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, 6 insertions, 4 deletions

diff --git a/lib/diameter/src/base/diameter_codec.erl b/lib/diameter/src/base/diameter_codec.erl
index f900bb0c5e..aab8b5887e 100644
--- a/lib/diameter/src/base/diameter_codec.erl
+++ b/lib/diameter/src/base/diameter_codec.erl
@@ -76,9 +76,10 @@ setopts(Opts)
   when is_list(Opts) ->
     lists:foreach(fun setopt/1, Opts).
 
-%% Decode stringish types to string()? The default true is for
-%% backwards compatibility.
-setopt({string_decode = K, false = B}) ->
+%% The default string_decode true is for backwards compatibility.
+setopt({K, false = B})
+  when K == string_decode;
+       K == strict_mbit ->
     setopt(K, B);
 
 %% Regard anything but the generated RFC 3588 dictionary as modern.
@@ -96,7 +97,8 @@ setopt(Key, Value) ->
 
 getopt(Key) ->
     case get({diameter, Key}) of
-        undefined when Key == string_decode ->
+        undefined when Key == string_decode;
+                       Key == strict_mbit ->
             true;
         undefined when Key == rfc ->
             6733;