RSIP (Realm-Specific Internet Protocol)
Like NAT, the experimental protocol RSIP translates between public and private IP addresses. But instead of requiring a boundary router to translate, RSIP uses a simple protocol between a user's desktop PC and a boundary router to perform preparatory signaling. Through this signaling, the PC is able to prepare each packet in a way that removes the translation burden.
The RSIP protocol works via a simple challenge-response structure, and employs a vocabulary consisting of "parameters" and "messages."
Operation begins when RSIP client software in a PC signals the RSIP server software in a boundary router or gateway. Through this exchange, the RSIP client requests a public IP address, plus one or more of the router/gateway's ports.
In reply, the router/gateway's RSIP server software assigns a public IP address and one or more port numbers, in addition to lease time, tunnel type and other parameters.When the packet hits the RSIP server/ gateway, the packet's uniqueness is identified by the combination of assigned IP address and port numbers.
As with NAT, the RSIP server uses a reserved IP address, such as 10.0.0.4, for its own internal-enterprise addressing scheme. But unlike NAT, the boundary device gateway does not have to possess the intelligence to perform the translation; instead, the RSIP server/gateway sees the information it needs in the packet header, then consults its RSIP table to determine where the packet should go.
From Protocol helps stretch IPv4 addresses, Network World Tech Update, 01/17/00.
NOTE: The Internet Engineering Steering Group states:
The set of documents describing the RSIP technology imply significant host and gateway changes for a complete implementation. In addition, the floating of port numbers can cause problems for some applications, preventing an RSIP-enabled host from interoperating transparently with existing applications in some cases (e.g., IPsec). Finally, there may be significant operational complexities associated with using RSIP. ... Accordingly, the costs and benefits of using RSIP should be carefully weighed against other means of relieving address shortage.
Additional resources
Realm Specific IP: Framework
RFC 3102.
Realm Specific IP: Protocol Specification
RFC 3103.
RSIP Support for End-to-end IPsec
RFC 3104.
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