Internet Engineering Task Force IPTEL WG
Internet Draft Lennox/Schulzrinne
ietf-iptel-cpl-requirements-00.txt Lucent Bell Labs/Columbia University
July 30, 1998
Expires: February 1999
Call Processing Language Requirements
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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
architecture for such a method, 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
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and dramatic decentralization of the provisioning of telephone
services so they can be under the user's control.
Many of these services may reside on end devices. A broad set of
services, however -- those involving user location, call
distribution, behavior-on-busy, and the like -- are independent of a
particular end device, or need to be operational even when an end
device is unavailable. These still best reside in a network device
rather than an end system. To allow user control over such devices,
we need a standardized way for end-users to specify the precise
behavior of the servers. This document proposes an architecture in
which network devices or end systems respond to call signalling
events by triggering user-created programs which control the reaction
to the events.
For reasons discussed in section 3.7, this document proposes a
relatively static, non-expressively-complete language to solve this
problem. We call this a call processing language. However, most of
the requirements this document lists apply equally well to a library
of call processing routines for an existing language.
2 Motivating examples
These are some specific examples of services which we want to be able
to create programatically. They are arranged roughly in order of
increasing requirements they impose. 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 superviser, in which case it
should be proxied to the user's cellphone if it has registered.
o Administrative screening -- firewall
An outgoing call should be rejected if it is going to any
destination that is on a "banned" list. Otherwise, it should be
forwarded on to the appropriate destination; if the destination
accepts the call, the firewall should be told to open up the UDP
host/port pairs the two endpoints specified for their media. The
same thing should be done for incoming calls, checking the
origination address.
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o Central phone server
If a call comes in for a specific person, it should be
redirected to the locations where they can currently be found.
If a call comes in for the general "information" address we've
advertised, 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 gets a recorded message indicating what times they can
call.
o Intelligent user location
When a call comes in, it should ring at every station from which
the user has registered. 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 Intelligent user location with mixer (home phone)
When a call comes in, it should ring at every station from which
the user has registered. If the call is picked up from more than
one station, the media from each station should be transparently
mixed together and sent to the caller.
o Third-party registration control
When a registration arrives for a user, make sure that the
registration was authenticated, the person performing the
registration has permission to perform the registration for the
specified user, and the location registered is allowed for the
registered user. If so, enter it in the registration database;
if not, reject it.
o Calendarbook access
When a call comes in, the user's on-line calendar should be
consulted. If it specifies that the user has a meeting scheduled
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for this time, the caller should get a busy indication.
Otherwise, the call should be directed to the user's office
telephone.
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 and the time and
duration of the call are logged. If no corresponding client is
found, the call is forwarded to the lawyer's secretary.
o End system busy
When a new call comes in, if the user is currently in a call, a
call-waiting tone is generated, unless one of the calls
currently in progress is with the user's boss or he or she has
set "Do Not Disturb" in the user interface, in which case the
caller gets a busy indication.
o Phone bank (call distribution/queueing)
Incoming calls should be distributed to the phone-bank workers,
so that each worker handles approximately the same total number
of calls. If all the phone-bank workers are busy, calls should
be queued until someone is available. Calls coming from
preferred customers should get priority in the queue. If the
length of the queue grows to twice the size of the phone bank,
calls should be directed to management as well until the queue
length has decreased again. Each caller should be given an
approximate indication of waiting time or number of calls ahead
of them in the queue as they wait. Callers should be given the
option of listening to the music-on-hold of their choice.
3 Architecture
3.1 Network components
A network which supports Internet telephony consists of two types of
components: end systems, which originate and/or receive media, and
network systems, which relay signalling information.
End systems are either user agents, which reach actual people, or
automated systems, which do not; this document will deal primarily
with the former. Network systems, in SIP, are proxy servers,
redirect servers, or registrars; in H.323 they are gatekeepers. The
functionality between the two protocols is largely equivalent, and
this document will generally use the SIP names. Proxy servers are
network systems which receive a request and forward it on. Redirect
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servers tell the originating location an alternate location to try.
Registrars track users' current locations; they will usually be the
same devices as proxy or redirect servers, but do not necessarily
have to be. See the illustrations in [1]. End systems may also have
some properties of network systems, most likely the ability to
perform redirection.
3.2 Model of normal use
Local Signalling Server Remote Signalling Server
locates destination finds location for
permanent address
_______ Local server contacts _______ _______
| | permanent address | | | |
| | --------------------------------------> | | ----->| |
|_____| Local address |_____| |_____|
^ ----- server con- permanent -------> \ Search Another
Send to / \-----------\ /---------/ \ for Terminal
local / X \ User
server/ contacts / \ tacts terminal _|
_______ Originator ---------/ \----------- address _______
| | ----------/ \-------> | |
| | ----------------------------------------------> | |
|_____| Direct connection to terminal address |_____|
Terminal
Originator has terminal address
Figure 1: Illustration of call signalling messages
Internet telephony addresses can be divided into two broad
categories: terminal addresses and permanent user addresses. A
terminal address is one that refers to a particular device, whose
network-level (IP) address does not change. A permanent user address,
on the other hand, refers on a more abstract level to an individual
user, whose current location and network address may change. When a
user becomes available at a location, his or her end system registers
itself with a network server indicating this fact. (In SIP, users
register via the REGISTER message; in H.323, via RRQ and related RAS
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messages.) A user may register from more than one location
simultaneously.
Figure 1 shows how call signalling may flow. Calls may be placed
either to terminal or permanent user addresses. Calls to terminal
addresses may contact the corresponding device directly, or may
travel through some signalling server. Calls to permanent user
addresses must pass through the signalling server, which locates the
user and proxies or redirects the call to the appropriate terminal
addresses which the user has registered.
Signalling servers may also be used on the originating side. Rather
than locate a call's destination on its own, an end system, when
originating a call, may have been configured to transfer all its call
requests through a single, presumably local, server. This server can
then perform the somewhat complex task of actually locating the
destination, as well as other tasks such as firewall penetration or
encryption of signalling information.
Call requests may be forwarded between multiple signalling servers on
both the origination and destination ends of a call. For example, a
corporation could have a large company-wide server which forwards
incoming call requests to individual departmental servers, which then
perform the task of actually locating the desired user. This is
similar to a typical configuration of e-mail forwarding.
Different call invitations for a particular end system might travel
through a different set of signalling servers; for instance, a user
with several addresses might register his current end system with
several different servers. Similarly, an end system placing a call
might have several different outgoing signalling servers through
which it could place the call. Thus, in general, a signalling server
does not see all the signalling events for a particular end system;
and so it does not have enough information to be able to determine
the end system's state.
3.3 Purpose of a call processing language
A call processing language (CPL) is primarily intended to allow the
user to modify the way an Internet telephony system handles call
events. Call events include signalling events such as call setup,
termination, or parameter changes, and also, for servers with an
appropriate media path, in-band events such as DTMF tones. The user
can modify either incoming or outgoing calls.
The most common sort of modification will be for incoming call
setups. Some ways a user might want to alter the call setup process
include: to search terminal addresses for a given user address in an
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alternate way; to specificy what happens when the initial search
fails, either when it receives some sort of negative response (e.g.,
busy), or does not receive any definitive response within a fixed
time period (e.g., no answer); or to handle certain origination
addresses specially, for instance by informing the caller that the
call was refused. The useful changes to the outgoing call setup are
somewhat more limited in scope, but one example is to translate a
user's abbreviated addresses into an address specified with a fully-
qualified domain name. The transformations to parameter changes or
call terminations are generally only useful to complete the actions
begun at call setup time; see for instance the lawyer's office
example in section 2.
Once a language with this level of power has been introduced, other
applications of it present themselves. An administrator might wish to
perform administrative restrictions on users' calls, for instance
blocking incoming or outgoing calls from certain domains. The
language could also be scripted on an end system; with minimal
extensions, behavior specific to end systems, such as the specifics
of how the user is alerted to incoming calls, could also be made
programmable.
3.4 Creation and transport of a call processing language script
Users create call processing language scripts, typically on end
devices, and transmit them through the network to network systems.
Scripts persist in network devices until changed or deleted, unless
they are specifically given an expiration time; a network device
which supports CPL scripting will need stable storage.
The exact means by which the end device transmits the script to the
server remains to be determined; it is likely that many solutions
will be able to co-exist. This method will need to be authenticated
in almost all cases. The methods that have been suggested include
web access, SIP REGISTER message payloads, remote method invocation,
SNMP, LDAP, and remote file systems such as NFS.
Creation of a CPL script may be through the creation of a text file;
or for a simpler user experience, a graphical user interface which
allows the manipulation of some basic rules.
The end device on which the user creates the CPL script need not bear
any relationship to the end devices to which calls are actually
placed. For example, a CPL script might be created on a PC, whereas
calls might be intended to be received on a simple audio-only
telephone. The CPL also might not necessarily be created on a device
near either the end device or the signalling server in network terms;
a user might, for example, decide to forward his or her calls to a
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remote location only after arriving at that location.
Users can also retrieve their current script from the network to an
end system so it can be edited. The signalling server should also be
able to report errors related to the script to the user, both static
errors that could be detected at upload time, and any run-time errors
that occur.
If a user's calls will pass through multiple local signalling servers
which know about that user (as discussed in section 3.2), the user
may choose to upload scripts to any or all of those servers. These
scripts can be entirely independent; see section 3.6 for some
implications of this.
If, as discussed in section 3.3, the call processing language is
extended to control end systems, the script-creation mechanism
described above should also be able to create such end-system
scripts. It may be possible that the end system on which the script
executes (the simple telephone mentioned before) is not the same
device as the end system on which the script is created; in this
case, the script should be transmitted from the script creation site
to the end system in the same way it is transmitted from creation
sites to network systems.
3.5 Execution process of a CPL script
When a call event arrives, a CPL server considers the information in
the request and determines if any of the scripts it has stored are
applicable to the call in question. If so, it performs the actions
corresponding to the matching scripts.
The most common type of script defines a set of actions to be taken
for the entire process of call set-up -- from the time a call request
is initially received, to the time that (from the point of view of
this device) the call is either definitively accepted or definitively
rejected. This could be near-instantaneous, if, for instance, the
script decides to reject the call; or it could be an arbitrarily long
time, if we are waiting for a call pick-up without a timeout.
Generally, we expect a script to be structured as a list of
condition/action pairs; if an incoming invitation matches a given
condition, then the corresponding action (or more properly set of
actions) will be taken. Whether this should be explicit in the
language or just implicit in the normal usage remains to be seen. If
no condition matches the invitation, the signalling server's standard
action should be taken.
Other types of scripts may define sets of actions to be taken for
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other call events: call termination; changes to media format or other
call parameters (re-invitations, in SIP); or in-band call events,
such as a user sending DTMF tones. However, it is important to note
that many, if not most, network servers cannot expect to be able to
observe such events; subsequent signalling information may short-cut
past the server, as media information almost certainly will.
While many of the uses of a CPL script are specific to one particular
user, there are a number of circumstances in which an administrator
of a signalling server would wish to provide a script which applies
to all users of the server, or a large set of them. For instance, a
system might be configured to prevent calls from or to a list of
banned incoming or outgoing addresses; these should presumably be
configured for everyone, but users still need to be able to have
their own custom scripts as well. Similarly, an administrative script
might perform the necessary operations to allow media to traverse a
firewall; but individual users' scripts should not have permission to
perform these operations. See the next section for some implications
of this.
3.6 Feature interaction behavior
Feature interaction is the term used in telephony systems when two or
more requested features produce ambiguous or conflicting behavior
[3]. Feature interaction issues for features implemented with a call
processing language can be roughly divided into three categories:
feature-to-feature in one server, script-to-script in one server, and
server-to-server.
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.
Script-to-script interactions can arise if both an originator and a
destination have scripts specified on the same signalling server, or
if an administrative script and a user's script are both specified.
In the former case, the correct behavior is fairly obvious: a server
should first execute the originator's script, and then, if that
script placed a call to a destination, call the destination script
with the appropriate conditions.
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The correct behavior in the latter case depends on the scope of the
administrative script; however, normally, the administrator's script
should run after origination scripts, intercepting any proxy or
redirection decisions, and before recipient scripts, to avoid a
user's script evading administrative restrictions.
The third case -- server-to-server interactions -- is the most
complex of these three. Many such problems are unsolvable in an
administratively heterogeneous network, even a "lightly"
heterogeneous network such as current telephone systems. The
canonical example of this is the interaction 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.
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.
As an aside, [3] discusses a fourth type of feature interaction for
traditional telephone networks, signalling ambiguity. This can arise
when several features overload the same operation in the limited
signal path from an end station to the network, for example, flashing
the switch-hook can mean both "add a party to a three-way call" and
"switch to call waiting." Because of the explicit nature of
signalling in both the Internet telephony protocols discussed here,
this issue does not arise.
3.7 Relationship with existing languages
This document's description of the CPL as a "language" is not
intended to imply that a new language necessarily needs to be
implemented from scratch. A server could potentially implement all
the functionality described here as a library or set of extensions
for an existing language; Java, or the various freely-available
scripting languages (Tcl, Perl, Python, Guile), are obvious
possibilities.
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However, there are motivations for creating a new language. All the
existing languages are, naturally, expressively complete; this has
two inherent disadvantages. The first is that any function
implemented in them can take an arbitrarily long time, use an
arbitrarily large amount of memory, and may never terminate. For call
processing, this sort of resource usage is probably not necessary,
and as described in section 5.1, may in fact be undesirable. One
model for this is the electronic mail filtering language Sieve [4],
which deliberately restricts itself from being Turing-complete. The
second disadvantage with expressively complete languages is that they
make automatic generation and parsing very difficult; an analogy can
be drawn with the difference between markup languages like HTML or
XML, which can easily be manipulated by smart editors, and powerful
document programming languages such as Latex or Postscript which
usually cannot be.
4 Related work
A future revision of this document will discuss such items as
decision tree languages, AT&T's TOPS language, the IN service
creation language, timed state diagrams, the Java servlet API, cgi-
bin, and active networks.
5 Necessary language features
This section lists those properties of a call processing language
which we believe to be necessary to have in order to implement the
motivating examples, in line with the described architecture.
5.1 Language characteristics
These are some abstract attributes which any proposed call processing
language should possess.
o Light-weight, efficient, easy to implement
In addition to the general reasons why this is desirable, a
network server might conceivably handle very large call volumes,
and we don't want CPL execution to be a major bottleneck. One
way to achieve this might be to compile scripts before
execution.
o Easily verifiable for correctness
For a script which runs in a server, mis-configurations can
result in a user becoming unreachable, making it difficult to
indicate run-time errors to a user (though a second-channel
error reporting mechanism such as e-mail could ameliorate this).
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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
servers can easily recognize features they don't understand and
safely inform the user of this fact.
o Independent of underlying signalling details
The same scripts should be usable whether the underlying
protocol is SIP, H.323, a traditional telephone network, or any
other means of setting up calls. It should also be agnostic to
address formats. (We use SIP terminology in our descriptions of
requirements, but this should map fairly easily to other
systems.) It may also be useful to have the language extend to
processing of other sorts of communication, such as e-mail or
fax.
5.2 Base features -- call signalling
To be useful, a call processing language obviously should be able to
react to and initiate call signalling events.
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o Should execute an action script when a call request arrives
See section 3, particularly 3.5.
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:
- Event type
It should be possible to handle call invitations, call
terminations, user registrations, OPTIONS requests, and
other distinct call events separately.
- Originator address
We want to be able to do originator-based screening or
routing.
- Destination address
Similarly, we want to be able to do destination-based
screening or routing. Note that in SIP we want to be able
to filter on any or all of the addresses in the To header,
the Location header, and the Request-URI.
- Information about caller or call
SIP has textual fields such as Subject, Organization,
Priority, etc., and a display name for addresses; users can
also add non-standard additional headers. H.323 has a
single Display field.
- Media description
Requests specify the types of media that will flow, their
bandwidth usage, their network destination addresses, etc.
- Authentication/encryption status
Requests can be authenticated. Many properties of the
authentication are relevant: the method of
authentication/encryption, who performed the
authentication, which specific fields were encrypted, etc.
o Should be able to take action based on a request
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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, 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 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 all its message fields
This consists of a similar set of fields as appear in
a request.
- relay it on to the requestor
If the response is satisfactory, it should be
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returned to the sender.
- for a fork, choose one of several responses to
relay back
If we forked a request, we obviously expect to
receive several responses. There are several issues
here -- choosing among the responses, and how long to
wait if we've received responses from some but not
all destinations.
- initiate other actions
If we didn't get a response, or any we liked, we
should be able to try something else instead (e.g.,
call forward on busy).
5.3 Base features -- non-signalling
A number of other features that a call processing language should
have do not refer to call signalling per se; however, they are still
extremely desirable to implement many useful features.
The servers which provide these features might reside in other
Internet devices, or might be local to the server (or other
possibilities). The language should be independent of the location of
these servers, at least at a high level.
o Logging
In addition to the CPL server's natural logging of events, the
user will also want to be able to log arbitrary other items. The
actual storage for this logging information might live either
locally or remotely.
o Error reporting
If an unexpected error occurs, the script should be able to
report the error to the script's owner. This should use the same
mechanism as the script server uses to report language errors to
the user (see section 5.9).
o Access to user-location info
Proxies will often collect information on users' current
location, either through SIP REGISTER messages, the H.323 RRQ
family of RAS messages, or some other mechanism (see section
3.2). The CPL should be able to refer to this information so a
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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.
o Creation of and access to local state
A CPL script may wish to store state information, so that
scripts invoked for future transactions related to this call or
this user can have access to decisions made by an earlier
invocation. For instance, a SIP re-invitation should be proxied
to the same location as accepted the original invitation,
regardless of the usual forwarding sequence; a server may wish
to log the termination of a call in the same way it logged its
initiation; or a user might want to limit the number of
concurrent calls or calls per day allowed. The persistence of
this state information for a call should be time-limited, either
explicitly or by default. See section 3.5 for some caveats for a
network system of expecting to receive events other than call
initiation.
5.4 Higher-level features
There are some, more complex services which it would be quite useful
to be able to describe with a CPL, but which require considerably
more maintenance of state, elaborate inter-call event triggering, and
so forth, than the features described earlier.
It is not clear whether these features should be specified as
primitives of the language, or whether they should be assembled from
lower-level features. In the latter case, the language may need to
have additional low-level features added, beyond those specified in
the previous sections, so these features can be constructed.
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o Queueing
Calls which should go to end systems which aren't accepting
calls currently can be queued to await delivery. This requires
inter-call synchronization to know when to take calls off the
queue.
It should be possible for the CPL to specify a priority for a
queue entry.
o Call distribution
Calls can be spread to a number of end systems, in a "phone
bank" style set-up. Calls need to be directed to exactly one,
currently available destination, in some fair manner (e.g.,
hierarchical, round-robin, randomly distributed, or weighted
fair queueing). In many cases, this means that the proxy system
needs to be able to track the state of end systems.
This should also be able to interface with queueing -- if all
end systems are busy, the call is queued, and when one becomes
free, the call is taken off the queue.
5.5 Contingent features
Some features are only useful if other network entities are
available.
o Access to media servers
We want to be able to connect a remote call to recorded audio
(or video) messages.
o Firewall control
If we are working in an environment with a firewall, we need to
be able to tell it to open up a specific host/port 5-tuple for
our media to flow through.
We should be able to specify authentication so the firewall
knows it can trust the proxy.
o Mixer/translator control
If we have a media mixer or translator available, we want to be
able to tell it to mix media between several addresses, with
fine-grained control over what media flows to where, in what
formats.
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5.6 End-system specific features
Some features can only be implemented in end-systems, either because
some end-system state is not generally communicated over the network,
or because there is no protocol to signal that actions need to be
performed. If we want these features to be implementable with the
CPL, these additional operations will be necessary.
o Access to current calling state
We want to know how many other calls are in progress, who they
are with, etc.
o Access to additional user interface state
The end system's user interface might present options which the
user could specify on a per-call basis (for example, setting "do
not disturb").
o Control of user notification UI
This will allow such features as custom distinctive ringing,
call-waiting tones (in combination with the call-state query),
"reminder ring" for call forwarding, etc.
5.7 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 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.
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5.8 Future features
A number of services which don't exist yet or aren't widely deployed
in the Internet will be relevant for Internet telephony. Once they
are available, a CPL script should be able to control them.
o Ability to specify quality-of-service
Certain calls -- either on the basis of importance, or for
known-troublesome destinations -- should be able to have their
desired quality of service specified by the language.
o Access to wide-area service location information
A proxy doing call distribution might want to locate the service
"closest" (in any one of a number of senses) to the caller; or
we might want to find a PSTN gateway close to the destination of
a PSTN-style call. In either case a script should be able to
control these operations.
o Control of payment authorization
If any kind of per-call billing is required, a CPL might want to
be able to decide whether to accept charges. This is obviously a
rather delicate operation from a security standpoint.
5.9 Control
As described in section 3.4, we must have a mechanism to send and
retrieve CPL scripts, and associated data, to and from a signalling
server. This method should support reporting upload-time errors to
users; we also need some mechanism to report errors to users at
script execution time. Authentication is vital, and encryption is
very useful. The specification of this mechanism can be (and probably
ought to be) a separate specification from that of the call
processing language itself.
6 Security considerations
The security considerations of transferring CPL scripts are discussed
in sections 3.4 and 5.9. Some considerations about the execution of
the language are discussed in section 5.1.
7 Acknowledgments
We would like to thank Tom La Porta and Jonathan Rosenberg for their
comments and suggestions.
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8 Authors' Addresses
Jonathan Lennox
Lucent Technologies, Bell Laboratories
Rm. 4F-520
101 Crawfords Corner Road
Holmdel, NJ 07733
USA
electronic mail: lennox@dnrc.bell-labs.com
Henning Schulzrinne
Dept. of Computer Science
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA
electronic mail: schulzrinne@cs.columbia.edu
9 Bibliography
[1] M. Handley, H. Schulzrinne, and E. Schooler, "SIP: session
initiation protocol," Internet Draft, Internet Engineering Task
Force, May 1998. Work in progress.
[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] E. J. Cameron, N. D. Griffeth, Y.-J. Lin, M. E. Nilson, and et
al, "A feature interaction benchmark for IN and beyond," Feature
Interactions in Telecommunications Systems, IOS Press , pp. 1--23,
1994.
[4] T. Showalter, "Sieve -- a mail filtering language," Internet
Draft, Internet Engineering Task Force, Jan. 1998. Work in progress.
Full Copyright Statement
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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
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Internet Draft CPL-R July 30, 1998
document itself may not be modified in any way, such as by removing
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Table of Contents
1 Introduction ........................................ 1
2 Motivating examples ................................. 2
3 Architecture ........................................ 4
3.1 Network components .................................. 4
3.2 Model of normal use ................................. 5
3.3 Purpose of a call processing language ............... 6
3.4 Creation and transport of a call processing
language script ................................................ 7
3.5 Execution process of a CPL script ................... 8
3.6 Feature interaction behavior ........................ 9
3.7 Relationship with existing languages ................ 10
4 Related work ........................................ 11
5 Necessary language features ......................... 11
5.1 Language characteristics ............................ 11
5.2 Base features -- call signalling .................... 12
5.3 Base features -- non-signalling ............ 15
5.4 Higher-level features ............................... 16
5.5 Contingent features ................................. 17
5.6 End-system specific features ........................ 18
5.7 Language features ................................... 18
5.8 Future features ..................................... 19
5.9 Control ............................................. 19
6 Security considerations ............................. 19
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7 Acknowledgments ..................................... 19
8 Authors' Addresses .................................. 20
9 Bibliography ........................................ 20
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