The Quality for Service Protocol
draft-aranda-dispatch-q4s-01
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8802.
Expired & archived
|
|
|---|---|---|---|
| Authors | Carlos Barcenilla , Luis Migu Vizcaino , Jacobo Lajo , Monica Cortes , Jose Javier Garcia Aranda , Joaquin Salvachua , Juan Quemada | ||
| Last updated | 2011-06-27 (Latest revision 2011-01-25) | ||
| RFC stream | (None) | ||
| Formats | |||
| IETF conflict review | conflict-review-aranda-dispatch-q4s, conflict-review-aranda-dispatch-q4s, conflict-review-aranda-dispatch-q4s, conflict-review-aranda-dispatch-q4s, conflict-review-aranda-dispatch-q4s, conflict-review-aranda-dispatch-q4s | ||
| Additional resources | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | Became RFC 8802 (Informational) | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-aranda-dispatch-q4s-01
DISPATCH Working Group J. J. Garcia Aranda
J. Perez Lajo
Internet Draft L. M. Diaz Vizcaino
Intended status: Standards Track Alcatel-Lucent
Expires: December 2011 C. Barcenilla
M. Cortes
J. Salvachua
J. Quemada
Univ. Politecnica de Madrid
June 27, 2011
The Quality for Service Protocol
draft-aranda-dispatch-q4s-01.txt
Status of this Memo
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carefully, as they describe your rights and restrictions with
respect to this document.
Abstract
This memo describes an application level protocol for the standard
communication of e2e QoS compliance information using a protocol
based on Hypertext Transfer Protocol (HTTP), which forms the basis
for the World Wide Web, and Session Description Protocol (SDP).
Quality for Service Protocol (Q4S) provides a mechanism for latency,
jitter, bandwidth an packet loss negotiation and monitoring,
alerting whenever one of the negotiated conditions is violated.
Implementation details on the actions to be triggered upon
reception/detection of QoS alerts exchanged by the protocol are out
of scope of this draft, it is application dependant (e.g. increase
quality, reduce bit-rate) or even network dependant (e.g. change
connection's quality profile).
Table of Contents
1. Introduction......................................... 4
1.1. Motivation...................................... 6
1.2. Summary of Features............................... 7
2. Terminology ......................................... 8
3. Overview of Operation................................. 8
4. Protocol........................................... 12
4.1. Protocol Phases................................. 12
4.2. SDP Structure................................... 14
4.2.1. qos-level attribute.......................... 15
4.2.2. public-address attributes..................... 15
4.2.3. latency attribute........................... 16
4.2.4. jitter attribute............................ 16
4.2.5. bandwidth attribute.......................... 16
4.2.6. packetloss attribute......................... 16
4.2.7. flow attributes............................. 16
4.2.8. Measurement attributes....................... 18
4.3. Measurements ................................... 19
4.3.1. Latency ................................... 19
4.3.2. Jitter.................................... 20
4.3.3. Bandwidth.................................. 21
4.3.4. Packet loss................................ 23
4.4. Handshake Phase................................. 24
4.5. Negotiation phase ............................... 26
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4.5.1. Stage 0: Measurement of latencies and jitters.... 27
4.5.2. Stage 1: Measurement of bandwidth and packet loss. 33
4.5.3. Constraints not reached...................... 37
4.5.3.1. Policy server is present ................. 42
4.5.4. QoS Level changes........................... 44
4.5.4.1. QoS Level increments without changes in network
behaviour..................................... 45
4.6. Continuity phase................................ 45
4.6.1.1. Normal mode............................ 46
4.6.1.2. Sliding window mode...................... 48
4.7. Termination Phase ............................... 51
4.8. Dynamic constraints and flows...................... 51
4.9. Qos-level downgrade operation...................... 53
4.10. Sanity check of Quality sessions.................. 54
5. Q4S messages........................................ 54
5.1. Requests....................................... 55
5.2. Responses...................................... 56
5.3. Header Fields................................... 57
5.3.1. Specific Q4S Request Header Fields............. 57
5.3.2. Specific Q4S Response Header Fields ............ 58
5.4. Bodies ........................................ 59
5.4.1. Encoding................................... 60
6. General User Agent behavior. .......................... 60
6.1. Roles......................................... 60
6.2. Multiple Quality sessions in parallel............... 61
6.3. General client behavior .......................... 62
6.3.1. Generating requests.......................... 63
6.4. General server behavior .......................... 63
7. Q4S method definitions ............................... 64
7.1. BEGIN......................................... 65
7.2. GET........................................... 65
7.3. READY......................................... 66
7.4. PING.......................................... 66
7.5. DATA.......................................... 66
7.6. QOS-ALERT...................................... 67
7.7. CANCEL ........................................ 67
8. Response codes...................................... 68
8.1. 100 Trying..................................... 68
8.2. 200 OK ........................................ 68
8.3. Redirection 3xx................................. 68
8.4. Request Failure 4xx.............................. 68
8.4.1. 400 Bad Request............................. 68
8.4.2. 404 Not Found............................... 68
8.4.3. 405 Method Not Allowed....................... 69
8.4.4. 406 Not Acceptable .......................... 69
8.4.5. 408 Request Timeout.......................... 69
8.4.6. 412 A precondition has not been met ............ 69
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8.4.7. 413 Request Entity Too Large.................. 69
8.4.8. 414 Request-URI Too Long...................... 69
8.4.9. 415 Unsupported Media Type.................... 69
8.4.10. 416 Unsupported URI Scheme................... 70
8.5. Server Failure 5xx............................... 70
8.5.1. 500 Server Internal Error..................... 70
8.5.2. 501 Not Implemented.......................... 70
8.5.3. 503 Service Unavailable...................... 70
8.5.4. 504 Server Time-out.......................... 70
8.5.5. 505 Version Not Supported..................... 71
8.5.6. 513 Message Too Large........................ 71
8.6. Global Failures 6xx.............................. 71
8.6.1. 600 session not exist........................ 71
8.6.2. 601 quality level not allowed ................. 71
8.6.3. 603 Session not allowed...................... 71
8.6.4. 604 authorization not allowed ................. 71
9. Implementation Recommendations......................... 71
9.1. Default client constraints........................ 71
9.2. Bandwidth measurements........................... 72
9.3. Packet loss measurement resolution ................. 72
9.4. Measurements and reactions........................ 72
9.5. Instability treatments........................... 73
9.6. Scenarios...................................... 74
9.6.1. Client to ACP............................... 74
9.6.2. Client to client............................ 75
10. Security Considerations.............................. 76
11. IANA Considerations................................. 76
12. Conclusions........................................ 79
13. References ........................................ 80
13.1. Normative References............................ 80
13.2. Informative References .......................... 81
14. Acknowledgments.................................... 82
15. Authors' Addresses.................................. 83
1. Introduction
The World Wide Web (WWW) is a distributed hypermedia system which
has gained widespread acceptance among Internet users. Although WWW
browsers support other, preexisting Internet application protocols,
the native and primary protocol used between WWW clients and servers
is the HyperText Transfer Protocol (HTTP) (RFC 2616 [1]). The ease
of use of the Web has prompted its widespread employment as a
client/server architecture for many applications. Many of such
applications require the client and the server to be able to
communicate each other and exchange information with certain quality
constraints.
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Quality in communications at application level consists of four
measurable parameters:
o Latency: The time a message takes to travel from source to
destination. It may be approximated to RTT/2 (Round trip time),
assuming the networks are symmetrical.
o Jitter: latency variation. There are some formulas to calculate
Jitter, and in this context we will consider the statistical
variance formula.
o Bandwidth: To assure the quality, a protocol MUST assure the
availability of 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.
Q4S provides a mechanism for quality monitoring based on HTTP and
SDP in order to be easily integrated in WWW, but it may be used by
any type of application, not only those based on HTTP. Quality
requirements may be needed by any type of application that
communicates using any kind of protocol, especially those which have
real-time constraints. Depending on the nature of each application
the constraints may be different leading to different parameter
thresholds that need to be met.
Q4S is an application level Client/Server protocol that continuously
measures session quality for a given flow (or set of flows), end-to-
end and in real-time; raising an alert if quality parameters are
below a given pre-negotiated threshold. Q4S describes when these
alerts 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, in order to
minimize the impact on them. To perform the measurements, two
control flows are created on both communication paths (forward and
reverse direction).
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1.1. Motivation
Monitoring quality of service (QoS) in computer networks is useful
for several reasons:
o Enable real-time services and applications to verify whether
network resources achieve a certain QoS level.
o Monitoring 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 Monitoring also applies to Peer to Peer (P2P) real-time
applications
o Enable ISPs to offer QoS to any ACP or final user application
in an accountable way
o Enable e2e negotiation of QoS parameters, independently of the
Internet service providers of both endpoints.
A protocol to monitor QoS must address the following issues:
o Must be ready to be used in conjunction with current standard
protocols and applications, without forcing a change on them.
o Must have a formal and compact way to specify quality
constraints of the desired application to run.
o Must have measurement mechanisms avoiding application
disruption.
o Must have specific messages to alert about the violation of
quality constraints in different directions (forward and
reverse), because network routing may not be symmetrical, and
of course, quality constraints may not be symmetrical.
o Must protect the data (constrains, measurements, QoS levels
demanded from the network) in order to avoid the injection of
malicious data in the measurements.
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1.2. Summary of Features
Quality for Service is a message-oriented communication protocol
that can be used in conjunction with any other application-level
protocol.
The benefits in quality enhancement 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 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
constraints. It will raise alerts if the new constraints are
violated.
Q4S session lifetime is composed of four phases with different
purposes. Two phases perform network parameter measurements as per a
negotiated measurement procedure. Different measurement procedures
COULD be used inside Q4S, although one default measurement mechanism
is needed for compatibility reasons and is the one defined in this
draft. Basically, Q4S defines how to transport application quality
requirements SLA and measurement results between client and server
and provides, as well, monitoring and alerting.
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.
During the lifetime of the application, the protocol periodically
renews the session measurements and alerts if the measured values of
quality parameters do not meet the negotiated application
requirements.
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The quality parameters can be suggested by the client in the first
message of the handshake phase, but it's the server that accepts
these parameter values or forces others. The server is in charge of
deciding the final values of quality connection.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119 [3].
3. Overview of Operation
This section introduces the basic operation of Q4S using simple
examples. This section is of tutorial nature and does not contain
any normative statements.
The first example shows the basic functions of a Q4S: communication
establishment between a client and a server, quality requirement
negotiations for the requested application, application start and
continuous quality parameter measurements, and finally communication
termination. The message exchange is depicted in Figure 1.
The client triggers the establishment of the communication
requesting a specific service or application from the server. This
first message must have a special URI (RFC 3986), which forces 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 initial communication quality requirements in an
SDP body.
This request is answered by the server with an Q4S 200 OK method
letting the client know that it accepts the request. This message
MUST contain an SDP body with the quality constraints required by
the requested application and the measurement procedure to use.
Once the communication has been established, 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. This 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.
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Measurements are being taken and communicated to each other through
Q4S PING messages sent both from the client and the server. All Q4S
PING requests will be answered by Q4S 200 OK messages to allow for
bidirectional measurements.
After a pre-agreed number of measurements, determined by the
measurement procedure as sent by the server, have been performed,
the client will send a Q4S GET message to the server containing the
measured values of all quality parameters from its point of view.
The server receives this message and if the values meet the
necessary requirements answers with a Q4S 200 OK method.
As the communication meets the conditions, the application will
start. Q4S will continue to measure the communication parameters
verifying that the real-time application can keep executing under
quality conditions. This will be done through the Q4S PING message
exchange on both connection paths.
Once the client wants to terminate the communication it sends a Q4S
CANCEL message, which will be acknowledged by the server with
another Q4S CANCEL message.
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+------------------------------------------------+
| |
| Client Server |
| |
| --------- Q4S BEGIN -----------> |
| <-------- Q4S 200 OK ----------- |
| |
| --------- Q4S READY -----------> |
| <-------- Q4S 200 OK ----------- |
| |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| --------- Q4S PING ------------> |
| <-------- Q4S PING ------------- |
| --------- Q4S 200 OK ----------> |
| <-------- Q4S 200 OK ----------- |
| ... |
| |
| --------- Q4S GET -------------> |
| <-------- Q4S 200 OK ----------- | application start
| |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| |
| --------- Q4S CANCEL ----------> | application end
| <-------- Q4S CANCEL ----------- |
| |
+------------------------------------------------+
Figure 1 Basic Q4S message exchange.
The second example shows the behavior of Q4S protocol in the event
of the loss of quality for a given parameter constraint: detection
of a the violation of specific parameter constraints after
continuous measurements of network parameters, the raising of an
alert, the termination in case of non recovery of the quality
communication. This message exchange is depicted in Figure 2.
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+------------------------------------------------+
| |
| Policy |
| Client Server Server |
| |
| --------- Q4S BEGIN -----------> |
| <-------- Q4S 200 OK ----------- |
| |
| --------- Q4S READY -----------> |
| <-------- Q4S 200 OK ----------- |
| |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| ... |
| --------- Q4S GET -------------> |
| <-------- Q4S 412 -------------- |
| |
| --- QOS-ALERT ----> |
| <-- 100 trying ---- |
| |
| ---- QOS-ALERT ----> |
| <--- QOS-ALERT ----- |
| <--- QOS-ALERT ---- |
| |
| (waiting period) |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| ... |
| --------- Q4S GET -------------> |
| <-------- Q4S 412 -------------- |
| |
| --------- Q4S CANCEL ----------> |
| <-------- Q4S CANCEL ----------- |
| |
+------------------------------------------------+
Figure 2 Q4S message exchange with quality constraints not reached.
The initiation of the communication happens in the same way as in
the first example establishing communication from the client to the
server through a Q4S BEGIN message answered by the server if ready
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with a Q4S 200 OK. In this case, the server indicates the presence
of a policy server, which will receive the alerts. The policy server
is an entity that can act on the network. It has a set of different
quality levels defined and pre-agreed upon between the ACD and the
ISP.
Then the measurements are taken in order to verify if the
communication conditions meet the quality constraints of the
application through an exchange of Q4S PING requests and Q4S 200 OK
responses. The client sends the server the measured values in a Q4S
GET message and waits for the answer with the actions to take.
In this case, some or many of the quality constraints are not met
and the server sends a Q4S 412 message indicating that a pre-
condition as not been met.
The client will then send a Q4S QOS-ALERT message to the policy
server. The policy server responds to the client with a Q4S 100
trying message and notifies the server of the received alert. The
policy server will verify the authorization credentials and raise
the quality of service level acting on the network elements as per
SLA agreement between the ACP and the ISP, which are out of scope of
this protocol description.
After a delay, the client and server will initiate the quality
parameter measurements through more Q4S PING and Q4S 200 OK
messages. If the quality of the communication cannot be met, the
client closes the connection with a Q4S CANCEL message acknowledged
by the server with another Q4S CANCEL.
4. 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.
4.1. 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
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communication with enough quality for the application to function
flawlessly.
To monitor quality constraints of the application, four phases are
defined and can be shown in Figure 3.
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.
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 match the constraints. This phase
is iterative until quality constraints are reached or the
session is cancelled after a number of measurement cycles
without meeting the quality constraints. Just after reaching
the quality requirements, Q4S provides a simple optional
mechanism to start the application, which will benefit from
quality connection from the beginning, using HTTP.
o Continuity phase: in which quality is continuously measured. If
quality becomes degraded, an alert shall be issued. New
measurements may follow up to a negotiated maximum before
moving to Termination phase if constraints cannot be met. In
this phase the measurements MUST avoid disturbing application
by consuming network resources.
o Termination phase: in which the session is terminated.
+---------------------------------------------------------------+
| constraints not reached |
| +------------------+ |
| | V |
| Handshake ---> Negotiation +--> Continuity -+-+-> Termination |
| A | | A | | |
| | | | | | | |
| +--+ | +----------+ | |
| | V |
| +->Application Application |
| start end |
| |
+---------------------------------------------------------------+
Figure 3 Session lifetime phases.
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4.2. 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 this parameter and 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.
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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=public-address:client IP4 192.0.2.33
a=public-address:server IP4 198.51.100.58
a=latency:40/35
a=jitter:10/10
a=bandwidth:20/6000
a=packetloss:5/5
a=flow:data downlink TCP/10000-20000
a=flow:control downlink UDP/55000
a=flow:control downlink TCP/55001
a=flow:data uplink TCP/56000
a=flow:control uplink UDP/56000
a=flow:control uplink TCP/56001
a=measurement:procedure default,50/50,75/75,,0
a=measurement:latency 10000/10000
a=measurement:jitter 10000/10000
a=measurement:bandwidth 0/0
a=measurement:packetloss 0/0
4.2.1. qos-level attribute
The "qos-level" attribute contains the QoS level for uplink and
downlink. Default values are 0 for both directions. The meaning of
each level is out of scope of Q4S, but a higher level SHOULD
correspond to a better service quality.
The "qos-level" attribute may be changed during the protocol
lifetime raising or lowering the value as necessary following the
network measurements and the application needs.
4.2.2. 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.
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4.2.3. latency attribute
The maximum uplink and downlink latency tolerance are specified in
the "latency" attribute, expressed in milliseconds.
4.2.4. jitter attribute
The maximum uplink and downlink jitter tolerance are specified in
the "jitter" attribute, expressed in milliseconds.
4.2.5. bandwidth attribute
The minimum uplink and downlink bandwidth are specified in the
"bandwidth" attribute, expressed in kbps.
4.2.6. packetloss attribute
The maximum uplink and downlink packet loss tolerance are specified
in the "packetloss" attribute expressed in percentage.
4.2.7. flow attributes
These attributes specify the flows (protocol, source IP, source Port
+ destination IP, destination port) of data over TCP and UDP ports
to be used in uplink and downlink communications.
Several "flow" attributes can be defined. The goal is to monitor
each flow to verify that the quality constraints are met. These
flows identify the direction (uplink or downlink), the protocol (TCP
or UDP) (RFC 761 [8] and RFC 768 [9]) and the ports that are going
to be used by the application data and, of course, by the Q4S
control flows (for quality measurements), because the quality
measurements MUST be achieved over the same quality session for each
direction. All defined flows will be considered within the same
quality profile, which is determined by the qos-level attribute in
each direction.
During negotiation phase the specified control ports will be used
for Q4S messages, and this is the reason to separate application
data ports from Q4S control ports, otherwise they could collide.
The control should involve two UDP flows, one uplink and one
downlink, and two TCP flows, again one uplink and one downlink.
Application data MAY consist of many flows, depending on the nature
of the application. The handshake phase takes place through the
Contact URI, using TCP port 80 for example. However, the negotiation
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phase will take place on the specified control ports (UDP and TCP)
using the Session URI.
The "downlink port" is a port in which the client listens for server
requests and MUST be used as origin port of client responses. The
"uplink port" is a port in which server is listening for incoming
messages from the client and MUST be used as origin port of server
responses.
If the server's "downlink" port is null (a=flow:control downlink
TCP/0), the client May choose one randomly as per OS standard rules.
"Downlink" 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.
+------------------------------------------------+
| |
| Client Server |
| |
| downlink port uplink port |
| A | |
| | | |
| +-----------------------------+ |
| |
| |
+------------------------------------------------+
Figure 4 Downlink flow.
+------------------------------------------------+
| |
| Client Server |
| |
| downlink port uplink port |
| | A |
| | | |
| +-----------------------------+ |
| |
| |
+------------------------------------------------+
Figure 5 Uplink flow.
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4.2.8. Measurement attributes
These attributes contain the measurement procedure and the results
of the quality measurements.
Measurement parameters are included using the session attribute
"measurement". The first measurement parameter is the procedure. Q4S
provides a "default" procedure for measurements, but others like
RTP/RTCP might be used and defined later. This draft will only
define and explain the "default" procedure.
In the initial client request a set of measurement procedures can be
sent to the server for negotiation. One measurement procedure line
MUST be included in the SDP message for each proposed method. The
server MUST answer with only one line with the chosen procedure.
For each procedure, a set of values of parameters separated by ","
can be included in the same attribute line. The amount and type of
parameters depends on the procedure type.
In the following example the "default" procedure type is chosen:
a=measurement:procedure default,50/50,75/75,5000,1,40/80,100/256
In the "default" procedure, the meaning of these parameters is:
o The first parameter is the interval of time (in milliseconds)
between PING requests during the negotiation phase. Uplink and
downlink values from the client's point of view are separated
by "/". This allows having different responsiveness values
depending on the control resources used in each direction.
o The second parameter is the time interval (in milliseconds)
between PING requests during the continuity phase. Forward and
reverse 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. If not present, a
default value of 5000 ms MUST be assumed. Forward and reverse
values are separated by "/".
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o The fourth parameter indicates the operation mode during the
continuity phase. Two values are defined: 0 means "normal" mode
and 1 means "sliding window" mode. If not present, normal mode
(default value of 0) will be assumed. These modes of operation
will be described in [4.7].
o The fifth parameter is only applicable in sliding window mode.
It indicates the window size for jitter and latency
calculations. If not present, a value of 256 MUST be assumed.
Forward and reverse values are separated by "/".
o The sixth parameter is only applicable in sliding window mode.
It indicates the window size for packet loss calculations. If
not present, a value of 256 MUST be assumed. Forward and
reverse values are separated by "/".
There are four more measurement attributes:
a=measurement:latency 10000/10000
a=measurement:jitter 10000/10000
a=measurement:bandwidth 0/0
a=measurement:packetloss 0/0
The latency, jitter, bandwidth and packetloss measurement attributes
contain the values measured for each of these quality parameters in
uplink and downlink directions. Quality parameters values in these
measurement attributes provide a snapshot of the quality level
reached in each measurement stage.
They can be omitted during the initial protocol phases as no
measurements have been taken, but they MUST be filled in when
sending GET requests and 412 responses.
4.3. Measurements
This section describes the way quality parameters are measured as
defined by the "default" procedure.
4.3.1. Latency
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
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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. As the
forward and reverse latencies are unknown the client and server will
assume that the network is symmetric. The latency will therefore be
calculated as the average value of all the RTT samples divided by 2.
4.3.2. Jitter
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 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 statistical variance formula:
Jitter Statistical = SquareRootOf(SumOf((ElapsedTime[i]-
Average)^2)/(ReceivedPacketCount-1))
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.
After 255 samples the client has the values of RTT and downlink
jitter and the server has RTT and uplink jitter.
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4.3.3. Bandwidth
In order to measure the available bandwidth, both the client and the
server MUST start sending DATA messages simultaneously using the UDP
control ports exchanged during the handshake phase in the SDP
message, at the needed rate to verify the availability of the
bandwidth constraint in each direction using messages of 1 Kbyte in
length. The messages are sent during the period of time defined in
the third parameter of the SDP measurement procedure attribute in
millisecond units. If this parameter is not present, a value of 5
seconds SHOULD be used by default.
a=measurement:procedure default,50/50,75/75,5000,0
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+------------------------------------------------+
| Rate |
| A |
| | |
|downlink rate-|-------------------+ <-- traffic |
| | | sent by |
| | | server |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| | | |
| uplink rate-|-------------------+ <-- traffic |
| | | sent by |
| | | client |
| | | |
| | | |
| |---|---|---|---|---|----> time |
| 0 1 2 3 4 5 (sec.) |
| |
+------------------------------------------------+
Figure 6 Bandwidth and packet loss measurements.
The goal of these measurements is not to identify the bandwidth of
the Internet connection but to determine if the required bandwidth
is available, meeting the application's constraints. Therefore, the
requested bandwidth MUST be measured sending only that bit rate.
When measuring bandwidth, all DATA requests sent MUST be 1 kilobyte
in length (UDP payload length), and MUST include a Sequence-Number
header with a sequential number starting at 0. The Sequence-Number
MUST be incremented by 1 with each sent DATA packet. If any
measurement stage needs to be repeated, the values MUST start at
zero again. DATA requests MUST NOT be answered. Examples:
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Client message:
=========================
DATA 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
aaaaaaaaaaaaa ( to complete 1024 bytes UDP payload length)
=========================
The client MUST send DATA packets to the server to allow the server
to measure the uplink bandwidth. The server MUST send DATA packets
to the client to allow the client to measure the downlink bandwidth.
server message:
=========================
DATA q4s://www.example.com Q4S/1.0
Session-Id: 53655765
Sequence-Number: 0
Content-Type: text
Content-Length: XXXX
aaaaaaaaaaaaa ( to complete 1024 bytes UDP payload length)
=========================
When the measurement time interval is over, the client and the
server have a collection of server messages and can calculate the
downlink and uplink bandwidth respectively.
4.3.4. Packet loss
Packet loss and bandwidth are measured simultaneously using the DATA
packets sent by both the client and the server. Because the DATA
packets contain a Sequence-Number header incremented sequentially
with each sent packet, lost packets can be easily identified. The
lost packets have to be counted during the measurement time.
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4.4. Handshake Phase
The first phase consists of a Q4S BEGIN method issued from the
client to the server.
The first Q4S message MUST have a special URI (RFC 3986 [4]), which
forces the use of the Q4S protocol if it is implemented in a
standard web browser.
This URI, named "Contact URI", is used to request the start of a
session. Its scheme MUST be:
"q4s:" "//" host [":" port] [path["?" query]
Optionally, the client can send the desired quality parameters
(enclosed in the body of the message as an SDP document) and the
server can take them into account when it builds the answer with the
final values, following a request / response schema (RFC 3464 [5]).
The description of these quality parameters is attached in an SDP
document.
If the request is accepted, the server MUST answer with a Q4S 200 OK
message, and in the body of the answer message, an SDP document MUST
be included (RFC 4566 [2]), with information about the required
quality constraints. Q4S responses should use the protocol
designator "Q4S/1.0".
After these two messages are exchanged, the first phase is
completed. The quality parameters have been sent to the client. Next
step is to measure the quality of the communication path between the
client and the server and alert if the SLA is being violated.
+------------------------------------------------+
| |
| Client Server |
| |
| ------- Q4S BEGIN ------------> |
| |
| <------ Q4S 200 OK ------------ |
| |
| |
+------------------------------------------------+
Figure 7 Handshake phase.
Example of Client Request and Server Answer:
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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
Q4S-Resource-Server: \
q4s://www.example.com/example/util/agent?num=666
Q4S-Policy-Server: q4s://www.qosmanager.com/agent
Signature: 6ec1ba40e2adf2d783de530ae254acd4f3477ac4
Content-Length: 131
(SDP not shown)
=========================
The "Expires" header purpose is to provide a sanity check and
enables the server to close inactive sessions. If the client does
not send a new request before the expiration time, the server can
close the session.
The "Signature" header contains a digital signature that can be used
by the network to validate the SDP, preventing security attacks.
The signature is an optional header generated by the server using a
hash and encryption method such as MD5 (RFC 1321 [6]) and RSA (RFC
2437 [7]), but it depends on the certificate used by the server.
This certificate is supposed to be delivered by a Certification
Authority (CA) or policy owner to the server. The signature is
applied to the SDP body.
Signature= RSA ( MD5 (<sdp>), <certificate> )
If the signature is not present, other validation mechanism may be
implemented in order to provide assured quality with security and
control.
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The optional response header "Q4S-Resource-Server" contains the
Session URI, which is in charge of this session. This URI MUST be
invoked by the client in all later requests. Example:
Q4S-Resource-server: \
q4s://www.example.com/example/util/agent?num=666
If this header is not present, the client will continue sending all
requests to the original Contact URI, but if it is present, its use
is mandatory.
The last optional response header is "Q4S-Policy-Server" which
contains the "Policy Server URI" towards which client MUST send the
later QOS-ALERT messages. If the "Q4S-Policy-Server" header is
present, the "Q4S-Resource-server" header is mandatory, as the
policy server MUST notify the Q4S server about the Q4S-ALERT
received from the client and this information in not available to
the policy server except through the "Q4S-Resource-server" header.
During the next phases of the protocol, the client role will perform
a mix of client and server role. Hence, the client can specify a
"Q4S-Resource-Client" header in the BEGIN request, indicating the
Resource Client URI, a relative URI in charge of the server requests
when client receives requests from the server. Example:
Q4S-Resource-Client: /example/useragent
This URI MUST be relative because user agents may not have an
associated domain, or its IP address is unknown.
4.5. Negotiation phase
The negotiation phase is in charge of measuring the quality
parameters and verifying that the communication paths meet the
required quality constraints. If the quality session is compliant
with the minimum quality constraints the application can start. If
not, a higher quality service level will be demanded and the results
on the network parameters will be measured again. If after some time
the quality constraints cannot be met the quality session is
terminated due.
The steps to be taken in this phase depend on the measurement
procedure exchanged during the handshake phase. This document only
describes the "default" procedure, but others can be used, like
RTP/RTCP (RFC 3550 [10]).
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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
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 control 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.
4.5.1. Stage 0: Measurement of latencies and jitters
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+------------------------------------------------+
| |
| Client Server |
| |
| ------- Q4S READY -----------> |
| |
| <----- Q4S 200 OK ----------- |
| |
| |
+------------------------------------------------+
Figure 8 Beginning of Stage 0 of the Negotiation phase.
The Stage 0 MUST start with a synchronization message exchange
initiated with the client's READY message. This allows the
synchronization of negotiation phases in multiple quality sessions
enabling the possibility to repeat a successful stage.
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 as described in section 4.3.
The client MUST send its PING requests using the UDP control flow
ports defined in the SDP negotiated during the handshake phase. The
downlink port is set as destination and the uplink port is set as
origin (according to the example given in the SDP structure, from
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client UDP port 56000 to server UDP port 55000). The Sequence-Number
header MUST be set to 0.
At the same time the server must begin to do exactly the same, using
the downlink UDP control ports to send PING requests towards the
client.
+------------------------------------------------+
| |
| Client Server |
| |
| --------- Q4S READY -----------> |
| <-------- Q4S 200 OK ----------- |
| |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| --------- Q4S PING ------------> |
| <-------- Q4S PING ------------- |
| --------- Q4S 200 OK ----------> |
| <-------- Q4S 200 OK ----------- |
| ... |
| |
+------------------------------------------------+
Figure 9 Simultaneous exchange of PING request and responses.
This is an example of the PING request sent from the client and the
server's response:
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Client Request:
=========================
PING q4s://www.example.com Q4S/1.0
Session-Id: 53655765
Sequence-Number: 0
User-Agent: q4s-ua-experimental-1.0
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 control ports and incrementing
the Sequence-Number with each message and MUST NOT wait for a
response to send the next PING request. The "Sequence-Number" header
value is a sequential integer number and MUST start at zero. If this
stage is repeated, the initial Sequence-Number MUST start again at
zero.
Optionally the PING request can include:
- a "Timestamp" header, with the time in which the message has
been sent. In case the header is present, the server MUST
include the header in the response without changing the value.
- a "Measurements" header, with the values of the jitter and
packet loss measured by each entity. The client will send its
measurements to the server and the server his measurements to
the client. Example : Measurements: 13, 1
- a "Qualimeter" header, with the value in percentage of
experienced versus desired quality.
In 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).
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a=measurement:procedure default,50/60,50/50,5000,0
A minimum of 256 PING messages MUST be exchanged in order to be able
to measure latency, jitter and packet-loss. Both the client and the
server calculate the respective measured parameter values. The
mechanisms to calculate the different parameters are described in
section 4.3.
Then the client MUST send a GET request to the server using the TCP
uplink control port exchanged in the handshake phase in order to
communicate the measured parameters to the server. This message MUST
always be sent, independently of the measurement procedure used. The
GET request contains a body with updated downlink values for the
latency, jitter and packet loss measurement attributes.
An example of a GET request can be seen below.
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Client Request:
=========================
GET q4s://www.example.com Q4S/1.0
Host: www.example.com
User-Agent: q4s-ua-experimental-1.0
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:0/0
a=public-address:client IP4 192.0.2.33
a=public-address:server IP4 198.51.100.58
a=latency:40/35
a=jitter:10/10
a=bandwidth:20/6000
a=packetloss:5/5
a=flow:data downlink TCP/10000-20000
a=flow:control downlink UDP/55000
a=flow:control downlink TCP/55001
a=flow:data uplink TCP/56000
a=flow:control uplink UDP/56000
a=flow:control uplink TCP/56001
a=measurement:procedure default,50/50,75/75,5000,0
a=measurement:latency 40/40
a=measurement:jitter 0/10
a=measurement:bandwidth 0/0
a=measurement:packetloss 0/2
=========================
When the server receives this message, it compares the latency value
(RTT/2) with its own measurements, in order to avoid inconsistencies
and giving priority to the latency value measured by server.
At this point there are two possibilities
o The latency, jitter and packet loss constraints are reached
o The latency, jitter and packet loss constraints are not reached
In the first case, the server verifies that all three parameters are
within acceptable values and then MUST answer with an
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acknowledgement, a QOS 200 OK message. This message contains an SDP
body with the server's measured data.
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)
=========================
It means that the client and the server have finalized Stage 0 and
are ready for Stage 1, bandwidth and packet loss measurement.
If the bandwidth constraints are empty or with value zero, the
negotiation phase MUST terminate and both client and server initiate
the Continuity Phase.
The second case, in which some constraint has not been met is
detailed in section 4.5.3. Constraints not reached.
4.5.2. Stage 1: Measurement of bandwidth and packet loss
This stage begins in a similar way to stage 0, sending a READY
request over TCP control ports. This READY message "Stage" header
value is 1. The server answers with a Q4S 200 OK message to
synchronize the initiation of the measurements.
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+------------------------------------------------+
| |
| Client Server |
| |
| --------- Q4S READY -----------> |
| <-------- Q4S 200 OK ----------- |
| |
| --------- Q4S DATA -----------> |
| <-------- Q4S DATA _----------- |
| --------- Q4S DATA -----------> |
| <-------- Q4S DATA _----------- |
| ... |
| --------- Q4S GET -------------> |
| <-------- Q4S 200 OK ----------- |
| |
+------------------------------------------------+
Figure 10 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 DATA messages simultaneously using the UDP control
ports. Section 4.3.3 describes the bandwidth measurement in detail.
After the measurements have been performed the client MUST send a
GET message to the server using the uplink TCP control port
including in the body of the message the SDP data with the parameter
measurement attributes filling the downlink fields of the bandwidth
and packet loss.
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Client Request:
=========================
GET 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:1/1
a=public-address:client IP4 192.0.2.33
a=public-address:server IP4 198.51.100.58
a=latency:40/35
a=jitter:10/10
a=bandwidth:20/6000
a=packetloss:5/5
a=flow:data downlink TCP/10000-20000
a=flow:control downlink UDP/55000
a=flow:control downlink TCP/55001
a=flow:data uplink TCP/56000
a=flow:control uplink UDP/56000
a=flow:control uplink TCP/56001
a=measurement:procedure default,50/50,50/50,5000,0
a=measurement:latency 30/30
a=measurement:jitter 6/4
a=measurement:bandwidth 0/4000
a=measurement:packetloss 0/3
==============================
At this point there are two possibilities:
o The bandwidth and packet loss constraints are reached in both
directions.
o The bandwidth and packet loss constraints are not reached in
one both directions.
The second case, with violated constraints is explained in 4.6.3
Constraints not reached.
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If measurements match the constraints, the negotiation phase is
successful, the client and server have verified that all constraints
are met and the application can be started. An optional simple
mechanism, based on HTTP, is defined to trigger the application
using the "Trigger-URI" header.
The server answer MUST be 200 OK, and MAY include the URI for
triggering the application using an optional "Trigger-URI" header.
Server Answer:
=========================
Q4S/1.0 200 OK
Date: Mon, 10 Jun 2010 10:00:01 GMT
Trigger-URI: http://www.example.com/app_start
Expires: 3000
Content-Type: application/sdp
Signature: 6ec1ba40e2adf2d783de530ae254acd4f3477ac4
Content-Length: 131
(SDP not shown)
=========================
The application SHOULD be triggered using an URI, by means of an
HTTP request, specified in the Q4S header "Trigger-URI". Other
mechanisms, such as including a "Location" header in the Q4S
message, to force redirection is not recommended because these
mechanisms are achieved without parsing the body of the message.
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+------------------------------------------------+
| |
| Client Server |
| |
| --------- HTTP GET ----------------> |
| <-------- redirect to q4s ---------- |
| |
| ------- Q4S BEGIN ----------------> |
| |
| (Handshake Phase) |
| (Negotiation Phase) |
| |
| <---- Q4S 200 OK with trigger URI-- |
| |
| --------- HTTP GET ----------------> |
| |
| (Application starts) |
| |
+------------------------------------------------+
Figure 11 Trigger the application using HTTP URI
Figure 12 shows a usage example; an integration of HTTP and Q4S is
shown. First, the client contacts the server using HTTP, a
redirection to a Q4S URI is achieved and the User Agent starts the
Q4S handshake phase. After negotiation phase succeeds, the client
trigger the application using the URI indicated in the Q4S 200 OK
message.
4.5.3. Constraints not reached
After a measurement period the client sends the measured parameters
in the SDP body of a GET request to the server.
If there is any parameter that does not comply with the uplink or
downlink quality constraints required, the server MUST answer the
client's GET request with a 412 message (a precondition setting
required by the client or server has not been met) indicating in the
method type the parameter or parameters that violate the
constraints. This message MUST contain an SDP body with all data,
the client's and the server's parameter measurements.
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The 412 message MUST include two additional headers: Cause and
Signature headers. The Cause: header includes information about the
direction and the parameter that did not meet the constraints. The
Signature header contains the signed hash value of the SDP body in
order to protect all the SDP the data.
Server's 412 Answer:
=========================
Q4S/1.0 412 downlink_bandwidth
Date: Mon, 10 Jun 2010 10:00:01 GMT
Content-Type: application/sdp
Expires: 3000
Cause:downlink_bandwidth
Signature: 6ec1ba40e2adf2d783de530ae254acd4f3477ac4
Content-Length: 131
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=public-address:client IP4 192.0.2.33
a=public-address:server IP4 198.51.100.58
a=latency:40/35
a=jitter:10/10
a=bandwidth:20/6000
a=packetloss:5/5
a=flow:data downlink TCP/10000-20000
a=flow:control downlink UDP/55000
a=flow:control downlink TCP/55001
a=flow:data uplink TCP/56000
a=flow:control uplink UDP/56000
a=flow:control uplink TCP/56001
a=measurement:procedure default,50/50,50/50,5000,0
a=measurement:latency 30/30
a=measurement:jitter 6/4
a=measurement:bandwidth 200/4000
a=measurement:packetloss 2/3
=========================
In the body of the 412 message, the server MAY also rise the "qos-
level" SDP session-level attribute of the affected direction (uplink
or downlink). The maximum qos-level allowed is 9, both uplink and
downlink. If this value is reached without meeting the constraints,
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the Q4S protocol initiates the Termination phase, quality session is
aborted using the CANCEL method.
After a Q4S 412 message the client MUST send a QOS-ALERT request to
acknowledge the SLA violation (using TCP control port). Notice that
the server's signature header is present in the client QOS-ALERT, in
order to allow integrity validation. An example of a QOS-ALERT
message follows.
Client Request:
=========================
QOS-ALERT q4s://www.example.com Q4S/1.0
Host: www.example.com
User-Agent: q4s-ua-experimental-1.0
Signature: 6ec1ba40e2adf2d783de530ae254acd4f3477ac4
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=public-address:client IP4 192.0.2.33
a=public-address:server IP4 198.51.100.58
a=latency:40/35
a=jitter:10/10
a=bandwidth:20/6000
a=packetloss:5/5
a=flow:data downlink TCP/10000-20000
a=flow:control downlink UDP/55000
a=flow:control downlink TCP/55001
a=flow:data uplink TCP/56000
a=flow:control uplink UDP/56000
a=flow:control uplink TCP/56001
a=measurement:procedure default,50/50,50/50,5000,0
a=measurement:latency 30/30
a=measurement:jitter 6/4
a=measurement:bandwidth 200/4000
a=measurement:packetloss 2/3
=========================
If during the handshake phase the optional header Q4S-policy-server
is included in the server response, the QOS-ALERT message MUST be
sent to the policy server, otherwise, the client will send this
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message to the server directly. The scenario with an existing policy
server will be explained in 4.6.3.1.
Upon receiving the QOS-ALERT request from the client, the server
will acknowledge the alert issuing another QOS-ALERT request towards
the client. This message MUST include a new header: "Guard-time"
shown in the example below.
Server Answer:
=========================
QOS-ALERT q4s://www.example.com Q4S/1.0
Date: Mon, 10 Jun 2010 10:00:01 GMT
Content-Type: application/sdp
Expires: 3000
Cause: latency
Guard-time: 5000
Signature: 6ec1ba40e2adf2d783de530ae254acd4f3477ac4
Content-Length: 131
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=public-address:client IP4 192.0.2.33
a=public-address:server IP4 198.51.100.58
a=latency:40/35
a=jitter:10/10
a=bandwidth:20/6000
a=packetloss:5/5
a=flow:data downlink TCP/10000-20000
a=flow:control downlink UDP/55000
a=flow:control downlink TCP/55001
a=flow:data uplink TCP/56000
a=flow:control uplink UDP/56000
a=flow:control uplink TCP/56001
a=measurement:procedure default,50/50,50/50,5000,0
a=measurement:latency 30/30
a=measurement:jitter 6/4
a=measurement:bandwidth 200/4000
a=measurement:packetloss 2/3
=========================
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At this point, both client and server wait for "Guard-Time" seconds
as specified by the header before attempting to re-initiate the
measurements that caused the alert. This time SHOULD be set such
that enough time has been given to allow the server to take any
actions notifying the application or allowing for the network to
recover. (5 seconds should be enough, but this depends on each
case). After "Guard-Time" seconds the measurement process MUST be
initiated by the client sending a READY request.
If the client does not obey the "Guard-time", and sends a READY
message before this time elapses, the server MUST wait and not
answer the READY message until the guard time has elapsed.
If during the measurement process some interference disturbs or
affects the measurement results, it is better to repeat the process
again rather than alerting of an SLA violation. This is always
possible by sending current values of parameter "qos-level" without
changes, and in this case a header Guard-time can be set to "0". It
is a good practice to repeat the measurements before reporting a
violation.
An example of a message exchange with several guard-times is shown
below.
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+------------------------------------------------+
| |
| Client Server |
| |
| <-------- (DATA packets) ------------> |
| ... |
| --------- Q4S GET ----------------> |
| <-------- Q4S 412 ----------------- |
| ---- Q4S QOS-ALERT ---------------> |
| <--- Q4S QOS-ALERT ---------------- |
| (guard-time) |
| --------- Q4S READY --------------> |
| <-------- Q4S 200 OK -------------- |
| <-------- (DATA packets) ------------> |
| ... |
| --------- Q4S GET ----------------> |
| <-------- Q4S 412 ----------------- |
| ---- Q4S QOS-ALERT ---------------> |
| <--- Q4S QOS-ALERT ---------------- |
| (guard time) |
| --------- Q4S READY---------------> |
| <-------- Q4S 200 OK -------------- |
| <-------- (DATA packets) ------------> |
| ... |
| --------- Q4S GET ----------------> |
| <-------- Q4S 200 OK--------------- |
| |
| |
| |
+------------------------------------------------+
Figure 12 Several measurements with alerts and final success.
4.5.3.1. Policy server is present
If during handshake phase the optional header Q4S-Policy-Server is
included in the server response, the QOS-ALERT request MUST be sent
to the policy server, which can implement all or some of these
features (but not exclusive to):
o Client and server validation in terms of SLA.
o Authentication (Signature validation) and security (block
malicious clients)
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o Policy rules (following rules are only examples):
- Maximum quality level allowed for the ACP
- Time bands allowed for provide quality sessions for the ACP
- Number of simultaneous quality sessions allowed
- Maximum time used by quality sessions allowed
- Etc.
With policy server, the QOS-ALERT message sent by the client MUST
contain the URIs of the server and the client to be contacted later
by the policy server. Therefore the following headers MUST be
included in the client request: "Q4S-Resource-server" and
"Q4S-Resource-client"
Depending on the results of the operations achieved by the policy
server, the client could receive different types of errors or CANCEL
messages.
The flows of messages in this case are in the following figure:
+------------------------------------------------+
| |
| Client Policy Server |
| Server |
| |
| --- QOS-ALERT -----> |
| <-- 100 trying ----- |
| |
| ---- QOS-ALERT ----> |
| <--- QOS-ALERT ----- |
| <--- QOS-ALERT ----- |
| |
+------------------------------------------------+
Figure 13 Policy server.
If the validation or authentication of the QOS-ALERT operation
fails, the policy server will send a CANCEL request to the client
without contacting the server.
If any of the policy rules fail, the server will send a 6XX error to
the client, indicating the rule that is not satisfied.
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Only if the validation, authentication and policy checking are
successful, the server is contacted by the policy server and the
QOS-ALERT message is forwarded to it.
4.5.4. QoS Level changes
If any constraint was violated, the server may raise by one the qos-
level attribute of the 412 message sent to the client. 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 receipt 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.
Client Request:
=========================
CANCEL q4s://www.example.com Q4S/1.0
Host: www.example.com
User-Agent: q4s-ua-experimental-1.0
Signature: 6ec1ba40e2adf2d783de530ae254acd4f3477ac4
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
Date: Mon, 10 Jun 2010 10:00:01 GMT
Expires: 0
Content-Type: application/sdp
Signature: 6ec1ba40e2adf2d783de530ae254acd4f3477ac4
Content-Length: 131
(SDP not shown)
=========================
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+------------------------------------------------+
| |
| Client Server |
| |
| <-------- (measurements) ------------> |
| |
| --------- Q4S GET ----------------> |
| <-------- Q4S 412 ----------------- |
| ---- Q4S QOS-ALERT ---------------> |
| <--- Q4S QOS-ALERT --------------- |
| --------- Q4S READY --------------> |
| <-------- Q4S 200 OK -------------- |
| |
| <-------- (measurements) ------------> |
| |
| --------- Q4S GET ----------------> |
| <-------- Q4S 412 ----------------- |
| --------- Q4S CANCEL -------------> |
| <-------- Q4S CANCEL -------------- |
| |
| |
+------------------------------------------------+
Figure 14 Failed negotiation phase.
4.5.4.1. QoS Level increments without changes in network behaviour
If the qos-level has not reached the maximum value (9) but after 3
QOS-ALERT messages (with increments in qos-level) the network
remains with the same quality values, the client and the server MUST
assume that the network can not reach the desired quality and abort
the session in order to save resources (time and traffic). To do
that, the client MUST send a CANCEL request and the server MUST
react to it sending a CANCEL request too.
If the client does not send a CANCEL request but a request using a
different method, the server MUST react to it sending a CANCEL
request.
4.6. Continuity phase
During the negotiation phase, latency, jitter, bandwidth and packet
loss have been measured. During continuity phase bandwidth will not
be measured again because bandwidth measurements may disturb
application performance.
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This phase is supposed to be executed at the same time as the real
time application is being used.
In the default measurement procedure, two working modes are defined
for this phase: normal and sliding window modes. This draft only
covers the default procedure.
4.6.1.1. Normal mode
The server can force the use of normal mode by setting the fourth
parameter of "procedure" SDP attribute to 0. If this parameter is
not set, the default value is assumed (zero), and normal mode will
be used.
Example:
a=measurement:procedure default,50/50,50/50,5000,0
Considering that network conditions can change, the client may
periodically check network conditions against negotiated
constraints. The maximum interval expected between network testing
is indicated in the Q4S Expires header.
However, the measurements can be carried out periodically in a
smaller period of time than "Expires" header value. Intense
interactive applications, like arcade videogames, the period to
repeat the measurements may be very small (even zero), in order to
measure continuously the quality and assure the best reaction time.
To reach the best reaction time, the use of the sliding window mode
is recommended.
To start the continuity phase, the client sends a Q4S READY method,
using the TCP control port specified in the handshake phase,
indicating the new Stage header value for continuity phase (value
2).
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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 Response:
=========================
Q4S/1.0 200 OK
Session-Id: 53655765
Stage: 2
Content-Length: 0
=========================
After these messages are exchanged, latency, jitter and packet loss
measurement are started. The measurement procedure is identical to
the one carried out in the negotiation phase and is explained in the
Measurements section 3.2, except for the time elapsed between
salmples. If the default measurement method is being used, it is
recommended to use a larger interval for PING messages than the one
used in the negotiation phase, but the same number of samples will
be taken to check quality. The goal of incrementing the interval of
PING messages is to minimize the load of the server, which would be
running lots of connections in parallel.
The interval used for this phase is indicated in the second
parameter of the attribute line for the procedure. In this example,
the interval is 75 milliseconds.
a=measurement:procedure default,50/50,75/75,5000,0
A value larger than the one used in the negotiation phase is
recommended.
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+------------------------------------------------+
| |
| Client Server |
| |
| |
| --------- Q4S READY -----------> |
| <-------- Q4S 200 OK ----------- |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| --------- Q4S PING ------------> |
| <-------- Q4S PING ------------- |
| --------- Q4S 200 OK ----------> |
| <-------- Q4S 200 OK ----------- |
| ... |
| --------- Q4S GET -------------> |
| <-------- Q4S 412 -------------- |
| --------- Q4S QOS-ALERT -------> |
| <-------- Q4S QOS-ALERT -------- |
| (delay) |
| --------- Q4S READY -----------> |
| <-------- Q4S 200 OK ----------- |
| --------- Q4S PING ------------> |
| <-------- Q4S 200 OK ----------- |
| <-------- Q4S PING ------------- |
| -------- Q4S 200 OK ----------> |
| --------- Q4S PING ------------> |
| <-------- Q4S PING ------------- |
| --------- Q4S 200 OK ----------> |
| <-------- Q4S 200 OK ----------- |
| ... |
| --------- Q4S GET -------------> |
| <-------- Q4S 200 OK ----------- |
| |
+------------------------------------------------+
Figure 15 Continuity.
4.6.1.2. Sliding window mode
In order to improve the reaction time when network conditions
degrade quickly, the server can force the use of the sliding window
mode by setting the fourth parameter of the "procedure" SDP
attribute to 1.
Example:
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a=measurement:procedure default,50/50,50/50,5000,1
The sliding window mode applies a sliding window of samples instead
cycles of samples. The number of samples involved in the sliding
window may be different for jitter and latency calculations than for
packet-loss calculations according to the fifth and sixth parameters
of the measurement attribute. In this example, the jitter and
latency sliding window comprises 40 samples whereas the size of the
packet-loss sliding window is 100 samples:
a=measurement:procedure default,50/50,75/75,5000,1,40/40,100/100
In addition, the sizes of these windows are configurable per
direction.
In the sliding window mode, PING requests are sent continuously (in
both directions) and when the Sequence-Number header reaches the
value of 255, the client MUST NOT send a GET message for
instructions, but continues sending PING messages with the Sequence-
Number header starting again at zero. When the server PING Sequence-
Number header reaches 255, 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 16 Sliding samples window
Only when the client detects that the measured values (downlink
jitter, downlink packet loss and latency) are not reaching the
constraints, a GET request is sent to the server.
When the server receives the GET request, it stops sending PING
requests and answers the GET request just received. If the response
code is 412, then a QOS-ALERT will be requested by the client,
exactly in the same way as described in normal mode.
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On the other hand, if the server detects that the measured values
(uplink jitter, uplink packet loss and latency) are not reaching the
constraints, it MUST choose between the following alternatives:
o The server stops sending PING request to the client. In this
case the client MUST notice this lack of PING requests using a
timeout (Expire header) at reception. If so, the client reacts
stopping the PING requests to the server and sending a GET
request for instructions, exactly in the same way as described
in normal mode.
o It continues sending PING requests but all of them with
Sequence-Number set to -1 till a client GET request is
received. Then the server stops sending PING messages and
answers the GET request with the corresponding 412 error,
exactly in the same way as described in normal mode. The client
reacts sending this GET request when it receives a PING request
with Sequence-Number header set to -1. This behavior allows the
shortest reaction time under degraded network conditions.
Both alternatives MUST be implemented by the Q4S client.
In Sliding-window mode, the optional header "measurements" of the
PING message may be quite useful, because it allows the server to
obtain all measurements without needing to receive a GET message
from client. In this mode, the server may raise alerts directly,
independently of which direction didn't meet a quality constraint,
and send them to a policy server without client intervention.
To avoid further alerts from the client, the SDP message sent by
server to client may have high thresholds. The following diagram
represents this scenario
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+------------------------------------------------+
| |
| Client Server Policy |
| Server |
| |
| --- PING ----------> |
| <-- 200 OK---------- |
| <----- PING -------- |
| --- 200 OK --------> ---- QOS-ALERT ----> |
| --- PING ----------> <--- QOS-ALERT ----- |
| ... |
| |
+------------------------------------------------+
Figure 17 Direct Alert sending by the server
4.7. Termination Phase
The Termination phase is not a phase itself but an end point for the
established Q4S session. This phase 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 application has stopped
running and alerts the server about its termination so that the
server can terminate the Q4S session.
. 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.
4.8. Dynamic constraints and flows
Depending on the nature of the application, the constraints to be
reached may evolve, changing some or all constraint values in any
direction.
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The client MUST be able to deal with this possibility. When the
server sends an SDP document attached to a reply (200 OK, or 412,
etc), the client MUST assume all the new received values, overriding
any previous value in use.
The dynamic changes on the constraints can be as a result of two
possibilities:
o The application communicates to the Q4S a change in the
constraints. In this case the application requirements can
evolve and the Q4S server will be aware of them.
o The application uses TCP flows. In that case, in order to
guarantee a constant throughput, the nature of TCP behavior
forces the use of a composite constraint function, which
depends on RTT, packet loss and window control mechanism
implemented in each TCP stack.
TCP throughput can be less than actual bandwidth if the
Bandwidth-Delay Product (BDP) is large or if the network suffers
from a high packet loss rate. In both cases, TCP congestion control
algorithms may result in a suboptimal performance.
Different TCP congestion control implementations like Reno [14],
High Speed TCP (RFC 3649 [15]), CUBIC [16], Compound TCP (CTCP
[17]), etc. reach different throughputs under the same network
conditions of RTT and packet loss. In all cases, depending on the
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 viceversa (use packetloss measurement
to change dynamically latency constraints).
A general guideline to calculate the packetloss constraint and RTT
constraint consists in approximating the throughput using a
simplified formula, which should take into account the TCP stack
implementation of the receiver, in addition to RTT and packet loss:
Th= Function( RTT, packet loss, ...)
Then, depending on RTT measured values, set dynamically the
packetloss constraint.
It is possible to easily calculate a worst-case boundary for the
Reno algorithm which should ensure for all algorithms that the
target throughput is actually achieved. Except that, high-speed
algorithms will then have even a larger throughput, if more
bandwidth is available.
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For the Reno algorithm, the Mathis' formula may be used [15] for the
upper bound on the throughput:
Th <= (MSS/RTT)*(1 / sqrt{p})
In absence of packet loss, a practical limit for the TCP throughput
is the receiver_window_size divided by the round-trip time. However,
if the TCP implementation uses a window scale option, this limit can
reach the available bandwidth value.
4.9. Qos-level downgrade operation
During the continuity phase it might be desirable to downgrade the
current qos-level SDP parameter.
The strategy to carry out downgrades must include the possibility to
exclude specific data flows from SDP dynamically. A Q4S client MUST
allow this kind of SDP modifications by server.
Periodically (every several minutes, depending on the
implementation) the server could force a QOS-ALERT, in which the
level is downgraded for control flows, excluding application data
flows from the embedded SDP of that request. To set the new SDP, the
server MUST include the modified SDP in the 412 error message.
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 constraints,
then a new QOS-ALERT to this lower qos-level can be forced by
the server, in which the SDP includes the application data
flows in addition to control flows.
o If the measurements in the lower level do not meet the
constraints, then a new QOS-ALERT to the previous qos-level
MUST be forced by the server, in which the SDP includes only
the control flows.
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+------------------------------------------------+
| |
| qos-level |
| A |
| | |
| 4| |
| | |
| 3| +------+ |
| | | | |
| 2| +----+ +----+ +--- |
| | | | | |
| 1| +----+ +-----+ |
| | | |
| 0+---+---------------------------------> time |
| |
+------------------------------------------------+
Figure 18 Possible evolution of qos-level
This mechanism avoids the risk of disturbing the application, while
the measurements are being run in lower levels. However, this
optimization of resources is optional, and MUST be used carefully.
The chosen period to measure a lower qos level is implementation
dependant. Therefore it is not included as a measurement procedure
parameter. It is recommended to use a large value, such as 20
minutes.
4.10. Sanity check of Quality sessions
A session may finish by 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 that a
client can wait to repeat the continuity phase (in normal mode).
5. Q4S messages
Q4S is a text-based protocol and uses the UTF-8 charset (RFC 3629
[11]). A Q4S message is either a request or a response.
Both Request and Response messages use the basic format of Internet
Message Format (RFC 5322 [12]). Both types of messages consist of a
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start-line, one or more header fields, an empty line indicating the
end of the header fields, and an optional message-body.
generic-message = start-line
*message-header
CRLF
[ message-body ]
start-line = Request-Line / Status-Line
The start-line, each message-header line, and the empty line MUST be
terminated by a carriage-return line-feed sequence (CRLF). Note
that the empty line MUST be present even if the message-body is not.
Much of Q4S's messages and header field syntax are identical to
HTTP/1.1. However, Q4S is not an extension of HTTP.
5.1. 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
Method: This specification defines five methods: GET for getting
information and sending quality reports, PING and DATA for
quality measurements purpose, CANCEL for terminating
sessions, and QOS-ALERT for querying ISPs for quality
upgrades.
Request-URI: The Request-URI is a Q4S URI (RFC 2396) as described in
2.2.1 It Normally indicates the user or service to which this
request is being addressed to, but in the Q4S context, there
are some methods whose URI only reflects the service on the
server side, but nothing more. This is the case of the QOS-
ALERT method, because the real address of a QoS upgrade
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request is the network, and therefore in this case the URI
only reflects the server address. In addition the CANCEL
method has the same treatment, and in the ECHO and DATA
methods invoked by the server to the client the meaning of
the URI is only the URI of the service, but not the
destination of the request. 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.
5.2. 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 2616 [1]). In
the same way as HTTP, Q4S applications are not required to
understand the meaning of all registered status codes, though such
understanding is obviously desirable. However, applications MUST
understand the class of any status code, as indicated by the first
digit, and treat any unrecognized response as being equivalent to
the x00 status code of that class.
The Q4S-ALERT and CANCEL requests do not have to be responded.
5.3. 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.
5.3.1. Specific Q4S Request Header Fields
In addition to HTTP header fields, these are the specific Q4S
request header fields
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o Session-Id: the value for this header is the same session id
used in SDP 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 integer number assigned to PING and
DATA messages.
o Timestamp: this optional header contains the system time (with
the best possible accuracy). Indicates the time in which the
request was sent.
o Signature: this header contains a digital signature that can be
used by the network to validate the SDP. The signature is
always generated by the server. It is optional.
o Q4S-Resource-Client: this optional header contains the relative
URI in charge of this session at the client side. In The case
of being included, it MUST appear in the GET request of
handshake phase. This URI MUST be invoked by the server in all
later requests. It is optional, but it should be present, it
becomes mandatory for the counterpart. This URI MUST be
relative because user agents can not have associated domain, in
addition to ignore their public IP address.
5.3.2. Specific Q4S Response Header Fields
o Expires: the 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 can close
the session. The value MUST be an integer and the measurement
unit are milliseconds.
o Guard-time: A time interval in milliseconds left vacant (i.e.,
during which no data is sent) during the quality session. The
guard time provides a safety margin before re-starting each
measurement process when a QOS-ALERT has been raised. This
header is optional in all messages but mandatory in the QOS-
ALERT sent by the server.
o Sequence-Number: same meaning as Request Header Fields
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o Timestamp: UTC time in nanoseconds. Indicates the time in which
the request was sent. If the server (or a client) receives a
Timestamp header in a request, MUST include the same header
with the same value in the response. The purpose of this header
is simplify the RTT calculation.
o Signature: same meaning as Request Header Fields
o Q4S-Resource-Server: this optional header contains the URI in
charge of this session (Session URI). In case of being
included, it MUST appear in the response to the BEGIN request
of the handshake phase. This URI MUST be invoked by the client
in all later requests. It is optional, but if present, it
becomes mandatory for the counterpart.
o Q4S-Policy-Server: this optional header contains the URI
towards which the client and MUST send the QOS-ALERT messages
(Policy Server URI). In case this header is present, the Q4S-
Resource-Server header is mandatory, and MUST be included in
the QOS-ALERT messages sent by the client to the policy server.
In addition, the QOS-ALERT sent to the policy server MUST
contain the header Q4S-Resource-client
o Cause: This header field is a comma-separated list which
contains the cause(s) for which the connection constraints were
not reached after measurement process. Current defined values
are:
. Downlink_latency
. Uplink_latency
. Downlink_jitter
. Uplink_jitter
. Downlink_bw
. Uplink_bw
5.4. 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.
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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.
5.4.1. Encoding
The body MUST NOT be compressed. This mechanism is valid for
other protocols such as HTTP and SIP (RFC 3261 [13]), but
a compression/coding scheme will limit certain logical
implementations of the way the request is parsed, thus, making the
protocol concept more implementation dependant. In addition,
bandwidth calculation may not be valid if compression is used.
Therefore, the HTTP request header "Accept-Encoding" can not be used
in Q4S with different values than "identity" and if it is present in
a request, the server MUST ignore it. In addition, the response
header "Content-Encoding" is optional, but if present, the unique
permitted value is "identity".
The body length in bytes is provided by the Content-Length header
field. The "chunked" transfer encoding of HTTP/1.1 MUST NOT be used
for Q4S (Note: The chunked encoding modifies the body of a message
in order to transfer it as a series of chunks, each one with its own
size indicator.)
6. General User Agent behavior.
6.1. Roles
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.
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 19 P2P roles.
6.2. Multiple Quality sessions in parallel
A quality session is intended to be used for a single application
(or application instance). It means that for using the application,
the client MUST establish only one quality session against the
server. Indeed, the relation between Session-Id and application is 1
to 1.
If a user wants to raise several independent quality sessions
simultaneously against different servers (or against the same
server) it can execute multiple Q4S clients to establish separate
quality sessions. However, this is not recommended because:
o The establishment of a new quality session may affect other
running applications over other quality sessions. Thus, minimum
quality level may not be achieved depending on individual
requirements of each application.
o If the negotiation phase is executed separately before running
any application, the quality requirements could not be assured
when the applications are running in parallel.
o Flow identification (Protocol, SourceIP, Source Port +
Destination IP, Destination Port) must always be unique for
each application/application instance, to ensure that each one
of them is using their QoS constraints.
For running different applications in parallel it is highly
recommended to execute the negotiation phase of all of them
simultaneously, in order to assure the quality constraints of all
applications in parallel. To do that, a single User Agent MUST be
used, and this User Agent MUST be able to launch several quality
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session negotiations in parallel, synchronizing the beginning of
each negotiation phase, and running again the negotiation phase of
all applications in parallel until all of them succeed.
In order to repeat the execution of a negotiation phase that has
been succeeded, both, client and server MUST allow using the READY
method with a Stage header value already succeeded.
6.3. General client behavior
A Q4S Client has different behaviors. We will use letters X,Y,Z to
designate each different behavior (follow the letter bullets in the
figure below).
X) When it sends messages over TCP (methods GET, QOS-ALERT and
CANCEL) it behaves strictly like a state machine that sends
requests and waits for responses. Depending on the response type
it enters in a new state.
When it sends UDP messages (methods PING and DATA), 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 assumes the role
of a server.
Z) At bandwidth and packet loss measurement stage, the client does
not expect to receive responses when sending DATA requests to the
server. In addition, it MUST receive and process all server
messages in order to achieve the downlink measurement.
The QOS-ALERT and CANCEL do not need to be answered. However, these
methods may have a conventional answer if an error is produced.
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+-----------+------------------------+-----------+-----------+
| Handshake | Negotiation |Continuity |Termination|
| Phase | Phase | Phase | Phase |
| | | | |
| X ---------> Y --> X --> Z --> X ---> Y --> X ---> X |
| | A | A | | A | | |
| | | | | | | | | | |
| | +-----+ +-----+ | +-----+ | |
| | | | |
+------------------------------------------------+-----------+
Figure 20 Phases & client behaviors.
6.3.1. Generating requests
A valid Q4S request formulated by a Client MUST, at a minimum,
contain the following header fields:
If no SDP is included: This is the case of PING and DATA messages.
The header Session-Id and Sequence-Number are mandatory.
If SDP is included: this is the case of GET, QOS-ALERT and CANCEL
messages. Inside SDP is included Session-Id, therefore the inclusion
of Session-Id header is optional.
6.4. General server behavior
If a server does not understand a header field in a request (that
is, the header field is not defined in this specification or in
any supported extension), the server MUST ignore that header field
and continue processing the message.
The role of the server is changed at negotiation and continuity
phases, in which server MUST send packets to measure jitter, latency
and bandwidth. Therefore, the different behaviors of server are
(follow the letter bullets in the figure below):
R) When the client sends messages over TCP (methods GET, QOS-
ALERT and CANCEL) it behaves strictly like a state machine that
receives messages and sends responses.
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When the client begins to send UDP messages (methods PING and DATA),
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 as a client, sending PING
requests toward the client and receiving responses.
T) At bandwidth and packet loss measurement, the server sends
DATA requests to the client. In addition, it MUST receive and
process client messages in order to achieve the uplink
measurement.
The QOS-ALERT and CANCEL do not need to be answered. However, these
methods may have a conventional answer if an error is produced.
+-----------+------------------------+-----------+-----------+
| Handshake | Negotiation |Continuity |Termination|
| Phase | Phase | Phase | Phase |
| | | | |
| R ---------> S --> R --> T --> R ---> S --> R ---> R |
| | A | A | | A | | |
| | | | | | | | | | |
| | +-----+ +-----+ | +-----+ | |
| | | | |
+------------------------------------------------+-----------+
Figure 21 Phases & server behaviours.
7. 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.
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Method = "BEGIN" | "GET" | "READY" | "PING" | "DATA" |
"QOS-ALERT" | "CANCEL" | extension-method
extension-method = token
The list of methods allowed by a resource can be specified in an
"Allow" header field (RFC 2616 [1] section 14.7). The return code of
the response always notifies the client when a method is currently
allowed on a resource, since the set of allowed methods can change
dynamically. Any server application SHOULD return the status code
405 (Method Not Allowed) if the method is known, but not allowed for
the requested resource, and 501 (Not Implemented) if the method is
unrecognized or not implemented by the server.
7.1. BEGIN
The BEGIN method means request 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 all quality parameters for the desired application to
run.
When a BEGIN message is received by the server, any current quality
session is cancelled and a new session should be created.
The response to a Q4S BEGIN request is not cacheable.
7.2. GET
The GET method means retrieve information from a resource identified
by a Q4S URI.
In the negotiation and continuity phases, this method is used to
check if the server considers the quality good enough to execute the
desired application. If the measured quality is not enough, the
server will return a 412 error.
The response to a Q4S GET request is not cacheable.
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7.3. READY
The READY method is used to synchronize the starting time for
sending of PING and DATA messages over UDP between clients and
servers.
In addition, the Stage header included in this method is mandatory
and allows clients to repeat a test, which is needed in scenarios
with multiple quality sessions between one client and different
servers.
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.
7.4. 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 control port. If a server receives this message in
another port it MUST ignore it.
The fundamental difference between the PING and DATA requests is
reflected in the different measurements achieved with them. PING is
a short message, and MUST be answered in order to measure RTT,
whereas DATA is a long message (1 Kbyte) and MUST NOT be answered.
PING is a request method that can be originated by client but also
by server. Client MUST answer the server PINGs, assuming a "server
role" for these messages during measurement process.
7.5. DATA
This message is used only during the continuity phase to measure the
bandwidth and packet loss of a session. The message MUST be sent
only over UDP control port. If a server receives this message in
other port it MUST ignore it.
The fundamental difference between the PING and DATA requests is
reflected in the different measurements achieved with them. PING is
a short message, and MUST be answered in order to measure RTT,
whereas DATA is a long message (1 Kbyte) and MUST NOT be answered.
DATA is a request method that can be originated by the client but
also by server. Both (client and server) MUST NOT answer DATA
messages.
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7.6. QOS-ALERT
This is the request message that Q4S generates when the measurements
indicate that quality SLA is 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 and the SLA). The QOS-ALERT message does
not contain any detail on the actions to be taken, which depends on
the agreements between all involved parties.
A QOS-ALERT request does not have to be answered unless there is an
error condition. However, after receiving a QOS-ALERT request, the
server sends a QOS-ALERT request to the client.
This method can be initiated by the client only after a 412 error
coming from server, and with enough information to build the
QOS-ALERT message.
If the "Q4S-Policy-Server" header was included in the server
response of the handshake phase, the QOS-ALERT message MUST be sent
to the URI indicated in this header, otherwise the QOS-ALERT message
MUST be sent to the server.
With policy server, the QOS-ALERT message sent by the client MUST
contain the URIs of the server and the client to be contacted later
by the policy server. Therefore the following headers MUST be
included in the client request: "Q4S-Resource-Server" and "Q4S-
Resource-Client".
The response to a Q4S QOS-ALERT request is not cacheable.
7.7. CANCEL
Like QOS-ALERT, this message is used for communication with the
network resources. The semantics in this case is the release of the
special resources assigned to the session.
In the same way as QOS-ALERT, CANCEL does not need to be answered.
However, if the server receives a CANCEL message, it should send a
new CANCEL request towards the client acknowledging the reception.
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8. Response codes
Q4S response codes are used for TCP and UDP. However, in UDP only
the response code 200 is used.
8.1. 100 Trying
This response indicates that the request has been received by the
next-hop server (the policy 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 QOS-ALERT by the client.
8.2. 200 OK
The request has succeeded.
8.3. 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.
8.4. 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.
8.4.1. 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".
8.4.2. 404 Not Found
The server has definitive information that the user does not exist
at the domain specified in the Request-URI. This status is also
returned if the domain in the Request-URI does not match any of the
domains handled by the recipient of the request.
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8.4.3. 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.
8.4.4. 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.
8.4.5. 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
8.4.6. 412 A precondition has not been met
The server is indicating that the SLA is being violated.
8.4.7. 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.
8.4.8. 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.
8.4.9. 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.
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8.4.10. 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.
8.5. Server Failure 5xx
5xx responses are failure responses given when a server itself is
having trouble.
8.5.1. 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.
8.5.2. 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.
8.5.3. 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).
8.5.4. 504 Server Time-out
The server did not receive a timely response from an external server
it accessed in attempting to process the request.
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8.5.5. 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.
8.5.6. 513 Message Too Large
The server was unable to process the request since the message
length exceeded its capabilities.
8.6. Global Failures 6xx
6xx responses indicate that a server has definitive information
about a particular policy not satisfied for processing the request.
8.6.1. 600 session not exist
The Session-Id is not valid
8.6.2. 601 quality level not allowed
The QOS level requested is not allowed for the pair client/server
8.6.3. 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 QOS-
ALERT is not allowed for the pair client/server, etc)
8.6.4. 604 authorization not allowed
The policy server does not authorize the QOS-ALERT operation because
any internal or external reason.
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 browser settings
menu. Otherwise these quality headers will not be present in the
first message.
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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. Bandwidth measurements
In programming languages or Operating Systems with timers or limited
clock resolution, it is recommended to use an approach based on
several intervals to send messages of 1KB, in order to reach the
required bandwidth consumption using a rate as close as possible to
a constant rate.
For example, if the resolution is 1 millisecond, and the bandwidth
to reach is 11Mbps, a good approach consists of sending:
1 message of 1KB every 1 millisecond +
1 message of 1KB every 3 milliseconds +
1 message of 1KB every 23 milliseconds
The number of intervals depends on required bandwidth and accuracy
that the programmer wants to achieve.
9.3. Packet loss measurement resolution
Depending on application nature and network conditions, a packet
loss resolution less than 1% may be needed. In such case, there is
no limit to the number of samples used for this calculation. A
tradeoff between time and resolution should be reached in each case.
For example, in order to have a resolution of 1/10000, the last
10000 samples should be considered in the packetloss measured value.
The problem of this approach is the reliability of old samples. If
the interval used between PING messages is 50ms, then to have a
resolution of 1/1000 it takes 50 seconds and a resolution of 1/10000
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.
9.4. Measurements and reactions
Q4S can be used as a mechanism for measure and trigger actions (i.e.
lowering video bit-rate) in real-time in order to reach the
application constraints, addressing measured possible network
degradation.
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The trigger is based on message QOS-ALERT, which is always forced by
the server response 412 error. A server can avoid these QOS-REQUEST
messages sending 200 OK when a GET message is received from server,
independently whether the constraints are met or not.
9.5. Instability treatments
There are two scenarios in which Q4S can be affected by network
problems: loss of control packets and outlier samples
9.5.1. Loss of control packets
Lost UDP packets (PING or DATA messages) don't cause any problems
for the Q4S state machine, but if control packets like READY, 200
OK, 412 error, or GET messages are lost, some undesirable
consequences could arise.
Q4S does have protection mechanisms to overcome these situations.
Examples:
. If a READY packet is lost, after a certain timeout, the client
SHOULD resend another READY packet.
. If the server's expected 200 OK answer to the client's READY
message is lost, but PING packets begin to arrive, we assume
that the initial received PING packet is enough and the client
SHOULD start sending PING messages.
. If a GET packet is lost, the client will not receive any
response by the server. After a certain timeout, the client
SHOULD resend another GET message.
. If the 412 error message is lost, the server will receive a
resent GET message instead a QoS-REQUEST message.
9.5.2. Outlier samples
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Outlier samples are those jitter or latency values far from the
general/average values of most samples.
In order to get rid of some outlier samples, that due to a bad
spurious sample or an error on a measurement. The recommendation is
to implement a median one dimension filter. This is a very common
filtering for noise or errors on signal and image processing.
This is a very simple algorithm, where we keep a small window of
previous measurements. We recommend a small window of 5 to 10
values. The new value is to get the median from the sorted tuple of
window stored values.
This small window is not the same as the window used for calculating
the average latency or jitter, but a simple mechanism to add a new
(and more reliable) sample to the list.
9.6. Scenarios
Q4S could be used in two scenarios:
o client to ACP (Application content provider)
o client to client.
9.6.1. Client to ACP
In this scenario, the policy server is optional. If it exists, the
QOS-ALERT messages MUST be sent to this policy server which acts as
a proxy for this type of messages and validates them (plus any other
actions out of scope of this document).
In order to avoid useless load on the server, the policy server
could receive the BEGIN messages of handshake phase. For this
purpose, the policy server MUST know the URI of the Q4S servers.
In this scenario a client could send the BEGIN to the policy server,
with an additional parameter in the URI requested, which identifies
the server, like:
Q4s://www.policy.com/listofservers?id=xtiwn28821ho4
Then the Policy Server validates the request and forward the BEGIN
to the Q4S server, adding the Q4S-Resource-Server to the response
for the client in the 200 OK response.
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+------------------------------------------------+
| |
| Client Policy Server |
| Server |
| --- BEGIN ---------> |
| <-- 100 trying ----- |
| |
| --- BEGIN ----------> |
| <--- 200 OK ---------- |
| <--- 200 OK----- --- |
| |
+------------------------------------------------+
Figure 22 Policy server.
In this scenario the client MUST send further messages directly to
the server without passing through policy server.
There is a possible scenario in which the policy server is contacted
only by the Q4S server, enabled through the reception of the client
measurements in PING messages that include the "measurements"
header. In this case, the QOS-ALERT message does not cause an
interruption of the sliding-window during continuity phase.
+------------------------------------------------+
| |
| Client Server Policy |
| Server |
| |
| --- PING ----------> |
| <-- 200 OK---------- |
| <----- PING -------- |
| --- 200 OK --------> ---- QOS-ALERT ----> |
| --- PING ----------> <--- QOS-ALERT ----- |
| ... |
| |
+------------------------------------------------+
Figure 23 Alerts are sent by the server directly to the policy
server
9.6.2. 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
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and their public IP address to make possible the assumption of
server role.
The register function is out of scope of this protocol version,
because different HTTP mechanisms can be used and Q4S MUST NOT force
any.
10. Security Considerations
Different types of attacks can be avoided:
o Spoofing of server IP address can be avoided using the
digital signature mechanism. The network can easily
validate this digital signature using the public key of the
server certificate.
o The client could try to send QOS-ALERT requests constantly,
trying to enter in the negotiation phase continuously. In
this case, the server MUST answer a message "CANCEL", in
order to release the all levels reached and return to plain
access without enhanced quality.
This protocol could be supported over IPSec to increase privacy,
although it is out of scope of this proposal.
11. IANA Considerations
A specific port for Q4S TCP control flow mechanism could be
assigned. It could simplify the network implementation. Other
possibility is to use any other port (like 80, HTTP). In this case
the network could use the protocol designator "Q4S" as the mark for
distinguish and treat the packets.
Q4S uses SDP as a container for session information, in which
quality attributes have been added as extended "session-level"
attributes. These set of new attributes should be registered (in
order to avoid the prefix "X-"). In this document, this set of
attributes has been presented as registered attributes.
This is the list of attribute field names to register:
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Attribute name: qos-level
Type of attribute: session level
Subject to the charset attribute: NO
Explanation of purpose: defines the current QoS profile in uplink
and downlink for the communication between client and server. The
exact meaning of each level is implementation dependant but in
general, a higher qos-level value corresponds to a better quality
network profile.
Appropriate attribute values: [0..9] "/" [0..9]
Attribute name: public-address
Type of attribute: session level
Subject to the charset attribute: NO
Explanation of purpose: contains the public IP address of the client
or the server.
Appropriate attribute values:<"client"|"server"><"IP4"|"IP6"> <value
of IP address>
Attribute name: latency
Type of attribute: session level
Subject to the charset attribute: NO
Explanation of purpose: defines the latency constraints in
milliseconds in uplink and downlink for the communication between
client and server. Appropriate attribute values: [0..9999] "/"
[0..9999]
If there is no constraint in some direction (uplink, downlink or
both) the value can be empty in that direction
Attribute name: jitter
Type of attribute: session level
Subject to the charset attribute: NO
Explanation of purpose: defines the jitter constraints in
milliseconds in uplink and downlink for the communication between
client and server.
Appropriate attribute values: [0..9999] "/" [0..9999]
Attribute name: bandwidth
Type of attribute: session level
Subject to the charset attribute: NO
Explanation of purpose: define the bandwidth constraints in kbps in
uplink and downlink for the communication between client and server.
Appropriate attribute values: [0..99999] "/" [0..99999]
Attribute name: packetloss
Type of attribute: session level
Subject to the charset attribute: NO
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Explanation of purpose: define the packet loss tolerance constraints
in 100% in uplink and downlink for the communication between client
and server.
Appropriate attribute values: [0..99] "/" [0..99]
Attribute name: flow
Type of attribute: session level
Subject to the charset attribute: NO
Explanation of purpose: define a flow between a client and a server.
The flow involves purpose (data or control), direction (uplink or
downlink) protocol (UDP or TCP) and port or range or ports
Attribute values:
<"control"|"data"> <"uplink"|"downlink"> <"UDP"|"TCP">
<0..65535>[ "-" [0..65535]]
Attribute name: measurement
Type of attribute: session level
Subject to the charset attribute: NO
Explanation of purpose: define the procedure to measure the quality
and the different values for each measurement
Attribute values: "procedure/" <procedure> |
"latency "[0..9999] "/" [0..9999] |
"jitter "[0..9999] "/" [0..9999] |
"bandwidth "[0..99999] "/" [0..99999] |
"packetloss "[0..255] "/" [0..255]
If the attribute value is "procedure", the rest of the line MUST
contain the name of the procedure and optional parameters, separated
by ",".
In the case of procedure "default", the valid values are:
a=measurement:procedure default,[0..999]"/" [0..999] "," [0..999]
"/" [0..999] "," [0..9999] "," [0|1 "," [0..999]/[0..999] ","
[0..255]/[0..255]]
where:
o The first parameter is the interval of time (in milliseconds)
between PING messages in the negotiation phase. Forward (client
to server) and reverse (server to client) values separated by
"/".
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o The second parameter is the interval of time (in milliseconds)
between PING messages in the continuity phase. Forward (client
to server) and reverse (server to client) values separated by
"/".
o The third parameter is the time used to measure bandwidth
during negotiation phase. In case of not present, a default
value of 5000 ms will be assumed.
o The fourth parameter indicates the mode for continuity phase (0
means "normal" and 1 means "sliding window"). In case of not be
present, normal mode (default value of 0) will be assumed.
o The fifth parameter is only applicable in sliding window mode.
It indicates the window size for the jitter and latency
calculation on both forward and reverse directions. If not
present, a value of 256 MUST be assumed.
o The sixth parameter is only applicable in sliding window mode.
It indicates the window size for packet loss calculations on
both forward and reverse directions. If not present, a value of
256 MUST be assumed.
Other procedure names are allowed, but at least "default" procedure
implementation is mandatory in client and servers.
12. Conclusions
Q4S defines four phases with different purposes, and inside these
phases the negotiated measurement procedure is used. Different
measurement procedures can be used (even RTCP itself) inside Q4S.
Basically, Q4S only defines how to transport SLA information and
measurement results as well as providing some mechanisms for
alerting. Q4S does not ask for resources. Q4S only alerts if one (or
some) of SLA quality parameters are being violated. Depends on
server (Application content provider) to do something with this
information and return it back to a SLA-compliant state.
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13. References
13.1. Normative References
[1] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,Masinter, L.,
Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1" RFC 2616, June 1999.
[2] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 4566, July 2006.
[3] Bradner, S., "Key words for use in RFCs to Indicate
RequirementLevels", BCP 14, RFC 2119, March 1997.
[4] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 3986, January
2005.
[5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
SDP", RFC 3264, June 2002.
[6] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[7] Johnsson, J., B. Kaliski, "Public-Key Cryptography Standards
(PCS) #1: RSA Cryptography Specifications version 2.1", RFC
3447, February 2003.
[8] Postel, J., "DoD Standard Transmission Control Protocol", RFC
761, January 1980.
[9] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
1980.
[10] Schulzrinne, H., Casner, S., Frederick, R., Jacobson, V. "RTP:
A Transport Protocol for Real-Time Applications", RFC 3550,
july 2003.
[11] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
RFC 3629, November 2003.
[12] Resnick, P., "Internet Message Format", RFC 5322, October 2008
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13.2. Informative References
[13] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.
Peterson, J., Sparks, R., Handley, M. and Schooler, E. , "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[14] Mathis, M., Semke, J., Mahdavi, J., Ott, T., "The Macroscopic
Behavior of the TCP Congestion Avoidance Algorithm", Computer
Communications Review, 27(3), July 1997.
[15] Floyd, S., "HighSpeed TCP for a Large Congestion Windows", RFC
3649, December 2003.
[16] Rhee, I., Xu, L., Ha, S., "CUBIC for Fast Long-Distance
Networks", Internet-draft draft-rhee-tcpm-cubic-02, February
2009.
[17] Sridharan, M., Tan, K., Bansal, D., Thaler, D., "Compound TCP:
A New TCP Congestion Control for High-Speed and Long Distance
Networks", Internet-draft draft-sridharan-tcpm-ctcp-02,
November, 2008.
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14. Acknowledgments
Many people have made comments and suggestions contributing to this
document. In particular, we would like to thank:
Sonia Herranz Pablo, Clara Cubillo Pastor, Francisco Duran Pina,
Ignacio Moreno Lopez, Michael Scharf, Jesus Soto Viso and Federico
Guillen.
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15. Authors' Addresses
Jose Javier Garcia Aranda
Alcatel-Lucent
C/Maria Tubau 9
28050 Madrid
Spain
Phone: +34 91 330 4348
Email: Jose_Javier.Garcia_Aranda@alcatel-lucent.com
Jacobo Perez Lajo
Alcatel-Lucent
C/Maria Tubau 9
28050 Madrid
Spain
Phone: +34 91 330 4165
Email: jacobo.perez@alcatel-lucent.com
Luis Miguel Diaz Vizcaino
Alcatel-Lucent
C/Maria Tubau 9
28050 Madrid
Spain
Phone: +34 91 330 4871
Email: Luismi.Diaz@alcatel-lucent.com
Carlos Barcenilla
Universidad Politecnica de Madrid
Avenida Complutense 30
28040 Madrid
Spain
Phone: +34 91 549 5700 - 3032
Email: barcenilla@dit.upm.es
Monica Cortes
Universidad Politecnica de Madrid
Avenida Complutense 30
28040 Madrid
Spain
Phone: +34 91 336 5700 - 3044
Email: cortesm@dit.upm.es
Joaquin Salvachua
Universidad Politecnica de Madrid
Avenida Complutense 30
28040 Madrid
Spain
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Phone: +34 91 549 5700 - 3056
Email: jsr@dit.upm.es
Juan Quemada
Universidad Politecnica de Madrid
Avenida Complutense 30
28040 Madrid
Spain
Phone: +34 91 336 7331
Email: jquemada@dit.upm.es
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