flags: add 'luminar' endpoints support

Currently, upon the reception of an ADD_ADDR (and when the fullmesh flag
is not used), the in-kernel PM will create new subflows using the local
address the routing configuration will pick.

It would be easier to pick local addresses from a selected list of
endpoints, and use it only once, than relying on routing rules.

Use case: both the client (C) and the server (S) have two addresses (a
and b). The client establishes the connection between C(a) and S(a).
Once established, the server announces its additional address S(b). Once
received, the client connects to it using its second address C(b).
Compared to a situation without the 'laminar' endpoint for C(b), the
client didn't use this address C(b) to establish a subflow to the
server's primary address S(a). So at the end, we have:

   C        S
  C(a) --- S(a)
  C(b) --- S(b)

In case of a 3rd address on each side (C(c) and S(c)), upon the
reception of an ADD_ADDR with S(c), the client should not pick C(b)
because it has already been used. C(c) should then be used.

Note that this situation is currently possible if C doesn't add any
endpoint, but configure the routing in order to pick C(b) for the route
to S(b), and pick C(c) for the route to S(c). That doesn't sound very
practical because it means knowing in advance the IP addresses that
will be used and announced by the server.

'laminar', like the idea of laminar flows: the different subflows don't
mix with each other on an endpoint, unlike the "turbulent" way traffic
is mixed by 'fullmesh'.

This new flag is then added to mptcpd as well.

Link: multipath-tcp/mptcp_net-next#503
Link: https://git.kernel.org/netdev/net-next/c/539f6b9de39e
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>

Author: matttbe

etc/mptcpd.conf.in

@@ -37,6 +37,7 @@ path-manager=@mptcpd_default_pm@
 #   signal    (do not use with the "fullmesh" flag)
 #   backup
 #   fullmesh  (do not use with the "signal" flag)
+#   laminar
 #
 # Plugins that deal with the in-kernel path manager may use these
 # flags when advertising addresses.

include/linux/mptcp_upstream.h

@@ -39,6 +39,7 @@
 #define MPTCP_PM_ADDR_FLAG_BACKUP		_BITUL(2)
 #define MPTCP_PM_ADDR_FLAG_FULLMESH		_BITUL(3)
 #define MPTCP_PM_ADDR_FLAG_IMPLICIT		_BITUL(4)
+#define MPTCP_PM_ADDR_FLAG_LAMINAR		_BITUL(5)
 
 struct mptcp_info {
 	__u8	mptcpi_subflows;
@@ -68,7 +69,8 @@ struct mptcp_info {
 	__u64	mptcpi_bytes_received;
 	__u64	mptcpi_bytes_acked;
 	__u8	mptcpi_subflows_total;
-	__u8	reserved[3];
+	__u8	mptcpi_endp_laminar_max;
+	__u8	reserved[2];
 	__u32	mptcpi_last_data_sent;
 	__u32	mptcpi_last_data_recv;
 	__u32	mptcpi_last_ack_recv;

include/mptcpd/types.h

@@ -73,6 +73,9 @@ typedef uint32_t mptcpd_flags_t;
  * @note Do not use with @c MPTCPD_ADDR_FLAG_SIGNAL.
  */
 #define MPTCPD_ADDR_FLAG_FULLMESH (1U << 3)
+
+/// Use this endpoint in reaction to ADD_ADDR, but only once.
+#define MPTCPD_ADDR_FLAG_LAMINAR (1U << 5)
 ///@}
 
 /**

man/mptcpd.8.in

@@ -72,8 +72,9 @@ is a comma separated list containing one or more of the flags
 .IR subflow ,
 .IR signal ,
 .IR backup ,
+.IR fullmesh ,
 and
-.I fullmesh
+.IR laminar
 that plugins that deal with the in-kernel path manager may use when
 advertising addresses, e.g.
 .B --addr-flags=subflow

plugins/path_managers/addr_adv.c

@@ -54,7 +54,8 @@ static void update_limits(struct mptcpd_pm *pm, int delta)
           If the pm creates outgoing subflows, we assume this is
           the client side, and accepts add_addrs from the server.
          */
-        if (pm->config->addr_flags & MPTCPD_ADDR_FLAG_SUBFLOW)
+        if (pm->config->addr_flags &
+            (MPTCPD_ADDR_FLAG_SUBFLOW | MPTCPD_ADDR_FLAG_LAMINAR))
                 _limits[1].limit = _limits[0].limit;
 
         int const result = mptcpd_kpm_set_limits(pm,

src/configuration.c

@@ -121,6 +121,7 @@ static struct tok_entry const addr_flags_toks[] = {
         { MPTCPD_ADDR_FLAG_SIGNAL,   "signal"   },
         { MPTCPD_ADDR_FLAG_BACKUP,   "backup"   },
         { MPTCPD_ADDR_FLAG_FULLMESH, "fullmesh" },
+        { MPTCPD_ADDR_FLAG_LAMINAR,  "laminar"  },
         { 0, NULL },
 };
 

src/netlink_pm_upstream.c

@@ -34,7 +34,8 @@
 #if MPTCPD_ADDR_FLAG_SIGNAL != MPTCP_PM_ADDR_FLAG_SIGNAL                \
         || MPTCPD_ADDR_FLAG_SUBFLOW != MPTCP_PM_ADDR_FLAG_SUBFLOW       \
         || MPTCPD_ADDR_FLAG_BACKUP != MPTCP_PM_ADDR_FLAG_BACKUP         \
-        || MPTCPD_ADDR_FLAG_FULLMESH != MPTCP_PM_ADDR_FLAG_FULLMESH
+        || MPTCPD_ADDR_FLAG_FULLMESH != MPTCP_PM_ADDR_FLAG_FULLMESH     \
+        || MPTCPD_ADDR_FLAG_LAMINAR != MPTCP_PM_ADDR_FLAG_LAMINAR
 # error Mismatch between mptcpd and upstream kernel addr flags.
 #endif