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DISPATCH Working Group                               J.J. Garcia Aranda
                                                          J. Perez Lajo
Internet Draft                                       L.M. Diaz Vizcaino
                                                      G. Munoz Fernandez
Intended status: Standards Track                         Alcatel-Lucent
Expires: January 2018                                     C. Barcenilla
                                                              M. Cortes
                                                           J. Salvachua
                                                             J. Quemada
                                            Univ. Politecnica de Madrid
                                                    M. Narganes Quijano
                                                               Tecnalia
                                                  I. Martinez Sarriegui
                                                      L. Fajardo Ibanez
                                                        P. Lopez Zapico
                                                                 Optiva


                                                          July 18, 2017

                     The Quality for Service Protocol
                     draft-aranda-dispatch-q4s-04.txt

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with
   the provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

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   http://www.ietf.org/ietf/1id-abstracts.txt

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   This Internet-Draft will expire on January 18, 2018.






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Copyright Notice

   Copyright (c) 2012 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document. Please review these documents
   carefully, as they describe your rights and restrictions with
   respect to this document.

Abstract

   This memo describes an application level protocol for the standard
   communication of e2e QoS compliance information using a protocol
   based on Hypertext Transfer Protocol (HTTP), which forms the basis
   for the World Wide Web, and Session Description Protocol (SDP).
   Quality for Service Protocol (Q4S) provides a mechanism for latency,
   jitter, bandwidth and packet loss negotiation and monitoring,
   alerting whenever one of the negotiated conditions is violated.

   Implementation details on the actions to be triggered upon
   reception/detection of QoS alerts exchanged by the protocol are out
   of scope of this draft, it is application dependent (e.g. increase
   quality, reduce bit-rate) or even network dependent (e.g. change
   connection's quality profile).



Table of Contents


   1. Introduction...................................................5
      1.1. Motivation................................................6
      1.2. Summary of Features.......................................7
   2. Terminology....................................................8
   3. Overview of Operation..........................................8
   4. Q4S messages..................................................20
      4.1. Requests.................................................20
      4.2. Responses................................................21
      4.3. Header Fields............................................23
         4.3.1. Common Q4S Header Fields............................23
         4.3.2. Specific Q4S Request Header Fields..................23
         4.3.3. Specific Q4S Response Header Fields.................25
      4.4. Bodies...................................................25
         4.4.1. Encoding............................................25


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   5. Q4S method definitions........................................26
      5.1. BEGIN....................................................26
      5.2. READY....................................................26
      5.3. PING.....................................................27
      5.4. BWIDTH...................................................27
      5.5. Q4S-ALERT................................................27
      5.6. Q4S-RECOVERY.............................................28
      5.7. CANCEL...................................................29
   6. Response codes................................................29
      6.1. 100 Trying...............................................29
      6.2. Success 2xx..............................................29
         6.2.1. 200 OK..............................................29
      6.3. Redirection 3xx..........................................30
      6.4. Request Failure 4xx......................................30
         6.4.1. 400 Bad Request.....................................30
         6.4.2. 404 Not Found.......................................30
         6.4.3. 405 Method Not Allowed..............................30
         6.4.4. 406 Not Acceptable..................................30
         6.4.5. 408 Request Timeout.................................31
         6.4.6. 413 Request Entity Too Large........................31
         6.4.7. 414 Request-URI Too Long............................31
         6.4.8. 415 Unsupported Media Type..........................31
         6.4.9. 416 Unsupported URI Scheme..........................31
      6.5. Server Failure 5xx.......................................31
         6.5.1. 500 Server Internal Error...........................31
         6.5.2. 501 Not Implemented.................................31
         6.5.3. 503 Service Unavailable.............................32
         6.5.4. 504 Server Time-out.................................32
         6.5.5. 505 Version Not Supported...........................32
         6.5.6. 513 Message Too Large...............................32
      6.6. Global Failures 6xx......................................32
         6.6.1. 600 session does not exist..........................33
         6.6.2. 601 quality level not allowed.......................33
         6.6.3. 603 Session not allowed.............................33
         6.6.4. 604 authorization not allowed.......................33
   7. Protocol......................................................33
      7.1. Protocol Phases..........................................33
      7.2. SDP Structure............................................35
         7.2.1. "qos-level" attribute...............................36
         7.2.2. "alerting-mode" attribute...........................37
         7.2.3. "alert-pause" attribute.............................37
         7.2.4. "recovery-pause" attribute..........................37
         7.2.5. "public-address" attributes.........................38
         7.2.6. "latency" attribute.................................38
         7.2.7. "jitter" attribute..................................38
         7.2.8. "bandwidth" attribute...............................38
         7.2.9. "packetloss" attribute..............................38


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         7.2.10. "flow" attributes..................................39
         7.2.11. Measurement attributes.............................40
      7.3. Measurements.............................................41
         7.3.1. Latency.............................................42
         7.3.2. Jitter..............................................42
         7.3.3. Bandwidth...........................................43
         7.3.4. Packet loss.........................................45
      7.4. Handshake Phase..........................................46
      7.5. Negotiation phase........................................47
         7.5.1. Stage 0: Measurement of latencies and jitters.......49
         7.5.2. Stage 1: Measurement of bandwidth and packet loss...52
         7.5.3. Quality constraints not reached.....................55
            7.5.3.1. Actuator role..................................58
            7.5.3.2. Policy server role.............................59
         7.5.4. QoS Level changes...................................59
      7.6. Continuity phase.........................................60
      7.7. Termination Phase........................................63
      7.8. Dynamic constraints and flows............................64
      7.9. Qos-level upgrade and downgrade operation................65
   8. General User Agent behavior...................................67
      8.1. Roles in peer to peer scenarios..........................67
      8.2. Multiple Quality sessions in parallel....................68
      8.3. General client behavior..................................69
         8.3.1. Generating requests.................................70
      8.4. General server behavior..................................70
   9. Implementation Recommendations................................71
      9.1. Default client constraints...............................71
      9.2. Latency and Jitter measurements..........................71
      9.3. Bandwidth measurements...................................72
      9.4. Packet loss measurement resolution.......................72
      9.5. Measurements and reactions...............................73
      9.6. Instability treatments...................................73
      9.7. Scenarios................................................74
         9.7.1. Client to ACP.......................................74
         9.7.2. Client to client....................................75
   10. Security Considerations......................................75
   11. IANA Considerations..........................................75
   12. Conclusions..................................................79
   13. References...................................................80
      13.1. Normative References....................................80
      13.2. Informative References..................................81
   14. Acknowledgments..............................................82
   15. Authors' Addresses...........................................83






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1. Introduction

      The World Wide Web (WWW) is a distributed hypermedia system which
   has gained widespread acceptance among Internet users. Although WWW
   browsers support other, preexisting Internet application protocols,
   the native and primary protocol used between WWW clients and servers
   is the HyperText Transfer Protocol (HTTP) (RFC 2616 [1]).  The ease
   of use of the Web has prompted its widespread employment as a
   client/server architecture for many applications.  Many of such
   applications require the client and the server to be able to
   communicate each other and exchange information with certain quality
   constraints.

   Quality in communications at application level consists of four
   measurable parameters:

     o  Latency: The time a message takes to travel from source to
        destination. It may be approximated to RTT/2 (Round trip time),
        assuming the networks are symmetrical. In this context we will
        consider the statistical median formula.

     o  Jitter: latency variation. There are some formulas to calculate
        Jitter, and in this context we will consider the arithmetic
        mean formula.

     o  Bandwidth: bit rate of communication. To assure quality, a
        protocol MUST assure the availability of the bandwidth needed
        by the application.

     o  Packet loss: The percentage of packet loss is closely related
        to bandwidth and jitter. Affects bandwidth because a high
        packet loss implies sometimes retransmissions that also
        consumes extra bandwidth, other times the retransmissions are
        not achieved (for example in video streaming over UDP) and the
        information received is less than the required bandwidth. In
        terms of jitter, a packet loss sometimes is seen by the
        destination like a larger time between arrivals, causing a
        jitter growth.

   Any other communication parameter such as throughput is not a
   network parameter because depends on protocol window size and other
   implementation-dependent aspects.

   Q4S provides a mechanism for quality monitoring based on an HTTP
   syntax and SDP in order to be easily integrated in WWW, but it may
   be used by any type of application, not only those based on HTTP.
   Quality requirements may be needed by any type of application that


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   communicates using any kind of protocol, especially those with real-
   time constraints. Depending on the nature of each application the
   constraints may be different leading to different parameter
   thresholds that need to be met.

   Q4S is an application level Client/Server protocol that continuously
   measures session quality for a given flow (or set of flows), end-to-
   end and in real-time; raising alerts if quality parameters are below
   a given pre-negotiated threshold and sending recoveries when quality
   parameters are restored. Q4S describes when these notifications,
   alerts and recoveries, need to be sent and the entity receiving
   them. The actions undertaken by the receiver of the alert are out of
   scope of the protocol.

   Q4S is session-independent from the application flows, to minimize
   the impact on them. To perform the measurements, two control flows
   are created on both communication paths (forward and reverse
   directions).

        1.1. Motivation

   Monitoring quality of service (QoS) in computer networks is useful
   for several reasons:

     o  Enable real-time services and applications to verify whether
        network resources achieve a certain QoS level. This helps real-
        time services and applications to run through the Internet,
        allowing the existence of Application Content providers (ACPs)
        which offer guaranteed real-time services to the final users.

     o  Real-time monitoring allows applications to adapt themselves to
        network conditions (Application-based QoS) and/or request more
        network quality to the ISP (if the ISP offers this
        possibility).

     o  Monitoring may also be required by Peer to Peer (P2P) real-time
        applications for which Q4S can be used

     o  Enable ISPs to offer QoS to any ACP or final user application
        in an accountable way

     o  Enable e2e negotiation of QoS parameters, independently of the
        Internet service providers of both endpoints.

   A protocol to monitor QoS must address the following issues:




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     o  Must be ready to be used in conjunction with current standard
        protocols and applications, without forcing a change on them.

     o  Must have a formal and compact way to specify quality
        constraints of the desired application to run.

     o  Must have measurement mechanisms avoiding application
        disruption, minimizing network resources consumption.

     o  Must have specific messages to alert about the violation of
        quality constraints in different directions (forward and
        reverse), because network routing may not be symmetrical, and
        of course, quality constraints may not be symmetrical.

     o  After having alerted about the violation of quality
        constraints, must have specific messages to inform about
        recovery of quality constraints in corresponding directions
        (forward and reverse).

     o  Must protect the data (constrains, measurements, QoS levels
        demanded from the network) in order to avoid the injection of
        malicious data in the measurements.



        1.2. Summary of Features

      Quality for Service (Q4S) is a message-oriented communication
   protocol that can be used in conjunction with any other application-
   level protocol. Q4S protocol is a measurement protocol. Any taken
   action derived from its measurements are out of scope of the
   protocol. These actions depend on application provider and may be
   application-level adaptive reactions, may involve requests to ISP,
   or whatever application provider decide.

   The benefits in quality measurements provided by Q4S can be used by
   any type of application that uses any type of protocol for data
   transport. It provides a quality monitoring scheme for any
   communication that takes place between the client and the server,
   not only for the Q4S communication itself.

   Q4S does not establish multimedia sessions and it does not transport
   application data. It monitors the fulfillment of the quality
   requirements of the communication between the client and the server,
   and therefore does not impose any restrictions on the type of
   application, protocol or the type of usage of the monitored quality
   connection.


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   Some applications may vary their quality requirements dynamically
   for any given quality parameter. Q4S is able to adapt to the
   changing application needs modifying the parameter thresholds to the
   new values and monitoring the network quality according to the new
   quality constraints. It will raise alerts if the new constraints are
   violated.

   Q4S session lifetime is composed of four phases with different
   purposes: Handshake, Negotiation, Continuity and Termination.
   Negotiation and Continuity phases perform network parameter
   measurements as per a negotiated measurement procedure. Different
   measurement procedures COULD be used inside Q4S, although one
   default measurement mechanism is needed for compatibility reasons
   and is the one defined in this draft. Basically, Q4S defines how to
   transport application quality requirements and measurement results
   between client and server and providing monitoring and alerting too.

   Q4S MUST be executed just before starting a client-server
   application which needs a quality connection in terms of latency,
   jitter, bandwidth and/or packet loss. Once client and server have
   succeeded in establishing communication under quality constraints,
   the application can start, and Q4S continues measuring and alerting
   if necessary.

   The quality parameters can be suggested by the client in the first
   message of the handshake phase, but it's the server that accepts
   these parameter values or forces others. The server is in charge of
   deciding the final values of quality connection.



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 RFC 2119 [3].

3. Overview of Operation

      This section introduces the basic operation of Q4S using simple
   examples.  This section is of tutorial nature and does not contain
   any normative statements.

   The first example shows the basic functions of a Q4S: communication
   establishment between a client and a server, quality requirement
   negotiations for the requested application, application start and



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   continuous quality parameter measurements, and finally communication
   termination.

   The client triggers the establishment of the communication
   requesting a specific service or application from the server. This
   first message must have a special URI (RFC 3986), which may force
   the use of the Q4S protocol if it is implemented in a standard web
   browser. This message consists of a Q4S BEGIN method, which can
   optionally include a proposal for the communication quality
   requirements in an SDP body. This option gives the client a certain
   negotiation capacity about quality requirements, but it will be the
   server who finally decides about the stated requirements.

   This request is answered by the server with a Q4S 200 OK response
   letting the client know that it accepts the request. This response
   message MUST contain an SDP body with:

     o  The assigned Q4S session id.

     o  The quality constraints required by the requested application.

     o  The measurement procedure to use.

     o  The alerting mode: there are two different scenarios for
        sending alerts that trigger actions either on the network or in
        the application when measurements identify violated quality
        constraints. In both cases, alerts are triggered by the server.

          o a) Q4S-aware-network scenario: the network is Q4S aware,
            and reacts by itself to these alerts. In this scenario Q4S
            ALERT messages are sent by the server to the client, and
            network elements inspect and process these alert message.
            The alerting mode in this scenario is called Q4S-aware-
            network alerting mode.

          o b) Reactive scenario: the network is not Q4S aware. In this
            scenario alert notifications are sent to a specific node,
            called Actuator, which is in charge of taking decisions
            regarding what actions to trigger: either to change
            application behavior to adapt it to network conditions
            and/or invoke a network policy server in order to
            reconfigure the network and request more quality for
            application flows.






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            +------+                           +-----------+
            |  App |<----- app flows---------->|Application|
            |Client|                           +-----------+
            +------+                                 A
                                                     |
            +------+             +------+       +--------+
            | Q4S  |<----Q4S---->| Q4S  |<----->|Actuator|
            |Client|             |Server|       +--------+
            +------+             +------+            |
                                                     V
                                              +-------------+
                                              |policy server|
                                              +-------------+

                        Figure 1 Reactive scenario


     o  The format of messages exchanged between the server stack and
        the Actuator, doesn't follow Q4S codification rules, but their
        format will be implementation dependent. In this way, we will
        call "notifications" to the messages sent from the server stack
        to the Actuator (e.g. alert notifications), and "acknowledges"
        to messages sent from the Actuator to the server stack in
        response to notifications (e.g. alert acknowledges).alert-
        pause: the amount of time between consecutive alerts. In the
        Q4S-aware-network scenario, the server has to wait this period
        of time between Q4S ALERT messages sent to the client. In the
        Reactive scenario, the server stack has to wait this period of
        time between alert notifications sent to the Actuator.
        Measurements are not stopped in Negotiation or Continuity
        Phases during this period of time, but no alerts are fired even
        with violated network quality constraints in order to leave
        time for network reconfiguration or for application adjustments

     o  recovery-pause: The amount of time the Q4S server waits before
        trying to recover initial qos-level. After having detected
        violation of quality constraints several times, the qos-level
        will have been increased accordingly. If this violation
        detection finally stops, the server waits for a period of time
        (recovery time) and if the situation persists, it tries to
        recover to previous qos-level values gradually by sending Q4S
        RECOVERY messages to the client, in the Q4S-aware-network
        scenario, or recovery notifications to the Actuator, in the
        Reactive scenario.




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   It is important to highlight that any Q4S 200 OK response sent by
   the server to the client at any time during the life of a quality
   session may contain an SDP body with new values of quality
   constraints required by the application. Depending on the phase and
   the state of the measurement procedure within the specific phase,
   the client will react accordingly so as to take into account the new
   quality constraints in the measurement procedure.

   Once the communication has been established (handshake phase is
   finished), the protocol will verify that the communication path
   between the client and the server meets the quality constraints on
   both directions, from and to the server (negotiation phase). This
   negotiation phase requires taking measurements of the quality
   parameters: latencies, jitter, bandwidth and packet loss. This phase
   is initiated with a client message containing a Q4S READY method,
   which will be answered by the server with a Q4S 200 OK response.

   Negotiation measurements are achieved in two sequential stages:

     o  Stage 0: latency and jitter measurements

     o  Stage 1: bandwidth and packet loss measurements

   Stage 0 measurements are being taken through Q4S PING messages sent
   both from both the client and the server. All Q4S PING requests will
   be answered by Q4S 200 OK messages to allow for bidirectional
   measurements.

   After a pre-agreed number of measurements have been performed,
   determined by the measurement procedure sent by the server, three
   scenarios may be possible:

   a) Measurements do not meet the requirements: in this case the stage
      0 is repeated after sending an alert from the server to the
      client or from the server stack to the Actuator, depending on the
      alerting mode defined in the Handshake phase. Notice that
      measurements continue to be taken but no alerts are fired during
      the alert-pause time. In the Reactive scenario, the Actuator will
      decide either to forward the alert notification to the network
      policy server or to the application, depending on where
      reconfiguration actions have to be taken.

   b) Measurements do meet the requirements: in this case client moves
      to stage 1 sending a new READY message.

   c) At any time during the measurement procedure, the Q4S 200 OK
      message sent by the server to the client, in response to a Q4S


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      PING message, contains an SDP body with new values of quality
      constraints required by the application; this means the
      application has varied their quality requirements dynamically and
      therefore quality thresholds used while monitoring quality
      parameters have to be changed to the new constraints. In this
      case the client moves to the beginning of the Stage 0 for
      initiating the negotiation measurements again.

   Stage 1 is optional. Its purpose is to measure the availability of
   application needed bandwidth. This stage can be skipped by client
   sending a "READY 2" message after completion of stage 0 when
   bandwidth requirements is set to cero kbps in the SDP. Stage 1
   measurements are achieved through Q4S BWIDTH messages sent both from
   the client and the server. Unlike PING messages, Q4S BWIDTH requests
   will not be answered.

   If Stage 0 and 1 meet the application quality constraints, the
   application may start. Q4S will enter the continuity phase measuring
   the network quality parameters through the Q4S PING message exchange
   on both connection paths, and raising alerts in case of violation. .

   Once the client wants to terminate the quality session it sends a
   Q4S CANCEL message, which will be acknowledged by the server with
   another Q4S CANCEL message. Termination of quality sessions are
   always initiated by the client because Q4S TCP requests follow the
   client server schema.

   This figure depicts the message exchange in a successful scenario.





















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               +-------------------------------------------+
               |                                           |
               | Client                             Server |
               |                                           |
   Handshake   |     --------- Q4S BEGIN ----------->      |
               |     <-------- Q4S 200 OK -----------      |
               |                                           |
   Negotiation |                                           |
   (Stage 0)   |     --------- Q4S READY 0---------->      |
               |     <-------- Q4S 200 OK -----------      |
               |                                           |
               |     --------- Q4S PING ------------>      |
               |     <-------- Q4S 200 OK -----------      |
               |     <-------- Q4S PING -------------      |
               |      -------- Q4S 200 OK ---------->      |
               |     --------- Q4S PING ------------>      |
               |     <-------- Q4S PING -------------      |
               |     --------- Q4S 200 OK ---------->      |
               |     <-------- Q4S 200 OK -----------      |
               |                    ...                    |
   Negotiation |                                           |
   (Stage 1)   |     --------- Q4S READY 1---------->      |
               |     <-------- Q4S 200 OK -----------      |
               |                                           |
               |     --------- Q4S BWITDH ---------->      |
               |     <-------- Q4S BWIDTH------------      |
               |     --------- Q4S BWITDH ---------->      |
               |     <-------- Q4S BWIDTH------------      |
               |                    ...                    |
   Continuity  |     --------- Q4S READY 2 --------->      |
               |     <-------- Q4S 200 OK -----------      | app start
               |                                           |
               |     --------- Q4S PING ------------>      |
               |     <-------- Q4S 200 OK -----------      |
               |     <-------- Q4S PING -------------      |
               |      -------- Q4S 200 OK ---------->      |
               |                                           |
   Termination |     --------- Q4S CANCEL ---------->      | app end
               |     <-------- Q4S CANCEL -----------      |
               |                                           |
               +-------------------------------------------+
                 Figure 2 Successful Q4S message exchange.







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   Client and server measurements are included into PING and BWIDTH
   messages, allowing both sides of the communication to be are aware
   of all measurements in both directions.

   The following two examples show the behavior of the Q4S protocol
   when: quality constraints are violated, alerts are generated; and,
   later on, violation of quality constraints stops leadig to the
   execution of the recovery process. The first example shows the Q4S-
   aware-network alerting mode scenario:








































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               +-------------------------------------------+
               |                                           |
               | Client                             Server |
               |                                           |
   Handshake   |     --------- Q4S BEGIN ----------->      |
               |     <-------- Q4S 200 OK -----------      |
               |                                           |
   Negotiation |                                           |
   (Stage 0)   |     --------- Q4S READY 0---------->      |
               |     <-------- Q4S 200 OK -----------      |
               |                                           |
               |     --------- Q4S PING ------------>      |
               |     <-------- Q4S 200 OK -----------      |
               |     <-------- Q4S PING -------------      |
               |      -------- Q4S 200 OK ---------->      |
               |                    ...                    |
               |                                           |
               |     <-------- Q4S ALERT ------------      |
               |     -------- Q4S ALERT ------------>      |
               |          (alert-pause start)              |
   Repetition  |                                           |
   of Stage 0  |     --------- Q4S READY 0---------->      |
               |     <-------- Q4S 200 OK -----------      |
               |                                           |
               |     --------- Q4S PING ------------>      |
               |     <-------- Q4S 200 OK -----------      |
               |     <-------- Q4S PING -------------      |
               |                    ...                    |
   Negotiation |                                           |
   (Stage 1)   |     --------- Q4S READY 1---------->      |
               |     <-------- Q4S 200 OK -----------      |
               |                                           |
               |     --------- Q4S BWITDH ---------->      |
               |     <-------- Q4S BWIDTH------------      |
               |                    ...                    |
               |                                           |
   Continuity  |     --------- Q4S READY 2 --------->      |
               |     <-------- Q4S 200 OK -----------      | app start
               |                                           |
               |     --------- Q4S PING ------------>      |
               |     <-------- Q4S 200 OK -----------      |
               |     <-------- Q4S PING -------------      |
               |      -------- Q4S 200 OK ---------->      |
               |                    ...                    |
               |(alert-pause expires &                     |
               |                   violated constraints)   |
               |     <-------- Q4S ALERT ------------      |


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               |     --------- Q4S ALERT ----------->      |
               |                                           |
               |           (alert-pause start)             |
               |     --------- Q4S PING ------------>      |
               |     <-------- Q4S 200 OK -----------      |
               |     <-------- Q4S PING -------------      |
               |     --------- Q4S 200 OK ---------->      |
               |                    ...                    |
               |(alert-pause expires &                     |
               |                   violated constraints)   |
               |     <-------- Q4S ALERT ------------      |
               |     --------- Q4S ALERT ----------->      |
               |           (alert-pause)                   |
               |     --------- Q4S PING ------------>      |
               |     <-------- Q4S 200 OK -----------      |
               |     <-------- Q4S PING -------------      |
               |      -------- Q4S 200 OK ---------->      |
               |                    ...                    |
               |(alert-pause expires &                     |
               |                 Fullfilled constraints)   |
               |                                           |
               |           (recovery-pause start)          |
               |                                           |
               |     --------- Q4S PING ------------>      |
               |     <-------- Q4S 200 OK -----------      |
               |     <-------- Q4S PING -------------      |
               |      -------- Q4S 200 OK ---------->      |
               |                    ...                    |
               |(recovery-pause expires &                  |
               |                 Fullfilled constraints)   |
               |     --------- Q4S RECOVERY --------->     |
               |     <-------- Q4S RECOVERY -----------    |
               |                                           |
               |          (recovery-pause start)           |
               |     --------- Q4S PING ------------>      |
               |     <-------- Q4S 200 OK -----------      |
               |     <-------- Q4S PING -------------      |
               |      -------- Q4S 200 OK ---------->      |
               |                    ...                    |
               |                                           |
   Termination |     --------- Q4S CANCEL ---------->      | app end
               |     <-------- Q4S CANCEL -----------      |
               |                                           |
               +-------------------------------------------+
                 Figure 3 Q4S-aware-network alerting mode.




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   In this Q4S-aware-network alerting mode scenario, the server may
   send Q4S alerts to the client at any time on detection of violated
   quality constraints. This alerting exchange must not interrupt the
   continuity quality parameter measurements between client and server.


   The second example depicted in the following figure represents the
   Reactive scenario, in which alert notifications are sent from the
   server stack to the Actuator who is in charge of deciding either to
   act over application behavior and/or invoke a network policy server.
   The Actuator is an entity that has a pre-defined set of different
   quality levels and decides how to act depending on the actions
   stated for each of these levels; it can take actions for making
   adjustments on the application or it can send a request to the
   policy server for acting on the network. The policy server also has
   a pre-defined set of different quality levels pre-agreed upon
   between the Application Content Provider and the ISP. The Reactive
   alerting mode is the default mode.































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               +-------------------------------------------+
               |                                           |
               | Client               Server      Actuator |
   Handshake   |   ----- Q4S BEGIN ----->                  |
               |   <---- Q4S 200 OK -----                  |
               |                                           |
   Negotiation |                                           |
   (Stage 0)   |   ----- Q4S READY 0---->                  |
               |   <---- Q4S 200 OK -----                  |
               |                                           |
               |   ----- Q4S PING ------>                  |
               |   <---- Q4S 200 OK -----                  |
               |   <---- Q4S PING -------                  |
               |    ---- Q4S 200 OK ---->                  |
               |              ...                          |
               |  (alert-pause start)                      |
               |                          --alert          |
               |                         notification-->   |
               |                                           |
               |                         <--alert          |
               |                          acknowledge---   |
               |                                           |
   Repetition  |                                           |
   of Stage 0  |   ----- Q4S READY 0---->                  |
               |   <---- Q4S 200 OK -----                  |
               |                                           |
               |   ----- Q4S PING ------>                  |
               |   <---- Q4S 200 OK -----                  |
               |   <---- Q4S PING -------                  |
               |              ...                          |
               |(alert-pause expires &                     |
               |                   violated constraints)   |
               |                                           |
               |                         --alert           |
               |                         notification-->   |
               |                                           |
               |                         <--alert          |
               |                          acknowledge---   |
               |                                           |
               |   ----- Q4S PING ------>                  |
               |   <---- Q4S 200 OK -----                  |
               |   <---- Q4S PING -------                  |
               |              ...                          |
   Negotiation |                                           |
   (Stage 1)   |   ----- Q4S READY 1---->                  |
               |   <---- Q4S 200 OK -----                  |


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               |                                           |
               |   ----- Q4S BWITDH ---->                  |
               |   <---- Q4S BWIDTH------                  |
               |              ...                          |
   Continuity  |   ----- Q4S READY 2 --->                  |
               |   <---- Q4S 200 OK -----                  | app start
               |                                           |
               |(alert-pause expires &                     |
               |                  fulfilled constraints)   |
               |                                           |
               |(recovery-pause start)                     |
               |   ----- Q4S PING ------>                  |
               |   <---- Q4S 200 OK -----                  |
               |   <---- Q4S PING -------                  |
               |   ----- Q4S PING ------>                  |
               |                                           |
               |(recovery-pause expires &                  |
               |                  fulfilled constraints)   |
               |                                           |
               |                         --alert           |
               |                         notification-->   |
               |                                           |
               |                         <--alert          |
               |                          acknowledge---   |
               |                                           |
               |(recovery-pause start)                     |
               |   <---- Q4S 200 OK -----                  |
               |   <---- Q4S PING -------                  |
               |   ----- Q4S 200 OK ---->                  |
               |   ----- Q4S PING ------>                  |
               |              ...                          |
               |                                           |
   Termination |   ----- Q4S CANCEL ---->                  | app end
               |                          --cancel         |
               |                          notification-->  |
               |                                           |
               |                          <--cancel        |
               |                           acknowledge--   |
               |   <---- Q4S CANCEL -----                  |
               |                                           |
               +-------------------------------------------+
                     Figure 4 Reactive alerting mode.

   At the end of any Negotiation phase stage, the server sends an alert
   notification to the Actuator if quality constraints are violated.
   During the period of time defined by the alert-pause parameter, no
   further alert notifications are sent, but measurements are not


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   interrupted. This way, both the client and the server will detect
   network improvements as soon as possible. In a similar way, during
   the continuity phase, the server may send alert notifications at any
   time to the Actuator on detection of violated quality constraints.
   This alerting exchange must not interrupt the continuity
   measurements between client and server.

   Finally, in the Termination phase, Q4S CANCEL messages sent from the
   client to the server must be forwarded from the server to the
   Actuator in order to release possible assigned resources for the
   session.



4. Q4S messages

   Q4S is a text-based protocol and uses the UTF-8 charset (RFC 3629
   [11]). A Q4S message is either a request or a response.

   Both Request and Response messages use the basic format of Internet
   Message Format (RFC 5322 [12]). Both types of messages consist of a
   start-line, one or more header fields, an empty line indicating the
   end of the header fields, and an optional message-body.


         generic-message  =  start-line
                             *message-header
                             CRLF
                             [ message-body ]
         start-line       =  Request-Line / Status-Line

   The start-line, each message-header line, and the empty line MUST be
   terminated by a carriage-return line-feed sequence (CRLF).  Note
   that the empty line MUST be present even if the message-body is not.

      Much of Q4S's messages and header field syntax are identical to
   HTTP/1.1. However, Q4S is not an extension of HTTP.



        4.1. Requests

   Q4S requests are distinguished by having a Request-Line for a start-
   line. A Request-Line contains a method name, a Request-URI, and the
   protocol version separated by a single space (SP) character.



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   The Request-Line ends with CRLF. No CR or LF are allowed except in
   the end-of-line CRLF sequence. No linear whitespace (LWS) is allowed
   in any of the elements.

         Request-Line  =  Method SP Request-URI SP Q4S-Version CRLF

   Method: This specification defines seven methods: BEGIN for starting
          and negotiate quality sessions, READY for synchronization of
          measurements, PING and BWIDTH for quality measurements
          purpose, CANCEL for terminating sessions, Q4S-ALERT for
          quality violations reporting, and Q4S-RECOVERY for quality
          recovery reporting.

   Request-URI: The Request-URI is a Q4S URI (RFC 2396) as described in
          7.4. The Request-URI MUST NOT contain unescaped spaces or
          control characters and MUST NOT be enclosed in "<>".

   Q4S-Version: Both request and response messages include the version
          of Q4S in use. To be compliant with this specification,
          applications sending Q4S messages MUST include a Q4S-Version
          of "Q4S/1.0".  The Q4S-Version string is case-insensitive,
          but implementations MUST send upper-case. Unlike HTTP/1.1,
          Q4S treats the version number as a literal string.  In
          practice, this should make no difference.

        4.2. Responses

   Q4S responses are distinguished from requests by having a Status-
   Line as their start-line. A Status-Line consists of the protocol
   version followed by a numeric Status-Code and its associated textual
   phrase, with each element separated by a single SP character. No CR
   or LF is allowed except in the final CRLF sequence.

     Status-Line  =  Q4S-Version SP Status-Code SP Reason-Phrase CRLF

   The Status-Code is a 3-digit integer result code that indicates the
   outcome of an attempt to understand and satisfy a request. The
   Reason-Phrase is intended to give a short textual description of the
   Status-Code.  The Status-Code is intended for use by automata,
   whereas the Reason-Phrase is intended for the human user. A client
   is not required to examine or display the Reason-Phrase.

   While this specification suggests specific wording for the reason
   phrase, implementations MAY choose other text, for example, in the


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   language indicated in the Accept-Language header field of the
   request.

   The first digit of the Status-Code defines the class of response.
   The last two digits do not have any categorization role.  For this
   reason, any response with a status code between 100 and 199 is
   referred to as a "1xx response", any response with a status code
   between 200 and 299 as a "2xx response", and so on.  Q4S/1.0 allows
   following values for the first digit:


         1xx: Provisional -- request received, continuing to process
   the request;

         2xx: Success -- the action was successfully received,
              understood, and accepted;

         3xx: Redirection -- further action needs to be taken in order
              to complete the request;

         4xx: Request Failure -- the request contains bad syntax or
              cannot be fulfilled at this server;

         5xx: Server Error -- the server failed to fulfill an
              apparently valid request;

         6xx: Global Failure -- the request cannot be fulfilled at any
              server.

   The status codes are the same described in HTTP (RFC 2616 [1]). In
   the same way as HTTP, Q4S applications are not required to
   understand the meaning of all registered status codes, though such
   understanding is obviously desirable. However, applications MUST
   understand the class of any status code, as indicated by the first
   digit, and treat any unrecognized response as being equivalent to
   the x00 status code of that class.

   The Q4S-ALERT, Q4S-RECOVERY and CANCEL requests do not have to be
   responded. However, after receiving a Q4S-ALERT, Q4S-RECOVERY or
   CANCEL request, the server SHOULD sends a Q4S-ALERT, Q4S-RECOVERY or
   CANCEL request to the client








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        4.3. Header Fields

   Q4S header fields are identical to HTTP header fields in both syntax
   and semantics.

   Some header fields only make sense in requests or responses. These
   are called request header fields and response header fields,
   respectively.  If a header field appears in a message not matching
   its category (such as a request header field in a response), it MUST
   be ignored.



      4.3.1. Common Q4S Header Fields

   These fields may appear in Request and Response messages.

     o  Session-Id: the value for this header is the same session id
        used in SDP (embedded in "o" SDP parameter) and is assigned by
        the server. The messages without SDP MUST include this header.
        If a message has and SDP body, this header is optional. The
        method of <session id> allocation is up to the creating tool,
        but it is suggested that a UTC timestamp be used to ensure
        uniqueness.

     o  Sequence-Number: sequential and cyclic positive integer number
        assigned to PING and BWIDTH messages, and acknowledged in 200
        OK responses.

     o  Timestamp: this optional header contains the system time (with
        the best possible accuracy). It indicates the time in which the
        PING request was sent. If this header is present in PING
        messages, then the 200 OK response messages include this value.

     o  Stage: this is used in client's READY requests and server's
        200 OK responses during the Negotiation and Continuity phases
        in order to synchronize the initiation of the measurements.
        Example:  Stage: 0



      4.3.2. Specific Q4S Request Header Fields

   In addition to HTTP header fields, these are the specific Q4S
   request header fields




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     o  User-Agent: this header contains information about the
        implementation of the user agent. This is for statistical
        purposes, the tracing of protocol violations, and the automated
        recognition of user agents for the sake of tailoring responses
        to avoid particular user agent limitations. User agents SHOULD
        include this field with requests. The field can contain
        multiple product tokens and comments identifying the agent and
        any sub-products which form a significant part of the user
        agent. By convention, the product tokens are listed in order of
        their significance for identifying the application.

     o  Signature: this header contains a digital signature that can be
        used by the network or policy server to validate the SDP,
        preventing security attacks. The signature is an optional
        header generated by the server according to the pre-agreed
        security policies between the Application Content Provider and
        the ISP. For example, a hash algorithm and encryption method
        such as MD5 (RFC 1321 [6]) and RSA (RFC 2437 [7]) based on the
        server certificate could be used. This certificate is supposed
        to be delivered by a Certification Authority (CA) or policy
        owner to the server. The signature is applied to the SDP body.

         Signature= RSA ( MD5 (<sdp>), <certificate> )

        If the signature is not present, other validation mechanism may
        be implemented in order to provide assured quality with
        security and control.

     o  Measurements: this header carries the measurements of the
        quality parameters in PING and BWIDTH requests. The format is:

        Measurements: "l=" " "|[0..9999] ", j=" " "|[0..9999] ", pl=" "
        "|[0.00 .. 100.00] ", bw=" " "|[0..9999]

        Where "l" stands for latency followed by the measured value or
        an empty space, "j" stands for jitter followed by the measured
        value or an empty space, "pl" stands for packetloss followed by
        the measured value in % or an empty space and "bw" stands for
        bandwidth followed by the measured value or an empty space.










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      4.3.3. Specific Q4S Response Header Fields

     o  Expires: its purpose is to provide a sanity check and allow the
        server to close inactive sessions. If the client does not send
        a new request before the expiration time, the server MAY close
        the session. The value MUST be an integer and the measurement
        units are milliseconds.

        In order to keep the session open the server MUST send a Q4S
        alert before the session expiration (Expires header), with the
        same quality levels and an alert cause of "keep-alive". The
        purpose of this alert is to avoid TCP sockets (which were
        opened with READY message) from being closed, specially in NAT
        scenarios.



        4.4. Bodies

     Requests, including new requests defined in extensions to this
   specification, MAY contain message bodies unless otherwise noted.
   The interpretation of the body depends on the request method.

   For response messages, the request method and the response status
   code determine the type and interpretation of any message body.  All
   responses MAY include a body.

   The Internet media type of the message body MUST be given by the
   Content-Type header field.

      4.4.1. Encoding

      The body MUST NOT be compressed. This mechanism is valid for
   other protocols such as HTTP and SIP (RFC 3261 [13]), but
   a compression/coding scheme will limit certain logical
   implementations of the way the request is parsed, thus, making the
   protocol concept more implementation dependent. In addition,
   bandwidth calculation may not be valid if compression is used.
   Therefore, the HTTP request header "Accept-Encoding" cannot be used
   in Q4S with different values than "identity" and if it is present in
   a request, the server MUST ignore it. In addition, the response
   header "Content-Encoding" is optional, but if present, the unique
   permitted value is "identity".

   The body length in bytes is provided by the Content-Length header
   field. The "chunked" transfer encoding of HTTP/1.1 MUST NOT be used
   for Q4S (Note: The chunked encoding modifies the body of a message


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   in order to transfer it as a series of chunks, each one with its own
   size indicator.)



5. Q4S method definitions

   The Method token indicates the method to be performed on the
   resource identified by the Request-URI. The method is case-
   sensitive.

          Method  = "BEGIN" | "READY" | "PING" | "BWIDTH" |
                    "Q4S-ALERT" | "Q4S-RECOVERY" | "CANCEL" |
   extension-method

          extension-method = token

   The list of methods allowed by a resource can be specified in an
   "Allow" header field (RFC 2616 [1] section 14.7). The return code of
   the response always notifies the client when a method is currently
   allowed on a resource, since the set of allowed methods can change
   dynamically. Any server application SHOULD return the status code
   405 (Method Not Allowed) if the method is known, but not allowed for
   the requested resource, and 501 (Not Implemented) if the method is
   unrecognized or not implemented by the server.

        5.1. BEGIN

   The BEGIN method requests information from a resource identified by
   a Q4S URI. The semantics of this method is the starting of a quality
   session.

   This method is only used during the handshake phase to retrieve the
   SDP containing session id and all quality and operation parameters
   for the desired application to run.

   When a BEGIN message is received by the server, any current quality
   session is cancelled and a new session should be created.

   The response to a Q4S BEGIN request is not cacheable.

        5.2. READY

   The READY method is used to synchronize the starting time for
   sending of PING and BWIDTH messages over UDP between clients and
   servers. The stage header included in this method is mandatory.



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   This message is only used in negotiation and continuity phases, and
   only just before making a measurement. Otherwise (out of this
   context), the server MUST ignore this method.

        5.3. PING

   This message is used during the negotiation and continuity phases to
   measure the RTT and jitter of a session. The message MUST be sent
   only over UDP ports.

   The fundamental difference between the PING and BWIDTH requests is
   reflected in the different measurements achieved with them. PING is
   a short message, and MUST be answered in order to measure RTT and
   jitter, whereas BWIDTH is a long message (1 Kbyte or more) and MUST
   NOT be answered.

   PING is a request method that can be originated by client but also
   by server. Client MUST also answer the server PING messages,
   assuming a "server role" for these messages during measurement
   process.

   The Measurements header included in this method is mandatory, and
   provides updated measurements values for latency, jitter and packet
   loss to the counterpart.

        5.4. BWIDTH

   This message is used only during the Negotiation phase to measure
   the bandwidth and packet loss of a session. The message MUST be sent
   only over UDP ports.

   BWIDTH is a request method that can be originated by the client but
   also by server. Both (client and server) MUST NOT answer BWIDTH
   messages.

   The Measurements header included in this method is mandatory, and
   provides updated measurements values for bandwidth and packet loss
   to the counterpart.



        5.5. Q4S-ALERT

   This is the request message that Q4S generates when the measurements
   indicate that quality constraints are being violated. It is used
   during the negotiation and continuity phases.



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   This informative message indicates that the user experience is being
   degraded and includes the details of the problem (bandwidth, jitter,
   packet loss measurements). The Q4S-ALERT message does not contain
   any detail on the actions to be taken, which depends on the
   agreements between all involved parties.

   Q4S-ALERT request does not have to be answered with a response
   message unless there is an error condition. However, after receiving
   a Q4S-ALERT request, the counterpart answers with a Q4S-ALERT
   request. The response to a Q4S-ALERT request is not cacheable.

   This method is always initiated by the server in both alerting
   modes. In Q4S-aware-network alerting mode, the Q4S-ALERT messages
   are fired by the server and sent to the client, advising the network
   to react by itself. In Reactive alerting mode. alert notifications
   are triggered by the server stack and sent to the Actuator.(see
   Figure1 "Reactive Scenario")

   Client----q4s----SERVER STACK--->ACTUATOR-->APP OR POLICY SERVER

   The way in which the server stack notifies the Actuator is
   implementation dependent, and the communication between the Actuator
   and the network policy server is defined by the protocol and API
   that the policy server implements.

        5.6. Q4S-RECOVERY

   This is the request message that Q4S generates when the measurements
   indicate that quality constraints were being violated but they have
   been fulfilled during a period of time already (recovery pause). It
   is used during the negotiation and continuity phases.

   This informative message indicates that the qos-level could be
   increased gradually until the initial qos-level is recovered (the
   qos-level established at the beginning os the session of that was
   decreased during violation of constraints). The Q4S-RECOVERY message
   does not contain any detail on the actions to be taken, which
   depends on the agreements between all involved parties.

   Q4S- RECOVERY request does not have to be answered with a response
   message unless there is an error condition. However, after receiving
   a Q4S- RECOVERY request, the counterpart answers with a Q4S-
   RECOVERY request. The response to a Q4S- RECOVERY request is not
   cacheable.

   As for the Q4S-ALERT message, the Q4S-RECOVERY method is always
   initiated by the server in both alerting modes. In Q4S-aware-network


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   alerting mode, the Q4S-RECOVERY messages are fired by the server and
   sent to the client, advising the network to react by itself. In
   Reactive alerting mode, recovery notifications are triggered by the
   server stack and sent to the Actuator.(see Figure1 "Reactive
   Scenario")

        5.7. CANCEL

   The semantics of CANCEL message is the release of the Q4S session id
   and the possible resources assigned to the session. This message
   could be triggered by Q4S stack or by the application using the
   stack (through an implementation dependent API).

   In the same way as Q4S-ALERT, CANCEL must not be answered with a
   response message. However, if the server receives a CANCEL message,
   it must answer with a CANCEL request message towards the client,
   acknowledging the reception.

   In the Reactive scenario, the server stack MUST react to the Q4S
   CANCEL messages received from the client by forwarding a cancel
   notification to the Actuator, in order to release possible assigned
   resources for the session (at application or at policy server). The
   Actuator must answer the cancel notification with a cancel
   acknowledge towards the server stack, acknowledging the reception.



6. Response codes

   Q4S response codes are used for TCP and UDP. However, in UDP only
   the response code 200 is used.

        6.1. 100 Trying

   This response indicates that the request has been received by the
   next-hop server and that some unspecified action is being taken on
   behalf of this request (for example, a database is being consulted).
   This response, like all other provisional responses, stops
   retransmissions of a Q4S-ALERT during the alert-pause time.

        6.2. Success 2xx

   2xx responses give information about success of a request.

      6.2.1. 200 OK

   The request has succeeded.


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        6.3. Redirection 3xx

   3xx responses give information about the user's new location, or
   about alternative services that might be able to satisfy the
   request.

   The requesting client SHOULD retry the request at the new
   address(es) given by the Location header field.

        6.4. Request Failure 4xx

   4xx responses are definite failure responses from a particular
   server. The client SHOULD NOT retry the same request without
   modification (for example, adding appropriate headers or SDP
   values). However, the same request to a different server might be
   successful.

      6.4.1. 400 Bad Request

   The request could not be understood due to malformed syntax. The
   Reason-Phrase SHOULD identify the syntax problem in more detail, for
   example, "Missing Sequence-Number header field".

      6.4.2. 404 Not Found

   The server has definitive information that the user does not exist
   at the domain specified in the Request-URI. This status is also
   returned if the domain in the Request-URI does not match any of the
   domains handled by the recipient of the request.

      6.4.3. 405 Method Not Allowed

   The method specified in the Request-Line is understood, but not
   allowed for the address identified by the Request-URI.

   The response MUST include an Allow header field containing a list of
   valid methods for the indicated address.

      6.4.4. 406 Not Acceptable

   The resource identified by the request is only able of generating
   response entities that have content characteristics not acceptable
   according to the Accept header field sent in the request.




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      6.4.5. 408 Request Timeout

   The server could not produce a response within a suitable amount of
   time, and the client MAY repeat the request without modifications at
   any later time

      6.4.6. 413 Request Entity Too Large

   The server is refusing to process a request because the request
   entity-body is larger than the one that the server is willing or
   able to process. The server MAY close the connection to prevent the
   client from continuing the request.

      6.4.7. 414 Request-URI Too Long

   The server is refusing to process the request because the Request-
   URI is longer than the one that the server accepts.

      6.4.8. 415 Unsupported Media Type

   The server is refusing to process the request because the message
   body of the request is in a format not supported by the server for
   the requested method. The server MUST return a list of acceptable
   formats using the Accept, Accept-Encoding, or Accept-Language header
   field, depending on the specific problem with the content.

      6.4.9. 416 Unsupported URI Scheme

   The server cannot process the request because the scheme of the URI
   in the Request-URI is unknown to the server.

        6.5. Server Failure 5xx

   5xx responses are failure responses given when a server itself is
   having trouble.

      6.5.1. 500 Server Internal Error

   The server encountered an unexpected condition that prevented it
   from fulfilling the request. The client MAY display the specific
   error condition and MAY retry the request after several seconds.

      6.5.2. 501 Not Implemented

   The server does not support the functionality required to fulfill
   the request. This is the appropriate response when a Server does not



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   recognize the request method and it is not capable of supporting it
   for any user.

   Note that a 405 (Method Not Allowed) is sent when the server
   recognizes the request method, but that method is not allowed or
   supported.

      6.5.3. 503 Service Unavailable

   The server is temporarily unable to process the request due to a
   temporary overloading or maintenance of the server. The server MAY
   indicate when the client should retry the request in a Retry-After
   header field. If no Retry-After is given, the client MUST act as if
   it had received a 500 (Server Internal Error) response.

   A client receiving a 503 (Service Unavailable) SHOULD attempt to
   forward the request to an alternate server. It SHOULD NOT forward
   any other requests to that server for the duration specified in the
   Retry-After header field, if present.

   Servers MAY refuse the connection or drop the request instead of
   responding with 503 (Service Unavailable).

      6.5.4. 504 Server Time-out

   The server did not receive a timely response from an external server
   it accessed in attempting to process the request.

      6.5.5. 505 Version Not Supported

   The server does not support, or refuses to support, the Q4S protocol
   version that was used in the request. The server is indicating that
   it is unable or unwilling to complete the request using the same
   major version as the client, other than with this error message.

      6.5.6. 513 Message Too Large

   The server was unable to process the request since the message
   length exceeded its capabilities.

        6.6. Global Failures 6xx

   6xx responses indicate that a server has definitive information
   about a particular policy not satisfied for processing the request.





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      6.6.1. 600 session does not exist

   The Session-Id is not valid

      6.6.2. 601 quality level not allowed

   The QOS level requested is not allowed for the pair client/server

      6.6.3. 603 Session not allowed

   The session is not allowed due to some policy (number of sessions
   allowed for the server is exceeded, or the time band of the Q4S-
   ALERT is not allowed for the pair client/server, etc)

      6.6.4. 604 authorization not allowed

   The policy server does not authorize the Q4S-ALERT quality session
   improvement operation due to an internal or external reason.

7. Protocol

   This section describes the measurement procedures, the SDP structure
   of the Q4S messages, the different Q4S protocol phases and the
   messages exchanged in them.



        7.1. Protocol Phases

      All elements of the IP network contribute to the quality in terms
   of latency, jitter, bandwidth and packet loss. All these elements
   have their own quality policies in terms of priorities, traffic
   mode, etc. and each element has its own way to manage the quality.
   The purpose of a quality connection is to establish an end-to-end
   communication with enough quality for the application to function
   flawlessly.

   To monitor quality constraints of the application, four phases are
   defined and can be seen in the following figure:










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   +---------------------------------------------------------------+
   |                                                               |
   |                                                               |
   | Handshake ---> Negotiation -+--> Continuity ----> Termination |
   |                   A         |    (app start) |    (app end)   |
   |                   |         |        A       |       A        |
   |                   |     violated     |     violated  |        |
   |                   |    constraints   |   constraints |        |
   |                   |      |     |     |_______|   ____|        |
   |                   |      |     |     +-------+       |        |
   |                   |      |     |                     |        |
   |                   +------+     +---------------------+        |
   |                                                               |
   +---------------------------------------------------------------+

                     Figure 5 Session lifetime phases.



     o  Handshake phase: in which the server is contacted by the client
        and in the answer message the quality constraints for the
        application is communicated embedded in an SDP.

     o  Negotiation phase: in which the quality of the connection is
        measured in both directions (latency, jitter, bandwidth and
        packet loss), and Q4S messages may be sent in order to alert if
        the measured quality does not meet the constraints. This phase
        is iterative until quality constraints are reached or the
        session is cancelled after a number of measurement cycles with
        consistent violation of the quality constraints. The number of
        measurement cycles executed depends on the qos-level which is
        incremented in each cycle until a maximum qos-level value is
        reached. Just after reaching the quality requirements, Q4S
        provides a simple optional mechanism using HTTP to start the
        application.

     o  Continuity phase: in which quality is continuously measured. In
        this phase the measurements MUST avoid disturbing the
        application by consuming network resources. If quality
        constraints are not met, the server stack will notify the
        Actuator with an alert notification. If later the quality
        improves, the server stack will notify the Actuator, in this
        case with a recovery notification. After several alert
        notifications with no quality improvements, the Q4S stack
        should move to Termination phase.




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     o  Termination phase: in which the Q4S session is terminated. The
        application may be closed too or may not start.



        7.2. SDP Structure

   The original goal of SDP was to announce necessary information for
   the participants and multicast MBONE (Multicast Backbone)
   applications. Right now, its use has been extended to the
   announcement and the negotiation of multimedia sessions. The purpose
   of Q4S is not to establish media stream sessions, but to monitor a
   quality connection. This connection may be later used to establish
   any type of session including media sessions; Q4S does not impose
   any conditions on the type of communication requiring quality
   parameters.

   SDP will be used by Q4S to exchange quality constraints and will
   therefore always have all the media attributes ("m") set to zero.

   The SDP embedded in the messages is the container of the quality
   parameters. As these may vary depending on the direction of the
   communication (to and from the client) all quality parameters need
   to specify the uplink and downlink values: <uplink> / <downlink>.
   When one or both of these values are empty, it MUST be understood as
   needing no constraint on that parameter and/or that direction.

   The uplink direction MUST be considered as being the communication
   from the client to the server. The downlink direction MUST be
   considered as being the communication from the server to the client.

   The SDP information can comprise all or some of the following
   parameters shown in the example below. This is an example of an SDP
   message used by Q4S included in the 200 OK response to a Q4S BEGIN
   request.














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   v=0
   o=q4s-UA 53655765 2353687637 IN IP4 192.0.2.33
   s=Q4S
   i=Q4S parameters
   t=0 0
   a=qos-level:0/0
   a=alerting-mode:Reactive
   a= alert-pause:5000
   a=public-address:client IP4 80.0.2.33
   a=public-address:server IP4 198.51.100.58
   a= latency:40
   a= jitter:10/10
   a= bandwidth:20/6000
   a= packetloss:0.50/0.50
   a=flow:app clientListeningPort TCP/10000-20000
   a=flow:app clientListeningPort UDP/15000-18000
   a=flow:app serverListeningPort TCP/56000
   a=flow:app serverListeningPort UDP/56000
   a=flow:q4s clientListeningPort UDP/55000
   a=flow:q4s clientListeningPort TCP/55001
   a=flow:q4s serverListeningPort UDP/56000
   a=flow:q4s serverListeningPort TCP/56001

   Notice that in this SDP example there are no measurement attributes,
   these will only be included in the SDP body in Q4S-ALERT messages.

   As quality constraints may be changed by applications at any time
   during the Q4S session lifetime, any Q4S 200 OK response sent by the
   server to the client in the Negotiation and Continuity phases could
   also include an SDP body with the new quality requirements stated by
   the applications from then on. Therefore, in response to any PING
   request sent by the client to the server, the server could send a
   Q4S 200 OK with an SDP message embedded that specifies new quality
   constraints requested by the application.


      7.2.1. "qos-level" attribute

   The "qos-level" attribute contains the QoS level for uplink and
   downlink. Default values are 0 for both directions. The meaning of
   each level is out of scope of Q4S, but a higher level SHOULD
   correspond to a better service quality.

   The "qos-level" attribute may be changed during the session lifetime
   raising or lowering the value as necessary following the network
   measurements and the application needs.



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      7.2.2. "alerting-mode" attribute

   The "alerting-mode" attribute specifies the player in charge of
   triggering Q4S alerts in case of constraint violation. There are two
   possible values:

   a) Q4S-aware-network: Q4S ALERT messages are triggered by the server
      to the client. In this case the network is supposed to be Q4S
      aware, and reacts by itself to these alerts.

   b) Reactive: alert notifications are sent by the server stack to the
      Actuator. In this case the network is not Q4S aware and a
      specific node (Actuator) is in charge of triggering tuning
      mechanisms., either on the network or in the application.

   The "alerting-mode" attribute is optional and if not present
   Reactive alerting mode is assumed.

      7.2.3. "alert-pause" attribute

   In the Q4S-aware-network scenario, the "alert-pause" attribute
   specifies the amount of time (in milliseconds) the server waits
   between consecutive Q4S ALERT messages sent to the client. In the
   Reactive scenario, the "alert-pause" attribute specifies the amount
   of time (in milliseconds) the server stack waits between consecutive
   alert notifications sent to the Actuator. Measurements are not
   stopped in Negotiation or Continuity Phases during this period of
   time, but no Q4S ALERT messages or alert notifications are fired,
   even with violated quality constraints, allowing either network
   reconfigurations or application adjustments.



      7.2.4.  "recovery-pause" attribute

   In the Q4S-aware-network scenario, the "recovery-pause" attribute
   specifies the amount of time (in milliseconds) the server waits for
   initiating the qos-level recovery process. Once the recovery process
   has started, the "recovery-pause" attribute also states the amount
   of time (in milliseconds) between consecutive Q4S-RECOVERY messages
   sent by the server to the client (in the Q4S-aware-network
   scenario), or between recovery notifications sent by the server
   stack to the Actuator (in the Reactive scenario).






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      7.2.5. "public-address" attributes

   This attribute contains the public IP address of the client and the
   server. The server fills these attributes with his own public IP
   address and the public IP address of the first message received from
   the client in the handshake phase.

   The purpose of these attributes is to make available the addressing
   information to network policy server or other external entities in
   charge of processing Q4S-ALERT messages.

      7.2.6. "latency" attribute

   The maximum latency (considered equal for uplink and downlink)
   tolerance are specified in the "latency" attribute, expressed in
   milliseconds. In the Q4S-aware-network scenario, if the latency
   constraints are not met, a Q4S-ALERT method will be sent to the
   client. In the Reactive scenario, if the latency constraints are not
   met, an alert notification will be sent to the Actuator. If the
   "latency" attribute is not present or has a 0 value, no latency
   constraints need to be met and no measurements MAY be taken.

      7.2.7. "jitter" attribute

   The maximum uplink and downlink jitter tolerance are specified in
   the "jitter" attribute, expressed in milliseconds. In the Q4S-aware-
   network scenario, if the jitter constraints are not met, a Q4S-ALERT
   method will be sent to the client. In the Reactive scenario, if the
   latency constraints are not met, an alert notification will be sent
   to the Actuator. If "jitter" attribute is not present or has a 0
   value, no jitter constraints need to be met and no measurements MAY
   be taken.

      7.2.8. "bandwidth" attribute

   The minimum uplink and downlink bandwidth are specified in the
   "bandwidth" attribute, expressed in kbps. In the Q4S-aware-network
   scenario, if the bandwidth constraints are not met, a Q4S-ALERT
   method will be sent to the client. In the Reactive scenario, an
   alert notification will be sent to the Actuator. If "bandwidth"
   attribute is not present or has a 0 value, no bandwidth constraints
   need to be met and no measurements MAY be taken.

      7.2.9. "packetloss" attribute

   The maximum uplink and downlink packet loss tolerance are specified
   in the "packetloss" attribute expressed in percentage (two decimal


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   accuracy). In the Q4S-aware-network scenario, if the packetloss
   constraints are not met, a Q4S-ALERT method will be sent to the
   client. In the Reactive scenario, an alert notification will be sent
   to the Actuator. If " packetloss" attribute is not present or has a
   0 value, no packetloss constraints need to be met and no
   measurements MAY be taken.





      7.2.10. "flow" attributes

   These attributes specify the flows (protocol, destination IP/ports)
   of data over TCP and UDP ports to be used in uplink and downlink
   communications.

   Several "flow" attributes can be defined. These flows identify the
   listening port (client or server), the protocol (TCP or UDP) (RFC
   761 [8] and RFC 768 [9]) with the range of ports that are going to
   be used by the application and, of course, by the Q4S protocol (for
   quality measurements). All defined flows (app and q4s) will be
   considered within the same quality profile, which is determined by
   the qos-level attribute in each direction. This allows to assume
   that measurements on q4s flows are the same experimented by the
   application which is using app flows.

   During negotiation and continuity phases the specified Q4S ports in
   the "flow:q4s" attributes of SDP will be used for Q4S messages.

   The Q4S flows comprise two UDP flows and two TCP flows (one uplink
   and one downlink for each one) whereas application traffic MAY
   consist of many flows, depending on its nature. The handshake phase
   takes place through the Q4S Contact URI, using TCP port 80, for
   example. However the negotiation and continuity phases will take
   place on the specified Q4S ports (UDP and TCP) specified in the SDP.













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   The "clientListeningPort" is a port in which the client listens for
   server requests and MUST be used as origin port of client responses.
   The "serverListeningPort" is a port in which server is listening for
   incoming messages from the client. The origin port of server
   responses may be different than "serverListeningPort" value.

   If "clientListeningPort" is zero (a=flow:q4s clientListeningPort
   TCP/0), the client MAY choose one randomly as per OS standard rules.
   Client ports inside the SDP must always be matched against actual
   received port values on the server side in order to deal with
   NAT/NATP devices. If zero value or incorrect value is present,
   server must set the value to the received origin port in the next
   message with SDP (200 OK, ALERT and CANCEL messages).


      7.2.11. Measurement attributes

   These attributes contain the measurement procedure and the results
   of the quality measurements.

   Measurement parameters are included using the session attribute
   "measurement". The first measurement parameter is the procedure. Q4S
   provides a "default" procedure for measurements, but others like
   RTP/RTCP might be used and defined later. This draft will only
   define and explain the "default" procedure.

   In the initial client request a set of measurement procedures can be
   sent to the server for negotiation. One measurement procedure line
   MUST be included in the SDP message for each proposed method. The
   server MUST answer with only one line with the chosen procedure.

   For each procedure, a set of values of parameters separated by ","
   can be included in the same attribute line. The amount and type of
   parameters depends on the procedure type.

   In the following example the "default" procedure type is chosen:

   a=measurement:procedure default(50/50,75/75,5000,40/80,100/256)


   In the "default" procedure, the meaning of these parameters is:

     o  The first parameter is the interval of time (in milliseconds)
        between PING requests during the negotiation phase. Uplink and
        downlink values from the client's point of view are separated
        by "/". This allows having different responsiveness values
        depending on the control resources used in each direction.


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     o  The second parameter is the time interval (in milliseconds)
        between PING requests during the continuity phase. Uplink and
        downlink values are separated by "/". This allows having two
        different responsiveness values depending on the control
        resources used in each direction.

     o  The third parameter is the time interval to be used to measure
        bandwidth during the negotiation phase..

     o  The fourth parameter indicates the window size for jitter and
        latency calculations. Uplink and downlink values are separated
        by "/".

     o  The fifth parameter indicates the window size for packet loss
        calculations. Uplink and downlink values are separated by "/".

   There are four more measurement attributes:

   a=measurement:latency 45
   a=measurement:jitter 3/12
   a=measurement:bandwidth 200/9800
   a=measurement:packetloss 0.00/1.00


   The latency, jitter, bandwidth and packetloss measurement attributes
   contain the values measured for each of these quality parameters in
   uplink and downlink directions. Notice that latency is considered
   equal for uplink and downlink directions. Quality parameter values
   in these measurement attributes provide a snapshot of the quality
   reached and MUST only be included in Q4S-ALERT messages in the SDP
   body such that they can be protected from malicious attacks as these
   alerts include a signature of the SDP body in the header. The rest
   of messages will include the measured values in the Measurements
   header.



        7.3. Measurements

   This section describes the way quality parameters are measured as
   defined by the "default" procedure. Measurements MUST be taken for
   any quality parameter with constraints, that is, specified in the
   SDP attributes with non-zero values. For non-present attributes
   measurements MAY be omitted.





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      7.3.1. Latency

   Latency measurements will be performed if the latency attribute
   and/or the application latency attribute are present and with non-
   zero values.

   Q4S defines a PING method in order to exchange packets between the
   client and the server. Based on this PING exchange the client and
   the server are able to calculate the round-trip time (RTT). The RTT
   is the sum of downlink latency (normally named "reverse latency")
   and uplink latency (normally named "forward latency").

   At least 255 samples of RTT MUST be taken by the client and server.
   As the forward and reverse latencies are impossible to measure,
   client and server will assume that both latencies are identical
   (symmetric network assumption). The latency will therefore be
   calculated as the statistical median value of all the RTT samples
   divided by 2.



      7.3.2. Jitter

   Jitter measurements will be performed if the jitter attribute and/or
   the application jitter attribute are present and with non-zero
   values.

   The jitter can be calculated independently by the client and by the
   server. The downlink jitter is calculated by the client taking into
   account the time interval between PING requests as defined by the
   measurement procedure attribute in the first or second parameter
   depending on the Q4S protocol phase. The client and the server MUST
   send these PING requests at the specified intervals. The client
   measures the downlink jitter whereas the server measures the uplink
   jitter. Note that PING responses are not taken into account when
   calculating jitter values.

   Every time a PING request message is received by an endpoint (either
   server or client), the corresponding jitter value is updated using
   the Statistical Jitter value calculated on the first 255 packets
   received using the arithmetic mean of the absolute values of elapsed
   times.

   Each endpoint sends a PING periodically with a fixed interval, each
   value of "elapsed time" (ET) should be very close to this interval.
   If a PING message is lost, the elapsed time value is doubled.
   Identifying lost PING messages, however, is not an issue because all


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   PING messages are labeled with a Sequence-Number header. Therefore
   the receiver can discard this elapsed time value.

   In order to have the first jitter sample, the receiver MUST wait
   until it receives 3 PING requests, because each ET is the time
   between two PINGs and a Jitter needs at least two ET.

   The client measures the values of RTT and downlink jitter and the
   server measures RTT and uplink jitter, but all measurements are
   shared with the counterpart by means of "Measurements" header of
   PING message.

      7.3.3. Bandwidth

   Bandwidth measurements will be performed if the bandwidth attribute
   and/or the application bandwidth attribute is present and with non-
   zero values.

   In order to measure the available bandwidth, both the client and the
   server MUST start sending BWIDTH messages simultaneously using the
   UDP control ports exchanged during the handshake phase in the SDP
   message, at the needed rate to verify the availability of the
   bandwidth constraint in each direction using messages of 1 Kbyte or
   more in length. The messages are sent during the period of time
   defined in the third parameter of the SDP measurement default
   procedure attribute in millisecond units.

   a=measurement:procedure default(50/50,75/75,5000,256/256,256/256)





















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            +------------------------------------------------+
            |             Rate                               |
            |              A                                 |
            |              |                                 |
            |downlink rate-|-------------------+ <-- traffic |
            |              |                   |     sent by |
            |              |                   |     server  |
            |              |                   |             |
            |              |                   |             |
            |              |                   |             |
            |              |                   |             |
            |              |                   |             |
            |              |                   |             |
            |              |                   |             |
            |              |                   |             |
            |              |                   |             |
            |              |                   |             |
            |              |                   |             |
            |              |                   |             |
            |  uplink rate-|-------------------+ <-- traffic |
            |              |                   |     sent by |
            |              |                   |     client  |
            |              |                   |             |
            |              |                   |             |
            |              |---|---|---|---|---|----> time   |
            |              0   1   2   3   4   5     (sec.)  |
            |                                                |
            +------------------------------------------------+

             Figure 6  Bandwidth and packet loss measurements.

   The goal of these measurements is not to identify the available
   bandwidth of the communication path but to determine if the required
   bandwidth is available, meeting the application's constraints.
   Therefore, the requested bandwidth MUST be measured sending only the
   highest bit rate required by the bandwidth attribute.

   When measuring bandwidth, all BWIDTH requests sent MUST be 1
   kilobyte in length (UDP payload length), and MUST include a
   Sequence-Number header with a sequential number starting at 0. The
   Sequence-Number MUST be incremented by 1 with each BWIDTH packet
   sent. If any measurement stage needs to be repeated, the sequence
   number MUST start at zero again. BWIDTH requests MUST NOT be
   answered. Examples:





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   Client message:
   =========================
          BWIDTH q4s://www.example.com Q4S/1.0
          User-Agent: q4s-ua-experimental-1.0
          Session-Id: 53655765
          Sequence-Number: 0
          Content-Type: text
          Content-Length: XXXX
          Measurements: l=22, j=10, pl=0.00, bw=3000


          aaaaaaaaaaaaa ( to complete 1024 bytes UDP payload length)
   =========================

   The client MUST send BWIDTH packets to the server to allow the
   server to measure the uplink bandwidth. The server MUST send BWIDTH
   packets to the client to allow the client to measure the downlink
   bandwidth.

   Server message:
   =========================
          BWIDTH q4s://www.example.com Q4S/1.0
          Session-Id: 53655765
          Sequence-Number: 0
          Content-Type: text
          Content-Length: XXXX
          Measurements: l=22, j=7, pl=0.00, bw=200


          aaaaaaaaaaaaa ( to complete 1024 bytes UDP payload length)
   =========================




      7.3.4. Packet loss

   Packet loss and bandwidth are measured simultaneously using the
   BWIDTH packets sent by both the client and the server. Because the
   BWIDTH packets contain a Sequence-Number header incremented
   sequentially with each sent packet, lost packets can be easily
   identified. The lost packets have to be counted during the
   measurement time.






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        7.4. Handshake Phase

   The first phase consists of a Q4S BEGIN method issued from the
   client to the server.

   The first Q4S message MUST have a special URI (RFC 3986 [4]), which
   forces the use of the Q4S protocol if it is implemented in a
   standard web browser.

   This URI, named "Contact URI", is used to request the start of a
   session. Its scheme MUST be:

         "q4s:" "//" host [":" port] [path["?" query]

   Optionally, the client can send the desired quality parameters
   enclosed in the body of the message as an SDP document. The server
   MAY take them into account when building the answer message with the
   final values in the SDP body, following a request / response schema
   (RFC 3464 [5]).

   If the request is accepted, the server MUST answer it with a Q4S 200
   OK message, which MUST contain an SDP body (RFC 4566 [2]) with the
   assigned session id (embedded in the "o" SDP parameter), the IP
   addresses to be used, the flow ports to be used, the measurement
   procedure to be followed and information about the required quality
   constraints. Additionally, the alerting-mode and alert-pause time
   parameters may be included. Q4S responses should use the protocol
   designator "Q4S/1.0".

   After these two messages are exchanged, the first phase is
   completed. The quality parameter thresholds have been sent to the
   client. The next step is to measure the actual quality of the
   communication path between the client and the server and alert if
   the SLA is being violated.

            +------------------------------------------------+
            |                                                |
            | Client                            Server       |
            |                                                |
            |     ------- Q4S BEGIN ------------>            |
            |                                                |
            |     <------ Q4S 200 OK ------------            |
            |                                                |
            |                                                |
            +------------------------------------------------+

                         Figure 7 Handshake phase.


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   Example of Client Request and Server Answer:

   Client Request:
   =========================
   BEGIN q4s://www.example.com Q4S/1.0
   Content-Type: application/sdp
   User-Agent: q4s-ua-experimental-1.0
   Content-Length: 142

   (SDP not shown)
   =========================


   Server Answer:
   =========================
   Q4S/1.0 200 OK
   Date: Mon, 10 Jun 2010 10:00:01 GMT
   Content-Type: application/sdp
   Expires: 3000
   Signature: 6ec1ba40e2adf2d783de530ae254acd4f3477ac4
   Content-Length: 131

   (SDP not shown)
   =========================


   The headers used are explained in section 4.3.

        7.5. Negotiation phase

   The negotiation phase is in charge of measuring the quality
   parameters and verifying that the communication paths meet the
   required quality constraints on both directions as specified in the
   SDP body.

   The measured parameters will be compared with the quality
   constraints specified in the SDP body. If the quality session is
   compliant with all the quality constraints the application can
   start.










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     o  If the quality constraints are not met, a higher quality
        service level will be demanded. Depending on the scenario, this
        quality upgrade will be managed as follows: In the Q4S-aware-
        network scenario: a Q4S-ALERT method will be triggered by the
        server to the client and the client will answer with the same
        Q4S-ALERT method. After receiving the same Q4S-ALERT from the
        counterpart, no other alerts will be triggered by the server
        during the "alert-pause" period of time, in order to allow the
        network to react, but measurements will continue to be taken to
        achieve early detection of improved network quality conditions
        and a fast application start.

     o  In the Reactive scenario: an alert notification will be sent by
        the server stack to the Actuator, and the Actuator will answer
        with an alert acknowledgement. After receiving the alert
        acknowledgement from the Actuator, the server stack will not
        send other alert notifications during the "alert-pause" period
        of time, in order to allow the Actuator to react and trigger
        actions on the application or on the policy server, but
        measurements will continue to be taken to achieve early
        detection of improved network quality conditions and a fast
        application start.

   In both scenarios stated above, if after several measurement cycles,
   the network constraints cannot be met the quality session is
   terminated. Concretely when under all possible actions taken by
   Actuator the quality remains below requirements, the session must be
   terminated.

   The steps to be taken in this phase depend on the measurement
   procedure exchanged during the handshake phase. This document only
   describes the "default" procedure, but others can be used, like
   RTP/RTCP (RFC 3550 [10]).

   Measurements of latency and jitter are done calculating the
   differences in arrival times of packets and can be achieved with
   little bandwidth consumption. The bandwidth measurement, on the
   other hand, involves higher bandwidth consumption in both directions
   (uplink and downlink).

   To avoid wasting unnecessary network resources these two types of
   measurements will be performed in two separate stages. If the
   required latencies and jitters cannot be reached, it makes no sense
   to waste network resources measuring bandwidth. In addition, if
   achieving the required latency and jitter thresholds implies
   upgrading the quality session level, the chance of obtaining
   compliant bandwidth measurements without retries is higher, saving


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   network traffic again. Therefore, the default procedure, determines
   that the measurements are taken in two stages:

     o  Stage 0: Measurement of latencies, jitters and packet loss

     o  Stage 1: Measurement of bandwidths and packet loss

   Notice that packet loss can be measured in both stages, as all
   messages exchanged include a sequence-number header that allows for
   easy packet loss detection.

   The client starts the negotiation phase sending a READY request
   using the TCP Q4S ports defined in the SDP. This READY request
   includes a "Stage" header that indicates the measurement stage.

   If either jitter, latency or both are specified, the negotiation
   phase begins with the measurement of latencies and jitters (stage
   0). If none of those attributes are specified, stage 0 is skipped.



      7.5.1. Stage 0: Measurement of latencies and jitters

   The Stage 0 MUST start with a synchronization message exchange
   initiated with the client's READY message.


   Client request, READY message:
   =========================
          READY q4s://www.example.com Q4S/1.0
          Stage: 0
          Session-Id: 53655765
          User-Agent: q4s-ua-experimental-1.0
          Content-Length: 0
   =========================

   Server Response:
   =========================
     Q4S/1.0 200 OK
          Session-Id: 53655765
          Stage:0
          Content-Length: 0
   =========================






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   This triggers the exchange of a sequence of PING requests and
   responses that will lead to the calculation of RTT (latency), jitter
   and packet loss.

   After receiving 200 OK, the client must send the first PING message
   and the server will wait to send PINGs until the reception of this
   first client PING.

   Client and server MUST send PING requests to each other. The
   Sequence-Number header of the first PING MUST be set to 0. Client
   and server will manage their own sequence numbers.



            +------------------------------------------------+
            |                                                |
            | Client                                Server   |
            |                                                |
            |      --------- Q4S READY 0 --------->          |
            |      <-------- Q4S 200 OK -----------          |
            |                                                |
            |      --------- Q4S PING ------------>          |
            |      <-------- Q4S 200 OK -----------          |
            |      <-------- Q4S PING -------------          |
            |       -------- Q4S 200 OK ---------->          |
            |      --------- Q4S PING ------------>          |
            |      <-------- Q4S PING -------------          |
            |      --------- Q4S 200 OK ---------->          |
            |      <-------- Q4S 200 OK -----------          |
            |                     ...                        |
            |                                                |
            +------------------------------------------------+

       Figure 8 Simultaneous exchange of PING request and responses.



   This is an example of the PING request sent from the client and the
   server's response:










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   Client Request:
   =========================
          PING q4s://www.example.com Q4S/1.0
          Session-Id: 53655765
          Sequence-Number: 0
          User-Agent: q4s-ua-experimental-1.0
          Measurements: l=22, j=12, pl=0.20, bw=
          Content-Length: 0
   =========================

   Server Response:
   =========================
     Q4S/1.0 200 OK
          Session-Id: 53655765
          Sequence-Number: 0
          Content-Length: 0
   =========================

   The function of the PING method is similar to the ICMP echo request
   message. The server MUST answer as soon as it receives the message.

   Both endpoints MUST send Q4S PING messages with the periodicity
   specified in the first parameter of SDP measurement procedure
   attribute, using always the same UDP ports and incrementing the
   Sequence-Number with each message.

   In the following example, the SDP measurement procedure attribute,
   this value is 50 milliseconds (from the client to the server) and
   60ms (from the server to the client).

   a=measurement:procedure default(50/60,50/50,5000,256/256,256/256)

   They MUST NOT wait for a response to send the next PING request. The
   "Sequence-Number" header value is incremented sequentially and MUST
   start at zero. If this stage is repeated, the initial Sequence-
   Number MUST start at zero again.

   All PING requests MUST contain a "Measurements" header, with the
   values of the latency, jitter and packet loss measured by each
   entity up to that moment. The client will send its measurements to
   the server and the server his measurements to the client. Example:

      Measurements: l=22, j=13, pl=0.10, bw=

   Where l stands for latency, j for jitter, pl for packetloss and bw
   for bandwidth. The bandwidth value is omitted, as it is not measured
   at this stage.


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   Optionally the PING request can include a "Timestamp" header, with
   the time in which the message has been sent. In case the header is
   present, the server MUST include the header in the response without
   changing the value.

   A minimum number of PING messages MUST be exchanged in order to be
   able to measure latency, jitter and packet-loss with certain
   accuracy (at least 256 samples are recommended to get a accurate
   packet loss measurement). Both the client and the server calculate
   the respective measured parameter values. The mechanisms to
   calculate the different parameters are described in section 7.3.

   At the end of this stage 0, there are three possibilities:

     o  The latency, jitter and packet loss constraints are reached in
        both directions

     o  The latency, jitter and packet loss constraints are not reached
        in one or both directions



   In the first case, Stage 0 is finished. Client and server are ready
   for Stage 1: bandwidth and packet loss measurement. The client moves
   to stage 1 by sending a READY message including the header "Stage:
   1".


   If the bandwidth constraints are empty or with value zero, the
   negotiation phase MUST terminate and both client and server may
   initiate the Continuity Phase. In this case client moves to
   Continuity phase by sending a READY message including the header
   "Stage: 2".

   The second case, in which one or more quality constraints have not
   been met, is detailed in section 7.5.4. Quality constraints not
   reached.



      7.5.2. Stage 1: Measurement of bandwidth and packet loss

   This stage begins in a similar way to stage 0, sending a READY
   request over TCP. This READY message "Stage" header value is 1. The
   server answers with a Q4S 200 OK message to synchronize the
   initiation of the measurements.



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            +------------------------------------------------+
            |                                                |
            | Client                                Server   |
            |                                                |
            |      --------- Q4S READY 1 ----------->        |
            |      <-------- Q4S 200 OK -------------        |
            |                                                |
            |      --------- Q4S BWIDTH  ----------->        |
            |      <-------- Q4S BWIDTH  ------------        |
            |      --------- Q4S BWIDTH  ----------->        |
            |      <-------- Q4S BWIDTH  ------------        |
            |                  ...                           |
            |                                                |
            +------------------------------------------------+
          Figure 9 Starting bandwidth and packet loss measurement



   Client Request:
   =========================
          READY q4s://www.example.com Q4S/1.0
          User-Agent: q4s-ua-experimental-1.0
          Stage: 1
          Session-Id: 53655765
          Content-Length: 0

   =========================

   Server Response:
   =========================
     Q4S/1.0 200 OK
          Session-Id: 53655765
          Stage: 1
          Content-Length: 0

   =========================


   Just after receiving the 200 OK, both the client and the server MUST
   start sending BWIDTH messages simultaneously using the UDP q4s
   ports. Section 7.3.3 describes the bandwidth measurement in detail.



   At the end of this stage 1, there are three possibilities:




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     o  The bandwidth and packet loss constraints are reached in both
        directions

     o  The bandwidth and packet loss constraints are not reached in
        one both directions.

   In the first case, Stage 1 is finished. Client and server are ready
   for Continuity phase. The client moves to this phase by sending a
   READY message including the header "Stage: 2". The server answer
   MUST be 200 OK.


            +------------------------------------------------+
            |                                                |
            | Client                                Server   |
            |                                                |
            |     ---------  Q4S READY 2 -------------->     |
            |     <---- Q4S 200 OK with trigger URI-----     |
            |                                                |
            |     ---------   HTTP GET ---------------->     |
            |                                                |
            |            (Application starts)                |
            |                                                |
            +------------------------------------------------+

            Figure 10    Trigger the application using HTTP URI























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   Client Request:
   =========================
   READY q4s://www.example.com Q4S/1.0
   User-Agent: q4s-ua-experimental-1.0
   Stage: 2
   Session-Id: 53655765
   Content-Length: 0

   =========================


   Server Answer:
   =========================
   Q4S/1.0 200 OK
   Date: Mon, 10 Jun 2010 10:00:01 GMT
   Session-Id: 53655765
   Trigger-URI: http://www.example.com/app_start
   Expires: 3000
   Content-Type: application/sdp
   Signature: 6ec1ba40e2adf2d783de530ae254acd4f3477ac4
   Content-Length: 131

   (SDP not shown)
   =========================


   If the "Trigger-URI" header is present, the client SHOULD send an
   HTTP request to this URI.

   The second case, with violated network constraints is explained in
   7.5.4 Quality constraints not reached.




      7.5.3. Quality constraints not reached

   After finishing Stage 1 of the Negotiation phase, the client and the
   server have each other measured parameter values as these have been
   exchanged in the "Measurements" headers of the PING and BWIDTH
   messages. If there is one or more parameters that do not comply with
   the uplink or downlink application constraints required both the
   server and the client are aware of it.



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   If there is any quality parameter that does not meet the uplink or
   downlink quality constraints specified in the SDP message, two
   scenarios are possible depending on the specified alerting-mode (if
   not present, default value is "Reactive" alerting mode):

   a) Q4S-aware-network alerting mode: the server MUST send a Q4S-ALERT
      message to the client including the digital signature header, and
      the client MUST answer with the same Q4S-ALERT message. The
      Signature header contains the signed hash value of the SDP body
      in order to protect all the SDP the data and therefore it MUST
      contain the measurement parameters in the body.






































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   Server request
   =========================
   Q4S-ALERT q4s://www.example.com Q4S/1.0
   Host: www.example.com
   User-Agent: q4s-ua-experimental-1.0
   Session-Id: 53655765
   Content-Type: application/sdp
   Content-Length: 142

   v=0
   o=q4s-UA 53655765 2353687637 IN IP4 192.0.2.33
   s=Q4S
   i=Q4S parameters
   t=0 0
   a=qos-level:1/2
   a=alerting-mode: Q4S-aware-network
   a= alert-pause:5000
   a=public-address:client IP4 80.0.2.33
   a=public-address:server IP4 198.51.100.58
   a= latency:40
   a= jitter:10/10
   a= bandwidth:20/6000
   a= packetloss:0.50/0.50
   a=flow:app downlink TCP/10000-20000
   a=flow:app uplink TCP/56000
   a=flow:q4s downlink UDP/55000
   a=flow:q4s downlink TCP/55001
   a=flow:q4s uplink UDP/56000
   a=flow:q4s uplink TCP/56001
   a=measurement:procedure default(50/50,50/50,5000,256/256,256/256)
   a=measurement:latency 30
   a=measurement:jitter 6/4
   a=measurement:bandwidth 200/4000
   a=measurement:packetloss 0.20/0.33
   =========================


   At this point, both client and server keep on measuring but without
   sending new Q4S ALERT messages during the "alert-pause"
   milliseconds.





   b) Reactive alerting mode: the server stack MUST send an alert
      notification to the Actuator, and the Actuator MUST answer with


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      an acknowledgement to the received alert notification. The alert
      notification sent to the Actuator by the server stack doesn't
      follow Q4S message style but should have all the information the
      Actuator will need for the actions to be taken, which will be
      implementation dependent.

   At this point, during Negotiation phase, both client and server keep
   on measuring without sending new alert notifications to the Actuator
   during the "alert-pause" milliseconds specified in the SDP. This
   way, both client and server will detect any improvement in network
   conditions as soon as the network reacts. The application can start
   as soon as the number of measurements indicated in the measurement
   procedure attribute indicates that the quality parameters are met.

   Same applies to Continuity phase: the measurement dialog between
   client and server must not be interrupted by any possible ALERT
   message.



          7.5.3.1. Actuator role

   Actuator receives notifications of unmet requirements from Q4S
   server stack, and act over the application or over the network
   policy server, according with certain logic out of scope of this
   protocol.

   The Actuator logic activates mechanisms at application level or/and
   network level based on a quality level dictionary, in which each
   level meaning is implementation dependent and each level involve
   different actions based on rules to keep certain user experience
   quality.

   The type of actions that an actuator can take at application level
   are application dependent and may involve:

     o  Reduction of application functionalities, such as limitation of
        application speed or application options.

     o  Reduction of application resources usage, such as reduction of
        frames per second in a video app or any other parameter
        modification in order to adapt to network conditions.

   Apart from actuate at application level, the actuator may act at
   network level if a network policy server is available.




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          7.5.3.2. Policy server role

   A network policy server may be part of the reactive scenario and it
   is in charge of managing network quality provision. Network policy
   server may implement all or some of these features (but not
   exclusive to):

     o  Server validation in terms of quality constraints.

     o  Authentication (Signature validation) and security (block
        malicious clients)

     o  Policy rules (following rules are only examples):

          - Maximum quality level allowed for the ACP

          - Time bands allowed for providing quality sessions

          - Number of simultaneous quality sessions allowed

          - Maximum time used by allowed quality sessions

          - Etc.

   If any of the policy rules fail, a Q4S-ALERT message must be
   answered by a 6XX error, indicating the cause.



      7.5.4. QoS Level changes

   If any constraint was violated, client or server (depending on
   alerting mode) MAY trigger a Q4S-ALERT asking for higher qos-level
   attribute. The maximum qos-level allowed is 9, both uplink and
   downlink.

   If the qos-level has reached the maximum value for downlink or
   uplink without matching the constraints, then a CANCEL request MUST
   be sent by the client using the TCP port determined in the handshake
   phase in order to release the session. In reaction to the reception
   of the CANCEL request, the server MUST send a CANCEL request too. If
   no CANCEL request is received, the expiration time cancels the
   session at server side.






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   Client Request:
   =========================
   CANCEL q4s://www.example.com Q4S/1.0
   User-Agent: q4s-ua-experimental-1.0
   Session-Id: 53655765
   Content-Type: application/sdp
   Content-Length: 142

   (SDP not shown)
   =========================

   Server Request in reaction to Client Request:
   =========================
   CANCEL q4s://www.example.com Q4S/1.0
   Session-Id: 53655765
   Expires: 0
   Content-Type: application/sdp
   Signature: 6ec1ba40e2adf2d783de530ae254acd4f3477ac4
   Content-Length: 131

   (SDP not shown)
   =========================


        7.6. Continuity phase

   During the negotiation phase, latency, jitter, bandwidth and packet
   loss have been measured. During the continuity phase bandwidth will
   not be measured again because bandwidth measurements may disturb
   application performance.

   This phase is supposed to be executed at the same time as the real
   time application is being used.




   This draft only covers the default procedure. The continuity
   operation with default procedure is based on a sliding window of
   samples. The number of samples involved in the sliding window may be
   different for jitter and latency than for packet-loss calculations
   according to the fifth and sixth parameters of the measurement
   procedure attribute. In this example, the jitter and latency sliding
   window comprises 40 samples whereas the size of the packet-loss
   sliding window is 100 samples:

   a=measurement:procedure default(50/50,75/75,5000,40/40,100/100)


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   In addition, the sizes of these windows are configurable per
   direction: uplink and downlink values may differ.

   PING requests are sent continuously (in both directions) and when
   the Sequence-Number header reaches the maximum value, the client
   continues sending PING messages with the Sequence-Number header
   starting again at zero. When the server PING Sequence-Number header
   reaches the maximum value, it does the same, starting again from
   zero.

   On the client side, the measured values of downlink jitter, downlink
   packet loss and latency are calculated using the last samples,
   discarding older ones, in a sliding window schema.

           +--------------------------------------------------+
           |                                                  |
           | 55 56 57 . . . 253 254 255 0 1 2 . . . 55 56     |
           |        A                                   A     |
           |        |                                   |     |
           |        +-----------------------------------+     |
           |                                                  |
           +--------------------------------------------------+

                    Figure 11    Sliding samples window

   Only if the server detects that the measured values (downlink or
   uplink jitter, packet loss or latency) are not reaching the quality
   constraints, a Q4S ALERT is triggered and sent either to the client
   or to the Actuator, depending on the alerting mode, and the alert-
   pause timer is started.

   In Q4S-aware-network alerting mode, if the client receives a Q4S
   ALERT message, it MUST answer sending the Q4S ALERT request message
   back to the server including the SDP (with its corresponding digital
   signature).

   Both client and server will keep performing measurements but no
   other Q4S ALERT message MUST be sent during "alert-pause"
   milliseconds.  The operations needed to act on the network and the
   agents in charge of them are out of scope of this draft.









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            +------------------------------------------------+
            |                                                |
            | Client                      Server             |
            |                                                |
            |               ...                              |
            |   ----------- PING ---------->                 |
            |   <--------- 200 OK ----------                 |
            |   <------- Q4S-ALERT ---------                 |
            |   -------- Q4S-ALERT -------->                 |
            |   <---------- PING -----------                 |
            |   ---------- 200 OK --------->                 |
            |   ----------- PING ---------->                 |
            |   <--------- 200 OK ----------                 |
            |   <---------- PING -----------                 |
            |   ---------- 200 OK --------->                 |
            |        ...                                     |
            |                                                |
            +------------------------------------------------+

         Figure 12   Continuity in Q4S-aware-network alerting mode



   In the Reactive scenario, if the server detects that the measured
   values (downlink or uplink jitter, packet loss or latency) are not
   reaching the quality constraints, an alert notification is triggered
   and sent to the Actuator. The Actuator must then answer to the
   server stack with an alert acknowledgement

   The measurement dialog between the client and the server must not be
   interrupted by any possible ALERT message.


















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            +------------------------------------------------+
            |                                                |
            | Client             Server             Actuator |
            |        ...                                     |
            |   --- PING ---------->                         |
            |   <-- 200 OK----------                         |
            |   <----- PING --------                         |
            |   <--- 200 OK -------- ---- alert              |
            |                            notification -->    |
            |                                                |
            |   --- PING ----------> <--- alert              |
            |                             acknowledge ---    |
            |   <-- 200 OK----------                         |
            |   <----- PING --------                         |
            |   --- 200 OK -------->                         |
            |        ...                                     |
            |                                                |
            +------------------------------------------------+

   Figure 13   Continuity in Reactive alerting mode


        7.7. Termination Phase

   The Termination phase is end point for the established Q4S session
   that is reached in the following cases:

     .  A CANCEL message has been received. The client sends a CANCEL
        message due to the impossibility of the network to meet the
        required quality constraints. The client and server application
        will be notified by the respective Q4S stack.

     .  Session expires: if after the Expires time no client or server
        activity is detected, that end cancels the session.

     .  A BEGIN message has been received by the server. The pre-
        existing Q4S quality session is cancelled and a new session
        will be initiated.

   The meaning of Termination phase in terms of release of resources or
   accounting is application dependent and out of scope of the Q4S
   protocol.

   In Reactive alerting mode, Q4S CANCEL messages received by the Q4S
   server must cause the sending of cancel notifications sent from the
   server stack to the Actuator in order to release possible assigned
   resources for the session.


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      7.7.1. Sanity check of Quality sessions

   A session may finish due to several reasons (client shutdown, client
   CANCEL request, constraints not reached, etc), and any session
   finished MUST release the assigned resources.

   In order to release the assigned server resources for the session,
   the "Expires" header indicates the maximum interval of time without
   exchanging any Q4S message.





        7.8. Dynamic constraints and flows

   Depending on the nature of the application, the quality constraints
   to be reached may evolve, changing some or all quality constraint
   values in any direction.

   The client MUST be able to deal with this possibility. When the
   server sends an SDP document attached to a response (200 OK, or Q4S-
   ALERT, etc), the client MUST take all the new received values,
   overriding any previous value in use.

   The dynamic changes on the quality constraints can be as a result of
   two possibilities:

     o  The application communicates to the Q4S server a change in the
        constraints. In this case the application requirements can
        evolve and the Q4S server will be aware of them.

     o  The application uses TCP flows. In that case, in order to
        guarantee a constant throughput, the nature of TCP behavior
        forces the use of a composite constraint function, which
        depends on RTT, packet loss and window control mechanism
        implemented in each TCP stack.

   TCP throughput can be less than actual bandwidth if the
   Bandwidth-Delay Product (BDP) is large or if the network suffers
   from a high packet loss rate. In both cases, TCP congestion control
   algorithms may result in a suboptimal performance.

   Different TCP congestion control implementations like Reno [14],
   High Speed TCP (RFC 3649 [15]), CUBIC [16], Compound TCP (CTCP
   [17]), etc. reach different throughputs under the same network
   conditions of RTT and packet loss. In all cases, depending on the


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   RTT measured value, the Q4S server could change dynamically the
   packetloss constraints (defined in SDP) in order to make possible to
   reach a required throughput or vice versa (use packetloss
   measurement to change dynamically latency constraints).

   A general guideline to calculate the packetloss constraint and RTT
   constraint consists in approximating the throughput using a
   simplified formula, which should take into account the TCP stack
   implementation of the receiver, in addition to RTT and packet loss:

             Th= Function( RTT, packet loss, ...)

   Then, depending on RTT measured values, set dynamically the
   packetloss constraint.

   It is possible to easily calculate a worst-case boundary for the
   Reno algorithm, which should ensure for all algorithms that the
   target throughput is actually achieved. Except that, high-speed
   algorithms will then have even a larger throughput, if more
   bandwidth is available.

   For the Reno algorithm, the Mathis' formula may be used [15] for the
   upper bound on the throughput:

            Th <= (MSS/RTT)*(1 / sqrt{p})

   In absence of packet loss, a practical limit for the TCP throughput
   is the receiver_window_size divided by the round-trip time. However,
   if the TCP implementation uses a window scale option, this limit can
   reach the available bandwidth value.

        7.9. Qos-level upgrade and downgrade operation

   Each time the server detects violation of constraints, the alert
   mechanism is triggered, the alert-pause timer is started and the
   qos-level is increased. When this happens repeatedly and the qos-
   level reaches its maximum value (value 9), the session is cancelled.
   But when the violation of constraints stops before reaching qos-
   level maximum value, the recovery mechanism allows for the qos-level
   upgrade gradually.

   Following, this downgrade and upgrade of qos-level is explained with
   an example:

     1. A Q4S session is initiated successfully with qos-level=0.




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     2. During the continuity phase, violation of constraints is
        detected; qos-level is increased to 1, a Q4S-ALERT is sent by
        the server to the client and alert-pause timer is started.

     3. Alert-pause timer expires and still violation of constraints
        is detected; qos-level is increased to 2, a Q4S-ALERT is sent
        by the server to the client and alert-pause timer is started.

     4. Alert-pause timer expires but violation of constraints has
        stopped; recovery-pause is started.

     5. Recovery-pause timer expires and no violation of constraints
        has been detected meanwhile; qos-level is decreased to 1, a
        Q4S-RECOVERY is sent by the server to the client and recovery-
        pause timer is started again.

     6. Recovery-pause timer expires again and no violation of
        constraints has been detected meanwhile; qos-level is decreased
        to 0 and a Q4S-RECOVERY is sent by the server to the client;
        recovery-pause timer is not started this time as qos-level has
        reached its initial value.



   When the network configuration allows for the possibility of
   managing Q4S flows and application flows independently (either is a
   network-based QoS or a Q4S aware network), the qos-level downgrade
   process could be managed more efficiently using a strategy that
   allows for carrying out qos-level downgrades excluding app flows
   from SDP dynamically. The Q4S flows would be downgraded to allow for
   measurements on a lower quality level without interference of the
   application flows. A Q4S client MUST allow this kind of SDP
   modifications by the server.

   Periodically (every several minutes, depending on the
   implementation) a Q4S-ALERT could be triggered, in which the level
   is downgraded for Q4S flows, excluding application flows from the
   embedded SDP of that request.

   This mechanism allows to measure at lower levels of quality while
   application flows continue using a higher qos level value.

     o  If the measurements in the lower level meet the quality
        constraints, then a Q4S-RECOVERY message to this lower qos-
        level may be triggered, in which the SDP includes the
        application flows in addition to Q4S flows.



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     o  If the measurements in the lower level do not meet the
        constraints, then a new Q4S-ALERT to the previous qos-level
        MUST be triggered, in which the SDP includes only the Q4S
        flows.

            +------------------------------------------------+
            |                                                |
            | qos-level                                      |
            |   A                                            |
            |   |                                            |
            |  4|                                            |
            |   |                                            |
            |  3|             +------+                       |
            |   |             |      |                       |
            |  2|        +----+      +----+     +---         |
            |   |        |                |     |            |
            |  1|   +----+                +-----+            |
            |   |   |                                        |
            |  0+---+---------------------------------> time |
            |                                                |
            +------------------------------------------------+

               Figure 14    Possible evolution of qos-level

   This mechanism avoids the risk of disturbing the application, while
   the measurements are being run in lower levels. However, this
   optional optimization of resources MUST be used carefully.

   The chosen period to measure a lower qos level is implementation
   dependent. Therefore it is not included as a measurement procedure
   parameter. It is recommended to use a large value, such as 20
   minutes.



8. General User Agent behavior

        8.1. Roles in peer to peer scenarios

   In order to allow peer to peer applications, a Q4S User Agent (UA)
   MUST be able to assume both client and server role. The role assumed
   depends on who sends the first message.

   In a communication between two UAs, the UA that sends the Q4S BEGIN
   request in the first place, for starting the handshake phase, shall
   assume the client role.



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   If both UASs send the BEGIN request at the same time, they will wait
   for a random time to restart again.

   Otherwise, an UA may be configured to act only as server (e.g.,
   content provider's side).

             +-----------------------------------------------+
             |                                               |
             | UA(Client)                         UA(Server) |
             |                                               |
             |     -------- Q4S BEGIN ------------->         |
             |     <------- Q4S BEGIN --------------         |
             |                                               |
             |     ------- Q4S BEGIN -------------->         |
             |     <------ Q4S 200 OK --------------         |
             |                                               |
             |                                               |
             +-----------------------------------------------+

                          Figure 15    P2P roles.

        8.2. Multiple Quality sessions in parallel

   A Q4S session is intended to be used for an application. It means
   that for using the application, the client MUST establish only one
   Q4S session against the server. Indeed, the relation between
   session-id and application is 1 to 1.

   If a user wants to participate in several independent Q4S sessions
   simultaneously against different servers (or against the same
   server) can execute different Q4S clients to establish  separately
   different Q4S sessions but it is not recommended, because:

     o  The establishment of a new Q4S session may affect other
        running applications over other Q4S sessions during bandwidth
        measurement.

     o  If the negotiation phase is executed separately before running
        any application, the summation of bandwidth requirements could
        not be met when the applications are running in parallel.









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        8.3. General client behavior

   A Q4S Client has different behaviors. We will use letters X,Y,Z to
   designate each different behavior (follow the letter bullets in the
   figure below).

     X) When it sends messages over TCP (methods BEGIN, READY, Q4S-
     ALERT, Q4S-RECOVERY and CANCEL) it behaves strictly like a state
     machine that sends requests and waits for responses. Depending on
     the response type it enters in a new state.

   When it sends UDP messages (methods PING and BWIDTH), a Q4S client
   is not strictly a state machine that sends messages and waits for
   responses because:


     Y) At latency, jitter and packet loss measurement, the PING
     requests are sent periodically, not after receiving the response
     to the previous request. In addition, the client MUST answer the
     PING requests coming from the server, therefore the client assumes
     temporarily the role of a server.

     Z) At bandwidth and packet loss measurement stage, the client does
     not expect to receive responses when sending BWIDTH requests to
     the server. In addition, it MUST receive and process all server
     messages in order to achieve the downlink measurement.

   The Q4S-ALERT and CANCEL may have a conventional answer if an error
   is produced, otherwise the corresponding answer is formatted as a
   request message.


      +-----------+------------------------+-----------+-----------+
      | Handshake |    Negotiation         |Continuity |Termination|
      |   Phase   |      Phase             |   Phase   |  Phase    |
      |           |                        |           |           |
      | X ---------> Y --> X --> Z --> X ---> Y --> X ---> X       |
      |           |  A     |     A     |   |  A     |  |           |
      |           |  |     |     |     |   |  |     |  |           |
      |           |  +-----+     +-----+   |  +-----+  |           |
      |           |                        |           |           |
      +------------------------------------------------+-----------+

                  Figure 16    Phases & client behaviors.





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      8.3.1. Generating requests

   A valid Q4S request formulated by a Client MUST, at a minimum,
   contains the following header fields:

     o  If no SDP is included: the header Session-Id and Sequence-
        Number are mandatory.

     o  If SDP is included: Session-Id is embedded into SDP, therefore
        the inclusion of Session-Id header is optional but if present
        must have the same value. Measurements are embedded into the
        SDP only for Q4S-ALERT messages in order to be signed.

   At any time, if the server sends a new SDP with updated values,
   client MUST take it into account.

        8.4. General server behavior

   If a server does not understand a header field in a request (that
   is, the header field is not defined in this specification or in any
   supported extension), the server MUST ignore that header field and
   continue processing the message.

   The role of the server is changed at negotiation and continuity
   phases, in which server MUST send packets to measure jitter, latency
   and bandwidth. Therefore, the different behaviors of server are
   (follow the letter bullets in the figure below):

      R) When the client sends messages over TCP (methods BEGIN, READY
      Q4S-ALERT, Q4S-RECOVERY and CANCEL) it behaves strictly like a
      state machine that receives messages and sends responses.

   When the client begins to send UDP messages (methods PING and
   BWIDTH), a Q4S server is not strictly a state machine that receives
   messages and sends responses because:

      S) At latency, jitter and packet loss measurement, the PING
      requests are sent periodically by the client but also by the
      server. In this case the server behaves as a server answering
      client requests but also behaves temporarily as a client, sending
      PING requests toward the client and receiving responses.

      T) At bandwidth and packet loss measurement, the server sends
      BWIDTH requests to the client. In addition, it MUST receive and
      process client messages in order to achieve the uplink
      measurement.



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   The Q4S-ALERT and CANCEL may have a conventional answer if an error
   is produced, otherwise the corresponding answer is formatted as a
   request message.

   .


      +-----------+------------------------+-----------+-----------+
      | Handshake |    Negotiation         |Continuity |Termination|
      |   Phase   |      Phase             |   Phase   |  Phase    |
      |           |                        |           |           |
      | R ---------> S --> R --> T --> R ---> S --> R ---> R       |
      |           |  A     |     A     |   |  A     |  |           |
      |           |  |     |     |     |   |  |     |  |           |
      |           |  +-----+     +-----+   |  +-----+  |           |
      |           |                        |           |           |
      +------------------------------------------------+-----------+

                 Figure 17    Phases & server behaviours.




9. Implementation Recommendations

        9.1. Default client constraints

   To provide a default configuration, it would be good that the client
   had a configurable set of Quality headers in the implementation
   settings menu. Otherwise these quality headers will not be present
   in the first message.

   Different business models (out of scope of this proposal) may be
   achieved: depending on who pays for the quality session, the server
   can accept certain Client parameters sent in the first message, or
   force billing parameters on the server side.

        9.2. Latency and Jitter measurements

   In execution systems, where the timers are not accurate, a
   recommended approach consists of including the optional header
   "Timestamp" in the PING request with the time in which the message
   has been sent. This allows an accurate measurement of the jitter
   even with no identical intervals of time between PINGs.





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        9.3. Bandwidth measurements

   In programming languages or Operating Systems with timers or limited
   clock resolution, it is recommended to use an approach based on
   several intervals to send messages of 1KB, in order to reach the
   required bandwidth consumption using a rate as close as possible to
   a constant rate.

   For example, if the resolution is 1 millisecond, and the bandwidth
   to reach is 11Mbps, a good approach consists of sending:

     1 message of 1KB every 1 millisecond +

     1 message of 1KB every 3 milliseconds +

     1 message of 1KB every 23 milliseconds

   The number of intervals depends on required bandwidth and accuracy
   that the programmer wants to achieve.

   In execution systems where the timers are not accurate, a
   recommended approach consists of checking at each interval, the
   number of packets that should have been sent at this timestamp since
   origin and send the needed number of packets in order to reach the
   required bandwidth.

   The shorter packets are used, the more constant is the rate of
   bandwidth measurement. However, this may stress the execution system
   in charge of receiving and processing packets. As a consequence,
   some packets may be lost because of stack overflows. To deal with
   this potential issue, a larger packet is recommended (2KB or more)
   taking into account the overhead produced by the chunks headers.



        9.4. Packet loss measurement resolution

   Depending on application nature and network conditions, a packet
   loss resolution less than 1% may be needed. In such case, there is
   no limit to the number of samples used for this calculation. A
   tradeoff between time and resolution should be reached in each case.
   For example, in order to have a resolution of 1/10000, the last
   10000 samples should be considered in the packetloss measured value.

   The problem of this approach is the reliability of old samples. If
   the interval used between PING messages is 50ms, then to have a
   resolution of 1/1000 it takes 50 seconds and a resolution of 1/10000


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   takes 500 seconds (more than 8 minutes). The reliability of a packet
   loss calculation based on a sliding window of 8 minutes depends on
   how fast network conditions evolve.

        9.5. Measurements and reactions

   Q4S can be used as a mechanism for measure and trigger network
   tuning and application level actions (i.e. lowering video bit-rate,
   reduce multiplayer interaction speed, etc) in real-time in order to
   reach the application constraints, addressing measured possible
   network degradation.



        9.6. Instability treatments

   There are two scenarios in which Q4S can be affected by network
   problems: loss of Q4S packets and outlier samples



   9.5.1. Loss of control packets

   Lost UDP packets (PING or BWIDTH messages) don't cause any problems
   for the Q4S state machine, but if TCP packets are lost, some
   undesirable consequences could arise.

   Q4S does have protection mechanisms to overcome these situations.
   Examples:

     o  If a BEGIN packet is lost or its corresponding answer, after a
        certain timeout, the client SHOULD resend another BEGIN packet,
        resetting the session

     o  If a READY packet is lost, after a certain timeout, the client
        SHOULD resend another READY packet.

     o  If a QOS ALERT request is lost or its corresponding answer,
        after a certain timeout, the originator SHOULD resend another
        Q4S-ALERT packet.

     o  If CANCEL request is lost or its corresponding answer, after a
        certain timeout, the originator SHOULD resend another CANCEL
        packet.





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   9.5.2. Outlier samples



   Outlier samples are those jitter or latency values far from the
   general/average values of most samples.

   Hence Q4S default measurement method uses the statistical median
   formula for latency calculation, the outlier samples are
   neutralized. This is a very common filtering for noise or errors on
   signal and image processing.



        9.7. Scenarios

   Q4S could be used in two scenarios:

     o  client to ACP (Application content provider)

     o  client to client (peer to peer scenario)



      9.7.1. Client to ACP

   One server:

   It is the common scenario in which client contact server to
   establish a Q4S session.



   N servers:

   In Content Delivery Networks and in general applications where
   delivery of contents can be achieved by different delivery nodes,
   two working mechanisms can be defined

     o  Starting mode: End-user may run Q4S against several delivery
        nodes and after some seconds choose the best one to start the
        multimedia session

     o  Prevention mode:During streaming session, user keeps several
        Q4S dialogs against different alternative delivery nodes. In
        case of congestion, end-user may change to the best alternative
        delivery node


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      9.7.2. Client to client

   In order to solve the client to client scenario, a Q4S register
   function MUST be implemented. This allows clients contact each other
   for sending the BEGIN message. In this scenario, the Register server
   would be used by peers to publish their Q4S-Resource-Server header
   and their public IP address to make possible the assumption of
   server role.

   The register function is out of scope of this protocol version,
   because different HTTP mechanisms can be used and Q4S MUST NOT force
   any.

10. Security Considerations

   Different types of attacks can be avoided. For example, Spoofing of
   server IP address can be avoided using the digital signature
   mechanism. The network can easily validate this digital signature
   using the public key of the server certificate.

   This protocol could be supported over IPSec to increase privacy,
   although it is out of scope of this proposal.



11. IANA Considerations

   A specific port for Q4S TCP control flow mechanism could be
   assigned. It could simplify the network implementation. Other
   possibility is to use any other port (like 80, HTTP). In this case
   the network could use the protocol designator "Q4S" as the mark for
   distinguish and treat the packets.

   Q4S uses SDP as a container for session information, in which
   quality attributes have been added as extended "session-level"
   attributes. These set of new attributes should be registered (in
   order to avoid the prefix "X-"). In this document, this set of
   attributes has been presented as registered attributes.

   This is the list of attribute field names to register:







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   Attribute name: qos-level
   Type of attribute: session level
   Subject to the charset attribute: NO
   Explanation of purpose: defines the current QoS profile in uplink
   and downlink for the communication between client and server. The
   exact meaning of each level is implementation dependant but in
   general, a higher qos-level value corresponds to a better quality
   network profile.
   Appropriate attribute values: [0..9] "/" [0..9]

   Attribute name: alerting-mode
   Type of attribute: session level
   Subject to the charset attribute: NO
   Explanation of purpose: defines the receiver of the Q4S alerts sent
   by the server. In Q4S-aware-network alerting mode, Q4S alerts are
   sent to the client. In this case the network is supposed to be Q4S
   aware, and reacts by itself to these alerts. In Reactive alerting
   mode, Q4S alerts sent to the network policy server. In this case the
   network is not Q4S aware and a specific node (policy server) is
   supposed to be in charge of achieving network tuning mechanisms. The
   "alerting-mode" attribute is optional, and its default value, when
   it is not present, is "Reactive".
   Appropriate attribute values: <"Q4S-aware-network"|"Reactive">


   Attribute name: alert-pause
   Type of attribute: session level
   Subject to the charset attribute: NO
   Explanation of purpose: interval of time in milliseconds that the
   server must wait between Q4S-ALERT messages in order to allow
   network tuning operations. Measurements are not affected by this
   pause.
   Appropriate attribute values: [0..60000]



   Attribute name: public-address
   Type of attribute: session level
   Subject to the charset attribute: NO
   Explanation of purpose: contains the public IP address of the client
   or the server.
   Appropriate attribute values:<"client"|"server"><"IP4"|"IP6"> <value
   of IP address>

   Attribute name: latency
   Type of attribute: session level
   Subject to the charset attribute: NO


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   Explanation of purpose: defines the latency constraints in
   milliseconds for the communication between client and server.
   Appropriate attribute values: [0..9999]

   Attribute name: jitter
   Type of attribute: session level
   Subject to the charset attribute: NO
   Explanation of purpose: defines the jitter constraints in
   milliseconds in uplink and downlink for the communication between
   client and server.
   Appropriate attribute values: [0..9999] "/" [0..9999]

   Attribute name: bandwidth
   Type of attribute: session level
   Subject to the charset attribute: NO
   Explanation of purpose: define the bandwidth constraints in kbps in
   uplink and downlink for the communication between client and server.
   Appropriate attribute values: [0..99999] "/" [0..99999]

   Attribute name: packetloss
   Type of attribute: session level
   Subject to the charset attribute: NO
   Explanation of purpose: define the packet loss tolerance constraints
   in 100% in uplink and downlink for the communication between client
   and server.
   Appropriate attribute values: [0.00 ..100.00] "/"[0.00 ..100.00]

   Attribute name: application
   Type of attribute: session level
   Subject to the charset attribute: NO
   Explanation of purpose: define the quality parameter tolerance
   constraints with uplink and downlink values for the communication
   between the client and the server for four different parameters:
   latency, jitter, bandwidth and packetloss.
   Attribute values:
      <"latency:"> [0..9999]
      <"jitter:"|"bandwidth:"> [0..99999] "/" [0..99999]]
      <"packetloss:" [0.00 ..100.00] "/"[0.00 ..100.00]]

   Attribute name: flow
   Type of attribute: session level
   Subject to the charset attribute: NO
   Explanation of purpose: define a flow between a client and a server.
   The flow involves purpose (application or q4s -control-),
   destination port (server or client) protocol (UDP or TCP) and port
   or range or ports



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   The "clientListeningPort" is a port in which the client listens for
   server requests and MUST be used as origin port of client responses.
   The "serverListeningPort" is a port in which server is listening for
   incoming messages from the client. The origin port of server
   responses may be different than "serverListeningPort" value.
   Attribute values:
      <"q4s"|"app"> <"serverListeningPort"|"clientListeningPort">
   <"UDP"|"TCP"> <0..65535>[ "-" [0..65535]]

   Attribute name: measurement
   Type of attribute: session level
   Subject to the charset attribute: NO
   Explanation of purpose: define the procedure to measure the quality
   and the different values for each measurement
   Attribute values:  "procedure/" <procedure> |
                      "latency "[0..9999] "/" [0..9999] |
                      "jitter "[0..9999] "/" [0..9999] |
                      "bandwidth "[0..99999] "/" [0..99999] |
                      "packetloss "[0.00..100.00] "/" [0.00..100.00]

   If the attribute value is "procedure", the rest of the line MUST
   contain the name of the procedure and optional parameters, separated
   by ",".

   In the case of procedure "default", the valid values are:

   a=measurement:procedure default,[0..999]"/" [0..999]  "," [0..999]
   "/" [0..999] "," [0..9999] "," [0..999]/[0..999] ","
   [0..999]/[0..999]

    where:

     o  The first parameter is the interval of time (in milliseconds)
        between PING requests during the negotiation phase. Uplink and
        downlink values from the client's point of view are separated
        by "/". This allows having different responsiveness values
        depending on the control resources used in each direction.

     o  The second parameter is the time interval (in milliseconds)
        between PING requests during the continuity phase. Uplink and
        downlink values are separated by "/". This allows having two
        different responsiveness values depending on the control
        resources used in each direction.

     o  The third parameter is the time interval to be used to measure
        bandwidth during the negotiation phase.



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     o  The fourth parameter indicates the window size for jitter and
        latency calculations. Forward and reverse values are separated
        by "/".

     o  The fifth parameter indicates the window size for packet loss
        calculations. Forward and reverse values are separated by "/".



   Other procedure names are allowed, but at least "default" procedure
   implementation is mandatory in client and servers.

12. Conclusions

   Q4S defines four phases with different purposes, and inside these
   phases the negotiated measurement procedure is used. Basically, Q4S
   only defines how to transport SLA information and measurement
   results as well as providing some mechanisms for alerting network
   and/or application.

   Q4S does not ask for resources neither define actions to do in case
   quality is degraded. Q4S only alerts if one (or some) of SLA quality
   parameters are being violated. Depends on server (Application
   content provider) to do something with this information and return
   it back to a SLA-compliant state.
























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13. References

        13.1. Normative References

   [1]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,Masinter, L.,
         Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
         HTTP/1.1" RFC 2616, June 1999.

   [2]   Handley, M. and V. Jacobson, "SDP: Session Description
         Protocol", RFC 4566, July 2006.

   [3]   Bradner, S., "Key words for use in RFCs to Indicate
         RequirementLevels", BCP 14, RFC 2119, March 1997.

   [4]   Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
         Resource Identifiers (URI): Generic Syntax", RFC 3986, January
         2005.

   [5]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
         SDP", RFC 3264, June 2002.

   [6]   Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
         April 1992.

   [7]   Johnsson, J., B. Kaliski, "Public-Key Cryptography Standards
         (PCS) #1: RSA Cryptography Specifications version 2.1", RFC
         3447, February 2003.

   [8]   Postel, J., "DoD Standard Transmission Control Protocol", RFC
         761, January 1980.

   [9]   Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
         1980.

   [10]  Schulzrinne, H., Casner, S., Frederick, R., Jacobson, V. "RTP:
         A Transport Protocol for Real-Time Applications", RFC 3550,
         july 2003.

   [11]  Yergeau, F., "UTF-8, a transformation format of ISO 10646",
         RFC        3629, November 2003.

   [12]  Resnick, P., "Internet Message Format", RFC 5322, October 2008





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        13.2. Informative References

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

   [14]  Mathis, M., Semke, J., Mahdavi, J., Ott, T., "The Macroscopic
         Behavior of the TCP Congestion Avoidance Algorithm", Computer
         Communications Review, 27(3), July 1997.

   [15]  Floyd, S., "HighSpeed TCP for a Large Congestion Windows", RFC
         3649, December 2003.

   [16]  Rhee, I., Xu, L., Ha, S., "CUBIC for Fast Long-Distance
         Networks", Internet-draft draft-rhee-tcpm-cubic-02, February
         2009.

   [17]  Sridharan, M., Tan, K., Bansal, D., Thaler, D., "Compound TCP:
         A New TCP Congestion Control for High-Speed and Long Distance
         Networks", Internet-draft draft-sridharan-tcpm-ctcp-02,
         November, 2008.




























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14. Acknowledgments

   Many people have made comments and suggestions contributing to this
   document. In particular, we would like to thank:

   Sonia Herranz Pablo, Clara Cubillo Pastor, Francisco Duran Pina,
   Michael Scharf, Jesus Soto Viso and Federico Guillen.








































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15. Authors' Addresses

   Jose Javier Garcia Aranda
   Alcatel-Lucent
   C/Maria Tubau 9
   28050 Madrid
   Spain
   Phone: +34 91 330 4348
   Email: jose_javier.garcia_granda@nokia.com

   Jacobo Perez Lajo
   Alcatel-Lucent
   C/Maria Tubau 9
   28050 Madrid
   Spain
   Phone: +34 91 330 4165
   Email: jacobo.perez@nokia.com

   Luis Miguel Diaz Vizcaino
   Alcatel-Lucent
   C/Maria Tubau 9
   28050 Madrid
   Spain
   Phone: +34 91 330 4871
   Email: Luismi.Diaz@nokia.com

   Gonzalo Munoz Fernandez
   Alcatel-Lucent
   C/Maria Tubau 9
   28050 Madrid
   Spain
   Phone: +34 91 330 4000
   Email: gonzalo.munoz_fernandez.ext@nokia.com

   Carlos Barcenilla
   Universidad Politecnica de Madrid
   Avenida Complutense 30
   28040 Madrid
   Spain
   Phone: +34 91 549 5700 - 3032
   Email: barcenilla@dit.upm.es

   Monica Cortes
   Universidad Politecnica de Madrid
   Avenida Complutense 30
   28040 Madrid
   Spain


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   Phone: +34 91 336 5700 - 3044
   Email: cortesm@dit.upm.es

   Joaquin Salvachua
   Universidad Politecnica de Madrid
   Avenida Complutense 30
   28040 Madrid
   Spain
   Phone: +34 91 549 5700 - 3056
   Email: jsr@dit.upm.es

   Juan Quemada
   Universidad Politecnica de Madrid
   Avenida Complutense 30
   28040 Madrid
   Spain
   Phone: +34 91 336 7331
   Email: jquemada@dit.upm.es

   Maribel Narganes
   Tecnalia Research & Innovation
   Parque Cientifico y Tecnologico de Bizkaia,
   Geldo Auzoa, 700 Edificio
   48160 Derio, Vizcaya
   Spain
   Phone: +34 91 336 7331
   Email: maribel.narganes@tecnalia.com

   Inaki Martinez Sarriegui
   Optiva Media
   Edificio Europa II,
   Calle Musgo 2, 1G,
   28023 Madrid
   Spain
   Phone: +34 91 297 7271
   Email: inaki.martinez@optivamedia.com

   Lara Fajardo Ibanez
   Optiva Media
   Edificio Europa II,
   Calle Musgo 2, 1G,
   28023 Madrid
   Spain
   Phone: +34 91 297 7271
   Email: lara.fajardo@optivamedia.com

   Pablo Lopez Zapico


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   Optiva Media
   Edificio Europa II,
   Calle Musgo 2, 1G,
   28023 Madrid
   Spain
   Phone: +34 91 297 7271
   Email: Pablo.lopez@optivamedia.com










































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