SIP
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... management of
communications sessions between users. SIP end systems are called
user agents, and intermediate elements ...
... elements are known as proxy servers. A
typical SIP configuration, referred to as the SIP "trapezoid", is
shown in Figure 1. In this diagram, a caller ...
... proxy servers. A
typical SIP configuration, referred to as the SIP "trapezoid", is
shown in Figure 1. In this diagram, a caller in domain ...
... As part of this call flow, proxy 1 needs to determine a SIP server
for domain B. To do this, proxy ...
... NAPTR [3] records. This document describes the
specific problems that SIP uses DNS to help solve, and provides a
solution.
...
... described in the Introduction. The first is for proxy 1 to discover
the SIP server in domain B, in order to forward the call for joe@B.
The second is for proxy ...
... The choice of transport protocol is particularly noteworthy. Unlike
many other protocols, SIP can run over a variety of transport
protocols, including TCP, UDP ...
... proxy 1 to discover the available
transport protocols for SIP services at domain B, and the relative
...
... ............................ ..............................
Figure 1: The SIP trapezoid
It is important to note that DNS lookups ...
... elements.
Since SIP is used for the establishment of interactive communications
services, the time it takes to complete a transaction between a
...
... Scalability and high availability are important in SIP. SIP services
scale up through clustering techniques. Typically, in a realistic
...
... identify a backup for proxy 1 that it can send the response to. This
problem is more realistic in SIP than it is in other transactional
protocols. The reason is that some SIP responses can take a long
...
... problem is more realistic in SIP than it is in other transactional
protocols. The reason is that some SIP responses can take a long
time to be generated, because a human user frequently needs to be
...
... The procedures here are invoked when a client needs to send a request
to a resource identified by a SIP or SIPS (secure SIP) URI. This URI ...
... client needs to send a request
to a resource identified by a SIP or SIPS (secure SIP) URI. This URI
...
... 1], the request can be sent to a
specific intermediate proxy not identified by a SIP URI, but rather,
by a hostname or numeric IP address. In that case, a temporary URI ...
... next-hop proxy. As a result, in all cases, the
problem boils down to resolution of a SIP or SIPS URI in DNS to
...
... transaction is as defined in [1]. That is, once a SIP server has
successfully been contacted (success is defined below), all
retransmissions ...
... successfully been contacted (success is defined below), all
retransmissions of the SIP request and the ACK for non-2xx SIP
...
... retransmissions of the SIP request and the ACK for non-2xx SIP
responses to INVITE MUST be sent to the same host ...
... INVITE MUST be sent to the same host. Furthermore, a
CANCEL for a particular SIP request MUST be sent to the same SIP
server that the SIP request ...
... host. Furthermore, a
CANCEL for a particular SIP request MUST be sent to the same SIP
server that the SIP request was delivered to.
...
... SIP request MUST be sent to the same SIP
server that the SIP request was delivered to.
Because the ACK ...
... URI. It identifies the domain to be contacted. A description of the
SIP and SIPS URIs and a definition of these parameters can be found
in [1 ...
... 2543(-> 3265prop | 3264prop | 3263prop | 3262prop | 3261prop) [6], and thus
the only one guaranteed to be interoperable for a SIP URI. It was
also specified as the default transport in RFC 2543(-> 3265prop | 3264prop | 3263prop | 3262prop | 3261prop) ...
... 2543(-> 3265prop | 3264prop | 3263prop | 3262prop | 3261prop) when no transport
was present in the SIP URI. However, another transport, such as TCP,
...
... transport, such as TCP,
MAY be used if the guidelines of SIP mandate it for this particular
request. That is the case, for example, for requests that exceed the
path MTU ...
... NAPTR service fields
with values "SIP+D2X" and "SIPS+D2X", where X is a letter that
corresponds to a transport protocol supported by the domain ...
... service field. The
converse is not true, however. A client resolving a SIP URI SHOULD
retain records with "SIPS" as the protocol, if the client supports
...
... IN NAPTR 100 50 "s" "SIP+D2U" "" _sip._udp.example.com.
This indicates that the server supports TLS ...
... SRV 0 2 5060 server2.example.com
If a SIP proxy, redirect server, or registrar is to be contacted
...
... lookup of NAPTR records, there MUST be at least three
records - one with a "SIP+D2T" service field, one with a "SIP+D2U"
...
... records - one with a "SIP+D2T" service field, one with a "SIP+D2U"
service field, and one with a "SIPS+D2T" service field ...
... with SIPS as the protocol in the service field SHOULD be preferred
(i.e., have a lower value of the order field) above records with SIP
as the protocol in the service field. A record ...
... transport protocol,
such as TCP, MAY be used if the guidelines of SIP mandate it for this
particular request. That is the case, for example, for requests that
exceed the path MTU ...
... identifier "_sips" for SIPS URIs. For a
SIP URI, if the client wishes to use TLS, it also uses the service ...
... service)" without
giving any details on what happens in the event of failure. Those
details are described here for SIP.
For SIP requests ...
...
has a different value of the Via branch ID than the previous (and
therefore constitutes a new SIP transaction). That request is sent
to the next element ...
... "session identifier" (a complete dialog identifier in SIP terms), a
new dialog would be created identically at each server. This could,
...
...
In many cases, an element needs to construct a SIP URI for inclusion
in a Contact header in a REGISTER ...
... context, frequently on a business card or secure web page, or within
a SIP message which has already been secured with TLS. See RFC 3261prop
...
... is truly needed, but we allow TLS to be used for requests resolved by
a SIP URI to allow security that is better than no TLS at all.
...
... is called the Transport Determination Application, and its goal is to
map an incoming SIP or SIPS URI to a set of SRV records for the
...
... from the AUS. For this application, the first well known rule
extracts the host portion of the SIP or SIPS URI.
...
... for the service fields for each transport supported by SIP. The
table of mappings from service field values to transport protocols ...
... a new fictitious transport protocol called NCTP might be
"SIP+D2N".
Protocol: The specific transport protocol ...
... Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261prop, June 2002. ...
... Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg, "SIP: Session Initiation Protocol", RFC 2543(-> 3265prop | 3264prop | 3263prop | 3262prop | 3261prop), March 1999. ...