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					Multimedia Communication in
        the Internet
       VoIP and SIP
                  Dr. Dorgham Sisalem



     These presentation materials describe Tekelec's present plans to develop and make available to its customers certain
     products, features and functionality. Tekelec is only obligated to provide those deliverables specifically included in a
     written agreement signed by Tekelec and customer.
Description of Multimedia




        These presentation materials describe Tekelec's present plans to develop and make available to its customers certain
        products, features and functionality. Tekelec is only obligated to provide those deliverables specifically included in a
        written agreement signed by Tekelec and customer.
Session Description Protocol (SDP)

•   Convey sufficient information to enable participation in a multimedia
    session
•   SDP includes description of:
        Media to use (codec, sampling rate)
        Media destination (IP address and port number)
        Session name and purpose
        Times the session is active
      Contact information
•   Note: indeed SDP is a data format rather than a protocol.



    RFC2327

                                      Tekelec Confidential /
                                 For Discussion Purposes Only /
                                Tekelec Confidential
                                          Non-Binding
Example: Session Description Protocol (SDP)


  v=0
  o=sisalem 28908044538 289080890 IN IP4 193.175.132.118
  s=SIP Tutorial
  e=sisalem@fokus.gmd.de
  c=IN IP4 126.16.69.4
                                                          Receiver’s IP address
  t=28908044900 28908045000
  m=audio 49170 RTP/AVP 0 98
  a=rtpmap:98 L16/11025/2                               Supported RTP payload types


                                                        Receiver’s port
                                                        number

   “I am ready to receive G.711 (0) and L16 (98) audio
   at IP address 126.16.69.4, port number 49170”.
                                     Tekelec Confidential /
                                For Discussion Purposes Only /
                               Tekelec Confidential
                                         Non-Binding
IP Telephony
What is IP-Telephony?




  These presentation materials describe Tekelec's present plans to develop and make available to its customers certain
  products, features and functionality. Tekelec is only obligated to provide those deliverables specifically included in a
  written agreement signed by Tekelec and customer.
Why Another Phone System?

 •   Carrying voice on IP-based packet networks first identified by Cohen
     in 1977*
 •   Commercialization and standardization began in 1995; Vocaltec the
     first company to ship IP2PSTN gateways (proprietary)
 •   ITU-T standardized H.323 to recreate ISDN
 •   SIP standardization began in IETF in 1995
 •   Adoption of SIP for use in 3GPP in late nineties
 •   Motivation:
       Cost saving through telco by-passing
       Service Integration




                                         Tekelec Confidential /
                                    For Discussion Purposes Only /
                                   Tekelec Confidential
                                             Non-Binding
Why Another Phone System?


 •   Lower transport costs:
       Routing is cheaper than switching (lower equipment and line prices)
             Network is not only dedicated to voice
       More efficient bandwidth usage:
             Variable compression (often lower than 64kb/s)
             Usage of silence suppression
       Application integration
             Support voice, video or text all at the same time with the same network
             Integrate intelligent applications easily (initiate a call when something
              happens)




                                            Tekelec Confidential /
                                       For Discussion Purposes Only /
                                      Tekelec Confidential
                                                Non-Binding
IP-Telephony is not just Telephony!!!!

 •   IP-Telephony signaling is not about
         Reengineering PSTN
         Carrying PSTN over IP
         Providing QoS
         Solving the business model dilemma
         Introducing world peace
 •   IP-Telephony is just another Internet application but with special requirements
       More stringent QoS requirements but so would video conferencing and distributed
        gaming
       Interaction with legacy technology
       Regulatory issues
       Comparisons to legacy services




                                         Tekelec Confidential /
                                    For Discussion Purposes Only /
                                   Tekelec Confidential
                                             Non-Binding
Basic Internet Telephony Scenarios




 PC-to-PC (MS NetMeeting, appliances)
                                                                       Internet
                                                    IP Phone

 PC-to-phone (net2phone.com)
                                                            Internet
                                   IP Phone
                                                                         Egress PSTN Gateway PSTN Phone
 phone-to-phone (Paegas)
                                                     Internet

          PSTN Phone                                              Egress PSTN Gateway
                       Ingress PSTN Gateway                                             PSTN Phone
 phone-to-PC as
 well

                                     Tekelec Confidential /
                                For Discussion Purposes Only /
                               Tekelec Confidential
                                         Non-Binding
PC-to-phone/PC Scenario


                          Egress PSTN Gateway
                                                                        PSTN Phone

                        Internet




         IP SoftPhone

                                   IP Hard-Phone
 Benefits:
 •   Cost-savings                                                                    IP SoftPhone
 •   Service integration: Many today’s installation serve as PBX replacement
     today.




                                            Tekelec Confidential /
                                       For Discussion Purposes Only /
                                      Tekelec Confidential
                                                Non-Binding
Phone-2-phone Scenario



                                    Internet

PSTN Phone   Ingress PSTN Gateway                                                  PSTN Phone
                                                             Egress PSTN Gateway
 Benefit:
 •   Cost savings through telco bypass. Most of the voice path in the
     Internet which has no notion of distance and duration in its charging
     model.




                                             Tekelec Confidential /
                                        For Discussion Purposes Only /
                                       Tekelec Confidential
                                                 Non-Binding
Shift in Motivation Since Nineties



  •   Telco prices dropping, cost saving not as appealing as in the past:
        Berlin-Prague before market liberalization in 1995: 1DM a minute
        Berlin-Prague today*: 4.50 cents (factor of 11)
  •   Expectations set high on service integration
        Voicemail2email, click-to-dial, conferencing, instant messaging and
         presence, etc.
  •   Shift in business mode: “be telco on your own” doable without large
      investments and operational costs. Seen as an opportunity for small
      businesses.




       * Source: http://www.billigertelefonieren.de/
                                      Tekelec Confidential /
                                 For Discussion Purposes Only /
                            Tekelec Confidential
                                          Non-Binding
Completed Work

 •   Since beginning, lot of focus has been paid to quality of service to assure
     “nice voice”:
         Network provisioning
         QoS Signaling (RSVP, diffserv)
         Packet-loss concealment
         Less concern today – backbones overprovisioned, end-devices improved and
          burned in hardware
 •   Signaling for IP Telephony Well Understood Today:
       Session Initiation Protocol (RFC3261)
       Gateways to PSTN




                                         Tekelec Confidential /
                                    For Discussion Purposes Only /
                                   Tekelec Confidential
                                             Non-Binding
       SIP Implementations Widely Available




   SNOM          Pingtel    Allied Telesyn                      Cisco




    Intertex     Cisco            Siemens                      Mitel    Microsoft
                           Free SIP server with capacity that can
                           server VoIP signaling for Bay Area.

… and many more …http://www.iptel.org/info/products/
                                   Tekelec Confidential /
                              For Discussion Purposes Only /
                             Tekelec Confidential
                                       Non-Binding
Integrated Applications

 •   Distributed games                           •      Video conferencing
         SIP Quake sighted!
                                                 •      Instant Messaging
 •   Virtual reality
                                                            voicemail notifications
 •   Web-pages and applets                                  stock notifications
 •   Links in e-mails                                       callback notification
 •   Web-IVRs                                    •      Calendars
 •   Click-to-dial                                          pre-setup conference calls
 •   Directory Services                          •      Unified Messaging
                                                            voicemail2email




                                etc.
                                     Tekelec Confidential /
                                For Discussion Purposes Only /
                               Tekelec Confidential
                                         Non-Binding
Example:                                          Trial Site


 •   Provides just signaling services
       gives users a unique globally reachable address
       resembles Web-hosting in IP world or NetCentrex in PSTN world
       no media transport -- only signaling relayed, media does not hit the server at all

 •   To set it up, we needed
       PC
       Freely available software
       IP access
       one part-time undergraduate student

 •   Users need
       IP phone (either in SW or HW)
 •   Complimentary services may be easily provided by other parties, users just
     need to set up their signaling preferences:
       bridging to PSTN, voicemail--2-email, etc.
Protocols




These presentation materials describe Tekelec's present plans to develop and make available to its customers certain
products, features and functionality. Tekelec is only obligated to provide those deliverables specifically included in a
written agreement signed by Tekelec and customer.
What Problems Do Need to Be Solved for VoIP?


  •   Session management
        Users may move from terminal to terminal with different capabilities and change
         their willingness to communicate
        To set-up a communication session between two or more users, a signaling
         protocol is needed
        Session Initiation Protocol (SIP) supports locating users, session negotiation
         (audio/video/instant messaging, etc.) and changing session state
  •   Media Transport
        Getting packetized voice over lossy and congested network in real-time
        RTP – protocol for transmitting real-time data such as audio, video and games
  •   End-to-end delivery: underlying IP connects the whole world
  •   Supporting protocols: DNS, IP , routing protocol, AAA, gateway location,
      QoS, etc.
  •   IETF Practice: Decomposition Principle; Separate protocols are used for
      separate purposes. Signaling protocol separate from media and QoS
      signaling. All of them on top of IP.



                                          Tekelec Confidential /
                                     For Discussion Purposes Only /
                                    Tekelec Confidential
                                              Non-Binding
Protocol Zoo (Hourglass Model)



                                 Media

  HTTP      H.323     SIP        RTP      RTCP

           TCP      SCTP            UDP

                     IPv4/IPv6

                           PPP            AALx

Ethernet     GPRS     SONET       V.x      ATM
 Packetized Communication

          Signaling Protocol
                                             Call Server
          Media Transport
       End Users                                           End Users



                       IP Router


Note:
•Every packet may take a completely different path
•Signaling takes typically different path than media does
•Both signaling and media as well as other applications (FTP, web,
email, … ) look “alike” up to transport layer and share the same fate
 Components Integration with SIP


           PSTN Gateway
                           SMS Gateway                  •   All components use SIP
                                                        •   They are glued together
                                                            using a proxy server that
                                                            implements a routing
IP Phone Pool                                               policy for signaling




                                         Softphones
                     SIP proxy           And
                                         Messaging
                                         applications


                Applications

                                              Other domains
SIP (RFC3261) - General Purpose Presence Protocol

 •   SIP is not limited to Internet telephony
       SIP establishes user presence
       SIP messages can convey arbitrary signaling payload: session
         description, instant messages, JPEGs, any MIME types
 •   Suitable for applications having a notion of session
       distributed virtual reality systems,
       network games (Quake II/III implementations),
       video conferencing, etc.
 •   Applications may leverage SIP infrastructure (Call Processing, User Location,
     Authentication)
       Instant Messaging and Presence
       SIP for Appliances




                                        Tekelec Confidential /
                                   For Discussion Purposes Only /
                                  Tekelec Confidential
                                            Non-Binding
SIP Is Not

 •   Transport Protocol
 •   QoS Reservation Protocol
 •   Gateway Control Protocol


 •   It does NOT dictate ...
        Product features and services (color of your phone and distinctive ringing
          melodies, number of simultaneous calls your phone can handle, don’t
          disturb feature, ...)
       network configuration




                                        Tekelec Confidential /
                                   For Discussion Purposes Only /
                                  Tekelec Confidential
                                            Non-Binding
SIP Workhorses

  • SIP Proxy Server
       relays call signaling, i.e. acts as both client and server
       operates in a transactional manner, i.e., it keeps no session
        state
       transparent to end-devices
       does not generate messages on its own (except ACK and
        CANCEL)
       Allows for additional services (call forwarding, AAA, forking,
        etc.)
  • SIP Redirect Server
       redirects callers to other servers
  • SIP Registrar
       accept registration requests from users
       maintains user’s whereabouts at a Location Server
      (like GSM HLR)

                                   Tekelec Confidential /
                              For Discussion Purposes Only /
                             Tekelec Confidential
                                       Non-Binding
SIP End-devices

 •   User Agent (user application)
       UA Client (originates calls)
       UA Server (listens for incoming calls)
 •   Types of UAs:
       Softphone and hardphones
       Messaging clients
       PSTN gateways
       Media servers (voicemail)
       Etc.




                                       Tekelec Confidential /
                                  For Discussion Purposes Only /
                                 Tekelec Confidential
                                           Non-Binding
SIP Addresses

 •   SIP gives you a globally reachable address.
       Callees bind their temporary address to the global one using SIP REGISTER
        method.
       Callers use this address to establish real-time communication with callees.

 •   URLs used as address data format; examples:
       sip:jiri@iptel.org
       sip:voicemail@iptel.org?subject=callme
       sip:sales@hotel.xy; geo.position:=48.54_-123.84_120
 •   must include host, may include user name, port number, parameters (e.g.,
     transport), etc.
 •   may be embedded in Webpages, email signatures, printed on your business
     card, etc.
 •   address space unlimited
 •   non-SIP URLs can be used as well (mailto:, http:, ...)

                                         Tekelec Confidential /
                                    For Discussion Purposes Only /
                                   Tekelec Confidential
                                             Non-Binding
SIP RFC3261 Methods


  •   INVITE initiates sessions
        session description included in message body
        re-INVITEs used to change session state
  •   ACK confirms session establishment
        can only be used with INVITE
  •   BYE terminates sessions
  •   CANCEL cancels a pending INVITE
  •   OPTIONS capability inquiry
  •   REGISTER binds a permanent address to current location; may convey
      user data (CPL scripts)




                                      Tekelec Confidential /
                                 For Discussion Purposes Only /
                                Tekelec Confidential
                                          Non-Binding
SIP Extension Methods

 •    UPDATE               precondition met
                           (draft-ietf-sip-manyfolks-resource)
 •    PRACK                provisional reliable responses
                           acknowledgement
                           (draft-ietf-sip-100rel)
 •    INFO        mid-call signaling
                           (RFC 2976)
 •    SUBSCRIBE/ instant messaging
     NOTIFY/               (draft-ietf-simple-*)
 •    MESSAGE




                                      Tekelec Confidential /
                                 For Discussion Purposes Only /
                                Tekelec Confidential
                                          Non-Binding
SIP Response Codes
 •   Borrowed from HTTP: xyz explanatory text
 •   Receivers need to understand response class (“x”)
 •   x80 and higher codes avoid conflicts with future HTTP response
     codes
 •   1yz Informational
      100 Trying
      180 Ringing (processed locally)
      181 Call is Being Forwarded
 •   2yz Success
      200 ok
 •   3yz Redirection
      300 Multiple Choices
      301 Moved Permanently
      302 Moved Temporarily


                                    Tekelec Confidential /
                               For Discussion Purposes Only /
                              Tekelec Confidential
                                        Non-Binding
SIP Response Codes (cont.)

•   4yz Client error
      400 Bad Request
      401 Unauthorized
      482 Loop Detected
      486 Busy Here
•   5yz Server failure
      500 Server Internal Error
•   6yz Global Failure
      600 Busy Everywhere




                                         Tekelec Confidential /
                                    For Discussion Purposes Only /
                                   Tekelec Confidential
                                             Non-Binding
SIP Message Structure
                 Request                                                                       Response
                                                                    SIP/2.0 200 OK
INVITE sip:UserB@there.com SIP/2.0

                                                                   Via: SIP/2.0/UDP here.com:5060
Via: SIP/2.0/UDP here.com:5060                                     From: BigGuy <sip:UserA@here.com>

From: BigGuy <sip:UserA@here.com>                                  To: LittleGuy <sip:UserB@there.com>;tag=65a35
                                                                   Call-ID: 12345601@here.com
To: LittleGuy <sip:UserB@there.com>        Message                 CSeq: 1 INVITE
Call-ID: 12345600@here.com                 Header                  Subject: Happy Christmas
CSeq: 1 INVITE
                                           Fields                  Contact: LittleGuy <sip:UserB@there.com>
Subject: Happy Christmas                                           Content-Type: application/sdp

Contact: BigGuy <sip:UserA@here.com>
                                                                   Content-Length: 134


Content-Type: application/sdp
Content-Length: 147


                                                                   v=0
 v=0
                                                                   o=UserB 2890844527 2890844527 IN IP4 there.com
 o=UserA 2890844526 2890844526 IN IP4 here.com
                                                                   s=Session SDP
 s=Session SDP
                                                                   c=IN IP4 110.111.112.113
 c=IN IP4 100.101.102.103
                                                                   t=0 0
 t=0 0
 m=audio 49172 RTP/AVP 0
                                      Payload                      m=audio 3456 RTP/AVP 0
                                                                   a=rtpmap:0 PCMU/8000
 a=rtpmap:0 PCMU/8000

                                 “receive RTP G.711-encoded audio at 100.101.102.103:49172”
                                                       Tekelec Confidential /
                                                  For Discussion Purposes Only /
                                                 Tekelec Confidential
                                                           Non-Binding
SIP Registration

 •   Bind a SIP address to an IP number
 •   Registration sent to SIP registrar responsible for the domain
       Sisalem@iptel.org -> registrar: iptel.org
       Find registrar through DNS
 •   Registration is needed to receive calls
       Not necessarily needed for initiating calls




                                        Tekelec Confidential /
                                   For Discussion Purposes Only /
                                  Tekelec Confidential
                                            Non-Binding
SIP Registrar
                                                                   This registration example
                                   Location Server                 establishes presence of
                                                                   user with address jiri@iptel.org
            Jiri @ 195.37.78.173


                                                                   and binds this address to user’s
                                                                   current location 195.37.78.173.
       #2


                                      REGISTER sip:iptel.org SIP/2.0
                                      From: sip:jiri@iptel.org
                                      To: sip:jiri@iptel.org         #1
                                      Contact: <sip:195.37.78.173>
                                      Expires: 3600


                                           #3        SIP/2.0 200 OK
       SIP Registrar
    (domain iptel.org)

                                                      Tekelec Confidential /
                                                 For Discussion Purposes Only /
                                                Tekelec Confidential
                                                          Non-Binding
Basic SIP Call-Flow (Proxy Mode)

                                                                                        Location Server
                                            #0




                                                                       jiri@195.37.78.173
DNS SRV Query ? iptel.org                             #2                                    #3
Reply: IP Address of iptel.org SIP Server
                                                                                             INVITE sip:jiri@195.37.78.173




                                                           jiri
          INVITE sip:jiri@iptel.org                                                          From: sip:Caller@sip.com
          From: sip:Caller@sip.com                                                           To: sip: jiri@iptel.org       #4
          To: sip: jiri@iptel.org                #1                                          Call-ID: 345678@sip.com
          Call-ID: 345678@sip.com

                           OK 200                                                                  OK 200
                     #6
                           From: sip:Caller@sip.com                                                From: sip:Caller@sip.com #5
                           To: sip: jiri@iptel.org                Proxy                            To: sip: jiri@iptel.org
                           Call-ID: 345678@sip.com                                                 Call-ID: 345678@sip.com

             #7     ACK sip:jiri@iptel.org

 Caller@sip.com                                                                                                     sip:jiri@195.37.78.173


                  Media streams #8



                                                             Tekelec Confidential /
                                                        For Discussion Purposes Only /
                                                       Tekelec Confidential
                                                                 Non-Binding
Proxy Server Functionality

 • Serve as rendezvous point at which callees are globally
   reachable
 • Perform routing function, i.e., determine to which SIP hop
   (UA/proxy/redirect) signaling should be relayed
 • Allow for call processing logic.
       user’s signaling preferences
       AAA
       firewall control
       etc.

 • Forking: Several destinations may be tried for a request
   sequentially or in parallel.

                                    Tekelec Confidential /
                               For Discussion Purposes Only /
                              Tekelec Confidential
                                        Non-Binding
Proxy Chaining

 •   SIP proxies may be arbitrarily chained and add different values along
     the signaling path
         least-cost gateway routing
         load distribution
         processing users’ preferences
         etc.
 •   In some case a local outbound proxy may be involved
       provides locally important call processing logic (e.g., identifying nearest
        911)
       manages firewall
       provides least-gateway-cost routing service
       IP phones must know address of the proxy:may be configured manually
        or with a configuration protocol (DHCP, TFTP, ... )
 •   Servers have to avoid loops and recognize spirals


                                        Tekelec Confidential /
                                   For Discussion Purposes Only /
                                  Tekelec Confidential
                                            Non-Binding
Proxy Chaining - an Example

Caller’s administrative domain   Administrative domain of a PSTN gateway operator
                                                                                    gw01.asia.pstn.com
                                 pstn.com                           asia.pstn.com

                            #2                             #3                       #4
         #1

    Caller’s outbound       Destination’s       Proxy in the target
    proxy accomplishes      “first-hit proxy”   area distributes load
    firewall traversal.     identifies a proxy in a gateway farm.
                            serving dialed
                            area.
         Note: signaling (in red) may take a completely different
         path from media (in blue).


                                        Tekelec Confidential /
                                   For Discussion Purposes Only /
                                  Tekelec Confidential
                                            Non-Binding
Ability to Try Multiple Destinations: Forking
 •   A proxy may fork a request to multiple destinations either in parallel (“reach
     me everywhere”) or serially (“forward no reply”).
 •   A proxy can cancel pending parallel searches after a successful response is
     received.
 •   A proxy can iterate through redirection responses (“recursive forking”).
 •   The first “OK” is taken.

           #1 INVITE
          #2 Trying
                                       #3 INVITE
                                           #4 Ringing


                                             #5 CANCEL
                                          #6 OK
                                                                     #7 INVITE




                                         Tekelec Confidential /
                                    For Discussion Purposes Only /
                                   Tekelec Confidential
                                             Non-Binding
Stateful versus Stateless Proxy Operational Mode

   • SIP Proxies may operate either in stateful or
     stateless mode; which of the modes is used depends
     on implementation or configuration.
   • stateless mode:
       Usage: good for heavy-load scenarios -- works well for
        example if they act as application-layer load distributors.
       Behavior:
             proxies just receive messages, perform routing logic, send
              messages out and forget anything they knew;
             they should cache results of SIP routing logic as it is not able
              to distinguish between retransmissions and new requests --
              and would result in new execution of SIP routing logic for
              every retransmission



                                       Tekelec Confidential /
                                  For Discussion Purposes Only /
                                 Tekelec Confidential
                                           Non-Binding
Stateful versus Stateless Proxy Operational Mode

   • stateful mode:
       Usage: good for implementing some services (e.g., “forward
        on no reply”)
       Behavior:
            proxies maintain state during entire transaction; they
             remember outgoing requests as well as incoming requests that
             generated them until transaction is over;
            a forking proxy should be stateful
            reduce retransmission time by acting on behalf of sender
             closer to destination




                                    Tekelec Confidential /
                               For Discussion Purposes Only /
                              Tekelec Confidential
                                        Non-Binding
“Stateful” Proxy Refers to Transactions
                                                                                    Frequently
                        SIP state                                                    Misunderstood
                        forgotten                                                       Issue
                        as soon as
                                              •     SIP proxies deliver a “one-time
                     transaction over
                                                    rendezvous service” (as opposed to
                                                    state storage service).
                                              •     Thus a stateful proxy just keeps state
INVITE                                              during a SIP “rendezvous transaction”
a@a.com                                             and completely forgets it afterwards.
                                              •     A SIP proxy is not aware of existing
                    OK                              calls. In case of failure, existing calls are
                                                    NOT affected!

    Legend
    SIP signaling
    SIP state
    media

                                        Tekelec Confidential /
                                   For Discussion Purposes Only /
                                  Tekelec Confidential
                                            Non-Binding
Tell Other Parties Where You Are: Contact Header Field

  •   In general, it indicates where users may be reached for subsequent requests
        Minimize proxy’s involvement in transactions: once all parties know each other’s
         contact address they can communicate directly and proxy server is no more
         needed
        used for the “next-try” in 3xx redirection responses
        may include multiple weighted addresses
        if behind a firewall, the Contact header field carries public server’s address
  •   In REGISTER method
        indicates current SIP address of requestor
              if different from current registration, it is added to current registration list
              if equal, contact parameters including updated and expiration timer updated
              2xx replies return a list of current registrations
        “Expires: 0” removes an entry, if along with “Contact: *” all entries are
         removed

        Note: Multiple header fields with the same field-name may be present in a
         message if and only if the entire field-value for that header field is defined
                         as a comma-separated list (i.e., #(values)).


                                                Tekelec Confidential /
                                           For Discussion Purposes Only /
                                          Tekelec Confidential
                                                    Non-Binding
 SIP Operation in Redirect Mode


                                                                         Location Server
                                         #2




                                                       Callee@home.com
                                          Callee
                                                                         #3


Caller@sip.com
       #1 INVITE Callee@example.com


            302 moved temporarily
       #4
            Contact: Callee@home.com
                                                   Proxy
      #5 ACK Callee@example.com

                                                                              Callee@home.com
            #6 INVITE Callee@home.com

                         #7 OK 200

               #8 ACK Callee@home.com


                                              Tekelec Confidential /
                                         For Discussion Purposes Only /
                                        Tekelec Confidential
                                                  Non-Binding
SIP Server -- Proxy versus Redirection

 •   A SIP server may either proxy or redirect a request
 •   Which of the two method applies is an implementation or configuration issue.
     It may be statically configured or dynamically determined (CPL).
 •   Redirection useful if a user moves or changes her provider (PSTN: “The
     number you have dialed is not available.”) -- caller does not need to try the
     original server next time.
 •   Redirection stateless.
 •   Proxy useful if forking, AAA, firewall control needed. In general, proxying
     grants more control to the server.
 •   Signaling using a proxy faster if RTT between call originator and destination
     domain is the major delay contributor.




                                        Tekelec Confidential /
                                   For Discussion Purposes Only /
                                  Tekelec Confidential
                                            Non-Binding
Routing Basics

 •   Requests could be sent directly to the contact address
 •   Media addresses are exchanged in SDP
 •   Replies follow the same path as the requests
       Each proxy adds a VIA header indicating its address
       Receiver gets a list in the form of:
              VIA: proxy_1, proxy_2 .....proxy_n
       Receiver sends the reply back to le last proxy in the via header (proxy_n) which
        forwards the reply to proxy_n-1
 •   Proxies that want to see all future requests of a user add a RecordRoute
     header
       Receiver get a list of RR: proxy_1, proxy_2 ..proxy_n
       RR list is included in reply
       Sender directs its future requests to proxy_1, which forwards it to proxy_2 ....




                                               Tekelec Confidential /
                                          For Discussion Purposes Only /
                                         Tekelec Confidential
                                                   Non-Binding
Summary of SIP Properties
  •   Textual (HTTP-like) client-server protocol
        Easy to debug, extend and process with textual operating systems
  •   End-2-end
        It puts most of intelligence into end-devices – good for scalability and
         extensibility
  •   Internet addressing using URIs
        E.g., sip:jiri@iptel.org
        Non-SIP URIs possible to (e.g., they may be used to redirect a caller
         to webpage)
        Address space unlimited and may be used to create services
         (sip:sales@hotel.xy; geo.position:=48.54_-123.84_120)
  •   It delivers mobility: User can register from one or more locations
      with IP connectivity




                                       Tekelec Confidential /
                                  For Discussion Purposes Only /
                                 Tekelec Confidential
                                           Non-Binding

				
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