Internet DRAFT - draft-aranda-dispatch-q4s
draft-aranda-dispatch-q4s
Internet Draft
Intended status: Informational J.J. Garcia Aranda
Expires: January 2020 Nokia
M. Cortes
J. Salvachua
Univ. Politecnica de Madrid
M. Narganes
Tecnalia
I. Martinez Sarriegui
Optiva Media
July 5, 2019
The Quality for Service Protocol
draft-aranda-dispatch-q4s-10.txt
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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 Protocol
(Q4S) provides a mechanism to negotiate and monitor latency,
jitter, bandwidth, and packet, 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 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, 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.
Table of Contents
1 Introduction...................................................5
1.1 Scope....................................................6
1.2 Motivation...............................................7
1.3 Summary of Features......................................8
1.4 Differences with OWAMP/TWAMP.............................9
2 Terminology....................................................9
3 Overview of Operation.........................................10
4 Q4S Messages..................................................21
4.1 Requests................................................21
4.2 Responses...............................................22
4.3 Header Fields...........................................23
4.3.1 Common Q4S Header Fields..........................23
4.3.2 Specific Q4S Request Header Fields................24
4.3.3 Specific Q4S Response Header Fields...............25
4.4 Bodies..................................................26
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4.4.1 Encoding..........................................26
5 Q4S Method Definitions........................................26
5.1 BEGIN...................................................27
5.2 READY...................................................27
5.3 PING....................................................27
5.4 BWIDTH..................................................28
5.5 Q4S-ALERT...............................................28
5.6 Q4S-RECOVERY............................................28
5.7 CANCEL..................................................29
6 Response Codes................................................29
6.1 100 Trying..............................................30
6.2 Success 2xx.............................................30
6.2.1 200 OK............................................30
6.3 Redirection 3xx.........................................30
6.4 Request Failure 4xx.....................................30
6.4.1 400 Bad Request...................................30
6.4.2 404 Not Found.....................................30
6.4.3 405 Method Not Allowed............................31
6.4.4 406 Not Acceptable................................31
6.4.5 408 Request Timeout...............................31
6.4.6 413 Request Entity Too Large......................31
6.4.7 414 Request-URI Too Long..........................31
6.4.8 415 Unsupported Media Type........................31
6.4.9 416 Unsupported URI Scheme........................31
6.5 Server Failure 5xx......................................32
6.5.1 500 Server Internal Error.........................32
6.5.2 501 Not Implemented...............................32
6.5.3 503 Service Unavailable...........................32
6.5.4 504 Server Time-out...............................32
6.5.5 505 Version Not Supported.........................32
6.5.6 513 Message Too Large.............................33
6.6 Global Failures 6xx.....................................33
6.6.1 600 session does not exist........................33
6.6.2 601 quality level not allowed.....................33
6.6.3 603 Session not allowed...........................33
6.6.4 604 authorization not allowed.....................33
7 Protocol......................................................33
7.1 Protocol Phases.........................................33
7.2 SDP Structure...........................................35
7.2.1 "qos-level" attribute.............................36
7.2.2 "alerting-mode" attribute.........................37
7.2.3 "alert-pause" attribute...........................37
7.2.4 "recovery-pause" attribute........................37
7.2.5 "public-address" attributes.......................38
7.2.6 "latency" attribute...............................38
7.2.7 "jitter" attribute................................38
7.2.8 "bandwidth" attribute.............................38
7.2.9 "packetloss" attribute............................39
7.2.10 "flow" attributes.................................39
7.2.11 "measurement" attributes..........................40
7.2.12 "max-content-length" attribute....................42
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7.3 Measurements............................................42
7.3.1 Latency...........................................42
7.3.2 Jitter............................................43
7.3.3 Bandwidth.........................................43
7.3.4 Packet loss.......................................46
7.4 Handshake Phase.........................................46
7.5 Negotiation Phase.......................................47
7.5.1 Stage 0: Measurement of Latencies and Jitter......49
7.5.2 Stage 1: Measurement of Bandwidth and Packet Loss.52
7.5.3 Quality Constraints Not Reached...................55
7.5.3.1 Actuator Role..................................57
7.5.3.2 Policy Server Role.............................58
7.5.4 QoS Level Changes.................................58
7.6 Continuity Phase........................................59
7.7 Termination Phase.......................................62
7.8 Dynamic Constraints And Flows...........................63
7.9 Qos-level Upgrade And Downgrade Operation...............64
8 General User Agent Behavior...................................66
8.1 Roles in Peer-to-Peer Scenarios.........................66
8.2 Multiple Quality Sessions in Parallel...................67
8.3 General Client bBhavior.................................67
8.3.1 Generating Requests...............................68
8.4 General Server Behavior.................................69
9 Implementation Recommendations................................70
9.1 Default Client Constraints..............................70
9.2 Latency and Jitter Measurements.........................70
9.3 Bandwidth Measurements..................................71
9.4 Packet Loss Measurement Resolution......................72
9.5 Measurements and Reactions..............................72
9.6 Instability Treatments..................................72
9.6.1 Loss of Control Packets...........................73
9.6.2 Outlier Samples...................................73
9.7 Scenarios...............................................73
9.7.1 Client to ACP.....................................74
9.7.2 Client to Client..................................74
10 Security Considerations.......................................74
10.1 Confidentiality Issues..................................74
10.2 Integrity of Measurements and Authentication............75
10.3 Privacy of Measurements.................................75
10.4 Availability Issues.....................................75
10.5 Bandwidth Occupancy Issues..............................75
11 Future Code Point Requirements................................76
11.1 Service Port............................................76
12 References....................................................77
12.1 Normative References....................................77
12.2 Informative References..................................78
13 Acknowledgments...............................................80
14 Contributors..................................................81
15 Authors' Addresses............................................83
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1 Introduction
The World Wide Web (WWW) is a distributed hypermedia system
which has gained widespread acceptance among Internet users.
Although WWW browsers support other, preexisting Internet
application protocols, the primary protocol used between WWW
clients and servers became the HyperText Transfer Protocol (HTTP)
(RFC 7230 [1], RFC 7231 [2], RFC 7232 [3], RFC 7233 [4], RFC 7234
[5], and RFC 7235 [6]). Since then, HTTP over TLS (known as HTTPS
and described in RFC 2818 [7]) 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 each other and exchange information with certain
quality constraints.
Quality in communications at the application level consists of
four measurable parameters:
o Latency: The time a message takes to travel from source to
destination. It may be approximated to RTT/2 (Round trip
time), assuming the networks are symmetrical. In this context
we will consider the statistical median formula.
o Jitter: latency variation. There are some formulas to
calculate Jitter, and in this context we will consider the
arithmetic mean formula.
o Bandwidth: bit rate of communication. To assure quality, a
protocol must assure the availability of the bandwidth needed
by the application.
o Packet loss: The percentage of packet loss is closely related
to bandwidth and jitter. Affects bandwidth because a high
packet loss implies sometimes retransmissions that also
consumes extra bandwidth, other times the retransmissions are
not achieved (for example in video streaming over UDP) and
the information received is less than the required bandwidth.
In terms of jitter, a packet loss sometimes is seen by the
destination like a larger time between arrivals, causing a
jitter growth.
Any other communication parameter such as throughput, is not a
network parameter because it depends on protocol window size and
other implementation-dependent aspects.
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The Quality for Service Protocol (Q4S) provides a mechanism for
quality monitoring based on an HTTP syntax and the Session
Description protocol (SDP) in order to be easily integrated in
WWW, but it may be used by any type of application, not only those
based on HTTP. Quality requirements may be needed by any type of
application that 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 pre-negotiated threshold and
sending recoveries when quality parameters are restored. Q4S
describes when these notifications, alerts and recoveries, need to
be sent and the entity receiving them. The actions undertaken by
the receiver of the alert are out of scope of the protocol.
Q4S is session-independent from the application flows, to minimize
the impact on them. To perform the measurements, two control flows
are created on both communication paths (forward and reverse
directions).
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. It means
that Q4S is designed to be used jointly with other transport
protocols such as Real Time Protocol (RTP)(RFC 3550 [8]),
Transmission Control Protocol (TCP) (RFC 793 [16]), Quick UDP
Internet Connections (QUIC)[9] , HTTP [1], etc.
Some existent transport protocols are focused in real-time media
transport and certain connection metrics are available, which is
the case of RTP and Real Time Control Protocol (RTCP)[8]. Other
protocols such as QUIC provide low connection latencies as well as
advanced congestion control. These protocols transport data
efficiently and provide 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 OWAMP and TWAMP quality measurement protocols.
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Q4S enable applications to become reactive under e2e network
quality changes. To achieve it, an independent Q4S stack
application must run in parallel to target application. Then, Q4S
metrics may be used to trigger actions on 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 target application allow.
1.2 Motivation
Monitoring quality of service (QoS) in computer networks is useful
for several reasons:
o Enable real-time services and applications to verify whether
network resources achieve a certain QoS level. This helps
real-time services and applications to run through the
Internet, allowing the existence of Application Content
Providers (ACPs) which offer guaranteed real-time services to
the final users.
o Real-time monitoring allows applications to adapt themselves
to network conditions (Application-based QoS) and/or request
more network quality to the Internet Service Provider (ISP)
(if the ISP offers this possibility).
o Monitoring may also be required by Peer to Peer (P2P) real-
time applications for which Q4S can be used
o Enable ISPs to offer QoS to any ACP or final user application
in an accountable way
o Enable e2e negotiation of QoS parameters, independently of
the ISPs of both endpoints.
A protocol to monitor QoS must address the following issues:
o Must be ready to be used in conjunction with current standard
protocols and applications, without forcing a change on them.
o Must have a formal and compact way to specify quality
constraints desired by the application to run.
o Must have measurement mechanisms avoiding application
disruption and minimizing network resources consumption.
o Must have specific messages to alert about the violation of
quality constraints in different directions (forward and
reverse), because network routing may not be symmetrical, and
of course, quality constraints may not be symmetrical.
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o After having alerted about the violation of quality
constraints, must have specific messages to inform about
recovery of quality constraints in corresponding directions
(forward and reverse).
o Must protect the data (constrains, measurements, QoS levels
demanded from the network) in order to avoid the injection of
malicious data in the measurements.
1.3 Summary of Features
The Quality for Service Protocol (Q4S) 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 application provider and may
be application-level adaptive reactions, may involve requests to
ISP, or whatever application provider decide.
The benefits in quality measurements provided by Q4S can be used
by any type of application that uses any type of protocol for data
transport. It provides a quality monitoring scheme for any
communication that takes place between the client and the server,
not only for the Q4S communication itself.
Q4S does not establish multimedia sessions and it does not
transport application data. It monitors the fulfillment of the
quality requirements of the communication between the client and
the server, and therefore does not impose any restrictions on the
type of application, protocol or the type of usage of the
monitored quality connection.
Some applications may vary their quality requirements dynamically
for any given quality parameter. Q4S is able to adapt to the
changing application needs modifying the parameter thresholds to
the new values and monitoring the network quality according to the
new quality constraints. It will raise alerts if the new
constraints are violated.
Q4S session lifetime is composed of four phases with different
purposes: Handshake, Negotiation, Continuity and Termination.
Negotiation and Continuity phases perform network parameter
measurements as per a negotiated measurement procedure. Different
measurement procedures could be used inside Q4S, although one
default measurement mechanism is needed for compatibility reasons
and is the one defined in this document. Basically, Q4S defines
how to transport application quality requirements and measurement
results between client and server and providing monitoring and
alerting too.
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Q4S must be executed just before starting a client-server
application which needs a quality connection in terms of latency,
jitter, bandwidth and/or packet loss. Once client and server have
succeeded in establishing communication under quality constraints,
the application can start, and Q4S continues measuring and
alerting if necessary.
The quality parameters can be suggested by the client in the first
message of the handshake phase, but it's the server that accepts
these parameter values or forces others. The server is in charge
of deciding the final values of quality connection.
1.4 Differences with OWAMP/TWAMP
OWAMP (RFC 4656) [27] and TWAMP (RFC 5357) [28] are two protocols
to measure network quality in terms of RTT, but has a different
goal than Q4S. The main difference is the scope: Q4S is designed
to assist reactive applications, while OWAMP/TWAMP is designed
just to measure network delay.
Differences can be summarized in the following points:
. OWAMP/TWAMP is 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
required bandwidth for specific application.
. OWAMP/TWAMP does not have responsivity control (which
defines the speed of protocol reactions under network quality
changes), because this protocol is designed to measure
network performance, not to assist reactive applications and
does not detect the fluctuations of quality in certain time
intervals to take reactive actions. However, responsivity
control is a key feature of Q4S.
. OWAMP/TWAMP is not intended to run in parallel with reactive
applications, but Q4S' goal is to run in parallel and assist
reactive applications to take 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
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described in BCP 14 RFC 2119 [11] RFC 8174 [21] 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 tutorial nature and does not contain
any normative statements.
The first example shows the basic functions of a Q4S:
communication establishment between a client and a server, quality
requirement negotiations for the requested application,
application start and continuous quality parameter measurements,
and finally communication termination.
The client triggers the establishment of the communication
requesting a specific service or application from the server. This
first message must have a special URI (RFC 3986)[12], which may
force the use of the Q4S protocol if it is implemented in a
standard web browser. This message consists of a Q4S BEGIN method,
which can optionally include a proposal for the communication
quality requirements in an SDP body. This option gives the client
a certain negotiation capacity about quality requirements, but it
will be the server who finally decides about the stated
requirements.
This request is answered by the server with a Q4S 200 OK response
letting the client know that it accepts the request. This response
message must contain an SDP body with:
o The assigned Q4S session id.
o The quality constraints required by the requested
application.
o The measurement procedure to use.
o The alerting mode: there are two different scenarios for
sending alerts that trigger actions either on the network or
in the application when measurements identify violated
quality constraints. In both cases, alerts are triggered by
the server.
o a) Q4S-aware-network scenario: the network is Q4S aware,
and reacts by itself to these alerts. In this scenario
Q4S ALERT messages are sent by the server to the client,
and network elements inspect and process these alert
messages. The alerting mode in this scenario is called
Q4S-aware-network alerting mode.
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o 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 taking decisions regarding what actions to
trigger: either to change application behavior to adapt
it to network conditions and/or invoke a network policy
server in order to reconfigure the network and request
more quality for application flows.
+------+ +-----------+
| App |<----- app flows---------->|Application|
|Client| +-----------+
+------+ A
|
+------+ +------+ +--------+
| Q4S |<----Q4S---->| Q4S |<----->|Actuator|
|Client| |Server| +--------+
+------+ +------+ |
V
+-------------+
|policy server|
+-------------+
Figure 1 Reactive scenario
o The format of messages exchanged between the server stack and
the Actuator, doesn't follow Q4S codification rules, but
their format will be implementation dependent. In this way,
we will call 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).
o alert-pause: The amount of time between consecutive alerts.
In the Q4S-aware-network scenario, the server has to wait
this period of time between Q4S ALERT messages sent to the
client. In the Reactive scenario, the server stack has to
wait this period of time between alert notifications sent to
the Actuator. Measurements are not stopped in Negotiation or
Continuity Phases during this period of time, but no alerts
are sent even with violated network quality constraints in
order to leave time for network reconfiguration or for
application adjustments.
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o recovery-pause: The amount of time the Q4S server waits
before trying to recover the initial qos-level. After having
detected violation of quality constraints several times, the
qos-level will have been increased accordingly. If this
violation detection finally stops, the server waits for a
period of time (recovery time) and if the situation persists,
it tries to recover to previous qos-level values gradually by
sending Q4S RECOVERY messages to the client, in the Q4S-
aware-network scenario, or recovery notifications to the
Actuator, in the Reactive scenario.
It is important to highlight that any Q4S 200 OK response sent by
the server to the client at any time during the life of a quality
session may contain an SDP body with new values of quality
constraints required by the application. Depending on the phase
and the state of the measurement procedure within the specific
phase, the client will react accordingly so as to take into
account the new quality constraints in the measurement procedure.
Once the communication has been established (handshake phase is
finished), the protocol will verify that the communication path
between the client and the server meets the quality constraints on
both directions, from and to the server (negotiation phase). This
negotiation phase requires taking measurements of the quality
parameters: latencies, jitter, bandwidth and packet loss. This
phase is initiated with a client message containing a Q4S READY
method, which will be answered by the server with a Q4S 200 OK
response.
Negotiation measurements are achieved in two sequential stages:
o Stage 0: latency and jitter measurements
o Stage 1: bandwidth and packet loss measurements
Stage 0 measurements are being taken through Q4S PING messages
sent both from both the client and the server. All Q4S PING
requests will be answered by Q4S 200 OK messages to allow for
bidirectional measurements.
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, client can send a
"READY 1" message.
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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 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
and therefore quality thresholds used while monitoring quality
parameters have to be changed to the new constraints. In this
case the client moves to the beginning of the Stage 0 for
initiating the negotiation measurements again.
Stage 1 is optional. Its purpose is to measure the availability of
application needed bandwidth. This stage can be skipped by client
sending a "READY 2" message after completion of stage 0 when
bandwidth requirements is set to cero kbps in the SDP. Stage 1
measurements are achieved through Q4S BWIDTH messages sent both
from the client and the server. Unlike PING messages, Q4S BWIDTH
requests will not be answered.
If Stage 0 and 1 meet the application quality constraints, the
application may start. Q4S will enter the continuity phase
measuring the network quality parameters through the Q4S PING
message exchange on both connection paths, and raising alerts in
case of violation. .
Once the client wants to terminate the quality session it sends a
Q4S CANCEL message, which will be acknowledged by the server with
another Q4S CANCEL message. Termination of quality sessions are
always initiated by the client because Q4S TCP requests follow the
client server schema.
Figure 2 depicts the message exchange in a successful scenario.
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+-------------------------------------------+
| |
| Client Server |
| |
Handshake | --------- Q4S BEGIN -----------> |
| <-------- Q4S 200 OK ----------- |
| |
Negotiation | |
(Stage 0) | --------- Q4S READY 0----------> |
| <-------- Q4S 200 OK ----------- |
| |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| --------- Q4S PING ------------> |
| <-------- Q4S PING ------------- |
| --------- Q4S 200 OK ----------> |
| <-------- Q4S 200 OK ----------- |
| ... |
Negotiation | |
(Stage 1) | --------- Q4S READY 1----------> |
| <-------- Q4S 200 OK ----------- |
| |
| --------- Q4S BWITDH ----------> |
| <-------- Q4S BWIDTH------------ |
| --------- Q4S BWITDH ----------> |
| <-------- Q4S BWIDTH------------ |
| ... |
Continuity | --------- Q4S READY 2 ---------> |
| <-------- Q4S 200 OK ----------- | app
start
| |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| |
Termination | --------- Q4S CANCEL ----------> | app end
| <-------- Q4S CANCEL ----------- |
| |
+-------------------------------------------+
Figure 2 Successful Q4S message exchange.
Client and server measurements are included into PING and BWIDTH
messages, allowing both sides of the communication to be are aware
of all measurements in both directions.
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The following two examples show the behavior of the Q4S protocol
when: quality constraints are violated, alerts are generated; and,
later on, violation of quality constraints stops leading to the
execution of the recovery process. The first example (Figure 3)
shows the Q4S-aware-network alerting mode scenario:
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+-------------------------------------------+
| |
| Client Server |
| |
Handshake | --------- Q4S BEGIN -----------> |
| <-------- Q4S 200 OK ----------- |
| |
Negotiation | |
(Stage 0) | --------- Q4S READY 0----------> |
| <-------- Q4S 200 OK ----------- |
| |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| ... |
| |
| <-------- Q4S ALERT ------------ |
| -------- Q4S ALERT ------------> |
| (alert-pause start) |
Repetition | |
of Stage 0 | --------- Q4S READY 0----------> |
| <-------- Q4S 200 OK ----------- |
| |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| ... |
Negotiation | |
(Stage 1) | --------- Q4S READY 1----------> |
| <-------- Q4S 200 OK ----------- |
| |
| --------- Q4S BWITDH ----------> |
| <-------- Q4S BWIDTH------------ |
| ... |
| |
Continuity | --------- Q4S READY 2 ---------> |
| <-------- Q4S 200 OK ----------- | app
start
| |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| ... |
|(alert-pause expires & |
| violated constraints) |
| <-------- Q4S ALERT ------------ |
| --------- Q4S ALERT -----------> |
| |
| (alert-pause start) |
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| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| --------- Q4S 200 OK ----------> |
| ... |
|(alert-pause expires & |
| violated constraints) |
| <-------- Q4S ALERT ------------ |
| --------- Q4S ALERT -----------> |
| (alert-pause) |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| ... |
|(alert-pause expires & |
| Fullfilled constraints) |
| |
| (recovery-pause start) |
| |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| ... |
|(recovery-pause expires & |
| Fullfilled constraints) |
| <--------- Q4S RECOVERY --------- |
| -------- Q4S RECOVERY -----------> |
| |
| (recovery-pause start) |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| ... |
| |
Termination | --------- Q4S CANCEL ----------> | app end
| <-------- Q4S CANCEL ----------- |
| |
+-------------------------------------------+
Figure 3 Q4S-aware-network alerting mode.
In this Q4S-aware-network alerting mode scenario, the server may
send Q4S alerts to the client at any time on detection of violated
quality constraints. This alerting exchange must not interrupt the
continuity quality parameter measurements between client and
server.
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The second example depicted in the 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 either
to act over application behavior and/or invoke a network policy
server. The Actuator is an entity that has a pre-defined set of
different quality levels and decides how to act depending on the
actions stated for each of these levels; it can take actions for
making adjustments on the application or it can send a request to
the policy server for acting on the network. The policy server
also has a pre-defined set of different quality levels pre-agreed
upon between the Application Content Provider and the ISP. The
Reactive alerting mode is the default mode.
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+-------------------------------------------+
| |
| Client Server Actuator |
Handshake | ----- Q4S BEGIN -----> |
| <---- Q4S 200 OK ----- |
| |
Negotiation | |
(Stage 0) | ----- Q4S READY 0----> |
| <---- Q4S 200 OK ----- |
| |
| ----- Q4S PING ------> |
| <---- Q4S 200 OK ----- |
| <---- Q4S PING ------- |
| ---- Q4S 200 OK ----> |
| ... |
| (alert-pause start) |
| --alert |
| notification--> |
| |
| <--alert |
| acknowledge--- |
| |
Repetition | |
of Stage 0 | ----- Q4S READY 0----> |
| <---- Q4S 200 OK ----- |
| |
| ----- Q4S PING ------> |
| <---- Q4S 200 OK ----- |
| <---- Q4S PING ------- |
| ... |
|(alert-pause expires & |
| violated constraints) |
| |
| --alert |
| notification--> |
| |
| <--alert |
| acknowledge--- |
| |
| ----- Q4S PING ------> |
| <---- Q4S 200 OK ----- |
| <---- Q4S PING ------- |
| ... |
Negotiation | |
(Stage 1) | ----- Q4S READY 1----> |
| <---- Q4S 200 OK ----- |
| |
| ----- Q4S BWITDH ----> |
| <---- Q4S BWIDTH------ |
| ... |
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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 Negotiation phase stage, the server sends an
alert notification to the Actuator if quality constraints are
violated. During the period of time defined by the alert-pause
parameter, no further alert notifications are sent, but
measurements are not interrupted. This way, both the client and
the server will detect network improvements as soon as possible.
In a similar way, during the continuity phase, the server may send
alert notifications at any time to the Actuator on detection of
violated quality constraints. This alerting exchange must not
interrupt the continuity measurements between client and server.
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Finally, in the Termination phase, Q4S CANCEL messages sent from
the client to the server must be forwarded from the server to the
Actuator in order to release possible assigned resources for the
session.
4 Q4S Messages
Q4S is a text-based protocol and uses the UTF-8 charset (RFC 3629
[19]). A Q4S message is either a request or a response.
Both Request and Response messages use the basic format of
Internet Message Format (RFC 5322 [20]). 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.
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 (LWS) is
allowed
in any of the elements.
Request-Line = Method SP Request-URI SP Q4S-Version CRLF
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Method: This specification defines seven methods: BEGIN for
starting and negotiate quality sessions, READY for
synchronization of measurements, PING and BWIDTH for
quality measurements purpose, CANCEL for terminating
sessions, Q4S-ALERT for quality violations reporting, and
Q4S-RECOVERY for quality recovery reporting.
Request-URI: The Request-URI is a Q4S URI (RFC 2396) as described
in 7.4. The Request-URI MUST NOT contain unescaped spaces
or control characters and MUST NOT be enclosed in "<>".
Q4S-Version: Both request and response messages include the
version of Q4S in use. To be compliant with this
specification, applications sending Q4S messages MUST
include a Q4S-Version of "Q4S/1.0". The Q4S-Version string
is case-insensitive, but implementations MUST send upper-
case. Unlike HTTP/1.1, Q4S treats the version number as a
literal string. In practice, this should make no
difference.
4.2 Responses
Q4S responses are distinguished from requests by having a Status-
Line as their start-line. A Status-Line consists of the protocol
version followed by a numeric Status-Code and its associated
textual phrase, with each element separated by a single SP
character. No CR or LF is allowed except in the final CRLF
sequence.
Status-Line = Q4S-Version SP Status-Code SP Reason-Phrase CRLF
The Status-Code is a 3-digit integer result code that indicates
the outcome of an attempt to understand and satisfy a request. The
Reason-Phrase is intended to give a short textual description of
the Status-Code. The Status-Code is intended for use by automata,
whereas the Reason-Phrase is intended for the human user. A client
is not required to examine or display the Reason-Phrase.
While this specification suggests specific wording for the reason
phrase, implementations MAY choose other text, for example, in the
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:
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1xx: Provisional -- request received, continuing to process
the request;
2xx: Success -- the action was successfully received,
understood, and accepted;
3xx: Redirection -- further action needs to be taken in
order
to complete the request;
4xx: Request Failure -- the request contains bad syntax or
cannot be fulfilled at this server;
5xx: Server Error -- the server failed to fulfill an
apparently valid request;
6xx: Global Failure -- the request cannot be fulfilled at
any
server.
The status codes are the same described in HTTP (RFC 7231 [2]). In
the same way as HTTP, Q4S applications are not required to
understand the meaning of all registered status codes, though such
understanding is obviously desirable. However, applications MUST
understand the class of any status code, as indicated by the first
digit, and treat any unrecognized response as being equivalent to
the x00 status code of that class.
The Q4S-ALERT, Q4S-RECOVERY and CANCEL requests do not have to be
responded. However, after receiving a Q4S-ALERT, Q4S-RECOVERY or
CANCEL request, the server SHOULD 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 not matching
its category (such as a request header field in a response), it
MUST be ignored.
4.3.1 Common Q4S Header Fields
These fields may appear in Request and Response messages.
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o Session-Id: the value for this header is the same session id
used in SDP (embedded in "o" SDP parameter) and is assigned
by the server. The messages without SDP MUST include this
header. If a message has and SDP body, this header is
optional. The method of <session id> allocation is up to the
creating tool, but it is suggested that a UTC timestamp be
used to ensure uniqueness.
o Sequence-Number: sequential and cyclic positive integer
number assigned to PING and BWIDTH messages, and acknowledged
in 200 OK responses.
o Timestamp: this optional header contains the system time
(with the best possible accuracy). It indicates the time in
which the PING request was sent. If this header is present in
PING messages, then the 200 OK response messages MUST include
this value.
o Stage: this is used in client's READY requests and server's
200 OK responses during the Negotiation and Continuity phases
in order to synchronize the initiation of the measurements.
Example: Stage: 0
4.3.2 Specific Q4S Request Header Fields
In addition to HTTP header fields, these are the specific Q4S
request header fields
o User-Agent: this header contains information about the
implementation of the user agent. This is for statistical
purposes, the tracing of protocol violations, and the
automated recognition of user agents for the sake of
tailoring responses to avoid particular user agent
limitations. User agents SHOULD include this field with
requests. The field MAY contain multiple product tokens and
comments identifying the agent and any sub-products which
form a significant part of the user agent. By convention, the
product tokens are listed in order of their significance for
identifying the application.
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o Signature: this header 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 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 (RFC 4634 [14]) and RSA (RFC
8017 [15]) 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 is not present, other validation mechanism
MAY be implemented in order to provide assured quality with
security and control.
o Measurements: this header carries the measurements of the
quality parameters in PING and BWIDTH requests. The format
is:
Measurements: "l=" " "|[0..9999] ", j=" " "|[0..9999] ", pl="
" "|[0.00 .. 100.00] ", bw=" " "|[0..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 packetloss 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
o Expires: its purpose is to provide a sanity check and allow
the server to close inactive sessions. If the client does not
send a new request before the expiration time, the server MAY
close the session. The value MUST be an integer and the
measurement units are milliseconds.
In order to keep the session open the server MUST send a Q4S
alert before the session expiration (Expires header), with
the same quality levels and an alert cause of "keep-alive".
The purpose of this alert is to avoid TCP sockets (which were
opened with READY message) from being closed, specially in
NAT scenarios.
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4.4 Bodies
Requests, including new requests defined in extensions to this
specification, MAY contain message bodies unless otherwise noted.
The interpretation of the body depends on the request method.
For response messages, the request method and the response status
code determine the type and interpretation of any message body.
All responses MAY include a body.
The Internet media type of the message body MUST be given by the
Content-Type header field.
4.4.1 Encoding
The body MUST NOT be compressed. This mechanism is valid for
other protocols such as HTTP and SIP (RFC 3261 [22]), but
a compression/coding scheme will limit certain logical
implementations of the way the request is parsed, thus, making the
protocol concept more implementation dependent. In addition,
bandwidth calculation may not be valid if compression is used.
Therefore, the HTTP request header "Accept-Encoding" cannot be
used in Q4S with different values than "identity" and if it is
present in a request, the server MUST ignore it. In addition, the
response header "Content-Encoding" is optional, but if present,
the unique permitted value is "identity".
The body length in bytes 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 (RFC 7231 [2]). 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
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dynamically. Any server application SHOULD return the status code
405 (Method Not Allowed) if the method is known, but not allowed
for the requested resource, and 501 (Not Implemented) if the
method is unrecognized or not implemented by the server.
5.1 BEGIN
The BEGIN method requests information from a resource identified
by a Q4S URI. The semantics of this method is the starting of a
quality session.
This method is only used during the handshake phase to retrieve
the SDP containing session id and all quality and operation
parameters for the desired application to run.
When a BEGIN message is received by the server, any current
quality session MUST be cancelled, and a new session should be
created.
The response to a Q4S BEGIN request is not cacheable.
5.2 READY
The READY method is used to synchronize the starting time for
sending of PING and BWIDTH messages over UDP between clients and
servers. The stage header included in this method is mandatory.
This message is only used in negotiation and continuity phases,
and only just before making a measurement. Otherwise (out of this
context), the server MUST ignore this method.
5.3 PING
This message is used during the negotiation and continuity phases
to measure the RTT and jitter of a session. The message MUST be
sent only over UDP ports.
The fundamental difference between the PING and BWIDTH requests is
reflected in the different measurements achieved with them. PING
is a short message, and MUST be answered in order to measure RTT
and jitter, whereas BWIDTH is a long message and MUST NOT be
answered.
PING is a request method that can be originated by client but also
by server. Client MUST also answer the server PING messages,
assuming a "server role" for these messages during measurement
process.
The Measurements header included in this method is mandatory, and
provides updated measurements values for latency, jitter and
packet loss to the counterpart.
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5.4 BWIDTH
This message is used only during the Negotiation phase to measure
the bandwidth and packet loss of a session. The message MUST be
sent only over UDP ports.
BWIDTH is a request method that can be originated by the client
but also by server. Both (client and server) MUST NOT answer
BWIDTH messages.
The Measurements header included in this method is mandatory and
provides updated measurements values for bandwidth and packet loss
to the counterpart.
5.5 Q4S-ALERT
This is the request message that Q4S generates when the
measurements indicate that quality constraints are being violated.
It is used during the negotiation and continuity phases.
This informative message indicates that the user experience is
being degraded and includes the details of the problem (bandwidth,
jitter, packet loss measurements). The Q4S-ALERT message does not
contain any detail on the actions to be taken, which depends on
the agreements between all involved parties.
Q4S-ALERT request does not have to be answered with a response
message unless there is an error condition, but with an answer
formatted as a request Q4S-ALERT message. The response to a Q4S-
ALERT request is not cacheable.
This method MUST be initiated by the server in both alerting
modes. In Q4S-aware-network alerting mode, the Q4S-ALERT messages
are fired by the server and sent to the client, advising the
network to react by itself. In Reactive alerting mode, alert
notifications are triggered by the server stack and sent to the
Actuator(see Figure1 "Reactive Scenario").
Client----q4s----SERVER STACK--->ACTUATOR-->APP OR POLICY SERVER
The way in which the server stack notifies the Actuator is
implementation dependent, and the communication between the
Actuator and the network policy server is defined by the protocol
and API that the policy server implements.
5.6 Q4S-RECOVERY
This is the request message that Q4S generates when the
measurements indicate that quality constraints were being violated
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but they have been fulfilled during a period of time already
(recovery pause). It is used during the negotiation and continuity
phases.
This informative message indicates that the qos-level could be
increased gradually until the initial qos-level is recovered (the
qos-level established at the beginning os the session of that was
decreased during violation of constraints). The Q4S-RECOVERY
message does not contain any detail on the actions to be taken,
which depends on the agreements between all involved parties.
Q4S-RECOVERY request MUST NOT be answered with a response message
unless there is an error condition, but with an answer formatted
as a request Q4S-RECOVERY message. The response to a Q4S-RECOVERY
request is not cacheable.
As for the Q4S-ALERT message, the Q4S-RECOVERY method is always
initiated by the server in both alerting modes. In Q4S-aware-
network alerting mode, the Q4S-RECOVERY messages are fired by the
server and sent to the client, advising the network to react by
itself. In Reactive alerting mode, recovery notifications are
triggered by the server stack and sent to the Actuator(see Figure1
"Reactive Scenario").
5.7 CANCEL
The semantics of CANCEL message is the release of the Q4S session
id and the possible resources assigned to the session. This
message could be triggered by Q4S stack or by the application
using the stack (through an implementation dependent API).
In the same way as Q4S-ALERT, CANCEL must not be answered with a
response message, 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 application or at policy
server). The Actuator MUST answer the cancel notification with a
cancel acknowledge towards the server stack, acknowledging the
reception.
6 Response Codes
Q4S response codes are used for TCP and UDP. However, in UDP only
the response code 200 is used.
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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 unknown error group, the expected action should
be the same as with 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 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 headers or SDP
values). However, the same request to a different server might be
successful.
6.4.1 400 Bad Request
The request could not be understood due to malformed syntax. The
Reason-Phrase SHOULD identify the syntax problem in more detail,
for example, "Missing Sequence-Number header field".
6.4.2 404 Not Found
The server has definitive information that the user does not exist
at the domain specified in the Request-URI. This status is also
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returned if the domain in the Request-URI does not match any of
the domains handled by the recipient of the request.
6.4.3 405 Method Not Allowed
The method specified in the Request-Line is understood, but not
allowed for the address identified by the Request-URI.
The response MUST include an Allow header field containing a list
of valid methods for the indicated address.
6.4.4 406 Not Acceptable
The resource identified by the request is only able of generating
response entities that have content characteristics not acceptable
according to the Accept header field sent in the request.
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.
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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.
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In the case that Q4S version is not supported, this error may be
sent by the server in 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 since the message
length exceeded its capabilities.
6.6 Global Failures 6xx
6xx responses indicate that a server has definitive information
about a particular policy not satisfied for processing the
request.
6.6.1 600 session does not exist
The Session-Id is not valid
6.6.2 601 quality level not allowed
The QOS level requested is not allowed for the pair client/server
6.6.3 603 Session not allowed
The session is not allowed due to some policy (number of sessions
allowed for the server is exceeded, or the time band of the Q4S-
ALERT is not allowed for the pair client/server, etc.).
6.6.4 604 authorization not allowed
The policy server does not authorize the Q4S-ALERT quality session
improvement operation due to an internal or external reason.
7 Protocol
This section describes the measurement procedures, the SDP
structure of the Q4S messages, the different Q4S protocol phases
and the messages exchanged in them.
7.1 Protocol Phases
All elements of the IP network contribute to the quality in
terms of latency, jitter, bandwidth and packet loss. All these
elements have their own quality policies in terms of priorities,
traffic mode, etc. and each element has its own way to manage the
quality. The purpose of a quality connection is to establish an
end-to-end communication with enough quality for the application
to function flawlessly.
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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.
o Handshake phase: in which the server is contacted by the
client and in the answer message the quality constraints for
the application is communicated embedded in an SDP.
o Negotiation phase: in which the quality of the connection is
measured in both directions (latency, jitter, bandwidth and
packet loss), and Q4S messages may be sent in order to alert
if the measured quality does not meet the constraints. This
phase is iterative until quality constraints are reached, or
the session is cancelled after a number of measurement cycles
with consistent violation of the quality constraints. The
number of measurement cycles executed depends on the qos-
level which is incremented in each cycle until a maximum qos-
level value is reached. Just after reaching the quality
requirements, Q4S provides a simple optional mechanism using
HTTP to start the application.
o Continuity phase: in which quality is continuously measured.
In this phase the measurements MUST avoid disturbing the
application by consuming network resources. If quality
constraints are not met, the server stack will notify the
Actuator with an alert notification. If later the quality
improves, the server stack will notify the Actuator, in this
case with a recovery notification. After several alert
notifications with no quality improvements, the Q4S stack
SHOULD move to Termination phase.
o Termination phase: in which the Q4S session is terminated.
The application may be closed too or may not start.
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7.2 SDP Structure
The original goal of SDP was to announce necessary information for
the participants and multicast MBONE (Multicast Backbone)
applications. Right now, its use has been extended to the
announcement and the negotiation of multimedia sessions. The
purpose of Q4S is not to establish media stream sessions, but to
monitor a quality connection. This connection may be later used to
establish any type of session including media sessions; Q4S does
not impose any conditions on the type of communication requiring
quality parameters.
SDP will be used by Q4S to exchange quality constraints and will
therefore always have all the media attributes ("m") set to zero.
The SDP embedded in the messages is the container of the quality
parameters. As these may vary depending on the direction of the
communication (to and from the client) all quality parameters need
to specify the uplink and downlink values: <uplink> / <downlink>.
When one or both of these values are empty, it MUST be understood
as needing no constraint on that parameter and/or that direction.
The uplink direction MUST be considered as being the communication
from the client to the server. The downlink direction MUST be
considered as being the communication from the server to the
client.
The SDP information can comprise all or some of the following
parameters shown in the example below. This is an example of an
SDP message used by Q4S included in the 200 OK response to a Q4S
BEGIN request.
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v=0
o=q4s-UA 53655765 2353687637 IN IP4 192.0.2.33
s=Q4S
i=Q4S parameters
t=0 0
a=qos-level:0/0
a=alerting-mode:Reactive
a=alert-pause:5000
a=public-address:client IP4 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 SDP message embedded that
specifies new quality constraints requested by the application.
7.2.1 "qos-level" attribute
The "qos-level" attribute contains the QoS level for uplink and
downlink. Default values are 0 for both directions. The meaning of
each level is out of scope of Q4S, but a higher level SHOULD
correspond to a better service quality.
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.
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7.2.2 "alerting-mode" attribute
The "alerting-mode" attribute specifies the player in charge of
triggering Q4S alerts in case of constraint violation. There are
two possible values:
Appropriate attribute values: <"Q4S-aware-network"|"Reactive">
a) Q4S-aware-network: Q4S ALERT messages are triggered by the
server to the client. In this case the network is supposed to
be Q4S aware, and reacts by itself to these alerts.
b) Reactive: alert notifications are sent by the server stack to
the Actuator. In this case the network is not Q4S aware and a
specific node (Actuator) is in charge of triggering tuning
mechanisms., either on the network or in the application.
The "alerting-mode" attribute is optional and if not present
Reactive alerting mode is assumed.
7.2.3 "alert-pause" attribute
In the Q4S-aware-network scenario, the "alert-pause" attribute
specifies the amount of time (in milliseconds) the server waits
between consecutive Q4S ALERT messages sent to the client. In the
Reactive scenario, the "alert-pause" attribute specifies the
amount of time (in milliseconds) the server stack waits between
consecutive alert notifications sent to the Actuator. Measurements
are not stopped in Negotiation or Continuity Phases during this
period of time, but no Q4S ALERT messages or alert notifications
are fired, even with violated quality constraints, allowing either
network reconfigurations or application adjustments.
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).
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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 his own public
IP address and the public IP address of the first message received
from the client in the handshake phase.
The purpose of these attributes is to make available the
addressing information to network policy server or other external
entities in charge of processing Q4S-ALERT messages.
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 are specified in the "latency" attribute, expressed in
milliseconds. In the Q4S-aware-network scenario, if the latency
constraints are not met, a Q4S-ALERT method will be sent to the
client. In the Reactive scenario, if the latency constraints are
not met, an alert notification will be sent to the Actuator. If
the "latency" attribute is not present or has a 0 value, no
latency constraints need to be met and no measurements MAY be
taken.
Appropriate attribute values: [0..9999]
7.2.7 "jitter" attribute
The maximum uplink and downlink jitter tolerance are specified in
the "jitter" attribute, expressed in milliseconds. In the Q4S-
aware-network scenario, if the jitter constraints are not met, a
Q4S-ALERT method will be sent to the client. In the Reactive
scenario, if the latency constraints are not met, an alert
notification will be sent to the Actuator. If "jitter" attribute
is not present or has a 0 value, no jitter constraints need to be
met and no measurements MAY be taken.
Appropriate attribute values: [0..9999] "/" [0..9999]
7.2.8 "bandwidth" attribute
The minimum uplink and downlink bandwidth are specified in the
"bandwidth" attribute, expressed in kbps. In the Q4S-aware-network
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scenario, if the bandwidth constraints are not met, a Q4S-ALERT
method will be sent to the client. In the Reactive scenario, an
alert notification will be sent to the Actuator. If "bandwidth"
attribute is not present or has a 0 value, no bandwidth
constraints need to be met and no measurements MAY be taken.
Appropriate attribute values: [0..99999] "/" [0..99999]
7.2.9 "packetloss" attribute
The maximum uplink and downlink packet loss tolerance are
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 "packetloss"
attribute is not present or has a 0 value, no packetloss
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 or UDP) (RFC
793 [16] and RFC 768 [17]) with the range of ports that are going
to be used by the application and, of course, by the Q4S protocol
(for quality measurements). All defined flows (app and q4s) will
be considered within the same quality profile, which is determined
by the qos-level attribute in each direction. This allows to
assume that measurements on q4s flows are the same experimented by
the application which is using app flows.
During negotiation and continuity phases the specified Q4S ports
in the "flow:q4s" attributes of SDP will be used for Q4S messages.
The Q4S flows comprise two UDP flows and two TCP flows (one uplink
and one downlink for each one) whereas application traffic MAY
consist of many flows, depending on its nature. The handshake
phase takes place through the Q4S Contact URI, using the standard
Q4S TCP port. However, the negotiation and continuity phases will
take place on the specified Q4S ports (UDP and TCP) specified in
the SDP.
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The "clientListeningPort" is a port in which the client listens
for server requests and MUST be used as origin port of client
responses. The "serverListeningPort" is a port in which server is
listening for incoming messages from the client. The origin port
of server responses may be different than "serverListeningPort"
value.
If "clientListeningPort" is zero (a=flow:q4s clientListeningPort
TCP/0), the client MAY choose one randomly as per OS standard
rules. Client ports inside the SDP must always be matched against
actual received port values on the server side in order to deal
with NAT/NATP devices. If zero value or incorrect value is
present, server must set the value to the received origin port in
the next message with SDP (200 OK, ALERT and CANCEL messages).
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:
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o The first parameter is the interval of time (in milliseconds)
between PING requests during the negotiation phase. Uplink
and downlink values from the client's point of view are
separated by "/". This allows having different responsiveness
values depending on the control resources used in each
direction.
o The second parameter is the time interval (in milliseconds)
between PING requests during the continuity phase. Uplink and
downlink values are separated by "/". This allows having two
different responsiveness values depending on the control
resources used in each direction.
o The third parameter is the time interval to be used to
measure bandwidth during the negotiation phase.
o The fourth parameter indicates the window size for jitter and
latency calculations. Uplink and downlink values are
separated by "/".
o The fifth parameter indicates the window size for packet loss
calculations. Uplink and downlink values are separated by
"/".
There are four more measurement attributes:
a=measurement:latency 45
a=measurement:jitter 3/12
a=measurement:bandwidth 200/9800
a=measurement:packetloss 0.00/1.00
The latency, jitter, bandwidth and packet-loss measurement
attributes contain the values measured for each of these quality
parameters in uplink and downlink directions. Notice that latency
is considered equal for uplink and downlink directions. Quality
parameter values in these measurement attributes provide a
snapshot of the quality reached and MUST only be included in Q4S-
ALERT messages in the SDP body such that they can be protected
from malicious attacks as these alerts include a signature of the
SDP body in the header. The rest of messages will include the
measured values in the Measurements header.
In the case of procedure "default", the valid values are:
a=measurement:procedure default,[0..999]"/" [0..999] "," [0..999]
"/" [0..999] "," [0..9999] "," [0..999]/[0..999] ","
[0..999]/[0..999]
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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 inform about MTU limits
in end points, 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 non-present attributes
measurements MAY be omitted.
7.3.1 Latency
Latency measurements will be performed if the latency attribute
and/or the application latency attribute are present and with non-
zero values.
Q4S defines a PING method in order to exchange packets between the
client and the server. Based on this PING exchange the client and
the server are able to calculate the round-trip time (RTT). The
RTT is the sum of downlink latency (normally named "reverse
latency") and uplink latency (normally named "forward latency").
At least 255 samples of RTT MUST be taken by the client and
server. As the forward and reverse latencies are impossible to
measure, client and server will assume that both latencies are
identical (symmetric network assumption). The latency will
therefore be calculated as the statistical median value of all the
RTT samples divided by 2.
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7.3.2 Jitter
Jitter measurements will be performed if the jitter attribute
and/or the application jitter attribute are present and with non-
zero values.
The jitter can be calculated independently by the client and by
the server. The downlink jitter is calculated by the client taking
into account the time interval between PING requests as defined by
the measurement procedure attribute in the first or second
parameter depending on the Q4S protocol phase. The client and the
server MUST send these PING requests at the specified intervals.
The client measures the downlink jitter whereas the server
measures the uplink jitter. Note that PING responses are not taken
into account when calculating jitter values.
Every time a PING request message is received by an endpoint
(either server or client), the corresponding jitter value is
updated using the Statistical Jitter value calculated on the first
255 packets received using the arithmetic mean of the absolute
values of elapsed times.
Each endpoint sends a PING periodically with a fixed interval,
each value of "elapsed time" (ET) should be very close to this
interval. If a PING message is lost, the elapsed time value is
doubled. Identifying lost PING messages, however, is not an issue
because all PING messages are labeled with a Sequence-Number
header. Therefore the receiver can discard this elapsed time
value.
In order to have the first jitter sample, the receiver MUST wait
until it receives 3 PING requests, because each ET is the time
between two PINGs and a Jitter needs at least two ET.
The client measures the values of RTT and downlink jitter and the
server measures RTT and uplink jitter, but all measurements are
shared with the counterpart by means of "Measurements" header of
PING message.
7.3.3 Bandwidth
Bandwidth measurements will be performed if the bandwidth
attribute and/or the application bandwidth attribute is present
and with non-zero values.
In order to measure the available bandwidth, both the client and
the server MUST start sending BWIDTH messages simultaneously using
the UDP control ports exchanged during the handshake phase in the
SDP message, at the needed rate to verify the availability of the
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bandwidth constraint in each direction. The messages are sent
during the period of time defined in the third parameter of the
SDP measurement default procedure attribute in millisecond units.
a=measurement:procedure default(50/50,75/75,5000,256/256,256/256)
+------------------------------------------------+
| Rate |
| A |
| | |
|downlink rate-|-------------------+ <-- traffic |
| | | sent by |
| | | server |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| uplink rate-|-------------------+ <-- traffic |
| | | sent by |
| | | client |
| | | |
| | | |
| |---|---|---|---|---|----> time |
| 0 1 2 3 4 5 (sec.) |
| |
+------------------------------------------------+
Figure 6 Bandwidth and packet loss measurements.
The goal of these measurements is not to identify the available
bandwidth of the communication path but to determine if the
required bandwidth is available, meeting the application's
constraints. Therefore, the requested bandwidth MUST be measured
sending only the highest bit rate required by the bandwidth
attribute. This is illustrated in Figure 6.
During bandwidth measurement time, ALERTS are not expected, but
only at the end of the measurement time.
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When measuring bandwidth, all BWIDTH requests sent MUST be 1
kilobyte in length (UDP payload length by default), and MUST
include a Sequence-Number header 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)
=========================
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7.3.4 Packet loss
Packet loss and bandwidth are measured simultaneously using the
BWIDTH packets sent by both the client and the server. Because the
BWIDTH packets contain a Sequence-Number header incremented
sequentially with each sent packet, lost packets can be easily
identified. The lost packets 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 (RFC 3986 [12]),
which forces the use of the Q4S protocol if it is implemented in a
standard web browser.
This URI, named "Contact URI", is used to request the start of a
session. Its scheme MUST be:
"q4s:" "//" host [":" port] [path["?" query]
Optionally, the client can send the desired quality parameters
enclosed in the body of the message as an SDP document. The server
MAY take them into account when building the answer message with
the final values in the SDP body, following a request / response
schema (RFC 3264 [13]).
If the request is accepted, the server MUST answer it with a Q4S
200 OK message, which MUST contain an SDP body (RFC 4566 [10])
with the assigned session id (embedded in the "o" SDP parameter),
the IP addresses to be used, the flow ports to be used, the
measurement procedure to be followed and information about the
required quality constraints. Additionally, the alerting-mode and
alert-pause time parameters may be included. Q4S responses should
use the protocol designator "Q4S/1.0".
After these two messages are exchanged, the first phase is
completed. The quality parameter thresholds have been sent to the
client. The next step is to measure the actual quality of the
communication path between the client and the server and alert if
the Service Level Agreement (SLA) is being violated.
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+------------------------------------------------+
| |
| Client Server |
| |
| ------- Q4S BEGIN ------------> |
| |
| <------ Q4S 200 OK ------------ |
| |
| |
+------------------------------------------------+
Figure 7 Handshake phase.
Example of Client Request and Server Answer:
Client Request:
=========================
BEGIN q4s://www.example.com Q4S/1.0
Content-Type: application/sdp
User-Agent: q4s-ua-experimental-1.0
Content-Length: 142
(SDP not shown)
=========================
Server Answer:
=========================
Q4S/1.0 200 OK
Date: Mon, 10 Jun 2010 10:00:01 GMT
Content-Type: application/sdp
Expires: 3000
Signature: 6ec1ba40e2adf2d783de530ae254acd4f3477ac4
Content-Length: 131
(SDP not shown)
=========================
The headers used are explained in section 4.3.
7.5 Negotiation Phase
The negotiation phase is in charge of measuring the quality
parameters and verifying that the communication paths meet the
required quality constraints on both directions as specified in
the SDP body.
The measured parameters will be compared with the quality
constraints specified in the SDP body. If the quality session is
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compliant with all the quality constraints the application can
start.
o If the quality constraints are not met, a higher quality
service level will be demanded. Depending on the scenario,
this quality upgrade will be managed as follows: In the Q4S-
aware-network scenario: a Q4S-ALERT method will be triggered
by the server to the client and the client will answer with
the same Q4S-ALERT method. After receiving the same Q4S-ALERT
from the counterpart, no other alerts will be triggered by
the server during the "alert-pause" period of time, in order
to allow the network to react, but measurements will continue
to be taken to achieve early detection of improved network
quality conditions and a fast application start.
o In the Reactive scenario: an alert notification will be sent
by the server stack to the Actuator, and the Actuator will
answer with an alert acknowledgement. After receiving the
alert acknowledgement from the Actuator, the server stack
will not send other alert notifications during the "alert-
pause" period of time, in order to allow the Actuator to
react and trigger actions on the application or on the policy
server, but measurements will continue to be taken to achieve
early detection of improved network quality conditions and a
fast application start.
In both scenarios stated above, if after several measurement
cycles, the network constraints cannot be met the quality session
is terminated. Concretely when under all possible actions taken by
Actuator the quality remains below requirements, the session must
be terminated.
The steps to be taken in this phase depend on the measurement
procedure exchanged during the handshake phase. This document only
describes the "default" procedure, but others can be used, like
RTP/RTCP (RFC 3550 [18]).
Measurements of latency and jitter are done calculating the
differences in arrival times of packets and can be achieved with
little bandwidth consumption. The bandwidth measurement, on the
other hand, involves higher bandwidth consumption in both
directions (uplink and downlink).
To avoid wasting unnecessary network resources these two types of
measurements will be performed in two separate stages. If the
required latencies and jitters cannot be reached, it makes no
sense to waste network resources measuring bandwidth. In addition,
if achieving the required latency and jitter thresholds implies
upgrading the quality session level, the chance of obtaining
compliant bandwidth measurements without retries is higher, saving
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network traffic again. Therefore, the default procedure,
determines that the measurements are taken in two stages:
o Stage 0: Measurement of latencies, jitters and packet loss
o Stage 1: Measurement of bandwidths and packet loss
Notice that packet loss can be measured in both stages, as all
messages exchanged include a sequence-number header that allows
for easy packet loss detection.
The client starts the negotiation phase sending a READY request
using the TCP Q4S ports defined in the SDP. This READY request
includes a "Stage" header that indicates the measurement stage.
If either jitter, latency or both are specified, the negotiation
phase begins with the measurement of latencies and jitters (stage
0). If none of those attributes are specified, stage 0 is skipped.
7.5.1 Stage 0: Measurement of Latencies and 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.
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After receiving 200 OK, the client must send the first PING
message and the server will wait to send PINGs until the reception
of this first client PING.
Client and server MUST send PING requests to each other. The
Sequence-Number header of the first PING MUST be set to 0. Client
and server will manage their own sequence numbers.
+------------------------------------------------+
| |
| Client Server |
| |
| --------- Q4S READY 0 ---------> |
| <-------- Q4S 200 OK ----------- |
| |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| --------- Q4S PING ------------> |
| <-------- Q4S PING ------------- |
| --------- Q4S 200 OK ----------> |
| <-------- Q4S 200 OK ----------- |
| ... |
| |
+------------------------------------------------+
Figure 8 Simultaneous exchange of PING request and responses.
Figure 8 shows an example of the PING request sent from the client
and the server's response:
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Client Request:
=========================
PING q4s://www.example.com Q4S/1.0
Session-Id: 53655765
Sequence-Number: 0
User-Agent: q4s-ua-experimental-1.0
Measurements: l=22, j=12, pl=0.20, bw=
Content-Length: 0
=========================
Server Response:
=========================
Q4S/1.0 200 OK
Session-Id: 53655765
Sequence-Number: 0
Content-Length: 0
=========================
The function of the PING method is similar to the ICMP echo
request message. The server MUST answer as soon as it receives the
message.
Both endpoints MUST send Q4S PING messages with the periodicity
specified in the first parameter of SDP measurement procedure
attribute, using always the same UDP ports and incrementing the
Sequence-Number with each message.
In the following example, the SDP measurement procedure attribute,
this value is 50 milliseconds (from the client to the server) and
60ms (from the server to the client).
a=measurement:procedure default(50/60,50/50,5000,256/256,256/256)
They MUST NOT wait for a response to send the next PING request.
The "Sequence-Number" header value is incremented sequentially and
MUST start at zero. If this stage is repeated, the initial
Sequence-Number MUST start at zero again.
All PING requests MUST contain a "Measurements" header, with the
values of the latency, jitter and packet loss measured by each
entity up to that moment. The client will send its measurements to
the server and the server his measurements to the client. Example:
Measurements: l=22, j=13, pl=0.10, bw=
Where l stands for latency, j for jitter, pl for packetloss and bw
for bandwidth. The bandwidth value is omitted, as it is not
measured at this stage.
Optionally the PING request can include a "Timestamp" header, with
the time in which the message has been sent. In case the header is
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present, the server MUST include the header in the response
without changing the value.
A minimum number of PING messages MUST be exchanged in order to be
able to measure latency, jitter and packet-loss with certain
accuracy (at least 256 samples are RECOMMENDED to get a accurate
packet loss measurement). Both the client and the server calculate
the respective measured parameter values. The mechanisms to
calculate the different parameters are described in section 7.3.
At the end of this stage 0, there are three possibilities:
o The latency, jitter and packet loss constraints are reached
in both directions
o The latency, jitter and packet loss constraints are not
reached in one or both directions
In the first case, Stage 0 is finished. Client and server are
ready for Stage 1: bandwidth and packet loss measurement. The
client moves to stage 1 by sending a READY message including the
header "Stage: 1".
If the bandwidth constraints are empty or with value zero, the
negotiation phase MUST terminate and both client and server may
initiate the Continuity Phase. In this case client moves to
Continuity phase by sending a READY message including the header
"Stage: 2".
The second case, in which one or more quality constraints have not
been met, is detailed in section 7.5.4.
7.5.2 Stage 1: Measurement of Bandwidth and Packet Loss
This stage begins in a similar way to stage 0, sending a READY
request over TCP. This READY message "Stage" header value is 1.
The server answers with a Q4S 200 OK message to synchronize the
initiation of the measurements as shown in Figure 9.
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+------------------------------------------------+
| |
| Client Server |
| |
| --------- Q4S READY 1 -----------> |
| <-------- Q4S 200 OK ------------- |
| |
| --------- Q4S BWIDTH -----------> |
| <-------- Q4S BWIDTH ------------ |
| --------- Q4S BWIDTH -----------> |
| <-------- Q4S BWIDTH ------------ |
| ... |
| |
+------------------------------------------------+
Figure 9 Starting bandwidth and packet loss measurement
Client Request:
=========================
READY q4s://www.example.com Q4S/1.0
User-Agent: q4s-ua-experimental-1.0
Stage: 1
Session-Id: 53655765
Content-Length: 0
=========================
Server Response:
=========================
Q4S/1.0 200 OK
Session-Id: 53655765
Stage: 1
Content-Length: 0
=========================
Just after receiving the 200 OK, both the client and the server
MUST start sending BWIDTH messages simultaneously using the UDP
q4s ports. Section 7.3.3 describes the bandwidth measurement in
detail.
At the end of this stage 1, there are three possibilities:
o The bandwidth and packet loss constraints are reached in both
directions
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o The bandwidth and packet loss constraints are not reached in
one both directions.
In the first case, Stage 1 is finished. Client and server are
ready for Continuity phase. The client moves to this phase by
sending a READY message including the header "Stage: 2". The
server answer MUST be 200 OK 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
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Client Request:
=========================
READY q4s://www.example.com Q4S/1.0
User-Agent: q4s-ua-experimental-1.0
Stage: 2
Session-Id: 53655765
Content-Length: 0
=========================
Server Answer:
=========================
Q4S/1.0 200 OK
Date: Mon, 10 Jun 2010 10:00:01 GMT
Session-Id: 53655765
Trigger-URI: http://www.example.com/app_start
Expires: 3000
Content-Type: application/sdp
Signature: 6ec1ba40e2adf2d783de530ae254acd4f3477ac4
Content-Length: 131
(SDP not shown)
=========================
If the "Trigger-URI" header is present, the client SHOULD send an
HTTP request to this URI.
The second case, with violated network constraints is explained in
7.5.4.
7.5.3 Quality Constraints Not Reached
After finishing Stage 1 of the Negotiation phase, the client and
the server have each other measured parameter values as these have
been exchanged in the "Measurements" headers of the PING and
BWIDTH messages. If there is one or more parameters that do not
comply with the uplink or downlink application constraints
required both the server and the client are aware of it.
If there is any quality parameter that does not meet the uplink or
downlink quality constraints specified in the SDP message, two
scenarios are possible depending on the specified alerting-mode
(if not present, default value is "Reactive" alerting mode):
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a) Q4S-aware-network alerting mode: the server MUST send a Q4S-
ALERT message to the client including the digital signature
header, and the client MUST answer with the same Q4S-ALERT
message. The Signature header contains the signed hash value of
the SDP body in order to protect all the SDP the data and
therefore it MUST contain the measurement parameters in the
body.
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 client and server keep on measuring but
without sending new Q4S ALERT messages during the "alert-pause"
milliseconds.
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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 client and server
keep on measuring without sending new alert notifications to the
Actuator during the "alert-pause" milliseconds specified in the
SDP. This way, both client and server will detect any improvement
in network conditions as soon as the network reacts. The
application can start as soon as the number of measurements
indicated in the measurement procedure attribute indicates that
the quality parameters are met.
Same applies to Continuity phase: the measurement dialog between
client and server must not be interrupted by any possible ALERT
message.
7.5.3.1 Actuator Role
Actuator receives notifications of unmet requirements from the Q4S
server stack, and act upon the application or the network policy
server, according to logic out of scope of this protocol.
The Actuator logic activates mechanisms at application level
or/and network level based on a quality level dictionary, in which
each level meaning is implementation dependent and each level
involve different actions based on rules to keep certain user
experience quality.
The type of actions that an Actuator can take at application level
are application dependent and MAY involve:
o Reduction of application functionalities, such as limitation
of application speed or application options.
o Reduction of application resources usage, such as reduction
of frames per second in a video app or any other parameter
modification in order to adapt to network conditions.
Apart from actions at application level, the Actuator MAY act at
network level if a network policy server is available.
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7.5.3.2 Policy Server Role
A network policy server may be part of the reactive scenario and
it is in charge of managing network quality provision. Network
policy server may implement all or some of these features (but not
exclusive to):
o Server validation in terms of quality constraints.
o Authentication (Signature validation) and security (block
malicious clients)
o Policy rules (following rules are only examples):
- Maximum quality level allowed for the ACP
- Time bands allowed for providing quality sessions
- Number of simultaneous quality sessions allowed
- Maximum time used by allowed quality sessions
- Etc.
If any of the policy rules fail, a Q4S-ALERT message MUST be
answered by a 6XX error, indicating the cause.
7.5.4 QoS Level Changes
If any constraint was violated, server MAY trigger a Q4S-ALERT
asking for higher qos-level attribute. The maximum qos-level
allowed is 9, both uplink and downlink.
If the qos-level has reached the maximum value for downlink or
uplink without matching the constraints, then a CANCEL request
MUST be sent by the client using the TCP port determined in the
handshake phase in order to release the session. In reaction to
the reception of the CANCEL request, the server MUST send a CANCEL
request too. If no CANCEL request is received, the expiration time
cancels the session at server side.
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Client Request:
=========================
CANCEL q4s://www.example.com Q4S/1.0
User-Agent: q4s-ua-experimental-1.0
Session-Id: 53655765
Content-Type: application/sdp
Content-Length: 142
(SDP not shown)
=========================
Server Request in reaction to Client Request:
=========================
CANCEL q4s://www.example.com Q4S/1.0
Session-Id: 53655765
Expires: 0
Content-Type: application/sdp
Signature: 6ec1ba40e2adf2d783de530ae254acd4f3477ac4
Content-Length: 131
(SDP not shown)
=========================
7.6 Continuity Phase
During the negotiation phase, latency, jitter, bandwidth and
packet loss have been measured. During the continuity phase
bandwidth will not be measured again because bandwidth
measurements may disturb application performance.
This phase is supposed to be executed at the same time as the
real-time application is being used.
This document only covers the default procedure. The continuity
operation with default procedure is based on a sliding window of
samples. The number of samples involved in the sliding window may
be different for jitter and latency than for packet-loss
calculations according to the fifth and sixth parameters of the
measurement procedure attribute. In 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 reaches the maximum value, the client
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continues sending PING messages with the Sequence-Number header
starting again at zero. When the server PING Sequence-Number
header reaches the maximum value, it does the same, starting again
from zero.
On the client side, the measured values of downlink jitter,
downlink packet loss and latency are calculated using the last
samples, discarding older ones, in a sliding window schema.
+--------------------------------------------------+
| |
| 55 56 57 . . . 253 254 255 0 1 2 . . . 55 56 |
| A A |
| | | |
| +-----------------------------------+ |
| |
+--------------------------------------------------+
Figure 11 Sliding samples window
Only if the server detects that the measured values (downlink or
uplink jitter, packet loss or latency) are not reaching the
quality constraints, a Q4S ALERT is triggered and sent either to
the client or to the Actuator, depending on the alerting mode, and
the alert-pause timer is started.
In Q4S-aware-network alerting mode shown in Figure 12, if the
client receives a Q4S ALERT message, it MUST answer sending the
Q4S ALERT request message back to the server including the SDP
(with its corresponding digital signature).
Both client and server will keep performing measurements but no
other Q4S ALERT message MUST be sent during "alert-pause"
milliseconds. The operations needed to act on the network and the
agents in charge of them are out of scope of this draft.
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+------------------------------------------------+
| |
| Client Server |
| |
| ... |
| ----------- PING ----------> |
| <--------- 200 OK ---------- |
| <------- Q4S-ALERT --------- |
| -------- Q4S-ALERT --------> |
| <---------- PING ----------- |
| ---------- 200 OK ---------> |
| ----------- PING ----------> |
| <--------- 200 OK ---------- |
| <---------- PING ----------- |
| ---------- 200 OK ---------> |
| ... |
| |
+------------------------------------------------+
Figure 12 Continuity in Q4S-aware-network alerting mode
In the Reactive scenario 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.
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+------------------------------------------------+
| |
| Client Server Actuator |
| ... |
| --- PING ----------> |
| <-- 200 OK---------- |
| <----- PING -------- |
| <--- 200 OK -------- ---- alert |
| notification --> |
| |
| --- PING ----------> <--- alert |
| acknowledge --- |
| <-- 200 OK---------- |
| <----- PING -------- |
| --- 200 OK --------> |
| ... |
| |
+------------------------------------------------+
Figure 13 Continuity in Reactive alerting mode
7.7 Termination Phase
The Termination phase is the end point for the established Q4S
session that is reached in the following cases:
. A CANCEL message has been received. The client sends a
CANCEL message due to the impossibility of the network to
meet the required quality constraints. The client and server
application will be notified by the respective Q4S stack.
. Session expires: if after the Expires time no client or
server activity is detected, that end cancels the session.
. A BEGIN message has been received by the server. The pre-
existing Q4S quality session is cancelled and a new session
will be initiated.
The meaning of Termination phase in terms of release of resources
or accounting is application dependent and out of scope of the Q4S
protocol.
In Reactive alerting mode, Q4S CANCEL messages received by the Q4S
server must cause the sending of cancel notifications sent from
the server stack to the Actuator in order to release possible
assigned resources for the session.
7.7.1. Sanity Check of Quality Sessions
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A session may finish due to several reasons (client shutdown,
client CANCEL request, constraints not reached, etc), and any
session finished MUST release the assigned resources.
In order to release the assigned server resources for the session,
the "Expires" header indicates the maximum interval of time
without exchanging any Q4S message.
7.8 Dynamic Constraints And Flows
Depending on the nature of the application, the quality
constraints to be reached may evolve, changing some or all quality
constraint values in any direction.
The client MUST be able to deal with this possibility. When the
server sends an SDP document attached to a response (200 OK, or
Q4S-ALERT, etc), the client MUST take all the new received values,
overriding any previous value in use.
The dynamic changes on the quality constraints can be as a result
of two possibilities:
o The application communicates to the Q4S server a change in
the constraints. In this case the application requirements
can evolve and the Q4S server will be aware of them.
o The application uses TCP flows. In that case, in order to
guarantee a constant throughput, the nature of TCP behavior
forces the use of a composite constraint function, which
depends on RTT, packet loss and window control mechanism
implemented in each TCP stack.
TCP throughput can be less than actual bandwidth if the
Bandwidth-Delay Product (BDP) is large or if the network suffers
from a high packet loss rate. In both cases, TCP congestion
control algorithms may result in a suboptimal performance.
Different TCP congestion control implementations like Reno [23],
High Speed TCP (RFC 3649 [24]), CUBIC [25], Compound TCP (CTCP
[26]), 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 change dynamically the
packetloss constraints (defined in SDP) in order to make possible
to reach a required throughput or vice versa (use packetloss
measurement to change dynamically latency constraints).
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A general guideline to calculate the packetloss constraint and RTT
constraint consists in approximating the throughput using a
simplified formula, which should take into account the TCP stack
implementation of the receiver, in addition to RTT and packet
loss:
Th= Function( RTT, packet loss, ...)
Then, depending on RTT measured values, set dynamically the
packetloss constraint.
It is possible to easily calculate a worst-case boundary for the
Reno algorithm, which should ensure for all algorithms that the
target throughput is actually achieved. Except that, high-speed
algorithms will then have even a larger throughput, if more
bandwidth is available.
For the Reno algorithm, the Mathis' formula may be used [23] for
the upper bound on the throughput:
Th <= (MSS/RTT)*(1 / sqrt{p})
In absence of packet loss, a practical limit for the TCP
throughput is the receiver_window_size divided by the round-trip
time. However, if the TCP implementation uses a window scale
option, this limit can reach the available bandwidth value.
7.9 Qos-level Upgrade And Downgrade Operation
Each time the server detects violation of constraints, the alert
mechanism is triggered, the alert-pause timer is started, and the
qos-level is increased. When this happens repeatedly, and the qos-
level reaches its maximum value (value 9), the session is
cancelled. But when the violation of constraints stops before
reaching qos-level maximum value, the recovery mechanism allows
for the qos-level upgrade gradually.
Following, this downgrade and upgrade of qos-level is explained
with an example:
1. A Q4S session is initiated successfully with qos-level=0.
2. During the continuity phase, violation of constraints is
detected; qos-level is increased to 1, a Q4S-ALERT is sent by
the server to the client and alert-pause timer is started.
3. Alert-pause timer expires and still violation of constraints
is detected; qos-level is increased to 2, a Q4S-ALERT is sent
by the server to the client and alert-pause timer is started.
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4. Alert-pause timer expires but violation of constraints has
stopped; recovery-pause is started.
5. Recovery-pause timer expires, and no violation of
constraints has been detected meanwhile; qos-level is
decreased to 1, a Q4S-RECOVERY is sent by the server to the
client and recovery-pause timer is started again.
6. Recovery-pause timer expires again and no violation of
constraints has been detected meanwhile; qos-level is
decreased to 0 and a Q4S-RECOVERY is sent by the server to
the client; recovery-pause timer is not started this time as
qos-level has reached its initial value.
When the network configuration allows for the possibility of
managing Q4S flows and application flows independently (either is
a network-based QoS or a Q4S aware network), the qos-level
downgrade process could be managed more efficiently using a
strategy that allows for carrying out qos-level downgrades
excluding app flows from SDP dynamically. The Q4S flows would be
downgraded to allow for measurements on a lower quality level
without interference of the application flows. A Q4S client MUST
allow this kind of SDP modifications by the server.
Periodically (every several minutes, depending on the
implementation) a Q4S-ALERT could be triggered, in which the level
is downgraded for Q4S flows, excluding application flows from the
embedded SDP of that request.
This mechanism allows to measure at lower levels of quality while
application flows continue using a higher qos level value.
o If the measurements in the lower level meet the quality
constraints, then a Q4S-RECOVERY message to this lower qos-
level may be triggered, in which the SDP includes the
application flows in addition to Q4S flows.
o If the measurements in the lower level do not meet the
constraints, then a new Q4S-ALERT to the previous qos-level
MUST be triggered, in which the SDP includes only the Q4S
flows.
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+------------------------------------------------+
| |
| 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 client and server role. The role
assumed depends on who sends the first message.
In a communication between two UAs, the UA that sends the Q4S
BEGIN request in the first place, for starting the handshake
phase, shall assume the client role.
If both UASs 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).
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+-----------------------------------------------+
| |
| UA(Client) UA(Server) |
| |
| -------- Q4S BEGIN -------------> |
| <------- Q4S BEGIN -------------- |
| |
| ------- Q4S BEGIN --------------> |
| <------ Q4S 200 OK -------------- |
| |
| |
+-----------------------------------------------+
Figure 15 P2P roles.
8.2 Multiple Quality Sessions in Parallel
A Q4S session is intended to be used for an application. It means
that for using the application, the client MUST establish only one
Q4S session against the server. Indeed, the relation between
session-id and application is 1 to 1.
If a user wants to participate in several independent Q4S sessions
simultaneously against different servers (or against the same
server) it can execute different Q4S clients to establish
separately different Q4S sessions but it is NOT RECOMMENDED,
because:
o The establishment of a new Q4S session may affect other
running applications over other Q4S sessions during bandwidth
measurement.
o If the negotiation phase is executed separately before
running any application, the summation of bandwidth
requirements could not be met when the applications are
running in parallel.
8.3 General Client bBhavior
A Q4S Client has different behaviors. We will use letters X,Y,Z to
designate each different behavior (follow the letter bullets in
figure 16).
X) When it sends messages over TCP (methods BEGIN, READY, Q4S-
ALERT, Q4S-RECOVERY and CANCEL) it behaves strictly like a state
machine that sends requests and waits for responses. Depending
on the response type it enters in a new state.
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When it sends UDP messages (methods PING and BWIDTH), a Q4S client
is not strictly a state machine that sends messages and waits for
responses because:
Y) At latency, jitter and packet loss measurement, the PING
requests are sent periodically, not after receiving the response
to the previous request. In addition, the client MUST answer the
PING requests coming from the server, therefore the client
assumes temporarily the role of a server.
Z) At bandwidth and packet loss measurement stage, the client
does not expect to receive responses when sending BWIDTH
requests to the server. In addition, it MUST receive and process
all server messages in order to achieve the downlink
measurement.
The Q4S-ALERT and CANCEL may have a conventional answer if an
error is produced, otherwise the corresponding answer is formatted
as a request message.
+-----------+------------------------+-----------+-----------+
| Handshake | Negotiation |Continuity |Termination|
| Phase | Phase | Phase | Phase |
| | | | |
| X ---------> Y --> X --> Z --> X ---> Y --> X ---> X |
| | A | A | | A | | |
| | | | | | | | | | |
| | +-----+ +-----+ | +-----+ | |
| | | | |
+------------------------------------------------+-----------+
Figure 16 Phases & client behaviors.
8.3.1 Generating Requests
A valid Q4S request formulated by a Client MUST, at a minimum,
contains the following header fields:
o If no SDP is included: the header Session-Id and Sequence-
Number are mandatory.
o If SDP is included: Session-Id is embedded into SDP,
therefore the inclusion of Session-Id header is optional but
if present must have the same value. Measurements are
embedded into the SDP only for Q4S-ALERT messages in order to
be signed.
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At any time, if the server sends a new SDP with updated values,
client MUST take it into account.
8.4 General Server Behavior
If a server does not understand a header field in a request (that
is, the header field is not defined in this specification or in
any supported extension), the server MUST ignore that header field
and continue processing the message.
The role of the server is changed at negotiation and continuity
phases, in which server MUST send packets to measure jitter,
latency and bandwidth. Therefore, the different behaviors of
server are (follow the letter bullets in the figure 17):
R) When the client sends messages over TCP (methods BEGIN,
READY Q4S-ALERT, Q4S-RECOVERY and CANCEL) it behaves strictly
like a state machine that receives messages and sends
responses.
When the client begins to send UDP messages (methods PING and
BWIDTH), a Q4S server is not strictly a state machine that
receives messages and sends responses because:
S) At latency, jitter and packet loss measurement, the PING
requests are sent periodically by the client but also by the
server. In this case the server behaves as a server answering
client requests but also behaves temporarily as a client,
sending PING requests toward the client and receiving
responses.
T) At bandwidth and packet loss measurement, the server sends
BWIDTH requests to the client. In addition, it MUST receive and
process client messages in order to achieve the uplink
measurement.
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.
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+-----------+------------------------+-----------+-----------+
| Handshake | Negotiation |Continuity |Termination|
| Phase | Phase | Phase | Phase |
| | | | |
| R ---------> S --> R --> T --> R ---> S --> R ---> R |
| | A | A | | A | | |
| | | | | | | | | | |
| | +-----+ +-----+ | +-----+ | |
| | | | |
+------------------------------------------------+-----------+
Figure 17 Phases & server behaviours.
9 Implementation Recommendations
9.1 Default Client Constraints
To provide a default configuration, it would be good that the
client had a configurable set of Quality headers in the
implementation settings menu. Otherwise these quality headers will
not be present in the first message.
Different business models (out of scope of this proposal) may be
achieved: depending on who pays for the quality session, the
server can accept certain Client parameters sent in the first
message, or force billing parameters on the server side.
9.2 Latency and Jitter Measurements
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, client can send a
"READY 1" message.
In execution systems, where the timers are not accurate, a
recommended approach consists of including the optional header
"Timestamp" in the PING request with the time in which the message
has been sent. This allows an accurate measurement of the jitter
even with no identical intervals of time between PINGs.
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9.3 Bandwidth Measurements
In programming languages or Operating Systems with 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 11Mbps, a good approach consists of sending:
1 message of 1KB every 1 millisecond +
1 message of 1KB every 3 milliseconds +
1 message of 1KB every 23 milliseconds
The number of intervals depends on required bandwidth and accuracy
that the programmer wants to achieve.
Considering messages of 1KB (= 8000 bits), a general approach to
determine these intervals is
1) Compute Target bandwidth / 8000 bits. In the example above is
11Mbps/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 sub-multiple 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:
1000/22 = 45 >42
1000/23 = 43 >42
1000 / 24 = 41.6 < 42
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We can choose 24 ms but then we need to cover 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 packets are used, the more constant is the rate of
bandwidth measurement. However, this may stress the execution
system in charge of receiving and processing packets. As a
consequence, some packets may be lost because of stack overflows.
To deal with this potential issue, a larger packet is RECOMMENDED
(2KB or more) taking into account the overhead produced by the
chunks headers.
9.4 Packet Loss Measurement Resolution
Depending on application nature and network conditions, a packet
loss resolution less than 1% may be needed. In such cases, there
is no limit to the number of samples used for this calculation. A
tradeoff between time and resolution should be reached in each
case. For example, in order to have a resolution of 1/10000, the
last 10000 samples should be considered in the packet loss
measured value.
The problem of this approach is the reliability of old samples. If
the interval used between PING messages is 50ms, then to have a
resolution of 1/1000 it takes 50 seconds and a resolution of
1/10000 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, reduce multiplayer interaction speed, etc) in real-time in
order to reach the application constraints, addressing measured
possible network degradation.
9.6 Instability Treatments
There are two scenarios in which Q4S can be affected by network
problems: loss of Q4S packets and outlier samples.
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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:
o If a BEGIN packet is lost or its corresponding answer, after
a certain timeout, the client SHOULD resend another BEGIN
packet, resetting the session
o If a READY packet is lost, after a certain timeout, the
client SHOULD resend another READY packet.
o If a QOS ALERT request is lost or its corresponding answer,
after a certain timeout, the originator SHOULD resend another
Q4S-ALERT packet.
o If a CANCEL request is lost or its corresponding answer,
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 Q4S default measurement method uses the statistical median
formula for latency calculation, the outlier samples are
neutralized. This is a very common filtering for noise or errors
on signal and image processing.
9.7 Scenarios
Q4S could be used in two scenarios:
o client to ACP (Application content provider)
o client to client (peer to peer scenario)
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9.7.1 Client to ACP
One server:
It is the common scenario in which client contact server to
establish a Q4S session.
N servers:
In Content Delivery Networks and in general applications where
delivery of contents can be achieved by different delivery nodes,
two working mechanisms can be defined
o Starting mode: End-user may run Q4S against several delivery
nodes and after some seconds choose the best one to start the
multimedia session
o Prevention mode: During streaming session, user keeps several
Q4S dialogs against different alternative delivery nodes. In
case of congestion, end-user MAY change to the best
alternative delivery node
9.7.2 Client to Client
In order to solve the client to client scenario, a Q4S register
function MUST be implemented. This allows clients contact each
other for sending the BEGIN message. In this scenario, the
Register server would be used by peers to publish their Q4S-
Resource-Server header and their public IP address to make
possible the assumption of server role.
The register function is out of scope of this protocol version,
because different HTTP mechanisms can be used and Q4S MUST NOT
force any.
10 Security Considerations
10.1 Confidentiality Issues
Hence 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.
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10.2 Integrity of Measurements and Authentication
Identity impersonation could potentially produce anomalous Q4S
measurements. If this attack is based on spoofing of 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 Man-in-the-Middle (MITM) attack) relay on the same
mechanism, the SDP signature.
The Signature header 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 relays on UDP and
TCP, and IPSec supports both. If Q4S is used for application-based
QoS, then IPsec is operationally valid but if Q4S is used to
trigger network-based actions, then measurements could be wrong,
unless IPSec ports be considered at any potential action over the
network (such as prioritization of certain application flows).
10.4 Availability Issues
Any loss of connectivity may interrupt the availability of Q4S
service, and results 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 Q4S
service only for authenticated users. Q4S can be launched after
user is authenticated by the application. At this moment, his IP
address is known and the Q4S service may be enabled for this IP
address. Otherwise 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 normally the rest of available bandwidth.
However, a malicious Q4S client could re-starts Q4S sessions just
after finishing the negotiation phase. The consequence would be to
waste bandwidth for nothing.
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In order to mitigate this possible anomalous behavior, it is
RECOMMENDED to configure the server to reject sessions from the
same end-point 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
The need for an assigned PORT is to make possible a future Q4S
aware network, capable of react by itself to Q4S alerts. A
specific port would simplify the identification of the protocol by
network elements in charge of take possible reactive decisions.
Therefore, the need for a port by IANA may be postponed to 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 pass to a new protocol stage (handshake,
negotiation and continuity) as well as inform of alerts when
measurements do not meet the requirements.
Reference : this document. This service does not use IP-layer
broadcast, multicast, or anycast communication.
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12 References
12.1 Normative References
[1] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Message Syntax
and Routing", RFC 7230, June 2014.
[2] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Semantics and
Content", RFC 7231, June 2014.
[3] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Conditional
Requests", RFC 7232, June 2014.
[4] Fielding, R., Ed., Y. Lafon, Ed. and J. Reschke, Ed.
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
RFC 7233, June 2014.
[5] Fielding, R., Ed., M. Nottingham, Ed. and J. Reschke, Ed.
"Hypertext Transfer Protocol (HTTP/1.1): Caching", RFC 7234,
June 2014.
[6] Fielding, R., Ed. and J. Reschke, Ed. "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235, June 2014.
[7] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000
[8] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[9] Thomson, M., "Version-Independent Properties of QUIC",
April 2019
[10] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 4566, July 2006.
[11] Bradner, S., "Key words for use in RFCs to Indicate
RequirementLevels", BCP 14, RFC 2119, March 1997.
[12] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 3986,
January 2005.
[13] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with SDP", RFC 3264, June 2002.
[14] Eastlake, D. and Hansen, T. "US Secure Hash Algorithms",
RFC 4634, May 1992.
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[15] Moriarty, K., Johnsson, J., B. Kaliski, "Public-Key
Cryptography Standards (PKCS) #1: RSA Cryptography
Specifications version 2.2", RFC 8017, November 2016.
[16] Defense Advanced Research Projects Agency, " Transmission
Control Protocol", RFC 793, September 1981.
[17] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[18] Schulzrinne, H., Casner, S., Frederick, R., Jacobson, V.
"RTP: A Transport Protocol for Real-Time Applications", RFC
3550, july 2003.
[19] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, November 2003.
[20] Resnick, P., "Internet Message Format", RFC 5322, October
2008
[21] Leiba, S., "Ambiguity of Uppercase vs Lowercase in RFC 2119
Key Words", RFC 8174, May 2007
12.2 Informative References
[22] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A. Peterson, J., Sparks, R., Handley, M. and Schooler, E. ,
"SIP: Session Initiation Protocol", RFC 3261, June 2002.
[23] Mathis, M., Semke, J., Mahdavi, J., Ott, T., "The
Macroscopic Behavior of the TCP Congestion Avoidance
Algorithm", Computer Communications Review, 27(3), July
1997.
[24] Floyd, S., "HighSpeed TCP for a Large Congestion
Windows", RFC 3649, December 2003.
[25] Rhee, I., Xu, L., Ha, S., "CUBIC for Fast Long-Distance
Networks", Internet-draft draft-rhee-tcpm-cubic-02, February
2009.
[26] Sridharan, M., Tan, K., Bansal, D., Thaler, D., "Compound
TCP: A New TCP Congestion Control for High-Speed and Long
Distance Networks", Internet-draft draft-sridharan-tcpm-
ctcp-02, November, 2008.
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[27] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
Zekauskas, "A One-way Active Measurement Protocol (OWAMP)",
RFC 4656, September 2006.
[28] Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
RFC 5357, October 2008.
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13 Acknowledgments
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.
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14 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 Ibanez
Optiva Media
Email: lara.fajardo@optivamedia.com
Pablo Lopez Zapico
Optiva Media
Email: Pablo.lopez@optivamedia.com
David Muelas Recuenco
Universidad Autonoma de Madrid
Email: dav.muelas@uam.es
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Jesus Molina Merchan
Universidad Autonoma de Madrid
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
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15 Authors' Addresses
Jose Javier Garcia Aranda
Nokia
C/Maria Tubau 9
28050 Madrid
Spain
Phone: +34 91 330 4348
Email: jose_javier.garcia_aranda@nokia.com
Monica Cortes
Universidad Politecnica de Madrid
Avenida Complutense 30
28040 Madrid
Spain
Email: cortesm@dit.upm.es
Joaquin Salvachua
Universidad Politecnica de Madrid
Avenida Complutense 30
28040 Madrid
Spain
Phone: +34 91 0672134
Email: jsalvachua@dit.upm.es
Maribel Narganes
Tecnalia Research & Innovation
Parque Cientifico y Tecnologico de Bizkaia
Geldo Auzoa, Edificio 700
E-48160 Derio (Bizkaia)
Spain
Phone: +34 946 430 850
Email: maribel.narganes@tecnalia.com
Inaki Martinez Sarriegui
Optiva Media
Edificio Europa II,
Calle Musgo 2, 1G,
28023 Madrid
Spain
Phone: +34 91 297 7271
Email: inaki.martinez@optivamedia.com
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