Updating Messaging ch file to RST

Migrating only Messaging ch file
Renamed ch_messaging.xml to messaging.rst
Follow Up: convert Messaging sections

Change-Id: I582a4f8ba20fdde2f73c60ef9cc8ba0644936e54

security-guide-rst/source/messaging.rst

@@ -1,3 +1,35 @@
 ===============
-Message Queue
+Message queuing
 ===============
+
+Message queuing services facilitate inter-process communication in
+OpenStack. OpenStack supports these message queuing service back ends:
+
+-  RabbitMQ
+
+-  Qpid
+
+-  ZeroMQ or 0MQ
+
+Both RabbitMQ and Qpid are Advanced Message Queuing Protocol (AMQP)
+frameworks, which provide message queues for peer-to-peer communication.
+Queue implementations are typically deployed as a centralized or
+decentralized pool of queue servers. ZeroMQ provides direct peer-to-peer
+communication through TCP sockets.
+
+Message queues effectively facilitate command and control functions
+across OpenStack deployments. Once access to the queue is permitted no
+further authorization checks are performed. Services accessible through
+the queue do validate the contexts and tokens within the actual message
+payload. However, you must note the expiration date of the token because
+tokens are potentially re-playable and can authorize other services in
+the infrastructure.
+
+OpenStack does not support message-level confidence, such as message
+signing. Consequently, you must secure and authenticate the message
+transport itself. For high-availability (HA) configurations, you must
+perform queue-to-queue authentication and encryption.
+
+With ZeroMQ messaging, IPC sockets are used on individual machines.
+Because these sockets are vulnerable to attack, ensure that the cloud
+operator has secured them.