Network Working Group S. Dhesikan
Internet-Draft D. Druta, Ed.
Intended status: Standards Track ATT
Expires: September 10, 2014 P. Jones
J. Polk
March 09, 2014

DSCP and other packet markings for RTCWeb QoS


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.

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Table of Contents

1. Introduction

Differentiated Services Code Points (DSCP)[RFC2474] 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.

2. Relation to Other Standards

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.

3. Terminology

The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" in this document are to be interpreted as described in [RFC2119].

4. Inputs

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:

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.

5. DSCP Mappings

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 [RFC4594]. A web browser SHOULD use these values to mark the appropriate media packets. More information on EF can be found in [RFC3246]. More information on AF can be found in [RFC2597].

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.

6. Security Considerations

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.

7. IANA Considerations

This specification does not require any actions from IANA.

8. Downward References

This specification contains a downwards reference to [RFC4594] however the parts of that RFC used by this specification are sufficiently stable for this downward reference.

9. Acknowledgements

Cullen Jennings was one of the authors of this text in the original individual submission but was unceremoniously kicked off by the chairs when it became a WG version. Thanks To David Black, Magnus Westerland, Paolo Severini, Jim Hasselbrook, Joe Marcus, and Erik Nordmark for their help.

10. Document History

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.

11. References

11.1. Normative References

[RFC4594] Babiarz, J., Chan, K. and F. Baker, "Configuration Guidelines for DiffServ Service Classes", RFC 4594, August 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

11.2. Informative References

[RFC3246] Davie, B., Charny, A., Bennet, J.C.R., Benson, K., Le Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V. and D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246, March 2002.
[RFC2474] Nichols, K., Blake, S., Baker, F. and D.L. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998.
[RFC2597] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski, "Assured Forwarding PHB Group", RFC 2597, June 1999.

Authors' Addresses

Subha Dhesikan Cisco EMail:
Dan Druta (editor) ATT EMail:
Paul Jones Cisco EMail:
James Polk Cisco EMail: