Information About the NAT Optimized SIP Media Path with SDP Feature
Restrictions for NAT Optimized SIP Media Path with SDP
SIP messages may or may not have SDP. This feature processes SIP messages with SDP only. If a call exchange with SDP is certain to occur, this feature should be used.
Use the “NAT - Optimized SIP Media without SPD” feature for SIP messages without SPD. This feature processes all packets sent through the NAT-enabled router but is more CPU intensive than processing SIP messages with SPD.
Benefits of NAT Optimized SIP Media Path with SDP
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The media path can be shortened, decreasing voice delay.
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More control of voice policy is possible because the media path is closer to the customer domain and not deep within the service provider cloud.
NAT Optimized SIP Media Path with SDP Feature Design
The NAT Optimized SIP Media Path with SDP feature provides the ability to optimize the media path taken by a SIP VoIP session when NAT is used. NAT forces the VoIP traffic to take at least one extra hop in the network, which usually results in several additional hops being added to the path between two IP hosts.
Cisco IOS NAT will add the relevant translation information per SIP session within the SIP protocol messages. The SIP Application Layer Gateway support within Cisco IOS NAT will extract this translation information from the SIP packets and create NAT table entries.
The “piggybacking” of NAT translation information within the SIP call flows, the design of how users interact with the application when they talk to it, will allow the media path of a SIP VoIP session between two calling parties to take the optimized routing path between each other.