The link kernel supports this endpoint flag since v5.15, let's
expose it to user-space. It allows creation on fullmesh topolgy
via MPTCP subflow.

Additionally update the related man-page, clarifying the behavior
of related options.

Acked-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 ip/ipmptcp.c        |  3 ++-
 man/man8/ip-mptcp.8 | 61 ++++++++++++++++++++++++++++++++++-----------
 2 files changed, 49 insertions(+), 15 deletions(-)

diff --git a/ip/ipmptcp.c b/ip/ipmptcp.c
index 0f5b6e2d..433fa68d 100644
--- a/ip/ipmptcp.c
+++ b/ip/ipmptcp.c
@@ -31,7 +31,7 @@ static void usage(void)
 		"	ip mptcp limits show\n"
 		"	ip mptcp monitor\n"
 		"FLAG-LIST := [ FLAG-LIST ] FLAG\n"
-		"FLAG  := [ signal | subflow | backup ]\n");
+		"FLAG  := [ signal | subflow | backup | fullmesh ]\n");
 
 	exit(-1);
 }
@@ -53,6 +53,7 @@ static const struct {
 	{ "signal",		MPTCP_PM_ADDR_FLAG_SIGNAL },
 	{ "subflow",		MPTCP_PM_ADDR_FLAG_SUBFLOW },
 	{ "backup",		MPTCP_PM_ADDR_FLAG_BACKUP },
+	{ "fullmesh",		MPTCP_PM_ADDR_FLAG_FULLMESH },
 };
 
 static void print_mptcp_addr_flags(unsigned int flags)
diff --git a/man/man8/ip-mptcp.8 b/man/man8/ip-mptcp.8
index 22335b61..019debe2 100644
--- a/man/man8/ip-mptcp.8
+++ b/man/man8/ip-mptcp.8
@@ -53,6 +53,8 @@ ip-mptcp \- MPTCP path manager configuration
 .B subflow
 .RB "|"
 .B backup
+.RB "|"
+.B fullmesh
 .RB  "]"
 
 .ti -8
@@ -103,22 +105,41 @@ is a unique numeric identifier for the given endpoint
 
 .TP
 .BR signal
-the endpoint will be announced/signalled to each peer via an ADD_ADDR MPTCP
-sub-option
+The endpoint will be announced/signaled to each peer via an MPTCP ADD_ADDR
+sub-option. Upon reception of an ADD_ADDR sub-option, the peer can try to
+create additional subflows, see
+.BR ADD_ADDR_ACCEPTED_NR.
 
 .TP
 .BR subflow
-if additional subflow creation is allowed by MPTCP limits, the endpoint will
-be used as the source address to create an additional subflow after that
-the MPTCP connection is established.
+If additional subflow creation is allowed by the MPTCP limits, the MPTCP
+path manager will try to create an additional subflow using this endpoint
+as the source address after the MPTCP connection is established.
 
 .TP
 .BR backup
-the endpoint will be announced as a backup address, if this is a
-.BR signal
-endpoint, or the subflow will be created as a backup one if this is a
+If this is a
+.BR subflow
+endpoint, the subflows created using this endpoint will have the backup
+flag set during the connection process. This flag instructs the peer to
+only send data on a given subflow when all non-backup subflows are
+unavailable. This does not affect outgoing data, where subflow priority
+is determined by the backup/non-backup flag received from the peer
+
+.TP
+.BR fullmesh
+If this is a
+.BR subflow
+endpoint and additional subflow creation is allowed by the MPTCP limits,
+the MPTCP path manager will try to create an additional subflow for each
+known peer address, using this endpoint as the source address. This will
+occur after the MPTCP connection is established. If the peer did not
+announce any additional addresses using the MPTCP ADD_ADDR sub-option,
+this will behave the same as a plain
 .BR subflow
-endpoint
+endpoint. When the peer does announce addresses, each received ADD_ADDR
+sub-option will trigger creation of an additional subflow to generate a
+full mesh topology.
 
 .sp
 .PP
@@ -136,17 +157,29 @@ ip mptcp limits set	change the MPTCP subflow creation limits
 .IR SUBFLOW_NR
 specifies the maximum number of additional subflows allowed for each MPTCP
 connection. Additional subflows can be created due to: incoming accepted
-ADD_ADDR option, local
+ADD_ADDR sub-option, local
 .BR subflow
 endpoints, additional subflows started by the peer.
 
 .TP
 .IR ADD_ADDR_ACCEPTED_NR
-specifies the maximum number of ADD_ADDR suboptions accepted for each MPTCP
-connection. The MPTCP path manager will try to create a new subflow for
-each accepted ADD_ADDR option, respecting the
+specifies the maximum number of incoming ADD_ADDR sub-options accepted for
+each MPTCP connection. After receiving the specified number of ADD_ADDR
+sub-options, any other incoming one will be ignored for the MPTCP connection
+lifetime. When an ADD_ADDR sub-option is accepted and there are no local
+.IR fullmesh
+endpoints, the MPTCP path manager will try to create a new subflow using the
+address in the ADD_ADDR sub-option as the destination address and a source
+address determined using local routing resolution
+When
+.IR fullmesh
+endpoints are available, the MPTCP path manager will try to create new subflows
+using each
+.IR fullmesh
+endpoint as a source address and the peer's ADD_ADDR address as the destination.
+In both cases the
 .IR SUBFLOW_NR
-limit.
+limit is enforced.
 
 .sp
 .PP
-- 
2.33.1

Hello:

This patch was applied to iproute2/iproute2-next.git (main)
by David Ahern <dsahern@kernel.org>:

On Fri, 26 Nov 2021 11:35:44 +0100 you wrote:
> The link kernel supports this endpoint flag since v5.15, let's
> expose it to user-space. It allows creation on fullmesh topolgy
> via MPTCP subflow.
> 
> Additionally update the related man-page, clarifying the behavior
> of related options.
> 
> [...]

Here is the summary with links:
  - [iproute2-next] mptcp: add support for fullmesh flag
    https://git.kernel.org/pub/scm/network/iproute2/iproute2-next.git/commit/?id=432cb06b453a

You are awesome, thank you!
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