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RFC 5631 - Session Initiation Protocol (SIP) Session Mobility

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Network Working Group                                         R. Shacham
Request for Comments: 5631                                H. Schulzrinne
Category: Informational                              Columbia University
                                                            S. Thakolsri
                                                             W. Kellerer
                                                        DoCoMo Euro-Labs
                                                            October 2009

           Session Initiation Protocol (SIP) Session Mobility


   Session mobility is the transfer of media of an ongoing communication
   session from one device to another.  This document describes the
   basic approaches and shows the signaling and media flow examples for
   providing this service using the Session Initiation Protocol (SIP).
   Service discovery is essential to locate targets for session transfer
   and is discussed using the Service Location Protocol (SLP) as an
   example.  This document is an informational document.

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1. Overview ........................................................3
   2. Requirements ....................................................4
   3. Roles of Entities ...............................................4
   4. Device Discovery ................................................5
   5. Session Mobility ................................................7
      5.1. Options for Session Mobility ...............................7
           5.1.1. Transfer and Retrieval ..............................7
           5.1.2. Whole and Split Transfer ............................7
           5.1.3. Transfer Modes ......................................8
         Mobile Node Control (MNC) Mode .............8
         Session Handoff (SH) Mode ..................8
           5.1.4. Types of Transferred Media ..........................8
      5.2. Addressing of Local Devices ................................9
      5.3. Mobile Node Control Mode ..................................10
           5.3.1. Transferring a Session to a Single Local Device ....10
         RTP Media .................................10
         MSRP Sessions .............................11
           5.3.2. Transfer to Multiple Devices .......................13
           5.3.3. Retrieval of a Session .............................16
      5.4. Session Handoff (SH) mode .................................16
           5.4.1. Transferring a Session to a Single Local Device ....16
           5.4.2. Retrieval of a Session .............................19
           5.4.3. Transfer to Multiple Devices .......................21
      5.5. Distributing Sessions for Incoming Call ...................23
      5.6. Use of ICE in Session Mobility ............................24
   6. Reconciling Device Capability Differences ......................25
      6.1. Codec Differences .........................................25
      6.2. Display Resolution and Bandwidth Differences ..............27
   7. Simultaneous Session Transfer ..................................27
   8. Session Termination ............................................28
   9. Security Considerations ........................................29
      9.1. Authorization for Using Local Devices .....................29
      9.2. Maintaining Media Security During Session Mobility ........29
           9.2.1. Establishing Secure RTP Using SDP ..................29
           9.2.2. Securing Media Using the Transport Layer ...........31
      9.3. Flooding Attacks in MNC Mode ..............................31
   10. Acknowledgments ...............................................32
   11. References ....................................................32
      11.1. Normative References .....................................32
      11.2. Informative References ...................................33

1.  Overview

   As mobile devices improve and include more enhanced capabilities for
   IP-based multimedia communications, they will remain limited in terms
   of bandwidth, display size, and computational power.  Stationary IP
   multimedia endpoints (including hardware IP phones, videoconferencing
   units, embedded devices and software phones) allow more convenience
   of use, but are not mobile.  Moving active multimedia sessions
   between these devices allows mobile and stationary devices to be used
   concurrently or interchangeably in mid-session, combining their
   advantages into a single "virtual device".  An approach to session
   mobility based on the Session Initiation Protocol (SIP) [1] was
   described first in [20], where two different methods are proposed:
   third-party call control (3pcc) [2] and the REFER method [3].

   This document expands on this concept, defining a framework for
   session mobility that allows a Mobile Node to discover available
   devices and to include them in an active session.  In particular, the
   framework for session mobility presented in this document describes
   basic approaches for using existing protocols and shows the signaling
   and media flow examples for providing session mobility using SIP.  It
   is intended as an informational document.

   The devices selected as session transfer targets may be either
   personal or public.  Personal devices are ones used by a single
   individual, such as one's PC or phone.  Public devices are ones
   available for use by a large group of people and include large
   conference-room displays.  Two capabilities are required to transfer

      Device Discovery - At all times, a user is aware of the devices
      that are available in his local area, along with their

      Session Mobility - While in a session with a remote participant,
      the user may transfer any subset of the active media sessions to
      one or more devices.

   This document describes session mobility examples for SIP.  It does
   not mandate any particular protocol for device discovery.  Many
   different protocols exist and we discuss the tradeoffs involved in
   choosing between them.  For our examples, we use the Service Location
   Protocol (SLP) [17], primarily because it is the only such protocol
   standardized by the IETF.

2.  Requirements

   This session mobility framework seeks to fulfill the following

   o  REQ 1: Backward Compatibility - We distinguish two kinds of
      devices.  Enhanced devices support the call flows described in
      Section 5 and can perform discovery, while basic devices can do
      neither and only have basic SIP capabilities.  Devices initiating
      session mobility must have enhanced functionality, while all
      others can be either basic or enhanced devices.  This includes the
      transfer destinations, such as the local video camera, as well as
      the device being used by the remote participant.

   o  REQ 2: Flexibility - Differences in device capabilities should be
      reconciled.  Transfer should be possible to devices that do not
      support the codec being used in the session, and even to devices
      that do not have a codec in common with the remote participant.  A
      transfer should also take into account device differences in
      display resolution and bandwidth.

   o  REQ 3: Minimal Disruption - Session transfer should involve
      minimal disruption of the media flow and should not appear to the
      remote participant as a new call.

3.  Roles of Entities

   Session mobility involves five types of components: A Correspondent
   Node (CN), a Mobile Node (MN), one or more local devices used as
   targets for session transfer, an SLP [17] Directory Agent (DA), and,
   optionally, a transcoder.  The Correspondent Node (CN) is a basic
   multimedia endpoint being used by a remote participant and may be
   located anywhere.  It may be a SIP User Agent (UA), or a Public
   Switched Telephone Network (PSTN) phone reachable through a gateway.
   The Mobile Node (MN) is a mobile device, containing a SIP UA for
   standard SIP call setup, as well as specialized SIP-handling
   capabilities for session mobility, and an SLP [17] User Agent (UA)
   for discovering local devices.  The local devices are located in the
   user's local environment for discovery and use in his current
   session.  They may be mobility-enhanced or basic.  Basic devices,
   such as IP phones, are SIP-enabled, but have no other special
   capabilities.  Mobility-enhanced devices have SLP Service Agent
   capabilities for advertising their services and session mobility
   handling.  They also contain an SLP UA, whose purpose will be
   explained in the discussion of multi-device systems in Section 5.4.3.
   The SLP Directory Agent (DA) keeps track of devices, including their
   locations and capabilities.  The use of SLP will be described in more
   detail in Section 4.  SIP-based transcoding services [18] are used,

   when necessary, to translate between media streams, as described in
   Section 6.

4.  Device Discovery

   A Mobile Node must be able to discover suitable devices in its
   vicinity.  This is outside the scope of SIP, and a separate service
   location protocol is needed.  It is outside the scope of this
   document to define any service location protocol.  This section
   discusses various options, and describes one of them in more detail.

   While having a global infrastructure for discovering devices or other
   services in any location would be desirable, nothing of this sort is
   currently deployed or standardized.  However, this document assumes
   that such an infrastructure is unnecessary for discovering devices
   that are in close proximity, such as in the same room.  It is
   possible for such devices to be discovered through direct
   communication over a short-range wireless protocol such as the
   Bluetooth Session Description Protocol (SDP).  Two other categories
   of service discovery protocols may be used, assuming that devices
   that are physically close to each other are also within the same
   network and/or part of the same DNS domain.  Multicast-based
   protocols, such as SLP [17] (in its serverless mode) or Bonjour
   (mDNS-SD [30]), may be used as long as the Mobile Node is within the
   same subnet as the local devices.  When devices are part of the same
   DNS domain, server-mode SLP or non-multicast DNS Service Discovery
   (DNS-SD) [29] are possible solutions.  Such protocols can discover
   devices within a larger geographical area, and have the advantage
   over the first category in that they allow for the discovery of
   devices at different location granularities, such as at the room or
   building level, and in locations other than the current one.  In
   order to discover devices in a specific location, location
   attributes, such as room number, must be used in the search, e.g., as
   service attributes in SLP or as a domain name in DNS-SD.  The mobile
   device must ascertain its location in order to perform this search.
   We note that many of these techniques could be difficult to implement
   in practice.  For example, different wireless networks may be
   deployed by different organizations, which could make it unrealistic
   to have a common DNS setup.

   We describe here how SLP is used in server mode in general, then how
   it may be used to discover devices based on their location.  As
   mentioned before, this is only one of many ways to perform service
   discovery.  SLP identifies services by a "service type", a "service
   URL", which can be any URL, and a set of attributes, defined as
   name-value pairs.  The attributes may be information such as vendor,
   supported media codecs, and display resolution.  SLP defines three
   roles: Service Agents (SAs), which send descriptions of services;

   User Agents (UAs), which query for services; and Directory Agents
   (DAs), which receive the registrations and queries.  An SA registers
   a service description to a DA with a service registration (SrvReg)
   message that includes its service type, service URL, and attribute-
   value set.  A UA queries for services by sending a service request
   (SrvRqst) message, narrowing the query based on service type and
   attribute values.  It receives a reply (SrvRply) that contains a list
   of URLs of services that match the query.  It may then ascertain the
   specific attributes of each service using an attribute request
   (AttrRqst) message.

   This document assumes the following use of SLP for discovering local
   devices.  Devices have a service type of "sip-device" and a SIP URI
   as the service URI.  Section 5.2 describes the form of this URI.
   Attributes specify device characteristics such as vendor, supported
   media codec, display resolution, as well as location coordinates,
   such as street address and room number.  SAs are co-located with SIP
   UAs on session-mobility enhanced devices, while a separate SA is
   available to send SrvReg messages on behalf of basic devices, which
   do not have integrated SLP SAs.

   The Mobile Node includes an SLP UA that discovers available local
   devices and displays them to the user, showing, for example, a map of
   all devices in a building or a list of devices in a current room.
   Once the MN receives its current location in some manner, its SLP UA
   issues a SrvRqst message to the DA requesting all SIP devices, using
   the location attributes to filter out those that are not in the
   current room.  A SrvRply message is sent to the mobile device with a
   list of SIP URIs for all devices in the room.  A separate attribute
   request (AttrRqst) is then sent for each URL to get the attributes of
   the service.  The MN displays for the user the available devices in
   the room and their attributes.  Figure 1 shows this protocol flow.

           Device                       DA                      MN
             |(1) SrvReg                |                       |
             |------------------------->|                       |
             |(2) SrvRply               |                       |
             |<-------------------------|                       |
             |                          |                       |
             |                          |                       |
             |                          |(3) SrvRqst            |
             |                          |<----------------------|
             |                          |(4) SrvRply  URL list  |
             |                          |---------------------->|
             |                          |(5) AttrRqst URL1      |
             |                          |<----------------------|
             |                          |(6) AttrRply           |
             |                          |---------------------->|
             |                          |     ...               |
             |                          |                       |

      Figure 1.  SLP message flow for the device to register its service
                 and the MN to discover it, along with its attributes.

5.  Session Mobility

5.1.  Options for Session Mobility

5.1.1.  Transfer and Retrieval

   Session mobility involves both transfer and retrieval of an active
   session.  A transfer means to move the session on the current device
   to one or more other devices.  A retrieval causes a session currently
   on another device to be transferred to the local device.  This may
   mean returning a session to the device on which it had originally
   been before it was transferred to another device.  For example, after
   discovering a large video monitor, a user transfers the video output
   stream to that device; when he walks away, he returns the stream to
   his mobile device for continued communication.  One may also move a
   session to a device that had not previously carried it.  For example,
   a participant in an audio call on his stationary phone may leave his
   office in the middle of the call and transfer the call to the mobile
   device as he is running out the door.

5.1.2.  Whole and Split Transfer

   The set of session media may either be transferred completely to a
   single device or split across multiple devices.  For instance, a user
   may only wish to transfer the video component of his session while
   maintaining the audio component on his PDA.  Alternatively, he may
   find separate video and audio devices and wish to transfer one media

   type to each.  Furthermore, even the two directions of a full-duplex
   session may be split across devices.  For example, a PDA's display
   may be too small for a good view of the other call participant, so
   the user may transfer video output to a projector and continue to use
   the PDA camera.

5.1.3.  Transfer Modes

   Two different modes are possible for session transfer, Mobile Node
   Control (MNC) mode and Session Handoff (SH) mode.  We describe them
   below in turn.  Mobile Node Control (MNC) Mode

   In Mobile Node Control mode, the Mobile Node uses third-party call
   control [2].  It establishes a SIP session with each device used in
   the transfer and updates its session with the CN, using the SDP
   parameters to establish media sessions between the CN and each
   device, which take the place of the current media sessions with the
   CN.  The shortcoming of this approach is that it requires the MN to
   remain active to maintain the sessions.  Session Handoff (SH) Mode

   A user may need to transfer a session completely because, for
   example, the battery on his mobile device is running out or he is
   losing radio connectivity.  Alternatively, the user of a stationary
   device who leaves the area and wishes to transfer the session to his
   mobile device will not want the session to remain on the stationary
   device when he is away, since it will allow others to easily tamper
   with his call.  In such a case, Session Handoff mode, which
   completely transfers the session signaling and media to another
   device, is useful.

   Based on our experiments, we have found MNC mode to be more
   interoperable with existing devices used on the CN's side.  The
   remainder of this section describes the transfer, retrieval, and
   splitting of sessions in each of the two session transfer modes.

5.1.4.  Types of Transferred Media

   A communication session may consist of a number of media types, and a
   user should be able to transfer any of them to his device of choice.
   This document considers audio, video, and messaging.  Audio and video
   are carried by RTP and negotiated in the SDP body of the SIP requests
   and responses.  Three different methods exist for carrying text
   messages, and possibly other MIME types, that are suitable for SIP
   endpoints.  RTP may be used to transport text payloads in real time,

   based on [9].  Any examples given for audio and video will work
   identically for text, as only the payloads differ.  For the transfer
   of entire messages (as opposed to a small number of characters in
   RTP), either the SIP MESSAGE method [21] or the Message Session Relay
   Protocol (MSRP) [7] may be used.  MESSAGE is used to send individual
   page-mode messages.  The messages are not associated with a session,
   and no negotiation is done to establish a session.  Typically, a SIP
   UA will allow the user to send MESSAGE requests during an established
   dialog, and they are sent to the same contact address as all
   signaling messages are sent in mid-session.  We discuss later how
   these messages are affected by session mobility.  MSRP, on the other
   hand, is based on sessions that are established like the real-time
   media sessions previously described.  As such, transferring them is
   similar to transferring other media sessions.  However, this document
   will point out where special handling is necessary for these types of

5.2.  Addressing of Local Devices

   As stated before, this document assumes both personal and public
   devices.  We assume that public devices use a dedicated Address of
   Record (AOR), such as sip:device11@example.com.  A personal device
   already uses the owner's AOR, so that he should be reachable there;
   that AOR could also be used for transferring sessions.  However, it
   is preferable to distinguish the role of a device as a transfer
   target from its existing role.  Therefore, all devices are assumed to
   have dedicated AORs.

   Since every transfer device has its own AOR, there is a one-to-one
   mapping between AOR and device.  Therefore, a transfer request could
   be addressed to the AOR, which would resolve to the device.  However,
   in Section 5.4.3, we present a model where devices create multi-
   device systems to pool their capabilities.  Therefore, a single
   device must be reachable by multiple URIs representing different
   combinations of devices.  The appropriate solution is to define each
   combination of devices with a Globally Routable UA URI (GRUU) [12].

   Therefore, we assume the following addressing for the remainder of
   the document.  As mentioned earlier, a device has a unique AOR.  It
   registers a separate contact URI for itself and for each system of
   devices that it controls.  Each contact has an associated GRUU, which
   is registered with SLP as the Service URI, and may be directly
   addressed by another device in a request sent through the proxy.
   When the proxy forwards the request to the device, it will replace
   the GRUU with the contact URI, as described in [12].  Therefore, the
   contact URI, not the associated GRUU, will be used by devices to
   determine whether the request is for the device itself or for a
   multi-device system.  We assume that the public GRUU is used.

5.3.  Mobile Node Control Mode

5.3.1.  Transferring a Session to a Single Local Device  RTP Media

         local device                MN                        CN
           |(1) INVITE no sdp        |                         |
           |<------------------------|                         |
           |(2) 200 OK local params  |                         |
           |------------------------>|                         |
           |                         |(3) INVITE local params  |
           |                         |------------------------>|
           |    RTP                  |                         |
           |                         |(4) 200 OK CN params     |
           |                         |<------------------------|
           |                         |(5) ACK                  |
           |                         |------------------------>|
           |(6) ACK CN params        |                         |
           |<------------------------|        RTP              |
           |                         |                         |
           |                         |                         |

      Figure 2.  Mobile Node Control mode flow for transfer to a single

   Figure 2 shows the message flow for transferring a session to a
   single local device.  It follows Third Party Call Control Flow I
   (specified in [2]), which is recommended as long as the endpoints
   will immediately answer.  The MN sends a SIP INVITE request to the
   local device used for the transfer, requesting that a new session be
   established, but does not include an SDP body.  The local device's
   response contains an SDP body that includes the address and port it
   will use for any media, as well as a list of codecs it supports for
   each.  The MN updates the session with the CN by sending an INVITE
   request (re-INVITE) containing the local device's media parameters in
   the SDP body, as follows:

      c=IN IP4 av_device.example.com
      m=audio 4400 RTP/AVP 0 8
      a=rtpmap:0 PCMU/8000
      a=rtpmap:8 PCMA/8000
      m=video 5400 RTP/AVP 31 34
      a=rtpmap:31 H261/90000
      a=rtpmap:34 H263/90000

   Sending both audio and video media lines will transfer both media
   sessions of an existing audio/video call to the local device.
   Alternatively, the MN may select a subset of the media available on
   the local device, and use the local device's parameters for those
   media in the request sent to the CN, while continuing to use its own
   parameters for the rest of the media.  For example, if it only wishes
   to transfer an audio session to a local device that supports audio
   and video, it will isolate the appropriate media line for audio from
   the response received from the local device and put it in the request
   sent to the CN, along with its own video parameters.  The CN will
   send a response and includes, in its body, the media parameters that
   it will use, which may or may not be the same as the ones used in the
   existing session.  The MN will send an ACK message to the local
   device, which includes these parameters in the body.  The MN will
   establish a session with the local device and maintain its session
   with the CN, while the media flow will be established directly
   between the CN and the local device.  Only the MN, who will be in an
   ongoing session with the CN, will later be allowed to retrieve the
   media session.  Parsing of unknown SDP attributes by the controller
   is discussed in [2].  MSRP Sessions

   In figure 2, the message sequence for transferring an MSRP message
   session using MNC mode is identical to that used for audio or video,
   although the contents of the messages differ.  To simplify the
   example, we assume that an MSRP session, with no other media, is
   being transferred to a local messaging node, MSGN.  In the following
   flow, we refer to the corresponding messages in Figure 2.  An empty
   INVITE request (1) is sent to the local messaging node, MSGN, as

   INVITE sip:msgn@example.com;gr=urn:uuid:jtr5623n SIP/2.0
   To: <sip:msgn@example.com;gr=urn:uuid:jtr5623n>
   From: <sip:bob@example.com>;tag=786
   Call-ID: 893rty@mn.example.com
   Content-Type: application/sdp

   The messaging node responds with all of its media capabilities,
   including MSRP, as follows (2):

   SIP/2.0 200 OK
   To: <sip:msgn@example.com;gr=urn:uuid:jtr5623n;tag=087js>;tag=087js
   From: <sip:bob@example.com>;tag=786
   Call-ID: 893rty@mn.example.com
   Content-Type: application/sdp

   c=IN IP4 msgn.example.com
   m=message 52000 msrp/tcp *
   m=audio 4400 RTP/AVP 0 8
   a=rtpmap:0 PCMU/8000
   a=rtpmap:8 PCMA/8000
   m=video 5400 RTP/AVP 31 34
   a=rtpmap:31 H261/90000
   a=rtpmap:34 H263/90000

   The same request is then sent by the MN to the CN (3), but containing
   the MSRP media and attribute lines with the path given in the MSGN
   response above.  The CN responds (4) with its own path.  The MN
   includes this in the ACK that it sends to the MSGN (6).

   MSRP sessions are carried over a reliable connection, using TCP or
   TLS (Transport Layer Security).  Therefore, unlike in the case of
   real-time media, this connection must be established.  According to
   the MSRP specifications, the initiator of a message session, known as
   the "offerer", must be the active endpoint, and open the TCP
   connection between them.  In this transfer scenario, the offerer of
   both sessions is the MN, who is on neither end of the desired TCP
   connection.  As such, neither endpoint will establish the connection.
   A negotiation mechanism could be used to assign the role of active
   endpoint during session setup.  However, while MSRP leaves open this
   possibility, it is not currently included in this document due to
   complexity.  The only other way that such session transfer would be
   possible is if both the CN and the local device ordinarily use an
   MSRP relay [8], since no direct connection must be established
   between them.  When each new endpoint receives the INVITE request
   from the MN, it will create a TLS connection with one of its
   preconfigured relays if such a connection does not yet exist (the CN
   will already have one because of its session with the MN) and receive
   the path of the relay.  In its response to the MN, it will include
   the entire path that must be traversed, including its relay, in the
   path attribute.  For instance, the response from the MSGN will look
   as follows:

   SIP/2.0 200 OK
   To: <sip:msgn@example.com;gr=urn:uuid:jtr5623n;tag=087js>;tag=087js
   From: <sip:bob@example.com>;tag=786
   Call-ID: 893rty@mn.example.com
   Content-Type: application/sdp

   c=IN IP4 msgn.example.com
   m=message 52000 msrp/tcp *
   a=path:msrp://relayA.example.com:12000/kjhd37s2s2;tcp \

   Since the CN and the local device each establish a TLS connection
   with their relay, as they would for any session, and the relays will
   establish a connection between them when a subsequent MSRP message is
   sent, neither party needs to establish any special connection.  The
   existing protocol may therefore be used for session transfer.

5.3.2.  Transfer to Multiple Devices

   In order to split the session across multiple devices, the MN
   establishes a new session with each local device through a separate
   INVITE request, and updates the existing session with the CN with an
   SDP body that combines appropriate media parameters it receives in
   their responses.  For instance, in order to transfer an audio and
   video call to two devices, the MN initiates separate sessions with
   each of them, combines the audio media line from one response and the
   video media line from the other, and sends them together as the
   request to the CN, as follows:

   m=audio 48400 RTP/AVP 0
   c= IN IP4 audio_dev.example.com
   a=rtpmap:0 PCMU/8000
   m=video 58400 RTP/AVP 34
   c= IN IP4 video_dev.example.com
   a=rtpmap:34 H263/90000

   The CN responds with its own parameters for audio and video.  The MN
   splits them and sends one to each local device in the ACK that
   completes each session setup.

  video_dev          audio_dev                MN                      CN
     |                   |(1) INVITE no sdp   |                       |
     |                   |<-------------------|   RTP Audio           |
     |                   |(2) 200  params     |                       |
     |                   |------------------->|                       |
     |                   |(3) INVITE no sdp   |                       |
     |<---------------------------------------|                       |
     |                   |(4) 200    params   |                       |
     |--------------------------------------->|                       |
     |                   |                    |(5) INVITE  a/v  params|
     |                   |                    |---------------------->|
     |                   |         RTP Audio  |                       |
     |   RTP Video       |<...........................................|
     |                   |                    |(6) 200 OK             |
     |                   |                    |<----------------------|
     |                   |                    |(7) ACK                |
     |                   |                    |---------------------->|
     |                   |(8) ACK CN audio    |                       |
     |                   |<-------------------|   RTP Audio           |
     |                   |...........................................>|
     |                   |(9) ACK CN video    |                       |
     |<---------------------------------------|   RTP Video           |
     |                   |                    |                       |
     |                   |                    |                       |

      Figure 3.  Mobile Node Control mode flow for transfer to multiple

   Splitting a full-duplex media service, such as video, across an input
   and an output device, such as a camera and a video display, is a
   simple extension of this approach.  The signaling is identical to
   that of Figure 3, with the audio and video devices replaced by a
   video output and a video input device.  The SDP, however, is slightly
   different.  The MN invites the local devices into two different
   sessions, but does not include any SDP body.  They each respond with
   all of their available media.  If they only support unidirectional
   media, as is the case for a camera or display-only device, they will
   include the "sendonly" or "recvonly" attributes.  Otherwise, the MN
   will have to append the appropriate attribute to each one's media
   line before sending the combined SDP body to the CN.  That body will
   look as follows:

   m=video 50900 RTP/AVP 34
   a=rtpmap:34 H263/90000
   c=IN IP4 camera.example.com
   m=video 50800 RTP/AVP 34
   a=rtpmap:34 H263/90000
   c=IN IP4 display.example.com

   In updating an SDP session, according to Section 8 of [4], the i-th
   media line in the new SDP corresponds to the i-th media line in the
   previous SDP.  In the above cases, if a media type is added during
   the transfer, the media line(s) should follow the existing ones.
   When an existing media is transferred to a different device, the
   media line should appear in the same place that it did in the
   previous SDP, as should the lines for all media that have not been
   altered.  When a duplex media stream is being split across an input
   and output device, the stream corresponding to the input device
   should appear in place of the duplex media stream.  Since this new
   stream is the one that will be received by the CN, including it in
   place of the old one ensures that the CN views the new stream as a
   replacement of the old one.  The media line corresponding to the
   output device must appear after all existing media lines.  In the
   last example, if the SDP had initially contained a video line
   followed by an audio line, the updated SDP sent to the CN would look
   as follows:

   m=video 50900 RTP/AVP 34
   a=rtpmap:34 H263/90000
   c=IN IP4 camera.example.com
   m=audio 45000 RTP/AVP 0
   a=rtpmap:0 PCMU/8000
   m=video 50800 RTP/AVP 34
   a=rtpmap:34 H263/90000
   c=IN IP4 display.example.com

   During the course of the session, the CN may send a MESSAGE request
   to the MN containing text conversation from the remote user.  If the
   mobile user wishes to have such messages displayed on a device other
   than the MN, the request is simply forwarded to that device.  The
   forwarded message should be composed as though it were any other
   message from the MN to the local device, and include the body of the
   received message.  The local device will send any MESSAGE request to
   the MN, who will forward it to the CN.

5.3.3.  Retrieval of a Session

   The MN may later retrieve the session by sending an INVITE request to
   the CN with its own media parameters, causing the media streams to
   return.  It then sends a BYE message to each local device to
   terminate the session.

5.4.  Session Handoff (SH) mode

5.4.1.  Transferring a Session to a Single Local Device

   Session Handoff mode uses the SIP REFER method [3].  This message is
   a request sent by a "referrer" to a "referee", which "refers" it to
   another URI, the "refer target", which may be a SIP URI to be
   contacted with an INVITE or other request, or a non-SIP URI, such as
   a web page.  This URI is specified in the Refer-To header.  The
   Referred-By [5] header is used to give the referrer's identity, which
   is sent to the refer target for authorization.  Essential headers
   from this message may also be encrypted and sent in the message body
   as Secure/Multipurpose Internet Mail Extensions (S/MIME) to
   authenticate the REFER request.  Figure 4 shows the flow for
   transferring a session.

         device15                        MN                    CN
           |(1) REFER                    |                     |
           |<----------------------------|                     |
           |(2) 202 Accepted             |                     |
           |---------------------------->|                     |
           |(3) INVITE, Replaces         |                     |
           |           RTP                                     |
           |(4) 200 OK                   |                     |
           |           RTP                                     |
           |(5) ACK                      |                     |
           |                             |(6) BYE              |
           |                             |<--------------------|
           |                             |(7) 200 OK           |
           |                             |-------------------->|
           |(8) NOTIFY                   |                     |
           |---------------------------->|                     |
           |(9) 200 OK                   |                     |
           |<----------------------------|                     |
           |                             |                     |
           |                             |                     |

      Figure 4.  Session Handoff mode flow for transfer to a single

   The MN sends the following REFER request (1) to a local device:

      REFER sip:device15@example.com;gr=urn:uuid:qfnb443ccui SIP/2.0
      To: <sip:device15@example.com;gr=urn:uuid:qfnb443ccui>
      From: <sip:bob@example.com>
      Referred-By: <sip:bob@example.com>

          [S/MIME authentication body]

   This message refers the local device to invite the refer target, the
   CN, into a session.  The "audio" and "video" tokens in the Refer-To
   URI are callee capabilities [10].  Here they are used to inform the
   referee that it should initiate an audio and video session with the
   CN.  Also included in the URI is the Replaces header field,
   specifying that a Replaces header field should be included with the
   specified value in the subsequent INVITE request.  The Replaces

   header identifies an existing session that should be replaced by the
   new session.  Here, the local device will request that the CN
   replaces its current session with the MN with the new session.
   According to [6], the CN should only accept a request to replace a
   session from certain authorized categories of users.  One such type
   of user is the current participant in the session.  The MN may,
   therefore, refer the local device to replace its current session with
   the CN.  However, it provides authentication by encrypting several
   headers from the original REFER request in an S/MIME body that it
   sends in the REFER.  The local device sends this body to the CN.
   This keeps a malicious user from indiscriminately replacing another
   user's session.  Once the local device receives the REFER request, it
   sends an INVITE request to the CN, and a normal session setup ensues.
   The CN then tears down its session with the MN.

   Once the local device has established a session with the CN, it sends
   a NOTIFY request to the MN, as specified in [3].  This NOTIFY
   contains the To (including tag), From (including tag), and Call-ID
   header fields from the established session to allow the MN to
   subsequently retrieve the session, as described in Section 5.4.2.

   Once a session is transferred, the destination for MESSAGE requests
   moves automatically.  Since a new session is established between the
   CN and the local device, any subsequent MESSAGE requests will be sent
   to that device.

   The transfer flow described above for media sessions may also be used
   to transfer an MSRP session.  The local device will initiate an MSRP
   session in message (4), along with the other sessions.  The REFER
   request (1) indicates that an MSRP session should be established
   using callee capabilities in the Refer-To header field, as it does
   for audio and video.  Such a media feature tag, "message" has already
   been defined [11].  Once the local device receives the REFER request,
   it initiates an MSRP session with the CN.  As the initiator, it will
   establish a TCP connection in order to carry the session (as
   specified in [7]), or will set up the session through its relay if
   configured to do so.

5.4.2.  Retrieval of a Session

        device15                          MN                    CN
            |(1) REFER                    |                     |
            |<----------------------------|                     |
            |(2) 202 Accepted             |                     |
            |---------------------------->|                     |
            |(3) REFER                    |                     |
            |---------------------------->|                     |
            |(4) 202 Accepted             |                     |
            |<----------------------------|                     |
            |                             |(5) INVITE, Replaces |
            |                             |-------------------->|
            |                             |   RTP               |
            |                             |<....................|
            |                             |(6) 200 OK           |
            |                             |<--------------------|
            |                             |   RTP               |
            |                             |....................>|
            |                             |(7) ACK              |
            |                             |-------------------->|
            |           (8) BYE           |                     |
            |           (9) 200 OK        |                     |
            |                             |                     |
            |                             |                     |

      Figure 5.  Session Handoff mode flow for session retrieval.

   Figure 5 shows the flow for retrieval by the MN of a session
   currently on a local device.  In order to better motivate the message
   flow, we start by describing the final INVITE (5) and work backwards.
   In order for a device to retrieve a session in Session Handoff mode,
   it must initiate a session with the CN that replaces the CN's
   existing session.  The following message is sent by the MN to the CN

   INVITE sip:corresp@example.com;gr=urn:uuid:bbb6981 SIP/2.0
   To: <sip:corresp@example.com;gr=urn:uuid:bbb6981>
   From: <sip:bob@example.com>
   Replaces: 1@device15.example.com;to-tag=aaa;from-tag=bbb
   Referred-By: <sip:device15@example.com>

      [S/MIME authentication body]

   Since the users on the MN and the local device are different
   identities, the MN needs to be referred by the local device and
   include its URI in the Referred-By header, in addition to including
   an S/MIME authentication body from the local device, in order to be
   permitted to replace the session.  Therefore, the MN must receive a
   REFER request from the local device referring it to send this INVITE
   request.  The user could use the user interface of the local device
   to send this REFER message.  However, such an interface may not be
   available.  Also, the user may wish to execute the transfer while
   running out of the office with mobile device in hand.  In order for
   the MN to prompt the REFER from the local device, it sends a "nested
   REFER" [5], a REFER request for another REFER.  In this case, the
   second REFER is sent back to the Mobile Node.  That REFER must
   specify that the Replaces header be included in the subsequent INVITE
   request.  The REFER request from the local device to the MN (3) is
   composed as follows:

REFER sip:bob@example.com;gr=urn:uuid:ytav223h67gb3 SIP/2.0
To: <sip:bob@example.com;gr=urn:uuid:ytav223h67gb3>
From: <sip:device15@example.com>
Refer-To: <sip:correspondent@example.com;gr=urn:uuid:bbb6981;audio;
Referred-By: <sip:device15@example.com>

    [S/MIME authentication body]

   A header field is included in the Refer-To URI to specify the value
   of the Replaces header in the target INVITE request.  In order to
   have this message sent to it, the MN must send the following REFER
   request (1):

REFER sip:device15@example.com;gr=urn:uuid:qfnb443ccui SIP/2.0
To: <sip:device15@example.com;gr=urn:uuid:qfnb443ccui>
From: <sip:bob@example.com>

   The Refer-To header specifies that the MN is the refer target and
   that the referral be in the form of a REFER request.  The header
   field specifies that the REFER request contains a Refer-To header
   containing the URI of the CN.  That URI, itself, contains the "audio"
   and "video" callee capabilities that will tell the MN to initiate an
   audio and video call, and a header field specifying that the ultimate
   INVITE request contains a Replaces header.  If the MN had previously
   transferred the session to the local device, it would have received

   these in the NOTIFY sent by the local device following the
   establishment of the session.  If, on the other hand, the MN is
   retrieving a session it had not previously held, as mentioned above
   in Section 5.1.1, it gets these parameters by subscribing to the
   Dialog Event Package [13] of the local device.  Such a subscription
   would only be granted, for instance, to the owner of the original
   device that carried the session.  Even when these parameters are
   provided in the Replaces header, the local device does not accept the
   REFER request from anybody except the original participant in the
   session or the owner of the device.  The MN receives the REFER
   request from the local device, sends the INVITE request to the CN,
   which accepts it, and, once the session is established, terminates
   its session with the local device.

5.4.3.  Transfer to Multiple Devices

   Splitting a session in SH mode requires multiple media sessions to be
   established between the CN and local devices, without the MN
   controlling the signaling.  This could be done by sending multiple
   REFER requests to the local devices, referring each to the CN.  The
   disadvantage of this method is that there is currently no standard
   way to associate multiple sessions as part of a single call in SIP.
   Therefore, each session between the CN and a local device will be
   treated as a separate call.  They may occupy different parts of the
   user interface, their media may not be available simultaneously, and
   they may have to be terminated separately.  This certainly does not
   fulfill the requirement of seamlessness.

   This document describes the use of multi-device systems to overcome
   this problem.  A local device's SLP UA queries for other devices and
   joins with them to create a "virtual device", or a Multi-Device
   System (MDS).  We refer to the controlling device as the Multi-Device
   System Manager (MDSM).  In a system that includes at least one
   mobility-enhanced device, one of them may act as the MDSM.  In a
   system consisting entirely of basic devices, either a dedicated host
   or another local device from outside of the system acts as MDSM.
   When the MDSM subsequently receives a REFER request, it uses third-
   party call control to set up media sessions between the CN and each
   device in the system.  Specifically, it invites each local device
   into a separate session, and uses their media parameters (and
   possibly its own) in the INVITE request it sends to the CN.

   A single device may act as an MDSM for several different groups of
   devices, and also act as an ordinary device with only its native
   capabilities.  There must be a way to address a request to a device
   and specify whether it is to the device itself or one of the multi-
   device systems it controls.  As mentioned above in Section 5.2, a
   device registers a separate contact for itself and for each of its

   multi-device systems.  For example, the device with AOR
   "sip:device11@example.com" and hostname "device11.example.com" will
   register a contact "sip:device11@device11.example.com" that
   represents its own capabilities.  Once it discovers other devices and
   creates an MDS, it will register a new contact,
   "sip:av1@device11.example.com".  It associates a GRUU with each of
   these contacts.  The device itself and any new system is registered
   in SLP using the GRUU.  When the proxy receives a request addressed
   to a GRUU, it will rewrite it as the contact URI before forwarding
   the request to the device.  The device will use this unique contact
   to determine whether to handle the request natively or with one of
   its systems.

   Figure 6 shows the transfer of a session to a multi-device system.
   The audio device has previously discovered the video device and
   created a multi-device system.  The REFER request sent to
   "sip:device11@example.com;gr=urn:uuid:893eeeyuinm981" prompts the
   audio device to invite the video device into a session to ascertain
   its SDP, and then to invite the CN into a session using its own SDP
   and that of the video device.

   video                  audio                   MN           CN
     |                      |(1) REFER            |            |
     |                      |<--------------------|            |
     |                      |(2) 202 Trying       |            |
     | (3) INVITE no sdp    |-------------------->|            |
     |<---------------------|                     |            |
     | (4) 200 OK    SDP    |                     |            |
     |--------------------->|                     |            |
     |                      |(5) INVITE a/v SDP, Replaces      |
     |                      |--------------------------------->|
     |                      |         RTP Audio                |
     |                      |<.................................|
     |                      |               RTP Video          |
     |                      |(6) 200 OK CN SDP                 |
     |                      |<---------------------------------|
     |                      |                RTP Audio         |
     | (7) ACK CN Video SDP |.................................>|
     |<---------------------|                     |            |
     | RTP Video            |                     |            |
     |                      |(8) ACK              |            |
     |                      |--------------------------------->|
     |                      |                     |(9) BYE     |
     |                      |                     |<-----------|
     |                      |                     |(10) 200 OK |
     |                      |                     |----------->|
     |                      |                     |            |
     |                      |                     |            |

      Figure 6.  Session Handoff to a multi-device system.

5.5.  Distributing Sessions for Incoming Call

   The examples presented above have involved an established session
   that a user transfers to one or more devices.  Another scenario would
   be for an incoming call to be immediately distributed between
   multiple devices when the user accepts the call.  In such a case, the
   initial session would not yet be established when the transfer takes

   The transfer could be carried out in either of the transfer modes.
   However, complete handoff to a separate device, which is done in
   Session Handoff mode, could be achieved through existing means, such
   as proxying or redirection.  MNC mode would be useful in a case where
   the user wishes to automatically include an additional device in a
   call.  For instance, a user with a desk IP phone and a PC with a
   video camera could join the two into a single logical device.  The

   SIP UA on the PC would, for any incoming call, send an INVITE request
   to the desk phone, setting the display name in the From header field
   to "Bob Jones (audio portion)", for instance, so that the user can
   identify the caller on the phone.  The user could then either accept
   or reject, as he would with a call coming directly to the phone.  If
   he accepts, the PC UA, acting as the controller, would respond to the
   caller with its video parameters and the phone's audio parameters in
   the SDP body.  The final ACK from the Correspondent Node would then
   complete the session establishment.

   If the desk phone is registered as a contact for the user, it would
   also ring in response to the direct call being proxied there, in
   addition to the INVITE request sent by the controller, causing
   confusion to the user.  The use of caller preferences can solve this
   problem, as the caller would indicate that the call should
   preferentially be proxied to devices with audio and video
   capabilities.  It is likely that the caller would use caller
   preferences in any case, if they were available to him, to avoid the
   callee unknowingly picking up the IP phone when he has a video-
   capable device available.  However, since caller preferences are not
   yet widely supported on commercial devices, the callee must ensure
   the proper routing of the call.  One solution would be for the PC to
   register its contact with a higher priority than the one given to the
   phone.  The Call Processing Language (CPL) [22] (the "proxy" node)
   could then be used to specify that forking should be done to the set
   of user devices in sequence, rather than in parallel.  Since all
   calls would first be sent to the PC as long as it were online, it
   would redirect any request that included only audio in its SDP.

5.6.  Use of ICE in Session Mobility

   Interactive Connectivity Establishment (ICE) [27] is a protocol for
   Network Address Translator (NAT) traversal that may be used with SIP.
   Rather than negotiating addresses and ports used for media sessions
   directly in SDP, a list of possible address/ports (candidates) is
   exchanged, and the Session Traversal Utilities for NAT (STUN) [28]
   protocol is used to check which pairs of candidates may be used.  ICE
   could be used in the call flows described in this section.  In MNC
   mode, the candidates would be sent by each local device to the MN,
   who would exchange them with the CN.  Afterward, each device would
   perform checks with the CN to determine an appropriate candidate.  In
   SH mode, where the local device establishes a session with the CN,
   ICE would work no differently than in the standard case.

6.  Reconciling Device Capability Differences

   Session mobility sometimes involves the transfer of a session between
   devices with different capabilities.  For example, the codec being
   used in the current session may not be available on the new device.
   Furthermore, that device may not support any codec that is supported
   by the CN.  In addition to codecs, devices may have different
   resolutions or bandwidth limitations that must be taken into account
   when carrying out a session transfer.

6.1.  Codec Differences

   Before executing a session transfer, the device checks the
   capabilities of the CN and the new device.  These may be found
   through either the SIP OPTIONS method, used in SIP to query a
   device's media capabilities, or may be included as SLP service
   attributes.  Since the OPTIONS method is standard, it is suggested to
   be used to query the CN, while SLP is suggested to be used to get the
   media capabilities of local devices, since it is already being used
   for them.

   If the CN and the local device are found to have a common codec, the
   transfer flow will negotiate that this should become the codec used
   in the media session.  In MNC mode, the MN forwards the response from
   the local device to the CN, who will choose a codec it supports from
   those available.  In Session Handoff mode, the MN sends a REFER
   request to the local device and allows it to negotiate a common codec
   with the CN during their session establishment.  No special behavior
   of the MN is required.

   If the MN sees that a common codec does not exist, it executes the
   transfer through an intermediate transcoding service.  Rather than
   establishing a direct media session between the CN and the local
   device, separate sessions are established between the transcoder and
   each of them, with the transcoder translating between the streams.
   The Mobile Node discovers available transcoders through some means,
   including SLP.

   Using transcoding services in SIP is defined in [18] using third-
   party call control.  In MNC mode, the Mobile Node establishes one
   media session between the transcoder and the CN, and one between the
   transcoder and the local device.  This differs from the normal
   transcoding case, where one party establishes a media session between
   itself and the transcoder and one between the transcoder and the
   other party.  The MN starts by sending an INVITE request to the local
   device with no body; it receives in the response the list of codecs
   that the device can use.  It then repeats this for the CN, and
   receives its available codecs.  It chooses one codec from each side,

   along with the address and port of each device, and combines them in
   an INVITE request sent to the transcoder.  The transcoder responds
   with the ports on which it will accept each stream.  The appropriate
   port information is sent individually to the CN and the local device.
   Once the three sessions have been established, two media sessions
   exist, and the transcoder translates between them.  This flow is
   shown in Figure 7.

  AN       Transcoder                      MN                      CN
(codec A)                                                      (codec B)
   |           |(1) INVITE no sdp           |                       |
   |<---------------------------------------|                       |
   |           |(2) 200 AN params           |                       |
   |--------------------------------------->|                       |
   |           |                            |(3) INVITE no sdp      |
   |           |                            |---------------------->|
   |           |                            |(4) 200 OK CN params   |
   |           |                            |<----------------------|
   |           |(5) INVITE AN, CN params    |                       |
   |           |<---------------------------|                       |
   |           |(6) 200 OK TA, TB params    |                       |
   |           |--------------------------->|                       |
   |           |(7) ACK                     |                       |
   |           |<---------------------------|                       |
   |           |(8) ACK TA params           |                       |
   |<---------------------------------------|                       |
   |   RTP     |                            |                       |
   |..........>|          RTP               |                       |
   |           |...................................................>|
   |           |                            | (9) ACK TB params     |
   |           |                            |---------------------->|
   |           |                            |  RTP                  |
   |   RTP     |<...................................................|
   |<..........|                            |                       |
   |           |                            |                       |

      Figure 7.  Transfer of a session in Mobile Node Control mode
                 through a transcoder to translate between native codecs
                 of CN and an audio node AN, where they share no common

   In Session Handoff mode, the local device itself establishes a
   session with the CN through the transcoder.  After receiving the
   REFER request, it uses the OPTIONS method to find the capabilities of
   the CN.  It will then use a common codec, if available, in the
   session setup, or set up the transcoded session using third-party
   call control as in [18].

6.2.  Display Resolution and Bandwidth Differences

   Other differences in device capabilities, such as display resolution
   and bandwidth limitations, are also suggested to be reconciled during
   transfer.  For example, a mobile device, limited both in its display
   size and bandwidth, will likely be receiving the video stream from
   the other call participant at a low resolution and frame rate.  When
   the user transfers his video output to a large-screen display, he may
   start viewing much higher-quality video at the higher native
   resolution of the display and at a higher frame rate.

   Changing the image resolution and frame rate requires no special
   handling by the MN.  An SDP format is defined [19] for specifying
   these and other parameters for the H.263+ codec, for example.  The
   suitable image formats and corresponding MPIs (Minimum Picture
   Interval, related to the frame rate) supported for the given codec
   are listed following the media line, in order of preference.  For
   example, the following lines in SDP would indicate that a device
   supports the H.263 codec (value 34) with the image sizes of 16CIF,
   4CIF, CIF, and QCIF (with the MPI for each format following the "="):

      m=video 60300 RTP/AVP 34
      a=fmtp:34 16CIF=8;4CIF=6;CIF=4;QCIF=3

   In MNC mode, the response by the local device (Figure 2, message 2)
   to the initial INVITE request sent by the MN includes this line in
   the SDP body, and the MN then includes it in the INVITE request sent
   to the CN (3).  In Session Handoff mode, the local device includes
   this parameter in the INVITE request sent to the CN (Figure 4,
   message 3) after receiving the REFER request.  If the local device is
   not configured to include the supported image sizes during session
   establishment, the information could be made available through SLP.
   The MN then includes it in the INVITE request sent to the CN in
   Mobile Node Control mode.  However, this information is not sent in
   Session Handoff mode unless the local device was configured to send
   it.  In both modes, the MN sends its own resolution and frame rate
   preferences in the body of the INVITE request sent to retrieve the

7.  Simultaneous Session Transfer

   A session transfer may be carried out by one call participant after
   the other participant has transferred the session on his side.  If
   the first transfer was done in MNC mode, a subset of the original
   session media is now on local devices.  The MN receives either a
   re-INVITE from the other participant or an INVITE request from a
   local device on the other side.  This message carries the new media
   parameters of the session.  The MN, therefore, must send a re-INVITE

   to any local devices with these parameters.  It then includes the
   parameters returned from these devices in the 200 OK response.  If
   the first transfer was done in SH mode, the local device will
   directly receive the session transfer message from the other party
   and will follow the normal procedure for responding to an INVITE
   request.  If it is controlling other local devices for this session
   as part of an MDS, it follows the procedure above, where the first
   transfer was done in MNC mode.

   It may occur that both participants attempt a transfer at the same
   time.  In MNC mode, each node initiates a session with a local
   device, then sends a re-INVITE to the other node.  Section 14.2 in
   [1] mandates a 491 response when a re-INVITE is received for a dialog
   once another re-INVITE has already been sent.  Once both parties
   receive this response, they each generate a random timer with
   staggered intervals.  Once its timer fires, each participant attempts
   the re-INVITE again.  The first to receive it from the other
   participant responds to it with the SDP parameters of its local
   device.  Both participants then send an ACK request to their local
   device containing the new parameters obtained from the other one
   during the re-INVITE process.

   In SH mode, if both participants attempt a transfer at the same time,
   after one node sends a REFER request to the local device, it receives
   the INVITE request from the local device on the other end.  The
   appropriate protocol definition could mandate that a 491 response be
   sent in this case, as well.  This response would be returned to the
   referrer in a NOTIFY indicating the status of the referred session
   establishment.  The staggered timer solution described above could
   work.  The MN would cancel the REFER request sent to the local
   device, then wait a random amount of time before sending it again.

8.  Session Termination

   Once a session has been transferred, the user may terminate it by
   hanging up the current device, as he would do in a call originating
   on that device.  This should be true even when the session is using
   several local devices.  In MNC mode, when the user hangs up the
   current device, a BYE request is sent to the controller.  The
   controller must then send a BYE request to each device used in the
   transfer and a BYE request to the CN.  An MDSM used for SH mode must
   follow the same procedure.  In SH mode, the current device has
   previously initiated an ordinary session with the CN in response to
   the REFER request, and the BYE it sends to the CN on hang-up requires
   no special handling.

9.  Security Considerations

   As this work is based heavily on the work in [2], [3], and [5], the
   security considerations described in those documents apply.  We
   discuss here the particular issues of authorizing use of local
   devices, providing media-level security following transfer, and the
   issue of flooding attacks in MNC mode.

9.1.  Authorization for Using Local Devices

   It is necessary that the use of a local device be limited to
   authorized parties.  As stated earlier, this document assumes both
   personal and public devices, and these have different authorization
   policies.  A personal device only accepts transfer requests from a
   single identity, the device owner.  Therefore, the most appropriate
   means of access control is to maintain a list of identities
   representing the device owner authorized to transfer sessions to the
   device.  As mentioned before, the device is configured with an AOR
   representing its status as a transfer device, in addition to the
   user's AOR.  Only requests made to the device AOR follow the access
   list, while incoming requests to the user's AOR are accepted from
   anyone (provided that a white or blacklist or other policy does not
   preclude their request from being accepted).  The SIP-Identity header
   [25] is used to securely identify the initiator of a SIP request.
   That specification can be used in our use-cases when the local device
   must ensure that the INVITE or REFER request in MNC or SH mode,
   respectively, is indeed from the owner of the device.

   Public devices accept transfer requests from a large number of
   identities.  Access lists may be used for this purpose.
   Alternatively, since devices are often available to categories of
   users, such as "manager" or "faculty member", an appropriate solution
   may be to use trait-based authorization [23].  Using this mechanism,
   a user may acquire, from a trusted authorization service, an
   "assertion" of his user status and permissions.  The assertion, or a
   reference to it, is included in the request to use the device.

9.2.  Maintaining Media Security During Session Mobility

9.2.1.  Establishing Secure RTP Using SDP

   Confidentiality, message authentication, and replay protection are
   necessary in internet protocols, including those used for real-time
   multimedia communications.  The Secure Real-time Transfer Protocol
   (SRTP) [14] provides these for RTP streams.  Since SRTP may be used
   to carry the media sessions of SIP devices, such as the MN and CN, we

   discuss how to ensure that the session continues to use SRTP
   following the transfer to another device.  This is also discussed in
   less detail in [2].

   The establishment of secure RTP communications through SDP is defined
   by two documents.  The "crypto" attribute [15] is a media-level
   attribute whose value includes the desired cryptographic suite and
   key parameters used to perform symmetric encryption on the RTP
   packets.  Since the key information is sent in the SDP body with no
   dedicated encryption or integrity protection, a separate protocol
   such as S/MIME must be used to protect the signaling messages.
   Another document [16] specifies the "key-mgmt" attribute used to
   provide parameters for a key management protocol, such as MIKEY.
   Using this attribute, the two participants exchange keys encrypted by
   a public or shared key, or negotiate a key using the Diffie-Hellman

   The use of cryptographic parameters in SDP does not change the
   message flows described earlier in this document.  For instance, in
   MNC mode shown in Figure 2, the response from the local device (2)
   will include, in addition to any supported media type, cryptographic
   information for each type.  This cryptographic information will be a
   list of attribute lines describing the crypto suite and key
   parameters using either of the two attributes mentioned.  These lines
   will be sent by the MN to the CN in the subsequent request (3).  The
   CN will choose a cryptographic method and return its own key
   information in the response (4).  Maintaining a secure media session
   in SH mode requires the local device to negotiate a cryptographic
   relationship in the session that it establishes following its receipt
   of the REFER request.

   It is noted in [2] that establishing media security in third party
   call control depends on the cooperation of the controller.  In this
   document, the Mobile Node (MN) in Mobile Node Control mode (MNC) has
   the role of controller in 3pcc, while in the Session Handoff (SH)
   mode, MN uses the REFER method instead.  The following is an excerpt
   from that document:

      End-to-end media security is based on the exchange of keying
      material within SDP.  The proper operation of these mechanisms
      with third party call control depends on the controller behaving
      properly.  So long as it is not attempting to explicitly disable
      these mechanisms, the protocols will properly operate between the
      participants, resulting in a secure media session that even the
      controller cannot eavesdrop or modify.  Since third party call
      control is based on a model of trust between the users and the
      controller, it is reasonable to assume it is operating in a well-
      behaved manner.  However, there is no cryptographic means that can

      prevent the controller from interfering with the initial exchanges
      of keying materials.  As a result, it is trivially possibl[e] for
      the controller to insert itself as an intermediary on the media
      exchange, if it should so desire.

   We note here that given the model presented in this document, where
   the controller is operated by the same person that uses the local
   device, i.e., the MN user, there is even more reason to believe that
   the controller will be well-behaved and will not interfere with the
   initial transfer of key exchanges.

9.2.2.  Securing Media Using the Transport Layer

   The exchange of media could alternatively be secured at the transport
   layer, using either TLS or Datagram Transport Layer Security (DTLS)
   [24].  The one consideration for use of these protocols in session
   mobility would be assigning the client and server roles.  In SH mode,
   it may be assumed that the local device, the referee, would act as
   the client, since it is initiating the signaling session with the CN.
   However, in MNC mode, these roles would be unclear.  The same problem
   was mentioned above in establishing a secure connection for an MSRP
   session transferred in MNC mode.  This problem could be solved
   through the use of Connection-Oriented Media (COMEDIA) [26], which
   specifies the "setup" SDP attribute to negotiate these roles.

   We describe here briefly how this is done.  In the MNC exchange shown
   in Figure 2, the local device chooses whether to specify a media
   session over a secured transport in its response to the MN.  If so,
   it includes under the media line a "setup" attribute set to either
   "active", "passive", or "actpass".  This is sent on to the CN.
   Assuming it agreed to such a session, it responds with a "setup"
   attribute, as per the COMEDIA specifications.  This is then sent by
   the MN to the local device.  If the local device and CN agreed on
   their roles, the appropriate session could be established, through
   which the media would be transmitted.  Before they transmit media
   between them, the CN and local device exchange messages to establish
   the TLS or DTLS session.  This same approach could be used to
   establish an SRTP security context over DTLS, as per [31].

9.3.  Flooding Attacks in MNC Mode

   The MNC call flows in this document, where one device instructs
   another device to send an RTP flow to a third one, present the
   possibility of a flooding attack.  This is a general problem that
   relates to any use of 3pcc.  In this document, it is only a concern
   where the device is public, as described at the beginning of this
   section, and a large group of people can transfer media to it, since
   there may not be a very strong trust relationship between the device

   owner (e.g., an institution) and the users.  Obviously, where a
   device is private and only its owner can transfer to it, the concern
   does not exist, given the use of the Identity header mentioned
   earlier.  A possible solution may be the use of ICE [27], since both
   sides confirm that they want to receive each other's media.

10.  Acknowledgments

   We would like to acknowledge the helpful comments made about this
   document by the SIP community, in particular Jon Peterson, Joerg Ott,
   and Cullen Jennings.

11.  References

11.1.  Normative References

   [1]   Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
         Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
         Session Initiation Protocol", RFC 3261, June 2002.

   [2]   Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
         "Best Current Practices for Third Party Call Control (3pcc) in
         the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April

   [3]   Sparks, R., "The Session Initiation Protocol (SIP) Refer
         Method", RFC 3515, April 2003.

   [4]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         Session Description Protocol (SDP)", RFC 3264, June 2002.

   [5]   Sparks, R., "The Session Initiation Protocol (SIP) Referred-By
         Mechanism", RFC 3892, September 2004.

   [6]   Mahy, R., Biggs, B., and R. Dean, "The Session Initiation
         Protocol (SIP) "Replaces" Header", RFC 3891, September 2004.

   [7]   Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed., "The
         Message Session Relay Protocol (MSRP)", RFC 4975, September

   [8]   Jennings, C., Mahy, R., and A. Roach, "Relay Extensions for the
         Message Sessions Relay Protocol (MSRP)", RFC 4976, September

   [9]   Hellstrom, G. and P. Jones, "RTP Payload for Text
         Conversation", RFC 4103, June 2005.

   [10]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
         User Agent Capabilities in the Session Initiation Protocol
         (SIP)", RFC 3840, August 2004.

   [11]  Camarillo, G., "Internet Assigned Number Authority (IANA)
         Registration of the Message Media Feature Tag", RFC 4569, July

   [12]  Rosenberg, J., "Obtaining and Using Globally Routable User
         Agent URIs (GRUU) in the Session Initiation Protocol (SIP)",
         RFC 5627, October 2009.

11.2.  Informative References

   [13]  Rosenberg, J., Schulzrinne, H., and R. Mahy, Ed., "An INVITE-
         Initiated Dialog Event Package for the Session Initiation
         Protocol (SIP)", RFC 4235, November 2005.

   [14]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
         Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
         3711, March 2004.

   [15]  Andreasen, F., Baugher, M., and D. Wing, "Session Description
         Protocol (SDP) Security Descriptions for Media Streams", RFC
         4568, July 2006.

   [16]  Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
         Carrara, "Key Management Extensions for Session Description
         Protocol (SDP) and Real Time Streaming Protocol (RTSP)", RFC
         4567, July 2006.

   [17]  Guttman, E., Perkins, C., Veizades, J., and M. Day, "Service
         Location Protocol, Version 2", RFC 2608, June 1999.

   [18]  Camarillo, G., Burger, E., Schulzrinne, H., and A. van Wijk,
         "Transcoding Services Invocation in the Session Initiation
         Protocol (SIP) Using Third Party Call Control (3pcc)", RFC
         4117, June 2005.

   [19]  Ott, J., Bormann, C., Sullivan, G., Wenger, S., and R. Even,
         Ed., "RTP Payload Format for ITU-T Rec. H.263 Video", RFC 4629,
         January 2007.

   [20]  Schulzrinne, H. and E. Wedlund, "Application-Layer Mobility
         Using SIP", ACM SIGMOBILE Mobile Computing and Communications
         Review, Vol. 4, No. 3, July 2000.

   [21]  Campbell, B., Ed., Rosenberg, J., Schulzrinne, H., Huitema, C.,
         and D. Gurle, "Session Initiation Protocol (SIP) Extension for
         Instant Messaging", RFC 3428, December 2002.

   [22]  Lennox, J., Wu, X., and H. Schulzrinne, "Call Processing
         Language (CPL): A Language for User Control of Internet
         Telephony Services", RFC 3880, October 2004.

   [23]  Peterson, J., Polk, J., Sicker, D., and H. Tschofenig, "Trait-
         Based Authorization Requirements for the Session Initiation
         Protocol (SIP)", RFC 4484, August 2006.

   [24]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
         Security", RFC 4347, April 2006.

   [25]  Peterson, J. and C. Jennings, "Enhancements for Authenticated
         Identity Management in the Session Initiation Protocol (SIP)",
         RFC 4474, August 2006.

   [26]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
         Session Description Protocol (SDP)", RFC 4145, September 2005.

   [27]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Protocol for Network Address Translator (NAT) Traversal for
         Offer/Answer Protocols", Work in Progress, October 2007.

   [28]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, "Session
         Traversal Utilities for NAT (STUN)", RFC 5389, October 2008.

   [29]  Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery",
         Work in Progress, September 2008.

   [30]  Cheshire, S. and M. Krochmal, "Multicast DNS", Work in
         Progress, September 2008.

   [31]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework for
         Establishing an SRTP Security Context using DTLS", Work in
         Progress, March 2009.

Authors' Addresses

   Ron Shacham
   Columbia University
   1214 Amsterdam Avenue, MC 0401
   New York, NY  10027

   EMail: shacham@cs.columbia.edu

   Henning Schulzrinne
   Columbia University
   1214 Amsterdam Avenue, MC 0401
   New York, NY  10027

   EMail: hgs@cs.columbia.edu

   Srisakul Thakolsri
   DoCoMo Communications Laboratories Europe
   Landsberger Str. 312
   Munich  80687

   EMail: thakolsri@docomolab-euro.com

   Wolfgang Kellerer
   DoCoMo Communications Laboratories Europe
   Landsberger Str. 312
   Munich  80687

   EMail: kellerer@docomolab-euro.com


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