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Network Working Group                                        S. ShenkerRequest for Comments: 2215                                J. WroclawskiCategory: Standards Track                            Xerox PARC/MIT LCS                                                         September 1997General Characterization Parameters forIntegrated Service Network ElementsStatus 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 defines a set of general control and characterization   parameters for network elements supporting the IETF integrated   services QoS control framework. General parameters are those with   common, shared definitions across all QoS control services.1. Introduction   This memo defines the set of general control and characterization   parameters used by network elements supporting the integrated   services framework.  "General" means that the parameter has a common   definition and shared meaning across all QoS control services.   Control parameters are used by applications to provide information to   the network related to QoS control requests. An example is the   traffic specification (TSpec) generated by application senders and   receivers.   Characterization parameters are used to discover or characterize the   QoS management environment along the path of a packet flow requesting   active end-to-end QoS control.  These characterizations may   eventually be used by the application requesting QoS control, or by   other network elements along the path. Examples include information   about which QoS control services are available along a network path   and estimates of the available path bandwidth.   Individual QoS control service specifications may refer to these   parameter definitions as well as defining additional parameters   specific to the needs of that service.Shenker & Wroclawski        Standards Track                     [Page 1]

RFC 2215          General Characterization Parameters     September 1997   Parameters are assigned machine-oriented ID's using a method   described in [RFC 2216] and summarized here.  These ID's may be used   within protocol messages (e.g., as described in [RFC 2210]) or   management interfaces to describe the parameter values present. Each   parameter ID is composed from two numerical fields, one identifying   the service associated with the parameter (the <service_number>), and   the other (the <parameter_number>) identifying the parameter itself.   Because the definitions of the parameters defined in this note are   common to all QoS control services, the <parameter_number> values for   the parameters defined here are assigned from the "general   parameters" range (1 - 127).      NOTE: <parameter_numbers> in the range 128 - 254 name parameters      with definitions specific to a particular QoS control service. In      contrast to the general parameters described here, it is necessary      to consider both the <service_number> and <parameter_number> to      determine the meaning of the parameter.      Service number 1 is reserved for use as described inSection 2 of      this note. Service numbers 2 through 254 will be allocated to      individual QoS control services. Currently, Guaranteed service      [RFC 2212] is allocated number 2, and Controlled-load service [RFC      2211] is allocated number 5.   In this note, the textual form                    <service_number, parameter_number>   is used to write a service_number, parameter_number pair.  The range   of possible of service_number and parameter_number values specified   in [RFC 2216] allow the parameter ID to directly form the tail   portion of a MIB object ID representing the parameter. This   simplifies the task of making parameter values available to network   management applications.   The definition of each parameter used to characterize a path through   the network describes two types of values; local and composed.  A   Local value gives information about a single network element.   Composed values reflect the running composition of local values along   a path, specified by some composition rule.  Each parameter   definition specifies the composition rule for that parameter. The   composition rule tells how to combine an incoming composed value   (from the already-traversed portion of the path) and the local value,   to give a new composed value which is passed to the next network   element in the path. Note that the composition may proceed eitherShenker & Wroclawski        Standards Track                     [Page 2]

RFC 2215          General Characterization Parameters     September 1997   downstream, toward the receiver(s), or upstream, toward the sender.   Each parameter may give only one definition for the local value, but   may potentially give more than one definition for composition rules   and composed values. This is because it may be useful to compose the   same local value several times following different composition rules.   Because characterization parameters are used to compute the   properties of a specific path through the internetwork, all   characterization parameter definitions are conceptually "per-next-   hop", as opposed to "per interface" or "per network element".  In   cases where the network element is (or is controlling) a shared media   or large-cloud subnet, the element may need to provide different   values for different next-hops within the cloud.  In practice, it may   be appropriate for vendors to choose and document a tolerance range,   such that if all next-hop values are within the tolerance range only   a single value need be stored and provided.   Local and composed characterization parameter values have distinct   ID's so that a network management entity can examine the value of   either a local or path-composed parameter at any point within the   network.   Each parameter definition includes a description of the minimal   properties, such as range and precision, required of any wire   representation of that parameter's values. Each definition also   includes an XDR [RFC 1832] description of the parameter, describing   an appropriate external (wire) data representation for the   parameter's values. This dual definition is intended to encourage a   common wire representation format whenever possible, while still   allowing other representations when required by the specific   circumstances (e.g., ASN.1 within SNMP).   The message formats specified in [RFC 2210] for use with the RSVP   setup protocol use the XDR data representation parameters.   All of the parameters described in this note are mandatory, in the   sense that a network element claiming to support integrated service   must recognize arriving values in setup and management protocol   messages, process them correctly, and export a reasonable value in   response. For some parameters, the specification requires that the   network element compute and export an *accurate* local value. For   other parameters, it is acceptable for the network element to   indicate that it cannot compute and export an accurate local value.   The definition of these parameters provides a reserved value which   indicates "indeterminate" or "invalid". This value signals that an   element cannot process the parameter accurately, and consequently   that the result of the end-to-end composition is also questionable.Shenker & Wroclawski        Standards Track                     [Page 3]

RFC 2215          General Characterization Parameters     September 1997      NOTE (temporary): Previous versions of this and the RSVP use      document used both the reserved-value approach and a separate      INVALID flag to record this fact.  Now, the reserved-value      approach is used exclusively. This is so that any protocol which      retrieves a parameter value, including SNMP, can carry the invalid      indication without needing a separate flag. The INVALID flag      remains in the RSVP message format but is reserved for use only      with a possible future service-composition scheme.2. Default and Service-Specific Values for General Parameters   General parameters have a common *definition* across all QoS control   services. Frequently, the same *value* of a general parameter will be   correct for all QoS control services offered by a network element. In   this circumstance, there is no need to export a separate copy of the   value for each QoS control service; instead the node can export one   number which applies to all supported services.   A general parameter value which applies to all services supported at   a network node is called a default or global value. For example, if   all of the QoS control services provided at a node support the same   maximum packet size, the node may export a single default value for   the PATH_MTU parameter described inSection 3, rather than providing   a separate copy of the value for each QoS control service. In the   common case, this reduces both message size and processing overhead   for the setup protocol.   Occasionally an individual service needs to report a value differing   from the default value for a particular general parameter. For   example, if the implementation of Guaranteed Service [RFC 2212] at a   router is restricted by scheduler or hardware considerations to a   maximum packet size smaller than supported by the router's best-   effort forwarding path, the implementation may wish to export a   "service-specific" value of the PATH_MTU parameter so that   applications using the Guaranteed service will function correctly.   In the example above, the router might supply a value of 1500 for the   default PATH_MTU parameter, and a value of 250 for the PATH_MTU   parameter applying to guaranteed service. In this case, the setup   protocol providing path characterization carries (and delivers to the   application) both a value for Guaranteed service and a value for   other services.   The distinction between default and service-specific parameter values   makes no sense for non-general parameters (those defined by a   specific QoS control service, rather than this note), because both   the definition and value of the parameter are always specific to the   particular service.Shenker & Wroclawski        Standards Track                     [Page 4]

RFC 2215          General Characterization Parameters     September 1997   The distinction between default and service-specific values for   general parameters is reflected in the parameter ID name space.  This   allows network nodes, setup protocols, and network management tools   to distinguish default from service-specific values, and to determine   which service a service-specific parameter value is associated with.   Service number 1 is used to indicate the default value. A parameter   value identified by the ID:                           <1, parameter_number>   is a default value, which applies to all services unless it is   overridden by a service-specific value for the same parameter.   A parameter value identified by the ID:                    <service_number, parameter_number>   where service_number is not equal to 1, is a service-specific value.   It applies only to the service identified by service_number.   These service-specific values are also called override values.  This   is because when both service-specific and default values are present   for a parameter, the service-specific value overrides the default   value (for the service to which it applies). The rules for composing   service-specific and global general parameters support this override   capability.  The basic rule is to use the service-specific value if   it exists, and otherwise the global value.   A complete summary of the characterization parameter composition   process is given below. In this summary, the "arriving value" is the   incompletely composed parameter value arriving from a neighbor node.   The "local value" is the (global or service-specific) value made   available by the local node. The "result" is the newly composed value   to be sent to the next node on the data path.     1. Examine the <service_number, parameter_number> pair associated     with the arriving value. This information is conveyed by the setup     protocol together with the arriving value.     2. If the arriving value is for a parameter specific to a single     service (this is true when the parameter_number is larger than     128), compose the arriving value with the local value exported by     the specified service, and pass the result to the next hop. In this     case there is no need to consider global values, because the     parameter itself is specific to just one service.Shenker & Wroclawski        Standards Track                     [Page 5]

RFC 2215          General Characterization Parameters     September 1997     3. If the arriving value is a service-specific value for a     generally defined parameter (the parameter_number is 127 or less,     and the service_number is other than 1), and the local     implementation of that service also exports a service-specific     value for the parameter, compose the service-specific arriving     value and the service-specific local value of the parameter, and     pass the result as a service-specific value to the next-hop node.     4. If the arriving value is a service-specific value for a general     parameter (the parameter_number is 127 or less, and the     service_number is other than 1), and the local implementation of     that service does *not* export a service-specific value, compose     the service-specific arriving value with the global value for that     parameter exported by the local node, and pass the result as a     service-specific value to the next-hop node.     5. If the arriving value is a global value for a general parameter     (parameter_number is 127 or less, and the service_number is 1), and     the local implementation of *any* service exports a service-     specific value for that general parameter, compose the arriving     (global) value with the service-specific value for that parameter     exported by the local service, and pass the result as a service-     specific value to the next-hop node. This will require adding a new     data field to the message passed to the next hop, to hold the newly     generated service-specific value. Repeat this process for each     service that exports a service-specific value for the parameter.     6. If the arriving value is a global value for a general parameter     (the service_number is 1, and the parameter_number is 127 or less),     compose the arriving (global) value with the global parameter value     exported by the local node, and pass the result as a global     (service 1) value to the next-hop node. This step is performed     whether or not any service-specific values were generated and     exported in step 5.3. General Parameter Definitions 3.1 NON-IS_HOP flag parameter   This parameter provides information about the presence of network   elements which do not implement QoS control services along the data   path.   The local value of the parameter is 1 if the network element does not   implement the relevant QoS control service, or knows that there is a   break in the chain of elements which implement the service.  The   local parameter is 0 otherwise.  The local parameter is assigned   parameter_number 1.Shenker & Wroclawski        Standards Track                     [Page 6]

RFC 2215          General Characterization Parameters     September 1997   The composition rule for this parameter is the OR function. A   composed parameter value of 1 arriving at the endpoint of a path   indicates that at least one point along the path does not offer the   indicated QoS control service.  The parameter_number for the composed   quantity is 2.   The global NON_IS_HOP flag parameter thus has the ID <1,2>. If this   flag is set, it indicates that one or more network elements along the   application's data path does not support the integrated services   framework at all. An example of such an element would be an IP router   offering only best-effort packet delivery and not supporting any   resource reservation requests.   Obviously, a network element which does not support this   specification will not know to set this flag.  The actual   responsibility for determining that a network node does not support   integrated services may fall to the network element, the setup   protocol, or a manual configuration operation and is dependent on   implementation and usage.  This calculation must be conservative.   For example, a router sending packets into an IP tunnel must assume   that the tunneled packets will not receive QoS control services   unless it or the setup protocol can prove otherwise.   Service-specific versions of the NON_IS_HOP flag indicate that one or   more network elements along a path don't support the particular   service. For example, the flag parameter identified by ID <2,2> being   set indicates that some network element along the path does not   support the Guaranteed service, though it might support another   service such as Controlled-Load.   If the global NON_IS_HOP flag <1,2> is set for a path, the receiver   (network element or application) should consider the values of all   other parameters defined in this specification, including service-   specific NON_IS_HOP flags, as possibly inaccurate. If a service   specific NON_IS_HOP flag is set for a path, the receiver should   consider the values of all other parameters associated with that   service as possibly inaccurate.   The NON_IS_HOP parameter may be represented in any form which can   express boolean true and false. However, note that a network element   must set this flag precisely when it does *not* fully understand the   format or data representation of an arriving protocol message   (because it does not support the specified service). Therefore, the   data representation used for this parameter by setup and management   protocols must allow the parameter value to be read and set even if   the network element cannot otherwise parse the protocol message.Shenker & Wroclawski        Standards Track                     [Page 7]

RFC 2215          General Characterization Parameters     September 1997   An appropriate XDR description of this parameter is:                             bool NON_IS_HOP;   However, the standard XDR data encoding for this description will not   meet the requirement described above unless other restrictions are   placed on message formats. An alternative data representation may be   more appropriate.      NOTE: The message format described for RSVP in [RFC 2210] carries      this parameter as a single-bit flag, referred to as the "break      bit". 3.2 NUMBER_OF_IS_HOPS   IS stands for "integrated services aware".  An integrated services   aware network element is one that conforms to the various   requirements described in this and other referenced documents.  The   network element need not offer a specific service, but if it does it   must support and characterize the service in conformance with the   relevant specification, and if it does not it must correctly set the   NON_IS_HOP flag parameter for the service. For completeness, the   local parameter is assigned the parameter_number 3.   The composition rule for this parameter is to increment the counter   by one at each IS-aware hop.  This quantity, when composed end-to-   end, informs the endpoint of the number of integrated-services aware   network elements traversed along the path.  The parameter_number for   this composed parameter is 4.   Values of the composed parameter will range from 1 to 255, limited by   the bound on IP hop count.   The XDR representation of this parameter is:                      unsigned int NUMBER_OF_IS_HOPS; 3.3. AVAILABLE_PATH_BANDWIDTH   This parameter provides information about the bandwidth available   along the path followed by a data flow.  The local parameter is an   estimate of the bandwidth the network element has available for   packets following the path.  Computation of the value of this   parameter should take into account all information available to the   network element about the path, taking into consideration   administrative and policy controls on bandwidth, as well as physical   resources.Shenker & Wroclawski        Standards Track                     [Page 8]

RFC 2215          General Characterization Parameters     September 1997      NOTE: This parameter should reflect, as closely as possible, the      actual bandwidth available to packets following a path. However,      the bandwidth available may depend on a number of factors not      known to the network element until a specific QoS request is in      place, such as the destination(s) of the packet flow, the service      to be requested by the flow, or external policy information      associated with a reservation request.  Because the parameter must      in fact be provided before any specific QoS request is made, it is      frequently difficult to provide the parameter accurately. In      circumstances where the parameter cannot be provided accurately,      the network element should make the best attempt possible, but it      is acceptable to overestimate the available bandwidth by a      significant amount.   The parameter_number for AVAILABLE_PATH_BANDWIDTH is 5. The global   parameter <1, 5> is an estimate of the bandwidth available to any   packet following the path, without consideration of which (if any)   QoS control service the packets may be subject to.   In cases where a particular service is administratively or   technically restricted to a limited portion of the overall available   bandwidth, the service module may wish to export an override   parameter which specifies this smaller bandwidth value.   The composition rule for this parameter is the MIN function. The   composed value is the minimum of the network element's value and the   previously composed value. This quantity, when composed end-to-end,   informs the endpoint of the minimal bandwidth link along the path   from sender to receiver.  The parameter_number for the composed   minimal bandwidth along the path is 6.   Values of this parameter are measured in bytes per second.  The   representation must be able to express values ranging from 1 byte per   second to 40 terabytes per second, about what is believed to be the   maximum theoretical bandwidth of a single strand of fiber.   Particularly for large bandwidths, only the first few digits are   significant, so the use of a floating point representation, accurate   to at least 0.1%, is encouraged.   The XDR representation for this parameter is:                      float AVAILABLE_PATH_BANDWIDTH;   For values of this parameter only valid non-negative floating point   numbers are allowed. Negative numbers (including "negative zero"),   infinities, and NAN's are not allowed.Shenker & Wroclawski        Standards Track                     [Page 9]

RFC 2215          General Characterization Parameters     September 1997      NOTE: An implementation which utilizes general-purpose hardware or      software IEEE floating-point support may wish to verify that      arriving parameter values meet these requirements before using the      values in floating-point computations, in order to avoid      unexpected exceptions or traps.   If the network element cannot or chooses not to provide an estimate   of path bandwidth, it may export a local value of zero for this   parameter.  A network element or application receiving a composed   value of zero for this parameter must assume that the actual   bandwidth available is unknown. 3.4 MINIMUM_PATH_LATENCY   The local parameter is the latency of the packet forwarding process   associated with the network element, where the latency is defined to   be the *smallest* possible packet delay added by the network element.   This delay results from speed-of-light propagation delay, from packet   processing limitations, or both. It does not include any variable   queuing delay which may be present.   The purpose of this parameter is to provide a baseline minimum path   latency for use with services which provide estimates or bounds on   additional path delay, such as Guaranteed [RFC 2212].  Together with   the queuing delay bound offered by Guaranteed and similar services,   this parameter gives the application knowledge of both the minimum   and maximum packet delivery delay.  Knowing both the minimum and   maximum latency experienced by data packets allows the receiving   application to accurately compute its de-jitter buffer requirements.   Note that the quantity characterized by this parameter is the   absolute smallest possible value for the packet processing and   transmission latency of the network element. This value is the   quantity required to provide the end hosts with jitter bounds. The   parameter does *not* provide an upper-bound estimate of minimum   latency, which might be of interest for best-effort traffic and QoS   control services which do not explicitly offer delay bounds. In other   words, the parameter will always underestimate, rather than   overestimate, latency, particularly in multicast and large cloud   situations.   When packets traversing a network element may experience different   minimal latencies over different paths, this parameter should, if   possible, report an accurate latency value for each path. For   example, when an ATM point-multipoint virtual circuit is used to   implement IP multicast, the mechanism that implements this parameter   for the ATM cloud should ideally compute a separate value for each   destination. Doing this may require cooperation between the ingressShenker & Wroclawski        Standards Track                    [Page 10]

RFC 2215          General Characterization Parameters     September 1997   and egress elements bounding the multi-access communication cloud.   The method by which this cooperation is achieved, and the choice of   which IP-level network element actually provides and composes the   value, is technology-dependent.   An alternative choice is to provide the same value of this parameter   for all paths through the cloud. The value reported must be the   smallest latency for any possible path. Note that in this situation,   QoS control services (e.g., Guaranteed) which provide an upper bound   on latency cannot simply add their queuing delay to the value   computed by this parameter; they must also compensate for path delays   above the minimum. In this case the range between the minimum and   maximum packet delays reported to the application may be larger than   actually occurs, because the application will be told about the   minimum delay along the shortest path and the maximum delay along the   actual path.  This is acceptable in most situations.   A third alternative is to report the "indeterminate" value, as   specified below.  In this circumstance the client application may   either deduce a minimum path latency through measurement, or assume a   value of zero.   The composition rule for this parameter is summation with a clamp of   (2**32 - 1) on the maximum value. This quantity, when composed end-   to-end, informs the endpoint of the minimal packet delay along the   path from sender to receiver. The parameter_number for the latency of   the network element's link is 7. The parameter_number for the   cumulative latency along the path is 8.   The latencies are reported in units of one microsecond. An individual   element can advertise a latency value between 1 and 2**28 (somewhat   over two minutes) and the total latency added across all elements can   range as high as (2**32)-2. If the sum of the different elements   delays exceeds (2**32)-2, the end-to-end advertised delay should be   reported as indeterminate. This is described below.   Note that while the granularity of measurement is microseconds, a   conforming element is free to actually measure delays more loosely.   The minimum requirement is that the element estimate its delay   accurately to the nearest 100 microsecond granularity. Elements that   can measure more accurately are, of course, encouraged to do so.      NOTE: Measuring in milliseconds is not acceptable, because if the      minimum delay value is a millisecond, a path with several hops      will lead to a composed delay of at least several milliseconds,      which is likely to be misleading.Shenker & Wroclawski        Standards Track                    [Page 11]

RFC 2215          General Characterization Parameters     September 1997   The XDR description of this parameter is:                    unsigned int MINIMUM_PATH_LATENCY;   The distinguished value (2**32)-1 is taken to mean "indeterminate   latency". A network element which cannot accurately predict the   latency of packets it is processing should set its local parameter to   this value. Because the composition function limits the composed sum   to this value, receipt of this value at a network element or   application indicates that the true path latency is not known. This   may happen because one or more network elements could not supply a   value, or because the range of the composition calculation was   exceeded. 3.5. PATH_MTU   This parameter computes the maximum transmission unit (MTU) for   packets following a data path.  This value is required to invoke QoS   control services which require that IP packet size be strictly   limited to a specific MTU. Existing MTU discovery mechanisms cannot   be used because they provide information only to the sender and they   do not directly allow for QoS control services to specify MTU's   smaller than the physical MTU.   The local characterization parameter is the IP MTU, where the MTU of   a network element is defined to be the maximum transmission unit the   network element can accommodate without fragmentation, including IP   and upper-layer protocol headers but not including link level   headers.  The composition rule is to take the minimum of the network   element's MTU and the previously composed value.  This quantity, when   composed end-to-end, informs the endpoint of the maximum transmission   unit that can traverse the path from sender to receiver without   fragmentation.  The parameter_number for the MTU of the network   element's link is 9.  The parameter_number for the composed MTU along   the path is 10.   A correct and valid value of this parameter must be provided by all   IS-aware network elements.   A specific service module may specify an MTU smaller than that of the   overall network element by overriding this parameter with one giving   the service's MTU value. A service module may not specify an MTU   value larger than that given by the global parameter.   Values of this parameter are measured in bytes.  The representation   must be able to express values ranging from 1 byte to 2**32-1 bytes.Shenker & Wroclawski        Standards Track                    [Page 12]

RFC 2215          General Characterization Parameters     September 1997   The XDR description of this parameter is:                          unsigned int PATH_MTU; 3.6. TOKEN_BUCKET_TSPEC   This parameter is used to describe data traffic parameters using a   simple token bucket filter. This parameter is used by data senders to   describe the traffic parameters of traffic it expects to generate,   and by QoS control services to describe the parameters of traffic for   which the reservation should apply. It is defined as a general rather   than service-specific parameter because the same traffic description   may be used by several QoS control services in some situations.      NOTE: All previous definitions in this note have described      "characterization parameters", with local values set by network      elements to characterize their behavior and composition rules to      give the resulting end-to-end behavior. The TOKEN_BUCKET_TSPEC is      not a characterization parameter, because intermediate nodes      within the network do not export local values for      TOKEN_BUCKET_TSPECs. The TOKEN_BUCKET_TSPEC is simply a data      structure definition given here because it is common to more than      one QoS control service.   The TOKEN_BUCKET_TSPEC parameter is assigned parameter_number 127.   The TOKEN_BUCKET_TSPEC takes the form of a token bucket specification   plus a peak rate [p], minimum policed unit [m], and a maximum packet   size [M].   The token bucket specification includes an average or token rate [r]   and a bucket depth [b].  Both [r] and [b] must be positive.   The token 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. In practice, only the first few digits of the   [r] and [p] parameters 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. 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 peak traffic rate [p] 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. In practice, only the first few digits ofShenker & Wroclawski        Standards Track                    [Page 13]

RFC 2215          General Characterization Parameters     September 1997   the [r] and [p] parameters are significant, so the use of floating   point representations, accurate to at least 0.1% is encouraged. The   peak rate value may be set to positive infinity, indicating that it   is unknown or unspecified.   The range of values allowed for these parameters is intentionally   large to allow for future network technologies. A particular network   element is not expected to support the full range of values.   The minimum policed unit, [m], is an integer measured in bytes.  This   size includes the application data and all protocol headers at or   above the IP level (IP, TCP, UDP, RTP, etc.). It does not include the   link-level header size, because these headers will change in size as   the packet crosses different portions of the internetwork.   All IP datagrams less than size [m] are treated as being of size m   for purposes of resource allocation and policing. The purpose of this   parameter is to allow reasonable estimation of the per-packet   resources needed to process a flow's packets (maximum packet rate can   be computed from the [b] and [m] terms) and to reasonably bound the   bandwidth overhead consumed by the flow's link-level packet headers.   The maximum bandwidth overhead consumed by link-level headers when   carrying a flow's packets is bounded by the ratio of the link-level   header size to [m]. Without the [m] term, it would be necessary to   compute this bandwidth overhead assuming that every flow was always   sending minimum-sized packets, which is unacceptable.   The maximum packet size, [M], is the biggest packet that will conform   to the traffic specification; it is also measured in bytes.  Any   packets of larger size sent into the network may not receive QoS-   controlled service, since they are considered to not meet the traffic   specification.   Both [m] and [M] must be positive, and [m] must be less then or equal   to [M].   The XDR description of this parameter is:         struct {           float r;           float b;           float p;           unsigned m;           unsigned M;         } TOKEN_BUCKET_TSPEC;Shenker & Wroclawski        Standards Track                    [Page 14]

RFC 2215          General Characterization Parameters     September 1997   For the fields [r] and [b] only valid non-negative floating point   numbers are allowed. Negative numbers (including "negative zero),   infinities, and NAN's are not allowed.   For the field [p], only valid non-negative floating point numbers or   positive infinity are allowed. Negative numbers (including "negative   zero), negative infinities, 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 parameter values meet these requirements before using the      values in floating-point computations, in order to avoid      unexpected exceptions or traps.4. Security Considerations   Implementation of the characterization parameters described in this   memo creates no known new avenues for malicious attack on the network   infrastructure.  Implementation of these characterization parameters   does, of necessity, reveal some additional information about a   network's performance, which in extremely rare circumstances could be   viewed as a security matter by the network provider.5. References   [RFC 2005] Braden, R., Ed., et. al., "Resource Reservation Protocol   (RSVP) - Version 1 Functional Specification",RFC 2205, September   1997.   [RFC 2210] Wroclawski, J., "The Use of RSVP with IETF Integrated   Services",RFC 2210, September 1997.   [RFC 2216] Shenker, S., and J. Wroclawski, "Network Element QoS   Control Service Specification Template",RFC 2216, September 1997.   [RFC 2212] Shenker, S., Partridge, C., and R. Guerin "Specification   of the Guaranteed Quality of Service",RFC 2212, September 1997.   [RFC 2211] Wroclawski, J., "Specification of the Controlled Load   Quality of Service",RFC 2211, September 1997.   [RFC 1832] Srinivansan, R., "XDR: External Data Representation   Standard",RFC 1832, August 1995.Shenker & Wroclawski        Standards Track                    [Page 15]

RFC 2215          General Characterization Parameters     September 1997Authors' Addresses   Scott Shenker   Xerox PARC   3333 Coyote Hill Road   Palo Alto, CA 94304-1314   Phone: 415-812-4840   Fax:   415-812-4471   EMail: shenker@parc.xerox.com   John Wroclawski   MIT Laboratory for Computer Science   545 Technology Sq.   Cambridge, MA  02139   Phone: 617-253-7885   Ffax:  617-253-2673 (FAX)   EMail: jtw@lcs.mit.eduShenker & Wroclawski        Standards Track                    [Page 16]