Applying fair queueing and traffic shaping for Internet applications on top of ATM The LB_SCFQ algorithm

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1 Applying air queueing and traic shaping or Internet applications on top o ATM The LB_SCFQ algorithm Abstract Fair queueing mechanisms give very promising results or ATM networks. Combining a air queueing scheme with a leaky bucket admission policy can give an upper-bound on end-to-end delay. In this paper, we propose a combined algorithm, based on a Sel Clocked Fair Queueing scheme (SCFQ) and a Leaky Bucket policing mechanism (LB) called the LB_SCFQ algorithm. It aims at implementing a traic shaping and a air resource scheduling between connections at the user network interace or Internet applications. We show how conorming parameters can be evaluated to ensure airness in resource sharing as well. We explain how they are related to the ATM traic contract parameters negociated at the ATM connection setup. Results o implementation in a Solaris environment are inally shown. 1 Introduction: Leila Lamti Leila.Lamti@enst-bretagne.r Hossam Aii Hossam.Aii@enst-bretagne.r Télécom Bretagne, Networks and Multimedia Services Department 2 rue de la Châtaigneraie, BP 78, Cesson Sévigné, France A primary role o Queueing mechanisms in ATM switches aims at protecting the network and hence end-systems rom congestion. An additionnal role is to promote eicient use o network resources. Reservation manages resource allocation in each node to meet QoS requirements per connection. This does not under-evaluate the importance o traic shaping at user side. Usage Parameter Control veriies that users conorm their data lows to the negociated traic contract. As mentionned in [4] a well shaped traic source does not require any additionnal re-shaping in intermediate nodes. On the other hand, end-to-end delay depends on the source traic characteristics and the scheduling algorithm at the network switches. Several studies have proved that combining a traic enorcement mechanism with a air queueing algorithm at each ATM switch, can provide an upper bound on delay and another on decrepancy in service among users [1]. Since ATM switches implement CAC and UPC [7], with an appropriate scheduling algorithm, guaranted QoS and perormances can be met [6]. The Sel Clocked Fair Queueing introduced by Golestani [2] is recognized to be one o the most eicient mechanisms or resource scheduling. In act, by serving traic rom dierent sessions in proportion to the assigned service share, the SCFQ creates a mechanism or resource sharing while maintaining a minimum throughput or each backlogged user [1]. In the IP over ATM solution, dierent UDP and TCP lows are usually multiplexed into one unique ATM virtual circuit. It has been shown in several ield trials [9] and conirmed by simulation that these connections suer rom unair bandwidth sharing. Although the multiplexed lows may remain conormant to the UNI/NNI UPC, there is still an unairness problem in the resource allocation or each low. This is due to 1) the application o process scheduling that is independent o available network parameters. 2) The multiplexing o two dierent kinds o data streams having either a closed loop TCP low control or none (UDP). 3) The eect o time lag in TCP connection establishement which advantages early sessions. In this paper, we propose a solution to guarantee airness among users and to ensure traic enorcement to the negociated ATM contract. The idea is to combine a traic shaper with a air queueing algorithm into one single mechanism called the LB_SCFQ. It is based on the Leaky Bucket or traic shaping and on the Sel Clocked Fair Queueing or scheduling. With a traic control implemented at the user side, shaped lows will be submitted to the ATM network. Moreover, air queueing guarantees a air share o local resources among active connections. This paper is organized as ollows: In Section 2, we present the Sel Clocked Fair Queueing algorithm (SCFQ) and the LB policing mechanism. In Section 3, we show how we have combined the SCFQ with the LB into a one unique algorithm (LB_SCFQ). In Section 4, implementation details in the Solaris Operating System are described. We outline diiculties that were encountered due to the system limitations. We show how the LB_SCFQ parameters are related to the ATM traic contract ones. Implementation results are presented at the end o this section. Finally, Section 5 summarizes and concludes this paper. 1

2 2 The basic schemes o the LB_SCFQ algorithm 2.1 The SCFQ algorithm: The SCFQ is based on the notion o system s virtual clock which is viewed as an indicator o work progress in the system. To describe the SCFQ scheme, consider a set o active lows at the user side. We denote R the service share (or weight) assigned to the low. This algorithm relies on computing a service tag or each arriving packet and on serving the queued packets in increasing order o their service tags. It is based on the adoption o an internally generated virtual time as the index o work progress. A ormal proo o the airness properties o SCFQ can be ound in [3]. For a given low, the service tags o dierent packets are recursively computed. For a packet P k : S( P k ) = Max { v(a( P k ) ), S( P k 1 k ) } + L / R (1) where: v( t ) = The service tag o the packet in service at time t. S( P 0 ) = 0. k L = Length o packet P k In the SCFQ algorithm, the current virtual time is simply extracted rom the packet stored at the head o the queue. The only major complexity in the implementation o the SCFQ algorithm is hence the maintenance o a sorted list o packets. Another study has shown, with simulation results, that the air queueing algorithm, when coupled with buering policies, succeeds in guaranteeing the negociated bandwidth to conorming connections. It was also shown that without a proper policing, the beneicial eects o air queueing may be seriously endangered [5]. With the LB algorithm, the burstiness o the total traic at the network edge is controlled. It can hence be used to perorm the necessary policing here-above mentioned. 2.2 The LB agorithm: Within the UNI, connections are policed with the UPC mechanism. A connection momentarily exceeding its traic parameters, may see its cells dropped even i it obeys to its contract over large intervals. To avoid such losses, we propose to introduce at the user side a traic shaper. This shaper is based on the Leaky Bucket scheme. With such an algorithm, we can compute or each packet a theoretical departure time. "Early" packets are hence delayed. The original version o the leaky bucket algorithm LB(I,L) [7] is deined with two parameters: I (Increment) and L (Limit). It can be viewed as a inite-capacity bucket whose real-valued content drains out at a continuous rate o 1 unit o content per time-unit and increases by I or each conorming packet arrival. I at a packet arrival, the content o the bucket is less than or equal to the limit value (L), the packet is conorming. Otherwise, it is not conorming. 3 The LB_SCFQ algorithm We propose or each incoming packet P k o low a Theoretical Departure Time TDT( P k ) and S( P k ), designating its service tag. TDT( P k ) depends on the connection s shaping rate. S( P k ) depends on the system s virtual time and on the the connection weight. Let: T i = Minimum Interarrival between packets or a connection. (The computation o this parameter is explained Section 4.4). Packet P k coming T i seconds ater P k 1 is accepted and blocked otherwise. We can express the Theoretical Departure Time as: TDT( P k ) = Max { A( P k ), TDT( P k 1 ) } + T i (2) 2

3 where: A ( P k ) = Arrival time o packet P k and TDT( P 0 ) = A( P 0 ). The service tag is computed like in (1). It is used as an insertion rank in the packet queue. I two packets P k and P m are j assigned the same service tag, they are stored in a FIFO queue ( see Figure 1 ). At the arrival o a packet, the algorithm ollows these steps: 1)- It computes the service tag and the Theoretical Departure Time o the arriving packet. 2)- It stores the packet in the queue. 3)- The departure server picks rom the queue head the irst packet and compares its TDT to the current system clock. I current TDT is greater or equal to the system clock, then the packet is sent and deleted rom the queue. Otherwise, it is put back into the queue at the same position (note that we deal only with pointers within the kernel space, no eective copies are perormed). System_clock Incoming packets Tagging service Timing service n 1 Departure server Figure 1 The LB_SCFQ algorithm 4 Implementation: A multi-task operating system supports our implementation (SOLARIS 2.5.1). In order to guarantee a minimal number o modiications in existing applications, we have implemented our solution in the kernel space. A brie description o this environment is hence necessary to later discuss the context limitations. 4.1 The streams architecture: A stream is a ull-duplex bidirectionnel processing and data-transer path between a stream driver in the kernel space and a process in the user space. Each connection request initiated by the user process (an tp application or example) is processed by the stream head [8]. A TCP thread (process) is created. In the example (Figure 2), 3 TCP processes are created; each one corresponding to a connection initiated at the user space. All packets (messages in STREAMS terminology) are multiplexed into one unique IP process. This latter creates a hash table entry or each connection to store speciic inormation elements ( source port, destination port etc.. ). 4.2 Operating system limitations: In a previous work [3], we have implemented a separate Leaky Bucket per TCP connection. With such a solution, at each non-conorming packet, the TCP process is blocked via a special kernel routine till the arrival o the theoretical departure time o the packet. Our results have demonstrated that the system clock granularity is not accurate enough to guarantee good perormances. Moreover, a delay was measured between the timer expiration and the real TCP activation ( due to context switching, workstation load etc..). A periodical regulator was then proposed. It was shown to be equivalent to the Leaky Bucket algorithm over long durations and allowed to send periodiodically short term bursts. Since these bursts may be discarded at the UNI by stringent UPC, we propose to improve our solution (accuracy and equivalence towards the LB algorithm) as explained in the ollowing section. 3

4 user process user process user process User Space Kernel Space stream head TCP TCP TCP IP device driver (SA) Figure 2The streams architecture 4.3 Implementation details: As explained in Section 4.1, the IP module multiplexes all arriving packets. In order to ensure a traic shaping and a air share o the available resources (CPU and bandwidth), we propose to implement the LB_SCFQ at the IP module. It is to be noted that inormation relative to a particular low ( R, S( P k 1 ) etc... ) are stored in a hash table in the IP module. Since we can identiy connections to which each packet belongs (source port, destination port,...), we can determine the corresponding hash entry and recuperate inormation about the low. The scheduler processes IP packets. It picks up the irst one P k rom the queue head and compares the current system clock system_clock to TDT( P k ). i TDT( P k ) >= system_clock, then the packet is sent downstream to the ATM driver. Otherwise, it is put back into the queue at the same position. 4.4 Parameters choice: As we implement the LB_SCFQ algorithm at the IP module, we have to take into consideration that an IP packet will be ragmented into a burst o ATM cells by the SAR layer in the ATM card. IP packet T T i Figure 3 : Fragmentation o an IP packet into ATM cells Since we control the inter-arrival time between packets, we must consider an IP packet as an ATM burst o cells. At the ATM level, these cells (resulting rom the ragmentation o an IP packet) arrive with a T time interval (introduced by AAL5). To implement accurate traic shaping, we have choosen traic contract parameters so that bursts can be tolerated. Suppose that: T = 1/PCR (in seconds). = Maximum Burst Size (in cells). 4

5 T s = 1/SCR (in seconds). BT = Burst Tolerance (in seconds). S = An IP packet size. I we consider an IP packet as a burst, then : The Leaky Bucket algorithm is viewed in Figure 4, = S (3) PCR SCR Figure 4 :The LB algorithm This algorithm absorbs packets with a maximum burst size o : = BT Ts T I X(t) designates the instantaneous state o the buer (in cells), then X(t) can be viewed in Figure 5: (4) X(t) t (s) t 1 t 2 Figure 5 : State o the LB buer Ater t 1, the buer reaches its maximum size. As the bucket drains continouesly (with a rate o SCR), it becomes empty when packets are sent at a SCR rate. Ater t 2 rom t 1, the buer is empty. We conclude that: t 1 = ( PCR SCR) t 2 = T s Finally, we have to wait T i = t 1 + t 2 between 2 IP packets. This minimum interarrival time was used in (2) to insure conormity with the Leaky Lucket i implemented at the packet level. To recapitulate, suppose that a VBR connection speciies an ATM contract with the set o parameters PCR, CDVT and SCR 1. Thus, given, Ts and T, BT is set equal to: BT = ( 1) ( Ts T ) (7) As we have to consider an IP packet as a burst, is calculated according to (3). The BT parameter is then computed according to (7). We denote such a connection by C(PCR, SCR, BT); PCR and SCR are expressed in cell/s and BT is expressed in microseconds. Given equations (5) and (6), we can now compute the Ti parameter used in the LB_SCFQ algorithm: Ti = ( 1) T + s ( PCR SCR) (5) (6) (8) 1.Note that in the signalling message, the BT parameter is conveyed through the which is coded as a number o cells. 5

6 4.5 Results: The aim o the LB_SCFQ implementation is to oer the QoS support or Internet applications on top o ATM networks. The challenge here is to oer QoS acilities with a minimal number o modiications to the existent TLI (Transport Layer Interace) or Socket API. Since ATM connections require at least an ATM address and a traic contract, it was necessary to modiy the structure, used in TCP connect() system call, known as sockaddr_in. This structure was modiied to include QoS support and ATM address identiication. The parameters set covers CBR and VBR service classes. At the IP driver, the contract is extracted and the LB_SCFQ parameters are computed. To validate the implementation o our solution, we initiated 3 connections on a client/server architecture across an ATM network. All o the 3 connections are supposed to have very long data transer. IP packets o (S=1400 bytes) are sent. Equation (7) is used to determine the BT parameter (expressed in microseconds). As a result, connections are assigned to these QoS parameters { C1(1000, 2000, 23.2), C2 (2000, 3000, 5.8), C3(9000, 10000, 0.29) }. Equation (8) was used to compute the interarrival between IP packets. Results o such an implementation can be viewed in Figure 6. As shown, air allocation o resource among the three users is ensured. Moreover, each o the 3 connections received its reserved bandwidth. Traic shaping and air scheduling goals are both reached with such an implementation. 5 Conclusion In this paper, we have presented the LB_SCFQ algorithm which oers both traic shaping and air scheduling at the user network interace. We irst explained the SCFQ and the LB algorithms and showed how they were combined into one unique mechanism to insure the two unctionnalities. We have also presented the relation o the LB_SCFQ parameters with ATM traic contract ones. Results o implementation in a Solaris environment are shown. They demonstrate the eectiveness o the proposed algorithm by providing guaranted QoS to dierent connections through a air allocation, as well as a traic shaping ensuring conormance at the UNI. 6 Reerences Bandwidth with LB_SCFQ algorithm (in cell/s) Figure 6: Number o sent IP packets Figure 6 Fair allocation and traic shaping with the LB_SCFQ algorithm. Connection C1 Connection C2 Connection C3 [1] S.J. Golestani: Fair Queuing Algorithms or Packet Scheduling in BISDN. In Proceedings o IZS 96. [2] S.J. Golestani: A Sel-Clocked Fair Queueing Scheme For Broadband Applications - In Proceedings o INFOCOM 94. [3] L.Lamti, H.Aii, M.Hamdi: Design and implementation o a lexible traic controller or ATM connections - Will appear in Proceedings o IFIP HPN 97. [4] J.Y. Le Boudec: Network Calculus Made Easy - Technical Report December 1996 (DI 96/218). [5] A.J. Marsan, C.Casetti, M.Munao, J.Valdes: Fair queueing in ATM networks: A Simulation Study. 6

7 [6] A.K. Parekh and R.G. Gallager: : A generalized processor sharing approach to low control in integrated services networks. The multiple node case. In proceedings IEEE INFOCOM 93 pages , [7] S.Sathaye: The ATM Forum Traic Management Speciications, version 4.0 ATM Forum/ R6 April 96. [8] SOLARIS Streams Programmer s guide (2.5) - SunSot. [9] t-ten - Available at URL 7