One document matched: draft-rosenberg-sipping-service-identification-03.txt
Differences from draft-rosenberg-sipping-service-identification-02.txt
SIPPING J. Rosenberg
Internet-Draft Cisco
Intended status: Best Current July 9, 2007
Practice
Expires: January 10, 2008
Identification of Communications Services in the Session Initiation
Protocol (SIP)
draft-rosenberg-sipping-service-identification-03
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Abstract
This document considers the problem of service identification in the
Session Initiation Protocol (SIP). Service identification is the
process of determining the user-level use case that is driving the
signaling being utilized by the user agent. While seemingly simple,
this process is quite complex, and when not addressed properly, can
lead to fraud, interoperability problems, and stifling of innovation.
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This document discusses these problems and makes recommendations on
how to address them.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Services and Service Identification . . . . . . . . . . . . . 4
4. Example Services . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. IPTV vs. Multimedia . . . . . . . . . . . . . . . . . . . 6
4.2. Gaming vs. Voice Chat . . . . . . . . . . . . . . . . . . 7
4.3. Configuration vs. Pager Messaging . . . . . . . . . . . . 7
5. Using Service Identification . . . . . . . . . . . . . . . . . 7
5.1. Application Invocation in the User Agent . . . . . . . . . 8
5.2. Application Invocation in the Network . . . . . . . . . . 9
5.3. Network Quality of Service Authorization . . . . . . . . . 9
5.4. Service Authorization . . . . . . . . . . . . . . . . . . 10
5.5. Accounting and Billing . . . . . . . . . . . . . . . . . . 10
5.6. Negotiation of Service . . . . . . . . . . . . . . . . . . 10
5.7. Dispatch to Devices . . . . . . . . . . . . . . . . . . . 11
6. Key Principles of Service Identification . . . . . . . . . . . 11
6.1. Services are a By-Product of Signaling . . . . . . . . . . 11
6.2. Perils of Explicit Identifiers . . . . . . . . . . . . . . 13
6.2.1. Fraud . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2.2. Systematic Interoperability Failures . . . . . . . . . 14
6.2.3. Stifling of Service Innovation . . . . . . . . . . . . 16
7. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. Normative References . . . . . . . . . . . . . . . . . . . 18
11.2. Informational References . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . . . 20
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1. Introduction
The Session Initiation Protocol (SIP) [2] defines mechanisms for
initiating and managing communications sessions between agents. SIP
allows for a broad array of session types between agents. It can
manage audio sessions, ranging from low bitrate voice-only up to
multi-channel hi fidelity music. It can manage video sessions,
ranging from small, "talking-head" style video chat, up to high
definition multipoint video conferencing, to low bandwidth user-
generated content, up to high definition movie and TV content. SIP
endpoints can be anything - adaptors that convert an old analog
telephone to Voice over IP (VoIP), dedicated hardphones, fancy
hardphones with rich displays and user entry capabilities, softphones
on a PC, buddylist and presence applications on a PC, dedicated
videoconferencing peripherals, and speakerphones.
This breadth of applicability is SIPs greatest asset, but it also
introduces numerous challenges. One of these is that, when an
endpoint generates a SIP INVITE for a session, or receives one, that
session can potentially be within the context of any number of
different use cases and endpoint types. For example, a SIP INVITE
with a single audio stream could represent a Push-To-Talk session
between mobile devices, a VoIP session between softphones, or audio-
based access to stored content on a server.
These differing use cases have driven implementors and system
designers to seek techniques for service identification. Service
identification is the process of determining and/or signaling the
specific use case that is driving the signaling being generated by a
user agent. At first glance, this seems harmless and easy enough.
It is tempting to define a new header, "Service-ID", for example, and
have a user agent populate it with any number of well-known tokens
which define what the service is. This information could then be
consumed for any number of purposes.
However, as this document will demonstrate, service identification is
a very complex and difficult process, and can very easily lead to
fraud, systemic interoperability failures, and a complete stifling of
the innovation that SIP was meant to achieve.
Section 3 begins by defining a service and the service identification
problem. Section 4 gives some concrete examples of services and why
they can be challenging to identify. Section 5 explores the ways in
which a service identification can be utilized within a network.
Next, Section 6 discusses the key architectural principles of service
identification, and how explicit service identifiers can lead to
fraud, interoperability failures, and stifling of service innovation.
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2. Terminology
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 RFC 2119 [1].
3. Services and Service Identification
The problem of identifying services within SIP is not a new one. The
problem has been considered extensively in the context of presence.
In particular, the presence data model for SIP [3] defines the
concept of a service as one of the core notions that presence
describes. Services are described in Section 3.3 of RFC 4479, which
has this to say on the topic:
3.3. Service
Each presentity has access to a number of services. Each of these
represents a point of reachability for communications that can be
used to interact with the user. Examples of services are telephony
(that is, traditional circuit-based telephone service), push-to-talk,
instant messaging, Short Message Service (SMS), and Multimedia
Message Service (MMS).
It is difficult to give a precise definition for service. One
reasonable approach is to model each software or hardware agent in
the system as a service. If a user starts a softphone application on
their PC, then that represents a service. If a user has a videophone
device, then that represents another service. This is effectively a
physical view of services. This definition, however, starts to fall
apart when a service is spread across multiple software agents or
devices. For example, a SIP URI representing an address-of-record
can be routed to a softphone or a videophone, or both. In that case,
one might attempt instead to define a service based on its address on
the network. This definition also falls apart when modeling devices
or applications that receive calls and dispatch them to different
"helpers" based on potentially complex logic. For example, a
cellular telephone might house multiple SIP applications, each of
which can "register" different handlers based on the method or even
body type of the request. Each of those applications or handlers can
rightfully be considered a service, but it doesn't have an address on
the network distinct from the others.
Because of this inherent difficulty in precisely defining a service,
the data model doesn't try to constrain what can be considered a
service. Rather, anything can be considered a service so long as it
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exhibits a set of key properties defined by this model. In
particular, each service is associated with characteristics that
identify the nature and capabilities of that service, with reach
information that indicates how to connect to the service, with status
information representing the state of that service, and relative
information that describes the ways in which that service relates to
others associated with the presentity.
As a consequence, in this model, services are not explicitly
enumerated. There is no central registry where one finds identifiers
for each service. Consequently, each service does not have a single
"service" attribute with values such as "ptt" or "telephony". That
doesn't mean that these consolidated monikers aren't useful; indeed,
they represent an essential summary of what the service is. Such
summarization is useful in creating icons that allow a user to choose
one service over another. A watcher is free to create such
summarization information from any of the information associated with
a service. The reach information often provides valuable information
for creating such a summarization. Oftentimes, the scheme of the URI
is synonymous with the view of what a service is. An "sms" URI [14]
clearly indicates SMS, for example. For some URIs, there may be many
services available, for example, SIP or tel [15], in which case the
scheme is less meaningful as a way of creating a summary. The reach
information could also indicate that certain application software has
to be invoked (such as a videogame), in which case that aspect of the
reach information would be useful for generating an iconic
representation of the game.
Essentially, the service is the user-visible use case that is driving
the behavior of the user-agents and servers in the SIP network.
Being user-visible means that there is a difference in user
experience between two services that are different. That user
experience can be part of the call, or outside of the call. Within a
call, the user experience can be based on different media types (an
audio call vs. a video chat), different content within a particular
media type (stored content, such as a movie or TV session), different
devices (a wireless device for "telephony" vs. a PC application for
"voice-chat"), different user interfaces (a buddy list view of voice
on a PC application vs. a software emulation of a hard phone),
different communities that can be accessed (voice chat with other
users that have the same voice chat client, vs. voice communications
with any endpoint on the PSTN), or different applications that are
invoked by the user (manually selecting a push-to-talk application
from a wireless phone vs. a telephony application). Outside of a
call, the difference in user experience can be a billing one (cheaper
for one service than other), a notification feature for one and not
another (for example, an IM that gets sent whenever a user makes a
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call), and so on.
In some cases, there is very little difference in the underlying
technology that will support two different services, and in other
cases, there are big differences. However, for purposes of this
discussion, the key definition is that two services are distinct when
there is a perceived difference by the user in the two services.
This leads naturally to the desire to perform service identification.
Service identification is defined as the process of (1) determination
of the underlying service which is driving a particular signaling
exchange, (2) associating that service with some kind of moniker, and
(3) attaching that moniker to a signaling message (typically a SIP
INVITE), and then utilizing it for various purposes within the
network. Service identification can be done in the endpoints, in
which case the UA would insert the moniker directly into the
signaling message based on its awareness of the service. Or, it can
be done within a proxy in the network, based on inspection of the SIP
message, or based on hints placed into the message by the user.
4. Example Services
It is very useful to consider several example services, especially
ones that appear difficult to differentiate from each other.
4.1. IPTV vs. Multimedia
IP Television (IPTV) is the usage of IP networks to access
traditional television content, such as movies and shows. SIP can be
utilized to establish a session to a media server in a network, which
then serves up multimedia content and streams it as an audio and
video stream towards the client. Whether SIP is ideal for IPTV is,
in itself, a good question. However, such a discussion is outside
the scope of this document.
Consider multimedia conferencing. The user accesses a voice and
video conference at a conference server. The user might join in
listen-only mode, in which case the user receives audio and video
streams, but does not send.
These two services - IPTV and multimedia conferencing, clearly appear
as different services. They have different user experiences and
applications. A user is unlikely to ever be confused about whether a
session is IPTV or multimedia conferencing. Indeed, they are likely
to have different software applications or endpoints for the two
services.
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However, these two services look remarkably alike based on the
signaling. Both utilize audio and video. Both could utilize the
same codecs. Both are unidirectional streams (from a server in the
network to the client). Thus, it would appear on the surface that
there is no way to differentiate them, based on inspection of the
signaling alone.
4.2. Gaming vs. Voice Chat
Consider an interactive game, played between two users from their
mobile devices. The game involves the users sending each other game
moves, using a messaging channel, in addition to voice. In another
service, users have a voice and IM chat conversation using a buddy
list application on their PC.
In both services, there are two media streams - audio and messaging.
The audio uses the same codecs. Both use the Message Session Relay
Protocol (MSRP) [5]. In both cases, the caller would send an INVITE
to the AOR of the target user. However, these represent fairly
different services, in terms of user experience.
4.3. Configuration vs. Pager Messaging
The SIP MESSAGE method [8] provides a way to send one-shot messages
to a particular AOR. This specification is primarily aimed at Short
Message Service (SMS) style messaging, commonly found in wireless
phones. Receipt of a MESSAGE request would cause the messaging
application on a phone to launch, allowing the user to browse message
history and respond.
However, MESSAGE is sometimes used for the delivery of content to a
device for other purposes. For example, some providers use it to
deliver configuration updates, such as new phone settings or
parameters, or to indicate that a new version of firmware is
available. Though not designed for this purpose, MESSAGE gets used
since, in existing wireless networks, SMS are used for this purpose,
and MESSAGE is the SIP equivalent of SMS.
Consequently, the MESSAGE request sent to a phone can be for two
different services. One would require invocation of a messaging app,
whereas the other would be consumed by the software in the phone,
without any user interaction at all.
5. Using Service Identification
It is important to understand what the service identity would be
utilized for, if known. The discussions in Section 4 give some hints
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to the possible usages. Here, we explicitly discuss them.
5.1. Application Invocation in the User Agent
In some of the examples above, there were multiple software
applications running within a single user agent. When an incoming
INVITE or MESSAGE arrives, it must be delivered to the appropriate
application software. When each service is bound to a distinct
software application, it would seem that the service identity is
needed to dispatch the message to the appropriate piece of software.
This is shown in Figure 2.
+---------------------------------+
| |
| +-------------+ +-------------+ |
| | UI | | UI | |
| +-------------+ +-------------+ |
| +-------------+ +-------------+ |
| | | | | |
| | Service 1 | | Service 2 | |
| | | | | |
| +-------------+ +-------------+ |
| +-----------------------------+ |
| | | |
| | SIP | |
| | Layer | |
| | | |
| +-----------------------------+ |
| |
+---------------------------------+
Physical Device
Figure 2
The role of the SIP layer is to parse incoming messages, handle the
SIP state machinery for transactions and dialogs, and then dispatch
request to the appropriate service. For the example services in
Section 4.2, an incoming INVITE for the gaming service would be
delivered to the gaming application software. An incoming INVITE for
the voice chat service would be delivered to the voice chat
application software. For the examples in Section 4.3, a MESSAGE
request for user to user messaging would be delivered to the
messaging or SMS app, and a MESSAGE request containing configuration
data would be delivered to a configuration update application.
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5.2. Application Invocation in the Network
Another usage of a service identifier would be to cause servers in
the SIP network to provide additional processing, based on the
service. For example, an INVITE issued by a user agent for IPTV
would pass through a server that does some kind of content rights
management, authorizing whether the user is allowed to access that
content. On the other hand, an INVITE issued by a user for
multimedia conferencing would pass through a server providing
"traditional" telephony features, such as outbound call screening and
call recording. It would make no sense for the INVITE associated
with IPTV to have outbound call screening and call recording applied,
and it would make no sense for the multimedia conferencing INVITE to
be processed by the content rights management server. Indeed, in
these cases, its not just an efficiency issue (invoking servers when
not needed), but rather, truly incorrect behavior can occur. For
example, if an outbound call screening application is set to block
outbound calls to everything except for the phone numbers of friends
and family, an IPTV request that gets processed by such a server
would be blocked (as its not targeted to the AOR of a friend or
family member). This would block a user's attempt to access IPTV
services, when that was not the goal at all.
Similarly, a MESSAGE request from Section 4.3 might need to pass
through a message server for filtering when it is associated with
chat, but not when it is associated with config. Consider a filter
which gets applied to MESSAGE requests, and that filter runs in a
server in the network. The filter operation prevents user Joe from
sending messages to user Bob that contain the words "stock" or
"purchase", due to some regulations that disallow Joe and Bob from
discussing stock trading. However, a MESSAGE for configuration
purposes might contain an XML document that uses the token "stock" as
some kind of attribute. This configuration update would be discarded
by the filtering server, when it should not have been.
5.3. Network Quality of Service Authorization
The IP network can provide differing levels of Quality of Service
(QoS) to IP packets. This service can include guaranteed throughput,
latency, or loss characteristics. Typically, the user agent will
make some kind of QoS request, either using explicit signaling
protocols (such as RSVP) or through marking of Diffserv value in
packets. The network will need to make a policy decision based on
whether these QoS treatments are authorized or not. One common
authorization policy is to check if the user has invoked a service
using SIP that they are authorized to invoke, and that this service
requires the level of QoS treatment the user has requested.
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For example, consider IPTV and multimedia conferencing as described
in Section 4.1. IPTV is a non-real time service. Consequently,
media traffic for IPTV would be authorized for bandwidth guarantees,
but not for latency or loss guarantees. On the other hand,
multimedia conferencing is real time. Its traffic would require
bandwidth, loss and latency guarantees from the network.
Consequently, if a user should make an RSVP reservation for a media
stream, and ask for latency guarantees for that stream, the network
would like to be able to authorize it if the service was multimedia
conferencing, but not if it was IPTV. This would require the server
performing the QoS authorization to know the service associated with
the INVITE that set up the session.
5.4. Service Authorization
Frequently, a network administrator will want to authorize whether a
user is allowed to invoke a particular service. Not all users will
be authorized to use all services that are provided. For example, a
user may not be authorized to access IPTV services, whereas they are
authorized to utilize multimedia processing. A user might not be
able to utilize a multiplayer gaming service, whereas they are
authorized to utilize voice chat services.
Consequently, when an INVITE arrives at a proxy in the network, the
proxy will need to determine what the requested service is, so that
the proxy can make an authorization decision.
5.5. Accounting and Billing
Service authorization and accounting/billing go hand in hand.
Presumably, one of the primary reasons for authorizing that a user
can utilize a service is that they are being billed differently based
on the type of service. Consequently, one of the goals of a service
identity is to be able to include it in accounting records, so that
the appropriate billing model can be applied.
For example, in the case of IPTV, a service provider can bill based
on the content (US $5 per movie, perhaps), whereas for multimedia
conferencing, they can bill by the minute. This requires the
accounting streams to indicate which service was invoked for the
particular session.
5.6. Negotiation of Service
In some cases, when the caller initiates a session, they don't
actually know which service will be utilized. Rather, they might
like to offer up all of the services they have available to the
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called party, and then let the called party decide, or let the system
make a decision based on overlapping service capabilities.
As an example, s user can do both the game and the voice chat service
of Section 4.2. They initiate a session to a target AOR, but the
devices used by that user can only support voice chat. Consequently,
voice chat gets utilized for the session.
5.7. Dispatch to Devices
When a user has multiple devices, each with varying capabilities in
terms of service, it is useful to dispatch an incoming request to the
right device based on whether the device can support the service that
has been requested.
For example, if a user initiates a gaming session with voice chat,
and the target user has two devices - one that can support the gaming
service, and the other that cannot, the INVITE should be dispatched
to the device which supports the gaming session.
6. Key Principles of Service Identification
In this section, we describe some of the key principles of performing
service identification.
6.1. Services are a By-Product of Signaling
Almost always, the first solution that people consider is to add some
kind of field to the signaling messages which indicates what the
service is. This field would then be inserted by the user agent, and
then can be used by the proxies and other user agent as a service
identifier.
This approach, however, misses a key point, which cannot be stressed
enough, and which represents the core architectural principle to be
understood here:
A service is the by-product of the signaling and the context
around it (the user profile, time-of-day and so on) - the effects
of the signaling message once launched into the network. The
service identity is therefore always derivable from the signaling
and its context without additional identifiers.
When a user sends an INVITE request to the network, and targets that
request at an IPTV server, and includes SDP for audio and video
streaming, the *result* of sending such an INVITE is that an IPTV
session occurs. The entire purpose of the INVITE is to establish
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such a session, and therefore, invoke the service. Thus, a service
is not something that is different from the rest of the signaling
message. A service is what the user gets after the network and other
user agents have processed a signaling message.
This principle leads to another important conclusion:
If two services are different, but their signaling appears to be
the same, it is because there is in fact something different that
has been overlooked, or something has been implied from the
signaling which should have been signaled explicitly.
This makes sense; if a service is the byproduct of signaling, how can
a user have different experiences and different services when the
signaling message is the same? There has to be something different
in the messages, if the user experience was in fact different.
To illustrate this, let us take each of the example services in
Section 4 and investigate whether there is, or should be, something
different in the signaling in each case.
IPTV vs. Multimedia Conferencing: The two services in Section 4.1
appear to have identical signaling. They both involve audio and
video streams, both of which are unidirectional. Both might
utilize the same codecs. However, there is another important
difference in the signaling - the target URI. In the case of
IPTV, the request is targeted at a media server or to a particular
piece of content to be viewed. In the case of multimedia
conferencing, the target is a conference server. The
administrator of the domain can therefore examine the two Request-
URI, and figure out whether it is targeted for a conference server
or a content server, and use that to derive the service associated
with the request.
Gaming vs. Voice Chat: Though both sessions involve MSRP and voice,
and both are targeted to the same AOR of the called user, there is
a difference. The MSRP messages for the gaming session carry
content which is game specific, whereas the MSRP messages for the
voice chat are just regular text, meant for rendering to a user.
Thus, the MSRP session in the SDP will indicate the specific
content type that MSRP is carrying, and this type will differ in
both cases. Even if the game moves look like text, since they are
being consumed by an automata there is an underlying schema that
dictates their content, and therefore, this schema represents the
actual content type that should be signaled.
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Configuration vs. Pager Messaging: Just as in the case of gaming vs.
voice chat, the content type of the messages differentiates the
service that occurs as a consequence of the messages.
This is ultimately an expression of the principle of DWIM vs. DWIS
(Do-What-I-Mean vs. Do-What-I-Say). Explicit signaling is DWIS - the
user is asking for a service by invoking the signaling that results
in the desired effect. A service identifier is DWIM - an unspecific
request for something that is ill-defined and non-interoperable.
6.2. Perils of Explicit Identifiers
Given that the information in the signaling message always conveys
enough information to identify the service, another important
conclusion can be drawn:
Inclusion of an explicit service identifier within a message is,
at best, redundant, and at worst, an avenue for fraud, loss of
interoperability, and stifling of service innovation.
By "explicit service identifier", we mean a field included in the
signaling message that contains a token whose value indicates the
specific service invoked by the calling user. This would be "IPTV"
or "voice chat" or "shoot-em game" or "short message service". This
explicit identifier would typically be inserted by the originating
user agent, and carried in the signaling message.
Clearly, if the signaling message itself contains enough information
to identify the service, inclusion of an extra field to say the same
thing is going to be redundant. Redundancy by itself is not a big
deal. However, redundancy can lead to other,more significant
problems.
6.2.1. Fraud
First and foremost, it can lead to fraud. If a provider uses the
service identifier for billing and accounting purposes, or for
authorization purposes, it opens an avenue for attack. The user can
construct the signaling message so that its actual effect (which is
the service the user will receive), is what the user desires, but the
service identity (which is what is used for billing and
authorization) doesn't match, and indicates a cheaper service, or one
that the user is authorized to receive. If, however, the service
identity used by the domain admistrator is derived from the signaling
itself, the user cannot lie. If they did lie, they wouldn't get the
desired service.
Consider the example of IPTV vs. multimedia conferencing. If
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multimedia conferencing is cheaper, the user could send an INVITE for
an IPTV session, but include a service identifier which indicates
multimedia conferencing. They get the service associated with IPTV,
but at the cost of multimedia conferencing.
This same principle shows up in other places. For example, in the
identification of an emergency services call [6]. It is desirable to
give emergency services calls special treatment, such as being free,
authorized even when the user cannot otherwise make calls, and to
give them priority. If emergency calls where indicated through
something other than the target of the call being an emergency
services URN [7], it would open an avenue for fraud. The user could
place any desired URI in the request-URI, and indicate that the call
is an emergency services call. This could would then get special
treatment, but of course get routed to the target URI. The only way
to prevent this fraud is to consider an emergency call as any call
whose target is an emergency services URN. Thus, the service
identification here is based on the target of the request. When the
target is an emergency services URN, the request can get special
treatment. The user cannot lie, since there is no way to separately
indicate this is an emergency call, besides targeting it to an
emergency URN.
6.2.2. Systematic Interoperability Failures
How can inclusion of an explicit service identifier cause loss of
interoperability? When such an identifier is used to drive
functionality - such as dispatch on the phones, in the network, or
QoS authorization, it means that the wrong thing can happen when this
field is not set properly. Consider a user in domain 1, calling a
user in domain 2. Domain 1 provides the user with a service they
call "voice chat", which utilizes voice and IM for real time
conversation, driven off of a buddy list application on a PC. Domain
2 provides their users with a service they call, "text telephony",
which is a voice service on a wireless device that also allows the
user to send text messages. Consider the case where domain 1 and
domain 2 both have their user agents insert a service identifiers
into the request, and then use that to derive QoS authorization,
accounting, and invocation of applications in the network and in the
device. The user in domain 1 calls the user in domain 2, and inserts
the identifier "Voice Chat" into the INVITE. When this arrives at
the proxy in domain 2, the service is unknown. Consequently, the
request does not get the proper QoS treatment. When it gets
delivered to the User Agent of the user in domain 2, the user agent
does not see a service it understands, and so consequently, does not
know to dispatch the request to the right application software.
Thus, this call has completely failed, even when it could have
succeeded. This illustrates the following key point:
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Explicit service identifiers, used between domains, cause
interoperability failures unless all interconnected domains agree
on exactly the same set of services and how to name them.
Of course, lack of service identifiers does not guarantee service
interoperability. However, SIP was built with rich tools for
negotiation of capabilities at a finely granular level. One user
agent can make a call using audio and video, but if the receiving UA
only supports audio, SIP allows both sides to negotiate down to the
lowest common denominator. Thus, communications is still provided.
As another example, if one agent initiates a Push-To-Talk session
(which is audio with a companion floor control mechanism), and the
other side only did regular audio, SIP would be able to negotiate
back down to a regular voice call. As another example, if a calling
user agent is running a high-definition video conferencing endpoint,
and the called user agent supports just a regular video endpoint, the
codecs themselves can negotiate downward to a lower rate, picture
size, and so on. Thus, interoperability is achieved. Interestingly,
the final "service" may no longer be well characterized by the
service identifier that would have been placed in the original
INVITE. For example, in this case, of the original INVITE from the
caller had contained the service identifier, "hi-fi video", but the
video gets negotiated down to a lower rate and picture size, the
service identifier is no longer really appropriate.
This illustrates another key aspect of the interoperability problem:
Usage of explicit service identifiers in the request will result
in inconsistencies with results of any SIP negotiation that might
otherwise be applied in the session.
Of course, there are cases where negotiating to a common baseline is
not what is desired. SIP provides tools (such as Require), to force
the call to fail unless the desired capabilities are supported.
However, this is not recommended as a general rule [4].
When a service identifier becomes something that both proxies and the
user agent need to understand in order to properly treat a request,
it becomes equivalent to including a token in the Proxy-Require and
Require header fields of every single SIP request. The very reason
that RFC 4485 frowns upon usage of Require and certainly Proxy-
Require is the huge impact on interoperability it causes. It is for
this same reason that explicit service identifiers need to be
avoided:
The usage of explicit service identifiers is equivalent to the
usage of Require and Proxy-Require in the request, and has the
same negative impact on interoperability as those headers have.
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6.2.3. Stifling of Service Innovation
The probability that any two pair of service providers end up with
the same set of services, and give them the same names, becomes
decreasingly small as the number of providers grow. Indeed, it would
almost certainly require a centralized authority to identify what the
services are, how they work, and what they are named. This, in turn,
leads to a requirement for complete homogeneity in order to
facilitate interconnection. Two providers cannot usefully
interconnect unless they agree on the set of services they are
offering to their customers, and each do the same thing. This is, in
a very real sense, anathema to the entire notion of SIP, which is
built on the idea that heterogeneous domains can interconnect and
still get interoperability:
Explicit service identifiers lead to a requirement for homogeneity
in service definitions across providers that interconnect, ruining
the very service heterogeneity that SIP was meant to bring.
Indeed, Metcalfe's law says that the value of a network grows with
the square of the number of participants. As a consequence of this,
once a bunch of large domains did get together, agree on a set of
services, and then a set of well-known identifiers for those
services, it would force other providers to also deploy the same
services, in order to obtain the value that interconnection brings.
This, in turn, will stifle innovation, and quickly force the set of
services in SIP to become fixed and never expand beyond the ones
initially agreed upon. This, too, is anathema to the very framework
on which SIP is built, and defeats much of the purpose of why
providers have chosen to deploy SIP in their own networks:
Metcalfe's law, when combined with explicit service identifiers,
will stifle the ability of providers to develop new SIP services,
since they have no hope of interconnecting them with anyone else.
Consider the following example. Several providers get together, and
standardize on a bunch of service identifiers. One of these uses
audio and video (say, "multimedia conversation"). This service is
successful, and is widely utilized. Endpoints look for this
identifier to dispatch calls to the right software applications, and
the network looks for it to invoke features, perform accouting, and
QoS. A new provider gets the idea for a new service, say, avatar-
enhanced multimedia conversation. In this service, there is audio
and video, but there is a third stream, which renders an avatar. A
caller can press buttons on their phone, to cause the avatar on the
other person's device to show emotion, make noise, and so on. This
is similar to the way emoticons are used today in IM. This service
is enabled by adding a third media stream (and consequently, third
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m-line) to the SDP.
Normally, this service would be backwards compatible with a regular
audio-video endpoint, which would just reject the third media stream.
However, because a large network has been deployed that is expecting
to see the token, "multimedia conversation" and its associated audio+
video service, it is nearly impossible for the new provider to roll
out this new service. If they did, it would fail completely, or
partially fail, when their users call users in other provider
domains.
7. Recommendations
From these principles, several recommendations can be made:
o Systems needing to perform service identification must examine
existing signaling constructs to identify the service based on
fields that exist within the signaling message already.
o If it appears that the signaling currently defined in standards is
not sufficient to identify the service, it may be due to lack of
sufficient signaling to convey what is needed, and new standards
work should be undertaken to fill this gap.
o The usage of an explicit service identifier does make sense as a
way to cache a decision made by a network element, for usage by
another network element within the same domain. However, service
identifiers are fundamentally useful within a particular domain,
and any such header must be stripped at a network boundary.
8. Security Considerations
Oftentimes, the service associated with a request is utilized for
purposes such as authorization, accounting, and billing. When
service identification is not done properly, the possibility of
network fraud is introduced. It is for this reason, discussed
extensively in Section 6.2.1, that the usage of explicit service
identifiers inserted by a UA is NOT RECOMMENDED.
9. IANA Considerations
There are no IANA considerations associated with this specification.
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10. Acknowledgements
This document is based on discussions with Paul Kyzivat and Andrew
Allen, who contributed significantly to the ideas here. Much of the
content in this draft is a result of discussions amongst participants
in the SIPPING mailing list, including Dean Willis, Tom Taylor, Eric
Burger, Dale Worley, Christer Holmberg, and John Elwell, amongst many
others.
11. References
11.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
11.2. Informational References
[3] Rosenberg, J., "A Data Model for Presence", RFC 4479, July 2006.
[4] Rosenberg, J. and H. Schulzrinne, "Guidelines for Authors of
Extensions to the Session Initiation Protocol (SIP)", RFC 4485,
May 2006.
[5] Campbell, B., "The Message Session Relay Protocol",
draft-ietf-simple-message-sessions-19 (work in progress),
February 2007.
[6] Rosen, B., "Framework for Emergency Calling in Internet
Multimedia", draft-ietf-ecrit-framework-01 (work in progress),
March 2007.
[7] Schulzrinne, H., "A Uniform Resource Name (URN) for Services",
draft-ietf-ecrit-service-urn-06 (work in progress), March 2007.
[8] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and
D. Gurle, "Session Initiation Protocol (SIP) Extension for
Instant Messaging", RFC 3428, December 2002.
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Author's Address
Jonathan Rosenberg
Cisco
Edison, NJ
US
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
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