One document matched: draft-ietf-diffserv-framework-01.txt
Differences from draft-ietf-diffserv-framework-00.txt
Differentiated Services S.Blake, et al
Internet Draft October, 1998
Document: draft-ietf-diffserv-framework-01.txt
Yoram Bernet, Microsoft
James Binder, 3-Com
Steven Blake, Torrent Networking Technologies
Mark Carlson, Redcape Software
Srinivasan Keshav, Cornell University
Elwyn Davies, Nortel UK
Borje Ohlman, Ericsson
Dinesh Verma, IBM
Zheng Wang, Bell Labs Lucent Technologies
Walter Weiss, Lucent Technologies
A Framework for Differentiated Services
<draft-ietf-diffserv-framework-01.txt>
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
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A revised version of this draft document will be submitted to the
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Discussion and suggestions for improvement are requested. This
document will expire before April, 1999. Distribution of this draft
is unlimited.
1. Abstract
This document provides a general description of issues related to
the definition, configuration and management of services enabled by
the differentiated services architecture [DSARCH]. This document
should be read along with its companion documents, the
differentiated services architecture [DSARCH] and the definition of
the DS field [DSHEAD]. A glossary of specialist terms used may be
found in [DSARCH].
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2. Structure of this Draft
Section 3 defines Differentiated Services and explains the
motivation behind its deployment. Section 4 defines the concept of a
service and the components that comprise a service. Section 5
discusses several service examples. Section 6 examines intra-domain
provisioning, configuration and management issues. Section 7
examines inter-domain provisioning, configuration and management.
Section 8 addresses interoperability with Integrated Services and
RSVP. Section 9 discusses the interaction of differentiated services
with multicast and tunnelling. Section 10 addresses security
concerns.
3. Differentiated Services - Motivation and Definition
Traditionally, network service providers (both enterprise and
traditional ISPs) provide all customers with the same level of
performance (best-effort service). Most service differentiation has
been in the pricing structure (individual vs. business rates) or the
connectivity type (dial-up access vs. leased line, etc.). However,
in recent years, increased usage of the Internet has resulted in
scarcity of network capacity, compromising performance of
traditional, mission critical applications. At the same time, new
applications have emerged which demand much improved service
quality. As a result, service providers are finding it necessary to
offer their customers alternative levels of service. As well as
meeting new customer expectations, this allows service providers to
improve their revenues through premium pricing and competitive
differentiation of service offerings, which in turn can fund the
necessary expansion of the network.
The Differentiated Services architecture offers a framework within
which service providers can offer each customer a range of network
services which are differentiated on the basis of performance in
addition to pricing tiers used in the past. Customers request a
specific performance level on a packet by packet basis, by marking
the DS field of each packet with a specific value (see [DSHEAD] for
more details). This value specifies the Per-hop Behavior (PHB) to be
allotted to the packet within the provider's network. Typically, the
customer and provider negotiate a profile (policing profile)
describing the rate at which traffic can be submitted at each
service level. Packets submitted in excess of this profile may not
be allotted the service level requested. A salient feature of
differentiated services is its scalability, which allows it to be
deployed in very large networks. This scalability is achieved by
forcing as much complexity out of the core of the network into edge
devices which process lower volumes of traffic and lesser numbers of
flows, and offering services for aggregated traffic rather than on a
per-micro-flow basis.
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4. Services
[DSARCH] defines a Service as "the overall treatment of a defined
subset of a customer's traffic within a DS-domain, or end-to-end".
Although PHBs are at the heart of the differentiated services
architecture, it is the service obtained as a result of marking
traffic for a specific PHB, which is of value to the customer. PHBs
are merely building blocks for services. Service providers combine
PHB implementations with traffic conditioners, provisioning
strategies and billing models which enable them to offer services to
their customers. Providers and customers negotiate agreements with
respect to the services to be provided at each customer/provider
boundary. These take the form of Service Level Agreements (SLAs).
Bear in mind when considering the services that are offered in a DS-
domain that:
* DS services are all for unidirectional traffic only
* DS services are for traffic aggregates, not individual micro-flows
4.1 Customer/Provider Boundaries
Present day network traffic generally traverses a concatenation of
networks which may include hosts, home or office networks, campus or
corporate networks and several large transit networks. Home and
office networks are typically customers of campus or corporate
networks, which are in turn customers of large transit networks.
While one would expect the initial deployment of differentiated
services to be within large transit networks, its deployment may
also be extended to the smaller campus and corporate networks and in
special cases, all the way to individual hosts. As such, there may
be numerous customer/provider boundaries at which the concept of a
'service' applies.
4.2 SLAs and TCAs
At each differentiated service customer/provider boundary, the
service provided is defined in the form of an SLA. The SLA is a
contract which specifies the overall features and performance which
can be expected by the customer. Because DS services are
unidirectional the two directions of flow across the boundary will
need to be considered separately. An important subset of the SLA is
the traffic conditioning agreement, or TCA. The TCA specifies
detailed service parameters for each service level. Such parameters
include:
1. Detailed service performance parameters such as expected
throughput, drop probability, latency.
2. Traffic profiles which must be adhered to for the requested
service to be provided, such as token bucket parameters.
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3. Disposition of traffic submitted in excess of the specified
profile.
4. Marking services provided.
5. Shaping services provided.
In addition to the details in the TCA, the SLA may specify more
general service characteristics such as:
1. Availability/Reliability, which may include behavior in the event
of failures resulting in rerouting of traffic
2. Encryption services
3. Routing constraints
4. Authentication mechanisms
5. Mechanisms for monitoring and auditing the service
6. Responsibilities such as location of the equipment and
functionality, action if the contract is broken, support
capabilities
7. Pricing and billing mechanisms
These additional characteristics are important, and in the case of
pricing and billing, fundamental to the service offering but they do
not affect the service itself and are not considered further here.
4.3 Service Taxonomy: Quantitative through Qualitative and alternatives
The Differentiated Services architecture can support a broad
spectrum of different kinds of service. Categorizing these services
provides some constraints on the corresponding SLAs that can be
offered for the service.
Some services can be clearly categorized as qualitative or
quantitative depending on the type of performance parameters
offered. Examples of qualitative services are as follows:
1. Traffic offered at service level A will be delivered with low
latency.
2. Traffic offered at service level B will be delivered with low
loss.
The assurances offered in examples 1 and 2 are relative and can only
be verified by comparison.
Examples of quantitative services are as follows:
3. 90% of in profile traffic delivered at service level C will
experience no more than 50 msec latency.
4. 95% of in profile traffic delivered at service level D will be
delivered.
Examples 3 and 4 both provide concrete guarantees that could be
verified by suitable measurements on the example service
irrespective of any other services offered in parallel with it.
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There are also services which are not readily categorized as
qualitative or quantitative as in the following examples:
5. Traffic offered at service level E will be allotted twice the
bandwidth of traffic delivered at service level F.
6. Traffic with drop precedence AF12 has a lower probability of
delivery than traffic with drop precedence AF13.
In example 5, the provider is quantifying the relative benefit of
submitting traffic at service level E vs. service level F, but the
customer cannot expect any particular quantifiable throughput. This
can be described as a `Relative Quantification Service'.
In general, when a provider offers a quantitative service, it will
be necessary to specify quantitative policing profiles. In many
cases, quantitative policing profiles will be specified even for
services that do not offer quantitative performance.
Determining how to monitor and audit the delivery of a qualitative
or relative quantification service in such a way as to convince the
user that he has received fair measure requires careful attention.
It will be up to the customer to determine if the advantage offered
is sufficient to make the service worthwhile. The SLA must clearly
avoid making quantitative commitments for these services.
4.4 The Scope of a Service
The scope of a service refers to the topological extent over which
the service is offered. For example, assume that a provider offers a
service to a customer which connects to their network at ingress
point A. The service may apply to:
1. all traffic from ingress point A to any egress point
2. all traffic between ingress point A and egress point B
3. all traffic from ingress point A to a set of egress points
Egress points may be in the same domain as the ingress point or may
be in other domains which are either directly or indirectly
connected to the ingress domain. If the egress point is in another
domain, it will be necessary for the ingress provider to negotiate
SLAs with the relevant peer domains, which will recursively
negotiate with their peers to ensure that the service offered at
ingress point A can indeed be extended to the egress points
specified. The scope of a service is part of the SLA governing
ingress point A.
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In general, providers will be able to offer quantitative services
most efficiently when a specific set of egress points is specified.
Quantitative services which span multiple domains also require
tighter coupling between the SLA offered to the customer at ingress
point A and the SLAs negotiated with intermediate domains.
Qualitative services can more readily be offered to arbitrary sets
of egress points and require looser coupling between the SLA at
ingress point A and SLAs at intermediate domain boundaries.
4.4.1 Services Governing Received Traffic
A special case of service scope is a service that governs all
traffic between any ingress point and egress point B. The SLA that
defines this service would be at egress point B and would
effectively allow the customer to control the mix of traffic
received from the provider. While such a service is theoretically
possible, it conflicts with the more traditional use of diffserv
which governs the quality with which traffic is sent, rather than
received.
A number of concerns would have to be addressed by such a service,
including:
Traffic going to point B from an ingress point A under the terms
of the SLA of this service may also be governed by an SLA for
traffic submitted at point A. The SLAs may conflict and it will
not, in general, be possible to resolve all such conflicts across
all the ingress points.
- Establishing a traffic profile for this service at every possible
ingress which prevents overload of the receiver can be more
complex than for other service scopes: Static profiles are likely
to be either inefficient (e.g. dividing the egress profile into
fixed proportions) or risky (e.g. allowing every ingress to send
the whole profile) whilst dynamic profiles require processes and
communication mechanisms to coordinate the settings.
- Without effective ingress profiles for the service, denial of
service attacks will be a serious problem.
Some of the characteristics of receiver oriented services can be
provided by local policies and the SLA for the domain to which
traffic is sent via the egress point as described in Sec. 4.6.4.
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4.5 Dynamic vs. Static SLAs
SLAs may be static or dynamic. Static SLAs are the norm at the
present time. These are instantiated as a result of negotiation
between human agents representing provider and customer. A static
SLA is first instantiated at the agreed upon service start date and
may periodically be renegotiated (on the order of days or weeks or
months). The SLA may specify that service levels change at certain
times of day or certain days of the week, but the agreement itself
remains static.
Dynamic SLAs, on the other hand, may change frequently. Such changes
may result for example, from variations in offered traffic load
relative to preset thresholds or from changes in pricing offered by
the provider as the traffic load fluctuates. Dynamic SLAs change
without human intervention and thus require an automated agent and
protocol, in effect, a bandwidth broker to represent the
differentiated service provider's domain (such as suggested in
[BB]).
Dynamic SLAs also present challenging problems to both end users and
network providers:
- Network providers have to balance frequently changing loads on
different routes within the provider network. This requires the
provider to adopt dynamic, automated resource provisioning
mechanisms rather than relying on static provisioning.
- Customer equipment will have to adapt to dynamic SLAs in order to
make the most out of the changing SLA.
- End user applications may have to adapt their behavior during a
session to make the most of (or even, cope with) dynamic SLAs.
It is worth reiterating that the SLAs in Differentiated Services
apply to aggregates of traffic and not individual flows. For
scalability, it is undesirable to envisage modifying an SLA every
time a new micro-flow is added or removed from an aggregate.
4.6 Provisioning Traffic Conditioners in Boundary Devices to Provide
Services
Once an SLA has been negotiated, the service provider (and
optionally the customer) will configure traffic conditioning
components at the boundary between the two networks. The service
provider does so with the goal of protecting the provider's network
such that the resources granted to the customer meet but do not
exceed the terms of the TCA. The customer does so with the goal of
making the best use of the service purchased from the provider. In
this section, we will briefly describe configuration of traffic
conditioners in boundary devices.
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Note that the provider's self interests require only that the
provider identify
- for which service level specific traffic is submitted,
- by which customer it is submitted, and
- for traffic with double-ended SLAs (i.e. SLA scope is type 2 or 3
of Sec. 4.4) only, the destination address to which the traffic
is directed.
Customer traffic may be authenticated either by the physical
connection on which it arrives or by some sophisticated
cryptographic means which is beyond the scope of this draft. The
provider need not be concerned with the customer's individual micro-
flows in delivering basic Differentiated Services (see Sec. 4.6.3
for additional services).
[DSARCH] identifies four traffic conditioning components:
1. Meters
2. Markers
3. Shapers
4. Droppers
The combination and interaction of the traffic conditioning
components is selected on a packet-by-packet basis by the DS
codepoint. The configuration parameters for the components at each
codepoint are determined by the policies and profiles applied, so
that the conditioner polices the traffic in the BA specified by the
codepoint. Meters measure submitted traffic for conformance to a
profile, providing control input for the other components which
implement the policing:
- Shapers police by delaying submitted traffic such that it does
not exceed the traffic rate specified in a profile.
- Droppers police by dropping traffic that is submitted at a rate
exceeding that specified in a profile.
- Markers police by re-marking the traffic with a different
codepoint either
- to demote out-of-profile traffic to a different PHB,
- as a result of an SLA which specifies codepoint mutation, or
- to ensure that only valid codepoints are used within the
domain.
In addition to these four components, traffic classifiers are
required in order to separate submitted traffic into different
classes. Classifiers may separate traffic based only on the DS-field
of submitted packets (BA classifiers) or may do so based on multiple
fields within the packet header and even the packet payload (MF
classifiers). MF classifiers may be used at boundaries to provide
certain per-micro-flow services to customers. Examples of such
services include per-flow marking or shaping. Typically, traffic
will arrive at the boundary of a DS domain pre-marked and
pre-shaped. However, at interfaces with some non-DS customer
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networks, it is possible that traffic will require marking and
shaping.
Even if a customer has pre-marked and pre-shaped, the service
provider will wish to police the traffic at the ingress boundary to
meet the domain's self-interests. This may result in traffic being
re-marked or dropped.
Traffic conditioning components (in particular, meters) will also be
the primary source of billing information for a differentiated
Services network.
4.6.1 Minimal Functionality at Provider's Ingress
At the very least, the service provider must limit traffic carried
on behalf of the customer to the constraints specified in the TCA. A
simplified TCA can be represented in the form of a table wherein
each row has the format:
DS-Mark : Profile : Disposition of non-conforming traffic
This row indicates that the provider commits to carry traffic marked
with 'DS-Mark' at the corresponding service level, provided that it
conforms to the 'Profile'. Traffic that is submitted with 'DS-Mark'
and which does not conform to the 'Profile', is subjected to
'Disposition of non-conforming traffic'. This is generally a
policing action such as re-marking to a lower service level,
delaying in a shaper, or dropping. Alternatively, it may be carried
at the requested service level, but subjected to a surcharge. The
SLA for this type of service would normally be expected to be of
type 1 of Sec. 4.4.1, where the traffic destination can take it
through any egress point of the domain.
To provide this minimal functionality, the provider must configure a
BA classifier to separate traffic into the different service level
requested, based on DS-Mark. Following the BA classifier, each class
must be metered for conformance to the corresponding profile.
Following the profiler, either a dropper, shaper or re-marker is
likely to be employed. The Better than Best Efforts service
described in Sec. 5.1 is an example of a service for which this
functionality is sufficient.
4.6.2 Functionality at Provider's Ingress for Double-ended SLAs
If quantitative or other services needing double-ended SLAs (types 2
and 3 of Sec. 4.4.1) are implemented in a DS Domain, these services
specify the possible egress port(s) for traffic conforming to the
SLA. The traffic conditioner needs to consider the destination
address of the packet as additional input to the policing process,
so that traffic is not accepted for egress ports for which an SLA
does not exist. The Virtual Leased Line service described in
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Sec. 5.2 is an example of a service that would require this
functionality.
A QoS VPN can be constructed by provisioning multiple instances
o
f
services of type 2, building in effect, a mesh of point to point QoS
links.
Services of type 3 are most likely to be used for multicast
applications (see Sec. 9).
4.6.3 Added Value Functionality at Provider's Ingress
The functionality described in Secs. 4.6.1 and 4.6.2 serves only to
protect the provider's network resources in line with the terms of
the TCA. It provides no assistance to the customer. The burden of
marking packets and shaping traffic falls entirely on the customer.
In some cases, the SLA may call for the provider to provide
additional services to the customer. Such services may include:
1. Marking traffic from specific micro-flows to a specific behaviour
aggregate (marking the DS-field).
2. Policing traffic from specific micro-flows or sets of micro-
flows, either in the form of dropping or shaping.
In order to provide such services, the provider must generally
employ an MF classifier in addition to the BA classifier. The need
for an MF classifier arises only when the customer requires the
provider to provide some form of traffic separation or
authentication on behalf of the customer. The provider may charge
dearly for these services depending on the degree of granularity and
the amount of work required. For example, shaping thousands of
customer micro-flows might consume considerable resources in the
provider's edge device. On the other hand marking based on source
subnet addresses would consume considerably fewer resources.
4.6.4 Functionality at Customer's Egress
Strictly speaking, the customer need not apply any specific traffic
conditioning. In this case, the customer relies on the provider to
mark as per negotiated MF classification criteria. In many cases it
is preferable for the customer to mark. Customer marking may be
necessary when customer packets are encrypted (as in the case of
end-to-end IPSec). Customer marking enables the customer to direct
specific traffic from specific users or applications to specific
service classes. This may be difficult or impossible for a provider
to do on behalf of a customer when, for example, applications use
volatile ports and users are assigned IP addresses based on DHCP.
In addition to marking, it is in the customer's best interest to at
least shape per service level, at the customer network's egress
point. Otherwise, customer traffic may be policed by the service
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provider with undesirable consequences (e.g. dropped packets).
Shaping per service level does not however, provide for micro-flow
traffic separation. As a consequence, a renegade traffic source may
cause the profile to be exceeded for a specific service level,
negatively impacting all customer flows which are marked for that
service level. Therefore, it is often in the customer's interest to
shape or at least to police, with micro-flow granularity.
4.6.5 Functionality at Provider's Egress
At the egress from a provider's domain there may be an SLA in place
with a peer DS domain, which might be either another provider or an
end user domain. As in Sec. 4.6.4, it is in the provider's best
interests to shape the traffic leaving the domain.
Depending on the SLA, the egress may be required to remark and/or
police or shape the traffic. Note that the forwarding treatment
applied to the packet in the egress node of the domain would be that
selected by the codepoint before it was remarked (otherwise, the
egress node has to support multiple codepoint to PHB mappings).
The provider may also wish to offer additional services to a
customer by policing egress traffic with micro-flow granularity if
the customer might expect to receive excessive traffic in a single
BA and wishes to apply greater control than could be achieved by
normal policing of the aggregate. This would be specified via an
SLA in the usual way.
4.7 Internal Provisioning
The provider must provision internal nodes in the provider network
to meet the assurances offered by SLAs negotiated at the boundaries
of the network. To do so, the provider may use similar traffic
conditioning mechanisms to those used at the network boundaries.
However, providers are unlikely to apply MF classification within
the interior of the network. The provider may police periodically
within the network, by reshaping, remarking or discarding traffic.
Service providers are experienced in provisioning large networks
which offer uniform service, assisted by predictive tools, traffic
modeling tools and real time measurements. Current techniques will
likely be applied to differentiated services networks, although, the
complexity of provisioning will increase significantly. In a
differentiated service network, the provider must ensure that
resources granted to traffic of one service level does not
inappropriately compromise assurances regarding traffic at other
service levels (for example, in example service 6, traffic in AF13
can legitimately compromise traffic in AF11 if an increase in AF13
traffic causes more AF11 traffic to be dropped). As mentioned
previously, internal provisioning in the case of dynamic SLAs will
likely require dynamic resource allocation protocols.
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4.8 End-to-End Service Construction
The Differentiated Services architecture proposes that an end-to-end
service can be constructed by the concatenation of domain services
and their associated customer-provider SLAs for each of the domains
which the service traffic has to cross.
Clearly, not all PHBs and services can be meaningfully concatenated,
and the definition of suitable services and their associated PHBs
will be a major focus of future Differentiated Services development.
This is discussed at greater length in Sect. 7.
5. Service Examples
In this section, we describe service examples and show how they can
be supported by specific PHBs. We base these examples on PHBs which
are defined in [AF]and [EF]. These examples are intended to be
illustrative of the wide range of services that can be employed
using the differentiated services model, and are not intended to be
an exhaustive list.
5.1 Better than Best-Effort (BBE) Service
This is a qualitative service which promises to carry specific web
server traffic at a higher priority than competing best-effort
traffic. Such a service offers relatively loose (not quantifiable)
performance from a given ingress point to any egress point. Such a
service is suitable for example for businesses offering access to
web based content. The BBE service enables the web content provider
to provide content at a generally higher rate than other content
providers are able to, in so reducing the latency experienced by
consumers of the web site.
5.1.1 Service Implementation
In this example, we assume that there is an SLA which defines the
service at the customer's ingress point. This is the point at which
the customer injects web server responses into the differentiated
services network. The information in the TCA can be represented in
the following form [AF]:
AF13 Mark : 1 Mbps : Any egress point : Excess traffic handled by
marking with AF11 mark.
Packets submitted for the BBE service should be marked with the DS-
field codepoint corresponding to the AF13 PHB. The provider is
promising to carry up to 1 Mbps of traffic from the ingress point to
any egress point at a higher priority than best-effort traffic. A
lesser class of service corresponding to the AF11 PHB will be
applied to traffic submitted for the AF13 PHB, in excess of 1 Mbps.
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The provider will provision a policer at the ingress point. Traffic
submitted up to the 1 Mbps limit will be directed to the AF13 PHB.
Traffic submitted in excess of 1 Mbps will be remarked for the AF11
PHB. Note that the scheme will preserve ordering of packets since
AF13 and AF11 use a single queue..
In order to provide this service, the provider will have to
implement the AF13 and AF11 PHBs in core network equipment. The AF13
and AF11 PHBs can be implemented for example, using a single RIO
queue. The provider will also have to provision equipment within the
core of the provider's network to provide the AF13/AF11 service. By
provisioning the appropriate RED parameters, for example, the
provider is able to control the priority of AF13 traffic relative to
AF11 traffic at each network node. Since there are no quantitative
guarantees, the provider can be quite liberal in its provisioning
strategy and may realize significant statistical multiplexing gains.
Also, the absence of quantitative guarantees makes it easy to
provide this type of service across multiple DS provider domains.
This is because is not necessary to negotiate, then provision and
enforce quantitative guarantees at multiple boundaries.
5.2 Leased Line Emulation Service
This is a quantitative service which emulates traditional leased
line service. As such, it promises to deliver customer traffic with
very low latency and very low drop probability, up to a negotiated
rate. Above this rate, traffic is dropped. Such a service is
typically offered between two specific points. It is suitable for
many customer applications. However, due to the high quality
guarantees, it is likely to be priced higher than alternate services
and therefore, to be used only for applications which really require
this type of service. An example of such an application is IP
telephony. A corporate customer might purchase leased line emulation
service between each pair of a number of corporate network sites.
5.2.1 Service Implementation
In this example, we consider a customer with three geographically
dispersed networks interconnected via a single provider network.
Customer attachment points are represented as A, B and C. At each
attachment point, an SLA describes the leased line service to be
provided to the other two points. The table below represents the
information required in the TCA at attachment point A:
EF-Mark : 100 Kbps : Egress point B : Discard non-conforming traffic
EF-Mark : 50 Kbps : Egress point C : Discard non-conforming traffic
Packets submitted for leased line service should be marked with the
DS-field codepoint corresponding to the EF PHB [EF]. From the
ingress point A, to the egress point B, the provider is promising to
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carry up to 100 Kbps of traffic. Excess traffic will be discarded.
From ingress point A, to egress point C, the provider promises to
carry 50 Kbps of traffic. Of course, there is some tolerance
required in policing the traffic and thus, there may be a
specification of tolerated jitter or burst size. However, for a
leased line service, the primary traffic profile parameter would be
the sustained traffic rate.
The provider will provision a policer at ingress point A to limit
traffic destined for egress point B, to 100 Kbps. Similarly, a
policer will be configured to limit traffic destined for egress
point C, to 50 Kbps. These policers will require classification
based on the DS-Mark and the destination address in each packet.
In order to provide this service, the provider will have to
implement the EF PHB in core network equipment. The EF PHB can be
implemented using strict priority queuing or alternatively, by
assigning EF marked packets to a heavily weighted queue in a WFQ
scheme. The provider will have to provision equipment within the
core of the provider's network. For example, routers carrying
traffic between point A and points B and/or C will have to be
provisioned considering the resources committed by the TCA at point
A. This means that a router which is both in the path from A to B
and from A to C, will have to be considered to have committed 150
Kbps of bandwidth as a result of the TCA in place at A. A router
that is only in the path from A to B, will have to be considered to
have committed 100 Kbps as a result of this TCA, and so on. Of
course, routing is subject to change and so, failover paths may have
to be provisioned as well. These may be provisioned to provide some
fraction of the service in the case of failover or alternatively,
the SLA at point A might reflect appropriate service availability
parameters. To enhance the assurances offered by EF service,
providers may employ route pinning mechanisms or QoS routing
mechanisms.
5.3 Quantitative Assured Media Playback Service
This service offers looser assurances than the leased line service
described above, but is still considered a quantitative service. In
particular, it promises to deliver traffic with a high degree of
reliability and with variable but bounded latency, up to a
negotiated rate. Above this rate, traffic is subject to significant
delay or drop. Such a service is typically offered between a
specific set of points. It is suitable for many customer
applications. It would likely be priced lower than a leased line
service, due to the latency variability. However, due to the latency
bound and high degree of delivery, it is likely to be priced higher
than alternate services. This service is particularly suitable for
video or audio playback, in which considerable bandwidth is required
on a continual basis, but the non-interactive nature of the traffic
makes it somewhat delay tolerant.
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5.3.1 Service Implementation
In this example, we again consider a customer with three
geographically dispersed networks interconnected via a single
provider network. The table below represents the information
required in the TCA at attachment point A:
AF13-Mark : 100 Kbps sustained, 100 Kb bursts tolerated at up to 200
Kbps : Egress point B : Excess burst traffic over sustained rate
marked with AF12-mark : Non-conforming traffic marked with AF11-mark
: Max latency = 1 second
AF13-Mark : 50 Kbps sustained, 100 Kb bursts tolerated at up to 100
Kbps : Egress point C : Excess burst traffic over sustained rate
marked with AF12-mark : Non-conforming traffic marked with AF11-mark
: Max latency = 2 seconds
Packets submitted for the assured playback service should be marked
with the DS-field codepoint corresponding to the AF13 PHB. From the
ingress point A, to the egress point B, the provider is promising to
carry up to 100 Kbps of sustained traffic with bursts of 100 Kb in
size at a peak rate of 200 Kbps. Excess burst traffic will be marked
with the codepoint for AF12 and out of profile traffic will be
carried but with the AF13 codepoint. So long as these conditions are
met, latency will be limited to 1 second. Note that for this
service, the traffic profile is described using a full set of token
bucket parameters. Since the latency bounds for such a service are
less strict than those required for the leased line service, a
certain degree of traffic burstiness can be tolerated.
The provider must support the AF11, AF12 and AF13 PHBs in core
network routers. These PHBs might be provided, for example, by
assigning AF11, AF12 and AF13 marked traffic to a single RIO queue
with high drop thresholds. The policers at the edge would limit
competing traffic in line with the TCA, in order to assure that the
latency bounds can be met. In addition, the service provider will
have to provision devices in the core of the network. The
provisioning considerations discussed in the context of the leased
line service apply here as well, however, in general, the service
provider has the liberty of being less conservative in provisioning
and realizing better statistical gains.
5.4 Superposition of Quantitative and Qualitative Services in the Same
Network
A compelling model would provide both quantitative and qualitative
services in the same differentiated service network(s) as follows. A
number of corporate campus networks would be interconnected by a
differentiated service network providing quantitative services
between the sites. For example, a mesh of leased line services would
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enable IP telephony between the sites. A mesh of media playback
service using the AF11 PHB would enable audio/video playback between
the sites. In addition, each corporate site would be allotted some
level of BBE service to arbitrary destinations. In this model, the
differentiated service network is effectively providing a mesh of
quantitative services between fixed locations (similar to a VPN).
This mesh is superimposed on a cloud supporting BBE service.
6. Provisioning and Configuration
The provision of differentiated services requires careful network
wide provisioning and configuration. Provisioning refers to the
determination and allocation of the resources needed at various
points in the network. Provisioning may dictate the addition or
removal of physical resources at various points (physical
provisioning). Provisioning may also dictate the modification of
operating parameters within existing physical network equipment to
alter the relative share of the equipment's resources which are
allotted to one or another class of traffic (logical provisioning).
Configuration refers to the distribution of the appropriate
operating parameters to network equipment to realize the
provisioning objectives.
In Secs. 4.6 and 4.7, we briefly discussed provisioning and
configuration requirements both at the network boundaries and in the
network interior. In this section we will focus primarily on the
coordination of provisioning and configuration throughout the
network, such that end-to-end services can be provided reliably. We
will discuss the roles of protocols such as SNMP, CLI, RSVP, COPS
and LDAP in the provisioning process.
6.1 Boundary vs. Interior Provisioning and Configuration
For the sake of brevity, consider the term 'provisioning' to refer
both to provisioning and configuration, except where otherwise
noted. It is helpful to consider provisioning at the network
boundaries, separately from provisioning of the interior. Since the
differentiated service provider is selling a contract (SLA) at the
network boundary, we can consider the boundary provisioning which
supports SLAs, to drive the interior provisioning. The two are not
entirely separable in that each affects the other. For example, a
network operator cannot offer an SLA which cannot be met by the
resources available in the interior of the network. In general, the
overall provisioning process iterates between the boundaries and the
interior. From here on we will refer to provisioning with respect to
the TCA rather than the SLA, since the TCA is the component of the
SLA that defines detailed traffic handling parameters.
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6.1.1 Boundary Provisioning
Boundary provisioning was considered briefly in Sec. 2.6. We
discussed the minimal provisioning that a provider must implement to
enforce a TCA. We also discussed additional configuration which a
provider may use to provide additional (especially per-flow)
services to a customer. The latter is not actually related to the
provisioning of resources within the differentiated services
network, but rather assists the customer by determining which
subsets of the customer's traffic make use of the resources
provisioned within the differentiated services network. As such, it
is out of the scope of this section. Here, we consider only the
minimal provisioning required at the boundary.
At a minimum, the provider must assure that sufficient physical
resources are provisioned at the boundary to meet the requirements
of the TCA. For example, if the sum of the profiles supported at a
particular ingress point would allow 10 Mbps of traffic to be
supported, it is unacceptable to provision a T-1 access link. A T-3
however, would be sufficient. Once the physical provisioning is
implemented, it is necessary to apply the appropriate logical
provisioning. This is achieved by configuring policers which limit
the amount of traffic accepted from the T-3 access link, at each
service level. It may also be necessary to set up the amount of
buffering available for the queues used for the service. Similar
provisioning is also appropriate at each egress point if the
aggregate of profiles provisioned to the egress exceeds the capacity
of the output link.
6.1.2 Interior Provisioning
For the purpose of provisioning the interior of the network, it is
desirable to understand or to control the volume of traffic of each
class which traverses each network node. The greater this
understanding, the more efficiently the network can be provisioned
while still meeting the requirements of the TCAs. It is feasible to
understand the volume of traffic traversing each node if this
traffic is admitted according to a TCA which dictates egress point
as well as ingress point. (This case generally applies to
quantitative services and was discussed in the context of the EF PHB
and the leased line service in Sec. 3.2.1). While traffic volumes
cannot be anticipated with 100% accuracy, it is possible to
approximate them quite well, especially with the help of route
pinning mechanisms. It is therefore possible to provision the
network reasonably accurately for traffic submitted for quantitative
services. Although such provisioning may be quite difficult in a
large network, it is nonetheless a tractable problem. We will refer
to this component of the provisioning problem as quantitative
provisioning.
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On the other hand, many (if not most) of the services offered by
differentiated service networks will not specify egress points (as
is the case for qualitative services) and will not restrict
submitted traffic to specific egress points, let alone specific
routes. Thus, interior nodes will have to be provisioned without an
a-priori understanding of the volume of traffic submitted for
qualitative services which will arrive at each node. It is necessary
to be able to provision differentiated service networks to support
both quantitative services with specific egress points as well as
qualitative services, which do not have specific egress points on
the same physical resources. To this end, it is necessary to isolate
the impact of qualitative traffic on the resources reserved for
quantitative traffic. This can only be achieved if the former is
treated with lower priority than the latter. Thus, in general,
resources will have to be provisioned first for quantitative
traffic, using quantitative provisioning mechanisms. Then,
qualitative provisioning can be used to allocate remaining resources
to qualitative traffic. Qualitative provisioning can also be
applied to services which offer a relative quantification of traffic
volumes.
The impact of the two types of traffic will have to be isolated by
ensuring that they do not share PHB codepoints. PHBs used for
quantitative services will always have higher priority access to
resources than those used for qualitative services. As a result, it
is necessary to carefully police traffic submitted for quantitative
PHBs. Failure to do so can result in the starvation of lower
priority traffic. In general it can be expected that only a small
fraction of the resources at each node will be provisioned for
quantitative traffic.
Similarly, a significant fraction of the traffic capacity should
remain for best-efforts service to provide a 'soft' target for
traffic dropping if congestion occurs or it is necessary to redirect
non-best efforts traffic in the event of failure.
6.1.2.1 Quantitative Provisioning of the Interior
As discussed previously, quantitative provisioning is a difficult
but tractable problem. With knowledge of the network routing
topology and the TCAs at the boundaries, it is possible to compute
the resources required at each interior node to carry the
quantitative traffic offered at the edges. Based on the results of
this computation, interior nodes must be provisioned and configured
with sufficient capacity to accommodate the quantitative traffic
which will arrive at the node, while leaving sufficient capacity
remaining to accommodate some amount of qualitative traffic.
The provisioning mechanism described assumes a top-down approach, in
which the network administrator studies the network topology and
traffic routing and computes the provisioning requirements. An
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alternative approach uses signalling to automate the process [MPLS].
For example, RSVP messages could be launched along the paths that
will be followed by submitted quantitative traffic. If a TCA calls
for 100 Kbps of leased-line service from ingress point A to egress
point B, an RSVP message could be transmitted from point A towards
point B, with a flowspec specifying 100 Kbps. This message would
traverse each node at which resources would have to be committed.
Conventional RSVP routers would install a reservation. In a
differentiated service network, RSVP could be adapted to provision
the resources required per the differentiated services model. In a
network which offers a number of static TCAs, such RSVP messages
could be launched from the TCA ingress point at the time the TCA is
initially instantiated with the effect of instantiating the
appropriate cumulative provisioning in routers along the various
routes. The advantage of this approach is that it does not require
explicit knowledge of the network topology. We will revisit these
two approaches to quantitative provisioning of the interior in a
later section.
Once the resources required for quantitative traffic at each node
have been determined, provisioning of the node consists of
installing or configuring interfaces of the appropriate capacity to
easily accommodate the quantitative traffic that will traverse the
node. Note that we do not state the precise meaning of 'to easily
accommodate'. A number of factors must be considered when
determining the appropriate capacity, given a certain volume of
predicted quantitative traffic. These include:
1. Margin of error
2. Statistical gain desired
3. Capacity remaining for qualitative (including best efforts)
traffic
The first, margin of error, accommodates mistakes in computation,
effects of transient route changes which are not otherwise accounted
for, effects of traffic clustering as it moves through the network
and so on. The statistical gain desired refers to the degree to
which a provider is willing to gamble that not all sources of
quantitative traffic will be simultaneously active at the limit
dictated by the TCAs at the ingress points (vs. the penalty the
provider would be willing to pay in terms of refunded charges or
lost customers). Finally, the provider must determine how much
capacity will be reserved for qualitative traffic at each node.
Thus, if it is determined that 1 Mbps of quantitative traffic might
traverse a specific node in a specific direction, the provider might
install a 10 Mbps interface in the node, to serve the corresponding
traffic direction. This would leave 9 Mbps of capacity quite safely
for qualitative traffic. In this case, the provider would be
assuming that statistical gains which might be realized will be used
to offset the margin of error which would compromise the resources
available.
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In addition to installing or configuring the appropriate capacity at
each interface, it may be desirable to configure policers to assure
that the resources actually consumed by the higher priority
quantitative traffic do not exceed expectations. This is especially
important if the provider is attempting to achieve a high degree of
statistical gain or has not allowed for a reasonable margin of
error. Policers need not be configured at each interior node, but
should probably be configured at certain key nodes. It may also be
necessary to configure the internal resources of the router (queues
and buffers) to deliver the services required.
6.1.2.2 Qualitative Provisioning of the Interior
As explained previously, it is necessary first to determine the
resources which must be provisioned at each node for quantitative
traffic. Once these have been determined, interfaces must be
installed or provisioned to accommodate the required resources while
leaving sufficient capacity for qualitative traffic. In order to do
so, it is necessary to determine the resources required at the node
for qualitative traffic. Since qualitative traffic cannot be assumed
to follow specific routes with the same degree of predictability as
quantitative traffic, this provisioning problem is far more
difficult and provisioning parameters must be estimated based on
heuristics, experience and possibly on real time measurement.
Once physical interfaces have been selected to accommodate the
resources required by the computed quantitative traffic load and the
estimated qualitative traffic load, additional configuration is
required to support qualitative traffic. Such configuration amounts
to the selection of relative weights for queues for different
service levels (in a WFQ scheme), or the selection of RIO or RED
thresholds or alternate logical resource provisioning parameters. It
is assumed that if quantitative traffic is accommodated via similar
queuing mechanisms (as opposed to strict priority queuing), that the
weighting parameters chosen for quantitative traffic isolate it
effectively from the effects of qualitative traffic. However, the
configuration parameters which differentiate the various qualitative
services may not provide such a degree of isolation among the
qualitative services. Thus, it may be necessary to attempt to
estimate the relative traffic arriving for each qualitative service
and to anticipate the interaction between traffic of different
qualitative services. It may be impossible to both efficiently and
conservatively provision a network for certain combinations of
qualitative services. To aid in the provisioning of a network for
qualitative services, it may be useful to configure policers to
control the volume of traffic arriving at a given node. However,
such policing might have to be restricted to shaping (rather than
discarding) in order to avoid violating TCAs in place at the network
boundaries.
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6.2. Static vs. Dynamic Provisioning
So far, we have considered static provisioning techniques. Even the
example of RSVP usage for provisioning assumed that the RSVP
messages were launched at the time a TCA was instantiated as opposed
to dynamically. In the case that TCAs are static, static
provisioning is adequate for quantitative traffic. However, since
qualitative traffic [e.b.1] offers less predictable patterns, it is
likely to cause traffic volumes at different nodes in the network to
change dynamically, even when the TCA is static. For this reason,
dynamic provisioning techniques are desirable and may assist the
service provider in making better use of network resources. In
addition, dynamic provisioning may enable the service provider to
provision more liberally for quantitative services, realizing
statistical gains. If we consider further, that it may be desirable
to provide dynamically changing TCAs, then the appeal of dynamic
provisioning techniques is even stronger.
Dynamic provisioning may be signalling based, measurement based or
both. For example, a conventional RSVP router supports signalling
based dynamic provisioning. Hosts signal the router to request more
or less resources and the router adjusts accordingly. The host may
or may not actually submit traffic at the rate at which it signalled
it would, but regardless, the resources are committed in case it
does. Measurement based provisioning would adjust the resources
committed in response to the traffic loads actually measured at the
device. While differentiated services does not specify any form of
signalled or measurement based provisioning, both may be useful.
6.3 Distributing Configuration Information
The process of physical provisioning is by necessity relatively
static and cannot be automated since it requires installation of
physical equipment. However, logical provisioning and configuration
can and should be automated to the degree possible. In this section,
we look at techniques for distributing configuration information.
6.3.1 Top Down Distribution of Configuration Information
In the simplest case, TCAs are static and both the boundaries and
interior of the network are provisioned statically by 'pushing'
configuration information down to the appropriate network nodes.
Configuration of boundary nodes requires primarily the pushing of
policing information to enforce the TCAs in place. (Additional fine
grain information may be pushed to provide traffic separation
services on behalf of the customer, but these are not addressed in
this context). Configuration information for boundary nodes is
determined at the time the TCA is negotiated. At this time, the
nodes are configured by the provider. The network administrator may
use one of several protocols to do so, including for example SNMP or
CLI.
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In order to accommodate the traffic submitted by the provisioning of
new TCAs, it is necessary to provision the interior of the network.
As discussed previously, it is possible to compute the resources
required for quantitative traffic. Assuming that sufficient physical
capacity has been provisioned, configuration amounts to logically
provisioning sufficient capacity at each interior interface and to
configuring policers for the quantitative traffic at various
interior nodes. In addition, qualitative provisioning requires the
configuration of queues, WFQ weights and/or RIO parameters at
various interior nodes, and may also include the configuration of
some number of policers. In the case, of static, top down
configuration, interior configuration information is also pushed
down via a configuration protocol such as SNMP or CLI.
The difficulty of such top down provisioning is that it requires the
network administrator to coordinate the provisioning of each network
node, at boundaries as well as in the interior, such that the
network is provisioned end-to-end in a consistent manner and is able
to efficiently deliver the services promised by the TCAs. In order
to assist the network administrator in this task, it is useful to
consider a database which holds both current topology information as
well as the current TCAs instantiated at the network boundaries.
This information is stored in a format dictated by a standard schema
as suggested in [Elleson].
Of course, the database is ideally maintained in a way which is
logically centralized (for ease of programming and modifying) but is
physically distributed (for the sake of robustness and fault
tolerance). Policy servers may be used to extract information from
the database and to convert it to configuration information which is
pushed down to individual nodes. In this scenario, policy servers
would likely use a directory access protocol such as LDAP to
retrieve information from the directory and would use a
configuration protocol such as SNMP or CLI to push the configuration
information down to the network nodes. Note that in this example,
the policy servers and the directory schemas are in effect
fulfilling the role of bandwidth broker [BB]. In particular, the
policy servers use an awareness of the network topology to provision
interior nodes such that certain end-to-end QoS routes can be
constructed and assurances implied by the TCAs at the boundaries can
be delivered.
6.3.2 Distribution of Configuration Information via Signalling
An alternate mechanism of distributing configuration information is
via signalling messages transmitted between boundary nodes of the
same differentiated service domain (intra-domain signalling). It is
also interesting to consider inter-domain signalling, but this will
be addressed separately. An example of such signalling was described
previously, in the usage of a modified form of RSVP. Such signalling
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is particularly useful for the purpose of installing configuration
information for quantitative services which affect specific paths
and is somewhat less useful (though not useless) for the purpose of
configuring qualitative services. It is likely that such a
signalling approach would be used in conjunction with top down
provisioning. For example, the directory schema might dictate the
amount of resources to be available for high priority quantitative
services at each node. These limits might be pushed down to
individual nodes a- priori. Signalling from the network boundaries,
at TCA instantiation time, would then be used to claim resources
from the pool of quantitative resources available at each node.
Alternatively, nodes might consult policy servers as the signalling
resource requests arrive at each node. The latter model is similar
to the use of per- flow RSVP signalling and PEP/PDP policy usage in
traditional RSVP networks. Qualitative configuration information
would still be pushed in a top down manner. The advantage of the
latter model is that policy servers would be dynamically updated
with information regarding the current usage of network resources.
In this model, it is likely that a variant of COPS would be used to
communicate between network nodes and the policy servers. Note that
COPS may be used for distribution of top down configuration
information as well, though it is not specifically designed for this
purpose.
One of the advantages of configuration via signalling, is that it
facilitates the support of dynamic TCAs. TCAs could be dynamically
renegotiated using inter-domain signalling. Such renegotiation would
require dynamically modifying the provisioning within the affected
domain, a process which requires some automated signalling protocol
such as an aggregated form of RSVP signalling between boundary nodes
in a provider's domain. This protocol would in effect, represent a
distributed bandwidth broker [BB] for the domain.
6.3.3 Modification of Configuration Information Based on Real-Time
Measurement
A third mechanism for the configuration of interior nodes would be
based on measurement of current traffic loads at key network nodes.
Measurement based configuration is less necessary for quantitative
provisioning, since quantitative traffic patterns are relatively
predictable. However, it can significantly enhance the efficiency
with which qualitative provisioning can be achieved. For example,
network nodes may feed policy servers with current qualitative
traffic load measurements. In response, bandwidth brokers and policy
servers might recompute the relative weights for different service
queues in a WFQ node and push the new configuration information to
the routers. It is likely that measurement based configuration for
qualitative services would be used in conjunction with signalling
based configuration for quantitative services.
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7. Inter-Domain Considerations and End-to-End Services
So far we have considered differentiated service primarily in the
context of a single DS domain providing service to a single
customer. The ultimate customers of the differentiated service
network are hosts and end users residing on peripheral stub
networks. In general, these are interconnected by multiple domains
and require service which spans these domains. Therefore, it is
important to consider the interaction of services provided by a
concatenation of differentiated service domains and the peripheral
stub networks, rather than the service provided by a single domain.
In this section, we discus inter-domain issues related to TCAs,
provisioning and service and PHB mapping.
7.1 TCAs
Each service provider is expected to negotiate bilateral agreements
at each boundary node at which it connects to an adjacent provider's
network. Such agreements are captured in the form of two TCAs, one
governing the services provided to provider A's traffic by provider
B and the other governing the services provided to provider B's
traffic by provider A. Note that provider A serves as a provider to
provider B with respect to traffic flowing from provider B to
provider A. On the other hand provider A is a customer of provider B
with respect to traffic flowing from provider A to provider B. The
two TCAs can be considered separately.
In general, the TCAs negotiated by a provider at any boundary will
be dictated by TCAs negotiated at other boundaries. For example,
assume that provider A offers leased line service to a customer with
an ingress point in provider A's domain, but an egress point in
provider B's domain. In this case, it is necessary that the TCA
between provider A and provider B be sufficient to accommodate the
assurance made by provider A to its leased line service customer.
Provider A may serve a number of customers with leased line services
terminating at various boundary points in provider B's network.
Thus, the TCA between provider A and provider B must represent the
aggregate requirements of the TCAs of all of provider A's customers.
7.2 Inter-Domain Provisioning
The inter-domain provisioning problem is not unlike the intra-
domain provisioning problem. The provider would generally begin by
evaluating the TCAs it has negotiated with its customers, and then
computing the impact of each of these TCAs on the TCAs it has
negotiated with its providers. For quantitative services, the
provider can compute the quantitative requirements of TCAs at each
of its provider's boundary nodes, as described above in the context
of the leased line service. For qualitative services, the process of
determining the requirements from its providers is fuzzier, since
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the volume of qualitative traffic expected to be carried through any
boundary is less deterministic.
In the simplest case, provisioning is based on static TCAs. In this
case, provisioning is an iterative process in which providers
negotiate TCAs with each of their customers, then apply the
appropriate internal provisioning techniques to meet these
requirements. In the process of internal provisioning, a provider
might determine that a particular TCA cannot be met due to internal
resource constraints. The provider would then either have to add
internal resources or renegotiate one or more customer TCAs.
Although the process may be somewhat iterative, it is relatively
static in that changes in boundary TCAs and internal provisioning
occur relatively infrequently (on the order of hours, days or
months) and require human intervention.
Internal provisioning to meet the requirements of TCAs relies on
provisioning techniques described previously. As TCAs are
negotiated, the provider must check that the existing internal
provisioning is sufficient to meet the requirements of the new TCA,
or must alter the internal provisioning. Recall that internal
provisioning might be pushed in a top down manner, from a domain's
logically centralized point of administration, or alternatively
might be distributed from the edges via signalling. In the former
case, some form of a bandwidth broker would be directly consulted or
notified regarding changes in TCAs negotiated at the domain
boundaries. In the case that signalling is used, provisioning
messages (such as described previously) would be launched from the
boundary at which the new TCA is negotiated. These would claim a
share of existing provisioned resources, or would notify the
bandwidth broker in the case that additional resources are required.
A more sophisticated model would allow TCAs to be renegotiated
dynamically. In this case, the process would be automatic, and would
not require human intervention. Each domain would in effect,
represent a bandwidth broker, via one protocol or another. A
specific inter-domain protocol might be used to communicate between
centralized bandwidth broker agents, or alternatively, an inter-
domain variant of RSVP might be used. In the latter case, there is
no direct interaction with a bandwidth broker per-se. However, the
collection of network nodes, policy servers and directory behave
collectively as a bandwidth broker which communicates using RSVP. In
either case, TCA renegotiations would be triggered by load
measurements at boundary nodes. These could be in the form of
changes in actual measured traffic volume, or alternatively, based
on explicit fine grain RSVP resource requests from hosts at the
periphery. Domains would approve renegotiations based both on
resource constraints as well as predetermined policy constraints.
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7.3 Service, PHB and Codepoint Mapping
In order to provide end-to-end service to customers, it must be
possible to extend services across multiple domains. Several
complexities may arise at inter-domain boundaries, as follows:
1. The services provided by a certain domain may not be compatible
with the services provided by a neighbour domain.
2. The services provided by a certain domain may be compatible with
those provided by the neighbour domain, but the PHB used to
obtain the service might be different.
3. The PHB might be the same, but the codepoint used to request the
PHB might be different.
4. The PHB and codepoint are the same but differences in
provisioning and charging models results in different services.
Resolution of these complexities requires determination of the
compatible services and negotiation of the PHB codepoints which will
be used to request the services. This process is greatly simplified
by the provision of a set of universal services using universally
recognized codepoints. The leased line service and EF codepoint is
likely to be one such example. Generally, extension of quantitative
services across multiple domains will require more uniformity in the
nature of the services provided. Qualitative services on the other
hand, may be extended end-to-end by a concatenation of services
which vary from domain to domain. For example, one domain may base a
qualitative service on a WFQ scheme with RED while another may use
priority queuing with RIO. Since the assurances provided by
qualitative services tend to be looser, it is possible that a
meaningful service can be provided end-to-end by concatenating these
two service types.
7.4 Host-Domain Boundaries
In certain cases, a host may be directly attached to a
differentiated service domain. This is likely both in the case of
campus networks that provide differentiated services within the
network or in the case of dial-up users connecting to a
differentiated service provider. In these cases, the host can be
considered the customer of the differentiated service network.
Legacy hosts are unlikely to mark their own packets for the
appropriate DS-field and are also unlikely to shape or police their
traffic. In the case of legacy hosts, the differentiated service
provider will have to provide these services on behalf of the
customer. In the case of campus networks, some network wide policy
would likely be used to configure these services in the DS boundary
devices. In the case of dial-up hosts, marking, shaping and
resources provided would likely be negotiated at the time the
customer signs up with the provider.
Newer hosts may be capable both of marking and of traffic shaping.
In this case, the overall per-host resource constraints are still
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likely to be somewhat static. However, the manner in which the host
shares these resources among its various traffic flows is determined
by the host. Of course, the provider will have to configure policers
to assure that the host does not seize more than its share of
resources in the differentiated service network.
8. Inter-operability with RSVP/Integrated Services
In this section, we discuss alternatives for inter-operability
between differentiated services and RSVP/Integrated services.
8.1 RSVP/Integrated Services Over Differentiated Services
This scenario is discussed in detail in [E2EQOS]. It assumes a model
in which peripheral stub networks are RSVP and Intserv aware. These
are interconnected by differentiated service networks. In this
model, the scalability of differentiated service networks helps to
extend the reach of RSVP/Integrated service (Intserv)networks.
Intervening differentiated service networks appear as a single RSVP
hop to the RSVP/Intserv networks. Hosts attached to the peripheral
RSVP/Intserv networks signal to each other for per-flow resource
requests across the differentiated service networks. Standard
RSVP/Intserv processing is applied within the RSVP/Intserv
peripheral networks. RSVP signalling messages are carried
transparently through the differentiated service networks. Devices
at the boundaries between the RSVP/Intserv networks and the
differentiated service networks process the RSVP messages and
provide admission control based on the availability of appropriate
resources within the differentiated service network.
This model is predicated on the availability of services within the
differentiated service network which can extend the reach of intserv
type services. For example, the leased line service can extend the
intserv guaranteed service across a differentiated service network.
Multiple guaranteed service micro-flows which exist in peripheral
networks are aggregated into the EF behaviour aggregate at the edge
of the diffserv network. When an RSVP request for guaranteed service
arrives at the edge of a differentiated service network, RSVP style
admission control is applied based on the amount of resources
requested in the intserv flowspec and the availability of
differentiated services at the corresponding service level (per the
TCA). If admission control succeeds, the originating host (or its
agent) marks traffic on the signalled microflow, for the appropriate
differentiated service level.
The RSVP/Intserv over differentiated service model is especially
suitable for providing quantitative end-to-end services. The use of
differentiated services eliminates the scalability concerns of
RSVP/Intserv networks. The use of RSVP signalling provides admission
control to the differentiated service network, based on resource
availability and policy decisions. It also greatly simplifies the
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configuration of differentiated service classifiers, policers and
other traffic conditioning components.
Variations on this theme would enable some number of nodes within
the differentiated service networks to process the per-flow RSVP
messages passing through. These could be used to aid in dynamic
provisioning without necessarily requiring any per-flow state or
processing within the differentiated service network. In yet another
model, the transition of per-flow RSVP messages through the
differentiated service network might trigger aggregated RSVP
signalling between differentiated service domain edges, for the
purpose of renegotiating TCAs and adjusting provisioning dynamically
[GBH97, CLASSY].
8.2 Parallel Operation
Another alternative for the interoperation of differentiated service
and RSVP/Intserv networks is simple parallel operation. In this
mode, each node within the differentiated service network may also
be an RSVP capable node. Some strategy would have to be selected for
determining which packets are handled using RSVP and which are
handled using differentiated services. For example, those that
classify to an RSVP installed filter might be handled using RSVP,
while those not classifying to specific RSVP filters would be
handled according to the DS-field using differentiated service
mechanisms. Such a model is likely to be deployed in smaller
networks (since the RSVP/Intserv component is less suited for large
networks). In particular, the stub networks cited in [E2EQOS] would
likely provide differentiated services for those qualitative
applications which do not signal, while providing RSVP/Intserv
services for those quantitative applications which do signal.
9. Multicast Services
Because the Differentiated Services Architecture deals only with
unidirectional flows, a 'multicast' service in a DS network will in
fact offer a point-to-multipoint unidirectional service. Each
source of traffic that wishes to send to the multicast group using
this service needs a separate SLA which applies at the ingress point
where the traffic enters the network.
The network resources that must be provisioned for a multicast
service will be affected by the mechanisms used by the routers to
provide the service. Depending on the capabilities of the routers
and the multicast routing protocol employed, sub-optimal replication
of a packet may result in multiple copies travelling over the same
link.
If receivers can be added dynamically to a multicast group whilst a
flow is in progress, the complexity of provisioning grows
considerably: The amount of network resources that will be consumed
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by multicast traffic originating from a particular upstream network
may be difficult to forecast in advance. Consequently, it may not
be possible to offer quantitative services where dynamic addition of
receivers adds to the paths through the network already used by the
flow.
9.1 Codepoints and PHBs for Multicast Services
To achieve resource isolation of multicast traffic from unicast
traffic, it may be necessary to use separate codepoints and separate
instances of a PHB or different PHBs for the multicast and unicast
services. If the multicast traffic is not adequately isolated,
dynamic addition of new members of the multicast group can adversely
affect existing unicast traffic.
Because a multicast service traffic flow can exit from a domain to
several peer domains, care must be taken to use a codepoint and PHB
that is compatible with the peering SLAs at the egress points. This
may be a more stringent requirement than for a unicast service where
a flow need only be compatible with a single egress point SLA.
9.2 Provisioning Multicast Services
The scope of a multicast service would normally be either case 1
(any egress point) or case 3 (a pre-defined set of egress points) of
Sec. 4.4.
For a quantitative service the scope will, in general, need to be
case 3. The service can be provisioned in a similar way to
corresponding unicast services with the same volume of traffic along
each of the paths from ingress to egress, but taking into account
that all paths will be used simultaneously and allowing for multiple
copies of traffic if necessary. If the multicast routing protocol
used can generate different multicast trees depending on the order
in which members join the group, provisioning may not be possible.
Solving this problem may require pinning of the multicast tree
branch points; the solution of this problem is outside the scope of
this framework.
For a qualitative service, provisioning is essentially the same as
the unicast case, but statistical multiplexing gains are likely to
be less because all paths may be used at once.
The traffic conditioning mechanisms for multicast services are not
significantly different from those for the unicast services but
multiple shapers may be required where traffic exits from several
interfaces on a single router or multiple replicas exit from one
interface.
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10. Security and Tunnelling Considerations
The security and tunnelling implications for the actual data
transport of the services of the Differentiated Services
Architecture have been extensively discussed in {DSARCH] and
[DSHEAD] to which the reader is referred.
Additional security considerations arise from the services overlaid
on the data transport:
1. The services are the subject of differential charging.
Accordingly, the service users have to be authenticated and
authorised, and the accounting data needed must be secured.
2. The mechanisms used to create and distribute the policy and
resource allocations must be secured.
3. Statistical data needed to audit service delivery must be
secured.
The mechanisms used to provide this security are outside the scope
of this framework, but are under consideration by the AAA working
group.
The use of tunnels in general and IPsec tunnels in particular
impedes the work of MF Classifiers by concealing the fields used by
L4 and higher layer classifiers. Thus traffic conditioners within
the area where IPsec encryption is used will need to rely only on IP
header fields, including the DS field (BA Classifiers will work
normally). If more sophisticated Mf classification is required it
will have to take place before the tunnel ingress and the
application of IPsec encryption. If IPsec encryption is used end-
to-end, then Differentiated Services may require host marking.
If a tunnel carries multiple flows with different traffic types,
they may be marked with different DS codepoints so that they are
subjected to appropriate behaviors in the network interior. This
may be considered to be a security breach as it allows traffic
patterns to become visible. If just one codepoint is used for all
traffic it should be selected carefully to be appropriate for all
the traffic in the tunnel.
11. Acknowledgements
The authors would like to acknowledge the helpful comments and
suggestions of the following individuals: Kathleen Nichols, Brian
Carpenter, David Black, Konstantinos Dovrolis, Shivkumar Kalyana,
Wu-chang Feng, Marty Borden, and Ronald Bonica.
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12. References
[BB] K. Nichols, V. Jacobson, and L. Zhang, "A Two-bit
Differentiated Services Architecture for the Internet", Internet
Draft
[CLARK] D. Clark and J. Wroclawski, "An Approach to Service
Allocation in the Internet", Internet Draft
[CLASSY] S. Berson and S. Vincent, "Aggregation of Internet
Integrated Services State", Internet Draft, November 1997.
[COPS] J. Boyle, R. Cohen, D. Durham, S. Herzog, R. Rajan, and A.
Sastry, "COPS (Common Open Policy Service) Protocol", March 1998.
[DSARCH] D. Black, S. Blake, M. Carlson, E. Davies, Z. Wang, and W.
Weiss, "An Architecture for Differentiated Services", Internet
Draft, May 1998.
[DSHEAD] K. Nichols and S. Blake, "Definition of the Differentiated
Services Field (DS Byte) in the IPv4 and IPv6 Headers", Internet
Draft, May 1998.
[AF] J.Heinanen, _Assured Forwarding PHB Group_Internet Draft,
August 1998.
[EF] V.Jacobson, _Expedited Fowarding Per Hop Behavior_, Internet
Draft, August 1998.
[Ellesson] E. Ellesson and S. Blake, "A Proposal for the Format and
Semantics of the TOS Byte and Traffic Class Byte in IPv4 and IPv6",
Internet Draft, November 1997.
[E2EQOS] Y. Bernet, R. Yavatkar, P. Ford, F. Baker, and L. Zhang,
"A Framework for End-to-End QoS Combining RSVP/Intserv and
Differentiated Services", Internet Draft, March 1998.
[GBH97] R. Guerin, S. Blake, and S. Herzog, "Aggregating RSVP- based
QoS Requests", Internet Draft, November 1997.
[IntServ] R. Braden, D. Clark, and S. Shenker, "Integrated Services
in the Internet Architecture: An Overview", Internet RFC 1633, July
1994.
[RSVP] B. Braden et. al., "Resource ReSerVation Protocol (RSVP) --
Version 1 Functional Specification", Internet RFC 2205, September
1997.
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11. Author's Addresses
Bernet, Yoram
Microsoft
One Microsoft Way
Redmond, WA 98052
Phone: +1 (425) 936-9568
Email: yoramb@microsoft.com
Binder, James
3Com Corp.
5400 Bayfront Plaza
Santa Clara, CA 95052
Phone: +1 (408) 326-6051
Email: james_binder@3com.com
Blake, Steven
Torrent Networking Technologies
3000 Aerial Center, Suite 140
Morrisville, NC 27560
Phone: +1-919-468-8466 x232
Fax: +1-919-468-0174
Email: slblake@torrentnet.com
Carlson, Mark
RedCape Software Inc.
2990 Center Green Court South
Boulder, CO 80301
Phone: +1 (303) 448-0048 x115
Email: mac@redcape.com
Davies, Elwyn
Nortel UK
London Road
Harlow, Essex CM17 9NA, UA
Phone: +44-1279-405498
Email: elwynd@nortel.co.uk
Ohlman, Borje
Ericsson Radio
Dialoggatan 1 (Kungens Kurva)
S-126 25 Stockholm
Sweden
Phone: +46-8-719 3187
Email: Borje.Ohlman@ericsson.com
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Draft-ietf-diffserv-framework-01.txt October, 1998
Srinivasan Keshav
4107B Uspon Hall
Cornell University
Ithaca, NY 14853
Phone: +607-255-5395
Email: skeshav@cs.cornell.edu
Dinesh Verma IBM T. J. Watson Research Center
P.O. Box 704
Yorktown Heights, NY 10598
Phone: +1 (914) 784-7466
Email: dverma@watson.ibm.com
Zheng Wang
Bell Labs Lucent Tech
101 Crawfords Corner Road
Holmdel, NJ 07733
Email: zhwang@bell-labs.com
Walter Weiss
Lucent Technologies
300 Baker Avenue, Suite 100 Concord, MA 01742-2168
Email: wweiss@lucent.com
Blake, et al Expires: April 1999 33
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