Call Processing Language Framework and Requirements
draft-ietf-iptel-cpl-framework-01
The information below is for an old version of the document that is already published as an RFC.
| Document | Type | RFC Internet-Draft (iptel WG) | |
|---|---|---|---|
| Authors | Jonathan Lennox , Henning Schulzrinne | ||
| Last updated | 2013-03-02 (Latest revision 1999-10-28) | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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| Document shepherd | (None) | ||
| IESG | IESG state | RFC 2824 (Informational) | |
| Consensus boilerplate | Unknown | ||
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| Send notices to | (None) |
draft-ietf-iptel-cpl-framework-01
Internet Engineering Task Force IPTEL WG
Internet Draft Lennox/Schulzrinne
draft-ietf-iptel-cpl-framework-01.txt Columbia University
October 22, 1999
Expires: April 2000
Call Processing Language Framework and Requirements
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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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. In the Internet
environment, however, this changes. Global connectivity and open
protocols allow end users or third parties to design and implement
new or customized services, and to deploy and modify their services
dynamically without requiring a service provider to act as an
intermediary.
A number of Internet applications have such customization
environments -- the web has CGI [3], for instance, and e-mail has
Sieve [4] or procmail. To create such an open customization
environment for Internet telephony, 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.
The development of this document has been substantially informed by
the development of a particular call processing language, as
described in [5]. In general, when this document refers to "a call
processing language," it is referring to a generic language that
fills this role; "the call processing language" or "the CPL" refers
to this particular language.
2 Example services
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
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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 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 Usage scenarios
A CPL would be useful for implementing services in a number of
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different scenarios.
o Script creation by end user
In the most direct approach for creating a service with a CPL,
an end user simply creates a script describing their service.
He or she simply decides what service he or she wants,
describes it using a CPL script, and then uploads it to a
server.
o Third party outsourcing
Because a CPL is a standardized language, it can also be used
to allow third parties to create or customize services for
clients. These scripts can then be run on servers owned by the
end user or the user's service provider.
o Administrator service definition
A CPL can also be used by server administrators to create
simple services or describe policy for servers they control.
If a server is implementing CPL services in any case,
extending the service architecture to allow administrators as
well as users to create scripts is a simple extension.
o Web middleware
Finally, there have been a number of proposals for service
creation or customization using web interfaces. A CPL could be
used as the back-end to such environments: a web application
could create a CPL script on behalf of a user, and the
telephony server could then implement the services without
either compotent having to be aware of the specifics of the
other.
4 CPL creation
There are also a number of means by which CPL scripts could be
created. Like HTML, which can be created in a number of different
manners, we envision multiple creation styles for a CPL script.
o Hand authoring
Most directly, CPL scripts can be created by hand, by
knowledgable users. The CPL described in [5] has a text
format with an uncomplicated syntax, so hand authoring will be
straightforward.
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o Automated scripts
CPL features can be created by automated means, such as in the
example of the web middleware described in the previous
section. With a simple, text-based syntax, standard text-
processing languages will be able to create and edit CPL
scripts easily.
o GUI tools
Finally, users will be able to use GUI tools to create and
edit CPL scripts. We expect that most average-experience
users will take this approach once the CPL gains popularity.
The CPL will be designed with this application in mind, so
that the full expressive power of scripts can be represented
simply and straightforwardly in a graphical manner.
5 Network model
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. This document generally uses SIP terminology,
as its authors' experience has mainly been with that protocol.
5.1 Model components
In the Call Processing Language's network model, an Internet
telephony network contains two types of components.
5.1.1 End systems
End systems are devices which originate and/or receive signalling
information and media. These include simple and complex telephone
devices, PC telephony clients, and automated voice systems. The CPL
abstracts away the details of the capabilities of these devices. An
end system can originate a call; and it can accept, reject, or
forward incoming calls. The details of this process (ringing, multi-
line telephones, and so forth) are not important for the CPL.
For the purposes of the CPL, gateways -- for example, a device which
connects calls between an IP telephony network and the PSTN -- are
also considered to be end systems. Other devices, such as mixers or
firewalls, are not directly dealt with by the CPL, and they will not
be discussed here.
5.1.2 Signalling servers
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Signalling servers are devices which relay or control signalling
information. In SIP, they are proxy servers, redirect servers, or
registrars; in H.323, they are gatekeepers.
Signalling servers 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.
RFC 2543 [1] has illustrations of proxy and redirect functionality.
End systems may also be able to perform some of these actions: almost
certainly rejection, and possibly redirection.
Signalling servers also normally maintain information about user
location. Whether by means of registrations (SIP REGISTER or H.323
RAS messages), static configuration, or dynamic searches, signalling
servers must have some means by which they can determine where a user
is currently located, in order to make intelligent choices about
their proxying or redirection behavior.
Signalling servers are also usually able to keep logs of transactions
that pass through them, and to send e-mail to desinations on the
Internet, under programatic control.
5.2 Component interactions
When an end system places a call, the call establishment request can
proceed by a variety of routes through components of the network. To
begin with, the originating end system must decide where to send its
requests. There are two possibilities here: the originator may be
configured so that all its requests go to a single local server; or
it may resolve the destination address to locate a remote signalling
server or end system to which it can send the request directly.
Once the request arrives at a signalling server, that server uses its
user location database, its local policy, DNS resolution, or other
methods, to determine the next signalling server or end system to
which the request should be sent. A request may pass through any
number of signalling servers: from zero (in the case when end systems
communicate directly) to, in principle, every server on the network.
What's more, any end system or signalling server can (in principle)
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receive requests from or send them to any other.
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
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
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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.
We see, then, that in Internet telephony signalling servers 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.
6 Interaction of CPL with network model
6.1 What a script does
A CPL script runs in a signalling server, 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
signalling servers, or to describe features on a "Global Functional
Plane" as in the Intelligent Network architecture [6].
More specifically, a script replaces the user location functionality
of a signalling server. As described in section 5.1.2, a signalling
server typically maintains a database of locations where a user can
be reached; it makes its proxy, redirect, and rejection decisions
based on the contents of that database. A CPL script replaces this
basic database lookup functionality; it takes the registration
information, the specifics of a call request, and other external
information it wants to reference, and chooses the signalling actions
to perform.
Abstractly, a script can be considered as a list of condition/action
pairs; if some attribute of the registration, request, and external
information matches a given condition, then the corresponding action
(or more properly set of actions) is taken. In some circumstances,
additional actions can be taken based on the consequences of the
first action and additional conditions. If no condition matches the
invitation, the signalling server's standard action -- its location
database lookup, for example -- is taken.
6.2 Which script is executed
CPL scripts are usually associated with a particular Internet
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telephony address. When a call establishment request arrives at a
signalling server which is a CPL server, that server associates the
source and destination addresses specified in the request with its
database of CPL scripts; if one matches, the corresponding script is
executed.
Once the script has executed, if it has choosen to perform a proxy
action, a new Internet telephony address will result as the
destination of that proxying. Once this has occured, the server again
checks its database of scripts to see if any of them are associated
with the new address; if one is, that script as well is executed
(assuming that a script has not attempted to proxy to an address
which the server has already tried). For more details of this
recursion process, and a description of what happens when a server
has scripts that correspond both to a scripts origination address and
its destination address, see section 8.2.
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. The recursion process described above allows scripts to be
associated with several of a user's addresses; thus, a user script
could specify an action "try me at my cell phone," whereas a device
script could say "I don't want to accept cell phone calls while I'm
out of my home area."
It is also possible for a CPL script to be associated not with one
specific Internet telephony address, but rather with all addresses
handled by a signalling server, or a large set of them. For instance,
an administrator might configure a system to prevent calls from or to
a list of banned incoming or outgoing addresses; these should
presumably be configured for everyone, but users should still to be
able to have their own custom scripts as well. Exactly when such
scripts should be executed in the recursion process depends on the
precise nature of the administrative script. See section 8.2 for
further discussion of this.
6.3 Where a script runs
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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 in figure 1, the
originating user could have a script on the outgoing proxy, and 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 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 8.3 for some implications of a scenario like this.
7 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 signalling servers.
Scripts persist in signalling servers until changed or deleted,
unless they are specifically given an expiration time; a network
system which supports CPL scripting will need stable storage.
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. Indeed, the device on which the script is created may not
be an "end device" in the sense described in section 5.1.1 at all;
for instance, a user could create and upload a CPL script from a
non-multimedia-capable web terminal.
The CPL also might not necessarily be created on a device near either
the end device or the signalling server in network terms. For
example, a user might decide to forward his or her calls to a remote
location only after arriving at that location.
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.
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.
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If a user has trust relationships with multiple signalling servers
(as discussed in section 6.3), the user may choose to upload scripts
to any or all of those servers. These scripts can be entirely
independent.
8 Feature interaction behavior
Feature interaction is the term used in telephony systems when two or
more requested features produce ambiguous or conflicting behavior
[7]. 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.
8.1 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
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.
8.2 Script-to-script interactions
Script-to-script interactions arise when a server invokes multiple
scripts for a single call, as described in section 6.2. This can
occur in a number of 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.
The solution to this interaction is to determine an ordering among
the scripts to be executed. In this ordering, the "first" script is
executed first; if this script allows or permits the call to be
proxied, the script corresponding to the next address is executed.
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 request arrived over the network. Note that in some
cases, forwarding can be recursive; a CPL server must be careful to
prevent forwarding loops.
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Abstractly, this can be viewed as equivalent to having each script
execute on a separate signalling server. Since the CPL architecture
is designed to allow scripts to be executed on multiple signalling
servers in the course of locating a user, we can conceptually
transform script-to-script interactions into the server-to-server
interactions described in the next section, reducing the number of
types of interactions we need to concern ourselves with.
The question, then, is to determine the correct ordering of the
scripts. For the case of a script forwarding to an address which
also has a script, the ordering is obvious; the other two cases are
somewhat more subtle. When both originator and destination scripts
exist, the originator's script should be executed before the
destination script; this allows the originator to perform address
translation, call filtering, etc., before a destination address is
determined and a corresponding script is chosen.
Even more complicated is the case of the ordering of administrative
scripts. Many administrative scripts, such as ones that restrict
source and destination addresses, need to be run after originator
scripts, but before destination scripts, to avoid a user's script
evading administrative restrictions through clever forwarding;
however, others, such as a global address book translation function,
would need to be run earlier or later. Servers which allow
administrative scripts to be run will need to allow the administrator
to configure when in the script execution process a particular
administrative script should fall.
8.3 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.
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
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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.
8.4 Signalling ambiguity
As an aside, [7] 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.
9 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 11.1, 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.
Similar levels of safety and protection (though not automatic
generation and parsing) could also be achieved through the use of a
"sandbox" such as is used by Java applets, where strict bounds are
imposed on the amount of memory, cpu time, stack space, etc., that a
program can use. The difficulty with this approach is primarily in
its lack of transparency and portability: unless the levels of these
bounds are imposed by the standard, a bad idea so long as available
resources are increasing exponentially with Moore's Law, a user can
never be sure whether a particular program can sucessfully be
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executed on a given server without running into the server's resource
limits, and a program which executes sucessfully on one server may
fail unexpectedly on another. Non-expressively-complete languages, on
the other hand, allow an implicit contract between the script writer
and the server: so long as the script stays within the rules of the
language, the server will guarantee that it will execute the script.
The second disadvantage with expressively complete languages is that
they make automatic generation and parsing of scripts very difficult,
as every parsing tool must be a full interpreter for the language. An
analogy can be drawn from the document-creation world: while text
markup languages like HTML or XML can be, and are, easily manipulated
by smart editors, powerful document programming languages such as
LaTeX or Postscript usually cannot be. While there are word
processors that can save their documents in LaTeX form, they cannot
accept as input arbitrary LaTeX documents, let alone preserve the
structure of the original document in an edited form. By contrast,
essentially any HTML editor can edit any HTML document from the web,
and the high-quality ones preserve the structure of the original
documents in the course of editing them.
10 Related work
10.1 IN service creation environments
The ITU's IN series describe, on an abstract level, service creation
environments [6]. 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.
10.2 SIP CGI
SIP CGI [8] 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.
The paper "Programming Internet Telephony Services" [9] discusses the
similarties and contrasts between SIP CGI and CPL in more detail.
11 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.
11.1 Language characteristics
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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.
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
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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.
11.2 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 6, particularly 6.1.
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 [10]. The script should be able
to make decisions based on this information.
- Information about caller or call
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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:
- 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
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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 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).
11.3 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.
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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 11.5).
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
5.2). 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.
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.
11.4 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.
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o Address filtering
Once a set of addresses has been retrieved through one of the
methods in section 11.3, 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.
11.5 Control
As described in section 7, 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.
12 Security considerations
The security considerations of transferring CPL scripts are discussed
in sections 7 and 11.5. Some considerations about the execution of
the language are discussed in section 11.1.
13 Changes from previous versions
13.1 Changes from draft-ietf-iptel-cpl-framework-00
The changebars in the Postscript and PDF versions of this document
indicate changes from this version.
o Added "Usage scenarios" section.
o Added "CPL creation" section.
o Reorganized old "Architecture" section into "Network model"
and "Interaction," and significantly re-organized and re-
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Internet Draft CPL-F October 22, 1999
worked it.
o Added discussion of "sandbox" execution environments, and
contrasted their merits with those of non-expressively-
complete languages.
o Changed the term "network systems" to "signalling servers"
throughout, to align the terminology used to that used in the
gateway location protocol.
o Significant re-writing and clarification throughout the
document.
13.2 Changes from draft-ietf-iptel-cpl-requirements-00
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.
14 To Do / Open Issues
o Add Terminology section.
15 Acknowledgments
We would like to thank Tom La Porta and Jonathan Rosenberg for their
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comments and suggestions.
16 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
17 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.
[3] K. Coar and D. Robinson, "The WWW common gateway interface
version 1.1," Internet Draft, Internet Engineering Task Force, Apr.
1999. Work in progress.
[4] T. Showalter, "Sieve: A mail filtering language," Internet Draft,
Internet Engineering Task Force, Mar. 1999. Work in progress.
[5] J. Lennox and H. Schulzrinne, "CPL: a language for user control
of internet telephony services," Internet Draft, Internet Engineering
Task Force, Mar. 1999. Work in progress.
[6] 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.
Lennox/Schulzrinne [Page 22]
Internet Draft CPL-F October 22, 1999
[7] 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.
[8] J. Lennox, J. Rosenberg, and H. Schulzrinne, "Common gateway
interface for SIP," Internet Draft, Internet Engineering Task Force,
May 1999. Work in progress.
[9] J. Rosenberg, J. Lennox, and H. Schulzrinne, "Programming
internet telephony services," Technical Report CUCS-010-99, Columbia
University, New York, New York, Mar. 1999.
[10] H. Schulzrinne and J. Rosenberg, "SIP caller preferences and
callee capabilities," Internet Draft, Internet Engineering Task
Force, Mar. 1999. Work in progress.
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
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This document and the information contained herein is provided on an
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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
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Lennox/Schulzrinne [Page 23]
Internet Draft CPL-F October 22, 1999
Table of Contents
1 Introduction ........................................ 1
2 Example services .................................... 2
3 Usage scenarios ..................................... 3
4 CPL creation ........................................ 4
5 Network model ....................................... 5
5.1 Model components .................................... 5
5.1.1 End systems ......................................... 5
5.1.2 Signalling servers .................................. 5
5.2 Component interactions .............................. 6
6 Interaction of CPL with network model ............... 8
6.1 What a script does .................................. 8
6.2 Which script is executed ............................ 8
6.3 Where a script runs ................................. 9
7 Creation and transport of a call processing
language script ................................................ 10
8 Feature interaction behavior ........................ 11
8.1 Feature-to-feature interactions ..................... 11
8.2 Script-to-script interactions ....................... 11
8.3 Server-to-server interactions ....................... 12
8.4 Signalling ambiguity ................................ 13
9 Relationship with existing languages ................ 13
10 Related work ........................................ 14
10.1 IN service creation environments .................... 14
10.2 SIP CGI ............................................. 14
11 Necessary language features ......................... 14
11.1 Language characteristics ............................ 14
11.2 Base features -- call signalling .................... 16
11.3 Base features -- non-signalling ..................... 18
11.4 Language features ................................... 19
11.5 Control ............................................. 20
12 Security considerations ............................. 20
13 Changes from previous versions ...................... 20
13.1 Changes from draft-ietf-iptel-cpl-framework-00 ...... 20
13.2 Changes from draft-ietf-iptel-cpl-requirements-00
................................................................ 21
14 To Do / Open Issues ................................. 21
15 Acknowledgments ..................................... 21
16 Authors' Addresses .................................. 22
17 Bibliography ........................................ 22
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