Traffic Control Functions in ATM Networks Byung G. Kim Payoff

Transcription

1 Traffic Control Functions in ATM Networks Byung G. Kim Payoff A standard monitoring algorithm for two traffic types--constant bit rate (e.g., voice) and variable bit rate (e.g., compressed video)--can be used to improve traffic flow control in asynchronous transfer mode (ATM) networks. Various control algorithms for available bit rate traffic (e.g., data) are also suggested. Introduction Asynchronous transfer mode (ATM) is the universal transport vehicle for multimedia traffic in the context of the Broadband Integrated Services Digital Network(B-ISDN) specification. Asynchronous Transfer Mode is a connection-oriented technology that transports data in 53-byte cells. To accommodate different types of connections, four service classes are currently defined in ATM networks: Constant bit rate (CBR). Variable bit rate (VBR). Available bit rate (ABR). Unspecified bit rate (UBR). ATM Services Constant Bit Rate Service CBR service is designed for connections that require a fixed amount of bandwidth for the entire connection time. The amount of bandwidth is equivalent to the peak rate of the connection. CBR service is intended to support real-time applications with stringent requirements for transfer delays and delay variations (e.g., digitized voice and Px64 video). Variable Bit Rate Service VBR service, on the other hand, is designed for those sources that generate cells at timevarying rates. Real-time Variable Bit Rate service provides tight bounds on cell delays and delay variations, whereas non-real-time VBR service provides a low cell loss ratio. Examples of VBR connections are a Motion Picture Experts Group Full-Motion Video source producing heavier traffic at scene changes and a bursty file transfer connection, occasionally sending a large amount of data. VBR service uses statistical multiplexing, by which the available bandwidth not used by light-traffic connections is taken advantage of by connections with temporarily heavy traffic. However, there is always a risk that the given VBR capacity may be saturated when many sources generate heavier traffic than expected. The VBR capacity and the buffer space should be carefully designed and managed so that occurrences of such capacity saturation are strictly controlled. Because of the statistical nature of traffic

2 fluctuation in VBR sources, the management of network resources is usually difficult for VBR service. Available Bit Rate Service The ABR service class is designed primarily for LAN traffic across an Asynchronous Transfer Mode network. The bandwidth left unallocated to CBR and VBR sources is made available to ABR sources for sharing. An ABR source is required to regulate its transmission rate according to the prevailing congestion condition in the network. Control cells that carry information about congestion, called resource management (RM) cells, are periodically inserted into the cell stream so that the network condition can be acquired from returning RM cells. A network guarantees a low cell loss ratio and a fair sharing of ABR bandwidth for those end systems that adapt their transmission rate according to the feedback information in RM cells. Unspecified Bit Rate Service UBR service demands no specific Quality Of Service from the network and is thus handled only after service requirements of CBR, VBR, and ABR connections are satisfied. CBR and VBR Traffic Control An Asynchronous Transfer Mode source declares its expected traffic pattern in the source traffic descriptor. The source traffic descriptor includes the peak cell rate (PCR), the sustainable cell rate (SCR), and burst tolerance (BT). The source then requests a connection by submitting connection traffic parameters to the network, which consist of the source traffic descriptor, the cell delay variation (CDV) tolerance, and the conformance definition. Conformance Definition Using GCRA As soon as a connection is established, the ATM network starts the usage parameter control function to monitor the actual traffic generated from the source against the source traffic parameters declared for the connection request. This function enforces the service contract so that network resources are protected from malicious or unintentional misbehavior of the source. For an unambiguous specification of the conformance definition, the generic cell rate algorithm (GCRA) is devised. For each cell arrival, the GCRA determines whether the cell conforms to the declared source traffic parameters. The GCRA can thus be used as a formal definition of traffic conformance test for a connection. However, the network is not obliged to use this algorithm for usage parameter control (UPC) as long as the operation UPC does not violate the Quality Of Service objectives of compliant connections. The GCRA defines the relationship between the peak cell rate and the cell delay variation tolerance, as well as the relationship between the sustainable cell rate and the burst tolerance. The GCRA is based on two parameters, the increment I and the limit L, and is denoted by GCRA(I,L). For example, let t a (k) denote the arrival time of the k-th cell in a connection. The basic idea of the GCRA is to have a variable the theoretical arrival time (TAT) keep track of the expected arrival time of the next subsequent cell. When TAT is updated for the k-th cell, three cases are handled:

3 Case 1: < or = TAT ta(k). The cell arrival time is on or after the expected arrival time. This cell is arriving at a slower pace than expected and is thus conforming to the source traffic parameters. TAT is updated to t a (k) + I. Case 2: TAT-L < or = t a (k) < TAT. The cell is arriving before the expected arrival time but within the limit of L. Although the cell is generated slightly ahead of the schedule, the cell is considered to be conforming. TAT is updated to TAT + I. Case 3: t a (k) < TAT - L. The cell arrived too early and is not conforming. TAT remains unchanged. The nonconforming cell may be either discarded or tagged to a lower priority. Traffic Policing This section explains how the generic cell rate algorithm is used to police cells from a CBR source. For simplicity, the time to transmit an ATM cell is set to 1. The GCRA is specified by GCRA(T c, [τ]), where T c and [τ] denote the contracted cell interarrival times and the cell delay variation tolerance, respectively. Suppose the CBR source violates the contract by generating cells at the fixed intervals T i, though slightly faster than the contracted interval T c (T i < T c ). If [τ] = 0, then Exhibit 1 illustrates that TAT is updated by T c and that cells are arriving at every T i. One extra cell appears within the increment interval of T c. Because the CDV tolerance ([τ]) is 0, this extra cell is policed, resulting in policing of every other cell. When policed cells are discarded, the cell loss ratio (CLR) becomes 0.5. For [τ] > 0, suppose TAT is updated from the arrival time of the k-th cell. If the (k+d)-th cell is to be policed by GCRA (T c, [τ]),the arrival time of the (k+d)-th cell should be within the margin of CDV tolerance [τ] of TAT for the (k+d)-th cell. An example with d = 3 is depicted in Exhibit 2. Cell k+3 is shown to be policed because its arrival time is earlier than the GCRA limit [τ]. At an arbitrary value of d, the (k+d)-th cell is discarded when d Ti < d T c [τ]. When policed cells are discarded, this amounts to a single cell loss in (d+1) cell arrivals. CLR changes in a stepwise fashion as T i decreases, as shown in Exhibit 3. Exhibit 1. Interval Updating with GCRA Exhibit 2. Interval Updating for a CBR Source With GCRA Exhibit 3. Cell Loss Rate of a CBR Source by GCRA Policing of a Variable Bit Rate source is performed on the basis of the sustainable cell rate (SCR). The SCR specifies the upper bound on the average cell rate and can be significantly lower than the peak cell rate. The time intervals between two successive cells arriving at the average and the peak rates are denoted by T s and T p, respectively. They are inverses of sustainable and peak rates. A burst is defined as a group of cells generated at the peak cell rate. The burst tolerance determines the maximum burst size, b, that may be transmitted at the peak rate.

4 Assessment of Monitoring Techniques During a connection establishment process, the ATM network has to reserve sufficient resources to meet the quality of service demand for the connection. Once admitted, the actual incoming traffic is policed to keep the traffic entering the network in conformance to the connection traffic parameters. The usage parameter control, or UPC, procedure is a preventive control mechanism so that a potential overload from a source is not permitted beyond the preestablished limits set by the connection parameters. However, a cell stream from a source may be altered along the path as it is mixed with other cell streams. The UPC procedure has to be flexible enough to allow for some deviations from the connection parameters by including cell delay variation tolerance and burst tolerance parameters. Because a cell stream conforming to the admission contract can be altered after a multiplexing, traffic shaping may be necessary to preserve the characteristics of the original cell stream. For example, a nonzero CDV tolerance allows the UPC to accept some cells from a CBR source, although they are spaced closer than the contracted interval. After the UPC checks for the conformance, it may be desirable to restore cell stream into the one matching the admission contract. A need for traffic shaping is strengthened from some results, which indicate that individual cell streams become burstier after a series of GCRA control actions. Although a usage parameter control algorithm such as the GCRA may be easily implemented, the traffic shaping is a costly operation since some history of a cell stream has to be maintained if a cell stream is to be reconstructed. Furthermore, some buffering may be needed as the cell stream is restored. The buffering per virtual channel (VC) is exactly what an ATM node wanted to avoid, however. ABR Traffic Control ABR traffic control relies on cooperative interworking among three components: The source end system (SES). The Asynchronous Transfer Mode switch. The destination end system (DES). Briefly, their control actions are as follows: An SES keeps track of the best estimate for its cell transmission rate, or allowed cell rate (ACR),according to the perceived congestion status in the network. To obtain the congestion status in the network, the SES periodically inserts a resource management cell in its data cell stream. The DES returns the RM cell back to the SES so that the network status is conveyed back to the SES. In the backward RM cell, a single congestion indication (CI) bit may be set for the SES to adjust its cell transmission rate. Optionally, an ATM switch may determine the best cell rate for an SES and convey it as the explicit rate in the backward resource management cell. Source and Destination Behavior When a source sends the first cell after connection setup, or after not sending any cells for T tm seconds or longer, it sends a forward RM cell to obtain the network congestion status. After every N rm -1 data cells, the source inserts a forward RM cell according to the prevailing ACR. When a backward RM cell is received with CI = 1, then ACR (i.e., the

5 allowed cell rate) is reduced at least to ACR RDF, where RDF is the rate reduction factor and is a constant (1/16, for example). On the other hand, if CI = 0 in the RM cell, ACR is increased by a constant additive increase rate. After the ACR is adjusted according to the CI bit, the new ACR is taken as the smaller of the updated ACR and the explicit rate stored in the RM cell. Destination behavior is relatively simple. Every ATM cell has an explicit forward congestion indication (EFCI) bit in its header. An ATM switch may set the EFCI bit if its buffer occupancy exceeds a certain threshold. The destination end system saves the EFCI state stored in the ATM header (i.e., the payload type field) of an incoming data cell. Upon receiving a forward RM cell, the DES retransmits the RM cell back to the SES after setting the direction of the cell from forward to backward and copying the saved EFCI state to the CI bit field in the RM cell. Namely, the CI bit from DES is set by the congestion status experienced by the most recent data cell. Switch Behavior A switch may use at least one feedback mechanism to control congestion. It must be pointed out that this is not an area of standardization and that it is purely up to the switch manufacturer or service provider to decide which scheme to implement. Binary Feedback The binary feedback scheme is based on the EFCI bit. A switch monitors the queue length and marks the EFCI bits of passing ATM cells, as long as the queue length exceeds a predefined queue threshold value. The binary feedback scheme is known to suffer from a potential unfairness when all connections share a common queueing buffer. Typically, a connection with more hops has a better chance of running into a congested switch than those with a smaller number of hops. Explicit Rate Feedback The explicit rate feedback scheme can provide the fairness by having each switch determine the suitable rate for an SES not to overload the switch. This rate is sent to the SES as the explicit rate. At the same time, each SES declared its prevailing cell rate in the current cell rate (CCR) field of an RM cell. What is the entity for this symbol? The enhanced proportional rate control algorithm (EPRCA) computes the mean ACR (MACR) among all connections as a running exponential weighted average(macr = (1 α)macr + αccr). A typical value of α is chosen to be 1/16 and CCR is taken from the passing RM cell. The fair share is then taken as a fraction (e.g., 7/8) of MACR, and any SES sending more than the fair share is asked to reduce its rate by setting the explicit rate field in the backward RM cell to the fair share. Congestion Avoidance Congestion avoidance schemes monitor the prevailing load according to the load factor z = Input_Rate/Target_Rate. The input rate is measured over a fixed averaging interval, whereas the target rate is set slightly below the ABR bandwidth. There are two variations of this scheme that differ in the manner by which the fair share or the explicit rate is computed. In the explicit rate indication for congestion avoidance (ERICA) algorithm, the fair share is given as the target rate divided by the

6 number of active connections. A switch determines the explicit rate as ER = max(ccr/z, Fair-Share). ER is updated periodically using current cell rate information from the forward RM cells and the load during the averaging interval. The ERICA algorithm attempts to guarantee at least the fair share amount of capacity to each active connection. Any excess capacity is distributed to active sources in proportion to their rates. The congestion avoidance using proportional control (CAPC) algorithm, on the other hand, does not update ER for each connection, but instead uses the same ER for every connection. ER in CAPC is computed and updated from the fair share. If z < 1, Fair- Share = Fair-Share *(1 + (1-z)*R up ). Otherwise, Fair-Share *(1 (z-1)*r dn ). In these cases, R up and R dn are constant slope parameters to increase and decrease the rate (the amount of changes allowed in each update is limited as well). Assessment of ABR Control In general, it is difficult to draw a general comparison among a number of ABR control schemes, since source and switch behaviors are affected by many factors, including the network topology. In order to have some idea about the effectiveness of different control strategies, three control algorithms are considered: 1. Binary feedback control. 2. EPRCA. 3. ERICA. Three ABR sources send cells to the switch buffer. They are located at 1, 10, and 100 km from the switch. Exhibits 4, 5, and 6 plot changes in ACRs in three sources. Exhibit 4. ACR According to the Binary Feedback Control Exhibit 5. ACR Changes in EPRCA Exhibit 6. ACR Changes in ERICA Clearly, the simple binary feedback algorithm shows a significant oscillation, whereas the oscillation is eliminated in ERICA. The cause of oscillation is that in a high-speed network such as an ATM network, the latency for feedback information can be significant. By the time, the source recognizes congestion in the network, a few megabytes of data may already have been sent out. When in congestion, every source attempts to reduce its rate, producing an excess capacity a short time later. When not in congestion, every source attempts to transmit a little bit more, producing a congestion a short time later. In general, however, oscillation of cell rates does not produce an adverse effect. Oscillation simply means that a source cannot send at a constant rate for an extended period of time. Both the latency and the rate oscillation will not be of any concern in a LAN environment where the delay and reaction time are considerably short. As soon as LANs

7 are connected over a long-distance, however, control parameters should be tuned to account for potential oscillatory behavior. Conclusion In summary, network techniques to recover from congestion and limit it, such as usage parameter control and traffic policing, are successfully applied to Asynchronous Transfer Mode Constant Bit Rate and Variable Bit Rate service. The generic cell rate algorithm, as specified by the ATM Forum, achieves traffic shaping, ensuring that traffic matches the service negotiated between users and the network during connection establishment. Techniques to avoid congestion, recover from congestion, and ensure flow control in ATM available bit rate service include the use of resource management cells and switch behavior. Author Biographies Byung G. Kim is an associate professor in the Computer Science Department at the University of Massachusetts at Lowell MA. He can be reached at kim@cs.uml.edu.

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