INTERNET-DRAFT Sanjeev Rampal, Expires June 20th 1997 IBM Corpn. December 20th, 1996. 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 guaranteed service class of the Integrated Services model is based on the Weighted Fair Queueing model and requires rate reservation in order to provide end-to-end delay guarantees. Often, the amount of reservation needed to satisfy specified delay bounds can be quite high leading to low network utilization. This draft demonstrates a property of the Weighted Fair Queueing formulation which enables grouping of flows (which use the guaranteed service and are routed over the same path) in such a way that a lower reservation is needed than would be required if each flow were treated individually, and the delay bounds of each flow can still be provably satisfied. 1.0 Introduction This draft presents a technique for improving the bandwidth utilization for connections/ flows belonging to the guaranteed service class [1] of the integrated services (IntServ) internet specification [2]. Rampal Expires 20th June, 1997 [Page 1] INTERNET DRAFT Flow Grouping for Guaranteed Service Dec, 1996 The guaranteed service class ensures no packet loss and 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. This service requires each network element in the path of a flow to reserve a certain bandwidth for this flow. One problem with supporting this service class is that the amount of bandwidth reservation required can be quite high (and could be much greater than even the peak data arrival rate of the flow). This results in poor network utilization. This draft addresses this utilization problem for guaranteed service flows. A method is presented for reducing the amount of bandwidth reservation needed to support such flows (thereby improving network utilization). This is done while retaining the same delay guarantees that the service already provides to each flow. The reduction in reservation is achieved through grouping some of the guaranteed service flows in a network in such a way that 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. In this draft, first, we outline how to perform this flow grouping. We then show that utilization can be improved while still provably satisfying delay guarantees for each flow. The main contribution of this draft is to demonstrate this property of Weighted Fair Queueing. We then outline an approach for implementing this technique in an internetwork. Note. The proposed flow grouping technique is completely independent of the reservation merging using filters such as "wild card filters" proposed in RSVP specifications. 2.0 Outline of Flow Grouping The proposed approach works by forming groups from among flows which use the guaranteed service and which are routed over the same path in a network (that is flows which use the exact same set of links for their path in the network). Current IP routing algorithms do not account for dynamic link loads and in the absence of failures, most or all traffic between any two nodes will usually be routed over the same set of links. With the use of more QoS or load sensitive routing algorithms in the future this technique can still be used however, additional state will need to be maintained in routers to indicate which flows are using the same path. This draft does not concentrate on such implementaiton details at this time. Rampal Expires 20th June 1997 [Page 2] INTERNET DRAFT Flow Grouping for Guaranteed Service Dec, 1996 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 recharacterize the traffic of each flow after the paths diverge, possibly using reshaping and adding delay. We do not address that problem. The main contribution of this draft is to introduce the grouping technique. Even if grouping is restricted only to flows which share the same path, this will always result in some improvement in the utilization over existing approaches which do not employ any grouping at all. 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 explained first using some examples. Next a procedure for implementing this in an internetwork 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. Rampal Expires 20th June, 1997 [Page 3] INTERNET DRAFT Flow Grouping for Guaranteed Service Dec, 1996 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. 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) Rampal Expires 20th June, 1997 [Page 4] INTERNET DRAFT Flow Grouping for Guaranteed Service Dec, 1996 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. 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) Rampal Expires 20th June, 1997 [Page 5] INTERNET DRAFT Flow Grouping for Guaranteed Service Dec, 1996 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. 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. 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) 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)). Rampal Expires 20th June, 1997 [Page 6] INTERNET DRAFT Flow Grouping for Guaranteed Service Dec, 1996 3.0 Implementing flow grouping in a network In a network supporting guaranteed service, this technique is implemented as follows. At each source, corresponding to each receiver node r, and each unique path p between these two nodes which has at least one guaranteed service flow, the following information is maintained. The triplet (b_i, r_i, P_i) and the delay requirement D_i is maintained for each such flow i. In addition, for each flow group between these nodes and using this path p, a list of flows is maintained which form this group. Flows that have not been combined with any other flow are treated as groups containing only one flow. For each flow group, an aggregated set of traffic parameters as well as amount of reservation and the delay guarantee (calculated according to equations (3), (4), (5) and (7) above) are maintained. Note that the information needed to be maintained can be greatly reduced if there is always only one path between any two nodes as is the characteristic of most existing IP routing algorithms which are not load sensitive. We now consider three possible cases that can cause the amount of reservation on a path to change. 1. A new flow enters the network. When a new flow enters the network and is routed over a specific path, it is tested whether it can be combined with each existing flow group between the same source receiver pair and using the same path (using the test outlined above). If it cannot be successfully combined with any existing group, then a separate reservation is created for this flow and a new group is created containing only this flow. Else, it is combined with that flow group which will result in the smallest overall increase in reservation. on this path. Finally, the parameters of the selected flow group are updated to reflect the addition of the new flow to the group (again according to equations (3) through (7)). Hence the test for grouping will be performed with every existing flow group on the same path which in the worst case equals the number of flows in the path. 2. An existing flow leaves the network. In practice, this may occur because the reservation refresh message for a particular flow was not received within a specified timeout period or an explicit `disconnect' message was received. Regardless of the disconnecting procedure the same techniques apply. Rampal Expires 20th June, 1997 [Page 7] INTERNET DRAFT Flow Grouping for Guaranteed Service Dec, 1996 The parameters of the group to which this flow was assigned are recalculated taking into consideration only the flows which continue to exist in the group. The new token generation rate r_g and bucket size b_g are calculated by subtracting out the contribution of the flow which is leaving the group. The new P_g and D_g can be calculated by recomputing the maximum and minimum respectively over the remaining flows. An improvement in efficiency can be obtained by using a heap data structure (one each for the maximum packet size and tolerable delay for flows in a flow group). This will speed up the calculations required everytime a flow enters or leaves a group. 3. An existing flow changes its traffic parameters or delay requirements The case of a change in the traffic parameters of a flow can be treated simply as a combination of the above two cases (it is equivalent to a flow with the old parameters leaving the network and another flow with the new parameters subsequently entering the network). 3.1 Performance control policies In order to obtain maximal bandwidth reduction, the network should always attempt to re-assign resources in any of the three cases above 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. 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. As another example, a network may choose to always perform separate reservations when a flow enters a network (in order to obtain fast connection setup), but then, perform the calculations for optimally performing the flow grouping in the background once the flow has already been setup. Several such policies can be defined but are not the main focus of this draft. This contribution of this draft is the flow grouping technique to show that it will always result in some improvement in the utilization for guaranteed service flows as compared to an implementation which does not perform grouping. Rampal Expires 20th June 1997 [Page 8] INTERNET DRAFT Flow Grouping for Guaranteed Service Dec, 1996 An additional comment is that the proposed scheme does not claim to achieve the optimal grouping of flows resulting in the absolute minimal possible reservation for a set of flows. The main point of this contribution is that use of this technique will result in a reservation which is never greater and often lower than when it is not used. 3.2 Some Additional Issues There is nothing in the flow grouping technique that restricts it to be performed only in the case of source-initiated reservation environments (such as ATM [5]) or in receiver-initiated environments (such as RSVP signaling over IP [3]). In any environment where the Guaranteed Service as defined in [1] is provided, this technique can be used to improve utilization. Policing and Implementation Issues 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. An additional issue is that the Guaranteed Service additionally includes two parameters in the traffic specification viz. a minimum policed unit and a peak rate. These parameters are not involved in the delay calculations [1], hence have not been addressed in this draft. We note for the sake of completeness that if needed the peak rate of a flow group should be calculated as being the sum of the peak rates of the individual flows in it, while the minimum policed unit should be calculated as the minimum of the corresponding quantity from all flows in the group. Rampal Expires 20th June 1997 [Page 9] INTERNET DRAFT Flow Grouping for Guaranteed Service Dec, 1996 4.0 References [1] S. Shenker, C. Partridge, R. Guerin "Specification of Guaranteed Service," draft-ietf-intserv-guaranteed-svc-06.txt, Integrated Services Working Group, Internet Engineering Task Force (IETF), August 1996. [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-14.txt, November 1996. [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. 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. Rampal Expires 20th June, 1997 [Page 10] INTERNET DRAFT Flow Grouping for Guaranteed Service Dec, 1996 6.0 Author Information Sanjeev Rampal C305/B664, IBM Networking Division, Research Triangle Park, NC 27709. Voice 919-254-4801 email sanjeev@raleigh.ibm.com Rampal Expires 20th June, 1997 [Page 11] INTERNET DRAFT Flow Grouping for Guaranteed Service Dec, 1996