DSCP and other packet markings for RTCWeb
QoS
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Many networks, such as service provider and enterprise networks, can
provide per packet treatments based on Differentiated Services Code
Points (DSCP) on a per hop basis. This document provides the recommended
DSCP values for browsers to use for various classes of traffic.
Differentiated Services Code Points (DSCP)
style packet marking can help provide QoS in some environments. There are
many use cases where such marking does not help, but it seldom makes
things worse if packets are marked appropriately. In other words, if too many packets, say all
audio or all audio and video, are marked for a given network condition then it can prevent desirable results. Either too
much other traffic will be starved, or there is not enough capacity for
the preferentially marked packets (i.e., audio and/or video).
This draft proposes how WebRTC applications can
mark packets. This draft does not contradict or redefine any advice from
previous IETF RFCs but simply provides a simple set of recommendations
for implementers based on the previous RFCs.
There are some environments where priority markings frequently help.
These include:
1. Private networks (Wide Area).
2. If the congested link is the broadband uplink in a Cable or DSL
scenario, often residential routers/NAT support preferential treatment
based on DSCP.
3. If the congested link is a local WiFi network, marking may
help.
Traditionally DSCP values have been thought of as being site
specific, with each site selecting its own code points for each QoS
level. However in the RTCWeb use cases, the browsers need to set them to
something when there is no site specific information. This document
describes a reasonable default set of DSCP code point values drawn from
existing RFCs and common usage. These code points are solely defaults.
Future drafts may define mechanisms for site specific mappings to
override the values provided in this draft.
This draft defines some inputs that the browser in an WebRTC application can look at to
determine how to set the various packet markings and defines the
mapping from abstract QoS policies (data type, priority level) to those
packet markings.
This specification does not change or override the advice in any
other standards about setting packet markings. It simply provides a
summary of them and provides the context of how they
relate into the RTCWeb context.
In some cases, such as DSCP where the normative RFC
leaves open multiple options to choose from, this clarifies which choice
should be used in the RTCWeb context. This document also specifies the
inputs that are needed by the browser to provide to the media engine.
The DSCP value set by the endpoint is not always trusted by the network.
Therefore, the DSCP value may be remarked to any other DSCP, even to best effort
at the network edge through policy. The mitigation for such action is through an authorization
mechanism. Such authorization mechanism is outside the scope of this document.
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY"
in this document are to be interpreted as described in .
The below uses the concept of a media flow, however these are
commonly not equivalent to a transport flow, i.e. as defined by a
5-tuple (source address, destination address, source port,
destination port, and protocol). Instead each media flow contains
all the packets associated with an independent media entity within
one 5-tuple. There may be multiple media flows within the same 5-
tuple. These media flows might be consisting of different media
types and have different priorities.
The following are the inputs that the browser provides to the media engine:
Data 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.
Priority: Another input is the relative treatment of the flow
within that data type. Many applications have multiple media
flows of the same data type and often some are more important
than others. Likewise, in a video conference where the flows in
the conference is of the same data type but contains different
media types, the flow for audio may be more important than the
video flow. JavaScript applications can tell the browser
whether a particular media flow is high, medium, low or very low
importance to the application.
When it comes to data transmission, a media (data) flow is the
SCTP stream under a common congestion control (currently within
the same SCTP association).
[I-D.ietf-rtcweb-transports] defines in more detail what an
individual media flow is within the WebRTC context.
Below is a table of DSCP markings for each data type of interest to
RTCWeb. These DSCPs for each data type listed are a reasonable
default set of code point values taken from . A
web browser SHOULD use these values to mark the appropriate media
packets. More information on EF can be found in .
More information on AF can be found in .
Data Type
Very Low
Low
Medium
High
Audio
CS1 (8)
BE (0)
EF (46)
EF (46)
Interactive Video with or without audio
CS1 (8)
BE (0)
AF42, AF43 (36, 38)
AF41, AF42 (34, 36)
Non-Interactive Video with or without audio
CS1 (8)
BE (0)
AF32, AF33 (28, 30)
AF31, AF32 (26, 28)
Data
CS1 (8)
BE (0)
AF1x (10, 12, 14)
AF2x (18, 20, 22)
The columns "very low", "low", "Medium" and "high" are the priority
levels. The browser app SHOULD first select the data type of the
media flow. Within the data type, the priority of the media flow
SHOULD be selected. All packets within a media flow SHOULD have
the same priority. In some cases, the selected cell may have
multiple DSCP values, such as AF41 and AF42. These offer different
drop precedences. One may select difference drop precedences for
the different packets in the media flow. Therefore, all packets in
the stream SHOULD be marked with the same priority but can have
difference drop precedences.
The combination of data type and priority provides specificity and
helps in selecting the right DSCP value for the media flow. In
some cases, the different drop precedence values provides
additional granularity in classifying packets within a media flow.
For example: In a video conference, the video media flow may be
medium 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.
The above table assumes that packets marked with CS1 is treated as
"less than best effort". However, the treatment of CS1 is
implementation dependent. If an implementation treats CS1 as other
than "less than best effort", then the priority of the packets may
be changed from what is intended.
If a packet enters a QoS domain that has no support for the above
defined Data Types/Application classes, then the network node at
the edge will remark the DSCP value based on policies.
Subsequently, if the packet enters a QoS domain that supports a
larger number of Data types/Application (service) classes, 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.
This draft does not add any additional security implication other than
the normal application use of DSCP. For security implications on use of DSCP,
please refer to Section 6 of RFC 4594 . Please also see work-in-progress draft
draft-ietf-rtcweb-security-04 as an additional reference.
This specification does not require any actions from IANA.
This specification contains a downwards reference to however the parts of that RFC used by this
specification are sufficiently stable for this downward reference.
Thanks To David Black, Magnus Westerland, Paolo Severini,
Jim Hasselbrook, Joe Marcus, and Erik Nordmark for their help.
Note to RFC Editor: Please remove this section.
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. This document is now being submitted as
individual submission to the TSVWG with the hope that WG will select it as
a WG draft and move it forward to an RFC.
Configuration Guidelines for DiffServ Service Classes
Key words for use in RFCs to Indicate
Requirement Levels
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