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INTERNET DRAFT draft-ietf-qosr-framework-00.txt March, 21, 1996 A Framework for QoS-based Routing in the Internet Eric Crawley Raj Nair Bala Rajagopalan Hal Sandick Bay Networks Ascom Nexion NEC USA IBM Status of this Memo This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress." To learn the current status of any Internet Draft, please check the 1id-abstracts.txt listing contained in the Internet Drafts Shadow Directories on ds.internic.net, nic.nordu.net, ftp.nisc.sri.com or munnari.oz.au. Distribution of this memo is unlimited. This Internet Draft expires on September, 26, 1997. ABSTRACT QoS-based routing is being recognized as the missing piece in the evolution of QoS-based service offerings in the Internet. This document describes some of the QoS-based routing issues and proposes a framework for QoS-based routing in the Internet. 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 might be best to employ the following two-pronged approach be 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. 2. Specify simple, consistent and stable interactions between ASs implementing routing solutions developed as above. draft-ietf-qosr-framework-00.txt [Page 1] This approach follows the traditional separation between intra and interdomain routing. It allows solutions like QOSPF [GOW96, ZSSC96], 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. The aim of this draft is to describe the QoS-based routing issues, begin identifying the basic requirements on intra and interdomain routing, and describe a model for interdomain routing. It is not an objective of this draft to dictate 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 how particular signaling protocols such as RSVP should interact with QoS-based routing. The specific interactions needed, however, would be clear from the intra and interdomain routing solutions devised. In the intradomain area, the goal is to develop consensus on the basic routing requirements while allowing maximum freedom for the development of solutions. In the interdomain area, the objectives are to begin identifying the QoS-based routing functions, and facilitate the development of a routing protocol that allows relatively simple interaction between domains. The views presented in this draft are expected to evolve as consensus emerges on Internet-wide QoS- based routing needs. In the next section, a glossary of relevant terminology is given. In Section 3, the objectives of QoS-based routing are described and past work in related areas is reviewed. The issues that must be dealt with by QoS-based Internet routing efforts are briefly outlined in Section 4. In Section 5, a start is made in defining the intradomain routing requirements. These requirements are purposely broad, putting few constraints on solution approaches. The interdomain routing model and issues are described in Section 6. Among topics that deserve special attention are routing metrics and path computation, and multicast routing. These are discussed in Sections 7 and 8, respectively. The interaction between QoS-based routing and signaling is briefly considered in Section 9. Finally, summary and conclusions are presented in Section 10. 2. GLOSSARY The following glossary lists the terminology used in this draft and an explanation of what is meant. Some of these terms may have different connotations, but when used in this draft, 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 intradomain routing protocol and administrative authority. 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. draft-ietf-qosr-framework-00.txt [Page 2] 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 [W96] and Guaranteed Service [SPG97]. RSVP: The ReSerVation Protocol [BZBH96]. A QoS signaling protocol for the Internet. Path: A unicast or multicast path. 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 determining the path for a flow, and attempting to establish state in routers along the flow path to satisfy its QoS requirement. 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 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. draft-ietf-qosr-framework-00.txt [Page 3] 3. BACKGROUND 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]. "Adaptive" routing, which has similar goals as above, 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. 3.1 Related Work 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. 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. draft-ietf-qosr-framework-00.txt [Page 4] 3.1.1 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. 3.1.2 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 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. draft-ietf-qosr-framework-00.txt [Page 5] "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 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. 3.1.3 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 [RSR95, GKR96]. To do this, 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 draft-ietf-qosr-framework-00.txt [Page 6] 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. 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. 3.1.4 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, draft-ietf-qosr-framework-00.txt [Page 7] 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. Subarea 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 applications 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. Then in 1995 IBM introduced an extension to APPN, High Performance Routing (HPR)[IBM97]. HPR uses a highly responsive 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. 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. 4. QOS-BASED ROUTING ISSUES Based on the discussion so far, we can identify a number of issues with regard to QoS-based Internet routing. While some of these are general concerns with any QoS-based routing scheme, others are specific to the Internet environment: draft-ietf-qosr-framework-00.txt [Page 8] - 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 routing metrics are used and how is flow admission control done? - What is the granularity of routing decision (i.e., destination-based, source and destination-based, or flow-based)? - With flow-based routing, how are QoS-accommodating paths computed by routers for unicast flows? - How are QoS-accommodating paths computed for multicast flows with different reservation styles and receiver heterogeneity? - What are the administrative control issues? - What factors affect the routing overheads?, and - What are the scalability issues? Some of these issues are discussed briefly next. Metrics and path computation is discussed in Section 7 and interdomain routing is discussed in Section 6. 4.1 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. The ISSLL working group and the IEEE 802.1 group are attempting to resolve these issues. 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 with some complexity. 4.2 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 destination. This set of routing granularities presents a problem for QoS routing solutions. draft-ietf-qosr-framework-00.txt [Page 9] If routing based only on destination address is considered, then all flows between any source and the destination will be routed over the same path. This is fine if the path has adequate capacity but it can be a problem if there are multiple flows to a destination that exceed the capacity of the link. One version of QOSPF [ZSSC96] 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 is also increased since the routing tables must include source/destination pairs instead of just destination. This amount of state increases rapidly as the traditional routes are summarized. The best granularity is found when routing is based on individual flows but this has a tremendous cost for routing state. Each QoS flow can be routed separately between any source and destination. Use of the IPv6 flow label can help in identifying or classifying flows. Both source/destination and flow based routing also have a dangerous property when it comes to route loop detection. If a node along a flow or source/destination based path loses the state information for the flow and the flow based route is different from the destination only based routing, the potential exists for a route loop to form when the node forwards the packet based on destination routing towards a node earlier on the path. 4.3 Unicast Flow Path Computation Algorithms With flow-based routing, how should paths be computed for unicast flows? The answer to this question depends on the performance objectives of a QoS-based routing scheme. One common objective 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 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 [RSR95]. Such a mechanism is referred to in this draft 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 interesting 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 acceptance rate for "larger" flows, without over- engineering the network, requires a fair higher level admission control mechanism. Path computation with multiple QoS constraints on a flow is a difficult problem [WC96]. The determination of allowable combination of QoS parameters, the performance objectives, and algorithms for path computation based on these are issues that must be addressed by the routing scheme. draft-ietf-qosr-framework-00.txt [Page 10] 4.4 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 6. 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. 4.4.1 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. 4.4.2 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 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. 4.5 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 [BZBH96], may require new algorithms. Multicast routing is discussed in some detail in Section 8. draft-ietf-qosr-framework-00.txt [Page 11] 4.6 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). 4.7 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 as discussed before, 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, along with a higher level admission control mechanism. 5. 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. draft-ietf-qosr-framework-00.txt [Page 12] 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. - 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 signaling requirement. 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 should support QoS-based multicasting with receiver heterogeneity and shared reservation styles. - Etc. 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. - Etc. 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. 6. INTERDOMAIN ROUTING The interdomain routing model is depicted below. AS1 AS2 AS3 ___________ _____________ ____________ | | | | | | | B------B B----B | | | | | | | -----B----- B------------- --B--------- \ / / \ / / ____B_____B____ _________B______ | | | | | B-------B | | | | | | B-------B | --------------- ---------------- AS4 AS5 draft-ietf-qosr-framework-00.txt [Page 13] 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. Finally, 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 the link state routing paradigm, 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 by a hierarchical link state representation, with appropriate metric aggregation. Link state routing is not necessarily 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 may also allow the implementation QoS-enhanced versions of traditional interdomain protocols such as IDRP. While limiting the information flow between domains results in routing simplicity and scalability, the information exchanged must enable certain basic functions: draft-ietf-qosr-framework-00.txt [Page 14] - determination of reachability to various destinations - loop-free flow routes - address aggregation when possible - 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. - determination, optionally, of multiple paths for a given destination, based on service classes. - expression of routing policies, including monetary cost, as a function of flow parameters, usage and administrative factors. These capabilities may be realized using a QoS-based path vector, link state or some other interdomain routing scheme. With any interdomain routing scheme the exact nature of the QoS and policy information exchanged between domains, as well as triggers for changes in interdomain routes and QoS indications, are to be determined. The next section discusses some general issues with metrics and path computation. This discussion is relevant to both intra and interdomain routing. 7. METRICS AND PATH COMPUTATION 7.1 Background Routing a flow represents the intent to use a collection of resources that could be distributed throughout the network. Most of these resources are usually concentrated along the path of the flow. Consequently, it is important to determine the path of a flow with the least impact on network performance by considering flow attributes such as priority, level of guaranteed service and possibly the estimated life of the connection. When signaling is involved, these needs have to be communicated to the router through a well-defined interface with the upper-layer software. The use of suitable metrics will ensure that the computed paths are both consistent with the requirements of the flow and those of the network. To allow a consistent interpretation of the metrics, a uniform representation of common metrics such a delay, residual bandwidth, etc., is required. Encoding of the maximum, minimum, range, and granularity 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 an update to be generated. The stability of the QoS routes would depend on the ability to control the generation of updates. It is essential to obtain a fairly stable view of the interconnection among the routing domains. Two classes of allocation schemes must be considered: link-by-link and path- by-path. The former refers to policies that consider a link in isolation to the rest of the network. The latter optimizes allocation of resources along an entire path with respect to the entire network or subnetwork. draft-ietf-qosr-framework-00.txt [Page 15] A link-by-link scheme implies that the traffic characteristics of a flow does not significantly impact route selection. This is valid for bursty traffic streams where individual bursts are not correlated to each other significantly. While this may be true for LAN-interconnections, this condition does not apply to real-time traffic such as voice/video where traffic streams exhibit a high degree of auto-correlation. In the rest of this discussion on metrics, we refer to terms such as "link" and "trunk" in a generic way that would include both physical and logical links. 7.2 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. 7.3 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. 7.4 Datagram Flows A delay-sensitive metric is the 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, we could use a recursive filtering technique that is based on a simple and efficient weighted averaging algorithm [NC94]. 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. 7.5 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 draft-ietf-qosr-framework-00.txt [Page 16] 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 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]. 7.6 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. 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. In addition to this, the routing cost for a flow has to capture the effects of different classes of service supported on the path taken by a flow. That is, a flow may be routed on a link supporting multiple COS. How resources are reserved for and shared among various flows on the link should be reflected draft-ietf-qosr-framework-00.txt [Page 17] on the cost of using the link. Specifically, the interactions between the new flow and the existing flows (or future flows where appropriate) on the link should be accounted for. For example, a new real-time flow of priority K over a trunk may impact existing real-time flows of lower priority sharing the same trunk. The lower priority flows are impacted, for instance, if they get preempted in order to route the higher priority flow. Or, flows may sometimes experience worse QoS than originally contracted at setup time. 8. 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. One possible 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. However, this model corresponds to the present RSVP signaling model. 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. 8.1 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. 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 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 [ZSSC96] 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. draft-ietf-qosr-framework-00.txt [Page 18] 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 IDMR 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 broken up into multiple, concurrent unicast 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. 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 some thought. 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. 8.2 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 to/ 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. draft-ietf-qosr-framework-00.txt [Page 19] 9. QOS-BASED ROUTING AND SIGNALING There must clearly be a well-defined interface between routing and signaling. The nature of this interface, and the interaction between routing and signaling has to be determined through joint work by the routing and signaling efforts. Lack of proper coordination could result in incompatibilities. This can be readily illustrated in the case of RSVP. 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. The receiver- oriented multicast routing model described above is an example. Clearly, 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. 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. Thus, the issue of routing-signaling interaction can be quite involved. 10. SUMMARY AND CONCLUSIONS In this draft, a framework for QoS-based Internet routing was defined. This framework emphasizes 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 draft was to encourage the development of different solution approaches for intradomain routing, subject to some broad requirements, while consensus on interdomain routing is achieved. To this end, a start was made on defining the intradomain routing requirements and the interdomain routing philosophy. Two areas that are particularly important for both intra and interdomain routing, metrics and path computation, and QoS-based multicast routing, were discussed in some detail. A detailed review of related work and QoS-based routing issues was also presented. REFERENCES [A79] V. Ahuja, "Routing and Flow Control in SNA" IBM Systems Journal, 18 No. 2, pp. 298-314, 1979. [A84] J. M. 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Rajagopalan, R. Srikant and K. G. Ramakrishnan, "An Efficient ATM VC Routing Scheme," Draft, 1995 (Available from braja@ccrl.nj.nec.com) [SD95] S. Sibal and A. Desimone, "Controlling Alternate Routing in General- Mesh Packet Flow Networks," Proceedings of ACM SIGCOMM `95, 1995. [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. [W96] J. Wroclawski. Specification of the Controlled-Load Network Element Service. Internet Draft, draft-ietf-intserv-ctrl-load-svc-04.txt, November 1996. [WC96] Z. Wang and J. Crowcroft, "QoS Routing for Supporting Resource Reservation," available at http://boom.cs.ucl.ac.uk/staff/zwang/pub.htm. draft-ietf-qosr-framework-00.txt [Page 22] [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. [ZSSC96] Z. Zhang, C. Sanchez, B. Salkewicz, and E. Crawley, "QoS Extensions to OSPF," Internet Draft, draft-zhang-qos-ospf-00.txt, June, 1996. AUTHORS' ADDRESSES Bala Rajagopalan Raj Nair NEC USA, C&C Research Labs Ascom Nexion 4 Independence Way 289 Great Rd. Princeton, NJ 08540 Acton, MA 01720 U.S.A U.S.A Ph: +1-609-951-2969 Ph: +1-508-266-4536 Email: braja@ccrl.nj.nec.com Email: nair@nexen.com Hal Sandick Eric S. Crawley IBM ND, E95/B664 Bay Networks Inc. 800 Park Offices Drive 3 Federal Street, BL3-04 RTP, NC 27705 Billerica, MA 01821 U.S.A U.S.A Ph: +1-919-254-4614 Ph: +1-508-670-8888 Email: sandick@vnet.ibm.com Email: esc@baynetworks.com ******* This draft expires on September, 26, 1997 ******** draft-ietf-qosr-framework-00.txt [Page 23]