Introduction to CPL

www.dynamicsoft.comWinter 2001 VoN Developers Conference -- February 25, 2001Introduction to CPL

Introduction to CPL

Jonathan Rosenberg

Chief Scientist


CPL Problem Statement

Would like to allow end users to Define new services

GUI tools or web Hand edit Back and forth

Customize vanilla services Have their services be enabled instantly

Service providers require Platform independence

Decouple service creation tool from execution Decouple service from signaling protocol

Assurances on safety


What are the Options?

Web interfaces populate customer profile data Not cross platform Not flexible

End users write Java applications and upload Moving between GUI tools and hand editing is impossible Safety issue still exists; sandboxing helps but not completely

BIG IDEA - Define an XML based Call Processing Language (CPL)


Advantages of XML for call services

Easily edited by hand or by GUI tools

Cross platform

Allows us to define our own language primitives Can engineer language to be safe Can make it easy to build many common services

Can be manipulated by off the shelf XML tools

Readily transported in HTTP, SIP, FTP, other means

Small in size (compared to Java)

Fast operation


History of CPL

Concept originated as end user uploading of service logic in SIP REGISTER JDR and HGS, 1997

Proposed as one of main work items for new IP Telephony (iptel) working group when it was formed in Nov 1997

Jonathan Lennox, Columbia U. took role as primary driver and author

Framework and Requirements done first RFC2824 issued May 2000

CPL specification First draft April 1999 Submitted for consideration as

RFC Dec. 2000

Implementations First one by JDR, mid 1999 at Bell

Labs Second by dynamicsoft Numerous ones exist or are in

progress now Indigo Software Ubiquity Netcentrex


CPL FAQ

Where do CPLs get executed? In network elements

SIP Proxies SIP Application Servers H.323 Gatekeepers

Can be used in clients, not designed for it

Who writes CPLs? Engineered for end users to write Can be written by administrators

and third parties as well

How do CPLs get into the network? Uploaded in REGISTER (next

slide)

Uploaded in http POST Sent through any other means

Direct database writes FTP etc.

What kinds of services are best suited for CPL? Call routing services Call screening services Call logging services


Uploading CPL in REGISTER

Include CPL in body of REGISTER message

MIME type application/cpl+xml

Content-Disposition Header Indicates purpose of bodies Indicates that content contains call

logic

Content-Action indicates what is desired Add the CPL to the server Delete CPL

CPLs DO NOT get refreshed

REGISTER sip:sip.example.com SIP/2.0From: Joe User <sip:[email protected]>To: "J. User" <sip:[email protected]>CSeq: 18 REGISTERExpires: 1800Call-ID: [email protected]: sip:[email protected]: application/cpl+xmlAuthorization: Basic am9lOnBhc3N3b3JkAFBXContent-Type: application/cpl+xmlContent-Length: 137Content-Disposition: scriptContent-Action: add

<cpl> <incoming> <reject status=“400”/> </incoming></cpl>


CPL Lifecycle

CPL Execution starts on arrival of INVITE message at a proxy

Governs operation of proxy for INVITE transaction ONLY

When final response is sent, CPL execution terminates Does not stay “in the call”

This is why CPL is good at pre-call services like routing and screening

Proxy UA1 UA2

INVINV

INV

300

ACK

600

ACK

600

CPLLifecycle


Proxies and CPL

SIP call setups traverse many paths

Which ones does a user need to put their CPL on?

CPL guides behavior of a single proxy Incoming calls, CPL is generally

on proxy that accesses registrations for that user

Outgoing calls, CPL is generally on local outbound proxy

CPLs can be put anywhere else Up to user/system to decide

where they need to go

CPLsfor caller

CPLsfor called user


CPL Structure CPL represents a tree

Trees have nodes and links Each corresponds to a tag in CPL

Nodes Specify actions to take or decisions to

make

Links Specify results of actions Specify which decision was made

CPL contains subroutines, each of which is a tree

Incoming - tree to execute for incoming calls

Outgoing - tree to execute for outgoing calls

Subroutines - can be used by either incoming or outgoing

<?xml version="1.0" ?> <!DOCTYPE cpl PUBLIC "-//IETF//DTD RFCxxxx CPL 1.0//EN" "cpl.dtd">

<cpl> <subaction id="voicemail"> SOME STUFF HERE </subaction>

<subaction id=“email”> SOME STUFF HERE </subaction>

<incoming> SOME STUFF HERE </incoming>

<outgoing> SOME STUFF HERE </outgoing> </cpl>


Beginning a CPL

First line Specifies XML version

Next line Specifies DTD to be used for validation

All CPLs then followed by CPL tag

Within CPL tag, you can have subactions zero or one incoming zero or one outgoing

<?xml version="1.0" ?> <!DOCTYPE cpl PUBLIC "-//IETF//DTD RFCxxxx CPL 1.0//EN" "cpl.dtd">

<cpl> <subaction id="voicemail"> SOME STUFF HERE </subaction>

<subaction id=“email”> SOME STUFF HERE </subaction>

<incoming> SOME STUFF HERE </incoming>

<outgoing> SOME STUFF HERE </outgoing> </cpl>


Mapping Trees to CPLs

lookup

proxy

What isthe date?

Reject Redirect

succ

ess

otherwise

Mon

day

otherwise

<lookup source=“registration”> <success> <proxy/> </success> <otherwise> <time-switch> <time dtstart=20001001T000000” duration=“24H” freq=“weekly” byday=“MO”> <reject status=“400”/> </time> <otherwise> <redirect/> </otherwise> </time-switch> </otherwise></lookup>


CPL Execution Model

Processing begins at incoming or outgoing tag Depends on system

Server traverses tree, making decisions or performing actions

Implicit global variable - the list of locations

Certain tags modify the location list location, lookup, location-filter

Other tags use the location list proxy, redirect


Available tags

Signaling Actions proxy redirect reject

Decisions address-switch string-switch time-switch priority-switch

Other actions mail log

Location modifiers location lookup remove-location

Subroutine calls subaction sub


Proxy tag

Node

Proxies to current location list

Outputs are result of proxy busy (486) noanswer (408 or timeout) redirection (3xx) failure (4xx,5xx,6xx)

Attributes timeout: how long to try before

giving up recurse: whether to recurse or not ordering: sequential, parallel, first-

only

<proxy recurse=“yes”> <busy> <reject/> </busy> <noanswer> <reject/> </noanswer></proxy>


Proxy tag Details

Tried elements are removed from global location list for parallel and sequential, that’s

all sip URLs for first only, that’s the top sip URL

Default timeout 20s if a no-answer tag is present system defined otherwise why? If there is a no-answer,

there is something else to do

Default recurse is yes

Default ordering is parallel

Recursing Contacts from 3xx are adding to

implicit global location list List is always kept in sorted order

based on q value


Redirect tag

Node

Redirects to the current location list Placed in Contact headers OK if there are no locations

No outputs allowed; script execution terminates

Single attribute: permanent yes or no Indicates whether redirection is a

permanent or temporary condition For SIP, 301 in one case, 302 in

other

<location url=“sip:foo@bar”> <redirect/></location>


Reject tag

Node

Rejects the call

Status code to use is provided as an attribute busy,notfound, reject, error SIP servers can also allow

numeric codes optional reason phrase can also

be used

CPL processing terminates

<reject status=“busy”/>


Switch Statements

Switch statements are the primary decision making components in CPL

They support a common structure Start with X-switch, where X is the

element to check Subtags are all called X, and each

of them represents a link with a specific value of the element

otherwise is defined as an allowed subtag of X-switch

not-present as well - when element is not in the message

<foo-switch> <foo is=“hello”> <reject/> </foo> <foo is=“goodbye”> <redirect/> </foo> <otherwise> <proxy/> </otherwise> <not-present> <log/> </not-present></foo-switch>


Address-Switch

Used to make decisions based on the value of address fields in the message

Defines two attributes, field and subfield

Field is origin (From field) original-destination (To field) destination (Request URI)

Subfield defines component of the URI in the field address-type (scheme of URL) user

Host port tel

If it’s a tel URL, or a sip URL with a telephone number, this element is defined

display display name; undefined for

request URI password


Address

Outputs of address-switch are address tags

Singe attribute, which can be one of is, contains, subdomain-of

is used for exact match case sensitivity depends on

subfield

contains substring match case sensitivity depends on

subfield

Subdomain-of only defined for tel and for host for host, it matches if the domain

in the subfield is a subdomain of the listed domain

for tel, it matches if the telephone number contains the listed number as a prefix


Examples

The given address tags to the right all match

Assume the request is

INVITE sip:[email protected] SIP/2.0From: FDisplay <sip:[email protected]>To: TDisplay <sip:[email protected]>

<address-switch field=“origin” subfield=“user”> <address is=“ruser”/></address-switch>

<address-switch field=“destination” subfield=“port”> <address is=“5060”/></address-switch>

<address-switch field=“original-destination” subfield=“host”> <address subdomain-of=“com”/></address-switch>

<address-switch field=“origin” subfield=“display”> <address contains=“FDisp”/></address-switch>


Address-Switch motivations

Older CPL drafts used string-switch for this

Motivations for change Hard to match on specific fields through glob matching Glob matching was hard to implement Take advantage of inherent structure Didn’t work for H.323


String-Switch

String matching on textual fields in request

Mandatory attribute “field” indicates whats being matched subject user-agent organization language

Subtags of string-switch are string

String contains either “is” or “contains” for exact or substring matching

<string-switch field=“subject”> <string is=“Sales”> <location url=“sip:[email protected]”> <proxy/> </location> </string> <string contains=“marketing”> <location url=“sip:[email protected]”> <proxy/> </location> </string> <otherwise> <location url=“sip:[email protected]”> <proxy/> </location> </otherwise></string-switch>


String Switch Details

Matching Issue Want case insensitive These fields are freeform UTF-8!

Case insensitivity depends on language and locale Nearly impossible to do

Solution is a new Compatibility Composition form defined for Unicode

Language is Accept-Language header in SIP Good for routing to operator based on language of caller Only substring matching is useful

Accept language has parameters, commas, etc.


Priority-Switch

Allows matching based on priority of request

Subtags are priority tags, which have attribute less, equal, greater

Comparisons are against defined SIP values of emergency, urgent, normal, non-urgent

If nothing is in request, priority is normal

unknown priorities mapped to normal for greater, less comparisons

<priority-switch> <priority equals=“newpriority”> <location url=“sip:[email protected]”> <proxy/> </location> </priority> <priority less=“normal”> <location url=“sip:[email protected]”> <proxy/> </location> </priority> <priority greater=“urgent”> <location url=“sip:[email protected]”> <proxy/> </location> </priority></priority-switch>


Time Switch

Significantly different and more complex than initial version Initially crontab based Now iCal based

Why iCal? Makes integration with calendaring programs easier More complete, more modern

-03 based on an iCal subset proven to be implementable in O(1) time!

Powerful representation of repeating events Every third Tuesday in February and March First Monday of the month etc.


Handling Timezones

User and server may be in different timezone

CPL times should refer to timezone of user

How to indicate timezone of user?

Offset from GMT not sufficient Want scripts to be valid for years Offset varies as DST comes and goes!!

Solution time-switch contains either tzurl or tzid tzurl refers to an object that defines the timezone tzid refers to either a to-be-established registry, or a server specific name names are things like EST and CST Java has a standard set of names


Time tags

Time tags contain many attributes that define repeating events

Basic event description dtstart: event start date and time dtend: event end time duration: instead of dtend

Defining repititions through periodicity freq: frequency of occurrence

daily, weekly, monthly, yearly interval: how often recurrence

repeats

Limiting repititions until: date/time when repititions

end

Defining exceptions byday List of days of the

week bymonthday List of days of the

month byyearday List of days of the

year byweekno List of weeks of the

year bymonth List of months of the

year


Exceptions

Expansion If frequency is a greater unit than largest exception Adds more events Example: freq=“yearly” bymonth=“jan,feb” defines an event that repeats

twice a year - once in jan, once in feb

Contraction If frequency is a smaller unit than largest exception Reduces number of events Example: freq=“daily” byday=“MO,WE” defines only Mondays and

Wednesdays


Example time-switch

Time of day forwarding/screening application

Proxy calls to work during 9-5 during the week

Proxy calls to home during the evening of the week

Reject calls on weekends

<time-switch tzid="America/New-York”> <time dtstart="20000703T090000" duration="8H" freq="weekly" byday="MO,TU,WE,TH,FR"> <location url=“sip:[email protected]”> <proxy/> </location> </time> <time dtstart=“20000703T170000” dtend=“20000703T210000” freq=“weekly” byday=“MO,TU,WE,TH,FR”> <location url=“sip:[email protected]”> <proxy/> </location> </time> <time dtstart=“20000703T000000” duration=“24H” freq=“weekly” byday=“SA,SU”> <reject reason=“it’s the weekend”/> </time></time-switch>


Location Addition Location

Defines a location to be added to the global location list

Single attribute, URL, contains the URL to add to the list

Lookup Obtains the URI to add through

indirection source attribute specifies place to

go to registration looks in registration

DB if source is a URL, fetches the

URL and that contains a document with just a URL to add

success, failure, notfound outputs

<location url=“sip:foo@bar”> <redirect/></location>

<lookup source=“registration”> <success> <proxy/> </success></lookup>


remove-location

Allows for removal of locations from global location list

Based on caller preferences specification

Caller prefs Associates parameters with

contacts Example: mobile=fixed,

media=video INVITEs contain headers that ask

to reach specific contacts by parameter

Parameter filtering two main attributes, param and

value both are comma separated lists param contains the name of a

parameter (e.g., mobile) value contains the value for that

parameter (e.g., fixed) same number of elements in

param and value lists causes matching contacts to be

removed

Address filtering location attribute contains a URL

pattern to match


Example Parameter filtering

<remove-location field=“mobility,feature” value=“mobile,voicemail”>

Sip:user@foo;mobility=“fixed”sip:joe@bar;feature=“voicemail”

Sip:user@foo;mobility=“fixed”


Mail tag

Allows CPL to send email

Single parameter, url, contains mailto URL to use

CPL server will Set the From field to its own

address Set the body to contain

information about the call, unless a body is specified by CPL

Set the Subject to something useful, unless subject is specified by CPL

Caveats Ampersand must be with an

entity!

<mail url=“mailto:user@host?subject=foo &body=hello%20there”>


Log tag

Allows CPL to generate log output

How output is logged is system dependent

Two parameters name: specifies name of log to

use interpretation of name is system

dependent comment: text to place in log

CPL should also generate time of day and other params in log

<log comment=“rejected call”>


Subroutines

CPL can call a subroutine defined by subaction

Subactions have an id

To call subroutine, use sub tag with that Id as the value of ref attribute

Subroutines can only be called if they are physically above in the CPL Avoids loops!

<subaction id=“vmail”> <location url=“sip:[email protected]”> <proxy/> </location></subaction>

<incoming> <lookup source=“registration”> <success> <proxy> <notfound> <sub ref=“vmail”/> </notfound> </proxy> </success> <otherwise> <sub ref=“vmail”/> </otherwise> </lookup></incoming>


Default Actions

What happens if a script reaches a point where there are no more tags?

Default Action is taken

Default action depends on how far script has gotten

No signaling actions, no locations defined, location set empty lookup in database, proxy or

redirect as normal

No signaling actions, no locations, location set non-empty

Happens for outgoing calls Proxy to location set

locations or lookups performed, no signaling proxy or redirect there

proxy action invoked, noanswer output not present let phone ring forever, or until

system defined timeout

proxy performed, responses received forward best response upstream


CPL Extensions

Uses XML namespaces

Each extension defines its own namespace

Will render DTD validation useless! General problem with

namespaces

<incoming> <lookup source=“registration”> <success> <proxy xmlns:foo=“http://bar”> <foo:screwup> <sub ref=“vmail”/> </foo:screwup> </proxy> </success> <otherwise> <sub ref=“vmail”/> </otherwise> </lookup></incoming>


Information Resource Jonathan Rosenberg

[email protected] +1 973.952.5000

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