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INFORMATIONAL
Network Working Group K. Nichols
Request for Comments: 3086 Packet Design
Category: Informational B. Carpenter
IBM
April 2001
Definition of Differentiated Services Per Domain Behaviors
and Rules for their Specification
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 (2001). All Rights Reserved.
Abstract
The differentiated services framework enables quality-of-service
provisioning within a network domain by applying rules at the edges
to create traffic aggregates and coupling each of these with a
specific forwarding path treatment in the domain through use of a
codepoint in the IP header. The diffserv WG has defined the general
architecture for differentiated services and has focused on the
forwarding path behavior required in routers, known as "per-hop
forwarding behaviors" (or PHBs). The WG has also discussed
functionality required at diffserv (DS) domain edges to select
(classifiers) and condition (e.g., policing and shaping) traffic
according to the rules. Short-term changes in the QoS goals for a DS
domain are implemented by changing only the configuration of these
edge behaviors without necessarily reconfiguring the behavior of
interior network nodes.
The next step is to formulate examples of how forwarding path
components (PHBs, classifiers, and traffic conditioners) can be used
to compose traffic aggregates whose packets experience specific
forwarding characteristics as they transit a differentiated services
domain. The WG has decided to use the term per-domain behavior, or
PDB, to describe the behavior experienced by a particular set of
packets as they cross a DS domain. A PDB is characterized by
specific metrics that quantify the treatment a set of packets with a
particular DSCP (or set of DSCPs) will receive as it crosses a DS
domain. A PDB specifies a forwarding path treatment for a traffic
aggregate and, due to the role that particular choices of edge and
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PHB configuration play in its resulting attributes, it is where the
forwarding path and the control plane interact. The measurable
parameters of a PDB should be suitable for use in Service Level
Specifications at the network edge.
This document defines and discusses Per-Domain Behaviors in detail
and lays out the format and required content for contributions to the
Diffserv WG on PDBs and the procedure that will be applied for
individual PDB specifications to advance as WG products. This format
is specified to expedite working group review of PDB submissions.
Table of Contents
1. Introduction ................................................ 2
2. Definitions ................................................. 4
3. The Value of Defining Edge-to-Edge Behavior ................. 5
4. Understanding PDBs .......................................... 7
5. Format for Specification of Diffserv Per-Domain Behaviors ...13
6. On PDB Attributes ...........................................16
7. A Reference Per-Domain Behavior .............................19
8. Guidelines for Advancing PDB Specifications .................21
9. Security Considerations .....................................22
10. Acknowledgements ............................................22
References ..................................................22
Authors' Addresses ..........................................23
Full Copyright Statement ....................................24
1 Introduction
Differentiated Services allows an approach to IP Quality of Service
that is modular, incrementally deployable, and scalable while
introducing minimal per-node complexity [RFC2475]. From the end
user's point of view, QoS should be supported end-to-end between any
pair of hosts. However, this goal is not immediately attainable. It
will require interdomain QoS support, and many untaken steps remain
on the road to achieving this. One essential step, the evolution of
the business models for interdomain QoS, will necessarily develop
outside of the IETF. A goal of the diffserv WG is to provide the
firm technical foundation that allows these business models to
develop. The first major step will be to support edge-to-edge or
intradomain QoS between the ingress and egress of a single network,
i.e., a DS Domain in the terminology of RFC 2474. The intention is
that this edge-to-edge QoS should be composable, in a purely
technical sense, to a quantifiable QoS across a DS Region composed of
multiple DS domains.
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The Diffserv WG has finished the first phase of standardizing the
behaviors required in the forwarding path of all network nodes, the
per-hop forwarding behaviors or PHBs. The PHBs defined in RFCs 2474,
2597 and 2598 give a rich toolbox for differential packet handling by
individual boxes. The general architectural model for diffserv has
been documented in RFC 2475. An informal router model [MODEL]
describes a model of traffic conditioning and other forwarding
behaviors. However, technical issues remain in moving "beyond the
box" to intradomain QoS models.
The ultimate goal of creating scalable end-to-end QoS in the Internet
requires that we can identify and quantify behavior for a group of
packets that is preserved when they are aggregated with other packets
as they traverse the Internet. The step of specifying forwarding
path attributes on a per-domain basis for a set of packets
distinguished only by the mark in the DS field of individual packets
is critical in the evolution of Diffserv QoS and should provide the
technical input that will aid in the construction of business models.
This document defines and specifies the term "Per-Domain Behavior" or
PDB to describe QoS attributes across a DS domain.
Diffserv classification and traffic conditioning are applied to
packets arriving at the boundary of a DS domain to impose
restrictions on the composition of the resultant traffic aggregates,
as distinguished by the DSCP marking , inside the domain. The
classifiers and traffic conditioners are set to reflect the policy
and traffic goals for that domain and may be specified in a TCA
(Traffic Conditioning Agreement). Once packets have crossed the DS
boundary, adherence to diffserv principles makes it possible to group
packets solely according to the behavior they receive at each hop (as
selected by the DSCP). This approach has well-known scaling
advantages, both in the forwarding path and in the control plane.
Less well recognized is that these scaling properties only result if
the per-hop behavior definition gives rise to a particular type of
invariance under aggregation. Since the per-hop behavior must be
equivalent for every node in the domain, while the set of packets
marked for that PHB may be different at every node, PHBs should be
defined such that their characteristics do not depend on the traffic
volume of the associated BA on a router's ingress link nor on a
particular path through the DS domain taken by the packets.
Specifically, different streams of traffic that belong to the same
traffic aggregate merge and split as they traverse the network. If
the properties of a PDB using a particular PHB hold regardless of how
the temporal characteristics of the marked traffic aggregate change
as it traverses the domain, then that PDB scales. (Clearly this
assumes that numerical parameters such as bandwidth allocated to the
particular PDB may be different at different points in the network,
and may be adjusted dynamically as traffic volume varies.) If there
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are limits to where the properties hold, that translates to a limit
on the size or topology of a DS domain that can use that PDB.
Although useful single-link DS domains might exist, PDBs that are
invariant with network size or that have simple relationships with
network size and whose properties can be recovered by reapplying
rules (that is, forming another diffserv boundary or edge to re-
enforce the rules for the traffic aggregate) are needed for building
scalable end-to-end quality of service.
There is a clear distinction between the definition of a Per-Domain
Behavior in a DS domain and a service that might be specified in a
Service Level Agreement. The PDB definition is a technical building
block that permits the coupling of classifiers, traffic conditioners,
specific PHBs, and particular configurations with a resulting set of
specific observable attributes which may be characterized in a
variety of ways. These definitions are intended to be useful tools
in configuring DS domains, but the PDB (or PDBs) used by a provider
is not expected to be visible to customers any more than the specific
PHBs employed in the provider's network would be. Network providers
are expected to select their own measures to make customer-visible in
contracts and these may be stated quite differently from the
technical attributes specified in a PDB definition, though the
configuration of a PDB might be taken from a Service Level
Specification (SLS). Similarly, specific PDBs are intended as tools
for ISPs to construct differentiated services offerings; each may
choose different sets of tools, or even develop their own, in order
to achieve particular externally observable metrics. Nevertheless,
the measurable parameters of a PDB are expected to be among the
parameters cited directly or indirectly in the Service Level
Specification component of a corresponding SLA.
This document defines Differentiated Services Per-Domain Behaviors
and specifies the format that must be used for submissions of
particular PDBs to the Diffserv WG.
2 Definitions
The following definitions are stated in RFCs 2474 and 2475 and are
repeated here for easy reference:
" Behavior Aggregate: a collection of packets with the same codepoint
crossing a link in a particular direction.
" Differentiated Services Domain: a contiguous portion of the
Internet over which a consistent set of differentiated services
policies are administered in a coordinated fashion. A
differentiated services domain can represent different
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administrative domains or autonomous systems, different trust
regions, different network technologies (e.g., cell/frame), hosts
and routers, etc. Also DS domain.
" Differentiated Services Boundary: the edge of a DS domain, where
classifiers and traffic conditioners are likely to be deployed. A
differentiated services boundary can be further sub-divided into
ingress and egress nodes, where the ingress/egress nodes are the
downstream/upstream nodes of a boundary link in a given traffic
direction. A differentiated services boundary typically is found
at the ingress to the first-hop differentiated services-compliant
router (or network node) that a host's packets traverse, or at the
egress of the last-hop differentiated services-compliant router or
network node that packets traverse before arriving at a host. This
is sometimes referred to as the boundary at a leaf router. A
differentiated services boundary may be co-located with a host,
subject to local policy. Also DS boundary.
To these we add:
" Traffic Aggregate: a collection of packets with a codepoint that
maps to the same PHB, usually in a DS domain or some subset of a DS
domain. A traffic aggregate marked for the foo PHB is referred to
as the "foo traffic aggregate" or "foo aggregate" interchangeably.
This generalizes the concept of Behavior Aggregate from a link to a
network.
" Per-Domain Behavior: the expected treatment that an identifiable or
target group of packets will receive from "edge-to-edge" of a DS
domain. (Also PDB.) A particular PHB (or, if applicable, list of
PHBs) and traffic conditioning requirements are associated with
each PDB.
" A Service Level Specification (SLS) is a set of parameters and
their values which together define the service offered to a traffic
stream by a DS domain. It is expected to include specific values
or bounds for PDB parameters.
3 The Value of Defining Edge-to-Edge Behavior
As defined in section 2, a PDB describes the edge-to-edge behavior
across a DS domain's "cloud." Specification of the transit
expectations of packets matching a target for a particular diffserv
behavior across a DS domain will both assist in the deployment of
single-domain QoS and will help enable the composition of end-to-end,
cross-domain services. Networks of DS domains can be connected to
create end-to-end services by building on the PDB characteristics
without regard to the particular PHBs used. This level of
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abstraction makes it easier to compose cross-domain services as well
as making it possible to hide details of a network's internals while
exposing information sufficient to enable QoS.
Today's Internet is composed of multiple independently administered
domains or Autonomous Systems (ASs), represented by the "clouds" in
figure 1. To deploy ubiquitous end-to-end quality of service in the
Internet, business models must evolve that include issues of charging
and reporting that are not in scope for the IETF. In the meantime,
there are many possible uses of quality of service within an AS and
the IETF can address the technical issues in creating an intradomain
QoS within a Differentiated Services framework. In fact, this
approach is quite amenable to incremental deployment strategies.
Where DS domains are independently administered, the evolution of the
necessary business agreements and future signaling arrangements will
take some time, thus, early deployments will be within a single
administrative domain. Putting aside the business issues, the same
technical issues that arise in interconnecting DS domains with
homogeneous administration will arise in interconnecting the
autonomous systems (ASs) of the Internet.
----------------------------------------
| AS2 |
| |
------- | ------------ ------------ |
| AS1 |------|-----X | | | |
------- | | | Y | | -------
| | | /| X----|--------| AS3 |
| | | / | | | -------
| | | / ------------ |
| | Y | |
| | | \ ------------ |
------- | | | \ | | |
| AS4 |------|-----X | \| | |
------- | | | Y X----|------
| | | | | |
| ------------ ------------ |
| |
| |
----------------------------------------
Figure 1: Interconnection of ASs and DS Domains
A single AS (e.g., AS2 in figure 1) may be composed of subnetworks
and, as the definition allows, these can be separate DS domains. An
AS might have multiple DS domains for a number of reasons, most
notable being to follow topological and/or technological boundaries
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and to separate the allocation of resources. If we confine ourselves
to the DS boundaries between these "interior" DS domains, we avoid
the non-technical problems of setting up a service and can address
the issues of creating characterizable PDBs.
The incentive structure for differentiated services is based on
upstream domains ensuring their traffic conforms to the Traffic
Conditioning Agreements (TCAs) with downstream domains and downstream
domains enforcing that TCA, thus metrics associated with PDBs can be
sensibly computed. The letters "X" and "Y" in figure 1 represent the
DS boundary routers containing traffic conditioners that ensure and
enforce conformance (e.g., shapers and policers). Although policers
and shapers are expected at the DS boundaries of ASs (the "X" boxes),
they might appear anywhere, or nowhere, inside the AS. Specifically,
the boxes at the DS boundaries internal to the AS (the "Y" boxes) may
or may not condition traffic. Technical guidelines for the placement
and configuration of DS boundaries should derive from the attributes
of a particular PDB under aggregation and multiple hops.
This definition of PDB continues the separation of forwarding path
and control plane described in RFC 2474. The forwarding path
characteristics are addressed by considering how the behavior at
every hop of a packet's path is affected by the merging and branching
of traffic aggregates through multiple hops. Per-hop behaviors in
nodes are configured infrequently, representing a change in network
infrastructure. More frequent quality-of-service changes come from
employing control plane functions in the configuration of the DS
boundaries. A PDB provides a link between the DS domain level at
which control is exercised to form traffic aggregates with quality-
of-service goals across the domain and the per-hop and per-link
treatments packets receive that results in meeting the quality-of-
service goals.
4 Understanding PDBs
4.1 Defining PDBs
RFCs 2474 and 2475 define a Differentiated Services Behavior
Aggregate as "a collection of packets with the same DS codepoint
crossing a link in a particular direction" and further state that
packets with the same DSCP get the same per-hop forwarding treatment
(or PHB) everywhere inside a single DS domain. Note that even if
multiple DSCPs map to the same PHB, this must hold for each DSCP
individually. In section 2 of this document, we introduced a more
general definition of a traffic aggregate in the diffserv sense so
that we might easily refer to the packets which are mapped to the
same PHB everywhere within a DS domain. Section 2 also presented a
short definition of PDBs which we expand upon in this section:
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Per-Domain Behavior: the expected treatment that an identifiable or
target group of packets will receive from "edge to edge" of a DS
domain. A particular PHB (or, if applicable, list of PHBs) and
traffic conditioning requirements are associated with each PDB.
Each PDB has measurable, quantifiable, attributes that can be used to
describe what happens to its packets as they enter and cross the DS
domain. These derive from the characteristics of the traffic
aggregate that results from application of classification and traffic
conditioning during the entry of packets into the DS domain and the
forwarding treatment (PHB) the packets get inside the domain, but can
also depend on the entering traffic loads and the domain's topology.
PDB attributes may be absolute or statistical and they may be
parameterized by network properties. For example, a loss attribute
might be expressed as "no more than 0.1% of packets will be dropped
when measured over any time period larger than T", a delay attribute
might be expressed as "50% of delivered packets will see less than a
delay of d milliseconds, 30% will see a delay less than 2d ms, 20%
will see a delay of less than 3d ms." A wide range of metrics is
possible. In general they will be expressed as bounds or percentiles
rather than as absolute values.
A PDB is applied to a target group of packets arriving at the edge of
the DS domain. The target group is distinguished from all arriving
packets by use of packet classifiers [RFC2475] (where the classifier
may be "null"). The action of the PDB on the target group has two
parts. The first part is the the use of traffic conditioning to
create a traffic aggregate. During traffic conditioning, conformant
packets are marked with a DSCP for the PHB associated with the PDB
(see figure 2). The second part is the treatment experienced by
packets from the same traffic aggregate transiting the interior of a
DS domain, between and inside of DS domain boundaries. The following
subsections further discuss these two effects on the target group
that arrives at the DS domain boundary.
----------- ------------ -------------------- foo
arriving _|classifiers|_|target group|_|traffic conditioning|_ traffic
packets | | |of packets | |& marking (for foo) | aggregate
----------- ------------ --------------------
Figure 2: Relationship of the traffic aggregate associated
with a PDB to arriving packets
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4.1.1 Crossing the DS edge: the effects of traffic conditioning on the
target group
This effect is quantified by the relationship of the emerging traffic
aggregate to the entering target group. That relationship can depend
on the arriving traffic pattern as well as the configuration of the
traffic conditioners. For example, if the EF PHB [RFC2598] and a
strict policer of rate R are associated with the foo PDB, then the
first part of characterizing the foo PDB is to write the relationship
between the arriving target packets and the departing foo traffic
aggregate. In this case, "the rate of the emerging foo traffic
aggregate is less than or equal to the smaller of R and the arrival
rate of the target group of packets" and additional temporal
characteristics of the packets (e.g., burst) may be specified as
desired. Thus, there is a "loss rate" on the arriving target group
that results from sending too much traffic or the traffic with the
wrong temporal characteristics. This loss rate should be entirely
preventable (or controllable) by the upstream sender conforming to
the traffic conditioning associated with the PDB specification.
The issue of "who is in control" of the loss (or demotion) rate helps
to clearly delineate this component of PDB performance from that
associated with transiting the domain. The latter is completely
under control of the operator of the DS domain and the former is used
to ensure that the entering traffic aggregate conforms to the traffic
profile to which the operator has provisioned the network. Further,
the effects of traffic conditioning on the target group can usually
be expressed more simply than the effects of transiting the DS domain
on the traffic aggregate's traffic profile.
A PDB may also apply traffic conditioning at DS domain egress. The
effect of this conditioning on the overall PDB attributes would be
treated similarly to the ingress characteristics (the authors may
develop more text on this in the future, but it does not materially
affect the ideas presented in this document.)
4.1.2 Crossing the DS domain: transit effects
The second component of PDB performance is the metrics that
characterize the transit of a packet of the PDB's traffic aggregate
between any two edges of the DS domain boundary shown in figure 3.
Note that the DS domain boundary runs through the DS boundary routers
since the traffic aggregate is generally formed in the boundary
router before the packets are queued and scheduled for output. (In
most cases, this distinction is expected to be important.)
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DSCPs should not change in the interior of a DS domain as there is no
traffic conditioning being applied. If it is necessary to reapply
the kind of traffic conditioning that could result in remarking,
there should be a DS domain boundary at that point, though such an
"interior" boundary can have "lighter weight" rules in its TCA.
Thus, when measuring attributes between locations as indicated in
figure 3, the DSCP at the egress side can be assumed to have held
throughout the domain.
-------------
| |
-----X |
| |
| DS |
| domain X----
| |
-----X |
| |
-------------
Figure 3: Range of applicability of attributes of a traffic
aggregate associated with a PDB (is between the
points marked "X")
Though a DS domain may be as small as a single node, more complex
topologies are expected to be the norm, thus the PDB definition must
hold as its traffic aggregate is split and merged on the interior
links of a DS domain. Packet flow in a network is not part of the
PDB definition; the application of traffic conditioning as packets
enter the DS domain and the consistent PHB through the DS domain must
suffice. A PDB's definition does not have to hold for arbitrary
topologies of networks, but the limits on the range of applicability
for a specific PDB must be clearly specified.
In general, a PDB operates between N ingress points and M egress
points at the DS domain boundary. Even in the degenerate case where
N=M=1, PDB attributes are more complex than the definition of PHB
attributes since the concatenation of the behavior of intermediate
nodes affects the former. A complex case with N and M both greater
than one involves splits and merges in the traffic path and is non-
trivial to analyze. Analytic, simulation, and experimental work will
all be necessary to understand even the simplest PDBs.
4.2 Constructing PDBs
A DS domain is configured to meet the network operator's traffic
engineering goals for the domain independently of the performance
goals for a particular flow of a traffic aggregate. Once the
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interior routers are configured for the number of distinct traffic
aggregates that the network will handle, each PDB's allocation at the
edge comes from meeting the desired performance goals for the PDB's
traffic aggregate subject to that configuration of packet schedulers
and bandwidth capacity. The configuration of traffic conditioners at
the edge may be altered by provisioning or admission control but the
decision about which PDB to use and how to apply classification and
traffic conditioning comes from matching performance to goals.
For example, consider the DS domain of figure 3. A PDB with an
explicit bound on loss must apply traffic conditioning at the
boundary to ensure that on the average no more packets are admitted
than can emerge. Though, queueing internal to the network may result
in a difference between input and output traffic over some
timescales, the averaging timescale should not exceed what might be
expected for reasonably sized buffering inside the network. Thus if
bursts are allowed to arrive into the interior of the network, there
must be enough capacity to ensure that losses don't exceed the bound.
Note that explicit bounds on the loss level can be particularly
difficult as the exact way in which packets merge inside the network
affects the burstiness of the PDB's traffic aggregate and hence,
loss.
PHBs give explicit expressions of the treatment a traffic aggregate
can expect at each hop. For a PDB, this behavior must apply to
merging and diverging traffic aggregates, thus characterizing a PDB
requires understanding what happens to a PHB under aggregation. That
is, PHBs recursively applied must result in a known behavior. As an
example, since maximum burst sizes grow with the number of microflows
or traffic aggregate streams merged, a PDB specification must address
this. A clear advantage of constructing behaviors that aggregate is
the ease of concatenating PDBs so that the associated traffic
aggregate has known attributes that span interior DS domains and,
eventually, farther. For example, in figure 1 assume that we have
configured the foo PDB on the interior DS domains of AS2. Then
traffic aggregates associated with the foo PDB in each interior DS
domain of AS2 can be merged at the shaded interior boundary routers.
If the same (or fewer) traffic conditioners as applied at the
entrance to AS2 are applied at these interior boundaries, the
attributes of the foo PDB should continue to be used to quantify the
expected behavior. Explicit expressions of what happens to the
behavior under aggregation, possibly parameterized by node in-degrees
or network diameters, are necessary to determine what to do at the
internal aggregation points. One approach might be to completely
reapply the traffic conditioning at these points; another might
employ some limited rate-based remarking only.
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Multiple PDBs may use the same PHB. The specification of a PDB can
contain a list of PHBs and their required configuration, all of which
would result in the same PDB. In operation, it is expected that a
single domain will use a single PHB to implement a particular PDB,
though different domains may select different PHBs. Recall that in
the diffserv definition [RFC2474], a single PHB might be selected
within a domain by a list of DSCPs. Multiple PDBs might use the same
PHB in which case the transit performance of traffic aggregates of
these PDBs will, of necessity, be the same. Yet, the particular
characteristics that the PDB designer wishes to claim as attributes
may vary, so two PDBs that use the same PHB might not be specified
with the same list of attributes.
The specification of the transit expectations of PDBs across domains
both assists in the deployment of QoS within a DS domain and helps
enable the composition of end-to-end, cross-domain services to
proceed by making it possible to hide details of a domain's internals
while exposing characteristics necessary for QoS.
4.3 PDBs using PHB Groups
The use of PHB groups to construct PDBs can be done in several ways.
A single PHB member of a PHB group might be used to construct a
single PDB. For example, a PDB could be defined using just one of
the Class Selector Compliant PHBs [RFC2474]. The traffic
conditioning for that PDB and the required configuration of the
particular PHB would be defined in such a way that there was no
dependence or relationship with the manner in which other PHBs of the
group are used or, indeed, whether they are used in that DS domain.
In this case, the reasonable approach would be to specify this PDB
alone in a document which expressly called out the conditions and
configuration of the Class Selector PHB required.
A single PDB can be constructed using more than one PHB from the same
PHB group. For example, the traffic conditioner described in RFC
2698 might be used to mark a particular entering traffic aggregate
for one of the three AF1x PHBs [RFC2597] while the transit
performance of the resultant PDB is specified, statistically, across
all the packets marked with one of those PHBs.
A set of related PDBs might be defined using a PHB group. In this
case, the related PDBs should be defined in the same document. This
is appropriate when the traffic conditioners that create the traffic
aggregates associated with each PDB have some relationships and
interdependencies such that the traffic aggregates for these PDBs
should be described and characterized together. The transit
attributes will depend on the PHB associated with the PDB and will
not be the same for all PHBs in the group, though there may be some
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parameterized interrelationship between the attributes of each of
these PDBs. In this case, each PDB should have a clearly separate
description of its transit attributes (delineated in a separate
subsection) within the document. For example, the traffic
conditioner described in RFC 2698 might be used to mark arriving
packets for three different AF1x PHBs, each of which is to be treated
as a separate traffic aggregate in terms of transit properties. Then
a single document could be used to define and quantify the
relationship between the arriving packets and the emerging traffic
aggregates as they relate to one another. The transit
characteristics of packets of each separate AF1x traffic aggregate
should be described separately within the document.
Another way in which a PHB group might be used to create one PDB per
PHB might have decoupled traffic conditioners, but some relationship
between the PHBs of the group. For example, a set of PDBs might be
defined using Class Selector Compliant PHBs [RFC2474] in such a way
that the traffic conditioners that create the traffic aggregates are
not related, but the transit performance of each traffic aggregate
has some parametric relationship to the other. If it makes sense to
specify them in the same document, then the author(s) should do so.
4.4 Forwarding path vs. control plane
A PDB's associated PHB, classifiers, and traffic conditioners are all
in the packet forwarding path and operate at line rates. PHBs,
classifiers, and traffic conditioners are configured in response to
control plane activity which takes place across a range of time
scales, but, even at the shortest time scale, control plane actions
are not expected to happen per-packet. Classifiers and traffic
conditioners at the DS domain boundary are used to enforce who gets
to use the PDB and how the PDB should behave temporally.
Reconfiguration of PHBs might occur monthly, quarterly, or only when
the network is upgraded. Classifiers and traffic conditioners might
be reconfigured at a few regular intervals during the day or might
happen in response to signalling decisions thousands of times a day.
Much of the control plane work is still evolving and is outside the
charter of the Diffserv WG. We note that this is quite appropriate
since the manner in which the configuration is done and the time
scale at which it is done should not affect the PDB attributes.
5 Format for Specification of Diffserv Per-Domain Behaviors
PDBs arise from a particular relationship between edge and interior
(which may be parameterized). The quantifiable characteristics of a
PDB must be independent of whether the network edge is configured
statically or dynamically. The particular configuration of traffic
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conditioners at the DS domain edge is critical to how a PDB performs,
but the act(s) of configuring the edge is a control plane action
which can be separated from the specification of the PDB.
The following sections must be present in any specification of a
Differentiated Services PDB. Of necessity, their length and content
will vary greatly.
5.1 Applicability Statement
All PDB specs must have an applicability statement that outlines the
intended use of this PDB and the limits to its use.
5.2 Technical specification
This section specifies the rules or guidelines to create this PDB,
each distinguished with "may", "must" and "should." The technical
specification must list the classification and traffic conditioning
required (if any) and the PHB (or PHBs) to be used with any
additional requirements on their configuration beyond that contained
in RFCs. Classification can reflect the results of an admission
control process. Traffic conditioning may include marking, traffic
shaping, and policing. A Service Provisioning Policy might be used
to describe the technical specification of a particular PDB.
5.3 Attributes
A PDB's attributes tell how it behaves under ideal conditions if
configured in a specified manner (where the specification may be
parameterized). These might include drop rate, throughput, delay
bounds measured over some time period. They may be bounds,
statistical bounds, or percentiles (e.g., "90% of all packets
measured over intervals of at least 5 minutes will cross the DS
domain in less than 5 milliseconds"). A wide variety of
characteristics may be used but they must be explicit, quantifiable,
and defensible. Where particular statistics are used, the document
must be precise about how they are to be measured and about how the
characteristics were derived.
Advice to a network operator would be to use these as guidelines in
creating a service specification rather than use them directly. For
example, a "loss-free" PDB would probably not be sold as such, but
rather as a service with a very small packet loss probability.
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5.4 Parameters
The definition and characteristics of a PDB may be parameterized by
network-specific features; for example, maximum number of hops,
minimum bandwidth, total number of entry/exit points of the PDB
to/from the diffserv network, maximum transit delay of network
elements, minimum buffer size available for the PDB at a network
node, etc.
5.5 Assumptions
In most cases, PDBs will be specified assuming lossless links, no
link failures, and relatively stable routing. This is reasonable
since otherwise it would be very difficult to quantify behavior and
this is the operating conditions for which most operators strive.
However, these assumptions must be clearly stated. Since PDBs with
specific bandwidth parameters require that bandwidth to be available,
the assumptions to be stated may include standby capacity. Some PDBs
may be specifically targeted for cases where these assumptions do not
hold, e.g., for high loss rate links, and such targeting must also be
made explicit. If additional restrictions, especially specific
traffic engineering measures, are required, these must be stated.
Further, if any assumptions are made about the allocation of
resources within a diffserv network in the creation of the PDB, these
must be made explicit.
5.6 Example Uses
A PDB specification must give example uses to motivate the
understanding of ways in which a diffserv network could make use of
the PDB although these are not expected to be detailed. For example,
"A bulk handling PDB may be used for all packets which should not
take any resources from the network unless they would otherwise go
unused. This might be useful for Netnews traffic or for traffic
rejected from some other PDB by traffic policers."
5.7 Environmental Concerns (media, topology, etc.)
Note that it is not necessary for a provider to expose which PDB (if
a commonly defined one) is being used nor is it necessary for a
provider to specify a service by the PDB's attributes. For example,
a service provider might use a PDB with a "no queueing loss"
characteristic in order to specify a "very low loss" service.
This section is to inject realism into the characteristics described
above. Detail the assumptions made there and what constraints that
puts on topology or type of physical media or allocation.
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5.8 Security Considerations for each PDB
This section should include any security considerations that are
specific to the PDB. Is it subject to any unusual theft-of-service
or denial-of-service attacks? Are any unusual security precautions
needed?
It is not necessary to repeat the general security discussions in
[RFC2474] and [RFC2475], but a reference should be included. Also
refer to any special security considerations for the PHB or PHBs
used.
6 On PDB Attributes
As discussed in section 4, measurable, quantifiable attributes
associated with each PDB can be used to describe what will happen to
packets using that PDB as they cross the domain. In its role as a
building block for the construction of interdomain quality-of-
service, a PDB specification should provide the answer to the
question: Under what conditions can we join the output of this domain
to another under the same traffic conditioning and expectations?
Although there are many ways in which traffic might be distributed,
creating quantifiable, realizable PDBs that can be concatenated into
multi-domain services limits the realistic scenarios. A PDB's
attributes with a clear statement of the conditions under which the
attributes hold is critical to the composition of multi-domain
services.
There is a clear correlation between the strictness of the traffic
conditioning and the quality of the PDB's attributes. As indicated
earlier, numerical bounds are likely to be statistical or expressed
as a percentile. Parameters expressed as strict bounds will require
very precise mathematical analysis, while those expressed
statistically can to some extent rely on experiment. Section 7 gives
the example of a PDB without strict traffic conditioning and
concurrent work on a PDB with strict traffic conditioning and
attributes is also in front of the WG [VW]. This section gives some
general considerations for characterizing PDB attributes.
There are two ways to characterize PDBs with respect to time. First
are properties over "long" time periods, or average behaviors. A PDB
specification should report these as the rates or throughput seen
over some specified time period. In addition, there are properties
of "short" time behavior, usually expressed as the allowable
burstiness in a traffic aggregate. The short time behavior is
important in understanding buffering requirements (and associated
loss characteristics) and for metering and conditioning
considerations at DS boundaries. For short-time behavior, we are
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interested primarily in two things: 1) how many back-to-back packets
of the PDB's traffic aggregate will we see at any point (this would
be metered as a burst) and 2) how large a burst of packets of this
PDB's traffic aggregate can appear in a queue at once (gives queue
overflow and loss). If other PDBs are using the same PHB within the
domain, that must be taken into account.
6.1 Considerations in specifying long-term or average PDB attributes
To characterize the average or long-term behavior for the foo PDB we
must explore a number of questions, for instance: Can the DS domain
handle the average foo traffic flow? Is that answer topology
dependent or are there some specific assumptions on routing which
must hold for the foo PDB to preserve its "adequately provisioned"
capability? In other words, if the topology of D changes suddenly,
will the foo PDB's attributes change? Will its loss rate
dramatically increase?
Let domain D in figure 4 be an ISP ringing the U.S. with links of
bandwidth B and with N tails to various metropolitan areas. Inside
D, if the link between the node connected to A and the node connected
to Z goes down, all the foo traffic aggregate between the two nodes
must transit the entire ring: Would the bounded behavior of the foo
PDB change? If this outage results in some node of the ring now
having a larger arrival rate to one of its links than the capacity of
the link for foo's traffic aggregate, clearly the loss rate would
change dramatically. In this case, topological assumptions were made
about the path of the traffic from A to Z that affected the
characteristics of the foo PDB. If these topological assumptions no
longer hold, the loss rate of packets of the foo traffic aggregate
transiting the domain could change; for example, a characteristic
such as "loss rate no greater than 1% over any interval larger than
10 minutes." A PDB specification should spell out the assumptions
made on preserving the attributes.
____X________X_________X___________ /
/ \ L |
A<---->X X<----->| E
| | |
| D | \
Z<---->X |
| |
\___________________________________/
X X
Figure 4: ISP and DS domain D connected in a ring and
connected to DS domain E
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6.2 Considerations in specifying short-term or bursty PDB attributes
Next, consider the short-time behavior of the traffic aggregate
associated with a PDB, specifically whether permitting the maximum
bursts to add in the same manner as the average rates will lead to
properties that aggregate or under what conditions this will lead to
properties that aggregate. In our example, if domain D allows each
of the uplinks to burst p packets into the foo traffic aggregate, the
bursts could accumulate as they transit the ring. Packets headed for
link L can come from both directions of the ring and back-to-back
packets from foo's traffic aggregate can arrive at the same time. If
the bandwidth of link L is the same as the links of the ring, this
probably does not present a buffering problem. If there are two
input links that can send packets to queue for L, at worst, two
packets can arrive simultaneously for L. If the bandwidth of link L
equals or exceeds twice B, the packets won't accumulate. Further, if
p is limited to one, and the bandwidth of L exceeds the rate of
arrival (over the longer term) of foo packets (required for bounding
the loss) then the queue of foo packets for link L will empty before
new packets arrive. If the bandwidth of L is equal to B, one foo
packet must queue while the other is transmitted. This would result
in N x p back-to- back packets of this traffic aggregate arriving
over L during the same time scale as the bursts of p were permitted
on the uplinks. Thus, configuring the PDB so that link L can handle
the sum of the rates that ingress to the foo PDB doesn't guarantee
that L can handle the sum of the N bursts into the foo PDB.
If the bandwidth of L is less than B, then the link must buffer
Nxpx(B-L)/B foo packets to avoid loss. If the PDB is getting less
than the full bandwidth L, this number is larger. For probabilistic
bounds, a smaller buffer might do if the probability of exceeding it
can be bounded.
More generally, for router indegree of d, bursts of foo packets might
arrive on each input. Then, in the absence of any additional traffic
conditioning, it is possible that dxpx(# of uplinks) back-to-back foo
packets can be sent across link L to domain E. Thus the DS domain E
must permit these much larger bursts into the foo PDB than domain D
permits on the N uplinks or else the foo traffic aggregate must be
made to conform to the TCA for entering E (e.g., by shaping).
What conditions should be imposed on a PDB and on the associated PHB
in order to ensure PDBs can be concatenated, as across the interior
DS domains of figure 1? Traffic conditioning for constructing a PDB
that has certain attributes across a DS domain should apply
independently of the origin of the packets. With reference to the
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example we've been exploring, the TCA for the PDB's traffic aggregate
entering link L into domain E should not depend on the number of
uplinks into domain D.
6.3 Remarks
This section has been provided as motivational food for thought for
PDB specifiers. It is by no means an exhaustive catalog of possible
PDB attributes or what kind of analysis must be done. We expect this
to be an interesting and evolutionary part of the work of
understanding and deploying differentiated services in the Internet.
There is a potential for much interesting research work. However, in
submitting a PDB specification to the Diffserv WG, a PDB must also
meet the test of being useful and relevant by a deployment
experience, described in section 8.
7 A Reference Per-Domain Behavior
The intent of this section is to define as a reference a Best Effort
PDB, a PDB that has little in the way of rules or expectations.
7.1 Best Effort PDB
7.1.1 Applicability
A Best Effort (BE) PDB is for sending "normal internet traffic"
across a diffserv network. That is, the definition and use of this
PDB is to preserve, to a reasonable extent, the pre-diffserv delivery
expectation for packets in a diffserv network that do not require any
special differentiation. Although the PDB itself does not include
bounds on availability, latency, and packet loss, this does not
preclude Service Providers from engineering their networks so as to
result in commercially viable bounds on services that utilize the BE
PDB. This would be analogous to the Service Level Guarantees that
are provided in today's single-service Internet.
In the present single-service commercial Internet, Service Level
Guarantees for availability, latency, and packet delivery can be
found on the web sites of ISPs [WCG, PSI, UU]. For example, a
typical North American round-trip latency bound is 85 milliseconds,
with each service provider's site information specifying the method
of measurement of the bounds and the terms associated with these
bounds contractually.
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7.1.2 TCS and PHB configurations
There are no restrictions governing rate and bursts of packets beyond
the limits imposed by the ingress link. The network edge ensures
that packets using the PDB are marked for the Default PHB (as defined
in [RFC2474]), but no other traffic conditioning is required.
Interior network nodes apply the Default PHB on these packets.
7.1.3 Attributes of this PDB
"As much as possible as soon as possible".
Packets of this PDB will not be completely starved and when resources
are available (i.e., not required by packets from any other traffic
aggregate), network elements should be configured to permit packets
of this PDB to consume them.
Network operators may bound the delay and loss rate for services
constructed from this PDB given knowledge about their network, but
such attributes are not part of the definition.
7.1.4 Parameters
None.
7.1.5 Assumptions
A properly functioning network, i.e., packets may be delivered from
any ingress to any egress.
7.1.6 Example uses
1. For the normal Internet traffic connection of an organization.
2. For the "non-critical" Internet traffic of an organization.
3. For standard domestic consumer connections
7.1.7 Environmental Concerns
There are no environmental concerns specific to this PDB.
7.1.8 Security Considerations for BE PDB
There are no specific security exposures for this PDB. See the
general security considerations in [RFC2474] and [RFC2475].
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8 Guidelines for writing PDB specifications
G1. Following the format given in this document, write a draft and
submit it as an Internet Draft. The document should have "diffserv"
as some part of the name. Either as an appendix to the draft, or in
a separate document, provide details of deployment experience with
measured results on a network of non-trivial size carrying realistic
traffic and/or convincing simulation results (simulation of a range
of modern traffic patterns and network topologies as applicable).
The document should be brought to the attention of the diffserv WG
mailing list, if active.
G2. Initial discussion should focus primarily on the merits of the
PDB, though comments and questions on the claimed attributes are
reasonable. This is in line with the Differentiated Services goal to
put relevance before academic interest in the specification of PDBs.
Academically interesting PDBs are encouraged, but would be more
appropriate for technical publications and conferences, not for
submission to the IETF. (An "academically interesting" PDB might
become a PDB of interest for deployment over time.)
The implementation of the following guidelines varies, depending on
whether there is an active diffserv working group or not.
Active Diffserv Working Group path:
G3. Once consensus has been reached on a version of a draft that it
is a useful PDB and that the characteristics "appear" to be correct
(i.e., not egregiously wrong) that version of the draft goes to a
review panel the WG co-chairs set up to audit and report on the
characteristics. The review panel will be given a deadline for the
review. The exact timing of the deadline will be set on a case-by-
case basis by the co-chairs to reflect the complexity of the task and
other constraints (IETF meetings, major holidays) but is expected to
be in the 4-8 week range. During that time, the panel may correspond
with the authors directly (cc'ing the WG co-chairs) to get
clarifications. This process should result in a revised draft and/or
a report to the WG from the panel that either endorses or disputes
the claimed characteristics.
G4. If/when endorsed by the panel, that draft goes to WG last call.
If not endorsed, the author(s) can give an itemized response to the
panel's report and ask for a WG Last Call.
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G5. If/when passes Last Call, goes to ADs for publication as a WG
Informational RFC in our "PDB series".
If no active Diffserv Working Group exists:
G3. Following discussion on relevant mailing lists, the authors
should revise the Internet Draft and contact the IESG for "Expert
Review" as defined in section 2 of RFC 2434 [RFC2434].
G4. Subsequent to the review, the IESG may recommend publication of
the Draft as an RFC, request revisions, or decline to publish as an
Informational RFC in the "PDB series".
9 Security Considerations
The general security considerations of [RFC2474] and [RFC2475] apply
to all PDBs. Individual PDB definitions may require additional
security considerations.
10 Acknowledgements
The ideas in this document have been heavily influenced by the
Diffserv WG and, in particular, by discussions with Van Jacobson,
Dave Clark, Lixia Zhang, Geoff Huston, Scott Bradner, Randy Bush,
Frank Kastenholz, Aaron Falk, and a host of other people who should
be acknowledged for their useful input but not be held accountable
for our mangling of it. Grenville Armitage coined "per domain
behavior (PDB)" though some have suggested similar terms prior to
that. Dan Grossman, Bob Enger, Jung-Bong Suk, and John Dullaert
reviewed the document and commented so as to improve its form.
References
[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 Services",
December 1998.
[RFC2597] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597, June 1999.
[RFC2598] Jacobson, V., Nichols, K. and K. Poduri, "An Expedited
Forwarding PHB", RFC 2598, June 1999.
Nichols & Carpenter Informational [Page 22]
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[RFC2698] Heinanen, J. and R. Geurin, "A Two Rate Three Color
Marker", RFC 2698, June 1999.
[MODEL] Bernet, Y., Blake, S., Grossman, D. and A. Smith, "An
Informal Management Model for Diffserv Routers", Work in
Progress.
[MIB] Baker, F., Chan, K. and A. Smith, "Management Information
Base for the Differentiated Services Architecture", Work in
Progress.
[VW] Jacobson, V., Nichols, K. and K. Poduri, "The 'Virtual
Wire' Per-Domain Behavior", Work in Progress.
[WCG] Worldcom, "Internet Service Level Guarantee",
http://www.worldcom.com/terms/service_level_guarantee/
t_sla_terms.phtml
[PSI] PSINet, "Service Level Agreements",
http://www.psinet.com/sla/
[UU] UUNET USA Web site, "Service Level Agreements",
http://www.us.uu.net/support/sla/
[RFC2434] Alvestrand, H. and T. Narten, "Guidelines for IANA
Considerations", BCP 26, RFC 2434, October 1998.
Authors' Addresses
Kathleen Nichols
Packet Design, LLC
2465 Latham Street, Third Floor
Mountain View, CA 94040
USA
EMail: nichols@packetdesign.com
Brian Carpenter
IBM
c/o iCAIR
Suite 150
1890 Maple Avenue
Evanston, IL 60201
USA
EMail: brian@icair.org
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Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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