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_service.erl b/lib/diameter/src/base/diameter_service.erl
index a31cef2c8c..7d4a24f7a0 100644
--- a/lib/diameter/src/base/diameter_service.erl
+++ b/lib/diameter/src/base/diameter_service.erl
@@ -128,6 +128,7 @@
              | {share_peers, diameter:remotes()}       %% broadcast to
              | {use_shared_peers, diameter:remotes()}  %% use from
              | {restrict_connections, diameter:restriction()}
+             | {strict_mbit, boolean()}
              | {string_decode, boolean()}
              | {incoming_maxlen, diameter:message_length()}]}).
 %% shared_peers reflects the peers broadcast from remote nodes.
@@ -697,7 +698,8 @@ service_options(Opts) ->
                                                 ?RESTRICT)},
      {spawn_opt, proplists:get_value(spawn_opt, Opts, [])},
      {string_decode, proplists:get_value(string_decode, Opts, true)},
-     {incoming_maxlen, proplists:get_value(incoming_maxlen, Opts, 16#FFFFFF)}].
+     {incoming_maxlen, proplists:get_value(incoming_maxlen, Opts, 16#FFFFFF)},
+     {strict_mbit, proplists:get_value(strict_mbit, Opts, true)}].
 %% The order of options is significant since we match against the list.
 
 mref(false = No) ->