The communications establishment capability of the Session Initiation Protocol is being expanded by the IETF to include establishing network layer connectivity for UDP for a range of scenarios, including where hosts are behind NAT boxes, and host are running IPv6. So far, this work has been limited to UDP because of the assumed impossibility of establishing TCP connections through NAT, and because of the difficulty of predicting port assignments on certain common types of NATs. This paper reports on preliminary success in establishing TCP connections through NAT, and on port prediction. In so doing, we suggest that it may be appropriate for SIP to take a broader architectural role in P2P network layer connectivity for both IPv4 and IPv6
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