mptcp: clarify when options can be used

RFC8684 doesn't seem to clearly specify which MPTCP options can be used
together.

Some options are mutually exclusive -- e.g. MP_CAPABLE and MP_JOIN --,
some can be used together -- e.g. DSS + MP_PRIO --, some can but we
prefer not to -- e.g. DSS + ADD_ADDR -- and some have to be used
together at some points -- e.g. MP_FAIL and DSS.

We need to clarify this as a base before allowing other modifications.

For example, does it make sense to send a RM_ADDR with an MPC or MPJ?
This remains open for possible future discussions.

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index ab054c3..7345f28 100644 (file)
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -1267,8 +1267,27 @@ void mptcp_write_options(__be32 *ptr, const struct tcp_sock *tp,
        const struct sock *ssk = (const struct sock *)tp;
        struct mptcp_subflow_context *subflow;
 
-       /* DSS, MPC, MPJ, ADD_ADDR, FASTCLOSE and RST are mutually exclusive,
-        * see mptcp_established_options*()
+       /* Which options can be used together?
+        *
+        * X: mutually exclusive
+        * O: often used together
+        * C: can be used together in some cases
+        * P: could be used together but we prefer not to (optimisations)
+        *
+        *  Opt: | MPC  | MPJ  | DSS  | ADD  |  RM  | PRIO | FAIL |  FC  |
+        * ------|------|------|------|------|------|------|------|------|
+        *  MPC  |------|------|------|------|------|------|------|------|
+        *  MPJ  |  X   |------|------|------|------|------|------|------|
+        *  DSS  |  X   |  X   |------|------|------|------|------|------|
+        *  ADD  |  X   |  X   |  P   |------|------|------|------|------|
+        *  RM   |  C   |  C   |  C   |  P   |------|------|------|------|
+        *  PRIO |  X   |  C   |  C   |  C   |  C   |------|------|------|
+        *  FAIL |  X   |  X   |  C   |  X   |  X   |  X   |------|------|
+        *  FC   |  X   |  X   |  X   |  X   |  X   |  X   |  X   |------|
+        *  RST  |  X   |  X   |  X   |  X   |  X   |  X   |  O   |  O   |
+        * ------|------|------|------|------|------|------|------|------|
+        *
+        * The same applies in mptcp_established_options() function.
         */
        if (likely(OPTION_MPTCP_DSS & opts->suboptions)) {
                struct mptcp_ext *mpext = &opts->ext_copy;