Add service_opt() strict_mbit

There are differing opinions on whether or not reception of an arbitrary
AVP setting the M-bit is an error. 1.3.4 of RFC 6733 says this about
how an existing Diameter application may be modified:

   o  The M-bit allows the sender to indicate to the receiver whether or
      not understanding the semantics of an AVP and its content is
      mandatory.  If the M-bit is set by the sender and the receiver
      does not understand the AVP or the values carried within that AVP,
      then a failure is generated (see Section 7).

   It is the decision of the protocol designer when to develop a new
   Diameter application rather than extending Diameter in other ways.
   However, a new Diameter application MUST be created when one or more
   of the following criteria are met:

   M-bit Setting

      An AVP with the M-bit in the MUST column of the AVP flag table is
      added to an existing Command/Application.  An AVP with the M-bit
      in the MAY column of the AVP flag table is added to an existing
      Command/Application.

The point here is presumably interoperability: that the command grammar
should specify explicitly what mandatory AVPs much be understood, and
that anything more is an error.

On the other hand, 3.2 says thus about command grammars:

   avp-name         = avp-spec / "AVP"
                      ; The string "AVP" stands for *any* arbitrary AVP
                      ; Name, not otherwise listed in that Command Code
                      ; definition.  The inclusion of this string
                      ; is recommended for all CCFs to allow for
                      ; extensibility.

This renders 1.3.4 pointless unless "*any* AVP" is qualified by "not
setting the M-bit", since the sender can effectively violate 1.3.4
without this necessitating an error at the receiver. If clients add
arbitrary AVPs setting the M-bit then request handling becomes more
implementation-dependent.

The current interpretation in diameter is strict: if a command grammar
doesn't explicitly allow an AVP setting the M-bit then reception of such
an AVP is regarded as an error. The strict_mbit option now allows this
behaviour to be changed, false turning all responsibility for the M-bit
over to the user.

1 files changed, 3 insertions, 1 deletions

diff --git a/lib/diameter/src/base/diameter_traffic.erl b/lib/diameter/src/base/diameter_traffic.erl
index 230a05fa11..6be2c35d48 100644
--- a/lib/diameter/src/base/diameter_traffic.erl
+++ b/lib/diameter/src/base/diameter_traffic.erl
@@ -79,6 +79,7 @@
          apps         :: [#diameter_app{}],
          sequence     :: diameter:sequence(),
          codec        :: [{string_decode, boolean()}
+                          | {strict_mbit, boolean()}
                           | {incoming_maxlen, diameter:message_length()}]}).
 %% Note that incoming_maxlen is currently handled in diameter_peer_fsm,
 %% so that any message exceeding the maximum is discarded. Retain the
@@ -108,7 +109,8 @@ make_recvdata([SvcName, PeerT, Apps, SvcOpts | _]) ->
               sequence = Mask,
               codec = [T || {K,_} = T <- SvcOpts,
                             lists:member(K, [string_decode,
-                                             incoming_maxlen])]}.
+                                             incoming_maxlen,
+                                             strict_mbit])]}.
 
 %% ---------------------------------------------------------------------------
 %% peer_up/1