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For this RFC, original HTML is available from the RFC-Editor: RFC8802



Independent Submission                                       J.J. Aranda
Request for Comments: 8802                                     M. Cortés
Category: Informational                                            Nokia
ISSN: 2070-1721                                             J. Salvachúa
                                             Univ. Politecnica de Madrid
                                                             M. Narganes
                                                                Tecnalia
                                                   I. Martínez-Sarriegui
                                                            Optiva Media
                                                               July 2020


                 The Quality for Service (Q4S) Protocol

Abstract

   This memo describes an application-level protocol for the
   communication of end-to-end QoS compliance information based on the
   HyperText Transfer Protocol (HTTP) and the Session Description
   Protocol (SDP).  The Quality for Service (Q4S) protocol provides a
   mechanism to negotiate and monitor latency, jitter, bandwidth, and
   packet loss, and to alert 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 document; it is either application dependent (e.g., act to
   increase quality or reduce bit-rate) or network dependent (e.g.,
   change connection's quality profile).

   This protocol specification is the product of research conducted over
   a number of years; it is presented here as a permanent record and to
   offer a foundation for future similar work.  It does not represent a
   standard protocol and does not have IETF consensus.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates for any level of Internet Standard;
   see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8802.

Copyright Notice

   Copyright (c) 2020 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
   (https://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.

Table of Contents

   1.  Introduction
     1.1.  Scope
     1.2.  Motivation
     1.3.  Summary of Features
     1.4.  Differences from OWAMP/TWAMP
   2.  Terminology
   3.  Overview of Operation
   4.  Q4S Messages
     4.1.  Requests
     4.2.  Responses
     4.3.  Header Fields
       4.3.1.  Common Q4S Header Fields
       4.3.2.  Specific Q4S Request Header Fields
       4.3.3.  Specific Q4S Response Header Fields
     4.4.  Bodies
       4.4.1.  Encoding
   5.  Q4S Method Definitions
     5.1.  BEGIN
     5.2.  READY
     5.3.  PING
     5.4.  BWIDTH
     5.5.  Q4S-ALERT
     5.6.  Q4S-RECOVERY
     5.7.  CANCEL
   6.  Response Codes
     6.1.  100 Trying
     6.2.  Success 2xx
       6.2.1.  200 OK
     6.3.  Redirection 3xx
     6.4.  Request Failure 4xx
       6.4.1.  400 Bad Request
       6.4.2.  404 Not Found
       6.4.3.  405 Method Not Allowed
       6.4.4.  406 Not Acceptable
       6.4.5.  408 Request Timeout
       6.4.6.  413 Request Entity Too Large
       6.4.7.  414 Request-URI Too Long
       6.4.8.  415 Unsupported Media Type
       6.4.9.  416 Unsupported URI Scheme
     6.5.  Server Failure 5xx
       6.5.1.  500 Server Internal Error
       6.5.2.  501 Not Implemented
       6.5.3.  503 Service Unavailable
       6.5.4.  504 Server Time-Out
       6.5.5.  505 Version Not Supported
       6.5.6.  513 Message Too Large
     6.6.  Global Failures 6xx
       6.6.1.  600 Session Does Not Exist
       6.6.2.  601 Quality Level Not Allowed
       6.6.3.  603 Session Not Allowed
       6.6.4.  604 Authorization Not Allowed
   7.  Protocol
     7.1.  Protocol Phases
     7.2.  SDP Structure
       7.2.1.  "qos-level" Attribute
       7.2.2.  "alerting-mode" Attribute
       7.2.3.  "alert-pause" Attribute
       7.2.4.  "recovery-pause" Attribute
       7.2.5.  "public-address" Attributes
       7.2.6.  "latency" Attribute
       7.2.7.  "jitter" Attribute
       7.2.8.  "bandwidth" Attribute
       7.2.9.  "packetloss" Attribute
       7.2.10. "flow" Attributes
       7.2.11. "measurement" Attributes
       7.2.12. "max-content-length" Attribute
     7.3.  Measurements
       7.3.1.  Latency
       7.3.2.  Jitter
       7.3.3.  Bandwidth
       7.3.4.  Packet Loss
     7.4.  Handshake Phase
     7.5.  Negotiation Phase
       7.5.1.  Stage 0: Measurement of Latencies and Jitter
       7.5.2.  Stage 1: Measurement of Bandwidth and Packet Loss
       7.5.3.  Quality Constraints Not Reached
         7.5.3.1.  Actuator Role
         7.5.3.2.  Policy Server Role
       7.5.4.  "qos-level" Changes
     7.6.  Continuity Phase
     7.7.  Termination Phase
       7.7.1.  Sanity Check of Quality Sessions
     7.8.  Dynamic Constraints and Flows
     7.9.  "qos-level" Upgrade and Downgrade Operation
   8.  General User Agent Behavior
     8.1.  Roles in Peer-to-Peer Scenarios
     8.2.  Multiple Quality Sessions in Parallel
     8.3.  General Client Behavior
       8.3.1.  Generating Requests
     8.4.  General Server Behavior
   9.  Implementation Recommendations
     9.1.  Default Client Constraints
     9.2.  Latency and Jitter Measurements
     9.3.  Bandwidth Measurements
     9.4.  Packet Loss Measurement Resolution
     9.5.  Measurements and Reactions
     9.6.  Instability Treatments
       9.6.1.  Loss of Control Packets
       9.6.2.  Outlier Samples
     9.7.  Scenarios
       9.7.1.  Client to ACP
       9.7.2.  Client to Client
   10. Security Considerations
     10.1.  Confidentiality Issues
     10.2.  Integrity of Measurements and Authentication
     10.3.  Privacy of Measurements
     10.4.  Availability Issues
     10.5.  Bandwidth Occupancy Issues
   11. Future Code Point Requirements
     11.1.  Service Port
   12. IANA Considerations
   13. References
     13.1.  Normative References
     13.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   The World Wide Web (WWW) is a distributed hypermedia system that has
   gained widespread acceptance among Internet users.  Although WWW
   browsers support other, preexisting Internet application protocols,
   the primary protocol used between WWW clients and servers became the
   HyperText Transfer Protocol (HTTP) ([RFC7230], [RFC7231], [RFC7232],
   [RFC7233], [RFC7234], and [RFC7235]).  Since then, HTTP over TLS
   (known as HTTPS and described in [RFC2818]) has become an imperative
   for providing secure and authenticated WWW access.  The mechanisms
   described in this document are equally applicable to HTTP and HTTPS.

   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 with each other and exchange information with certain
   quality constraints.

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

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

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

   Bandwidth:  Bit rate of communication.  To ensure quality, a protocol
         must ensure the availability of the bandwidth needed by the
         application.

   Packet loss:  The percentage of packet loss is closely related to
         bandwidth and jitter.  Packet loss affects bandwidth because a
         high packet loss sometimes implies 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 as a larger time between arrivals, causing a jitter
         growth.

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

   The Q4S protocol provides a mechanism for quality monitoring based on
   an HTTP syntax and the Session Description Protocol (SDP) in order to
   be easily integrated in the 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 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 (e2e) and in real time; raising alerts if quality parameters are
   below a given 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).

   This protocol specification is the product of research conducted over
   a number of years and is presented here as a permanent record and to
   offer a foundation for future similar work.  It does not represent a
   standard protocol and does not have IETF consensus.

1.1.  Scope

   The purpose of Q4S is to measure end-to-end network quality in real
   time.  Q4S does not transport any application data.  This means that
   Q4S is designed to be used jointly with other transport protocols
   such as Real-time Transport Protocol (RTP) [RFC3550], Transmission
   Control Protocol (TCP) [RFC0793], QUIC [QUIC], HTTP [RFC7230], etc.

   Some existent transport protocols are focused on real-time media
   transport and certain connection metrics are available, which is the
   case of RTP and RTP Control Protocol (RTCP) [RFC3550].  Other
   protocols such as QUIC provide low connection latencies as well as
   advanced congestion control.  These protocols transport data
   efficiently and provide a lot of functionalities.  However, there are
   currently no other quality measurement protocols offering the same
   level of function as Q4S.  See Section 1.4 for a discussion of the
   IETF's quality measurement protocols, One-Way Active Measurement
   Protocol (OWAMP) and Two-Way Active Measurement Protocol (TWAMP).

   Q4S enables applications to become reactive under e2e network quality
   changes.  To achieve it, an independent Q4S stack application must
   run in parallel with the target application.  Then, Q4S metrics may
   be used to trigger actions on the target application, such as speed
   adaptation to latency in multiuser games, bitrate control at
   streaming services, intelligent commutation of delivery node at
   Content Delivery Networks, and whatever the target application
   allows.

1.2.  Motivation

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

   *  It enables real-time services and applications to verify whether
      network resources achieve a certain QoS level.  This helps real-
      time services and applications to run over the Internet, allowing
      the existence of Application Content Providers (ACPs), which offer
      guaranteed real-time services to the end users.

   *  Real-time monitoring allows applications to adapt themselves to
      network conditions (application-based QoS) and/or request more
      network quality from the Internet Service Provider (ISP) (if the
      ISP offers this possibility).

   *  Monitoring may also be required by peer-to-peer (P2P) real-time
      applications for which Q4S can be used.

   *  Monitoring enables ISPs to offer QoS to any ACP or end user
      application in an accountable way.

   *  Monitoring enables e2e negotiation of QoS parameters,
      independently of the ISPs of both endpoints.

   A protocol to monitor QoS must address the following issues:

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

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

   *  Must have measurement mechanisms that avoid application disruption
      and minimize network resources consumption.

   *  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.

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

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

1.3.  Summary of Features

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

   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;
   therefore, it does not impose any restrictions on the type of
   application, protocol, or usage of the monitored quality connection.

   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.

   The Q4S session lifetime is composed of four phases with different
   purposes: Handshake, Negotiation, Continuity, and Termination.
   Negotiation and Continuity phases perform network parameter
   measurements 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 document.  Basically, Q4S defines how to
   transport application quality requirements and measurement results
   between a client and server and how to provide monitoring and
   alerting, too.

   Q4S must be executed just before starting a client-server application
   that needs a quality connection in terms of latency, jitter,
   bandwidth, and/or packet loss.  Once the 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 is the server that accepts
   these parameter values or forces others.  The server is in charge of
   deciding the final values of quality connection.

1.4.  Differences from OWAMP/TWAMP

   OWAMP [RFC4656] and TWAMP [RFC5357] are two protocols to measure
   network quality in terms of RTT, but they have a different goal than
   Q4S.  The main difference is the scope: Q4S is designed to assist
   reactive applications, whereas OWAMP/TWAMP is designed to measure
   just network delay.

   The differences can be summarized in the following points:

   *  OWAMP and TWAMP are not intended for measuring availability of
      resources (certain bandwidth availability, for example) but only
      RTT.  However, Q4S is intended for measuring required bandwidth,
      packet loss, jitter, and latency in both directions.  Available
      bandwidth is not measured by Q4S, but bandwidth required for a
      specific application is.

   *  OWAMP and TWAMP do not have responsivity control (which defines
      the speed of protocol reactions under network quality changes)
      because these protocols are designed to measure network
      performance, not to assist reactive applications, and do not
      detect the fluctuations of quality within certain time intervals
      to take reactive actions.  However, responsivity control is a key
      feature of Q4S.

   *  OWAMP and TWAMP are not intended to run in parallel with reactive
      applications, but the Q4S protocol's goal is to run in parallel
      and assist reactive applications in making decisions based on Q4S-
      ALERT packets, which may trigger actions.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Overview of Operation

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

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

   The client triggers the establishment of the communication by
   requesting a specific service or application from the server.  This
   first message must have a special URI [RFC3986], 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 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 the following:

   *  The assigned Q4S sess-id.

   *  The quality constraints required by the requested application.

   *  The measurement procedure to use.

   *  "alerting-mode" attribute: 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.

      (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 messages.
           The alerting mode in this scenario is called Q4S-aware-
           network alerting mode.

      (b)  Reactive scenario: As shown in Figure 1, the network is not
           Q4S aware.  In this scenario, alert notifications are sent to
           a specific node, called an Actuator, which is in charge of
           making 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 better quality for application flows.

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

                            Figure 1: Reactive Scenario

           The format of messages exchanged between the server stack and
           the Actuator doesn't follow Q4S codification rules; their
           format will be implementation dependent.  In this way, we
           will call the messages sent from the server stack to the
           Actuator "notifications" (e.g., alert notifications) and the
           messages sent from the Actuator to the server stack in
           response to notifications "acknowledges" (e.g., alert
           acknowledges).

   *  "alert-pause" attribute: 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 sent, even with
      violated network quality constraints, in order to leave time for
      network reconfiguration or for application adjustments.

   *  "recovery-pause" attribute: The amount of time the Q4S server
      waits before trying to recover the initial "qos-level"
      (Section 7.2.1).  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 (Section 7.9).

   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 to take into account the new quality constraints in
   the measurement procedure.

   Once the communication has been established (i.e., the Handshake
   phase is finished), the protocol will verify that the communication
   path between the client and the server meets the quality constraints
   in 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:

   Stage 0:  latency and jitter measurements

   Stage 1:  bandwidth and packet loss measurements

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

   Different client and server implementations may send a different
   number of PING messages for measuring, although at least 255 messages
   should be considered to perform the latency measurement.  The Stage 0
   measurements only may be considered ended when neither client nor
   server receive new PING messages after an implementation-dependent
   guard time.  Only after Stage 0 has ended, can the client send a
   "READY 1" message.

   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 sent
        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 by 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
        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;
        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 Stage 0 for
        initiating the negotiation measurements again.

   Stage 1 is optional.  Its purpose is to measure the availability of
   application-needed bandwidth.  If the "bandwidth" attribute is set to
   zero kbps in the SDP, the client can skip stage 1 by sending a "READY
   2" message after completion of stage 0.  Stage 1 measurements are
   achieved through Q4S BWIDTH messages sent from both 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 by
   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.

   Figure 2 depicts the message exchange in a successful scenario.

               +-------------------------------------------+
               |                                           |
               | 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

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

   The following two examples show the behavior of the Q4S protocol when
   quality constraints are violated, and alerts are generated; and,
   later on, when the violation of quality constraints stops leading to
   the execution of the recovery process.  The first example (Figure 3)
   shows the Q4S-aware-network alerting mode scenario:

               +-------------------------------------------+
               |                                           |
               | 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 ------------      |
               |     --------- 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 &                     |
               |                 Fulfilled constraints)    |
               |                                           |
               |           (recovery-pause start)          |
               |                                           |
               |     --------- Q4S PING ------------>      |
               |     <-------- Q4S 200 OK -----------      |
               |     <-------- Q4S PING -------------      |
               |      -------- Q4S 200 OK ---------->      |
               |                    ...                    |
               |(recovery-pause expires &                  |
               |                 Fulfilled 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

   In this Q4S-aware-network alerting mode scenario, the server may send
   Q4S alerts to the client at any time upon 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 Figure 4 represents the Reactive
   scenario, in which alert notifications are sent from the server stack
   to the Actuator, which is in charge of deciding to act over
   application behavior and/or to invoke a network policy server.  The
   Actuator is an entity that has a 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 defined set of
   different quality levels previously agreed upon between the
   Application Content Provider and the ISP.  The Reactive alerting mode
   is the default mode.

               +-------------------------------------------+
               |                                           |
               | 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 -----                  |
               |                                           |
               |   ----- 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)   |
               |                                           |
               |                         --recovery        |
               |                         notification-->   |
               |                                           |
               |                         <--recovery       |
               |                          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 stage of the Negotiation phase, the server sends an
   alert notification to the Actuator if quality constraints are
   violated.  During the period of time defined by the "alert-pause"
   attribute, no further alert notifications are sent, but measurements
   are not 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 upon 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 [RFC3629].  A
   Q4S message is either a request or a response.

   Both request and response messages use the basic format of Internet
   Message Format [RFC5322].  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.  This document uses
   ABNF notation [RFC5234] for the definitions of the syntax of
   messages.

   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.

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

   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.

   The Request-Line ends with CRLF.  No CR or LF are allowed except in
   the end-of-line CRLF sequence.  No linear whitespace (LWSP) 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 negotiating quality sessions, READY for synchronization of
         measurements, PING and BWIDTH for quality measurements
         purposes, CANCEL for terminating sessions, Q4S-ALERT for
         reporting quality violations, and Q4S-RECOVERY for reporting
         quality recovery.

   Request-URI:  The Request-URI is a Q4S URI [RFC3986] as described in
         Section 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 uppercase.  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
   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 as described in HTTP [RFC7231].  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 to.  However, after receiving a Q4S-ALERT, Q4S-RECOVERY, or
   CANCEL request, the server SHOULD send a Q4S-ALERT, Q4S-RECOVERY, or
   CANCEL request to the client.

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 that does not
   match 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.

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

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

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

   Stage:  this is used in the client's READY requests and the 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:

   User-Agent:  this header field 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 MAY contain
         multiple product tokens and comments identifying the agent and
         any sub-products that form a significant part of the user
         agent.  By convention, the product tokens are listed in order
         of their significance for identifying the application.

   Signature:  this header field contains a digital signature that can
         be used by the network, Actuator, or policy server to validate
         the SDP, preventing security attacks.  The Signature is an
         optional header field 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 SHA256 [RFC6234] and RSA [RFC8017]
         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 ( SHA256 (<sdp>), <certificate> )

         If the Signature header field is not present, other validation
         mechanisms MAY be implemented in order to provide assured
         quality with security and control.

   Measurements:  this header field 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..999999]

         Where "l" stands for latency followed by the measured value (in
         milliseconds) or an empty space, "j" stands for jitter followed
         by the measured value (in milliseconds) or an empty space, "pl"
         stands for packet loss followed by the measured value (in
         percentage with two decimals) or an empty space, and "bw"
         stands for bandwidth followed by the measured value (in kbps)
         or an empty space.

4.3.3.  Specific Q4S Response Header Fields

   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 field),
         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 [RFC3261], but a compression/coding
   scheme will limit the way the request is parsed to certain logical
   implementations, thus making the protocol concept more implementation
   dependent.  In addition, the bandwidth calculation may not be valid
   if compression is used.  Therefore, the HTTP Accept-Encoding request
   header field cannot be used in Q4S with values different from
   "identity", and if it is present in a request, the server MUST ignore
   it.  In addition, the response header field Content-Encoding is
   optional, but if present, the unique permitted value is "identity".

   The body length in bytes MUST be 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 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 [RFC7231].  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 purpose of this method is to start the quality session.

   This method is used only during the Handshake phase to retrieve the
   SDP containing the sess-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 MUST be canceled, 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 the
   sending of PING and BWIDTH messages over UDP between clients and
   servers.  Including the Stage header field in this method is
   mandatory.

   This message is used only in Negotiation and Continuity phases, and
   only just before making a measurement.  Otherwise (outside 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 it MUST be answered in order to measure RTT and
   jitter, whereas BWIDTH is a long message and MUST NOT be answered.

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

   Including the Measurements header field 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 either the
   client or the server.  Both client and server MUST NOT answer BWIDTH
   messages.

   Including the Measurements header field 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.

   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 depend on the agreements
   between all involved parties.

   Unless there is an error condition, an answer to a Q4S-ALERT request
   is optional and is formatted as a request Q4S-ALERT message.  If
   there is an error condition, then a response message is sent.  The
   response to a Q4S-ALERT request is not cacheable.

   This method MUST be initiated by the server in both alerting modes.
   In the Q4S-aware-network alerting mode, the Q4S-ALERT messages are
   sent by the server to the client, advising the network to react by
   itself.  In the Reactive alerting mode, alert notifications are
   triggered by the server stack and sent to the Actuator (see Figure 1,
   "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, which had been violated, have been
   fulfilled during a period of time ("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 of the session that was
   decreased during violation of constraints.  See Section 7.9).  The
   Q4S-RECOVERY message does not contain any detail on the actions to be
   taken, which depends on the agreements between all involved parties.

   The answer to a Q4S-RECOVERY request is formatted as a request Q4S-
   RECOVERY message.  A Q4S-RECOVERY request MUST NOT be answered with a
   response message unless there is an error condition.  The response to
   a Q4S-RECOVERY request is not cacheable.

   Like the Q4S-ALERT message, the Q4S-RECOVERY method is always
   initiated by the server in both alerting modes.  In the Q4S-aware-
   network alerting mode, the Q4S-RECOVERY messages are sent by the
   server to the client, advising the network to react by itself.  In
   the Reactive alerting mode, recovery notifications are triggered by
   the server stack and sent to the Actuator (see Figure 1, "Reactive
   Scenario").

5.7.  CANCEL

   The purpose of the CANCEL message is the release of the Q4S Session-
   Id and the possible resources assigned to the session.  This message
   could be triggered by the 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, but with an answer formatted as a request Q4S-
   CANCEL message.

   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 the application or at the 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.

   The receiver of an unknown response code must take a generic action
   for the received error group (1xx, 2xx, 3xx, 4xx, 5xx, 6xx).  In case
   of an unknown error group, the expected action should be the same as
   with the 6xx error group.

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 the success of a request.

6.2.1.  200 OK

   The request has succeeded.

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 header fields 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 to generate
   response entities that have content characteristics that are not
   acceptable according to the Accept header field sent in the request.

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
   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.

   In the case that the Q4S version is not supported, this error may be
   sent by the server in the Handshake phase just after receiving the
   first BEGIN message from client.

6.5.6.  513 Message Too Large

   The server was unable to process the request because 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.

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 client/server pair.

6.6.3.  603 Session Not Allowed

   The session is not allowed due to some policy (the number of sessions
   allowed for the server is exceeded, or the time band of the Q4S-ALERT
   is not allowed for the client/server pair, 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 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 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 Figure 5:

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

                     Figure 5: Session Lifetime Phases

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

   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
      canceled 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.

   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 the Termination phase.

   Termination phase:  in which the Q4S session is terminated.  The
      application may be closed also 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 descriptions ("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> (see
   Section 7.5.3 for an example).  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.

   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 198.51.100.51
   a=public-address:server IP4 198.51.100.58
   a=measurement:procedure default(50/50,75/75,5000,40/80,100/256)
   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

   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 embedded SDP message 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.

   Appropriate attribute values: [0..9] "/" [0..9]

   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.

7.2.2.  "alerting-mode" Attribute

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

   Appropriate attribute values: <"Q4S-aware-network"|"Reactive">

   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.

   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 for either network
   reconfigurations or application adjustments.

   Appropriate attribute values: [0..60000]

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).

   Appropriate attribute values: [0..60000]

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 its 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 the network policy server or other external entities
   in charge of processing Q4S-ALERT messages.

   Appropriate attribute values: <"client"|"server"> <"IP4"|"IP6">
   <value of IP address>

7.2.6.  "latency" Attribute

   The maximum latency (considered equal for uplink and downlink)
   tolerance is 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.

   Appropriate attribute values: [0..9999]

7.2.7.  "jitter" Attribute

   The maximum uplink and downlink jitter tolerance is 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 the "jitter" attribute is not present or has a 0
   value, no jitter constraints need to be met, and no measurements MAY
   be taken.

   Appropriate attribute values: [0..9999] "/" [0..9999]

7.2.8.  "bandwidth" Attribute

   The minimum uplink and downlink bandwidth is 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 the "bandwidth"
   attribute is not present or has a 0 value, no bandwidth constraints
   need to be met, and no measurements MAY be taken.

   Appropriate attribute values: [0..99999] "/" [0..99999]

7.2.9.  "packetloss" Attribute

   The maximum uplink and downlink packet loss tolerance is specified in
   the "packetloss" attribute expressed in percentage (two decimal
   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 the "packetloss" attribute is not present or has
   a 0 value, no packet loss constraints need to be met, and no
   measurements MAY be taken.

   Appropriate attribute values: [0.00 ..100.00] "/"[0.00 ..100.00]

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 [RFC0793] or UDP
   [RFC0768]) 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 us to
   assume that measurements on "q4s" flows are the same as experienced
   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 the standard Q4S TCP
   port.  However, the Negotiation and Continuity phases will take place
   on the Q4S ports (UDP and TCP) specified in the SDP.

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

   If "clientListeningPort" is zero ("a=flow:q4s clientListeningPort
   TCP/0"), the client MAY choose one randomly 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/
   NAPT devices.  If a zero value or incorrect value is present, the
   server must set the value to the received origin port in the next
   message with SDP (200 OK, ALERT, and CANCEL messages).

   Attribute values:
      <"q4s"|"app"> <"serverListeningPort"|"clientListeningPort">
   <"UDP"|"TCP"> <0..65535> [ "-" [0..65535]]

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 document 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 the
   following:

   *  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 different responsiveness values depending on the
      control resources used in each direction.

   *  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 two different
      responsiveness values depending on the control resources used in
      each direction.

   *  The third parameter is the time interval to be used to measure
      bandwidth during the Negotiation phase.

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

   *  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 "measurement:latency", "measurement:jitter",
   "measurement:bandwidth", and "measurement:packetloss" 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 the messages will include the measured values in the Measurements
   header field.

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

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

7.2.12.  "max-content-length" Attribute

   The adaptation of measurement traffic to approximate the actual data
   streams' characteristics is convenient to accurately estimate the
   expected QoS for applications.  Particularly, packet size can have a
   remarkable effect on bandwidth estimations.  Moreover, this can
   produce problems depending on the MTU of the end hosts and links
   along the path.

   Therefore, the maximum content length MAY be set in an attribute
   denoted as "max-content-length".  Its value MUST be given in bytes
   and MUST NOT include application, transport, network, or link layer
   headers, i.e., size of the content length at the application layer.
   If not set, the value MUST be 1000 bytes.

   Furthermore, this attribute MAY be used to communicate MTU limits in
   endpoints, hence reducing possible bias as a result of network-layer
   fragmentation.

   For instance:

   a=max-content-length:1300

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 absent attributes, measurements
   MAY be omitted.

7.3.1.  Latency

   Latency measurements will be performed if the "latency" attribute
   and/or the "a=measurement:latency" attribute are present and have
   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, the
   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 "a=measurement:jitter" attribute are present and have 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 is received by an endpoint (either server
   or client), the corresponding jitter value is updated with the
   statistical jitter value, which is the arithmetic mean of the
   absolute values of elapsed times calculated on the first 255 packets
   received.

   Each endpoint sends a PING periodically with a fixed interval, and
   each value of "elapsed time" (ET) should be very close to this
   interval.  If a PING message is lost, the ET value is doubled.
   Identifying lost PING messages, however, is not an issue because all
   PING messages are labeled with a Sequence-Number header field.
   Therefore, the receiver can discard this ET 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 measurement 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 the Measurements header field
   of the PING message.

7.3.3.  Bandwidth

   Bandwidth measurements will be performed if the "bandwidth" attribute
   and/or the "a=measurement:bandwidth" attribute is present and has
   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.  The messages are sent during
   the period of time defined in the third parameter of the SDP
   "measurement:procedure default" attribute in milliseconds.

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

           +------------------------------------------------+
           |             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 bitrate required by the bandwidth attribute.  This is
   illustrated in Figure 6.

   ALERTS are not expected during bandwidth measurement, but only at the
   end of the measurement time.

   When measuring bandwidth, all BWIDTH requests sent MUST be 1 kilobyte
   in length (UDP payload length by default), they MUST include a
   Sequence-Number header field with a sequential number starting at 0,
   and their content MUST consist of randomly generated values to
   minimize the effect of compression elements along the path.  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:

   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

          VkZaU1FrNVZNVlZSV0doT1ZrZ (to complete up to "max-content-
                                    length" 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

          ZY0VaT1ZURlZVVmhyUFE9PQ (to complete up to max-content-
                                  length 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 field incremented
   sequentially with each sent packet, lost packets can be easily
   identified.  The lost packets MUST be counted during the measurement
   time.

7.4.  Handshake Phase

   The first phase consists of a Q4S BEGIN method issued from the client
   to the server as shown in Figure 7.

   The first Q4S message MUST have a special URI [RFC3986], 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
   [RFC3264].

   If the request is accepted, the server MUST answer it with a Q4S 200
   OK message, which MUST contain an SDP body [RFC4566] with the
   assigned sess-id (embedded in the SDP "o=" line), 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 attributes
   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 Service Level
   Agreement (SLA) is being violated.

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

                         Figure 7: Handshake Phase

   The following is an example of a client request and a 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 header fields 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 in 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.

   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.

   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 [RFC3550].

   Measurements of latency and jitter are made by calculating the
   differences in the 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
   network traffic again.  Therefore, the "default" procedure determines
   that the measurements are taken in two stages:

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

   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 field that allows
   for easy packet loss detection.

   The client starts the Negotiation phase by sending a READY request
   using the TCP Q4S ports defined in the SDP.  This READY request
   includes a Stage header field 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 is specified, stage 0 is skipped.

7.5.1.  Stage 0: Measurement of Latencies and Jitter

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

   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 a 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.

   The client and server MUST send PING requests to each other.  The
   Sequence-Number header field of the first PING MUST be set to 0.  The
   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

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

   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 [RFC0792].  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, always using the same UDP ports and incrementing the
   Sequence-Number with each message.

   In the following example, the value of the first parameter of the SDP
   "measurement:procedure" attribute is 50 milliseconds (from the client
   to the server) and 60 ms (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 field 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 field 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 will send its measurements to the client.
   Example:

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

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

   Optionally the PING request can include a Timestamp header field with
   the time in which the message has been sent.  In the case that the
   header field is present, the server MUST include the header field 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 an 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:

   *  The latency, jitter, and packetloss constraints are reached in
      both directions

   *  The latency, jitter, and packetloss constraints are not reached in
      one or both directions

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

   If the bandwidth constraints are either empty or have a value of
   zero, the Negotiation phase MUST terminate, and both client and
   server may initiate the Continuity phase.  In this case, client moves
   to the Continuity phase by sending a READY message that includes the
   header field, "Stage: 2".

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

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.  The value of the READY message's Stage header
   field is 1.  The server answers with a Q4S 200 OK message to
   synchronize the initiation of the measurements as shown in Figure 9.

           +------------------------------------------------+
           |                                                |
           | 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:

   *  The bandwidth and packetloss constraints are reached in both
      directions.

   *  The bandwidth and packetloss constraints are not reached in one or
      both directions.

   In the first case, Stage 1 is finished.  The client and server are
   ready for the Continuity phase.  The client moves to this phase by
   sending a READY message that includes the header field, "Stage: 2".
   The server answer MUST be 200 OK as shown in Figure 10.

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

             Figure 10: Trigger the Application Using HTTP URI

   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 field is present, the client SHOULD send an
   HTTP request to this URI.

   The second case, with violated network constraints, is explained in
   Section 7.5.4.

7.5.3.  Quality Constraints Not Reached

   After finishing Stage 1 of the Negotiation phase, the client and the
   server have each other's measured parameter values as these have been
   exchanged in the Measurements header fields 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.

   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, the 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 field, and the client MUST answer with the same Q4S-ALERT
        message.  The Signature header field contains the signed hash
        value of the SDP body in order to protect all the SDP data, and
        therefore it MUST contain the "measurement" parameters in the
        body.

      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 198.51.100.51
      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 the 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
        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 the 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.

   The 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

   The actuator receives notifications of unmet requirements from the
   Q4S server stack and acts upon the application or the network policy
   server, according to logic out of scope of this protocol.

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

   The type of actions that an Actuator can take at the application
   level are application dependent and MAY involve:

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

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

   Apart from actions at the application level, the Actuator MAY act at
   the network level if a network policy server is available.

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.  A network policy
   server may implement all or some of these features (but
   implementation is not exclusive to):

   *  Server validation in terms of quality constraints

   *  Authentication (Signature validation) and security (blocking of
      malicious clients)

   *  Policy rules (the 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, the server MAY trigger a Q4S-ALERT
   asking for a higher "qos-level" attribute.  The maximum "qos-level"
   allowed is 9 for both uplink and downlink.

   If the "qos-level" has reached the maximum value for the 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 on the server side.

   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 document only covers the "default" procedure.  The continuity
   operation with the "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 the example, shown in
   Figure 11, 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)

   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 field reaches the maximum value, the
   client continues sending PING messages with the Sequence-Number
   header field starting again at zero.  When the server PING Sequence-
   Number header field 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 the Q4S-aware-network alerting mode shown in Figure 12, if the
   client receives a Q4S-ALERT message, it MUST answer by sending the
   Q4S-ALERT request message including the SDP (with its corresponding
   digital signature) back to the server.

   Both client and server will keep performing measurements, but Q4S-
   ALERT messages MUST NOT 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 document.

           +------------------------------------------------+
           |                                                |
           | 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 shown in Figure 13, 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.

           +------------------------------------------------+
           |                                                |
           | 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 the endpoint 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 network's inability to meet the required
      quality constraints.  The client and server application will be
      notified by their respective Q4S stacks.

   *  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 canceled, and a new session will be
      initiated.

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

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

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 field 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 a result of two
   possibilities:

   *  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.

   *  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 a 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 [RENO],
   High Speed TCP [RFC3649], CUBIC [CUBIC], Compound TCP (CTCP) [CTCP],
   etc., reach different throughputs under the same network conditions
   of RTT and packet loss.  In all cases, depending on the RTT-measured
   value, the Q4S server could dynamically change the packetloss
   constraints (defined in the SDP) in order to make it possible to
   reach a required throughput or vice versa (using
   "measurement:packetloss" to change dynamically the latency
   constraints).

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

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

   Then, depending on RTT-measured values, set dynamically the packet
   loss 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 larger throughput if more bandwidth is available.

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

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

   In the absence of packet loss, a practical limit for the TCP
   throughput is the receiver_window_size divided by the RTT.  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 a 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
   canceled.  But when the violation of constraints stops before
   reaching "qos-level" maximum value, the recovery mechanism allows for
   the "qos-level" upgrade gradually.

   This downgrade and upgrade of "qos-level" is explained with the
   following example:

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

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

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

   4.  The "alert-pause" timer expires, but the violation of constraints
       has stopped; the "recovery-pause" timer is started.

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

   6.  The "recovery-pause" timer expires again, and no violation of
       constraints has been detected.  Meanwhile, the "qos-level" is
       decreased to 0, and a Q4S-RECOVERY is sent by the server to the
       client.  The "recovery-pause" timer is not started this time as
       the "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 application 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
   modification 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 the measurement at lower levels of quality
   while application flows continue using a higher "qos-level" value.

   *  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 the Q4S flows.

   *  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, illustrated in Figure 14, 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 the client and server role.  The role
   assumed depends on who sends the first message.

   In a communication between two UAs, the UA that first sends the Q4S
   BEGIN request to start the Handshake phase shall assume the client
   role.

   If both UAs send the BEGIN request at the same time, they will wait
   for a random time to restart again as shown in Figure 15.

   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.  This means
   that for using the application, the client MUST establish only one
   Q4S session against the server.  Indeed, the relation between the
   Session-Id and the 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), it can execute different Q4S clients to establish separately
   different Q4S sessions, but it is NOT RECOMMENDED because:

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

   *  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.

8.3.  General Client Behavior

   A Q4S client has different behaviors.  We will use letters X, Y, and
   Z to designate each different behavior (follow the letters in
   Figure 16 and their descriptions 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 into 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 of the following:

   Y)  During the measurement of latency, jitter, and packet loss, the
       PING requests are sent periodically, not just 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)  During the 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 and Client Behaviors

8.3.1.  Generating Requests

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

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

   If SDP is included:  the Session-Id is embedded into the SDP,
      therefore the inclusion of the Session-Id header field 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 new SDP with updated values, the
   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 the server MUST send packets to measure jitter,
   latency, and bandwidth.  Therefore, the different behaviors of the
   server are (follow the letters in Figure 17 and their descriptions
   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 of the following:

   S)  During the measurement of latency, jitter, and packet loss, the
       PING requests are sent periodically by the client and 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)  During 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.

   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 and Server Behaviors

9.  Implementation Recommendations

9.1.  Default Client Constraints

   To provide a default configuration, it would be good if 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

   Different client and server implementations may send a different
   number of PING messages for measuring, although at least 255 messages
   should be considered to perform the latency measurement.  The Stage 0
   measurements may be considered ended only when neither the client nor
   server receive new PING messages after an implementation-dependent
   guard time.  Only after, the client can send a "READY 1" message.

   In execution systems, where the timers are not accurate, a
   recommended approach consists of including the optional Timestamp
   header field 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.

9.3.  Bandwidth Measurements

   In programming languages or operating systems with limited timers or
   clock resolution, it is recommended to use an approach based on
   several intervals to send messages of 1KB (= 8000 bits) 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 11 Mbps, 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 the required bandwidth and
   accuracy that the programmer wants to achieve.

   Considering messages of 1KB (= 8000 bits), a general approach to
   determine these intervals is the following:

   (1)  Compute target bandwidth / 8000 bits.  In the example above, it
        is 11 Mbps / 8000 = 1375 messages per second.

   (2)  Divide the number of messages per second by 1000 to determine
        the number of messages per millisecond: 1375 / 1000 = 1.375.
        The integer value is the number of messages per millisecond (in
        this case, one).  The pending bandwidth is now 375 messages per
        second.

   (3)  To achieve the 375 messages per second, use a submultiple of
        1000, which must be less than 375:

            1000 / 2 = 500 > 375

            1000 / 3 = 333 < 375

        In this case, a message every 3 ms is suitable.  The new pending
        target bandwidth is 375 - 333 = 42 messages per second.

   (4)  Repeat the same strategy as point 3 to reach the pending
        bandwidth.  In this case, 23 ms is suitable because of the
        following:

            1000 / 22 = 45 > 42

            1000 / 23 = 43 > 42

            1000 / 24 = 41.6 < 42

   We can choose 24 ms, but then we need to cover an additional 0.4
   messages per second (42 - 41.6 = 0.4), and 43 is a number higher than
   42 but very close to it.

   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 the packets used, the more constant 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 the application nature and network conditions, a packet
   loss resolution less than 1% may be needed.  In such cases, there is
   no limit to the number of samples used for this calculation.  A
   trade-off 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 packet loss measured value.

   The problem of this approach is the reliability of old samples.  If
   the interval used between PING messages is 50 ms, then to have a
   resolution of 1/1000, it takes 50 seconds, and a resolution of
   1/10000 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 to measure and trigger network tuning
   and application-level actions (i.e. lowering video bit-rate, reducing
   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.

9.6.1.  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 delivered too late
   (which we will consider as "lost"), some undesirable consequences
   could arise.

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

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

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

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

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

9.6.2.  Outlier Samples

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

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

9.7.  Scenarios

   Q4S could be used in two scenarios:

   *  client to ACP

   *  client to client (peer-to-peer scenario)

9.7.1.  Client to ACP

   One server:

   It is the common scenario in which the client contacts the 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:

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

   Prevention mode:  during a streaming session, the user keeps several
      Q4S dialogs against different alternative delivery nodes.  In case
      of congestion, the end user MAY change to the best alternative
      delivery node.

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 to 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 enable the assumption of the
   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

10.1.  Confidentiality Issues

   Because Q4S does not transport any application data, Q4S does not
   jeopardize the security of application data.  However, other certain
   considerations may take place, like identity impersonation and
   measurements privacy and integrity.

10.2.  Integrity of Measurements and Authentication

   Identity impersonation could potentially produce anomalous Q4S
   measurements.  If this attack is based on spoofing of the server IP
   address, it can be avoided using the digital signature mechanism
   included in the SDP.  The network can easily validate this digital
   signature using the public key of the server certificate.

   Integrity of Q4S measurements under any malicious manipulation (such
   as a Man-in-the-Middle (MITM) attack) relies on the same mechanism,
   the SDP signature.

   The Signature header field contains the signed hash value of the SDP
   body in order to protect all the SDP data, including the
   measurements.  This signature not only protects the integrity of data
   but also authenticates the server.

10.3.  Privacy of Measurements

   This protocol could be supported over IPsec.  Q4S relies on UDP and
   TCP, and IPsec supports both.  If Q4S is used for application-based
   QoS, then IPsec is operationally valid; however, if Q4S is used to
   trigger network-based actions, then measurements could be incorrect
   unless the IPsec ports can be a target of potential action over the
   network (such as prioritizing IPsec flows to measure the new,
   upgraded state of certain application flows).

10.4.  Availability Issues

   Any loss of connectivity may interrupt the availability of the Q4S
   service and may result in higher packet loss measurements, which is
   just the desired behavior in these situations.

   In order to mitigate availability issues caused by malicious attacks
   (such as DoS and DDoS), a good practice is to enable the Q4S service
   only for authenticated users.  Q4S can be launched after the user is
   authenticated by the application.  At this moment, the user's IP
   address is known, and the Q4S service may be enabled for this IP
   address.  Otherwise, the Q4S service should appear unreachable.

10.5.  Bandwidth Occupancy Issues

   Q4S bandwidth measurement is limited to the application needs.  It
   means that all available bandwidth is not measured, but only the
   fraction required by the application.  This allows other applications
   to use the rest of available bandwidth normally.

   However, a malicious Q4S client could restart Q4S sessions just after
   finishing the Negotiation phase.  The consequence would be to waste
   bandwidth for nothing.

   In order to mitigate this possible anomalous behavior, it is
   RECOMMENDED to configure the server to reject sessions from the same
   endpoint when this situation is detected.

11.  Future Code Point Requirements

   If the ideas described in this document are pursued to become a
   protocol specification, then the code points described in this
   document will need to be assigned by IANA.

11.1.  Service Port

   An assigned port would make possible a future Q4S-aware network
   capable of reacting by itself to Q4S alerts.  A specific port would
   simplify the identification of the protocol by network elements in
   charge of making possible reactive decisions.  Therefore, the need
   for a port assignment by IANA may be postponed until there is the
   need for a future Q4S-aware network.

   Service Name: Q4S

   Transport Protocol(s): TCP

   Assignee:
      Name: Jose Javier Garcia Aranda

      Email: jose_javier.garcia_aranda@nokia.com

   Contact:
      Name: Jose Javier Garcia Aranda

      Email: jose_javier.garcia_aranda@nokia.com

   Description:  The service associated with this request is in charge
         of the establishment of new Q4S sessions, and during the
         session, manages the handoff to a new protocol phase
         (Handshake, Negotiation and Continuity) as well as sends alerts
         when measurements do not meet the requirements.

   Reference:  This document.  This service does not use IP-layer
         broadcast, multicast, or anycast communication.

12.  IANA Considerations

   This document has no IANA actions.

13.  References

13.1.  Normative References

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,
              <https://www.rfc-editor.org/info/rfc7232>.

   [RFC7233]  Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
              "Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
              RFC 7233, DOI 10.17487/RFC7233, June 2014,
              <https://www.rfc-editor.org/info/rfc7233>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <https://www.rfc-editor.org/info/rfc7235>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,
              <https://www.rfc-editor.org/info/rfc2818>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <https://www.rfc-editor.org/info/rfc3264>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.

13.2.  Informative References

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/info/rfc792>.

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-29, 9 June 2020,
              <https://tools.ietf.org/html/draft-ietf-quic-transport-
              29>.

   [RFC4656]  Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
              Zekauskas, "A One-way Active Measurement Protocol
              (OWAMP)", RFC 4656, DOI 10.17487/RFC4656, September 2006,
              <https://www.rfc-editor.org/info/rfc4656>.

   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
              Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, DOI 10.17487/RFC5357, October 2008,
              <https://www.rfc-editor.org/info/rfc5357>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

   [RENO]     Mathis, M., Semke, J., Mahdavi, J., and T. Ott, "The
              Macroscopic Behavior of the TCP Congestion Avoidance
              Algorithm", ACM SIGCOMM Computer Communication Review, pp.
              67-82, DOI 10.1145/263932.264023, July 1997,
              <https://doi.org/10.1145/263932.264023>.

   [RFC3649]  Floyd, S., "HighSpeed TCP for Large Congestion Windows",
              RFC 3649, DOI 10.17487/RFC3649, December 2003,
              <https://www.rfc-editor.org/info/rfc3649>.

   [CUBIC]    Rhee, I., Xu, L., and S. Ha, "CUBIC for Fast Long-Distance
              Networks", Work in Progress, Internet-Draft, draft-rhee-
              tcpm-cubic-02, 26 August 2008,
              <https://tools.ietf.org/html/draft-rhee-tcpm-cubic-02>.

   [CTCP]     Sridharan, M., Tan, K., Bansal, D., and D. Thaler,
              "Compound TCP: A New TCP Congestion Control for High-Speed
              and Long Distance Networks", Work in Progress, Internet-
              Draft, draft-sridharan-tcpm-ctcp-02, 11 November 2008,
              <https://tools.ietf.org/html/draft-sridharan-tcpm-ctcp-
              02>.

Acknowledgements

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

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

   Additionally, we want to thank the Spanish Centre for the Development
   of Industrial Technology (CDTI) as well as the Spanish Science and
   Tech Ministry, which funds this initiative through their innovation
   programs.

Contributors

   Jacobo Perez Lajo
   Nokia Spain

   Email: jacobo.perez@nokia.com


   Luis Miguel Diaz Vizcaino
   Nokia Spain

   Email: Luismi.Diaz@nokia.com


   Gonzalo Munoz Fernandez
   Nokia Spain

   Email: gonzalo.munoz_fernandez.ext@nokia.com


   Manuel Alarcon Granero
   Nokia Spain

   Email: manuel.alarcon_granero.ext@nokia.com


   Francisco Jose Juan Quintanilla
   Nokia Spain

   Email: francisco_jose.juan_quintanilla.ext@nokia.com


   Carlos Barcenilla
   Universidad Politecnica de Madrid


   Juan Quemada
   Universidad Politecnica de Madrid

   Email: jquemada@dit.upm.es


   Ignacio Maestro
   Tecnalia Research & Innovation

   Email: ignacio.maestro@tecnalia.com


   Lara Fajardo Ibañez
   Optiva Media

   Email: lara.fajardo@optivamedia.com


   Pablo López Zapico
   Optiva Media

   Email: Pablo.lopez@optivamedia.com


   David Muelas Recuenco
   Universidad Autonoma de Madrid

   Email: dav.muelas@uam.es


   Jesus Molina Merchan
   Universidad Autonoma de Madrid

   Email: jesus.molina@uam.es


   Jorge E. Lopez de Vergara Mendez
   Universidad Autonoma de Madrid

   Email: jorge.lopez_vergara@uam.es


   Victor Manuel Maroto Ortega
   Optiva Media

   Email: victor.maroto@optivamedia.com


Authors' Addresses

   Jose Javier Garcia Aranda
   Nokia
   María Tubau 9
   28050 Madrid
   Spain

   Phone: +34 91 330 4348
   Email: jose_javier.garcia_aranda@nokia.com


   Mónica Cortés
   Nokia
   María Tubau 9
   28050 Madrid
   Spain

   Email: monica.cortes_sack@nokia.com


   Joaquín Salvachúa
   Universidad Politecnica de Madrid
   Avenida Complutense 30
   28040 Madrid
   Spain

   Phone: +34 91 0672134
   Email: Joaquin.salvachua@upm.es


   Maribel Narganes
   Tecnalia Research & Innovation
   Parque Científico y Tecnológico de Bizkaia
   Astondo Bidea, Edificio 700
   E-48160 Derio Bizkaia
   Spain

   Phone: +34 946 430 850
   Email: maribel.narganes@tecnalia.com


   Iñaki Martínez-Sarriegui
   Optiva Media
   Edificio Europa II,
   Calle Musgo 2, 1G,
   28023 Madrid
   Spain

   Phone: +34 91 297 7271
   Email: inaki.martinez@optivamedia.com


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