Requirements for Floor Control Protocols

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Copyright © The Internet Society (2006).

Abstract

Floor control is a means to manage joint or exclusive access to shared resources in a (multiparty) conferencing environment. Thereby, floor control complements other functions -- such as conference and media session setup, conference policy manipulation, and media control -- that are realized by other protocols. This document defines the requirements for a floor control protocol for multiparty conferences in the context of an existing framework.

Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................3
   3. Terminology .....................................................3
   4. Model ...........................................................4
   5. Integration with Conferencing ...................................5
   6. Assumptions about a Conference Policy ...........................6
   7. Floor Control Protocol Requirements .............................7
      7.1. Communication between Participant and Server ...............7
      7.2. Communication between Chair and Server .....................9
      7.3. General Protocol Requirements ..............................9
   8. Security Considerations ........................................10
   9. Acknowledgements ...............................................11
   10. References ....................................................12
      10.1. Normative References .....................................12
      10.2. Informative References ...................................12

1. Introduction

Conference applications often have shared resources such as the right to talk, input access to a limited-bandwidth video channel, or a pointer or input focus in a shared application.

In many cases, it is desirable to be able to control who can provide input (send/write/control, depending on the application) to the shared resource.

Floor control enables applications or users to gain safe and mutually exclusive or non-exclusive input access to the shared object or resource. The floor is an individual temporary access or manipulation permission for a specific shared resource (or group of resources) [6].

Floor control is an optional feature for conferencing applications. SIP [2] conferencing applications may also decide not to support this feature at all. Two-party applications may use floor control outside conferencing, although the usefulness of this kind of scenario is limited. Floor control may be used together with the conference policy control protocol (CPCP) [7], or it may be used as an independent stand-alone protocol, e.g., with SIP but without CPCP.

   Floor control has been studied extensively over the years (e.g., [8],
   [6], and [5]); therefore, earlier work can be leveraged here.

The present document describes the requirements for a floor control protocol. As a requirements specification, the document makes no assumptions about the later implementation of the respective requirements as parts of one or more protocols or about the entities implementing them and their roles.

This document may be used in conjunction with other documents, such as the conferencing framework document [3]. In particular, when speaking about a floor control server, this entity may be identical to or co-located with the focus or a conference policy server defined in the framework document, while participants and floor chairs referred to in this specification may be regular participants as introduced in the conferencing framework document. In this specification, the term "floor control protocol" is used in an abstract sense and may ultimately be mapped to any of the existing conference control or other signaling protocols (including CPCP and SIP). However, defining those relationships is left to a concrete floor control protocol specification.

2. Conventions Used in This Document

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 RFC 2119 [1].

3. Terminology

This document uses the definitions from [3].

The following additional definitions apply:

Floor: A permission to access or manipulate a specific shared resource or set of resources temporarily.

Conference owner: A privileged user who controls the conference, creates floors, and assigns and deassigns floor chairs. The conference owner does not have to be a member in a conference.

Floor chair: A user (or an entity) who manages one floor (grants, denies, or revokes a floor). The floor chair does not have to be a member in a conference.

Floor control: A mechanism that enables applications or users to gain safe and mutually exclusive or non-exclusive input access to the shared object or resource.

Floor control server: A logical entity that maintains the state of the floor(s) including which floors exists, who the floor chairs are, who holds a floor, etc. Requests to manipulate a floor are directed at the floor control server.

Floor request set: A logical data structure holding all requests for a particular floor at a given point in time.

Floor holder set: A logical data structure identifying all participants who currently hold the floor.

4. Model

The model for floor control is composed of three logical entities: a single floor control server, one or more floor chairs (moderators), and any number of regular conference participants.

A floor control protocol is used to convey the floor control messages among the floor chairs (moderators) of the conference, the floor control server, and the participants of the conference. A centralized architecture is assumed in which all messages go via one point, the floor control server. Processing (granting or rejecting) floor control requests is done by the one or more floor chairs or by the server itself, depending on the policy.

Floor requests from the participants are received by the floor control server and kept (at the level of the floor control protocol) in a floor request set (i.e., are not ordered in any particular fashion). The current floor holders are reflected in a current floor holder set. Floor chairs are capable of manipulating both sets to grant, revoke, reject, and pass the floor (for example).

The order in which requests are processed, whether they are granted or rejected, and how many participants obtain a floor simultaneously are determined by a higher-layer application operating on these sets and are not confined by the floor control protocol.

A floor is associated with one or more media sessions. The centralized conference server manages the floors and thus controls access to the media sessions. There are two aspects to this: 1) The server maintains and distributes consistent state information about who has a certain floor at a certain point in time and does so following some rule set. This provides all participants with the necessary information about who is allowed to speak (for example), but relies on a cooperative behavior among all participants. 2) In addition, to prevent individuals from ignoring the "hints" given by the floor control server, the latter may (e.g., in cooperation with other functional entities) enforce compliance with floor status, e.g., by blocking media streams from participants not entitled to speak. The floor control server controls the floors at least at the signaling level. In addition, actively controlling the actual (physical) media resources is highly recommended, but beyond the scope of this document.

As noted in the introduction, an actual protocol specification fulfilling the requirements defined in this memo may map the components of the above model onto the conferencing components defined in the conferencing framework document. Some of these aspects are discussed briefly in the next section.

5. Integration with Conferencing

Floor control itself does not support privileges such as creating floors and appointing floor chairs and handing over chair privileges to other users (or taking them away). Instead, some external mechanism, such as conference management (e.g., CPCP or web interface for policy manipulation) is used for that.

The conference policy (and thus the conference owner or creator) defines whether floor control is in use or not. Actually enforcing conference media distribution in line with the respective media's floor status (e.g., controlling an audio bridge) is beyond the scope of this document. Floor control itself does not define media enforcement. It is up to the conference and media policies to define which media streams may be used in a conference and which ones are floor controlled.

Typically, the conference owner creates the floor(s) using the conference policy control protocol (or some other mechanism) and appoints the floor chair. The conference owner can remove the floor anytime (so that a media session is not floor-controlled anymore) or change the floor chair or floor parameters.

The floor chair just controls the access to the floor(s), according to the conference policy.

A floor control server is a separate logical entity, typically co-located with focus and/or conference policy server. Therefore, the floor control server can interact with the focus and conference policy server and media servers as needed. Communication mechanisms between the floor control server and other central conferencing entities are not within the scope of the floor control protocol requirements described in this document.

Conferences may be cascaded, and hence a single participant in one conference may represent a second conference (called subconference). From a floor control perspective, there is no difference between a participant (identified by its URI) representing a single person or another (set of) subconference(s).

Note: In the latter case, it is the responsibility of the subconference to negotiate floor requests internally before passing on a request to the conference and to assign a floor internally upon receiving a floor grant. This may be done recursively by employing the floor control protocol with a different floor control server in the subconference.

6. Assumptions about a Conference Policy

The floor control protocol is supposed to be used to manage access to shared resources in the context of a conference. It is up to this conference -- more precisely, its conference policy [4] -- to define the rules for the operation of the floor control protocol. Furthermore, a conference policy control protocol [4] may define mechanisms that alter those rules during the course of a conference. This section briefly outlines the assumptions made by a floor control protocol about the conference policy and means for its modification.

The conference policy is expected to define the rules for floor control, which implies in particular that it is not the responsibility of the floor control protocol to establish or communicate those rules.

In general, it is assumed that the conference policy also defines who is allowed to create, change, and remove a floor in a conference.

Conference participants and floor chairs should be able to get and set floor-related parameters. The conference policy may restrict who may access or alter which parameters. Note that not all parameters maintained for a floor are also interpreted by the floor control protocol (e.g., floor policy descriptions may be stored associated with a floor but may be interpreted by a higher-layer application). Note also that changes to the floor control policy are outside the scope of the floor control protocol and are (for example) to be carried out by a conference policy control protocol.

(For example, it may be useful to see who the floor chair is, what kind of policy is in use, time limits, number of simultaneous floor holders, and current floor holder.)

The following requirements on a conference policy related to floor control are identified in [4]:

REQ-F1: It MUST be possible to define whether floor control is in use or not.

REQ-F2: It MUST be possible to define the algorithm to be used in granting the floor. (Note: Examples of algorithms are moderator- controlled, FCFS, or random.)

Note: It must be possible to use an automated floor policy where the floor control server decides autonomously about granting and rejecting floor requests as well as revoking the floor. It must also be possible to use a chair-controlled floor policy in which the floor control server notifies the floor chair and waits for the chair to make a decision. This enables the chair to fully control who has the floor. The server MAY forward all requests immediately to the floor chair, or it may do filtering and send only occasional notifications to the chair.

REQ-F3: It MUST be possible to define how many users can have the floor at the same time.

REQ-F4: It MUST be possible to have one floor for one or more media types.

REQ-F5:

It MUST be possible to have multiple floors in a conference.

REQ-F6: It MUST be possible to define whether a floor is moderator- controlled or not.

REQ-F7: If the floor is moderator-controlled, it MUST be possible to assign and replace the floor moderator.

7. Floor Control Protocol Requirements

This section covers the requirements on a floor control protocol. The requirements are grouped as follows: 1) floor control protocol between participant and server; 2) floor control protocol between floor chairs and server; 3) floor control server management; and 4) general protocol requirements.

7.1. Communication between Participant and Server

REQ-PS-1:

Participants MUST be able to request (claim) a floor.

REQ-PS-2: It SHOULD be possible for a participant requesting a floor to give additional information about the request, such as the topic of the question for an audio floor. Note: In some scenarios, the floor control server or the floor chair may use this information when granting the floor to the user, or when manipulating the floor sets at the server.

REQ-PS-3: It MUST be possible for a participant to modify (e.g., cancel) a previously placed floor request.

REQ-PS-4: It SHOULD be possible for a participant to initiate a floor control operation (e.g., floor request, release) on behalf of another participant (third-party floor control) provided that he is authorized to do so.

REQ-PS-5: A participant MUST be informed that she has been granted the floor.

REQ-PS-6: A participant MUST be informed that his floor request has been rejected.

REQ-PS-7: A participant MUST be informed that the floor was revoked from her.

REQ-PS-8: A participant SHOULD be informed that her floor request is pending and will be processed later.

REQ-PS-9:

A floor holder MUST be able to release a floor.

REQ-PS-10: It MUST be possible to notify conference participants of (changes to) the floor holder(s).

REQ-PS-11: It MUST be possible to notify conference participants when a new floor request is being made.

REQ-PS-12: It MUST be possible for a floor requester to request privacy for claiming the floor.

anonymous: The participants (including the floor chair) cannot see the floor requester's identity. The floor chairs grant the floor based on the claim id and the topic of the claim.

known to the floor chair: Only the floor chair is able to see the floor requester's identity; all other participants do not obtain this information.

public: All the participants can see the floor requester's identity.

REQ-PS-13: It MUST be possible for a participant to request privacy for holding the floor along with a floor request. Note that identity information about the participant may become available to others through different means (e.g., application/media protocols or the media itself such as the voice).

7.2. Communication between Chair and Server

REQ-CS-1: It MUST be possible to inform the floor chairs, if present, about a participant's floor request.

It SHOULD be possible to convey additional information the participant may have provided along with her request.

It MUST be possible to hide the requesting participant's identity from the chair, i.e., not to include this identity information in the floor request.

REQ-CS-2:

It MUST be possible to grant a floor to a participant.

REQ-CS-3: It MUST be possible to reject a participant's floor request.

REQ-CS-4: The floor chair MUST be able to revoke a floor from (one of) its current holder(s). Note that the floor chair may also remove pending floor requests from the request set (by rejecting them).

REQ-CS-5: It MUST be possible to notify floor chairs about changes to the floor holder(s).

REQ-CS-6: There SHOULD be operations to manipulate the request set available for floor chair(s). Such a request set SHOULD at least include creating, maintaining, and re-ordering floor requests in a queue and clearing the floor control queue.

REQ-CS-7: It MUST be possible to hide the identity of a floor chair from a subset or all participants of a conference.

REQ-CS-8: It MUST be possible for a newly assigned floor chair to learn (e.g., inquire) about the existing floor request set.

7.3. General Protocol Requirements

REQ-GEN-1: Bandwidth and terminal limitations SHOULD be taken into account in order to ensure that floor control can be efficiently used in mobile environments.

Note that efficient communication by means of minimal-sized messages may contradict the desire to express reasons for requesting a floor along with other information. Therefore, a floor control protocol SHOULD be designed in a way that it allows for expressive as well as minimal messaging, as a (negotiable) configuration option and/or selectable on a per-message basis.

REQ-GEN-2: The floor control MUST be a reliable client-server protocol. Hence, it MUST provide a positive response indicating that a request has been received or an error response if an error has occurred.

REQ-GEN-3: It MUST be possible for the floor control server to authenticate participants and chairs.

REQ-GEN-4: It MUST be possible for the participants and chairs to authenticate the server.

REQ-GEN-5: It MUST be possible to ensure message integrity between participants and chairs and the floor control server.

REQ-GEN-6: It MUST be possible to ensure the privacy of messages exchanged between participants and chairs and the floor control server.

8. Security Considerations

Floor control messages are exchanged on one hand between regular participants and the floor control server and on the other hand between the floor control server and the floor chair(s).

If enabled, floor control mechanisms are used to control who may contribute to a conference in arbitrary ways (speak, be seen, write, etc., as supported by the conferencing applications). It is important that floor control messages be protected because otherwise an attacker could prevent participants from being "heard" in the conference (e.g., in scenarios where silence is considered consent) or make participants be heard in a conference without their knowledge (e.g., eavesdropping on the participant's microphone). Such considerations are particularly relevant when floor control is used in conjunction with one or more (central) entities (e.g., a media mixer) controlled by the floor control server to enforce floor control decisions that may allow an attacker to "mute" a participant completely.

Communications between a conference participant and the floor control server are vulnerable to all kinds of masquerading attacks. If an attacker can spoof the identity of the participant or inject messages on his behalf, it may generate floor requests (e.g., floor release) and prevent proper participation of the participant. If an attacker can inject messages to the participant, it may generate arbitrary responses and false status information. If an attacker can impersonate the floor control server, a participant's requests may never reach the actual floor control server. If an attacker can intercept either side's messages (and hence become a man in the middle (MITM)), it may suppress, alter, or inject messages and thus manipulate a participant's view of the conference floor status as well as the floor control server's view of a participant.

Similar considerations apply to the communications between the floor control server and the floor chair(s). If an attacker can intercept messages from either side, it may defer or prevent responses to floor control requests (from a particular floor chair). If it can inject messages (particularly in the direction from the floor chair to the floor control server), it may steer the assignment of conference floors. If interception and injection is possible (man-in-the-middle scenario), an attacker can create an arbitrary image of the conference for the floor chair. If an attacker can impersonate a floor chair, it may rule the conference floor assignment (if there is only a single chair) or disrupt the conference course by means of arbitrary and potentially conflicting requests/responses/assignments (if there are multiple floor chairs). In the latter case, the amount of damage a single attacker can do depends on the floor control policy.

Finally, attackers may eavesdrop on the floor control communications and learn which participants are present, how active they are, who are the floor chairs, etc.

To mitigate the above threats, conference participants, floor control servers, and floor chairs SHOULD be authenticated upon initial contact. All floor control messages SHOULD be authenticated and integrity-protected to prevent third-party intervention and MITM attacks. Floor control messages SHOULD be encrypted to prevent eavesdropping.

9. Acknowledgements

The authors would like to thank IETF conferencing design team and Keith Drage, Marcus Brunner, Sanjoy Sen, Eric Burger, Brian Rosen, and Nermeen Ismail for their feedback.

10. References

10.1. Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
   
        Levels", RFC 2119, BCD 14, March 1997.
   
   [2]  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.

10.2. Informative References

   [3]  Rosenberg, J., "A Framework for Conferencing with the Session
        Initiation Protocol (SIP)", RFC 4353, February 2006.
   
   [4]  Koskelainen, P. and H. Khartabil, "Additional Requirements to
        Conferencing", Work in Progress, August 2004.
   
   [5]  Koskelainen, P., Schulzrinne, H., and X. Wu, "A SIP-based
        conference control framework", NOSSDAV 2002, Miami Beach,
        May 2002.
   
   [6]  Dommel, H. and J. Garcia-Luna-Aceves, "Floor control for
        activity coordination in networked multimedia applications",
        Proc. of 2nd Asian-pacific Conference on Communications APPC,
        Osaka Japan, June 1995.
   
   [7]  Koskelainen, P., Khartabil, H., and A. Niemi, "The Conference
        Policy Control Protocol (CPCP)", Work in Progress, October 2004.
   
   [8]  Borman, C., Kutscher, D., Ott, J., and D. Trossen, "Simple
        conference control protocol service specification", Work in
        Progress, March 2001.

Authors' Addresses

Petri Koskelainen
Nokia
102 Corporate Park Drive
White Plains, NY 10604
USA

   EMail: petri.koskelainen@nokia.com

Joerg Ott
Helsinki University of Technology
Networking Laboratory
Otakaari 5A
02150 Espoo
Finland

   EMail: jo@netlab.hut.fi
   
   Henning Schulzrinne
   Columbia University
   1214 Amsterdam Avenue
   New York  10027
   USA
   
   EMail: hgs@cs.columbia.edu
   
   Xiaotao Wu
   Columbia University
   1214 Amsterdam Avenue
   New York  10027
   USA
   
   EMail: xiaotaow@cs.columbia.edu

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