ATM Traffic Management, Congestion Control, Traffic Engineering

1. ATM Traffic Management, Congestion Control, Traffic Engineering

2. ATM Traffic Management and Control • Traffic Contract • Traffic Control • Congestion Control • Traffic Engineering BISDN/黃仁竑副教授

3. Traffic Contract • The traffic contract is an agreement between a user and a network accross a UNI regarding the following spaects of any VPC or VCC ATM cell flow: • QOS that a network is expected to provide • Traffic parameters that specify characteristics of the cell flow • The conformance checking rules • The network’s definition of a compliant connection BISDN/黃仁竑副教授

4. Reference Model • End-to-end QOS reference model • QOS commitments are probabilistic in nature and are approximated public/ private UNI public/ private UNI public ATM network public ATM network End User End User Network A QOS NetworkBQOS End-to-end QOS BISDN/黃仁竑副教授

5. Quality of Service • QOS is defined by specific parametersfor cells conforming to the traffic contract • QOS is defined in terms of following cell transfer outcomes • Successful cell transfer outcome • succefully received within a specified time • Errored cell outcome: received cell content differs from the origin • Lost cell outcome: original cell is not received • Misinserted cell outcome • a received cell has no corresponding transmitted cell • Severly-errored cell block outcome • more than M errored cell, lost cell, or misinserted cell outcomes are observed in a received cell block of N BISDN/黃仁竑副教授

6. QOS Parameters • Negotiable parameters • Peak-to-peak Cell Delay Variation (CDV) • Maximum Cell Transfer Delay (Max CTD) • Mean Cell Transfer Delay (Mean CTD) • Cell Loss Ratio (CLR) • Non-negotiable parameters • Cell Error Ratio (CER) • Severely Errored Cell Block Ratio (SECBR) • Cell Misinsertion Ratio (CMR) • QOS parameters are specified individually in UNI 4.0 • They were specified as classes in UNI 3.1 BISDN/黃仁竑副教授

7. CDV and CTD • CTD • fixed delay: propagation delay, transmission delay, fixed switch processing delay • random delay: queueing(buffering), scheduling delays • CDV • induced by buffering and scheduling prob. density fixed delay cell transfer delay Peak-to-peak CDV Max. CTD BISDN/黃仁竑副教授

8. CTD • Maximum CTD • (1-a) quantile of CTD • a is recommended to be CLR • granularity: • Mean CTD • arithmetic average taken over a sample of cell population BISDN/黃仁竑副教授

9. CDV • One-point cell delay variation • the difference between the cell’s reference arrival time (c) and actual arrival time (a) at the measurement point • Two-point cell delay variation • the difference between the absolute cell transfer delay between two MPs and a defined reference cell transfer delay • Peak-to-peak cell delay variation • (1-a) quantile of the CTD (worst case) minus the fixed CTD (best case) BISDN/黃仁竑副教授

10. CLR, CER, SECBR, CMR • Cell Loss Ratio (CLR) • Lost Cells/Total Transmitted Cells • Cell Error Ratio (CER) • Errored Cells/(Succ. Trans. Cells + Errored Cells) • Severely-Errored Cell Block Ratio (SECBR) • Severely Errored Cell Blocks/Total Transmitted Cell Blocks • Cell Misinsertion Rate • Misinserted Cells/Time Interval BISDN/黃仁竑副教授

11. Reference Model • Equivalent terminal reference model for traffic contract s1 TB TS PHY SAP Other CPE Functions Generating Traffic Deviations Physical Layer Functions UPC MUX Shaper Private UNI Public UNI sN ATN Layer PHY Layer Equivalent Terminal BISDN/黃仁竑副教授

12. Traffic Descriptor • A list of parameters which captures intrinsic source traffic characteristics • Peak Cell Rate (PCR) • 1/T, where T is the minimum intercell spacing in seconds • Cell Delay Variation Tolerance (CDVT): • number of cells that can be sent back-to-back at line speed is • Substainable Cell Rate (SCR) • maximum average cell rate • Maximum Burst Size (MBS) • maximum number of cells can be sent at peak rate • Minimum Cell Rate (MCR) • minimum cell rate that needs to be guaranteed BISDN/黃仁竑副教授

13. ATM Service Architecture (ATM Forum) • Constant Bit Rate (CBR) • require tightly constrainted delay and delay variation • Interactive Video/Audio, Video/Audio retrieval • Real-Time Variable Bit Rate (rt-VBR) • same as CBR but allows statistical multiplexing to impreove efficiency • Non-Real-Time Variable Bit Rate (nrt-VBR) • allows low cell lost rate and a bound of transfer delay • Response time critical transaction processing • Unspecified Bit Rate (UBR) • Available Bit Rate (ABR) • low cell loss rate but cell delay variation is not guaranteed BISDN/黃仁竑副教授

14. QOS Parameters, Traffic Descriptors, and Service Category • CBR • Traffic descriptor • PCR, CDVT • QOS • peak-to-peak CDV, Maximum CTD, CLR • rt-VBR • Traffic descriptor • PCR, SCR, CDVT(for PCR and SCR), MBS • QOS • peak-to-peak CDV, Maximum CTD, CLR BISDN/黃仁竑副教授

15. QOS Parameters, Traffic Descriptors, and Service Category • nrt-VBR • Traffic descriptor • PCR, SCR, CDVT(for PCR and SCR), MBS • QOS • Mean CTD, CLR • ABR • Traffic descriptor • PCR, CDVT, MCR • QOS • CLR • UBR • Specify PCR, CDVT, no QOS is specified BISDN/黃仁竑副教授

16. Conformance • Generic Cell Rate Algorithm • Virtual Scheduling Algorithm cell arrives at ta(k) TAT: Theoretical Arrival Time I: Increment L: Limit YES TAT<ta(k)? NO TAT=ta(k) YES Non Conforming Cell TAT> ta(k)+L? NO TAT = TAT + I Conforming Cell BISDN/黃仁竑副教授

17. Genetic Cell Rate Algorithm (GCRA) • Continuous-State Leaky Bucket Algorithm cell arrives at ta(k) X’ = X - (ta(k)-LCT) TAT: Theoretical Arrival Time X: Value of Leaky Bucket Counter X’: Updated Counter LCT: Last Compliance Time Note that capacity of Leaky Bucket is L+I YES X’<0? NO X’=0 YES Non Conforming Cell X’>L? NO X=X’+I, LCT= ta(k) Conforming Cell BISDN/黃仁竑副教授

18. PCR Reference Model s1 TB TS PHY SAP Other CPE Functions Generating Traffic Deviations Physical Layer Functions UPC MUX Shaper Private UNI Public UNI sN ATN Layer PHY Layer Equivalent Terminal BISDN/黃仁竑副教授

19. SCR Reference Model s1 TB TS PHY SAP Other CPE Functions Generating Traffic Deviations Physical Layer Functions UPC MUX Shaper Private UNI Public UNI sN ATN Layer PHY Layer Equivalent Terminal BISDN/黃仁竑副教授

20. CDV Tolerance • The maximum number of cells can be sent back-to-back at the full link rate BISDN/黃仁竑副教授

21. SCR and MBS • MBS is given by • Given MBS, Ts, T • Number of Cells can be sent in time t BISDN/黃仁竑副教授

22. Usage/Network Parameter Control • Cell flows must be checked, or “policed”, by Usage Parameter Control (UPC) at UNI or Network Parameter Control (NPC) at NNI. • UPC/NPC is also called policing. • Performance of UPC/NPC implementation is specified in relation to the leaky bucket conformance checking algorithm. • Possible UPC/NPC implementations • Leaky Bucket UPC/NPC • Sliding Window UPC • Jumping Window UPC BISDN/黃仁竑副教授

23. Leaky Bucket UPC • Same algorithm as the conforming check • no data buffer • For non-conforming cells • Monitor • Keeps track of how many cells were nonconforming • Tag • Tags non-conforming cells with CLP=1 • Discard • Disacrds non-conforming cells BISDN/黃仁竑副教授

24. Leaky Bucket UPC BISDN/黃仁竑副教授

25. Window-based UPC • Conforming criteria: no more than M cells in N cell times can be delivered • A window consist of N cell times • Sliding window • Window is moved to the right by Slide each cell time • Jumping window • Window is moved to the right by N units every N cell times • Exponentially Weighted Moving Average Mechanism (EWMA) • The max. number of accepted in the I-th window is a function of the allowed mean number of cells per interval and an exponentially weighted sum of the number of accepted cells in the preceeding intervals BISDN/黃仁竑副教授

26. Window-based UPC BISDN/黃仁竑副教授

27. UPC Differences Further Reading: E. P. Rathgeb, “Policing mechanisms for ATM Networks Modeling and Performance Comparison,” ITC, 1990. BISDN/黃仁竑副教授

28. Priority Control • Traffic with different QOS guarantees may require different priority queueing and scheduling policies. BISDN/黃仁竑副教授

29. Generic Flow Control • GFC is performed at UNI • Four bits are allocated for GFC; the first two bits are for control commands (start, stop, null) and the last two bits are for type • The multiplexer command terminals and terminals echo if they understand the commands. BISDN/黃仁竑副教授

30. Traffic Shaping • The source cell stream is shaped to conform the traffic contract (can be done at UNI and/or NNI) • Shaping Methods • Buffering (+ leaky bucket) • Spacing • Peak Cell Rate Reduction • Scheduling • Burst Length Limiting • Source Rate Limitation (circuit emulation) • Priority Queueing • Framing BISDN/黃仁竑副教授

31. Leaky Bucket with Data Buffer bucket size: b C source Maximum cells can be sent during time t: BISDN/黃仁竑副教授

32. Connection Admission Control • A connection request defines the source traffic parameters and requested QOS. The CAC function decides whether a connection request is admitted or denied. • Basic requirement for acceptance • The requested QOS can be satisfied • The QOS of existing connections can still be met • Can be done node-by-node or in a centralized system • Correlated to QOS guarantee and routing BISDN/黃仁竑副教授

33. Resource Management • Two Critical resources: buffer and bandwidth • Fast resource management • Managing resources dynamically (at burst level) • Fast bandwidth reservation • Bandwidth is reserved at each node along an end-to-end route • A request is sent for each new burst • Burst of cells is sent when granted ack is received • Bandwidth is then released after the busrt is sent • Works well only when the burst length is much larger than the RTD • Fast buffer reservation • Buffer is reserved instead of bandwidth BISDN/黃仁竑副教授

34. Congestion Control • Congestion • demand for resources exceeds the available resources • Impact of congestion depends on • Application characteristics • connection mode, retransmission policy, acknowledgement policy, responsiveness, flow control, ... • Network characteristics • queueing strategy, service scheduling policy, discard policy, route selection, connection mode, processing delay, propagation delay • Congestion occurs at three levels • cell level • burst level • call level BISDN/黃仁竑副教授

35. Congestion Control • Congestion may involve a single node or multiple nodes • Congestion control performance • Userful throughput (goodput) • Effective delay • Congestion collapse • When network is severely congested, goodput decreases as offered load increases. • An ideal congestion control scheme • No congestion collapse • When severely congested, the system maintains constant goodput (at maximum available capacity) and effective delay. BISDN/黃仁竑副教授

36. Congestion Control Categories and Levels Category Cell Level Burst Level Call Level Manage- ment Resource Allocation Network Engineering UPC Discard Overbooked CAC, Call Blocking Window, Rate, or Credit based Flow Control EFCI, UPC Tagging Avoidance Selective Cell Discard, Dynamic UPC Call Disconnect- ion, Operations Procedures Loss Feedback Recovery BISDN/黃仁竑副教授

37. Congestion Management • Attempts to ensure that congestion is never experienced • Methods • Resource allocation • Proper allocation of trunk capacity, buffer space • Resulting network utilization may be very low • UPC discard • Full booked CAC • CAC is done based on worst case • Network Engineering • Allocating resources based on long-term, historical trending and projections. BISDN/黃仁竑副教授

38. Congestion Avoidance • Attempts to avoid severe congestion, but continues to push the offered load into the mildly congested region • Methods • Explicit Forward Congestion Indication (EFCI) • EFCI bit is set if buffer exceeds a threshold • Destination should inform sender when EFCI is set (no BECN in ATM) • Relatively long round trip delay is a problem in high speed networks • UPC Tagging + selective discard (or dynamic UPC) • Overbooked CAC • Statisticaly QOS guarantee • Call Blocking BISDN/黃仁竑副教授

39. Congestion Avoidance by Flow Control • Desirable properities • Utilize bandwidth as much as possible • Fair to user access to the available bandwidth • Isolate conforming users from overloading type users • Flow control methods • Window-based flow control • TCP-like congestion control • Flow control window: determined by receiver’s available buffer size • Congestion window • Slow start + Multiplicative decrease • Rate-based flow control • Credit-based flow control BISDN/黃仁竑副教授

40. Rate-based Flow Control • End-to-end based control of transmitting rate at source • It can be augmented by adding intermediate nodes (domains) • Max-min fairness • RM cell fields • DIR (forward or return) CI (congestion indication) • CCR (current cell rate) ER (explicit rate) • Control parameters • ICR (initial cell rate) ACR (actual cell rate) • PCR (peak cell rate) MCR (minimum cell rate) • RIF, AIR (increase factor) RDF (rate decrease factor) • Two types of switch • EFCI, ER BISDN/黃仁竑副教授

41. Rate-based Segmented Flow Control Rate-based end-to-end feedback loop Destn Source Rate-based segmented control loop BISDN/黃仁竑副教授

42. Max-Min Flow Control • Max-min Fairness • All sessions shared a link should have the same bandwidth while the link utilization can be maximized • Max-Min  Maximizing network usage allocated to users with minimum allocation • Intuition: Maximize the allocation of each user i s.t. an incremental increase is its allocation does not cause a decrease of some other user’s allocation that is already as small as i’s or smaller. S0 C=1 C=1 C=3 r0=0.5 r1=0.5 r2=0.5 r3=2.5 S2 S3 S1 BISDN/黃仁竑副教授

43. Max-Min Flow Control • Model • G = (N,A) is a directed graph with |N| nodes and |A| links • P is a set of sessions, p denotes a session as well as the path traversed by p • rp : rate for session p • Fa : Flow on link a, Ca is the capacity of link a • problem: find rp such that max-min fairness is satisfied BISDN/黃仁竑副教授

44. Max-Min Flow Control • Definition • A rate vector r is max-min fair if it satisfies rp0, FaCa and for each p of P, rp cannot be increased without decreasing rp’ for some session p’ for which rp’  rp. More formally, let r* be some vector satisfies r*p0, FaCa . If rp < r*p then rp rp’ and rp’ > r*p’ • Property • For each session p, there is a bottleneck link a, s.t. Fa=Ca • rp is at least as large as the rate of any other session using that bottleneck link. • Max-Min fairness for ABR traffic? • Rate of ABR traffic is maintained between PCR and MCR BISDN/黃仁竑副教授

45. Max-Min Flow Control Algorithm Initialization Loop BISDN/黃仁竑副教授

46. Example S0 C=1 C=1 C=3 r0=0.5 r1=0.5 r2=0.5 r3=2.5 S2 S3 S1 Initial: Iteration 1: Iteration 2: BISDN/黃仁竑副教授

47. Explicit Forward Congestion Indication (EFCI) • At regular intervals, the destination would check if CI bit was set in the last received cell and send a backward RM cell to the sender to indicate congestion or not. • Linear increase • Sender receives an RM cell with CI=0, it would increase its rate by a fixed increment. ACR = min (ACR+AIR, PCR) • Multiplicative decrease • Sender receives an RM cell with CI=1, or does not receive RM cells for an interval, it would decrease its rate by an amount proportional to its current rate ACR = max (ACR/RDF, MCR) BISDN/黃仁竑副教授

48. Congestion Indication • How to determine a path is congested? • At least one of the switches on the path is congested • How to determine a switch is congested? • On-set threshold: turn the congestion on • Abatement threshold: turn the congestion off • Set the EFCI bit (2nd bit of PTI) data cell or RM cell traversed • How does a destination determine a VC is congested? • If the EFCI bit of the most recently received data cell is set BISDN/黃仁竑副教授

49. Explicit Rate (ER) Flow Control • ER switches • Set explicit rate in forward/backward RM cells • Algorithm • Source behavior • Destination behavior • Switch behavior • Many proposed algorithms • PRCA • EPRCA • ERICA • CAPC • Max-Min fairness (Charny, Tsang, …) BISDN/黃仁竑副教授

50. Proportional Rate Control Algorithm (PRCA) • Proportional Rate Control Algorithm (PRCA) • Rate of ABR traffic is maintained between PCR and MCR • Source behavior • Source starts with ICR (initial cell rate) • Source sends a RM cell to destination every Nrm data cells • After source received the returned RM cell • If CI is not set, increase rate by AIR and compensate the rate reduced additively in last cycle, but not exceeds ER or PCR • If CI is set, reduce rate by ACR/RDF, but not less than MCR • For each cell sent, source reduces rate by a fixed amount, ADR • Destination behavior • Destination sets CI bit if EFCI of last received data cell was set, change ER to desired rate and returns it to source • Switch behavior • Set CI bit, reduce ER rate if necessary BISDN/黃仁竑副教授