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Network Working Group E. Crawley Request for Comments: 2386 Argon Networks Category: Informational R. Nair

                                                          Arrowpoint
                                                      B. Rajagopalan
                                                             NEC USA
                                                          H. Sandick
                                                        Bay Networks
                                                         August 1998
         A Framework for QoS-based Routing in the Internet

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1998).  All Rights Reserved.

ABSTRACT

 QoS-based routing has been recognized as a missing piece in the
 evolution of QoS-based service offerings in the Internet. This
 document describes some of the QoS-based routing issues and
 requirements, and proposes a framework for QoS-based routing in the
 Internet. This framework is based on extending the current Internet
 routing model of intra and interdomain routing to support QoS.

1. SCOPE OF DOCUMENT & PHILOSOPHY

 This document proposes a framework for QoS-based routing, with the
 objective of fostering the development of an Internet-wide solution
 while encouraging innovations in solving the many problems that
 arise.  QoS-based routing has many complex facets and it is
 recommended that the following two-pronged approach be employed
 towards its development:
  1. Encourage the growth and evolution of novel intradomain QoS-based
     routing architectures. This is to allow the development of
     independent, innovative solutions that address the many QoS-based
     routing issues. Such solutions may be deployed in autonomous
     systems (ASs), large and small, based on their specific needs.

Crawley, et. al. Informational [Page 1] RFC 2386 A Framework for QoS-based Routing August 1998

  2. Encourage simple, consistent and stable interactions between ASs
     implementing routing solutions developed as above.
 This approach follows the traditional separation between intra and
 interdomain routing. It allows solutions like QOSPF [GKOP98, ZSSC97],
 Integrated PNNI [IPNNI] or other schemes to be deployed for
 intradomain routing without any restriction, other than their ability
 to interact with a common, and perhaps simple, interdomain routing
 protocol. The need to develop a single, all encompassing solution to
 the complex problem of QoS-based routing is therefore obviated. As a
 practical matter, there are many different views on how QoS-based
 routing should be done. Much overall progress can be made if an
 opportunity exists for various ideas to be developed and deployed
 concurrently, while some consensus on the interdomain routing
 architecture is being developed.  Finally, this routing model is
 perhaps the most practical from an evolution point of view. It is
 superfluous to say that the eventual success of a QoS-based Internet
 routing architecture would depend on the ease of evolution.
 The aim of this document is to describe the QoS-based routing issues,
 identify basic requirements on intra and interdomain routing, and
 describe an extension of the current interdomain routing model to
 support QoS. It is not an objective of this document to specify the
 details of intradomain QoS-based routing architectures.  This is left
 up to the various intradomain routing efforts that might follow.  Nor
 is it an objective to specify the details of the interface between
 reservation protocols such as RSVP and QoS-based routing. The
 specific interface functionality needed, however, would be clear from
 the intra and interdomain routing solutions devised.  In the
 intradomain area, the goal is to develop the basic routing
 requirements while allowing maximum freedom for the development of
 solutions. In the interdomain area, the objectives are to identify
 the QoS-based routing functions, and facilitate the development or
 enhancement of a routing protocol that allows relatively simple
 interaction between domains.
 In the next section, a glossary of relevant terminology is given. In
 Section 3, the objectives of QoS-based routing are described and the
 issues that must be dealt with by QoS-based Internet routing efforts
 are outlined. In Section 4, some requirements on intradomain routing
 are defined. These requirements are purposely broad, putting few
 constraints on solution approaches. The interdomain routing model and
 issues are described in Section 5 and QoS-based multicast routing is
 discussed in Section 6.  The interaction between QoS-based routing
 and resource reservation protocols is briefly considered in Section
 7. Security considerations are listed in Section 8 and related work
 is described in Section 9. Finally, summary and conclusions are
 presented in Section 10.

Crawley, et. al. Informational [Page 2] RFC 2386 A Framework for QoS-based Routing August 1998

2. GLOSSARY

 The following glossary lists the terminology used in this document
 and an explanation of what is meant. Some of these terms may have
 different connotations, but when used in this document, their meaning
 is as given.
 Alternate Path Routing : A routing technique where multiple paths,
 rather than just the shortest path, between a source and a
 destination are utilized to route traffic. One of the objectives of
 alternate path routing is to distribute load among multiple paths in
 the network.
 Autonomous System (AS): A routing domain which has a common
 administrative authority and consistent internal routing policy. An
 AS may employ multiple intradomain routing protocols internally and
 interfaces to other ASs via a common interdomain routing protocol.
 Source: A host or router that can be identified by a unique unicast
 IP address.
 Unicast destination: A host or router that can be identified by a
 unique unicast IP address.
 Multicast destination: A multicast IP address indicating all hosts
 and routers that are members of the corresponding group.
 IP flow (or simply "flow"): An IP packet stream from a source to a
 destination (unicast or multicast) with an associated Quality of
 Service (QoS) (see below) and higher level demultiplexing
 information. The associated QoS could be "best-effort".
 Quality-of-Service (QoS): A set of service requirements to be met by
 the network while transporting a flow.
 Service class: The definitions of the semantics and parameters of a
 specific type of QoS.
 Integrated services:  The Integrated Services model for the Internet
 defined in RFC 1633 allows for integration of QoS services with the
 best effort services of the Internet.  The Integrated Services
 (IntServ) working group in the IETF has defined two service classes,
 Controlled Load Service [W97] and Guaranteed Service [SPG97].
 RSVP:  The ReSerVation Protocol [BZBH97].  A QoS signaling protocol
 for the Internet.
 Path: A unicast or multicast path.

Crawley, et. al. Informational [Page 3] RFC 2386 A Framework for QoS-based Routing August 1998

 Unicast path: A sequence of links from an IP source to a unicast IP
 destination, determined by the routing scheme for forwarding packets.
 Multicast path (or Multicast Tree): A subtree of the network topology
 in which all the leaves and zero or more interior nodes are members
 of the same multicast group. A multicast path may be per-source, in
 which case the subtree is rooted at the source.
 Flow set-up: The act of establishing state in routers along a path to
 satisfy the QoS requirement of a flow.
 Crankback: A technique where a flow setup is recursively backtracked
 along the partial flow path up to the first node that can determine
 an alternative path to the destination.
 QoS-based routing: A routing mechanism under which paths for flows
 are determined based on some knowledge of resource availability in
 the network as well as the QoS requirement of flows.
 Route pinning: A mechanism to keep a flow path fixed for a duration
 of time.
 Flow Admission Control (FAC): A process by which it is determined
 whether a link or a node has sufficient resources to satisfy the QoS
 required for a flow. FAC is typically applied by each node in the
 path of a flow during flow set-up to check local resource
 availability.
 Higher-level admission control: A process by which it is determined
 whether or not a flow set-up should proceed, based on estimates and
 policy requirements of the overall resource usage by the flow.
 Higher-level admission control may result in the failure of a flow
 set-up even when FAC at each node along the flow path indicates
 resource availability.

3. QOS-BASED ROUTING: BACKGROUND AND ISSUES

3.1 Best-Effort and QoS-Based Routing

 Routing deployed in today's Internet is focused on connectivity and
 typically supports only one type of datagram service called "best
 effort" [WC96]. Current Internet routing protocols, e.g. OSPF, RIP,
 use "shortest path routing", i.e. routing that is optimized for a
 single arbitrary metric, administrative weight or hop count. These
 routing protocols are also "opportunistic," using the current
 shortest path or route to a destination. Alternate paths with
 acceptable but non-optimal cost can not be used to route traffic
 (shortest path routing protocols do allow a router to alternate among

Crawley, et. al. Informational [Page 4] RFC 2386 A Framework for QoS-based Routing August 1998

 several equal cost paths to a destination).
 QoS-based routing must extend the current routing paradigm in three
 basic ways.  First, to support traffic using integrated-services
 class of services, multiple paths between node pairs will have to be
 calculated. Some of these new classes of service will require the
 distribution of additional routing metrics, e.g. delay, and available
 bandwidth. If any of these metrics change frequently, routing updates
 can become more frequent thereby consuming network bandwidth and
 router CPU cycles.
 Second, today's opportunistic routing will shift traffic from one
 path to another as soon as a "better" path is found.  The traffic
 will be shifted even if the existing path can meet the service
 requirements of the existing traffic.  If routing calculation is tied
 to frequently changing consumable resources (e.g. available
 bandwidth) this change will happen more often and can introduce
 routing oscillations as traffic shifts back and forth between
 alternate paths. Furthermore, frequently changing routes can increase
 the variation in the delay and jitter experienced by the end users.
 Third, as mentioned earlier, today's optimal path routing algorithms
 do not support alternate routing.   If the best existing path cannot
 admit a new flow, the associated traffic cannot be forwarded even if
 an adequate alternate path exists.

3.2 QoS-Based Routing and Resource Reservation

 It is important to understand the difference between QoS-based
 routing and resource reservation.  While resource reservation
 protocols such as RSVP [BZBH97] provide a method for requesting and
 reserving network resources, they do not provide a mechanism for
 determining a network path that has adequate resources to accommodate
 the requested QoS.  Conversely, QoS-based routing allows the
 determination of a path that has a good chance of accommodating the
 requested QoS, but it does not include a mechanism to reserve the
 required resources.
 Consequently, QoS-based routing is usually used in conjunction with
 some form of resource reservation or resource allocation mechanism.
 Simple forms of QoS-based routing have been used in the past for Type
 of Service (TOS) routing [M98].  In the case of OSPF, a different
 shortest-path tree can be computed for each of the 8 TOS values in
 the IP header [ISI81]. Such mechanisms can be used to select
 specially provisioned paths but do not completely assure that
 resources are not overbooked along the path.  As long as strict
 resource management and control are not needed, mechanisms such as
 TOS-based routing are useful for separating whole classes of traffic

Crawley, et. al. Informational [Page 5] RFC 2386 A Framework for QoS-based Routing August 1998

 over multiple routes.  Such mechanisms might work well with the
 emerging Differential Services efforts [BBCD98].
 Combining a resource reservation protocol with QoS-based routing
 allows fine control over the route and resources at the cost of
 additional state and setup time. For example, a protocol such as RSVP
 may be used to trigger QoS-based routing calculations to meet the
 needs of a specific flow.

3.3 QoS-Based Routing: Objectives

 Under QoS-based routing,  paths for flows would be determined based
 on some knowledge of resource availability in the network, as well as
 the QoS requirement of flows. The main objectives of QoS-based
 routing are:
 1.  Dynamic determination of feasible paths:  QoS-based routing can
     determine a path, from among possibly many choices, that has a
     good chance of accommodating the QoS of the given flow. Feasible
     path selection may be subject to policy constraints, such as path
     cost, provider selection, etc.
 2.  Optimization of resource usage: A network state-dependent QoS-
     based routing scheme can aid in the efficient utilization of
     network resources by improving the total network throughput. Such
     a routing scheme can be the basis for efficient network
     engineering.
 3.  Graceful performance degradation: State-dependent routing can
     compensate for transient inadequacies in network engineering
     (e.g., during focused overload conditions), giving better
     throughput and a more graceful performance degradation as
     compared to a state-insensitive routing scheme [A84].
 QoS-based routing in the Internet, however, raises many issues:
  1. How do routers determine the QoS capability of each outgoing link

and reserve link resources? Note that some of these links may be

    virtual, over ATM networks and others may be broadcast multi-
    access links.
  1. What is the granularity of routing decision (i.e., destination-

based, source and destination-based, or flow-based)?

  1. What routing metrics are used and how are QoS-accommodating paths

computed for unicast flows?

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  1. How are QoS-accommodating paths computed for multicast flows with

different reservation styles and receiver heterogeneity?

  1. What are the performance objectives while computing QoS-based

paths?

  1. What are the administrative control issues?
  1. What factors affect the routing overheads?, and
  1. How is scalability achieved?
 Some of these issues are discussed briefly next. Interdomain routing
 is discussed in Section 5.

3.4 QoS Determination and Resource Reservation

 To determine whether the QoS requirements of a flow can be
 accommodated on a link, a router must be able to determine the QoS
 available on the link. It is still an open issue as to how the QoS
 availability is determined for broadcast multiple access links (e.g.,
 Ethernet). A related problem is the reservation of resources over
 such links.  Solutions to these problems are just emerging [GPSS98].
 Similar problems arise when a router is connected to a large non-
 broadcast multiple access network, such as ATM. In this case, if the
 destination of a flow is outside the ATM network, the router may have
 multiple egress choices. Furthermore, the QoS availability on the ATM
 paths to each egress point may be different. The issues then are,
    o   how does a router determine all the egress choices across the
        ATM network?
    o   how  does it determine what QoS is available over the path to
        each egress point?, and
    o   what QoS value does the router advertise for the ATM link.
 Typically, IP routing over ATM (e.g., NHRP) allows the selection of a
 single egress point in the ATM network, and the procedure does not
 incorporate any knowledge of the QoS required over the path. An
 approach like I-PNNI [IPNNI] would be helpful here, although it
 introduces some complexity.
 An additional problem with resource reservation is how to determine
 what resources have already been allocated to a multicast flow. The
 availability of this information during path computation improves the
 chances of finding a path to add a new receiver to a multicast flow.
 QOSPF [ZSSC97] handles this problem by letting routers broadcast
 reserved resource information to other routers in their area.

Crawley, et. al. Informational [Page 7] RFC 2386 A Framework for QoS-based Routing August 1998

 Alternate path routing [ZES97] deals with this issue by using probe
 messages to find a path with sufficient resources. Path QoS
 Computation (PQC) method, proposed in [GOA97], propagates bandwidth
 allocation information in RSVP PATH messages. A router receiving the
 PATH message gets an indication of the resource allocation only on
 those links in the path to itself from the source.  Allocation for
 the same flow on other remote branches of the multicast tree is not
 available. Thus, the PQC method may not be sufficient to find
 feasible QoS-accommodating paths to all receivers.

3.5 Granularity of Routing Decision

 Routing in the Internet is currently based only on the destination
 address of a packet.  Many multicast routing protocols require
 routing based on the source AND destination of a packet. The
 Integrated Services architecture and RSVP allow QoS determination for
 an individual flow between a source and a destination. This set of
 routing granularities presents a problem for QoS routing solutions.
 If routing based only on destination address is considered, then an
 intermediate router will route all flows between different sources
 and a given destination along the same path. This is acceptable if
 the path has adequate capacity but a problem arises if there are
 multiple flows to a destination that exceed the capacity of the link.
 One version of QOSPF [ZSSC97] determines QoS routes based on source
 and destination address.  This implies that all traffic between a
 given source and destination, regardless of the flow, will travel
 down the same route.  Again, the route must have capacity for all the
 QoS traffic for the source/destination pair.  The amount of routing
 state also increases since the routing tables must include
 source/destination pairs instead of just the destination.
 The best granularity is found when routing is based on individual
 flows but this incurs a tremendous cost in terms of the routing
 state.  Each QoS flow can be routed separately between any source and
 destination. PQC [GOA97] and alternate path routing [ZES97], are
 examples of solutions which operate at the flow level.
 Both source/destination and flow-based routing may be susceptible to
 packet looping under hop-by-hop forwarding. Suppose a node along a
 flow or source/destination-based path loses the state information for
 the flow.  Also suppose that the flow-based route is different from
 the regular destination-based route. The potential then exists for a
 routing loop to form when the node forwards a packet belonging to the
 flow using its destination-based routing table to a node that occurs

Crawley, et. al. Informational [Page 8] RFC 2386 A Framework for QoS-based Routing August 1998

 earlier on the flow-based path. This is because the latter node may
 use its flow-based routing table to forward the packet again to the
 former and this can go on indefinitely.

3.6 Metrics and Path Computation

3.6.1 Metric Selection and Representation

 There are some considerations in defining suitable link and node
 metrics [WC96]. First, the metrics must represent the basic network
 properties of interest. Such metrics include residual bandwidth,
 delay and jitter.  Since the flow QoS requirements have to be mapped
 onto path metrics, the metrics define the types of QoS guarantees the
 network can support.  Alternatively, QoS-based routing cannot support
 QoS requirements that cannot be meaningfully mapped onto a reasonable
 combination of path metrics.  Second, path computation based on a
 metric or a combination of metrics must not be too complex as to
 render them impractical. In this regard, it is worthwhile to note
 that path computation based on certain combinations of metrics (e.g.,
 delay and jitter) is theoretically hard. Thus, the allowable
 combinations of metrics must be determined while taking into account
 the complexity of computing paths based on these metrics and the QoS
 needs of flows. A common strategy to allow flexible combinations of
 metrics while at the same time reduce the path computation complexity
 is to utilize "sequential filtering". Under this approach, a
 combination of metrics is ordered in some fashion, reflecting the
 importance of different metrics (e.g., cost followed by delay, etc.).
 Paths based on the primary metric are computed first (using a simple
 algorithm, e.g., shortest path) and a subset of them are eliminated
 based on the secondary metric and so forth until a single path is
 found. This is an approximation technique and it trades off global
 optimality for path computation simplicity (The filtering technique
 may be simpler, depending on the set of metrics used. For example,
 with bandwidth and cost as metrics, it is possible to first eliminate
 the set of links that do not have the requested bandwidth and then
 compute the least cost path using the remaining links.)
 Now, once suitable link and node metrics are defined, a uniform
 representation of them is required across independent domains -
 employing possibly different routing schemes - in order to derive
 path metrics consistently (path metrics are obtained by the
 composition of link and node metrics). Encoding of the maximum,
 minimum, range, and granularity of the metrics are needed. Also, the
 definitions of comparison and accumulation operators are required. In
 addition, suitable triggers must be defined for indicating a
 significant change from a minor change.  The former will cause a
 routing update to be generated. The stability of the QoS routes would

Crawley, et. al. Informational [Page 9] RFC 2386 A Framework for QoS-based Routing August 1998

 depend on the ability to control the generation of updates. With
 interdomain routing, it is essential to obtain a fairly stable view
 of the interconnection among the ASs.

3.6.2 Metric Hierarchy

 A hierarchy can be defined among various classes of service based on
 the degree to which traffic from one class can potentially degrade
 service of traffic from lower classes that traverse the same link. In
 this hierarchy, guaranteed constant bit rate traffic is at the top
 and "best-effort" datagram traffic at the bottom.  Classes providing
 service higher in the hierarchy impact classes providing service in
 lower levels. The same situation is not true in the other direction.
 For example, a datagram flow cannot affect a real-time service. Thus,
 it may be necessary to distribute and update different metrics for
 each type of service in the worst case.  But, several advantages
 result by identifying a single default metric.  For example, one
 could derive a single metric combining the availability of datagram
 and real-time service over a common substrate.

3.6.3 Datagram Flows

 A delay-sensitive metric is probably the most obvious type of metric
 suitable for datagram flows. However, it requires careful analysis to
 avoid instabilities and to reduce storage and bandwidth requirements.
 For example, a recursive filtering technique based on a simple and
 efficient weighted averaging algorithm [NC94] could be used. This
 filter is used to stabilize the metric. While it is adequate for
 smoothing most loading patterns, it will not distinguish between
 patterns consisting of regular bursts of traffic and random loading.
 Among other stabilizing tools, is a minimum time between updates that
 can help filter out high-frequency oscillations.

3.6.4 Real-time Flows

 In real-time quality-of-service, delay variation is generally more
 critical than delay as long as the delay is not too high.  Clearly,
 voice-based applications cannot tolerate more than a certain level of
 delay. The condition of varying delays may be expected to a greater
 degree in a shared medium environment with datagrams, than in a
 network implemented over a switched substrate.  Routing a real-time
 flow therefore reduces to an exercise in allocating the required
 network resources while minimizing fragmentation of bandwidth. The
 resulting situation is a bandwidth-limited minimum hop path from a
 source to the destination.  In other words, the router performs an
 ordered search through paths of increasing hop count until it finds
 one that meets all the bandwidth needs of the flow. To reduce
 contention and the probability of false probes (due to inaccuracy in

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 route tables), the router could select a path randomly from a
 "window" of paths which meet the needs of the flow and satisfy one of
 three additional criteria: best-fit, first-fit or worst-fit. Note
 that there is a similarity between the allocation of bandwidth and
 the allocation of memory in a multiprocessing system. First-fit seems
 to be appropriate for a system with a high real-time flow arrival
 rates; and worst-fit is ideal for real-time flows with high holding
 times.  This rather nonintuitive result was shown in [NC94].

3.6.5 Path Properties

 Path computation by itself is merely a search technique, e.g.,
 Shortest Path First (SPF) is a search technique based on dynamic
 programming. The usefulness of the paths computed depends to a large
 extent on the metrics used in evaluating the cost of a path with
 respect to a flow.
 Each link considered by the path computation engine must be evaluated
 against the requirements of the flow, i.e., the cost of providing the
 services required by the flow must be estimated with respect to the
 capabilities of the link. This requires a uniform method of combining
 features such as delay, bandwidth, priority and other service
 features.  Furthermore, the costs must reflect the lost opportunity
 of using each link after routing the flow.

3.6.6 Performance Objectives

 One common objective during path computation is to improve the total
 network throughput.  In this regard, merely routing a flow on any
 path that accommodates its QoS requirement is not a good strategy. In
 fact, this corresponds to uncontrolled alternate routing [SD95] and
 may adversely impact performance at higher traffic loads.  It is
 therefore necessary to consider the total resource allocation for a
 flow along a path, in relation to available resources, to determine
 whether or not the flow should be routed on the path.  Such a
 mechanism is referred to in this document as "higher level admission
 control". The goal of this is to ensure that the "cost" incurred by
 the network in routing a flow with a given QoS is never more than the
 revenue gained.  The routing cost in this regard may be the lost
 revenue in potentially blocking other flows that contend for the same
 resources. The formulation of the higher level admission control
 strategy, with suitable administrative hooks and with fairness to all
 flows desiring entry to the network, is an issue.  The fairness
 problem arises because flows with smaller reservations tend to be
 more successfully routed than flows with large reservations, for a
 given engineered capacity.  To guarantee a certain level of

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 acceptance rate for "larger" flows, without over-engineering the
 network, requires a fair higher level admission control mechanism.
 The application of higher level admission control to multicast
 routing is discussed later.

3.7 Administrative Control

 There are several administrative control issues. First, within an AS
 employing state-dependent routing, administrative control of routing
 behavior may be necessary. One example discussed earlier was higher
 level admission control. Some others are described in this section.
 Second, the control of interdomain routing based on policy is an
 issue.  The discussion of interdomain routing is defered to Section
 5.
 Two areas that need administrative control, in addition to
 appropriate routing mechanisms, are handling flow priority with
 preemption, and resource allocation for multiple service classes.

3.7.1 Flow Priorities and Preemption

 If there are critical flows that must be accorded higher priority
 than other types of flows, a mechanism must be implemented in the
 network to recognize flow priorities. There are two aspects to
 prioritizing flows.  First, there must be a policy to decide how
 different users are allowed to set priorities for flows they
 originate. The network must be able to verify that a given flow is
 allowed to claim a priority level signaled for it. Second, the
 routing scheme must ensure that a path with the requested QoS will be
 found for a flow with a probability that increases with the priority
 of the flow. In other words, for a given network load, a high
 priority flow should be more likely to get a certain QoS from the
 network than a lower priority flow requesting the same QoS. Routing
 procedures for flow prioritization can be complex.  Identification
 and evaluation of different procedures are areas that require
 investigation.

3.7.2 Resource Control

 If there are multiple service classes, it is necessary to engineer a
 network to carry the forecasted traffic demands of each class. To do
 this, router and link resources may be logically partitioned among
 various service classes. It is desirable to have dynamic partitioning
 whereby unused resources in various partitions are dynamically
 shifted to other partitions on demand [ACFH92]. Dynamic sharing,
 however, must be done in a controlled  fashion in order to prevent
 traffic under some service class from taking up more resources than

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 what was engineered for it for prolonged periods of time. The design
 of such a resource sharing scheme, and its incorporation into the
 QoS-based routing scheme are significant issues.

3.8 QoS-Based Routing for Multicast Flows

 QoS-based multicast routing is an important problem, especially if
 the notion of higher level admission control is included. The
 dynamism in the receiver set allowed by IP multicast, and receiver
 heterogeneity add to the problem. With straightforward implementation
 of distributed heuristic algorithms for multicast path computation
 [W88, C91], the difficulty is essentially one of scalability. To
 accommodate QoS, multicast path computation at a router must have
 knowledge of not only the id of subnets where group members are
 present, but also the identity of branches in the existing tree. In
 other words, routers must keep flow-specific state information. Also,
 computing optimal shared trees based on the shared reservation style
 [BZBH97], may require new algorithms.  Multicast routing is discussed
 in some detail in Section 6.

3.9 Routing Overheads

 The overheads incurred by a routing scheme depend on the type of the
 routing scheme, as well as the implementation. There are three types
 of overheads to be considered: computation, storage and
 communication. It is necessary to understand the implications of
 choosing a routing mechanism in terms of these overheads.
 For example, considering link state routing, the choice of the update
 propagation mechanism is important since network state is dynamic and
 changes relatively frequently. Specifically, a flooding mechanism
 would result in many unnecessary message transmissions and
 processing.  Alternative techniques, such as tree-based forwarding
 [R96], have to be considered. A related issue is the quantization of
 state information to prevent frequent updating of dynamic state.
 While coarse quantization reduces updating overheads, it may affect
 the performance of the routing scheme.  The tradeoff has to be
 carefully evaluated.  QoS-based routing incurs certain overheads
 during flow establishment, for example, computing a source route.
 Whether this overhead is disproportionate compared to the length of
 the sessions is an issue. In general, techniques for the minimization
 of routing-related overheads during flow establishment must be
 investigated. Approaches that are useful include pre-computation of
 routes, caching recently used routes, and TOS routing based on hints
 in packets (e.g., the TOS field).

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3.10 Scaling by Hierarchical Aggregation

 QoS-based routing should be scalable, and hierarchical aggregation is
 a common technique for scaling (e.g., [PNNI96]). But this introduces
 problems with regard to the accuracy of the aggregated state
 information [L95]. Also, the aggregation of paths under multiple
 constraints is difficult. One of the difficulties is the risk of
 accepting a flow based on inaccurate information, but not being able
 to support the QoS requirements of flow because the capabilities of
 the actual paths that are aggregated are not known during route
 computation.  Performance impacts of aggregating path metric
 information must therefore be understood. A way to compensate for
 inaccuracies is to use crankback, i.e., dynamic search for alternate
 paths as a flow is being routed. But crankback increases the time to
 set up a flow, and may adversely affect the performance of the
 routing scheme under some circumstances. Thus, crankback must be used
 judiciously, if at all, along with a higher level admission control
 mechanism.

4. INTRADOMAIN ROUTING REQUIREMENTS

 At the intradomain level, the objective is to allow as much latitude
 as possible in addressing the QoS-based routing issues. Indeed, there
 are many ideas about how QoS-based routing services can be
 provisioned within ASs. These range from on-demand path computation
 based on current state information, to statically provisioned paths
 supporting a few service classes.
 Another aspect that might invite differing solutions is performance
 optimization. Based on the technique used for this, intradomain
 routing could be very sophisticated or rather simple. Finally, the
 service classes supported, as well as the specific QoS engineered for
 a service class, could differ from AS to AS. For instance, some ASs
 may not support guaranteed service, while others may. Also, some ASs
 supporting the service may be engineered for a better delay bound
 than others. Thus, it requires considerable thought to determine the
 high level requirements for intradomain routing that both supports
 the overall view of QoS-based routing in the Internet and allows
 maximum autonomy in developing solutions.
 Our view is that certain minimum requirements must be satisfied by
 intradomain routing in order to be qualified as "QoS-based" routing.
 These are:
  1. The routing scheme must route a flow along a path that can

accommodate its QoS requirements, or indicate that the flow cannot

   be admitted with the QoS currently being requested.

Crawley, et. al. Informational [Page 14] RFC 2386 A Framework for QoS-based Routing August 1998

  1. The routing scheme must indicate disruptions to the current route

of a flow due to topological changes.

  1. The routing scheme must accommodate best-effort flows without any

resource reservation requirements. That is, present best effort

   applications and protocol stacks need not have to change to run in
   a domain employing QoS-based routing.
  1. The routing scheme may optionally support QoS-based multicasting

with receiver heterogeneity and shared reservation styles.

 In addition, the following capabilities are also recommended:
  1. Capabilities to optimize resource usage.
  1. Implementation of higher level admission control procedures to

limit the overall resource utilization by individual flows.

 Further requirements along these lines may be specified. The
 requirements should capture the consensus view of QoS-based routing,
 but should not preclude particular approaches (e.g., TOS-based
 routing) from being implemented. Thus, the intradomain requirements
 are expected to be rather broad.

5. INTERDOMAIN ROUTING

 The fundamental requirement on interdomain QoS-based routing is
 scalability.  This implies that interdomain routing cannot be based
 on highly dynamic network state information. Rather, such routing
 must be aided by sound network engineering and relatively sparse
 information exchange between independent routing domains. This
 approach has the advantage that it can be realized by straightforward
 extensions of the present Internet interdomain routing model. A
 number of issues, however, need to be addressed to achieve this, as
 discussed below.

Crawley, et. al. Informational [Page 15] RFC 2386 A Framework for QoS-based Routing August 1998

5.1 Interdomain QoS-Based Routing Model

 The interdomain QoS-based routing model is depicted below:
        AS1                   AS2             AS3
    ___________        _____________      ____________
   |           |      |             |    |            |
   |           B------B             B----B            |
   |           |      |             |    |            |
    -----B-----       B-------------      --B---------
          \         /                      /
           \       /                      /
        ____B_____B____         _________B______
       |               |       |                |
       |               B-------B                |
       |               |       |                |
       |               B-------B                |
        ---------------         ----------------
             AS4                           AS5
 Here, ASs exchange standardized routing information via border nodes
 B.  Under this model, each AS can itself consist of a set of
 interconnected ASs, with standardized routing interaction. Thus, the
 interdomain routing model is hierarchical.  Also, each lowest level
 AS employs an intradomain QoS-based routing scheme (proprietary or
 standardized by intradomain routing efforts such as QOSPF). Given
 this structure, some questions that arise are:
  1. What information is exchanged between ASs?
  1. What routing capabilities does the information exchange lead to?

(E.g., source routing, on-demand path computation, etc.)

  1. How is the external routing information represented within an AS?
  1. How are interdomain paths computed?
  1. What sort of policy controls may be exerted on interdomain path

computation and flow routing?, and

  1. How is interdomain QoS-based multicast routing accomplished?
 At a high level, the answers to these questions depend on the routing
 paradigm. Specifically, considering link state routing, the
 information exchanged between domains would consist of an abstract
 representation of the domains in the form of logical nodes and links,
 along with metrics that quantify their properties and resource
 availability.  The hierarchical structure of the ASs may be handled

Crawley, et. al. Informational [Page 16] RFC 2386 A Framework for QoS-based Routing August 1998

 by a hierarchical link state representation, with appropriate metric
 aggregation.
 Link state routing may not necessarily be advantageous for
 interdomain routing for the following reasons:
  1. One advantage of intradomain link state routing is that it would

allow fairly detailed link state information be used to compute

   paths on demand for flows requiring QoS. The state and metric
   aggregation used in interdomain routing, on the other hand, erodes
   this property to a great degree.
  1. The usefulness of keeping track of the abstract topology and

metrics of a remote domain, or the interconnection between remote

   domains is not obvious. This is especially the case when the remote
   topology and metric encoding are lossy.
  1. ASs may not want to advertise any details of their internal

topology or resource availability.

  1. Scalability in interdomain routing can be achieved only if

information exchange between domains is relatively infrequent.

   Thus, it seems practical to limit information flow between domains
   as much as possible.
 Compact information flow allows the implementation QoS-enhanced
 versions of existing interdomain protocols such as BGP-4. We look at
 the interdomain routing issues in this context.

5.2 Interdomain Information Flow

 The information flow between routing domains must enable certain
 basic functions:
 1.  Determination of reachability to various destinations
 2.  Loop-free flow routes
 3.  Address aggregation whenever possible
 4.  Determination of the QoS that will be supported on the path to a
     destination. The QoS information should be relatively static,
     determined from the engineered topology and capacity of an AS
     rather than ephemeral fluctuations in traffic load through the
     AS. Ideally, the QoS supported in a transit AS should be allowed
     to vary significantly only under exceptional circumstances, such
     as failures or focused overload.

Crawley, et. al. Informational [Page 17] RFC 2386 A Framework for QoS-based Routing August 1998

 5.  Determination, optionally, of multiple paths for a given
     destination, based on service classes.
 6.  Expression of routing policies, including monetary cost, as a
     function of flow parameters, usage and administrative factors.
 Items 1-3 are already part of existing interdomain routing. Item 5 is
 also a straightfoward extension of the current model. The main
 problem areas are therefore items 4 and 6.
 The QoS of an end-to-end path is obtained by composing the QoS
 available in each transit AS.  Thus, border routers must first
 determine what the locally available QoS is in order to advertise
 routes to both internal and external destinations. The determination
 of local "AS metrics" (corresponding to link metrics in the
 intradomain case) should not be subject to too much dynamism. Thus,
 the issue is how to define such metrics and what triggers an
 occasional change that results in re-advertisements of routes.
 The approach suggested in this document is not to compute paths based
 on residual or instantaneous values of AS metics (which can be
 dynamic), but utilize only the QoS capabilities engineered for
 aggregate transit flows.  Such engineering may be based on the
 knowledge of traffic to be expected from each neighboring ASs and the
 corresponding QOS needs.  This information may be obtained based on
 contracts agreed upon prior to the provisioning of services. The AS
 metric then corresponds to the QoS capabilities of the "virtual path"
 engineered through the AS (for transit traffic) and a different
 metric may be used for different neighbors. This is illustrated in
 the following figure.
        AS1                   AS2             AS3
    ___________        _____________      ____________
   |           |      |             |    |            |
   |           B------B1           B2----B            |
   |           |      |             |    |            |
    -----B-----       B3------------      --B---------
          \         /
           \       /
        ____B_____B____
       |               |
       |               |
       |               |
       |               |
        ---------------
             AS4

Crawley, et. al. Informational [Page 18] RFC 2386 A Framework for QoS-based Routing August 1998

 Here, B1 may utilize an AS metric specific for AS1 when computing
 path metrics to be  advertised to AS1. This metric is based on the
 resources engineered in AS2 for transit traffic from AS1. Similarly,
 B3 may utilize a different metric when computing path metrics to be
 advertised to AS4.  Now, it is assumed that as long as traffic flow
 into AS2 from AS1 or AS4 does not exceed the engineered values, these
 path metrics would hold.  Excess traffic due to transient
 fluctuations, however, may be handled as best effort or marked with a
 discard bit.
 Thus, this model is different from the intradomain model, where end
 nodes pick a path dynamically based on the QoS needs of the flow to
 be routed.  Here, paths within ASs are engineered based on presumed,
 measured or declared traffic and QoS requirements. Under this model,
 an AS can contract for routes via multiple transit ASs with different
 QoS requirements. For instance, AS4 above can use both AS1 and AS2 as
 transits for same or different destinations. Also, a QoS contract
 between one AS and another may generate another contract between the
 second and a third AS and so forth.
 An issue is what triggers the recomputation of path metrics within an
 AS.  Failures or other events that prevent engineered resource
 allocation should certainly trigger recomputation. Recomputation
 should not be triggered in response to arrival of flows within the
 engineered limit.

5.3 Path Computation

 Path computation for an external destination at a border node is
 based on reachability, path metrics and local policies of selection.
 If there are multiple selection criteria (e.g., delay, bandwidth,
 cost, etc.), mutiple alternaives may have to be maintained as well as
 propagated by border nodes. Selection of a path from among many
 alternatives would depend on the QoS requests of flows, as well as
 policies. Path computation may also utilze any heuristics for
 optimizing resource usage.

5.4 Flow Aggregation

 An important issue in interdomain routing is the amount of flow state
 to be processed by transit ASs. Reducing the flow state by
 aggregation techniques must therefore be seriously considered. Flow
 aggregation means that transit traffic through an AS is classified
 into a few aggregated streams rather than being routed at the
 individual flow level. For example, an entry border router may
 classify various transit flows entering an AS into a few coarse
 categories, based on the egress node and QoS requirements of the
 flows.  Then, the aggregated stream for a given traffic class may be

Crawley, et. al. Informational [Page 19] RFC 2386 A Framework for QoS-based Routing August 1998

 routed as a single flow inside the AS to the exit border router. This
 router may then present individual flows to different neighboring ASs
 and the process repeats at each entry border router. Under this
 scenario, it is essential that entry border routers keep track of the
 resource requirements for each transit flow and apply admission
 control to determine whether the aggregate requirement from any
 neighbor exceeds the engineered limit. If so, some policy must be
 invoked to deal with the excess traffic. Otherwise, it may be assumed
 that aggregated flows are routed over paths that have adequate
 resources to guarantee QoS for the member flows. Finally, it is
 possible that entry border routers at a transit AS may prefer not to
 aggregate flows if finer grain routing within the AS may be more
 efficient (e.g., to aid load balancing within the AS).

5.5 Path Cost Determination

 It is hoped that the integrated services Internet architecture would
 allow providers to charge for IP flows based on their QoS
 requirements.  A QoS-based routing architecture can aid in
 distributing information on expected costs of routing flows to
 various destinations via different domains. Clearly, from a
 provider's point of view, there is a cost incurred in guaranteeing
 QoS to flows.  This cost could be a function of several parameters,
 some related to flow parameters, others based on policy. From a
 user's point of view, the consequence of requesting a particular QoS
 for a flow is the cost incurred, and hence the selection of providers
 may be based on cost. A routing scheme can aid a provider in
 distributing the costs in routing to various destinations, as a
 function of several parameters, to other providers or to end users.
 In the interdomain routing model described earlier, the costs to a
 destination will change as routing updates are passed through a
 transit domain. One of the goals of the routing scheme should be to
 maintain a uniform semantics for cost values (or functions) as they
 are handled by intermediate domains. As an example, consider the cost
 function generated by border node B1 in domain A and passed to node
 B2 in domain B below.  The routing update may be injected into domain
 B by B2 and finally passed to B4 in domain C by router B3. Domain B
 may interpret the cost value received from domain A in any way it
 wants, for instance, adding a locally significant component to it.
 But when this cost value is passed to domain C, the meaning of it
 must be what domain A intended, plus the incremental cost of
 transiting domain B, but not what domain B uses internally.

Crawley, et. al. Informational [Page 20] RFC 2386 A Framework for QoS-based Routing August 1998

  Domain A                    Domain B           Domain C
   ____________          ___________      ____________
  |            |        |           |    |            |
  |            B1------B2          B3---B4            |
  |            |        |           |    |            |
   ------------          -----------      ------------
 A problem with charging for a flow is the determination of the cost
 when the QoS promised for the flow was not actually delivered.
 Clearly, when a flow is routed via multiple domains, it must be
 determined whether each domain delivers the QoS it declares possible
 for traffic through it.

6. QOS-BASED MULTICAST ROUTING

 The goals of QoS-based multicast routing are as follows:
  1. Scalability to large groups with dynamic membership
  1. Robustness in the presence of topological changes
  1. Support for receiver-initiated, heterogeneous reservations
  1. Support for shared reservation styles, and
  1. Support for "global" admission control, i.e., administrative

control of resource consumption by the multicast flow.

 The RSVP multicast flow model is as follows. The sender of a
 multicast flow advertises the traffic characteristics periodically to
 the receivers.  On receipt of an advertisement, a receiver may
 generate a message to reserve resources along the flow path from the
 sender. Receiver reservations may be heterogeneous. Other multicast
 models may be considered.
 The multicast routing scheme attempts to determine a path from the
 sender to each receiver that can accommodate the requested
 reservation.  The routing scheme may attempt to maximize network
 resource utilization by minimizing the total bandwidth allocated to
 the multicast flow, or by optimizing some other measure.

6.1 Scalability, Robustness and Heterogeneity

 When addressing scalability, two aspects must be considered:
   1.  The overheads associated with receiver discovery. This overhead
       is incurred when determining the multicast tree for forwarding
       best-effort sender traffic characterization to receivers.

Crawley, et. al. Informational [Page 21] RFC 2386 A Framework for QoS-based Routing August 1998

   2.  The overheads associated with QoS-based multicast path
       computation.  This overhead is incurred when flow-specific
       state information has to be collected by a router to determine
       QoS-accommodating paths to a receiver.
 Depending on the multicast routing scheme, one or both of these
 aspects become important. For instance, under the present RSVP model,
 reservations are established on the same path over which sender
 traffic characterizations are sent, and hence there is no path
 computation overhead. On the other hand, under the proposed QOSPF
 model [ZSSC97] of multicast source routing, receiver discovery
 overheads are incurred by MOSPF [M94] receiver location broadcasts,
 and additional path computation overheads are incurred due to the
 need to keep track of existing flow paths. Scaling of QoS-based
 multicast depends on both these scaling issues. However, scalable
 best-effort multicasting is really not in the domain of QoS-based
 routing work (solutions for this are being devised by the IDMR WG
 [BCF94, DEFV94]). QoS-based multicast routing may build on these
 solutions to achieve overall scalability.
 There are several options for QoS-based multicast routing. Multicast
 source routing is one under which multicast trees are computed by the
 first-hop router from the source, based on sender traffic
 advertisements.  The advantage of this is that it blends nicely with
 the present RSVP signaling model. Also, this scheme works well when
 receiver reservations are homogeneous and the same as the maximum
 reservation derived from sender advertisement.  The disadvantages of
 this scheme are the extra effort needed to accommodate heterogeneous
 reservations and the difficulties in optimizing resource allocation
 based on shared reservations.
 In these regards, a receiver-oriented multicast routing model seems
 to have some advantage over multicast source routing. Under this
 model:
   1.  Sender traffic advertisements are multicast over a best-effort
       tree which can be different from the QoS-accommodating tree for
       sender data.
   2.  Receiver discovery overheads are minimized by utilizing a
       scalable scheme (e.g., PIM, CBT), to multicast sender traffic
       characterization.
   3.  Each receiver-side router independently computes a QoS-
       accommodating path from the source, based on the receiver
       reservation. This path can be computed based on unicast routing
       information only, or with additional multicast flow-specific
       state information. In any case, multicast path computation is

Crawley, et. al. Informational [Page 22] RFC 2386 A Framework for QoS-based Routing August 1998

       broken up into multiple, concurrent nunicast path computations.
   4.  Routers processing unicast reserve messages from receivers
       aggregate resource reservations from multiple receivers.
 Flow-specific state information may be limited in Step 3 to achieve
 scalability [RN98]. In general, limiting flow-specific information in
 making multicast routing decisions is important in any routing model.
 The advantages of this model are the ease with which heterogeneous
 reservations can be accommodated, and the ability to handle shared
 reservations. The disadvantages are the incompatibility with the
 present RSVP signaling model, and the need to rely on reverse paths
 when link state routing is not used. Both multicast source routing
 and the receiver-oriented routing model described above utilize per-
 source trees to route multicast flows. Another possibility is the
 utilization of shared, per-group trees for routing flows. The
 computation and usage of such trees require further work.
 Finally, scalability at the interdomain level may be achieved if
 QoS-based multicast paths are computed independently in each domain.
 This principle is illustrated by the QOSPF multicast source routing
 scheme which allows independent path computation in different OSPF
 areas. It is easy to incorporate this idea in the receiver-oriented
 model also. An evaluation of multicast routing strategies must take
 into account the relative advantages and disadvantages of various
 approaches, in terms of scalability features and functionality
 supported.

6.2 Multicast Admission Control

 Higher level admission control, as defined for unicast, prevents
 excessive resource consumption by flows when traffic load is high.
 Such an admission control strategy must be applied to multicast flows
 when the flow path computation is receiver-oriented or sender-
 oriented. In essence, a router computing a path for a receiver must
 determine whether the incremental resource allocation for the
 receiver is excessive under some administratively determined
 admission control policy. Other admission control criteria, based on
 the total resource consumption of a tree may be defined.

7. QOS-BASED ROUTING AND RESOURCE RESERVATION PROTOCOLS

 There must clearly be a well-defined interface between routing and
 resource reservation protocols. The nature of this interface, and the
 interaction between routing and resource reservation has to be
 determined carefully to avoid incompatibilities. The importance of
 this can be readily illustrated in the case of RSVP.

Crawley, et. al. Informational [Page 23] RFC 2386 A Framework for QoS-based Routing August 1998

 RSVP has been designed to operate independent of the underlying
 routing scheme. Under this model, RSVP PATH messages establish the
 reverse path for RESV messages.  In essence, this model is not
 compatible with QoS-based routing schemes that compute paths after
 receiver reservations are received. While this incompatibility can be
 resolved in a simple manner for unicast flows, multicast with
 heterogeneous receiver requirements is a more difficult case.  For
 this, reconciliation between RSVP and QoS-based routing models is
 necessary. Such a reconciliation, however, may require some changes
 to the RSVP model depending on the QoS-based routing model [ZES97,
 ZSSC97, GOA97]. On the other hand, QoS-based routing schemes may be
 designed with RSVP compatibility as a necessary goal. How this
 affects scalability and other performance measures must be
 considered.

8. SECURITY CONSIDERATIONS

 Security issues that arise with routing in general are about
 maintaining the integrity of the routing protocol in the presence of
 unintentional or malicious introduction of information that may lead
 to protocol failure [P88]. QoS-based routing requires additional
 security measures both to validate QoS requests for flows and to
 prevent resource-depletion type of threats that can arise when flows
 are allowed to make arbitratry resource requests along various paths
 in the network. Excessive resource consumption by an errant flow
 results in denial of resources to legitimate flows. While these
 situations may be prevented by setting up proper policy constraints,
 charging models and policing at various points in the network, the
 formalization of such protection requires work [BCCH94].

9. RELATED WORK

 "Adaptive" routing, based on network state, has a long history,
 especially in circuit-switched networks. Such routing has also been
 implemented in early datagram and virtual circuit packet networks.
 More recently, this type of routing has been the subject of study in
 the context of ATM networks, where the traffic characteristics and
 topology are substantially different from those of circuit-switched
 networks [MMR96]. It is instructive to review the adaptive routing
 methodologies, both to understand the problems encountered and
 possible solutions.
 Fundamentally, there are two aspects to adaptive, network state-
 dependent routing:
   1.  Measuring and gathering network state information, and
   2.  Computing routes based on the available information.

Crawley, et. al. Informational [Page 24] RFC 2386 A Framework for QoS-based Routing August 1998

 Depending on how these two steps are implemented, a variety of
 routing techniques are possible. These differ in the following
 respects:
  1. what state information is used
  2. whether local or global state is used
  3. what triggers the propagation of state information
  4. whether routes are computed in a distributed or centralized manner
  5. whether routes are computed on-demand, pre-computed, or in a

hybrid manner

  1. what optimization criteria, if any, are used in computing routes
  2. whether source routing or hop by hop routing is used, and
  3. how alternate route choices are explored
 It should be noted that most of the adaptive routing work has focused
 on unicast routing. Multicast routing is one of the areas that would
 be prominent with Internet QoS-based routing. We treat this
 separately, and the following review considers only unicast routing.
 This review is not exhaustive, but gives a brief overview of some of
 the approaches.

9.1 Optimization Criteria

 The most common optimization criteria used in adaptive routing is
 throughput maximization or delay minimization. A general formulation
 of the optimization problem is the one in which the network revenue
 is maximized, given that there is a cost associated with routing a
 flow over a given path [MMR96, K88]. In general, global optimization
 solutions are difficult to implement, and they rely on a number of
 assumptions on the characteristics of the traffic being routed
 [MMR96]. Thus, the practical approach has been to treat the routing
 of each flow (VC, circuit or packet stream to a given destination)
 independently of the routing of other flows. Many such routing
 schemes have been implemented.

9.2 Circuit Switched Networks

 Many adaptive routing concepts have been proposed for circuit-
 switched networks. An example of a simple adaptive routing scheme is
 sequential alternate routing [T88]. This is a hop-by-hop
 destination-based routing scheme where only local state information
 is utilized.  Under this scheme, a routing table is computed for each
 node, which lists multiple output link choices for each destination.
 When a call set-up request is received by a node, it tries each
 output link choice in sequence, until it finds one that can
 accommodate the call. Resources are reserved on this link, and the
 call set-up is forwarded to the next node. The set-up either reaches
 the destination, or is blocked at some node. In the latter case, the

Crawley, et. al. Informational [Page 25] RFC 2386 A Framework for QoS-based Routing August 1998

 set-up can be cranked back to the previous node or a failure
 declared. Crankback allows the previous node to try an alternate
 path.  The routing table under this scheme can be computed in a
 centralized or distributed manner, based only on the topology of the
 network. For instance, a k-shortest-path algorithm can be used to
 determine k alternate paths from a node with distinct initial links
 [T88]. Some mechanism must be implemented during path computation or
 call set-up to prevent looping.
 Performance studies of this scheme illustrate some of the pitfalls of
 alternate routing in general, and crankback in particular [A84, M86,
 YS87]. Specifically, alternate routing improves the throughput when
 traffic load is relatively light, but adversely affects the
 performance when traffic load is heavy. Crankback could further
 degrade the performance under these conditions. In general,
 uncontrolled alternate routing (with or without crankback) can be
 harmful in a heavily utilized network, since circuits tend to be
 routed along longer paths thereby utilizing more capacity. This is an
 obvious, but important result that applies to QoS-based Internet
 routing also.
 The problem with alternate routing is that both direct routed (i.e.,
 over shortest paths) and alternate routed calls compete for the same
 resource.  At higher loads, allocating these resources to alternate
 routed calls result in the displacement of direct routed calls and
 hence the alternate routing of these calls. Therefore, many
 approaches have been proposed to limit the flow of alternate routed
 calls under high traffic loads. These schemes are designed for the
 fully-connected logical topology of long distance telephone networks
 (i.e., there is a logical link between every pair of nodes). In this
 topology, direct routed calls always traverse a 1-hop path to the
 destination and alternate routed calls traverse at most a 2-hop path.
 "Trunk reservation" is a scheme whereby on each link a certain
 bandwidth is reserved for direct routed calls [MS91]. Alternate
 routed calls are allowed on a trunk as long as the remaining trunk
 bandwidth is greater than the reserved capacity. Thus, alternate
 routed calls cannot totally displace direct routed calls on a trunk.
 This strategy has been shown to be very effective in preventing the
 adverse effects of alternate routing.
 "Dynamic alternate routing" (DAR) is a strategy whereby alternate
 routing is controlled by limiting the number of choices, in addition
 to trunk reservation [MS91]. Under DAR, the source first attempts to
 use the direct link to the destination. When blocked, the source
 attempts to alternate route the call via a pre-selected neighbor. If
 the call is still blocked, a different neighbor is selected for
 alternate routing to this destination in the future. The present call

Crawley, et. al. Informational [Page 26] RFC 2386 A Framework for QoS-based Routing August 1998

 is dropped. DAR thus requires only local state information. Also, it
 "learns" of good alternate paths by random sampling and sticks to
 them as long as possible.
 More recent circuit-switched routing schemes utilize global state to
 select routes for calls. An example is AT&T's Real-Time Network
 Routing (RTNR) scheme [ACFH92]. Unlike schemes like DAR, RTNR handles
 multiple classes of service, including voice and data at fixed rates.
 RTNR utilizes a sophisticated per-class trunk reservation mechanism
 with dynamic bandwidth sharing between classes. Also, when alternate
 routing a call, RTNR utilizes the loading on all trunks in the
 network to select a path. Because of the fully-connected topology,
 disseminating status information is simple under RTNR; each node
 simply exchanges status information directly with all others.
 From the point of view of designing QoS-based Internet routing
 schemes, there is much to be learned from circuit-switched routing.
 For example, alternate routing and its control, and dynamic resource
 sharing among different classes of traffic. It is, however, not
 simple to apply some of the results to a general topology network
 with heterogeneous multirate traffic. Work in the area of ATM network
 routing described next illustrates this.

9.3 ATM Networks

 The VC routing problem in ATM networks presents issues similar to
 that encountered in circuit-switched networks. Not surprisingly, some
 extensions of circuit-switched routing have been proposed. The goal
 of these routing schemes is to achieve higher throughput as compared
 to traditional shortest-path routing. The flows considered usually
 have a single QoS requirement, i.e., bandwidth.
 The first idea is to extend alternate routing with trunk reservation
 to general topologies [SD95].  Under this scheme, a distance vector
 routing protocol is used to build routing tables at each node with
 multiple choices of increasing hop count to each destination. A VC
 set-up is first routed along the primary ("direct") path. If
 sufficient resources are not available along this path, alternate
 paths are tried in the order of increasing hop count. A flag in the
 VC set-up message indicates primary or alternate routing, and
 bandwidth on links along an alternate path is allocated subject to
 trunk reservation. The trunk reservation values are determined based
 on some assumptions on traffic characteristics. Because the scheme
 works only for a single data rate, the practical utility of it is
 limited.
 The next idea is to import the notion of controlled alternate routing
 into traditional link state QoS-based routing [GKR96]. To do this,

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 first each VC is associated with a maximum permissible routing cost.
 This cost can be set based on expected revenues in carrying the VC or
 simply based on the length of the shortest path to the destination.
 Each link is associated with a metric that increases exponentially
 with its utilization. A switch computing a path for a VC simply
 determines a least-cost feasible path based on the link metric and
 the VC's QoS requirement.  The VC is admitted if the cost of the path
 is less than or equal to the maximum permissible routing cost. This
 routing scheme thus limits the extent of "detour" a VC experiences,
 thus preventing excessive resource consumption. This is a practical
 scheme and the basic idea can be extended to hierarchical routing.
 But the performance of this scheme has not been analyzed thoroughly.
 A similar notion of admission control based on the connection route
 was also incorporated in a routing scheme presented in [ACG92].
 Considering the ATM Forum PNNI protocol [PNNI96], a partial list of
 its stated characteristics are as follows:
          o   Scales to very large networks
          o   Supports hierarchical routing
          o   Supports QoS
          o   Uses source routed connection setup
          o   Supports multiple metrics and attributes
          o   Provides dynamic routing
 The PNNI specification is sub-divided into two protocols: a signaling
 and a routing protocol. The PNNI signaling protocol is used to
 establish point-to-point and point to multipoint connections and
 supports source routing, crankback and alternate routing. PNNI source
 routing allows loop free paths.  Also, it allows each implementation
 to use its own path computation algorithm. Furthermore, source
 routing is expected to support incremental deployment of future
 enhancements such as policy routing.
 The PNNI routing protocol is a dynamic, hierarchical link state
 protocol that propagates topology information by flooding it through
 the network.  The topology information is the set of resources (e.g.,
 nodes, links and addresses) which define the network. Resources are
 qualified by defined sets of metrics and attributes (delay, available
 bandwidth, jitter, etc.) which are grouped by supported traffic
 class.  Since some of the metrics used will change frequently, e.g.,
 available bandwidth, threshold algorithms are used to determine if
 the change in a metric or attribute is significant enough to require
 propagation of updated information.  Other features include, auto
 configuration of the routing hierarchy, connection admission control
 (as part of path calculation) and aggregation and summarization of
 topology and reachability information.

Crawley, et. al. Informational [Page 28] RFC 2386 A Framework for QoS-based Routing August 1998

 Despite its functionality, the PNNI routing protocol does not address
 the issues of multicast routing, policy routing and control of
 alternate routing. A problem in general with link state QoS-based
 routing is that of efficient broadcasting of state information. While
 flooding is a reasonable choice with static link metrics it may
 impact the performance adversely with dynamic metrics.
 Finally, Integrated PNNI [I-PNNI] has been designed from the start to
 take advantage of the QoS Routing capabilities that are available in
 PNNI and integrate them with routing for layer 3.  This would provide
 an integrated layer 2 and layer 3 routing protocol for networks that
 include PNNI in the ATM core.  The I-PNNI specification has been
 under development in the ATM Forum and, at this time, has not yet
 incorporated QoS routing mechanisms for layer 3.

9.4 Packet Networks

 Early attempts at adaptive routing in packet networks had the
 objective of delay minimization by dynamically adapting to network
 congestion.  Alternate routing based on k-shortest path tables, with
 route selection based on some local measure (e.g., shortest output
 queue) has been described [R76, YS81]. The original ARPAnet routing
 scheme was a distance vector protocol with delay-based cost metric
 [MW77]. Such a scheme was shown to be prone to route oscillations
 [B82]. For this and other reasons, a link state delay-based routing
 scheme was later developed for the ARPAnet [MRR80]. This scheme
 demonstrated a number of techniques such as triggered updates,
 flooding, etc., which are being used in OSPF and PNNI routing today.
 Although none of these schemes can be called QoS-based routing
 schemes, they had features that are relevant to QoS-based routing.
 IBM's System Network Architecture (SNA) introduced the concept of
 Class of Service (COS)-based routing [A79, GM79].  There were several
 classes of service:  interactive, batch, and network control.  In
 addition, users could define other classes. When starting a data
 session an application or device would request a COS.  Routing would
 then map the COS into a statically configured route which marked a
 path across the physical network.  Since SNA is connection oriented,
 a session was set up along this path and the application's or
 device's data would traverse this path for the life of the session.
 Initially, the service delivered to a session was based on the
 network engineering and current state of network congestion. Later,
 transmission priority was added to subarea SNA.  Transmission
 priority allowed more important traffic (e.g. interactive) to proceed
 before less time-critical traffic (e.g. batch) and improved link and
 network utilization. Transmission priority of a session was based on
 its COS.

Crawley, et. al. Informational [Page 29] RFC 2386 A Framework for QoS-based Routing August 1998

 SNA later evolved to support multiple or alternate paths between
 nodes.  But, although assisted by network design tools, the network
 administrator still had to statically configure routes. IBM later
 introduced SNA's Advanced Peer to Peer Networking (APPN) [B85]. APPN
 added new features to SNA including dynamic routing based on a link
 state database. An application would use COS to indicate it traffic
 requirements and APPN would calculate a path capable of meeting these
 requirements.  Each COS was mapped to a table of acceptable metrics
 and parameters that qualified the nodes and links contained in the
 APPN topology Database.  Metrics and parameters used as part of the
 APPN route calculation include, but are not limited to:  delay, cost
 per minute, node congestion and security.  The dynamic nature of APPN
 allowed it to route around failures and reduce network configuration.
 The service delivered by APPN was still based on the network
 engineering, transmission priority and network congestion.  IBM later
 introduced an extension to APPN, High Performance Routing
 (HPR)[IBM97]. HPR uses a congestion avoidance algorithm called
 adaptive rate based (ARB) congestion control.  Using predictive
 feedback methods, the ARB algorithm prevents congestion and improves
 network utilization.  Most recently, an extension to the COS table
 has been defined so that HPR routing could recognize and take
 advantage of ATM QoS capabilities.
 Considering IP routing, both IDRP [R92] and OSPF support  type of
 service (TOS)-based routing. While the IP header has a TOS field,
 there is no standardized way of utilizing it for TOS specification
 and routing. It seems possible to make use of the IP TOS feature,
 along with TOS-based routing and proper network engineering, to do
 QoS-based routing. The emerging differentiated services model is
 generating renewed interest in TOS support. Among the newer schemes,
 Source Demand Routing (SDR) [ELRV96] allows  on-demand path
 computation by routers and the implementation of strict and loose
 source routing. The Nimrod architecture [CCM96] has a number of
 concepts built in to handle scalability and specialized path
 computation. Recently, some work has been done on QoS-based routing
 schemes for the integrated services Internet. For example, in [M98],
 heuristic schemes for efficient routing of flows with bandwidth
 and/or delay constraints is described and evaluated.

9. SUMMARY AND CONCLUSIONS

 In this document, a framework for QoS-based Internet routing was
 defined.  This framework adopts the traditional separation between
 intra and interdomain routing. This approach is especially meaningful
 in the case of QoS-based routing, since there are many views on how
 QoS-based routing should be accomplished and many different needs.
 The objective of this document was to encourage the development of

Crawley, et. al. Informational [Page 30] RFC 2386 A Framework for QoS-based Routing August 1998

 different solution approaches for intradomain routing, subject to
 some broad requirements, while consensus on interdomain routing is
 achieved. To this end, the QoS-based routing issues were described,
 and some broad intradomain routing requirements and an interdomain
 routing model were defined. In addition, QoS-based multicast routing
 was discussed and a detailed review of related work was presented.
 The deployment of QoS-based routing across multiple administrative
 domains requires both the development of intradomain routing schemes
 and a standard way for them to interact via a well-defined
 interdomain routing mechanism. This document, while outlining the
 issues that must be addressed, did not engage in the specification of
 the actual features of the interdomain routing scheme. This would be
 the next step in the evolution of wide-area, multidomain QoS-based
 routing.

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Crawley, et. al. Informational [Page 32] RFC 2386 A Framework for QoS-based Routing August 1998

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Crawley, et. al. Informational [Page 33] RFC 2386 A Framework for QoS-based Routing August 1998

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Crawley, et. al. Informational [Page 34] RFC 2386 A Framework for QoS-based Routing August 1998

 [W97]   Wroclawski, J., "Specification of the Controlled-Load Network
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          Extensions to OSPF", Work in Progress.

Crawley, et. al. Informational [Page 35] RFC 2386 A Framework for QoS-based Routing August 1998

AUTHORS' ADDRESSES

 Bala Rajagopalan
 NEC USA, C&C Research Labs
 4 Independence Way
 Princeton, NJ 08540
 U.S.A
 Phone: +1-609-951-2969
 EMail: braja@ccrl.nj.nec.com
 Raj Nair
 Arrowpoint
 235 Littleton Rd.
 Westford, MA 01886
 U.S.A
 Phone: +1-508-692-5875, x29
 EMail: nair@arrowpoint.com
 Hal Sandick
 Bay Networks, Inc.
 1009 Slater Rd., Suite 220
 Durham, NC 27703
 U.S.A
 Phone: +1-919-941-1739
 EMail: Hsandick@baynetworks.com
 Eric S. Crawley
 Argon Networks, Inc.
 25 Porter Rd.
 Littelton, MA 01460
 U.S.A
 Phone: +1-508-486-0665
 EMail: esc@argon.com

Crawley, et. al. Informational [Page 36] RFC 2386 A Framework for QoS-based Routing August 1998

Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
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Crawley, et. al. Informational [Page 37]

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