Internet Engineering Task Force Y. Bernet Diffserv Working Group Microsoft INTERNET-DRAFT S. Blake Expires January 2001 Ericsson draft-ietf-diffserv-model-04.txt D. Grossman Motorola A. Smith <editor> July 2000 An Informal Management Model for Diffserv Routers Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This document is a product of the IETF's Differentiated Services Working Group. Comments should be addressed to WG's mailing list at diffserv@ietf.org. The charter for Differentiated Services may be found at http://www.ietf.org/html.charters/diffserv-charter.html Copyright (C) The Internet Society (2000). All Rights Reserved. Distribution of this memo is unlimited. Abstract This memo proposes an informal management model of Differentiated Services (Diffserv) routers for use in their management and configuration. This model defines functional datapath elements (e.g. classifiers, meters, actions (e.g. marking, absolute dropping, counting, multiplexing), algorithmic droppers, queues and schedulers. It describes possible configuration parameters for these elements and how they might be interconnected to realize the range of traffic conditioning and per- Bernet et al. Expires January 2001 [Page 1]
Internet Draft Diffserv Informal Management Model July 2000 hop behavior (PHB) functionalities described in the Diffserv Architecture [DSARCH]. The model is intended to be abstract and capable of representing the configuration parameters important to Diffserv functionality for a variety of specific router implementations. It is not intended as a guide to system implementation nor as a formal modelling description. This model serves as the rationale for the design of an SNMP MIB [DSMIB] and for other configuration interfaces (e.g. [DSPIB]) and, possibly, more detailed formal models (e.g. [QOSDEVMOD]): these should all be consistent with this model. 1. Introduction Differentiated Services (Diffserv) [DSARCH] is a set of technologies which allow network service providers to offer services with different kinds of network quality-of-service (QoS) objectives to different customers and their traffic streams. The premise of Diffserv networks is that routers within the core of the network handle packets in different traffic streams by forwarding them using different per-hop behaviors (PHBs). The PHB to be applied is indicated by a Diffserv codepoint (DSCP) in the IP header of each packet [DSFIELD]. Note that this document uses the terminology defined in [DSARCH, DSTERMS] and in Section 2. The advantage of such a scheme is that many traffic streams can be aggregated to one of a small number of behavior aggregates (BA) which are each forwarded using the same PHB at the router, thereby simplifying the processing and associated storage. In addition, there is no signaling, other than what is carried in the DSCP of each packet, and no other related processing that is required in the core of the Diffserv network since QoS is invoked on a packet-by- packet basis. The Diffserv architecture enables a variety of possible services which could be deployed in a network. These services are reflected to customers at the edges of the Diffserv network in the form of a Service Level Specification (SLS) [DSTERMS]. The ability to provide these services depends on the availability of cohesive management and configuration tools that can be used to provision and monitor a set of Diffserv routers in a coordinated manner. To facilitate the development of such configuration and management tools it is helpful to define a conceptual model of a Diffserv router that abstracts away implementation details of particular Diffserv routers from the parameters of interest for configuration and management. The purpose of this memo is to define such a model. Bernet et al. Expires January 2001 [Page 2]
Internet Draft Diffserv Informal Management Model July 2000 The basic forwarding functionality of a Diffserv router is defined in other specifications; e.g., [DSARCH, DSFIELD, AF-PHB, EF-PHB]. This document is not intended in any way to constrain or to dictate the implementation alternatives of Diffserv routers. It is expected that router implementers will demonstrate a great deal of variability in their implementations. To the extent that implementers are able to model their implementations using the abstractions described in this memo, configuration and management tools will more readily be able to configure and manage networks incorporating Diffserv routers of assorted origins. o Section 3 starts by describing the basic high-level blocks of a Diffserv router. It explains the concepts used in the model, including the hierarchical management model for these blocks which uses low-level functional datapath elements such as Classifiers, Actions, Queues. o Section 4 describes Classifier elements. o Section 5 discusses Meter elements. o Section 6 discusses Action elements. o Section 7 discusses the basic queueing elements of Algorithmic Droppers, Queues and Schedulers and their functional behaviors (e.g. traffic shaping). o Section 8 shows how the low-level elements can be combined to build modules called Traffic Conditioning Blocks (TCBs) which are useful for management purposes. o Section 9 discusses security concerns. 2. Glossary This memo uses terminology which is defined in [DSARCH] and in [DSTERMS]. Some of the terms defined there are defined again here in order to provide additional detail, along with some new terms specific to this document. Absolute A functional datapath element which simply discards all Dropper packets arriving at its input. Algorithmic A functional datapath element which selectively discards Dropper packets that arrive at its input, based on a discarding Bernet et al. Expires January 2001 [Page 3]
Internet Draft Diffserv Informal Management Model July 2000 algorithm. It has one data input and one output. Classifier A functional datapath element which consists of filters which select matching and non-matching packets and forwards them along a particular datapath within the router. A classifier splits a single incoming traffic stream into multiple outgoing ones. Counter A functional datapath element which updates a packet counter and also an octet counter for every packet that passes through it. Used for collecting statistics. Filter A set of wildcard, prefix, masked, range and/or exact match conditions on the content of a packet's headers or other data, and/or on implicit or derived attributes associated with the packet. A filter is said to match only if each condition is satisfied. Functional A basic building block of the conceptual router. Datapath Typical elements are Classifiers, Meters, Actions, Element Algorithmic Droppers, Queues and Schedulers. Multiplexer A multiplexor. (Mux) Multiplexor A functional datapath element that merges multiple (Mux) traffic streams (datapaths) into a single traffic stream (datapath). Non-work- A property of a scheduling algorithm such that it conserving services packets no sooner than a scheduled departure time, even if this means leaving packets in a FIFO while the link is idle. Policing The process of comparing the arrival of data packets against a temporal profile and forwarding, delaying or dropping them so as to make the output stream conformant to the profile. Policing is modelled here as the combination of either a meter or a scheduler with either an absolute dropper or an algorithmic dropper. Queueing A combination of functional datapath elements Block that modulates the transmission of packets belonging to a traffic streams and determines their ordering, possibly storing them temporarily or Bernet et al. Expires January 2001 [Page 4]
Internet Draft Diffserv Informal Management Model July 2000 discarding them. Scheduling An algorithm which determines which queue of a set algorithm of queues to service next. This may be based on the relative priority of the queues, on a weighted fair bandwidth sharing policy or some other policy. Such an algorithm may be either work-conserving or non- work-conserving. Shaping The process of delaying packets within a traffic stream to cause it to conform to some defined temporal profile. Shaping can be implemented using a queue serviced by a non-work-conserving scheduling algorithm. Traffic A logical datapath entity consisting of a number of Conditioning functional datapath elements interconnected in Block (TCB) such a way as to perform a specific set of traffic conditioning functions on an incoming traffic stream. A TCB can be thought of as an entity with one input and one or more outputs and a set of control parameters. Work- A property of a scheduling algorithm such that it conserving services a packet, if one is available, at every transmission opportunity." 3. Conceptual Model This section introduces a block diagram of a Diffserv router and describes the various components illustrated in Figure 1. Note that a Diffserv core router is assumed to include only a subset of these components: the model presented here is intended to cover the case of both Diffserv edge and core routers. 3.1. Elements of a Diffserv Router The conceptual model includes abstract definitions for the following: o Traffic Classification elements. o Metering functions. o Actions of Marking, Absolute Dropping, Counting and Multiplexing. Bernet et al. Expires January 2001 [Page 5]
Internet Draft Diffserv Informal Management Model July 2000 o Queueing elements, including capabilities of algorithmic dropping and scheduling. o Certain combinations of the above functional datapath elements into higher-level blocks known as Traffic Conditioning Blocks (TCBs). The components and combinations of components described in this document form building blocks that need to be manageable by Diffserv configuration and management tools. One of the goals of this document is to show how a model of a Diffserv device can be built using these component blocks. This model is in the form of a connected directed acyclic graph (DAG) of functional datapath elements that describes the traffic conditioning and queueing behaviors that any particular packet will experience when forwarded to the Diffserv router. Figure 1 illustrates the major functional blocks of a Diffserv router: 3.1.1. Datapath An ingress interface, routing core and egress interface are illustrated at the center of the diagram. In actual router implementations, there may be an arbitrary number of ingress and egress interfaces interconnected by the routing core. The routing core element serves as an abstraction of a router's normal routing and switching functionality. The routing core moves packets between interfaces according to policies outside the scope of Diffserv. The actual queueing delay and packet loss behavior of a specific router's switching fabric/backplane is not modeled by the routing core; these should be modeled using the functional datapath elements described later. The routing core should be thought of as an infinite bandwidth, zero- delay backplane connecting ingress and egress interfaces. The components of interest on the ingress/egress interfaces are the functional datapath elements (e.g. classifiers, queueing elements) that support Diffserv traffic conditioning and per-hop behaviors [DSARCH]. These are the fundamental components comprising a Diffserv router and will be the focal point of our conceptual model. 3.1.2. Configuration and Management Interface Diffserv operating parameters are monitored and provisioned through this interface. Monitored parameters include statistics regarding traffic carried at various Diffserv service levels. These statistics may be important for accounting purposes and/or for tracking compliance to Traffic Conditioning Specifications (TCSs) [DSTERMS] negotiated with Bernet et al. Expires January 2001 [Page 6]
Internet Draft Diffserv Informal Management Model July 2000 +---------------+ | Diffserv | Mgmt | configuration | <----+-->| & management |------------------+ SNMP,| | interface | | COPS | +---------------+ | etc. | | | | | | | v v | +-------------+ +-------------+ | | ingress i/f | +---------+ | egress i/f | --------->| classify, |-->| routing |-->| classify, |----> data | | meter, | | core | | meter |data out in | | action, | +---------+ | action, | | | queueing | | queueing | | +-------------+ +-------------+ | ^ ^ | | | | | | | +------------+ | +-->| QOS agent | | -------->| (optional) |---------------------+ QOS | (e.g. RSVP)| cntl +------------+ msgs Figure 1: Diffserv Router Major Functional Blocks customers. Provisioned parameters are primarily classification rules, TC and PHB configuration parameters. The network administrator interacts with the Diffserv configuration and management interface via one or more management protocols, such as SNMP or COPS, or through other router configuration tools such as serial terminal or telnet consoles. Specific policy rules and goals governing the Diffserv behaviour of a router are presumed to be installed by policy management mechanisms. However, Diffserv routers are subject to implementation decisions which form a meta- policy that scopes the kinds of policies which can be successfully implemented by the router. External reporting of such implementation capabilities are considered out of scope for this document. 3.1.3. Optional QoS Agent Module Diffserv routers may snoop or participate in either per-microflow or per-flow-aggregate signaling of QoS requirements [E2E] e.g. using the RSVP protocol. Snooping of RSVP messages may be used, for example, to Bernet et al. Expires January 2001 [Page 7]
Internet Draft Diffserv Informal Management Model July 2000 learn how to classify traffic without actually participating as a RSVP protocol peer. Diffserv routers may reject or admit RSVP reservation requests to provide a means of admission control to Diffserv-based services or they may use these requests to trigger provisioning changes for a flow-aggregation in the Diffserv network. A flow-aggregation in this context might be equivalent to a Diffserv BA or it may be more fine-grained, relying on a MF classifier [DSARCH]. Note that the conceptual model of such a router implements the Integrated Services Model as described in [INTSERV], applying the control plane controls to the data classified and conditioned in the data plane, as desribed in [E2E]. Note that a QoS Agent component of a Diffserv router, if present, might be active only in the control plane and not in the data plane. In this scenario, RSVP could be used merely to signal reservation state without installing any actual reservations in the data plane of the Diffserv router: the data plane could still act purely on Diffserv DSCPs and provide PHBs for handling data traffic without the normal per-microflow handling expected to support some Intserv services. 3.2. Hierarchical Model of Diffserv Components This document focuses on the Diffserv-specific components of the router. Figure 2 shows a high-level view of ingress and egress interfaces of a router. The diagram illustrates two Diffserv router interfaces, each having an ingress and an egress component. It shows classification, metering, action and queueing functions which might be instantiated on each interface's ingress and egress component. In principle, if one were to construct a network entirely out of two- port routers (in appropriate places connected by LANs or similar media), then it would be necessary for each router to perform four QoS control functions in the datapath on traffic in each direction: - Classify each message according to some set of rules, possibly just a "match everything" rule. - If necessary, determine whether the data stream the message is part of is within or outside its rate by metering the stream. - Perform a set of resulting actions, including applying a drop policy appropriate to the classification and queue in question and perhaps additionally marking the traffic with a Differentiated Services Code Point (DSCP) as defined in [DSCP]. Bernet et al. Expires January 2001 [Page 8]
Internet Draft Diffserv Informal Management Model July 2000 Interface A Interface B +-------------+ +---------+ +-------------+ | ingress i/f | | | | egress i/f | | classify, | | | | classify, | --->| meter, |---->| |---->| meter, |---> | action, | | | | action, | | queueing | | | | queueing | +-------------+ | routing | +-------------+ | core | +-------------+ | | +-------------+ | egress i/f | | | | ingress i/f | | classify, | | | | classify, | <---| meter, |<----| |<----| meter, |<--- | action, | | | | action, | | queueing | +---------+ | queueing | +-------------+ +-------------+ Figure 2. Traffic Conditioning and Queueing Elements - Enqueue the traffic for output in the appropriate queue, which may either shape the traffic or simply forward it with some minimum rate or maximum latency. If the network is now built out of N-port routers, the expected behavior of the network should be identical. Therefore, this model must provide for essentially the same set of functions on the ingress as on the egress port of the router. Some interfaces will be "edge" interfaces and some will be "interior" to the Differentiated Services domain. The one point of difference between an ingress and an egress interface is that all traffic on an egress interface is queued, while traffic on an ingress interface will typically be queued only for shaping purposes, if at all. Therefore, equivalent functional datapath elements may be modelled on both the ingress and egress components of an interface. Note that it is not mandatory that each of these functional datapath elements be implemented on both ingress and egress components; equally, the model allows that multiple sets of these elements may be placed in series and/or in parallel at ingress or at egress. The arrangement of elements is dependent on the service requirements on a particular interface on a particular router. By modelling these elements on both ingress and egress components, it is not implied that they must be implemented in this way in a specific router. For example, a router may implement all shaping and PHB queueing on the interface egress component or may instead implement it only on the ingress component. Furthermore, the classification needed to map a packet to an egress component queue (if present) need not be implemented on the egress component but instead Bernet et al. Expires January 2001 [Page 9]
Internet Draft Diffserv Informal Management Model July 2000 may be implemented on the ingress component, with the packet passed through the routing core with in-band control information to allow for egress queue selection. >From a device-level configuration management perspective, the following hierarchy exists: At the lowest level considered here, are individual functional datapath elements, each with their own configuration parameters and management counters and flags. At the next level, the network administrator manages groupings of these functional datapath elements interconnected in a DAG. These functional datapath elements are organized in self-contained TCBs which are used to implement some desired network policy (see Section 8). One or more TCBs may be instantiated on each ingress or egress component; they may be connected in series and/or in parallel configurations on the multiple outputs of a preceding TCB. A TCB can be thought of as a "black box" with one input and one or more outputs (in the data path). Each interface (ingress or egress) may have a different TCB configuration. At the topmost level considered here, the network administrator manages interfaces. Each interface consists of an ingress component and an egress component. Each component may contain one or more TCBs. 4. Classifiers 4.1. Definition Classification is performed by a classifier element. Classifiers are 1:N (fan-out) devices: they take a single traffic stream as input and generate N logically separate traffic streams as output. Classifiers are parameterized by filters and output streams. Packets from the input stream are sorted into various output streams by filters which match the contents of the packet or possibly match other attributes associated with the packet. Various types of classifiers using different filters are described in the following sections. Figure 3 illustrates a classifier, where the outputs connect to succeeding functional datapath elements. The simplest possible Classifier element is one that matches all packets that are applied at its input. In this case, the Classifier element is just a no-op and may be omitted. Bernet et al. Expires January 2001 [Page 10]
Internet Draft Diffserv Informal Management Model July 2000 unclassified classified traffic traffic +------------+ | |--> match Filter1 --> OutputA ------->| classifier |--> match Filter2 --> OutputB | |--> no match --> OutputC +------------+ Figure 3. An Example Classifier Note that we allow a multiplexor (see Section 6.5) before the classifier to allow input from multiple traffic streams. For example, if multiple ingress sub-interfaces feed through a single classifier then the interface number can be considered by the classifier as a packet attribute and be included in the packet's classification key. This optimization may be important for scalability in the management plane. Another example of a packet attribute could be an integer representing the BGP community string associated with the packet's best-matching route. The following classifier separates traffic into one of three output streams based on three filters: Filter Matched Output Stream -------------- --------------- Filter1 A Filter2 B Filter3 (no match) C Where Filters1 and Filter2 are defined to be the following BA filters ([DSARCH], Section 4.2.1 ): Filter DSCP ------ ------ 1 101010 2 111111 3 ****** (wildcard) 4.1.1. Filters A filter consists of a set of conditions on the component values of a packet's classification key (the header values, contents, and attributes relevant for classification). In the BA classifier example above, the classification key consists of one packet header field, the DSCP, and both Filter1 and Filter2 specify exact-match conditions on the value of Bernet et al. Expires January 2001 [Page 11]
Internet Draft Diffserv Informal Management Model July 2000 the DSCP. Filter3 is a wildcard default filter which matches every packet, but which is only selected in the event that no other more specific filter matches. In general there are a set of possible component conditions including exact, prefix, range, masked and wildcard matches. Note that ranges can be represented (with less efficiency) as a set of prefixes and that prefix matches are just a special case of both masked and range matches. In the case of a MF classifier [DSARCH], the classification key consists of a number of packet header fields. The filter may specify a different condition for each key component, as illustrated in the example below for a IPv4/TCP classifier: Filter IP Src Addr IP Dest Addr TCP SrcPort TCP DestPort ------ ------------- ------------- ----------- ------------ Filter4 172.31.8.1/32 172.31.3.X/24 X 5003 In this example, the fourth octet of the destination IPv4 address and the source TCP port are wildcard or "don't cares". MF classification of fragmented packets is impossible if the filter uses transport-layer port numbers e.g. TCP port numbers. MTU-discovery is therefore a prerequisite for proper operation of a Diffserv network that uses such classifiers. 4.1.2. Overlapping Filters Note that it is easy to define sets of overlapping filters in a classifier. For example: Filter5: Type: Masked-DSCP Value: 111000 Mask: 111000 Filter6: Type: Masked-DSCP Value: 000111 (binary) Mask: 000111 (binary) A packet containing DSCP = 111111 cannot be uniquely classified by this pair of filters and so a precedence must be established between Filter5 and Filter6 in order to break the tie. This precedence must be established either (a) by a manager which knows that the router can accomplish this particular ordering e.g. by means of reported capabilities, or (b) by the router along with a mechanism to report to a Bernet et al. Expires January 2001 [Page 12]
Internet Draft Diffserv Informal Management Model July 2000 manager which precedence is being used. Such precedence mechanisms must be supported in any translation of this model into specific syntax for configuration and management protocols. As another example, one might want first to disallow certain applications from using the network at all, or to classify some individual traffic streams that are not Diffserv-marked. Traffic that is not classified by those tests might then be inspected for a DSCP. The word "then" implies sequence and this must be specified by means of precedence. An unambiguous classifier requires that every possible classification key match at least one filter (possibly the wildcard default) and that any ambiguity between overlapping filters be resolved by precedence. Therefore, the classifiers on any given interface must be "complete" and will often include an "everything else" filter as the lowest precedence element in order for the result of classification to be deterministic. Note that this completeness is only required of the first classifier that incoming traffic will meet as it enters an interface - subsequent classifiers on an interface only need to handle the traffic that it is known that they will receive. This model of classifier operation makes the assumption that all filters of the same precedence be applied simultaneously. Whilst convenient from a modelling point-of-view, this may or may not be how the classifier is actually implemented - this assumption is not intended to dictate how the implementation actually handles this, merely to clearly define the required end result. 4.2. Examples 4.2.1. Behaviour Aggregate (BA) Classifier The simplest Diffserv classifier is a behavior aggregate (BA) classifier [DSARCH]. A BA classifier uses only the Diffserv codepoint (DSCP) in a packet's IP header to determine the logical output stream to which the packet should be directed. We allow only an exact-match condition on this field because the assigned DSCP values have no structure, and therefore no subset of DSCP bits are significant. The following defines a possible BA filter: Filter8: Type: BA Value: 111000 Bernet et al. Expires January 2001 [Page 13]
Internet Draft Diffserv Informal Management Model July 2000 4.2.2. Multi-Field (MF) Classifier Another type of classifier is a multi-field (MF) classifier [DSARCH]. This classifies packets based on one or more fields in the packet (possibly including the DSCP). A common type of MF classifier is a 6- tuple classifier that classifies based on six fields from the IP and TCP or UDP headers (destination address, source address, IP protocol, source port, destination port, and DSCP). MF classifiers may classify on other fields such as MAC addresses, VLAN tags, link-layer traffic class fields or other higher-layer protocol fields. The following defines a possible MF filter: Filter9: Type: IPv4-6-tuple IPv4DestAddrValue: 0.0.0.0 IPv4DestAddrMask: 0.0.0.0 IPv4SrcAddrValue: 172.31.8.0 IPv4SrcAddrMask: 255.255.255.0 IPv4DSCP: 28 IPv4Protocol: 6 IPv4DestL4PortMin: 0 IPv4DestL4PortMax: 65535 IPv4SrcL4PortMin: 20 IPv4SrcL4PortMax: 20 A similar type of classifier can be defined for IPv6. 4.2.3. Free-form Classifier A Free-form classifier is made up of a set of user definable arbitrary filters each made up of {bit-field size, offset (from head of packet), mask}: Classifier2: Filter12: OutputA Filter13: OutputB Default: OutputC Filter12: Type: FreeForm SizeBits: 3 (bits) Offset: 16 (bytes) Value: 100 (binary) Mask: 101 (binary) Filter13: Bernet et al. Expires January 2001 [Page 14]
Internet Draft Diffserv Informal Management Model July 2000 Type: FreeForm SizeBits: 12 (bits) Offset: 16 (bytes) Value: 100100000000 (binary) Mask: 111111111111 (binary) Free-form filters can be combined into filter groups to form very powerful filters. 4.2.4. Other Possible Classifiers Classification may also be performed based on information at the datalink layer below IP (e.g. VLAN or datalink-layer priority) or perhaps on the ingress or egress IP, logical or physical interface identifier. (e.g. the incoming channel number on a channelized interface). A classifier that filters based on IEEE 802.1p Priority and on 802.1Q VLAN-ID might be represented as: Classifier3: Filter14 AND Filter15: OutputA Default: OutputB Filter14: -- priority 4 or 5 Type: Ieee8021pPriority Value: 100 (binary) Mask: 110 (binary) Filter15: -- VLAN 2304 Type: Ieee8021QVlan Value: 100100000000 (binary) Mask: 111111111111 (binary) Such classifiers may be subject of other standards or may be enterprise- specific but are not discussed further here. 5. Meters Metering is defined in [DSARCH]. Diffserv network providers may choose to offer services to customers based on a temporal (i.e., rate) profile within which the customer submits traffic for the service. In this event, a meter might be used to trigger real-time traffic conditioning actions (e.g., marking) by routing a non-conforming packet through an appropriate next-stage action element. Alternatively, by counting conforming and/or non-conforming traffic, it might also be used for collecting data for out-of-band management functions such as billing applications. Bernet et al. Expires January 2001 [Page 15]
Internet Draft Diffserv Informal Management Model July 2000 Meters are logically 1:N (fan-out) devices (although a multiplexor can be used in front of a meter). Meters are parameterized by a temporal profile and by conformance levels, each of which is associated with a meter's output. Each output can be connected to another functional element. Note that this model of a meter differs slightly from that described in [DSARCH]. In that description the meter is not a datapath element but is instead used to monitor the traffic stream and send control signals to action elements to dynamically modulate their behavior based on the conformance of the packet. Figure 4 illustrates a meter with 3 levels of conformance. In some Diffserv examples, three levels of conformance are discussed in terms of colors, with green representing conforming, yellow representing partially conforming and red representing non-conforming [AF-PHB]. These different conformance levels may be used to trigger different queueing, marking or dropping treatment later on in the processing. Other example meters use a binary notion of conformance; in the general case N levels of conformance can be supported. In general there is no constraint on the type of functional datapath element following a meter output, but care must be taken not to inadvertently configure a datapath that results in packet reordering that is not consistent with PHB requirements. A meter, according to this model, measures the rate at which packets making up a stream of traffic pass it, compares the rate to some set of thresholds and produces some number (two or more) potential results: a given packet is said to "conform" to the meter if, at the time that the packet is being looked at, the stream appears to be within the meter's limit rate. unmetered metered traffic traffic +---------+ | |--------> conformance A --------->| meter |--------> conformance B | |--------> conformance C +---------+ Figure 4. A Generic Meter Bernet et al. Expires January 2001 [Page 16]
Internet Draft Diffserv Informal Management Model July 2000 5.1. Token-Bucket Model The concept of conformance to a meter bears comment. The concept applied in several rate-control architectures, including ATM, Frame Relay, Integrated Services and Differentiated Services, is variously described as a "leaky bucket" or a "token bucket". Both token buckets and leaky buckets are, by definition, theoretical relationships between some defined burst_size, rate and interval: rate = burst_size/interval Thus, a token bucket or leaky bucket might specify an information rate of 1.2 Mbps with a burst size of 1500 bytes. In this case, the token rate is 1,200,000 bits per second, the token burst is 12,000 bits and the token interval is 10 milliseconds. The specification says that conforming traffic will in the worst case come in 100 bursts per second of 1500 bytes and at a rate not to exceed 1.2 Mbps. A leaky bucket algorithm is primarily used for traffic shaping (handled under Queues and Schedulers in this model). Traffic theoretically departs from a device at a rate of one bit every so many time units (in the example, one bit every 0.83 microseconds) but, in fact, departs in multi-bit units (packets) at a rate approximating that. In the example, it might send one 1500 byte packet every 10 ms, or one 500 byte packet every 3.3 ms. It is also possible to build multi-rate leaky buckets, in which traffic departs from the switch at varying rates depending on recent activity or inactivity. Implementations generally seek as constant a transmission rate as achievable. In theory, a 10 Mbps shaped transmission stream from an algorithmic implementation and a stream which is running at 10 Mbps because its bottleneck link has been a 10 Mbps Ethernet link should be indistinguishable. Depending on configuration, the approximation to theoretical smoothness may vary by moving as much as an MTU from one token interval to another. Traffic may also be jostled by traffic competing for the same transmission resources. A token bucket measures the arrival rate of traffic from another system, which may have originally been shaped using a leaky bucket shaper or its equivalent, and determines whether it (still) conforms to the specification. Multi-rate token buckets (token buckets with both a peak and a mean rate, and sometimes more rates) are commonly used, such as described in [SRTCM] and [TRTCM]. In this case, absolute smoothness is not expected, but conformance to one or more of the specified rates is expected. Bernet et al. Expires January 2001 [Page 17]
Internet Draft Diffserv Informal Management Model July 2000 Simplistically, a data stream is said to conform to a simple token bucket parameterised by a {rate, burst_size} if the system receives in any time interval, t, at most, an amount of data not exceeding (rate * t) + burst_size. For the multi-rate token bucket case, the data stream is said to conform if, for each of the rates, the stream conforms to the token-bucket profile appropriate for traffic of that class. For example, received traffic that arrives pre-classified as one of the "excess" rates (e.g. AF12 or AF13 traffic for a device implementing the AF1x PHB) is only compared to the relevant "excess" token bucket profile. When used as a leaky bucket shaper, this definition interacts with clock granularity in ways one might not expect. A leaky bucket releases a packet only when all of its bits would have been allowed: it does not borrow from future capacity. If the clock is very fine grain, on the order of the bit rate or faster, this is not an issue. But if the clock is relatively slow (and millisecond or multi-millisecond clocks are not unusual in networking equipment), this can introduce jitter to the shaped stream. The fact that data is organized into variable length packets introduces some uncertainty in this conformance decision. When used in a Scheduler, a leaky bucket releases a packet only when all of its bits would have been allowed: it does not borrow from future capacity. Ideally, when used in a Meter, a token bucket accepts a packet only if all of its bits would have been accepted and does not borrow excess capacity required from future capacity. This is consistent with [SRTCM] and [TRTCM]. In real-world deployment, where MTUs are often larger than the burst size offered by a link-layer network service provider and TCP is more commonly ACK-paced than shaped using a leaky bucket, the loose model offers a solution to the problems that arise. For a more detailed look at the practical issues, see Appendix A. 5.2. Examples The following are some examples of possible meters. 5.2.1. Average Rate Meter An example of a very simple meter is an average rate meter. This type of meter measures the average rate at which packets are submitted to it over a specified averaging time. An average rate profile may take the following form: Meter1: Bernet et al. Expires January 2001 [Page 18]
Internet Draft Diffserv Informal Management Model July 2000 Type: AverageRate Profile: Profile1 ConformingOutput: Queue1 NonConformingOutput: Counter1 Profile1: Type: AverageRate AverageRate: 120 kbps Delta: 100 msec A meter measuring against this profile would continually maintain a count that indicates the total number of packets arriving between time T (now) and time T - 100 msecs. So long as an arriving packet does not push the count over 12 kbits in the last 100 msec then the packet would be deemed conforming. Any packet that pushes the count over 12 kbits would be deemed non-conforming. Thus, this meter deems packets to correspond to one of two conformance levels: conforming or non- conforming and sends them on for the appropriate subsequent treatment. 5.2.2. Exponential Weighted Moving Average (EWMA) Meter The EWMA form of meter is easy to implement in hardware and can be parameterized as follows: avg_rate(t) = (1 - Gain) * avg_rate(t') + Gain * rate(t) t = t' + Delta For a packet arriving at time t: if (avg_rate(t) > AverageRate) non-conforming else conforming "Gain" controls the time constant (e.g. frequency response) of what is essentially a simple IIR low-pass filter. "rate(t)" measures the number of incoming bytes in a small fixed sampling interval, Delta. Any packet that arrives and pushes the average rate over a predefined rate AverageRate is deemed non-conforming. An EWMA meter profile might look something like the following: Meter2: Type: ExpWeightedMovingAvg Profile: Profile2 ConformingOutput: Queue1 NonConformingOutput: AbsoluteDropper1 Bernet et al. Expires January 2001 [Page 19]
Internet Draft Diffserv Informal Management Model July 2000 Profile2: Type: ExpWeightedMovingAvg AverageRate: 25 kbps Delta: 10 usec Gain: 1/16 5.2.3. Two-Parameter Token Bucket Meter A more sophisticated meter might measure conformance to a token bucket (TB) profile. A TB profile generally has two parameters, an average token rate and a burst size. TB meters compare the arrival rate of packets to the average rate specified by the TB profile. Logically, tokens accumulate in a bucket at the average rate, up to a maximum credit which is the burst size. Packets of length L bytes are considered conforming if any tokens are available in the bucket at the time of packet arrival: up to L bytes may then be borrowed from future token allocations. Packets are allowed to exceed the average rate in bursts up to the burst size. Packets which arrive to find a bucket with no tokens in it are deemed non-conforming. A two-parameter TB meter has exactly two possible conformance levels (conforming, non-conforming). TB implementation details are discussed in Appendix A. Note that this is a "lenient" meter that allows some borrowing, as discussed above. A two-parameter TB meter might appear as follows: Meter3: Type: SimpleTokenBucket Profile: Profile3 ConformingOutput: Queue1 NonConformingOutput: AbsoluteDropper1 Profile3: Type: SimpleTokenBucket AverageRate: 200 kbps BurstSize: 100 kbytes 5.2.4. Multi-Stage Token Bucket Meter More complicated TB meters might define two burst sizes and three conformance levels. Packets found to exceed the larger burst size are deemed non-conforming. Packets found to exceed the smaller burst size are deemed partially conforming. Packets exceeding neither are deemed conforming. Token bucket meters designed for Diffserv networks are described in more detail in [SRTCM, TRTCM, GTC]; in some of these references three levels of conformance are discussed in terms of colors, with green representing conforming, yellow representing partially conforming and red representing non- conforming. Often these multi- Bernet et al. Expires January 2001 [Page 20]
Internet Draft Diffserv Informal Management Model July 2000 conformance level meters can be implemented using an appropriate configuration of multiple two- parameter TB meters. A profile for a multi-stage TB meter with three levels of conformance might look as follows: Meter4: Type: TwoRateTokenBucket ProfileA: Profile4 ConformingOutputA: Queue1 ProfileB: Profile5 ConformingOutputB: Marker1 NonConformingOutput: AbsoluteDropper1 Profile4: Type: SimpleTokenBucket AverageRate: 100 kbps BurstSize: 20 kbytes Profile5: Type: SimpleTokenBucket AverageRate: 100 kbps BurstSize: 100 kbytes 5.2.5. Null Meter A null meter has only one output: always conforming, and no associated temporal profile. Such a meter is useful to define in the event that the configuration or management interface does not have the flexibility to omit a meter in a datapath segment. Meter5: Type: NullMeter Output: Queue1 6. Action Elements The classifiers and meters described up to this point are fan-out elements which are generally used to determine the appropriate action to apply to a packet. The set of possible actions that can then be applied include: - Marking - Absolute Dropping Bernet et al. Expires January 2001 [Page 21]
Internet Draft Diffserv Informal Management Model July 2000 - Multiplexing - Counting - Null action - do nothing The corresponding action elements are described in the following sections. Diffserv nodes may apply shaping, policing and/or marking to traffic streams that exceed the bounds of their TCS in order to prevent a traffic stream from seizing more than its share of resources from a Diffserv network. Shaping, sometimes considered as a TC action, is treated as a part of the queueing module in this model, as is the use of Algorithmic Dropping techniques - see section 7. Policing is modelled as either a concatenation of a Meter with an Absolute Dropper or as a concatenation of an Algorithmic Dropper with a Scheduler. These elements will discard packets which exceed the TCS. Marking is performed by a Marker Action, which (in this context) alters the DSCP, and thus the PHB, of the packet to give it a lower-grade treatment at subsequent Diffserv nodes. 6.1. Marker Markers are 1:1 elements which set a codepoint (e.g. the DSCP in an IP header). Markers may also act on unmarked packets (e.g. those submitted with DSCP of zero) or may re-mark previously marked packets. In particular, the model supports the application of marking based on a preceding classifier match. The mark set in a packet will determine its subsequent treatment in downstream nodes of a network and possibly also in subsequent processing stages within this router. DSCP Markers for Diffserv are normally parameterized by a single parameter: the 6-bit DSCP to be marked in the packet header. Marker1: Type: DSCPMarker Mark: 010010 6.2. Absolute Dropper Absolute Droppers simply discard packets. There are no parameters for these droppers. Because this Algorithmic Dropper is a terminating point of the datapath and has no outputs, it is probably desirable to forward the packet through a Counter Action first for instrumentation purposes. AbsoluteDropper1: Bernet et al. Expires January 2001 [Page 22]
Internet Draft Diffserv Informal Management Model July 2000 Type: AbsoluteDropper Absolute Droppers are not the only elements than can cause a packet to be discarded: another element is an Algorithmic Dropper element (see Section 7.1.3). However, since this element's behavior is closely tied the state of one or more queues, we choose to distinguish it as a separate functional datapath element. 6.3. Multiplexor It is occasionally necessary to multiplex traffic streams into a functional datapath element with a single input. A M:1 (fan-in) multiplexor is a simple logical device for merging traffic streams. It is parameterized by its number of incoming ports. Mux1: Type: Multiplexor Output: Queue2 6.4. Counter One passive action is to account for the fact that a data packet was processed. The statistics that result might be used later for customer billing, service verification or network engineering purposes. Counters are 1:1 functional datapath elements which update a counter by L and a packet counter by 1 every time a L-byte sized packet passes through them. Counters can be used to count packets about to be be dropped by an Absolute Dropper or to count packets arriving at or departing from some other functional element. Counter1: Type: Counter Output: Queue1 6.5. Null Action A null action has one input and one output. The element performs no action on the packet. Such an element is useful to define in the event that the configuration or management interface does not have the flexibility to omit an action element in a datapath segment. Null1: Type: Null Output: Queue1 Bernet et al. Expires January 2001 [Page 23]
Internet Draft Diffserv Informal Management Model July 2000 7. Queueing Elements Queueing elements modulate the transmission of packets belonging to the different traffic streams and determine their ordering, possibly storing them temporarily or discarding them. Packets are usually stored either because there is a resource constraint (e.g., available bandwidth) which prevents immediate forwarding, or because the queueing block is being used to alter the temporal properties of a traffic stream (i.e. shaping). Packets are discarded either because of buffering limitations, because a buffer threshold is exceeded (including when shaping is performed), as a feedback control signal to reactive control protocols such as TCP, because a meter exceeds a configured profile (i.e. policing). The queueing elements in this model represent a logical abstraction of a queueing system, which is used to configure PHB-related parameters. The model can be used to represent a broad variety of possible implementations. However, it need not necessarily map one-to-one with physical queueing systems in a specific router implementation. Implementors should map the configurable parameters of the implementation's queueing systems to these queueing element parameters as appropriate to achieve equivalent behaviors. 7.1. Queueing Model Queueing is a function a which lends itself to innovation. It must be modelled to allow a broad range of possible implementations to be represented using common structures and parameters. This model uses functional decomposition as a tool to permit the needed lattitude. Queueing systems perform three distinct, but related, functions: they store packets, they modulate the departure of packets belonging to various traffic streams and they selectively discard packets. This model decomposes the queueing block into the component elements that perform each of these functions: Queues, Schedulers and Algorithmic Droppers, respectively. These elements may be connected together as part of a TCB containing one or more Queues, zero or more Algorithmic Droppers and one or more Schedulers. The remainder of this section discusses FIFO Queues: typically, the Queue element of this model will be implemented as a FIFO data structure. However, this does not preclude implementations which are not strictly FIFO, in that they also support operations that remove or examine packets (e.g., for use by discarders) other than at the head or tail. However, such operations MUST NOT have the effect of reordering packets belonging to the same microflow. Bernet et al. Expires January 2001 [Page 24]
Internet Draft Diffserv Informal Management Model July 2000 Note that the term FIFO has multiple different common usages: it is sometimes taken to mean, among other things, a data structure that permits items to be removed only in the order in which they were inserted or a service discipline which is non- reordering. 7.1.1. FIFO Queue In this model, a FIFO Queue element is a data structure which at any time may contain zero or more packets. It may have one or more thresholds associated with it. A FIFO has one or more inputs and exactly one output. It must support an enqueue operation to add a packet to the tail of the queue, and a dequeue operation to remove a packet from the head of the queue. Packets must be dequeued in the order in which they were enqueued. A FIFO has a current depth, which indicates the number of packets that it contains at a particular time. FIFOs in this model are modelled without inherent limits on their depth - obviously this does not reflect the reality of implementations: FIFO size limits are modelled here by an algorithmic dropper associated with the FIFO, typically at its input. It is quite likely that, every FIFO will be preceded by an algorithmic dropper. One exception might be the case where the packet stream has already been policed to a profile that can never exceed the scheduler bandwidth available at the FIFO's output - this would not need an algorithmic dropper at the input to the FIFO. This representation of a FIFO allows for one common type of depth limit, one that results from a FIFO supplied from a limited pool of buffers, shared between multiple FIFOs. In an implementation, packets are presumably stored in one or more buffers. Buffers are allocated from one or more free buffer pools. If there are multiple instances of a FIFO, their packet buffers may or may not be allocated out of the same free buffer pool. Free buffer pools may also have one or more threshold associated with them, which may affect discarding and/or scheduling. Other than this, buffering mechanisms are implementation specific and not part of this model. A FIFO might be represented using the following parameters: Queue1: Type: FIFO Output: Scheduler1 Note that a FIFO must provide triggers and/or current state information to other elements upstream and downstream from it: in particular, it is likely that the current depth will need to be used by Algorithmic Dropper elements placed before or after the FIFO. It will also likely need to provide an implicit "I have packets for you" signal to Bernet et al. Expires January 2001 [Page 25]
Internet Draft Diffserv Informal Management Model July 2000 downstream Scheduler elements. 7.1.2. Scheduler A scheduler is an element which gates the departure of each packet that arrives at one of its inputs, based on a service discipline. It has one or more input and exactly one output. Each input has an upstream element to which it is connected, and a set of parameters that affects the scheduling of packets received at that input. The service discipline (also known as a scheduling algorithm) is an algorithm which might take any of the following as its input(s): a) static parameters such as relative priority associated with each of the scheduler's inputs. b) absolute token bucket parameters for maximum or minimum rates associated with each of the scheduler's inputs. c) parameters, such as packet length or DSCP, associated with the packet currently present at its input. d) absolute time and/or local state. Possible service disciplines fall into a number of categories, including (but not limited to) first come, first served (FCFS), strict priority, weighted fair bandwidth sharing (e.g., WFQ, WRR, etc.), rate-limited strict priority and rate-based. Service disciplines can be further distinguished by whether they are work-conserving or non-work-conserving (see Glossary). Non-work-conserving schedulers can be used to shape traffic streams to match some profile by delaying packets that might be deemed non-conforming by some downstream node: a packet is delayed until such time as it would conform to a downstream meter using the same profile. [DSARCH] defines PHBs without specifying required scheduling algorithms. However, PHBs such as the class selectors [DSFIELD], EF [EF-PHB] and AF [AF-PHB] have descriptions or configuration parameters which strongly suggest the sort of scheduling discipline needed to implement them. This memo discusses a minimal set of queue parameters to enable realization of these per- hop behaviors. It does not attempt to specify an all- embracing set of parameters to cover all possible implementation models. A mimimal set includes: a) a minimum service rate profile which allows rate guarantees for each traffic stream as required by EF and AF without specifying the details of how excess bandwidth between these traffic streams is Bernet et al. Expires January 2001 [Page 26]
Internet Draft Diffserv Informal Management Model July 2000 shared. Additional parameters to control this behavior should be made available, but are dependent on the particular scheduling algorithm implemented. b) a service priority, used only after the minimum rate profiles of all inputs have been satisfied, to decide how to allocate any remaining bandwidth. c) a maximum service rate profile, for use only with a non-work- conserving service discipline. For an implementation of the EF PHB using a strict priority scheduling algorithm that assumes that the aggregate EF rate has been appropriately bounded to avoid starvation, the minimum rate profile would be reported as zero and the maximum service rate would be reported as line rate. Such an implementation, with multiple priority classes, could also be used for the Diffserv class selectors [DSFIELD]. Alternatively, setting the service priority values for each input to the scheduler to the same value enables the scheduler to satisfy the minimum service rates for each input, so long as the sum of all minimum service rates is less than or equal to the line rate. For example, a non-work-conserving scheduler, allocating spare bandwidth equally between all its inputs, might be represented using the following parameters: Scheduler1: Type: Scheduler2Input Input1: MaxRateProfile: Profile1 MinRateProfile: Profile2 Priority: none Input2: MaxRateProfile: Profile3 MinRateProfile: Profile4 Priority: none A work-conserving scheduler might be represented using the following parameters: Scheduler2: Type: Scheduler3Input Input1: Bernet et al. Expires January 2001 [Page 27]
Internet Draft Diffserv Informal Management Model July 2000 MaxRateProfile: WorkConserving MinRateProfile: Profile5 Priority: 1 Input2: MaxRateProfile: WorkConserving MinRateProfile: Profile6 Priority: 2 Input3: MaxRateProfile: WorkConserving MinRateProfile: none Priority: 3 7.1.3. Algorithmic Dropper An Algorithmic Dropper is an element which selectively discards packets that arrive at its input, based on a discarding algorithm. It has one data input and one output. In this model (but not necessarily in a real implementation), a packet enters the dropper at its input and either its buffer is returned to a free buffer pool or the packet exits the dropper at the output. Alternatively, an Algorithmic Dropper may invoke operations on a FIFO which selectively removes a packet, then return its buffer to the free buffer pool, based on a discarding algorithm. In this case, the operation ould be modelled as a side-effect on the FIFO upon which it operated, rather than as having a discrete input and output. These two treatments are equivalent and we choose the former here. The Algorithmic Dropper is modelled as having a single input. It is possible that packets which were classified differently by a Classifier in this TCB will end up passing through the same dropper. The dropper's algorithm may need to apply different calculations based on characteristics of the incoming packet e.g. its DSCP. So there is a need, in implementations of this model, to be able to relate information about which classifier element was matched by a packet from a Classifier to an Algorithmic Dropper. In the rare cases where this is required, the chosen model is to insert another Classifier element at this point in the flow and for it to feed into multiple Algorithmic Dropper elements, each one implementing a drop calculation that is independent of any classification keys of the packet: this will likely require the creation of a new TCB to contain the Classifier and the Algorithmic Dropper elements. There are many other formulations of a model that could represent this linkage, other than the one described above: one formulation would have Bernet et al. Expires January 2001 [Page 28]
Internet Draft Diffserv Informal Management Model July 2000 been to have a pointer from one of the drop probability calculation algorithms inside the dropper to the original Classifier element that selects this algorithm. Another way would have been to have multiple "inputs" to the Algorithmic Dropper element fed from the preceding elements, leading eventually back to the Classifier elements that matched the packet. Yet another formulation might have been for the Classifier to (logically) include some sort of "classification identifier" along with the packet along its path, for use by any subsequent element. And yet another could have been to include a classifier inside the dropper, in order for it to pick out the drop algorithm to be applied. All of these other approaches were deemed to be more clumsy or less useful than the approach taken here. An Algorithmic Dropper, shown in Figure 5, has one or more triggers that cause it to make a decision whether or not to drop one (or possibly more than one) packet. A trigger may be internal (the arrival of a packet at the input to the dropper) or it may be external (resulting from one or more state changes at another element, such as a FIFO depth exceeding a threshold or a scheduling event). It is likely that an instantaneous FIFO depth will need to be smoothed over some averaging interval. Some dropping algorithms may require several trigger inputs feeding back from events elsewhere in the system e.g. smoothing functions that calculate averages over more than one time interval. Smoothing functions are outside the scope of this document and are not modelled here, we merely indicate where they might be added in the model. A trigger may be a boolean combination of events (e.g. a FIFO depth exceeding a threshold OR a buffer pool depth falling below a threshold). The dropping algorithm makes a decision on whether to forward or to discard a packet. It takes as its parameters some set of dynamic parameters (e.g. averaged or instantaneous FIFO depth) and some set of static parameters (e.g. thresholds) and possibly parameters associated with the packet (e.g. its PHB, as determined by a classifier, which will determine on which of the droppers inputs the packet arrives). It may also have internal state and is likely to keep counters regarding the dropped packets (there is no appropriate place here to include a Counter Action element). RED, RED-on-In-and-Out (RIO) and Drop-on-threshold are examples of dropping algorithms. Tail-dropping and head-dropping are effected by the location of the dropper relative to the FIFO. Note that, although an Algorithmic Dropper may require knowledge of data fields in a packet, as discovered by a Classifier in the same TCB, it may not modify the packet (i.e. it is not a marker). Bernet et al. Expires January 2001 [Page 29]
Internet Draft Diffserv Informal Management Model July 2000 +--------------------------------------+ | +------------+ +-----------+ |Algorithmic | | smoothing | n |trigger & | |Dropper | | function(s)|---/--->|discard | | | | (optional) | |calc. | | | +------------+ +-----------+ | | ^ TailDrop| |HeadDrop | +------------|-------------|-|---------+ | | | +---|-------------+ | | | | v |Depth v Input ----------------------+ to Scheduler -----------------------------> |x|x|x|x|x|x|x|-------------------> ----------------------+ FIFO | | | | | | v | bit-bucket +---+ Figure 5. Algorithmic Dropper + Queue An Algorithmic Dropper which uses a RIO algorithm might be represented using the following parameters: AlgorithmicDropper1: Type: AlgorithmicDropper Discipline: RIO Trigger: Internal Output: Fifo1 InputA: (in profile) MinThresh: Fifo1.Depth > 20 kbyte MaxThresh: Fifo1.Depth > 30 kbyte InputB: (out of profile) MinThresh: Fifo1.Depth > 10 kbyte MaxThresh: Fifo1.Depth > 20 kbyte SampleWeight .002 MaxDropProb 1% Bernet et al. Expires January 2001 [Page 30]
Internet Draft Diffserv Informal Management Model July 2000 Another form of Algorithmic Dropper, a threshold-dropper, might be represented using the following parameters: AlgorithmicDropper2: Type: AlgorithmicDropper Discipline: Drop-on-threshold Trigger: Fifo2.Depth > 20 kbyte Output: Fifo1 Yet another Algorithmic Dropper which drops all out-of-profile packets whenever the FIFO threshold exceeds a certain depth (this Algorithmic Dropper is not part of the larger TCB example) might be represented with the following parameters: AlgorithmicDropper3: Type: AlgorithmicDropper2Input Discipline: Drop-out-packets-on-threshold Output: Fifo3 InputA: (in profile) Trigger: none InputB: (out of profile) Trigger: Fifo3.Depth > 100 kbyte 7.1.4. Constructing queueing blocks from the elements A queueing block is constructed by concatenation of these functional datapath elements. Elements of the same type may appear more than once in a queueing block, either in parallel or in series. Typically, a queueing block will have relatively many elements in parallel and few in series. Iteration and recursion are not supported constructs in this grammar. A queueing block must have at least one Queue, zero or more Algorithmic Droppers and at least one Scheduler. The following inter- connections are allowed: 1) The input of a Queue may be the input of the queueing block or it may be connected to the output of an Algorithmic Dropper or to an output of a Scheduler. 2) Each input of a Scheduler may be connected to the output of a Queue, to the output of an Algorithmic Dropper or to the output of another Scheduler. 3) The input of an Algorithmic Dropper must be the input of the queueing block. Bernet et al. Expires January 2001 [Page 31]
Internet Draft Diffserv Informal Management Model July 2000 4) The output of the queueing block may be the output of a Queue, an Algorithmic Dropper or a Scheduler. Note, in particular, that Schedulers may operate in series such that a packet at the head of a Queue feeding the concatenated Schedulers is serviced only after all of the scheduling criteria are met. For example, a Queue which carries EF traffic streams may be served first by a non- work-conserving Scheduler to shape the stream to a maximum rate, then by a work-conserving Scheduler to mix EF traffic streams with other traffic streams. Alternatively, there might be a Queue and/or a dropper between the two Schedulers. 7.2. Sharing load among traffic streams using queueing Queues are used, in Differentiated Services, for a number of purposes. In essence, they are simply places to store traffic until it is transmitted. However, when several queues are used together in a queueing system, they can also achieve effects beyond that for given traffic streams. They can be used to limit variation in delay or impose a maximum rate (shaping), to permit several streams to share a link in a semi-predictable fashion (load sharing), or to move variation in delay from some streams to other streams. Traffic shaping is often used to condition traffic such that packets arriving in a burst will be "smoothed" and deemed conforming by subsequent downstream meters in this or other nodes. Shaping may also be used to isolate certain traffic streams from the effects of other traffic streams of the same BA. In [DSARCH] a shaper is described as a queueing element controlled by a meter which defines its temporal profile. However, this representation of a shaper differs substantially from typical shaper implementations. In this conceptual model, a shaper is realized by using a non-work- conserving Scheduler. Some implementations may elect to have queues whose sole purpose is shaping, while others may integrate the shaping function with other buffering, discarding and scheduling associated with access to a resource. Shapers operate by delaying the departure of packets that would be deemed non-conforming by a meter configured to the shaper's maximum service rate profile. The packet is scheduled to depart no sooner than such time that it would become conforming. 7.2.1. Load Sharing Load sharing is the traditional use of queues. It was theoretically explored in a paper by Floyd [FLOYD] in 1993, but has been in use in communications systems since the 1970's. Bernet et al. Expires January 2001 [Page 32]
Internet Draft Diffserv Informal Management Model July 2000 [DSARCH] discusses load sharing as dividing an interface among traffic classes predictably or applying a minimum rate to each of a set of traffic classes, which might be measured as an absolute lower bound on the rate a traffic stream achieves, or a fraction of the rate an interface offers. It is generally implemented as some form of weighted round robin among a set of FIFO queues or WFQ system. This has interesting side-effects. A key effect sought is to ensure that the mean rate the traffic in a stream experiences is never lower than some threshold when there is at least that much traffic to send. When there is less traffic than this, the queue tends to be starved for traffic, meaning that the queuing system will not delay its traffic by very much. When there is significantly more and the queue fills, packets in this class will be delayed significantly more than traffic in other classes that are under- using their available capacity. This form of queuing system therefore tends to move delay and variation in delay from under-used classes of traffic to heavier users, as well as managing the rates of the traffic streams. A side-effect of a WRR or WFQ implementation is that between any two packets in a given traffic class, the scheduler may emit one or more packets from each of the other classes in the queuing system. In cases where average behavior is in view, this is perfectly acceptable. In cases where traffic is very intolerant of jitter and there are a number of competing classes, this may have undesirable consequences. 7.2.2. Traffic Priority Traffic Prioritization is a special case of load sharing, wherein a certain traffic class is deemed so jitter-intolerant that if it has traffic present, that traffic must be sent at the earliest possible time. By extension, several priorities might be defined, such that traffic in each of several classes is given preferential service over any traffic of a lower class. It is the obvious implementation of IP Precedence as described in [RFC 791], of 802.1p traffic classes [802.1D], and other similar technologies. Priority is often abused in real networks; people tend to think that traffic which has a business priority deserves this treatment, and talk more about the business imperatives than the actual application requirements. This can have severe consequences; networks have been configured which placed business-critical traffic at a higher priority than routing traffic, resulting in congestive collapse of the networks. However, it has a legitimate use in services like EF, where it is absolutely known, due to policing, that a traffic stream does not abuse its rate, and the application is indeed jitter-intolerant enough to Bernet et al. Expires January 2001 [Page 33]
Internet Draft Diffserv Informal Management Model July 2000 merit this type of handling. 8. Traffic Conditioning Blocks (TCBs) The Classifier, Meter, Action, Algorithmic Dropper, Queue and Scheduler functional datapath elements described above can be combined into Traffic Conditioning Blocks (TCBs). A TCB is an abstraction of a set of functional datapath elements that may be used to facilitate the definition of specific traffic conditioning functionality e.g. it might be likened to a template which can be replicated multiple times for different traffic streams or different customers. It has no likely physical representation in the implementation of the data path: it is invented purely as an abstraction for use by management tools. This model describes the configuration and management of a Diffserv interface in terms of a TCB that contains, by definition, zero or more Classifier, Meter, Action, Algorithmic Dropper, Queue and Scheduler elements. These elements are arranged arbitrarily according to the policy being expressed, but always in the order here. Traffic may be classified; classified traffic may be metered; each stream of traffic identified by a combination of classifiers and meters may have some set of actions performed on it, followed by drop algorithms; packets of the traffic stream may ultimately be stored into a queue and then be scheduled out to the next TCB or physical interface. It is possible to omit elements or include null elements of any type, or to concatenate multiple functional datapath elements of the same type. When the Diffserv treatment for a given packet needs to have those building blocks repeated, this is performed by cascading multiple TCBs: an output of one TCB may drive the input of a succeeding one. For example, consider the case where traffic of a set of classes is shaped to a set of rates, but the total output rate of the group of classes must also be limited to a rate. One might imagine a set of network news feeds, each with a certain maximum rate, and a policy that their aggregate may not exceed some figure. This may be simply accomplished by cascading two TCBs. The first classifies the traffic into its separate feeds and queues each feed separately. The feeds (or a subset of them) are now fed into a second TCB, which places all input (these news feeds) into a single queue with a certain maximum rate. In implementation, one could imagine this as the several literal queues, a CBQ or WFQ system with an appropriate (and complex) weighting scheme, or a number of other approaches. But they would have the same externally measurable effect on the traffic as if they had been literally implemented with separate TCBs. A generalised TCB might consist of the following stages: - Classification stage Bernet et al. Expires January 2001 [Page 34]
Internet Draft Diffserv Informal Management Model July 2000 - Metering stage - Action stage - Algorithmic Dropping stage - Queueing stage - Scheduling stage where each stage may consist of a set of parallel datapaths consisting of pipelined elements. A Classifier or a Meter is typically a 1:N element, an Action, Algorithmic Dropper or Queue is typically a 1:1 element and a Scheduler is a N:1 element. A complete TCB should, however, result in a 1:1 or 1:N abstract element. Note that the fan-in or fan-out of an element is not an important defining characteristic of this taxonomy. 8.1. An Example TCB A SLS is presumed to have been negotiated between the customer and the provider which specifies the handling of the customer's traffic by the provider's network. The agreement might be of the following form: DSCP PHB Profile Treatment ---- --- ------- ---------------------- 001001 EF Profile4 Discard non-conforming. 001100 AF11 Profile5 Shape to profile, tail-drop when full. 001101 AF21 Profile3 Re-mark non-conforming to DSCP 001000, tail-drop when full. other BE none Apply RED-like dropping. This SLS specifies that the customer may submit packets marked for DSCP 001001 which will get EF treatment so long as they remain conforming to Profile1 and will be discarded if they exceed this profile. The discarded packets are counted in this example, perhaps for use by the provider's sales department in convincing the customer to buy a larger SLS. Packets marked for DSCP 001100 will be shaped to Profile2 before forwarding. Packets marked for DSCP 001101 will be metered to Profile3 with non-conforming packets "downgraded" by being re-marked with a DSCP of 001000. It is implicit in this agreement that conforming packets are given the PHB originally indicated by the packets' DSCP field. Figures 6 and 7 illustrates a TCB that might be used to handle this SLS at an ingress interface at the customer/provider boundary. The Classification stage of this example consists of a single BA classifier. The BA classifier is used to separate traffic based on the Diffserv service level requested by the customer (as indicated by the Bernet et al. Expires January 2001 [Page 35]
Internet Draft Diffserv Informal Management Model July 2000 +-----+ | A|---------------------------> to Queue1 +->| | | | B|--+ +-----+ +-----+ | +-----+ | | | | | | Meter1 +->| |--->| | | | | | | | +-----+ +-----+ | Counter1 Absolute submitted +-----+ | Dropper1 traffic | A|-----+ --------->| B|----------------------------------------> to Dropper1 | C|-----+ | X|--+ | +-----+ | | +-----+ +-----+ Classifier1| | | A|--------------->|A | (BA) | +->| | | |--> to Dropper2 | | B|--+ +-----+ +->|B | | +-----+ | | | | +-----+ | Meter2 +->| |-+ Mux1 | | | | +-----+ | Marker1 +-------------------------------------> to Dropper3 Figure 6: An Example Traffic Conditioning Block (Part 1) DSCP in each submitted packet's IP header). We illustrate three DSCP filter values: A, B and C. The 'X' in the BA classifier is a wildcard filter that matches every packet not otherwise matched. The path for DSCP 001100 proceeds directly to Dropper1 whilst the paths for DSCP 001001 and 001101 include a metering stage. All other traffic is passed directly on to Dropper3. There is a separate meter for each set of packets corresponding to classifier outputs A and C. Each meter uses a specific profile, as specified in the TCS, for the corresponding Diffserv service level. The meters in this example each indicate one of two conformance levels: conforming or non-conforming. Following the Metering stage is an Action stage in some of the branches. Packets submitted for DSCP 001001 (Classifier output A) that are deemed non- conforming by Meter1 are counted and discarded while packets that are conforming are passed on to Queue1. Packets submitted for DSCP 001101 (Classifier output C) that are deemed non-conforming by Meter2 are re-marked and then both conforming and non-conforming packets are multiplexed together before being passed on to Dropper2/Queue3. Bernet et al. Expires January 2001 [Page 36]
Internet Draft Diffserv Informal Management Model July 2000 The Algorithmic Dropping, Queueing and Scheduling stages are realised as follows, illustrated in figure 7. Note that the figure does not show any of the implicit control linkages between elements that allow e.g. an Algorithmic Dropper to sense the current state of a succeeding Queue. Conforming DSCP 001001 packets from Meter1 are passed directly to Queue1: there is no way, with a configuration of the following Scheduler that patches the metering, for these packets to overflow the depth of Queue1 so there is never a requirement for dropping at this point. Packets marked for DSCP 001100 must be passed through a tail-dropper, Dropper1, which serves to limit the depth of the following queue, Queue2: packets that arrive to a full queue will be discarded - this is likely to be an error case: the customer is obviously not sticking to its agreed profile. Similarly, all packets from the original DSCP 001101 stream (some may have been re-marked by this stage) are passed to Dropper2 and Queue3. Packets marked for all other DSCPs are passed to Dropper3 which is a RED-like algorithmic dropper: based on feedback of the current depth of Queue4, this dropper is likely to discard enough packets from its input stream to keep the queue depth under control. These four Queue elements are then serviced by a Scheduler element Scheduler1: tis must be configured to give each of its inputs an appropriate priority and/or bandwidth share. Inputs A and C are given guarantees of bandwidth, as appropriate for the contracted profiles. Input B is given a limit on the bandwidth it can use i.e. a non-work- conserving discipline in order to achieve the desired shaping of this stream. Input D is given no limits or guarantees but a lower priority than the other queues, appropriate for its best-effort status. Traffic then exits the Scheduler in a single orderly stream. The interconnections of the TCB elements illustrated in Figures 6 and 7 can be represented as follows: TCB1: Classifier1: FilterA: Meter1 FilterB: Dropper1 FilterC: Meter2 Default: Dropper3 Meter1: Type: AverageRate Profile: Profile1 ConformingOutput: Queue1 NonConformingOutput: Counter1 Counter1: Bernet et al. Expires January 2001 [Page 37]
Internet Draft Diffserv Informal Management Model July 2000 from Meter1 +-----+ ------------------------------->| |----+ | | | +-----+ | Queue1 | | +-----+ from Classifier1 +-----+ +-----+ +->|A | ---------------->| |------->| |------>|B |-------> | | | | +--->|C | exiting +-----+ +-----+ | +->|D | traffic AlgDropper1 Queue2 | | +-----+ | | Scheduler1 from Mux1 +-----+ +-----+ | | ---------------->| |------->| |--+ | | | | | | +-----+ +-----+ | AlgDropper2 Queue3 | | from Classifier1 +-----+ +-----+ | ---------------->| |------->| |----+ | | | | +-----+ +-----+ AlgDropper3 Queue4 Figure 7: An Example Traffic Conditioning Block (Part 2) Output: AbsoluteDropper1 Meter2: Type: AverageRate Profile: Profile3 ConformingOutput: Mux1.InputA NonConformingOutput: Marker1 Marker1: Type: DSCPMarker Mark: 001000 Output: Mux1.InputB Mux1: Output: Dropper2 AlgDropper1: Type: AlgorithmicDropper Discipline: Drop-on-threshold Bernet et al. Expires January 2001 [Page 38]
Internet Draft Diffserv Informal Management Model July 2000 Trigger: Queue2.Depth > 10kbyte Output: Queue2 AlgDropper2: Type: AlgorithmicDropper Discipline: Drop-on-threshold Trigger: Queue3.Depth > 20kbyte Output: Queue3 AlgDropper3: Type: AlgorithmicDropper Discipline: RED93 Trigger: Internal Output: Queue3 MinThresh: Queue3.Depth > 20 kbyte MaxThresh: Queue3.Depth > 40 kbyte <other RED parms too> Queue1: Type: FIFO Output: Scheduler1.InputA Queue2: Type: FIFO Output: Scheduler1.InputB Queue3: Type: FIFO Output: Scheduler1.InputC Queue4: Type: FIFO Output: Scheduler1.InputD Scheduler1: Type: Scheduler4Input InputA: MaxRateProfile: none MinRateProfile: Profile4 Priority: 20 InputB: MaxRateProfile: Profile5 MinRateProfile: none Priority: 40 InputC: MaxRateProfile: none MinRateProfile: Profile3 Bernet et al. Expires January 2001 [Page 39]
Internet Draft Diffserv Informal Management Model July 2000 Priority: 20 InputD: MaxRateProfile: none MinRateProfile: none Priority: 10 8.2. An Example TCB to Support Multiple Customers The TCB described above can be installed on an ingress interface to implement a provider/customer TCS if the interface is dedicated to the customer. However, if a single interface is shared between multiple customers, then the TCB above will not suffice, since it does not differentiate among traffic from different customers. Its classification stage uses only BA classifiers. The configuration is readily modified to support the case of multiple customers per interface, as follows. First, a TCB is defined for each customer to reflect the TCS with that customer: TCB1, defined above is the TCB for customer 1 and elements are created for TCB2 and for TCB3 which reflect the agreements with customers 2 and 3 respectively. These 3 TCBs may or may not share the same elements and parameters. Finally, a classifier is added to the front end to separate the traffic from the three different customers. This forms a new TCB, TCB4, which is illustrated in Figure 8. A representation of this multi-customer TCB might be: TCB4: Classifier4: submitted +-----+ traffic | A|--------> TCB1 --------->| B|--------> TCB2 | C|--------> TCB3 | X|------+ +-----+ +-----+ +-->| | Classifier4 +-----+ AbsoluteDrop4 Figure 8: An Example of a Multi-Customer TCB Bernet et al. Expires January 2001 [Page 40]
Internet Draft Diffserv Informal Management Model July 2000 Filter1: to TCB1 Filter2: to TCB2 Filter3: to TCB3 No Match: AbsoluteDropper4 TCB1: (as defined above) TCB2: (similar to TCB1, perhaps with different elements or numeric parameters) TCB3: (similar to TCB1, perhaps with different elements or numeric parameters) and the filters, based on each customer's source MAC address, could be defined as follows: Filter1: Type: MacAddress SrcValue: 01-02-03-04-05-06 (source MAC address of customer 1) SrcMask: FF-FF-FF-FF-FF-FF DestValue: 00-00-00-00-00-00 DestMask: 00-00-00-00-00-00 Filter2: (similar to Filter1 but with customer 2's source MAC address as SrcValue) Filter3: (similar to Filter1 but with customer 3's source MAC address as SrcValue) In this example, Classifier4 separates traffic submitted from different customers based on the source MAC address in submitted packets. Those packets with recognized source MAC addresses are passed to the TCB implementing the TCS with the corresponding customer. Those packets with unrecognized source MAC addresses are passed to a dropper. TCB4 has a Classifier stage and an Action element stage performing dropping of all unmatched traffic. 8.3. TCBs Supporting Microflow-based Services The TCB illustrated above describes a configuration that might be suitable for enforcing a SLS at a router's ingress. It assumes that the Bernet et al. Expires January 2001 [Page 41]
Internet Draft Diffserv Informal Management Model July 2000 customer marks its own traffic for the appropriate service level. It then limits the rate of aggregate traffic submitted at each service level, thereby protecting the resources of the Diffserv network. It does not provide any isolation between the customer's individual microflows. A more complex example might be a TCB configuration that offers additional functionality to the customer. It recognizes individual customer microflows and marks each one independently. It also isolates the customer's individual microflows from each other in order to prevent a single microflow from seizing an unfair share of the resources available to the customer at a certain service level. This is illustrated in Figure 9. Suppose that the customer has an SLS which specifices 2 service levels, to be identifed to the provider by DSCP A and DSCP B. Traffic is first directed to a MF classifier which classifies traffic based on miscellaneous classification criteria, to a granularity sufficient to identify individual customer microflows. Each microflow can then be +-----+ +-----+ Classifier1 | | | |---------------+ (MF) +->| |-->| | +-----+ | +-----+ | | | | |---->| | | | A|------ +-----+ +-----+ +-----+ | --->| B|-----+ Marker1 Meter1 Absolute | | C|---+ | Dropper1 | +-----+ | X|-+ | | +-----+ +-----+ +-->|A | +-----+ | | | | | | |------------------>|B |---> | | +->| |-->| | +-----+ +-->|C | to TCB2 | | | | | |---->| | | +-----+ | | +-----+ +-----+ +-----+ | Mux1 | | Marker2 Meter2 Absolute | | | Dropper2 | | | +-----+ +-----+ | | | | | | |---------------+ | |--->| |-->| | +-----+ | | | | |---->| | | +-----+ +-----+ +-----+ | Marker3 Meter3 Absolute | Dropper3 V etc. Figure 9: An Example of a Marking and Traffic Isolation TCB Bernet et al. Expires January 2001 [Page 42]
Internet Draft Diffserv Informal Management Model July 2000 marked for a specific DSCP The metering elements limit the contribution of each of the customer's microflows to the service level for which it was marked. Packets exceeding the allowable limit for the microflow are dropped. This TCB could be formally specified as follows: TCB1: Classifier1: (MF) FilterA: Marker1 FilterB: Marker2 FilterC: Marker3 etc. Marker1: Output: Meter1 Marker2: Output: Meter2 Marker3: Output: Meter3 Meter1: ConformingOutput: Mux1.InputA NonConformingOutput: AbsoluteDropper1 Meter2: ConformingOutput: Mux1.InputB NonConformingOutput: AbsoluteDropper2 Meter3: ConformingOutput: Mux1.InputC NonConformingOutput: AbsoluteDropper3 etc. Mux1: Output: to TCB2 Note that the detailed traffic element declarations are not shown here. Traffic is either dropped by TCB1 or emerges marked for one of two DSCPs. This traffic is then passed to TCB2 which is illustrated in Figure 10. Bernet et al. Expires January 2001 [Page 43]
Internet Draft Diffserv Informal Management Model July 2000 +-----+ | |---------------> to Queue1 +->| | +-----+ +-----+ | | |---->| | | A|---+ +-----+ +-----+ ->| | Meter5 AbsoluteDropper4 | B|---+ +-----+ +-----+ | | |---------------> to Queue2 Classifier2 +->| | +-----+ (BA) | |---->| | +-----+ +-----+ Meter6 AbsoluteDropper5 Figure 10: Additional Example: TCB2 TCB2 could then be specified as follows: Classifier2: (BA) FilterA: Meter5 FilterB: Meter6 Meter5: ConformingOutput: Queue1 NonConformingOutput: AbsoluteDropper4 Meter6: ConformingOutput: Queue2 NonConformingOutput: AbsoluteDropper5 8.4. Cascaded TCBs Nothing in this model prevents more complex scenarios in which one microflow TCB precedes another (e.g. for TCBs implementing separate TCSs for the source and for a set of destinations). 9. Security Considerations Security vulnerabilities of Diffserv network operation are discussed in [DSARCH]. This document describes an abstract functional model of Diffserv router elements. Certain denial-of-service attacks such as those resulting from resource starvation may be mitigated by appropriate configuration of these router elements; for example, by rate limiting certain traffic streams or by authenticating traffic marked for higher quality-of-service. Bernet et al. Expires January 2001 [Page 44]
Internet Draft Diffserv Informal Management Model July 2000 One particular theft- or denial-of-service issue may arise where a token-bucket meter, with an absolute dropper for non-conforming traffic, is used in a TCB to police a stream to a given TCS: the definition of the token-bucket meter in section 5 indicates that it should be lenient in accepting a packet whenever any bits of the packet would have been within the profile; the definition of the leaky-bucket scheduler is conservative in that a packet is to be transmitted only if the whole packet fits within the profile. This difference may be exploited by a malicious scheduler either to obtain QoS treatment for more octets than allowed in the TCS or to disrupt (perhaps only slightly) the QoS guarantees promised to other traffic streams. 10. Acknowledgments Concepts, terminology, and text have been borrowed liberally from [POLTERM], [DSMIB] and [DSPIB]. We wish to thank the authors of those documents: Fred Baker, Michael Fine, Keith McCloghrie, John Seligson, Kwok Chan and Scott Hahn for their contributions. This document has benefitted from the comments and suggestions of several participants of the Diffserv working group. 11. References [AF-PHB] J. Heinanen, F. Baker, W. Weiss, and J. Wroclawski, "Assured Forwarding PHB Group", RFC 2597, June 1999. [DSARCH] M. Carlson, W. Weiss, S. Blake, Z. Wang, D. Black, and E. Davies, "An Architecture for Differentiated Services", RFC 2475, December 1998 [DSFIELD] K. Nichols, S. Blake, F. Baker, and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998. [DSMIB] F. Baker, A. Smith, K. Chan, "Differentiated Services MIB", Internet Draft <http://www.ietf.org/internet-drafts/draft-ietf- diffserv-mib-04.txt>, July 2000. [DSPIB] M. Fine, K. McCloghrie, J. Seligson, K. Chan, S. Hahn, and A. Bernet et al. Expires January 2001 [Page 45]
Internet Draft Diffserv Informal Management Model July 2000 Smith, "Quality of Service Policy Information Base", Internet Draft <draft-ietf-diffserv-pib-00.txt>, March 2000. [DSTERMS] D. Grossman, "New Terminology for Diffserv", Internet Draft <draft- ietf-diffserv-new-terms-02.txt>, November 1999. [E2E] Y. Bernet, R. Yavatkar, P. Ford, F. Baker, L. Zhang, M. Speer, K. Nichols, R. Braden, B. Davie, J. Wroclawski, and E. Felstaine, "Integrated Services Operation over Diffserv Networks", Internet Draft <http://www.ietf.org/internet-drafts/draft-ietf-issll- diffserv-rsvp-04.txt>, March 2000. [EF-PHB] V. Jacobson, K. Nichols, and K. Poduri, "An Expedited Forwarding PHB", RFC 2598, June 1999. [FLOYD] S. Floyd, "General Load Sharing", 1993. [GTC] L. Lin, J. Lo, and F. Ou, "A Generic Traffic Conditioner", Internet Draft <http://www.ietf.org/internet-drafts/draft-lin-diffserv- gtc-01.txt>, August 1999. [INTSERV] R. Braden, D. Clark and S. Shenker, "Integrated Services in the Internet Architecture: an Overview" RFC 1633, June 1994. [POLTERM] F. Reichmeyer, D. Grossman, J. Strassner, M. Condell, "A Common Terminology for Policy Management", Internet Draft <http://www.ietf.org/internet-drafts/draft-reichmeyer-polterm- terminology-00.txt>, March 2000 [QOSDEVMOD] J. Strassner, W. Weiss, D. Durham, A. Westerinen, "Information Model for Describing Network Device QoS Mechanisms", Internet Draft <http://www.ietf.org/internet-drafts/draft-ietf-policy-qos-device- info-model-00.txt>, April 2000 [QUEUEMGMT] B. Braden et al., "Recommendations on Queue Management and Congestion Avoidance in the Internet", RFC 2309, April 1998. Bernet et al. Expires January 2001 [Page 46]
Internet Draft Diffserv Informal Management Model July 2000 [SRTCM] J. Heinanen, and R. Guerin, "A Single Rate Three Color Marker", RFC 2697, September 1999. [TRTCM] J. Heinanen, R. Guerin, "A Two Rate Three Color Marker", RFC 2698, September 1999. [VIC] McCanne, S. and Jacobson, V., "vic: A Flexible Framework for Packet Video", ACM Multimedia '95, November 1995, San Francisco, CA, pp. 511-522. <ftp://ftp.ee.lbl.gov/papers/vic-mm95.ps.Z> [802.1D] "Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Common specifications - Part 3: Media Access Control (MAC) Bridges: Revision. This is a revision of ISO/IEC 10038: 1993, 802.1j-1992 and 802.6k-1992. It incorporates P802.11c, P802.1p and P802.12e.", ISO/IEC 15802-3: 1998. 12. Appendix A. Simple Token Bucket Discussion and Definition Internet Protocol (IP) packets are of variable-length but theoretical token buckets operate using fixed-length time intervals or pieces of data. This leaves an implementor of a token bucket scheme with a dilemma. When the amount of bandwidth tokens, TB, left in the token bucket is positive but less than the size of the packet being operated on, one of three things can be done: (1) The whole size of the packet can be substracted from the bucket, leaving it negative, remembering that the token bucket size must be added to TB rather than simply setting it "full". This potentially puts more than the token bucket size into this token bucket interval and less into the next. It does, however, make the average amount accepted per token bucket interval equal to the token burst. This approach accepts traffic if any bit in the packet would be accepted and borrows up to one MTU of capacity from one or more subsequent intervals when necessary. Such a token bucket implementation is said to be a "loose" token bucket. (2) Alternatively, the amount can be left unchanged (and maybe an attempt could be made to accept the packet under another threshold in another bucket), remembering that the token bucket size must be added to the TB variable rather than simply setting it "full". This potentially puts less than the token bucket size Bernet et al. Expires January 2001 [Page 47]
Internet Draft Diffserv Informal Management Model July 2000 into this token bucket interval and more into the next. Like the first option, it makes the average amount accepted per token bucket interval equal to the token burst. This approach accepts traffic if every bit in the packet would be accepted and borrows up to one MTU of capacity from one or more previous intervals when necessary. Such a token bucket implementation is said to be a "strict" (or perhaps "stricter") token bucket. (3) The TB variable can be set to zero to account for the first part of the packet and the remainder of the packet size can be taken out of the next-colored bucket. This, of course, has another bug: the same packet cannot have both conforming and nonconforming components in the Diffserv architecture and so is not really appropriate here. Unfortunately, the thing that cannot be done is exactly to fit the token burst specification with random sized packets: therefore token buckets in a variable length packet environment always have a some variance from theoretical reality. This has also been observed in the ATM Guaranteed Frame Rate (GFR) service category specification and Frame Relay. Some find the behavior of a "loose" token bucket unacceptable, as it is significantly different than the token bucket description for ATM and for Frame Relay. However, the "strict" token bucket approach has three characteristics which are important to keep in mind: (1) First, if the maximum token burst is smaller than the MTU, it is possible that traffic never matches the specification. This may be avoided by not allowing such a specification. (2) Second, the strict token bucket specifications [SRTCM] and [TRTCM], as specified, are subject to a persistent under-run. These accumulate burst capacity over time, up to the maximum burst size. Suppose that the maximum burst size is exactly the size of the packets being sent - which one might call the "strictest" token bucket implementation. In such a case, when one packet has been accepted, the token depth becomes zero, and starts to accumulate. If the next packet is received any time earlier than a token interval later, it will not be accepted. If the next packet arrives exactly on time, it will be accepted and the token depth again set to zero. If it arrives later, however, the token depth will stop accumulating, as it is capped by the maximum burst size, and tokens that would have accumulated between the end of that token interval and the actual arrival of the packet are lost. As a result, natural jitter in the network conspires against the algorithm to reduce the actual acceptance rate. Overcoming this error requires the maximum token bucket Bernet et al. Expires January 2001 [Page 48]
Internet Draft Diffserv Informal Management Model July 2000 size to be significantly greater than the MTU. (3) Third, operationally, a strict token bucket is reasonable for traffic which has been shaped by a leaky bucket shaper or a serial line. However, traffic in the Internet is rarely shaped in that way. TCP applies no shaping to its traffic, but rather depends on longer-range Ack Clocking behavior to help it approximate a certain rate and explicitly sends traffic bursts during slow start, retransmission and fast recovery. Video-on-IP implementations such as [VIC] may have a leaky bucket shaper available to them, but often do not, and simply enqueue the output of their codec for transmission on the appropriate interface. As a result, in each of these cases, a strict shaper may reject traffic in the short term (single token interval) which it would have accepted if it had a longer time in view and which it needs to accept for the application to work properly. To work around this, the token interval must approximate or exceed the RTT of the session or sessions in question and the burst size must accommodate the largest burst that the originator might send. The behavior defined in [SRTCM] and [TRTCM] is not mandatory for compliance, but we give here a mathematical definition of two- parameter token bucket operation which is consistent with those documents and which can be used to define a shaping profile. Define a token bucket with bucket size BS, token accumulation rate R and instantaneous token occupancy T(t). Assume that T(0) = BS. Then after an arbitrary interval with no packet arrivals, T(t) will not change since the bucket is already full of tokens. Assume a packet of size B bytes at time t'. The bucket capacity T(t'-) = BS still. Then, as long as B <= BS, the packet conforms to the meter, and T(t') = BS - B. Assume an interval v = t - t' elapses before the next packet, of size C <= BS, arrives. T(t-) is given by the following equation: T(t-) = min { BS, T(t') + v*R } maximum of BS tokens). If T(t-) - C = 0, the packet conforms and T(t) = T(t-) - C. Otherwise, the packet does not conform and T(t) = T(t-). Bernet et al. Expires January 2001 [Page 49]
Internet Draft Diffserv Informal Management Model July 2000 This function can be used to define a shaping profile. If a packet of size C arrives at time t, it will be eligible for transmission at time te given as follows (we still assume C <= BS): te = max { t, t" } where t" = (C - T(t') + t'*R)/R, T(t") = C, the time when C credits have accumulated in the bucket, and when the packet would conform if the token bucket were a meter. te != t" only if t != t". 13. Authors' Addresses Yoram Bernet Microsoft One Microsoft Way Redmond, WA 98052 Phone: +1 425 936 9568 E-mail: yoramb@microsoft.com Andrew Smith FAX: +1 414 345 1827 E-mail: ah_smith@pacbell.net Steven Blake Ericsson 920 Main Campus Drive, Suite 500 Raleigh, NC 27606 Phone: +1 919 472 9913 E-mail: slblake@torrentnet.com Daniel Grossman Motorola Inc. 20 Cabot Blvd. Mansfield, MA 02048 Phone: +1 508 261 5312 E-mail: dan@dma.isg.mot.com Table of Contents 1 Introduction .................................................... 2 2 Glossary ........................................................ 3 3 Conceptual Model ................................................ 5 3.1 Elements of a Diffserv Router ................................. 5 3.1.1 Datapath .................................................... 6 3.1.2 Configuration and Management Interface ...................... 6 Bernet et al. Expires January 2001 [Page 50]
Internet Draft Diffserv Informal Management Model July 2000 3.1.3 Optional QoS Agent Module ................................... 7 3.2 Hierarchical Model of Diffserv Components ..................... 8 4 Classifiers ..................................................... 10 4.1 Definition .................................................... 10 4.1.1 Filters ..................................................... 11 4.1.2 Overlapping Filters ......................................... 12 4.2 Examples ...................................................... 13 4.2.1 Behaviour Aggregate (BA) Classifier ......................... 13 4.2.2 Multi-Field (MF) Classifier ................................. 14 4.2.3 Free-form Classifier ........................................ 14 4.2.4 Other Possible Classifiers .................................. 15 5 Meters .......................................................... 15 5.1 Token-Bucket Model ............................................ 17 5.2 Examples ...................................................... 18 5.2.1 Average Rate Meter .......................................... 18 5.2.2 Exponential Weighted Moving Average (EWMA) Meter ............ 19 5.2.3 Two-Parameter Token Bucket Meter ............................ 20 5.2.4 Multi-Stage Token Bucket Meter .............................. 20 5.2.5 Null Meter .................................................. 21 6 Action Elements ................................................. 21 6.1 Marker ........................................................ 22 6.2 Absolute Dropper .............................................. 22 6.3 Multiplexor ................................................... 23 6.4 Counter ....................................................... 23 6.5 Null Action ................................................... 23 7 Queueing Elements ............................................... 24 7.1 Queueing Model ................................................ 24 7.1.1 FIFO Queue .................................................. 25 7.1.2 Scheduler ................................................... 26 7.1.3 Algorithmic Dropper ......................................... 28 7.1.4 Constructing queueing blocks from the elements .............. 31 7.2 Sharing load among traffic streams using queueing ............. 32 7.2.1 Load Sharing ................................................ 32 7.2.2 Traffic Priority ............................................ 33 8 Traffic Conditioning Blocks (TCBs) .............................. 34 8.1 An Example TCB ................................................ 35 8.2 An Example TCB to Support Multiple Customers .................. 40 8.3 TCBs Supporting Microflow-based Services ...................... 41 8.4 Cascaded TCBs ................................................. 44 9 Security Considerations ......................................... 44 10 Acknowledgments ................................................ 45 11 References ..................................................... 45 12 Appendix A. Simple Token Bucket Discussion and Definition ...... 47 13 Authors' Addresses ............................................. 50 Bernet et al. Expires January 2001 [Page 51]
Internet Draft Diffserv Informal Management Model July 2000 14. Full Copyright Copyright (C) The Internet Society (2000). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implmentation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Bernet et al. Expires January 2001 [Page 52]
