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Network Working Group                                     C. PartridgeRequest for Comments: 1363                                         BBN                                                        September 1992A Proposed Flow SpecificationStatus of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard.  Distribution of this memo is   unlimited.Abstract   A flow specification (or "flow spec") is a data structure used by   internetwork hosts to request special services of the internetwork,   often guarantees about how the internetwork will handle some of the   hosts' traffic.  In the future, hosts are expected to have to request   such services on behalf of distributed applications such as   multimedia conferencing.   The flow specification defined in this memo is intended for   information and possible experimentation (i.e., experimental use by   consenting routers and applications only).  This RFC is a product of   the Internet Research Task Force (IRTF).Introduction   The Internet research community is currently studying the problems of   supporting a new suite of distributed applications over   internetworks.  These applications, which include multimedia   conferencing, data fusion, visualization, and virtual reality, have   the property that they require the distributed system (the collection   of hosts that support the applications along with the internetwork to   which they are attached) be able to provide guarantees about the   quality of communication between applications.  For example, a video   conference may require a certain minimum bandwidth to be sure that   the video images are delivered in a timely way to all recipients.   One way for the distributed system to provide guarantees is for hosts   to negotiate with the internetwork for rights to use a certain part   of the internetwork's resources.  (An alternative is to have the   internetwork infer the hosts' needs from information embedded in the   data traffic each host injects into the network.  Currently, it is   not clear how to make this scheme work except for a rather limited   set of traffic classes.)Partridge                                                       [Page 1]

RFC 1363             A Proposed Flow Specification        September 1992   There are a number of ways to effect a negotiation.  For example a   negotiation can be done in-band or out-of-band.  It can also be done   in advance of sending data (possibly days in advance), as the first   part of a connection setup, or concurrently with sending (i.e., a   host starts sending data and starts a negotiation to try to ensure   that it will allowed to continue sending).  Insofar as is possible,   this memo is agnostic with regard to the variety of negotiation that   is to be done.   The purpose of this memo is to define a data structure, called a flow   specification or flow spec, that can be used as part of the   negotiation to describe the type of service that the hosts need from   the internetwork.  This memo defines the format of the fields of the   data structure and their interpretation.  It also briefly describes   what purpose the different fields fill, and discusses why this set of   fields is thought to be both necessary and sufficient.   It is important to note that the goal of this flow spec is to able to   describe *any* flow requirement, both for guaranteed flows and for   applications that simply want to give hints to the internetwork about   their requirements.Format of the Flow Spec       0                   1                   2                   3       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |              Version          |    Maximum Transmission Unit  |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |      Token Bucket Rate        |        Token Bucket Size      |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |  Maximum Transmission Rate    |     Minimum Delay Noticed     |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |     Maximum Delay Variation   |        Loss Sensitivity       |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |     Burst Loss Sensitivity    |          Loss Interval        |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+      |    Quality of Guarantee       |      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Discussion of the Flow Spec   The flow spec indicates service requirements for a single direction.   Multidirectional flows will need to request services in both   directions (using two flow specs).   To characterize a unidirectional flow, the flow spec needs to do four   things.Partridge                                                       [Page 2]

RFC 1363             A Proposed Flow Specification        September 1992   First, it needs to characterize how the flow's traffic will be   injected into the internetwork.  If the internetwork doesn't know   what to expect (is it a gigabit-per-second flow or a three kilobit-   per-second flow?) then it is difficult for the internetwork to make   guarantees.  (Note the word "difficult" rather than "impossible."  It   may be possible to statistically manage traffic or over-engineer the   network so well that the network can accept almost all flows, without   setup.  But this problem looks far harder than asking the sender to   approximate its behavior so the network can plan.)  In this flow   spec, injected traffic is characterized as having a sustainable rate   (the token bucket rate) a peak rate (the maximum transmission rate),   and an approximate burst size (the token bucket size).  A more   precise definition of each of these fields is given below.  The   characterization is based, in part, on the work done in [1].   Second, the flow spec needs to characterize sensitivity to delay.   Some applications are more sensitive than others.  At the same time,   the internetwork will likely have a choice of routes with various   delays available from the source to destination.  For example, both   routes using satellites (which have very long delays) and routes   using terrestrial lines (which will have shorter delays) may be   available.  So the sending host needs to indicate the flow's   sensitivity to delay.  However, this field is only advisory.  It only   tells the network when to stop trying to reduce the delay - it does   not specify a maximum acceptable delay.   There are two problems with allowing applications to specify the   maximum acceptable delay.   First, observe that an application would probably be happy with a   maximum delay of 100 ms between the US and Japan but very unhappy   with a delay of 100 ms within the same city.  This observation   suggests that the maximum delay is actually variable, and is a   function of the delay that is considered achievable.  But the   achievable delay is largely determined by the geographic distance   between the two peers, and this sort of geographical information is   usually not available from a network.  Worse yet, the advent of   mobile hosts makes such information increasingly hard to provide.  So   there is reason to believe that applications may have difficulty   choosing a rational maximum delay.   The second problem with maximum delays is that they are an attempt to   quantify what performance is acceptable to users, and an application   usually does not know what performance will be acceptable its user.   For example, a common justification for specifying a maximum   acceptable delay is that human users find it difficult to talk to   each other over a link with more than about 100 ms of delay.   Certainly such delays can make the conversation less pleasant, but itPartridge                                                       [Page 3]

RFC 1363             A Proposed Flow Specification        September 1992   is still possible to converse when delays are several seconds long,   and given a choice between no connection and a long delay, many users   will pick the delay.  (The phone call may involve an important matter   that must be resolved.)   As part of specifying a flow's delay sensitivity, the flow spec must   also characterize how sensitive the flow is to the distortion of its   data stream.   Packets injected into a network according to some pattern will not   normally come out of the network still conforming to the pattern.   Instead, the pattern will have been distorted by queueing effects in   the network.  Since there is reason to believe that it may make   network design easier to continue to allow the networks slightly   distort traffic patterns, it is expected that those applications   which are sensitive to distortion will require their hosts to use   some amount of buffering to reshape the flow back into its original   form.  It seems reasonable to assume that buffer space is not   infinite and that a receiving system will wish to limit the amount of   buffering that a single flow can use.   The amount of buffer space required for removing distortion at the   receiving system is determined by the variation in end-to-end   transmission delays for data sent over the flow.  If the transmission   delay is a mean delay, D, plus or minus a variance, V, the receiving   system needs buffer space equivalent to 2 * V * the transmission   rate.  To see why this is so, consider two packets, A and B, sent T   time units apart which must be delivered to the receiving application   T time units apart.  In the worst case, A arrives after a delay of   D-V time units (the minimum delay) and B arrives after a delay of D+V   time units (the maximum delay).  The receiver cannot deliver B until   it arrives, which is T + 2 * V time units after A.  To ensure that A   is delivered T time units before B, A must be buffered for 2 * V time   units.  The delay variance field is the value of 2 * V, and allows   the receiver to indicate how much buffering it is willing to provide.   A third function of the flow spec is to signal sensitivity to loss of   data.  Some applications are more sensitive to the loss of their data   than other applications.  Some real-time applications are both   sensitive to loss and unable to wait for retransmissions of data.   For these particularly sensitive applications, hosts may implement   forward error correction on a flow to try to absolutely minimize   loss.  The loss fields allow hosts to request loss properties   appropriate for the application's requirements.   Finally, it is expected that the internetwork may be able to provide   a range of service guarantees.  At the best, the internetwork may be   asked to guarantee (with tight probability bounds) the quality ofPartridge                                                       [Page 4]

RFC 1363             A Proposed Flow Specification        September 1992   service it will provide.  Or the internetwork may simply be asked to   ensure that packets sent over the flow take a terrestrial path.  The   quality of guarantee field indicates what type of service guarantee   the application desires.Definition of Individual FieldsGeneral Format of Fields   With a few exceptions, fields of the flow spec are expressed using a   common 16-bit format.  This format has two forms.  The first form is   shown below.               0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+              |0|  Exponent   |     Value     |              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   In this format, the first bit is 0, followed by 7 bits of an exponent   (E), and an 8-bit value (V).  This format encodes a number, of the   form V * (2**E).  This representation was chosen to allow easy   representation of a wide range of values, while avoiding over-precise   representations.   In some case, systems will not wish to request a precise value but   rather simply indicate some sensitivity.  For example, a virtual   terminal application like Telnet will likely want to indicate that it   is sensitive to delay, but it may not be worth expressing particular   delay values for the network to try to achieve.  For these cases,   instead of a number, the field in the flow spec will take the   following form:               0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+              |1|   Well-defined Constant     |              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   The first bit of the field is one, and is followed by a 15-bit   constant.  The values of the constants for given fields are defined   below.  Any additional values can be requested from the Internet   Assigned Numbers Authority (IANA).   Version Field      This field is a 16-bit integer in Internet byte order.  It is the      version number of the flow specification.  The version number of      the flow specification defined in this document is 1.  The IANA is      responsible for assigning future version numbers for any proposedPartridge                                                       [Page 5]

RFC 1363             A Proposed Flow Specification        September 1992      revisions of this flow specification.      This field does not use the general field format.   Maximum Transmission Unit (MTU)      A 16-bit integer in Internet byte order which is the maximum      number of bytes in the largest possible packet to be transmitted      over this flow.      This field does not use the general field format.      The field serves two purposes.      It is a convenient unit for expressing loss properties.  Using the      default MTU of the internetwork is inappropriate since the      internetwork have very large MTU, such the 64Kbytes of IP, but      applications and hosts may be sensitive to losses of far less than      an MTU's amount of data -- for example, a voice application would      be sensitive to a loss of several consecutive small packets.      The MTU also bounds the amount of time that a flow can transmit,      uninterrupted, on a shared media.      Similarly, the loss rates of links that suffer bit errors will      vary dramatically based on the MTU size.   Token Bucket Rate      The token bucket rate is one of three fields used to define how      traffic will be injected into the internetwork by the sending      application.  (The other two fields are the token bucket size and      the maximum transmission rate.)      The token rate is the rate at which tokens (credits) are placed      into an imaginary token bucket.  For each flow, a separate bucket      is maintained.  To send a packet over the flow, a host must remove      a number of credits equal to the size of the packet from the token      bucket.  If there are not enough credits, the host must wait until      enough credits accumulate in the bucket.      Note that the fact that the rate is expressed in terms of a token      bucket rate does not mean that hosts must implement token buckets.      Any traffic management scheme that yields equivalent behavior is      permitted.      The field is in the general field format and counts the number of      byte credits (i.e., right to send a byte) per second which arePartridge                                                       [Page 6]

RFC 1363             A Proposed Flow Specification        September 1992      deposited into the token bucket.  The value must be a number (not      a well-known constant).      The value zero is slightly special.  It is used to indicate that      the application is not making a request for bandwidth guarantees.      If this field is zero, then the Token Bucket Size must also be      zero, and the type of guarantee requested may be no higher than      predicted service.   Token Bucket Size      The token bucket size controls the maximum amount of data that the      flow can send at the peak rate.  More formally, if the token      bucket size is B, and the token bucket rate is R, over any      arbitrarily chosen interval T in the life of the flow, the amount      of data that the flow sends cannot have exceeded B + (R * T)      bytes.      The token bucket is filled at the token bucket rate.  The bucket      size limits how many credits the flow may store.  When the bucket      is full, new credits are discarded.      The field is in the general field format and indicates the size of      the bucket in bytes.  The value must be a number.      Note that the bucket size must be greater than or equal to the MTU      size.      Zero is a legal value for the field and indicates that no credits      are saved.   Maximum Transmission Rate      The maximum transmission rate limits how fast packets may be sent      back to back from the host.  Consider that if the token bucket is      full, it is possible for the flow to send a series of back-to-back      packets equal to the size of the token bucket.  If the token      bucket size is large, this back-to-back run may be long enough to      significantly inhibit multiplexing.      To limit this effect, the maximum transmission rate bounds how      fast successive packets may be placed on the network.      One can think of the maximum transmission rate control as being a      form of a leaky bucket.  When a packet is sent, a number of      credits equal to the size of the packet is placed into an empty      bucket, which drains credits at the maximum transmission rate.  No      more packets may be sent until the bucket has emptied again.Partridge                                                       [Page 7]

RFC 1363             A Proposed Flow Specification        September 1992      The maximum transmission rate is the rate at which the bucket is      emptied.  The field is in the general field format and indicates      the size of the bucket in bytes.  The value must be a number and      must be greater than or equal to the token bucket rate.      Note that the MTU size can be used in conjunction with the maximum      transmission rate to bound how long an individual packet blocks      other transmissions.  The MTU specifies the maximum time an      individual packet may take.  The Maximum Transmission Rate, limits      the frequency with which packets may be placed on the network.   Minimum Delay Noticed      The minimum delay noticed field tells the internetwork that the      host and application are effectively insensitive to improvements      in end-to-end delay below this value.  The network is encouraged      to drive the delay down to this value but need not try to improve      the delay further.      The field is in the general field format.      If expressed as a number it is the number of microseconds of delay      below which the host and application do not care about      improvements.  Human users only care about delays in the      millisecond range but some applications will be computer to      computer and computers now have clock times measured in a handful      of nanoseconds.  For such computers, microseconds are an      appreciable time.  For this reason, this field measures in      microseconds, even though that may seem small.      If expressed as a well-known constant (first bit set), two field      values are accepted:         0 - the application is not sensitive to delay         1 - the application is moderately delay sensitive             e.g., avoid satellite links where possible).   Maximum Delay Variation      If a receiving application requires data to be delivered in the      same pattern that the data was transmitted, it may be necessary      for the receiving host to briefly buffer data as it is received so      that the receiver can restore the old transmission pattern.  (An      easy example of this is a case where an application wishes to send      and transmit data such as voice samples, which are generated and      played at regular intervals.  The regular intervals may be      distorted by queueing effects in the network and the receiver mayPartridge                                                       [Page 8]

RFC 1363             A Proposed Flow Specification        September 1992      have to restore the regular spacing.)      The amount of buffer space that the receiving host is willing to      provide determines the amount of variation in delay permitted for      individual packets within a given flow.  The maximum delay      variation field makes it possible to tell the network how much      variation is permitted.  (Implementors should note that the      restrictions on the maximum transmission rate may cause data      traffic patterns to be distorted before they are placed on the      network, and that this distortion must be accounted for in      determining the receiver buffer size.)      The field is in the general field format and must be a number.  It      is the difference, in microseconds, between the maximum and      minimum possible delay that a packet will experience.  (There is      some question about whether microsecond units are too large.  At a      terabit per second, one microsecond is a megabit.  Presumably if a      host is willing to receive data at terabit speeds it is willing to      provide megabits of buffer space.)      The value of 0, meaning the receiving host will not buffer out      delays, is acceptable but the receiving host must still have      enough buffer space to receive a maximum transmission unit sized      packet from the sending host.  Note that it is expected that a      value of 0 will make it unlikely that a flow can be established.   Loss Sensitivity      This field indicates how sensitive the flow's traffic is to      losses.  Loss sensitivity can be expressed in one of two ways:      either as a number of losses of MTU-sized packets in an interval,      or simply as a value indicating a level of sensitivity.      The field is in the general field format.      If the value is a number, then the value is the number of MTU-      sized packets that may be lost out of the number of MTU-sized      packets listed in the Loss Interval field.      If the value is a well-known constant, then one of two values is      permitted:         0 - the flow is insensitive to loss         1 - the flow is sensitive to loss (where possible             choose the path with the lowest loss rate).Partridge                                                       [Page 9]

RFC 1363             A Proposed Flow Specification        September 1992   Burst Loss Sensitivity      This field states how sensitive the flow is to losses of      consecutive packets.  The field enumerates the maximum number of      consecutive MTU-sized packets that may be lost.      The field is in the general field format.      If the value is a number, then the value is the number of      consecutive MTU-sized packets that may be lost.      If the value is a well-known constant, then the value 0 indicates      that the flow is insensitive to burst loss.      Note that it is permissible to set the loss sensitivity field to      simply indicate sensitivity to loss, and set a numerical limit on      the number of consecutive packets that can be lost.   Loss Interval      This field determines the period over which the maximum number of      losses per interval are measured.  In other words, given any      arbitrarily chosen interval of this length, the number of losses      may not exceed the number in the Loss Sensitivity field.      The field is in the general field format.      If the Loss Sensitivity field is a number, then this field must      also be a number and must indicate the number of MTU-sized packets      which constitutes a loss interval.      If the Loss Sensitivity field is not a number (i.e., is a well-      known constant) then this field must use the well-known constant      of 0 (i.e., first bit set, all other bits 0) indicating that no      loss interval is defined.   Quality of Guarantee      It is expected that the internetwork will likely have to offer      more than one type of guarantee.      There are two unrelated issues related to guarantees.      First, it may not be possible for the internetwork to make a firm      guarantee.  Consider a path through an internetwork in which the      last hop is an Ethernet.  Experience has shown (e.g., some of the      IETF conferencing experiments) that an Ethernet can often give      acceptable performance, but clearly the internetwork cannotPartridge                                                      [Page 10]

RFC 1363             A Proposed Flow Specification        September 1992      guarantee that the Ethernet will not saturate at some time during      a flow's lifetime.  Thus it must be possible to distinguish      between flows which cannot tolerate the small possibility of a      failure (and thus must guaranteed at every hop in the path) and      those that can tolerate islands of uncertainty.      Second, there is some preliminary work (see [2]) that suggests      that some applications will be able to adapt to modest variations      in internetwork performance and that network designers can exploit      this flexibility to allow better network utilization.  In this      model, the internetwork would be allowed to deviate slightly from      the promised flow parameters during periods of load.  This class      of service is called predicted service (to distinguish it from      guaranteed service).      The difference between predicted service and service which cannot      be perfectly guaranteed (e.g., the Ethernet example mentioned      above) is that the imperfect guarantee makes no statistical      promises about how it might mis-behave.  In the worst case, the      imperfect guarantee will not work at all, whereas predicted      service will give slightly degraded service.  Note too that      predicted service assumes that the routers and links in a path all      cooperate (to some degree) whereas an imperfect guarantee states      that some routers or links will not cooperate.      The field is a 16-bit field in Internet byte order.  There are six      legal values:         0 - no guarantee is required (the host is simply expressing             desired performance for the flow)         100 (hex) - an imperfect guarantee is requested.         200 (hex) - predicted service is requested and if unavailable,                     then no flow should be established.         201 (hex) - predicted service is requested but an imperfect                     guarantee is acceptable.         300 (hex) - guaranteed service is requested and if a firm                     guarantee cannot be given, then no flow should be                     established.         301 (hex) - guaranteed service is request and but an imperfect                     guarantee is acceptable.      It is expected that asking for predicted service or permitting an      imperfect guarantee will substantially increase the chance that aPartridge                                                      [Page 11]

RFC 1363             A Proposed Flow Specification        September 1992      flow request will be accepted.Possible Limitations in the Proposed Flow Spec   There are at least three places where the flow spec is arguably   imperfect, based on what we currently know about flow reservation.   In addition, since this is a first attempt at a flow spec, readers   should expect modifications as we learn more.   First, the loss model is not perfect.  Simply stating that an   application is sensitive to loss and to burst loss is a rather crude   indication of sensitivity.  However, explicitly enumerating loss   requirements within a cycle is also an imperfect mechanism.  The key   problem with the explicit values is that not all packets sent over a   flow will be a full MTU in size.  Expressed another way, the current   flow spec expects that an MTU-sized packet will be the unit of error   recovery.  If flows send packets in a range of sizes, then the loss   bounds may not be very useful.  However, the thought of allowing a   flow to request a set of loss models (one per packet size) is   sufficiently painful that I've limited the flow to one loss profile.   Further study of loss models is clearly needed.   Second, the minimum delay sensitivity field limits a flow to stating   that there is one point on a performance sensitivity curve below   which the flow is no longer interested in improved performance.  It   may be that a single point is insufficient to fully express a flow's   sensitivity.  For example, consider a flow for supporting part of a   two-way voice conversation.  Human users will notice improvements in   delay down to a few 10s of milliseconds.  However, the key point of   sensitivity is the delay at which normal conversation begins to   become awkward (about 100 milliseconds).  By allowing only one   sensitivity point, the flow spec forces the flow designer to either   ask for the best possible delay (e.g, a few 10's of ms) to try to get   maximum performance from the network, or state a sensitivity of about   95 ms, and accept the possibility that the internetwork will not try   to improve delay below that value, even if it could (and even though   the user would notice the improvement).  My expectation is that a   simple point is likely to be easier to deal with than attempting to   enumerate two (or three or four) points in the sensitivity curve.   Third, the models for service guarantees is still evolving and it is   by no means clear that the service choices provided are the correct   set.Partridge                                                      [Page 12]

RFC 1363             A Proposed Flow Specification        September 1992How an Internetwork is Expected to Handle a Flow Spec   There are at least two parts to the issue of how an internetwork is   expected to handle a flow spec.  The first part deals with how the   flow spec is interpreted so that the internetwork can find a route   which will allow the internetwork to match the flow's requirements.   The second part deals with how the network replies to the host's   request.   The precise mechanism for setting up a flow, given a flow spec, is a   large topic and beyond the scope of this memo.  The purpose of the   next few paragraphs is simply to sketch an argument that this flow   spec is sufficient to the requirements of the setup mechanisms known   to the author.   The key problem in setting up a flow is determining if there exist   one or more routes from the source to the destination(s) which might   be able to support the quality of service requested.  Once one has a   route (or set of candidate routes) one can take whatever actions may   be appropriate to confirm that the route is actually viable and to   cause the flow's data to follow that route.   There are a number of ways to find a route.  One might try to build a   route on the fly by establishing the flow hop-by-hop (as ST-II does)   or one might consult a route server which provides a set of candidate   source routes derived from a routing database.  However, whatever   system is used, some basic information about the flow needs to be   provided to the routing system.  This information is:      * How much bandwidth the flow may require.  There's no point        in routing a flow that expects to send at over 10 megabits per        second via a T1 (1.5 megabit per second) link.      * How delay sensitive the application is.  One does not wish        to route a delay-sensitive application over a satellite link,        unless the satellite link is the only possible route from here        to there.      * How much error can be tolerated.  Can we send this flow over        our microwave channel on a rainy day or is a more reliable link        required?      * How firm the guarantees need to be.  Can we put an Ethernet        in as one of the hops?      * How much delay variation is tolerated.  Again, can an Ethernet        be included in the path?  Does the routing system need to worry        if the addition of this flow will cause a few routers to runPartridge                                                      [Page 13]

RFC 1363             A Proposed Flow Specification        September 1992        at close to capacity?  (A side note: we assume that the routers        are running with priority queueing systems, so running the router        close to capacity doesn't mean that all flows get long and        variable delays.  Rather, running close to capacity means that        high priority flows will be unaffected, and low priority flows        will get hit with a lot of delay and variation.)   The flow spec provides all of this information.  So it seems   plausible to assume it provides enough information to make routing   decisions at setup time.   The flow spec was designed with the expectation that the network   would give a yes or no reply to a request for a guaranteed flow.   Some researchers have suggested that the negotiation to set up a flow   might be an extended negotiation, in which the requesting host   initially requests the best possible flow it could desire and then   haggles with the network until they agree on a flow with properties   that the network can actually provide and the application still finds   useful.  This notion bothers me for at least two reasons.  First, it   means setting up a flow is a potentially long process.  Second, the   general problem of finding all possible routes with a given set of   properties is a version of the traveling salesman problem, and I   don't want to embed traveling salesman algorithms into a network's   routing system.   The model used in designing this flow spec was that a system would   ask for the minimum level of service that was deemed acceptable and   the network would try to find a route that met that level of service.   If the network is unable to achieve the desired level of service, it   refuses the flow, otherwise it accepts the flow.The Flow Spec as a Return Value   This memo does not specify the data structures that the network uses   to accept or reject a flow.  However, the flow spec has been designed   so that it can be used to return the type of service being   guaranteed.   If the request is being accepted, the minimum delay field could be   set to the guaranteed or predicted delay, and the quality of   guarantee field could be set to no guarantee (0), imperfect guarantee   (100 hex), predicted service (200 hex), or guaranteed service (300   hex).   If the request is being rejected, the flow spec could be modified to   indicate what type of flow the network believes it could accept e.g.,   the traffic shape or delay characteristics could be adjusted or thePartridge                                                      [Page 14]

RFC 1363             A Proposed Flow Specification        September 1992   type of guarantee lowered).  Note that this returned flow spec would   likely be a hint, not a promised offer of service.Why Type of Service is not Good Enough   The flow spec proposed in this memo takes the form of a set of   parameters describing the properties and requirements of the flow.   An alternative approach which is sometimes mentioned (and which is   currently incorporated into IP) is to use a Type of Service (TOS)   value.   The TOS value is an integer (or bit pattern) whose values have been   predefined to represent requested quality of services.  Thus, a TOS   of 47 might request service for a flow using up to 1 gigabit per   second of bandwidth with a minimum delay sensitivity of 100   milliseconds.   TOS schemes work well if the different quality of services that may   be requested are both enumerable and reasonably small.   Unfortunately, these conditions do not appear to apply to future   internetworks.  The range of possible bandwidth requests alone is   huge.  Combine this range with several gradations of delay   requirements, and widely different sensitivities to errors and the   set of TOS values required becomes extremely large.  (At least one   person has suggested to the author that perhaps a TOS field combined   with a bandwidth parameter might be appropriate.  In other words, a   two parameter model.  That's a tempting idea but my gut feeling is   that it is not quite sufficient so I'm proposing a more complete   parametric model.)   Another reason to prefer parametric service is optimization issues.   A key issue in flow setup is trying to design the the routing system   to optimize its management of flows.  One can optimize on a number of   criteria.  A good example of an optimization problem is the following   question (expressed by Isidro Castineyra of BBN):     "Given a request to establish a flow, how can the internetwork     accept that request in such a way as to maximize the chance that     the internetwork will also be able to accept the next flow     request?"   The optimization goal here is call-completion - maximizing the chance   that requests to establish flows will succeed.  One might   alternatively try to maximize revenue (if one is charging for flows).   The internetwork is presumably in a better position to do   optimizations if it has more information about the flow's expected   behavior.  For example, if a TOS system says only that a flow isPartridge                                                      [Page 15]

RFC 1363             A Proposed Flow Specification        September 1992   delay sensitive, the routing system must seek out the most direct   route for the flow.  But if the routing system is told that the flow   is sensitive only to delays over 100 milliseconds, there may be a   number of routes other than the most direct route which can satisfy   this delay, thus leaving the most direct route available for a later   flow which needs a far lower delay.   In fairness, it should be noted that a danger of a parametric model   is that it is very easy to have too many parameters.  The yearn to   optimize can be overdone.  The goal of this flow spec is to enumerate   just enough parameters that it appears that essential needs can be   expressed, and the internetwork has some information it can use to   try to manage the flows.  Features that would simply be nice or   useful to have (but not essential) are left out to keep the parameter   space small.An Implication of the Flow Spec   It is important to observe that the there are fields in the flow spec   that are based on information from the sender (such as rate   information) and fields in the flow spec that are based on   information from the receiver (such as delay variation).  There are   also fields that may sender and receiver to negotiate in advance.   For example, the acceptable loss rate may depend on whether the   sender and receiver both support the same type of forward error   correction.  The delay sensitivity for a voice connection may depend,   in part, on whether both sender and receiver support echo cancelling.   The implication is that the internetwork must permit the sender and   receiver to communicate in advance of setting up a flow, because a   flow spec can only be defined once both sender and receiver have had   their say.  In other words, a reserved flow should not be the only   form of communication.   There must be some mechanism to perform a   short exchange of messages in preparation for setting up a flow.   (Another aside: it has been suggested that perhaps the solution to   this problem is to have the sender establish a flow with an   incomplete flow spec, and when the receiver gets the flow spec, have   the receiver send the completed flow spec back along the flow, so the   internetwork can "revise" the flow spec according to the receiver's   desires.  I have two problems with this approach.  First, it is   entirely possible that the receiver's information may lead the   internetwork to conclude that the flow established by the sender is   no good.  For example, the receiver may indicate it has a smaller   tolerance for delay variation than expected and force the flow to be   rerouted over a completely different path.  Second, if we try to   avoid having the receiver's information cause the flow to fail, then   we have to over-allocate the flow's during the preliminary setup.Partridge                                                      [Page 16]

RFC 1363             A Proposed Flow Specification        September 1992   But over allocating the resources requested may lead us to choose   better quality paths than we need for this flow.  In other words, our   attempts to optimize use of the network will fail.)Advance Reservations and Flow Duration   The primary purpose of a flow specification is to provide information   to the internetwork so the internetwork can properly manage the   proposed flow's traffic in the context of other traffic in the   internetwork.  One question is whether the flow should give the   network information about when the flow is expected to start and how   long the flow is expected to last.   Announcing when a flow will start is generally of interest for   advance reservations.  (If the flow is not be reserved substantially   in advance, the presentation of the flow spec to the internetwork can   be taken as an implicit request for a flow, now.)  It is my view that   advance reservation is a distinct problem from the describing the   properties of a flow.  Advanced reservations will require some   mechanism to maintain information in the network about flows which   are not currently active but are expected to be activated at some   time in the future.  I anticipate this will require some sort of   distributed database to ensure that information about advanced   reservations is not accidentally lost if parts of the internetwork   crash.  In other words, advance reservations will require   considerable additional supporting baggage that it would probably be   better to keep out of the average flow spec.   Deciding whether a flow spec should contain information about how   long the flow is expected to run is a harder decision to make.   Clearly if we anticipate that the internetwork will support advance   reservations, it will be necessary for elements of the internetwork   to predict their traffic load, so they can ensure that advance   reservations are not compromised by new flow requests.  However,   there is a school of thought that believes that estimating future   load from current behavior of existing flows is more accurate than   anything the flows may have declared in their flow specs.  For this   reason, I've left a duration field out of the flow spec.Examples   To illustrate how the flow spec values might be used, this section   presents three example flow specs.   Telnet      For the first example, consider using the flow spec to request      service for an existing application: Telnet.  Telnet is a virtualPartridge                                                      [Page 17]

RFC 1363             A Proposed Flow Specification        September 1992      terminal protocol, and one can think of it as stringing a virtual      wire across the network between the user's terminal and a remote      host.      Telnet has proved a very successful application without a need to      reserve bandwidth: the amount of data sent over any Telnet      connection tends to be quite small.  However, Telnet users are      often quite sensitive to delay, because delay can affect the time      it takes to echo characters.  This suggests that a Telnet      connection might benefit from asking the internetwork to avoid      long delay paths.  It could so so using the following flow spec      (for both directions):      Version=1      MTU=80 [40 bytes of overhead + 40 bytes user data]      Token Bucket Rate=0/0/0 [don't want a guarantee]      Token Bucket Size=0/0/0      Maximum Transmission Rate=0/0/0      Maximum Delay Noticed=1/1 [constant = delay sensitive]      Maximum Delay Variation=0/0/0 [not a concern]      Loss Sensitivity=1/0 [don't worry about loss]      Burst Loss Sensitivity=1/0      Loss Interval=1/0      Quality of Guarantee=1/0 [just asking]      It is worth noting that Telnet's flow spec is likely to be the      same for all instantiations of a Telnet connection.  As a result,      there may be some optimizations possible (such as just tagging      Telnet packets as being subject to the well-known Telnet flow      spec).   A Voice Flow      Now consider transmitting voice over the Internet.  Currently,      good quality voice can be delivered at rates of 32Kbit/s or      16Kbit/s.  Assuming the rate is 32Kbit/s and voice samples are 16      bit samples packaged into UDP datagrams (for a data rate of about      60 Kbyte/s), a flow spec might be:      Version=1      MTU=30 [2 byte sample in UDP datagram]      Token Bucket Rate=0/10/59 [60.4 Kbytes/s]      Token Bucket Size=0/0/30 [save enough to send immediately                                after pauses]      Maximum Transmission Rate=0/10/59 [peak same as mean]      Maximum Delay Noticed=0/10/100 [100 ms]      Maximum Delay Variation=0/10/10 [keep variation low]      Loss Sensitivity=1/1 [loss sensitive]Partridge                                                      [Page 18]

RFC 1363             A Proposed Flow Specification        September 1992      Burst Loss Sensitivity=0/0/5 [keep bursts small]      Loss Interval=1/0      Quality of Guarantee=1/201 [predicted service and I'll accept                                  worse]   A Variable Bit-Rate Video Flow      Variable bit-rate video transmissions vary the rate at which they      send data according to the amount of the video image that has      changed between frames.  In this example, we consider a one-way      broadcast of a picture.  If we assume 30 frames a second and that      a full frame is about 1 megabit of data, and that on average about      10% of the frame changes, but in the worst case the entire frame      changes, the flow spec might be:      Version=1      MTU=4096 [big so we can put lots of bits in each packet]      Token Bucket Rate=0/20/1 [8 Mbits/s]      Token Bucket Size=0/17/2 [2 Mbits/s]      Maximum Transmission Rate=0/20/30 [30 Mbits/s]      Maximum Delay Noticed=1/1 [somewhat delay sensitive]      Maximum Delay Variation=0/10/1 [no more than one second of                                      buffering]      Loss Sensitivity=0/0/1 [worst case, one loss per frame]      Burst Loss Sensitivity=0/0/1 [no burst errors please]      Loss Interval=0/0/33 [one frame in MTU sized packets]      Quality of Guarantee=1/300 [guaranteed service only]      The token bucket is sized to be two frames of data, and the bucket      rate will fill the bucket every 250 ms.  The expectation is that      full scene changes will be rare and that a fast rate with a large      bucket size should accommodate even a series of scene changes.   Disclaimer      In all cases, these examples are simply to sketch the use of the      flow spec.  The author makes no claims that the actual values used      are the correct ones for a particular application.Security Considerations   Security considerations definitely exist.  For example, one might   assume that users are charged for guaranteed flows.  In that case,   some mechanism must exist to ensure that a flow request (including   flow spec) is authenticated.  However I believe that such issues have   to be dealt with as part of designing a negotiation protocol, and are   not part of designing the flow spec data structure.Partridge                                                      [Page 19]

RFC 1363             A Proposed Flow Specification        September 1992Acknowledgements   I'd like to acknowledge the tremendous assistance of Steve Deering,   Scott Shenker and Lixia Zhang of XEROX PARC in writing this RFC.   Much of this flow spec was sketched out in two long meetings with   them at PARC.  Others who have offered notable advice and comments   include Isidro Castineyra, Deborah Estrin, and members of the End-   to-End Research Group chaired by Bob Braden.  All ideas that prove   misbegotten are the sole responsibility of the author.  This work was   funded under DARPA Contract No. MDA903-91-D-0019.  The views   expressed in this document are not necessarily those of the Defense   Advanced Research Projects Agency.References   1. Parekh, A., "A Generalized Processor Sharing Approach      to Flow Control in Integrated Services Networks",      MIT Laboratory for Information and Decision Systems,      Report No. LIDS-TH-2089.   2. Clark, D., Shenker, S., and L. Zhang, "Supporting Real-Time      Applications in an Integrated Services Packet Network:      Architecture and Mechanism", Proceedings of ACM SIGCOMM '92,      August 1992.Author's Address   Craig Partridge   BBN   824 Kipling St   Palo Alto, CA  94301   Phone: 415-325-4541   EMail: craig@aland.bbn.comPartridge                                                      [Page 20]