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IPv4 Residual Deployment (4rd) is an IPv6 transition mechanism for Internet service providers for deployment of Internet Protocol version 6 (IPv6), while maintaining IPv4 service to customers. The protocol and sample applications are specified in RFC 7600. ==Features== IPv4 Residual Deployment has three main features: *Mesh topology: between two endpoints, IPv4 packets take the same direct routes as IPv6 packets. *Shared IPv4 addresses: to deal with the unavoidable IPv4-address shortage, several customers can be assigned a common IPv4 address, with disjoint TCP/UDP port sets assigned to each (an application of the general A+P model of RFC 6346). *Stateless operation: conversions of IPv4 packets into IPv6 packets at domain entry, and the reverse at domain exit, are sateless (i.e., one where no per-customer state is needed in domain edge nodes). Compared to other IETF-specified mechanisms having the same main features, i.e., MAP-E (RFC 7597, RFC 7598, RFC 2473) and MAP-T (RFC 7599, RFC 7598, RFC 6145), its distinctive property is that it simultaneously supports: *Full IPv4-fragmentation transparency: with this feature, support of the path MTU Discovery of RFC 4821, recommended in RFC 6349, is preserved. Without it, wherever firewalls filter ICMP packets (as they too commonly do), end systems that support RFC 4821 lose their ability to take advantage of paths that support large packets . *Applicability of IPv6 packet inspections to IPv4: when traversing IPv6-only domains, converted IPv4 packets are ordinary IPv6 packets, with their contents unchanged and valid in IPv6. Thus, IPv6 filters that are performed within the IPv6-only domain, e.g. for Access Control Lists, Web caches, Deep packet inspections, are, implicitly and automatically, effective on domain-traversing IPv4 packets. MAP-E only supports the former, and MAP-T only supports the latter. If an ISP wants to offer residual IPv4 service across an IPv6-only domain, and provides customer-premises equipment to all its customers of this domain, it can chose any of MAP-E, MAP-T, or 4rd, with due awareness that MAP-E and MAP-T are specified in sandards-track RFCs, while 4rd is, at least so far, specified in an experimental-track RFC (see the History section below): the chosen mechanism remains purely internal to each domain. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「IPv4 Residual Deployment」の詳細全文を読む スポンサード リンク
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