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I notice that RFC 6157 updates RFC 3264, but does not mention changes to RFC 3263.
RFC 3263 - locating SIP servers - repeatedly says: "the client performs an A or AAAA record lookup of the domainvname". Note the "or"
RFC 6157, section 3.1 says:
Furthermore, there SHOULD exist both IPv6 and IPv4 DNS entries for outbound proxy servers. This allows the user agent to query DNS and obtain an IP address most appropriate for its use (i.e., an IPv4-only user agent will query DNS for A resource records (RRs), an IPv6-only user agent will query DNS for AAAA RRs, and a dual-stack user agent will query DNS for all RRs and choose a specific network.)
The clause about a dual-stack user agent clearly doesn't follow RFC 3263, since it implies "and" instead of "or". There's no MUST, SHOULD or MAY language applied here, so it seems like this is an oversight - not that RFC 6157 is wrong, but that there should have been a more clear update to RFC 3263.
Nitpicking, but I think it's important to clarify the DNS functionality in regards to dual stacks. In addition, I think there's a need for a BCP to explain how a domain can indicate
preference of address family - ipv4 or ipv6 - by using SRV entries.
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