IntServ Parameters: General Characterization Parameters for Integrated Service Network Elements

Network Working Group                                        S. Shenker
Request for Comments: 2215                                J. Wroclawski
Category: Standards Track                            Xerox PARC/MIT LCS
                                                         September 1997


                General Characterization Parameters for
                  Integrated Service Network Elements


Status 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.

   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 in Section 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 either
   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.

      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 in Section 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.

   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.

     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.

   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.

   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.

      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.

      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 ingress
   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.

   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.

   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 of
   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;

   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.

Authors' 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.edu