Extension to the Session Description Protocol (SDP) for Bypass of Border Gateways

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Last updated 2008-12-17
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This document describes an extension to the Session Description Protocol (SDP) that can be used by systems of cooperating networks using Application Level Gateways (ALG) to insert border gateways performing as Network Address Port Translators (NAPT) between their IP realms to identify when border gateways can be bypassed for more efficient media flow. This extension can be used by networks based on a protocol using the SDP offer/answer model, such as the IP Multimedia Subsystem (IMS) of the Third Generation Partnership Project (3GPP), which is based on the Session Initiation Protocol (SIP). ALGs using this extension can determine within a single SDP offer/answer transaction when the insertion of a new border gateway would cause the media path to re-enter an IP realm visited elsewhere within the media path, and to bypass one or more border gateways that would otherwise be included in the media path. This extension also works with hosted NAPT traversal schemes to establish a direct media path between endpoints within the same IP realm. Optional procedures provide additional means to improve media flow.


Richard Ejzak (ejzak@alcatel-lucent.com)

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