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Versions: 00 01 02 rfc2990                                              
Internet Architecture Board                                   G. Huston
Internet Draft                                                  Telstra
Document: draft-iab-qos-00.txt                               March 2000
Category: Informational


                 Next Steps for the IP QoS Architecture


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of
   six months and may be updated, replaced, or obsoleted by other
   documents at any time. It is inappropriate to use Internet- Drafts
   as reference material or to cite them other than as "work in
   progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


1. Abstract

   While there has been significant progress in the definition of IP
   QoS architecture, there are a number of aspects of QoS that appear
   to need further elaboration as they relate to translating a set of
   tools into a coherent platform for end-to-end service delivery. This
   document highlights the outstanding issues relating to the
   deployment and use of QoS mechanisms within the Internet, noting
   those areas where further standards work may be required.


2. Introduction

   The default service offering associated with the Internet is
   characterized as a best-effort variable service response. Within
   this service profile the network makes no attempt to actively
   differentiate its service response between the traffic streams
   generated by concurrent users of the network. As the load generated
   by the active traffic flows within the network varies, the network's
   best effort service response will also vary.

   The objective of various Internet Quality of Service (QoS) efforts
   is to augment this base service with a number of selectable service
   responses. These service responses may be distinguished from the

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   best-effort service by some form of superior service level, or they
   may be distinguished by providing a predictable service response
   which is unaffected by external conditions such as the number of
   concurrent traffic flows or their generated traffic load.

   Any network service response is an outcome of the resources
   available to service a load, and the level of the load itself. To
   offer such distinguished services there is not only a requirement to
   provide a differentiated service response within the network, there
   is also a requirement to control the service-qualified load admitted
   into the network, so that the resources allocated by the network to
   support a particular service response are capable of providing that
   response for the imposed load. This combination of load control
   elements and service management elements can be summarized as "rules
   plus behaviours".

   As a general observation of QoS architectures, the service load
   control aspect of QoS is perhaps the most troubling component of the
   architecture. While there are a wide array of well understood
   service response mechanisms that are available to IP networks,
   matching a set of such mechanisms within a controlled environment to
   respond to a set of service loads to achieve a completely consistent
   service response remains an area of weakness within existing IP QoS
   architectures. The control elements span a number of requirements,
   including end-to-end application signalling, end-to-network service
   signalling and resource management signalling to allow policy-based
   control of network resources.

   One way of implementing this control of imposed load to match the
   level of available resources is through service level negotiation.
   Here, the application first signals its service requirements to the
   network, and the network responds to this request. The application
   only proceeds if the network has indicated that it is able to carry
   the additional load at the requested service level. This can take
   the form of explicit negotiation and commitment, where there is a
   single negotiation phase, followed by a commitment to the service
   level on the part of the network. This approach is used by the
   Integrated Services architecture, where the application frames its
   service request within the resource reservation protocol (RSVP), and
   then passes this request into the network. The network can either
   respond positively in terms of its agreement to commit to this
   service profile, or it can reject the request. If the network
   commits to the request with a resource reservation, the application
   can then pass traffic into the network with the expectation that as
   long as the traffic remains within the traffic load profile that was
   originally associated with the request, the network will meet the
   requested service levels. There is no requirement for the
   application to reconfirm the service reservation itself, as the
   interaction between RSVP and the network constantly refresh the
   reservation while it remains active. The reservation remains in
   force until the application explicitly requests termination of the
   reservation, or the network signals to the application that it is
   unable to continue with a service commitment to the reservation. The

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   essential feature of this model is the "all or nothing" nature of
   the service model. Either the network commits to the reservation, in
   which case the application does not have to monitor the network's
   level of response to the service, or the network indicates that it
   cannot meet the reservation.

   An alternative approach to load control is to decouple the network
   load control function from the application. This is the basis of the
   Differentiated Services architecture. Here, the network implements a
   load control function as part of the function of admission of
   traffic into the network, admitting no more traffic within each
   service category as there are assumed to be resources in the network
   to deliver the intended service response. Necessarily there is some
   element of imprecision in this function given that traffic may take
   an arbitrary path through the network. In terms of the interaction
   between the network and the application, this takes the form of a
   service request without prior negotiation, where the application
   requests a particular service response by simply marking each packet
   with a code to indicate the desired service.


3. State and Stateless QoS

   These two approaches to load control can be characterized as state-
   based and stateless approaches respectively.

   In order for a resource reservation to be honored by the network,
   the network must maintain some form of remembered state to describe
   the resources that have been reserved, and the network path over
   which the reserved service will operate. This is to ensure integrity
   of the reservation. In addition, each active network element within
   the network path must maintain a local state that allows incoming IP
   packets to be correctly classified and then associated with an
   appropriate service response that is consistent with the end-to-end
   service reservation.

   In the second approach the packet is marked with a code to trigger
   the appropriate service response from every network element that
   handles the packet, so that there is no need to install a per-flow
   state on these network elements. Also,  the application is not
   required to provide the network with advance notice relating to the
   destination of the traffic, nor any indication of the intended
   traffic profile or the associated service profile, so any form of
   per-application or per-path resource reservation is not feasible. In
   this model there is no maintained per-flow state within the network.

   The state-based Integrated Services architectural model admits a
   greater level of accuracy, and a finer level of granularity on the
   part of the network to respond to service requests. Each service
   request can be used to generate a reservation state within the
   network that is intended to prevent the resources associated with
   the reservation to be pre-empted to service other reservations or to
   service best effort traffic loads. The state-based model is intended

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   to be exclusionary, where other traffic is displaced in order to
   meet the reservation's service targets.

   As noted in RFC2208 [2], there are several areas of concern about
   the deployment of this form of service architecture. With regard to
   concerns of per-flow service scalability, the resource requirements
   (computational processing and memory consumption) for running per-
   flow resource reservations on routers increase in direct proportion
   to the number of separate reservations that need to be accommodated.
   By the same token, router forwarding performance may be impacted
   adversely by the packet-classification and scheduling mechanisms
   intended to provide differentiated services for these resource-
   reserved flows. This service architecture also poses some challenges
   to the queuing mechanisms, where there is the requirement to
   allocate absolute levels of egress bandwidth to individual flows,
   while still supporting an unmanaged low priority best effort traffic
   class.

   The stateless approach to service management is more approximate in
   the nature of its outcomes. Here there is no explicit negotiation
   between the application's signaling of the service request and the
   network's capability to deliver a particular service response. If
   the network is incapable of meeting the service request, then the
   request simply will not be honored. In such a situation there is no
   requirement for the network to inform the application that the
   request cannot be honored, and it is left to the application to
   determine if the service has not been delivered. The major attribute
   of this approach is that it can possess excellent scaling
   properties. If the network is capable of supporting a limited number
   of discrete service responses, and the routers uses per-packet
   marking to trigger the service response, then the processor and
   memory requirements in each router do not increase in proportion to
   the level of traffic passed through the router.

   It is not intended to describe these service architectures in
   further detail within this document. The reader is referred to
   RFC1633 [3] for an overview of the Integrated Services Architecture
   (IntServ) and RFC2475 [4] for an overview of the Differentiated
   Services architecture (DiffServ).

4. Next Steps for QoS Architectures

   Both the Integrated Services architecture and the Differentiated
   Services architecture have some missing elements in terms of their
   current definition. The more critical elements are highlighted in
   this section, as they are likely to form the next steps in the
   refinement of the QoS architecture.

4.1 QoS-Enabled Applications

   One of the basic areas of uncertainty with QoS architectures is
   whether QoS is a per-application service, or whether QoS is a
   transport-layer option. Per-application services have obvious

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   implications of extending the QoS architecture into some form of
   Application Protocol Interface (API), so that applications could
   negotiate a QoS response from the network and alter their behaviour
   according to the outcome of the response. As a transport layer
   option, it could be envisaged that any application could have its
   traffic carried by some form of QoS-enabled network services by
   changing the host configuration, or by changing the configuration at
   some other network control point, without making any explicit
   changes to the application itself. The strength of the transport
   layer approach is that there is no requirement to substantially
   alter application behaviour, as the application is itself unaware of
   the administratively assigned QoS.

   In the case of the Integrated Services architecture, this transport
   layer approach does not appear to be an available option, as the
   application does require some alteration to function correctly in
   this environment. The application must be able to provide to the
   service reservation module a profile of its anticipated traffic, or
   in other words the application must be able to predict its traffic
   load. In addition the application must be able to share the
   reservation state with the network, so that if the network state
   fails, the application can be informed of the failure.

   In the case of the Differentiated Services architecture there is no
   explicit provision for the application to communicate with the
   network regarding service levels. This does allow the use of a
   transport level option within the end system that does not require
   explicit alteration of the application to mark its generated traffic
   with one of the available Differentiated Services service profiles.
   However, whether the application is aware of such service profiles
   or not, there is no level of service assurance to the application in
   such a model. If the Differentiated Services boundary traffic
   conditioners enter a load shedding state, the application is not
   signalled of this condition, and is not explicitly aware that the
   requested service response is not being provided by the network. If
   the network itself changes state and is unable to meet the
   cumulative traffic loads admitted by the ingress traffic
   conditioners, neither the ingress traffic conditioners, nor the
   client applications, are informed of this failure to maintain the
   associated service quality. While there is no explicit need to alter
   application behavior in this architecture, as the basic DiffServ
   mechanism is one that is managed within the network itself, the
   consequence is that an application may not be aware whether a
   particular service state is being delivered to the application.

   An additional factor for QoS enabled applications is that of
   receiver capability negotiation. There is no value in the sender
   establishing a QoS-enabled path across a network to the receiver if
   the receiver is incapable of absorbing the consequent data flow.
   This implies that QoS enabled applications also require some form of
   end-to-end capability negotiation, possibly through a generic
   protocol to allow the sender to match its QoS requirements to the
   minimum of the flow resources that can be provided by the network

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   and the flow resources that can be processed by the receiver. In the
   case of the Integrated services architecture the application end-to-
   end interaction can be integrated into the RSVP negotiation. In the
   case of the Differentiated Services architecture there is no clear
   path of integrating such receiver control into the signalling model
   of the architecture as it stands.

   For end-to-end service delivery it does appear that QoS
   architectures will need to extend to the level of the application
   requesting the service profile. It appears that further refinement
   of the QoS architecture is required to integrate DiffServ network
   services into an end-to-end service delivery model.

4.2 The Service Environment

   The outcome of the considerations of these two approaches to QoS
   architecture within the network is that there appears to be no
   single comprehensive service environment that possesses both service
   accuracy and scaling properties.

   The maintained reservation state of the Integrated Services
   architecture and the end-to-end signaling function of RSVP are part
   of a service management architecture, but it is not cost effective,
   or even feasible, to operate a per-application reservation and
   classification state across the high speed core of a network.

   While the aggregated behavior state of the Differentiated Services
   architecture does offer excellent scaling properties, the lack of
   end-to-end signaling facilities makes such an approach one that
   cannot operate in isolation within any environment. The
   Differentiated Services architecture can be characterized as a
   boundary-centric operational model. With this boundary-centric
   architecture, the signalling of resource availability from the
   interior of the network to the boundary traffic conditioners is not
   defined, nor is the signalling from the traffic conditioners to the
   application that is resident on the end system. What appears to be
   required within the Differentiated Services service model is both
   resource availability signaling from the core of the network to the
   DiffServ boundary and signaling from the boundary to the client
   application.

4.3 QoS Discovery

   There is no robust mechanism for network path discovery with
   specific service performance attributes. The assumption within both
   IntServ and DiffServ architectures is that the best effort routing
   path is used, where the path is either capable of sustaining the
   service load, or not.

   Assuming that the deployment of service differentiating
   infrastructure will be piecemeal, even if only in the initial stages
   of a QoS rollout, such an assumption may be unwarranted.  If this is
   the case, then how can a host application determine if there is a

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   distinguished service path to the destination? No mechanisms exist
   within either architecture to query the network for the potential to
   support a specific service profile. Such a query would potentially
   examine a number of candidate paths, rather than simply examining
   the lowest metric routing path, so that this discovery function is
   likely to be associated with some form of QoS routing functionality.
   From this perspective, there is still further refinement that may be
   required in the model of service discovery and the associated task
   of resource reservation.

4.4 QoS Routing and Resource Management

   To date QoS routing has been developed at some distance from the
   task of development of QoS architectures. The implicit assumption
   within the current QoS architectural models is that the routing best
   effort path will be used for both best effort traffic and
   distinguished service traffic.

   There is no explicit architectural option to allow the network
   service path to be aligned along other than the single best routing
   metric path, so that available network resources can be efficiently
   applied to meet service requests. Considerations of maximizing
   network efficiency would imply that some form of path selection is
   necessary within a QoS architecture, allowing the set of service
   requirements to be optimally supported within the network's
   aggregate resource capability.

   In addition to path selection, SPF-based interior routing protocols
   allow for the flooding of link metric information across all network
   elements. This mechanism appears to be a productive direction to
   provide the control-level signalling between the interior of the
   network and the network admission elements, allowing the admission
   systems to admit traffic based on current resource availability
   rather than on necessarily conservative statically defined admission
   criteria.

   There is a more fundamental issue here concerning resource
   management and traffic engineering. The approach of single path
   selection with static load characteristics does not match a
   networked environment which contains a richer mesh of connectivity
   and dynamic load characteristics. In order to make efficient use of
   a rich connectivity mesh, it is necessary to be able to direct
   traffic with a common ingress and egress point across a set of
   available network paths, spreading the load across a broader
   collection of network links. At its basic form this is essentially a
   traffic engineering problem. To support this function it is
   necessary to calculate per-path dynamic load metrics, and allow the
   network's ingress system the ability to distribute incoming traffic
   across these paths in accordance with some model of desired traffic
   balance. To apply this approach to a QoS architecture would imply
   that each path has some form of vector of quality attributes, and
   incoming traffic is balanced across a subset of available paths
   where the quality attribute of the traffic is matched with the

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   quality vector of each available path. This augmentation to the
   semantics of the traffic engineering is matched by a corresponding
   shift in the calculation and interpretation of the path's quality
   vector. In this approach what needs to be measured is not the path's
   resource availability level (or idle proportion), but the path's
   potential to carry additional traffic at a certain level of quality.
   This potential metric is one that allows existing lower priority
   traffic to be displaced to alternative paths. The path's quality
   metric can be interpreted as a metric describing the displacement
   capability of the path, rather than a resource availability metric.

   This area of active network resource management, coupled with
   dynamic network resource discovery, and the associated control level
   signalling to network admission systems appears to be a topic for
   further research at this point in time.


4.5 TCP and QoS

   A congestion-managed rate-adaptive traffic flow (such as used by
   TCP) uses the feedback from the ACK packet stream to time subsequent
   data transmissions. The resultant traffic flow rate is an outcome of
   the service quality provided to both the forward data packets and
   the reverse ACK packets. If the ACK stream is treated by the network
   with a different service profile to the outgoing data packets, it
   remains an open question as to what extent will the data forwarding
   service be compromised in terms of achievable throughput. High rates
   of jitter on the ACK stream can cause ACK compression, that in turn
   will cause high burst rates on the subsequent data send. Such bursts
   will stress the service capacity of the network and will compromise
   TCP throughput rates.

   One way to address this is to use some form of symmetric service,
   where the ACK packets are handled using the same service class as
   the forward data packets. If symmetric service profiles are
   important for TCP sessions, how can this be structured in a fashion
   that does not incorrectly account for service usage? In other words,
   how can both directions of a TCP flow be accurately accounted to one
   party?

   Additionally, there is the interaction between the routing system
   and the two TCP data flows. The Internet routing architecture does
   not intrinsically preserve TCP flow symmetry, and the network path
   taken by the forward packets of a TCP session may not exactly
   correspond to the path used by the reverse packet flow.

   TCP also exposes an additional performance constraint in the manner
   of the traffic conditioning elements in a QoS-enabled network.
   Traffic conditioners within QoS architectures are typically
   specified using a rate enforcement mechanism of token buckets. Token
   bucket traffic conditioners behave in a manner that is analogous to
   a First In First Out queue. Such traffic conditioning systems impose
   tail drop behavior on TCP streams. This tail drop behavior can

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   produce TCP timeout retransmission, unduly penalizing the average
   TCP goodput rate to a level that may be well below the level
   specified by the token bucket traffic conditioner. Token buckets can
   be considered as TCP-hostile network elements.

   The larger issue exposed in this consideration is that provision of
   some form of assured service to congestion-managed traffic flows
   requires traffic conditioning elements that operate using weighted
   RED-like control behaviors within the network, with less
   deterministic traffic patterns as an outcome. A requirement to
   manage TCP burst behavior through token bucket control mechanisms is
   most appropriately managed in the sender's TCP stack.

4.6 Per-Flow States and Per-Packet classifiers

   Both the IntServ and DiffServ architectures use packet classifiers
   as an intrinsic part of their architecture. In the case of DiffServ
   the classifiers are part of the network ingress element, while
   within the IntServ architecture every active network element is
   required to operate packet classifiers.

   Within the IntServ architecture the classifiers are defined to the
   level of granularity of an individual traffic flow, using the
   packet's 5-tuple of (source address, destination address, source
   port, destination port, protocol) as the means to identify an
   individual traffic flow. The DiffServ Multi-Field (MF) classifiers
   are also able to use this 5-tuple to map individual traffic flows
   into supported behavior aggregates.

   The use of IPSEC, NAT and various forms of IP tunnels result in a
   occlusion of the flow identification within the IP packet header,
   combining individual flows into a larger aggregate state that may be
   too coarse for the network's service policies. The issue with such
   mechanisms is that they may occur within the network path in a
   fashion that is not visible to the end application, compromising the
   ability for the application to determine whether the requested
   service profile is being delivered by the network.

   IP packet fragmentation also affects the ability of the network to
   identify individual flows, as the trailing fragments of the IP
   packet will not include the TCP or UDP port address information.
   This admits the possibility of trailing fragments of a packet within
   a distinguished service class being classified into the base best
   effort service category, and delaying the ultimate delivery of the
   IP packet to the destination until the trailing best effort
   delivered fragments have arrived.

   The observation made here is that QoS services do have a number of
   caveats that should be placed on both the application and the
   network. Applications should perform path MTU discovery in order to
   avoid packet fragmentation. Deployment of various forms of payload
   encryption, header address translation and header encapsulation

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   should be undertaken with due attention to their potential impacts
   on service delivery packet classifiers.

4.7 The Service Set

   The underlying question posed here is how many distinguished service
   responses are adequate to provide a functionally adequate range of
   service responses?

   The Differentiated Services architecture does not make any limiting
   restrictions on the number of potential services that a network
   operator can offer. The network operator may be limited to a choice
   of up to 64 discrete services in terms of the 6 bit service code
   point in the IP header but as the mapping from service to code point
   can be defined by each network operator, there can be any number of
   potential services.

   As always, there is such a thing as too much of a good thing, and a
   large number of potential services leads to a set of issues around
   end-to-end service coherency when spanning multiple network domains.
   A small set of distinguished services can be supported across a
   large set of service providers by equipment vendors and by
   application designers alike. An ill defined large set of potential
   services serves little productive purpose. This does point to a
   potential refinement of the QoS architecture to define a small core
   set of service profiles as well-known service profiles, and place
   all other profiles within a private use category.

4.8 Measuring Service Delivery

   There is a strong requirement within any QoS architecture for
   network management approaches that provide a coherent view of the
   operating state of the network. This differs from a conventional
   element-by-element management view of the network in that the desire
   here is to be able to provide a view of the available resources
   along a particular path within a network, and map this view to an
   admission control function which can determine whether to admit a
   service differentiated flow along the nominated network path.

   As well as managing the admission systems through resource
   availability measurement, there is a requirement to be able to
   measure the operating parameters of the delivered service. Such
   measurement methodologies are required in order to answer the
   question of how the network operator provides objective measurements
   to substantiate the claim that the delivered service quality
   conformed to the service specifications. Equally, there is a
   requirement for a measurement methodology to allow the client to
   measure the delivered service quality so that any additional expense
   that may be associated with the use of premium services can be
   justified in terms of superior application performance.

   Such measurement methodologies appear to fall within the realm of
   additional refinement to the QoS architecture.

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4.9 QoS Accounting

   It is reasonable to anticipate that such forms of premium service
   and customized service will attract an increment on the service
   tariff. The provision of a distinguished service is undertaken with
   some level of additional network resources to support the service,
   and the tariff premium should reflect this altered resource
   allocation. Not only does such an incremental tariff shift the added
   cost burden to those clients who are requesting a disproportionate
   level of resources, but it provides a means to control the level of
   demand for premium service levels.

   If there are to be incremental tariffs on the use of premium
   services, then some accounting of the use of the premium service
   would appear to be necessary relating use of the service to a
   particular client. So far there is no definition of such an
   accounting model nor a definition as to how to gather the data to
   support the resource accounting function.

   The impact of this QoS service model may be quite profound to the
   models of Internet service provision. The commonly adopted model in
   both the public internet and within enterprise networks is that of a
   model of access, where the clients service tariff is based on the
   characteristics of access to the services, rather than that of the
   actual use of the service. The introduction of QoS services creates
   a strong impetus to move to usage-based tariffs, where the tariff is
   based on the level of use of the network's resources. This, in turn,
   generates a requirement to meter resource use, which is a form of
   usage accounting.

4.10 QoS Deployment Diversity

   It is extremely improbable that any single form of service
   differentiation technology will be rolled out across the Internet
   and across all enterprise networks.

   Some networks will deploy some form of service differentiation
   technology while others will not. Some of these service platforms
   will interoperate seamlessly and other less so. To expect all
   applications, host systems, network routers, network policies, and
   inter-provider arrangements to coalesce into a single homogenous
   service environment that can support a broad range of service
   responses is an somewhat unlikely outcome given the diverse nature
   of the available technologies and industry business models. It is
   more likely that we will see a number of small scale deployment of
   service differentiation mechanisms and some efforts to bridge these
   environments together in some way.

   In this heterogeneous service environment the task of service
   capability discovery is as critical as being able to invoke service
   responses and measure the service outcomes. QoS architectures will

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   need to include protocol capabilities in supporting service
   discovery mechanisms.

   In addition, such a heterogeneous deployment environment will create
   further scaling pressure on the operational network as now there is
   an additional dimension to the size of the network. Each potential
   path to each host is potentially qualified by the service
   capabilities of the path. While one path may be considered as a
   candidate best effort path, another path may offer a more precise
   match between the desired service attributes and the capabilities of
   the path to sustain the service. Much of the brunt of such scaling
   pressures will be seen in the inter-domain and intra-domain routing
   domain where there are pressures to increase the number of
   attributes of a routing entry, and also to use the routing protocol
   in some form of service signaling role.


4.11 QoS Inter-Domain signalling

   QoS Path selection is both an intra-domain (interior) and an inter-
   domain (exterior) issue. Within the inter-domain space, the current
   routing technologies allow each domain to connect to a number of
   other domains, and to express its policies with respect to received
   traffic in terms of inter-domain route object attributes.
   Additionally, each domain may express its policies with respect to
   sending traffic through the use of boundary route object filters,
   allowing a domain to express its preference for selecting one
   domain's advertised routes over another. The inter-domain routing
   space is a state of dynamic equilibrium between these various route
   policies.

   The introduction of differentiated services adds a further dimension
   to this policy space. For example, while a providers may execute an
   interconnection agreement with one party to exchange best effort
   traffic, it may execute another agreement with a second party to
   exchange service qualified traffic. The outcome of this form of
   interconnection is that the service provider will require external
   route advertisements to be qualified by the accepted service
   profiles. Generalizing from this scenario, it is reasonable to
   suggest that we will require the qualification of routing
   advertisements with some form of service quality attributes. This
   implies that we will require some form of quality vector-based
   forwarding function, at least in the inter-domain space, and some
   associated routing protocol can pass a quality of service vector in
   an operationally stable fashion.

   The implication of this requirement is that the number of objects
   being managed by routing systems must expand dramatically, as the
   size and number of objects managed within the routing domain
   increases, and the calculation of a dynamic equilibrium of import
   and export policies between interconnected providers will also be
   subject to the same level of scaling pressure.

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   This has implications within the inter-domain forwarding space as
   well, as the forwarding decision in such a services differentiated
   environment is then qualified by some form of service quality
   vector. This is required in order to pass exterior traffic to the
   appropriate exterior interconnection gateway.

4.12 QoS Deployment Logistics

   How does the deployment of service-aware networks commence? Which
   gets built first - host applications or network infrastructure?

   This is another instance of the chicken and egg problem. No network
   operator will make the significant investment in distinguished
   service infrastructure unless there is a set of clients and
   applications available to make immediate use of such facilities. No
   application designer will attempt to integrate service quality
   features into the application unless there is a model of operation
   supported by widespread deployment that makes the additional
   investment in application complexity worthwhile. With both halves of
   the deployment scenario waiting for the other to move, deployment of
   distinguished services may require some other external impetus.

   Further aspects of this deployment picture lie in the issues of
   network provisioning and the associated task of traffic engineering.
   Engineering a network to meet the demands of best effort flows
   follows a well understood pattern of matching network points of user
   concentrations to content delivery network points with best effort
   paths. Integrating QoS-mediated traffic engineering into the
   provisioning model suggests a provisioning requirement that also
   requires input from a QoS demand model.

5. The objective of the QoS architecture

   What is the precise nature of the problem that QoS is attempting to
   solve? Perhaps this is one of the more fundamental questions
   underlying the QoS effort, and the diversity of potential responses
   is a pointer to the breadth of scope of the QoS effort.

   All of the following responses form a part of the QoS intention:

    1. To control the network service response such that the response
       to a specific service element is consistent and predictable.

    2. To control the network service response such that a service
       element is provided with a level of response equal to or above a
       guaranteed minimum.

    3. To allow a service element to establish in advance the service
       response that can or will be obtained from the network.

    4. To control the contention for network resources such that a
       service element is provided with a superior level of network
       resource.

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    5. To control the contention for network resources such that a
       service element does not obtain an unfair allocation of
       resources (to some definition of 'fairness').

   Broadly speaking, the first three responses can be regarded as
   'application-centric', and the latter as 'network-centric'. It is
   critical to bear in mind that none of these responses can be
   addressed in isolation within any effective QoS architecture. Within
   the end-to-end architectural model of the Internet, applications
   make minimal demands on the underlying IP network. In the case of
   TCP, the protocol uses an end-to-end control signal approach to
   dynamically adjust to the prevailing network state. QoS
   architectures add a critical addition to this

6. Towards an end-to-end QoS architecture

   The challenge facing the QoS architecture lies in addressing the
   weaknesses noted above, and in integrating the various elements of
   the architecture into a cohesive whole that is capable of sustaining
   end-to-end service models across a wide diversity of internet
   platforms. It should be noted that such an effort may not
   necessarily result in a single resultant architecture, and that it
   is possible to see a number of end-to-end approaches based on
   different combinations of the existing components.

   One approach is to attempt to combine both architectures into an
   end-to-end model, using IntServ as the architecture which allows
   applications to interact with the network, and DiffServ as the
   architecture to manage admission the network's resources. In this
   approach, the basic tension that needs to be resolved lies in
   difference between the per-application view of the IntServ
   architecture and the network boundary-centric view of the DiffServ
   architecture.

   One major building block for such an end-to-end service architecture
   is a service signalling protocol. The RSVP signalling protocol can
   address the needs of applications that require a per-service end-to-
   end service signalling environment. The abstracted model of RSVP is
   that of a discovery signalling protocol that allows an application
   to use a single transaction to communicate its service requirements
   to both the network and the remote party, and through the response
   mechanism, to allow these network elements to commit to the service
   requirements. The barriers to deployment for this model lie in an
   element-by element approach to service commitment, implying that
   each network element must undertake per-flow signalling and per-flow
   processing to support this service model. This approach is seen as
   imposing an unacceptable level of overhead on the central core
   elements of large carrier networks.

   The DiffServ approach uses a model of abstraction which attempts to
   create an external view of a compound network as a single

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   subnetwork. From this perspective the network can be perceived as
   two boundary service points, ingress and egress. The advantage of
   this approach is that there exists the potential to eliminate the
   requirement for per-flow state and per-flow processing on the
   interior elements of such a network.

   One approach is for applications to use RSVP to request that their
   flows be admitted into the network. If a request is accepted, it
   would imply that there is a committed resource reservation within
   the IntServ-capable components of the network, and that the service
   requirements have been mapped into a compatible aggregate service
   class within the DiffServ-capable network. The DiffServ core must be
   capable of carrying the RSVP messages across the DiffServ network,
   so that further resource reservation is possible within the IntServ
   network upon egress from the DiffServ environment. The approach
   calls for the DiffServ network to use per-flow multi-field (MF)
   classifier, where the MF classification is based on the RSVP-
   signalled flow specification. The service specification of the RSVP-
   signalled resource reservation is mapped into a compatible aggregate
   DiffServ behavior aggregate and the MF classifier marks packets
   according to the selected behavior. Alternatively the boundary of
   the IntServ and DiffServ networks can use the IntServ egress to mark
   the flow packets with the appropriate DSCP, allowing the DiffServ
   ingress element to use the BA classifier, and dispense with the per-
   flow MF classifier.

   An end-to-end QoS model requires that any admission failure within
   the DiffServ network be communicated to the end application via
   RSVP. This allows the application to take some form of corrective
   action, either by modifying it's service requirements or terminating
   the application. If the service agreement between the DiffServ
   network is statically provisioned, then this static information can
   be loaded into the IntServ boundary systems, and IntServ can manage
   the allocation of available DiffServ behavior aggregate resources.
   If the service agreement is dynamically variable, some form of
   signaling is required between the two networks to pass this resource
   availability information back into the RSVP signalling environment.


7. Conclusions

   None of these are any reason to condemn the QoS architectures as
   completely impractical, nor do they provide any reason to believe
   that the efforts of deploying QoS architectures will not come to
   fruition. What this is intended to illustrate is that there are
   still a number of activities that are essential precursors to
   widespread deployment and use of such QoS networks and that there is
   a need to fill in the missing sections with something more
   substantial than pixie dust and wishful thinking.

8. Security Considerations

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   The Internet is not an architecture that includes a strict
   implementation of fairness of access to the common transmission and
   switching resource. The introduction of any form of fairness, and,
   in the case of QoS, weighted fairness, implies a requirement for
   transparency in the implementation of the fairness contract between
   the network provider and the network's users. This requires some
   form of resource accounting and auditing, which, in turn, requires
   the use of authentication and access control. The balancing factor
   is that a shared resource should not overtly expose the level of
   resource usage of any one user to any other, so that some level of
   secrecy is required in this environment

   The QoS environment also exposes the potential of theft of resources
   through the unauthorized admission of traffic with an associated
   service profile. QoS signalling protocols which are intended to
   undertake resource management and admission control require the use
   of identity authentication and integrity protection in order to
   mitigate this potential for theft of resources.

   Both forms of QoS architecture require the internal elements of the
   network to be able to undertake classification of traffic based on
   some form of identification that is carried in the packet header in
   the clear. Classifications systems that use multi-field specifiers,
   or per-flow specifiers rely on the carriage of end-to-end packet
   header fields being carried in the clear. This has conflicting
   requirements for security architectures that attempt to mask such
   end-to-end identifiers within an encrypted payload.

   QoS architectures can be considered as a means of exerting control
   over network resource allocation. In the event of a rapid change in
   resource availability (e.g. disaster) it is an undesirable outcome
   if the remaining resources are completely allocated to a single
   class of service to the exclusion of all other classes. Such an
   outcome constitutes a denial of service, where the traffic control
   system (routing) selects paths that are incapable of carrying any
   traffic of a particular service class.



9. References


   1  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.



   [1]  Bradner, S., _The Internet Standards Process _ Revision 3_,
        BCP9, RFC2026, October 1996.

   [2]  Mankin, A., Baker, F., Braden, R., O'Dell, M., Romanow, A.,
   Weinrib, A., Zhang, L., _Resource ReSerVation Protocol (RSVP)
   Version 1 Applicability Statement_, RFC2208, September 1997.

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   [3]  Braden. R., Clark, D., Shenker, S., _Integrated Services in the
   Internet Architecture: an Overview_, RFC1633, June 1994.

   [4]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., Weiss,
   W., _An Architecture for Differentiated Services_, RFC2475, December
   1998.



10.  Acknowledgments

   Valuable contributions to this draft came from Brian Carpenter, Jon
   Crowcroft, Tony Hain and Henning Schulzrinne.


11. Author's Addresses

   Geoff Huston
   Telstra
   5/490 Northbourne Ave
   Dickson ACT 2602
   AUSTRALIA
   Email: gih@telstra.net

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