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BLISS J. Rosenberg
Internet-Draft Cisco
Intended status: Informational March 9, 2009
Expires: September 10, 2009
Basic Level of Interoperability for Session Initiation Protocol (SIP)
Services (BLISS) Problem Statement
draft-ietf-bliss-problem-statement-04
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Abstract
The Session Initiation Protocol (SIP) has been designed as a general
purpose protocol for establishing and managing multimedia sessions.
It provides many core functions and extensions in support of features
such as transferring of calls, parking calls, and so on. However,
interoperability of more advanced features between different vendors
has been poor. This document describes the reason behind these
interoperability problems, and presents a framework for addressing
them.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. The Confusion of Tongues . . . . . . . . . . . . . . . . . . . 5
3. Concrete Examples . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Call Forward No Answer . . . . . . . . . . . . . . . . . . 6
3.1.1. Approach 1: UA Redirects . . . . . . . . . . . . . . . 7
3.1.2. Approach II: Proxy Forwards . . . . . . . . . . . . . 8
3.1.3. Approach III: UA Proxies . . . . . . . . . . . . . . . 9
3.1.4. Approach IV: Call Processing Language . . . . . . . . 10
3.1.5. Failure Cases . . . . . . . . . . . . . . . . . . . . 11
3.1.5.1. No One Implements . . . . . . . . . . . . . . . . 12
3.1.5.2. Both Implement . . . . . . . . . . . . . . . . . . 12
3.1.5.3. Missing Half . . . . . . . . . . . . . . . . . . . 12
4. Solution Considerations . . . . . . . . . . . . . . . . . . . 12
5. BLISS Solution Framework . . . . . . . . . . . . . . . . . . . 14
5.1. Phase I - Identify a Feature Group . . . . . . . . . . . . 14
5.2. Phase II - Gather Data . . . . . . . . . . . . . . . . . . 16
5.3. BLISS Phase III - Problem Definition . . . . . . . . . . . 16
5.4. BLISS Phase 4 - Minimum Interop Definition . . . . . . . . 17
6. Structure of the BLISS Final Deliverable . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
10. Informative References . . . . . . . . . . . . . . . . . . . . 19
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
The Session Initiation Protocol (SIP) [RFC3261] has been designed as
a general purpose protocol for establishing and managing multimedia
sessions. In this role, it provides many core functions and
extensions to support "session management features". In this
context, session management features (or just features in this
specification) are operations, typically invoked by the user, that
provide some form value-added functionality within the context of a
multimedia session. Examples of features include putting a call on
hold (possibly with music), transferring calls, creating ad-hoc
conferences, having calls automatically forwarded, and so on.
The SIP specification itself includes primitives to support some of
these features. For example, RFC 3264 [RFC3264] defines SDP
signaling parameters for placing a call on hold. Numerous SIP
extensions have been developed which focus on functionality needed
for session management features. The REFER specification, RFC 3515
[RFC3515], defines a primitive operation for a user agent to ask
another user agent to send a SIP request, typically to initiate a
session. REFER is used to support many features, such as transfer,
park, and hold. The Replaces specification, RFC 3891 [RFC3891],
allows one dialog to replace another. This header field is useful
for consultation transfer features. The dialog event package, RFC
4235 [RFC4235], allows one UA to learn about the dialog states on
another UA. This package is useful for features such as shared line.
However, despite this veritable plethora of specifications that can
support session management features, in practice, interoperability
has been quite poor for these kinds of functions. When user agents
from one vendor are connected to servers and user agents from other
vendors, very few of these types of features actually work. In most
cases, call hold and basic transfer are broadly interoperable, but
more advanced features such as park and resume, music-on-hold, and
shared line appearances, do not work.
In some cases, these interoperability failures are the fault of poor
implementations. In other cases, they are purposeful failures, meant
to ensure that third party equipment is not utilized in a vendor's
solution. However, in many cases the problem is with the
specifications. There are two primary specification problems that
can cause interoperability failure:
o A feature requires functionality that is not defined in any
specification. Therefore, the feature cannot be implemented in an
interoperable way.
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o A feature can be implemented in many different ways, each one
using different specifications or different call flows, and
assuming different functionality in each participating component
of the system. However, each component in a particular deployment
each chose a different way, and therefore the overall system lacks
interoperability.
This latter problem is the primary focus of this document. Section 2
describes the problem in architectural and more abstract terms.
Section 3 then gives several concrete examples that demonstrate the
problem. Section 4 then proposes a general framework for resolving
the interoperability problem. Finally, Section 6 defines a template
that can be utilized by specifications for addressing this
interoperability problem.
2. The Confusion of Tongues
SIP is typically deployed in environments a large number of user
agents and some number of servers, such as proxy servers, registrars,
feature servers, and so on. Put together, these form a distributed
system used to realize a multimedia communications network.
Architecturally, a SIP-based multimedia network can be though of as a
distributed state machine. Each node in the network implements a
state machine, and messages sent by the protocol serve the purpose of
synchronizing the state machines across nodes. If one considers
these session management features (hold, transfer, park, etc.), each
of them is ultimately trying to achieve a state change in the state
machines of two or more nodes in the network. Call hold, for
example, attempts to change the state of media transfer between a
pair of user agents. More complex features, such as transfer, are an
attempt to synchronize dialog and call states across three or more
user agents. In all cases, SIP messaging is used between these
agents to change the state machinery of the protocol.
If we consider a particular feature, the protocol machinery for
accomplishing the feature requires logic on each node involved in the
feature. Let us say that feature X can be implemented using two
different techniques - X.1 and X.2. Each technique is composed of a
series of message exchanges and associated state machine processing
in each affected node. If all affected nodes implement the same
logic - say the logic for X.1 - the feature works. Similarly, if all
implement the logic for X.2, the feature works. However, if some of
the nodes implement the logic for X.1, and others have implemented
the logic for X.2, the outcome is unpredicable and the feature may
not interoperate.
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We call this problem "the confusion of tongues". It arises whenever
there is more than one way to implement a particular feature amongst
a set of nodes. While each approach is, by itself, conformant to the
specifications, there are interoperability failures because of a
heterogeneous selection of methodologies within a particular
deployment.
This problem is ameliorated when the logic required for a particular
feature exists almost entirely within a single node. Any feature
involving multiple parties ultimately requires some form of logic in
other nodes. However, when the logic required for a feature requires
that the other nodes only support for the basic SIP specs - [RFC3261]
and [RFC3263] - we call this a single ended feature. Single-ended
features tend to be more interoperable because they rely on just the
lingua franca - basic SIP - from everyone else. An example of a
single-ended feature is mute, which can be done locally within a node
without any signaling at all. Another feature is basic hold (without
music), which requires only that the other side support [RFC3263].
Unfortunately, many features are fundamentally not single ended. A
feature that is not single ended is called a multi-ended feature.
Examples include transfer (which relies on at least support for
REFER) and music-on-hold.
3. Concrete Examples
Several concrete examples can be demonstrated which demonstrate the
confusion of tongues.
3.1. Call Forward No Answer
Call Forward No Answer (CFNA), is a very basic feature. In this
feature, user X calls user Y. If user Y is not answering, the call is
forwarded to another user, user Z. Typically this forwarding takes
place after a certain amount of time.
Even for a simple feature like this, there are several ways of
implementing it. Consider the reference architecture in Figure 1.
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+---------+
| |
| |
| Proxy |
| |
| |
+---------+
// | \
// | \
// | \\
// | \
// | \
// | \\
/ | \
| \
+-------+ +-------+ +-------+
| | | | | |
| UA X | | UA Y | | UA Z |
| | | | | |
| | | | | |
+-------+ +-------+ +-------+
Figure 1: Call Forward Use Case
In this simple network, there are four "nodes" that are cooperating
to implement this feature. There are three user agents, UA X, UA Y
and UA Z. All three user agents are associated with a single proxy.
When UA X makes a call to UA Y, the INVITE is sent to the proxy which
delivers it to UA Y.
3.1.1. Approach 1: UA Redirects
In this approach, the call forwarding functionality is implemented in
the user agents. The user agents have a field on the user interface
that a user can enable to cause calls to be forwarded on no-answer.
The user can also set up the forward-to URI through the user
interface.
The basic call flow for this approach is shown in Figure 2.
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UA X Proxy UA Y UA Z
|(1) INVITE Y | | |
|------------->| | |
| |(2) INVITE Y | |
| |------------->| |
| | |No Answer |
| |(3) 302 Z | |
| |<-------------| |
| |(4) ACK | |
| |------------->| |
| |(5) INVITE Z | |
| |---------------------------->|
| |(6) 200 OK | |
| |<----------------------------|
|(7) 200 OK | | |
|<-------------| | |
|(8) ACK | | |
|------------------------------------------->|
Figure 2: CFNA Approach I
When the call from UA X arrives at the proxy, it is forwarded to UA
Y. User Y is not there, so UA Y rings for a time. After the call
forward timeout has elapsed, UA Y generates a 302 response. This
response contains a Contact header field containing the forward-to
URI (sip:Z@example.com). This is received by the proxy, which
recurses on the 3xx, causing the call to be forwarded to Z.
3.1.2. Approach II: Proxy Forwards
In this approach, the call forwarding functionality is implemented in
the proxy. The proxy has a web interface that allows the user to set
up the call forwarding feature and specify the forward-to URI.
The basic call flow for this approach is shown in Figure 3.
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UA X Proxy UA Y UA Z
|(1) INVITE Y | | |
|------------->| | |
| |(2) INVITE Y | |
| |------------->| |
| | |No Answer |
| |(3) CANCEL | |
| |------------->| |
| |(4) 200 CANCEL| |
| |<-------------| |
| |(5) 487 | |
| |<-------------| |
| |(6) ACK | |
| |------------->| |
| |(7) INVITE Z | |
| |---------------------------->|
| |(8) 200 OK | |
| |<----------------------------|
|(9) 200 OK | | |
|<-------------| | |
|(10) ACK | | |
|------------------------------------------->|
Figure 3: CFNA Approach II
When the call from UA X arives at the proxy, the proxy sends the
INVITE to UA Y. UA Y rings for a time. The call timeout timer runs
on the proxy. After the timeout has elapsed, the proxy generates a
CANCEL, causing the call to stop ringing at UA X. It then consults
its internal configuration, notes that call forwarding on no-answer
is configured for user Y. It obtains the forward-to URI, and sends an
INVITE to it. User Z ansers and the call proceeds.
3.1.3. Approach III: UA Proxies
In this last approach, the user agent implements the call forwarding,
but does so by acting as a proxy, forwarding the call to Z on its
own. As in Approach I, the UA would have an interface on its UI for
enabling call forwarding and entering the forward-to URI.
The basic call flow for this approach is shown in Figure 4.
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UA X Proxy UA Y UA Z
|(1) INVITE Y | | |
|------------->| | |
| |(2) INVITE Y | |
| |------------->| |
| | |No Answer |
| |(3) INVITE Z | |
| |<-------------| |
| |(4) INVITE Z | |
| |---------------------------->|
| |(5) 200 OK | |
| |<----------------------------|
| |(6) 200 OK | |
| |------------->| |
| |(7) 200 OK | |
| |<-------------| |
|(8) 200 OK | | |
|<-------------| | |
|(9) ACK | | |
|------------------------------------------->|
Figure 4: CFNA Approach III
UA X sends an INVITE to its proxy targeted for Y. The proxy sends
this INVITE to UA Y. The user does not answer. So, after a timeout,
the UA acts like a proxy and sends the INVITE back to P, this time
with a Request-URI identifying Z. The proxy forwards this to Z, and
the call completes.
3.1.4. Approach IV: Call Processing Language
In this approach, the proxy implements the call forwarding logic.
However, instead of the logic being configured through a web page, it
has been uploaded to the proxy server through a Call Processing
Language (CPL) [RFC3880] script that the UA included in its
registration request.
The basic call flow for this approach is shown in Figure 5.
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UA X Proxy UA Y UA Z
| |(1) REGISTER | |
| |with CPL | |
| |<-------------| |
| |(2) 200 OK | |
| |------------->| |
|(3) INVITE Y | | |
|------------->| | |
| |(4) INVITE Y | |
| |------------->| |
| | |No Answer |
| |(5) CANCEL | |
| |------------->| |
| |(6) 200 CANCEL| |
| |<-------------| |
| |(7) 487 | |
| |<-------------| |
| |(8) ACK | |
| |------------->| |
| |(9) INVITE Z | |
| |---------------------------->|
| |(10) 200 OK | |
| |<----------------------------|
|(11) 200 OK | | |
|<-------------| | |
|(12) ACK | | |
|------------------------------------------->|
Figure 5: CFNA Approach IV
This flow is nearly identical to the one in Figure 3, however, the
logic in the proxy is guided by the CPL script.
3.1.5. Failure Cases
We have now described four different call forwarding implementations.
All four are compliant to RFC 3261. All four assume some form of
"feature logic" in some of the components in order to realize this
feature. For Approach I, this logic is entirely in the UA, and
consists of the activation of the feature, configuration of the
forward-to URI, execution of the timer, and then causing of a
redirect to the forward-to URI. This implementation of the feature
is single ended. For approach II, the logic is entirely in the
proxy, and consists of the activation of the feature through the web,
configuration of the forward-to URI through the web, execution of the
timer, and then causing of CANCEL and sequential fork to the
forward-to URI. This implementation approach is also single-ended.
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In approach III, all of the logic exists on the UA, and consists of
the activation of the feature, configuration of the forward-to URI,
execution of the timer, and then causing of a proxy to the forward-to
URI. This approach is also single-ended. In approach IV, all of the
feature logic is in the proxy, but it is implemented by CPL, and the
UA has a CPL implementation that establishes the forwarding number
configuration. Consequently, this approach is multi-ended.
If one considers several different combinations of implementation,
several error cases arise.
3.1.5.1. No One Implements
In this case, the UA assumes approach II (that is, it assumes the
proxy handles call forwarding), while the proxy assumes approaches I
or III (that is, the UA handles call forwarding). In this case, the
call will arrive at the proxy, which forwards it to UA Y, where it
rings indefinitely. The feature does not get provided at all.
3.1.5.2. Both Implement
In this case, the UA assumes approach I (that is, it assumes that it
handles call forwarding), and the proxy assumes approach II (that it,
it assumes that it handles call forwarding). In this case, assuming
that the forwarding number ends up being provisioned in both places,
the actual behavior of the system is a race condition. If the timer
fires first at the proxy, the call is forwarded to the number
configured on the proxy. If the timer fires first on the UA, the
call is forwarded to the number configured on the UA. If these
forwarding numbers are different, this results in highly confusing
behavior.
3.1.5.3. Missing Half
In this case, the UA implements CPL, but the proxy does not. Or, the
proxy implements CPL, but the UA does not. In either case, the logic
for the forwarding feature cannot be configured, and the feature does
not work.
4. Solution Considerations
There are many ways this interoperability problem can be solved. The
most obvious solution is to actually enumerate every specific feature
that we wish to support with SIP (Call Forward No Answer, Call
Forward Busy, Hold, Music-on-hold, and so on). Then, for each
feature, identify a specific call flow that realizes it, and describe
the exact functionality required in each component of the system. In
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the case of call forward no answer, for example, we would choose one
of the four approaches, define the information that needs to be
configured (timeout, activation state, call forwarding URI), and
describe the timer and how it operates. This approach would actually
lead to excellent interoperability, but would come at high cost. The
set of interoperable features would be limited to only those which we
explicitly specify, and there would be little room for innovation.
To avoid this pitfall and others like it, a proper solution to the
interoperability has to be structured in such a way that it achieves
the following goals:
Covers Everything: Ultimately, the goal of the solution is to make
things work in reality. This means that the solution has to cover
all aspects of the feature that can be a source of
interoperability problems. This includes traditional signaling,
media, and even provisioning and configuration issues. For
example, the failure of Section 3.1.5.3 was caused by an
inconsistent provisioning mechanism between the UA and the server.
Consequently, interoperability requires this mechanism to be
agreed upon to the degree required for interop. The objective of
BLISS is that the resulting specifications ensure that you can
take a UA from one vendor, plug it into the server of another, and
it works - full stop.
Avoid Enumeration: One of the main goals of SIP is to provide a rich
set of features. If it requires a specification to be developed
for each and every feature, this goal of SIP is lost. Instead,
SIP will be limited to a small number of features and it will be
hard to add new ones. Therefore, any solution to the
interoperability problem must avoid the need to enumerate each and
every feature and document something about it.
Allow Variability in Definition: It should not be necessary to
rigorously define the behavior of any particular feature. It is
possible for variations to occur that do not affect
interoperability. For example, a variation on CFNA is that a
provisional response can be sent back to the originator informing
them that the call was forwarded. This variation can be
implemented without impacting interoperability at all; if the
originator can render or utilize the provisional response, things
work. If they can't things still work on the originator simply
doesn't get that part of the feature. We should allow this kind
of localized variability in what each feature does, to preserve
innovation.
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Assume Multimedia: Though many of the features discussed so far are
very telephony centric, they all apply and can be used with any
number of media types. In addition, it is important that the
solution to the interoperability problem not assume a particular
media type. Unless the feature is specifically about a media type
(instant message logging for example), it must be possible for it
to work with all media types.
Allow Variability in Implementation: Whenever possible, the solution
to the interoperability problem should strive to allow variations
in how the implementations work, while preserving
interoperability. For example, in the case of call forwarding,
the central source of interoperability failure is that is unclear
whether the UAs or proxies have responsibility for the forwarding
logic. If the decision was made that this logic is in the UA,
then either Approach I or Approach III will work. Consequently,
it is not necessary to specify which of those two approaches is to
be implemented; just that the UA performs the implementation.
Support a Multiplicity of Environments: SIP is utilized in a broad
set of environments. These include large service providers
targeted to consumers, enterprises with business phones, and peer-
to-peer systems where there is no central server at all. SIP is
utilized in wireless networks with limited bandwidth and high
packet loss, and in high-bandwidth wired environments. It is the
goal of this process that interoperability be possible using the
same set of specifications for all cases. The problem is not
restricted to just enterprises, even though many advanced features
typically get associated with enterprise.
5. BLISS Solution Framework
The framework for solving this interoperability dilemma is called
BLISS - Basic Level of Interoperability for SIP Services. This
solution is actually a process that a working group can follow to
identify interoperability problems and then develop solutions.
5.1. Phase I - Identify a Feature Group
The first step is to identify a feature or set of features which have
been known to be problematic in actual deployments. These features
are collected into bundles called a feature group. A feature group
is a collection of actual features that all have a similar flow, and
for which it is believed the source of interoperability failures may
be common. A feature group can also have just one feature. For
example, Call Forward No Answer, Call Forward Busy, Call Forward
Unconditional are all very similar, and clearly all have the same
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interoperability problem described in Section 3.1. However, the root
issue with these flows is that there needs to be a common
understanding of where call treatment feature logic is executed, and
how the desired treatment is signaled from the user to the place
where it is implemented. Thus, other features that are similar, in
that they make a decision on call handling based on user input or
conditions, will likely also benefit from consideration.
Thus, a feature group is defined by a characteristic that identifies
a large (and in fact, possibly infinite) number of actual "features"
that all belong to the group. This characteristic is called its
functional primitive. The first step in the BLISS process is to
identify feature groups and their functional primitives that are
narrow enough so they are meaningful, yet broad enough that they are
not overly constraining. This is not exact, and the initial
definitions do not need to be exact. They can be refined as the
BLISS process proceeds. Indeed, in many cases, investigations can
start with a single feature - for example call park - and analysis
can proceed with just one. As work proceeds, the definition of the
feature group can be broadened. In the case of CFNA, clearly a
functional primitive of "call forwarding features that execute on no-
answer" is too narrow. A functional primitive of "features that
handle an initial INVITE" is too broad. An ideal starting point
would probably be, "features that result in a retargeting or response
operation that depend on user-specified criteria". This covers all
of the call forwarding variations, but also includes features like
Do-Not-Disturb.
Each feature group should be defined in a similar way, through the
definition of a functional primitive by which one could decide
whether or not a particular feature was included. As part of this
definition, the group can consider specific features and agree
whether or not they are covered by the primitive. For example, would
"send call to voicemail" be covered by the functional primitive
"features that result in a retargeting or response operation that
depend on user-specified criteria"? The answer is yes in this case.
Discussion of what features are covered by a functional primitive is
part of the discussion in this phase.
Care must be taken not to define the functional primitive in such a
way as to eliminate the possibility of any but a defined and
enumerated set of features from being included. The functional
primitive should clearly cover features which are in existence today,
and of interest, but allow for future ones that could be covered by
the primitive. This avoids the perils of enumeration as discussed in
Section 4.
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5.2. Phase II - Gather Data
With the functional primitive identified and a shared understanding
of which features fit within it, the next step is for working group
participants to document how their implementations implement features
in the group.
This can be done any number of ways. Ideally, call flows would be
collected that document the mechanism implemented by each vendor.
However, experience has shown that vendors frequently consider this
information proprietary or sensitive. An alternate model is to
define a survey which asks high level questions about how the feature
or feature group is implemented. Yet another model is to merely ask
vendors to submit freeform text which describes their implementation.
It is a decision of the working group as to whether to actually
publish the collected information as an RFC, use them as a working
internet draft, or just keep them on a web page. The gathered data
is not an output of the BLISS process; they are only an intermediate
step. If the information is to be published as an RFC, it is
suggested that a single document be published for each functional
primitive. The title of the document would be something like,
"Enumeration of Existing Practices for Foo" where "Foo" is some
moniker for the functional primitive. Such a document must be clear
that it is NOT a best practice. It would strictly be informational.
5.3. BLISS Phase III - Problem Definition
With current practice for a particular feature group collected, the
next step in the process is to an analyze the data. The analysis
considers each permutation of implementation of logic from the data
gathered in the previous phase, and determines which combinations
work, and which ones do not.
General speaking, this analysis is performed by taking the components
associated with the feature (for example, in the case of CFNA, there
are four components - three UA and one proxy), and for each one
considering what happens when it implements one of the logical
behaviors identified in the cases identified from the previous phase.
Thus, if four variations on a feature have been submitted to the
group, and that feature has four components, there are 16 possible
deployment scenarios that can be considered. In practice, many of
these are equivalent or moot, and therefore the number in practice
will be much smaller. The group should work to identify those cases
that are going to be of interest, and then based on the logic in each
component, figure out where interoperability failures occur.
This phase can be accomplished using documents that contain flows, or
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can be purely a thinking exercise carried out on the mailing list or
in a design team. In all likelihood, it will depend on the feature
group and the level of complexity. Regardless of the intermediate
steps, the end goal of this phase should be an enumeration of
combinations with known interoperability problems. One possible
output would look exactly like the contents of Section 3.1.5, which
describe several failure modes that are possible.
5.4. BLISS Phase 4 - Minimum Interop Definition
The final step in the BLISS process is to repair the interopreability
failures identified in the previous phase. This is done by coming up
with a set of recommendations on behaviors of various components,
such that, were those rules to be followed, those interoperability
failure cases would not have occurred.
In some cases, these recommendations identify a place in the network
where something has to happen. Again, considering our CFNA example,
the primary recommendation that needs to be made is where the logic
for call handling should happen - in the UA, in the proxy, or both.
This is likely to be a contentious topic, and the right thing will
certainly be a function of participant preference and use cases that
are considered important. But, no one ever said life is easy.
In other cases, these recommendations take the form of a
specification that needs to be implemented. For example, CFNA can be
implemented using CPL, in which case both the UA and proxy need to
support it. If the group should decide that CPL is the main way to
implement these features, the recommendation should clearly state
that CPL is required in both places.
Indeed, if a particular functional primitive requires any
functionality to be present in any node that goes beyond the "common"
functions in RFC 3261, the recommendations need to state that. For
example, if a particular feature can be implemented using S/MIME, and
the group decides that S/MIME is the required everywhere for this
feature to work, that recommendation should be clearly stated.
In some cases, only a part of a specification is required in order
for the features in a feature group to be interoperable. In that
case, the group should identify which parts it is. In the example of
CPL, RFC 3880 [RFC3880], the ability to support non-signalling
controls is not neccesary to achieve an implementation of this
feature group. So, the recommendation could be that this part is not
required.
Another key part of the recommendations that get made in this phase,
are recommendations around capability discovery. If a decision is
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made that says there are multiple different ways that a feature can
work, and it is necessary to know which one is in use, some kind of
capability exchange is required. Consider once more CFNA. If the
recommendation of the group is that all proxies have to implement the
logic associated with the feature, but phones can also optionally do
it, the UA needs to determine whether it has to be responsible for
this feature or not. Otherwise, the failure mode in Section 3.1.5.2
may still happen. This particular problem can be resolved, for
example, by the use of a feature tag in the Require header field that
would inform the proxy whether it should or should not provide the
feature. The BLISS recommendations for this phase need to include
these kinds of things, if they are necessary for the feature group.
The recommendations in this phase, covering specific protocols or
pieces of protocols, places where functionality needs to reside, and
capability negotiations and controls, are all the final output of the
BLISS process. If the group has done its job well, with these
recommendations, a (potentially large) class of features will
interoperate, yet there will be room for innovation.
6. Structure of the BLISS Final Deliverable
This section describes a recommended template for the final BLISS
deliverable - the recommendations of Section 5.4.
There will typically be a document produced per functional primitive.
The title of the document must clearly articulate the functional
primitive that is being addressed. For example, if the functional
group is forwarding, an appropriate title would be, "Best Practices
for Interoperability of Forwarding Features in the Session Initiation
Protocol". It is important that the feature group be well
articulated in the title, so that implementors seeking guidance on
these features can find it.
Similarly, the abstract of the document is very important. It has to
contain several sentences that more clearly articulate the functional
primitive definition. In addition, the abstract should contain
example features, by name or description, that are defined by the
functional primitive. Again, this is important so that people
looking to understand why feature foo doesn't work, can find the
right specification that tells them what they need to do to make it
work.
The body of the document needs to first clearly and fully define the
functional primitive. It must then enumerate features that are in
the group. Next, the document should summarize the problems that
have arisen in practice that led to the interoperability failures.
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This would basically be a summarization of the results of phase III
of the BLISS process. If the feature group were call forwarding,
this part of the document would discuss how the primary problem is
where in the network the actual feature logic lives - UA or proxy,
and that the interop problems occur because of inconsistent choices
between UA and proxy. The final part of the document is explicit
recommendations. This would typically be broken out by component
types - a section for UA, a section for proxies or "servers" more
generally (so that it is clear that B2BUAs aren't excused from the
interoperability requirements). This section would clearly state the
requirements for this feature group - specifications, portions of
specifications, and capability behaviors that are required.
7. Security Considerations
Interoperability of security functions is also a critical part of the
overall interoperability problem, and must be considered as well.
8. IANA Considerations
There are no IANA considerations associated with this specification.
9. Acknowledgements
I'd like to thank Shida Schubert, Jason Fischl, and John Elwell for
actually running the BLISS process and providing feedback on its
effectiveness.
10. Informative References
[RFC3261] 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.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
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Method", RFC 3515, April 2003.
[RFC3891] Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
Protocol (SIP) "Replaces" Header", RFC 3891,
September 2004.
[RFC4235] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
[RFC3880] Lennox, J., Wu, X., and H. Schulzrinne, "Call Processing
Language (CPL): A Language for User Control of Internet
Telephony Services", RFC 3880, October 2004.
Author's Address
Jonathan Rosenberg
Cisco
Iselin, NJ
US
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
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| PAFTECH AB 2003-2026 | 2026-04-23 09:22:27 |