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Versions: 00 01 02 RFC 3086

Internet Engineering Task Force                 K. Nichols
Differentiated Services Working Group           Packet Design
Internet Draft                                  B. Carpenter
Expires in June, 2001                           IBM
draft-ietf-diffserv-pdb-def-01                  December, 2000


        Definition of Differentiated Services Per Domain Behaviors
                and Rules for their Specification

                <draft-ietf-diffserv-pdb-def-02>

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 working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

This document is a product of the Diffserv working group. Comments
on this draft should be directed to the Diffserv mailing list
<diffserv@ietf.org>. The list of current Internet-Drafts can be accessed
at www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft
Shadow Directories can be accessed at www.ietf.org/shadow.html.
Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2000). 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 [RFC2474]. The diffserv WG has defined
the general architecture for differentiated services [RFC2475]
and has focused on the forwarding path behavior required in routers,
known as "per-hop forwarding behaviors" (or PHBs) [RFC2474, RFC2597,
and RFC2598]. 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 [RFC2475,
MODEL, MIB]. 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 fowarding path treatment for a traffic aggregate
and, due to the role that particular choices of edge and 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.


A pdf version of this document is available at:
www.packetdesign.com/ietf/diffserv/pdb_def.pdf.

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.

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 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 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 Specfication (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 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.

              ----------------------------------------
              |                AS2                   |
              |                                      |
 -------      |     ------------     ------------    |
 | AS1 |------|-----X           |    |          |    |
 -------      |     |           |    Y          |    |        -------
              |     |           |   /|          X----|--------| AS3 |
              |     |           |  / |          |    |        -------
              |     |           | /  ------------    |
              |     |           Y      |             |
              |     |           | \  ------------    |
 -------      |     |           |  \ |          |    |
 | AS4 |------|-----X           |   \|          |    |
 -------      |     |           |    Y          X----|------
              |     |           |    |          |    |
              |     ------------     ------------    |
              |                                      |
              |                                      |
              ----------------------------------------

      Figure 1: Interconnection of ASs and DS Domains


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.

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
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 rectangular boxes 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 (dark
rectangles), they might appear anywhere, or nowhere, inside the
AS. Specifically, the boxes at the DS boundaries internal to the
AS (shaded rectangles) 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 decribed 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:

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


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 transitting
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.)

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 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.

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 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
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.

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.

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 build-
ing 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 character-
istics) and for metering and conditioning considerations at DS
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.

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 de-
pendent 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


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
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 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 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.

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

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.

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 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] 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 Services",
S. Blake, D. Black, M.Carlson, 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] "An Informal Management Model for Diffserv Routers",
draft-ietf-diffserv-model-05.txt, Y. Bernet, S. Blake, D. Grossman,
A. Smith

[MIB] "Management Information Base for the Differentiated Services
Architecture", draft-ietf-diffserv- mib-06.txt, F. Baker, K. Chan,
A. Smith

[VW] "The 'Virtual Wire' Per-Domain Behavior",
draft-ietf-diffserv-pdb-vw-00.txt, V. Jacobson, K. Nichols, and
K. Poduri

[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/

[RFC234] RFC 2434, "Guidelines for IANA Considerations", T. Narten,
H. Alverstrand. www.ietf.org/rfc/ rfc2434.txt


Authors' Addresses

 Kathleen Nichols                 Brian Carpenter
 Packet Design, Inc.              IBM
 66 Willow Place                  c/o iCAIR
 Menlo Park, CA 94025             Suite 150
 USA                              1890 Maple Avenue
                                  Evanston, IL 60201
                                  USA
 email: nichols@packetdesign.com  email: brian@icair.org


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