One document matched: draft-ietf-iptel-cpl-framework-00.txt
Internet Engineering Task Force IPTEL WG
Internet Draft Lennox/Schulzrinne
draft-ietf-iptel-cpl-framework-00.txt Columbia University
June 25, 1999
Expires: December 1999
Call Processing Language Framework and Requirements
STATUS OF THIS MEMO
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all provisions of Section 10 of RFC2026.
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Abstract
A large number of the services we wish to make possible for Internet
telephony require fairly elaborate combinations of signalling
operations, often in network devices, to complete. We want a simple
and standardized way to create such services to make them easier to
implement and deploy. This document describes an architectural
framework for such a mechanism, which we call a call processing
language. It also outlines requirements for such a language.
1 Introduction
Recently, several protocols have been created to allow telephone
calls to be made over IP networks, notably SIP [1] and H.323 [2].
These emerging standards have opened up the possibility of a broad
and dramatic decentralization of the provisioning of telephone
services so they can be under the user's control.
Many Internet telephony services can, and should, be implemented
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entirely on end devices. Multi-party calls, for instance, or call
waiting alert tones, or camp-on services, depend heavily on end-
system state and on the specific content of media streams,
information which often is only available to the end system. A
variety of services, however -- those involving user location, call
distribution, behavior when end systems are busy, and the like -- are
independent of a particular end device, or need to be operational
even when an end device is unavailable. These services are still best
located in a network device, rather than in an end system.
Traditionally, network-based services have been created only by
service providers. Service creation typically involved using
proprietary or restricted tools, and there was little range for
customization or enhancement by end users. Internet telephony,
however, provides an opportunity to open up the service creation
process to end users or third-party service designers. To accomplish
this however, we need a standardized, safe way for these new service
creators to describe the desired behavior of network servers.
This document describes an architecture in which network devices
respond to call signalling events by triggering user-created programs
written in a simple, static, non-expressively-complete language. We
call this language a call processing language
2 Motivating examples
To motivate the subsequent discussion, this section gives some
specific examples of services which we want users to be able to
create programmatically. Note that some of these examples are
deliberately somewhat complicated, so as to demonstrate the level of
decision logic that should be possible.
o Call forward on busy/no answer
When a new call comes in, the call should ring at the user's
desk telephone. If it is busy, the call should always be
redirected to the user's voicemail box. If, instead, there's no
answer after four rings, it should also be redirected to his or
her voicemail, unless it's from a supervisor, in which case it
should be proxied to the user's cell phone if it is currently
registered.
o Information address
A company advertises a general "information" address for
prospective customers. When a call comes in to this address, if
it's currently working hours, the caller should be given a list
of the people currently willing to accept general information
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calls. If it's outside of working hours, the caller should get a
webpage indicating what times they can call.
o Intelligent user location
When a call comes in, the list of locations where the user has
registered should be consulted. Depending on the type of call
(work, personal, etc.), the call should ring at an appropriate
subset of the registered locations, depending on information in
the registrations. If the user picks up from more than one
station, the pick-ups should be reported back separately to the
calling party.
o Intelligent user location with media knowledge
When a call comes in, the call should be proxied to the station
the user has registered from whose media capabilities best match
those specified in the call request. If the user does not pick
up from that station within four rings, the call should be
proxied to the other stations from which he or she has
registered, sequentially, in order of decreasing closeness of
match.
o Client billing allocation -- lawyer's office
When a call comes in, the calling address is correlated with the
corresponding client, and client's name, address, and the time
of the call is logged. If no corresponding client is found, the
call is forwarded to the lawyer's secretary.
3 Architecture
The Call Processing Language operates on a generalized model of an
Internet telephony network. While the details of various protocols
differ, on an abstract level all major Internet telephony
architectures are sufficiently similar that their major features can
be described commonly.
3.1 Network components
In the view of the Call Processing Language, an Internet telephony
network consists of two types of components. End systems originate
and/or receive signalling information and media; network systems
relay or control signalling information. While in actual networks
other devices exist, such as mixers, media gateways, or firewalls,
the CPL does not deal with them directly, and they will not be
discussed here.
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Network systems, in SIP, are proxy servers, redirect servers, or
registrars; in H.323 they are gatekeepers. On the abstract level the
CPL deals with, the functionality of two protocols is largely
equivalent, and this document will generally use SIP terminology.
Network systems can perform three types of actions on call setup
information. They can:
proxy it: forward it on to one or more other network or end systems,
returning one of the responses received.
redirect it: return a response informing the sending system of a
different address to which it should send the request.
reject it: inform the sending system that the setup request could not
be completed. See RFC 2543 [1] for illustrations of proxy and
redirect functionality. End systems may also be able to perform
some of these actions: almost certainly rejection, and possibly
redirection.
3.2 Network model
An Internet telephony network contains a number of network systems
and a number of user agents. Call establishment requests can pass
through a series of network systems, and user agents can be contacted
by any of a number of network systems, or directly by other user
agents.
For example, in figure 1, there are two paths the call establishment
request information may take. For Route 1, the originator knows only
a user address for the user it is trying to contact, and it is
configured to send outgoing calls through a local outgoing proxy
server. Therefore, it forwards the request to its local server,
which finds the server of record for that address, and forwards it on
to that server.
In this case, the organization the destination user belongs to uses a
multi-stage setup to find users. The corporate server identifies
which department a user is part of, then forwards the request to the
appropriate departmental server, which actually locates the user.
(This is similar to the way e-mail forwarding is often configured.)
The response to the request will travel back along the same path.
For route 2, however, the originator knows the specific device
address it is trying to contact, and it is not configured to use a
local outgoing proxy. In this case, the originator can directly
contact the destination without having to communicate with any
network servers at all.
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Outgoing Corporate Departmental
Proxy Server Server
_______ Outgoing proxy contacts _______ _______
| | corporate server | | | |
| | -------------------------> | | ---------> | |
|_____| |_____| |_____|
Route 1 ^ \ Searches
/ \ for
Sends to/ \ User
proxy / _|
_______ _______
| | Route 2 | |
| | ----------------------------------------------------> | |
|_____| Originator directly contacts destination |_____|
Originator Destination
Figure 1: Possible paths of call setup messages
We see, then, that in Internet telephony network systems cannot in
general know the state of end systems they "control," since
signalling information may have bypassed them. This architectural
limitation implies a number of restrictions on how some services can
be implemented. For instance, a network system cannot reliably know
if an end system is currently busy or not; a call may have been
placed to the end system without traversing that network system.
Thus, signalling messages must explicitly travel to end systems to
find out their state; in the example, the end system must explicitly
return a "busy" indication.
Users can have CPL scripts on any network server which their call
establishment requests pass through and with which they have a trust
relationship. For instance, in the example above the destination user
could have scripts on both the corporate server and the departmental
server. These scripts would typically perform different functions,
related to the role of the server on which they reside; a script on
the corporate-wide server could be used to customize which department
the user wishes to be found at, for instance, whereas a script at the
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departmental server could be used for more fine-grained location
customization. Some services, such as filtering out unwanted calls,
could be located at either server. See section 3.6.3 for some
implications of a scenario like this.
3.3 Role of a CPL script
A CPL script runs in a network system, and controls that system's
proxy, redirect, or rejection actions for the set-up of a particular
call. It does not attempt to co-ordinate the behavior of multiple
network systems, or to describe features on a "Global Functional
Plane" as in the Intelligent Network architecture.
CPL scripts are associated with a particular Internet telephony
address. When a call establishment request arrives at a network
system which is a CPL server, that server associates the address
specified in the request with its database of CPL scripts; if one
matches, that corresponding script is executed. CPL scripts may be
associated either with the originator address or the destination
address of the call establishment request. For some discussion of
what happens if, for instance, a server has scripts for both an
originating and destination address, see section 3.6.2.
In general, in an Internet telephony network, an address will denote
one of two things: either a user, or a device. A user address refers
to a particular individual, for example sip:joe@example.com,
regardless of where that user actually is or what kind of device he
or she is using. A device address, by contrast, refers to a
particular physical device, such as sip:x26063@phones.example.com.
Other, intermediate sorts of addresses are also possible, and have
some use (such as an address for "my cell phone, wherever it
currently happens to be registered"), but we expect them to be less
common. A CPL script is agnostic to the type of address it is
associated with; while scripts associated with user addresses are
probably the most useful for most services, there is no reason that a
script could not be associated with any other type of address as
well.
By controlling basic call set-up actions, a user can achieve a number
of services. Many common services are implemented using a CPL script
for incoming calls to a user address. These include: searching for
the user's current location in some specialized way; specifying what
happens when this initial search fails, either because it received
some sort of negative response (e.g., busy) or did not receive any
definitive response within a fixed time period (e.g., no answer); or
handling certain originating addresses specifically, for instance by
informing the caller that the call was refused. Services that can be
implemented by a script triggered by an outgoing user address are
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somewhat more limited, but one example is to translate a user's
abbreviated addresses into addresses specified with a fully-qualified
domain name.
3.4 Creation and transport of a call processing language script
Users create call processing language scripts, typically on end
devices, and transmit them through the network to network systems.
Scripts persist in network systems until changed or deleted, unless
they are specifically given an expiration time; a network system
which supports CPL scripting will need stable storage.
The exact means by which the end device transmits the script to the
server remains to be determined; it is likely that many solutions
will be able to co-exist. This method will need to be authenticated
in almost all cases. The methods that have been suggested include
web file upload, SIP REGISTER message payloads, remote method
invocation, SNMP, ACAP, LDAP, and remote file systems such as NFS.
Creation of a CPL script may be through the creation of a text file;
or for a simpler user experience, a graphical user interface which
allows the manipulation of some basic rules.
The end device on which the user creates the CPL script need not bear
any relationship to the end devices to which calls are actually
placed. For example, a CPL script might be created on a PC, whereas
calls might be intended to be received on a simple audio-only
telephone. The CPL also might not necessarily be created on a device
near either the end device or the signalling server in network terms;
a user might, for example, decide to forward his or her calls to a
remote location only after arriving at that location.
Users can also retrieve their current script from the network to an
end system so it can be edited. The signalling server should also be
able to report errors related to the script to the user, both static
errors that could be detected at upload time, and any run-time errors
that occur.
If a user's calls can pass through multiple local signalling servers
which know about that user (as discussed in section 3.2), the user
may choose to upload scripts to any or all of those servers. These
scripts can be entirely independent.
3.5 Execution process of a CPL script
When a call event arrives, a CPL server considers the information in
the request and determines if any of the scripts it has stored are
applicable to the call in question. If so, it performs the actions
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corresponding to the matching scripts.
The most common type of script defines a set of actions to be taken
for the entire process of call set-up -- from the time a call request
is initially received, to the time that (from the point of view of
this device) the call is either definitively accepted or definitively
rejected. This could be near-instantaneous, if, for instance, the
script decides to reject the call; or it could be an arbitrarily long
time, if the server allows calls to wait for a call pick-up without
imposing a timeout.
Abstractly, a script can be considered as a list of condition/action
pairs; if an incoming invitation matches a given condition, then the
corresponding action (or more properly set of actions) will be taken.
In some circumstances, additional actions can be taken based on the
consequences of the first action, and possibly on additional
conditions. If no condition matches the invitation, the signalling
server's standard action should be taken.
While many of the uses of a CPL script are specific to one particular
user, there are a number of circumstances in which an administrator
of a signalling server would wish to provide a script which applies
to all users of the server, or a large set of them. For instance, a
system might be configured to prevent calls from or to a list of
banned incoming or outgoing addresses; these should presumably be
configured for everyone, but users still need to be able to have
their own custom scripts as well. Similarly, an administrative script
might perform the necessary operations to allow media to traverse a
firewall; but individual users' scripts should not have permission to
perform these operations. See the next section for some implications
of this.
3.6 Feature interaction behavior
Feature interaction is the term used in telephony systems when two or
more requested features produce ambiguous or conflicting behavior
[3]. Feature interaction issues for features implemented with a call
processing language can be roughly divided into three categories:
feature-to-feature in one server, script-to-script in one server, and
server-to-server.
3.6.1 Feature-to-feature interactions
Due to the explicit nature of event conditions discussed in the
previous section, feature-to-feature interaction is not likely to be
a problem in a call processing language environment. Whereas a
subscriber to traditional telephone features might unthinkingly
subscribe to both "call waiting" and "call forward on busy," a user
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creating a CPL script would only be able to trigger one action in
response to the condition "a call arrives while the line is busy."
Given a good user interface for creation, or a CPL server which can
check for unreachable code in an uploaded script, contradictory
condition/action pairs can be avoided.
3.6.2 Script-to-script interactions
Script-to-script interactions can arise if multiple scripts are
invoked for a single call. This can occur in a number of possible
cases: if both the call originator and the destination have scripts
specified on a single server; if a script forwards a request to
another address which also has a script; or if an administrative
script is specified as well as a user's individual script.
In the first two of these cases, the correct behavior is fairly
obvious: the server should first execute the actions specified by the
"first" script. In the first case, this is the originator's script;
in the second case, this is the script which triggered the request.
When the first script says to forward the request to some other
address, those actions are considered as new requests which arrive at
the second script. When the second script sends back a final
response, that response arrives at the first script in the same
manner as if a script arrived over the network. Note that for the
second type of these interactions, script forwarding can be
recursive; a CPL server much be careful to prevent forwarding loops.
The correct behavior for the third type of script-to-script
interaction depends on the scope of the administrative script.
Typically, the administrator's script should run after origination
scripts, intercepting any proxy or redirection decisions, and before
recipient scripts, to avoid a user's script evading administrative
restrictions.
3.6.3 Server-to-server interactions
The third case of feature interactions, server-to-server
interactions, is the most complex of these three. The canonical
example of this type of interaction is the combination of Originating
Call Screening and Call Forwarding: a user (or administrator) may
wish to prevent calls from being placed to a particular address, but
the local script has no way of knowing if a call placed to some
other, legitimate address will be proxied, by a remote server, to the
banned address. This type of problem is unsolvable in an
administratively heterogeneous network, even a "lightly"
heterogeneous network such as current telephone systems. CPL does not
claim to solve it, but the problem is not any worse for CPL scripts
than for any other means of deploying services.
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Another class of server-to-server interactions are best resolved by
the underlying signalling protocol, since they can arise whether the
signalling servers are being controlled by a call processing language
or by some entirely different means. One example of this is
forwarding loops, where user X may have calls forwarded to Y, who has
calls forwarded back to X. SIP has a mechanism to detect such loops.
A call processing language server thus does not need to define any
special mechanisms to prevent such occurrences; it should, however,
be possible to trigger a different set of call processing actions in
the event that a loop is detected, and/or to report back an error to
the owner of the script through some standardized run-time error
reporting mechanism.
3.6.4 Signalling ambiguity
As an aside, [3] discusses a fourth type of feature interaction for
traditional telephone networks, signalling ambiguity. This can arise
when several features overload the same operation in the limited
signal path from an end station to the network: for example, flashing
the switch-hook can mean both "add a party to a three-way call" and
"switch to call waiting." Because of the explicit nature of
signalling in both the Internet telephony protocols discussed here,
this issue does not arise.
3.7 Relationship with existing languages
This document's description of the CPL as a "language" is not
intended to imply that a new language necessarily needs to be
implemented from scratch. A server could potentially implement all
the functionality described here as a library or set of extensions
for an existing language; Java, or the various freely-available
scripting languages (Tcl, Perl, Python, Guile), are obvious
possibilities.
However, there are motivations for creating a new language. All the
existing languages are, naturally, expressively complete; this has
two inherent disadvantages. The first is that any function
implemented in them can take an arbitrarily long time, use an
arbitrarily large amount of memory, and may never terminate. For call
processing, this sort of resource usage is probably not necessary,
and as described in section 5.1, may in fact be undesirable. One
model for this is the electronic mail filtering language Sieve [4],
which deliberately restricts itself from being Turing-complete. The
second disadvantage with expressively complete languages is that they
make automatic generation and parsing very difficult; an analogy can
be drawn with the difference between markup languages like HTML or
XML, which can easily be manipulated by smart editors, and powerful
document programming languages such as Latex or Postscript which
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usually cannot be.
4 Related work
4.1 IN service creation environments
The ITU's IN series describe, on an abstract level, service creation
environments [5]. These describe services in a traditional circuit-
switched telephone network as a series of decisions and actions
arranged in a directed acyclic graph. Many vendors of IN services use
modified and extended versions of this for their proprietary service
creation environments.
4.2 SIP CGI
SIP CGI [6] is an interface for implementing services on SIP servers.
Unlike a CPL, it is a very low-level interface, and would not be
appropriate for services written by non-trusted users.
5 Necessary language features
This section lists those properties of a call processing language
which we believe to be necessary to have in order to implement the
motivating examples, in line with the described architecture.
5.1 Language characteristics
These are some abstract attributes which any proposed call processing
language should possess.
o Light-weight, efficient, easy to implement
In addition to the general reasons why this is desirable, a
network server might conceivably handle very large call volumes,
and we don't want CPL execution to be a major bottleneck. One
way to achieve this might be to compile scripts before
execution.
o Easily verifiable for correctness
For a script which runs in a server, mis-configurations can
result in a user becoming unreachable, making it difficult to
indicate run-time errors to a user (though a second-channel
error reporting mechanism such as e-mail could ameliorate this).
Thus, it should be possible to verify, when the script is
committed to the server, that it is at least syntactically
correct, does not have any obvious loops or other failure modes,
and does not use too many server resources.
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o Executable in a safe manner
No action the CPL script takes should be able to subvert
anything about the server which the user shouldn't have access
to, or affect the state of other users without permission.
Additionally, since CPL scripts will typically run on a server
on which users cannot normally run code, either the language or
its execution environment must be designed so that scripts
cannot use unlimited amounts of network resources, server CPU
time, storage, or memory.
o Easily writeable and parseable by both humans and machines.
For maximum flexibility, we want to allow humans to write their
own scripts, or to use and customize script libraries provided
by others. However, most users will want to have a more
intuitive user-interface for the same functionality, and so will
have a program which creates scripts for them. Both cases
should be easy; in particular, it should be easy for script
editors to read human-generated scripts, and vice-versa.
o Extensible
It should be possible to add additional features to a language
in a way that existing scripts continue to work, and existing
servers can easily recognize features they don't understand and
safely inform the user of this fact.
o Independent of underlying signalling details
The same scripts should be usable whether the underlying
protocol is SIP, H.323, a traditional telephone network, or any
other means of setting up calls. It should also be agnostic to
address formats. (We use SIP terminology in our descriptions of
requirements, but this should map fairly easily to other
systems.) It may also be useful to have the language extend to
processing of other sorts of communication, such as e-mail or
fax.
5.2 Base features -- call signalling
To be useful, a call processing language obviously should be able to
react to and initiate call signalling events.
o Should execute actions when a call request arrives
See section 3, particularly 3.5.
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o Should be able to make decisions based on event properties
A number of properties of a call event are relevant for a
script's decision process. These include, roughly in order of
importance:
- Destination address
We want to be able to do destination-based routing or
screening. Note that in SIP we want to be able to filter
on either or both of the addresses in the To header and the
Request-URI.
- Originator address
Similarly, we want to be able to do originator-based
screening or routing.
- Caller Preferences
In SIP, a caller can express preferences about the type of
device to be reached -- see [7]. The script should be able
to make decisions based on this information.
- Information about caller or call
SIP has textual fields such as Subject, Organization,
Priority, etc., and a display name for addresses; users can
also add non-standard additional headers. H.323 has a
single Display field.
- Media description
Requests specify the types of media that will flow, their
bandwidth usage, their network destination addresses, etc.
- Authentication/encryption status
Requests can be authenticated. Many properties of the
authentication are relevant: the method of
authentication/encryption, who performed the
authentication, which specific fields were encrypted, etc.
o Should be able to take action based on a request
There are a number of actions we can take in response to an
incoming request. We can:
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- reject it
We should be able to indicate that the call is not
acceptable or not able to be completed. We should also
be able to send more specific rejection codes
(including, for SIP, the associated textual string,
warning codes, or message payload).
- send a provisional response to it
While a call request is being processed, provisional
responses such as "Trying," "Ringing," and "Queued" are
sent back to the caller. It is not clear whether the
script should specify the sending of such responses
explicitly, or whether they should be implicit in other
actions performed.
- redirect it
We should be able to tell the request sender to try a
different location.
- proxy it
We should be able to send the request on to another
location, or to several other locations ("branching" the
request), and await the responses. It should also be
possible to specify a timeout value after which we give
up on receiving any definitive responses.
o Should be able to take action based a response to a
proxied or branched request
Once we have proxied requests, we need to be able to make
decisions based on the responses we receive to those
requests (or the lack thereof). We should be able to:
- consider its message fields
We should be able to consider the same fields of a
response as we consider in the initial request.
- relay it on to the requestor
If the response is satisfactory, it should be
returned to the sender.
- for a branch, choose one of several responses to
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relay back
If we branched a request, we obviously expect to
receive several responses. There are several issues
here -- choosing among the responses, and how long to
wait if we've received responses from some but not
all destinations.
- initiate other actions
If we didn't get a response, or any we liked, we
should be able to try something else instead (e.g.,
call forward on busy).
5.3 Base features -- non-signalling
A number of other features that a call processing language should
have do not refer to call signalling per se; however, they are still
extremely desirable to implement many useful features.
The servers which provide these features might reside in other
Internet devices, or might be local to the server (or other
possibilities). The language should be independent of the location of
these servers, at least at a high level.
o Logging
In addition to the CPL server's natural logging of events, the
user will also want to be able to log arbitrary other items. The
actual storage for this logging information might live either
locally or remotely.
o Error reporting
If an unexpected error occurs, the script should be able to
report the error to the script's owner. This should use the same
mechanism as the script server uses to report language errors to
the user (see section 5.5).
o Access to user-location info
Proxies will often collect information on users' current
location, either through SIP REGISTER messages, the H.323 RRQ
family of RAS messages, or some other mechanism (see section
3.2). The CPL should be able to refer to this information so a
call can be forwarded to the registered locations or some subset
of them.
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o Database access
Much information for CPL control might be stored in external
databases, for example a wide-area address database, or
authorization information, for a CPL under administrative
control. The language could specify some specific database
access protocols (such as SQL or LDAP), or could be more
generic.
o Other external information
Other external information the script should be able to access
includes web pages, which could be sent back in a SIP message
body; or a clean interface to remote procedure calls such as
Corba, RMI, or DCOM, for instance to access an external billing
database.
5.4 Language features
Some features do not involve any operations external to the CPL's
execution environment, but are still necessary to allow some standard
services to be implemented. (This list is not exhaustive.)
o Pattern-matching
It should be possible to give special treatment to addresses and
other text strings based not only on the full string but also on
more general or complex sub-patterns of them.
o Address filtering
Once a set of addresses has been retrieved through one of the
methods in section 5.3, the user needs to be able to choose a
sub-set of them, based on their address components or other
parameters.
o Randomization
Some forms of call distribution are randomized as to where they
actually end up.
o Date/time information
Users may wish to condition some services (e.g., call
forwarding, call distribution) on the current time of day, day
of the week, etc.
5.5 Control
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Internet Draft CPL-F June 25, 1999
As described in section 3.4, we must have a mechanism to send and
retrieve CPL scripts, and associated data, to and from a signalling
server. This method should support reporting upload-time errors to
users; we also need some mechanism to report errors to users at
script execution time. Authentication is vital, and encryption is
very useful. The specification of this mechanism can be (and probably
ought to be) a separate specification from that of the call
processing language itself.
6 Security considerations
The security considerations of transferring CPL scripts are discussed
in sections 3.4 and 5.5. Some considerations about the execution of
the language are discussed in section 5.1.
7 Changes from previous versions
7.1 Changes from draft-ietf-iptel-cpl-requirements-00
The changebars in the Postscript version of this document indicate
changes from this version.
o Changed the title of the draft from "...Requirements" to
"...Framework and Requirements," and changed the draft name,
to better reflect the content.
o Deleted a number of overambitious service examples that aren't
supported in the CPL as it has developed.
o Deleted discussion of end systems, media devices, and other
items that aren't supported in the CPL as it has developed.
o Reorganized the Architecture section.
o Clarified the Network Model section.
o Added Related Work section.
o Added requirement to support caller preferences.
o Deleted many requirements for higher-level and end-system
features that are not supported in the CPL as it has
developed.
o Re-worded many sections for clarity.
o Added To Do / Open Issues section.
Lennox/Schulzrinne [Page 17]
Internet Draft CPL-F June 25, 1999
8 To Do / Open Issues
o Add Terminology section.
o How do users find out which servers they should upload their
scripts to?
o Flesh out the Related Work sections, particularly describing
the different roles of CPL and SIP CGI. (As in [8].)
o The Control section needs to be fleshed out considerably.
o The entire document should be reorganized for clarity.
9 Acknowledgments
We would like to thank Tom La Porta and Jonathan Rosenberg for their
comments and suggestions.
10 Authors' Addresses
Jonathan Lennox
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
USA
electronic mail: lennox@cs.columbia.edu
Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
USA
electronic mail: schulzrinne@cs.columbia.edu
11 Bibliography
[1] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," Request for Comments (Proposed
Standard) 2543, Internet Engineering Task Force, Mar. 1999.
[2] International Telecommunication Union, "Visual telephone systems
and equipment for local area networks which provide a non-guaranteed
quality of service," Recommendation H.323, Telecommunication
Standardization Sector of ITU, Geneva, Switzerland, May 1996.
Lennox/Schulzrinne [Page 18]
Internet Draft CPL-F June 25, 1999
[3] E. J. Cameron, N. D. Griffeth, Y.-J. Lin, M. E. Nilson, W. K.
Schure, and H. Velthuijsen, "A feature interaction benchmark for IN
and beyond," Feature Interactions in Telecommunications Systems, IOS
Press , pp. 1--23, 1994.
[4] T. Showalter, "Sieve: A mail filtering language," Internet Draft,
Internet Engineering Task Force, Mar. 1999. Work in progress.
[5] International Telecommunication Union, "General recommendations
on telephone switching and signaling -- intelligent network:
Introduction to intelligent network capability set 1," Recommendation
Q.1211, Telecommunication Standardization Sector of ITU, Geneva,
Switzerland, Mar. 1993.
[6] J. Lennox, J. Rosenberg, and H. Schulzrinne, "Common gateway
interface for SIP," Internet Draft, Internet Engineering Task Force,
May 1999. Work in progress.
[7] H. Schulzrinne and J. Rosenberg, "SIP caller preferences and
callee capabilities," Internet Draft, Internet Engineering Task
Force, Mar. 1999. Work in progress.
[8] J. Rosenberg, J. Lennox, and H. Schulzrinne, "Programming
internet telephony services," Technical Report CUCS-010-99, Columbia
University, New York, New York, Mar. 1999.
Full Copyright Statement
Copyright (c) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
Lennox/Schulzrinne [Page 19]
Internet Draft CPL-F June 25, 1999
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Table of Contents
1 Introduction ........................................ 1
2 Motivating examples ................................. 2
3 Architecture ........................................ 3
3.1 Network components .................................. 3
3.2 Network model ....................................... 4
3.3 Role of a CPL script ................................ 6
3.4 Creation and transport of a call processing
language script ................................................ 7
3.5 Execution process of a CPL script ................... 7
3.6 Feature interaction behavior ........................ 8
3.6.1 Feature-to-feature interactions ..................... 8
3.6.2 Script-to-script interactions ....................... 9
3.6.3 Server-to-server interactions ....................... 9
3.6.4 Signalling ambiguity ................................ 10
3.7 Relationship with existing languages ................ 10
4 Related work ........................................ 11
4.1 IN service creation environments .................... 11
4.2 SIP CGI ............................................. 11
5 Necessary language features ......................... 11
5.1 Language characteristics ............................ 11
5.2 Base features -- call signalling .................... 12
5.3 Base features -- non-signalling ............ 15
5.4 Language features ................................... 16
5.5 Control ............................................. 16
6 Security considerations ............................. 17
7 Changes from previous versions ...................... 17
7.1 Changes from draft-ietf-iptel-cpl-requirements-00
................................................................ 17
8 To Do / Open Issues ................................. 18
9 Acknowledgments ..................................... 18
10 Authors' Addresses .................................. 18
11 Bibliography ........................................ 18
Lennox/Schulzrinne [Page 20]
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