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					IP Mobility                                                 1




               Mobile IP (multimedia)
              without Mobile IP: SIP and
                    RTP mobility
                         Henning Schulzrinne
                      Dept. of Computer Science
                         Columbia University
                         New York, New York
                    schulzrinne@cs.columbia.edu

                              Bell Laboratories

                                June 8, 1999


                      (Joint work Elin Wedlund)


                                                  June 9, 1999
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                                         Overview

              • mobility: which layer?

              • SIP overview

              • SIP mobility

              • RTP mobility

              • Open issues




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                                       Mobility

              • connected: move while connected

              • plug-in: lap top

              • synchronization: PalmPilot




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                                  Review: Mobile IP

              • goal: make IP address changes invisible to upper layers

              • “home address” of mobile host (MH)

        home agent (HA): tunnels IP packets to current location;

        corresponent host (CH): the host that talks to MH (mobile or
            fixed);

        foreign agent (FA): router/MH that decapsulates packets




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                           Mobile moves to new network

                                                            MH
                                     home network                MH   mobile host
                                         CN
                                                 HA
                                                                 CH   correspondent host
                                                                 HA   router with home agent
                      CH                                              functionality
                             data                                HA   router with foreign agent
                                                                      functionality
                                         FA
                                                tunnelled
                           data                 data



                                         data
                                    MH
                                         foreign
                                         network




              • extra HA–FA + encapsulation packet delay
              • IP-in-IP encapsulation overhead of 20 bytes
              • MH may send directly to CH, but may not get out


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                         Mobile IP with route optimization

              • CH tunnels directly to FA

              • UDP-based binding update from HA

              • old FA sends binding warning to HA « HA sends binding
                update




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                         Problems with route-optimized IP

              • CH must change IP stack (tunnel!)

              • CH must support binding update process

              • only HA can send update « delay

              • MH has to rely on old FA to forward packets

              • don’t send binding updates blindly




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                       Why is there no mobile IP deployment?

              • no incentive to install: few short-term visitors, few public
                access points beyond modems

              • address-based packet filtering for return traffic

              • operating system support lacking

              • many synchronization-based « partially disconnected
                operation




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                          Application-Layer Mobility

        Address assignment:

              • DHCP
              • IPv6 autoconfiguration (link-local addresses)

        Web: “designed for mobility”?

              • short transactions « client can close
              • restartable
              • stateless at server (cookies)
              • recoverable: “range” header



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                          Application-Layer Mobility

        Email:

              • POP for email retrieval, local storage
              • IMAP for server-storage
              • web-based email access (hotmail.com)
              • but: need “home” server for SMTP (spam prevention)




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                           Application-layer mobility

        News:

               • download and read
               • upload to local server

        ftp:

               • now: mostly large file transfers
               • recent: restart capability (“smart download”)




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                          Application-layer mobility

        file systems:

              • NFS: UDP based
              • even with TCP, stateless

        telnet:

              • type while walking/driving?
              • decreasing importance

        irc: needs continuous connectivity!




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                         Mobile applications: radio and TV

              • multicast: join at new location

              • main problems: IGMP leave latency, IGMP traffic

              • leave latency: depart without IGMP farewell

              • smooth handoff if listener in next local network




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                        What are truly mobile applications?

              • clients only

              • audio and video

                 – 38% at home, 41% cars, 21% at work
                 – three hours each weekday

              • telephone: “number of mobile-phone subscriptions in Finland
                (2.9 mio.) has surpassed the number of traditional fixed-phone
                subscriptions.”




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              SIP Basics




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          Light-weight signaling: Session Initiation Protocol (SIP)
        IETF MMUSIC working group (RFC 2543)

              • light-weight generic signaling protocol

              • part of IETF conference control architecture:

                 – SAP for “Internet TV Guide” announcements
                 – RTSP for media-on-demand
                 – SDP for describing media
                 – others: malloc, multicast, conference bus, . . .

              • typical post-dial delay: 1.5 round-trip time (with UDP)



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                                  SIP Features, cont’d.

              • provides call control (hold, forward, transfer, media changes,
                ...)

              • leverages web infrastructure: security, “cgi-bin”, electronic
                payments, PICS, cookies, . . .

              • web-oriented: return HTML pages (“web IVR”)

              • network-protocol independent: UDP or TCP (or AAL5 or
                X.25)

              • easily extends to presence information (“buddy lists”) and
                event notification



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                 SIP for H.323 experts
              H.323           SIP + SDP
              H.225.0 + RAS   SIP
              H.245           SDP, SMIL, . . .
              gatekeeper      proxy




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                                 SIP architecture: peer-to-peer
                                           SIP
                                     redirect server




                                                                   128.119.40.186
                                                       Internet

                                                       RTP audio



          128.59.19.141

                          CATV
                          Ethernet




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                                        SIP architecture: carrier
                                authenticate
                                anonymize

                                       SIP                                              SIP
                                      proxy                                            proxy

                                                                                                   10.4.5.6


                                      firewall               RTP audio                 firewall
                                       NAPT                                             NAPT
                                                  128.59.19.141     128.119.40.186
                                    edge router                                      edge router
              10.1.2.3
                         CATV
                         Ethernet




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                                 SIP addresses food chain

              yellow pages                       “president of the United States”
                                                     ↓ WWW search engines
              common names                         “Bill Clinton, Whitehouse”
                                                       ↓ directory services
              host-independent                    president@whitehouse.gov
                                             SIP                                SIP
              host-specific       sip:bubba@oval.eop.gov sip:+1-202-456-1111@net2ph.com
                                          ↓ DNS
              IP address              198.137.241.30




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                                       SIP: basic operation

              1. use directory service (e.g., LDAP) to map name to
                 user@domain

              2. locate SIP servers using DNS SRV, CNAME

              3. called server may map name to user@host

              4. callee accepts, rejects, forward (→ new address)

              5. if new address, go to step 2

              6. if accept, caller confirms

              7. . . . conversation . . .

              8. caller or callee sends BYE


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                                  SIP–DNS interaction
        extended email-like domain resolution « try until success:

              1. try SRV DNS record for “sip.udp” and “sip.tcp” in domain,
                 with priority and weights for randomized load balancing

              2. DNS CNAME or A record

              3. may try SMTP EXPN command to get new address; goto (1)

              4. if all else fails, send SIP request via MIME




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                                    SIP operation in proxy mode
                                                                                               cs.columbia.edu

                                                                          ? location server
                                   1
                                                                                                   4
                                   INVITE                                                                          5
                cs.tu-berlin.de                                                                  INVITE hgs@play




                                                                                hgs@play
                                                               henning
                                   henning@columbia.edu    2
                                                                                           3
                                                                                                       200 OK
                                   200 OK
              cz@cs.tu-berlin.de                                                                       6
                                   7                                                                               play


                                                                         tune                          9
                                   8
                                                                                                   ACK hgs@play
                                   ACK
                                   henning@colubmbia.edu




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                          SIP operation in redirect mode
                                                ieee.org
                                                                         ?                  location
                                                                                            server




                                                                             columbia.edu
                          1
              tu-berlin.de INVITE henning@ieee.org         2




                                                               henning
                                                                                             3

                         302 Moved temporarily
                         Contact: hgs@columbia.edu
                          4


                          5 ACK henning@ieee.org

                                                                    columbia.edu
                          6 INVITE hgs@columbia.edu


                          7 200 OK

                          8 ACK hgs@columbia.edu                                                 hgs



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                       SIP operation in redirect mode
                INVITE alice@ieee.org

                                     302 Moved temporarily


                ACK

                INVITE a12@columbia.edu


                                                     100 Trying


                                                       200 OK
                ACK

              UAC            redirect server                 UAS
                                ieee.org               a12@columbia.edu



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                     Interaction with resource reservation
        avoid “fast busy” after ringing « interleave
                         INVITE


                          YESSIR?
                                        SR
                                               reserve
                RR                             (no traffic - no charge)
                SR
                                         RR

                            200



                       ACK or BYE

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                                 Invitation modes
                    invitation            conference
                                  unicast    multicast
                    unicast       telephony    MBone session
                    multicast     reach first   dept. conference
        « SIP for all modes, SAP also for multicast/multicast




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                            SIP servers and clients

        UAC: user-agent client (caller application)

        UAS: user-agent server « accept, redirect, refuse call

        redirect server: redirect requests

        proxy server: server + client

        registrar: track user locations

        often combine registrar + proxy or redirect server




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                           Proxy and redirect servers

        proxy: may fork requests « parallel or sequential search

              • near-end proxy: outgoing calls « address lookup, policy,
                firewalls
              • far-end proxy: closer to callee « callee firewall, call path
                hiding

        redirect server: lower state overhead, more messages




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                              SIP requests and responses

              • HTTP look-alike

              • provisional and final responses:

                 – 1xx = searching, ringing, queueing, . . .
                 – 2xx = success
                 – 3xx = forwarding
                 – 4xx = client mistakes
                 – 5xx = server failures
                 – 6xx = busy, refuse, not available anywhere



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                              SIP protocol request
        INVITE sip:schulzrinne@cs.columbia.edu SIP/2.0
        From: Christian Zahl <sip:cz@cs.tu-berlin.de>
        To: Henning Schulzrinne <sip:schulzrinne@cs.columbia.edu>
        Via: SIP/2.0/UDP 131.215.131.131, SIP/2.0 foo.com
        Call-ID: 3678134014@cloud9.cs.tu-berlin.de
        Content-Type: application/sdp
        Content-Length: 187
        CSeq: 8348 INVITE
        Subject: New error codes

        session description




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                                      SIP URLs
        sip:[user:pw@]host:[port]
        ;transport=UDP;maddr=224.2.0.1

              • used in Request-URI, Contact headers (redirect,
                registration), web pages

              • transport and maddr specify transport

              • can specify methods, header and body in web pages, email

              • example: sip:a.g.bell@belltel.com




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                             SIP/RTP mobility overview

              • pre-call mobility « SIP proxy, redirect

              • mid-call mobility « SIP re-INVITE, RTP

              • recovery from disconnection




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                                   Review: DHCP

              1. client broadcasts DHCPDISCOVER

              2. server offers address: DHCPOFFER

              3. client broadcasts DHCPREQUEST

              4. server acks via DHCPACK




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                                   SIP mobility: pre-call

              • MH acquires IP address via DHCP
              • optional: MH finds SIP server via multicast REGISTER
              • MH updates home SIP server
              • optimization: hierarchical LR (later)

                                                redir                  MH      mobile host
                                                                  MH
                                                        home           CH      correspondent host
                                                        network        redir
                                                                               SIP redirect server
                                       1

                              CH       2                                1      SIP INVITE

                                                                        2      SIP 302 moved temporarily

                                                                        3      SIP INVITE
                                       3
                                   5   4                                4      SIP OK

                                                                        5      data
                                           MH
                                                foreign
                                                network




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                                       SIP mobility: mid-call

              • MH→CH: new INVITE, with Contact and updated SDP

              • re-registers with home registrar

                                                 redir
                                                                   MH   MH      mobile host
                                                         home           CH      correspondent host
                                                         network
                                                                        redir
                                                                                SIP redirect server

                             CH                                          1      SIP INVITE

                                                                         2      SIP OK

                                  2                                      3      data
                                       1
                             3

                                            MH
                                                 foreign
                                      MH
                                                 network




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                      SIP mobility: simultaneous moves
                                     MH
                                                   MH               MH




                                          INVITE           INVITE




                 MH                                MH               MH




        « need addition to protocol behavior: try well-known address




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                                SIP mobility: security

              • telephone-like: random Call-ID, sequential CSeq

              • password: HTTP “basic” authentication « plain-text password
                no better than Call-ID

              • challenge-response: HTTP “digest” authentication « possibly
                more messages for updates

              • true crypto: PGP, S/MIME

              • possibilities: shared secret, ipsec, Lamport’s hash (OTP)




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                       SIP mobility: multi-stage registration
        Send this to local registrar, ca.move.com
        REGISTER sip:ny.move.com SIP/2.0
        Contact: sip:me@ca.move.com
          ;expires=3600

              • gets proxied like INVITE to nyc.move.com « hierarchical
                LR

              • only updates home registrar when MH leaves Calif.

              • first INVITE goes to ca.move.com




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                                  Packet loss recovery

              • CH’s packets sent to old MH address lost

              • MH indicates seq. no. of last packet received in SDP

              • CH retransmits missing packets

              • artificial silence period?

              • proxy recovery?




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                                 RTP mobility support

              • RTP uses 32-bit random synchronization source (SSRC)

              • stays constant even if IP address changes

              • RTCP SDES CNAME binds to global identity

              • very fast, binding update retransmitted every 50 ms. . .

              • loop/collision detection: if two sources with same SSRC, keep
                one

              • need to modify RTP discard behavior: keep new IP address




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                                    RTP mobility

              • CH accepts RTP packets from new IP address

              • “mobile RTP” CH:

                 – send own RTP packets (also) to the new address
                 – allows smooth hand-offs
                 – terminate double sending when receiving SIP update




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                              Comparison SIP vs. IP mobility

                         IP             SIP                     RTP
              constant   IP address     SIP url user@home.com   SSRC/SDES
              update     binding upd.   re-INVITE               audio packet
              security   via HA         call-ID, crypto         crypto
              impl.      OS             app.                    app.
              reg.       one (*)        proxy                   none




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                                 Open Issues/Problems

              • latency: DHCP uses ARP for confirmation « 150 ms latency

              • soft handover « multiple IP addresses?

              • multi-stage location register

              • interaction with RTP loop detection

              • division of labor: do we need link-layer mobility?




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                                      Conclusion

              • mobile data mostly done

              • mobile IP: infrastructure changes, overhead

              • Internet telephone mobility « SIP

              • mobile media-on-demand terminals: RTP or RTSP
                REDIRECT




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                            More information

        Internet and telecom statistics:
            http://www.cs.columbia.edu/˜hgs/internet

        Papers: http://www.cs.columbia.edu/˜hgs/research/irt

        RTP: http://www.cs.columbia.edu/˜hgs/rtp

        RTSP: http://www.cs.columbia.edu/˜hgs/rtsp

        SIP: http://www.cs.columbia.edu/˜hgs/sip




                                                               June 9, 1999