SIPPING R. Shacham
Internet-Draft H. Schulzrinne
Intended status: Informational Columbia University
Expires: May 17, 2007 S. Thakolsri
W. Kellerer
DoCoMo Euro-Labs
November 13, 2006
Session Initiation Protocol (SIP) Session Mobility
draft-shacham-sipping-session-mobility-03
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Abstract
Session Mobility is the seamless transfer of media of an ongoing
communication session from one device to another. This document
describes the general methods and specifies the flows for providing
this service as part of the Session Initiation Protocol (SIP). The
basic steps involved in session mobility which are describe are
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service discovery to locate devices to use as transfer targets, for
which the Service Location Protocol (SLP) is used, session transfer,
and, sometimes, reconciliation of device capability differences. The
described session mobility methods include the possibility of
transferring any subset of the active media to one or more devices.
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Component Overview . . . . . . . . . . . . . . . . . . . . . . 5
4. Service Location . . . . . . . . . . . . . . . . . . . . . . . 5
5. Session Mobility . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Options for Session Mobility . . . . . . . . . . . . . . . 8
5.1.1. Transfer and Retrieval . . . . . . . . . . . . . . . . 8
5.1.2. Whole and Split Transfer . . . . . . . . . . . . . . . 8
5.1.3. Transfer Modes . . . . . . . . . . . . . . . . . . . . 9
5.1.3.1. Mobile Node Control (MNC) Mode . . . . . . . . . . 9
5.1.3.2. Session Handoff (SH) Mode . . . . . . . . . . . . 9
5.1.4. Types of Transferred Media . . . . . . . . . . . . . . 9
5.2. Mobile Node Control Mode . . . . . . . . . . . . . . . . . 10
5.2.1. Transfer to a Single Local Device . . . . . . . . . . 10
5.2.1.1. RTP Media . . . . . . . . . . . . . . . . . . . . 10
5.2.1.2. MSRP Sessions . . . . . . . . . . . . . . . . . . 11
5.2.2. Transfer to Multiple Devices . . . . . . . . . . . . . 13
5.2.3. Retrieval of a Session . . . . . . . . . . . . . . . . 16
5.3. Session Handoff (SH) mode . . . . . . . . . . . . . . . . 16
5.3.1. Transfer to a Single Device . . . . . . . . . . . . . 16
5.3.2. Retrieval of a Session . . . . . . . . . . . . . . . . 19
5.3.3. Transfer to Multiple Devices . . . . . . . . . . . . . 21
5.4. On Incoming Call . . . . . . . . . . . . . . . . . . . . . 22
6. Reconciling Device Capability Differences . . . . . . . . . . 23
6.1. Codec Differences . . . . . . . . . . . . . . . . . . . . 23
6.2. Display Resolution and Bandwidth Differences . . . . . . . 26
7. Simultaneous Session Transfer . . . . . . . . . . . . . . . . 27
8. Session Termination . . . . . . . . . . . . . . . . . . . . . 27
9. Security Considerations . . . . . . . . . . . . . . . . . . . 28
9.1. Authorization for Using Local Devices . . . . . . . . . . 28
9.2. Maintaining Media Security in Session Mobility . . . . . . 28
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
11. Change History . . . . . . . . . . . . . . . . . . . . . . . . 30
11.1. Changes from draft-shacham-sipping-session-mobility-00 . . 30
11.2. Changes from draft-shacham-sipping-session-mobility-01 . . 30
11.3. Changes from draft-shacham-sipping-session-mobility-02 . . 30
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
12.1. Normative References . . . . . . . . . . . . . . . . . . . 31
12.2. Informative References . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
Intellectual Property and Copyright Statements . . . . . . . . . . 34
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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 not mobility. The seamless transition between these devices
allows them to be used concurrently or interchangeably in mid-
session, combining the advantages of both into a single "virtual
device." Since the Session Initiation Protocol (SIP) [1] has been
chosen by the Third Generation Partnership Project (3GPP) as its
standard for session establishment in the Internet Multimedia
Subsystem (IMS) [16] and it is being deployed in hardware and
software IP multimedia clients, it is natural to specify an
architecture to provide this seamless, ubiquitous service using SIP.
Such a SIP-based approach is first given in [22], 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
complete system for session mobility. The aim of this system is to
allow a Mobile Node to discover available devices and to include them
in an active session. In order to accomplish this, two main
components are defined:
o Service Location - At all times, a user is aware of the devices
which are available in his local area, along with their
capabilities.
o Session Mobility - While in a session with a remote participant,
the user may transfer any subset of the active media services to
one or more devices.
As described above, this document describes Session Mobility for SIP.
Service location, on the other hand, does not depend on any
particular protocol. Many different models exist for providing this
service, and this document discusses the tradeoffs involved in
choosing between them. For our examples, however, we have decided to
use the Service Location Protocol (SLP) [17].
2. Requirements
This session mobility framework seeks to fulfill the following
requirements:
o REQ 1: Interoperability - No special capabilities should be
required of the remote participant in the call, as long as he is
using a SIP-compliant device or there is a PSTN gateway between
them. The device should be capable of handling a transfer by the
mobile user utilizing only features specified in Requests For
Comments (RFCs) and mature Internet Drafts.
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o REQ 2: Backward Compatibility - Both mobility-enhanced and basic
devices should be available as destinations for transfer.
Mobility-enhanced devices are those that are controlled by
software that is enhanced to support special call handling and
service discovery. Commercial IP phones and embedded devices are
basic since they cannot be enhanced to support such capabilities.
o REQ 3: 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 4: Seamlessness - Session transfer should be as seamless as
possible. It should involve minimal disruption of the media flow
and should not appear to the remote participant as a new call.
3. Component Overview
Session Mobility involves five basic components: The Correspondent
Node (CN), the Mobile Node (MN), the local devices, an SLP 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 UA, or a
PSTN phone reachable through a gateway. The Mobile Node (MN) is a
mobile device, containing a SIP User Agent (UA) for standard SIP call
setup, as well as specialized SIP-handling capabilities for session
mobility and an SLP [17] User Agent (UA) for service querying. 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 5.3.3. The SLP Directory Agent (DA) keeps track
of devices based on their location and capabilities. 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. Service Location
A mobile node must be able to discover suitable devices in its
vicinity. It is possible for devices in close proximity to be
discovered by direct methods such as Bluetooth without the use of
centralized servers. On the other hand, many centralized directory-
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based service discovery protocols exist, such as the Service Location
Protocol (SLP). These protocols are general and are not based on
physical locations of services, but they may be easily adapted by
adding location attributes to the service description [27]. They
have the advantage of allowing discovery of devices at different
location granularities, such as at the room or building level, and in
a location other than that of the device. They have the disadvantage
of requiring mobile devices to discover their location in order to
perform such queries. We have chosen to use a general protocol, SLP,
for device discovery in this document.
Many standard technologies may be used to update the mobile node of
its location for use in an SLP query. Indoors, the node can receive
its civic coordinates (ie., street address, room number, etc.) either
directly or indirectly. With direct methods, the location is sent to
the node itself by means such as a Bluetooth beacon. Indirect
methods use external location sources such as swipe cards to update
the user's location. The mobile node subscribes to the user's
location as part of his presence [23] which has been suggested by
[24] as a way to receive location updates in the format standardized
in [20]. Outdoors, a mobile device may use direct methods such as
the Global Positioning System (GPS) [21] to update it on its
geospatial coordinates. Online databases can translate these to
civic coordinates. The choice of location technology is beyond the
scope of this document.
Once the node has obtained its location, it uses SLP to query for
available devices in the vicinity that may be used as transfer
destinations. 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. For the SIP devices in this framework, we
assume a service type called "sip-device," whose SIP URL, such as
sip:audio_rm123@example.com or sip:audio@device1.example.com serves
as its service URL. The first URL mentioned is an address of record
that is used to connect to the device through the local proxy server,
while the second URL includes the name of the host on which the
service is provided, "device1.example.com," allowing a point-to-point
session to be established with the device. Either of these models
may be used, although the proxy model will likely make the device
available after an IP address change more quickly than in the point-
to-point model, where the DNS entry would take longer to be updated
unless dynamic DNS were used. The service description also includes
attributes specifying device characteristics (e.g., vendor, supported
media codec, display resolution) and location parameters, such as
street address and room number.
SLP defines Service Agents (SAs) which send descriptions of services
using the Service Registration (SrvReg) message, and User Agents (UA)
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which query for services using the Service Request (SrvRqst) message.
SAs are co-located with SIP UAs on 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. SLP provides two
models for a UA to query for services. In the distributed model, the
UA sends requests through multicast and SAs reply directly with the
details of the service they provide. In the centralized model, a
Directory Agent (DA) is used, to which the SAs register and from
which the UAs request services. For our examples, we use the
centralized model, though either could be used. The SA registers its
service description to the 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 the 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.
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 which are not in the
current room. A SrvRply message is sent to the mobile device with a
list of SIP URIs for all devices on the floor. A separate Attribute
Request (AttrRqst) is then sent for each URL to get the attributes of
the service, which include the room where the device is located. The
MN displays for the user the available devices in the room, and their
attributes. Figure 1 shows this protocol flow.
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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. Transfer means to move the session on the current device to
one or more other devices. Retrieval means to remotely transfer a
session currently on another device to the local device. This may
mean to return 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 retrieve
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
Session media may either be transferred completely to a single device
or be split across multiple devices. For instance, a user may only
wish to transfer the video of his session while maintaining the audio
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on his PDA. Alternatively, he may find separate video and audio
devices and wish to transfer one media service 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 mode and Session Handoff mode.
5.1.3.1. 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 session with the
CN. The shortcoming of this approach is that it requires the MN to
remain active to maintain the sessions.
5.1.3.2. Session Handoff (SH) Mode
A user may need to transfer a session completely because the battery
on his mobile device is running out. 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 case, Session Handoff mode,
which completely transfers the session signaling and media to another
device, is useful.
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 messaging
of text, and possibly other MIME types, that are suitable for SIP
endpoints. RTP may be used to transport text payloads based on [9].
Any examples given for audio and video will work identically for
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text, as only the payloads differ. For the transfer of whole
messages, either the SIP MESSAGE method [25] 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 as
part of 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 sessions.
5.2. Mobile Node Control Mode
5.2.1. Transfer to a Single Local Device
5.2.1.1. RTP Media
local 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
device.
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
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device used for the transfer, requesting that a new session be
established, but not including an SDP body. The local device's
response contains an SDP body that includes the address and ports 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
message (re-INVITE) containing the local device's media parameters in
the SDP body, as follows:
v=0
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
The MN may select a subset of the media returned, and use that in the
request sent to the CN. 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. The CN sends 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 sends an ACK message to the local device,
which includes these parameters in the body. The MN has established
separate SIP session with the CN and the local device, but a media
flow has been established between the CN and the local device.
5.2.1.2. MSRP Sessions
The message sequence for transferring an MSRP message session using
MNC mode is identical to that used for audio or video, in Figure 2,
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 device, MSGN. An empty INVITE
request (1) is sent to the local messaging node, MSGN, as follows:
INVITE sip:msgn@msgn.example.com SIP/2.0
To: <sip:msgn@msgn.example.com>
From: <sip:mn@mn.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):
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SIP/2.0 200 OK
To: <sip:msgn@msgn.example.com>;tag=087js
From: <sip:mn@mn.example.com>;tag=786
Call-ID: 893rty@mn.example.com
Content-Type: application/sdp
v=0
c=IN IP4 msgn.example.com
m=message 52000 msrp/tcp *
a=accept-types:text/plain
a=path:msrp://msgn.example.com:12000/kjhd37s2s2;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. 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, opening 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 end point 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 possiblity, it is not currently included 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 to it, including its relay, in the path attribute. For
instance, the response from the MSGN will look as follows:
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SIP/2.0 200 OK
To: <sip:msgn@msgn.example.com>;tag=087js
From: <sip:mn@mn.example.com>;tag=786
Call-ID: 893rty@mn.example.com
Content-Type: application/sdp
v=0
c=IN IP4 msgn.example.com
m=message 52000 msrp/tcp *
a=accept-types:text/plain
a=path:msrp://relayA.example.com:12000/kjhd37s2s2;tcp \
path:msrp://msgn.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.
5.2.2. 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, it initiates separate sessions with each
of them, combines the audio media line from one response and the
video line from the other, and sends them together as the request to
the CN, as follows:
v=0
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.
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VN AN MN CN
| |(1) INVITE no sdp | |
| |<-------------------| RTP Audio |
| |(2) 200 AN params | |
| |------------------->| |
| |(3) INVITE no sdp | |
|<---------------------------------------| |
| |(4) 200 VN params | |
| |------------------->| |
| | |(5) INVITE AN/VN 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
devices.
Splitting a full-duplex media service such as video across an input
and an output device 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 unidectional media, as is the case for a camera or
devoted display 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
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a=sendonly
c=IN IP4 camera.example.com
m=video 50800 RTP/AVP 34
a=rtpmap:34 H263/90000
a=recvonly
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 will only stop listening to
the old port (in case it changes) once it starts receiving on the new
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
a=sendonly
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
a=recvonly
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.
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5.2.3. 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.
5.3. Session Handoff (SH) mode
5.3.1. Transfer to a Single Device
Session Handoff mode uses the SIP REFER method [3]. This message is
a request sent by a "referer" 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 referer'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 S/MIME to authenticate the REFER request. Figure 4 shows the flow
for transferring a session.
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AN MN CN
|(1) REFER | |
|<----------------------------| |
|(2) 202 Accepted | |
|---------------------------->| |
|(3) INVITE, Replaces | |
|-------------------------------------------------->|
| RTP |
|<..................................................|
|(4) 200 OK | |
|<--------------------------------------------------|
| RTP |
|..................................................>|
|(5) ACK | |
|-------------------------------------------------->|
|(6) NOTIFY | |
|---------------------------->| |
|(7) 200 OK | |
|<----------------------------| |
| |(6) BYE |
| |-------------------->|
| |(7) 200 OK |
| |<--------------------|
| | |
| | |
Figure 4 : Session Handoff mode flow for transfer to a single device.
The MN sends the following REFER request (1) to a local device:
REFER sip:av@local_device.example.com SIP/2.0
To: <sip:av@local_device.example.com>
From: <sip:mn@example.com>
Refer-To:<sip:cn@host1.example.com;audio;video?
Replaces="1@mn.example.com;
to-tag=bbb;from-tag=aaa">
Referred-By: <sip:mn@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 following the 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 is the "Replaces" header which is to be included
in the INVITE request. The "Replaces" header identifies an existing
session that should be replaced by the new session. Here, the local
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device requests that the CN replace its current session with the MN
with the new session. According to [6], the CN should only accept a
request to replace a session by certain authorized groups 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 must provide 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.
AN MN CN
|(1) REFER | |
|<----------------------------| |
|(2) 202 Accepted | |
|---------------------------->| |
|(3) REFER | |
|---------------------------->| |
| |(4) INVITE, Replaces |
| |-------------------->|
| | RTP |
| |<....................|
| |(5) 200 OK |
| |<--------------------|
| | RTP |
| |....................>|
| |(6) ACK |
| |-------------------->|
| (7) BYE | |
|<--------------------------------------------------|
| (8) 200 OK | |
|-------------------------------------------------->|
| | |
| | |
Figure 5 : Session Handoff mode flow for session retrieval.
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 5.3.2.
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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) must indicate 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 field tag, "message" has already
been defined [12]. 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.
5.3.2. Retrieval of a Session
Figure 5 shows the flow for retrieval by the MN of a session
currently on a local device. 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 (4):
INVITE sip:cn@host1.example.com SIP/2.0
To: <sip:cn@host1.example.com>
From: <sip:mn@example.com>
Replaces: 1@local_device.example.com;to-tag=aaa;from-tag=bbb
Referred-By: <sip:av@local_device.example.com>
[S/MIME authentication body]
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, and the user may also wish to execute the transfer while
running out of the office with mobile device in hand. In order to
prompt the REFER from the Mobile Node, a "nested REFER," [5] a REFER
request for another REFER, is sent. 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 target INVITE request. The
REFER request from the local device to the MN (3) is composed as
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follows:
REFER sip:mn@example.com SIP/2.0
To: <sip:mn@ example.com>
From: <sip:av@local_device.example.com>
Refer-To: <sip:cn@host1.example.com;audio;video?
Replaces="1@local_device.example.com;to-tag=aaa;
from-tag=bbb">
Referred-By: <sip:av@local_device.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:av@local_device.example.com SIP/2.0
To: <sip:av@local_device.example.com>
From: <sip:mn@example.com>
Refer-To:<sip:mn@host1.example.com;method=REFER
?Refer-To="<sip:cn@host1.example.com;audio;
video?Replaces=%221@local_device.example.com;
to-tag=aaa;from-tag=bbb%22>">
The "Refer-To" header specifies the MN as the refer target and that
the referral be in the form of a REFER request. The header field
specifies that the REFER request should contain a "Refer-To" header
containing the URI of the CN. That URI, itself, should contain 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 should contain 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 must get these parameters by
subscribing to the Dialog Event Package [11] 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
must not accept the REFER request from anybody except for 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
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established, terminates its session with the local device.
5.3.3. 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". It then chooses a
single SIP URI to address it, such as sip:a_v@dev.example.com, and
registers the service in SLP. We refer to the controlling device as
the Multi-Device System Manager (MDSM). In a system that includes at
least one mobility-enhanced device, it 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 must act as MDSM.
Once the MN discovers a system of devices, it may hand off a session
by sending a REFER request to the MDSM URI. When the MDSM receives
the request sent to this URI, it uses third-party call control to set
up media sessions between the CN and each device in the system
(including itself). Specifically, it invites each local device into
a separate session, and uses their media parameters to invite the CN
into a session. Figure 6 shows the transfer of a session to a multi-
device system. The Audio Node (AN) has previously discovered the
Video Node (VN), and created a multi-device system. The REFER
request sent to sip:a_v@an.example.com prompts the Audio Node to
invite the Video Node 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 Node.
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VN AN MN CN
| |(1) REFER | |
| |<--------------------| |
| |(2) 202 Trying | |
| (3) INVITE no sdp |-------------------->| |
|<-------------------| | |
| (4) 200 OK VN SDP | | |
|------------------->| | |
| |(5) INVITE AN/VN SDP, Replaces |
| |--------------------------------->|
| | RTP Audio |
| |<.................................|
| | RTP Video |
|<......................................................|
| |(6) 200 OK CN SDP |
| |<---------------------------------|
| | RTP Audio |
| (7) ACK CN SDP |.................................>|
|<-------------------| | |
| RTP Video | | |
|......................................................>|
| |(8) ACK | |
| |--------------------------------->|
| | |(9) BYE |
| | |----------->|
| | |(10) 200 OK |
| | |<-----------|
| | | |
| | | |
Figure 6 : Session handoff to a multi-device system.
5.4. On Incoming Call
The examples presented above have involved an established session
which 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
place.
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 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
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PC would, for any incoming call, send an INVITE request to the desk
phone, setting the display name in the From header 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 caller 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 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 would alone need to ensure the proper routing of
the call. The desired behavior could be achieved by not registering
the desk phone and having all calls, whether video or audio-only go
through the PC. The PC would register two contacts, one representing
itself and the phone as a single audio-video device, and another
representing only the phone, but using the PC's host address. A
third solution would be to register both devices, give a higher
priority to the PC UA, and use CPL (the "proxy" node) [26] to specify
that routing should be done to the set of user devices in sequence,
rather than in parallel. Since all calls would first be proxied to
the PC as long as it were online, it would need to redirect any
request that included audio in its SDP.
6. Reconciling Device Capability Differences
Session mobility sometimes involves the transfer of a session between
devices with differing 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 which must be taken into account
when carrying out session transfer.
6.1. Codec Differences
Before executing a session transfer, the device must check the
capabilities of the CN and the new device. These may be found
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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 should be used
to query the CN, while SLP should 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 should be carried out so that it becomes the codec used in
the new media session. In MNC Mode, the flow is identical, but the
SDP bodies include the desired codec. 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.
If the CN and the local device are found to have a common codec, the
transfer should be carried out so that it becomes the codec used in
the new media session. In MNC Mode, the flow is identical, but the
SDP bodies include the desired codec. 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.
If a common codec does not exist, the MN must execute the transfer
through an intermediate transcoding service, which need not be
geographically local. 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 may
discover available transcoders through any means, including SLP.
Using transcoding services in SIP is defined in [18] using third-
party call control. The standard case involves a controller, party
A, that initiates a media session with party B through a transcoder.
The controller invites the transcoder into a session and provides its
own media parameters to indicate which media it would like
transcoded. The transcoder responds with a "200 OK" that includes
its own media parameters, namely the ports on which it will receive
each stream. The controller then establishes a separate session with
B, in which it gives it the address and port of the transcoder as the
destination of its media. It also establishes its own media session
with the transcoder. Once both sessions are established, two media
streams have been established through the transcoder.
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AN Transcoder MN CN
(codec A) (codec B)
| |(1) INVITE MN params | |
| |<---------------------------| |
| |(2) 200 TA, TB params | |
| |--------------------------->| |
| |(3) ACK | |
| |<---------------------------| |
| |(4) INVITE TA params | |
|<---------------------------------------| |
| RTP | | |
|..........>| | |
| |(5) 200 AN params | |
|--------------------------------------->| |
| |(6) ACK | |
|<---------------------------------------| |
| | |(7) INVITE TB params |
| | |---------------------->|
| | | RTP |
| |<...................................................|
| | |(8) 200 OK CN params |
| | |<----------------------|
| | |(9) ACK |
| | |---------------------->|
| |(8) INVITE AN, CN params | |
| RTP |<---------------------------| |
|<..........| | RTP |
| |...................................................>|
| |(9) 200 OK TA,TB params | |
| |--------------------------->| |
| |(10) ACK | |
| |<---------------------------| |
| | | |
Figure 7 : Transfer of a session in Mobile Node Control mode through
a transcoder to translate between native codecs of CN and an audio
device AN, where they share no common codec.
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. The initial INVITE sent by the MN to the transcoder
includes a session description referring to itself. The transcoder
responds with parameters for both streams that it will accept. The
MN invites the local device and the CN into separate sessions using
these provided parameters. They each provide their own parameters in
the responses, which the MN combines to update the session with the
transcoder. Once the three sessions have been established, two media
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sessions exist, and the transcoder translates between them. This
flow is shown in Figure 7.
In Session Handoff mode, the local device itself must establish 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, should also be reconciled during tranfer.
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 framerate. 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 framerate.
Changing the image resolution and framerate requires no special
handling by the MN. An SDP format is defined [19] for specifying
these and other parameters for the H.263+ codec. The suitable image
formats and corresponding MPIs (Minimum Picture Interval, related to
the framerate) 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 1)
to the initial INVITE request sent by the MN would include this line
in the SDP body, and the MN would then include it in the INVITE
request sent to the CN (3). In Session Handoff mode, the local
device would include this parameter in the INVITE request sent to the
CN (Figure 4, 3) after receiving the REFER request. If the local
device is not mobility-enhanced, and is, therefore, not configured to
include the supported image sizes during session establishment, the
information could be made available through SLP. The MN would then
include it in the INVITE request sent to the CN in mobile node
control mode. However, this information would not be sent in Session
Handoff mode unless the local device were configured to send it. In
both modes, the MN would send its own resolution and framerate
preferences in the body of the INVITE request sent to retrieve the
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session.
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 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
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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 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. A 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 the the REFER request,
and the BYE it sends to the CN on hang-up requires no special
handling.
9. Security Considerations
Many security concerns must be addressed in session mobility. 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 and providing media-level security following transfer.
9.1. Authorization for Using Local Devices
Use of a local device must be limited to authorized parties. We
assume in this document that these devices are private, accessible
for transfer only to their owners. They must accept these types of
INVITE and REFER requests only from identities representing the
owner. However, they may also be used to accept ordinary calls.
Therefore, there must be a distinction between requests to
communicate with the owner himself, which should be accepted from
anybody (except for those excluded through a white/black list policy)
and requests to use the device for transfer, which is only accepted
from the owner. The latter type of request should be addressed to a
different URI, such as phone@example.com. Digest or S/MIME may be
used to authenticate the owner's identity.
Deployment of public devices that are available to a group of users
for their transfers is beyond the scope of this document.
9.2. Maintaining Media Security in Session Mobility
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) [13] 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].
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The establishment of secure RTP communications through SDP is defined
by two documents. The "crypto" attribute [14], 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 [15] 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
method.
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 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. 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 possibly 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
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device, there is even more reason to believe that the controller will
be well-behaved and will not interfere with the transfer of key
exchanges.
10. IANA Considerations
This document has no actions for IANA.
11. Change History
[[RFC Editor: Please remove this entire section upon publication as
an RFC.]]
11.1. Changes from draft-shacham-sipping-session-mobility-00
o Discussed in 5.3.1 how MN receives To, From tags and Call-ID of
transferred session, and in 5.3.2 how it receives these if it had
not previously held the session.
o Defined in subsection 5.1.4 the different media that may be
transferred, introducing the three types of messaging
o Specified in 5.2.2 and 5.3.2 how transfer flows apply to messaging
using SIP MESSAGE and MSRP
o Added subsection 5.4 on incoming call
o Added section 7 on hangup behavior
o Divided up subsection 8.1 to discuss disruption for transfer of
continuous media (8.1.1) and MSRP sessions (8.1.2)
o Added Section 9.3 about privacy concerns for output devices
11.2. Changes from draft-shacham-sipping-session-mobility-01
o Clarified in Section 5.2 how media lines must be arranged in a re-
INVITE to the CN in MNC mode.
o Discussed in the Security Considerations section how security
settings are maintained after session transfer.
o Added Section 7 on Simultaneous Session Transfer.
11.3. Changes from draft-shacham-sipping-session-mobility-02
o Discussed the use of the "key-mgmt" attribute in the Security
Considerations section and made reference to the concern raised in
3pcc RFC about media security establishment.
o Removed sections on input and output devices in Security
Considerations.
o Removed section on performance.
12. References
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12.1. Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Sparks, R., Handley, A., 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)", RFC 3725, April 2004.
[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
the 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., Mahy, R., and C. Jennings, "The Message Session
Relay Protocol, IETF Internet Draft (Work in Progress)",
October 2006.
[8] Jennings, C. and R. Mahy, "Relay Extensions for the Message
Session Relay Protocol, IETF Internet Draft (Work in
Progress)", July 2006.
[9] Hellstrom, G., "RTP Payload for Text Conversation", RFC 2793,
May 2000.
[10] Rosenberg, J., Schulzrinne, H., and P. Kyzivat, "Indicating
User Agent Capabilities in the Session Initiation Protocol
(SIP)", RFC 3840, August 2004.
[11] Rosenberg, J., Schulzrinne, H., and R. Mahy, "An INVITE-
Initiated Dialog Event Package for the Session Initiation
Protocol (SIP)", RFC 4235, November 2005.
[12] Camarillo, G., "Internet Assigned Number Authority (IANA)
Registration of the Message Media Feature Tag Session
Initiation Protocol (SIP)", RFC 4569, July 2006.
[13] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
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[14] Andreasen, F., Baugher, M., and D. Wing, "Session Description
Protocol (SDP) Security Descriptions for Media Streams",
RFC 4568, July 2006.
[15] Arkko, J., Lindhold, 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.
12.2. Informative References
[16] "3GPP: TS 23.228: IP Multimedia Subsystem (IMS) (Stage 2),
Release 5", September 2002.
[17] Gutman, 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., Sullivan, G., Wenger, S., and R. Even, "RTP Payload
Format for the 1998 Version of ITU-T Rec. H.263 Video (H.263+),
IETF Internet Draft (Work in progress)", September 2005.
[20] Peterson, J., "A Presence-based GEOPRIV Location Object
Format", RFC 4119, December 2005.
[21] "Global Positioning System Standard Positioning Service
Specification, 2nd Edition", June 1995.
[22] Schulzrinne, H. and E. Wedlund, "Application-Layer Mobility
Using SIP, ACM Mobile Computing and Communications Review,
Vol.4, No.3", July 2000.
[23] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
[24] Peterson, J., "A Presence Architecture for the Distribution of
GEOPRIV Location Objects", RFC 4079, July 2005.
[25] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C., and
D. Gurle, "Session Initiation Protocol (SIP) Extension for
Instant Messaging", RFC 3428, December 2002.
[26] Lennox, J., Wu, X., and H. Schulzrinne, "Call Processing
Language (CPL): A Language for User Control of Internet
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Telephony Services", RFC 3880, October 2004.
[27] Berger, S., Schulzrinne, H., Sidiroglou, S., and X. Wu,
"Ubiquitous Computing Using SIP", October 2004.
Authors' Addresses
Ron Shacham
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
USA
Email: rs2194@cs.columbia.edu
Henning Schulzrinne
Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027
USA
Email: hgs@cs.columbia.edu
Srisakul Thakolsri
DoCoMo Communications Laboratories Europe
Landsberger Str. 312
Munich 80687
Germany
Email: thakolsri@docomolab-euro.com
Wolfgang Kellerer
DoCoMo Communications Laboratories Europe
Landsberger Str. 312
Munich 80687
Germany
Email: kellerer@docomolab-euro.com
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