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In an IMS scenario, the 

The key concept of this feature is that when a UA sends a REGISTER request, it creates a network "flow". A “flow” is a transport-layer association between two hosts that is represented by the network address and port number of both ends and by the transport protocol. For TCP, a flow is equivalent to a TCP connection. For UDP, a flow is a bidirectional stream of datagrams between a single pair of IP addresses and ports of both peers. The proxy can later use this network "flow", whether using a bidirectional stream of UDP datagrams, a TCP/TLS connection, or an analogous concept in another  transport protocol to forward any incoming requests to this UA in the context of the registration.

This outbound support capability also provides a means where the UA can simultaneously register through multiple outbound proxies to the same registrar and continue to receive service when one of the outbound proxies fails or the flow between the

The 

To enable outbound support, use configuration similar to the following example:

% set addressContext default zone ZONE_IAD sipTrunkGroup TG_PCSCF_INT_IP signaling keepSupport enabled outboundSupport enabled
% commit
% set addressContext default zone ZONE_IAD sipTrunkGroup TG_PCSCF_INT_IP services natTraversal outboundTcpKeepaliveTimer 240 outboundUdpKeepaliveTimer 60
% commit