Network Working Group J. Babiarz
Request for Comments: 4594 K. Chan
Category: Informational Nortel Networks
F. Baker
Cisco Systems
August 2006
Configuration Guidelines for DiffServ Service Classes
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes service classes configured with Diffserv and
recommends how they can be used and how to construct them using
Differentiated Services Code Points (DSCPs), traffic conditioners,
Per-Hop Behaviors (PHBs), and Active Queue Management (AQM)
mechanisms. There is no intrinsic requirement that particular DSCPs,
traffic conditioners, PHBs, and AQM be used for a certain service
class, but as a policy and for interoperability it is useful to apply
them consistently.
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Table of Contents
1. Introduction ....................................................3
1.1. Requirements Notation ......................................4
1.2. Expected Use in the Network ................................4
1.3. Service Class Definition ...................................5
1.4. Key Differentiated Services Concepts .......................5
1.4.1. Queuing .............................................6
1.4.1.1. Priority Queuing ...........................6
1.4.1.2. Rate Queuing ...............................6
1.4.2. Active Queue Management .............................7
1.4.3. Traffic Conditioning ................................7
1.4.4. Differentiated Services Code Point (DSCP) ...........8
1.4.5. Per-Hop Behavior (PHB) ..............................8
1.5. Key Service Concepts .......................................8
1.5.1. Default Forwarding (DF) .............................9
1.5.2. Assured Forwarding (AF) .............................9
1.5.3. Expedited Forwarding (EF) ..........................10
1.5.4. Class Selector (CS) ................................10
1.5.5. Admission Control ..................................11
2. Service Differentiation ........................................11
2.1. Service Classes ...........................................12
2.2. Categorization of User Service Classes ....................13
2.3. Service Class Characteristics .............................16
2.4. Deployment Scenarios ......................................21
2.4.1. Example 1 ..........................................21
2.4.2. Example 2 ..........................................23
2.4.3. Example 3 ..........................................25
3. Network Control Traffic ........................................27
3.1. Current Practice in the Internet ..........................27
3.2. Network Control Service Class .............................27
3.3. OAM Service Class .........................................29
4. User Traffic ...................................................30
4.1. Telephony Service Class ...................................31
4.2. Signaling Service Class ...................................33
4.3. Multimedia Conferencing Service Class .....................35
4.4. Real-Time Interactive Service Class .......................37
4.5. Multimedia Streaming Service Class ........................39
4.6. Broadcast Video Service Class .............................41
4.7. Low-Latency Data Service Class ............................43
4.8. High-Throughput Data Service Class ........................45
4.9. Standard Service Class ....................................47
4.10. Low-Priority Data ........................................48
5. Additional Information on Service Class Usage ..................49
5.1. Mapping for Signaling .....................................49
5.2. Mapping for NTP ...........................................50
5.3. VPN Service Mapping .......................................50
6. Security Considerations ........................................51
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7. Acknowledgements ...............................................52
8. Appendix A .....................................................53
8.1. Explanation of Ring Clipping ..............................53
9. References .....................................................54
9.1. Normative References ......................................54
9.2. Informative References ....................................55
1. Introduction
To aid in understanding the role of this document, we use an analogy:
the Differentiated Services specifications are fundamentally a
toolkit. The specifications provide the equivalent of band saws,
planers, drill presses, and other tools. In the hands of an expert,
there is no limit to what can be built, but such a toolkit can be
intimidating to the point of being inaccessible to a non-expert who
just wants to build a bookcase. This document should be viewed as a
set of "project plans" for building all the (diffserv) furniture that
one might want. The user may choose what to build (e.g., perhaps our
non-expert doesn't need a china cabinet right now), and how to go
about building it (e.g., plans for a non-expert probably won't employ
mortise/tenon construction, but that absence does not imply that
mortise/tenon construction is forbidden or unsound). The authors
hope that these diffserv "project plans" will provide a useful guide
to Network Administrators in the use of diffserv techniques to
implement quality-of-service measures appropriate for their network's
traffic.
This document describes service classes configured with Diffserv and
recommends how they can be used and how to construct them using
Differentiated Services Code Points (DSCPs), traffic conditioners,
Per-Hop Behaviors (PHBs), and Active Queue Management (AQM)
mechanisms. There is no intrinsic requirement that particular DSCPs,
traffic conditioners, PHBs, and AQM be used for a certain service
class, but as a policy and for interoperability it is useful to apply
them consistently.
Service class definitions are based on the different traffic
characteristics and required performance of the
applications/services. This approach allows us to map current and
future applications/services of similar traffic characteristics and
performance requirements into the same service class. Since the
applications'/services' characteristics and required performance are
end to end, the service class notion needs to be preserved end to
end. With this approach, a limited set of service classes is
required. For completeness, we have defined twelve different service
classes, two for network operation/administration and ten for
user/subscriber applications/services. However, we expect that
network administrators will implement a subset of these classes
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relevant to their customers and their service offerings. Network
Administrators may also find it of value to add locally defined
service classes, although these will not necessarily enjoy end-to-end
properties of the same type.
Section 1 provides an introduction and overview of technologies that
are used for service differentiation in IP networks. Section 2 is an
overview of how service classes are constructed to provide service
differentiation, with examples of deployment scenarios. Section 3
provides configuration guidelines of service classes that are used
for stable operation and administration of the network. Section 4
provides configuration guidelines of service classes that are used
for differentiation of user/subscriber traffic. Section 5 provides
additional guidance on mapping different applications/protocols to
service classes. Section 6 addresses security considerations.
1.1. Requirements Notation
The key words "SHOULD", "SHOULD NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in [RFC2119].
1.2. Expected Use in the Network
In the Internet today, corporate LANs and ISP WANs are generally not
heavily utilized. They are commonly 10% utilized at most. For this
reason, congestion, loss, and variation in delay within corporate
LANs and ISP backbones is virtually unknown. This clashes with user
perceptions, for three very good reasons.
o The industry moves through cycles of bandwidth boom and bandwidth
bust, depending on prevailing market conditions and the periodic
deployment of new bandwidth-hungry applications.
o In access networks, the state is often different. This may be
because throughput rates are artificially limited or over-
subscribed, or because of access network design trade-offs.
o Other characteristics, such as database design on web servers
(that may create contention points, e.g., in filestore) and
configuration of firewalls and routers, often look externally like
a bandwidth limitation.
The intent of this document is to provide a consistent marking,
conditioning, and packet treatment strategy so that it can be
configured and put into service on any link that is itself congested.
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1.3. Service Class Definition
A "service class" represents a set of traffic that requires specific
delay, loss, and jitter characteristics from the network.
Conceptually, a service class pertains to applications with similar
characteristics and performance requirements, such as a "High-
Throughput Data" service class for applications like the web and
electronic mail, or a "Telephony" service class for real-time traffic
such as voice and other telephony services. Such a service class may
be defined locally in a Differentiated Services (DS) domain, or
across multiple DS domains, possibly extending end to end.
A service class as defined here is essentially a statement of the
required characteristics of a traffic aggregate. The required
characteristics of these traffic aggregates can be realized by the
use of defined per-hop behavior (PHB) [RFC2474]. The actual
specification of the expected treatment of a traffic aggregate within
a domain may also be defined as a per-domain behavior (PDB)
[RFC3086].
Each domain may choose to implement different service classes or to
use different behaviors to implement the service classes or to
aggregate different kinds of traffic into the aggregates and still
achieve their required characteristics. For example, low delay,
loss, and jitter may be realized using the EF PHB, or with an over-
provisioned AF PHB. This must be done with care as it may disrupt
the end-to-end performance required by the applications/services.
This document provides recommendations on usage of PHBs for specific
service classes for their consistent implementation. These
recommendations are not to be construed as prohibiting use of other
PHBs that realize behaviors sufficient for the relevant class of
traffic.
The Default Forwarding "Standard" service class is REQUIRED; all
other service classes are OPTIONAL. It is expected that network
administrators will base their choice of the level of service
differentiation that they will support on their need, starting off
with three or four service classes for user traffic and adding others
as the need arises.
1.4. Key Differentiated Services Concepts
The reader SHOULD be familiar with the principles of the
Differentiated Services Architecture [RFC2474]. We recapitulate key
concepts here only to provide convenience for the reader, the
referenced RFCs providing the authoritative definitions.
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1.4.1. Queuing
A queue is a data structure that holds packets that are awaiting
transmission. The packets may be delayed while in the queue,
possibly due to lack of bandwidth, or because it is low in priority.
There are a number of ways to implement a queue. A simple model of a
queuing system, however, is a set of data structures for packet data,
which we will call queues, and a mechanism for selecting the next
packet from among them, which we call a scheduler.
1.4.1.1. Priority Queuing
A priority queuing system is a combination of a set of queues and a
scheduler that empties them in priority sequence. When asked for a
packet, the scheduler inspects the highest priority queue and, if
there is data present, returns a packet from that queue. Failing
that, it inspects the next highest priority queue, and so on. A
freeway onramp with a stoplight for one lane that allows vehicles in
the high-occupancy-vehicle lane to pass is an example of a priority
queuing system; the high-occupancy-vehicle lane represents the
"queue" having priority.
In a priority queuing system, a packet in the highest priority queue
will experience a readily calculated delay. This is proportional to
the amount of data remaining to be serialized when the packet arrived
plus the volume of the data already queued ahead of it in the same
queue. The technical reason for using a priority queue relates
exactly to this fact: it limits delay and variations in delay and
should be used for traffic that has that requirement.
A priority queue or queuing system needs to avoid starvation of
lower-priority queues. This may be achieved through a variety of
means, such as admission control, rate control, or network
engineering.
1.4.1.2. Rate Queuing
Similarly, a rate-based queuing system is a combination of a set of
queues and a scheduler that empties each at a specified rate. An
example of a rate-based queuing system is a road intersection with a
stoplight. The stoplight acts as a scheduler, giving each lane a
certain opportunity to pass traffic through the intersection.
In a rate-based queuing system, such as Weighted Fair Queuing (WFQ)
or Weighted Round Robin (WRR), the delay that a packet in any given
queue will experience depends on the parameters and occupancy of its
queue and the parameters and occupancy of the queues it is competing
with. A queue whose traffic arrival rate is much less than the rate
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at which it lets traffic depart will tend to be empty, and packets in
it will experience nominal delays. A queue whose traffic arrival
rate approximates or exceeds its departure rate will tend not to be
empty, and packets in it will experience greater delay. Such a
scheduler can impose a minimum rate, a maximum rate, or both, on any
queue it touches.
1.4.2. Active Queue Management
Active Queue Management, or AQM, is a generic name for any of a
variety of procedures that use packet dropping or marking to manage
the depth of a queue. The canonical example of such a procedure is
Random Early Detection (RED), in that a queue is assigned a minimum
and maximum threshold, and the queuing algorithm maintains a moving
average of the queue depth. While the mean queue depth exceeds the
maximum threshold, all arriving traffic is dropped. While the mean
queue depth exceeds the minimum threshold but not the maximum
threshold, a randomly selected subset of arriving traffic is marked
or dropped. This marking or dropping of traffic is intended to
communicate with the sending system, causing its congestion avoidance
algorithms to kick in. As a result of this behavior, it is
reasonable to expect that TCP's cyclic behavior is desynchronized and
that the mean queue depth (and therefore delay) should normally
approximate the minimum threshold.
A variation of the algorithm is applied in Assured Forwarding PHB
[RFC2597], in that the behavior aggregate consists of traffic with
multiple DSCP marks, which are intermingled in a common queue.
Different minima and maxima are configured for the several DSCPs
separately, such that traffic that exceeds a stated rate at ingress
is more likely to be dropped or marked than traffic that is within
its contracted rate.
1.4.3. Traffic Conditioning
In addition, at the first router in a network that a packet crosses,
arriving traffic may be measured and dropped or marked according to a
policy, or perhaps shaped on network ingress, as in "A Rate Adaptive
Shaper for Differentiated Services" [RFC2963]. This may be used to
bias feedback loops, as is done in "Assured Forwarding PHB"
[RFC2597], or to limit the amount of traffic in a system, as is done
in "Expedited Forwarding PHB" [RFC3246]. Such measurement procedures
are collectively referred to as "traffic conditioners". Traffic
conditioners are normally built using token bucket meters, for
example with a committed rate and burst size, as in Section 1.5.3 of
the DiffServ Model [RFC3290]. The Assured Forwarding PHB [RFC2597]
uses a variation on a meter with multiple rate and burst size
measurements to test and identify multiple levels of conformance.
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Multiple rates and burst sizes can be realized using multiple levels
of token buckets or more complex token buckets; these are
implementation details. The following are some traffic conditioners
that may be used in deployment of differentiated services:
o For Class Selector (CS) PHBs, a single token bucket meter to
provide a rate plus burst size control.
o For Expedited Forwarding (EF) PHB, a single token bucket meter to
provide a rate plus burst size control.
o For Assured Forwarding (AF) PHBs, usually two token bucket meters
configured to provide behavior as outlined in "Two Rate Three
Color Marker (trTCM)" [RFC2698] or "Single Rate Three Color Marker
(srTCM)" [RFC2697]. The two-rate, three-color marker is used to
enforce two rates, whereas the single-rate, three-color marker is
used to enforce a committed rate with two burst lengths.
1.4.4. Differentiated Services Code Point (DSCP)
The DSCP is a number in the range 0..63 that is placed into an IP
packet to mark it according to the class of traffic it belongs in.
Half of these values are earmarked for standardized services, and the
other half of them are available for local definition.
1.4.5. Per-Hop Behavior (PHB)
In the end, the mechanisms described above are combined to form a
specified set of characteristics for handling different kinds of
traffic, depending on the needs of the application. This document
seeks to identify useful traffic aggregates and to specify what PHB
should be applied to them.
1.5. Key Service Concepts
While Differentiated Services is a general architecture that may be
used to implement a variety of services, three fundamental forwarding
behaviors have been defined and characterized for general use. These
are basic Default Forwarding (DF) behavior for elastic traffic, the
Assured Forwarding (AF) behavior, and the Expedited Forwarding (EF)
behavior for real-time (inelastic) traffic. The facts that four code
points are recommended for AF and that one code point is recommended
for EF are arbitrary choices, and the architecture allows any
reasonable number of AF and EF classes simultaneously. The choice of
four AF classes and one EF class in the current document is also
arbitrary, and operators MAY choose to operate more or fewer of
either.
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The terms "elastic" and "real-time" are defined in [RFC1633], Section
3.1, as a way of understanding broad-brush application requirements.
This document should be reviewed to obtain a broad understanding of
the issues in quality of service, just as [RFC2475] should be
reviewed to understand the data plane architecture used in today's
Internet.
1.5.1. Default Forwarding (DF)
The basic forwarding behaviors applied to any class of traffic are
those described in [RFC2474] and [RFC2309]. Best-effort service may
be summarized as "I will accept your packets" and is typically
configured with some bandwidth guarantee. Packets in transit may be
lost, reordered, duplicated, or delayed at random. Generally,
networks are engineered to limit this behavior, but changing traffic
loads can push any network into such a state.
Application traffic in the internet that uses default forwarding is
expected to be "elastic" in nature. By this, we mean that the sender
of traffic will adjust its transmission rate in response to changes
in available rate, loss, or delay.
For the basic best-effort service, a single DSCP value is provided to
identify the traffic, a queue to store it, and active queue
management to protect the network from it and to limit delays.
1.5.2. Assured Forwarding (AF)
The Assured Forwarding PHB [RFC2597] behavior is explicitly modeled
on Frame Relay's Discard Eligible (DE) flag or ATM's Cell Loss
Priority (CLP) capability. It is intended for networks that offer
average-rate Service Level Agreements (SLAs) (as FR and ATM networks
do). This is an enhanced best-effort service; traffic is expected to
be "elastic" in nature. The receiver will detect loss or variation
in delay in the network and provide feedback such that the sender
adjusts its transmission rate to approximate available capacity.
For such behaviors, multiple DSCP values are provided (two or three,
perhaps more using local values) to identify the traffic, a common
queue to store the aggregate, and active queue management to protect
the network from it and to limit delays. Traffic is metered as it
enters the network, and traffic is variously marked depending on the
arrival rate of the aggregate. The premise is that it is normal for
users occasionally to use more capacity than their contract
stipulates, perhaps up to some bound. However, if traffic should be
marked or lost to manage the queue, this excess traffic will be
marked or lost first.
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1.5.3. Expedited Forwarding (EF)
The intent of Expedited Forwarding PHB [RFC3246] is to provide a
building block for low-loss, low-delay, and low-jitter services. It
can be used to build an enhanced best-effort service: traffic remains
subject to loss due to line errors and reordering during routing
changes. However, using queuing techniques, the probability of delay
or variation in delay is minimized. For this reason, it is generally
used to carry voice and for transport of data information that
requires "wire like" behavior through the IP network. Voice is an
inelastic "real-time" application that sends packets at the rate the
codec produces them, regardless of availability of capacity. As
such, this service has the potential to disrupt or congest a network
if not controlled. It also has the potential for abuse.
To protect the network, at minimum one SHOULD police traffic at
various points to ensure that the design of a queue is not overrun,
and then the traffic SHOULD be given a low-delay queue (often using
priority, although it is asserted that a rate-based queue can do
this) to ensure that variation in delay is not an issue, to meet
application needs.
1.5.4. Class Selector (CS)
Class Selector provides support for historical codepoint definitions
and PHB requirement. The Class Selector DS field provides a limited
backward compatibility with legacy (pre DiffServ) practice, as
described in [RFC2474], Section 4. Backward compatibility is
addressed in two ways. First, there are per-hop behaviors that are
already in widespread use (e.g., those satisfying the IPv4 Precedence
queuing requirements specified in [RFC1812]), and we wish to permit
their continued use in DS-compliant networks. In addition, there are
some codepoints that correspond to historical use of the IP
Precedence field, and we reserve these codepoints to map to PHBs that
meet the general requirements specified in [RFC2474], Section
4.2.2.2.
No attempt is made to maintain backward compatibility with the "DTR"
or Type of Service (TOS) bits of the IPv4 TOS octet, as defined in
[RFC0791] and [RFC1349].
A DS-compliant network can be deployed with a set of one or more
Class Selector-compliant PHB groups. Also, a network administrator
may configure the network nodes to map codepoints to PHBs,
irrespective of bits 3-5 of the DSCP field, to yield a network that
is compatible with historical IP Precedence use. Thus, for example,
codepoint '011000' would map to the same PHB as codepoint '011010'.
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1.5.5. Admission Control
Admission control (including refusal when policy thresholds are
crossed) can ensure high-quality communication by ensuring the
availability of bandwidth to carry a load. Inelastic real-time flows
such as Voice over Internet Protocol (VoIP) (telephony) or video
conferencing services can benefit from use of an admission control
mechanism, as generally the telephony service is configured with
over-subscription, meaning that some users may not be able to make a
call during peak periods.
For VoIP (telephony) service, a common approach is to use signaling
protocols such as SIP, H.323, H.248, MEGACO, and Resource Reservation
Protocol (RSVP) to negotiate admittance and use of network transport
capabilities. When a user has been authorized to send voice traffic,
this admission procedure has verified that data rates will be within
the capacity of the network that it will use. Many RTP voice
payloads are inelastic and cannot react to loss or delay in any
substantive way. For these voice payloads, the network SHOULD police
at ingress to ensure that the voice traffic stays within its
negotiated bounds. Having thus assured a predictable input rate, the
network may use a priority queue to ensure nominal delay and
variation in delay.
Another approach that may be used in small and bandwidth-constrained
networks for limited number of flows is RSVP [RFC2205] [RFC2996].
However, there is concern with the scalability of this solution in
large networks where aggregation of reservations [RFC3175] is
considered to be required.
2. Service Differentiation
There are practical limits on the level of service differentiation
that should be offered in the IP networks. We believe we have
defined a practical approach in delivering service differentiation by
defining different service classes that networks may choose to
support in order to provide the appropriate level of behaviors and
performance needed by current and future applications and services.
The defined structure for providing services allows several
applications having similar traffic characteristics and performance
requirements to be grouped into the same service class. This
approach provides a lot of flexibility in providing the appropriate
level of service differentiation for current and new, yet unknown
applications without introducing significant changes to routers or
network configurations when a new traffic type is added to the
network.
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2.1. Service Classes
Traffic flowing in a network can be classified in many different
ways. We have chosen to divide it into two groupings, network
control and user/subscriber traffic. To provide service
differentiation, different service classes are defined in each
grouping. The network control traffic group can further be divided
into two service classes (see Section 3 for detailed definition of
each service class):
o "Network Control" for routing and network control function.
o "OAM" (Operations, Administration, and Management) for network
configuration and management functions.
The user/subscriber traffic group is broken down into ten service
classes to provide service differentiation for all the different
types of applications/services (see Section 4 for detailed definition
of each service class):
o Telephony service class is best suited for applications that
require very low delay variation and are of constant rate, such as
IP telephony (VoIP) and circuit emulation over IP applications.
o Signaling service class is best suited for peer-to-peer and
client-server signaling and control functions using protocols such
as SIP, SIP-T, H.323, H.248, and Media Gateway Control Protocol
(MGCP).
o Multimedia Conferencing service class is best suited for
applications that require very low delay and have the ability to
change encoding rate (rate adaptive), such as H.323/V2 and later
video conferencing service.
o Real-Time Interactive service class is intended for interactive
variable rate inelastic applications that require low jitter and
loss and very low delay, such as interactive gaming applications
that use RTP/UDP streams for game control commands, and video
conferencing applications that do not have the ability to change
encoding rates or to mark packets with different importance
indications.
o Multimedia Streaming service class is best suited for variable
rate elastic streaming media applications where a human is waiting
for output and where the application has the capability to react
to packet loss by reducing its transmission rate, such as
streaming video and audio and webcast.
o Broadcast Video service class is best suited for inelastic
streaming media applications that may be of constant or variable
rate, requiring low jitter and very low packet loss, such as
broadcast TV and live events, video surveillance, and security.
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o Low-Latency Data service class is best suited for data processing
applications where a human is waiting for output, such as web-
based ordering or an Enterprise Resource Planning (ERP)
application.
o High-Throughput Data service class is best suited for store and
forward applications such as FTP and billing record transfer.
o Standard service class is for traffic that has not been identified
as requiring differentiated treatment and is normally referred to
as best effort.
o Low-Priority Data service class is intended for packet flows where
bandwidth assurance is not required.
2.2. Categorization of User Service Classes
The ten defined user/subscriber service classes listed above can be
grouped into a small number of application categories. For some
application categories, it was felt that more than one service class
was needed to provide service differentiation within that category
due to the different traffic characteristic of the applications,
control function, and the required flow behavior. Figure 1 provides
a summary of service class grouping into four application categories.
Application Control Category
o The Signaling service class is intended to be used to control
applications or user endpoints. Examples of protocols that would
use this service class are SIP or H.248 for IP telephone service
and SIP or Internet Group Management Protocol (IGMP) for control
of broadcast TV service to subscribers. Although user signaling
flows have similar performance requirements as Low-Latency Data,
they need to be distinguished and marked with a different DSCP.
The essential distinction is something like "administrative
control and management" of the traffic affected as the protocols
in this class tend to be tied to the media stream/session they
signal and control.
Media-Oriented Category
Due to the vast number of new (in process of being deployed) and
already-in-use media-oriented services in IP networks, five service
classes have been defined.
o Telephony service class is intended for IP telephony (VoIP)
service. It may also be used for other applications that meet the
defined traffic characteristics and performance requirements.
o Real-Time Interactive service class is intended for inelastic
video flows from applications such as SIP-based desktop video
conferencing applications and for interactive gaming.
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o Multimedia Conferencing service class is for video conferencing
solutions that have the ability to reduce their transmission rate
on detection of congestion. These flows can therefore be
classified as rate adaptive. As currently two types of video
conferencing equipment are used in IP networks (ones that generate
inelastic traffic and ones that generate rate-adaptive traffic),
two service class are needed. The Real-Time Interactive service
class should be used for equipment that generates inelastic video
flows and the Multimedia Conferencing service class for equipment
that generates rate-adaptive video flows.
o Broadcast Video service class is to be used for inelastic traffic
flows, which are intended for broadcast TV service and for
transport of live video and audio events.
o Multimedia Streaming service class is to be used for elastic
multimedia traffic flows. This multimedia content is typically
stored before being transmitted. It is also buffered at the
receiving end before being played out. The buffering is
sufficiently large to accommodate any variation in transmission
rate that is encountered in the network. Multimedia entertainment
over IP delivery services that are being developed can generate
both elastic and inelastic traffic flows; therefore, two service
classes are defined to address this space, respectively:
Multimedia Streaming and Broadcast Video.
Data Category
The data category is divided into three service classes.
o Low-Latency Data for applications/services that require low delay
or latency for bursty but short-lived flows.
o High-Throughput Data for applications/services that require good
throughput for long-lived bursty flows. High Throughput and
Multimedia Steaming are close in their traffic flow
characteristics with High Throughput being a bit more bursty and
not as long-lived as Multimedia Streaming.
o Low-Priority Data for applications or services that can tolerate
short or long interruptions of packet flows. The Low-Priority
Data service class can be viewed as "don't care" to some degree.
Best-Effort Category
o All traffic that is not differentiated in the network falls into
this category and is mapped into the Standard service class. If a
packet is marked with a DSCP value that is not supported in the
network, it SHOULD be forwarded using the Standard service class.
Babiarz, et al. Informational [Page 14]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Figure 1, below, provides a grouping of the defined user/subscriber
service classes into four categories, with indications of which ones
use an independent flow for signaling or control; type of flow
behavior (elastic, rate adaptive, or inelastic); and the last column
provides end user Quality of Service (QoS) rating as defined in ITU-T
Recommendation G.1010.
-----------------------------------------------------------------
| Application | Service | Signaled | Flow | G.1010 |
| Categories | Class | | Behavior | Rating |
|-------------+---------------+----------+-----------+------------|
| Application | Signaling | Not | Inelastic | Responsive |
| Control | |applicable| | |
|-------------+---------------+----------+-----------+------------|
| | Telephony | Yes | Inelastic | Interactive|
| |---------------+----------+-----------+------------|
| | Real-Time | Yes | Inelastic | Interactive|
| | Interactive | | | |
| |---------------+----------+-----------+------------|
| Media- | Multimedia | Yes | Rate | Interactive|
| Oriented | Conferencing | | Adaptive | |
| |---------------+----------+-----------+------------|
| |Broadcast Video| Yes | Inelastic | Responsive |
| |---------------+----------+-----------+------------|
| | Multimedia | Yes | Elastic | Timely |
| | Streaming | | | |
|-------------+---------------+----------+-----------+------------|
| | Low-Latency | No | Elastic | Responsive |
| | Data | | | |
| |---------------+----------+-----------+------------|
| Data |High-Throughput| No | Elastic | Timely |
| | Data | | | |
| |---------------+----------+-----------+------------|
| | Low-Priority | No | Elastic |Non-critical|
| | Data | | | |
|-------------+---------------+----------+-----------+------------|
| Best Effort | Standard | Not Specified |Non-critical|
-----------------------------------------------------------------
Figure 1. User/Subscriber Service Classes Grouping
Babiarz, et al. Informational [Page 15]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Here is a short explanation of the end user QoS category as defined
in ITU-T Recommendation G.1010. User traffic is divided into four
different categories, namely, interactive, responsive, timely, and
non-critical. An example of interactive traffic is between two
humans and is most sensitive to delay, loss, and jitter. Another
example of interactive traffic is between two servers where very low
delay and loss are needed. Responsive traffic is typically between a
human and a server but can also be between two servers. Responsive
traffic is less affected by jitter and can tolerate longer delays
than interactive traffic. Timely traffic is either between servers
or servers and humans and the delay tolerance is significantly longer
than responsive traffic. Non-critical traffic is normally between
servers/machines where delivery may be delay for period of time.
2.3. Service Class Characteristics
This document provides guidelines for network administrators in
configuring their network for the level of service differentiation
that is appropriate in their network to meet their QoS needs. It is
expected that network operators will configure and provide in their
networks a subset of the defined service classes. Our intent is to
provide guidelines for configuration of Differentiated Services for a
wide variety of applications, services, and network configurations.
In addition, network administrators may choose to define and deploy
other service classes in their network.
Figure 2 provides a behavior view for traffic serviced by each
service class. The traffic characteristics column defines the
characteristics and profile of flows serviced, and the tolerance to
loss, delay, and jitter columns define the treatment the flows will
receive. End-to-end quantitative performance requirements may be
obtained from ITU-T Recommendations Y.1541 and Y.1540.
Babiarz, et al. Informational [Page 16]
RFC 4594 Guidelines for DiffServ Service Classes August 2006
-------------------------------------------------------------------
|Service Class | | Tolerance to |
| Name | Traffic Characteristics | Loss |Delay |Jitter|
|===============+==============================+======+======+======|
| Network |Variable size packets, mostly | | | |
| Control |inelastic short messages, but | Low | Low | Yes |
| | traffic can also burst (BGP) | | | |
|---------------+------------------------------+------+------+------|
| | Fixed-size small packets, | Very | Very | Very |
| Telephony | constant emission rate, | Low | Low | Low |
| | inelastic and low-rate flows | | | |
|---------------+------------------------------+------+------+------|
| Signaling | Variable size packets, some | Low | Low | Yes |
| | what bursty short-lived flows| | | |
|---------------+------------------------------+------+------+------|
| Multimedia | Variable size packets, | Low | Very | |
| Conferencing | constant transmit interval, | - | Low | Low |
| |rate adaptive, reacts to loss |Medium| | |
|---------------+------------------------------+------+------+------|
| Real-Time | RTP/UDP streams, inelastic, | Low | Very | Low |
| Interactive | mostly variable rate | | Low | |
|---------------+------------------------------+------+------+------|
| Multimedia | Variable size packets, |Low - |Medium| Yes |
| Streaming | elastic with variable rate |Medium| | |
|---------------+------------------------------+------+------+------|
| Broadcast | Constant and variable rate, | Very |Medium| Low |
| Video | inelastic, non-bursty flows | Low | | |
|---------------+------------------------------+------+------+------|
| Low-Latency | Variable rate, bursty short- | Low |Low - | Yes |
| Data | lived elastic flows | |Medium| |
|---------------+------------------------------+------+------+------|
| OAM | Variable size packets, | Low |Medium| Yes |
| | elastic & inelastic flows | | | |
|---------------+------------------------------+------+------+------|
|High-Throughput| Variable rate, bursty long- | Low |Medium| Yes |
| Data | lived elastic flows | |- High| |
|---------------+------------------------------+------+------+------|
| Standard | A bit of everything | Not Specified |
|---------------+------------------------------+------+------+------|
| Low-Priority | Non-real-time and elastic | High | High | Yes |
| Data | | | | |
-------------------------------------------------------------------
Figure 2. Service Class Characteristics
Babiarz, et al. Informational [Page 17]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Notes for Figure 2: A "Yes" in the jitter-tolerant column implies
that data is buffered in the endpoint and that a moderate level of
network-induced variation in delay will not affect the application.
Applications that use TCP as a transport are generally good examples.
Routing protocols and peer-to-peer signaling also fall in this class;
although loss can create problems in setting up calls, a moderate
level of jitter merely makes call placement a little less predictable
in duration.
Service classes indicate the required traffic forwarding treatment in
order to meet user, application, or network expectations. Section 3
defines the service classes that MAY be used for forwarding network
control traffic, and Section 4 defines the service classes that MAY
be used for forwarding user traffic with examples of intended
application types mapped into each service class. Note that the
application types are only examples and are not meant to be all-
inclusive or prescriptive. Also, note that the service class naming
or ordering does not imply any priority ordering. They are simply
reference names that are used in this document with associated QoS
behaviors that are optimized for the particular application types
they support. Network administrators MAY choose to assign different
service class names to the service classes that they will support.
Figure 3 defines the RECOMMENDED relationship between service classes
and DS codepoint assignment with application examples. It is
RECOMMENDED that this relationship be preserved end to end.
Babiarz, et al. Informational [Page 18]
RFC 4594 Guidelines for DiffServ Service Classes August 2006
------------------------------------------------------------------
| Service | DSCP | DSCP | Application |
| Class Name | Name | Value | Examples |
|===============+=========+=============+==========================|
|Network Control| CS6 | 110000 | Network routing |
|---------------+---------+-------------+--------------------------|
| Telephony | EF | 101110 | IP Telephony bearer |
|---------------+---------+-------------+--------------------------|
| Signaling | CS5 | 101000 | IP Telephony signaling |
|---------------+---------+-------------+--------------------------|
| Multimedia |AF41,AF42|100010,100100| H.323/V2 video |
| Conferencing | AF43 | 100110 | conferencing (adaptive) |
|---------------+---------+-------------+--------------------------|
| Real-Time | CS4 | 100000 | Video conferencing and |
| Interactive | | | Interactive gaming |
|---------------+---------+-------------+--------------------------|
| Multimedia |AF31,AF32|011010,011100| Streaming video and |
| Streaming | AF33 | 011110 | audio on demand |
|---------------+---------+-------------+--------------------------|
|Broadcast Video| CS3 | 011000 |Broadcast TV & live events|
|---------------+---------+-------------+--------------------------|
| Low-Latency |AF21,AF22|010010,010100|Client/server transactions|
| Data | AF23 | 010110 | Web-based ordering |
|---------------+---------+-------------+--------------------------|
| OAM | CS2 | 010000 | OAM&P |
|---------------+---------+-------------+--------------------------|
|High-Throughput|AF11,AF12|001010,001100| Store and forward |
| Data | AF13 | 001110 | applications |
|---------------+---------+-------------+--------------------------|
| Standard | DF (CS0)| 000000 | Undifferentiated |
| | | | applications |
|---------------+---------+-------------+--------------------------|
| Low-Priority | CS1 | 001000 | Any flow that has no BW |
| Data | | | assurance |
------------------------------------------------------------------
Figure 3. DSCP to Service Class Mapping
Notes for Figure 3: Default Forwarding (DF) and Class Selector 0
(CS0) provide equivalent behavior and use the same DS codepoint,
'000000'.
It is expected that network administrators will base their choice of
the service classes that they will support on their need, starting
off with three or four service classes for user traffic and adding
others as the need arises.
Babiarz, et al. Informational [Page 19]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Figure 4 provides a summary of DiffServ QoS mechanisms that SHOULD be
used for the defined service classes that are further detailed in
Sections 3 and 4 of this document. According to what
applications/services need to be differentiated, network
administrators can choose the service class(es) that need to be
supported in their network.
------------------------------------------------------------------
| Service | DSCP | Conditioning at | PHB | Queuing| AQM|
| Class | | DS Edge | Used | | |
|===============+======+===================+=========+========+====|
|Network Control| CS6 | See Section 3.1 | RFC2474 | Rate | Yes|
|---------------+------+-------------------+---------+--------+----|
| Telephony | EF |Police using sr+bs | RFC3246 |Priority| No |
|---------------+------+-------------------+---------+--------+----|
| Signaling | CS5 |Police using sr+bs | RFC2474 | Rate | No |
|---------------+------+-------------------+---------+--------+----|
| Multimedia | AF41 | Using two-rate, | | | Yes|
| Conferencing | AF42 |three-color marker | RFC2597 | Rate | per|
| | AF43 | (such as RFC 2698)| | |DSCP|
|---------------+------+-------------------+---------+--------+----|
| Real-Time | CS4 |Police using sr+bs | RFC2474 | Rate | No |
| Interactive | | | | | |
|---------------+------+-------------------+---------|--------+----|
| Multimedia | AF31 | Using two-rate, | | | Yes|
| Streaming | AF32 |three-color marker | RFC2597 | Rate | per|
| | AF33 | (such as RFC 2698)| | |DSCP|
|---------------+------+-------------------+---------+--------+----|
|Broadcast Video| CS3 |Police using sr+bs | RFC2474 | Rate | No |
|---------------+------+-------------------+---------+--------+----|
| Low- | AF21 | Using single-rate,| | | Yes|
| Latency | AF22 |three-color marker | RFC2597 | Rate | per|
| Data | AF23 | (such as RFC 2697)| | |DSCP|
|---------------+------+-------------------+---------+--------+----|
| OAM | CS2 |Police using sr+bs | RFC2474 | Rate | Yes|
|---------------+------+-------------------+---------+--------+----|
| High- | AF11 | Using two-rate, | | | Yes|
| Throughput | AF12 |three-color marker | RFC2597 | Rate | per|
| Data | AF13 | (such as RFC 2698)| | |DSCP|
|---------------+------+-------------------+---------+--------+----|
| Standard | DF | Not applicable | RFC2474 | Rate | Yes|
|---------------+------+-------------------+---------+--------+----|
| Low-Priority | CS1 | Not applicable | RFC3662 | Rate | Yes|
| Data | | | | | |
------------------------------------------------------------------
Figure 4. Summary of QoS Mechanisms Used for Each Service Class
Babiarz, et al. Informational [Page 20]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Notes for Figure 4:
o Conditioning at DS edge means that traffic conditioning is
performed at the edge of the DiffServ network where untrusted user
devices are connected or between two DiffServ networks.
o "sr+bs" represents a policing mechanism that provides single rate
with burst size control.
o The single-rate, three-color marker (srTCM) behavior SHOULD be
equivalent to RFC 2697, and the two-rate, three-color marker
(trTCM) behavior SHOULD be equivalent to RFC 2698.
o The PHB for Real-Time Interactive service class SHOULD be
configured to provide high bandwidth assurance. It MAY be
configured as a second EF PHB that uses relaxed performance
parameters and a rate scheduler.
o The PHB for Broadcast Video service class SHOULD be configured to
provide high bandwidth assurance. It MAY be configured as a third
EF PHB that uses relaxed performance parameters and a rate
scheduler.
o In network segments that use IP precedence marking, only one of
the two service classes can be supported, High-Throughput Data or
Low-Priority Data. We RECOMMEND that the DSCP value(s) of the
unsupported service class be changed to 000xx1 on ingress and
changed back to original value(s) on egress of the network segment
that uses precedence marking. For example, if Low-Priority Data
is mapped to Standard service class, then 000001 DSCP marking MAY
be used to distinguish it from Standard marked packets on egress.
2.4. Deployment Scenarios
It is expected that network administrators will base their choice of
the service classes that they will support on their need, starting
off with three or four service classes for user traffic and adding
more service classes as the need arises. In this section, we provide
three examples of possible deployment scenarios.
2.4.1. Example 1
A network administrator determines that he needs to provide different
performance levels (quality of service) in his network for the
services that he will be offering to his customers. He needs to
enable his network to provide:
Babiarz, et al. Informational [Page 21]RFC 4594 Guidelines for DiffServ Service Classes August 2006
o Reliable VoIP (telephony) service, equivalent to Public Switched
Telephone Network (PSTN).
o A low-delay assured bandwidth data service.
o Support for current Internet services.
For this example, the network administrator's needs are addressed
with the deployment of the following six service classes:
o Network Control service class for routing and control traffic that
is needed for reliable operation of the provider's network.
o Standard service class for all traffic that will receive normal
(undifferentiated) forwarding treatment through the network for
support of current Internet service.
o Telephony service class for VoIP (telephony) bearer traffic.
o Signaling service class for Telephony signaling to control the
VoIP service.
o Low-Latency Data service class for the low-delay assured bandwidth
differentiated data service.
o OAM service class for operation and management of the network.
Figure 5 provides a summary of the mechanisms needed for delivery of
service differentiation for Example 1.
-------------------------------------------------------------------
| Service | DSCP | Conditioning at | PHB | | |
| Class | | DS Edge | Used | Queuing| AQM|
|===============+=======+===================+=========+========+====|
|Network Control| CS6 | See Section 3.1 | RFC2474 | Rate | Yes|
|---------------+-------+-------------------+---------+--------+----|
| Telephony | EF |Police using sr+bs | RFC3246 |Priority| No |
|---------------+-------+-------------------+---------+--------+----|
| Signaling | CS5 |Police using sr+bs | RFC2474 | Rate | No |
|---------------+-------+-------------------+---------+--------+----|
| Low- | AF21 | Using single-rate,| | | Yes|
| Latency | AF22 |three-color marker | RFC2597 | Rate | per|
| Data | AF23 | (such as RFC 2697)| | |DSCP|
|---------------+-------+-------------------+---------+--------+----|
| OAM | CS2 |Police using sr+bs | RFC2474 | Rate | Yes|
|---------------+-------+-------------------+---------+--------+----|
| Standard |DF(CS0)| Not applicable | RFC2474 | Rate | Yes|
| | +other| | | | |
-------------------------------------------------------------------
Figure 5. Service Provider Network Configuration Example 1
Babiarz, et al. Informational [Page 22]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Notes for Figure 5:
o "sr+bs" represents a policing mechanism that provides single rate
with burst size control.
o The single-rate, three-color marker (srTCM) behavior SHOULD be
equivalent to RFC 2697.
o Any packet that is marked with DSCP value that is not represented
by the supported service classes SHOULD be forwarded using the
Standard service class.
2.4.2. Example 2
With this example, we show how network operators with Example 1
capabilities can evolve their service offering to provide three new
additional services to their customers. The new additional service
capabilities that are to be added are:
o SIP-based desktop video conference capability to complement VoIP
(telephony) service.
o TV and on-demand movie viewing service to residential subscribers.
o Network-based data storage and file backup service to business
customers.
The new additional services that the network administrator would like
to offer are addressed with the deployment of the following four
additional service classes (these are additions to the six service
classes already defined in Example 1):
o Real-Time Interactive service class for transport of MPEG-4 real-
time video flows to support desktop video conferencing. The
control/signaling for video conferencing is done using the
Signaling service class.
o Broadcast Video service class for transport of IPTV broadcast
information. The channel selection and control is via IGMP mapped
into the Signaling service class.
o Multimedia Streaming service class for transport of stored MPEG-2
or MPEG-4 content. The selection and control of streaming
information is done using the Signaling service class. The
selection of Multimedia Streaming service class for on-demand
movie service was chosen as the set-top box used for this service
has local buffering capability to compensate for the bandwidth
variability of the elastic streaming information. Note that if
transport of on-demand movie service is inelastic, then the
Broadcast Video service class SHOULD be used.
o High-Throughput Data service class is for transport of bulk data
for network-based storage and file backup service to business
customers.
Babiarz, et al. Informational [Page 23]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Figure 6 provides a summary of the mechanisms needed for delivery of
service differentiation for all the service classes used in Example
2.
-------------------------------------------------------------------
| Service | DSCP | Conditioning at | PHB | | |
| Class | | DS Edge | Used | Queuing| AQM|
|===============+=======+===================+=========+========+====|
|Network Control| CS6 | See Section 3.1 | RFC2474 | Rate |Yes |
|---------------+-------+-------------------+---------+--------+----|
| Telephony | EF |Police using sr+bs | RFC3246 |Priority| No |
|---------------+-------+-------------------+---------+--------+----|
| Signaling | CS5 |Police using sr+bs | RFC2474 | Rate | No |
|---------------+-------+-------------------+---------+--------+----|
| Real-time | CS4 |Police using sr+bs | RFC2474 | Rate | No |
| Interactive | | | | | |
|---------------+-------+-------------------+---------+--------+----|
|Broadcast Video| CS3 |Police using sr+bs | RFC2474 | Rate | No |
|---------------+-------+-------------------+---------+--------+----|
| Multimedia | AF31 | Using two-rate, | | |Yes |
| Streaming | AF32 |three-color marker | RFC2597 | Rate |per |
| | AF33 | (such as RFC 2698)| | |DSCP|
|---------------+-------+-------------------+---------+--------+----|
| Low- | AF21 | Using single-rate,| | |Yes |
| Latency | AF22 |three-color marker | RFC2597 | Rate |per |
| Data | AF23 | (such as RFC 2697)| | |DSCP|
|---------------+-------+-------------------+---------+--------+----|
| OAM | CS2 |Police using sr+bs | RFC2474 | Rate |Yes |
|---------------+-------+-------------------+---------+--------+----|
| High- | AF11 | Using two-rate, | | |Yes |
| Throughput | AF12 |three-color marker | RFC2597 | Rate |per |
| Data | AF13 | (such as RFC 2698)| | |DSCP|
|---------------+-------+-------------------+---------+--------+----|
| Standard |DF(CS0)| Not applicable | RFC2474 | Rate |Yes |
| | +other| | | | |
-------------------------------------------------------------------
Figure 6. Service Provider Network Configuration Example 2
Notes for Figure 6:
o "sr+bs" represents a policing mechanism that provides single rate
with burst size control.
o The single-rate, three-color marker (srTCM) behavior SHOULD be
equivalent to RFC 2697, and the two-rate, three-color marker
(trTCM) behavior SHOULD be equivalent to RFC 2698.
Babiarz, et al. Informational [Page 24]RFC 4594 Guidelines for DiffServ Service Classes August 2006
o Any packet that is marked with DSCP value that is not represented
by the supported service classes SHOULD be forwarded using the
Standard service class.
2.4.3. Example 3
An enterprise network administrator determines that they need to
provide different performance levels (quality of service) in their
network for the new services that are being offered to corporate
users. The enterprise network needs to:
o Provide reliable corporate VoIP service.
o Provide video conferencing service to selected Conference Rooms.
o Support on-demand distribution of prerecorded audio and video
information to large number of users.
o Provide a priority data transfer capability for engineering teams
to share design information.
o Reduce or deny bandwidth during peak traffic periods for selected
applications.
o Continue to provide normal IP service to all remaining
applications and services.
For this example, the enterprise's network needs are addressed with
the deployment of the following nine service classes:
o Network Control service class for routing and control traffic that
is needed for reliable operation of the enterprise network.
o OAM service class for operation and management of the network.
o Standard service class for all traffic that will receive normal
(undifferentiated) forwarding treatment.
o Telephony service class for VoIP (telephony) bearer traffic.
o Signaling service class for Telephony signaling to control the
VoIP service.
o Multimedia Conferencing service class for support of inter-
Conference Room video conferencing service using H.323/V2 or
similar equipment.
o Multimedia Streaming service class for transfer of prerecorded
audio and video information.
o High-Throughput Data service class to provide bandwidth assurance
for timely transfer of large engineering files.
o Low-Priority Data service class for selected background
applications where data transfer can be delayed or suspended for a
period of time during peak network load conditions.
Babiarz, et al. Informational [Page 25]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Figure 7 provides a summary of the mechanisms needed for delivery of
service differentiation for Example 3.
-------------------------------------------------------------------
| Service | DSCP | Conditioning at | PHB | | |
| Class | | DS Edge | Used | Queuing| AQM|
|===============+=======+===================+=========+========+====|
|Network Control| CS6 | See Section 3.2 | RFC2474 | Rate | Yes|
|---------------+-------+-------------------+---------+--------+----|
| Telephony | EF |Police using sr+bs | RFC3246 |Priority| No |
|---------------+-------+-------------------+---------+--------+----|
| Signaling | CS5 |Police using sr+bs | RFC2474 | Rate | No |
|---------------+-------+-------------------+---------+--------+----|
| Multimedia | AF41 | Using two-rate, | | | Yes|
| Conferencing | AF42 | three-color marker| RFC2597 | Rate | per|
| | AF43 | (such as RFC 2698)| | |DSCP|
|---------------+-------+-------------------+---------+--------+----|
| Multimedia | AF31 | Using two-rate, | | | Yes|
| Streaming | AF32 | three-color marker| RFC2597 | Rate | per|
| | AF33 | (such as RFC 2698)| | |DSCP|
|---------------+-------+-------------------+---------+--------+----|
| OAM | CS2 |Police using sr+bs | RFC2474 | Rate | Yes|
|---------------+-------+-------------------+---------+--------+----|
| High- | AF11 | Using two-rate, | | |Yes |
| Throughput | AF12 |three-color marker | RFC2597 | Rate |per |
| Data | AF13 | (such as RFC 2698)| | |DSCP|
|---------------+-------+-------------------+---------+--------+----|
| Low-Priority | CS1 | Not applicable | RFC3662 | Rate | Yes|
| Data | | | | | |
|---------------+-------+-------------------+---------+--------+----|
| Standard |DF(CS0)| Not applicable | RFC2474 | Rate | Yes|
| | +other| | | | |
-------------------------------------------------------------------
Figure 7. Enterprise Network Configuration Example
Notes for Figure 7:
o "sr+bs" represents a policing mechanism that provides single rate
with burst size control.
o The single-rate, three-color marker (srTCM) behavior SHOULD be
equivalent to RFC 2697, and the two-rate, three-color marker
(trTCM) behavior SHOULD be equivalent to RFC 2698.
o Any packet that is marked with DSCP value that is not represented
by the supported service classes SHOULD be forwarded using the
Standard service class.
Babiarz, et al. Informational [Page 26]RFC 4594 Guidelines for DiffServ Service Classes August 2006
3. Network Control Traffic
Network control traffic is defined as packet flows that are essential
for stable operation of the administered network as well as for
information that may be exchanged between neighboring networks across
a peering point where SLAs are in place. Network control traffic is
different from user application control (signaling) that may be
generated by some applications or services. Network control traffic
is mostly between routers and network nodes that are used for
operating, administering, controlling, or managing the network
segments. Network Control Traffic may be split into two service
classes, i.e., Network Control and OAM.
3.1. Current Practice in the Internet
Based on today's routing protocols and network control procedures
that are used in the Internet, we have determined that CS6 DSCP value
SHOULD be used for routing and control and that CS7 DSCP value SHOULD
be reserved for future use, potentially for future routing or control
protocols. Network administrators MAY use a Local/Experimental DSCP;
therefore, they may use a locally defined service class within their
network to further differentiate their routing and control traffic.
RECOMMENDED Network Edge Conditioning for CS7 DSCP marked packets:
o Drop or remark CS7 packets at ingress to DiffServ network domain.
o CS7 marked packets SHOULD NOT be sent across peering points.
Exchange of control information across peering points SHOULD be
done using CS6 DSCP and the Network Control service class.
3.2. Network Control Service Class
The Network Control service class is used for transmitting packets
between network devices (routers) that require control (routing)
information to be exchanged between nodes within the administrative
domain as well as across a peering point between different
administrative domains. Traffic transmitted in this service class is
very important as it keeps the network operational, and it needs to
be forwarded in a timely manner.
The Network Control service class SHOULD be configured using the
DiffServ Class Selector (CS) PHB, defined in [RFC2474]. This service
class SHOULD be configured so that the traffic receives a minimum
bandwidth guarantee, to ensure that the packets always receive timely
service. The configured forwarding resources for Network Control
service class SHOULD be such that the probability of packet drop
under peak load is very low in this service class. The Network
Babiarz, et al. Informational [Page 27]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Control service class SHOULD be configured to use a Rate Queuing
system such as defined in Section 1.4.1.2 of this document.
The following are examples of protocols and applications that SHOULD
use the Network Control service class:
o Routing packet flows: OSPF, BGP, ISIS, RIP.
o Control information exchange within and between different
administrative domains across a peering point where SLAs are in
place.
o LSP setup using CR-LDP and RSVP-TE.
The following protocols and applications SHOULD NOT use the Network
Control service class:
o User traffic.
The following are traffic characteristics of packet flows in the
Network Control service class:
o Mostly messages sent between routers and network servers.
o Variable size packets, normally one packet at a time, but traffic
can also burst (BGP).
o User traffic is not allowed to use this service class. By user
traffic, we mean packet flows that originate from user-controlled
end points that are connected to the network.
The RECOMMENDED DSCP marking is CS6 (Class Selector 6).
RECOMMENDED Network Edge Conditioning:
o At peering points (between two DiffServ networks) where SLAs are
in place, CS6 marked packets SHOULD be policed, e.g., using a
single rate with burst size (sr+bs) token bucket policer to keep
the CS6 marked packet flows to within the traffic rate specified
in the SLA.
o CS6 marked packet flows from untrusted sources (for example, end
user devices) SHOULD be dropped or remarked at ingress to the
DiffServ network.
o Packets from users/subscribers are not permitted access to the
Network Control service classes.
The fundamental service offered to the Network Control service class
is enhanced best-effort service with high bandwidth assurance. Since
this service class is used to forward both elastic and inelastic
flows, the service SHOULD be engineered so that the Active Queue
Management (AQM) [RFC2309] is applied to CS6 marked packets.
Babiarz, et al. Informational [Page 28]RFC 4594 Guidelines for DiffServ Service Classes August 2006
If RED [RFC2309] is used as an AQM algorithm, the min-threshold
specifies a target queue depth, and the max-threshold specifies the
queue depth above which all traffic is dropped or ECN marked. Thus,
in this service class, the following inequality should hold in queue
configurations:
o min-threshold CS6 < max-threshold CS6
o max-threshold CS6 <= memory assigned to the queue
Note: Many other AQM algorithms exist and are used; they should be
configured to achieve a similar result.
3.3. OAM Service Class
The OAM (Operations, Administration, and Management) service class is
RECOMMENDED for OAM&P (Operations, Administration, and Management and
Provisioning) using protocols such as Simple Network Management
Protocol (SNMP), Trivial File Transfer Protocol (TFTP), FTP, Telnet,
and Common Open Policy Service (COPS). Applications using this
service class require a low packet loss but are relatively not
sensitive to delay. This service class is configured to provide good
packet delivery for intermittent flows.
The OAM service class SHOULD use the Class Selector (CS) PHB defined
in [RFC2474]. This service class SHOULD be configured to provide a
minimum bandwidth assurance for CS2 marked packets to ensure that
they get forwarded. The OAM service class SHOULD be configured to
use a Rate Queuing system such as defined in Section 1.4.1.2 of this
document.
The following applications SHOULD use the OAM service class:
o Provisioning and configuration of network elements.
o Performance monitoring of network elements.
o Any network operational alarms.
The following are traffic characteristics:
o Variable size packets.
o Intermittent traffic flows.
o Traffic may burst at times.
o Both elastic and inelastic flows.
o Traffic not sensitive to delays.
RECOMMENDED DSCP marking:
o All flows in this service class are marked with CS2 (Class
Selector 2).
Babiarz, et al. Informational [Page 29]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Applications or IP end points SHOULD pre-mark their packets with CS2
DSCP value. If the end point is not capable of setting the DSCP
value, then the router topologically closest to the end point SHOULD
perform Multifield (MF) Classification, as defined in [RFC2475].
RECOMMENDED conditioning performed at DiffServ network edge:
o Packet flow marking (DSCP setting) from untrusted sources (end
user devices) SHOULD be verified at ingress to DiffServ network
using Multifield (MF) Classification methods, defined in
[RFC2475].
o Packet flows from untrusted sources (end user devices) SHOULD be
policed at ingress to DiffServ network, e.g., using single rate
with burst size token bucket policer to ensure that the traffic
stays within its negotiated or engineered bounds.
o Packet flows from trusted sources (routers inside administered
network) MAY not require policing.
o Normally OAM&P CS2 marked packet flows are not allowed to flow
across peering points. If that is the case, then CS2 marked
packets SHOULD be policed (dropped) at both egress and ingress
peering interfaces.
The fundamental service offered to "OAM" traffic is enhanced best-
effort service with controlled rate. The service SHOULD be
engineered so that CS2 marked packet flows have sufficient bandwidth
in the network to provide high assurance of delivery. Since this
service class is used to forward both elastic and inelastic flows,
the service SHOULD be engineered so that Active Queue Management
[RFC2309] is applied to CS2 marked packets.
If RED [RFC2309] is used as an AQM algorithm, the min-threshold
specifies a target queue depth for each DSCP, and the max-threshold
specifies the queue depth above which all traffic with such a DSCP is
dropped or ECN marked. Thus, in this service class, the following
inequality should hold in queue configurations:
o min-threshold CS2 < max-threshold CS2
o max-threshold CS2 <= memory assigned to the queue
Note: Many other AQM algorithms exist and are used; they should be
configured to achieve a similar result.
4. User Traffic
User traffic is defined as packet flows between different users or
subscribers. It is the traffic that is sent to or from end-terminals
and that supports a very wide variety of applications and services.
User traffic can be differentiated in many different ways; therefore,
Babiarz, et al. Informational [Page 30]RFC 4594 Guidelines for DiffServ Service Classes August 2006
we investigated several different approaches to classifying user
traffic. We looked at differentiating user traffic as real-time
versus non-real-time, elastic or rate-adaptive versus inelastic,
sensitive versus insensitive to loss as well as traffic
categorization as interactive, responsive, timely, and non-critical,
as defined in ITU-T Recommendation G.1010. In the final analysis, we
used all of the above for service differentiation, mapping
application types that seemed to have different sets of performance
sensitivities, and requirements to different service classes.
Network administrators can categorize their applications according to
the type of behavior that they require and MAY choose to support all
or a subset of the defined service classes. Figure 3 provides some
common applications and the forwarding service classes that best
support them, based on their performance requirements.
4.1. Telephony Service Class
The Telephony service class is RECOMMENDED for applications that
require real-time, very low delay, very low jitter, and very low
packet loss for relatively constant-rate traffic sources (inelastic
traffic sources). This service class SHOULD be used for IP telephony
service.
The fundamental service offered to traffic in the Telephony service
class is minimum jitter, delay, and packet loss service up to a
specified upper bound. Operation is in some respect similar to an
ATM CBR service, which has guaranteed bandwidth and which, if it
stays within the negotiated rate, experiences nominal delay and no
loss. The EF PHB has a similar guarantee.
Typical configurations negotiate the setup of telephone calls over
IP, using protocols such as H.248, MEGACO, H.323, or SIP. When a
user has been authorized to send telephony traffic, the call
admission procedure should have verified that the newly admitted flow
will be within the capacity of the Telephony service class forwarding
capability in the network. For VoIP (telephony) service, call
admission control is usually performed by a telephony call server/
gatekeeper using signaling (SIP, H.323, H.248, MEGACO, etc.) on
access points to the network. The bandwidth in the core network and
the number of simultaneous VoIP sessions that can be supported needs
to be engineered and controlled so that there is no congestion for
this service. Since the inelastic types of RTP payloads in this
class do not react to loss or significant delay in any substantive
way, the Telephony service class SHOULD forward packets as soon as
possible. Some RTP payloads that may be used in telephony
applications are adaptive and will not be in this class.
Babiarz, et al. Informational [Page 31]
RFC 4594 Guidelines for DiffServ Service Classes August 2006
The Telephony service class SHOULD use Expedited Forwarding (EF) PHB,
as defined in [RFC3246], and SHOULD be configured to receive
guaranteed forwarding resources so that all packets are forwarded
quickly. The Telephony service class SHOULD be configured to use a
Priority Queuing system such as that defined in Section 1.4.1.1 of
this document.
The following applications SHOULD use the Telephony service class:
o VoIP (G.711, G.729 and other codecs).
o Voice-band data over IP (modem, fax).
o T.38 fax over IP.
o Circuit emulation over IP, virtual wire, etc.
o IP Virtual Private Network (VPN) service that specifies single-
rate, mean network delay that is slightly longer then network
propagation delay, very low jitter, and a very low packet loss.
The following are traffic characteristics:
o Mostly fixed-size packets for VoIP (60, 70, 120 or 200 bytes in
size).
o Packets emitted at constant time intervals.
o Admission control of new flows is provided by telephony call
server, media gateway, gatekeeper, edge router, end terminal, or
access node that provides flow admission control function.
Applications or IP end points SHOULD pre-mark their packets with EF
DSCP value. If the end point is not capable of setting the DSCP
value, then the router topologically closest to the end point SHOULD
perform Multifield (MF) Classification, as defined in [RFC2475].
The RECOMMENDED DSCP marking is EF for the following applications:
o VoIP (G.711, G.729 and other codecs).
o Voice-band data over IP (modem and fax).
o T.38 fax over IP.
o Circuit emulation over IP, virtual wire, etc.
RECOMMENDED Network Edge Conditioning:
o Packet flow marking (DSCP setting) from untrusted sources (end
user devices) SHOULD be verified at ingress to DiffServ network
using Multifield (MF) Classification methods, defined in
[RFC2475].
o Packet flows from untrusted sources (end user devices) SHOULD be
policed at ingress to DiffServ network, e.g., using single rate
with burst size token bucket policer to ensure that the telephony
traffic stays within its negotiated bounds.
Babiarz, et al. Informational [Page 32]RFC 4594 Guidelines for DiffServ Service Classes August 2006
o Policing is OPTIONAL for packet flows from trusted sources whose
behavior is ensured via other means (e.g., administrative controls
on those systems).
o Policing of Telephony packet flows across peering points where SLA
is in place is OPTIONAL as telephony traffic will be controlled by
admission control mechanism between peering points.
The fundamental service offered to "Telephony" traffic is enhanced
best-effort service with controlled rate, very low delay, and very
low loss. The service MUST be engineered so that EF marked packet
flows have sufficient bandwidth in the network to provide guaranteed
delivery. Normally traffic in this service class does not respond
dynamically to packet loss. As such, Active Queue Management
[RFC2309] SHOULD NOT be applied to EF marked packet flows.
4.2. Signaling Service Class
The Signaling service class is RECOMMENDED for delay-sensitive
client-server (traditional telephony) and peer-to-peer application
signaling. Telephony signaling includes signaling between IP phone
and soft-switch, soft-client and soft-switch, and media gateway and
soft-switch as well as peer-to-peer using various protocols. This
service class is intended to be used for control of sessions and
applications. Applications using this service class require a
relatively fast response, as there are typically several messages of
different sizes sent for control of the session. This service class
is configured to provide good response for short-lived, intermittent
flows that require real-time packet forwarding. To minimize the
possibility of ring clipping at start of call for VoIP service that
interfaces to a circuit switch Exchange in the Public Switched
Telephone Network (PSTN), the Signaling service class SHOULD be
configured so that the probability of packet drop or significant
queuing delay under peak load is very low in IP network segments that
provide this interface. The term "ring clipping" refers to those
instances where the front end of a ringing signal is altered because
the bearer path is not made available in time to carry all of the
audible ringing signal. This condition may occur due to a race
condition between when the tone generator in the circuit switch
Exchange is turned on and when the bearer path through the IP network
is enabled. See Section 8.1 for additional explanation of "ring
clipping" and Section 5.1 for explanation of mapping different
signaling methods to service classes.
The Signaling service class SHOULD use the Class Selector (CS) PHB,
defined in [RFC2474]. This service class SHOULD be configured to
provide a minimum bandwidth assurance for CS5 marked packets to
ensure that they get forwarded. The Signaling service class SHOULD
Babiarz, et al. Informational [Page 33]RFC 4594 Guidelines for DiffServ Service Classes August 2006
be configured to use a Rate Queuing system such as that defined in
Section 1.4.1.2 of this document.
The following applications SHOULD use the Signaling service class:
o Peer-to-peer IP telephony signaling (e.g., using SIP, H.323).
o Peer-to-peer signaling for multimedia applications (e.g., using
SIP, H.323).
o Peer-to-peer real-time control function.
o Client-server IP telephony signaling using H.248, MEGACO, MGCP, IP
encapsulated ISDN, or other proprietary protocols.
o Signaling to control IPTV applications using protocols such as
IGMP.
o Signaling flows between high-capacity telephony call servers or
soft switches using protocol such as SIP-T. Such high-capacity
devices may control thousands of telephony (VoIP) calls.
The following are traffic characteristics:
o Variable size packets, normally one packet at a time.
o Intermittent traffic flows.
o Traffic may burst at times.
o Delay-sensitive control messages sent between two end points.
RECOMMENDED DSCP marking:
o All flows in this service class are marked with CS5 (Class
Selector 5).
Applications or IP end points SHOULD pre-mark their packets with CS5
DSCP value. If the end point is not capable of setting the DSCP
value, then the router topologically closest to the end point SHOULD
perform Multifield (MF) Classification, as defined in [RFC2475].
RECOMMENDED conditioning performed at DiffServ network edge:
o Packet flow marking (DSCP setting) from untrusted sources (end
user devices) SHOULD be verified at ingress to DiffServ network
using Multifield (MF) Classification methods defined in [RFC2475].
o Packet flows from untrusted sources (end user devices) SHOULD be
policed at ingress to DiffServ network, e.g., using single rate
with burst size token bucket policer to ensure that the traffic
stays within its negotiated or engineered bounds.
o Packet flows from trusted sources (application servers inside
administered network) MAY not require policing.
o Policing of packet flows across peering points SHOULD be performed
to the Service Level Agreement (SLA).
Babiarz, et al. Informational [Page 34]RFC 4594 Guidelines for DiffServ Service Classes August 2006
The fundamental service offered to "Signaling" traffic is enhanced
best-effort service with controlled rate and delay. The service
SHOULD be engineered so that CS5 marked packet flows have sufficient
bandwidth in the network to provide high assurance of delivery and
low delay. Normally, traffic in this service class does not respond
dynamically to packet loss. As such, Active Queue Management
[RFC2309] SHOULD NOT be applied to CS5 marked packet flows.
4.3. Multimedia Conferencing Service Class
The Multimedia Conferencing service class is RECOMMENDED for
applications that require real-time service for rate-adaptive
traffic. H.323/V2 and later versions of video conferencing equipment
with dynamic bandwidth adjustment are such applications. The traffic
sources in this service class have the ability to dynamically change
their transmission rate based on feedback from the receiver. One
approach used in H.323/V2 equipment is, when the receiver detects a
pre-configured level of packet loss, it signals to the transmitter
the indication of possible on-path congestion. When available, the
transmitter then selects a lower rate encoding codec. Note that
today, many H.323/V2 video conferencing solutions implement fixed-
step bandwidth change (usually reducing the rate), traffic resembling
step-wise CBR.
Typical video conferencing configurations negotiate the setup of
multimedia session using protocols such as H.323. When a user/end-
point has been authorized to start a multimedia session, the
admission procedure should have verified that the newly admitted data
rate will be within the engineered capacity of the Multimedia
Conferencing service class. The bandwidth in the core network and
the number of simultaneous video conferencing sessions that can be
supported SHOULD be engineered to control traffic load for this
service.
The Multimedia Conferencing service class SHOULD use the Assured
Forwarding (AF) PHB, defined in [RFC2597]. This service class SHOULD
be configured to provide a bandwidth assurance for AF41, AF42, and
AF43 marked packets to ensure that they get forwarded. The
Multimedia Conferencing service class SHOULD be configured to use a
Rate Queuing system such as that defined in Section 1.4.1.2 of this
document.
The following applications SHOULD use the Multimedia Conferencing
service class:
o H.323/V2 and later versions of video conferencing applications
(interactive video).
Babiarz, et al. Informational [Page 35]RFC 4594 Guidelines for DiffServ Service Classes August 2006
o Video conferencing applications with rate control or traffic
content importance marking.
o Application server-to-application server non-bursty data transfer
requiring very low delay.
o IP VPN service that specifies two rates and mean network delay
that is slightly longer then network propagation delay.
o Interactive, time-critical, and mission-critical applications.
The following are traffic characteristics:
o Variable size packets.
o The higher the rate, the higher the density of large packets.
o Constant packet emission time interval.
o Variable rate.
o Source is capable of reducing its transmission rate based on
detection of packet loss at the receiver.
Applications or IP end points SHOULD pre-mark their packets with DSCP
values as shown below. If the end point is not capable of setting
the DSCP value, then the router topologically closest to the end
point SHOULD perform Multifield (MF) Classification, as defined in
[RFC2475] and mark all packets as AF4x. Note: In this case, the
two-rate, three-color marker will be configured to operate in Color-
Blind mode.
RECOMMENDED DSCP marking when performed by router closest to source:
o AF41 = up to specified rate "A".
o AF42 = in excess of specified rate "A" but below specified rate
"B".
o AF43 = in excess of specified rate "B".
o Where "A" < "B".
Note: One might expect "A" to approximate the sum of the mean rates
and "B" to approximate the sum of the peak rates.
RECOMMENDED DSCP marking when performed by H.323/V2 video
conferencing equipment:
o AF41 = H.323 video conferencing audio stream RTP/UDP.
o AF41 = H.323 video conferencing video control RTCP/TCP.
o AF41 = H.323 video conferencing video stream up to specified rate
"A".
o AF42 = H.323 video conferencing video stream in excess of
specified rate "A" but below specified rate "B".
o AF43 = H.323 video conferencing video stream in excess of
specified rate "B".
o Where "A" < "B".
Babiarz, et al. Informational [Page 36]RFC 4594 Guidelines for DiffServ Service Classes August 2006
RECOMMENDED conditioning performed at DiffServ network edge:
o The two-rate, three-color marker SHOULD be configured to provide
the behavior as defined in trTCM [RFC2698].
o If packets are marked by trusted sources or a previously trusted
DiffServ domain and the color marking is to be preserved, then the
two-rate, three-color marker SHOULD be configured to operate in
Color-Aware mode.
o If the packet marking is not trusted or the color marking is not
to be preserved, then the two-rate, three-color marker SHOULD be
configured to operate in Color-Blind mode.
The fundamental service offered to "Multimedia Conferencing" traffic
is enhanced best-effort service with controlled rate and delay. For
video conferencing service, typically a 1% packet loss detected at
the receiver triggers an encoding rate change, dropping to the next
lower provisioned video encoding rate. As such, Active Queue
Management [RFC2309] SHOULD be used primarily to switch the video
encoding rate under congestion, changing from high rate to lower
rate, i.e., 1472 kbps to 768 kbps. The probability of loss of AF41
traffic MUST NOT exceed the probability of loss of AF42 traffic,
which in turn MUST NOT exceed the probability of loss of AF43
traffic.
If RED [RFC2309] is used as an AQM algorithm, the min-threshold
specifies a target queue depth for each DSCP, and the max-threshold
specifies the queue depth above which all traffic with such a DSCP is
dropped or ECN marked. Thus, in this service class, the following
inequality should hold in queue configurations:
o min-threshold AF43 < max-threshold AF43
o max-threshold AF43 <= min-threshold AF42
o min-threshold AF42 < max-threshold AF42
o max-threshold AF42 <= min-threshold AF41
o min-threshold AF41 < max-threshold AF41
o max-threshold AF41 <= memory assigned to the queue
Note: This configuration tends to drop AF43 traffic before AF42 and
AF42 before AF41. Many other AQM algorithms exist and are used; they
should be configured to achieve a similar result.
4.4. Real-Time Interactive Service Class
The Real-Time Interactive service class is RECOMMENDED for
applications that require low loss and jitter and very low delay for
variable rate inelastic traffic sources. Interactive gaming and
video conferencing applications that do not have the ability to
change encoding rates or to mark packets with different importance
Babiarz, et al. Informational [Page 37]RFC 4594 Guidelines for DiffServ Service Classes August 2006
indications are such applications. The traffic sources in this
traffic class do not have the ability to reduce their transmission
rate according to feedback received from the receiving end.
Typically, applications in this service class are configured to
negotiate the setup of RTP/UDP control session. When a user/end-
point has been authorized to start a new session, the admission
procedure should have verified that the newly admitted data rates
will be within the engineered capacity of the Real-Time Interactive
service class. The bandwidth in the core network and the number of
simultaneous Real-time Interactive sessions that can be supported
SHOULD be engineered to control traffic load for this service.
The Real-Time Interactive service class SHOULD use the Class Selector
(CS) PHB, defined in [RFC2474]. This service class SHOULD be
configured to provide a high assurance for bandwidth for CS4 marked
packets to ensure that they get forwarded. The Real-Time Interactive
service class SHOULD be configured to use a Rate Queuing system such
as that defined in Section 1.4.1.2 of this document. Note that this
service class MAY be configured as a second EF PHB that uses relaxed
performance parameter, a rate scheduler, and CS4 DSCP value.
The following applications SHOULD use the Real-Time Interactive
service class:
o Interactive gaming and control.
o Video conferencing applications without rate control or traffic
content importance marking.
o IP VPN service that specifies single rate and mean network delay
that is slightly longer then network propagation delay.
o Inelastic, interactive, time-critical, and mission-critical
applications requiring very low delay.
The following are traffic characteristics:
o Variable size packets.
o Variable rate, non-bursty.
o Application is sensitive to delay variation between flows and
sessions.
o Lost packets, if any, are usually ignored by application.
RECOMMENDED DSCP marking:
o All flows in this service class are marked with CS4 (Class
Selector 4).
Babiarz, et al. Informational [Page 38]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Applications or IP end points SHOULD pre-mark their packets with CS4
DSCP value. If the end point is not capable of setting the DSCP
value, then the router topologically closest to the end point SHOULD
perform Multifield (MF) Classification, as defined in [RFC2475].
RECOMMENDED conditioning performed at DiffServ network edge:
o Packet flow marking (DSCP setting) from untrusted sources (end
user devices) SHOULD be verified at ingress to DiffServ network
using Multifield (MF) Classification methods defined in [RFC2475].
o Packet flows from untrusted sources (end user devices) SHOULD be
policed at ingress to DiffServ network, e.g., using single rate
with burst size token bucket policer to ensure that the traffic
stays within its negotiated or engineered bounds.
o Packet flows from trusted sources (application servers inside
administered network) MAY not require policing.
o Policing of packet flows across peering points SHOULD be performed
to the Service Level Agreement (SLA).
The fundamental service offered to "Real-Time Interactive" traffic is
enhanced best-effort service with controlled rate and delay. The
service SHOULD be engineered so that CS4 marked packet flows have
sufficient bandwidth in the network to provide high assurance of
delivery. Normally, traffic in this service class does not respond
dynamically to packet loss. As such, Active Queue Management
[RFC2309] SHOULD NOT be applied to CS4 marked packet flows.
4.5. Multimedia Streaming Service Class
The Multimedia Streaming service class is RECOMMENDED for
applications that require near-real-time packet forwarding of
variable rate elastic traffic sources that are not as delay sensitive
as applications using the Multimedia Conferencing service class.
Such applications include streaming audio and video, some video
(movies) on-demand applications, and webcasts. In general, the
Multimedia Streaming service class assumes that the traffic is
buffered at the source/destination; therefore, it is less sensitive
to delay and jitter.
The Multimedia Streaming service class SHOULD use the Assured
Forwarding (AF) PHB, defined in [RFC2597]. This service class SHOULD
be configured to provide a minimum bandwidth assurance for AF31,
AF32, and AF33 marked packets to ensure that they get forwarded. The
Multimedia Streaming service class SHOULD be configured to use Rate
Queuing system such as that defined in Section 1.4.1.2 of this
document.
Babiarz, et al. Informational [Page 39]RFC 4594 Guidelines for DiffServ Service Classes August 2006
The following applications SHOULD use the Multimedia Streaming
service class:
o Buffered streaming audio (unicast).
o Buffered streaming video (unicast).
o Webcasts.
o IP VPN service that specifies two rates and is less sensitive to
delay and jitter.
The following are traffic characteristics:
o Variable size packets.
o The higher the rate, the higher the density of large packets.
o Variable rate.
o Elastic flows.
o Some bursting at start of flow from some applications.
Applications or IP end points SHOULD pre-mark their packets with DSCP
values as shown below. If the end point is not capable of setting
the DSCP value, then the router topologically closest to the end
point SHOULD perform Multifield (MF) Classification, as defined in
[RFC2475], and mark all packets as AF3x. Note: In this case, the
two-rate, three-color marker will be configured to operate in Color-
Blind mode.
RECOMMENDED DSCP marking:
o AF31 = up to specified rate "A".
o AF32 = in excess of specified rate "A" but below specified rate
"B".
o AF33 = in excess of specified rate "B".
o Where "A" < "B".
Note: One might expect "A" to approximate the sum of the mean rates
and "B" to approximate the sum of the peak rates.
RECOMMENDED conditioning performed at DiffServ network edge:
o The two-rate, three-color marker SHOULD be configured to provide
the behavior as defined in trTCM [RFC2698].
o If packets are marked by trusted sources or a previously trusted
DiffServ domain and the color marking is to be preserved, then the
two-rate, three-color marker SHOULD be configured to operate in
Color-Aware mode.
o If the packet marking is not trusted or the color marking is not
to be preserved, then the two-rate, three-color marker SHOULD be
configured to operate in Color-Blind mode.
Babiarz, et al. Informational [Page 40]RFC 4594 Guidelines for DiffServ Service Classes August 2006
The fundamental service offered to "Multimedia Streaming" traffic is
enhanced best-effort service with controlled rate and delay. The
service SHOULD be engineered so that AF31 marked packet flows have
sufficient bandwidth in the network to provide high assurance of
delivery. Since the AF3x traffic is elastic and responds dynamically
to packet loss, Active Queue Management [RFC2309] SHOULD be used
primarily to reduce forwarding rate to the minimum assured rate at
congestion points. The probability of loss of AF31 traffic MUST NOT
exceed the probability of loss of AF32 traffic, which in turn MUST
NOT exceed the probability of loss of AF33.
If RED [RFC2309] is used as an AQM algorithm, the min-threshold
specifies a target queue depth for each DSCP, and the max-threshold
specifies the queue depth above which all traffic with such a DSCP is
dropped or ECN marked. Thus, in this service class, the following
inequality should hold in queue configurations:
o min-threshold AF33 < max-threshold AF33
o max-threshold AF33 <= min-threshold AF32
o min-threshold AF32 < max-threshold AF32
o max-threshold AF32 <= min-threshold AF31
o min-threshold AF31 < max-threshold AF31
o max-threshold AF31 <= memory assigned to the queue
Note: This configuration tends to drop AF33 traffic before AF32 and
AF32 before AF31. Note: Many other AQM algorithms exist and are
used; they should be configured to achieve a similar result.
4.6. Broadcast Video Service Class
The Broadcast Video service class is RECOMMENDED for applications
that require near-real-time packet forwarding with very low packet
loss of constant rate and variable rate inelastic traffic sources
that are not as delay sensitive as applications using the Real-Time
Interactive service class. Such applications include broadcast TV,
streaming of live audio and video events, some video-on-demand
applications, and video surveillance. In general, the Broadcast
Video service class assumes that the destination end point has a
dejitter buffer, for video application usually a 2 - 8 video-frame
buffer (66 to several hundred of milliseconds), and therefore that it
is less sensitive to delay and jitter.
The Broadcast Video service class SHOULD use the Class Selector (CS)
PHB, defined in [RFC2474]. This service class SHOULD be configured
to provide high assurance for bandwidth for CS3 marked packets to
ensure that they get forwarded. The Broadcast Video service class
SHOULD be configured to use Rate Queuing system such as that defined
in Section 1.4.1.2 of this document. Note that this service class
Babiarz, et al. Informational [Page 41]
RFC 4594 Guidelines for DiffServ Service Classes August 2006
MAY be configured as a third EF PHB that uses relaxed performance
parameter, a rate scheduler, and CS3 DSCP value.
The following applications SHOULD use the Broadcast Video service
class:
o Video surveillance and security (unicast).
o TV broadcast including HDTV (multicast).
o Video on demand (unicast) with control (virtual DVD).
o Streaming of live audio events (both unicast and multicast).
o Streaming of live video events (both unicast and multicast).
The following are traffic characteristics:
o Variable size packets.
o The higher the rate, the higher the density of large packets.
o Mixture of variable rate and constant rate flows.
o Fixed packet emission time intervals.
o Inelastic flows.
RECOMMENDED DSCP marking:
o All flows in this service class are marked with CS3 (Class
Selector 3).
o In some cases, such as those for security and video surveillance
applications, it may be desirable to use a different DSCP marking.
If so, then locally user definable (EXP/LU) codepoints in the
range '011xx1' MAY be used to provide unique traffic
identification. The locally user definable (EXP/LU) codepoint(s)
MAY be associated with the PHB that is used for CS3 traffic.
Furthermore, depending on the network scenario, additional network
edge conditioning policy MAY be needed for the EXP/LU codepoint(s)
used.
Applications or IP end points SHOULD pre-mark their packets with CS3
DSCP value. If the end point is not capable of setting the DSCP
value, then the router topologically closest to the end point SHOULD
perform Multifield (MF) Classification, as defined in [RFC2475].
RECOMMENDED conditioning performed at DiffServ network edge:
o Packet flow marking (DSCP setting) from untrusted sources (end
user devices) SHOULD be verified at ingress to DiffServ network
using Multifield (MF) Classification methods defined in [RFC2475].
o Packet flows from untrusted sources (end user devices) SHOULD be
policed at ingress to DiffServ network, e.g., using single rate
with burst size token bucket policer to ensure that the traffic
stays within its negotiated or engineered bounds.
Babiarz, et al. Informational [Page 42]RFC 4594 Guidelines for DiffServ Service Classes August 2006
o Packet flows from trusted sources (application servers inside
administered network) MAY not require policing.
o Policing of packet flows across peering points SHOULD be performed
to the Service Level Agreement (SLA).
The fundamental service offered to "Broadcast Video" traffic is
enhanced best-effort service with controlled rate and delay. The
service SHOULD be engineered so that CS3 marked packet flows have
sufficient bandwidth in the network to provide high assurance of
delivery. Normally, traffic in this service class does not respond
dynamically to packet loss. As such, Active Queue Management
[RFC2309] SHOULD NOT be applied to CS3 marked packet flows.
4.7. Low-Latency Data Service Class
The Low-Latency Data service class is RECOMMENDED for elastic and
responsive typically client-/server-based applications. Applications
forwarded by this service class are those that require a relatively
fast response and typically have asymmetrical bandwidth need, i.e.,
the client typically sends a short message to the server and the
server responds with a much larger data flow back to the client. The
most common example of this is when a user clicks a hyperlink (~ few
dozen bytes) on a web page, resulting in a new web page to be loaded
(Kbytes of data). This service class is configured to provide good
response for TCP [RFC1633] short-lived flows that require real-time
packet forwarding of variable rate traffic sources.
The Low-Latency Data service class SHOULD use the Assured Forwarding
(AF) PHB, defined in [RFC2597]. This service class SHOULD be
configured to provide a minimum bandwidth assurance for AF21, AF22,
and AF23 marked packets to ensure that they get forwarded. The Low-
Latency Data service class SHOULD be configured to use a Rate Queuing
system such as that defined in Section 1.4.1.2 of this document.
The following applications SHOULD use the Low-Latency Data service
class:
o Client/server applications.
o Systems Network Architecture (SNA) terminal to host transactions
(SNA over IP using Data Link Switching (DLSw)).
o Web-based transactions (E-commerce).
o Credit card transactions.
o Financial wire transfers.
o Enterprise Resource Planning (ERP) applications (e.g., SAP/BaaN).
o VPN service that supports Committed Information Rate (CIR) with up
to two burst sizes.
Babiarz, et al. Informational [Page 43]RFC 4594 Guidelines for DiffServ Service Classes August 2006
The following are traffic characteristics:
o Variable size packets.
o Variable packet emission rate.
o With packet bursts of TCP window size.
o Short traffic bursts.
o Source capable of reducing its transmission rate based on
detection of packet loss at the receiver or through explicit
congestion notification.
Applications or IP end points SHOULD pre-mark their packets with DSCP
values as shown below. If the end point is not capable of setting
the DSCP value, then the router topologically closest to the end
point SHOULD perform Multifield (MF) Classification, as defined in
[RFC2475] and mark all packets as AF2x. Note: In this case, the
single-rate, three-color marker will be configured to operate in
Color-Blind mode.
RECOMMENDED DSCP marking:
o AF21 = flow stream with packet burst size up to "A" bytes.
o AF22 = flow stream with packet burst size in excess of "A" but
below "B" bytes.
o AF23 = flow stream with packet burst size in excess of "B" bytes.
o Where "A" < "B".
RECOMMENDED conditioning performed at DiffServ network edge:
o The single-rate, three-color marker SHOULD be configured to
provide the behavior as defined in srTCM [RFC2697].
o If packets are marked by trusted sources or a previously trusted
DiffServ domain and the color marking is to be preserved, then the
single-rate, three-color marker SHOULD be configured to operate in
Color-Aware mode.
o If the packet marking is not trusted or the color marking is not
to be preserved, then the single-rate, three-color marker SHOULD
be configured to operate in Color-Blind mode.
The fundamental service offered to "Low-Latency Data" traffic is
enhanced best-effort service with controlled rate and delay. The
service SHOULD be engineered so that AF21 marked packet flows have
sufficient bandwidth in the network to provide high assurance of
delivery. Since the AF2x traffic is elastic and responds dynamically
to packet loss, Active Queue Management [RFC2309] SHOULD be used
primarily to control TCP flow rates at congestion points by dropping
packets from TCP flows that have large burst size. The probability
of loss of AF21 traffic MUST NOT exceed the probability of loss of
AF22 traffic, which in turn MUST NOT exceed the probability of loss
Babiarz, et al. Informational [Page 44]RFC 4594 Guidelines for DiffServ Service Classes August 2006
of AF23. Explicit Congestion Notification (ECN) [RFC3168] MAY also
be used with Active Queue Management.
If RED [RFC2309] is used as an AQM algorithm, the min-threshold
specifies a target queue depth for each DSCP, and the max-threshold
specifies the queue depth above which all traffic with such a DSCP is
dropped or ECN marked. Thus, in this service class, the following
inequality should hold in queue configurations:
o min-threshold AF23 < max-threshold AF23
o max-threshold AF23 <= min-threshold AF22
o min-threshold AF22 < max-threshold AF22
o max-threshold AF22 <= min-threshold AF21
o min-threshold AF21 < max-threshold AF21
o max-threshold AF21 <= memory assigned to the queue
Note: This configuration tends to drop AF23 traffic before AF22 and
AF22 before AF21. Many other AQM algorithms exist and are used; they
should be configured to achieve a similar result.
4.8. High-Throughput Data Service Class
The High-Throughput Data service class is RECOMMENDED for elastic
applications that require timely packet forwarding of variable rate
traffic sources and, more specifically, is configured to provide good
throughput for TCP longer-lived flows. TCP [RFC1633] or a transport
with a consistent Congestion Avoidance Procedure [RFC2581] [RFC3782]
normally will drive as high a data rate as it can obtain over a long
period of time. The FTP protocol is a common example, although one
cannot definitively say that all FTP transfers are moving data in
bulk.
The High-Throughput Data service class SHOULD use the Assured
Forwarding (AF) PHB, defined in [RFC2597]. This service class SHOULD
be configured to provide a minimum bandwidth assurance for AF11,
AF12, and AF13 marked packets to ensure that they are forwarded in a
timely manner. The High-Throughput Data service class SHOULD be
configured to use a Rate Queuing system such as that defined in
Section 1.4.1.2 of this document.
The following applications SHOULD use the High-Throughput Data
service class:
o Store and forward applications.
o File transfer applications.
o Email.
o VPN service that supports two rates (committed information rate
and excess or peak information rate).
Babiarz, et al. Informational [Page 45]RFC 4594 Guidelines for DiffServ Service Classes August 2006
The following are traffic characteristics:
o Variable size packets.
o Variable packet emission rate.
o Variable rate.
o With packet bursts of TCP window size.
o Source capable of reducing its transmission rate based on
detection of packet loss at the receiver or through explicit
congestion notification.
Applications or IP end points SHOULD pre-mark their packets with DSCP
values as shown below. If the end point is not capable of setting
the DSCP value, then the router topologically closest to the end
point SHOULD perform Multifield (MF) Classification, as defined in
[RFC2475], and mark all packets as AF1x. Note: In this case, the
two-rate, three-color marker will be configured to operate in Color-
Blind mode.
RECOMMENDED DSCP marking:
o AF11 = up to specified rate "A".
o AF12 = in excess of specified rate "A" but below specified rate
"B".
o AF13 = in excess of specified rate "B".
o Where "A" < "B".
RECOMMENDED conditioning performed at DiffServ network edge:
o The two-rate, three-color marker SHOULD be configured to provide
the behavior as defined in trTCM [RFC2698].
o If packets are marked by trusted sources or a previously trusted
DiffServ domain and the color marking is to be preserved, then the
two-rate, three-color marker SHOULD be configured to operate in
Color-Aware mode.
o If the packet marking is not trusted or the color marking is not
to be preserved, then the two-rate, three-color marker SHOULD be
configured to operate in Color-Blind mode.
The fundamental service offered to "High-Throughput Data" traffic is
enhanced best-effort service with a specified minimum rate. The
service SHOULD be engineered so that AF11 marked packet flows have
sufficient bandwidth in the network to provide assured delivery. It
can be assumed that this class will consume any available bandwidth
and that packets traversing congested links may experience higher
queuing delays or packet loss. Since the AF1x traffic is elastic and
responds dynamically to packet loss, Active Queue Management
[RFC2309] SHOULD be used primarily to control TCP flow rates at
congestion points by dropping packets from TCP flows that have higher
Babiarz, et al. Informational [Page 46]RFC 4594 Guidelines for DiffServ Service Classes August 2006
rates first. The probability of loss of AF11 traffic MUST NOT exceed
the probability of loss of AF12 traffic, which in turn MUST NOT
exceed the probability of loss of AF13. In such a case, if one
network customer is driving significant excess and another seeks to
use the link, any losses will be experienced by the high-rate user,
causing him to reduce his rate. Explicit Congestion Notification
(ECN) [RFC3168] MAY also be used with Active Queue Management.
If RED [RFC2309] is used as an AQM algorithm, the min-threshold
specifies a target queue depth for each DSCP, and the max-threshold
specifies the queue depth above which all traffic with such a DSCP is
dropped or ECN marked. Thus, in this service class, the following
inequality should hold in queue configurations:
o min-threshold AF13 < max-threshold AF13
o max-threshold AF13 <= min-threshold AF12
o min-threshold AF12 < max-threshold AF12
o max-threshold AF12 <= min-threshold AF11
o min-threshold AF11 < max-threshold AF11
o max-threshold AF11 <= memory assigned to the queue
Note: This configuration tends to drop AF13 traffic before AF12 and
AF12 before AF11. Many other AQM algorithms exist and are used; they
should be configured to achieve a similar result.
4.9. Standard Service Class
The Standard service class is RECOMMENDED for traffic that has not
been classified into one of the other supported forwarding service
classes in the DiffServ network domain. This service class provides
the Internet's "best-effort" forwarding behavior. This service class
typically has minimum bandwidth guarantee.
The Standard service class MUST use the Default Forwarding (DF) PHB,
defined in [RFC2474], and SHOULD be configured to receive at least a
small percentage of forwarding resources as a guaranteed minimum.
This service class SHOULD be configured to use a Rate Queuing system
such as that defined in Section 1.4.1.2 of this document.
The following applications SHOULD use the Standard service class:
o Network services, DNS, DHCP, BootP.
o Any undifferentiated application/packet flow transported through
the DiffServ enabled network.
The following is a traffic characteristic:
o Non-deterministic, mixture of everything.
Babiarz, et al. Informational [Page 47]RFC 4594 Guidelines for DiffServ Service Classes August 2006
The RECOMMENDED DSCP marking is DF (Default Forwarding) '000000'.
Network Edge Conditioning:
There is no requirement that conditioning of packet flows be
performed for this service class.
The fundamental service offered to the Standard service class is
best-effort service with active queue management to limit overall
delay. Typical configurations SHOULD use random packet dropping to
implement Active Queue Management [RFC2309] or Explicit Congestion
Notification [RFC3168], and MAY impose a minimum or maximum rate on
the queue.
If RED [RFC2309] is used as an AQM algorithm, the min-threshold
specifies a target queue depth, and the max-threshold specifies the
queue depth above which all traffic is dropped or ECN marked. Thus,
in this service class, the following inequality should hold in queue
configurations:
o min-threshold DF < max-threshold DF
o max-threshold DF <= memory assigned to the queue
Note: Many other AQM algorithms exist and are used; they should be
configured to achieve a similar result.
4.10. Low-Priority Data
The Low-Priority Data service class serves applications that run over
TCP [RFC0793] or a transport with consistent congestion avoidance
procedures [RFC2581] [RFC3782] and that the user is willing to accept
service without guarantees. This service class is specified in
[RFC3662] and [QBSS].
The following applications MAY use the Low-Priority Data service
class:
o Any TCP based-application/packet flow transported through the
DiffServ enabled network that does not require any bandwidth
assurances.
The following is a traffic characteristic:
o Non-real-time and elastic.
Babiarz, et al. Informational [Page 48]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Network Edge Conditioning:
There is no requirement that conditioning of packet flows be
performed for this service class.
The RECOMMENDED DSCP marking is CS1 (Class Selector 1).
The fundamental service offered to the Low-Priority Data service
class is best-effort service with zero bandwidth assurance. By
placing it into a separate queue or class, it may be treated in a
manner consistent with a specific Service Level Agreement.
Typical configurations SHOULD use Explicit Congestion Notification
[RFC3168] or random loss to implement Active Queue Management
[RFC2309].
If RED [RFC2309] is used as an AQM algorithm, the min-threshold
specifies a target queue depth, and the max-threshold specifies the
queue depth above which all traffic is dropped or ECN marked. Thus,
in this service class, the following inequality should hold in queue
configurations:
o min-threshold CS1 < max-threshold CS1
o max-threshold CS1 <= memory assigned to the queue
Note: Many other AQM algorithms exist and are used; they should be
configured to achieve a similar result.
5. Additional Information on Service Class Usage
In this section, we provide additional information on how some
specific applications should be configured to use the defined service
classes.
5.1. Mapping for Signaling
There are many different signaling protocols, ways that signaling is
used and performance requirements from applications that are
controlled by these protocols. We believe that different signaling
protocols should use the service class that best meets the objectives
of application or service they control. The following mapping is
recommended:
o Peer-to-peer signaling using SIP/H.323 is marked with CS5 DSCP
(use Signaling service class).
Babiarz, et al. Informational [Page 49]RFC 4594 Guidelines for DiffServ Service Classes August 2006
o Client-server signaling as used in many implementation for IP
telephony using H.248, MEGACO, MGCP, IP encapsulated ISDN, or
proprietary protocols is marked with CS5 DSCP (use Signaling
service class).
o Signaling between call servers or soft-switches in carrier's
network using SIP, SIP-T, or IP encapsulated ISUP is marked with
CS5 DSCP (use Signaling service class).
o RSVP signaling depends on the application. If RSVP signaling is
"on-path" as used in IntServ, then it needs to be forwarded from
the same queue (service class) and marked with the same DSCP value
as application data that it is controlling. This may also apply
to the "on-path" Next Steps in Signaling (NSIS) protocol.
o If IGMP is used for multicast session control such as channel
changing in IPTV systems, then IGMP packets should be marked with
CS5 DSCP (use Signaling service class). When IGMP is used only
for the normal multicast routing purpose, it should be marked with
CS6 DSCP (use Network Control service class).
5.2. Mapping for NTP
From tests that were performed, indications are that precise time
distribution requires a very low packet delay variation (jitter)
transport. Therefore, we suggest that the following guidelines for
Network Time Protocol (NTP) be used:
o When NTP is used for providing high-accuracy timing within an
administrator's (carrier's) network or to end users/clients, the
Telephony service class should be used, and NTP packets should be
marked with EF DSCP value.
o For applications that require "wall clock" timing accuracy, the
Standard service class should be used, and packets should be
marked with DF DSCP.
5.3. VPN Service Mapping
"Differentiated Services and Tunnels" [RFC2983] considers the
interaction of DiffServ architecture with IP tunnels of various
forms. Further to guidelines provided in RFC 2983, below are
additional guidelines for mapping service classes that are supported
in one part of the network into a VPN connection. This discussion is
limited to VPNs that use DiffServ technology for traffic
differentiation.
o The DSCP value(s) that is/are used to represent a PHB or a PHB
group should be the same for the networks at both ends of the VPN
tunnel, unless remarking of DSCP is done as ingress/egress
processing function of the tunnel. DSCP marking needs to be
preserved end to end.
Babiarz, et al. Informational [Page 50]RFC 4594 Guidelines for DiffServ Service Classes August 2006
o The VPN may be configured to support one or more service classes.
It is left up to the administrators of the two networks to agree
on the level of traffic differentiation that will be provided in
the network that supports VPN service. Service classes are then
mapped into the supported VPN traffic forwarding behaviors that
meet the traffic characteristics and performance requirements of
the encapsulated service classes.
o The traffic treatment in the network that is providing the VPN
service needs to be such that the encapsulated service class or
classes receive comparable behavior and performance in terms of
delay, jitter, and packet loss and that they are within the limits
of the service specified.
o The DSCP value in the external header of the packet forwarded
through the network providing the VPN service may be different
from the DSCP value that is used end to end for service
differentiation in the end network.
o The guidelines for aggregation of two or more service classes into
a single traffic forwarding treatment in the network that is
providing the VPN service is for further study.
6. Security Considerations
This document discusses policy and describes a common policy
configuration, for the use of a Differentiated Services Code Point by
transports and applications. If implemented as described, it should
require that the network do nothing that the network has not already
allowed. If that is the case, no new security issues should arise
from the use of such a policy.
It is possible for the policy to be applied incorrectly, or for a
wrong policy to be applied in the network for the defined service
class. In that case, a policy issue exists that the network SHOULD
detect, assess, and deal with. This is a known security issue in any
network dependent on policy-directed behavior.
A well-known flaw appears when bandwidth is reserved or enabled for a
service (for example, voice transport) and another service or an
attacking traffic stream uses it. This possibility is inherent in
DiffServ technology, which depends on appropriate packet markings.
When bandwidth reservation or a priority queuing system is used in a
vulnerable network, the use of authentication and flow admission is
recommended. To the author's knowledge, there is no known technical
way to respond to an unauthenticated data stream using service that
it is not intended to use, and such is the nature of the Internet.
The use of a service class by a user is not an issue when the SLA
between the user and the network permits him to use it, or to use it
up to a stated rate. In such cases, simple policing is used in the
Babiarz, et al. Informational [Page 51]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Differentiated Services Architecture. Some service classes, such as
Network Control, are not permitted to be used by users at all; such
traffic should be dropped or remarked by ingress filters. Where
service classes are available under the SLA only to an authenticated
user rather than to the entire population of users, authentication
and authorization services are required, such as those surveyed in
[AUTHMECH].
7. Acknowledgements
The authors thank the TSVWG reviewers, David Black, Brian E.
Carpenter, and Alan O'Neill for their review and input to this
document.
The authors acknowledge a great many inputs, most notably from Bruce
Davie, Dave Oran, Ralph Santitoro, Gary Kenward, Francois Audet,
Morgan Littlewood, Robert Milne, John Shuler, Nalin Mistry, Al
Morton, Mike Pierce, Ed Koehler Jr., Tim Rahrer, Fil Dickinson, Mike
Fidler, and Shane Amante. Kimberly King, Joe Zebarth, and Alistair
Munroe each did a thorough proofreading, and the document is better
for their contributions.
Babiarz, et al. Informational [Page 52]
RFC 4594 Guidelines for DiffServ Service Classes August 2006
8. Appendix A
8.1. Explanation of Ring Clipping
The term "ring clipping" refers to those instances where the front
end of a ringing signal is altered because the bearer channel is not
made available in time to carry all the audible ringing signal. This
condition may occur due to a race condition between when the tone
generator located in the circuit switch Exchange is turned on and
when the bearer path through the IP network is enabled. To reduce
ring clipping from occurring, delay of signaling path needs to be
minimized. Below is a more detailed explanation.
The bearer path setup delay target is defined as the ISUP Initial
Address Message (IAM) / Address Complete Message (ACM) round-trip
delay. ISUP refers to ISDN User Part of Signaling System No. 7
(SS7), as defined by ITU-T. This consists of the amount of time it
takes for the ISUP Initial Address Message (IAM) to leave the Transit
Exchange, travel through the SS7 network (including any applicable
STPs, or Signaling Transfer Points), and be processed by the End
Exchange thus generating the Address Complete Message (ACM) and for
the ACM to travel back through the SS7 network and return to the
Transit Exchange. If the bearer path has not been set up within the
soft-switch media gateway and the IP network that is performing the
Transit Exchange function by the time the ACM is forwarded to the
originating End Exchange, the phenomenon known as ring clipping may
occur. If ACM processing within the soft-switch media gateway and
delay through the IP network is excessive, it will delay the setup of
the bearer path, and therefore may cause clipping of the ring tone to
be heard.
The intra-exchange ISUP IAM signaling delay value should not exceed
240ms. This may include soft-switch, media gateway, router, and
propagation delay on the inter-exchange data path. This value
represents the threshold where ring clipping theoretically commences.
It is important to note that the 240ms delay objective as presented
is a maximum value. Service administrators are free to choose
specific IAM delay values according to their own preferences (i.e.,
they may wish to set a very low mean delay objective for strategic
reasons to differentiate themselves from other providers). In
summary, out of the 240-ms delay budget, 200ms is allocated as
cross-Exchange delay (soft-switch and media gateway) and 40ms for
network delay (queuing and distance).
Babiarz, et al. Informational [Page 53]
RFC 4594 Guidelines for DiffServ Service Classes August 2006
9. References
9.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC1349] Almquist, P., "Type of Service in the Internet Protocol
Suite", RFC 1349, July 1992.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers", RFC
1812, June 1995.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2309] Braden, B., Clark, D., Crowcroft, J., Davie, B., Deering,
S., Estrin, D., Floyd, S., Jacobson, V., Minshall, G.,
Partridge, C., Peterson, L., Ramakrishnan, K., Shenker,
S., Wroclawski, J., and L. Zhang, "Recommendations on
Queue Management and Congestion Avoidance in the
Internet", RFC 2309, April 1998.
[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.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Service", RFC 2475, 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.C., 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.
[RFC3662] Bless, R., Nichols, K., and K. Wehrle, "A Lower Effort
Per-Domain Behavior (PDB) for Differentiated Services",
RFC 3662, December 2003.
Babiarz, et al. Informational [Page 54]RFC 4594 Guidelines for DiffServ Service Classes August 2006
9.2. Informative References
[AUTHMECH] Rescorla, E., "A Survey of Authentication Mechanisms",
Work in Progress, September 2005.
[QBSS] "QBone Scavenger Service (QBSS) Definition", Internet2
Technical Report Proposed Service Definition, March 2001.
[RFC1633] Braden, R., Clark, D., and S. Shenker, "Integrated
Services in the Internet Architecture: an Overview", RFC
1633, June 1994.
[RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[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.
[RFC2963] Bonaventure, O. and S. De Cnodder, "A Rate Adaptive Shaper
for Differentiated Services", RFC 2963, October 2000.
[RFC2983] Black, D., "Differentiated Services and Tunnels", RFC
2983, October 2000.
[RFC2996] Bernet, Y., "Format of the RSVP DCLASS Object", RFC 2996,
November 2000.
[RFC3086] Nichols, K. and B. Carpenter, "Definition of
Differentiated Services Per Domain Behaviors and Rules for
their Specification", RFC 3086, April 2001.
[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC
3168, September 2001.
[RFC3175] Baker, F., Iturralde, C., Le Faucheur, F., and B. Davie,
"Aggregation of RSVP for IPv4 and IPv6 Reservations", RFC
3175, September 2001.
Babiarz, et al. Informational [Page 55]RFC 4594 Guidelines for DiffServ Service Classes August 2006
[RFC3290] Bernet, Y., Blake, S., Grossman, D., and A. Smith, "An
Informal Management Model for Diffserv Routers", RFC 3290,
May 2002.
[RFC3782] Floyd, S., Henderson, T., and A. Gurtov, "The NewReno
Modification to TCP's Fast Recovery Algorithm", RFC 3782,
April 2004.
Authors' Addresses
Jozef Babiarz
Nortel Networks
3500 Carling Avenue
Ottawa, Ont. K2H 8E9
Canada
Phone: +1-613-763-6098
Fax: +1-613-765-7462
EMail: babiarz@nortel.com
Kwok Ho Chan
Nortel Networks
600 Technology Park Drive
Billerica, MA 01821
US
Phone: +1-978-288-8175
Fax: +1-978-288-8700
EMail: khchan@nortel.com
Fred Baker
Cisco Systems
1121 Via Del Rey
Santa Barbara, CA 93117
US
Phone: +1-408-526-4257
Fax: +1-413-473-2403
EMail: fred@cisco.com
Babiarz, et al. Informational [Page 56]RFC 4594 Guidelines for DiffServ Service Classes August 2006
Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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Babiarz, et al. Informational [Page 57]
