One document matched: draft-ietf-tsvwg-rtcweb-qos-14.xml
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<rfc category="std" docName="draft-ietf-tsvwg-rtcweb-qos-14"
ipr="trust200902">
<front>
<title abbrev="WebRTC QoS">
DSCP and other packet markings for WebRTC QoS
</title>
<author fullname="Paul E. Jones" initials="P." surname="Jones">
<organization>Cisco Systems</organization>
<address>
<email>paulej@packetizer.com</email>
</address>
</author>
<author fullname="Subha Dhesikan" initials="S." surname="Dhesikan">
<organization>Cisco Systems</organization>
<address>
<email>sdhesika@cisco.com</email>
</address>
</author>
<author fullname="Cullen Jennings" initials="C."
surname="Jennings">
<organization>Cisco Systems</organization>
<address>
<email>fluffy@cisco.com</email>
</address>
</author>
<author fullname="Dan Druta" initials="D." surname="Druta">
<organization>AT&T</organization>
<address>
<email>dd5826@att.com</email>
</address>
</author>
<date/>
<abstract>
<t>
Many networks, such as service provider and enterprise networks,
can provide different forwarding treatments for individual
packets based on Differentiated Services Code Point (DSCP)
values on a per-hop basis. This document provides the
recommended DSCP values for web browsers to use for various
classes of WebRTC traffic.
</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>
Differentiated Services Code Point (DSCP) <xref target="RFC2474"/>
packet marking can help provide QoS in some environments.
This specification provides default packet marking for browsers
that support WebRTC applications, but does not change any advice
or requirements in existing IETF RFCs. The contents of this
specification are intended to be a simple set of implementation
recommendations based on the previous RFCs.
</t>
<t>
There are many use cases where such marking does not help, but it
seldom makes things worse if packets are marked appropriately.
There are some environments where DSCP markings frequently help,
though. These include:
</t>
<t>
<list style="numbers">
<t>
Private, wide-area networks.
</t>
<t>
Residential Networks. If the congested link is the
broadband uplink in a cable or DSL scenario, often
residential routers/NAT support preferential treatment based
on DSCP.
</t>
<t>
Wireless Networks. If the congested link is a local
wireless network, marking may help.
</t>
</list>
</t>
<t>
DSCP values are in principle site specific, with each site
selecting its own code points for controlling per-hop-behavior
to influence the QoS for transport-layer flows. However in the
WebRTC use cases, the browsers need to set them to something
when there is no site specific information. In this document,
"browsers" is used synonymously with "Interactive User Agent" as
defined in the HTML specification, <xref
target="W3C.REC-html5-20141028"/>. This document describes a
subset of DSCP code point values drawn from existing RFCs and
common usage for use with WebRTC applications. These code
points are solely defaults.
</t>
<t>
This specification defines inputs that are provided by the
WebRTC application hosted in the browser that aid the browser in
determining how to set the various packet markings. The
specification also defines the mapping from abstract QoS
policies (flow type, priority level) to those packet markings.
</t>
</section>
<section title="Relation to Other Specifications">
<t>
This document is a complement to <xref target="RFC7657"/>, which
describes the interaction between DSCP and real-time
communications. That RFC covers the implications of using
various DSCP values, particularly focusing on Real-time
Transport Protocol (RTP) <xref target="RFC3550"/> streams that
are multiplexed onto a single transport-layer flow.
</t>
<t>
There are a number of guidelines specified in
<xref target="RFC7657"/> that apply to marking traffic sent by
WebRTC applications, as it is common for multiple RTP streams to
be multiplexed on the same transport-layer flow. Generally, the
RTP streams would be marked with a value as appropriate from
<xref target="table-dscp"/>. A WebRTC application might also
multiplex data channel
<xref target="I-D.ietf-rtcweb-data-channel"/> traffic over the
same 5-tuple as RTP streams, which would also be marked as per
that table. The guidance in <xref target="RFC7657"/> says that
all data channel traffic would be marked with a single value
that is typically different than the value(s) used for RTP
streams multiplexed with the data channel traffic over the same
5-tuple, assuming RTP streams are marked with a value other than
default forwarding (DF). This is expanded upon further in the
next section.
</t>
<t>
This specification does not change or override the advice in any
other standards about setting packet markings. Rather, it
simply selects a subset of DSCP values that is relevant in the
WebRTC context.
</t>
<t>
The DSCP value set by the endpoint is not trusted by the
network. In addition, the DSCP value may be remarked at any
place in the network for a variety of reasons to any other DSCP
value, including default forwarding (DF) value to provide basic
best effort service. Even so, there is benefit in marking
traffic even if it only benefits the first few hops. The
implications are discussed in Secton 3.2 of
<xref target="RFC7657"/>. Further, a mitigation for such action
is through an authorization mechanism. Such an authorization
mechanism is outside the scope of this document.
</t>
</section>
<section title="Terminology">
<t>
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 <xref target="RFC2119"/>.
</t>
</section>
<section title="Inputs">
<t>
WebRTC applications send and receive two types of flows of
significance to this document:
<list style="symbols">
<t>
media flows which are RTP streams
<xref target="I-D.ietf-rtcweb-rtp-usage"/>
</t>
<t>
data flows which are data channels
<xref target="I-D.ietf-rtcweb-data-channel"/>
</t>
</list>
</t>
<t>
Each of the RTP streams and distinct data channels consists of
all of the packets associated with an independent media entity,
so an RTP stream or distinct data channel is not always
equivalent to a transport-layer flow defined by a 5-tuple
(source address, destination address, source port, destination
port, and protocol). There may be multiple RTP streams and data
channels multiplexed over the same 5-tuple, with each having a
different level of importance to the application and, therefore,
potentially marked using different DSCP values than another RTP
stream or data channel within the same transport-layer flow.
(Note that there are restrictions with respect to marking
different data channels carried within the same SCTP association
as outlined in <xref target="dscp-mappings"/>.)
</t>
<t>
The following are the inputs provided by the WebRTC application
to the browser:
<list style="symbols">
<t>
Flow Type: The browser provides this input as it knows if the
flow is audio, interactive video with or without audio,
non-interactive video with or without audio, or data.
</t>
<t>
Application Priority: Another input is the relative
importance of an RTP stream or data channel. Many
applications have multiple flows of the same Flow Type and
often some flows are more important than others. For
example, in a video conference where there are usually audio
and video flows, the audio flow may be more important than
the video flow. JavaScript applications can tell the
browser whether a particular flow is high, medium, low or
very low importance to the application.
</t>
</list>
</t>
<t>
<xref target="I-D.ietf-rtcweb-transports"/> defines in more
detail what an individual flow is within the WebRTC
context and priorities for media and data flows.
</t>
</section>
<section anchor="dscp-mappings" title="DSCP Mappings">
<t>
The DSCP values for each flow type of interest to WebRTC based
on application priority are shown in the following table. These
values are based on the framework and recommended values in
<xref target="RFC4594"/>. A web browser SHOULD use these values
to mark the appropriate media packets. More information on EF
can be found in <xref target="RFC3246"/>. More information on
AF can be found in <xref target="RFC2597"/>. DF is default
forwarding which provides the basic best effort service
<xref target="RFC2474"/>.
</t>
<texttable anchor="table-dscp"
title="Recommended DSCP Values for WebRTC Applications">
<ttcol align="center">Flow Type</ttcol>
<ttcol align="center">Very Low</ttcol>
<ttcol align="center">Low</ttcol>
<ttcol align="center">Medium</ttcol>
<ttcol align="center">High</ttcol>
<c>Audio</c>
<c>CS1 (8)</c>
<c>DF (0)</c>
<c>EF (46)</c>
<c>EF (46)</c>
<c> </c>
<c> </c>
<c> </c>
<c> </c>
<c> </c>
<c>Interactive Video with or without audio</c>
<c>CS1 (8)</c>
<c>DF (0)</c>
<c>AF42, AF43 (36, 38)</c>
<c>AF41, AF42 (34, 36)</c>
<c> </c>
<c> </c>
<c> </c>
<c> </c>
<c> </c>
<c>Non-Interactive Video with or without audio</c>
<c>CS1 (8)</c>
<c>DF (0)</c>
<c>AF32, AF33 (28, 30)</c>
<c>AF31, AF32 (26, 28)</c>
<c> </c>
<c> </c>
<c> </c>
<c> </c>
<c> </c>
<c>Data</c>
<c>CS1 (8)</c>
<c>DF (0)</c>
<c>AF11</c>
<c>AF21</c>
</texttable>
<t>
The application priority, indicated by the columns "very low",
"low", "Medium", and "high", signifies the relative importance
of the flow within the application. It is an input that the
browser receives to assist in selecting the DSCP value and
adjusting the network transport behavior.
</t>
<t>
The above table assumes that packets marked with CS1 are treated
as "less than best effort", such as the LE behavior described in
<xref target="RFC3662"/>. However, the treatment of CS1 is
implementation dependent. If an implementation treats CS1 as
other than "less than best effort", then the actual priority
(or, more precisely, the per-hop-behavior) of the packets may be
changed from what is intended. It is common for CS1 to be
treated the same as DF, so applications and browsers using CS1
cannot assume that CS1 will be treated differently than DF <xref
target="RFC7657"/>. However, it is also possible per
<xref target="RFC2474"/> for CS1 traffic to be given better
treatment than DF, thus caution should be exercised when
electing to use CS1.
</t>
<t>
Implementers should also note that excess EF traffic is dropped.
This could mean that a packet marked as EF may not get through
as opposed to a packet marked with a different DSCP value.
This is not a flaw, but how excess EF traffic is intended to be
treated.
</t>
<t>
The browser SHOULD first select the flow type of the flow.
Within the flow type, the relative importance of the flow
SHOULD be used to select the appropriate DSCP value.
</t>
<t>
The combination of flow type and application priority provides
specificity and helps in selecting the right DSCP value for the
flow. All packets within a flow SHOULD have the same application
priority. In some cases, the selected application priority cell
may have multiple DSCP values, such as AF41 and AF42. These offer
different drop precedences. The different drop precedence
values provides additional granularity in classifying packets
within a flow. For example, in a video conference, the video
flow may have medium application priority. If so, either AF42
or AF43 may be selected. If the I-frames in the stream are more
important than the P-frames, then the I-frames can be marked
with AF42 and the P-frames marked with AF43.
</t>
<t>
It is worth noting that the application priority is utilized by
the coupled congestion control mechanism for media flows per
<xref target="I-D.ietf-rmcat-coupled-cc"/> and the SCTP
scheduler for data channel traffic per
<xref target="I-D.ietf-rtcweb-data-channel"/>.
</t>
<t>
For reasons discussed in Section 6 of
<xref target="RFC7657"/>, if multiple flows are multiplexed
using a reliable transport (e.g., TCP) then all of the packets
for all flows multiplexed over that transport-layer flow MUST be
marked using the same DSCP value. Likewise, all WebRTC data
channel packets transmitted over an SCTP association MUST be
marked using the same DSCP value, regardless of how many data
channels (streams) exist or what kind of traffic is carried over
the various SCTP streams. In the event that the browser wishes
to change the DSCP value in use for an SCTP association, it MUST
reset the SCTP congestion controller after changing values.
Frequent changes in the DSCP value used for an SCTP association
are discouraged, though, as this would defeat any attempts at
effectively managing congestion. It should also be noted that
any change in DSCP value that results in a reset of the
congestion controller puts the SCTP association back into slow
start, which may have undesirable effects on application
performance.
</t>
<t>
For the data channel traffic multiplexed over an SCTP
association, it is RECOMMENDED that the DSCP value selected be
the one associated with the highest priority requested for all
data channels multiplexed over the SCTP association. Likewise,
when multiplexing multiple flows over a TCP connection,
the DCSP value selected should be the one associated with the
highest priority requested for all multiplexed flows.
</t>
<t>
If a packet enters a network that has no support for a flow
type-application priority combination specified in
<xref target="table-dscp"/> (above), then the network node at
the edge will remark the DSCP value based on policies. This
could result in the flow not getting the network treatment it
expects based on the original DSCP value in the packet.
Subsequently, if the packet enters a network that supports a
larger number of these combinations, there may not be sufficient
information in the packet to restore the original markings.
Mechanisms for restoring such original DSCP is outside the scope
of this document.
</t>
<t>
In summary, DSCP marking provides neither guarantees nor
promised levels of service. However, DSCP marking is expected
to provide a statistical improvement in real-time service as a
whole. The service provided to a packet is dependent upon the
network design along the path, as well as the network conditions
at every hop.
</t>
</section>
<section title="Security Considerations">
<t>
This specification does not add any additional security implication
other than the normal application use of DSCP not already
addressed by the following specifications. For security
implications on use of DSCP, please refer to Section 7 of
<xref target="RFC7657"/> and Section 6 of
<xref target="RFC4594"/>. Please also see
<xref target="I-D.ietf-rtcweb-security"/> as an additional reference.
</t>
</section>
<section title="IANA Considerations">
<t>
This specification does not require any actions from IANA.
</t>
</section>
<section title="Downward References">
<t>
This specification contains a downwards reference to <xref
target="RFC4594"/>. However, the parts of that RFC used by this
specification are sufficiently stable for this downward
reference.
</t>
</section>
<section title="Acknowledgements">
<t>
Thanks to David Black, Magnus Westerland, Paolo Severini, Jim
Hasselbrook, Joe Marcus, Erik Nordmark, Michael Tuexen, and
Brian Carpenter for their invaluable input.
</t>
</section>
<section title="Dedication">
<t>
This document is dedicated to the memory of James Polk, a
long-time friend and colleague. James made important
contributions to this specification, including being one of its
primary authors. The IETF global community mourns his loss and
he will be missed dearly.
</t>
</section>
<section title="Document History">
<t>
Note to RFC Editor: Please remove this section.
</t>
<t>
This document was originally an individual submission in RTCWeb WG.
The RTCWeb working group selected it to be become a WG document.
Later the transport ADs requested that this be moved to the TSVWG WG
as that seemed to be a better match.
</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include='reference.RFC.4594'?>
<?rfc include='reference.RFC.2119'?>
<?rfc include='reference.RFC.7657'?>
<?rfc include='reference.I-D.ietf-rtcweb-security'?>
<?rfc include='reference.I-D.ietf-rtcweb-transports'?>
<?rfc include='reference.I-D.ietf-rtcweb-rtp-usage'?>
<?rfc include='reference.I-D.ietf-rtcweb-data-channel'?>
</references>
<references title="Informative References">
<?rfc include='reference.RFC.2474'?>
<?rfc include='reference.RFC.2597'?>
<?rfc include='reference.RFC.3246'?>
<?rfc include='reference.RFC.3550'?>
<?rfc include='reference.RFC.3662'?>
<?rfc include='reference.W3C.REC-html5-20141028.xml'?>
<?rfc include='reference.I-D.ietf-rmcat-coupled-cc'?>
</references>
</back>
</rfc>
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