 = 21, IOAM ID of B.eth1 = 22
  (B) sysctl -w net.ipv6.ioam6_id=2
  (B) sysctl -w net.ipv6.conf.eth0.ioam6_id=21
  (B) sysctl -w net.ipv6.conf.eth1.ioam6_id=22

  # IOAM ID of C = 3, IOAM ID of C.eth0 = 31
  (C) sysctl -w net.ipv6.ioam6_id=3
  (C) sysctl -w net.ipv6.conf.eth0.ioam6_id=31

  Note that "_wide" IDs equivalents can be configured the same way.

2) Each node can be configured to form an IOAM domain. For instance,
   we allow IOAM from A to C only (not the reverse path), i.e. enable
   IOAM on ingress for B.eth0 and C.eth0:

  (B) sysctl -w net.ipv6.conf.eth0.ioam6_enabled=1
  (C) sysctl -w net.ipv6.conf.eth0.ioam6_enabled=1

3) An IOAM domain (e.g. ID=123) is defined and made known to each node:

  (A) ip ioam namespace add 123
  (B) ip ioam namespace add 123
  (C) ip ioam namespace add 123

4) Finally, an IOAM Pre-allocated Trace can be inserted in traffic sent
   by A when C (e.g. db02::2) is the destination:

  (A) ip -6 route add db02::2/128 encap ioam6 trace type 0x800000 ns 123
      size 12 dev eth0
Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ioam6-sysctl.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+IOAM6 Sysfs variables
+=====================
+
+
+/proc/sys/net/conf/<iface>/ioam6_* variables:
+=============================================
+
+ioam6_enabled - BOOL
+        Accept (= enabled) or ignore (= disabled) IPv6 IOAM options on ingress
+        for this interface.
+
+        * 0 - disabled (default)
+        * 1 - enabled
+
+ioam6_id - SHORT INTEGER
+        Define the IOAM id of this interface.
+
+        Default is ~0.
+
+ioam6_id_wide - INTEGER
+        Define the wide IOAM id of this interface.
+
+        Default is ~0.

Documentation/networking/ip-sysctl.rst

@@ -1926,6 +1926,23 @@ fib_notify_on_flag_change - INTEGER
         - 1 - Emit notifications.
         - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.
 
+ioam6_id - INTEGER
+        Define the IOAM id of this node. Uses only 24 bits out of 32 in total.
+
+        Min: 0
+        Max: 0xFFFFFF
+
+        Default: 0xFFFFFF
+
+ioam6_id_wide - LONG INTEGER
+        Define the wide IOAM id of this node. Uses only 56 bits out of 64 in
+        total. Can be different from ioam6_id.
+
+        Min: 0
+        Max: 0xFFFFFFFFFFFFFF
+
+        Default: 0xFFFFFFFFFFFFFF
+
 IPv6 Fragmentation:
 
 ip6frag_high_thresh - INTEGER