SIP
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... multimedia sessions include multimedia
conferences, Internet telephony and similar applications. SIP is one
of the key protocols used to implement Voice over IP (VoIP ...
... network cannot exist in isolation from
traditional telephone networks. It is vital for a SIP telephony
network to interwork with the PSTN ...
... The popularity of gateways that interwork between the PSTN and SIP
networks has motivated the publication of a set of common practices
that can assure consistent behavior across implementations. The
scarcity of SIP ...
... SIP
networks has motivated the publication of a set of common practices
that can assure consistent behavior across implementations. The
scarcity of SIP expertise outside the IETF suggests that the IETF is
...
... IETF suggests that the IETF is
the best place to stage this work, especially since SIP is in a
relative state of flux compared to the core protocols ...
... Moreover, the IETF working groups that focus on SIP (SIP and SIPPING)
are best positioned to ascertain whether or not any new extensions to
...
... SIPPING)
are best positioned to ascertain whether or not any new extensions to
SIP are justified for PSTN interworking. This framework ...
... interworking.
An important characteristic of any SIP telephony network is feature
transparency with respect to the PSTN ...
... SS7 [6]) should be offered
by a SIP network in a manner that precludes any debilitating
difference in user experience while not limiting the flexibility of
SIP ...
... SIP network in a manner that precludes any debilitating
difference in user experience while not limiting the flexibility of
SIP. On the one hand, it is necessary that SIP support the
primitives for the delivery ...
... difference in user experience while not limiting the flexibility of
SIP. On the one hand, it is necessary that SIP support the
primitives for the delivery of such services ...
... delivery of such services where the terminating
point is a regular SIP phone (see definition in Section 2 below)
rather than a device that is fluent in SS7. However, it is also
...
...
of SS7-SIP interconnection, to ensure transparency of features not
otherwise supported in SIP. If possible, SS7 ...
... SS7-SIP interconnection, to ensure transparency of features not
otherwise supported in SIP. If possible, SS7 information should be
available in its entirety and without any loss to trusted parties in
...
... SS7 information should be
available in its entirety and without any loss to trusted parties in
the SIP network across the PSTN-IP interface; one compelling need to
...
... networks.
Another important characteristic of a SIP telephony network is
routability of SIP requests ...
... SIP telephony network is
routability of SIP requests - a SIP request that sets up a telephone
call should contain sufficient information in its headers ...
... network is
routability of SIP requests - a SIP request that sets up a telephone
call should contain sufficient information in its headers to enable
...
... destination by proxy servers in
the SIP network. Most commonly this entails that parameters of a
call like the dialed number should be carried over from SS7 signaling ...
... to SIP requests. Routing in a SIP network may in turn be influenced
by mechanisms such as TRIP [8 ...
... framework for the
integration of legacy telephony signaling into SIP messages. SIP-T
provides the above two characteristics through techniques known as
...
... integration of legacy telephony signaling into SIP messages. SIP-T
provides the above two characteristics through techniques known as
'encapsulation ...
... provides the above two characteristics through techniques known as
'encapsulation' and 'translation' respectively. At a SIP-ISUP
gateway ...
... SS7 ISUP messages are encapsulated within SIP in order that
information necessary for services is not discarded in the SIP
request ...
... SIP in order that
information necessary for services is not discarded in the SIP
request. However, intermediaries like proxy servers that make
routing ...
... proxy servers that make
routing decisions for SIP requests cannot be expected to understand
ISUP, so simultaneously, some critical ...
... critical information is translated from
an ISUP message into the corresponding SIP headers in order to
determine how the SIP request ...
... SIP request will be routed.
While pure SIP has all the requisite instruments for the
establishment and termination of calls, it does not have any baseline
mechanism to carry any mid-call information (such as the ISUP ...
... session. This mid-
call information does not result in any change in the state of SIP
calls or the parameters of the sessions that SIP ...
... SIP
calls or the parameters of the sessions that SIP initiates. A
provision to transmit such optional application-layer information is
...
... SIP Interworking Requirements SIP-T Functions
==================================================================
Transparency of ISUP ...
... SIP body
Routability of SIP messages with Translation of ISUP information
dependencies on ISUP ...
... ISUP and aims
to specify the behavior across ISUP-SIP interfaces only. The scope
of the SIP-T ...
... SIP interfaces only. The scope
of the SIP-T enterprise may, over time, come to encompass other
signaling systems as well.
...
...
SIP-T is not a new protocol - it is a set of mechanisms for
interfacing traditional telephone ...
... interfacing traditional telephone signaling with SIP. The purpose of
SIP-T is to provide protocol translation ...
... signaling with SIP. The purpose of
SIP-T is to provide protocol translation and feature transparency
across points of PSTN ...
... protocol translation and feature transparency
across points of PSTN-SIP interconnection. It intended for use where
a VoIP network ...
... PSTN.
Using SIP-T, there are three basic models for how calls interact with
gateways. Calls that originate in the PSTN ...
... PSTN can traverse a gateway to
terminate at a SIP endpoint, such as an IP phone. Conversely, an IP
...
... gateways - a call may originate and terminate in the
PSTN, but cross a SIP-based network somewhere in the middle.
The SS7 ...
... o IP endpoints: Any SIP user agent that can act as an originator or
recipient of calls. Thus, the following devices are classified as
IP ...
... decomposed gateways and control logic that are frequently
deployed today. So for example, a SIP-ISUP gateway speaks ISUP ...
... ISUP
to the PSTN and SIP to the Internet and is responsible for
converting between the types of signaling ...
... audio media.
* SIP phones: The term used to represent all end-user devices
that originate or terminate SIP ...
... SIP phones: The term used to represent all end-user devices
that originate or terminate SIP VoIP calls.
...
... o Proxy Servers: A proxy server is a SIP intermediary that routes
SIP requests to their destinations ...
... proxy server is a SIP intermediary that routes
SIP requests to their destinations. For example, a proxy server
...
... destinations. For example, a proxy server
might direct a SIP request to another proxy, a gateway or a SIP ...
... -------- ********************* ---------
Figure 1: Motivation for SIP-T in ISUP-SIP interconnection
...
... cloud serves as a transit network for telephone
calls originating in a pair of LECs, where SIP is employed as the
VoIP protocol used to set up and tear down these VoIP ...
... network, an MGC converts the ISUP signals to SIP
requests, and sends them to a proxy server which in turn routes
calls on other MGCs. Although this figure depicts only two MGCs,
...
... PSTN rate
centers). For a call originating from LEC1 and be terminating in
LEC2, the originator in SIP-T is the gateway that generates the SIP
request for a VoIP ...
... LEC2, the originator in SIP-T is the gateway that generates the SIP
request for a VoIP call, and the terminator is the gateway that is
...
... VoIP call, and the terminator is the gateway that is
the consumer of the SIP request; MGC1 would thus be the originator
and MGC2, the terminator. Note that one or more proxies may be used
...
... PSTN, it is important to preserve the received SS7 information
within SIP requests at the originating gateway and reuse this SS7
...
... ISUP information in the SIP signaling, a SIP network
can ensure that no SS7 information that is critical ...
... SS7 information that is critical to the
instantiation of features is lost when SIP bridges calls between two
segments ...
... transport of signaling
information may be used to achieve this, obviating the need for SIP
and consequently that of SIP-T. SIP-T ...
... information may be used to achieve this, obviating the need for SIP
and consequently that of SIP-T. SIP-T is employed in order to
leverage the intrinsic benefits of utilizing SIP ...
... SIP
and consequently that of SIP-T. SIP-T is employed in order to
leverage the intrinsic benefits of utilizing SIP: request routing ...
... SIP-T. SIP-T is employed in order to
leverage the intrinsic benefits of utilizing SIP: request routing and
call control leveraging proxy servers ...
... proxy servers (including the use of forking),
ease of SIP service creation, SIP's capability negotiation ...
... ease of SIP service creation, SIP's capability negotiation systems,
and so on. Translation of information from the received ISUP ...
... and so on. Translation of information from the received ISUP message
parameters to SIP header fields enables SIP intermediaries to
consider this information as they handle requests. SIP-T ...
... ISUP message
parameters to SIP header fields enables SIP intermediaries to
consider this information as they handle requests. SIP-T thus
...
... SIP header fields enables SIP intermediaries to
consider this information as they handle requests. SIP-T thus
facilitates call establishment and the enabling of new telephony
services over the IP network ...
... Finally, the scenario in Figure 2 is just one of several flows in
which SIP-T can be used - voice calls do not always both originate
and terminate in the PSTN ...
... SIP phones can also be
endpoints in a SIP-T session. In subsequent sections, the following
possible flows ...
... PSTN and it preserves this information
(via encapsulation and translation) in the SIP messages that it
transmits towards the terminating gateway. The terminator
...
... gateway. The terminator
extracts the ISUP content from the SIP message that it receives
and it reuses this information in signaling sent to the PSTN ...
... PSTN and it preserves this ISUP
information in the SIP messages (via encapsulation and
translation) that it directs towards the terminating SIP user
agent ...
... SIP messages (via encapsulation and
translation) that it directs towards the terminating SIP user
agent. The terminator has no use for the encapsulated ISUP and
...
... 3. IP origination - PSTN termination: A SIP phone originates a VoIP
call that is routed by one or more proxy servers ...
... 4. IP origination - IP termination: This is a case for pure SIP.
SIP-T (either encapsulation ...
... IP termination: This is a case for pure SIP.
SIP-T (either encapsulation or translation of ISUP) does not come
...
...
The follow sections explore the essential SIP-T flows in detail.
Note that because proxy servers ...
... proxy servers are usually responsible for routing
SIP requests (based on the Request-URI) the eventual endpoints at
...
... Request-URI) the eventual endpoints at
which a SIP request will terminate is generally not known to the
originator. So the originator does not select from the flows
...
...
described in this section, as a matter of static configuration or on
a per-call basis - rather, each call is routed by the SIP network
independently, and it may instantiate any of the flows below as the
...
... segments of the PSTN
is referred to as 'SIP bridging'. When a call destined for the SIP
network originates in the PSTN ...
... is referred to as 'SIP bridging'. When a call destined for the SIP
network originates in the PSTN, an SS7 ISUP ...
... the PSTN network. This gateway is from the perspective of the SIP
protocol the user agent client for this call setup request.
...
... protocol the user agent client for this call setup request.
Traditional SIP routing is used in the IP network to determine the
...
... appropriate point of termination (in this instance a gateway) and to
establish a SIP dialog and begin negotiation of a media session
...
... encapsulated ISUP
present in the SIP request it receives as appropriate.
A very elementary call-flow ...
... -------- * * -------------
| PSTN | ** ** | SIP phone |
-------- ********************* -------------
...
...
A call originates from the PSTN and terminates at a SIP phone. Note
that in Figure 5, the proxy server acts as the registrar for the SIP ...
... SIP phone. Note
that in Figure 5, the proxy server acts as the registrar for the SIP
phone in question.
...
... / * * \
------------ * * ---------
|SIP phone | ** ** | PSTN |
------------ ********************* ---------
...
... PSTN termination
A call originates from a SIP phone and terminates in the PSTN.
Unlike the previous two flows ...
... encapsulation in the request - the terminating gateway therefore only
performs translation on the SIP headers to derive values for ISUP
...
... There are three distinct sorts of elements (from a functional point
of view) in a SIP VoIP network that interconnects with the PSTN ...
...
3. The intermediaries that route SIP requests from the originator to
the terminator
...
... Behavior for the Section 4.1, Section 4.2 and Section 4.3
intermediary roles in a SIP-T call are described in the following
sections.
...
... The function of the originating user agent client is to generate the
SIP Call setup requests (i.e., INVITEs). When a call originates in
the PSTN, a gateway ...
... PSTN, a gateway is the UAC; otherwise some native SIP endpoint is
the UAC. In either case, note that the originator generally cannot
...
... entity the terminator will be, i.e., whether
final destination of the request is in a SIP network or the PSTN.
...
... gateway takes the necessary steps to preserve the
ISUP information by encapsulating it in the SIP request it creates.
The originating gateway ...
... MIME body with appropriate MIME
headers). It then formulates the headers of the SIP INVITE request
from the parameters of the ISUP ...
... IAM.
In other cases (like Figure 7), a SIP phone is the originator of a
VoIP call. Usually, the SIP ...
... SIP phone is the originator of a
VoIP call. Usually, the SIP phone sends requests to a SIP proxy that
...
... VoIP call. Usually, the SIP phone sends requests to a SIP proxy that
is responsible for routing ...
... ISUP in order for that to take place, the
originator has no way to anticipate this and it would be foolhardy to
require that all SIP VoIP user agents have the capability to generate
...
... encapsulated ISUP. Thus, an originator must
generate the SIP signaling while performing ISUP encapsulation ...
... SIP-T terminator is a consumer of the SIP calls. The terminator
is a standard SIP UA that can be either a gateway that interworks
...
... signaling to the PSTN from the
incoming SIP message. Values for certain ISUP parameters may be
gleaned from the SIP ...
... SIP message. Values for certain ISUP parameters may be
gleaned from the SIP headers or extracted directly from an
encapsulated ...
... ISUP as a template for the message it will send, but it
overwrites parameter values in the template as it translates SIP
headers or adds any parameter values ...
... message. This does introduce a general requirement, however, that
devices like SIP phones handle multipart MIME messages and unknown
MIME types ...
... MIME messages and unknown
MIME types gracefully (this is a baseline SIP requirement, but also a
place where vendors ...
... routing messages to one another, as well as gateways and SIP phones.
Each proxy server makes a forwarding decision for a SIP request ...
... SIP phones.
Each proxy server makes a forwarding decision for a SIP request based
on values of various headers, or 'routable elements ...
... SIP request).
SIP-T does introduce some additional considerations for forwarding a
request that could lead to new features and requirements for
...
... intermediaries. Feature transparency of ISUP is central to the
notion of SIP-T. Compatibility between the ISUP variants of the
...
... simplicity of operation and cost. The requirement of procuring a
reasonable rate may dictate that a SIP-T call spans dissimilar PSTN
interfaces ...
... If the terminator does not understand ISUP, it ignores it while
performing standard SIP processing. If the terminator does
understand ISUP, and needs to signal to the PSTN ...
...
o Translate the headers of the SIP request into ISUP parameters,
overwriting any values in the message template.
...
... Components of the SIP-T Protocol ...
...
The mechanisms described in the following sections are the components
of SIP-T that provide the protocol functions entailed by the
requirements.
...
... Core SIP ...
... signaling is one of the major requirements
of SIP-T. SIP-T uses multipart MIME bodies to enable SIP messages ...
... SIP-T. SIP-T uses multipart MIME bodies to enable SIP messages to
contain multiple payloads (Session Description Protocol ...
... Translation encompasses all aspects of signaling protocol conversion
between SIP and ISUP. There are essentially two components to the
problem of translation:
...
... SIP message mapping: This describes a mapping between ISUP
and SIP at the message level. In SIP-T deployments gateways ...
... entrusted with the task of generating a specific ISUP message for
each SIP message received and vice versa. It is necessary to
specify the rules that govern the mapping between ISUP and SIP
messages ...
... SIP message received and vice versa. It is necessary to
specify the rules that govern the mapping between ISUP and SIP
messages (i.e., what ISUP messages is sent when a particular SIP
message is received: an IAM ...
... ISUP and SIP
messages (i.e., what ISUP messages is sent when a particular SIP
message is received: an IAM must be sent on receipt of an INVITE,
...
... a REL for BYE, and so on). A potential mapping between ISUP and
SIP messages has been described in [10].
...
... ISUP parameter-SIP header mapping: A SIP request that is used to
set up a telephone call should contain information that enables
...
... destination by proxy servers
in the SIP network - for example, the telephone number dialed by
the originating user. It is important to standardize a set of
...
... practices that defines the procedure for translation of
information from ISUP to SIP (for example, the Called Party
Number in an ISUP IAM ...
... Number in an ISUP IAM must be mapped onto the SIP 'To' header
field and Request-URI, etc.). This issue becomes inherently more
...
... Request-URI, etc.). This issue becomes inherently more
complicated by virtue of the fact that the headers of a SIP
request (especially an INVITE) may be transformed by
intermediaries, and that consequently, the SIP ...
... SIP
request (especially an INVITE) may be transformed by
intermediaries, and that consequently, the SIP headers and
encapsulated ...
...
Pure SIP does not have any provision for carrying any mid-call
control information that is generated during a session. The INFO [3 ...
... SIP Content Negotiation ...
... ISUP
elements into the same SIP message by using the MIME multipart
format. Traditionally in SIP ...
... SIP message by using the MIME multipart
format. Traditionally in SIP, if the terminating device does not
support a multipart payload (multipart/mixed ...
... multipart/mixed) and/or the ISUP MIME
type, it would then reject the SIP request with a 415 Unsupported
Media Type specifying the media types ...
... 'application/SDP'). The originator would subsequently have to re-
send the SIP request after stripping out the ISUP payload (i.e. with
...
... silently discard optional bodies that it does not understand
(allowing a SIP phone to ignore an ISUP payload when processing ISUP ...
... domains. In many legal environments, distribution of ISUP is
restricted to licensed carriers; SIP-T introduces some challenges in
so far as it bridges carrier signaling ...
... signaling. Any
administrative domain implementing SIP-T should have an adequate
security apparatus (including elements ...
...
Transporting ISUP in SIP bodies may provide opportunities for abuse,
fraud, and privacy concerns, especially when SIP-T ...
... SIP bodies may provide opportunities for abuse,
fraud, and privacy concerns, especially when SIP-T requests can be
generated, inspected or modified by arbitrary SIP endpoints. ISUP ...
... privacy concerns, especially when SIP-T requests can be
generated, inspected or modified by arbitrary SIP endpoints. ISUP
MIME ...
... 4]) to
alleviate this concern, as is described in the Security
Considerations of the core SIP specification [1]. Authentication
...
... Authentication
properties provided by S/MIME would allow the recipient of a SIP-T
message to ensure that the ISUP MIME ...
... Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261prop, May 2002. ...
... Camarillo, G., Roach, A., Peterson, J. and L. Ong, "Mapping of ISUP Overlap Signaling to SIP", Work in Progress. ...
... network (or vice versa) is questionable. Clearly, the strength
of SIP-T is realized when the encapsulated ISUP involves the
...
... Mankin, Scott Bradner and Steve Bellovin for their valuable comments.
The original 'SIP+' proposal for interconnecting portions of the PSTN
with SIP ...
... SIP+' proposal for interconnecting portions of the PSTN
with SIP bridging was developed by Eric Zimmerer.
...