DiffServ Applied to Real-time Transports D. Black, Ed.
Internet-Draft EMC
Intended status: Informational P. Jones
Expires: February 1, 2015 Cisco
July 31, 2014
Differentiated Services (DiffServ) and Real-time Communication
draft-ietf-dart-dscp-rtp-00
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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This Internet-Draft will expire on February 1, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
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. Drop Precedence . . . . . . . . . . . . . . . . . . . . . 10
2.7. Traffic Classifiers and DSCP Remarking . . . . . . . . . 11
3. RTP Multiplexing Background . . . . . . . . . . . . . . . . . 13
4. Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . 14
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
9.1. Normative References . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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
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].
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2. Background
[Editor's Note: Current section structure draft skips around topics
somewhat. The editor suggestsrestructuring to put real-time/RTP
material first (new section 2, consisting of current sections 2, 2.1
and 3), then DiffServ Background (new section 3, consisting of
current sections 2.2, 2.3, 2.6 and 2.7, followed by discussion of
interactions (new section 4, consisting of current sections 2.4, 2.5
and 5) and guidelines (current section 4, renumbered to new section
5).]
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 Stream: A stream of RTP packets containing media data, which may
be source data or redundant data. The RTP Packet Stream is
identified by an RTP synchronization source (SSRC) belonging to a
particular RTP session.
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Media encoding and packetization of a source stream results in a
source RTP stream plus zero or more redundancy RTP streams that
provide resilience against loss of packets from the source RTP stream
[I-D.ietf-avtext-rtp-grouping-taxonomy]. Redundancy information may
also be carried in the same RTP 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 streams.
This is referred to as RTP multiplexing.
The number of source streams and RTP 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
streams that provide protection to a source RTP stream, using
techniques such as Forward Error Correction. Another example is
simulcast transmission, where a video source stream can be
transmitted as high resolution and low resolution RTP streams at the
same time. In this case, a media processing function might choose to
send one or both RTP 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 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.
For the RTCWEB protocol suite [I-D.ietf-rtcweb-transports], 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 stream plus zero or more redundancy RTP
streams, so a MediaStream that consists of one MediaStreamTrack is
transmitted as a single source RTP stream plus zero or more
redundancy RTP streams. For more information on use of RTP in
RTCWEB, see [I-D.ietf-rtcweb-rtp-usage].
Other transport protocols may also be used to transmit real-time data
or near-real-time data. For example, SCTP [RFC4960] 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 encompasses a number of forms of
multiplexing:
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1. Individual source streams are carried in one or more individual
RTP streams that can be multiplexed into a single RTP session as
described in [RFC3550];
2. RTCP (see [RFC3550]) may be multplexed with the RTP session as
described in [RFC5761];
3. An RTP session could be multiplexed with other protocols via UDP
encapsulation over a common pair of UDP ports as described in
[RFC5764] as updated by
[I-D.petithuguenin-avtcore-rfc5764-mux-fixes]; and
4. The data may be further encapsulated via STUN [RFC5389] and TURN
[RFC5766] for NAT (Network Address Translator) traversal.
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. SDP bundle negotiation restrictions
[I-D.ietf-mmusic-sdp-bundle-negotiation] limit RTCWEB to using at
most a single DTLS session per UDP 5-tuple. In contrast, multple
SCTP associations can be mulitplexed over a single UDP 5-tuple
[RFC6951].
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.
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:
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
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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
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
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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 are intended 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. This generalizes to ranges of class selector
codepoints, but with limits - for example CS6 and CS7 are often
used for network control (e.g., routing) traffic [RFC4594] and
hence are likely to provide better forwarding behavior under
network load in order to prioritize network recovery from
disruptions.
o CS1 ('001000') was subsequently designated as the recommended
codepoint for the Lower Effort (LE) PHB [RFC3662]. An LE service
forwards traffic with "lower" priority than best effort and can be
"starved" by best effort and other "higher" priority traffic. Not
all networks offer an LE service. See [RFC3662] for further
discussion of the LE PHB and service.
Applications and traffic sources cannot rely upon different class
selector codepoints providing differentiated services or upon the
presence of an LE service that is selected by the CS1 DSCP. There is
no effective way for a network endpoint to determine whether the CS1
DSCP selects an LE service on a specific network, let alone end-to-
end. Packets marked with the CS1 DSCP may be forwarded with best
effort service or another "higher" priority service, 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 intended 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: Add a sentence or two on DCCP - 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 [RFC0793]
provides reliable in-order delivery of data with congestion control.
SCTP [RFC4960] 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 [RFC0768] 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, the protocol 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
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.
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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).
Reordering also affects other forms of congestion control, such as
techniques for RTP congestion control that were under development
when this document was published, see
[I-D.ietf-rmcat-cc-requirements] for requirements. These techniques
prefer use of a common (coupled) congestion controller for RTP
streams between the same endpoints in order to reduce packet loss and
delay by reducing competition for resources at any shared bottleneck.
Shared bottlenecks can be detected via correlations of measured
metrics such as one-way delay. An alternative approach assumes that
the set of packets on a single 5-tuple marked with DSCPs that do not
allow reordering will utilize a common network path and common
forwarding resources at each network node. Under that assumption,
any bottleneck encountered by such packets is shared among all of
them, making it safe to use a common (coupled) congestion controller,
see [I-D.welzl-rmcat-coupled-cc]. This is not a safe assumption when
the packets involved are marked with DSCP values that allow
reordering because a bottleneck may not be shared among all such
packets (e.g., if the DSCPs result in use of different queues at a
network node, only one of which is a bottleneck).
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 the specific usage of UDP by RTCWEB, 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 2.4). The degree of sensitivity depends on
protocol or stream timers, in contrast to reliable delivery protocols
that usually react to all reordering.
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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 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 discard data that is sufficiently late that
it cannot be rendered in source stream order, making
retransmission counterproductive. For this reason,
implementations of interactive real-time communication often do
not use retransmission.
o In contrast, replay of recorded media can tolerate signficantly
longer delays than interactive conversations, so replay is likely
to use larger jitter buffers than interactive conversations.
These larger jitter buffers increase the tolerance of replay to
reordering by comparison to 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
can be expected to 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. Drop Precedence
Each DiffServ AF class consists of three PHBs that differ solely in
drop precedence (e.g., AF3x consists of AF31, AF32 and AF33).
Reordering is prohibited among packets on the same 5-tuple that use
PHBs within a single AF class; further, these packets can be expected
to draw upon the same forwarding resources on network nodes (e.g.,
use the same router queue) and hence use of multple drop precedences
within an AF class is not expected to impact latency.
When PHBs within a single AF class are mixed for a protocol session,
the resulting drop likelihood is a mix of the drop likelihoods of the
PHBs involved. The primary effect of multiple drop precedences is to
influence decisions on what to drop with the goal that less important
packets are dropped in preference to more important packets.
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There are situations in which drop precedences should not be mixed.
A simple example is that there is little value in mixing drop
precedences iwthin a TCP connection, because TCP's ordered delivery
behavior results in any drop requiring the receiver to wait for the
dropped packet to be retransmitted. Any resulting delay depends on
the RTT and not the packe that was dropped. Hence a single PHB and
DSCP should be used for all packets in a TCP connection.
SCTP [RFC4960] differs from TCP in a number of ways, including the
ability to deliver messages in an order that differs from the order
in which they were sent and support for unreliable streams. However,
SCTP performs congestion control and retransmission across the entire
association, and not on a per-stream basis. Although there may be
advantages to using multiple drop precedence across SCTP streams or
within an SCTP stream that does not use reliable ordered delivery,
there is no practical operational experience in doing so (e.g., the
SCTP sockets API [RFC6458] does not support use of more than one DSCP
for an SCTP association). As a consequence, the impacts on SCTP
protocol and implementation behavior are unknown and difficult to
predict. Hence a single PHB and DSCP should be used for all packets
in an SCTP association, independent of the number or nature of
streams in that association.
RTCP multi-stream reporting optimizations for an RTP session
[I-D.ietf-avtcore-rtp-multi-stream-optimisation] assume that the RTP
streams involved experience the same packet loss behavior. This
mechanism is highly inappropriate if the RTP streams involved use
different PHBs, even if those PHBs differ solely in drop precedence.
2.7. 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 specific
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
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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
presented at the destination endpoint. Rather, 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 classifier for whatever 5-tuple it
carries. DiffServ classifiers generally ignore embedded protocol
headers (e.g., for SCTP or RTP embedded in UDP, header-based
classification is unlikely to look beyond 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
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].
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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 Network Address
Translator and Firewall (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.
The STUN [RFC5389]/ICE [RFC5245]/TURN [RFC5766]protocol family
provides NAT/FW traversal and port mapping for protocols (e.g., those
in the RTCWEB protocol suite) via communication with a relay server.
These protocols were originally designed for use of UDP, however,
they have been extended to use TCP as a transport for situations in
which UDP does not work [RFC6062].
When TCP is selected for NAT/FW traversal, a single PHB and DSCP
should be used for all traffic on that TCP connection for the reasons
discussed in Section 2.4 and Section 2.6 above. An additional reason
for this recommendation is that packetization for STUN/ICE/TURN
occurs before passing the resulting packets to TCP; TCP
resegmentation may result in a different packetization on the wire,
breaking any association between DSCPs and specific data to which
they are intended to apply.
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, especially when multiple protocol sessions
are involved. 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
likelihood of loss of a packet for one of these protocols; any such
loss adds delay to setting up a communication session. Further,
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reducing the number of STUN/ICE/TURN tasks places a lower burden 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. Guidelines
The only use of multiple standardized PHBs and DSCPs that prevents
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 allow reordering
within a single RTP 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 2.5 above. When a common (coupled)
congestion controller is used across multiple RTP streams, this
recommendation against use of PHBs and DSCPs that allow reordering
applies across all of the RTP streams that are within the scope of
a single common (coupled) congestion controller.
o Should use a single PHB and DSCP for an RTCP session, primarily to
avoid reordering for RTCP (and because there is no compelling
reason for use of different drop precedences. One of the PHBs and
associated DSCP used for the associated RTP traffic would be an
appropriate choice.
o Should not use different PHBs and DSCPs that allow reordering
within a reliable transport protocol session (e.g., TCP
connection, SCTP association). Receivers for such protocols
interpret reordering as indicating loss of some of the out-of-
order packets; see Section 2.4. For SCTP, this requirement
applies across the entire SCTP association, and not just to
individual streams within an association because SCTP's reliable
transmission functionality operates on the overall association.
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o Should use a a single PHB and DSCP for all packets in a single TCP
connection, and likewise for a single STP association. See
Section 2.6.
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 2.7 above.
o Cannot rely on end-to-end preservation of DSCPs as network node
remarking can change DSCPs and remove drop precedence distinctions
see Section 2.7 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 limit use of the CS1 codepoint to traffic for which best
effor forwarding is acceptable, 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).
[Editor's note: rtcweb-transports draft is not aligned with the
above. The rtcweb WG and the draft author will bring it into line.]
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
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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,
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 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 missing
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 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 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.
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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
monitors or inspects individual protocol flows, e.g., firewalls and
traffic monitoring systems. When implementations of such
functionality lack visibility into encapsulated traffic (likely for
many current implementations), it may be difficult or impossible to
apply network security policy and associated controls at a finer
granularity 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 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 stream.
8. Acknowledgements
This document is the result of many conversations that have occurred
within the dart working group and multiple other working groups in
the RAI and Transport areas. Many thanks to Harald Alvestrand, Erin
Bournival, Brian Carpenter, Keith Drage, Ruediger Geib, Cullen
Jennings, Jonathan Lennox, Karen Nielsen, Colin Perkins, James Polk,
Michael Welzl, Dan York and DART WG participants for their reviews
and comments.
[Editor's Note: Check which references should be Normative.]
9. References
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9.1. Normative References
[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.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[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.
[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.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol", RFC
4960, September 2007.
[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.
[RFC5865] Baker, F., Polk, J., and M. Dolly, "A Differentiated
Services Code Point (DSCP) for Capacity-Admitted Traffic",
RFC 5865, May 2010.
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[RFC6951] Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
Control Transmission Protocol (SCTP) Packets for End-Host
to End-Host Communication", RFC 6951, May 2013.
9.2. Informative References
[I-D.geib-tsvwg-diffserv-intercon]
Geib, R., "DiffServ interconnection classes and practice",
draft-geib-tsvwg-diffserv-intercon-06 (work in progress),
July 2014.
[I-D.ietf-avtcore-rtp-multi-stream-optimisation]
Lennox, J., Westerlund, M., Wu, W., and C. Perkins,
"Sending Multiple Media Streams in a Single RTP Session:
Grouping RTCP Reception Statistics and Other Feedback",
draft-ietf-avtcore-rtp-multi-stream-optimisation-03 (work
in progress), July 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-02 (work in progress), June 2014.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Negotiating Media Multiplexing Using the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
negotiation-07 (work in progress), April 2014.
[I-D.ietf-rmcat-cc-requirements]
Jesup, R., "Congestion Control Requirements For RMCAT",
draft-ietf-rmcat-cc-requirements-05 (work in progress),
July 2014.
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for
Browser-based Applications", draft-ietf-rtcweb-overview-10
(work in progress), June 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.
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[I-D.ietf-rtcweb-transports]
Alvestrand, H., "Transports for RTCWEB", draft-ietf-
rtcweb-transports-05 (work in progress), June 2014.
[I-D.welzl-rmcat-coupled-cc]
Welzl, M., Islam, S., and S. Gjessing, "Coupled congestion
control for RTP media", draft-welzl-rmcat-coupled-cc-03
(work in progress), May 2014.
[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.
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[RFC5129] Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
Marking in MPLS", RFC 5129, January 2008.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, February 2009.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC6062] Perreault, S. and J. Rosenberg, "Traversal Using Relays
around NAT (TURN) Extensions for TCP Allocations", RFC
6062, November 2010.
[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437, November 2011.
[RFC6458] Stewart, R., Tuexen, M., Poon, K., Lei, P., and V.
Yasevich, "Sockets API Extensions for the Stream Control
Transmission Protocol (SCTP)", RFC 6458, December 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)
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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
Changes from draft-york-dart-dscp-rtp-01 to -02
o More terminology alignment with RTP grouping taxonomy draft: "RTP
packet stream" -> "RTP stream"
o Aligned terminology for less-than-best-effort with RFC 3662 - LE
(Lower Effort) PHB and service
o Minor reference updates
Changes from draft-york-dart-dscp-rtp-02 to draft-ietf-dart-dscp-
rtp-00
o Reduce author list and convert to Informational - remove RFC 2119
reference and keywords
o Strengthen TCP and SCTP text.
o Add section 2.6 on drop precedence.
o Remove discussion of multiplexing multiple RTP sessions on a
single UDP 5-tuple
o Add discussions of RTCP,STUN/ICE/TURN and coupled congestion
control
o Many editorial changes.
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o Lots of additional references
Authors' Addresses
David Black (editor)
EMC
176 South Street
Hopkinton, MA 01748
USA
Phone: +1 508 293-7953
Email: david.black@emc.com
Paul Jones
Cisco
7025 Kit Creek Road
Research Triangle Park, MA 27502
USA
Phone: +1 919 476 2048
Email: paulej@packetizer.com
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