DiffServ Applied to Real-time Transports D. York
Internet-Draft Internet Society
Intended status: Standards Track D. Black, Ed.
Expires: January 4, 2015 EMC
C. Jennings
P. Jones
Cisco
July 3, 2014
Differentiated Services (DiffServ) and Real-time Communication
draft-york-dart-dscp-rtp-01
Abstract
This document describes the interaction between Differentiated
Services (DiffServ) network quality of service (QoS) functionality
and real-time network communication, including communication based on
the Real-time Transport Protocol (RTP). DiffServ is based on network
nodes applying different forwarding treatments to packets whose IP
headers are marked with different DiffServ Code Points (DSCPs). As a
result, use of different DSCPs within a single traffic stream may
cause transport protocol interactions (e.g., reordering). In
addition, DSCP markings may be changed or removed between the traffic
source and destination. This document covers the implications of
these DiffServ aspects for real-time network communication, including
RTCWEB.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 4, 2015.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. RTP Background . . . . . . . . . . . . . . . . . . . . . 3
2.2. Differentiated Services (DiffServ) Background . . . . . . 5
2.3. Diffserv PHBs (Per-Hop Behaviors) . . . . . . . . . . . . 7
2.4. DiffServ, Reordering and Transport Protocols . . . . . . 8
2.5. DiffServ, Reordering and Real-Time Communication . . . . 9
2.6. Traffic Classifiers and DSCP Remarking . . . . . . . . . 10
3. RTP Multiplexing Background . . . . . . . . . . . . . . . . . 11
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 12
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
This document describes the interactions between Differentiated
Services (DiffServ) network quality of service (QoS) functionality
[RFC2475] and real-time network communication, including
communication based on the Real-time Transport Protocol (RTP)
[RFC3550]. DiffServ is based on network nodes applying different
forwarding treatments to packets whose IP headers are marked with
different DiffServ Code Points (DSCPs)[RFC2474]. As a result use of
different DSCPs within a single traffic stream may cause transport
protocol interactions (e.g., reordering). In addition, DSCP markings
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may be changed or removed between the traffic's source and
destination. This document covers the implications of these DiffServ
aspects for real-time network communication, including RTCWEB traffic
[I-D.ietf-rtcweb-overview].
1.1. Requirements Language
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 [RFC2119].
2. Background
Real-time communications enables communication in real-time over an
IP network using voice, video, text, content sharing, etc. It is
possible to use one or more of these modalities in parallel in order
to provide a richer communication experience.
A simple example of real-time communications is a voice call placed
over the Internet wherein an audio stream is transmitted in each
direction between two users. A more complex example is an immersive
videoconferencing system that has multiple video screens, multiple
cameras, multiple microphones, and some means of sharing content.
For such complex systems, there may be multiple media streams that
may be transmitted via a single IP address and port or via multiple
IP addresses and ports.
2.1. RTP Background
The most common protocol used for real time media is the Real-Time
Transport Protocol (RTP) [RFC3550]. RTP defines a common
encapsulation format and handling rules for real-time data
transmitted over the Internet. Unfortunately, RTP terminology usage
has been inconsistent. For example, this document on RTP grouping
terminology [I-D.ietf-avtext-rtp-grouping-taxonomy] observes that:
RFC 3550 [RFC3550] uses the terms media stream, audio stream,
video stream and streams of (RTP) packets interchangeably.
Terminology in this document is based on that RTP grouping
terminology document with the following terms being of particular
importance (see that terminology document for full definitions):
Source Stream: A reference clock synchronized, time progressing,
digital media stream.
RTP Packet Stream: A stream of RTP packets containing media data,
which may be source data or redundant data. The RTP Packet Stream
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is identified by an RTP synchronization source (SSRC) belonging to
a particular RTP session.
Media encoding and packetization of a source stream results in a
source RTP packet stream plus zero or more redundancy RTP packet
streams that provide resilience against loss of packets from the
source RTP packet stream [I-D.ietf-avtext-rtp-grouping-taxonomy].
Redundancy information may also be carried in the same RTP packet
stream as the encoded source stream, e.g., see Section 7.2 of
[RFC5109]. With most applications, a single media type (e.g., audio)
is transmitted within a single RTP session. However, it is possible
to transmit multiple, distinct source streams over the same RTP
session as one or more individual RTP packet streams. This is
referred to as RTP multiplexing.
The number of source streams and RTP packet streams in an overall
real-time interaction can be surprisingly large. In addition to a
voice source stream and a video source stream, there could be
separate source streams for each of the cameras or microphones on a
videoconferencing system. As noted above, there might also be
separate redundancy RTP packet streams that provide protection to a
source RTP packet stream, using techniques such as Forward Error
Correction. Another example is simulcast transmission, where a video
source stream can be transmitted at high resolution and low
resolution RTP packet streams at the same time. In this case, a
media processing function might choose to send one or both RTP packet
streams onward to a receiver based on bandwidth availability or who
the active speaker is in a multipoint conference. Lastly, a
transmitter might send a the same media content concurrently as two
RTP packet streams using different encodings (e.g., VP8 in parallel
with H.264) to allow a media processing function to select a media
encoding that best matches the capabilities of the receiver.
Other transport protocols may also be used to transmit real-time data
or near-real-time data. For example, SCTP can be utilized to carry
application sharing or whiteboarding information as part of an
overall interaction that includes real time media. These additional
transport protocols can be multiplexed with an RTP session via UDP
encapsulation, thereby using a single pair of UDP ports.
The RTCWEB protocol suite [I-D.ietf-rtcweb-transports] employs two
layers of multiplexing:
1. Individual source streams are carried in one or more individual
RTP packet streams that can be multiplexed into a single RTP
session as described in [RFC3550]; and
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2. An RTP session could be multiplexed with other protocols via UDP
encapsulation over a common pair of UDP ports as described in
[RFC5764] and [I-D.petithuguenin-avtcore-rfc5764-mux-fixes]. The
resulting unidirectional UDP packet flow is identified by a
5-tuple, i.e., a combination of two IP addresses (source and
destination), two UDP ports (source and destination), and the use
of the UDP protocol.
For RTCWEB,an individual source stream is a MediaStreamTrack, and a
MediaStream contains one or more MediaStreamTracks
[W3C.WD-mediacapture-streams-20130903]. A MediaStreamTrack is
transmitted as a source RTP packet stream plus zero or more
redundancy RTP packet streams, so a MediaStream that consists of one
MediaStreamTrack is transmitted as a single source RTP packet stream
plus zero or more redundancy RTP packet streams.
For more information on use of RTP in RTCWEB, see
[I-D.ietf-rtcweb-rtp-usage].
[I-D.westerlund-avtcore-transport-multiplexing] proposes to allow
multiple RTP sessions to be multiplexed over a single UDP 5-tuple;
the future of that expired proposal is uncertain.
For IPv6, addition of the flow label [RFC6437] to 5-tuples results in
6-tuples, but in practice, use of a flow label is unlikely to result
in a finer-grain traffic subset than the corresponding 5-tuple (e.g.,
the flow label is likely to represent the combination of two ports
with use of the UDP protocol). For that reason, discussion in this
draft focuses on UDP 5-tuples.
[Editor's Note: Multiple RTP sessions cannot be multiplexed on the
same UDP 5-tuple, but what about multiple DTLS sessions for RTP? RFC
5764 appears to allow multiple DTLS sessions.]
[Editor's Note: Should RTCP multiplexing w/RTP be mentioned here, as
described in RFC 5761?]
2.2. Differentiated Services (DiffServ) Background
The DiffServ architecture is intended to enable scalable service
discrimination in the Internet without requiring each network node to
store per-flow state and participate in per-flow signaling. The
services may be end-to-end or within a network; they include both
those that can satisfy quantitative performance requirements (e.g.,
peak bandwidth) and those based on relative performance (e.g.,
"class" differentiation). Services can be constructed by a
combination of well-defined building blocks deployed in network nodes
that:
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o classify traffic and set bits in an IP header field at network
boundaries or hosts,
o use those bits to determine how packets are forwarded by the nodes
inside the network, and
o condition the marked packets (e.g., meter, mark, shape, police) at
network boundaries in accordance with the requirements or rules of
each service.
A network node that supports DiffServ includes a classifier that
selects packets based on the value of the DS field in IP headers,
along with buffer management and packet scheduling mechanisms capable
of delivering the specific packet forwarding treatment indicated by
the DS field value. Setting of the DS field and fine-grain
conditioning of marked packets need only be performed at network
boundaries; internal network nodes operate on traffic aggregates that
share a DS field value, or in some cases, a small set of related
values.
The DiffServ architecture[RFC2475] maintains distinctions among:
o the QoS service provided to a traffic aggregate,
o the conditioning functions and per-hop behaviors (PHBs) used to
realize services,
o the DS field value (DS codepoint, or DSCP) in the IP header used
to mark packets to select a per-hop behavior, and
o the particular implementation mechanisms that realize a per-hop
behavior.
This document focuses on PHBs and the usage of DSCPs to obtain those
behaviors. In a network node's forwarding path, the DSCP is used to
map a packet to a particular forwarding treatment, or per-hop
behavior (PHB) that specifies the forwarding treatment.
A per-hop behavior (PHB) is a description of the externally
observable forwarding behavior of a network node for network traffic
marked with a DSCP that selects that PHB. In this context,
"forwarding behavior" is a general concept - for example, if only one
DSCP is used for all traffic on a link, the observable forwarding
behavior (e.g., loss, delay, jitter) will often depend only on the
relative loading of the link. To obtain useful behavioral
differentiation,multiple traffic subsets are marked with different
DSCPs for different PHBs for which node resources such as buffer
space and bandwidth are allocated. PHBs provide the framework for a
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DiffServ network node to allocate resources to traffic subsets, with
network-scope differentiated services constructed on top of this
basic hop-by-hop (per-node) resource allocation mechanism.
The codepoints (DSCPs) may be chosen from a small set of fixed values
(the class selector codepoints), or from a set of recommended values
defined in PHB specifications, or from values that have purely local
meanings to a specific network that supports DiffServ; in general,
packets may be forwarded across multiple such networks between source
and destination.
The mandatory DSCPs are the class selector code points as specified
in [RFC2474]. The class selector codepoints (CS0-CS7) extend the
deprecated concept of IP Precedence in the IPv4 header; three bits
are added, so that the class selector DSCPs are of the form 'xxx000'.
The all-zero DSCP ('000000' or CS0) designates a Default PHB that
provides best-effort forwarding behavior and the remaining class
selector code points were originally specified to provide relatively
better per-hop-forwarding behavior in increasing numerical order,
but:
o There is no requirement that any two adjacent class selector
codepoints select different PHBs; adjacent class selector
codepoints may use the same pool of resources on each network node
in some networks.
o CS1 ('001000') was subsequently recommended for "less than best
effort" service when such a service is offered by a network
[RFC3662]. Not all networks offer such a service.
Applications and traffic sources cannot rely upon different class
selector codepoints providing differentiated services or upon the
presence of a "less than best effort" service that is selected by the
CS1 DSCP, and there is no effective way for a network endpoint to
determine whether the CS1 DSCP selects such a service on a specific
network, let alone end-to-end. Packets marked with the CS1 DSCP may
be carried as best effort or even "better than best effort", see
[RFC2474].
2.3. Diffserv PHBs (Per-Hop Behaviors)
Although Differentiated Services is a general architecture that may
be used to implement a variety of services, three fundamental
forwarding behaviors (PHBs) have been defined and characterized for
general use. These are:
1. Default Forwarding (DF) for elastic traffic [RFC2474]. The
Default PHB is always selected by the all-zero DSCP.
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2. Assured Forwarding (AF) [RFC2597] to provide differentiated
service to elastic traffic. Each instance of the AF behavior
consists of three PHBs that differ only in drop precedence, e.g.,
AF11, AF12 and AF13; such a set of three AF PHBs is referred to
as an AF class, e.g., AF1x. There are four defined AF classes,
AF1x through AF4x, with higher numbered classes expected to
receive better forwarding treatment than lower numbered classes.
3. Expedited Forwarding (EF) [RFC3246]intended for inelastic
traffic. Beyond the basic EF PHB, the VOICE-ADMIT PHB [RFC5865]
is an admission controlled variant of the EF PHB.
2.4. DiffServ, Reordering and Transport Protocols
[Editor's note: This section and the recommendations in Section 4 are
centered on TCP, UDP, and SCTP. They could use generalization to
include other transport protocols - DCCP is a likely one to include,
although it is not necessary to include every known transport
protocol.]
Transport protocols provide data communication behaviors beyond those
possible at the IP layer. An important example is that TCP provides
reliable in-order delivery of data with congestion control. SCTP
provides additional properties such as preservation of message
boundaries, and the ability to avoid head-of-line blocking that may
occur with TCP. In contrast, UDP is a basic unreliable datagram
protocol that provides port-based multiplexing and demultiplexing on
top of IP.
Transport protocols that provide reliable delivery (e.g., TCP, SCTP)
are sensitive to network reordering of traffic. When a protocol that
provides reliable delivery receives a packet other than the next
expected packet for an ordered connection or stream, it usually
assumes that the expected packet has been lost and respond with a
retransmission request for that packet. In addition, congestion
control functionality in transport protocols usually infers
congestion when packets are lost, creating an additional sensitivity
to significant reordering - such reordering may be (mis-)interpreted
as indicating congestion-caused packet loss, causing a reduction in
transmission rate. This remains true even when ECN [RFC3168] is in
use, as ECN receivers are required to treat missing packets as
potential indications of congestion. This requirement is based on
two factors:
o Severe congestion may cause ECN-capable network nodes to drop
packets, and
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o ECN traffic may be forwarded by network nodes that do not support
ECN and hence use packet drops to indicate congestion.
Congestion control is an important aspect of the Internet
architecture, see [RFC2914] for further discussion.
In general, marking packets with different DSCPs results in different
PHBs being applied at network nodes, making reordering possible due
to use of different pools of forwarding resources for each PHB. The
primary exception is that reordering is prohibited within each AF
class (e.g., AF1x), as the three PHBs in an AF class differ solely in
drop precedence. Reordering within a PHB or AF class may occur for
other transient reasons (e.g., route flap or ECMP rebalancing).
UDP is the primary transport protocol that is not sensitive to
reordering in the network, because it does not provide reliable
delivery or congestion control. On the other hand, when UDP is used
to encapsulate other protocols (e.g., as is the case for RTCWEB, see
Section 2.1), the reordering considerations for the encapsulated
protocols apply. For RTCWEB example in particular, every
encapsulated protocol (i.e., RTP, SCTP and TCP) is sensitive to
reordering as further discussed in this document.
2.5. DiffServ, Reordering and Real-Time Communication
Real-time communications are also sensitive to network reordering of
packets. Such reordering may lead to spurious NACK generation and
unneeded retransmission, as is the case for reliable delivery
protocols (see Section Section 2.4). The degree of sensitivity
depends on protocol or stream timers, in contrast to reliable
delivery protocols that usually react to all reordering.
Receiver jitter buffers have important roles in the effect of
reordering on real time communications:
o Minor packet reordering that is contained within a jitter buffer
usually has no effect on rendering of the received RTP packet
stream.
o Packet reordering that exceeds the capacity of a jitter buffer can
cause user-perceptible quality problems (e.g., glitches, noise)
for delay sensitive communication, such as interactive
conversations. Interactive real-time communication
implementations often choose to discard data that is sufficiently
late to prevent it from being rendered in source stream order,
making retransmission counterproductive. For this reason,
implementations of interactive real-time communication often do
not use retransmission.
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o In contrast, replay of recorded media can typically uses
significantly larger jitter buffers than can be tolerated for
interactive conversations, with the result that replay is more
tolerant to reordering than interactive conversations. The size
of the jitter buffer imposes an upper bound on replay tolerance to
reordering, but does enable retransmission to be used when the
jitter buffer is significantly larger than the amount of data that
will arrive during the round-trip latency for retransmission.
Network packet reordering caused by use of different DSCPs has no
effective upper bound, and can exceed the size of any reasonable
jitter buffer - in practice, the size of jitter buffers for replay is
limited by external factors such as the amount of time that a human
is willing to wait for replay to start.
2.6. Traffic Classifiers and DSCP Remarking
DSCP markings are not end-to-end in general. Each network can make
its own decisions about what PHBs to use and which DSCP maps to each
PHB. While every PHB specification includes a recommended DSCP, and
RFC 4594 [RFC4594] recommends their end-to-end usage, there is no
requirement that every network support any PHBs or use any DSCPs,
with the exception of the class selector codepoint requirements in
RFC 2474 [RFC2474]. When DiffServ is used, the edge or boundary
nodes of a network are responsible for ensuring that all traffic
entering that network conforms to that network's policies for DSCP
and PHB usage, and such nodes remark traffic (change the DSCP marking
as part of traffic conditioning) accordingly. As a result, DSCP
remarking is possible at any network boundary, including the first
network node that traffic sent by a host encounters. Remarking is
also possible within a network, e.g., for traffic shaping.
DSCP remarking is part of traffic conditioning; the traffic
conditioning functionality applied to packets at a network node is
determined by a traffic classifier [RFC2475]. Edge nodes of a
DiffServ network classify traffic based on selected packet header
fields; typical implementations do not look beyond the traffic's
5-tuple in the IP and transport protocol headers. As a result, when
multiple DSCPs are used for traffic that shares a 5-tuple, remarking
at a network boundary may result in all of the traffic being
forwarded with a single DSCP, thereby removing any differentiation
within the 5-tuple downstream of the remarking location. Network
nodes within a DiffServ network generally classify traffic based
solely on DSCPs, but may perform finer grain traffic conditioning
similar to that performed by edge nodes.
So, for two arbitrary network endpoints, there can be no assurance
that the DSCP set at the source endpoint will be preserved and
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presented at the destination endpoint. On the contrary, it is quite
likely that the DSCP will be set to zero (e.g., at the boundary of a
network operator that distrusts or does not use the DSCP field) or to
a value deemed suitable by an ingress (MF) classifier for whatever
5-tuple it carries. DiffServ classifiers generally ignore embedded
protocol headers (e.g., for SCTP or RTP embedded in UDP,
classification will be only on the outer UDP header).
In addition, remarking may remove application-level distinctions in
forwarding behavior - e.g., if multiple PHBs within an AF class are
used to distinguish different types of frames within a video RTP
packet stream, token-bucket-based remarkers operating in Color-Blind
mode (see [RFC2697] and [RFC2698] for examples) may remark solely
based on flow rate and burst behavior, removing the drop precedence
distinctions specified by the source.
Backbone and other carrier networks may employ a small number of
DSCPs (e.g., less than half a dozen) in order to manage a small
number of traffic aggregates; hosts that use a larger number of DSCPs
can expect to find that much of their intended differentiation is
removed by such networks. Better results may be achieved when DSCPs
are used to spread traffic among a smaller number of DiffServ-based
traffic subsets or aggregates, see [I-D.geib-tsvwg-diffserv-intercon]
for one proposal. This is of particular importance for MPLS-based
networks due to the limited size of the Traffic Class (TC) field in
an MPLS label [RFC5462] that is used to carry DiffServ information
and the use of that TC field for other purposes, e.g., ECN [RFC5129].
For further discussion on use of DiffServ with MPLS, see [RFC3270]
and [RFC5127].
3. RTP Multiplexing Background
Section 2 explains how source streams can be multiplexed over RTP
sessions which can in turn be multiplexed over UDP with packets
generated by other transport protocols. This section provides
background on why this level of multiplexing is desirable. The
rationale in this section applies both to multiplexing of source
streams in RTP sessions and multiplexing of an RTP session with
traffic from other transport protocols via UDP encapsulation.
Multiplexing reduces the number of ports utilized for real-time and
related communication in an overall interaction. While a single
endpoint might have plenty of ports available for communication, this
traffic often traverses points in the network that are constrained on
the number of available ports. A good example is a NAT/FW device
sitting at the network edge. As the number of simultaneous protocol
sessions increases, so does the burden placed on these devices in
order to provide port mapping.
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Another reason for multiplexing is to help reduce the time required
to establish bi-directional communication. Since any two
communicating users might be situated behind different NAT/FW
devices, it is necessary to employ techniques like STUN/ICE/TURN in
order to get traffic to flow between the two devices
[I-D.ietf-rtcweb-transports]. Performing the tasks required of
STUN/ICE/TURN take time and requiring an endpoint to perform these
tasks for multiple protocol sessions can increase the time required.
While tasks for different sessions can be performed in parallel, it
is nonetheless necessary for applications to wait for all sessions to
be opened before communication between to users can begin. Reducing
the number of STUN/ICE/TURN steps reduces the probability of losing a
packet and introducing delay in setting up a communication session.
Further, reducing the number of STUN/ICE/TURN tasks means that there
is a lower burden placed on the STUN and TURN servers.
Multiplexing may reduce the complexity and resulting load on an
endpoint. A single instance of STUN/ICE/TURN is simpler to execute
and manage than multiple instances STUN/ICE/TURN operations happening
in parallel, as the latter require synchronization and create more
complex failure situations that have to be cleaned up by additional
code.
4. Recommendations
The only standardized use of multiple PHBs and DSCPs that avoids
network reordering among packets marked with different DSCPs is use
of PHBs within a single AF class. All other uses of multiple PHBs
and/or the class selector DSCPs allow network reordering of packets
that are marked with different DSCPs. Based on this and the
foregoing discussion, the following requirements apply to use of
DiffServ with real-time communications - applications and other
traffic sources:
o SHOULD NOT use different PHBs and DSCPs that may cause reordering
within a single RTP packet stream. If this is not done,
significant network reordering may overwhelm implementation
assumptions about limits on reordering, e.g., jitter buffer size,
causing poor user experiences, see Section Section 2.5 above.
o SHOULD NOT use different PHBs and DSCPs that may cause reordering
within an ordered session for a reliable transport protocol (e.g.,
TCP, SCTP). Receivers for such protocols interpret reordering as
indicating loss of out-of-order packets causing undesired
retransmission requests, and will infer congestion from
significant reordering, causing throughput reduction. This
requirement applies to both unencapsulated and encapsulated (e.g.,
via UDP) uses of reliable transport protocols.
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o MAY use different PHBs and DSCPs that cause reordering within a
single UDP 5-tuple, subject to the above constraints. The service
differentiation provided by such usage is unreliable, as it may be
removed at network boundaries for the reasons described in
Section 2.6 above.
o MUST NOT rely on end-to-end preservation of DSCPs as network node
remarking can change DSCPs and remove drop precedence distinctions
see Section 2.6 above. For example, if a source uses drop
precedence distinctions within an AF class to identify different
types of video frames, using those DSCP values at the receiver to
identify frame type is inherently unreliable.
o SHOULD use the CS1 codepoint only for traffic that is acceptable
to forward as best effort traffic, as network support for use of
CS1 to select a "less than best effort" PHB is inconsistent.
Further, some networks may treat CS1 as providing "better than
best effort" forwarding behavior.
There is no requirement in this document for network operators to
differentiate traffic in any fashion. Networks may support all of
the PHBs discussed herein, classify EF and AFxx traffic identically,
or even remark all traffic to best effort at some ingress points.
Nonetheless, it is useful for network endpoints to provide finer
granularity DSCP marking on packets for the benefit of networks that
offer QoS service differentiation. A specific example is that
traffic originating from a browser may benefit from QoS service
differentiation in within-building and residential access networks,
even if the DSCP marking is subsequently removed or simplified. This
is because such networks and the boundaries between them are likely
traffic bottleneck locations (e.g., due to customer aggregation onto
common links and/or speed differences among links used by the same
traffic).
5. Examples
For real-time communications, one might want to mark the audio
packets using EF and the video packets as AF41. However, in a video
conference receiving the audio packets ahead of the video is not
useful because lip sync is necessary between audio and video. It may
still be desirable to send audio with a PHB that provides better
service, because early arrival of audio helps assure smooth audio
rendering, which is often more important than fully faithful video
rendering. There are also limits, as some devices have difficulties
in synchronizing voice and video when packets that need to be
rendered together arrive at significantly different times. It makes
more sense to use different PHBs when the audio and video source
streams do not share a strict timing relationship. For example,
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video content may be shared within a video conference via playback,
perhaps of an unedited video clip that is intended to become part of
a television advertisement. Such content sharing video does not need
precise synchronization with video conference audio, and could use a
different PHB, as content sharing video is more tolerant to jitter,
loss, and delay.
Within a layered video RTP packet stream, ordering of frame
communication is preferred, but importance of frame types varies,
making use of PHBs with different drop precedences appropriate. For
example, I-frames that contain an entire image are usually more
important than P-frames that contain only changes from the previous
image because loss of a P-frame (or part thereof) can be recovered
(at the latest) via the next I-frame, whereas loss of an I-frame (or
part thereof) may cause rendering problems for all of the P-frames
that depend on the I-frame. For this reason, it is appropriate to
mark I-frame packets with a PHB that has lower drop precedence than
the PHB used for P-frames, as long as the PHBs preserve ordering
among frames (e.g., are in an AF class) - AF41 for I-frames and AF43
for P-frames is one possibility. Additional spatial and temporal
layers beyond the base video layer could also be marked with higher
drop precedence than the base video layer, as their loss reduces
video quality, but does not disrupt video rendering.
Additional RTP packet streams in a real-time communication
interaction could be marked with CS0 and carried as best effort
traffic. One example is real-time text transmitted as specified in
RFC 4103[RFC4103]; best effort forwarding suffices when redundancy
encoding is used (as required by RFC 4103). Best effort forwarding
suffices because such real-time text has loose timing requirements;
RFC 4103 recommends sending text in chunks every 300ms. Such text is
technically real-time, but does not need a PHB promising better
service than best effort, in contrast to audio or video.
6. IANA Considerations
This document includes no request to IANA.
7. Security Considerations
The security considerations for all of the technologies discussed in
this document apply; in particular see the security considerations
for RTP in [RFC3550] and DiffServ in [RFC2474] and [RFC2475].
Multiplexing of multiple protocols onto a single UDP 5-tuple via
encapsulation has implications for network functionality that is
based on monitoring or inspection of individual protocol flows, e.g.,
firewalls and traffic monitoring systems. When implementations of
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such functionality lack visibility into encapsulated traffic (likely
for many current implementations), it may be difficult or impossible
to apply network security policy and controls at a finer grain than
the overall UDP 5-tuple.
Use of multiple DSCPs to provide differentiated QoS service may
reveal information about the encrypted traffic to which different
service levels are provided. For example, DSCP-based identification
of RTP packet streams combined with packet frequency and packet size
could reveal the type or nature of the encrypted source streams. The
IP header used for forwarding has to be unencrypted for obvious
reasons, and the DSCP likewise has to be unencrypted in order to
enable different IP forwarding behaviors to be applied to different
packets. The nature of encrypted traffic components can be disguised
via encrypted dummy data padding and encrypted dummy packets, e.g.,
see the discussion of traffic flow confidentiality in [RFC4303].
Encrypted dummy packets could even be added in a fashion that an
observer of the overall encrypted traffic might mistake for another
encrypted RTP packet stream.
8. Acknowledgements
This document is the result of many conversations that have occurred
within multiple working groups in the RAI and Transport areas. Many
thanks to Harald Alvestrand, Erin Bournival, Brian Carpenter,
Ruediger Geib and James Polk for their reviews and input.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. 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.
[RFC3246] Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
J., Courtney, W., Davari, S., Firoiu, V., and D.
Stiliadis, "An Expedited Forwarding PHB (Per-Hop
Behavior)", RFC 3246, March 2002.
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[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3662] Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort
Per-Domain Behavior (PDB) for Differentiated Services",
RFC 3662, December 2003.
[RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, May 2010.
9.2. Informative References
[I-D.geib-tsvwg-diffserv-intercon]
Geib, R., "DiffServ interconnection classes and practice",
draft-geib-tsvwg-diffserv-intercon-05 (work in progress),
February 2014.
[I-D.ietf-avtext-rtp-grouping-taxonomy]
Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro,
"A Taxonomy of Grouping Semantics and Mechanisms for Real-
Time Transport Protocol (RTP) Sources", draft-ietf-avtext-
rtp-grouping-taxonomy-01 (work in progress), February
2014.
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for Brower-
based Applications", draft-ietf-rtcweb-overview-09 (work
in progress), February 2014.
[I-D.ietf-rtcweb-rtp-usage]
Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time
Communication (WebRTC): Media Transport and Use of RTP",
draft-ietf-rtcweb-rtp-usage-15 (work in progress), May
2014.
[I-D.ietf-rtcweb-transports]
Alvestrand, H., "Transports for RTCWEB", draft-ietf-
rtcweb-transports-04 (work in progress), April 2014.
[I-D.petithuguenin-avtcore-rfc5764-mux-fixes]
Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
Updates for Secure Real-time Transport Protocol (SRTP)
Extension for Datagram Transport Layer Security (DTLS)",
draft-petithuguenin-avtcore-rfc5764-mux-fixes-00 (work in
progress), July 2014.
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[I-D.westerlund-avtcore-transport-multiplexing]
Westerlund, M. and C. Perkins, "Multiplexing Multiple RTP
Sessions onto a Single Lower-Layer Transport", draft-
westerlund-avtcore-transport-multiplexing-07 (work in
progress), October 2013.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC2697] Heinanen, J. and R. Guerin, "A Single Rate Three Color
Marker", RFC 2697, September 1999.
[RFC2698] Heinanen, J. and R. Guerin, "A Two Rate Three Color
Marker", RFC 2698, September 1999.
[RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
2914, September 2000.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC
3168, September 2001.
[RFC3270] Le Faucheur, F., Wu, L., Davie, B., Davari, S., Vaananen,
P., Krishnan, R., Cheval, P., and J. Heinanen, "Multi-
Protocol Label Switching (MPLS) Support of Differentiated
Services", RFC 3270, May 2002.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, June 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
[RFC4594] Babiarz, J., Chan, K., and F. Baker, "Configuration
Guidelines for DiffServ Service Classes", RFC 4594, August
2006.
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error
Correction", RFC 5109, December 2007.
[RFC5127] Chan, K., Babiarz, J., and F. Baker, "Aggregation of
Diffserv Service Classes", RFC 5127, February 2008.
[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, January 2008.
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[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, February 2009.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437, November 2011.
[W3C.WD-mediacapture-streams-20130903]
Burnett, D., Bergkvist, A., Jennings, C., and A.
Narayanan, "Media Capture and Streams", World Wide Web
Consortium WD WD-mediacapture-streams-20130903, September
2013, <http://www.w3.org/TR/2013/
WD-mediacapture-streams-20130903>.
Appendix A. Change History
[To be removed before RFC publication.]
Changes from draft-york-dart-dscp-rtp-00 to -01
o Added examples (Section 5)
o Reworked text on RTP session multiplexing, at most one RTP session
can be used per UDP 5-tuple.
o Initial terminology alignment with RTP grouping taxonomy draft.
o Added Section 2.5 on real-time communication interaction w/
reordering based on text from Harald Alvestrand.
o Strengthened warnings on loss of differentiation, but indicate
that differentiation may still be useful from source to point of
loss.
o Added a few sentences on DiffServ and MPLS.
o Added discussion of UDP-encapsulated protocols that are reordering
sensitive.
o Added initial security considerations.
o Many editorial changes
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Authors' Addresses
Dan York
Internet Society
Keene, N.H.
USA
Phone: +1-802-735-1624
Email: dyork@lodestar2.com
David Black (editor)
EMC
176 South Street
Hopkinton, MA 01748
USA
Phone: +1 508 293-7953
Email: david.black@emc.com
Cullen Jennings
Cisco
170 West Tasman Drive
MS: SJC-21/2
San Jose, CA 95134
USA
Phone: +1 408 421-9990
Email: fluffy@cisco.com
Paul Jones
Cisco
7025 Kit Creek Road
Research Triangle Park, MA 27502
USA
Phone: +1 919 476 2048
Email: paulej@cisco.com
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