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 ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Abstract 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. Rampal Expires February, 1998 [Page 1] INTERNET DRAFT Flow Grouping for Guaranteed Service July, 1997 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 individually. 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. Rampal Expires February, 1998 [Page 2] INTERNET DRAFT Flow Grouping for Guaranteed Service July, 1997 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 application. 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. Rampal Expires February, 1998 [Page 3] INTERNET DRAFT Flow Grouping for Guaranteed Service July, 1997 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. Rampal Expires February, 1998 [Page 4] INTERNET DRAFT Flow Grouping for Guaranteed Service July, 1997 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 gain ------------ ------- ----------- ------- ------ - ------- (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. Rampal Expires February, 1998 [Page 5] INTERNET DRAFT Flow Grouping for Guaranteed Service July, 1997 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) where 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) Rampal Expires February, 1998 [Page 6] INTERNET DRAFT Flow Grouping for Guaranteed Service July, 1997 3. If the reservation with grouping (from equation (6)) is smaller, grouping can be performed else it is better to provide individual reservations. 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 techniques. 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 flows. Rampal Expires February, 1998 [Page 7] INTERNET DRAFT Flow Grouping for Guaranteed Service July, 1997 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. Guerin Expires February, 1998 [Page 8] INTERNET DRAFT Flow Grouping for Guaranteed Service July, 1997 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 C305/B664, IBM Networking Division, Research Triangle Park, NC 27709. Voice 919-254-4801 email sanjeev@raleigh.ibm.com Roch Guerin IBM T.J. Watson Research Center Yorktown Heights, NY 10598 Voice 914-784-7038 email guerin@watson.ibm.com Guerin Expires February, 1998 [Page 9] INTERNET DRAFT Flow Grouping for Guaranteed Service July, 1997