Network Working Group G. Apostolopoulos Request for Comments: 2676 D. Williams Category: Experimental IBM S. Kamat Lucent R. Guerin UPenn A. Orda Technion T. Przygienda Siara Systems August 1999 QoS Routing Mechanisms and OSPF Extensions Status of this Memo This memo defines an Experimental Protocol for the Internet community. It does not specify an Internet standard of any kind. Discussion and suggestions for improvement are requested. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (1999). All Rights Reserved. Abstract This memo describes extensions to the OSPF [Moy98] protocol to support QoS routes. The focus of this document is on the algorithms used to compute QoS routes and on the necessary modifications to OSPF to support this function, e.g., the information needed, its format, how it is distributed, and how it is used by the QoS path selection process. Aspects related to how QoS routes are established and managed are also briefly discussed. The goal of this document is to identify a framework and possible approaches to allow deployment of QoS routing capabilities with the minimum possible impact to the existing routing infrastructure. In addition, experience from an implementation of the proposed extensions in the GateD environment [Con], along with performance measurements is presented.
Table of Contents 1. Introduction 3 1.1. Overall Framework . . . . . . . . . . . . . . . . . . . . 3 1.2. Simplifying Assumptions . . . . . . . . . . . . . . . . . 5 2. Path Selection Information and Algorithms 7 2.1. Metrics . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2. Advertisement of Link State Information . . . . . . . . . 8 2.3. Path Selection . . . . . . . . . . . . . . . . . . . . .10 2.3.1. Path Computation Algorithm . . . . . . . . . . .11 3. OSPF Protocol Extensions 16 3.1. QoS -- Optional Capabilities . . . . . . . . . . . . . .17 3.2. Encoding Resources as Extended TOS . . . . . . . . . . .17 3.2.1. Encoding bandwidth resource . . . . . . . . . . .19 3.2.2. Encoding Delay . . . . . . . . . . . . . . . . .21 3.3. Packet Formats . . . . . . . . . . . . . . . . . . . . .21 3.4. Calculating the Inter-area Routes . . . . . . . . . . . .22 3.5. Open Issues . . . . . . . . . . . . . . . . . . . . . . .22 4. A Reference Implementation based on GateD 22 4.1. The Gate Daemon (GateD) Program . . . . . . . . . . . . .22 4.2. Implementing the QoS Extensions of OSPF . . . . . . . . .23 4.2.1. Design Objectives and Scope . . . . . . . . . . .23 4.2.2. Architecture . . . . . . . . . . . . . . . . . .24 4.3. Major Implementation Issues . . . . . . . . . . . . . . .25 4.4. Bandwidth and Processing Overhead of QoS Routing . . . .29 5. Security Considerations 32 A. Pseudocode for the BF Based Pre-Computation Algorithm 33 B. On-Demand Dijkstra Algorithm for QoS Path Computation 36 C. Precomputation Using Dijkstra Algorithm 39 D. Explicit Routing Support 43 Endnotes 45 References 46 Authors' Addresses 48 Full Copyright Statement 50
1. Introduction In this document, we describe a set of proposed additions to the OSPF routing protocol (these additions have been implemented on top of the GateD [Con] implementation of OSPF V2 [Moy98]) to support Quality- of-Service (QoS) routing in IP networks. Support for QoS routing can be viewed as consisting of three major components: 1. Obtain the information needed to compute QoS paths and select a path capable of meeting the QoS requirements of a given request, 2. Establish the path selected to accommodate a new request, 3. Maintain the path assigned for use by a given request. Although we touch upon aspects related to the last two components, the focus of this document is on the first one. In particular, we discuss the metrics required to support QoS, the extension to the OSPF link state advertisement mechanism to propagate updates of QoS metrics, and the modifications to the path selection to accommodate QoS requests. The goal of the extensions described in this document is to improve performance for QoS flows (likelihood to be routed on a path capable of providing the requested QoS), with minimal impact on the existing OSPF protocol and its current implementation. Given the inherent complexity of QoS routing, achieving this goal obviously implies trading-off "optimality" for "simplicity", but we believe this to be required in order to facilitate deployment of QoS routing capabilities. In addition to describing the proposed extensions to the OSPF protocol, this document also reports experimental data based on performance measurements of an implementation done on the GateD platform (see Section 4). 1.1. Overall Framework We consider a network (1) that supports both best-effort packets and packets with QoS guarantees. The way in which the network resources are split between the two classes is irrelevant, except for the assumption that each QoS capable router in the network is able to dedicate some of its resources to satisfy the requirements of QoS packets. QoS capable routers are also assumed capable of identifying and advertising resources that remain available to new QoS flows. In addition, we limit ourselves to the case where all the routers involved support the QoS extensions described in this document, i.e., we do not consider the problem of establishing a route in a heterogeneous environment where some routers are QoS-capable and others are not. Furthermore, in this document, we focus on the case
of unicast flows, although many of the additions we define are applicable to multicast flows as well. We assume that a flow with QoS requirements specifies them in some fashion that is accessible to the routing protocol. For example, this could correspond to the arrival of an RSVP [RZB+97] PATH message, whose TSpec is passed to routing together with the destination address. After processing such a request, the routing protocol returns the path that it deems the most suitable given the flow's requirements. Depending on the scope of the path selection process, this returned path could range from simply identifying the best next hop, i.e., a hop-by-hop path selection model, to specifying all intermediate nodes to the destination, i.e., an explicit route model. The nature of the path being returned impacts the operation of the path selection algorithm as it translates into different requirements for constructing and returning the appropriate path information. However, it does not affect the basic operation of the path selection algorithm (2). For simplicity and also because it is the model currently supported in the implementation (see Section 4 for details), in the rest of this document we focus on the hop-by-hop path selection model. The additional modifications required to support an explicit routing model are discussed in appendix D, but are peripheral to the main focus of this document which concentrates on the specific extensions to the OPSF protocol to support computation of QoS routes. In addition to the problem of selecting a QoS path and possibly reserving the corresponding resources, one should note that the successful delivery of QoS guarantees requires that the packets of the associated "QoS flow" be forwarded on the selected path. This typically requires the installation of corresponding forwarding state in the router. For example, with RSVP [RZB+97] flows a classifier entry is created based on the filter specs contained in the RESV message. In the case of a Differentiated Service [KNB98] setting, the classifier entry may be based on the destination address (or prefix) and the corresponding value of the DS byte. The mechanisms described in this document are at the control path level and are, therefore, independent of data path mechanisms such as the packet classification method used. Nevertheless, it is important to notice that consistent delivery of QoS guarantees implies stability of the data path. In particular, while it is possible that after a path is first selected, network conditions change and result in the appearance of "better" paths, such changes should be prevented from unnecessarily affecting existing paths. In particular, switching over to a new (and better) path should be limited to specific conditions, e.g., when the initial selection turns out to be inadequate or extremely "expensive". This aspect is beyond the scope
of QoS routing and belongs to the realm of path management, which is outside the main focus of this document. However, because of its potentially significant impact on the usefulness of QoS routing, we briefly outline a possible approach to path management. Avoiding unnecessary changes to QoS paths requires that state information be maintained for each QoS path after it has been selected. This state information is used to track the validity of the path, i.e., is the current path adequate or should QoS routing be queried again to generate a new and potentially better path. We say that a path is "pinned" when its state specifies that QoS routing need not be queried anew, while a path is considered "un-pinned" otherwise. The main issue is then to define how, when, and where path pinning and un-pinning is to take place, and this will typically depend on the mechanism used to request QoS routes. For example, when the RSVP protocol is the mechanism being used, it is desirable that path management be kept as synergetic as possible with the existing RSVP state management. In other words, pinning and un- pinning of paths should be coordinated with RSVP soft states, and structured so as to require minimal changes to RSVP processing rules. A broad RSVP-routing interface that enables this is described in [GKR97]. Use of such an interface in the context of reserving resources along an explicit path with RSVP is discussed in [GLG+97]. Details of path management and a means for avoiding loops in case of hop-by-hop path setup can be found in [GKH97], and are not addressed further in this document. 1.2. Simplifying Assumptions In order to achieve our goal of minimizing impact to the existing protocol and implementation, we impose certain restrictions on the range of extensions we initially consider to support QoS. The first restriction is on the type of additional (QoS) metrics that will be added to Link State Advertisements (LSAs) for the purpose of distributing metrics updates. Specifically, the extensions to LSAs that we initially consider, include only available bandwidth and delay. In addition, path selection is itself limited to considering only bandwidth requirements. In particular, the path selection algorithm selects paths capable of satisfying the bandwidth requirement of flows, while at the same time trying to minimize the amount of network resources that need to be allocated, i.e., minimize the number of hops used. This focus on bandwidth is adequate in most instances, and meant to keep initial complexity at an acceptable level. However, it does not fully capture the complete range of potential QoS requirements. For example, a delay-sensitive flow of an interactive application could be put on a path using a satellite link, if that link provided a
direct path and had plenty of unused bandwidth. This would clearly be an undesirable choice. Our approach to preventing such poor choices, is to assign delay-sensitive flows to a "policy" that would eliminate from the network all links with high propagation delay, e.g., satellite links, before invoking the path selection algorithm. In general, multiple policies could be used to capture different requirements, each presenting to the path selection algorithm a correspondingly pruned network topology, on which the same algorithm would be used to generate an appropriate path. Alternatively, different algorithms could be used depending on the QoS requirements expressed by an incoming request. Such extensions are beyond the scope of this document, which limits itself to describing the case of a single metric, bandwidth. However, it is worth pointing out that a simple extension to the path selection algorithm proposed in this document allows us to directly account for delay, under certain conditions, when rate-based schedulers are employed, as in the Guaranteed Service proposal [SPG97]; details can be found in [GOW97]. Another important aspect to ensure that introducing support for QoS routing has the minimal possible impact, is to develop a solution that has the smallest possible computing overhead. Additional computations are unavoidable, but it is desirable to keep the computational cost of QoS routing at a level comparable to that of traditional routing algorithms. One possible approach to achieve this goal, is to allow pre-computation of QoS routes. This is the method that was chosen for the implementation of the QoS extensions to OSPF and is, therefore, the one described in detail in this document. Alternative approaches are briefly reviewed in appendices. However, it should be noted that although several alternative path selection algorithms are possible, the same algorithm should be used consistently within a given routing domain. This requirement may be relaxed when explicit routing is used, as the responsibility for selecting a QoS path lies with a single entity, the origin of the request, which then ensures consistency even if each router uses a different path selection algorithm. Nevertheless, the use of a common path selection algorithm within an AS is recommended, if not necessary, for proper operation. A last aspect of concern regarding the introduction of QoS routing, is to control the overhead associated with the additional link state updates caused by more frequent changes to link metrics. The goal is to minimize the amount of additional update traffic without adversely affecting the performance of path selection. In Section 2.2, we present a brief discussion of various alternatives that trade accuracy of link state information for protocol overhead. Potential enhancements to the path selection algorithm, which seek to (directly) account for the inaccuracies in link metrics, are described in [GOW97], while a comprehensive treatment of the subject
can be found in [LO98, GO99]. In Section 4, we also describe the design choices made in a reference implementation, to allow future extensions and experimentation with different link state update mechanisms. The rest of this document is structured as follows. In Section 2, we describe the general design choices and mechanisms we rely on to support QoS request. This includes details on the path selection metrics, link state update extensions, and the path selection algorithm itself. Section 3 focuses on the specific extensions that the OSPF protocol requires, while Section 4 describes their implementation in the GateD platform and also presents some experimental results. Section 5 briefly addresses security issues that the proposed schemes may raise. Finally, several appendices provide additional material of interest, e.g., alternative path selection algorithms and support for explicit routes, but somewhat outside the main focus of this document. 2. Path Selection Information and Algorithms This section reviews the basic building blocks of QoS path selection, namely the metrics on the which the routing algorithm operates, the mechanisms used to propagate updates for these metrics, and finally the path selection algorithm itself. 2.1. Metrics The process of selecting a path that can satisfy the QoS requirements of a new flow relies on both the knowledge of the flow's requirements and characteristics, and information about the availability of resources in the network. In addition, for purposes of efficiency, it is also important for the algorithm to account for the amount of resources the network has to allocate to support a new flow. In general, the network prefers to select the "cheapest" path among all paths suitable for a new flow, and it may even decide not to accept a new flow for which a feasible path exists, if the cost of the path is deemed too high. Accounting for these aspects involves several metrics on which the path selection process is based. They include: - Link available bandwidth: As mentioned earlier, we currently assume that most QoS requirements are derivable from a rate- related quantity, termed "bandwidth." We further assume that associated with each link is a maximal bandwidth value, e.g., the link physical bandwidth or some fraction thereof that has been set aside for QoS flows. Since for a link to be capable of accepting a new flow with given bandwidth requirements, at least that much bandwidth must be still available on the link, the relevant link metric is, therefore, the (current) amount of available (i.e.,
unallocated) bandwidth. Changes in this metric need to be advertised as part of extended LSAs, so that accurate information is available to the path selection algorithm. - Link propagation delay: This quantity is meant to identify high latency links, e.g., satellite links, which may be unsuitable for real-time requests. This quantity also needs to be advertised as part of extended LSAs, although timely dissemination of this information is not critical as this parameter is unlikely to change (significantly) over time. As mentioned earlier, link propagation delay can be used to decide on the pruning of specific links, when selecting a path for a delay sensitive request; also, it can be used to support a related extension, as described in [GOW97]. - Hop-count: This quantity is used as a measure of the path cost to the network. A path with a smaller number of hops (that can support a requested connection) is typically preferable, since it consumes fewer network resources. As a result, the path selection algorithm will attempt to find the minimum hop path capable of satisfying the requirements of a given request. Note that contrary to bandwidth and propagation delay, hop count is a metric that does not affect LSAs, and it is only used implicitly as part of the path selection algorithm. 2.2. Advertisement of Link State Information The new link metrics identified in the previous section need to be advertised across the network, so that each router can compute accurate and consistent QoS routes. It is assumed that each router maintains an updated database of the network topology, including the current state (available bandwidth and propagation delay) of each link. As mentioned before, the distribution of link state (metrics) information is based on extending OSPF mechanisms. The detailed format of those extensions is described in Section 3, but in addition to how link state information is distributed, another important aspect is when such distribution is to take place. One option is to mandate periodic updates, where the period of updates is determined based on a tolerable corresponding load on the network and the routers. The main disadvantage of such an approach is that major changes in the bandwidth available on a link could remain unknown for a full period and, therefore, result in many incorrect routing decisions. Ideally, routers should have the most current view of the bandwidth available on all links in the network, so that they can make the most accurate decision of which path to select. Unfortunately, this then calls for very frequent updates, e.g., each time the available bandwidth of a link changes, which is
neither scalable nor practical. In general, there is a trade-off between the protocol overhead of frequent updates and the accuracy of the network state information that the path selection algorithm depends on. We outline next a few possible link state update policies, which strike a practical compromise. The basic idea is to trigger link state advertisements only when there is a significant change in the value of metrics since the last advertisement. The notion of significance of a change can be based on an "absolute" scale or a "relative" one. An absolute scale means partitioning the range of values that a metric can take into equivalence classes and triggering an update whenever the metric changes sufficiently to cross a class boundary (3). A relative scale, on the other hand, triggers updates when the percentage change in the metric value exceeds a predefined threshold. Independent of whether a relative or an absolute change trigger mechanism is used, a periodic trigger constraint can also be added. This constraint can be in the form of a hold-down timer, which is used to force a minimum spacing between consecutive updates. Alternatively, a transmit timer can also be used to ensure the transmission of an update after a certain time has expired. Such a feature can be useful if link state updates advertising bandwidth changes are sent unreliably. The current protocol extensions described in Section 3 as well as the implementation of Section 4 do not consider such an option as metric updates are sent using the standard, and reliable, OSPF flooding mechanism. However, this is clearly an extension worth considering as it can help lower substantially the protocol overhead associated with metrics updates. In both the relative and absolute change approaches, the metric value advertised in an LSA can be either the actual or a quantized value. Advertising the actual metric value is more accurate and, therefore, preferable when metrics are frequently updated. On the other hand, when updates are less frequent, e.g., because of a low sensitivity trigger or the use of hold-down timers, advertising quantized values can be of benefit. This is because it can help increase the number of equal cost paths and, therefore, improve robustness to metrics inaccuracies. In general, there is a broad space of possible trade- offs between accuracy and overhead and selecting an appropriate design point is difficult and depends on many parameters (see [AGKT98] for a more detailed discussion of these issues). As a result, in order to help acquire a better understanding of these issues, the implementation described in Section 4 supports a range of options that allow exploration of the available design space. In addition, Section 4 also reports experimental data on the traffic load and processing overhead generated by links state updates for different configurations.
2.3. Path Selection There are two major aspects to computing paths for QoS requests. The first is the actual path selection algorithm itself, i.e., which metrics and criteria it relies on. The second is when the algorithm is actually invoked. The topology on which the algorithm is run is, as with the standard OSPF path selection, a directed graph where vertices (4) consist of routers and networks (transit vertices) as well as stub networks (non-transit vertices). When computing a path, stub networks are added as a post-processing step, which is essentially similar to what is done with the current OSPF routing protocol. The optimization criteria used by the path selection are reflected in the costs associated with each interface in the topology and how those costs are accounted for in the algorithm itself. As mentioned before, the cost of a path is a function of both its hop count and the amount of available bandwidth. As a result, each interface has associated with it a metric, which corresponds to the amount of bandwidth that remains available on this interface. This metric is combined with hop count information to provide a cost value, whose goal is to pick a path with the minimum possible number of hops among those that can support the requested bandwidth. When several such paths are available, the preference is for the path whose available bandwidth (i.e., the smallest value on any of the links in the path) is maximal. The rationale for the above rule is the following: we focus on feasible paths (as accounted by the available bandwidth metric) that consume a minimal amount of network resources (as accounted by the hop-count metric); and the rule for selecting among these paths is meant to balance load as well as maximize the likelihood that the required bandwidth is indeed available. It should be noted that standard routing algorithms are typically single objective optimizations, i.e., they may minimize the hop- count, or maximize the path bandwidth, but not both. Double objective path optimization is a more complex task, and, in general, it is an intractable problem [GJ79]. Nevertheless, because of the specific nature of the two objectives being optimized (bandwidth and hop count), the complexity of the above algorithm is competitive with even that of standard single-objective algorithms. For readers interested in a thorough treatment of the topic, with insights into the connection between the different algorithms, linear algebra and modification of metrics, [Car79] is recommended. Before proceeding with a more detailed description of the path selection algorithm itself, we briefly review the available options when it comes to deciding when to invoke the algorithm. The two main options are: 1) to perform on-demand computations, that is, trigger
a computation for each new request, and 2) to use some form of pre- computation. The on-demand case involves no additional issues in terms of when computations should be triggered, but running the path selection algorithm for each new request can be computationally expensive (see [AT98] for a discussion on this issue). On the other hand, pre-computing paths amortizes the computational cost over multiple requests, but each computation instance is usually more expensive than in the on-demand case (paths are computed to all destinations and for all possible bandwidth requests rather than for a single destination and a given bandwidth request). Furthermore, depending on how often paths are recomputed, the accuracy of the selected paths may be lower. In this document, we primarily focus on the case of pre-computed paths, which is also the only method currently supported in the reference implementation described in Section 4. In this case, clearly, an important issue is when such pre-computation should take place. The two main options we consider are periodic pre-computations and pre-computations after a given (N) number of updates have been received. The former has the benefit of ensuring a strict bound on the computational load associated with pre-computations, while the latter can provide for a more responsive solution (5). Section 4 provides some experimental results comparing the performance and cost of periodic pre-computations for different period values. 2.3.1. Path Computation Algorithm This section describes a path selection algorithm, which for a given network topology and link metrics (available bandwidth), pre-computes all possible QoS paths, while maintaining a reasonably low computational complexity. Specifically, the algorithm pre-computes for any destination a minimum hop count path with maximum bandwidth, and has a computational complexity comparable to that of a standard Bellman-Ford shortest path algorithm. The Bellman-Ford (BF) shortest path algorithm is adapted to compute paths of maximum available bandwidth for all hop counts. It is a property of the BF algorithm that, at its h-th iteration, it identifies the optimal (in our context: maximal bandwidth) path between the source and each destination, among paths of at most h hops. In other words, the cost of a path is a function of its available bandwidth, i.e., the smallest available bandwidth on all links of the path, and finding a minimum cost path amounts to finding a maximum bandwidth path. However, because the BF algorithm progresses by increasing hop count, it essentially provides for free the hop count of a path as a second optimization criteria. Specifically, at the kth (hop count) iteration of the algorithm, the maximum bandwidth available to all destinations on a path of no more than k hops is recorded (together with the corresponding routing
information). After the algorithm terminates, this information provides for all destinations and bandwidth requirements, the path with the smallest possible number of hops and sufficient bandwidth to accommodate the new request. Furthermore, this path is also the one with the maximal available bandwidth among all the feasible paths with at most these many hops. This is because for any hop count, the algorithm always selects the one with maximum available bandwidth. We now proceed with a more detailed description of the algorithm and the data structure used to record routing information, i.e., the QoS routing table that gets built as the algorithm progresses (the pseudo-code for the algorithm can be found in Appendix A). As mentioned before, the algorithm operates on a directed graph consisting only of transit vertices (routers and networks), with stub-networks subsequently added to the path(s) generated by the algorithm. The metric associated with each edge in the graph is the bandwidth available on the corresponding interface. Let us denote by b(n;m) the available bandwidth on the link from node n to m. The vertex corresponding to the router where the algorithm is being run, i.e., the computing router, is denoted as the "source node" for the purpose of path selection. The algorithm proceeds to pre-compute paths from this source node to all possible destination networks and for all possible bandwidth values. At each (hop count) iteration, intermediate results are recorded in a QoS routing table, which has the following structure: The QoS routing table: - a KxH matrix, where K is the number of destinations (vertices in the graph) and H is the maximal allowed (or possible) number of hops for a path. - The (n;h) entry is built during the hth iteration (hop count value) of the algorithm, and consists of two fields: * bw: the maximum available bandwidth, on a path of at most h hops between the source node (router) and destination node n; * neighbor: this is the routing information associated with the h (or less) hops path to destination node n, whose available bandwidth is bw. In the context of hop-by-hop path selection (6), the neighbor information is simply the identity of the node adjacent to the source node on that path. As a rule, the "neighbor" node must be a router and not a network, the only exception being the case where the network is the destination node (and the selected path is the single edge interconnecting the source to it).
Next, we provide additional details on the operation of the algorithm and how the entries in the routing table are updated as the algorithm proceeds. For simplicity, we first describe the simpler case where all edges count as "hops," and later explain how zero-hop edges are handled. Zero-hop edges arise in the case of transit networks vertices, where only one of the two incoming and outgoing edges should be counted in the hop count computation, as they both correspond to the same physical hop. Accounting for this aspect requires distinguishing between network and router nodes, and the steps involved are detailed later in this section as well as in the pseudo-code of Appendix A. When the algorithm is invoked, the routing table is first initialized with all bw fields set to 0 and neighbor fields cleared. Next, the entries in the first column (which corresponds to one-hop paths) of the neighbors of the computing router are modified in the following way: the bw field is set to the value of the available bandwidth on the direct edge from the source. The neighbor field is set to the identity of the neighbor of the computing router, i.e., the next router on the selected path. Afterwards, the algorithm iterates for at most H iterations (considering the above initial iteration as the first). The value of H could be implicit, i.e., the diameter of the network or, in order to better control the worst case complexity, it can be set explicitly thereby limiting path lengths to at most H hops. In the latter case, H must be assigned a value larger than the length of the minimum hop-count path to any node in the graph. At iteration h, we first copy column h-1 into column h. In addition, the algorithm keeps a list of nodes that changed their bw value in the previous iteration, i.e., during the (h-1)-th iteration. The algorithm then looks at each link (n;m) where n is a node whose bw value changed in the previous iteration, and checks the maximal available bandwidth on an (at most) h-hop path to node m whose final hop is that link. This amounts to taking the minimum between the bw field in entry (n;h-1) and the link metric value b(n;m) kept in the topology database. If this value is higher than the present value of the bw field in entry (m;h), then a better (larger bw value) path has been found for destination m and with at most h hops. The bw field of entry (m;h) is then updated to reflect this new value. In the case of hop-by-hop routing, the neighbor field of entry (m;h) is set to the same value as in entry (n;h-1). This records the identity of the first hop (next hop from the source) on the best path identified thus far for destination m and with h (or less) hops.
As mentioned earlier, extending the above algorithm to handle zero- hop edges is needed due to the possible use of multi-access networks, e.g., T/R, E/N, etc., to interconnect routers. Such entities are also represented by means of a vertex in the OSPF topology, but a network connecting two routers should clearly be considered as a single hop path rather than a two hop path. For example, consider three routers A, B, and C connected over an Ethernet network N, which the OSPF topology represents as in Figure 1. A----N----B | | C Figure 1: Zero-Hop Edges In the example of Figure 1, although there are directed edges in both directions, an edge from the network to any of the three routers must have zero "cost", so that it is not counted twice. It should be noted that when considering such environments in the context of QoS routing, it is assumed that some entity is responsible for determining the "available bandwidth" on the network, e.g., a subnet bandwidth manager. The specification and operation of such an entity is beyond the scope of this document. Accommodating zero-hop edges in the context of the path selection algorithm described above is done as follows: At each iteration h (starting with the first), whenever an entry (m;h) is modified, it is checked whether there are zero-cost edges (m;k) emerging from node m. This is the case when m is a transit network. In that case, we attempt to further improve the entry of node k within the current iteration, i.e., entry (k;h) (rather than entry (k;h+1)), since the edge (m;k) should not count as an additional hop. As with the regular operation of the algorithm, this amounts to taking the minimum between the bw field in entry (m;h) and the link metric value b(m;k) kept in the topology database (7). If this value is higher than the present value of the bw field in entry (k;h), then the bw field of entry (k;h) is updated to this new value. In the case of hop-by-hop routing, the neighbor field of entry (k;h) is set, as usual, to the same value as in entry (m;h) (which is also the value in entry (n;h-1)).
Note that while for simplicity of the exposition, the issue of equal cost, i.e., same hop count and available bandwidth, is not detailed in the above description, it can be easily supported. It only requires that the neighbor field be expanded to record the list of next (previous) hops, when multiple equal cost paths are present. Addition of Stub Networks As was mentioned earlier, the path selection algorithm is run on a graph whose vertices consist only of routers and transit networks and not stub networks. This is intended to keep the computational complexity as low as possible as stub networks can be added relatively easily through a post-processing step. This second processing step is similar to the one used in the current OSPF routing table calculation [Moy98], with some differences to account for the QoS nature of routes. Specifically, after the QoS routing table has been constructed, all the router vertices are again considered. For each router, stub networks whose links appear in the router's link advertisements will be processed to determine QoS routes available to them. The QoS routing information for a stub network is similar to that of routers and transit networks and consists of an extension to the QoS routing table in the form of an additional row. The columns in that new row again correspond to paths of different hop counts, and contain both bandwidth and next hop information. We also assume that an available bandwidth value has been advertised for the stub network. As before, how this value is determined is beyond the scope of this document. The QoS routes for a stub network S are constructed as follows: Each entry in the row corresponding to stub network S has its bw(s) field initialized to zero and its neighbor set to null. When a stub network S is found in the link advertisement of router V, the value bw(S,h) in the hth column of the row corresponding to stub network S is updated as follows: bw(S,h) = max ( bw(S,h) ; min ( bw(V,h) , b(V,S) ) ), where bw(V,h) is the bandwidth value of the corresponding column for the QoS routing table row associated with router V, i.e., the bandwidth available on an h hop path to V, and b(V,S) is the advertised available bandwidth on the link from V to S. The above expression essentially states that the bandwidth of a h hop path to stub network S is updated using a path through router V, only if the minimum of the bandwidth of the h hop path to V and the bandwidth on the link between V and S is larger than the current value.
Update of the neighbor field proceeds similarly whenever the bandwidth of a path through V is found to be larger than or equal to the current value. If it is larger, then the neighbor field of V in the corresponding column replaces the current neighbor field of S. If it is equal, then the neighbor field of V in the corresponding column is concatenated with the existing field for S, i.e., the current set of neighbors for V is added to the current set of neighbors for S. Extracting Forwarding Information from Routing Table When the QoS paths are precomputed, the forwarding information for a flow with given destination and bandwidth requirement needs to be extracted from the routing table. The case of hop-by-hop routing is simpler than that of explicit routing. This is because, only the next hop needs to be returned instead of an explicit route. Specifically, assume a new request to destination, say, d, and with bandwidth requirements B. The index of the destination vertex identifies the row in the QoS routing table that needs to be checked to generate a path. Assuming that the QoS routing table was constructed using the Bellman-Ford algorithm presented later in this section, the search then proceeds by increasing index (hop) count until an entry is found, say at hop count or column index of h, with a value of the bw field which is equal to or larger than B. This entry points to the initial information identifying the selected path. If the path computation algorithm stores multiple equal cost paths, then some degree of load balancing can be achieved at the time of path selection. A next hop from the list of equivalent next hops can be chosen in a round robin manner, or randomly with a probability that is weighted by the actual available bandwidth on the local interface. The latter is the method used in the implementation described in Section 4. The case of explicit routing is discussed in Appendix D. 3. OSPF Protocol Extensions As stated earlier, one of our goals is to limit the additions to the existing OSPF V2 protocol, while still providing the required level of support for QoS based routing. To this end, all of the existing OSPF mechanisms, data structures, advertisements, and data formats remain in place. The purpose of this section of the document is to describe the extensions to the OSPF protocol needed to support QoS as outlined in the previous sections.
3.1. QoS -- Optional Capabilities The OSPF Options field is present in OSPF Hello packets, Database Description packets and all LSAs. The Options field enables OSPF routers to support (or not support) optional capabilities, and to communicate their capability level to other OSPF routers. Through this mechanism, routers of differing capabilities can be mixed within an OSPF routing domain. Currently, the OSPF standard [Moy98] specifies the following 5 bits in the options octet: +-----------------------------------------------+ | * | * | DC | EA | N/P | MC | E | * | +-----------------------------------------------+ Note that the least significant bit (`T' bit) that was used to indicate TOS routing capability in the older OSPF specification [Moy94] has been removed. However, for backward compatibility with previous versions of the OSPF specification, TOS-specific information can be included in router-LSAs, summary-LSAs and AS-external-LSAs. We propose to reclaim the `T' bit as an indicator of router's QoS routing capability and refer to it as the `Q' bit. In fact, QoS capability can be viewed as an extension of the TOS-capabilities and QoS routing as a form of TOS-based routing. A router sets this bit in its hello packets to indicate that it is capable of supporting such routing. When this bit is set in a router or summary links link state advertisement, it means that there are QoS fields to process in the packet. When this bit is set in a network link state advertisement it means that the network described in the advertisement is QoS capable. We need to be careful in this approach so as to avoid confusing any old style (i.e., RFC 1583 based) TOS routing implementations. The TOS metric encoding rules of QoS fields introduced further in this section will show how this is achieved. Additionally, unlike the RFC 1583 specification that unadvertised TOS metrics be treated to have same cost as TOS 0, for the purpose of computing QOS routes, unadvertised TOS metrics (on a hop) indicate lack of connectivity for the specific TOS metrics (for that hop). 3.2. Encoding Resources as Extended TOS Introduction of QoS should ideally not influence the compatibility with existing OSPFv2 routers. To achieve this goal, necessary extensions in packet formats must be defined in a way that either is understood by OSPFv2 routers, ignored, or in the worst case "gracefully" misinterpreted. Encoding of QoS metrics in the TOS field which fortunately enough is longer in OSPF packets than
officially defined in [Alm92], allows us to mimic the new facility as extended TOS capability. OSPFv2 routers will either disregard these definitions or consider those unspecified. Specific precautions are taken to prevent careless OSPF implementations from influencing traditional TOS routers (if any) when misinterpreting the QoS extensions. For QoS resources, 32 combinations are available through the use of the fifth bit in TOS fields contained in different LSAs. Since [Alm92] defines TOS as being four bits long, this definition never conflicts with existing values. Additionally, to prevent naive implementations that do not take all bits of the TOS field in OSPF packets into considerations, the definitions of the `QoS encodings' is aligned in their semantics with the TOS encoding. Only bandwidth and delay are specified as of today and their values map onto `maximize throughput' and `minimize delay' if the most significant bit is not taken into account. Accordingly, link reliability and jitter could be defined later if necessary. OSPF encoding RFC 1349 TOS values ___________________________________________ 0 0000 normal service 2 0001 minimize monetary cost 4 0010 maximize reliability 6 0011 8 0100 maximize throughput 10 0101 12 0110 14 0111 16 1000 minimize delay 18 1001 20 1010 22 1011 24 1100 26 1101 28 1110 30 1111
OSPF encoding `QoS encoding values' ------------------------------------------- 32 10000 34 10001 36 10010 38 10011 40 10100 bandwidth 42 10101 44 10110 46 10111 48 11000 delay 50 11001 52 11010 54 11011 56 11100 58 11101 60 11110 62 11111 Representing TOS and QoS in OSPF. 3.2.1. Encoding bandwidth resource Given the fact that the actual metric field in OSPF packets only provides 16 bits to encode the value used and that links supporting bandwidth ranging into Gbits/s are becoming reality, linear representation of the available resource metric is not feasible. The solution is exponential encoding using appropriately chosen implicit base value and number bits for encoding mantissa and the exponent. Detailed considerations leading to the solution described are not presented here but can be found in [Prz95]. Given a base of 8, the 3 most significant bits should be reserved for the exponent part and the remaining 13 for the mantissa. This allows a simple comparison for two numbers encoded in this form, which is often useful during implementation. The following table shows bandwidth ranges covered when using different exponents and the granularity of possible reservations.
exponent value x range (2^13-1)*8^x step 8^x ------------------------------------------------- 0 8,191 1 1 65,528 8 2 524,224 64 3 4,193,792 512 4 33,550,336 4,096 5 268,402,688 32,768 6 2,147,221,504 262,144 7 17,177,772,032 2,097,152 Ranges of Exponent Values for 13 bits, base 8 Encoding, in Bytes/s The bandwidth encoding rule may be summarized as: "represent available bandwidth in 16 bit field as a 3 bit exponent (with assumed base of 8) followed by a 13 bit mantissa as shown below and advertise 2's complement of the above representation." 0 8 16 | | | ----------------- |EXP| MANT | ----------------- Thus, the above encoding advertises a numeric value that is 2^16 -1 -(exponential encoding of the available bandwidth): This has the property of advertising a higher numeric value for lower available bandwidth, a notion that is consistent with that of cost. Although it may seem slightly pedantic to insist on the property that less bandwidth is expressed higher values, it has, besides consistency, a robustness aspect in it. A router with a poor OSPF implementation could misuse or misunderstand bandwidth metric as normal administrative cost provided to it and compute spanning trees with a "normal" Dijkstra. The effect of a heavily congested link advertising numerically very low cost could be disastrous in such a scenario. It would raise the link's attractiveness for future traffic instead of lowering it. Evidence that such considerations are not speculative, but similar scenarios have been encountered, can be found in [Tan89].
Concluding with an example, assume a link with bandwidth of 8 Gbits/s = 1024^3 Bytes/s, its encoding would consist of an exponent value of 6 since 1024^3= 4,096*8^6, which would then have a granularity of 8^6 or approx. 260 kBytes/s. The associated binary representation would then be %(110) 0 1000 0000 0000% or 53,248 (8). The bandwidth cost (advertised value) of this link when it is idle, is then the 2's complement of the above binary representation, i.e., %(001) 1 0111 1111 1111% which corresponds to a decimal value of (2^16 - 1) - 53,248 = 12,287. Assuming now a current reservation level of 6;400 Mbits/s = 200 * 1024^2, there remains 1;600 Mbits/s of available bandwidth on the link. The encoding of this available bandwidth of 1'600 Mbits/s is 6,400 * 8^5, which corresponds to a granularity of 8^5 or approx. 30 kBytes/s, and has a binary representation of %(101) 1 1001 0000 0000% or decimal value of 47,360. The advertised cost of the link with this load level, is then %(010) 0 0110 1111 1111%, or (2^16-1) -47,360 = 18,175. Note that the cost function behaves as it should, i.e., the less bandwidth is available on a link, the higher the cost and the less attractive the link becomes. Furthermore, the targeted property of better granularity for links with less bandwidth available is also achieved. It should, however, be pointed out that the numbers given in the above examples match exactly the resolution of the proposed encoding, which is of course not always the case in practice. This leaves open the question of how to encode available bandwidth values when they do not exactly match the encoding. The standard practice is to round it to the closest number. Because we are ultimately interested in the cost value for which it may be better to be pessimistic than optimistic, we choose to round costs up and, therefore, bandwidth down. 3.2.2. Encoding Delay Delay is encoded in microseconds using the same exponential method as described for bandwidth except that the base is defined to be 4 instead of 8. Therefore, the maximum delay that can be expressed is (2^13-1) *4^7 i.e., approx. 134 seconds. 3.3. Packet Formats Given the extended TOS notation to account for QoS metrics, no changes in packet formats are necessary except for the (re)introduction of T-bit as the Q-bit in the options field. Routers not understanding the Q-bit should either not consider the QoS metrics distributed or consider those as `unknown' TOS.
To support QoS, there are additions to two Link State Advertisements, the Router Links Advertisement and the Summary Links Advertisement. As stated above, a router identifies itself as supporting QoS by setting the Q-bit in the options field of the Link State Header. When a router that supports QoS receives either the Router Links or Summary Links Advertisement, it should parse the QoS metrics encoded in the received Advertisement. 3.4. Calculating the Inter-area Routes This document proposes a very limited use of OSPF areas, that is, it is assumed that summary links advertisements exist for all networks in the area. This document does not discuss the problem of providing support for area address ranges and QoS metric aggregation. This is left for further studies. 3.5. Open Issues Support for AS External Links, Virtual Links, and incremental updates for summary link advertisements are not addressed in this document and are left for further study. For Virtual Links that do exist, it is assumed for path selection that these links are non-QoS capable even if the router advertises QoS capability. Also, as stated earlier, this document does not address the issue of non-QoS routers within a QoS domain.