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Versions: 00 01 02 03 RFC 3086

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


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

                <draft-ietf-diffserv-pdb-def-01>

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 rather than 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 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:
www.packetdesign.com/ietf/diffserv/pdb_def.pdf.

                Change log for -01 version

-Noted that PDB parameters may form part of SLS (1.0).

-Clarified that parameters are not absolute values (4.1, 5.3).

-Rewrote the PDB from PHB group section (4.3).

-Added traffic conditioning needs to PDB rules (5.2).

-Replaced reference to route pinning by general reference to traffic
engineering, added reference to assumption of standby capacity (5.5).

-Removed Example from Section 6 (6.3).

-Deleted Bulk Handling PDB (7.2).

-Add requirement for PDB deployment experience (section 8).

-General syntax cleanups etc. Shortened and tweaked the Abstract.
Tweaked the Introduction.

-Eliminated solo usage of "aggregate" since it seems to confuse people.
Using "aggregate" as an abbreviation for sometimes BA and sometimes
TA is definitely confusing. Removed the following from the definitions:
The terms "aggregate" and "behavior aggregate" are used interchangeably
in this document.

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
multi-domain QoS.

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.

In diffserv, rules are imposed on certain packets arriving at the
boundary of a DS domain through use of classification and traffic
conditioning 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, 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 marked traffic aggregate
mutates 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 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 customer-
visible in contracts and these may be stated quite differently from the
technical attributes specified in a PDB definition. 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 the "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.

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.


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

      Figure 1: Interconnection of ASs and DS Domains

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 "X's" 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, they
might appear anywhere, or nowhere, inside the AS. Specifically, the
X's at the DS boundaries internal to the AS 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 rules that are enforced 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 enforcement of rules (through the use
of traffic conditioning) to create a traffic aggregate. Packets that
conform to the rules 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 rules 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 is following the rules to
which the operator has provisioned the network. Further, the effects of
the enforcement of edge rules on the target group can usually be
expressed more simply than the traffic aggregate's transit attributes.

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.)

                            -------------
                            |           |
                       -----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")

DSCPs should not mutate in the interior of a DS domain as there are
no "rules" being applied to the traffic. If it is necessary to reapply
the kind of rules 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. 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.

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, 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 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 matching performance to goals.

For example, consider the DS domain of figure 3. A PDB with an explicit
bound on loss must have rules at the edge 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. 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 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. 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 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 edge rules 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 edge rules 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 application of the edge rules that create the
traffic aggregates associated with each PDB have some relationships and
interdependencies such that the traffic conditioning effects 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 edge rules, 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
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 configuration
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, reconfiguration of PHBs
might occur monthly, quarterly, or only when the network is upgraded.
The edge rules might be reconfigured at a few regular intervals during
the day or might happen in response to signalling decisions thousands
of times a day. Even at the shortest time scale, control plane actions
are not expected to happen per-packet. 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 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, including
whatever traffic conditioning is required, and the PHB (or PHBs) to be
used and any additional requirements on their configuration beyond that
contained in RFCs. Note that traffic conditioning may include
classification, admission control, marking, traffic shaping, and
policing.

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 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 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 itrole as a building
block for the construction of interdomain quality-of-service, a PDB
specification should provide the answer to the question: Under what
conditions can we join the output of this domain to another under the
same rules 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 rules 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, whereas those expressed statistically can to some extent
rely on experiment. Section 7 gives the example of a PDB without strict
rules and concurrent work on a PDB with strict rules 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 under-
standing buffering requirements (and associated loss characteristics)
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-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?


                 ____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


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.

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 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 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 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 associated 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 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 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 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.

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]), but no other traffic conditioning is required. 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 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 PDB 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

8 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. 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.

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 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.

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-04.txt, Y. Bernet, S. Blake, D. Grossman,
A. Smith

[MIB] "Management Information Base for the Differentiated Services
Architecture", draft-ietf-diffserv- mib-01.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.

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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