Programmable End System Services Using SIP

1
Programmable End System Services Using SIP

• Xiaotao Wu, Henning Schulzrinne
• 2nd New York Metro Area
• Networking Workshop
• September 3rd, 2002

2
Agenda

• Brief introduction to SIP
• SIP endpoints can perform service functions
• Network service creation techniques are not good
  for end system services
• Endpoint Service Markup Language (ESML)
• Comparison between ESML and the other XML based
  solutions for service creation
• Future work

3
SIP is , SIP is not

• SIP core protocol for establishing sessions in
  the Internet (peer-to-peer)
• Transports session description information from
  initiator (caller) to callee
• Allows change of parameters in mid-session
• Terminate session
• NOT for distribution of multimedia data
• NOT suitable for media gateway control

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Basic call setup

• Phone sends INVITE acts as UAC

INVITE sipalice_at_m2.home.com To
sipalice_at_home.com From Bob ltsipbob_at_office.com
gt cIN IP4 135.180.144.33 maudio 8000 RTP/AVP
0 5 8
office.com
alice
m2.home.com
bob
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Basic call setup

• Phone sends INVITE acts as UAC
• Alices phone rings acts as a UAS

User agent server
SIP/2.0 180 Ringing
office.com
alice
m2.home.com
bob
home.com
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Basic call setup

• Phone sends INVITE acts as UAC
• Alices phone rings acts as a UAS
• When Alice picks up, call is accepted and bobs
  phone sends ACK to complete the setup

office.com
alice
m2.home.com
bob
home.com
SIP/2.0 200 OK cIN IP4 135.180.144.32 maudio
9000 RTP/AVP 0 8
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Basic call setup

• Encoded audio packets sent over RTP

office.com
alice
bob
home.com
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Basic call setup

• Encoded audio packets sent over RTP
• When either party hangs up BYE is sent

BYE sipbob_at_office.com
office.com
alice
bob
home.com
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SIP beyond audio call

• Presence extensions
• SUBSCRIBE to request notifications
• NOTIFY when event occurs
• Instant messaging
• MESSAGE with text body
• Device control

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Todays Endpoints

• Processing ability
• Graphical rendering
• Storage capacity
• Converged information

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SIP endpoints are more than IP-based POTS phones
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SIP promotes end system services

• Two SIP UAs can talk to each other directly.
• Service-related information is explicitly
  contained in SIP headers.
• Contact sip1.2.3.4mobility"fixed"q0.8
• The simplicity of SIP makes it easier for end
  system to perform services.

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SIP CGI

• Programming language independent
• Maintains state via an opaque token
• For SIP proxies and endpoints
• call routing
• controlling forking
• call rejection
• call modification (Priority, Call-Info)
• RFC 3050

Priority.pl
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SIP CGI contd.

• Block _at_anonymous.com
• if (defined ENVSIP_FROM
• ENVSIP_FROM "sip_at_anonymous.com")
• print "SIP/2.0 600 I can't talk right
  now\n\n"
• Make calls from boss as urgent
• if (defined ENVSIP_FROM
• ENVSIP_FROM /sipboss_at_mycompany.com/)
• foreach reg (get_regs())
• print "CGI-PROXY-REQUEST reg SIP/2.0\n"
• print "Priority urgent\n\n"

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CPL

• XML-based interpreted
• Control
• Restricted to branching and subroutines
• no variables, no loops
• structured tree describing actions performed on
  call setup event
• Event
• incoming and outgoing events
• Switch
• address, language, time, priority
• Action
• Proxy, redirect, reject (busy)
• Graphical tools

• Protocol-independent
• SIP, H.323
• Extensions
• CPL for presence
• Logging
• Email

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CPL contd.

• lt?xml version"1.0" ?gt
• lt!DOCTYPE cpl PUBLIC "-//IETF//DTD RFC2824 CPL
  1.0//EN" "cpl.dtd"gt
• ltcplgt
• ltincominggt
• lttime-switchgt
• lttime dtstart"20020830T140000Z"
• dtend"20020830T150000Z"gt
• ltreject status"reject" reasonBusy" /gt
• lt/timegt
• lt/time-switchgt
• lt/incominggt
• lt/cplgt

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SIP servlet

• Java-based
• Runs in SIP server
• Receives SIP objects and processes them

• public class RejectServlet extends
  SipServletAdapter
• public boolean doInvite(SipRequest req)
• SipResponse res req.createResponse()
• res.setStatus(603)
• res.send()
• return true

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Services in sipc
SIP messages
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Need a language for end system service creation

• CPL
• Cannot originate a call
• Cannot be activated through non-call events
• SIP CGI
• An interface, not a language
• Can do nearly anything
• Existing techniques for network services are not
  appropriate for end system service creation

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Describe end system services

• Users pay for a bad service because they do not
  understand why it is not good
• Techniques for network services are not
  sufficient for end system services.

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Existing techniques for Network Services are not
sufficient for End System Services
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Existing techniques for Network Services are not
sufficient for End System Services
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Requirement for an end system service language

• Simple and easy to understand by non-programmers
• Platform neutral
• Express user interactions
• Control media and other end system applications
• Extensible to accommodate new services
• Restricted to certain class of services, not
  necessarily Turing-complete

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Endpoint Service Markup Language (ESML)

• XML based language
• Platform and underlying programming language
  neutral
• Readable by non-programmers
• Defined as an XML schema
• Derivation of new types
• Pre-defined types
• Tree-like structure
• Use packages to group events and actions

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ESML example

• ltesml name"online_call"
• require"generic presence ui"gt
• ltnotification status"online"
• priority"0.5"gt
• ltaddress-switch field"origin"gt
• ltaddress is"xyz_at_foo.com"gt
• ltcall /gt
• ltalert soundfoo.au"
• text"Calling xyz_at_foo.com" /gt
• lt/addressgt
• lt/address-switchgt
• lt/notificationgt
• lt/esmlgt

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ESML packages
email
web
im
conference
calendar
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Extend generic to sip

• ltxsschema targetNamespace"esmlsip"
• xmlnssip"esmlsip"
• xmlnsgeneric"esmlgeneric"
• ..........
• ltxscomplexType name"IncomingType"gt
• ltxscomplexContentgt
• ltxsextension
• base"genericIncomingType"gt
• ltxsattribute name"priority"
• type"PriorityType"/gt
• ..........
• lt/xsextensiongt
• lt/xscomplexContentgt
• lt/xscomplexTypegt

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ESML Service Creation
ESML editor
esml.xsl
service.esml (template)
xslif
address isvar
XSLT
configuration editor
service.html
translate.cgi
service_foo.esml
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Compare to other languages

• CPL
• Not able to initiate a call
• Not able to handle non-call events
• SCML
• Developed by the JAIN forum
• Closely tied to the JAIN Java Call Control (JCC)
  API. and defined using an XML Schema.
• Object model of JCC
• CCXML
• Defined in W3C, for voice browser call control,
  such as VoiceXML
• The states and events for CCXML is in a lower
  level abstraction than those for ESML and CPL.
  (like the events call.CALL_CONNECTED,
  call.CALL_ACTIVE, connection.CONNECTION_ALERTING)

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Future work

• Service partition between end system services and
  network services
• Feature interaction

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.NET

• connectivity really in two parts. One is how
  you represent the information and the other is
  how do you exchange that information. The first
  is about information formats and schemas and the
  second is about a rich set of protocols.
• SOAP and WSDL is not for telecommunication
  services
• How to share an ESML scripts among many endpoints
  fits in the .NET framework. However, how to
  represent the service logic in an endpoint is
  ESMLs own job.

ESML represent the service logic in an endpoint
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Security consideration

• Follow the same security consideration of CPL.
• It is designed to allow services to be specified
  in a manner which prevents potentially hostile or
  mis-configured scripts from launching security
  attacks, including denial-of-service attacks.
  Because script runtime is strictly bounded by
  acyclicity, and because the number of possible
  script operations are strictly limited, scripts
  should not be able to inflict damage upon a CPL
  server.

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End-to-end arguments

• End-to-end Arguments in System Design, Jerome
  H. Saltzer and David P. Reed and David D. Clark,
  ACM Transactions on Computer Systems, 1984The
  function in question can completely and correctly
  be implemented only with the knowledge and help
  of the application standing at the end points of
  the communication system. Therefore, providing
  that questioned function as a feature of the
  communication system itself is not possible.
  (Sometimes an incomplete version of the function
  provided by the communication system may be
  useful as a performance enhancement.)