One document matched: draft-jesske-dispatch-reason-in-responses-00.xml
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<rfc category="info" docName="draft-jesske-dispatch-reason-in-responses-00"
ipr="pre5378Trust200902">
<front>
<title abbrev="Reason Header">Use of the Reason header filed in Session
Initiation Protocol (SIP) responses</title>
<author fullname="Roland Jesske" initials="R.J." surname="Jesske">
<organization abbrev="Deutsche Telekom">Deutsche Telekom</organization>
<address>
<postal>
<street>Heinrich-Hertz-Strasse 3-7</street>
<city>Darmstadt</city>
<region></region>
<code>64307</code>
<country>Germany</country>
</postal>
<phone>+4961516282766</phone>
<email>r.jesske@telekom.de</email>
</address>
</author>
<author fullname="Laura Liess" initials="L.L." surname="Liess">
<organization abbrev="Deutsche Telekom">Deutsche Telekom</organization>
<address>
<postal>
<street>Heinrich-Hertz-Strasse 3-7</street>
<city>Darmstadt</city>
<region></region>
<code>64307</code>
<country>Germany</country>
</postal>
<phone>+4961516282761</phone>
<email>Laura.Liess@telekom.de</email>
</address>
</author>
<date day="19" month="August" year="2009" />
<area>rai</area>
<workgroup>dispatch</workgroup>
<abstract>
<t>Although the use of the Reason header in responses is considered in
RFC3326, doing so is not specified for any existing response code. This
document specifies the use of the Reason header field in SIP responses
to carry Q.850 reason codes for the failure of an INVITE. Although the
use of the Reason header field in responses is considered in RFC3326,
doing so is not specified for any existing response code. Nonetheless,
the Reason header field has been widely used in responses to carry Q.850
reason codes for failure responses to INVITEs that have been gatewayed
to PSTN systems. This document specifies and formally permits the use of
the Reason header field in SIP responses to carry Q.850 reason codes for
this and other purposes.</t>
</abstract>
</front>
<middle>
<section title="Terminology">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119"></xref>.</t>
<t>This document uses terms from <xref target="RFC3261"></xref>.</t>
</section>
<section title="Overview ">
<t>With introducing the Session Initiation Protocol <xref
target="RFC3261"></xref>into the IP Multimedia Subsystem (IMS) which is
defined by the 3rd Generation Partnership Project the it was required to
interoperate with the PSTN/ISDN. The European Telecommunication
Standards Institute (ETSI) has defined a Next Generation Network (NGN)
where a substantial part of it is based on the IMS.</t>
<t>ETSI has developed a number of requirements to support the usage of
SIP in Next Generation Networks that interoperate, at the service level,
with the Public Switched Telephone Network (PSTN), the Integrated
Services Digital Network (ISDN), the 3GPP IP Multimedia Subsystem (IMS),
and SIP networks and terminals that implement the service logic.</t>
<t>Also ITU-T has specified an interworking between SIP and PSTN/ISDN
networks <xref target="ITU.Q1912.5.2004"></xref> where reason within
responses are allready supported.</t>
<t>In order to provide full support in SIP of existing services,
extensions to SIP are needed.</t>
<t>This document proposes the use of the Reason header field in
responses. This is needed for creating services that must be
interoperable with the PSTN/ISDN network and the interoperability of
traversing communications through SIP not using SIP-I.</t>
<t>The main used case for reason header within responses are
interworking situations with PSTN/ISDN networks where the ISUP cause In
many cases the mapping of specific cause values will result in a generic
SIP Response like it is shown below.</t>
<t><xref target="RFC3398"></xref> and other Interworking specifications
like <xref target="3GPP.29.163"></xref> are describing the mapping of
ISUP Cause Values to SIP and vice versa. Looking on the specific mapping
shows that information will be lost when the call traverses ISUP without
using SIP-T.</t>
<t>Example:</t>
<t><xref target="RFC3398"></xref> describes the mapping of following
ISUP Causes to 503 and 408 like follows.</t>
<t><figure>
<preamble></preamble>
<artwork><![CDATA[
ISUP Cause value SIP response
---------------- ------------
34 no circuit available 503 Service unavailable
38 network out of order 503 Service unavailable
41 temporary failure 503 Service unavailable
42 switching equipment congestion 503 Service unavailable
47 resource unavailable 503 Service unavailable
58 bearer capability not presently 503 Service unavailable
Available
88 incompatible destination 503 Service unavailable
18 no user responding 408 Request Timeout
The mapping back is shown as follows:
Response received Cause value in the REL
----------------- ----------------------
503 Service unavailable 41 Temporary failure
408 Request timeout 102 Recovery on timer
expiry
]]></artwork>
<postamble></postamble>
</figure></t>
<t>The Example with 503 shows that a couple of different ISUP Cause
values are interworked to only one SIP response. With 408 the meaning of
the release cause is changed when interworked back to ISUP. Also
Services built on Cause 18 (e.G. a 2nd call attempt on an other number,
this service is like a sequential forking) will not work.</t>
</section>
<section title="Requirements">
<t>REQ-1:</t>
<t>It should be possible to support PSTN-SIP-PSTN scenarios where the
reason of a call release can be transferred though the SIP domain
without any loss of information and no change of reason.</t>
<t>REQ-2:</t>
<t>It should be possible to provide correct announcements to a SIP user
based on the reason for call clearing within the PSTN network or the
PSTN user. The PSTN does normally not provide announcements to
originating user when clearing the call.</t>
<t>REQ-3:</t>
<t>A UA may have the ability to display ISUP specific release causes or
show a equivalent text. A inclusion of <xref target="Q.850"></xref>
causes is out of scope.</t>
<t>REQ-4:</t>
<t>A application server providing specific PSTN like services may have
the possibility to include ISUP specific release causes</t>
<section title="Used Cases for the requirements">
<t>REQ-1 shows the scenario between two ISUP Gateways where SIP-I is
not a solution to be used. A IMS network defined by 3GPP where SIP-I
is not part of the framework is such an example. A used case is shown
in Figure 2.</t>
<t>REQ-2 identifies the scenario the gateway passes on the cause and
some announcement server translates the cause into the appropriate
announcement.</t>
<t>REQ-3 excludes that a SIP UA includes <xref target="Q.850"></xref>
cause values because that is not needed due to the different context
of a SIP UA and of course the existing SIP response codes. Also the
possibility of displaying Q.850 causes should be used very restrictive
and is not recommended. A used case could be a used case could be a
Integrated Access Device where a POTS Phone is connected to. It is
recommended to ignore the cause value if received be a UA.</t>
<t>REQ-4: allows an Apllication Server that is providing specific
Services simulating PSTN services to include Q.850 Causes. This is
needed specific for IMS operartors which are substituting their PSTN
to IMS. These operators may run Services in the SIP domain that are
provided to PSTN endusers.</t>
</section>
</section>
<section title="Overall Applicability">
<t>The SIP procedures specified in this document are foreseen for
networks providing simulation services and/or interworking to the
PSTN/ISDN.</t>
<t>The document is describing the use of the Reason header in SIP
responses. These procedures are only valuable if the reason contained in
the element "protocol" is "Q.850". A inclusion of a SIP reason
(protocol="SIP") is not helpful due to the fact that the response
already provides the SIP reason. The Release Causes are described within
<xref target="Q.850"></xref>. (Note: The ETSI specifications can be
downloaded under http://pda.etsi.org/pda/queryform.asp free of
charge.)</t>
<t>The appearance of the Reason header is applicable to final responses
4xx, 5xx and 6xx and in addition for 199 Responses as defined within
<xref target="I-D.ietf-sipcore-199"></xref>.</t>
</section>
<section title="Procedures">
<section title="Procedures at the UA">
<t>A UA that supports the Reason header field can process the <xref
target="Q.850"></xref> Cause Value and display it or an equivalent
text. The inclusion of a Reason header field by UA is only for B2B UA
interworking with the PSTN/ISDN or providing services foreseen.</t>
</section>
<section title="Procedures at a SIP proxy">
<t>SIP proxies that receive a response containing a Reason header
field is forwarding the response without changing the reason.</t>
<t>A SIP proxy receiving a request that includes a Reason header field
can route the request to an application server for further analysis
and base services on it.</t>
<t>Based on network policy a Proxy can remove a Reason header field
send from a UAC.</t>
</section>
<section title="Procedures at an application server">
<t>An application server that receives a SIP request that contains a
response including a Reason header MAY analyze the SIP Reason and base
further procedures on this analyses.</t>
<t>For Example the application server could use the reason for sending
a announcement towards the originating entity of the session.</t>
<t>As an example the Anonymous Communication Rejection (ACR) service
defined by ETSI Telecommunications and Internet converged Services and
Protocols for Advanced Networking (TISPAN)</t>
</section>
</section>
<section title="Procedures at an interworking point with ISUP">
<t>For interoperability reasons the Q.850 Cause Value of a Release shall
be mapped to the Reason Header.</t>
</section>
<section title="Example">
<t>Figure 1 shows the example of SIP interworking with the PSTN/ISDN.
Cause #87 is sent when the connecting user is not member of a Closed
User Group.</t>
<figure anchor="figur1" title="ISUP-SIP Call ">
<artwork><![CDATA[
A Gateway Proxy AS
| IAM | | |
|------------------>| INVITE | |
| |----------------->| INVITE |
| | 100 Trying |----------------->|
| |<-----------------| 100 Trying |
| | |<-----------------|
| ACK SDP held | | |
|<------------------| | 603 Decline |
| | 603 Decline | Reason Q850 #87 |
| | Reason Q850 #87 | |
| REL Cause #87 | |<-----------------|
| |<-----------------| |
|<----------------- | | |
| | | |
| | | |
| | | |
]]></artwork>
</figure>
<t>Figure 2 shows the example where the SIP network is used as transit
between PSTN/ISDN networks. This avoids that the Mapping back to the
Q.850 cause within ISUP change the meaning of the reason for release of
the call.</t>
<figure anchor="figur2" title="Transit case ">
<artwork><![CDATA[
A Gateway Gateway B
| IAM | | |
|------------------>| INVITE | |
| |----------------->| IAM |
| | 100 Trying |----------------->|
| |<-----------------| 100 Trying |
| | 603 Decline | |
| | Reason Q850 #87 | REL Cause #87 |
| REL Cause #87 | |<-----------------|
| |<-----------------| |
|<----------------- | | |
| | | |
| | | |
| | | |
]]></artwork>
</figure>
<t>Figure 3 shows the example where the SIP network puts an announcement
towards the UAB. The AS sends an announcement with a specific text back.
After some Time the Response will be sent back to the UA A and closes
all open transactions. With this possibility the SIP user can informed
with more specific information than only the Response code.</t>
<figure anchor="figur3" title="Transit case ">
<artwork><![CDATA[
A AS Gateway B
| INVITE | | |
|------------------>| INVITE | |
| |----------------->| IAM |
| | 100 Trying |----------------->|
| |<-----------------| |
| | 503 Decline | |
| | Reason Q850 #41 | REL Cause #41 |
| | |<-----------------|
| Announcement |<-----------------| |
|< ================ | | |
| | | |
| 503 after Timeout| | |
|<----------------- | | |
]]></artwork>
</figure>
<t>Figure 3: Call Release within the PSTN with an announce played within
the SIP network.</t>
</section>
<section title="Security Considerations">
<t>The presence of the Reason header in a response does not affect the
treatment of the response.</t>
<t>Including such a header by an untrusted entity could adulterate the
reactions of the originating entities. E.G. sending back a cause value
"87" can cause an announcement within the PSTN/ISDN saying that the call
was rejected due to the Closed User Group service.</t>
<t>Therefore it is RECOMMENDED to include the Reason header information
in Responses only by trusted entities as it is described within <xref
target="RFC3325"></xref>.</t>
</section>
<section title="IANA Considerations ">
<t>This document describes the use of the Reason header field described
within <xref target="RFC3326"></xref> . No additional SIP elements are
defined within this document. Therefore, this document does not provide
any action to IANA.</t>
</section>
<section title="Acknowledgments">
<t>The author would like to thank the members of the ETSI TISPAN WG3 for
their comments to this memo.</t>
</section>
</middle>
<back>
<references title="Normative References">
&rfc2119;
&rfc3261;
&rfc3325;
&rfc3326;
&rfc3398;
&threeGPP_29_163;
&draft-ietf-sipcore-199-00;
<reference anchor="ITU.Q1912.5.2004">
<front>
<title>Interworking between Session Initiation Protocol (SIP) and
Bearer Independent Call Control protocol or ISDN User Part [ITU-T
Recommendation Q.1912.5 (2005)]</title>
<author>
<organization>International Telecommunications
Union</organization>
</author>
<date month="April" year="2004" />
</front>
<seriesInfo name="ITU" value="Recommendation Q.1912.5" />
</reference>
<reference anchor="Q.850">
<front>
<title>Usage of cause and location in the Digital Subscriber
Signalling System No. 1 and the Signalling System No. 7 ISDN User
Part [ITU-T Recommendation Q.850]</title>
<author>
<organization></organization>
</author>
<date month="April" year="1998" />
</front>
<seriesInfo name="ITU" value="Recommendation Q.850" />
</reference>
</references>
</back>
</rfc>
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