Feature Interactions between SIP Call Control Services

1
Feature Interactions between SIP Call Control
Services

• Mario Kolberg
• University of Stirling

2
Introduction SIP

• SIP Session Initiation Protocol, IETF
• used for signalling, setup, modification and
  termination of a session
• media is separate
• text based protocol, based on HTTP
• 2 types of devices end devices and servers
• user agents initiate and respond to signalling,
  send and receive media, may provide services,
  e.g. SIP phones, soft phones
• servers redirect, proxy, registrar

3
Location of User

• SIP supports user mobility
• email like addresses sipmko_at_cs.stir.ac.uk
• but user may be located at discus.cs.stir.ac.uk
  and be logged on as mko0123
• public address vs. current location
• users need to register with a registrar
• work closely with proxy and redirect servers
• invitations are sent from UA via a number of
  servers to another UA
• can host services

4
A SIP Example
Bob
Laura
(1) INVITE
(2) 180 Ringing
(3) 200 OK
(4) ACK
Conversation
(5) BYE
(6) 200 OK
5
Feature Interactions

• truly distributed architecture
• highly programmable components
• servers may be under the control of different
  organisations
• end users may deploy their own services
• increased number of addresses

6
Applied Approach

• Triggering party
• Connection type
• Source, destination
• Original connection
• Connection after feature activation
• Parties Treatment

7
Applied Approach

• Analysis pairs of features
• Compare two feature descriptions according to
  four rules
• Single User Dual Feature Control
• Connection Looping
• Redirection and Treatment
• Diversion and Reversing

8
Single User Dual Control
CFB TP B (A, B) ? (A, C) CFU TP B (A, B) ?
(A, C)
AR TP A (B, A) ? (A, B) HL TP A (A, B) ?
(A, B)
9
Connection Looping
CFB TP B (A, B) ? (A, C) CFU TP C (A, C) ?
(A, B)
10
Redirection and Treatment
CFB TP C (A, C) ? (A, B) OCS TP A (A, B) ?
(A, Treat)
AR TP B (A, B) ? (B, A) OCS TP B (B, A) ?
(B, Treat)
11
Diversion and Reversing
CFB TP C (A, C) ? (A, B) AR TP B (A, B) ?
(B, A)
CFB TP A (B, A) ? (B, C) AR TP B (A, B) ?
(B, A)
12
Application to SIP

• SIP CGI (ish)
• when a service gets executed its description is
  included into the message
• descriptions are included using a new header
  (Contype)
• if there is already a description, apply the
  rules to find interactions
• services need to be surrounded by a cocoon which
  contains the description for that service and the
  algorithm

13
Resolution

• if an interaction is detected ?? resolution
• results from second service are discarded
• more elaborated approaches possible, policies
• approaches may be tuned towards various goals
• most services executed
• execute all services which are subscribed to by
  the party who pays for the call
• private environment services of an organisation
  may have priorities over services of an
  individual
• its all about priorities
• currently, the priority is with the services
  executed first

14
Example
Contype IDCFU TPsipbob_at_d254203.cs.stir.ac.uk
OrigFromchris_at_discus.cs.stir.ac.uk
OrigTobob_at_d254203.cs.stir.ac.uk FinalFromchris_at_
discus.cs.stir.ac.uk FinalToalice_at_d254203.cs.sti
r.ac.uk

• Not all services are triggered by INVITE messages
• some are triggered by responses (486 Busy)
• Contype header needs to be copied into responses

15
Experimentation Example 2

• SER, Pingtel phone, kphone

16
Experimentation

• SIP/2.0 200 OK
• Via SIP/2.0/UDP 139.153.254.50branchz9hG4bKafde
  .6e4854d4.0
• Via SIP/2.0/UDP 139.153.254.222
• ConType IDCFU TPsipalice_at_d254203.cs.stir.ac.u
  k
• OrigFromchris_at_discus.cs.stir.ac.uk
• OrigToalice_at_d254203.cs.stir.ac.uk
• FinalFromchris_at_discus.cs.stir.ac.uk
• FinalTobob_at_d254203.cs.stir.ac.uk
• Forwarded-To ltsipbob_at_d254203.cs.stir.ac.ukgt
• From ltsipchris_at_discus.cs.stir.ac.ukgttag1c8060
• To ltsipalice_at_d254203.cs.stir.ac.ukgttag1A8019B3
  
• Contact "root" ltsiproot_at_139.153.254.2035062tra
  nsportudpgt
• Record-Route ltsipalice_at_139.153.254.203ftag1c80
  60lrlrgt,
• ltsipalice_at_139.153.254.50ftag1c8060lrlrgt
• Who sent this reply Alice or Bob?
• Contact header
• Record route

17
Case Study Results

• 9 services
• CFU, CFB, OCS, TCS, VM, AR, DND, HL, GR

18
Discussion

• all distributed interactions are detected
• single component interactions are an issue
• if a service drops original INVITE which services
  is executed first?
• are single component interactions in fact
  interactions in SIP?
• usually a list of services to be executed on a
  component ? implicit ordering/priorities
• ISSUES multiple addresses for a party
• identifying a party from a response message

19
Discussion cont.

• some services are triggered differently than in
  the PSTN
• CFBTCS do not interact?

Alice
Proxy (CFB, TCS)
Bob
Chris
INVITE Bob
403 Screened
INVITE Bob
486 Busy
INVITE Chris