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-rw-r--r--lib/diameter/src/base/diameter_capx.erl43
1 files changed, 42 insertions, 1 deletions
diff --git a/lib/diameter/src/base/diameter_capx.erl b/lib/diameter/src/base/diameter_capx.erl
index 837125339a..62b05644b2 100644
--- a/lib/diameter/src/base/diameter_capx.erl
+++ b/lib/diameter/src/base/diameter_capx.erl
@@ -416,7 +416,48 @@ bcaps(N, Caps) ->
%% common_applications/3
%%
%% Identify the (local) applications to be supported on the connection
-%% in question.
+%% in question. The RFC says this:
+%%
+%% 2.4 Application Identifiers
+%%
+%% Relay and redirect agents MUST advertise the Relay Application ID,
+%% while all other Diameter nodes MUST advertise locally supported
+%% applications.
+%%
+%% Taken literally, every Diameter node should then advertise support
+%% for the Diameter common messages application, with id 0, since no
+%% node can perform capabilities exchange without it. Expecting this,
+%% or regarding the support as implicit, renders the Result-Code 5010
+%% (DIAMETER_NO_COMMON_APPLICATION) meaningless however, since every
+%% node would regard the common application as being in common with
+%% the peer. In practice, nodes may or may not advertise support for
+%% Diameter common messages.
+%%
+%% That only explicitly advertised applications should be considered
+%% when computing the intersection with the peer is supported here:
+%%
+%% 5.3. Capabilities Exchange
+%%
+%% The receiver of the Capabilities-Exchange-Request (CER) MUST
+%% determine common applications by computing the intersection of its
+%% own set of supported Application Ids against all of the
+%% Application-Id AVPs (Auth-Application-Id, Acct-Application-Id, and
+%% Vendor-Specific-Application-Id) present in the CER.
+%%
+%% The same section also has the following about capabilities exchange
+%% messages.
+%%
+%% The receiver only issues commands to its peers that have advertised
+%% support for the Diameter application that defines the command.
+%%
+%% This statement is also difficult to interpret literally since it
+%% would disallow D[WP]R and more when Diameter common messages isn't
+%% advertised. In practice, diameter lets requests be sent as long as
+%% there's a dictionary configured to support it, peer selection by
+%% advertised application being possible to preempt by passing
+%% candidate peers directly to diameter:call/4. The peer can always
+%% answer 3001 (DIAMETER_COMMAND_UNSUPPORTED) or 3007
+%% (DIAMETER_APPLICATION_UNSUPPORTED) if this is objectionable.
common_applications(LCaps, RCaps, #diameter_service{applications = Apps}) ->
LA = app_union(LCaps),