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Chapter 13

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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. Why Typical Tools Are Inadequate for ATM • Majority of ATM traffic sources are not amenable to flow control • Long-haul ATM ttrans << tprop … slow feedback • Broad range of application types, flow control may indiscriminately penalize some • bandwidth requirements • traffic patterns • service requirements • Very high-speed switching increases volatility re: control mechanisms Chapter 13: ATM Traffic & Congestion Control

  4. Latency/Speed Effects • Issue: rapid insertion rate of ATM cells vs. relatively long round-trip delays • small size (53 bytes) of ATM cell • small switching delays • high bandwidth links in ATM networks • Simplified Example: • 150 Mbps data rate • 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

  5. 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 queuing and processing • variation at UNI (user network interface) due to cell processing Chapter 13: ATM Traffic & Congestion Control

  6. General Procedure: Time Reassembly of 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

  7. 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

  8. 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 • Unspecified Bit Rate (UBR) • best-effort service • Available Bit Rate (ABR) • guaranteed minimum capacity, with bursts • Guaranteed Frame Rate (GFR) • like UBR/ABR, expressed in terms of frame rate Chapter 13: ATM Traffic & Congestion Control

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

  10. Traffic Parameters • Connection Traffic Descriptor • Source Traffic Descriptor: PCR, SCR, MBS, MCR, MFS • 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.) • Conformance Definition: unambiguous specification of conforming cells of a connection at the UNI (see GCRA, later) Chapter 13: ATM Traffic & Congestion Control

  11. 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

  12. QoS Parameters (as defined by the ATM Forum) • Peak-to-peak cell delay variation (CDV): acceptable delay variation at destination • 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

  13. Cell Transfer Delay Probability Density (real-time services) Variable component of delay, due to buffering and cell scheduling. Chapter 13: ATM Traffic & Congestion Control

  14. Congestion Control and Other Attributes • Congestion Control • defined only for ABR service 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

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

  16. Traffic & Congestion Control Function Classification 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

  17. Chapter 13 – Section 2 ATM Traffic Control (continued)

  18. Resource Management Using Virtual Paths • Multiple VCCs with different QoS requirements in same VPC • Cases to consider: • User-to-User application: VPC between pair of UNIs, QoS is user’s responsibility • User-to-network application: VPC between UNI and network node, QoS is network’s responsibility • Network-to-network application, QoS is network’s responsibility Chapter 13: ATM Traffic & Congestion Control

  19. Example VCC/VPC Configuration • How to allocate capacity to each VPC? • Aggregate peak demand • Statistical multiplexing Chapter 13: ATM Traffic & Congestion Control

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

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

  22. Usage Parameter Control Function Location Chapter 13: ATM Traffic & Congestion Control

  23. 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 to reduce “clumping” • Reduce delays, ensure fair resource allocation Chapter 13: ATM Traffic & Congestion Control

  24. 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

  25. Leaky Bucket Algorithm Chapter 13: ATM Traffic & Congestion Control

  26. GCRA: Continuous-State Leaky Bucket • Equivalent to virtual scheduling • Max Bucket capacity is T+ • Counter X is incremented by T for each compliant cell • Bucket decrements (drains at 1 unit/unit time GCRA (I, L): I = Increment L = Limit ta(k) = Time of arrival of a cell X =Value of leaky bucket counter X = Auxiliary variable LCT = Last compliance time At the time of arrival ta(1) of the first cell, X = 0 and LCT = ta(1) Chapter 13: ATM Traffic & Congestion Control

  27. Cell Arrival at UNI (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

  28. 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 are allowed • s is derived from the burstiness of the traffic stream: Burst Tolerance = s = (MBS-1) - Chapter 13: ATM Traffic & Congestion Control

  29. 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

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

  31. 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

  32. 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

  33. 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

  34. 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

  35. Variations in Allowed Cell Rate 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 RM Cell Format Chapter 13: ATM Traffic & Congestion Control

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

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