Network Working Group J. Lennox
Request for Comments: 2824 H. Schulzrinne
Category: Informational Columbia University
May 2000
Call Processing Language Framework and Requirements
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
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.
Table of Contents
1 Introduction ........................................ 2
2 Terminology ......................................... 3
3 Example services .................................... 4
4 Usage scenarios ..................................... 6
5 CPL creation ........................................ 6
6 Network model ....................................... 7
6.1 Model components .................................... 7
6.1.1 End systems ......................................... 7
6.1.2 Signalling servers .................................. 8
6.2 Component interactions .............................. 8
7 Interaction of CPL with network model ............... 10
7.1 What a script does .................................. 10
7.2 Which script is executed ............................ 11
7.3 Where a script runs ................................. 12
8 Creation and transport of a call processing
language script ..................................... 12
9 Feature interaction behavior ........................ 13
9.1 Feature-to-feature interactions ..................... 13
9.2 Script-to-script interactions ....................... 14
9.3 Server-to-server interactions ....................... 15
9.4 Signalling ambiguity ................................ 15
10 Relationship with existing languages ................ 15
11 Related work ........................................ 17
11.1 IN service creation environments .................... 17
11.2 SIP CGI ............................................. 17
12 Necessary language features ......................... 17
12.1 Language characteristics ............................ 17
12.2 Base features -- call signalling .................... 19
12.3 Base features -- non-signalling ..................... 21
12.4 Language features ................................... 22
12.5 Control ............................................. 23
13 Security Considerations ............................. 23
14 Acknowledgments ..................................... 23
15 Authors' Addresses .................................. 23
16 Bibliography ........................................ 24
17 Full Copyright Statement ............................ 25
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
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 Terminology
In this section we define some of the terminology used in this
document.
SIP [1] terminology used includes:
invitation: The initial INVITE request of a SIP transaction, by
which one party initiates a call with another.
redirect server: A SIP device which responds to invitations and
other requests by informing the request originator of an
alternate address to which the request should be sent.
proxy server: A SIP device which receives invitations and other
requests, and forwards them to other SIP devices. It then
receives the responses to the requests it forwarded, and
forwards them back to the sender of the initial request.
user agent: A SIP device which creates and receives requests, so
as to set up or otherwise affect the state of a call. This
may be, for example, a telephone or a voicemail system.
user agent client: The portion of a user agent which initiates
requests.
user agent server: The portion of a user agent which responds to
requests.
H.323 [2] terminology used includes:
terminal: An H.323 device which originates and receives calls, and
their associated media.
gatekeeper: An H.323 entity on the network that provides address
translation and controls Access to the network for H.323
terminals and other endpoints. The gatekeeper may also
provide other services to the endpoints such as bandwidth
management and locating gateways.
gateway: A device which translates calls between an H.323 network
and another network, typically the public-switched telephone
network.
RAS: The Registration, Admission and Status messages communicated
between two H.323 entities, for example between an endpoint
and a gatekeeper.
General terminology used in this document includes:
user location: The process by which an Internet telephony device
determines where a user named by a particular address can be
found.
CPL: A Call Processing Language, a simple language to describe how
Internet telephony call invitations should be processed.
script: A particular instance of a CPL, describing a particular
set of services desired.
end system: A device from which and to which calls are
established. It creates and receives the call's media
(audio, video, or the like). This may be a SIP user agent or
an H.323 terminal.
signalling server: A device which handles the routing of call
invitations. It does not process or interact with the media
of a call. It may be a SIP proxy or redirect server, or an
H.323 gatekeeper.
3 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
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.
4 Usage scenarios
A CPL would be useful for implementing services in a number of
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 component having to be aware of the specifics of the
other.
5 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
knowledgeable users. The CPL described in [5] has a text
format with an uncomplicated syntax, so hand authoring will be
straightforward.
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.
6 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.
6.1 Model components
In the Call Processing Language's network model, an Internet
telephony network contains two types of components.
6.1.1 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.
6.1.2 Signalling servers
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.
RFC2543 [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 destinations on the
Internet, under programmatic control.
6.2 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)
receive requests from or send them to any other.
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.
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.
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
7 Interaction of CPL with network model
7.1 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 coordinate 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 6.1.2, 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.
7.2 Which script is executed
CPL scripts are usually associated with a particular Internet
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 chosen to perform a proxy
action, a new Internet telephony address will result as the
destination of that proxying. Once this has occurred, 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 9.2.
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 9.2 for
further discussion of this.
7.3 Where a script runs
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 9.3 for some implications of a scenario like this.
This model does not specify the means by which users locate a CPL-
capable network server. In general, this will be through the same
means by which they locate a local Internet telephony server to
register themselves with; this may be through manual configuration,
or through automated means such as the Service Location Protocol [7].
It has been proposed that automated means of locating such servers
should include a field to indicate whether the server allows users to
upload CPLs.
8 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 6.1.1 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.
If a user has trust relationships with multiple signalling servers
(as discussed in section 7.3), the user may choose to upload scripts
to any or all of those servers. These scripts can be entirely
independent.
9 Feature interaction behavior
Feature interaction is the term used in telephony systems when two or
more requested features produce ambiguous or conflicting behavior
[8]. 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.
9.1 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.
9.2 Script-to-script interactions
Script-to-script interactions arise when a server invokes multiple
scripts for a single call, as described in section 7.2. 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.
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.
9.3 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
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.
9.4 Signalling ambiguity
As an aside, [8] 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.
10 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 12.1, 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 successfully be
executed on a given server without running into the server's resource
limits, and a program which executes successfully 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.
11 Related work
11.1 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.
11.2 SIP CGI
SIP CGI [9] 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" [10] discusses
the similarities and contrasts between SIP CGI and CPL in more
detail.
12 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.
12.1 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.
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 parsable 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.
12.2 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 7, particularly 7.1.
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 [11]. 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. The script should be able to make decisions
based on these parameters.
- Media description
Call invitations specify the types of media that will flow,
their bandwidth usage, their network destination addresses,
etc. The script should be able to make decisions based on
these media characteristics.
- Authentication/encryption status
Call invitations 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. The script
should be able to make decisions based on these security
parameters.
o Should be able to take action based on a call invitation
There are a number of actions we can take in response to an
incoming call setup 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).
- redirect it
We should be able to tell the call initiator sender to try a
different location.
- proxy it
We should be able to send the call invitation on to another
location, or to several other locations ("forking" the
invitation), 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
forked call invitation
Once we have proxied an invitation, we need to be able to make
decisions based on the responses we receive to that invitation
(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 invitation.
- relay it on to the call originator
If the response is satisfactory, it should be returned to
the sender.
- for a fork, choose one of several responses to relay back
If we forked an invitation, 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).
12.3 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 may use the same
mechanism as the script server uses to report language errors
to the user (see section 12.5).
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
6.2). 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 a script could 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.
However, for simplicity, these interfaces may not be in the
initial version of the protocol.
12.4 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 12.3, 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.
12.5 Control
As described in section 8, 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.
13 Security Considerations
The security considerations of transferring CPL scripts are discussed
in sections 8 and 12.5. Some considerations about the execution of
the language are discussed in section 12.1.
14 Acknowledgments
We would like to thank Tom La Porta and Jonathan Rosenberg for their
comments and suggestions.
15 Authors' Addresses
Jonathan Lennox
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
USA
EMail: lennox@cs.columbia.edu
Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
USA
EMail: schulzrinne@cs.columbia.edu
16 Bibliography
[1] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC2543, March 1999.
[2] International Telecommunication Union, "Packet based multimedia
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[3] K. Coar and D. Robinson, "The WWW common gateway interface
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[4] T. Showalter, "Sieve: A mail filtering language", Work in
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[5] J. Lennox and H. Schulzrinne, "CPL: a language for user control
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[6] International Telecommunication Union, "General recommendations
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[7] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
Location Protocol, Version 2", RFC2608, June 1999.
[8] E. J. Cameron, N. D. Griffeth, Y.-J. Lin, M. E. Nilson, W. K.
Schure, and H. Velthuijsen, "A feature interaction benchmark for
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[9] J. Lennox, J. Rosenberg, and H. Schulzrinne, "Common gateway
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[10] J. Rosenberg, J. Lennox, and H. Schulzrinne, "Programming
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Columbia University, New York, New York, Mar. 1999.
[11] H. Schulzrinne and J. Rosenberg, "SIP caller preferences and
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