Network Working Group A. Knauf
Internet-Draft G. Hege
Intended status: Standards Track T C. Schmidt
Expires: December 30, 2010 HAW Hamburg
M. Waehlisch
link-lab & FU Berlin
June 28, 2010
A RELOAD Usage for Distributed Conference Control (DisCo)
draft-knauf-p2psip-disco-00
Abstract
This document defines a RELOAD Usage for Distributed Conference
Control (DisCo) with SIP. DisCo splits the semantic of identifier
and locator of a SIP conference URI using a new Kind data structure.
Conference members are enabled to select conference controllers based
on proximity awareness. DisCo proposes call delegation to balance
load at focus peers. The document addresses also aspects of security
and trust, as well as compatibility for conference unaware clients.
Status of this Memo
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This Internet-Draft will expire on December 30, 2010.
Copyright Notice
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview of DisCo . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Reference Scenario . . . . . . . . . . . . . . . . . . . . 5
3.2. Initiating a Distributed Conference . . . . . . . . . . . 6
3.3. Joining a Conference . . . . . . . . . . . . . . . . . . . 7
3.4. Conference State Synchronization . . . . . . . . . . . . . 8
3.5. Call delegation . . . . . . . . . . . . . . . . . . . . . 9
3.6. Resiliance . . . . . . . . . . . . . . . . . . . . . . . . 9
3.7. Topology Awareness . . . . . . . . . . . . . . . . . . . . 9
4. RELOAD Usage for Distributed Conference Control . . . . . . . 11
4.1. Kind Data Structure . . . . . . . . . . . . . . . . . . . 11
4.2. Determining Coordinates . . . . . . . . . . . . . . . . . 12
4.3. Conference Creation . . . . . . . . . . . . . . . . . . . 12
4.4. Proximity-aware Conference Participation . . . . . . . . . 14
4.5. Advertising Focus Ability . . . . . . . . . . . . . . . . 16
4.6. Resiliance in a Distributed Conference . . . . . . . . . . 17
5. Focus Call Control Operations . . . . . . . . . . . . . . . . 19
5.1. Becoming an active Focus . . . . . . . . . . . . . . . . . 19
5.2. Delegating Calls . . . . . . . . . . . . . . . . . . . . . 21
5.3. Synchronizing the Conference State . . . . . . . . . . . . 22
6. DISCO Kind Definition . . . . . . . . . . . . . . . . . . . . 24
7. Conference State Event Package Extension . . . . . . . . . . . 25
7.1. The <focus-states> and <focus> elements . . . . . . . . . 25
7.2. XML Schema . . . . . . . . . . . . . . . . . . . . . . . . 26
8. Configuration Document Extension . . . . . . . . . . . . . . . 29
8.1. The <landmark> and <Landmark-host> elements . . . . . . . 29
8.2. Relax NG Grammar . . . . . . . . . . . . . . . . . . . . . 29
9. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
10. Security Considerations . . . . . . . . . . . . . . . . . . . 31
10.1. Layered Security . . . . . . . . . . . . . . . . . . . . . 31
10.2. Trust Aspects . . . . . . . . . . . . . . . . . . . . . . 31
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
12.1. Normative References . . . . . . . . . . . . . . . . . . . 33
12.2. Informative References . . . . . . . . . . . . . . . . . . 33
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34
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1. Introduction
This document describes a RELOAD Usage for distributed conference
control (DisCo) in a tightly coupled model with SIP [RFC3261]. The
Usage provides self-organizing and scalable signaling that allows
RELOAD peers and plain SIP user agents to participate in a managed
P2P conference. DisCo defines the following functions:
o A protocol scheme for distributed conference control
o RELOAD Usage and definition of conferencing Kind
o Mechanisms for conference synchronization and call delegation
o Mechanisms for proximity-aware routing for conference participants
o XML extension for the event package for conference state
o A graduated trust delegation system
In this document, the term distributed conferencing refers to a
multiparty conversation in a tightly coupled model in which the point
of control (i.e., the focus) is identified by unique URI, but the
focus service is located at many independent entities. Multiple SIP
[RFC3261] user agents uniformly control and manage a multiparty
session. This document defines a new Usage for RELOAD including an
additional Kind code point with a corresponding data structure that
complies the demands for distributed conferences. The data structure
stores the mapping of a single conference to multiple conference
controllers and thereby separates the conference URI from focus
instantiations.
Delay and jitter are critical issues in multimedia communications.
The proposed conferencing scheme supports mechanisms to build an
optimized interconnecting graph between conference participants and
their responsible conference controllers. Conference members will be
enabled to select the closest focus with respect to delay or jitter.
DisCo extends conference control mechanisms to provide a consistent
and reliable conferencing environment. Controlling peers maintain a
consistent view of the entire conference state. The multiparty
system can be re-structured based on call delegation operations.
To provide secure mechanisms, which allows users to join or even
control a distributed conference, this document describes a graduated
trust delegation system. The proposed system guides implementors how
to maintain privacy and trust to other peers in a distributed
multiparty system.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The terminology and definitions from der RELOAD base
[I-D.ietf-p2psip-base], the peer-to-peer SIP concepts draft
[I-D.ietf-p2psip-concepts] and the terminology formed by the
framework for conferencing with SIP [RFC4353]. Additionally the
following terms are used:
Coordinate Value: An opaque string that describes a host's relative
position in the network topology.
Focus peer: A RELOAD peer that provides SIP conferencing functions
and implements the Usage for distributed conferencing. It can be
'active' if is already in signaling relations to conference
participants. Otherwise it is 'potential' if it is only
registered in a distributed conference data structure but not
maintaining signaling relations yet.
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3. Overview of DisCo
3.1. Reference Scenario
The reference scenario for the Distributed Conference Control (DisCo)
is shown in Figure 1. Peers are connected via a RELOAD
[I-D.ietf-p2psip-base] instance, in which peers A and B are managing
a single multiparty conference. The conference is identified by a
unique conference URI, but located at peers A and B fulfilling the
role of focus. The mapping of the conference URI to one or more
responsible focus peers is stored in a new RELOAD Resource for
distributed conferencing within a data structure denoted as DisCo-
Registration. The owner O of the distributed conference resource
holds this data.
The focus peers A and B maintain SIP signaling relations to
conference participants, which may have different conference protocol
capabilities. In this example, peer A is the multiparty manager for
the RELOAD peer C and the plain SIP user agent E whereas focus peer B
serves for RELOAD peer D and the RELOAD client F.
RELOAD peers and clients obtain the contact information for the
conference from the owner O. In contrast, the user agent E receives
the conference URI not by RELOAD mechanisms, but resolves the ID and
joins the conference by plain SIP negotiation.
Focus peers establish a SIP signaling relation among each other used
for notification messages that synchronize the conference focus
peers' knowledge about the entire conference state. Additionally,
focus peers can transfer calls to each other by a call delegation
mechanism.
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+----------+
|DisCo Data|
+----------+
/
+-----+
|Owner|
# # # # # # # # # # |Peer | # # # # # # # # # # #
# | O | #
# +-----+ #
# #
# #
# #
+----+ +----+
|Peer| \ RELOAD Instance |Peer|
| C | \ | D |
+----+ \ +----+
# SIP #
# \ #
# \ #
# +-------+ +-------+ #(
# | Focus | | Focus | # )
# # | Peer | # # # # # # # # # # # | Peer | # # (
| A | <===Conf.Events/====> | B | )
+-------+ Call delegation +-------+ Overlay
/ \ Comm.
/ \ (
SIP SIP )
/ \ (
/ \ )
+----------+ +--------+
|User Agent| | Client |
| E | | F |
+----------+ +--------+
Figure 1: Reference Scenario: Focus peers A,B maintain a distributed
conference
3.2. Initiating a Distributed Conference
To create a conference the initiating user agent announces itself as
a focus for the conference. It stores its own contact information
(Address-of-Record or Destination List) in the RELOAD overlay under
DisCo-Registration Kind (cf., Figure Figure 2). The hashed
conference URI is used as the Resource-ID. This data structure will
later contain the contact IDs of all potential focus peers including
optionally topological descriptors.
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3.3. Joining a Conference
A RELOAD-aware node (cf., Bob in Figure Figure 2) intending to join
an existing conference retrieves the list of potential focus peers
stored in the DisCo-Registration under the conference's Resource-ID.
To join the conference it selects any of the focus peers (e.g.,
Alice) and establishes a connection using AppAttach. This transport
is then used to send an INVITE to the conference applying the chosen
focus as the contact. The selection of the focus peer to contact can
optionally be based on proximity information if available.
A node that is not aware of RELOAD uses common SIP signaling to
retrieve the conference URI.
A conference member proposes as a focus for subsequent participants
by storing a mapping of the conference URI to his Address-of-Record
or Destination List in the RELOAD overlay using the conference
Resource-ID. This decision should incorporate bandwidth, power, and
other constraints, but details are beyond the scope of this document.
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Alice RELOAD Bob
(initiating peer) (joining peer)
--------------------------------------------------------------------
| | |
| Alice stores her mapping to register a conference |
| Store mapping(ConfURI, Alice) | |
|------------------------------>| |
| | Lookup ConfURI |
| |<------------------------------|
| | Result ConfURI |
| |------------------------------>|
| | |
| Bob establishes transport connection to Alice |
| AppAttach |
|<--------------------------------------------------------------|
| AppAttach |
|-------------------------------------------------------------->|
| INVITE |
|<~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
| OK |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~>|
| ACK |
|<~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
| Media |
|<=============================================================>|
| | |
| Bob stores his mapping to become a focus peer too |
| | Store mapping(ConfURI, Bob) |
| |<------------------------------|
| | |
Figure 2: DisCo Usage generic Call Flow
3.4. Conference State Synchronization
Each focus of a conference maintains signaling connections to its
related participants independently from other conference controllers.
This distributed conference design effects that the entire SIP
conference state is jointly held by all conference focus peers. In
DisCo, state synchronization is based on SIP specific event
notifications [RFC3265].
Each focus peer can complete its view of the entire conference state
among the focus peers by subscribing all other focus peers for an XML
event package for distributed conferences. This is defined in this
document and based on the event package for conference state
[RFC4575]. Receivers of event notifications update their local
conference state document to regain a valid view of current
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conference state.
The event notification package for distributed conferences enables
focus peers to synchronize the entire conference state. It is
designed as an extension to the XML event package for conference
state, which provides signaling and media parameters for each peer
participating in the multiparty session. The extension defines
additional XML elements and complex types (see Section 7 for more
details), which allow views of the responsibilities of any focus peer
in the conference. By providing these views each focus peer is
enabled to perform additional load balancing operations and enhances
the robustness against departures of focus peers.
3.5. Call delegation
The call delegation (seeSection 5.2.) is a feature used to transfer
an incoming participation request to another focus peer. It can be
applied to prevent an overloading of focus peers reaching its limit
of serving new clients. Call delegation is realized through SIP
REFER requests, which carry signaling and session description
information of the callee to be transferred. A focus peer can decide
to refer an incoming call to a less loaded remote focus. This
feature is achieved transparently to the transferred user agent by
using a source routing mechanism at SIP dialog establishment.
Descriptions of overload detection are beyond the scope of this
document.
3.6. Resiliance
A focus peer can decide to leave the conference or may ungracefully
fail. In a traditional conferencing scenario, a loss of the
conference controller or the media distributor would cause a complete
fail of the multiparty conversation. Distributed conferencing uses
the redundancy by multiple focus peers to reconfigure a running
multiparty. Participants that lost their entry point to the
conference re-invite itself via the remaining focus peers or will be
re-invited by the controllers. This option is based on the
conference state and call delegation functions.
3.7. Topology Awareness
DisCo supports landmarking approaches based on an extension for the
RELOAD XML configuration document (see Section 8) to construct
topology-aware connections between focus and peers. Each peer
intending to create or participate in a distributed conference SHOULD
determine a topological descriptor that describes its relative
position in the n-dimensional Cartesian space. Focus peers store
these coordinate values as additional data field in the DisCo-
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Registration data structure. This enables peers joining the
conference to select the closest focus with respect to its coordinate
values.
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4. RELOAD Usage for Distributed Conference Control
4.1. Kind Data Structure
Each DisCo-Registration data structure stores the mappings for one
conference to many focus peers and for each focus peer the related
coordinates value. The data structure uses the RELOAD dictionary
type whereas the DictionaryKey value is the Node-ID of the focus peer
behind the dictionary entry. This allows a focus peer to update it
mappings. The DisCo data structure of type DisCoRegistration is
shown as follows:
enum {
sip_focus_uri (1),
sip_focus_node_id (2), (255)
} DisCoRegistrationtType;
struct {
opaque coordinate<0..2^16-1>
select (DisCoRegistrationtType.type) {
case sip_focus_uri: opaque uri<0..2^16-1>
case sip_focus_node_id: Destination destination_list<0..2^16-1>
/* This type can be extended */
}
} DisCoRegistrationData;
struct {
DisCoRegistrationtType type;
uint16 length;
DisCoRegistrationData data;
} DisCoRegistration;
The content of the DisCoRegistrationData structure are as follows:
type
type of the registration
length
the length of the registration PDU
data
the conference registration data
o If the DisCoRegistration is set to "sip_focus_uri", then it
contains an Address-of-Record (AOR) as an opaque string and opaque
"coordinates" string, that describes the relative network
position. See more in section 4.4.
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o If registration type is set to "sip_focus_node" then it contains
the Destination list for the peer and an opaque string
"coordinates" describing the focus' relative network position.
The structure is designed for enabling a peer to contact a focus of
the conference that is the nearest to itself. A joining peer MUST
select the focus peer, which coordinate value matches at most (see
section Section 4.4) to its own. In this manner it reduces the
problem of triangle inequality as without this feature a joining peer
could choose an inadequate remote conference controller causing large
signaling and may streaming delays.
4.2. Determining Coordinates
Each RELOAD peer within the context of a distributed conference
SHOULD be aware of it's relative position in the network topology.
Those position information can support a topology-aware conference
construction avoiding long signaling and media delays. Providing
this the Usage for distributed conference foresees the coordinates
value within the DisCo-Registration data structure that allows focus
peers to store a topological descriptor. It is a generic field that
describes a peer's relative position in the network as an N value
long position vector in the N-dimensional Cartesian space. Focus
peers store this coordinate value together with their announcement as
conference focus. Joining peers likewise SHOULD determine their
coordinates value and then select a focus peer whose relative
position matches at most (see section Section 4.4).
Many algorithms determine topology information by measuring Round-
Trip Times (RTT) towards a set on hosts serving as so called
landmarks. To support such algorithms this document describes an
extension to the RELOAD XML configuration document that allows to
configure the set of Landmark hosts that peer must use for position
estimation (see section Section 8). Once a focus peer has registered
its mapping in the DisCo data structure, it also stores the according
coordinates in the same mapping. These <Node-ID,coordinates> vectors
are used by peers at conference join to select the focus peer that is
relatively closest to itself.
Because topology-awareness can be obtained by many differnt
approaches a concrete algorithms is out of scope of this document.
4.3. Conference Creation
Before a peer registers to a new distributed conference, it is
RECOMMENDED to ensure the initiating peer has a most up to date copy
of the configuration document. In this way, the conference creator
assures that all joining peers will equally determine their
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coordinates value if such a alogithm is used. The first peer that
creates a distributed conference registers it in the RELOAD overlay
following the steps as described in Figure 3:
Enroll.Serv Alice Peer1 Overlay PeerN StoringPeer
-------------------------------------------------------------------
| | StatReq Res:Conf-URI | |
| |---------->|--------->|--------->|--------->|
| | StatAns | | |
| |<----------|<---------|<---------|<---------|
|<==Cert===| | | | |
| | | | | |
|===Cert==>| StoreReq Res:Conf-URI Kinds:DisCo[,SIP] |
| |---------->|--------->|--------->|--------->|
| | StoreAns | | |
| |<----------|<---------|<---------|<---------|
| | | | | |
Figure 3: Creation of a Distributed Conference
1. The peer MUST determine its own coordinate value (if used).
2. The peer MUST probe whether the desired conference URI is
available. It therefore generates the Resource-ID of the
conference URI with the overlay hash function and sends a RELOAD
StatReq towards this address. By the corresponding StatAns
response the peer knows whether the desired URI is occupied by
another a DisCo Kind or even a SIP-Registration Kind
[I-D.ietf-p2psip-sip]. If it is, the user MUST choose another
URI and repeat the availability checks. If no other DisCo or
SIP-Registration Kind are stored at this Resource-ID it proceeds
the registration.
3. Storing a conference registration is like to register a new
virtual user that has the conference URI as its Address-of-
Record. Therefore, the conference initiator MUST request the
enrollment server for a new overlay certificate that contains the
conference URI as user name. A sample certificate is shown
below:
User name: conference@dht.example.com
Node-ID: 013456789abcdef
Serial: 0815
4. The peer finally registers the DisCo data structure signed with
the above certificate by a Store request towards the storing peer
(the owner of the address space for the Resource-ID of the
conference URI).
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The additional certificate is needed for 2 major purposes:
o It separates the conference creator from the multiparty instance.
o It ensures the conference initiator's privacy. Because the DisCo
data structure will be accessed by many peers using the same
conference certificate. If they were using the conference
creators' certificate, they were permitted to write non-shared
Resources of the creator.
The conference creator MAY registers the conference URI as SIP-
Registration Kind as well. In this case, it also MUST sign the Store
request with the private key that matches to the certificate obtained
for the conference URI. This is necessary because in the case of the
departure of the conference creator, the other focus peers are
permitted to redirect the mapping to another focus peer still serving
the conference. The SIP-Registration SHOULD be sent in the same
StoreReq as the DisCo registration.
The creator of a distributed conference MUST select on the access
models as described in section Section 10.1 to define the desired
privacy level of the multiparty conference.
TODO: a description how a new certificate is generated in the RELOAD
instance without enrollment server
4.4. Proximity-aware Conference Participation
A RELOAD peer intending to join a distributed conference follows the
steps showed in Figure 5 :
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Bob Peer1 Overlay PeerN OwnerOfID Alice
--------------------------------------------------------------
| FetchReq Res:Conf-URI Kind:DisCo | |
|--------->|--------->|--------->|--------->| |
| |FetchAns | | | |
|<---------|<---------|<---------|<---------| |
| | | | | |
| Bob calculates Alice as closest Focus | |
| | | | | |
| |AppAttach--application:5060 | |
|--------->|--------->|--------->|--------->|--------->|
| |AppAttach--application:5060 | |
|<---------|<---------|<---------|<---------|<---------|
| | | | | |
|<-------------------ICE Checks----------------------->|
| | | | | |
| | INVITE sip:Alice | |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~>|
| | 200 OK | | |
|<~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
| | ACK | | |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~>|
| | | | | |
| Optinally, Alice passes writing permission |
| | | | | |
| |INFO content:Cert{DisCo-Resource} |
|<~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
| | | | | |
Figure 5: Participation of a Distributed Conference
1. The joining peer MUST determine its own coordinate value (if
used).
2. The joining peer sends a FetchReq message for the DisCo Kind to
the Resource-ID that corresponds to the hash over the conference
URI using the overlays hash-function. The FetchReq SHOULD NOT
include any specific dictionary keys thus it will receive all
potential -and active focus peers of the conference.
3. Once the joining peer received the Fetch results, it calculates
which of the focus peers is the relatively closest to itself by
making the following calculation for each dictionary entry:
* For each coordinate entry, calculate the each difference Di =
Fi - Pi, with Fi is the coordinates vector of the Focus peer
and Pi the coordinates vector of the joining peer.
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* For ech Di, calculate the scalar product of Di
4. The focus with the smallest scalar product SHOULD be chosen for
establishing a SIP signaling relation.
Depending on which DisCo-Registration type the selected focus has
stored its mapping, the joining Peer has the following 2
possibilities:
1. If the DisCoRegistrationType is sip_focus_node_id, the joining
peer uses RELOADs AppAttach request to establish a direct
transport connection to the selected focus peer. The application
field of the request MUST be set to 5060 indicating for SIP.
This transport connection SHOULD be used to a form an ordinary
SIP dialog. Further media session establishment is achieved by
usual SIP mechanisms.
2. If the DisCoRegistrationType is sip_focus_uri, the joining peer
MUST use the SIP-Registration [I-D.ietf-p2psip-sip] Usage to
resolve the URI and form connectivity to the selected focus.
Note that in the second case a focus peer can have multiple locations
for its SIP-registration. Therefore a focus MUST assure that its
coordinate value corresponds to its current mapping AoR to location.
Regardless of how the focus peer has registered its mapping in the
overlay a joining peer MUST add it's coordinate value base64 encoded
as URI-parameter in the contact-header field of the SIP INVITE
request. An example contact URI is
"sip:alice@example.com;coord=YWxpY2VAZXhhbXBsZS5jb20=". The
additional parameter is used by the requested focus peer as it is not
capable of serving additional conference participants. It then it
MUST delegate the call (see section Section 5.2) to the focus peer
whose coordinate value matches next best to the coordinates of the
joining peer. The focus peer therefore uses the same calculation as
described in the joining process.
After the final SIP ACK request completes the signaling relation, a
conference focus MAY passes the writing permission to the new
participant. It therefore sends a SIP INFO request carrying the
certificate for the DisCo Resource. The decision whether to pass
writing permission depends on the selected security model for the
distributed conference as described in section Section 10.1.
4.5. Advertising Focus Ability
All participants of a distributed conference can become a focus peer
for their multiparty. The decision can depend on the capacities of
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the joining peer like sufficient processing power (CPU, Memory) for
the desired media type and quality of the network connectivity.
Additionally, a peer intending to become focus of a conference SHOULD
NOT be located behind NAT or its IP SHOULD NOT belong to the private
address range. The information whether a participant is behind NAT
can be obtained by ICE connectivity checks during the conference
joining process.
If a participant is a candidate to become a focus of the conference
it stores its mapping (Destination List or AoR) and coordinate value
into the DisCo data structure. Because the DisCo Kind uses the USER-
MATCH access control policy, the shared certificate passed by the
participant's focus peer is sufficient to permit this peer to write
the DisCo Resource. By storing the mapping into the data structure a
participant becomes a potential focus.
TODO: What to do if the set of Landmark hosts changes during
conference?
4.6. Resiliance in a Distributed Conference
The decentralized character of distributed conferences provide
abilities to prevent the breakdown of the entire multiparty session
in the case that a focus peer disappears. Two possibilities of a
focus departure must be distinguished:
Friendly leave: A user whose peer is acting as conference focus
decides to quit coonference participation.
Unexpected leave: Any case in which a peer serving as conference
focus fails.
In the friendly case the leaving peer (lp) MUST accomplish the
following procedure:
o Lp deletes its mapping in the DisCo data structure by storing the
"nonexisting" value as described in the RELOAD base document
[I-D.ietf-p2psip-base].
o Lp searches the conference state XML document (see section
Section 7) for 'active' focus peers that have free capacities to
serve further participants. Additionally, it fetches the lastest
DisCo data structure for this conference to obtain all 'potential'
focus peers.
o The lp then calculates for all its related participants the
closest focus peer using the algorithm described in Section 4.2.
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o Based on the results from the previews step lp transfers all it's
participants to their ascertained focus peers using the call
delegation described in Section 5.2
If an unexpected leave is detected by a participant (e.g. missing
signaling and/or media packets) it MUST repeat the joining procedure
as described in Section 4.4.
Assuming unfavorably circumstances it can happen that the available
capacities over all potential and active focus peers are insufficient
to reassemble all lost participants. In this case it RECOMMENDED to
reassemble as many participants as possible in a first come first
serve algorithm and to fail the rest.
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5. Focus Call Control Operations
This section describes SIP call flows for third party call control
for distributed conferences. Those operation comprise the call
delegation and state synchronization mechanisms.
5.1. Becoming an active Focus
A conference participant that stored its mapping to the distributed
DisCo data structure serves as potential focus for further
participation requests by other peers. On incomming participation
request a potential focus becomes an active focus and is then
responsible to grant the joining peer access to the conference. Two
differnt scenarios for participaton requests must be distinguished:
o A joining peer requests the potential focus
o An already active focus peer transfers a participation request to
the potential focus
The second case will be dicussed as part of the call delegation in
Section 5.2.
For the case that a RELOAD peer directly requests a potential focus
for participation the call flow in Figure 6 describes the necessary
procedure. The joing peer (JP) has already established a transport
connection and sends a SIP INVITE request (1) to the contact address
(IP) to its selected potential focus (PF). Note that JP is thereby
unaware that PF does not serve any other participants yet. PF is
participating the conference through its own active focus (AF) and is
aware of the offered media types for the multiparty session. This PF
offers the available media parameter to JP in (2). After finalizing
the signaling in (3) and establishment of the media streams the PF in
charge to synchronize the distributed conference state. As the first
step of the pairwise subscrption (4) PF MUST send a SIP SUBSCRIBE
[RFC3265] request to the AF that is the focus peer respoonsible for
signaling with PF. It subscribes for the conference for the event
package for conference state[RFC4575] with 'multi-focus' extension
Section 7. This document therefore defines a new content type
"application/distributed-conference-info+xml" for a MIME entity that
contains conference state information for distributed conferences.
After confirming the subscription (5) AF informs PF about the entire
conference state by sending a 'full' XML document. It includes the
list of all participants, active focus peers and used media types.
In the second step for in the subscription procudure (8) AF MUST
subscribe PF for distributed conference. After confirmation in (9),
PF MUST inform AF about the arrival of JP and also MUST advertise its
own capacities. Pf therefore sends a NOTIFY request containing a
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'partial' conference state XML document that describes PF's local
state (e.g. capabilities, responsibilities for JP). This step
finalizes the promotion of PF to an active focus peer.
JoiningPeer PotentialFocus ActiveFocus
--------------------------------------------------------------------
| (1) INVITE pot.Focus | |
|------------------------> | |
| (2) 200 OK | |
|<--------------------------| |
| (3) ACK | |
|-------------------------->| |
| MEDIA | |
|<=========================>| |
| | |
| Pot.Focus requests conference state XML |
| | |
| | (4) SUBSCRIBE-event:disco |
| |---------------------------->|
| | (5) 200 OK |
| |<----------------------------|
| | (6) NOTFY conf.state XML |
| |<----------------------------|
| | (7) 200 OK |
| |---------------------------->|
| | |
| Active Focus completes pairwise subscription |
| | |
| | (8) SUBSCRIBE event.disco |
| |<----------------------------|
| | (9) 200 OK |
| |---------------------------->|
| | |
| Pot.Focus notifies about the new member JP |
| | |
| | (10) NOTIFY conf.state XML |
| |---------------------------->|
| | (11) 200 OK |
| |<----------------------------|
Figure 6: Pairwise subscription for conference state synchronization
Depending on whether AF has more subscriptions for the distributed
conference event package it will synchronize all other focus peers
sending notifications containing partial conference state XML
document received from PF.
Since PF is aware of the entire conference and MAY establishes more
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subscriptions to other active focus peers. This can reduce signaling
delays and could serve as guideline for new routes for the media
streams. A specification of how those new subscription should be
done is a TODO in this document.
5.2. Delegating Calls
The call delegation feature described in this document provides focus
peers the possibility to transfer an incoming participation request
to another focus peer. A focus peer SHOULD delegate incoming
participation requests if the number of participants it currently
maintains is equal to the 'max_participants' value the focus
advertised in the distributed conference XML document.
A sample scenario for call delegation in shown in Figure 7. A
joining peer (JP) requests the active focus (AF) for conference
participation (1). AF has no more capacity to serve JP as focus peer
and has to transfer the call. AF firstly temporally accepts the call
(2-3) and then selects an adequate focus peer for call delegation
based on JP's coordinate value (sent base64 encoded as URI parameter
in (1) see section Section 4.4). AF fetches the lastest DisCo data
structure (not shown in Figure 7) to obain all available potential
and active focus peers. As shown in the exmaple, AF determines the
potential focus (PF) as best candidate to become JP's focus and
transfers the participation request to PF sending a SIP REFER request
(4). The REFER request MUST contain the session identifier from JP
as payload in the request body and the call-ID of (1) as parameter in
the URI of the refer-to header field, e.g., 'Refer-To:
<sip:bob@dht.example;call-id=1234>'
Triggered by the REFER request PF is in charge to become JP's focus
peer and to enter the conference state sychronization process.
Because the call delegation operation should not interrupt JP's
participation request PF MUST use the signaling and session
information of (4). PF sends a re-INVITE request to JP that appears
as it were orginated by AF with an additional Record-Route header
field set to PF's contact address. By using this technique a
distributed conference appeares as one single entity. The additional
Record-Route header thereby ensures that further SIP signaling will
be routed to PF. After the signaling process the negotiated media
session can be established.
PF then is in charge to enter the conference state synchronization
mechanism by pairwise subscribting PF->AF, AF-->PF for the event
package for conference with multi-focus extension (9-14) as described
Section 5.1. Note that PF could subscribe another active focus peer
than AF since this is not necessarily the conference controller
responsible for PF.
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JoiningPeer ActiveFocus PotentialFocus
--------------------------------------------------------------------
| (1) INVITE act.Focus | |
|------------------------->| |
| (2) 200 OK | |
|<-------------------------| |
| (3) ACK | |
|------------------------->| |
| | |
| AF reached its threshold for serving new calls |
| | |
| | (4) REFER refer-to:JP |
| |-------------------------->|
| | (5) 200 OK |
| |<--------------------------|
| | |
| PF re-invites JP using AF's contact |
| | |
| (6) INVITE JP record-route:potentialFocus |
|<-----------------------------------------------------|
| (7) 200 OK | |
|----------------------------------------------------->|
| (8) ACK | |
|<-----------------------------------------------------|
| MEDIA |
|<====================================================>|
| | (9) SUBSCRIBE event:disco |
| |<------------------------- |
| | (10) 200 OK |
| |-------------------------> |
| | (11) NOTIFY conf.state XML|
| |-------------------------> |
| | (12) SUBSCRIBE event:disco|
| |-------------------------->|
| | (13) 200 OK |
| |<--------------------------|
| | (14) NOTIFY conf.state XML|
| |<--------------------------|
Figure 7: Call delegation to potential focus peer
5.3. Synchronizing the Conference State
The entire state of the distributed conference changes partly on
every event that happens at an active focus peer. Most commenly
those events refer to joins or lefts of conference participants. In
order to maintain a coherent conference state all focus peers MUST
send NOTIFY messages [RFC3265] to all subscribers (other focus peers)
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to synchronize the conference state. The payload of the
notifications MUST only contain a partial state information about the
changes in the state from the previews conference state.
If the connection graph build by the pairwise subscriptions between
the focus peers is not structured in a full mesh topology, state
notifations MUST be forwarded by intermediate focus peers. The
extension for the event package described in Section 7 therefore
provides an XML element that allows every focus to reconstruct the
connection graph among the focus peers.
If a state notification is received multiple times a focus peer MUST
NOT forward the dublicate state information for prevent loops.
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6. DISCO Kind Definition
This section formally defines the DisCo kind.
Name
DISCO-REGISTRATION
Kind IDs
The Resource name DISCO-REGISTRATION Kind-ID is the AOR of the
conference. The data stored is the DisCoRegistrationData, that
contains a coordinates value describing a peers relative network
position acting as focus for the conference. Additionally it
contains either the peers URI or a Destination list.
Data Model
The data model for the DISCO-REGISTRATION Kind-ID is dictionary.
The dictionary key is
the Node-ID of the peer action as focus.
Access Control
USER-MATCH
The data stored for the Kind-ID DISCO-REGISTRATION is of type
DisCoRegistration. It contains a "coordinates" value, that
describes the peers relative network position and
XOR one of the two following data:
sip_focus_uri
the URI of the peer action as focus
sip_focus_node_id
the Destination list of the peer acting as focus
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7. Conference State Event Package Extension
This section presents the XML extension for the event package for
conference state that enables a focus peer to have a view on the
responsibilities of each other focus peer. The additional
information by exentending the XML schema defined RFC4575 [RFC4575]
is can be used in the case of a focus departure and call delegations.
The new <focus-states> element as shown in Figure 8 is placed as
child-of the root-element <conference-info>. It's child element
<focus> represents every conference participant that is in the role
of an active focus peer to the conference.
conference-info
|
|-- conference-description
|
|-- host-info
|
|-- users
|
|-- focus-states
| |
| |-- focus
|
..
|-- ..
7.1. The <focus-states> and <focus> elements
The <focus-states> element serves as container of the <focus> sub-
elements, each describing the responsibilities of a conference
participant acting as focus peer.
The <focus> uses the following attributes:
entity: This attribute contains the AoR of the focus peer that is
declared in the user name field in the RELOAD certificate. This
AoR MUST correspond to the entity attribute defined in the focus
peer's <user> element in the base conference event package. A
user that whishes to lookup a focus peer's signaling information
can retrieve it by looking at the corresponding <user> element
with the same AoR in the entity attribute.
state: This attribute indicates whether the transmitted conference
state for this focus peer is 'full', 'partial' or 'deleted' and
have to be interpreted as defined in [RFC4575].
The <focus> element uses the complex focus-type that contains the
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following child-elements:
<focus-capacity> : This element describes a focus peer's maximal
number of participants it can serve respectively the maximal
number media connections to other focus peers it can handle.
<participant> : Each participant element describes a conference
member that has this focus peer as it's conference controller. It
uses the 'entity' attribute that contains the AoR that this RELOAD
peer uses as user name in the overlay certificate. It corresponds
to the AoR in the <user> element in the base conference event XML
document. Additionally, the <participants> element uses the
'state' attribute to provide the partial notification mechanism as
defined in [RFC4575].
<graph> : Each <graph> element describes a conference state
synchronization relation this focus peer maintains. By reference
to this element each conference controller has a view of the
entire synchronization topology over the focus peers. It uses the
'state' attribute as well.
7.2. XML Schema
The schema for XML extension is:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema
targetNamespace="http://www.example.org/inet-ci-multifocus-ext"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns="http://www.example.org/inet-ci-multifocus-ext"
elementFormDefault="qualified" >
<!--
FOCUS STATES ELEMENT
-->
<xs:element name="focus-states" type="focus-states-type"/>
<xs:complexType name="focus-states-type">
<xs:sequence>
<!--
FOCUS ELEMENT
-->
<xs:element name="focus" type="focus-type"
maxOccurs="unbounded" minOccurs="0"/>
<xs:any namespace="##other" processContents="lax"/>
</xs:sequence>
<xs:attribute name="state" type="state-type"/>
<xs:anyAttribute namespace="##other" processContents="lax"/>
</xs:complexType>
<!--
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FOCUS TYPE
-->
<xs:complexType name="focus-type">
<xs:sequence>
<xs:element name="focus-capacity"
type="focus-capacity-type" maxOccurs="1" minOccurs="0"/>
<xs:element name="participant" type="participant-type"
maxOccurs="unbounded" minOccurs="0"/>
<xs:element name="graph" type="graph-type"
maxOccurs="unbounded" minOccurs="0"/>
<xs:any namespace="##other" processContents="lax"/>
</xs:sequence>
<xs:attribute name="entity" type="xs:anyURI"/>
<xs:attribute name="state" type="state-type"/>
<xs:anyAttribute namespace="##other" processContents="lax"/>
</xs:complexType>
<!--
FOCUS-CAPACITY TYPE
-->
<xs:complexType name="focus-capacity-type">
<xs:sequence>
<xs:element name="max-participants" type="xs:int"
maxOccurs="1" minOccurs="0"/>
<xs:element name="max-focus-references" type="xs:int"
maxOccurs="1" minOccurs="0"/>
<xs:any namespace="##other" processContents="lax"/>
</xs:sequence>
<xs:attribute name="state" type="state-type"/>
<xs:anyAttribute namespace="##other" processContents="lax"/>
</xs:complexType>
<!--
PARTICIPANT TYPE
-->
<xs:complexType name="participant-type">
<xs:sequence>
<xs:any namespace="##other" processContents="lax"/>
</xs:sequence>
<xs:attribute name="entity" type="xs:anyURI"/>
<xs:attribute name="state" type="state-type"/>
<xs:anyAttribute namespace="##other" processContents="lax"/>
</xs:complexType>
<!--
GRAPH TYPE
-->
<xs:complexType name="graph-type">
<xs:sequence>
<xs:element name="ref-to-focus" type="xs:anyURI"
maxOccurs="1" minOccurs="0"/>
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<xs:any namespace="##other" processContents="lax"/>
</xs:sequence>
<xs:attribute name="state" type="state-type"/>
<xs:anyAttribute namespace="##other" processContents="lax"/>
</xs:complexType>
</xs:schema>
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8. Configuration Document Extension
This section defines an additional parameter for the <configuration>
element that extends the RELOAD XML configuration document. The
proposed <landmarks> element allows RELOAD provider to publish a set
of accessible and reliable hosts that SHOULD be used if RELOAD peers
use landmarking algorithms to determine relative position in the
network topology.
8.1. The <landmark> and <Landmark-host> elements
The <landmarks> element serves as container of the <landmark-host>
sub-elements each representing a single host that serves a landmark.
The <landmark-host> uses the following attributes:
address: The IP address (IPv4 or IPv6) of the landmark host.
port: The port on which the landmark host responses for distance
estimation.
More than one landmark hosts SHOULD be present in the configuration
document.
8.2. Relax NG Grammar
The grammar for the Landmark configuration document extension is:
<!--
LANDMARKS ELEMENT
-->
parameter &= element landmarks {
attribute version { xsd:int }
<!--
LANDMARK-HOST ELEMENT
-->
element landmark-host {
attribute address { xsd:string },
attribute port { xsd:int }
}*
}?
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9. Example
TODO: Call flow examples for joining, delegating
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10. Security Considerations
10.1. Layered Security
TODO: An ad hoc conference can be set up to a layered security model.
Three models: open access, focus authenticate, closed access model.
10.2. Trust Aspects
TODO: Describing the privacy level for a conference instance; define
whether a joining user is allowed to become a member or even focus of
a conference.
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11. IANA Considerations
TODO: register Kind-ID code point at the IANA
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12. References
12.1. Normative References
[I-D.ietf-p2psip-base]
Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
H. Schulzrinne, "REsource LOcation And Discovery (RELOAD)
Base Protocol", draft-ietf-p2psip-base-08 (work in
progress), March 2010.
[I-D.ietf-p2psip-sip]
Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
H. Schulzrinne, "A SIP Usage for RELOAD",
draft-ietf-p2psip-sip-04 (work in progress), March 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] 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.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, "A Session
Initiation Protocol (SIP) Event Package for Conference
State", RFC 4575, August 2006.
12.2. Informative References
[I-D.ietf-p2psip-concepts]
Bryan, D., Matthews, P., Shim, E., Willis, D., and S.
Dawkins, "Concepts and Terminology for Peer to Peer SIP",
draft-ietf-p2psip-concepts-03 (work in progress),
July 2008.
[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353,
February 2006.
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Authors' Addresses
Alexander Knauf
HAW Hamburg
Berliner Tor 7
Hamburg
Germany
Phone: +4940428758067
Email: alexander.knauf@haw-hamburg.de
URI: http://inet.cpt.haw-hamburg.de/members/knauf
Gabriel Hege
HAW Hamburg
Berliner Tor 7
Hamburg D-20099
Germany
Phone: +4940428758067
Email: hege@fhtw-berlin.de
URI: http://inet.cpt.haw-hamburg.de/members/hege
Thomas C. Schmidt
HAW Hamburg
Berliner Tor 7
Hamburg 20099
Germany
Email: schmidt@informatik.haw-hamburg.de
URI: http://inet.cpt.haw-hamburg.de/members/schmidt
Matthias Waehlisch
link-lab & FU Berlin
Hoenower Str. 35
Berlin D-10318
Germany
Email: mw@link-lab.net
URI: http://www.inf.fu-berlin.de/~waehl
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