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Issue #4: Different levels of support for DNS
queries
Category
Incomplete implementation of the specification
Description
RFC 3263 proposes
DNS procedures that allow a client to resolve a SIP Uniform Resource
Identifier (URI) to the IP address, port,
and transport protocol of the next hop to contact. A high percentage of UAs support only DNS A
query, few others have DNS SRV as an
advanced feature (which needs to be configured manually), and very few have
all of A, SRV and NAPTR queries by
default.
Implications
This difference in support for DNS queries could lead to situations
where two UAs select different transport protocols (e.g., TCP
rather than UDP) for the same next hop server (registrars or proxies). While this behavior should have no problem
in theory, in practice we have seen that SIP signaling over TCP
generally failing in a multi-vendor environment.
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