Network Working Group A. Knauf
Internet-Draft G. Hege
Intended status: Standards Track T C. Schmidt
Expires: July 3, 2011 HAW Hamburg
M. Waehlisch
link-lab & FU Berlin
December 30, 2010
A RELOAD Usage for Distributed Conference Control (DisCo)
draft-knauf-p2psip-disco-01
Abstract
This document defines a RELOAD Usage for Distributed Conference
Control (DisCo) with SIP. DisCo preserves conference addressing
through a single SIP URI by splitting its semantic of identifier and
locator using a new Kind data structure. Conference members are
enabled to select conference controllers based on proximity awareness
and to recover from failures of individual resource instances. DisCo
proposes call delegation to balance the load at focus peers. The
document also introduces methods to establish security and trust for
a shared resource access, as well as for compatibility with
conference unaware clients.
Status of this Memo
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This Internet-Draft will expire on July 3, 2011.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview of DisCo . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Reference Scenario . . . . . . . . . . . . . . . . . . . . 7
3.2. Initiating a Distributed Conference . . . . . . . . . . . 8
3.3. Joining a Conference . . . . . . . . . . . . . . . . . . . 9
3.4. Conference State Synchronization . . . . . . . . . . . . . 10
3.5. Call delegation . . . . . . . . . . . . . . . . . . . . . 11
3.6. Resilience . . . . . . . . . . . . . . . . . . . . . . . . 11
3.7. Topology Awareness . . . . . . . . . . . . . . . . . . . . 11
4. RELOAD Usage for Distributed Conference Control . . . . . . . 12
4.1. Kind Data Structure . . . . . . . . . . . . . . . . . . . 12
4.2. Determining Coordinates . . . . . . . . . . . . . . . . . 13
4.3. Conference Creation . . . . . . . . . . . . . . . . . . . 14
4.4. Obtaining a Conference Certificate . . . . . . . . . . . . 15
4.4.1. Self-generated . . . . . . . . . . . . . . . . . . . . 16
4.4.2. Using Enrollment Server . . . . . . . . . . . . . . . 16
4.5. Proximity-aware Conference Participation . . . . . . . . . 17
4.6. Advertising Focus Ability . . . . . . . . . . . . . . . . 19
4.7. Configuration Document Extension . . . . . . . . . . . . . 19
5. Conference State Synchronization . . . . . . . . . . . . . . . 20
5.1. Event Package Overview . . . . . . . . . . . . . . . . . . 20
5.2. <distributed-conference> . . . . . . . . . . . . . . . . . 22
5.3. <version-vector>/<version> . . . . . . . . . . . . . . . . 22
5.4. <conference-description> . . . . . . . . . . . . . . . . . 23
5.5. <focus> . . . . . . . . . . . . . . . . . . . . . . . . . 24
5.5.1. <focus-state> . . . . . . . . . . . . . . . . . . . . 25
5.5.2. <users>/<user> . . . . . . . . . . . . . . . . . . . . 25
5.5.3. <relations>/<relation> . . . . . . . . . . . . . . . . 26
5.6. Distribution of Change Events . . . . . . . . . . . . . . 26
5.7. Translation to Conference-Info Event Package . . . . . . . 27
5.7.1. <conference-info> . . . . . . . . . . . . . . . . . . 28
5.7.2. <conference-description> . . . . . . . . . . . . . . . 28
5.7.3. <host-info> . . . . . . . . . . . . . . . . . . . . . 28
5.7.4. <conference-state> . . . . . . . . . . . . . . . . . . 29
5.7.5. <users>/<user> . . . . . . . . . . . . . . . . . . . . 29
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5.7.6. <sidebars-by-ref>/<sidebars-by-value> . . . . . . . . 29
6. Distributed Conference Control with SIP . . . . . . . . . . . 30
6.1. Call delegation . . . . . . . . . . . . . . . . . . . . . 30
6.2. Conference Access . . . . . . . . . . . . . . . . . . . . 31
6.3. Media Negotiation and Distribution . . . . . . . . . . . . 32
6.3.1. Offer/Answer . . . . . . . . . . . . . . . . . . . . . 32
6.4. Restructuring a Conference . . . . . . . . . . . . . . . . 33
6.4.1. On Graceful Leave . . . . . . . . . . . . . . . . . . 33
6.4.2. On Unexpected Leave . . . . . . . . . . . . . . . . . 34
7. DisCo Kind Definition . . . . . . . . . . . . . . . . . . . . 35
8. Access Control Policy USER-CHAIN-MATCH . . . . . . . . . . . . 36
9. XML Schema . . . . . . . . . . . . . . . . . . . . . . . . . . 37
10. Relax NG Grammar . . . . . . . . . . . . . . . . . . . . . . . 41
11. Security Considerations . . . . . . . . . . . . . . . . . . . 42
11.1. Trust Aspects . . . . . . . . . . . . . . . . . . . . . . 42
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 43
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 44
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 45
14.1. Normative References . . . . . . . . . . . . . . . . . . . 45
14.2. Informative References . . . . . . . . . . . . . . . . . . 45
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 47
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 48
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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, clients 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 RELOAD Usage and definition of a Kind for shared Resources in
RELOAD
o Mechanisms for conference synchronization and call delegation
o Mechanism for proximity-aware routing within a conference
o An XML event package for distributed conferences
o Methods of establishing security and trust for shared resource
access
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 a 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 'DisCo' data
structure stores the mapping of a single conference to multiple
conference controllers and thereby separates the conference URI from
focus instantiations.
Authorized controllers of a conference are permitted to register
their mapping into the DisCo data structure independently. Thus, the
DisCo data structure represents a co-managed Resource in RELOAD. To
provide trusted and secure access to a co-managed Resource, this
document defines a Usage to share a specific RELOAD Resource. A
RELOAD user explicitly authorizes RELOAD peers within a RELOAD Kind
data structure called 'access list'.
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
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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.
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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 is
called 'active' if already involved in signaling relation to
conference participants. Otherwise, if only registered in a
distributed conference data structure, it is referred to as a
'potential' focus peer.
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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 the 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 storing peer SP 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 storing peer. 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|
+----------+
/
+-------+
|Storing|
# # # # # # # # # # | 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 as a
DisCo-Registration Kind (cf., 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 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 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 in the DisCo-Registration
data structure. 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 a SIP specific event
notifications mechanism [RFC3265].
Each focus peer can complete its view of the entire conference state
among the focus peers by subscribing the other focus peers for an XML
event package for distributed conferences. It is defined in this
document (see section Section 5) and is based on the event package
for conference state [RFC4575]. Receivers of event notifications
update their local conference state document to regain a valid view
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of current conference state.
The event notification package for distributed conferences enables
focus peers to synchronize the entire conference state. The event
package defines additional XML elements and complex types (see
Section 9 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 (see Section 4.2.) 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. Resilience
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 4.7) to construct
topology-aware connections between focus and peers. Each peer
intending to create or participate in a distributed conference MAY
determine a topological descriptor that describes its relative
position in the network. Focus peers store these coordinate values
as additional data field in the DisCo-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 mappings for one
conference instance 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 mapping. 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
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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.
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 MAY
select the focus peer, whose coordinate value matches at most 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 MAY
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. Focus peers
store this coordinate value together with their announcement as
conference focus. Joining peers likewise MAY determine their
coordinates value and then select a focus peer whose relative
position matches at most (see section Section 4.5).
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 4.7). 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 different
approaches a concrete algorithms is out of scope of this document.
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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
coordinates value if such a algorithm is used. The first peer that
creates a distributed conference registers it in the RELOAD overlay
following the steps as described in Figure 3:
CA 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 MAY determine its own coordinate value (if used).
2. The peer SHOULD 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 similar to registering a new
virtual user that has the conference URI as its Address-of-
Record. Two options for obtaining a conference certificate are
available as detailed in section Section 4.4. The first does not
require contacting the enrollment server, but only allows a set
of conference URIs closely related to the registering peer's user
name. The second option allows any form of conference URI but
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needs to contact the enrollment server. In both cases the
resulting certificate contains the conference URI as a user name.
4. The peer finally registers the DisCo data structure signed with
the private key corresponding to 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).
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 private key. If they were using the conference
creators' key, they were permitted to write non-shared Resources
of the creator.
When the conference creator has obtained the conference certificate
from the enrollment server, it MAY also register the conference URI
as a SIP-Registration Kind as well. In this case, it also MUST sign
the Store request with the private key that matches the certificate
obtained for the conference URI, since the SIP-Registration Kind uses
the USER-NODE-MATCH policy. In case of the conference creator's
departure, the remaining focus peers are permitted to redirect the
SIP mapping to another focus peer still serving the conference. The
SIP-Registration MAY be sent in the same StoreReq as the DisCo
registration.
4.4. Obtaining a Conference Certificate
In RELOAD each node uses a certificate to identify itself when
storing data at a specific location. Since the DisCo-Registration
needs to be written by multiple nodes, the private key of a group
certificate is distributed among all authorized focus peers. This
demands the assignment of a certificate for each conference ID which
is used for conferencing matters only. This document describes two
options for obtaining a conference certificate. The method to be
used depends on the desired pattern of the conference ID being
registered.
In both cases the certificate should have a sufficiently short
lifetime to prevent a compromised certificate from being used
continuously. The chance of a conference key leaking and thus being
compromised is much larger than for normal RELOAD certificates, since
the key is shared among the focus peers of a conference.
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4.4.1. Self-generated
A conference initiator can only register a conference ID that is
derived from the user name defined in it's own certificate, without
contacting an enrollment server. The conference ID must be closely
related to the user name of the creator to prevent malicious peers
not associated with the conference from generating a certificate for
the same name and registering themselves as a focus.
The building scheme for constructing legitimate conference URIs MUST
be specified in the overlay configuration document using a simple
pattern matching scheme.
Example:
URI pattern: *-conf-$USER@$DOMAIN
User Name: alice@example.com
XYZ-conf-alice@example.com is allowed
my-pretty-conf-alice@example.com is allowed
alice-conference@example.com is NOT allowed
The conference initiator generates a new public/private key pair and
signs the public key with his own private key, thus generating a
certificate for the conference. The conference certificate contains
the conference URI in the user name field, which MUST comply with the
URI pattern specified for DisCo-conferences in the configuration
document. Any peer validating the certificate MUST traverse the
certificate chain up to the enrollment server and only accept it if
the certificates of the creator and of the enrollment server are
valid. Additionally the user name in the certificate of the creator
MUST match the user name in the conference certificate using the
specified pattern.
This conference certificate is subsequently used to sign all entries
of the DisCo-Registration kind stored in the overlay. It MUST be
accepted by the responsible peer. This results in an extension of
the common USER-MATCH access control policy to USER-CHAIN-MATCH,
specified in Section 8.
4.4.2. Using Enrollment Server
An enrollment server MAY issue certificates for conferences with any
URI that is not related to the user name of the conference initiator.
Therefore a user agent intending to register a new conference
contacts the enrollment server and requests a certificate using the
standard mechanism defined in RELOAD [I-D.ietf-p2psip-base] Section
10.3.
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Conferences with certificates obtained from an enrollment server use
the USER-MATCH access control policy for the DisCo-Registration kind.
The enrollment server MUST NOT reuse the Node-ID of the conference
initiator for the conference certificate, since this certificate is
intended for distribution among the focus peers of a conference.
This would allow the focus peers to compromise any private resources
stored by the initiator using the NODE-MATCH policy.
4.5. Proximity-aware Conference Participation
A RELOAD peer intending to join a distributed conference follows the
steps shown in Figure 4:
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:Conf-Key{DisCo-Resource} |
|<~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
| | | | | |
Figure 4: Participation of a Distributed Conference
1. The joining peer MAY determine its own coordinate value (if
used).
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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.
4. The focus with the smallest distance MAY 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.
If a focus peer has not determined its relative network position, the
coordinates field MUST be left empty.
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 4.2) 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
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participant. It therefore sends a SIP INFO request carrying the
conference private key 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 6.2.
4.6. Advertising Focus Ability
All participants of a distributed conference can become a focus peer
for their conference. The decision can depend on the capacities of
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. Nevertheless, under special circumstances it might
be reasonable to locate a focus peer behind a NAT. For instance
within a companies network infrastructure.
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 either
the USER-MATCH or the USER-CHAIN-MATCH access control policy, the
shared 'private' key 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.
4.7. 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.
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.
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5. Conference State Synchronization
The global knowledge about signaling and media relations among the
conference participants and focus peers defines the global state of a
distributed conference. It is composed of the union of every local
state at the focus peers. To enable focus peers to provide
conference control operations that modify and/or require the global
state of a conference, this document defines a mechanism for inter-
focus state synchronization. It is based on mutual subscriptions for
an Event Package for Distributed Conferences and allows to preserve a
coherent knowledge of the global conference state. This XML based
event package named 'distributed-conference' MUST be supported by
each RELOAD peer that is registered within a DisCo-Registration.
Participants of a distributed conference MAY support it. To provide
backward compatibility to conference members that do not support the
distributed-conference event package, this document defines a
translation to the Event Package for Conference State [RFC4575].
5.1. Event Package Overview
The 'distributed-conference' event package is designed to convey
information about roles and relations of the conference participants.
Conference controllers obtain the global state of the conference and
use this information for load balancing or conference restructuring
mechanisms in case of a focus failure. Figure Figure 5 gives a
general overview of the document hierarchy.
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distributed-conference
|
|-- version-vector
| |-- version
| |-- version
|
|-- conference-description
|
|-- focus
| |-- focus-state
| | |-- user-count
| | |-- maximum-user-count
| | |-- active
| | |-- locked
| | |-- conf-uris
| | |-- available-media
| |
| |-- users
| | |-- user
| | | |-- endpoint
| | | | |-- media
| | | | |-- call-info
| |
| |-- relations
| | |-- relation
|-- focus
| |-- ...
Figure 5: Overview of the event package for distributed
conferences
The local state for each focus peer is described within a
corresponding 'focus' element. Each provides general information
about a focus peer, its signaling and media relations to participants
and focus peers. Conference participants are aggregated within
'users' elements, respectively, 'user' sub elements.
The document structure of 'distributed-conference' is designed to
allow concurrent occurring change events at several focus peers. For
that reason, each focus peer has exclusive writing permission to the
'focus' sub element that describes itself. The Event Package for
Distributed Conferences therefore imports the mechanisms for partial
notification and uses the 'Element Keys' definitions described in
[RFC4575] (see [RFC4575] sections 4.4-5.). A focus peer is only
allowed to update or change that <focus> sub element, whose 'entity'
Element Key contains the same AoR as the AoR of RELOAD user that is
acting focus peer. This restriction also applies to the child
elements of the 'version-vector' element. Each 'version' number is a
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property of a specific focus peer which is exclusively responsible to
increment the version number.
General information about the conference itself, is provided within a
'conference-description' element. In contrast to the 'focus' and
'version-vector' elements, the conference description is not meant
for concurrent updating. Instead, it provides static conference
descriptions that rarely change during a multiparty session.
More detailed descriptions about the event package and its elements
are provided in the following sections. The complete XML schema
definition is given in section Section 9.
5.2. <distributed-conference>
The <distributed-conference> element is the root of a distributed
conference XML document. It uses the following attributes:
entity: This attribute contains the conference URI that identifies
the distributed conference described by the XML document. A
SUBSCRIBE request addressed to this URI results in an ordinary
subscribe/notify relation between participants and their related
focus peer.
state: In accordance to [RFC4575], this attribute indicates whether
the content of a distributed conference document is a 'full',
'partial' or 'deleted' information.
The <distributed-conference> child elements are <vector-version>,
<conference-description> and the <focus> elements. An event
notification of state 'full' MUST include all these elements. An
event notification of state 'partial' MUST contain at least <version-
vector> and its sub elements.
5.3. <version-vector>/<version>
The Event Package for Distributed Conferences uses the <version-
vector>, respectively, <version> elements to enable a coherent
versioning scheme based on vector clocks as defined in
[timestamps-acsc88]. Each <version> element contains a unsigned
integer that describes the local version of a specific focus peer and
contains the following attribute:
entity This attribute contains the AoR of the focus peer. This AoR
corresponds to the 'user name' field, that is declared in the
certificate that the RELOAD user acting as focus peer obtained
while enrollment.
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If a focus peer originates a notification for a change event, the
focus peer MUST increment its associated <version> element by one.
Additionally, a change event notification MUST contain a complete
<version-vector> that carries each <version> element known by the
focus peer.
The recipient of a change event needs to update the <version> element
of the originator of the event notification in its local XML
document. All other <version> elements remain constant, although the
received <version-vector> might be different. Instead, a comparison
of both version vectors can indicate a different knowledge about the
global conference state.
As long as each <version> number in the recipients XML document is at
least lower than one to the received <version-vector> numbers, there
is no need for a subscription refresh. In this case, one or more
change event notifications might not reached all subscribers yet.
Otherwise, if any <version> number of the recipient is retarded more
than one, the recipient SHOULD refresh the subscription in order to
trigger a 'full' state notification. The recipient uses the full
state notification to recursively update every <focus> element, whose
corresponding <version> element is retarded more than one.
5.4. <conference-description>
The <conference-description> element provides general information and
links to auxiliary services for the conference. The following sub
elements provide human-readable text descriptions about the
conference:
<display-text>: Contains a short text description about the
conference
<subject>: Contains the subject of the conference
<free-text>: Contains a longer textual description about the
conference
<keywords>: Contains a list of keywords that match the conference
topic. The XML schema definition and semantic is imported from
the 'conference-info' event package [RFC4575].
In accordance with [RFC4575] a <service-uris> sub element enables
focus peers to advertise auxiliary services for the conference. The
XML schema definition and semantic is imported from the 'conference-
info' event package [RFC4575].
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The <conference-description> element uses the 'state' Element Key to
enable the partial notification mechanism.
5.5. <focus>
The <focus> element describes an active focus peer as whole. It
provides general information about a focus peer (e.g., display-text,
languages, etc.). Contains conference specific information about the
state of a focus peer (user-count, available media types, etc.).
Announces the signaling and media information about participants
maintained by this focus peer and describes Signaling or media
connections to other focus peers.
The <focus> element uses the following attributes:
entity: This attribute contains the AoR of the RELOAD user acting as
focus peer to the conference. This AoR corresponds to the 'user
name' field, that is declared in the certificate the user obtained
while RELOAD enrollment [I-D.ietf-p2psip-base]. The AoR in the
'entity' attribute uniquely identifies the focus peer, that is
allowed to update or change the sub elements of <focus>. All
other focus peers SHOULD NOT update or change sub elements of this
<focus> element. A SUBSCRIBE request addressed to this AoR MUST
be interpreted as the request for conference state synchronization
with another focus peer.
state: In accordance to RFC4575, this attribute indicates whether
the content of the <focus> element is a 'full', 'partial' or
'deleted' information. Since the event package for distributed
conferences uniquely restricts modifications on <focus> sub
elements to a specific focus peer, a 'partial' notification
contains a single <focus> element at maximum.
The following sub elements of <focus> provide general information
about a focus peer. The XML schema definition and semantic for
<associated-aors>, <roles> and <languages> are imported from the
'conference-info' event package [RFC4575].
<display-text>: Contains a short text description about the user
acting as focus peer.
<associated-aors>: In accordance with [RFC4575], this element
contains additional URIS that are associated with this user.
roles: In accordance with [RFC4575], this element MAY contain human-
readable text descriptions about the roles of the user in the
conference.
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<languages>: In accordance with [RFC4575], this element contains a
list of tokens, each describing a language understood by the user.
The following sections describe the remaining sub elements of <focus>
more detailed.
5.5.1. <focus-state>
The <focus-state> element aggregates a set of conference specific
information about the RELOAD user acting as focus peer. The
following attribute is defined for the <focus-state> element:
status: This attribute indicates whether the content of the <focus-
state> element is a 'full', 'partial' or 'deleted' information.
The <focus-state> element has the following sub elements:
<user-count>: This element contains the number of participants that
are connected to the conference via this focus peer at a certain
moment.
<maximum-user-count>: This number indicates a threshold of
participants a focus peer is able to serve. This value MAY
changes during a conference, depending on the focus peers current
load.
<conf-uris>: In accordance with the 'conference-info' event package
[RFC4575], this element MAY contains other conference URIs in
order to access the conference via different signaling means. The
XML schema definition and semantic is imported from [RFC4575].
<available-media>: This element is imports the <conference-media-
type> type XML scheme definitions from [RFC4575]. It allows a
focus peer to list its available media streams.
<active>: This boolean element indicates whether a focus peer is
currently active. Conference participation requests or a call
delegation request SHOULD succeed.
<locked>: In contrast to <active>, this element indicates that a
focus peer is not willing to accept any participation or call
delegation request.
5.5.2. <users>/<user>
The <users>, respectively, each <user> element describes a single
participant that is connected to the conference via the focus peer
which is described by the parent <focus> element. The <users>
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element XML schema definition and its semantic is imported from the
'conference-info' event package [RFC4575].
5.5.3. <relations>/<relation>
The <relations> element serves as container for <relation> elements,
each describing a specific connection between to another focus peer.
The parent element <relations> uses the 'state' attribute to enable
the partial notification mechanism. For the <relation> element the
following attribute is defined:
entity: This attribute contains the AoR of the remote focus.
The content of each <relation> is a comma separated string that
describes the tuple <CONNECTION-TYPE,IDENTIFIER>. The CONNECTION-
TYPE describes the type of connection (e.g. media, signaling, etc.)
and the IDENTIFIER contains a token that uniquely identifies the
connection in the conference. It is meant as a generic method to
announce any kind of connection to a remote focus. This
specification defines following tuples:
media,label: This tuple identifies a single media stream originated
in the focus peer described in the parent <focus> element. The
'media' part indicates that this connections belongs to any kind
of media connection. The 'label' part uniquely identifies to the
stream within the conference and corresponds to the SDP "label"
media attribute defined in [RFC4574].
sync,call-id: This tuple indicates a subscription for the event
package for distributed conferences. The remote focus peer
described in the 'entity' attribute is a subscriber for
distributed-conference event package for the purpose of conference
state synchronization. The 'call-id' part thereby corresponds to
the call-id of the SIP subscription dialog.
5.6. Distribution of Change Events
Maintaining a coherent conference state at each controller of a
distributed conference, requires a common protocol scheme to
communicate each conference change event to each focus peer. Using
SIP specific event notifications [RFC3265], this requirement would
result in an N to M relation (with N=M), between N notifiers and M
subscribers to synchronize the conference state. To avoid a 'full
meshed' subscription topology, where each focus peer has N
subscriptions to all other focus peers, this section describes a
'mutual' subscription scheme that constructs a spanning tree topology
among the focus peers.
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A member in a distributed conference that accepted an authorized
participation request and becomes a new focus peer, SHOULD join the
state synchronization process of a conference. Therefore, it sends a
SIP SUBSCRIBE to request the Event Package for Distributed
Conferences to its own focus peer. The subscription request SHOULD
be addressed to AoR of the active focus peer, thus it interprets this
subscription as a request for conference state synchronization. The
corresponding NOTIFY message contains a 'full' distributed-conference
state XML document (see section Section Section 5.1).
Following, the subscribed focus peer has to subscribe the new
upcoming focus peer for the distributed conference event package.
The corresponding notification by the new focus carries a 'partial'
conference state XML document. It contains the received <version-
vector> including a new <version> element for itself and contains a
new <focus> element that describes its local state (see section
Section 5.5).
Resulting by this subscription scheme, each focus peer has at least
one subscription to obtain updates for the conference state and is a
notifier for change events originated itself. In a incrementally
increasing conference, the 1st and 2nd focus peer have a mutual
subscription for conference change events. A 3rd focus could have a
mutual subscription with the 1st focus, a 4th focus to the 2nd focus
and so forth. The result is a spanning tree topology among the focus
peers in which each focus peer is a possible root for distribution
tree for conference change events.
However, the fact that event notifications need to traverse one or
more intermediate focus peers until conference-wide delivery, demands
a forwarding mechanism for change events. On receiving a change
event, a notified focus firstly validates based on the <version-
vector> whether the incoming state event is not a duplicate to
previews notifications. If its not a duplicate, the received change
event, secondly, triggers a new event notification process at the
receiver of the change event. It notifies all its subscribers with
excepting the sender of the event notification (which is not
necessarily the event originator). Thus, the change event will be
'flooded' among the focus peer of a conference.
5.7. Translation to Conference-Info Event Package
The Event Package for Distributed Conferences imports several XML
element definitions of the Event Package for Conference State
[RFC4575]. This is caused by two reasons. Firstly, the semantic of
these elements are fitting the demands to describe the global state
of a distributed conference and, secondly, it facilitates a re-
translation to [RFC4575] to enable a backward compatibility to DisCo-
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unaware clients. Therefore, each focus peer MAY provide a separate
[RFC4575] conform event notification service to its connected
participants.
The following sections describe the translation to the Event Package
for Conference State [RFC4575] by defining translation rules for the
root element and its direct sub elements. For a better
understanding, the following sections use a notation ci.<ELEMENT> to
refer to a sub element of the conference-info element, and
disco.<ELEMENT> to refer to an element of the distributed-conference
event package.
5.7.1. <conference-info>
The root element of Event Package for Conference State uses the
attributes 'entity', 'state' and 'version' and is the counterpart of
the <distributed-conference> root element in the DisCo Event Package.
The former two attributes 'entity' and 'state' are equal in both root
elements and can be seamlessly translated.
According to [RFC4575], the version attribute SHOULD be incremented
by one at any time a new notification being sent to a subscriber.
New notifications SHOULD be triggered by change events that are
originated within a focus peer and SHOULD be triggered by reception
of a 'distributed-conference' event state of another focus peer.
5.7.2. <conference-description>
The <conference-description> element exists in both event packages,
conference-info and distributed-conference. Thus, the following
elements are seamlessly translatable: <keywords>, <display-text>,
<subject>, <free-text> and <service-uris>.
The sub elements <conf-uris>, <maximum-user-count> and <available-
media> in conference-info have there counterparts below the
\focus\focus-state element of the distributed-conference event
package. Each describes a local state of a focus peer in the
conference. Hence, the intersection of every disco.<conf-uris>,
disco.<available-media> and the sum over each disco.<maximum-user-
count> element of each disco.<focus> element in distributed-
conference, specifies the content of the corresponding conference-
info elements.
5.7.3. <host-info>
According to [RFC4575] the ci.<host-info> element contains
information about the entity hosting the conference. For
participants in a distributed conference, the hosting entity is the
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focus peer through which they are connected to the conference. Thus,
the ci.<host-info> element contains information about a focus peer
that is serving its participants.
5.7.4. <conference-state>
The ci.<conference-state> element allows subscribers obtain
information about overall state of a conference. Its sub elements
ci.<user-count>, ci.<active> and ci.<locked> are reused as sub
elements of \focus\focus-state to describe the local state of a focus
peer in a distributed conference. The translation rules from the
distributed-conference to the conference-info event package are the
following:
<user-count>: The sum over each value of the disco.<user-count>
element defines the corresponding ci.<user-count>.
<active>: The boolean ci.<active> element is defined by the logical
concatenation over all disco.<active> elements by an OR-operator.
<locked> The boolean ci.<locked> element is defined by the logical
concatenation over all disco.<locked> elements by an AND-operator.
5.7.5. <users>/<user>
The distributed-conference event package imports the definitions of
the ci.<users> and ci.<user> elements under a parent disco.<focus>
element for each focus peer in a conference. Thus, the aggregation
over all disco.<users> elements specifies the content of the
corresponding ci.<users> element.
5.7.6. <sidebars-by-ref>/<sidebars-by-value>
In accordance to [RFC4575], if a participant is connected to a
sidebar, its responsible focus peer creates a new <user> by
referencing to the corresponding sidebar conference.
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6. Distributed Conference Control with SIP
6.1. Call delegation
Distributed conference control provides the possibility to delegate
the control for a conference participant from one focus to another
remote focus for some reason. In contrast to common call transfers
that are using the SIP REFER method as described in [RFC3515], call
delegations are achieved transparently to the transferred party.
However, a focus peer starts a call delegation by sending SIP REFER
request containing the URI of the participant in the Refer-To header
field. Additionally, the focus peer appends the following parameter
to the URI of the participant:
call-id: Contains the call-ID of the original SIP dialog
establishment between the referred participant and the referring
focus peer.
sess-id: Contains the 'session identifier' value of the original SDP
'o=' field of the original offer/answer process between referred
participant and referring focus peer.
If the recipient accepts the REFER request, it generates a re-INVITE
towards the referred party and sets the SIP call-id header and the
SDP 'session-identifier' field in the SDP offer, according to the URI
parameter values of the initial REFER request. The From header field
and contact header are set to the conference URI with setting the
'isfocus' tag to contact header. This identifies the peer as a focus
to the conference and identifies this re-INVITE as a request of the
SIP dialog between the party and the conference. To ensure that
further signaling messages will be routed correctly, the new focus
adds a Record-Route header field that contains the contact
information (URI, IP-address,..) of this peer.
An example call flow for call delegation is shown in Figure 6.
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Participant Referring Focus Remote Focus
--------------------------------------------------------------------
| Dialog | |
|<============>| |
| | |
| Delegating a participant to remote focus |
| | |
| |(1) REFER refer-to:<uri>?call-id=123&sess-id=456
| |------------------------------------>|
| |(2) 200 OK |
| |<------------------------------------|
| |(3) Notify: pending |
| |<------------------------------------|
| |(4) 200 OK |
| |------------------------------------>|
| | |
| Remote focus adds RR-header that carries its URI |
| | |
| (5) INVITE sip:<uri> record-route:<rem.focus> |
|<---------------------------------------------------|
| (6) 200 OK | |
|--------------------------------------------------->|
| (7) ACK | |
|<---------------------------------------------------|
| |(8) Notify: active |
| |<------------------------------------|
| |(9) 200 OK |
| |------------------------------------>|
Figure 6: Delegating a participant with SIP REFER
Note, subscriptions for the event packages 'distributed-conference'
and 'conference-info' are in scope of a specific focus peer and its
connected participants. Hence, after a successful call delegation,
the referring focus peer SHOULD terminate any subscription to the
referred participant. The notifier SHOULD include a reason parameter
"deactivated" to indicate a migration of the subscription as defined
in [RFC3265]. The new SUBSCRIBE request by the party MUST be sent
via the SIP dialog to the conference.
6.2. Conference Access
It is an important issue to define who is allowed to participate a
multimedia conference. In many cases, a group conversation can be an
open discussion free to participate, while in other occasions a
closed privacy of a multiparty session is demanded. Additionally, it
is an issue which of the conference participant is allowed to become
a controller of the multiparty session.
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To provide those constraints for distributed conferences in RELOAD,
this document defines a basic set of conference access that decide
whether:
o A peer is allowed to participate a conference
o A peer is allowed to become a focus of the conference
Both cases can be regulated with plain SIP authorization mechanisms
achieved by the focus peers asking for a participants credentials.
Those credentials and the allowed users can be setup at creation of
the conference. It is up by the conference to define different
access role deciding if a user is allowed to become a focus peer or
not.
6.3. Media Negotiation and Distribution
This section describes a basic scheme for media negotiation and
distribution, which is done in an ad-hoc fashion. Each focus peer
forwards all media streams it receives from the conference to all
connected peers it is responsible for and similarly all streams from
its peers to its responsible focus. This results in the media stream
naturally following the SIP signalling paths. Implementations MAY
choose to use a more sophisticated scheme, e.g. employing cross
connections between different sub-trees of the conference, but MUST
take measures to prevent loops in media routing.
When a new peer has been attached to a focus, new media steams may be
available to the focus, which need to be forwarded to the conference.
To accomplish this, the new media streams need to be signalled to the
other participants. This is usually done by sending a SIP re-INVITE
and modifying the media sessions, adding the new streams to the SDP.
This renegotiation can be costly since it needs to be propagated
through the whole conference. Also, distributing all media streams
separately to all participants can be quite bandwidth intensive.
Both problems can partially be mitigated by focus peers performing
mixing of media streams, thus trading bandwidth and signalling
overheads for computational load on focus peers.
6.3.1. Offer/Answer
A peer joining a conference negotiates media types and media
parameters with its designated focus using the standard SDP offer/
answer protocol [RFC3264]. The focus SHOULD offer all media streams
used in the conference.
A new participant does not necessarily know about all media streams
present in a conference beforehand, and thus some of the media
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streams might not be included in the initial SDP offer sent by the
joining peer. An SDP answer sent by the corresponding focus MUST NOT
contain any media streams not matching an offer (cf. [RFC3264]
Section 6). A joining peer which is aware of the distributed nature
of the conference, SHOULD NOT send an SDP offer in the initial INVITE
message, but instead send an empty INVITE to which the focus replies
with an OK, containing the offer. This prevents the need for a
second offer/answer-dialog to modify the session. But for
compatibility the normal behavior with the INVITE containing the
offer MUST be supported.
The focus SHOULD only offer media types and codecs which are already
used in the conference and which will probably be accepted when
forwarded to neighboring peers, unless the focus is prepared to do
transcoding and/or mixing of the received streams.
A focus has two options when distributing media streams from a new
participant to the conference. The focus can either mix the new
streams into his own, thus averting the need to modify the already
established media sessions with neighboring peers or in case the
focus is not willing or able to do mixing of the media streams, he
SHOULD modify the media sessions with all attached peers by sending a
re-INVITE, adding the new media streams coming from the newly joined
participant to the SDP. This SHOULD subsequently be done by all
other focus peers upon receiving the new stream, resulting in the
stream being distributed across the conference.
6.4. Restructuring a Conference
Distributed conference control provides the possibility to delegate
calls to remote focus peers. This feature is used to restructure a
conference in case of departure of a focus peer. Following, this
section presents restructuring schemes for graceful and unexpected
leaves of conference focus peers.
6.4.1. On Graceful Leave
In a graceful case, the leaving focus peer (LP) accomplishes the
following procedure:
o LP deletes its mapping in the DisCo-Registration by storing the
"non-existing" value as described in the RELOAD base document
[I-D.ietf-p2psip-base]. Afterwards, it fetches the lasted version
of the DisCo-Registration to obtain all potential focus peers.
o LP calculates for all its participants the closest focus among all
active and potential focus peer using the algorithm described in
Section 4.2. LP then delegates all participants to those focus
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peers.
o LP then announces its leave by sending a NOTIFY to all its
subscribers for the extended conference event package, setting its
<focus> state to 'deleted'. Thereafter, it ends its own SIP
conference dialog by sending by to its related focus peer.
Since the state synchronization topology in a distributed conference
is commonly arranged in a spanning tree, a leave of a focus peer
effects a gab in the tree structure. Those focus peers which had the
leaving focus peer as their parent focus, are supposed to reconnect
to the synchronization graph, by subscribing the leaving peer's
parent node.
6.4.2. On Unexpected Leave
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 4.5.
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7. 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
or
USER-CHAIN-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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8. Access Control Policy USER-CHAIN-MATCH
The base RELOAD document [I-D.ietf-p2psip-base] mandates that every
kind definition specifies an Access Control Model to use. The base
document defines four access control policies, of which none is
fitting for the purpose of shared write access to a resource. This
document defines a new access control model called USER-CHAIN-MATCH.
In the USER-CHAIN-MATCH policy, a given value MUST be written (or
overwritten) if and only if the request is singed with a key
associated with a certificate whose user name hashes (using the hash
function for the overlay) to the Resource-ID for the resource. Also
the user name of the certificate MUST match the user name of an
intermediary certificate in the chain to the root certificate, using
the matching pattern specified in the configuration document for the
corresponding kind.
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9. XML Schema
The XML schema for the event package for distributed conferences is:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:ci="urn:ietf:params:xml:ns:conference-info"
xmlns="urn:ietf:params:xml:ns:distributed-conference"
targetNamespace="urn:ietf:params:xml:ns:distributed-conference"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<!--
This imports the definitions in conference-info
-->
<xs:import namespace="urn:ietf:params:xml:ns:conference-info"
schemaLocation="http://www.iana.org/assignments/xml-registry/
schema/conference-info.xsd"/>
<xs:import namespace="http://www.w3.org/XML/1998/namespace"
schemaLocation="http://www.w3.org/2001/03/xml.xsd"/>
<!--
A DISTRIBUTED CONFERENCE ELEMENT
-->
<xs:element name="distributed-conference"
type="distributed-conference-type"/>
<!--
DISTRIBUTED CONFERENCE TYPE
-->
<xs:complexType name="distributed-conference-type">
<xs:sequence>
<xs:element name="version-vector"
type="version-vector-type" minOccurs="1"/>
<xs:element name="conference-description"
type="conference-description-type"
minOccurs="0" maxOccurs="1"/>
<xs:element name="focus"
type="focus-type"
minOccurs="0"
maxOccurs="unbounded"/>
<xs:any namespace="##other" processContents="lax"/>
</xs:sequence>
<xs:attribute name="state" type="ci:state-type"/>
<xs:attribute name="entity" type="xs:anyURI"/>
<xs:anyAttribute namespace="##other" processContents="lax"/>
</xs:complexType>
<!--
VERSION VECTOR TYPE
-->
<xs:complexType name="version-vector-type">
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<xs:sequence>
<xs:element name="version"
type="version-type"
minOccurs="1"
maxOccurs="unbounded"/>
<xs:any namespace="##other" processContents="lax"/>
</xs:sequence>
<xs:anyAttribute namespace="##other" processContents="lax"/>
</xs:complexType>
<!--
CONFERENCE DESCRIPTION TYPE
-->
<xs:complexType name="conference-description-type">
<xs:sequence>
<xs:element name="display-text"
type="xs:string" minOccurs="0"/>
<xs:element name="subject" type="xs:string" minOccurs="0"/>
<xs:element name="free" type="xs:string" minOccurs="0"/>
<xs:element name="keywords"
type="ci:keywords-type" minOccurs="0"/>
<xs:element name="service-uris"
type="ci:uris-type" minOccurs="0"/>
<xs:any namespace="##other" processContents="lax"/>
</xs:sequence>
<xs:attribute name="state" type="ci:state-type"/>
<xs:anyAttribute namespace="##other" processContents="lax"/>
</xs:complexType>
<!--
FOCUS TYPE
-->
<xs:complexType name="focus-type">
<xs:sequence>
<xs:element name="display-text"
type="xs:string" minOccurs="0"/>
<xs:element name="associated-aors"
type="ci:uris-type" minOccurs="0"/>
<xs:element name="roles"
type="ci:user-roles-type" minOccurs="0"/>
<xs:element name="languages"
type="ci:user-languages-type" minOccurs="0"/>
<xs:element name="focus-state"
type="focus-state-type" minOccurs="0"/>
<xs:element name="users"
type="ci:users-type" minOccurs="0"/>
<xs:element name="relations"
type="relations-type" minOccurs="0"/>
<xs:any namespace="#other" processContents="lax"/>
</xs:sequence>
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<xs:attribute name="entity" type="xs:anyURI"/>
<xs:attribute name="state" type="ci:state-type"/>
<xs:anyAttribute namespace="##other" processContents="lax"/>
</xs:complexType>
<!--
VERSION TYPE
-->
<xs:complexType name="version-type">
<xs:simpleContent>
<xs:extension base="xs:unsignedInt">
<xs:attribute name="entity" type="xs:anyURI"/>
<xs:anyAttribute namespace="##other" processContents="lax"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
<!--
FOCUS STATE TYPE
-->
<xs:complexType name="focus-state-type">
<xs:sequence>
<xs:element name="user-count"
type="xs:unsignedInt" minOccurs="0"/>
<xs:element name="maximal-user-count"
type="xs:unsignedInt" minOccurs="0"/>
<xs:element name="conf-uris"
type="ci:uris-type" minOccurs="0"/>
<xs:element name="available-media"
type="ci:conference-media-type" minOccurs="0"/>
<xs:element name="active" type="xs:boolean" minOccurs="0"/>
<xs:element name="locked" type="xs:boolean" minOccurs="0"/>
<xs:any namespace="##other" processContents="lax"/>
</xs:sequence>
<xs:attribute name="state" type="ci:state-type"/>
<xs:anyAttribute namespace="##other" processContents="lax"/>
</xs:complexType>
<!--
RELATIONS TYPE
-->
<xs:complexType name="relations-type">
<xs:sequence>
<xs:element name="relation"
type="relation-type"
minOccurs="0" maxOccurs="unbounded"/>
<xs:any namespace="##other" processContents="lax"/>
</xs:sequence>
<xs:attribute name="state" type="ci:state-type"/>
<xs:anyAttribute namespace="##other" processContents="lax"/>
</xs:complexType>
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<!--
RELATION TYPE
-->
<xs:complexType name="relation-type">
<xs:simpleContent>
<xs:extension base="xs:string">
<xs:attribute name="entity" type="xs:anyURI"/>
<xs:anyAttribute namespace="##other" processContents="lax"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:schema>
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10. 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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11. Security Considerations
11.1. 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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12. IANA Considerations
TODO: register Kind-ID code point at the IANA
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13. Acknowledgments
This work was stimulated by fruitful discussions in the P2PSIP
working group and SAM research group. We would like to thank all
active members for constructive thoughts and feedback. In
particular, the authors would like to thank (in alphabetical order)
David Bryan, Toerless Eckert, Lothar Grimm, Cullen Jennings, Peter
Musgrave, Joerg Ott, Peter Pogrzeba, Brian Rosen, and Jan Seedorf.
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14. References
14.1. 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-12 (work in
progress), November 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-05 (work in progress), July 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.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3515] Sparks, R., "The Session Initiation Protocol (SIP) Refer
Method", RFC 3515, April 2003.
[RFC4574] Levin, O. and G. Camarillo, "The Session Description
Protocol (SDP) Label Attribute", RFC 4574, August 2006.
[RFC4575] Rosenberg, J., Schulzrinne, H., and O. Levin, "A Session
Initiation Protocol (SIP) Event Package for Conference
State", RFC 4575, August 2006.
14.2. 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),
October 2010.
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[RFC4353] Rosenberg, J., "A Framework for Conferencing with the
Session Initiation Protocol (SIP)", RFC 4353,
February 2006.
[timestamps-acsc88]
Fidge, C., "Timestamps in Message-Passing Systems that
Preserve the Partial Ordering", Proceedings of 11th
Australian Computer Science Conference, pp. 56-66,
February 1988.
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Appendix A. Change Log
The following changes have been made from version
draft-knauf-p2psip-disco-00.
1. Updated references.
2. Corrected typos.
3. New Section: Conference State Synchronization
4. XML Event Package for Distributed Conferences
5. New mechanism for generating chained conference certificates
6. Allow shared writing of resources using Access Control Policy
USER-CHAIN-MATCH
7. Media Negotiation mechanism in a distributed conference
8. New Section: Distributed Conference Control with SIP
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Authors' Addresses
Alexander Knauf
HAW Hamburg
Berliner Tor 7
Hamburg D-20099
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 D-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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