RFC2475 - An Architecture for Differentiated Service

Network Working Group S. Blake
Request for Comments: 2475 Torrent Networking Technologies
Category: Informational D. Black
EMC Corporation
M. Carlson
Sun Microsystems
E. Davies
Nortel UK
Z. Wang
Bell Labs LUCent Technologies
W. Weiss
Lucent Technologies
December 1998
An Architecture for Differentiated Services
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This document defines an architecture for implementing scalable
service differentiation in the Internet. This architecture achieves
scalability by aggregating traffic classification state which is
conveyed by means of IP-layer packet marking using the DS field
[DSFIELD]. Packets are classified and marked to receive a particular
per-hop forwarding behavior on nodes along their path. Sophisticated
classification, marking, policing, and shaping operations need only
be implemented at network boundaries or hosts. Network resources are
allocated to traffic streams by service provisioning policies which
govern how traffic is marked and conditioned upon entry to a
differentiated services-capable network, and how that traffic is
forwarded within that network. A wide variety of services can be
implemented on top of these building blocks.
Table of Contents
1. Introduction ................................................. 2
1.1 Overview ................................................. 2
1.2 Terminology ............................................... 4
1.3 Requirements .............................................. 8
1.4 Comparisons with Other Approaches ......................... 9
2. Differentiated Services Architectural Model .................. 12
2.1 Differentiated Services Domain ............................ 12
2.1.1 DS Boundary Nodes and Interior Nodes .................. 12
2.1.2 DS Ingress Node and Egress Node ....................... 13
2.2 Differentiated Services Region ............................ 13
2.3 Traffic Classification and Conditioning ................... 14
2.3.1 Classifiers ........................................... 14
2.3.2 Traffic Profiles ...................................... 15
2.3.3 Traffic Conditioners .................................. 15
2.3.3.1 Meters ............................................ 16
2.3.3.2 Markers ........................................... 16
2.3.3.3 Shapers ........................................... 17
2.3.3.4 Droppers .......................................... 17
2.3.4 Location of Traffic Conditioners and MF Classifiers ... 17
2.3.4.1 Within the Source Domain .......................... 17
2.3.4.2 At the Boundary of a DS Domain .................... 18
2.3.4.3 In non-DS-Capable Domains ......................... 18
2.3.4.4 In Interior DS Nodes .............................. 19
2.4 Per-Hop Behaviors ......................................... 19
2.5 Network Resource Allocation ............................... 20
3. Per-Hop Behavior Specification Guidelines .................... 21
4. Interoperability with Non-Differentiated Services-Compliant
Nodes ........................................................ 25
5. Multicast Considerations ..................................... 26
6. Security and Tunneling Considerations ........................ 27
6.1 Theft and Denial of Service ............................... 28
6.2 IPsec and Tunneling Interactions .......................... 30
6.3 Auditing .................................................. 32
7. Acknowledgements ............................................. 32
8. References ................................................... 33
Authors" Addresses ............................................... 34
Full Copyright Statement ......................................... 36
1. Introduction
1.1 Overview
This document defines an architecture for implementing scalable
service differentiation in the Internet. A "Service" defines some
significant characteristics of packet transmission in one direction
across a set of one or more paths within a network. These
characteristics may be specified in quantitative or statistical terms
of throughput, delay, jitter, and/or loss, or may otherwise be
specified in terms of some relative priority of Access to network
resources. Service differentiation is desired to accommodate
heterogeneous application requirements and user eXPectations, and to
permit differentiated pricing of Internet service.
This architecture is composed of a number of functional elements
implemented in network nodes, including a small set of per-hop
forwarding behaviors, packet classification functions, and traffic
conditioning functions including metering, marking, shaping, and
policing. This architecture achieves scalability by implementing
complex classification and conditioning functions only at network
boundary nodes, and by applying per-hop behaviors to aggregates of
traffic which have been appropriately marked using the DS field in
the IPv4 or IPv6 headers [DSFIELD]. Per-hop behaviors are defined to
permit a reasonably granular means of allocating buffer and bandwidth
resources at each node among competing traffic streams. Per-
application flow or per-customer forwarding state need not be
maintained within the core of the network. A distinction is
maintained between:
o the service provided to a traffic aggregate,
o the conditioning functions and per-hop behaviors used to realize
services,
o the DS field value (DS codepoint) used to mark packets to select a
per-hop behavior, and
o the particular node implementation mechanisms which realize a
per-hop behavior.
Service provisioning and traffic conditioning policies are
sufficiently decoupled from the forwarding behaviors within the
network interior to permit implementation of a wide variety of
service behaviors, with room for future expansion.
This architecture only provides service differentiation in one
direction of traffic flow and is therefore asymmetric. Development
of a complementary symmetric architecture is a topic of current
research but is outside the scope of this document; see for example
[EXPLICIT].
Sect. 1.2 is a glossary of terms used within this document. Sec. 1.3
lists requirements addressed by this architecture, and Sec. 1.4
provides a brief comparison to other approaches for service
differentiation. Sec. 2 discusses the components of the architecture
in detail. Sec. 3 proposes guidelines for per-hop behavior
specifications. Sec. 4 discusses interoperability issues with nodes
and networks which do not implement differentiated services as
defined in this document and in [DSFIELD]. Sec. 5 discusses issues
with multicast service delivery. Sec. 6 addresses security and
tunnel considerations.
1.2 Terminology
This section gives a general conceptual overview of the terms used in
this document. Some of these terms are more precisely defined in
later sections of this document.
Behavior Aggregate (BA) a DS behavior aggregate.
BA classifier a classifier that selects packets based
only on the contents of the DS field.
Boundary link a link connecting the edge nodes of two
domains.
Classifier an entity which selects packets based on
the content of packet headers according to
defined rules.
DS behavior aggregate a collection of packets with the same DS
codepoint crossing a link in a particular
direction.
DS boundary node a DS node that connects one DS domain to a
node either in another DS domain or in a
domain that is not DS-capable.
DS-capable capable of implementing differentiated
services as described in this architecture;
usually used in reference to a domain
consisting of DS-compliant nodes.
DS codepoint a specific value of the DSCP portion of the
DS field, used to select a PHB.
DS-compliant enabled to support differentiated services
functions and behaviors as defined in
[DSFIELD], this document, and other
differentiated services documents; usually
used in reference to a node or device.
DS domain a DS-capable domain; a contiguous set of
nodes which operate with a common set of
service provisioning policies and PHB
definitions.
DS egress node a DS boundary node in its role in handling
traffic as it leaves a DS domain.
DS ingress node a DS boundary node in its role in handling
traffic as it enters a DS domain.
DS interior node a DS node that is not a DS boundary node.
DS field the IPv4 header TOS octet or the IPv6
Traffic Class octet when interpreted in
conformance with the definition given in
[DSFIELD]. The bits of the DSCP field
encode the DS codepoint, while the
remaining bits are currently unused.
DS node a DS-compliant node.
DS region a set of contiguous DS domains which can
offer differentiated services over paths
across those DS domains.
Downstream DS domain the DS domain downstream of traffic flow on
a boundary link.
Dropper a device that performs dropping.
Dropping the process of discarding packets based on
specified rules; policing.
Legacy node a node which implements IPv4 Precedence as
defined in [RFC791,RFC1812] but which is
otherwise not DS-compliant.
Marker a device that performs marking.
Marking the process of setting the DS codepoint in
a packet based on defined rules; pre-
marking, re-marking.
Mechanism a specific algorithm or operation (e.g.,
queueing discipline) that is implemented in
a node to realize a set of one or more per-
hop behaviors.
Meter a device that performs metering.
Metering the process of measuring the temporal
properties (e.g., rate) of a traffic stream
selected by a classifier. The
instantaneous state of this process may be
used to affect the operation of a marker,
shaper, or dropper, and/or may be used for
accounting and measurement purposes.
Microflow a single instance of an application-to-
application flow of packets which is
identified by source address, source port,
destination address, destination port and
protocol id.
MF Classifier a multi-field (MF) classifier which selects
packets based on the content of some
arbitrary number of header fields;
typically some combination of source
address, destination address, DS field,
protocol ID, source port and destination
port.
Per-Hop-Behavior (PHB) the externally observable forwarding
behavior applied at a DS-compliant node to
a DS behavior aggregate.
PHB group a set of one or more PHBs that can only be
meaningfully specified and implemented
simultaneously, due to a common constraint
applying to all PHBs in the set such as a
queue servicing or queue management policy.
A PHB group provides a service building
block that allows a set of related
forwarding behaviors to be specified
together (e.g., four dropping priorities).
A single PHB is a special case of a PHB
group.
Policing the process of discarding packets (by a
dropper) within a traffic stream in
accordance with the state of a
corresponding meter enforcing a traffic
profile.
Pre-mark to set the DS codepoint of a packet prior
to entry into a downstream DS domain.
Provider DS domain the DS-capable provider of services to a
source domain.
Re-mark to change the DS codepoint of a packet,
usually performed by a marker in accordance
with a TCA.
Service the overall treatment of a defined subset
of a customer"s traffic within a DS domain
or end-to-end.
Service Level Agreement a service contract between a customer and a
(SLA) service provider that specifies the
forwarding service a customer should
receive. A customer may be a user
organization (source domain) or another DS
domain (upstream domain). A SLA may
include traffic conditioning rules which
constitute a TCA in whole or in part.
Service Provisioning a policy which defines how traffic
Policy conditioners are configured on DS boundary
nodes and how traffic streams are mapped to
DS behavior aggregates to achieve a range
of services.
Shaper a device that performs shaping.
Shaping the process of delaying packets within a
traffic stream to cause it to conform to
some defined traffic profile.
Source domain a domain which contains the node(s)
originating the traffic receiving a
particular service.
Traffic conditioner an entity which performs traffic
conditioning functions and which may
contain meters, markers, droppers, and
shapers. Traffic conditioners are typically
deployed in DS boundary nodes only. A
traffic conditioner may re-mark a traffic
stream or may discard or shape packets to
alter the temporal characteristics of the
stream and bring it into compliance with a
traffic profile.
Traffic conditioning control functions performed to enforce
rules specified in a TCA, including
metering, marking, shaping, and policing.
Traffic Conditioning an agreement specifying classifier rules
Agreement (TCA) and any corresponding traffic profiles and
metering, marking, discarding and/or
shaping rules which are to apply to the
traffic streams selected by the classifier.
A TCA encompasses all of the traffic
conditioning rules explicitly specified
within a SLA along with all of the rules
implicit from the relevant service
requirements and/or from a DS domain"s
service provisioning policy.
Traffic profile a description of the temporal properties
of a traffic stream such as rate and burst
size.
Traffic stream an administratively significant set of one
or more microflows which traverse a path
segment. A traffic stream may consist of
the set of active microflows which are
selected by a particular classifier.
Upstream DS domain the DS domain upstream of traffic flow on a
boundary link.
1.3 Requirements
The history of the Internet has been one of continuous growth in the
number of hosts, the number and variety of applications, and the
capacity of the network infrastructure, and this growth is expected
to continue for the foreseeable future. A scalable architecture for
service differentiation must be able to accommodate this continued
growth.
The following requirements were identified and are addressed in this
architecture:
o should accommodate a wide variety of services and provisioning
policies, extending end-to-end or within a particular (set of)
network(s),
o should allow decoupling of the service from the particular
application in use,
o should work with existing applications without the need for
application programming interface changes or host software
modifications (assuming suitable deployment of classifiers,
markers, and other traffic conditioning functions),
o should decouple traffic conditioning and service provisioning
functions from forwarding behaviors implemented within the core
network nodes,
o should not depend on hop-by-hop application signaling,
o should require only a small set of forwarding behaviors whose
implementation complexity does not dominate the cost of a network
device, and which will not introduce bottlenecks for future high-
speed system implementations,
o should avoid per-microflow or per-customer state within core
network nodes,
o should utilize only aggregated classification state within the
network core,
o should permit simple packet classification implementations in core
network nodes (BA classifier),
o should permit reasonable interoperability with non-DS-compliant
network nodes,
o should accommodate incremental deployment.
1.4 Comparisons with Other Approaches
The differentiated services architecture specified in this document
can be contrasted with other existing models of service
differentiation. We classify these alternative models into the
following categories: relative priority marking, service marking,
label switching, Integrated Services/RSVP, and static per-hop
classification.
Examples of the relative priority marking model include IPv4
Precedence marking as defined in [RFC791], 802.5 Token Ring priority
[TR], and the default interpretation of 802.1p traffic classes
[802.1p]. In this model the application, host, or proxy node selects
a relative priority or "precedence" for a packet (e.g., delay or
discard priority), and the network nodes along the transit path apply
the appropriate priority forwarding behavior corresponding to the
priority value within the packet"s header. Our architecture can be
considered as a refinement to this model, since we more clearly
specify the role and importance of boundary nodes and traffic
conditioners, and since our per-hop behavior model permits more
general forwarding behaviors than relative delay or discard priority.
An example of a service marking model is IPv4 TOS as defined in
[RFC1349]. In this example each packet is marked with a request for
a "type of service", which may include "minimize delay", "maximize
throughput", "maximize reliability", or "minimize cost". Network
nodes may select routing paths or forwarding behaviors which are
suitably engineered to satisfy the service request. This model is
suBTly different from our architecture. Note that we do not describe
the use of the DS field as an input to route selection. The TOS
markings defined in [RFC1349] are very generic and do not span the
range of possible service semantics. Furthermore, the service
request is associated with each individual packet, whereas some
service semantics may depend on the aggregate forwarding behavior of
a sequence of packets. The service marking model does not easily
accommodate growth in the number and range of future services (since
the codepoint space is small) and involves configuration of the
"TOS->forwarding behavior" association in each core network node.
Standardizing service markings implies standardizing service
offerings, which is outside the scope of the IETF. Note that
provisions are made in the allocation of the DS codepoint space to
allow for locally significant codepoints which may be used by a
provider to support service marking semantics [DSFIELD].
Examples of the label switching (or virtual circuit) model include
Frame Relay, ATM, and MPLS [FRELAY, ATM]. In this model path
forwarding state and traffic management or QoS state is established
for traffic streams on each hop along a network path. Traffic
aggregates of varying granularity are associated with a label
switched path at an ingress node, and packets/cells within each label
switched path are marked with a forwarding label that is used to
lookup the next-hop node, the per-hop forwarding behavior, and the
replacement label at each hop. This model permits finer granularity
resource allocation to traffic streams, since label values are not
globally significant but are only significant on a single link;
therefore resources can be reserved for the aggregate of packets/
cells received on a link with a particular label, and the label
switching semantics govern the next-hop selection, allowing a traffic
stream to follow a specially engineered path through the network.
This improved granularity comes at the cost of additional management
and configuration requirements to establish and maintain the label
switched paths. In addition, the amount of forwarding state
maintained at each node scales in proportion to the number of edge
nodes of the network in the best case (assuming multipoint-to-point
label switched paths), and it scales in proportion with the square of
the number of edge nodes in the worst case, when edge-edge label
switched paths with provisioned resources are employed.
The Integrated Services/RSVP model relies upon traditional datagram
forwarding in the default case, but allows sources and receivers to
exchange signaling messages which establish additional packet
classification and forwarding state on each node along the path
between them [RFC1633, RSVP]. In the absence of state aggregation,
the amount of state on each node scales in proportion to the number
of concurrent reservations, which can be potentially large on high-
speed links. This model also requires application support for the
RSVP signaling protocol. Differentiated services mechanisms can be
utilized to aggregate Integrated Services/RSVP state in the core of
the network [Bernet].
A variant of the Integrated Services/RSVP model eliminates the
requirement for hop-by-hop signaling by utilizing only "static"
classification and forwarding policies which are implemented in each
node along a network path. These policies are updated on
administrative timescales and not in response to the instantaneous
mix of microflows active in the network. The state requirements for
this variant are potentially worse than those encountered when RSVP
is used, especially in backbone nodes, since the number of static
policies that might be applicable at a node over time may be larger
than the number of active sender-receiver sessions that might have
installed reservation state on a node. Although the support of large
numbers of classifier rules and forwarding policies may be
computationally feasible, the management burden associated with
installing and maintaining these rules on each node within a backbone
network which might be traversed by a traffic stream is substantial.
Although we contrast our architecture with these alternative models
of service differentiation, it should be noted that links and nodes
employing these techniques may be utilized to extend differentiated
services behaviors and semantics across a layer-2 switched
infrastructure (e.g., 802.1p LANs, Frame Relay/ATM backbones)
interconnecting DS nodes, and in the case of MPLS may be used as an
alternative intra-domain implementation technology. The constraints
imposed by the use of a specific link-layer technology in particular
regions of a DS domain (or in a network providing access to DS
domains) may imply the differentiation of traffic on a coarser grain
basis. Depending on the mapping of PHBs to different link-layer
services and the way in which packets are scheduled over a restricted
set of priority classes (or virtual circuits of different category
and capacity), all or a subset of the PHBs in use may be supportable
(or may be indistinguishable).
2. Differentiated Services Architectural Model
The differentiated services architecture is based on a simple model
where traffic entering a network is classified and possibly
conditioned at the boundaries of the network, and assigned to
different behavior aggregates. Each behavior aggregate is identified
by a single DS codepoint. Within the core of the network, packets
are forwarded according to the per-hop behavior associated with the
DS codepoint. In this section, we discuss the key components within
a differentiated services region, traffic classification and
conditioning functions, and how differentiated services are achieved
through the combination of traffic conditioning and PHB-based
forwarding.
2.1 Differentiated Services Domain
A DS domain is a contiguous set of DS nodes which operate with a
common service provisioning policy and set of PHB groups implemented
on each node. A DS domain has a well-defined boundary consisting of
DS boundary nodes which classify and possibly condition ingress
traffic to ensure that packets which transit the domain are
appropriately marked to select a PHB from one of the PHB groups
supported within the domain. Nodes within the DS domain select the
forwarding behavior for packets based on their DS codepoint, mapping
that value to one of the supported PHBs using either the recommended
codepoint->PHB mapping or a locally customized mapping [DSFIELD].
Inclusion of non-DS-compliant nodes within a DS domain may result in
unpredictable performance and may impede the ability to satisfy
service level agreements (SLAs).
A DS domain normally consists of one or more networks under the same
administration; for example, an organization"s intranet or an ISP.
The administration of the domain is responsible for ensuring that
adequate resources are provisioned and/or reserved to support the
SLAs offered by the domain.
2.1.1 DS Boundary Nodes and Interior Nodes
A DS domain consists of DS boundary nodes and DS interior nodes. DS
boundary nodes interconnect the DS domain to other DS or non-DS-
capable domains, whilst DS interior nodes only connect to other DS
interior or boundary nodes within the same DS domain.
Both DS boundary nodes and interior nodes must be able to apply the
appropriate PHB to packets based on the DS codepoint; otherwise
unpredictable behavior may result. In addition, DS boundary nodes
may be required to perform traffic conditioning functions as defined
by a traffic conditioning agreement (TCA) between their DS domain and
the peering domain which they connect to (see Sec. 2.3.3).
Interior nodes may be able to perform limited traffic conditioning
functions such as DS codepoint re-marking. Interior nodes which
implement more complex classification and traffic conditioning
functions are analogous to DS boundary nodes (see Sec. 2.3.4.4).
A host in a network containing a DS domain may act as a DS boundary
node for traffic from applications running on that host; we therefore
say that the host is within the DS domain. If a host does not act as
a boundary node, then the DS node topologically closest to that host
acts as the DS boundary node for that host"s traffic.
2.1.2 DS Ingress Node and Egress Node
DS boundary nodes act both as a DS ingress node and as a DS egress
node for different directions of traffic. Traffic enters a DS domain
at a DS ingress node and leaves a DS domain at a DS egress node. A
DS ingress node is responsible for ensuring that the traffic entering
the DS domain conforms to any TCA between it and the other domain to
which the ingress node is connected. A DS egress node may perform
traffic conditioning functions on traffic forwarded to a directly
connected peering domain, depending on the details of the TCA between
the two domains. Note that a DS boundary node may act as a DS
interior node for some set of interfaces.
2.2 Differentiated Services Region
A differentiated services region (DS Region) is a set of one or more
contiguous DS domains. DS regions are capable of supporting
differentiated services along paths which span the domains within the
region.
The DS domains in a DS region may support different PHB groups
internally and different codepoint->PHB mappings. However, to permit
services which span across the domains, the peering DS domains must
each establish a peering SLA which defines (either explicitly or
implicitly) a TCA which specifies how transit traffic from one DS
domain to another is conditioned at the boundary between the two DS
domains.
It is possible that several DS domains within a DS region may adopt a
common service provisioning policy and may support a common set of
PHB groups and codepoint mappings, thus eliminating the need for
traffic conditioning between those DS domains.
2.3 Traffic Classification and Conditioning
Differentiated services are extended across a DS domain boundary by
establishing a SLA between an upstream network and a downstream DS
domain. The SLA may specify packet classification and re-marking
rules and may also specify traffic profiles and actions to traffic
streams which are in- or out-of-profile (see Sec. 2.3.2). The TCA
between the domains is derived (explicitly or implicitly) from this
SLA.
The packet classification policy identifies the subset of traffic
which may receive a differentiated service by being conditioned and/
or mapped to one or more behavior aggregates (by DS codepoint re-
marking) within the DS domain.
Traffic conditioning performs metering, shaping, policing and/or re-
marking to ensure that the traffic entering the DS domain conforms to
the rules specified in the TCA, in accordance with the domain"s
service provisioning policy. The extent of traffic conditioning
required is dependent on the specifics of the service offering, and
may range from simple codepoint re-marking to complex policing and
shaping operations. The details of traffic conditioning policies
which are negotiated between networks is outside the scope of this
document.
2.3.1 Classifiers
Packet classifiers select packets in a traffic stream based on the
content of some portion of the packet header. We define two types of
classifiers. The BA (Behavior Aggregate) Classifier classifies
packets based on the DS codepoint only. The MF (Multi-Field)
classifier selects packets based on the value of a combination of one
or more header fields, such as source address, destination address,
DS field, protocol ID, source port and destination port numbers, and
other information such as incoming interface.
Classifiers are used to "steer" packets matching some specified rule
to an element of a traffic conditioner for further processing.
Classifiers must be configured by some management procedure in
accordance with the appropriate TCA.
The classifier should authenticate the information which it uses to
classify the packet (see Sec. 6).
Note that in the event of upstream packet fragmentation, MF
classifiers which examine the contents of transport-layer header
fields may incorrectly classify packet fragments subsequent to the
first. A possible solution to this problem is to maintain
fragmentation state; however, this is not a general solution due to
the possibility of upstream fragment re-ordering or divergent routing
paths. The policy to apply to packet fragments is outside the scope
of this document.
2.3.2 Traffic Profiles
A traffic profile specifies the temporal properties of a traffic
stream selected by a classifier. It provides rules for determining
whether a particular packet is in-profile or out-of-profile. For
example, a profile based on a token bucket may look like:
codepoint=X, use token-bucket r, b
The above profile indicates that all packets marked with DS codepoint
X should be measured against a token bucket meter with rate r and
burst size b. In this example out-of-profile packets are those
packets in the traffic stream which arrive when insufficient tokens
are available in the bucket. The concept of in- and out-of-profile
can be extended to more than two levels, e.g., multiple levels of
conformance with a profile may be defined and enforced.
Different conditioning actions may be applied to the in-profile
packets and out-of-profile packets, or different accounting actions
may be triggered. In-profile packets may be allowed to enter the DS
domain without further conditioning; or, alternatively, their DS
codepoint may be changed. The latter happens when the DS codepoint
is set to a non-Default value for the first time [DSFIELD], or when
the packets enter a DS domain that uses a different PHB group or
codepoint->PHB mapping policy for this traffic stream. Out-of-
profile packets may be queued until they are in-profile (shaped),
discarded (policed), marked with a new codepoint (re-marked), or
forwarded unchanged while triggering some accounting procedure.
Out-of-profile packets may be mapped to one or more behavior
aggregates that are "inferior" in some dimension of forwarding
performance to the BA into which in-profile packets are mapped.
Note that a traffic profile is an optional component of a TCA and its
use is dependent on the specifics of the service offering and the
domain"s service provisioning policy.
2.3.3 Traffic Conditioners
A traffic conditioner may contain the following elements: meter,
marker, shaper, and dropper. A traffic stream is selected by a
classifier, which steers the packets to a logical instance of a
traffic conditioner. A meter is used (where appropriate) to measure
the traffic stream against a traffic profile. The state of the meter
with respect to a particular packet (e.g., whether it is in- or out-
of-profile) may be used to affect a marking, dropping, or shaping
action.
When packets exit the traffic conditioner of a DS boundary node the
DS codepoint of each packet must be set to an appropriate value.
Fig. 1 shows the block diagram of a classifier and traffic
conditioner. Note that a traffic conditioner may not necessarily
contain all four elements. For example, in the case where no traffic
profile is in effect, packets may only pass through a classifier and
a marker.
+-------+
-------------------+
+-----> Meter
--+
+-------+
V V
+------------+ +--------+ +---------+
Shaper/
packets =====> Classifier =====> Marker =====> Dropper =====>

+------------+ +--------+ +---------+
Fig. 1: Logical View of a Packet Classifier and Traffic Conditioner
2.3.3.1 Meters
Traffic meters measure the temporal properties of the stream of
packets selected by a classifier against a traffic profile specified
in a TCA. A meter passes state information to other conditioning
functions to trigger a particular action for each packet which is
either in- or out-of-profile (to some extent).
2.3.3.2 Markers
Packet markers set the DS field of a packet to a particular
codepoint, adding the marked packet to a particular DS behavior
aggregate. The marker may be configured to mark all packets which
are steered to it to a single codepoint, or may be configured to mark
a packet to one of a set of codepoints used to select a PHB in a PHB
group, according to the state of a meter. When the marker changes
the codepoint in a packet it is said to have "re-marked" the packet.
2.3.3.3 Shapers
Shapers delay some or all of the packets in a traffic stream in order
to bring the stream into compliance with a traffic profile. A shaper
usually has a finite-size buffer, and packets may be discarded if
there is not sufficient buffer space to hold the delayed packets.
2.3.3.4 Droppers
Droppers discard some or all of the packets in a traffic stream in
order to bring the stream into compliance with a traffic profile.
This process is know as "policing" the stream. Note that a dropper
can be implemented as a special case of a shaper by setting the
shaper buffer size to zero (or a few) packets.
2.3.4 Location of Traffic Conditioners and MF Classifiers
Traffic conditioners are usually located within DS ingress and egress
boundary nodes, but may also be located in nodes within the interior
of a DS domain, or within a non-DS-capable domain.
2.3.4.1 Within the Source Domain
We define the source domain as the domain containing the node(s)
which originate the traffic receiving a particular service. Traffic
sources and intermediate nodes within a source domain may perform
traffic classification and conditioning functions. The traffic
originating from the source domain across a boundary may be marked by
the traffic sources directly or by intermediate nodes before leaving
the source domain. This is referred to as initial marking or "pre-
marking".
Consider the example of a company that has the policy that its CEO"s
packets should have higher priority. The CEO"s host may mark the DS
field of all outgoing packets with a DS codepoint that indicates
"higher priority". Alternatively, the first-hop router directly
connected to the CEO"s host may classify the traffic and mark the
CEO"s packets with the correct DS codepoint. Such high priority
traffic may also be conditioned near the source so that there is a
limit on the amount of high priority traffic forwarded from a
particular source.
There are some advantages to marking packets close to the traffic
source. First, a traffic source can more easily take an
application"s preferences into account when deciding which packets
should receive better forwarding treatment. Also, classification of
packets is much simpler before the traffic has been aggregated with
packets from other sources, since the number of classification rules
which need to be applied within a single node is reduced.
Since packet marking may be distributed across multiple nodes, the
source DS domain is responsible for ensuring that the aggregated
traffic towards its provider DS domain conforms to the appropriate
TCA. Additional allocation mechanisms such as bandwidth brokers or
RSVP may be used to dynamically allocate resources for a particular
DS behavior aggregate within the provider"s network [2BIT, Bernet].
The boundary node of the source domain should also monitor
conformance to the TCA, and may police, shape, or re-mark packets as
necessary.
2.3.4.2 At the Boundary of a DS Domain
Traffic streams may be classified, marked, and otherwise conditioned
on either end of a boundary link (the DS egress node of the upstream
domain or the DS ingress node of the downstream domain). The SLA
between the domains should specify which domain has responsibility
for mapping traffic streams to DS behavior aggregates and
conditioning those aggregates in conformance with the appropriate
TCA. However, a DS ingress node must assume that the incoming
traffic may not conform to the TCA and must be prepared to enforce
the TCA in accordance with local policy.
When packets are pre-marked and conditioned in the upstream domain,
potentially fewer classification and traffic conditioning rules need
to be supported in the downstream DS domain. In this circumstance
the downstream DS domain may only need to re-mark or police the
incoming behavior aggregates to enforce the TCA. However, more
sophisticated services which are path- or source-dependent may
require MF classification in the downstream DS domain"s ingress
nodes.
If a DS ingress node is connected to an upstream non-DS-capable
domain, the DS ingress node must be able to perform all necessary
traffic conditioning functions on the incoming traffic.
2.3.4.3 In non-DS-Capable Domains
Traffic sources or intermediate nodes in a non-DS-capable domain may
employ traffic conditioners to pre-mark traffic before it reaches the
ingress of a downstream DS domain. In this way the local policies
for classification and marking may be concealed.
2.3.4.4 In Interior DS Nodes
Although the basic architecture assumes that complex classification
and traffic conditioning functions are located only in a network"s
ingress and egress boundary nodes, deployment of these functions in
the interior of the network is not precluded. For example, more
restrictive access policies may be enforced on a transoceanic link,
requiring MF classification and traffic conditioning functionality in
the upstream node on the link. This approach may have scaling
limits, due to the potentially large number of classification and
conditioning rules that might need to be maintained.
2.4 Per-Hop Behaviors
A per-hop behavior (PHB) is a description of the externally
observable forwarding behavior of a DS node applied to a particular
DS behavior aggregate. "Forwarding behavior" is a general concept in
this context. For example, in the event that only one behavior
aggregate occupies a link, the observable forwarding behavior (i.e.,
loss, delay, jitter) will often depend only on the relative loading
of the link (i.e., in the event that the behavior assumes a work-
conserving scheduling discipline). Useful behavioral distinctions
are mainly observed when multiple behavior aggregates compete for
buffer and bandwidth resources on a node. The PHB is the means by
which a node allocates resources to behavior aggregates, and it is on
top of this basic hop-by-hop resource allocation mechanism that
useful differentiated services may be constructed.
The most simple example of a PHB is one which guarantees a minimal
bandwidth allocation of X% of a link (over some reasonable time
interval) to a behavior aggregate. This PHB can be fairly easily
measured under a variety of competing traffic conditions. A slightly
more complex PHB would guarantee a minimal bandwidth allocation of X%
of a link, with proportional fair sharing of any excess link
capacity. In general, the observable behavior of a PHB may depend on
certain constraints on the traffic characteristics of the associated
behavior aggregate, or the characteristics of other behavior
aggregates.
PHBs may be specified in terms of their resource (e.g., buffer,
bandwidth) priority relative to other PHBs, or in terms of their
relative observable traffic characteristics (e.g., delay, loss).
These PHBs may be used as building blocks to allocate resources and
should be specified as a group (PHB group) for consistency. PHB
groups will usually share a common constraint applying to each PHB
within the group, such as a packet scheduling or buffer management
policy. The relationship between PHBs in a group may be in terms of
absolute or relative priority (e.g., discard priority by means of
deterministic or stochastic thresholds), but this is not required
(e.g., N equal link shares). A single PHB defined in isolation is a
special case of a PHB group.
PHBs are implemented in nodes by means of some buffer management and
packet scheduling mechanisms. PHBs are defined in terms of behavior
characteristics relevant to service provisioning policies, and not in
terms of particular implementation mechanisms. In general, a variety
of implementation mechanisms may be suitable for implementing a
particular PHB group. Furthermore, it is likely that more than one
PHB group may be implemented on a node and utilized within a domain.
PHB groups should be defined such that the proper resource allocation
between groups can be inferred, and integrated mechanisms can be
implemented which can simultaneously support two or more groups. A
PHB group definition should indicate possible conflicts with
previously documented PHB groups which might prevent simultaneous
operation.
As described in [DSFIELD], a PHB is selected at a node by a mapping
of the DS codepoint in a received packet. Standardized PHBs have a
recommended codepoint. However, the total space of codepoints is
larger than the space available for recommended codepoints for
standardized PHBs, and [DSFIELD] leaves provisions for locally
configurable mappings. A codepoint->PHB mapping table may contain
both 1->1 and N->1 mappings. All codepoints must be mapped to some
PHB; in the absence of some local policy, codepoints which are not
mapped to a standardized PHB in accordance with that PHB"s
specification should be mapped to the Default PHB.
2.5 Network Resource Allocation
The implementation, configuration, operation and administration of
the supported PHB groups in the nodes of a DS Domain should
effectively partition the resources of those nodes and the inter-node
links between behavior aggregates, in accordance with the domain"s
service provisioning policy. Traffic conditioners can further
control the usage of these resources through enforcement of TCAs and
possibly through operational feedback from the nodes and traffic
conditioners in the domain. Although a range of services can be
deployed in the absence of complex traffic conditioning functions
(e.g., using only static marking policies), functions such as
policing, shaping, and dynamic re-marking enable the deployment of
services providing quantitative performance metrics.
The configuration of and interaction between traffic conditioners and
interior nodes should be managed by the administrative control of the
domain and may require operational control through protocols and a
control entity. There is a wide range of possible control models.
The precise nature and implementation of the interaction between
these components is outside the scope of this architecture. However,
scalability requires that the control of the domain does not require
micro-management of the network resources. The most scalable control
model would operate nodes in open-loop in the operational timeframe,
and would only require administrative-timescale management as SLAs
are varied. This simple model may be unsuitable in some
circumstances, and some automated but slowly varying operational
control (minutes rather than seconds) may be desirable to balance the
utilization of the network against the recent load profile.
3. Per-Hop Behavior Specification Guidelines
Basic requirements for per-hop behavior standardization are given in
[DSFIELD]. This section elaborates on that text by describing
additional guidelines for PHB (group) specifications. This is
intended to help foster implementation consistency. Before a PHB
group is proposed for standardization it should satisfy these
guidelines, as appropriate, to preserve the integrity of this
architecture.
G.1: A PHB standard must specify a recommended DS codepoint selected
from the codepoint space reserved for standard mappings [DSFIELD].
Recommended codepoints will be assigned by the IANA. A PHB proposal
may recommend a temporary codepoint from the EXP/LU space to
facilitate inter-domain experimentation. Determination of a packet"s
PHB must not require inspection of additional packet header fields
beyond the DS field.
G.2: The specification of each newly proposed PHB group should
include an overview of the behavior and the purpose of the behavior
being proposed. The overview should include a problem or problems
statement for which the PHB group is targeted. The overview should
include the basic concepts behind the PHB group. These concepts
should include, but are not restricted to, queueing behavior, discard
behavior, and output link selection behavior. Lastly, the overview
should specify the method by which the PHB group solves the problem
or problems specified in the problem statement.
G.3: A PHB group specification should indicate the number of
individual PHBs specified. In the event that multiple PHBs are
specified, the interactions between these PHBs and constraints that
must be respected globally by all the PHBs within the group should be
clearly specified. As an example, the specification must indicate
whether the probability of packet reordering within a microflow is
increased if different packets in that microflow are marked for
different PHBs within the group.
G.4: When proper functioning of a PHB group is dependent on
constraints such as a provisioning restriction, then the PHB
definition should describe the behavior when these constraints are
violated. Further, if actions such as packet discard or re-marking
are required when these constraints are violated, then these actions
should be specifically stipulated.
G.5: A PHB group may be specified for local use within a domain in
order to provide some domain-specific functionality or domain-
specific services. In this event, the PHB specification is useful
for providing vendors with a consistent definition of the PHB group.
However, any PHB group which is defined for local use should not be
considered for standardization, but may be published as an
Informational RFC. In contrast, a PHB group which is intended for
general use will follow a stricter standardization process.
Therefore all PHB proposals should specifically state whether they
are to be considered for general or local use.
It is recognized that PHB groups can be designed with the intent of
providing host-to-host, WAN edge-to-WAN edge, and/or domain edge-to-
domain edge services. Use of the term "end-to-end" in a PHB
definition should be interpreted to mean "host-to-host" for
consistency.
Other PHB groups may be defined and deployed locally within domains,
for experimental or operational purposes. There is no requirement
that these PHB groups must be publicly documented, but they should
utilize DS codepoints from one of the EXP/LU pools as defined in
[DSFIELD].
G.6: It may be possible or appropriate for a packet marked for a PHB
within a PHB group to be re-marked to select another PHB within the
group; either within a domain or across a domain boundary. Typically
there are three reasons for such PHB modification:
a. The codepoints associated with the PHB group are collectively
intended to carry state about the network,
b. Conditions exist which require PHB promotion or demotion of a
packet (this assumes that PHBs within the group can be ranked in
some order),
c. The boundary between two domains is not covered by a SLA. In this
case the codepoint/PHB to select when crossing the boundary link
will be determined by the local policy of the upstream domain.
A PHB specification should clearly state the circumstances under
which packets marked for a PHB within a PHB group may, or should be
modified (e.g., promoted or demoted) to another PHB within the group.
If it is undesirable for a packet"s PHB to be modified, the
specification should clearly state the consequent risks when the PHB
is modified. A possible risk to changing a packet"s PHB, either
within or outside a PHB group, is a higher probability of packet re-
ordering within a microflow. PHBs within a group may carry some
host-to-host, WAN edge-to-WAN edge, and/or domain edge-to-domain edge
semantics which may be difficult to duplicate if packets are re-
marked to select another PHB from the group (or otherwise).
For certain PHB groups, it may be appropriate to reflect a state
change in the node by re-marking packets to specify another PHB from
within the group. If a PHB group is designed to reflect the state of
a network, the PHB definition must adequately describe the
relationship between the PHBs and the states they reflect. Further,
if these PHBs limit the forwarding actions a node can perform in some
way, these constraints may be specified as actions the node should,
or must perform.
G.7: A PHB group specification should include a section defining the
implications of tunneling on the utility of the PHB group. This
section should specify the implications for the utility of the PHB
group of a newly created outer header when the original DS field of
the inner header is encapsulated in a tunnel. This section should
also discuss what possible changes should be applied to the inner
header at the egress of the tunnel, when both the codepoints from the
inner header and the outer header are accessible (see Sec. 6.2).
G.8: The process of specifying PHB groups is likely to be
incremental in nature. When new PHB groups are proposed, their known
interactions with previously specified PHB groups should be
documented. When a new PHB group is created, it can be entirely new
in scope or it can be an extension to an existing PHB group. If the
PHB group is entirely independent of some or all of the existing PHB
specifications, a section should be included in the PHB specification
which details how the new PHB group can co-exist with those PHB
groups already standardized. For example, this section might
indicate the possibility of packet re-ordering within a microflow for
packets marked by codepoints associated with two separate PHB groups.
If concurrent operation of two (or more) different PHB groups in the
same node is impossible or detrimental this should be stated. If the
concurrent operation of two (or more) different PHB groups requires
some specific behaviors by the node when packets marked for PHBs from
these different PHB groups are being processed by the node at the
same time, these behaviors should be stated.
Care should be taken to avoid circularity in the definitions of PHB
groups.
If the proposed PHB group is an extension to an existing PHB group, a
section should be included in the PHB group specification which
details how this extension interoperates with the behavior being
extended. Further, if the extension alters or more narrowly defines
the existing behavior in some way, this should also be clearly
indicated.
G.9: Each PHB specification should include a section specifying
minimal conformance requirements for implementations of the PHB
group. This conformance section is intended to provide a means for
specifying the details of a behavior while allowing for
implementation variation to the extent permitted by the PHB
specification. This conformance section can take the form of rules,
tables, pseudo-code, or tests.
G.10: A PHB specification should include a section detailing the
security implications of the behavior. This section should include a
discussion of the re-marking of the inner header"s codepoint at the
egress of a tunnel and its effect on the desired forwarding behavior.
Further, this section should also discuss how the proposed PHB group
could be used in denial-of-service attacks, reduction of service
contract attacks, and service contract violation attacks. Lastly,
this section should discuss possible means for detecting such attacks
as they are relevant to the proposed behavior.
G.11: A PHB specification should include a section detailing
configuration and management issues which may affect the operation of
the PHB and which may impact candidate services that might utilize
the PHB.
G.12: It is strongly recommended that an appendix be provided with
each PHB specification that considers the implications of the
proposed behavior on current and potential services. These services
could include but are not restricted to be user-specific, device-
specific, domain-specific or end-to-end services. It is also
strongly recommended that the appendix include a section describing
how the services are verified by users, devices, and/or domains.
G.13: It is recommended that an appendix be provided with each PHB
specification that is targeted for local use within a domain,
providing guidance for PHB selection for packets which are forwarded
into a peer domain which does not support the PHB group.
G.14: It is recommended that an appendix be provided with each PHB
specification which considers the impact of the proposed PHB group on
existing higher-layer protocols. Under some circumstances PHBs may
allow for possible changes to higher-layer protocols which may
increase or decrease the utility of the proposed PHB group.
G.15: It is recommended that an appendix be provided with each PHB
specification which recommends mappings to link-layer QoS mechanisms
to support the intended behavior of the PHB across a shared-medium or
switched link-layer. The determination of the most appropriate
mapping between a PHB and a link-layer QoS mechanism is dependent on
many factors and is outside the scope of this document; however, the
specification should attempt to offer some guidance.
4. Interoperability with Non-Differentiated Services-Compliant Nodes
We define a non-differentiated services-compliant node (non-DS-
compliant node) as any node which does not interpret the DS field as
specified in [DSFIELD] and/or does not implement some or all of the
standardized PHBs (or those in use within a particular DS domain).
This may be due to the capabilities or configuration of the node. We
define a legacy node as a special case of a non-DS-compliant node
which implements IPv4 Precedence classification and forwarding as
defined in [RFC791, RFC1812], but which is otherwise not DS-
compliant. The precedence values in the IPv4 TOS octet are
compatible by intention with the Class Selector Codepoints defined in
[DSFIELD], and the precedence forwarding behaviors defined in
[RFC791, RFC1812] comply with the Class Selector PHB Requirements
also defined in [DSFIELD]. A key distinction between a legacy node
and a DS-compliant node is that the legacy node may or may not
interpret bits 3-6 of the TOS octet as defined in [RFC1349] (the
"DTRC" bits); in practice it will not interpret these bit as
specified in [DSFIELD]. We assume that the use of the TOS markings
defined in [RFC1349] is deprecated. Nodes which are non-DS-compliant
and which are not legacy nodes may exhibit unpredictable forwarding
behaviors for packets with non-zero DS codepoints.
Differentiated services depend on the resource allocation mechanisms
provided by per-hop behavior implementations in nodes. The quality
or statistical assurance level of a service may break down in the
event that traffic transits a non-DS-compliant node, or a non-DS-
capable domain.
We will examine two separate cases. The first case concerns the use
of non-DS-compliant nodes within a DS domain. Note that PHB
forwarding is primarily useful for allocating scarce node and link
resources in a controlled manner. On high-speed, lightly loaded
links, the worst-case packet delay, jitter, and loss may be
negligible, and the use of a non-DS-compliant node on the upstream
end of such a link may not result in service degradation. In more
realistic circumstances, the lack of PHB forwarding in a node may
make it impossible to offer low-delay, low-loss, or provisioned
bandwidth services across paths which traverse the node. However,
use of a legacy node may be an acceptable alternative, assuming that
the DS domain restricts itself to using only the Class Selector
Codepoints defined in [DSFIELD], and assuming that the particular
precedence implementation in the legacy node provides forwarding
behaviors which are compatible with the services offered along paths
which traverse that node. Note that it is important to restrict the
codepoints in use to the Class Selector Codepoints, since the legacy
node may or may not interpret bits 3-5 in accordance with [RFC1349],
thereby resulting in unpredictable forwarding results.
The second case concerns the behavior of services which traverse
non-DS-capable domains. We assume for the sake of argument that a
non-DS-capable domain does not deploy traffic conditioning functions
on domain boundary nodes; therefore, even in the event that the
domain consists of legacy or DS-compliant interior nodes, the lack of
traffic enforcement at the boundaries will limit the ability to
consistently deliver some types of services across the domain. A DS
domain and a non-DS-capable domain may negotiate an agreement which
governs how egress traffic from the DS-domain should be marked before
entry into the non-DS-capable domain. This agreement might be
monitored for compliance by traffic sampling instead of by rigorous
traffic conditioning. Alternatively, where there is knowledge that
the non-DS-capable domain consists of legacy nodes, the upstream DS
domain may opportunistically re-mark differentiated services traffic
to one or more of the Class Selector Codepoints. Where there is no
knowledge of the traffic management capabilities of the downstream
domain, and no agreement in place, a DS domain egress node may choose
to re-mark DS codepoints to zero, under the assumption that the non-
DS-capable domain will treat the traffic uniformly with best-effort
service.
In the event that a non-DS-capable domain peers with a DS domain,
traffic flowing from the non-DS-capable domain should be conditioned
at the DS ingress node of the DS domain according to the appropriate
SLA or policy.
5. Multicast Considerations
Use of differentiated services by multicast traffic introduces a
number of issues for service provisioning. First, multicast packets
which enter a DS domain at an ingress node may simultaneously take
multiple paths through some segments of the domain due to multicast
packet replication. In this way they consume more network resources
than unicast packets. Where multicast group membership is dynamic,
it is difficult to predict in advance the amount of network resources
that may be consumed by multicast traffic originating from an
upstream network for a particular group. A consequence of this
uncertainty is that it may be difficult to provide quantitative
service guarantees to multicast senders. Further, it may be
necessary to reserve codepoints and PHBs for exclusive use by unicast
traffic, to provide resource isolation from multicast traffic.
The second issue is the selection of the DS codepoint for a multicast
packet arriving at a DS ingress node. Because that packet may exit
the DS domain at multiple DS egress nodes which peer with multiple
downstream domains, the DS codepoint used should not result in the
re