Internet Draft
Internet Engineering Task Force K. Nichols
Differentiated Services Working Group Packet Design
Internet Draft B. Carpenter
Expires in December, 2000 IBM
draft-ietf-diffserv-pdb-def-00.txt June, 2000
Definition of Differentiated Services Per Domain
Behaviors and Rules for their Specification
<draft-ietf-diffserv-pdb-def-00>
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026. Internet-Drafts are
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
The diffserv WG has defined the general architecture for differen-
tiated services (RFC 2475) and has been focused on the definition
and standardization of the forwarding path behavior required in
routers, known as "per-hop forwarding behaviors" (or PHBs)
(RFCs 2474, 2597, and 2598). The differentiated services frame-
work creates services within a network by applying rules at the
network edges to create traffic aggregates and coupling these with
a specific forwarding path treatment for the aggregate. The WG
has also discussed the behavior required at diffserv network edges
or boundaries for conditioning packet aggregates, such elements
as policers and shapers [MODEL, MIB]. A major feature of the
diffserv architecture is that only the components applying the
rules at the edge need to be changed in response to short-term
changes in QoS goals in the network, rather than reconfiguring
the interior behaviors.
The next step for the WG is to formulate examples of how the for-
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warding path components (PHBs, classifiers, and traffic condi-
tioners) can be used within the architectural framework 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 behav-
ior, or PDB, to describe the behavior experienced by packets of a
particular traffic aggregate as they cross a DS domain. PDBs can
be used to characterize, by specific metrics, the treatment individ-
ual packets with a particular DSCP (or set of DSCPs) will receive
as it crosses a DS domain. However, no microflow information
should be required as packets transit a differentiated services net-
work. A PDB is an expression of a fowarding path treatment, but
due to the role that particular choices of edge and PHB configura-
tion play in its resulting attributes, it is where the forwarding path
and the control plane interact.
This document defines and discusses Per Domain Behaviors in
detail and lays out the format and required content for contribu-
tions to the Diffserv WG on PDBs and the rules 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.
A pdf version of this document is available at: ftp://www.packet-
design.com/outgoing/ietf/pdb_def.pdf.
Table of Contents
1. Introduction ........................................ 2
2. Definitions ......................................... 3
3. The Value of Defining Edge-to-Edge Behavior ......... 4
4. Understanding Diffserv PDBs ......................... 5
5. Format for Specification of Diffserv PDBs ........... 8
6. PDB Attributes ..................................... 10
7. Reference Per-Domain Behaviors ..................... 13
8. Procedure for Submitting PDBs to Diffserv WG ....... 14
9. Acknowledgements ................................... 15
1.0 Introduction
Differentiated Services allows an approach to IP QoS that is mod-
ular, high performance, incrementally deployable, and scalable
[RFC2475]. Although an ultimate goal is interdomain quality of
service, there remain many untaken steps on the road to achieving
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this goal. 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 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. A diffserv Conceptual Model [MODEL]
describes a model of traffic conditioning and other forwarding
behaviors.
Although business models will have to evolve over time, there
also remain technical issues in moving "beyond the box" to QoS
models that apply within a single network domain. Providing
QoS on a per-domain basis is useful in itself and will provide use-
ful deployment experience for further IETF work as well as for
the evolution of business models. The step of specifying forward-
ing path attributes on a per-domain basis for a traffic aggregate
distinguished only by the mark in the DS field of individual pack-
ets is critical in the evolution of Diffserv QoS and should provide
the technical input that will aid in the construction of business
models. The ultimate goal of creating end to end QoS in the Inter-
net imposes the requirement that we can create and quantify a
behavior for a group of packets that is preserved when they are
aggregated with other packets. This document defines and speci-
fies the term "Per-Domain Behavior" or PDB to describe QoS
attributes across a DS domain.
In diffserv, rules are imposed on packets arriving at the boundary
of a DS domain through use of classification and traffic condi-
tioning which are set to reflect the policy and traffic goals for
that domain. 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. 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, a PHB should be defined such that its defining char-
acteristics don't depend on the volume of the associated BA on a
router's ingress link nor on a particular path through the DS
domain taken by the packets marked for it. If the properties of a
PDB using a particular PHB hold regardless of how the marked
aggregate mutates as it traverses the domain, then that PDB
scales. If there 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 sim-
ple relationships with network size and whose properties can be
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recovered by reapplying rules (that is, forming another diffserv
boundary or edge to re-enforce the rules for the 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 couples rules, specific PHBs, and
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 are 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 cus-
tomer-visible in contracts and these may be stated quite differ-
ently from the technical attributes specified in a PDB definition.
Similarly, specific PDBs are intended as tools for ISPs to con-
struct differentiated services offerings; each may choose different
sets of tools, or even develop their own, in order to achieve
particular externally observable metrics.
This document defines Differentiated Services Per-Domain
Behaviors and specifies the format that must be used for submis-
sions of particular PDBs to the Diffserv WG.
2.0 Definitions
The following definitions are stated in RFCs 2474 and 2475 and
are repeated here for easy reference:
o Behavior Aggregate: a collection of packets with the same codepoint
crossing a link in a particular direction. The terms "aggregate" and
"behavior aggregate" are used interchangeably in this document.
o 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 administrative domains or autono-
mous systems, different trust regions, different network technologies
(e.g., cell/frame), hosts and routers, etc. Also DS domain.
o 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 down-
stream/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
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be co-located with a host, subject to local policy. Also DS boundary.
To these we add:
o 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 a the foo PHB is referred to
as the "foo traffic aggregate" or the "foo aggregate" interchangeably.
o 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.
3.0 The Value of Defining Edge-to-Edge
Behavior
Networks of DS domains can be connected to create end-to-end
services, but where DS domains are independently administered,
the evolution of the necessary business agreements and future sig-
naling arrangements will take some time. Early deployments will
be within a single administrative domain. Specification of the
transit expectations of packets matching a target for a particular
diffserv behavior across a DS domain both assists in the deploy-
ment of single-domain QoS and will help enable the composition
of end-to-end, cross domain services to proceed. Putting aside the
business issues, the same technical issues that arise in intercon-
necting DS domains with homogeneous administration will arise
in interconnecting the autonomous systems (ASs) of the Internet.
Today's Internet is composed of multiple independently adminis-
tered domains or Autonomous Systems (ASs), represented by the
circles in figure 1. To deploy ubiquitous end-to-end quality of ser-
vice 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.
Figure 1: Interconnection of ASs and DS Domains
A single AS (for example, AS2 in figure 1) may be composed of
subnetworks and, as the definition allows, these can be separate
DS domains. For a number of reasons, it might be useful to have
multiple DS domains in an AS, most notable being to follow
topological and/or technological boundaries and to separate the
allocation of resources. If we confine ourselves to the DS bound-
aries 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.
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The incentive structure for differentiated services is based on
upstream domains ensuring their traffic conforms to agreed upon
rules and downstream domains enforcing that conformance, thus
metrics associated with PDBs can be sensibly computed. The
rectangular boxes in figure 1 represent the DS boundary routers
and thus would contain the traffic conditioners that ensure and
enforce conformance (e.g., shapers and policers). Although we
expect that policers and shapers will be required at the DS bound-
aries of ASs (dark rectangles), they might appear anywhere, or
nowhere, inside the AS. Thus, the boxes at the DS boundaries
internal to the AS (shaded rectangles) may or may not condition
traffic. Understanding a particular PDB's characteristics under
aggregation and multiple hops will result in guidelines for the
placement and configuration of DS boundaries.
This approach continues the separation of forwarding path and
control plane decribed in RFC 2474. The forwarding path charac-
teristics are addressed by considering what happens at every hop
of a packet's path and what behaviors can be characterized under
the merging and branching through multiple hops. The control
plane only needs to be employed 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 qual-
ity-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.
4.0 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:
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.
Measurable, quantifiable, attributes are associated with each PDB
and these can be used to describe what will happen to packets of
that PDB as they cross the DS domain. These derive from the
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rules that are enforced during the entry of packets into the DS
domain and the forwarding treatment (PHB) the packets get
inside the domain. PDB attributes may be absolute or statistical
and they may be parameterized by network properties. For exam-
ple, 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
deliverd 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.
Identification of the target group of packets is carried out using
classification. The Per-Domain Behavior applied to that group of
packet is characterized in two parts: 1) the relationship between
this target group of packets to the marked traffic aggregate which
results from the application of rules (through the use of traffic
conditioning) to the identified (classified) packets to create a traf-
fic aggregate marked for the associated PHB (see figure 2) and 2)
the attributes which result from the treatment experienced by
packets from the same traffic aggregate transiting the interior of a
DS domain, between and inside of DS boundaries.
Figure 2: Relationship of the traffic aggregate associated with a
PDB to arriving packets
The first part is more straightforward than the second, but might
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. This would be formulated as the
rate of the emerging foo traffic aggregate being less than or equal
to the smaller of R and the arrival rate of the target group of pack-
ets and additional temporal characteristics of the packets (e.g.,
burst) would be specified as desired. Thus, there is a "loss rate"
that results to the original target group from sending too much
traffic or the traffic with the wrong temporal characteristics that
should be entirely preventable (or controllable) by the upstream
sender conforming to the traffic conditioning associated with the
PDB specification. A PDB might also apply traffic conditioning
at egress at a DS boundary. This 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 pre-
sented in this document.) In section 4.3, we will revisit this dis-
cussion for PHB groups.
This aspect of "who is in control" of the loss (or demotion) rate
helps to clearly delineate the first part of characterizing packet
performance of a PDB from the second part. Further, the relation-
ship of the traffic aggregate to the arriving target packet group can
usually be expressed more simply that the traffic aggregate's tran-
sit attributes and depends on different elements. The second part
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is illustrated in figure 3 as the quantifiable metrics that can be
used to characterize the transit of any packet of a particular traffic
aggregate between any two edges of the DS domain boundary
shown in figure 3, including those indicated with arrows. Note
that the DS domain boundary runs through the DS boundary rout-
ers 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.)
Figure 3: Range of applicability of attributes of a traffic aggregate
associated with a PDB
The traffic aggregate associated with a PDB is formed by the
application of rules, through classification and traffic condition-
ing, to packets arriving at the DS boundary. Packets that conform
to the rules are marked with a DSCP that maps to a particular
PHB within a domain. DSCPs should not mutate in the interior of
a DS domain as there are no rules being applied. If it is necessary
to reapply the kind of rules that could result in remarking, there
should probably be a DS domain boundary at that point; an inte-
rior one that can have "lighter weight" rules. Thus, if measuring
attributes between locations as indicated in figure 3, the DSCP at
the egress side can be assumed to have held throughout the
domain.
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 rules 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, though, a PDB operates between N ingress points and
M egress points at the DS domain boundary. Even in the degener-
ate 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 under-
stand 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 perfor-
mance goals for a particular flow of a traffic aggregate. Once the
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
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the PDB's traffic aggretae subject to that configuration of link
schedulers and bandwidth. The rules at the edge may be altered
by provisioning or admission control but the decision about
which PDB to use and how to apply the rules comes from match-
ing performance to goals.
For example, consider the diffserv domain of figure 3. A PDB
with an attribute of an explicit bound on loss must have rules at
the edge to ensure that on the average no more packets are admit-
ted 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 net-
work. 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 pack-
ets 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 aggre-
gate can expect at each hop. For a PDB, this behavior must apply
to merging and diverging traffic aggregates, thus characterizing a
PDB requires exploring what happens to a PHB under aggrega-
tion. Rules must be recursively applied to result in a known
behavior. As an example, since maximum burst sizes grow with
the number of microflows or aggregate flows 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 traffi aggregate has known attributes that span inte-
rior 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. Using the same (or fewer) rules
as were applied to create the traffic aggregates at the entrance to
AS2, there should be confidence that the attributes of the foo
PDB can continue to be used to quantify by the expected behav-
ior. Explicit expressions of what happens to the behavior under
aggregation, possibly parameterized by node in-degrees or net-
work diameters are necessary to determine what to do at the inter-
nal aggregation points. One approach might be to completely
reapply the edge rules at these points. Another might employ
some limited rate-based remarking only.
Multiple PDBs might use the same PHB. In the specification of a
PDB, there might be a list of PHBs and their required configura-
tion, all of which would result in the same characteristics. In
operation, though, it is expected that a single domain will use a
single PHB to implement a particular PDB. A single PHB might
beselected 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.
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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 behavior aggre-
gates 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.
4.3 PDBs using PHB Groups
When a set of related PDBs are defined using a PHB group, they
should be defined in the same document. This would be particu-
larly appropriate if the application of the edge rules that create the
traffic aggregates associated with each PDB had some relation-
ships and interdependencies, as one would expect for the AF PHB
group [RFC2597]. Characterizing the traffic conditioning effects
should then be described for these PDBs 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, thus each should
have a clearly separate treatment, though there may be some
parameterized interrelationship between the attributes of each of
these PDBs.
For example, if the traffic conditioner described in RFC 2698 is
used to mark arriving packets for three different AF1x PHBs, then
the most reasonable approach is to define and quantify the rela-
tionship between the arriving packets and the emerging traffic
aggregates as they relate to one another. The transit characteris-
tics of packets of each separate AF1x traffic aggregate should be
described separately.
A set of PDBs might be defined using Class Selector Compliant
PHBs [RFC2474] in such a way that the edge rules that create the
traffic aggregates are not related, but the transit performance of
each traffic aggregate has some parametric relationship to the 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 and edge traffic conditioners are in the
packet forwarding path and operate at line rates while the config-
uration of the DS domain edge to enforce rules on who gets to use
the PDB and how the PDB should behave temporally is done by
the control plane on a very different time scale. For example, con-
figuration of PHBs might only occur monthly or quarterly. The
edge rules might be reconfigured at a few regular intervals during
the day or might happen in response to signalling decisions thou-
sands of times a day. Even at the shortest time scale, control plane
actions are not expected to happen per-packet. Much of the con-
trol plane work is still evolving and is outside the charter of the
Diffserv WG. We note that this is quite appropriate since the
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manner in which the configuration is done and the time scale at
which it is done should not affect the PDB attributes.
5.0 Format for Specification of Diffserv Per-Domain Behaviors
PDBs arise from a particular relationship between edge and inte-
rior (which may be parameterized). The quantifiable characteris-
tics of a PDB must be independent of whether the network edge is
configured statically or dynamically. The particular configuration
of traffic 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 con-
tent 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 Rules
This section describes the rules to be followed in the creation of
this PDB. Rules should be distinguished with "may", "must" and
"should." The rules specify the edge behavior and configuration
and the PHB (or PHBs) to be used and any additional require-
ments on their configuration beyond that contained in RFCs.
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 absolute
bounds or statistical bounds (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 characteris-
tics 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 proba-
bility.
5.4 Parameters
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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.
However, these assumptions must be clearly stated. Some PDBs
may be developed without these assumptions, e.g., for high loss
rate links, and these must also be made explicit. If additional
restrictions, e.g., route pinning, 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 behavior aggregate 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 due to
violation of that PDB's rules."
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 exam-
ple, 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.
6.0 PDB Attributes
Attributes are associated with each PDB: measurable, quantifi-
able, characteristics which can be used to describe what will hap-
pen to packets using that PDB as they cross the domain. These
expectations result directly from the application of edge rules
enforced during the creation of the PDB's traffic aggregate and/or
its entry into the domain and the forwarding treatment (PHB)
packets of that traffic aggregate get inside the domain. There are
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many ways in which traffic might be distributed, but creating a
quantifiable, realizable service across the DS domain will limit
the scenarios which can occur. There is a clear correlation
between the strictness of the rules and the quality of the charac-
terization of the PDB.
There are two ways to characterize PDBs with respect to time.
First are its properties over "long" time periods, or average
behaviors. In a PDB spec, these would be the rates or throughput
seen over some specified time period. In addition, there are prop-
erties of "short" time behavior, usually expressed as the allowable
burstiness in an aggregate. The short time behavior is important is
understanding the buffering (and associated loss characteristics)
and in quantifying how packets using the PDB aggregate, either
within a DS domain or at the boundaries. For short-time behavior,
we are 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.
Put simply, 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 rules and expectations?
6.1 Considerations in specifying long-term or average PDB attributes
To make this more concrete, consider the DS domain of figure 4
for which we will define the foo PDB. To characterize the average
or long-term behavior that must be specified 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?
Figure 4: ISP and DS domain D connected in a ring and connected
to DS domain E
Let figure 4 be an ISP ringing the U.S. with links of bandwidth B
and with N tails to various metropolitan areas. 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 hav-
ing 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 that case, there were topological assumptions
made about the path of the traffic from A to Z that affected the
characteristics of the foo PDB. Once these no longer hold, any
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assumptions on the loss rate of packets of the foo traffic aggregate
transiting the domain would change; for example, a characteristic
such as "loss rate no greater than 1% over any interval larger than
10 minutes" would no longer hold. A PDB specification should
spell out the assumptions made on preserving the attributes.
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 maxi-
mum bursts to add in the same manner as the average rates will
lead to properties that aggregate or under what rules 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 aggre-
gate, 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 band-
width 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 trans-
mitted. 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
rules, 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 rules for entering E
(e.g., by shaping).
What conditions should be imposed on a PDB and on the associ-
ated PHB in order to ensure PDBs can be concatenated, as across
the interior DS domains of figure 1? Edge rules for constructing a
PDB that has certain attributes across a DS domain should apply
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independently of the origin of the packets. With reference to the
example we've been exploring, the rules 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 Example
In this example, we will make the above more concrete. We
assume that only the foo PDB is using its associated traffic aggre-
gate and we use "foo agggregate" interchangeably with "the traf-
fic aggregate associated with the PDB foo." We also use "foo
packets" interchangeably with "the packets marked for the PHB
associated with PDB foo."
Assume the topology of figure 4 and that all the uplinks have the
same bandwidth B and link L has bandwidth L which is less than
or equal to B. The foo traffic aggregates from the N uplinks each
have average rate R and are destined to cross L. If only a fraction
a of link L is allocated to foo, then R =axL/N fits the average rate
constraint. If each of the N flows can have a burst of p packets
and half the flows transit the ring in each direction, then 2xp
packets can arrive at the foo queue for link L in the time it took to
transmit p packets on the ring, p/B. Although the link scheduler
for link L might allow the burst of packets to be transmitted at the
line rate L, after the burst allotment has been exceeded, the queue
should be expected to clear at only rate axL. Then consider the
packets that can accumulate. It takes 2xp/(axL) to clear the queue
of the foo packets. In that time, bursts of p packets from the other
uplinks can arrive from the ring, so the packets do not even have
to be back-to-back. Even if the packets do not arrive back-to-
back, but are spaced by less time than it takes to clear the queue
of foo packets, either the required buffer size can become large or
the burst size of foo packets entering E across L becomes large
and is a function of N, the number of uplinks of domain D.
Let L = 1.5 Mbps, B = 45 Mbps, a = 1/3, N=10, p = 3. Suppose
that the bursts from two streams of foo packets arrive at the queue
for link L very close together. Even if 3 of the packets are cleared
at the line rate of 1.5 Mbps, there will be 3 packets remaining to
be serviced at a 500 kbps rate. In the time allocated to send one of
these, 9 packets can arrive on each of the inputs from the ring. If
any non-zero number of these 18 packets are foo packets, the
queue size will not reduce. If two more bursts (6 of the 18 pack-
ets) arrive, the queue increases to 8 packets. Thus, it's possible to
build up quite a large queue, one likely to exceed the buffer allo-
cated for foo. The rate bound means that each of the uplinks will
be idle for the time to send three packets at 50 kbps, possibly by
policing at the ring egress, and thus the queue would eventually
decrease and clear, however, the queue at link L can still be very
large. PDBs where the intention is to permit loss should be con-
structed so as to provide a probabilistic bound for the queue size
to exceed a reasonable buffer size of one or two bandwidth-delay
products. Alternatively or additionally, rules can be used that
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bound the amount of foo packets that queue by limiting the burst
size at the ingress uplinks to one packet, resulting in a maximum
queue of N or 10 or to impose additional rules on the PDB. One
approach is to limit the domain over which the PDB applies so
that interior boundaries are placed at merge points (or between
every M merge points) so that a shaping edge conditioner can be
reapplied. Another approach is to use a PHB defined such that it
strictly limits the burstiness.
6.4 Remarks
This section has been provided to provide some 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 ser-
vices 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.
7.0 Reference Per-Domain Behaviors
The intent of this section is to define one or a few "reference"
PDBss; certainly a Best Effort PDB and perhaps others. This sec-
tion is very preliminary at this time and meant to be the starting
point for discussion rather than its end. These are PDBs that have
little in the way of rules or expectations.
7.1 Best Effort Behavior 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 deliv-
ery expectation for packets in a diffserv network that do not
require any special differentiation.
7.1.2 Rules
There are no rules 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]). Interior network nodes use 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
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other traffic aggregate), network elements should be configured
to permit packets of this PDB to consume them.
Although some network operators may bound the delay and loss
rate for this aggregate given knowledge about their network, these
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
7.2 Bulk Handling Behavior PDB
7.2.1 Applicability
A Bulk Handling (BH) PDB is for sending extremely non-critical
traffic across a diffserv network. There should be an expectation
that these packets may be delayed or dropped when other traffic is
present.
7.2.2 Rules
There are no rules governing rate and bursts of packets beyond
the limits imposed by the ingress link. The network edge ensures
that packets using this PDB are marked for either a CS or an AF
PHB. Interior network nodes must have this PHB configured so
that its packets may be starved when other traffic is present. For
example, using the PHB for Class Selector 1 (DSCP=001000), all
routers in the domain could be configured to queue such traffic
behind all other traffic, subject to tail drop.
7.2.3 Attributes of the BH PHB
Packets are forwarded when there are idle resources.
7.2.4 Parameters
None.
7.2.5 Assumptions
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A properly functioning network.
7.2.6 Example uses
1. For Netnews and other "bulk mail" of the Internet.
2. For "downgraded" traffic from some other PDB.
8.0 Procedure for submitting PDB
specifications to Diffserv WG
1. Following the guidelines of this document, write a draft and
submit it as an Internet Draft and bring it to the attention of the
WG mailing list.
2. Initial discussion on the WG should focus primarily on the
merits of the a PDB, though comments and questions on the
claimed attributes are reasonable. This is in line with our desire to
put relevance before academic interest in spending WG time on
PDBs. Academically interesting PDBs are encouraged, but not
for submission to the diffserv WG.
3. 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.
4. If/when endorsed by the panel, that draft goes to WG last call.
If not endorsed, the author(s) can give a itemized response to the
panel's report and ask for a WG Last Call.
5. If/when passes Last Call, goes to ADs for publication as a WG
Informational RFC in our "PDB series".
9.0 Acknowledgements
The ideas in this document have been heavily influenced by the
Diffserv WG and, in particular, by discussions with Van Jacob-
son, 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 sug-
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gested similar terms prior to that.
References
[RFC2474] RFC 2474, "Definition of the Differentiated Services
Field (DS Field) in the IPv4 and IPv6 Headers",
K.Nichols, S. Blake, F. Baker, D. Black, www.ietf.org/
rfc/rfc2474.txt
[RFC2475] RFC 2475, "An Architecture for Differentiated Ser-
vices", S. Blake, D. Black, M.Carl-
son,E.Davies,Z.Wang,W.Weiss, www.ietf.org/rfc/
rfc2475.txt
[RFC2597] RFC 2597, "Assured Forwarding PHB Group", F.
Baker, J. Heinanen, W. Weiss, J. Wroclawski,
www.ietf.org/rfc/rfc2597.txt
[RFC2598] RFC 2598, "An Expedited Forwarding PHB",
V.Jacobson, K.Nichols, K.Poduri, www.ietf.org/rfc/
rfc2598.txt
[RFC2698] RFC 2698, "A Two Rate Three Color Marker", J.
Heinanen, R. Guerin. www.ietf.org/rfc/rfc2698.txt
[MODEL] "A Conceptual Model for Diffserv Routers", draft-ietf-
diffserv-model-02.txt, Bernet et. al.
[MIB] "Management Information Base for the Differentiated
Services Architecture", draft-ietf-diffserv-mib-01.txt,
Baker et. al.
[VW] "The 'Virtual Wire' Behavior Aggregate", draft-ietf-diff-
serv-ba-vw-00.txt, V. Jacobson, K. Nichols, and K.
Poduri (being modified to reflect new terminology).
Authors' Addresses
Kathleen Nichols Brian E. Carpenter
Packet Design, Inc. IBM
66 Willow Place c/o iCAIR
Menlo Park, CA 94025 Suite 150
1890 Maple Avenue
Evanston, IL 60201
USA
email:
nichols@packetdesign.com brian@icair.org