element
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... SIP end systems are called
user agents, and intermediate elements are known as proxy servers. A
typical SIP ...
... It is important to note that DNS lookups can be used multiple times
throughout the processing of a call. In general, an element that
wishes to send a request (called a client) may need to perform DNS ...
... transport protocol
of a next hop element, called a server (it can be a proxy or a user
agent). Such processing could, in principle, occur at every hop
...
... proxy or a user
agent). Such processing could, in principle, occur at every hop
between elements.
Since SIP ...
... SIP assures high availability by having upstream elements detect
failures. For example, assume that proxy 2 is implemented as a
...
... intervals between communications between a pair of domains, it is not
generally possible for an element to maintain dynamic availability
state for the proxies ...
... Furthermore, the availability state must eventually be flushed in
order to redistribute load to recovered elements when they come back
online.
...
... online.
It is possible for elements to fail in the middle of a transaction.
For example, after proxy ...
... SIP transaction). That request is sent
to the next element in the list as specified by RFC 2782prop.
...
... request arrived on. However, it is important to provide failover
support when the client element fails between sending the request and
receiving the response.
...
... queries for A or AAAA records with that name. It
tries to send the response to each element on the resulting list of
IP addresses, using the port ...
... resulting list is sorted as described in [2], and the response is
sent to the topmost element on the new list described there. If that
results in a failure, the next entry on the list is tried.
...
... 1], these URIs have to have the
property that they resolve to the specific element that inserted
them. However, if they are constructed with just an IP address, for
...
... sip:1.2.3.4
then should the element fail, there is no way to route the request or
response through a backup.
...
... priority field
(indicating the preferred choice), and this record points to the
specific element that constructed the URI. However, there are
additional records with higher values of the priority ...
... additional records with higher values of the priority field that
point to backup elements that would be used in the event of failure.
This allows the constraint of RFC 3261prop ...