INTERNET-DRAFT                                        Sanjeev Rampal,
Expires January 15th 1998                             IBM Corpn.

                                                      Roch Guerin
                                                      IBM Corpn.
                                                      July 15th, 1997.

        Flow Grouping For Reducing Reservation Requirements for
                    Guaranteed Delay Service

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet- Drafts
   Shadow Directories on (Africa), (Europe), (Pacific Rim), (US East Coast), or (US West Coast).


   The purpose of this document is to illustrate when and how flow
   aggregation can be of benefit in the context of the Guaranteed
   Service.  Specifically, it identifies simple cases and provides
   generic rules for when grouping of flows allows a reduction in the
   amount of resources (bandwidth) needed to ensure the deterministic
   Guaranteed Service delay bounds of all flows.  The benefits of
   grouping should be especially of interest to, say, Internet Service
   Providers, wishing to interconnect sites with Guaranteed Service
   flows.  In particular, this document shows that in the case of flows
   with identical traffic characteristics and requirements, e.g.,
   Internet voice, grouping of flows is ALWAYS beneficial.

   This technique is not intended for IETF standardization and is
   being submitted for informational purposes only.

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1.0 Introduction

   This draft presents a technique for lowering the amount of bandwidth
   required by guaranteed service flows [1].

   The guaranteed service class provides a strict upper bound on
   the end-to-end delay that will be experienced by any packet of a
   flow belonging to this class (i.e. 100% of the packets will be
   delivered with a network delay of no more than the specified
   bound). This service requires each network element in the path
   of a flow to reserve a certain bandwidth for this flow.
   The provision of such deterministic guarantees does, however, come
   at a cost as the amount of bandwidth (service rate) required to
   ensure a given delay bound can be high, and in some instances even
   larger than the flow's peak rate.  As a result, techniques that
   offer the opportunity for lowering the required resources while
   providing the same guarantees are clearly desirable.

   This draft describes one set of such techniques, where reduction in
   the amount of bandwidth needed to guarantee the delay bounds of a
   given set of flows is achieved by grouping those flows.  In other
   words, the draft shows when the amount of
   bandwidth needed to be reserved for a group is less than the sum
   of the reservations that would be required for each flow

   The simplest and potentially most useful case where such a grouping
   is beneficial is that of a set of identical flows, i.e., same
   traffic characteristics and delay requirements, for which the
   bandwidth required by the grouped flows is ALWAYS less than the
   sum of the bandwidth needed by individual flows.  Such a scenario
   could be reasonably common in the case of an Internet Service
   Provider offering Internet telephony service between customer sites.
   In such a case, the application of grouping as described in this draft
   would result in lower bandwidth requirements to support a given
   number of voice calls.

   In the rest of this draft, we first review the criteria used when
   determining candidate flows for grouping, next we provide a simple
   example that illustrates both cases when grouping is beneficial
   and when it is not, and finally we outline a simple of rules for
   determining whether grouping should be attempted or not.

2.0 Outline of Flow Grouping

   The first requirement to identify guaranteed service flows that are
   potential candidate for grouping is that they all use the same path in
   the network.  In other words, the same set of flows should
   be present on all the links where grouping is performed.
   This constraint could be imposed on only a subset
   of the end-to-end path, e.g., grouping is only performed on the links
   between the ingress and egress routers when crossing a routing domain.
   However, this adds complexity to the determination of whether it is
   beneficial to group the flows or not.

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   As a result, in this document we only address the case where flows
   are group on their entire path.  The reason for choosing only flows
   which use the same path is that the delay bound calculated by the
   Guaranteed Service is an end-to-end value. If we try to group flows
   whose paths share some links initially but subsequently diverge
   for example, we will need to characterize the portion of the
   end-to-end delay that can be assigned to the common portion of the
   path.  As mentioned above, this adds complexity and we do not address
   this problem in this document.  The main contribution of the draft
   is to identify specific grouping criteria and guidelines, so that
   grouping always results in lowering the required bandwidth allocation.

   In particular, we show that by grouping some (but not all) of such
   flows appropriately and assigning a bandwidth reservation to the
   entire group instead of to the individual flows, we can reserve a
   lower amount of bandwidth for the group than the total that would be
   required if this grouping were not to be performed. Further we can
   still satisfy the delay bounds that are provided for each flow
   when such a grouping is not performed.  For a given set of flows,
   a simple test procedure is outlined using which the network
   determines whether the reservation with grouping will be less than
   without grouping. Accordingly flows will be grouped only if there
   is some bandwidth savings as compared to assigning a separate
   reservation for each flow as recommended by the specification [1].

2.1 Flow grouping technique

   The achievable improvement in utilization using flow grouping is
   first illustrated through some examples. Next a procedure for
   determining when grouping should be performed is outlined.

   Consider two flows(F1 and F2) using the guaranteed service class in
   a network as defined in [1] and [2]. These two flows are between the
   same source and receiver nodes and are routed over the exact
   same set of links (which we denote as L1, L2 ... Ln where n is the
   number of physical links/hops in the path). Let P denote the maximum
   packet size allowed for any connection in the network and let Cj
   denote the bandwidth of link Lj (j=1,2 ... n).

   Let the traffic characterization of these two flows be given as
   (b1, r1, P1) and (b2, r2, P2) respectively where the b term
   is the bucket size and the r term is the token generation rate of
   the leaky bucket characterizing the respective flows, while the P
   terms are the maximum packet sizes. Let D1 and D2 be the maximum
   tolerable delay for the respective flows as specified by the

   Consider a network in which separate rate reservations of R1 and
   R2 have been assigned to these two flows (we assume that
   the reservation for a flow is greater than its token generation rate
   which is a necessary condition for guaranteed service).  The total
   amount of bandwidth which needs to be reserved on each link in the
   path is hence R1 + R2.

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   In the guaranteed service formulation, the delay bound is inversely
   related to the amount of the reservation.  Hence to minimize the
   amount of reservation for this case, the delay guarantee provided by
   the network must equal that required by the flow (since any lower
   reservation will increase the delay bound provided by the network
   beyond the maximum tolerable limit specified by the application).

   Hence we have,

   D1 = (b1/R1) + (n-1) * (P1/ R1) + (sum from j=1 to n) P/Cj    (1)

   D2 = (b2/R2) + (n-1) * (P2/ R2) + (sum from j=1 to n) P/Cj    (2)

   The quantities on the right hand sides of equations (1) and (2)
   are the formulation of the end-to-end queueing delay bound as
   specified in [4]. The formulation in [1] is slightly different but
   is derived from the same. We restrict ourselves to the strict WFQ
   type formulation from [4] in this draft. The actual delay bound would
   equal the queueing delay bound plus the propagation delay. Given the
   route of a flow, the  propagation delay component is fixed and the
   same for all flows using the same route. Hence without any loss
   of generality, we restrict ourselves to queueing delay bounds in
   this draft.

   Now consider a flow created by grouping these two flows into one
   flow group which is characterized by the traffic model
   (b_g, r_g, P_g) (the bucket size, token generation rate and maximum
   packet size repectively of the resultant flow after grouping). Let
   this flow group be assigned a reservation of Rg. Physically, packets
   from both flows are simply multiplexed first come first served into
   one flow.  Packets which arrive at the same instant are reordered
   arbitrarily. If the scheduler uses separate queues for different
   flows for example, then we simply send packets from all flows in a
   flow group to the same queue.

   Consider a set of traffic characterization parameters for the flow
   group which are obtained as follows.

        b_g = b1 + b2                                           (3)

        r_g = r1 + r2                                           (4)

        Pg = max{P1, P2}                                        (5)

   In the appendix, we show that these values are sufficient to
   characterize the traffic of the flow group (which is the aggregate
   traffic consisting of packets from both flows). In other words, if
   the individual flows are deemed conforming by a leaky bucket derived
   from the individual traffic models, then the traffic of the flow
   group will also be deemed conforming by a leaky bucket based on
   traffic values for the flow group calculated as above.

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   The minimum value of the delay bound that can be guaranteed by
   the network to this combined flow can be calculated given this
   reservation.  We note that if the worst case delay bound guaranteed
   to the flow group is no more than the minimum of all delay bounds
   guaranteed to each individual flow, then the delay seen by any
   packet in the flow group is guaranteed to be less than the delay
   bound of the group and hence less than the delay bound guaranteed
   to its own flow.

   The table below shows the minimum delay bound for the individual
   and aggregate flows when Rg = R1 + R2 for two example values of
   the traffic parameters (n=2 and P/Cj = 1, j=1,2 are assumed).

   In the first case, the minimum delay bound that can be guaranteed
   for the combined flow with Rg = R1 + R2 (denoted
   as D_(1,2) is less than each of the bounds (D1 and D2) that can be
   guaranteed for the flows individually, using separate reservations.
   Hence a lower overall reservation (R') can be obtained
   when the flows are grouped. As shown, this results in a reduction of
   25 percent in the amount of bandwidth which needs to be reserved on
   each link in the path of these flows. On the other hand, in the
   second case, with Rg = R1 + R2,  the minimum delay bound that
   can be guaranteed for the combined flow is greater than that which
   can be guaranteed for flow F1 with separate reservations. In order
   for the combined flow to obtain a delay bound satisfactory for
   both flows, a reservation of greater than R1 + R2 would be required.
   Hence there is no gain in grouping for this case and better
   utilization would be obtained by keeping the two flows separate and
   maintaining individual reservations.

        Table 1: Examples of gain (or loss) using flow grouping

(b1, r1, P1)  (R1, D1)  (b2, r2, P2)   (R2, D2)   D_(1,2) R'   Percent
------------  -------   -----------    -------    ------  -    -------

(10, .5, 10)  (1, 22)   (10, .5, 10)   (1, 22)    17      1.5  25

(10, .5, 10)  (1, 22)   (10, .5, 100)  (1, 112)   62      -    Loss

n=2, P/Cj = 1, j=1,2

percentage gain is defined as 100 * (R1 + R2 - R')/(R1 + R2)

2.2. A procedure for testing whether grouping should be performed.

   As shown above, there may or may not be some gain in grouping
   two flows together. The procedure to test whether two flows can
   be grouped together is straightforward. The total reservations
   required with and without grouping are compared. Grouping can be
   performed only if it results in lower total reservation than the
   sum of the reservations with no grouping.

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   This test should be performed everytime a new flow using guaranteed
   service enters the network or an existing flow leaves the network
   or when the traffic characteristics of an existing flow change.
   At such a time the existing sets of flow grouping can possibly be
   changed to come up with another grouping which has a lower
   total reservation requirement. However this is also a policy
   decision which is implementation dependent.

   Searching exhaustively among all possible groupings of a set of
   flows for one that results in the absolute minimal total reservation
   is likely to be computationally intensive and intractable. A
   specific implementation may decide to tradeoff achieving the best
   possible overall reservation with limiting the scope of the search
   for an optimal combination of flows. Again, this draft does not
   detail the different such implementation techniques for performance
   control.  A simple technique is outlined below for checking whether
   two flows should be aggregated into a single flow for
   the purpose of reservation reduction.

   Given the two flows F1 and F2, and their traffic parameters and
   delay requirements as above, the procedure for testing whether
   they should be combined into a group is as follows.
   Typically, F1 would be a new incoming flow or an existing
   flow whose traffic characteristics have changed, while F2 represents
   an existing aggregated flow to which we are considering adding the flow
   F1 also.

   1. Calculate the total reservation required with grouping using
   equation (6) (which simply inverts the standard delay bound
   equation which has the form like equations (1) and (2)).

   Rg = max {r_g, (b_g+(n-1)* P_g)/(D_g - (sum j=1 to N) P/Cj)}     (6)

   where r_g, b_g and P_g are calculated according to equations (3)
   through (5).

   D_g = min(D1,  D2)                                               (7)

   2. Now calculate the total reservation required when grouping is not
   performed. This is given by equation (8) below.

   R_tot = R1 + R2                                                  (8)


   R1 = max{r_1, (b_1 + (n-1) * P_1)/(D1 - (sum j=1 to n) P/Cj)}    (9)

   R2 = max{r_2, (b_2 + (n-1) * P_2)/(D2 - (sum j=1 to n) P/Cj)}   (10)

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   3. If the reservation with grouping (from equation (6)) is smaller,
   grouping can be performed else it is better to provide individual

   4. If it is decided to group two flows, then the new parameters of
   the resultant flow group are calculated exactly as in equations (3)
   through (5) (and equation (7)).

2.3. The Case of Identical Flows

   As mentioned in the beginning for the special case of identical
   flows (i.e. identical traffic characteristics and delay requirements)
   we do not need to apply any checks to see whether aggregation will
   be useful. In fact aggregation will always be useful in such cases
   to reduce the reservation requirements. This can be seen by comparing
   equations (6), (9) and (10) for the case of b_g = b_1 + b_2,
   P_1 = P_2 = P_g, D1 = D2. Ignoring the pathological case
   where the reservation requirements are equal to the average data
   rates we will always have R1 + R2 > R_g.

2.4  Implementation of flow grouping

   The emphasis of this draft is to bring out the benefits of the flow
   grouping technique without outlining any implementation details.
   However we note briefly that one possible method of implementing
   this is by using IP-in-IP encapsulation to create an IP tunnel
   corresponding to each flow group. A signalling mechanism such as
   RSVP could then be used signal a single reservation for this flow
   group instead of for each flow within the group. Alternative methods
   also exist for creating some form of IP tunnel corresponding to
   each flow group so that a single low reservation can be requested
   for the single flow corresponding to the flow group rather than
   for each individual flow within it. This draft does not seek to
   go into any more detail on such possible implementation

3.0 Additional Issues

3.1 Performance control policies and optimal reservation reduction

   As mentioned earlier, in order to obtain maximal bandwidth
   reduction, the network should always attempt to re-assign
   flow groupings whenever any flow changes its characteristics
   or flows enter or leave the network, so that overall
   reservation is kept as low as possible. However, this
   requires performing the test for grouping frequently.
   Each such test can involve O(m) calculations where m is the
   number of flows in a path, since in the worst case, we have
   to compare an incoming or changing flow against all existing

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   The network can implement several policies which reduce the
   amount of calculations and resource re-allocations by
   compromising on the achievable reduction in reservation. For
   instance, if the only the tolerable delay for a flow increases,
   it may continue in its current group (thereby avoiding any
   resource re-allocation or calculations), even though a greater
   reduction in the amount of reservation could have been achieved
   if this flow was now assigned to another flow group.

   This draft does not present any algorithm to come up with
   the optimal grouping of a given set of flows (one that
   results in the lowest overall bandwidth reservation) since
   that is likely to be a computationally intractable problem.

3.2 Policing

   Even though flows are grouped, policing should still be
   performed on individual flows rather than on flow groups. This
   ensures that a flow which violates its traffic specification
   can be detected even if the combined traffic in a flow group may
   still be in comformance with the calculated parameters for
   the group.

4.0 References

   [i] S. Shenker, C. Partridge, R. Guerin "Specification of Guaranteed
   Service," draft-ietf-intserv-guaranteed-svc-08.txt, work in progress,
   Integrated Services Working Group, Internet Engineering Task Force
   (IETF), July 1997.

   [2] R. Braden, D. Clark, and S. Shenker, "Integrated Services
   in the Internet Architecture: an Overview," RFC 1633, June 1994.

   [3] R. Braden et. al., "Resource ReSerVation Protocol (RSVP) -
   Version 1 Functional Specification," draft-ietf-rsvp-spec-16.txt,
   work in progress, June 1997.

   [4] A.K. Parekh and R.G. Gallager, "A Generalized Processor Sharing
   Approach to Flow Control in Integrated Services Networks - The
   Multiple Node Case," Proc IEEE Infocom '93 pp.521-530.

   [5] The ATM Forum, "User-Network Interface (UNI) Specification
   v3.1,"  September 1994.

5.0 Appendix

   We show here that the traffic parameters calculated using equations
   (3), (4),  (5) and the delay bound of (7) are sufficient to
   characterize a flow group formed from two individual flows. Consider
   the flows F1 and F2 with traffic parameters (b1, r1, P1) and
   (b2, r2, P2) and delay upper bounds D1 and D2 as above.

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   If flow F1 conforms to its traffic specification,
   the amount of data provided by it to the source node over any time
   interval (t1 , t2) is at most b1 + r1*(t2 - t1). Similarly the amount
   of data provided by flow F2 in the same time interval cannot exceed
   b2 + r2*(t2 - t1).
   Now consider the flow obtained by grouping these two flows (Fg),
   with traffic parameters (b_g, r_g, P_g) and delay bound
   D_g, where these quantities are calculated according to
   equations (3), (4), (5) and (7) above. Over time interval (t1, t2)
   hence, the aggregate flow is required to generate no more
   than b_g + r_g*(t2 - t1). However, this is easily true because when
   equations (3) and (4) are true, we have

   b_g + r_g*(t2 - t1) = (b1 + r1*(t2 - t1)) + (b2 + r2*(t2 - t1))

   Also, the greater of P1 and P2 is clearly the
   maximum packet size of the combined flow. Finally, if every packet
   of the combined flow is guaranteed an end to end delay which is the
   minimum of the tolerable delays of all flows making up the flow group,
   clearly, every flow will see an acceptable delay even when it is
   combined with other flows in a group. Hence equations (3), (4), (5)
   and (7) represent valid parameter values for the flow obtained by
   grouping flows F1 and F2.

6.0 Acknowledgemnts

   The authors would like to thank Steve Blake of IBM for
   helpful comments and suggestions.

7.0 Author Information

   Sanjeev Rampal
   IBM Networking Division,
   Research Triangle Park, NC 27709.

   Voice 919-254-4801

   Roch Guerin
   IBM T.J. Watson Research Center
   Yorktown Heights, NY 10598

   Voice 914-784-7038

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