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Network Working Group                                       E. CrawleyRequest for Comments: 2386                              Argon NetworksCategory: Informational                                        R. Nair                                                            Arrowpoint                                                        B. Rajagopalan                                                               NEC USA                                                            H. Sandick                                                          Bay Networks                                                           August 1998A Framework for QoS-based Routing in the InternetStatus 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 OFDOCUMENT & 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. InSection 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. InSection 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 inSection 5 and QoS-based multicast routing is   discussed inSection 6.  The interaction between QoS-based routing   and resource reservation protocols is briefly considered inSection7. Security considerations are listed inSection 8 and related work   is described inSection 9. Finally, summary and conclusions are   presented inSection 10.Crawley, et. al.             Informational                      [Page 2]

RFC 2386           A Framework for QoS-based Routing         August 19982.  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 inRFC 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 ISSUES3.1  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 amongCrawley, 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 trafficCrawley, 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:   -  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.   -  What is the granularity of routing decision (i.e., destination-      based, source and destination-based, or flow-based)?   -  What routing metrics are used and how are QoS-accommodating paths      computed for unicast flows?Crawley, et. al.             Informational                      [Page 6]

RFC 2386           A Framework for QoS-based Routing         August 1998   -  How are QoS-accommodating paths computed for multicast flows with      different reservation styles and receiver heterogeneity?   -  What are the performance objectives while computing QoS-based      paths?   -  What are the administrative control issues?   -  What factors affect the routing overheads?, and   -  How is scalability achieved?   Some of these issues are discussed briefly next. Interdomain routing   is discussed inSection 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 occursCrawley, 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 Computation3.6.1 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 wouldCrawley, 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 inCrawley, et. al.             Informational                     [Page 10]

RFC 2386           A Framework for QoS-based Routing         August 1998   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 ofCrawley, et. al.             Informational                     [Page 11]

RFC 2386           A Framework for QoS-based Routing         August 1998   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 toSection5.   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 thanCrawley, et. al.             Informational                     [Page 12]

RFC 2386           A Framework for QoS-based Routing         August 1998   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 inSection 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).Crawley, et. al.             Informational                     [Page 13]

RFC 2386           A Framework for QoS-based Routing         August 19983.10   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:   - 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   - The routing scheme must indicate disruptions to the current route     of a flow due to topological changes.   - 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.   - The routing scheme may optionally support QoS-based multicasting     with receiver heterogeneity and shared reservation styles.   In addition, the following capabilities are also recommended:   - Capabilities to optimize resource usage.   - 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 19985.1 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:   - What information is exchanged between ASs?   - What routing capabilities does the information exchange lead to?     (E.g., source routing, on-demand path computation, etc.)   - How is the external routing information represented within an AS?   - How are interdomain paths computed?   - What sort of policy controls may be exerted on interdomain path     computation and flow routing?, and   - 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 handledCrawley, 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:   - 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.   - 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.   - ASs may not want to advertise any details of their internal     topology or resource availability.   - 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____         |               |         |               |         |               |         |               |          ---------------               AS4Crawley, 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 beCrawley, 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:   - Scalability to large groups with dynamic membership   - Robustness in the presence of topological changes   - Support for receiver-initiated, heterogeneous reservations   - Support for shared reservation styles, and   - 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 isCrawley, 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:   -  what state information is used   -  whether local or global state is used   -  what triggers the propagation of state information   -  whether routes are computed in a distributed or centralized manner   -  whether routes are computed on-demand, pre-computed, or in a      hybrid manner   -  what optimization criteria, if any, are used in computing routes   -  whether source routing or hop by hop routing is used, and   -  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, theCrawley, 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 callCrawley, 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,Crawley, et. al.             Informational                     [Page 27]

RFC 2386           A Framework for QoS-based Routing         August 1998   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 ofCrawley, 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.REFERENCES   [A79]    V. Ahuja, "Routing and Flow Control in SNA", IBM Systems            Journal, 18 No. 2, pp.  298-314, 1979.   [A84]    J. M. Akinpelu, "The Overload Performance of Engineered            Networks with Non-Hierarchical Routing", AT&T Technical            Journal, Vol. 63, pp. 1261-1281, 1984.   [ACFH92] G. R. Ash, J. S. Chen, A. E. Frey and B. D. Huang, "RealTime            Network Routing in a Dynamic Class-of-Service Network",            Proceedings of ITC 13, 1992.   [ACG92]  H. Ahmadi, J. Chen, and R. Guerin, "Dynamic Routing and Call            Control in High-Speed Integrated Networks", Proceedings of            ITC-13, pp. 397-403, 1992.   [B82]    D. P. Bertsekas, "Dynamic Behavior of Shortest Path Routing            Algorithms for Communication Networks", IEEE Trans. Auto.            Control, pp. 60-74, 1982.   [B85]    A. E. Baratz, "SNA Networks of Small Systems", IEEE JSAC,            May, 1985.   [BBCD98] Black, D., Blake, S., Carlson, M., Davies, E., Wang, Z., and            W. Weiss, "An Architecture for Differentiated Services",            Work in Progress.   [BCCH94] Braden, R., Clark, D., Crocker, D., and C. Huitema, "Report            of IAB Workshop on Security in the Internet Architecture",RFC 1636, June 1994.Crawley, et. al.             Informational                     [Page 31]

RFC 2386           A Framework for QoS-based Routing         August 1998   [BCF94]  A. Ballardie, J. Crowcroft and P. Francis, "Core-Based            Trees: A Scalable Multicast Routing Protocol", Proceedings            of SIGCOMM `94.   [BCS94]  Braden, R., Clark, D., and S. Shenker, "Integrated Services            in the Internet Architecture: An Overview",RFC 1633, July            1994.   [BZ92]   S. Bahk and M. El Zarki, "Dynamic Multi-Path Routing and How            it Compares with Other Dynamic Routing Algorithms for High            Speed Wide Area Networks", Proc. SIGCOMM `92, pp. 53-64,            1992.   [BZBH97] Braden, R., Zhang, L., Berson, S., Herzog, S., and S. Jamin,            "Resource ReSerVation Protocol (RSVP) -- Version 1            Functional Spec",RFC 2205, September 1997.   [C91]    C-H. Chow, "On Multicast Path Finding Algorithms",            Proceedings of the IEEE INFOCOM `91, pp. 1274-1283, 1991.   [CCM96]  Castineyra, I., Chiappa, J., and M. Steenstrup, "The Nimrod            Routing Architecture",RFC 1992, August 1996.   [DEFV94] S. E. Deering, D. Estrin, D. Farinnacci, V. Jacobson, C-G.            Liu, and L. Wei, "An Architecture for Wide-Area Multicast            Routing", Technical Report, 94-565, ISI, University of            Southern California, 1994.   [ELRV96] Estrin, D., Li, T., Rekhter, Y., Varadhan, K., and D.            Zappala, "Source Demand Routing: Packet Format and            Forwarding Specification (Version 1)",RFC 1940, May 1996.   [GKR96]  R. Gawlick, C. R. Kalmanek, and K. G. Ramakrishnan, "On-Line            Routing of Permanent Virtual Circuits", Computer            Communications, March, 1996.   [GPSS98] A. Ghanwani, J. W. Pace, V. Srinivasan, A. Smith and M.            Seaman, "A Framework for Providing Integrated Services over            Shared and Switched IEEE 802 LAN Technologies", Work in            Progress.   [GM79]   J. P. Gray, T. B. McNeil, "SNA Multi-System Networking", IBM            Systems Journal, 18 No. 2, pp.  263-297, 1979.   [GOA97]  Y. Goto, M. Ohta and K. Araki, "Path QoS Collection for            Stable Hop-by-Hop QoS Routing", Proc. INET '97, June, 1997.Crawley, et. al.             Informational                     [Page 32]

RFC 2386           A Framework for QoS-based Routing         August 1998   [GKOP98] R. Guerin, S. Kamat, A. Orda, T. Przygienda, and D.            Williams, "QoS Routing Mechanisms and OSPF extensions", work            in progress, March, 1998.   [IBM97]  IBM Corp, SNA APPN - High Performance Routing Architecture            Reference, Version 2.0, SV40-1018, February 1997.   [IPNNI]  ATM Forum Technical Committee. Integrated PNNI (I-PNNI) v1.0            Specification. af-96-0987r1, September 1996.   [ISI81]  Postel, J., "Internet Protocol", STD 5,RFC 791, September            1981.   [JMW83]  J. M. Jaffe, F. H. Moss, R. A. Weingarten, "SNA Routing:            Past, Present, and Possible Future", IBM Systems Journal,            pp.  417-435, 1983.   [K88]    F.P. Kelly, "Routing in Circuit-Switched Networks:            Optimization, Shadow Prices and Decentralization", Adv.            Applied Prob., pp. 112-144, March, 1988.   [L95]    W. C. Lee, "Topology Aggregation for Hierarchical Routing in            ATM Networks", ACM SIGCOMM Computer Communication Review,            1995.   [M86]    L. G. Mason, "On the Stability of Circuit-Switched Networks            with Non-hierarchical Routing", Proc. 25th Conf. On Decision            and Control, pp. 1345-1347, 1986.   [M98]    Moy, J., "OSPF Version 2", STD 54,RFC 2328, April 1998.   [M94]    Moy, J., "MOSPF: Analysis and Experience",RFC 1585,  March            1994.   [M98]    Q. Ma, "Quality-of-Service Routing in Integrated Services            Networks", PhD thesis, Computer Science Department, Carnegie            Mellon University, 1998.   [MMR96]  D. Mitra, J. Morrison, and K. G. Ramakrishnan, "ATM Network            Design and Optimization: A Multirate Loss Network            Framework", Proceedings of IEEE INFOCOM `96, 1996.   [MRR80]  J. M. McQuillan, I. Richer and E. C. Rosen, "The New Routing            Algorithm for the ARPANET", IEEE Trans.  Communications, pp.            711-719, May, 1980.Crawley, et. al.             Informational                     [Page 33]

RFC 2386           A Framework for QoS-based Routing         August 1998   [MS91]   D. Mitra and J. B. Seery, "Comparative Evaluations of            Randomized and Dynamic Routing Strategies for Circuit            Switched Networks", IEEE Trans. on Communications, pp. 102-            116, January, 1991.   [MW77]   J. M. McQuillan and D. C. Walden, "The ARPANET Design            Decisions", Computer Networks, August, 1977.   [NC94]   Nair, R. and Clemmensen, D. : "Routing in Integrated            Services Networks", Proc. 2nd International Conference on            Telecom.  Systems  Modeling and Analysis, March 1994.   [P88]    R. Perlman, "Network Layer Protocol with Byzantine            Robustness", Ph.D. Thesis, Dept. of EE and CS, MIT, August,            1988.   [PNNI96] ATM Forum PNNI subworking group, "Private Network-Network            Interface Spec.  v1.0 (PNNI 1.0)", afpnni-0055.00, March            1996.   [R76]    H. Rudin, "On Routing and "Delta Routing": A Taxonomy and            Performance Comparison of Techniques for Packet-Switched            Networks", IEEE Trans. Communications, pp. 43-59, January,            1996.   [R92]    Y. Rekhter, "IDRP Protocol Analysis: Storage Overhead", ACM            Comp.  Comm.  Review, April, 1992.   [R96]    B. Rajagopalan,"Efficient Link State Routing", Work in            Progress, available from braja@ccrl.nj.nec.com.   [RN98]   B. Rajagopalan and R. Nair, "Multicast Routing with Resource            Reservation", to appear in J. of High Speed Networks, 1998.   [SD95]   S. Sibal and A. Desimone, "Controlling Alternate Routing in            General-Mesh Packet Flow Networks", Proceedings of ACM            SIGCOMM, 1995.   [SPG97]  Shenker, S., Partridge, C., and R. Guerin, "Specification of            Guaranteed Quality of Service",RFC 2212, September 1997.   [T88]    D. M. Topkis, "A k-Shortest-Path Algorithm for Adaptive            Routing in Communications Networks", IEEE Trans.            Communications, pp.  855-859, July, 1988.   [W88]    B. M. Waxman, "Routing of Multipoint Connections", IEEE            JSAC, pp. 1617-1622, December, 1988.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            Element Service",RFC 2211, September 1997.   [WC96]   Z. Wang and J. Crowcroft, "QoS Routing for Supporting            Resource Reservation", IEEE JSAC, September, 1996.   [YS81]   T. P. Yum and M. Schwartz, "The Join-Based Queue Rule and            its Application to Routing in Computer Communications            Networks", IEEE Trans. Communications, pp. 505-511, 1981.   [YS87]   T. G. Yum and M. Schwartz, "Comparison of Routing Procedures            for Circuit-Switched Traffic in Nonhierarchical Networks",            IEEE Trans. Communications, pp. 535-544, May, 1987.   [ZES97]  Zappala, D., Estrin, D., and S. Shenker, "Alternate Path            Routing and Pinning for Interdomain Multicast Routing", USC            Computer Science Technical Report #97-655, USC, 1997.   [ZSSC97] Zhang, Z., Sanchez, C., Salkewicz, B., and E. Crawley, "QoS            Extensions to OSPF", Work in Progress.Crawley, et. al.             Informational                     [Page 35]

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

RFC 2386           A Framework for QoS-based Routing         August 1998Full Copyright Statement   Copyright (C) The Internet Society (1998).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Crawley, et. al.             Informational                     [Page 37]