domain
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... SIP "trapezoid", is
shown in Figure 1. In this diagram, a caller in domain A (UA1)
wishes to call Joe in domain B (joe@B). To do so, it communicates
...
... caller in domain A (UA1)
wishes to call Joe in domain B (joe@B). To do so, it communicates
with proxy 1 in its domain ...
... domain B (joe@B). To do so, it communicates
with proxy 1 in its domain (domain A). Proxy 1 forwards the request
...
... Proxy 1 forwards the request
to the proxy for the domain of the called party (domain B), which is
proxy ...
... proxy 1 needs to determine a SIP server
for domain B. To do this, proxy 1 makes use of DNS procedures, using
...
... proxy 1 to discover
the SIP server in domain B, in order to forward the call for joe@B.
The second is for proxy 2 to identify a backup for proxy ...
... IP
address, port, and transport protocol for the server in domain B.
The choice of transport protocol is particularly noteworthy. Unlike
...
... transport protocols for SIP services at domain B, and the relative
preferences of those transport protocols. Proxy ...
... . | | . . | | .
. +-------+ . . +-------+ .
. Domain A . . Domain B .
............................ ..............................
...
... . +-------+ . . +-------+ .
. Domain A . . Domain B .
............................ ..............................
...
... DNS needs to provide the ability
for domain B to configure a set of servers, along with prioritization
and weights, in order to provide a crude level of capacity-based load
balancing.
...
... proxy 2.2. In many cases, proxy 1 will not know which
domains it will ultimately communicate with. That information would
be known when a user actually makes a call to another user in that
domain ...
... domains it will ultimately communicate with. That information would
be known when a user actually makes a call to another user in that
domain. Proxy 1 may never communicate with that domain again after
...
... domain. Proxy 1 may never communicate with that domain again after
the call completes. Proxy 1 may communicate with thousands of
...
... the call completes. Proxy 1 may communicate with thousands of
different domains within a few minutes, and proxy 2 could receive
requests from thousands of different domains ...
... domains within a few minutes, and proxy 2 could receive
requests from thousands of different domains within a few minutes.
Because of this "many-to-many" relationship, and the possibly long
intervals between communications between a pair of domains ...
... domains within a few minutes.
Because of this "many-to-many" relationship, and the possibly long
intervals between communications between a pair of domains, it is not
generally possible for an element to maintain dynamic availability
...
... proxies it will communicate with. When a proxy gets
its first call with a particular domain, it will try the servers in
that domain in some order until it finds one that is available. The
...
... its first call with a particular domain, it will try the servers in
that domain in some order until it finds one that is available. The
identity of the available server would ideally be cached for some
...
... host value of the hostport component of the
URI. It identifies the domain to be contacted. A description of the
SIP and SIPS URIs ...
... SIP+D2X" and "SIPS+D2X", where X is a letter that
corresponds to a transport protocol supported by the domain. This
specification defines D2U for UDP, D2T for TCP ...
...
These NAPTR records provide a mapping from a domain to the SRV record
for contacting a server with the specific transport protocol ...
... UDP.
It is not necessary for the domain suffixes in the NAPTR replacement
field to match the domain ...
... domain suffixes in the NAPTR replacement
field to match the domain of the original query (i.e., example.com
above). However, for backwards compatibility ...
... above). However, for backwards compatibility with RFC 2543(-> 3265prop | 3264prop | 3263prop | 3262prop | 3261prop), a domain
MUST maintain SRV records for the domain ...
... domain
MUST maintain SRV records for the domain of the original query, even
if the NAPTR record ...
... query, even
if the NAPTR record is in a different domain. As an example, even
though the SRV record for TCP ...
... RFC 2543(-> 3265prop | 3264prop | 3263prop | 3262prop | 3261prop) will look up the SRV records for the domain directly. If
these do not exist because the NAPTR replacement points to a
...
... these do not exist because the NAPTR replacement points to a
different domain, the client will fail.
...
... protocol fields, (if the server is using
a site certificate), the domain name in the query and the domain name
...
... certificate), the domain name in the query and the domain name
in the replacement field MUST both be valid based on the site
...
... by the server in the TLS exchange. Similarly,
the domain name in the SRV query and the domain name ...
... attacker could modify the DNS records to contain
replacement values in a different domain, and the client could not
validate ...
... client performs an A or AAAA record lookup of the domain
name. The result will be a list of IP addresses, each of which can
be contacted at the specific port ...
... AAAA record
lookup of the domain name. The result will be a list of IP
addresses, each of which can be contacted using the transport
protocol determined previously, at the default port ...
... stateless proxies, it is
RECOMMENDED that domain administrators make the weights of SRV
records with equal priority ...
... TCP to send the response.
If, however, the sent-by field contained a domain name and a port
number, the server queries for A or AAAA records ...
... before it results in a failure.
If, however, the sent-by field contained a domain name and no port,
the server queries ...
... SRV provides a way to fix this. Instead of using an IP address, a
domain name that resolves to an SRV record can be used:
...
... attack can also be mitigated through caching. A client
which frequently contacts the same domain SHOULD cache whether or not
its NAPTR records ...