Set ZMQ_IDENTITY attribute of router sockets proposal

kernel_routing_id/kernel_routing_id.md

@@ -0,0 +1,25 @@
+# Set ZMQ\_IDENTITY of router sockets
+
+## Current specification
+
+A jupyter kernel uses router sockets to receive messages on the shell and control channel. The current specification
+does not put any constraint on the **ZMQ_IDENTITY** attribute of these sockets. Therefore, most of the implementations
+(if not all) rely on the default behavior of ZMQ which is to set a random value upon connection.
+
+This allows to use these sockets in the DEALER (client) - ROUTER (kernel) or REQ (client) - ROUTER (kernel) patterns,
+but prevents to use the ROUTER (client) - ROUTER (kernel) pattern.
+
+## Proposed enhancement
+
+We propose to set the **ZMQ_IDENTITY** to a value known by the clients, so that it is possible to implement
+a ROUTER - ROUTER pattern. This pattern is particularly useful when a single client wants to talk to many
+kernels and we want to avoid opening a lot of sockets. It is also a perfect solution for a multiplexer component
+that routes messages from many clients to different kernels.
+
+A common pattern is to use the socket's endpoint as its identity. Since the client already knows to which endpoint
+it will connect, this avoids complex additional exchanges between the client and the kernel, or complex additional
+configuration.
+
+## Impact on existing implementations
+
+This requires a minor change in all the existing kernels, but this is a single line change.