translator will see an IPv4-mapped address for the peer and use an IPv4-translatable address for its local address for that communication. When the IPv6-only node sends packets the IPv4-mapped address indicates that the translator needs to translate the packets. When the IPv4 node sends packets those will translated to have the IPv4-translatable address as a destination; it is not possible to use an IPv4-mapped or an IPv4-compatible address as a destination since that would either route the packet back to the translator (for the IPv4-mapped address) or make the packet be encapsulated in IPv4 (for the IPv4-compatible address). Thus this specification introduces the new notion of an IPv4-translatable address.
1.1. Applicability and Limitations
The use of this translation algorithm assumes that the IPv6 network is somehow well connected i.e. when an IPv6 node wants to communicate with another IPv6 node there is an IPv6 path between them. Various tunneling schemes exist that can provide such a path, but those mechanisms and their use is outside the scope of this document.
The IPv6 protocol [IPv6] has been designed so that the TCP and UDP pseudo-header checksums are not affected by the translations specified in this document, thus the translator does not need to modify normal TCP and UDP headers. The only exceptions are unfragmented IPv4 UDP packets which need to have a UDP checksum computed since a pseudo-header checksum is required for UDP in IPv6. Also, ICMPv6 include a pseudo-header checksum but it is not present in ICMPv4 thus the checksum in ICMP messages need to be modified by the translator. In addition, ICMP error messages contain an IP header as part of the payload thus the translator need to rewrite those parts of the packets to make the receiver be able to understand the included IP header. However, all of the translator's operations, including path MTU discovery, are stateless in the sense that the translator operates independently on each packet and does not retain any state from one packet to another. This allows redundant translator boxes without any coordination and a given TCP connection can have the two directions of packets go through different translator boxes.
The translating function as specified in this document does not translate any IPv4 options and it does not translate IPv6 routing headers, hop-by-hop extension headers, or destination options
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