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Chapter 13

Chapter 13. ATM Traffic & Congestion Control. Introduction. ATM congestion problem overview ITU-T and ATM Forum framework for control of delay-sensitive traffic ATM traffic control mechanisms ATM congestion control Congestion control schemes for bursty traffic (ABR and GFR).

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Chapter 13

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  1. Chapter 13 ATM Traffic & Congestion Control

  2. Introduction • ATM congestion problem overview • ITU-T and ATM Forum framework for control of delay-sensitive traffic • ATM traffic control mechanisms • ATM congestion control • Congestion control schemes for bursty traffic (ABR and GFR) Chapter 13: ATM Traffic & Congestion Control

  3. ATM Service Categories • Constant Bit Rate (CBR) • fixed data rate required at guaranteed capacity • Real-Time Variable Bit-Rate (rt-VBR) • tightly constrained delay and delay variation • sustained rate & guaranteed fast burst rate • Non-Real-Time Variable Bit-Rate (nrt-VBR) • no delay variation bound, cell loss ratio only • Available Bit Rate (ABR) • guaranteed minimum capacity, with bursts • Guaranteed Frame Rate (GFR) • like UBR/ABR, expressed in terms of frame rate • Unspecified Bit Rate (UBR) • best-effort service Chapter 13: ATM Traffic & Congestion Control

  4. Why Typical Traffic Control Schemes Are Inadequate for ATM • Majority of ATM traffic sources are time-sensitive, and not amenable to typical flow control schemes (e.g. CBR, rt-VBR) • For long-haul ATM: ttrans << tprop … slow feedback (latency/speed effects) • Due to the broad range of ATM application types, flow control may indiscriminately penalize some • bandwidth requirements (kbps to Mbps) • traffic patterns (CBR, VBR) • service requirements (delay/loss sensitivity) • Very high-speed switching increases volatility re: control mechanisms Chapter 13: ATM Traffic & Congestion Control

  5. ATM Performance considerations • Two key issues must be addressed for CBR and real-time VBR traffic… • Latency/speed effects for long-haul networks: Cause - ttrans << 2 x tprop (or, one RTT) Approach – fast feedback mechanisms • Cell delay variation: Cause - Variation at user-network interface and in network core Approach: time reassembly of CBR cells at receiver Chapter 13: ATM Traffic & Congestion Control

  6. Latency/Speed Effects • Issue: rapid insertion rate of ATM cells vs. relatively long round-trip delays • small size (53 bytes) of ATM cell • high bandwidth links in ATM networks • small switching delays • Simplified Example: • 150 Mbps data rate ( SONET OC-3) • tinsert = ttrans = = 2.8 x 10-6 seconds • U.S. coast-to-coast roundtrip dprop = 48 msec • Then the number of cells inserted (N) during RTT = = 1.7 x 104 cells = 7.2 million bits 53 x 8 bits 150 x 106 bps 48 x 10-3 seconds 2.8 x 10-6 seconds Chapter 13: ATM Traffic & Congestion Control

  7. Cell Delay Variation • General requirement: delay should be short • ATM designed to minimize delay • For some applications, rate of delivery of cells to destination must be constant (ATM’s CBR service level) • Contributors to cell delay variation • network contribution: queuing and processing variations • variation at UNI (user network interface) due to cell processing Chapter 13: ATM Traffic & Congestion Control

  8. Per I-371 Origins of Cell Delay Variation at UNI Interleaving prior to delivery to physical layer Further delays possible at the physical layer Chapter 13: ATM Traffic & Congestion Control

  9. Cell Delay Variation: CBR Cells  = 1/R = inverse of insertion rate V(0) = est. tolerable delay variation cell arrives late: discarded Slope = R cells/sec = 1/  cell insertion rate D(i) = end-to-end delay for the ith cell V(i) = V(i-1) – [ti – (ti-1 + )] Chapter 13: ATM Traffic & Congestion Control

  10. ATM Attributes How we describe an ATM traffic flow • Traffic parameters • QoS parameters • Congestion (for ABR) • Other (for UBR) Chapter 13: ATM Traffic & Congestion Control

  11. Traffic Parameters • Connection Traffic Descriptor • Source Traffic Descriptor: PCR, SCR, MBS, MCR, MFS (more on next slide) • Cell Delay Variation Tolerance (): upper bound on amount of cell delay that is introduced by the network interface and the UNI (due to interleaving, physical layer overhead, multiplexing, etc.) • Note- this is the value V(0) from our earlier discussion of CDV • Conformance Definition: unambiguous specification of conforming cells of a connection at the UNI (see GCRA, later) Chapter 13: ATM Traffic & Congestion Control

  12. Traffic Parameters • Source Traffic Descriptor • Peak Cell Rate (PCR): upper bound on traffic submitted by source (PCR = 1/T, where T = minimum cell spacing • Sustainable Cell Rate (SCR): upper bound on average rate of traffic submitted by source (over a larger T) • Maximum Burst Size (MBS): maximum number of cells sent continuously at PCR • Minimum Cell Rate (MCR): used with ABR and GFR… minimum cell rate requested, access to unused capacity up to PCR (elastic capacity = PCR-MCR?) • Maximum Frame Size (MFS): maximum size of a frame in cells available for GFR service Chapter 13: ATM Traffic & Congestion Control

  13. QoS Parameters (negotiated between user and network during connection set-up, as defined by the ATM Forum) • Peak-to-peak cell delay variation (CDV): acceptable delay variation at destination – the difference between the best case and worst case CTD • Maximum Cell Transfer Delay (maxCTD): maximum time between transmission of first bit of a cell at the source UNI to receipt of its last bit at the destination UNI • Cell Loss Ratio: ratio of lost cells to total transmitted cells on a connection Chapter 13: ATM Traffic & Congestion Control

  14. Variable component of delay, due to buffering and cell scheduling. Propagation through physical media. Fraction of cells that exceed Maximum and will be discarded or delivered late. Cell Transfer Delay Probability Density (real-time services) Chapter 13: ATM Traffic & Congestion Control

  15. Congestion Control and Other Traffic Attributes • Congestion Control • defined only for ABRservice category • uses network feedback controls • ABR flow control mechanism (more later) • Other Attributes (introduced July 2000) • Behavior class selector (BCS): • for IP differentiated services (DiffServ) • provides for different levels of service among UBR connections • implementation dependent, no guidance in specs • Minimum desired cell rate (MDCR): • UBR application minimum capacity objective Chapter 13: ATM Traffic & Congestion Control

  16. Service Category-Attribute Relationship Chapter 13: ATM Traffic & Congestion Control

  17. Traffic & Congestion Control Function Classification – A Framework affect more than one connection, effective over long timeframe determine if/how network can accommodate connection at a given QoS network responds within the round-trip lifetime of a cell react immediately to a cell as it is transmitted Chapter 13: ATM Traffic & Congestion Control

  18. Resource Management Using Virtual Paths • Multiple VCCs with various QoS requirements in same VPC • Cases to consider: • User-to-User application: VPCs between pairs of UNIs, VCC QoS is user’s responsibility… user must ensure that aggregate of VCCs does not exceed capacity allocated to VPC • User-to-network application: VPC between UNI and network node, network must accommodate QoS of individual VCCs • Network-to-network application:network must accommodate QoS of individual VCCs Chapter 13: ATM Traffic & Congestion Control

  19. Resource Management Using Virtual Paths • Performance (QoS) of a VCC depends on resources allocated to the VPC(s) through which the VCC extends • Network allocates capacity to each VPC based on performance objectives agreed between network and subscriber (contract). Two approaches: • Aggregate peak demand – VPC capacity (e.g. data rate) set to sum of peak data rates of all VCCs • Statistical multiplexing – VPC capacity set to be greater than or equal to average demand for all VCCs, but less than aggregate peak demand Chapter 13: ATM Traffic & Congestion Control

  20. Example VCC/VPC Configuration Chapter 13: ATM Traffic & Congestion Control

  21. Connection Admission Control (CAC) • Network accepts the connection only if it can commit resources - in both directions - that satisfy a given connection request: • Service category (CBR, rt-VBR, …) • Connection traffic descriptor (PCR, …, CDVT, conformance definition) • QoS (peak-to-peak CDV, max CTD, CLR) • Cell loss priority (CLP bit 0 or 0+1) • If connection is accepted, a “traffic contract” is awarded to the user Chapter 13: ATM Traffic & Congestion Control

  22. (a.k.a. jitter) Connection Admission Control (Traffic Contract) Chapter 13: ATM Traffic & Congestion Control

  23. Contract Contract Connection Admission Control (CAC) ATM Network Contract • Traffic Parameters • Peak cell rate • Sustainable cell rate • Burst tolerance • Etc. • Quality of Service • Delay • Jitter • Cell loss Chapter 13: ATM Traffic & Congestion Control

  24. Usage Parameter Control • Purpose: after a connection is established, protect the network’s resources from overload/abuse by violating connections • Monitors connection for conformance to the traffic contract • detect violation of assigned parameters based on conformance definition agreed to in contract • take appropriate action Chapter 13: ATM Traffic & Congestion Control

  25. Contract ?DECISION? • PASS • MARK CLP BIT • DROP ATM Network Usage Parameter Control Policing You are Not in Conformance with the Contract. What Should the Penalty Be?? REBEL APPLICATION Chapter 13: ATM Traffic & Congestion Control

  26. Usage Parameter Control Function Location Case 2 Case 1 Chapter 13: ATM Traffic & Congestion Control

  27. UPC Traffic Management • Peak Cell Rate Algorithm • Regulates the peak cell rate and the associated CDVT of a connection • Sustainable Cell Rate Algorithm • Regulates the sustainable cell rate and associated burst tolerance of a connection • Traffic Shaping • Smoothes out traffic at network entry points to reduce “clumping” • Reduce delays, ensure fair resource allocation Chapter 13: ATM Traffic & Congestion Control

  28. Generic Cell Rate Algorithm (GCRA): Virtual Scheduling GCRA (I, L): I = Increment L = Limit ta(k) = Time of arrival of a cell TAT = Theoretical arrival time At time of arrival ta(1) of the first cell of connection, TAT = ta(1) Late arrival OK Early arrival TEST Early arrival beyond limit NOT OK • Algorithm takes two arguments, I and L • With PCR of R, I = T = 1/R • CDVT limit,  = L • Then peak cell rate algorithm is expressed as: GCRA(T, ) I.e., this cell arrived too late. Early arrival within limit OK Reset TAT Chapter 13: ATM Traffic & Congestion Control

  29. Cell Clumping Cell Clumping Effect of CDVT on Cell Arrival at UNI (example: T=4.5) Ideal ( = 0.5) Possible ( = 1.5) Possible ( = 3.5) Note:  = T -  N = 1 + ( / T-) Possible ( = 7) Note:  > T -  N = 1 + ( / T-) Chapter 13: ATM Traffic & Congestion Control

  30. 1 1 SCR PCR Sustainable Cell Rate Algorithm • Uses GCRA (Ts, s), where: Ts = 1/Rsis the interarrival time at the sustainable cell rate, Rs, and s is the burst tolerance, or the time scale during which cell rate fluctuations (at PCR) are allowed • s is derived from the burstiness of the traffic stream: Burst Tolerance = s = (MBS-1) - Chapter 13: ATM Traffic & Congestion Control

  31. 1 1 SCR PCR Sustainable Cell Rate Algorithm • Note that, if the traffic stream is constrained by both GCRA (T, ) and GCRA (Ts, s) , then Maximum Burst Size (MBS) is: • s is derived from the burstiness of the traffic stream: Burst Tolerance = s = (MBS-1) - s Ts-T MBS = 1 + Chapter 13: ATM Traffic & Congestion Control

  32. UPC Function: Possible Actions based on CLP bit (dual CLP) Forward cell or discard it? (see p. 377) P? = Compliance test for parameter P Chapter 13: ATM Traffic & Congestion Control

  33. Token Bucket for Traffic Shaping Tokens are generated and fill the bucket at the constant rate, . To pass, a token is removed from the bucket. If bucket is empty, cell is queued to wait for next token. Departures rate is “smoothed” to . Chapter 13: ATM Traffic & Congestion Control

  34. ABR Traffic Management • CBR, rt-VBR, nrt-VBR: traffic contract with open-loop control • UBR: best effort sharing of unused capacity • ABR: share unused (available) capacity using closed-loop control of source • Allowed Cell Rate (ACR): current max. cell transmission rate • Minimum Cell Rate (MCR): network guaranteed minimum cell rate • Peak Cell Rate (PCR): max. value for ACR • Initial Cell Rate (ICR): initial value of ACR Chapter 13: ATM Traffic & Congestion Control

  35. ABR Traffic Management • ACR is dynamically adjusted based on feedback to the source in the form of Resource Management (RM) cells • RM cells contain three fields: • Congestion Indication (CI) bit • No Increase (NI) bit • Explicit Cell Rate (ER) field Chapter 13: ATM Traffic & Congestion Control

  36. Flow of Data and RM Cells – ABR Connection Nrm parameter usually set to 32 FRM cell flow  BRM Cell flow Chapter 13: ATM Traffic & Congestion Control

  37. ABR Source Reaction Rules RIF – Fixed rate increase factor (default 1/16) RDF – Fixed rate decrease factor (default 1/16) Chapter 13: ATM Traffic & Congestion Control

  38. Variations in Allowed Cell Rate RIF = 1/16 RDF = 1/4 Chapter 13: ATM Traffic & Congestion Control

  39. ABR RM Cell Format Chapter 13: ATM Traffic & Congestion Control

  40. Initial Values of Cell Fields Chapter 13: ATM Traffic & Congestion Control

  41. ABR Parameters Chapter 13: ATM Traffic & Congestion Control

  42. ABR Capacity Allocation • Two Functions of ATM Switches • Congestion Control: throttle back on rates based on buffer dynamics • Fairness: throttle back as required to ensure fair allocation of available capacity between connections • Two categories of switch algorithms • Binary: EFCI, CI and NI bits • Explicit rate: use of the ER field Chapter 13: ATM Traffic & Congestion Control

  43. Binary Feedback Schemes • Single FIFO queue at each output port buffer • switch issues EFCI, CI, NI based on threshold(s) in each queue • Multiple queues per port - separate queue for each VC, or group of VCs • uses threshold levels as above • Use selective feedback to dynamically allocate fair share of capacity • switch will mark cells that exceed their fair share of buffer capacity Chapter 13: ATM Traffic & Congestion Control

  44. Explicit Rate Feedback Schemes • Basic scheme at switch is: • compute fair share of capacity for each VC • determine the current load or degree of congestion • compute an explicit rate (ER) for each VC and send to the source in an RM cell • Several example of this scheme • Enhanced proportional rate control algorithm (EPRCA) • Explicit rate indication for congestion avoidance (ERICA) • Congestion Avoidance using proportional control (CAPC) Chapter 13: ATM Traffic & Congestion Control

  45. EPRCA • Switch calculates mean current load on each connection, called the MACR: MACR(I) = (1-) x MACR(I-1) +  x CCR(I) Note: typical value for  is 1/16 • When queue length at an output port exceeds the established threshold, update ER field in RMs for all VCs on that port as: ER  min[ER, DPF x MACR] where DPF is the down pressure factor parameter, typically set to 7/8. • Effect: lowers ERs of VCs that are consuming more than fair share of switch capacity Chapter 13: ATM Traffic & Congestion Control

  46. ERICA • Makes adjustments to ER based on switch load factor: Load Factor (LF) = Input rate /Target rate where input rate is averaged over a fixed interval, and target rate is typically 85-90% of link bandwidth • When LF > 1, congestion is threatened, and ERs are reduced by VC on a fair share basis: • Fairshare = target rate/number of VCs • Current VCshare = CCR/LF • newER = min[oldER, max[Fairshare, VCshare]] Chapter 13: ATM Traffic & Congestion Control

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