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Network Working Group                                      J. WroclawskiRequest For Comments: 2211                                       MIT LCSCategory: Standards Track                                 September 1997Specification of the Controlled-Load Network Element ServiceStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Abstract   This memo specifies the network element behavior required to deliver   Controlled-Load service in the Internet.  Controlled-load service   provides the client data flow with a quality of service closely   approximating the QoS that same flow would receive from an unloaded   network element, but uses capacity (admission) control to assure that   this service is received even when the network element is overloaded.1. Introduction   This document defines the requirements for network elements that   support the Controlled-Load service.  This memo is one of a series of   documents that specify the network element behavior required to   support various qualities of service in IP internetworks.  Services   described in these documents are useful both in the global Internet   and private IP networks.   This document is based on the service specification template given in   [1]. Please refer to that document for definitions and additional   information about the specification of qualities of service within   the IP protocol family.Wroclawski                 Standards Track                      [Page 1]

RFC 2211                Controlled-Load Network           September 19972. End-to-End Behavior   The end-to-end behavior provided to an application by a series of   network elements providing controlled-load service tightly   approximates the behavior visible to applications receiving best-   effort service *under unloaded conditions* from the same series of   network elements.  Assuming the network is functioning correctly,   these applications may assume that:     - A very high percentage of transmitted packets will be     successfully delivered by the network to the receiving end-nodes.     (The percentage of packets not successfully delivered must closely     approximate the basic packet error rate of the transmission     medium).     - The transit delay experienced by a very high percentage of the     delivered packets will not greatly exceed the minimum transmit     delay experienced by any successfully delivered packet. (This     minimum transit delay includes speed-of-light delay plus the fixed     processing time in routers and other communications devices along     the path.)   To ensure that these conditions are met, clients requesting   controlled-load service provide the intermediate network elements   with a estimation of the data traffic they will generate; the TSpec.   In return, the service ensures that network element resources   adequate to process traffic falling within this descriptive envelope   will be available to the client. Should the client's traffic   generation properties fall outside of the region described by the   TSpec parameters, the QoS provided to the client may exhibit   characteristics indicative of overload, including large numbers of   delayed or dropped packets. The service definition does not require   that the precise characteristics of this overload behavior match   those which would be received by a best-effort data flow traversing   the same path under overloaded conditions.      NOTE: In this memo, the term "unloaded" is used in the sense of      "not heavily loaded or congested" rather than in the sense of "no      other network traffic whatsoever".3. Motivation   The controlled load service is intended to support a broad class of   applications which have been developed for use in today's Internet,   but are highly sensitive to overloaded conditions.  Important members   of this class are the "adaptive real-time applications" currentlyWroclawski                 Standards Track                      [Page 2]

RFC 2211                Controlled-Load Network           September 1997   offered by a number of vendors and researchers. These applications   have been shown to work well on unloaded nets, but to degrade quickly   under overloaded conditions. A service which mimics unloaded nets   serves these applications well.   The controlled-load service is intentionally minimal, in that there   are no optional functions or capabilities in the specification. The   service offers only a single function, and system and application   designers can assume that all implementations will be identical in   this respect.   Internally, the controlled-load service is suited to a wide range of   implementation techniques, including evolving scheduling and   admission control algorithms that allow implementations to be highly   efficient in the use of network resources. It is equally amenable to   extremely simple implementation in circumstances where maximum   utilization of network resources is not the only concern.4. Network Element Data Handling Requirements   Each network element accepting a request for controlled-load service   must ensure that adequate bandwidth and packet processing resources   are available to handle the requested level of traffic, as given by   the requestor's TSpec. This must be accomplished through active   admission control. All resources important to the operation of the   network element must be considered when admitting a request. Common   examples of such resources include link bandwidth, router or switch   port buffer space, and computational capacity of the packet   forwarding engine.   The controlled-load service does not accept or make use of specific   target values for control parameters such as delay or loss. Instead,   acceptance of a request for controlled-load service is defined to   imply a commitment by the network element to provide the requestor   with service closely equivalent to that provided to uncontrolled   (best-effort) traffic under lightly loaded conditions.   The definition of "closely equivalent to unloaded best-effort   service" is necessarily imprecise. It is easiest to define this   quality of service by describing the events which are expected to   *not* occur with any frequency. A flow receiving controlled-load   service at a network element may expect to experience:Wroclawski                 Standards Track                      [Page 3]

RFC 2211                Controlled-Load Network           September 1997     - Little or no average packet queueing delay over all timescales     significantly larger than the "burst time". The burst time is     defined as the time required for the flow's maximum size data burst     to be transmitted at the flow's requested transmission rate, where     the burst size and rate are given by the flow's TSpec, as described     below.     - Little or no congestion loss over all timescales significantly     larger than the "burst time" defined above.  In this context,     congestion loss includes packet losses due to shortage of any     required processing resource, such as buffer space or link     bandwidth.  Although occasional congestion losses may occur, any     substantial sustained loss represents a failure of the admission     control algorithm.   The basic effect of this language is to establish an expectation on   the *duration* of a disruption in delivery service. Events of shorter   duration are viewed as statistical effects which may occur in normal   operation. Events of longer duration are indicative of failure to   allocate adequate capacity to the controlled-load flow.   A network element may employ statistical approaches to decide whether   adequate capacity is available to accept a service request. For   example, a network element processing a number of flows with long-   term characteristics predicted through measurement of past behavior   may be able to overallocate its resources to some extent without   reducing the level of service delivered to the flows.   A network element may employ any appropriate scheduling means to   ensure that admitted flows receive appropriate service.      NOTE: The flexibility implied by the above paragraph exists within      definite limits. Readers should observe that the specification's      requirement that the delay and loss behavior described above      imposes concrete requirements on implementations.      Perhaps the most important requirement is that the implementation      has to make bandwidth greater than the Tspec token rate available      to the flow in certain situations. The requirement for the      availability of extra bandwidth may be derived from the fluid      model of traffic scheduling (e.g. [7]). If a flow receives exactly      its promised token rate at all times, queueing caused by an over-      rate burst arriving at the network element may never clear,      causing the traffic queueing delay to permanantly increase. This      will happen if the flow continues to generate traffic at exactly      the token rate after emitting the burst.Wroclawski                 Standards Track                      [Page 4]

RFC 2211                Controlled-Load Network           September 1997      To control the long-term effects of traffic bursts, a Controlled      Load implementation has several options. At minimum, a mechanism      must be present to "borrow" bandwidth needed to clear bursts from      the network. There are a number of ways to implement such a      mechanism, ranging from explicit borrowing schemes within the      traffic scheduler to implicit schemes based on statistical      multiplexing and measurement-based admission control. The      specification does not prefer any method over any other, but does      require that some such mechanism must exist.      Similarly, the requirement for low congestion loss for in-Tspec      traffic implies that buffer management must have some flexibility.      Because the controlled-load service does not reshape traffic to      its token-bucket parameters at every node, traffic flowing through      the network will be distorted as it traverses queueing points.      This distortion is particularly likely to occur during traffic      bursts, precisely when buffering is most heavily used. In these      circumstances, rigidly restricting the buffering capacity to a      size equal to the flow's TSpec burst size may lead to congestion      loss. An implementaton should be prepared to make additional      buffering available to bursting flows. Again, this may be      accomplished in a number of ways. One obvious choice is      statistical multiplexing of a shared buffer pool.   Links are not permitted to fragment packets which receive the   controlled-load service. Packets larger than the MTU of the link must   be treated as nonconformant to the TSpec. This implies that they will   be forwarded according to the rules described in the Policing section   below.   Implementations of controlled-load service are not required to   provide any control of short-term packet delay jitter beyond that   described above. However, the use of packet scheduling algorithms   that provide additional jitter control is not prohibited by this   specification.   Packet losses due to non-congestion-related causes, such as link   errors, are not bounded by this service.5. Invocation Information   The controlled-load service is invoked by specifying the data flow's   desired traffic parameters (TSpec) to the network element. Requests   placed for a new flow will be accepted if the network element has the   capacity to forward the flow's packets as described above. Requests   to change the TSpec for an existing flow should be treated as a new   invocation, in the sense that admission control must be reapplied to   the flow. Requests that reduce the TSpec for an existing flow (in theWroclawski                 Standards Track                      [Page 5]

RFC 2211                Controlled-Load Network           September 1997   sense that the new TSpec is strictly smaller than the old TSpec   according to the ordering rules given below) should never be denied   service.   The Controlled-Load service uses the TOKEN_BUCKET_TSPEC defined in   Reference [5] to describe a data flow's traffic parameters. This   TSpec takes the form of a token bucket specification plus a peak rate   (p), a minimum policed unit (m) and a maximum packet size (M).   The token bucket specification includes a bucket rate r and a bucket   depth, b.  Both r and b must be positive.  The rate, r, is measured   in bytes of IP datagrams per second. Values of this parameter may   range from 1 byte per second to 40 terabytes per second. Network   elements MUST return an error for requests containing values outside   this range. Network elements MUST return an error for any request   containing a value within this range which cannot be supported by the   element. In practice, only the first few digits of the r parameter   are significant, so the use of floating point representations,   accurate to at least 0.1% is encouraged.   The bucket depth, b, is measured in bytes. Values of this parameter   may range from 1 byte to 250 gigabytes. Network elements MUST return   an error for requests containing values outside this range. Network   elements MUST return an error for any request containing a value   within this range which cannot be supported by the element. In   practice, only the first few digits of the b parameter are   significant, so the use of floating point representations, accurate   to at least 0.1% is encouraged.   The range of values allowed for these parameters is intentionally   large to allow for future network technologies. Any given network   element is not expected to support the full range of values.   The peak rate, p, is measured in bytes of IP datagrams per second and   has the same range and suggested representation as the bucket rate.   The peak rate parameter exists in this version of the specification   primarily for TSpec compatability with other QoS control services and   the shared TOKEN_BUCKET_TSPEC parameter. While some admission control   and buffer allocation algorithms may find the peak rate value useful,   the field may always be ignored by a Controlled-Load service   conforming to this version of the specification. That is, the service   module at a network element may always assume that the peak data rate   arriving at that element is the line rate of the incoming interface,   and the service's evaluation criteria do not require a network   element to consider the peak rate value. More explicit use of the   peak-rate parameter by a Controlled-Load service module may be added   to the specification in the future.Wroclawski                 Standards Track                      [Page 6]

RFC 2211                Controlled-Load Network           September 1997   The minimum policed unit, m, is an integer measured in bytes.  All IP   datagrams less than size m will be counted against the token bucket   as being of size m. The maximum packet size, M, is the biggest packet   that will conform to the traffic specification; it is also measured   in bytes.  Network elements MUST reject a service request if the   requested maximum packet size is larger than the MTU of the link.   Both m and M must be positive, and m must be less then or equal to M.   The preferred concrete representation for the TSpec is three floating   point numbers in single-precision IEEE floating point format followed   by two 32-bit integers in network byte order.  The first value is the   rate (r), the second value is the bucket size (b), the third is the   peak rate (p), the fourth is the minimum policed unit (m), and the   fifth is the maximum packet size (M). For the parameters (r) and (b),   only bit-patterns which represent valid non-negative floating point   numbers are allowed. Negative numbers (including "negative zero),   infinities, and NAN's are not allowed.  For the parameter (p) only   bit-patterns which represent valid non-negative floating point   numbers or positive infinity are allowed. Positive infinity is   represented with an exponent of all ones (255) and a sign bit and   mantissa of all zeroes. Negative numbers (including "negative zero"),   negative infinity, and NAN's are not allowed.      NOTE: An implementation which utilizes general-purpose hardware or      software IEEE floating-point support may wish to verify that      arriving parameters meet this requirement before using the      parameters in floating-point computations, in order to avoid      unexpected exceptions or traps.   The controlled-load service is assigned service_name 5.   The TOKEN_BUCKET_TSPEC parameter used by the Controlled-Load service   is general parameter number 127, as indicated in [5].6. Exported Information   The controlled-load service has no required characterization   parameters. Individual implementations may export appropriate   implementation-specific measurement and monitoring information.7. Policing   The controlled-load service is provided to a flow on the basis that   the flow's traffic conforms to a TSpec given at flow setup time. This   section defines the meaning of conformance to the controlled-load   TSpec, describes the circumstances under which a controlled-load   flow's traffic might *not* conform to the TSpec, and specifies the   network element's action in those circumstances.Wroclawski                 Standards Track                      [Page 7]

RFC 2211                Controlled-Load Network           September 1997   Controlled-load service modules provide QoS control for traffic   conforming to the TSpec given at setup time.  The TSpec's token   bucket parameters require that traffic must obey the rule that over   all time periods, the amount of data sent does not exceed rT+b, where   r and b are the token bucket parameters and T is the length of the   time period.  For the purposes of this accounting, links must count   packets that are smaller than the minimal policing unit m to be of   size m.  Packets that arrive at an element and cause a violation of   the the rT+b bound are considered nonconformant.   Additionally, packets bigger than the outgoing link MTU are   considered nonconformant.  It is expected that this situation will   not arise with any frequency, because flow setup mechanisms are   expected to notify the sending application of the appropriate path   MTU.   In the presence of nonconformant packets arriving for one or more   controlled-load flows, each network element must ensure locally that   the following requirements are met:     1) The network element MUST continue to provide the contracted     quality of service to those controlled-load flows not experiencing     excess traffic.     2) The network element SHOULD prevent excess controlled-load     traffic from unfairly impacting the handling of arriving best-     effort traffic.  This requirement is discussed further inSection 9     of this document (Guidelines for Implementors).     3) Consistent with points 1 and 2, the network element MUST attempt     to forward the excess traffic on a best-effort basis if sufficient     resources are available.   Network elements must not assume that that arrival of nonconformant   traffic for a specific controlled-load flow will be unusual, or   indicative of error.  In certain circumstances (particularly, routers   acting as the "split points" of a multicast distribution tree   supporting a shared reservation) large numbers of packets will fail   the conformance test *as a matter of normal operation*.   Network elements must not assume that data sources or upstream   elements have taken action to "police" controlled-load flows by   limiting their traffic to conform to the flow's TSpec.  Each network   element providing controlled-load service MUST independently ensure   that the requirements given above are met in the presence of   nonconformant arriving traffic for one or more controlled-load flows.Wroclawski                 Standards Track                      [Page 8]

RFC 2211                Controlled-Load Network           September 1997   Network elements may use any appropriate implementation mechanism to   meet the requirements given above.  Examples of such mechanisms   include token-bucket policing filters and per-flow scheduling   algorithms.  However, it is insufficient to simply place all   controlled-load flows into the same shared resource pool, without   first ensuring that non-conformant flows are prevented from starving   conformant flows of the necessary processing resources.   Further discussion of this issue may be found inSection 11 of this   note.   Beyond requirements 2 and 3 above, the controlled-load service does   not define the QoS behavior delivered to flows with non-conformant   arriving traffic.  Specifically, it is permissible either to degrade   the service delivered to all of the flow's packets equally, or to   sort the flow's packets into a conformant set and a nonconformant set   and deliver different levels of service to the two sets. This point   is discussed further inSection 9 of this note.   When resources are available, network elements at points within the   interior of the network SHOULD be prepared to accommodate packet   bursts somewhat larger than the actual TSpec. This requirement   derives from the traffic distortion effect described inSection 4. As   described there, it may be met either through explicit means or   statistical multiplexing of shared buffering resources.   When handling such traffic, it is permissible to allow some delaying   of a packet if that delay would allow it to pass the policing   function.  (In other words, to reshape the traffic).  However, the   overall requirement for limiting the duration of any such traffic   distortion must be considered. The challenge is to define a viable   reshaping function.   Intuitively, a plausible approach is to allow a delay of (roughly) up   to the maximum queueing delay experienced by completely conforming   packets before declaring that a packet has failed to pass the   policing function. The merit of this approach, and the precise   wording of the specification that describes it, require further   study.8. Ordering and Merging   The controlled-load service TSpec is ordered according to the   following rule: TSpec A is a substitute for ("as good or better than"   or "greater than or equal to") TSpec B if and only if:Wroclawski                 Standards Track                      [Page 9]

RFC 2211                Controlled-Load Network           September 1997     (1) the token bucket rate r for TSpec A is greater than or equal to     that of TSpec B,     (2) the token bucket depth b for TSpec A is greater than or equal     to that of TSpec B,     (3) the peak rate p for TSpec A is greater than or equal to that of     TSpec B,     (4) the minimum policed unit m for TSpec A is less than or equal to     that of TSpec B,     (5) the maximum packet size M of TSpec A is greater than or equal     to that of TSpec B.   Note that not all TSpecs can be ordered with respect to each other.   If two TSpecs differ but not all five of the points above are true,   then the TSpecs are unordered.   A merged TSpec is the TSpec used by the RSVP protocol when merging a   set of TSpecs to create a "merged" reservation. TSpec merging is   described further in [4] and [3]. The TSpec merge operation addresses   two requirements:     - The "merged" TSpec parameters are used as the traffic flow's     TSpec at the local node.     - The merged parameters are passed upstream to traffic source(s) to     describe characteristics of the actually installed reservation     along the data path.   For the controlled-load service, a merged TSpec may be calculated   over a set of TSpecs by taking:     (1) the largest token bucket rate r;     (2) the largest token bucket size b;     (3) the largest peak rate p;     (4) the smallest minimal policed unit m;     (5) the *smallest* maximum packet size M;   across all members of the set.Wroclawski                 Standards Track                     [Page 10]

RFC 2211                Controlled-Load Network           September 1997   A Least Common TSpec is a TSpec adequate to describe the traffic from   any one of a number of traffic flows. The least common TSpec may be   useful when creating a shared reservation for a number of flows using   SNMP or another management protocol. This differs from the merged   TSpec described above in that the computed parameters are not passed   upstream to the sources of traffic.   For the controlled-load service, the Least Common TSpec may be   calculated over a set of TSpecs by taking:     (1) the largest token bucket rate r;     (2) the largest token bucket size b;     (3) the largest peak rate p;     (4) the smallest minimal policed unit m;     (5) the largest maximum packet size M;   across all members of the set.   The sum of n controlled-load service TSpecs is used when computing   the TSpec for a shared reservation of n flows. It is computed by   taking:     - The sum across all TSpecs of the token bucket rate parameter r.     - The sum across all TSpecs of the token bucket size parameter b.     - The sum across all TSpecs of the peak rate parameter p.     - The minimum across all TSpecs of the minimum policed unit       parameter m.     - The maximum across all TSpecs of the maximum packet size       parameter M.   The minimum of two TSpecs differs according to whether the TSpecs can   be ordered according to the "greater than or equal to" rule above.   If one TSpec is less than the other TSpec, the smaller TSpec is the   minimum.  For unordered TSpecs, a different rule is used.  The   minimum of two unordered TSpecs is determined by comparing the   respective values in the two TSpecs and choosing:Wroclawski                 Standards Track                     [Page 11]

RFC 2211                Controlled-Load Network           September 1997     (1) the smaller token bucket rate r;     (2) the *larger* token bucket size b;     (3) the smaller peak rate p;     (4) the *smaller* minimum policed unit m;     (5) the smaller maximum packet size M;9. Guidelines for Implementors   REQUIREMENTS PLACED ON ADMISSION CONTROL ALGORITHM: The intention of   this service specification is that network elements deliver a level   of service closely approximating best-effort service under unloaded   conditions. As with best-effort service under these conditions, it is   not required that every single packet must be successfully delivered   with zero queueing delay. Network elements providing controlled-load   service are permitted to oversubscribe the available resources to   some extent, in the sense that the bandwidth and buffer requirements   indicated by summing the TSpec token buckets of all controlled-load   flows may exceed the maximum capabilities of the network element.   However, this oversubscription may only be done in cases where the   element is quite sure that actual utilization is less than the sum of   the token buckets would suggest, so that the implementor's   performance goals will be met. This information may come from   measurement of the aggregate traffic flow, specific knowledge of   application traffic statistics, or other means. The most conservative   approach, rejection of new flows whenever the addition of their   traffic would cause the strict sum of the token buckets to exceed the   capacity of the network element (including consideration of resources   needed to maintain the delay and loss characteristics specified by   the service) may be appropriate in other circumstances.   Specific issues related to this subject are discussed in the   "Evaluation Criteria" and "Examples of Implementation" sections   below.   INTERACTION WITH BEST-EFFORT TRAFFIC: Implementors of this service   should clearly understand that in certain circumstances (routers   acting as the "split points" of a multicast distribution tree   supporting a shared reservation) large numbers of a flow's packets   may fail the TSpec conformance test *as a matter of normal   operation*.  According to the requirements ofSection 7, these   packets should be forwarded on a best-effort basis if resources   permit.Wroclawski                 Standards Track                     [Page 12]

RFC 2211                Controlled-Load Network           September 1997   If the network element's best-effort queueing algorithm does not   distinguish between these packets and elastic best-effort traffic   such as TCP flows, THE EXCESS CONTROLLED-LOAD PACKETS WILL "BACK OFF"   THE ELASTIC TRAFFIC AND DOMINATE THE BEST-EFFORT BANDWIDTH USAGE. The   integrated services framework does not currently address this issue.   However, several possible solutions to the problem are known [RED,   xFQ].  Network elements supporting the controlled load service should   implement some mechanism in their best-effort queueing path to   discriminate between classes of best-effort traffic and provide   elastic traffic with protection from inelastic best-effort flows.   Two basic approaches are available to meet this requirement. The   network element can maintain separate resource allocations for   different classes of best-effort traffic, so that no one class will   excessively dominate the loaded best-effort mix. Alternatively, an   element can process excess controlled-load traffic at somewhat lower   priority than elastic best-effort traffic, so as to completely avoid   the back-off effect discussed above.   If most or all controlled-load traffic arises from non-rate-adaptive   real-time applications, the use of priority mechanisms might be   desirable. If most controlled-load traffic arises from rate-adaptive   realtime or elastic applications attempting to establish a bounded   minimum level of service, the use of separate resource classes might   be preferable. However, this is not a firm guideline. In practice,   the network element designer's choice of mechanism will depend   heavily on both the goals of the design and the implementation   techniques appropriate for the designer's platform. This version of   the service specification does not specify one or the other behavior,   but leaves the choice to the implementor.   FORWARDING BEHAVIOR IN PRESENCE OF NONCONFORMANT TRAFFIC: As   indicated inSection 7, the controlled-load service does not define   the QoS behavior delivered to flows with non-conformant arriving   traffic.  It is permissible either to degrade the service delivered   to all of the flow's packets equally, or to sort the flow's packets   into a conformant set and a nonconformant set and deliver different   levels of service to the two sets.   In the first case, expected queueing delay and packet loss   probability will rise for all packets in the flow, but packet   delivery reordering will, in general, remain at low levels. This   behavior is preferable for those applications or transport protocols   which are sensitive to excessive packet reordering. A possible   example is an unmodified TCP connection, which would see reordering   as lost packets, triggering duplicate acks and hence excessive   retransmissions.Wroclawski                 Standards Track                     [Page 13]

RFC 2211                Controlled-Load Network           September 1997   In the second case, some subset of the flow's packets will be   delivered with low loss and delay, while some other subset will be   delivered with higher loss and potentially higher delay. The delayed   packets will appear to the receiver to have been reordered in the   network, while the non-delayed packets will, on average, arrive in a   more timely fashion than if all packets were treated equally. This   might be preferable for applications which are highly time-sensitive,   such as interactive conferencing tools.10. Evaluation Criteria   The basic requirement placed on an implementation of controlled-load   service is that, under all conditions, it provide accepted data flows   with service closely similar to the service that same flow would   receive using best-effort service under unloaded conditions.   This suggests a simple two-step evaluation strategy. Step one is to   compare the service given best-effort traffic and controlled-load   traffic under underloaded conditions.     - Measure the packet loss rate and delay characteristics of a test     flow using best-effort service and with no load on the network     element.     - Compare those measurements with measurements of the same flow     receiving controlled-load service with no load on the network     element.     Closer measurements indicate higher evaluation ratings. A     substantial difference in the delay characteristics, such as the     smoothing which would be seen in an implementation which scheduled     the controlled-load flow using a fixed, constant-bitrate algorithm,     should result in a somewhat lower rating.   Step two is to observe the change in service received by a   controlled-load flow as the load increases.     - Increase the background traffic load on the network element,     while continuing to measuring the loss and delay characteristics of     the controlled-load flow. Characteristics which remain essentially     constant as the element is driven into overload indicate a high     evaluation rating. Minor changes in the delay distribution indicate     a somewhat lower rating. Significant increases in delay or loss     indicate a poor evaluation rating.Wroclawski                 Standards Track                     [Page 14]

RFC 2211                Controlled-Load Network           September 1997   This simple model is not adequate to fully evaluate the performance   of controlled-load service. Three additional variables affect the   evaluation. The first is the short-term burstiness of the traffic   stream used to perform the tests outlined above. The second is the   degree of long-term change in the controlled-load traffic within the   bounds of its TSpec.  (Changes in this characteristic will have great   effect on the effectiveness of certain admission control algorithms.)   The third is the ratio of controlled-load traffic to other traffic at   the network element (either best effort or other controlled   services).   The third variable should be specifically evaluated using the   following procedure.     With no controlled-load flows in place, overload the network     element with best-effort traffic (as indicated by substantial     packet loss and queueing delay).     Execute requests for controlled-load service giving TSpecs with     increasingly large rate and burst parameters. If the request is     accepted, verify that traffic matching the TSpec is in fact handled     with characteristics closely approximating the unloaded     measurements taken above.     Repeat these experiments to determine the range of traffic     parameter (rate, burst size) values successfully handled by the     network element. The useful range of each parameter must be     determined for several settings of the other parameter, to map out     a two-dimensional "region" of successfully handled TSpecs. When     compared with network elements providing similar capabilities, this     region indicates the relative ability of the elements to provide     controlled-load service under high load. A larger region indicates     a higher evaluation rating.11. Examples of Implementation   One possible implementation of controlled-load service is to provide   a queueing mechanism with two priority levels; a high priority one   for controlled-load and a lower priority one for best effort service.   An admission control algorithm is used to limit the amount of traffic   placed into the high-priority queue. This algorithm may be based   either on the specified characteristics of the high-priority flows   (using information provided by the TSpecs), or on the measured   characteristics of the existing high-priority flows and the TSpec of   the new request.   Another possible implementation of controlled-load service is based   on the existing capabilities of network elements which supportWroclawski                 Standards Track                     [Page 15]

RFC 2211                Controlled-Load Network           September 1997   "traffic classes" based on mechanisms such as weighted fair queueing   or class-based queueing [6]. In this case, it is sufficient to map   data flows accepted for controlled-load service into an existing   traffic class with adequate capacity to avoid overload. This   requirement is enforced by an admission control algorithm which   considers the characteristics of the traffic class, the   characteristics of the traffic already admitted to the class, and the   TSpec of the new flow requesting service. Again, the admission   control algorithm may be based either on the TSpec-specified or the   measured characteristics of the existing traffic.   A specific case of the above approach is to employ a scheduler which   implements weighted fair queueing or similar load-management scheme,   allocating a separate scheduling queue with correctly chosen weight   to each individual controlled-load flow.  In this circumstance, the   traffic scheduler also plays the role of the policing function, by   ensuring that nonconformant traffic arriving for one controlled-load   flow does not affect either other controlled-load flows or the best-   effort traffic. This elimination of mechanism is balanced by the   drawback that the approach does not benefit from any performance or   resource usage gain arising from statistical aggregation of several   flows into a single queueing class.   Admission control algorithms based on specified characteristics are   likely be appropriate when the number of flows in the high-priority   class is small, or the traffic characteristics of the flows appear   highly variable. In these situations the measured behavior of the   aggregate controlled-load traffic stream may not serve as an   effective predictor of future traffic, leading a measurement-based   admission control algorithm to produce incorrect results. Conversely,   in situations where the past behavior of the aggregate controlled-   load traffic *is* a good predictor of future behavior, a measurement-   based admission control algorithm may allow more traffic to be   admitted to the controlled-load service class with no degradation in   performance. An implementation may choose to switch between these two   approaches depending on the nature of the traffic stream at a given   time.   A variety of techniques may be used to provide the desired isolation   between excess (nonconformant) controlled-load traffic and other   best-effort traffic. Use of a low priority queue for nonconformant   controlled-load traffic is simple, but other approaches may provide   superior service or fit better into existing architectures.  Variants   of fair queueing or weighted fair queueing may be used to allocate a   percentage of the available resources to different best-effort   traffic classes. One approach would be to allocate each controlled-   load flow a a 1/N "fair share" percentage of the available best-Wroclawski                 Standards Track                     [Page 16]

RFC 2211                Controlled-Load Network           September 1997   effort bandwidth for its excess traffic. An alternate approach would   be to provide a single WFQ resource class for all excess controlled-   load traffic.  Finally, alternate mechanisms such as RED [xxx] may be   used to provide the same overall function.12. Examples of Use   The controlled-load service may be used by any application which can   make use of best-effort service, but is best suited to those   applications which can usefully characterize their traffic   requirements.  Applications based on the transport of "continuous   media" data, such as digitized audio or video, are an important   example of this class.   The controlled-load service is not isochronous and does not provide   any explicit information about transmission delay. For this reason,   applications with end-to-end timing requirements, including the   continuous-media class mentioned above, provide an application-   specific timing recovery mechanism, similar or identical to the   mechanisms required when these applications use best-effort service.   A protocol useful to applications requiring this capability is the   IETF Real-Time Transport Protocol [2].   Load-sensitive applications may choose to request controlled-load   service whenever they are run. Alternatively, these applications may   monitor their own performance and request controlled-load service   from the network only when best-effort service is not providing   acceptable performance. The first strategy provides higher assurance   that the level of quality delivered to the user will not change over   the lifetime of an application session. The second strategy provides   greated flexibility and offers cost savings in environments where   levels of service above best-effort incur a charge.13. Security Considerations   A network element implementing the service described here is   intentionally and explicitly expected to give preferential treatment   to selected packet traffic. This memo does not describe the mechanism   used to indicate which traffic is to receive the preferential   treatment - rather, the controlled-load service described here may be   invoked by a number of mechanisms, including RSVP, SNMP network   management software, or proprietary control software. However, any   mechanism used to invoke the controlled load service must provide   security sufficient to guard against use of this preferential   treatment capability by undesired or unauthorized traffic.  A correct   implementation of the controlled-load service is *not* susceptable to   a denial-of-service attack based on maliciously requesting a very   small resource allocation for the attacked traffic flow. This isWroclawski                 Standards Track                     [Page 17]

RFC 2211                Controlled-Load Network           September 1997   because the service specification requires that traffic in excess of   the requested level be carried on a best-effort basis, rather than   being dropped. This requirement is discussed further inSection 7 of   this memo.   Of necessity, giving preferential service to certain traffic flows   implies giving less service to other traffic flows.  Thus, it is   possible to conduct a denial of service attack by maliciously   reconfiguring the controlled-load "admission control algorithm" to   allow overallocation of available bandwidth or other forwarding   resources, starving non-controlled-load flows. In general, this is   unlikely to increase the network's vulnerability to attack, because   many other reconfigurations of a router or host can cause denial of   service. It is reasonable to assume that whatever means is used to   protect against other reconfiguration attacks will be adequate to   protect against this one as well.Appendix 1: Use of the Controlled-Load service with RSVP   The use of Controlled-Load service in conjunction with the RSVP   resource reservation setup protocol is specified in reference [4].   This document gives the format of RSVP FLOWSPEC, SENDER_TSPEC, and   ADSPEC objects needed to support applications desiring Controlled-   Load service and gives information about how RSVP processes those   objects. The RSVP protocol itself is specified in Reference [3].References   [1] Shenker, S., and J. Wroclawski. "Network Element Service   Specification Template",RFC 2216, September 1997.   [2] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson.   "RTP: A Transport Protocol for Real-Time Applications",RFC 1889,   January 1996.   [3] Braden, R., Ed., et. al., "Resource Reservation Protocol (RSVP) -   Version 1 Functional Specification",RFC 2205, September 1997.   [4] Wroclawski, J., "The use of RSVP with IETF Integrated Services",RFC 2210, September 1997.   [5] Shenker, S., and J. Wroclawski, "General Characterization   Parameters for Integrated Service Network Elements",RFC 2215,   September 1997.Wroclawski                 Standards Track                     [Page 18]

RFC 2211                Controlled-Load Network           September 1997   [6] S. Floyd, and V. Jacobson.  "Link-sharing and Resource Management   Models for Packet Networks," IEEE/ACM Transactions on Networking,   Vol. 3 No. 4, pp. 365-386, August 1995.   [7] A. K. J. Parekh. "A Generalized Processor Sharing Approach to   Flow Control in Integrated Service Networks". MIT Laboratory for   Information and Decision Systems, Report LIDS-TH-2089, February 1992Author's Address   John Wroclawski   MIT Laboratory for Computer Science   545 Technology Sq.   Cambridge, MA  02139   Phone: 617-253-7885   Fax:   617-253-2673 (FAX)   EMail: jtw@lcs.mit.eduWroclawski                 Standards Track                     [Page 19]